SSD planes were located upstream of the target and used to reconstruct beam ...... used to sweep the beam vertically over the calorimeter during dedicated cal-.
Fermi National
Accelerator
Laboratory
FERMILAR-Pub-93/007-E E709
Production of Direct-Photons and Neutral Mesons at Large Transverse Momenta by x- and p Beams at 500 GeV/c
G. Alverson et al The E706 Collaboration Fermi National Accelerator Laboratory P.O. Box 500, Batauia, Illinois 60510
January 1993
Submitted #
to Physical
Operated by Universities
Review D
Research Association
Inc. under Contract No. DE-ACOZ-76CH03000
wim the United States Department
of Energy
Disclaimer
This report was prepared as an account of work sponsored by an agency of the United States Gouernment. Neither the United States Government nor any agency thereof nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or seruice by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof: The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof,
Production
of Direct-Photons
at Large Transverse
and Neutral
Momenta
Mesons
by T- and p Beams
at 500 GeV/c G. Alverson,@)
W. F. Baker,c3) G. Balloccbi,(‘o+)
D. Berg,@) S. Blusk,@) C. Chandlee,(“‘)
C. Bromberg,@)
W. Dhgosz,(e)
W. Faissler,(“)
G. Fano~rakis,(‘~)
C. Johnstone,(3)
R. Miller,c4) E. Potbier,
P. Gutierrea,(‘)
A. Sinanidis,(‘)
The Fermilab (1) University
A. Lawa, A. MxI&(~)
J. Mans~ur,(‘~~~)
(3) Fermi National (4) Michigan
(8) Pennsylvania
and M. Zieliriski(‘o)
CA 95616
11 00 07, India Batavia,
East Lansing, Minneapolis,
IL 60510
MI 48824 MN 55455
University,
Boston,
MA 02115
of Oklahoma,
Norman,
OK 73019
State University,
University
of Pittsburgh,
Pittsburgh,
PA 15260
Rochester,
NY 14627
(10) University
of Rochester,
address:
address:
D. Weerasundara,@‘)
Laboratory,
of Minnesota,
(9) University
address:
P. Slattery,(“)
C. Yosef(‘,g)
Delhi
State University,
(7) University
D. Skow,(‘“,f)
- Davis, Davis,
of Delhi,
(6) Northeastern
K. Ruddick,(‘)
E706 Collaboration
Accelerator
(5) University
R. Roser,(“l
N. Va&s,(‘“)
of California
(2) University
(c) Current
J. Huston,(4)
I. Kourbanis,(ef)
G. Wu,(es’) T. Yasuda,(“)
(b) Cument
K. Hartman,@“)
B. M. Rajaram,(2~i)
W. Toothacker,@)
(a) Current
G. Ginther,@)
C. A. Nelson, Jr.,c3) B. Y. Oh,@) D. Orris,@~‘)
P. Shepard,@) R. K. Shivpuci,(‘)
J. Whitmore,@)
L. de Barbam,
D. Garelick,(@
P. Luken~,(~~‘) S. Mani,
E. Prebys,(‘“,h)
L. Smell,@)
T. Chand,@)
S. Easo,(R,d) E. Engels, Jr.,c9)
T. Ferbel,(‘o)
V. Kapoo~,(~)
F. Lobkowicz,(“)
D. Caky,
W. H. Chung,(B)
.I. Dunlea,
G. Glass,(e) M. Glaubman,(“)
C. L&&is,(e)
D. Brown,@)
B. C. Choudhary,(2’c)
W. DeSoi,(“)
R. Ber~son,(‘~~)
CERN,
LeCroy
1211 Geneva 23, Switzerland
Corporation,
University
Park, PA 16802
Chestnut
of California,
Ridge, NY 10977
Riverside,
CA 92521
(d) Current (e) Cumnt
(f) Current
address:
(g) Current
INFN,
address:
University
of Perugia,
address: Stanford University, Fermi
National
address: Michigan
(h) Current (i) Current
address:
Accelerator State Univemity,
address: Princeton
University,
Pemgia,
Stanford, CA 94309 Laboratory,
Batavia,
East Lansing, Princeton,
December
2
28, 1992)
IL 60510
MI 48824
NJ 08544
L. B. and S. B. 5’. College, Saga?, Karnatka (Received
Italy
57’7401, India
ABSTRACT We present
results
clude incident kinematic -0.7
from the initial
ranges of transverse
production,
momentumand
the nuclear
production
dependence
The cross sections are compared
different
rapidity
We have measured
as well as the v/r”
Cu, we have extracted A”.
experiment
with
choices of the QCD momentum
of 3.5 5 pi
Typeset
Using
13.85.Qk,
12.38.Qk,
ratio.
5 10 GeV/c
and
and direct-
parameterized
as
log QCD predictions
for
scales and several sets of parton
25.4O.Ve, 25.8O.L~; 13.85.Ni
3
and span the
From the data on Be and
of x0 production
next-to-leading
REVTEX
The data in-
cross sections for r”
functions.
PACS numbers:
E706.
and p beams at 500 GeV/ c on Be and Cu targets,
r-
5 ycm 5 0.7, respectively.
photon
run of Fermilab
distribution
I. INTRODUCTION The study of inclusive (pi) has been important perturbative with
methods
data.
single-hadron
to the development
functions
a quantitative
on the validity
of the parton
functions
aspects of direct, or prompt, in the literature
of direct-photons
provides
at large transverse
&CD, and can serve as a valuable
momentum
tool for probing
only the two processes shown in Fig. 1 contribute
calculations
and quark-glum
Compton
tons from the processes shown in Fig. directly
in which all particles
be individually
reconstructed
prompt
combination
To first order,
to the direct-photon
cross section,
scattering.
These diagrams capture unambiguous
higher-order
[4]. Since the pho-
at the elementary parton,
of the collision.
originating
photon produc-
structure.
of an outgoing
data are especially
with Deep Inelastic
sensitive
Scattering
vertex
This is in contrast
from the fragmenting
parton
to the gluon distribution
data can be used to map out the gluon distribution
four
diagram,
functions.
production,
function
to
must
parton
state gluon in the Compton
and Drell-Yan
of
the momentum
and then combined into a jet to determine
Because of the presence of an inital
photon
hadronic
1 are produced
reflects the kinematics
jet production, momenta.
of hadrons
[3]. The production
precise tests of theory
and not in the fragmentation
of the photon
ele-
a clean test of perturbative
of the process, and relatively
can be carried out to provide
the interaction,
of QCD matrix
distribution
tion at large pi have been addressed extensively
the essential characteristics
comparison
of partons.
and phenomenalogical
namely qTj annihilation
momentum
of QCD [1,2]. Large pr is a regime where
yield information
ments, and on the effective product Experimental
at large transverse
can be applied to QCD to provide
Such comparisons
and the fragmentation
production
In
direct-photon
over a wide range of
kinematics. Previous
measurements
center-of-mass the highest obtained
energies and incident
In the following photon
beam particles
[5-71. Our measurements results on proton experiments
sections, we describe the experimental
layout
and ?y” production publications
have covered a variety
at the ISR [6] an d in earlier fixed-target
used to extract
QCD calculations. recent
production
energy data for pion beams, and straddle
previously
procedures
of direct-photon
the #
and direct-photon
are compared
with
signals.
predictions
of
provide
interactions [7].
and the analysis
The results on direct-
from next-to-leading
log
The results presented in this paper expand upon more abbreviated of our work
[8], and provide 4
detailed
comparisons
with
QCD
predictions
at the next-to-leading
Fermilab
experiment
II. THE EXPERIMENTAL
SETUP
E706 is a second generation
fixed-target
to measure direct-photon end, a liquid
log level for both ?y”s and direct-photons.
production
and the associated
argon electromagnetic
was used to optimize
calorimeter
the reconstruction
was split into front charged-particle were initiated
shows the layout
of the various
spectrometer
was employed
spectrometer
components
To this
with
photons
simultaneously
(particularly
from
The EMLAC
against hadrons.
showers in the EMLAC
and to reconstruct
components
fine segmentation
signal).
discrimination
system was used to identify
by charged particles,
event structure.
to the direct-photon
and back sections to provide
tracking
designed
(EMLAC)
of overlapping
K’ decays, a large source of background
experiment
interaction
vertices.
that
Figure
of the Meson West spectrometer. by experiments
are discussed in the following
A 2
This
E706 and E672 [9]. The
subsections.
A. Beam Line The Meson West secondary tum recombining mass tagging. 800 GeV/c length
beam line.
It is capable of delivering
The secondary
protons,
aluminum
particles
extracted primary
beam line is a high resolution, utilized
a high intensity
in the experiment
from the Tevatron,
target.
two stage, momen-
incident
The components
beam with
were produced
by
upon an 0.77 interaction
of this beam line are shown in
Fig. 3. For the data presented 29 GeV/c
(RMS
here, the secondary
half-width).
The beam was occasionally
100 GeV/c for detector calibration at an angle of 0 mr to maximize particles,
purposes.
tuned
For positive
primary
proton
The negative secondaries were produced
the production
particle
yield was low4 per primary
beam from entering
per primary
angle was 1.4mr to prevent the intense
the secondary beam line aperture. proton.
to 500 &
to 25, 50 and
the yield, which for this run was 2 x lo-’
proton.
with a 23 set spill, during
beam energy was tuned
The Tevatron
which the experiment
typically
The positive
operated on a 57 set cycle, received 5 x lo7 secondary
beam particles. The intermediate plane.
The momentum
focus of the beam was momentum defining
The final focus, at the experiment
collimator
was located
target,
was momentum 5
dispersed in the horizontal at this intermediate recombined.
focus.
The second
stage of the beam line included were parallel differential
in both Cerenkov
The counter 7psia.
a 50m long straight
the horizontal counter
and vertical
used to identify
used a 42m long helium
planes.
This section
contained
the masses of the incident
gas radiator
a
particles.
in the pressure range from 4 to
Using this device, the negative beam was found to contain 97% x- and 3% K-
while the positive
beam was measured to contain
The beam line contained Their function upstream
five muon spoilers,
absorber,
of the target in the experimental
in Fig. 2) were positioned
[lo].
constructed
of magnetized
steel.
hall.
detectors
utilized
(SSDs),
Its purpose was to absorb hadrons
an analysis
magnet,
tracking
and proportional
wire chambers.
Six
beam tracks.
a transverse
to charged particles,
momentum was followed
wire chamber (PWC).
The SSD system was constructed
of seven X - Y modules,
The three modules upstream thick,
with two planes per
of the target, and the first one downstream,
consisted of 3 x 3 cm2 wafers, while the remaining the SSDs were 250 to 300 pm
of silicon
of the target and were used to reconstruct
plane) of N 450 MeV/c
by sixteen planes of proportional
6600 instrumented
system consisting
The analysis magnet, which imparted
(in the horizontal
to as veto walls
of the target and used to reconstruct
Eight SSD planes were located downstream vertex.
counters (referred
Tracking System
a charged particle
SSD planes were located upstream the interaction
the beam just
beam halo particles.
B. Charged-Particle The experiment
A
between the hadron absorber and the target to veto events
generated by any remaining
module [ll].
and < 2% K+
composed of 900 tons of steel, surrounded
in the beam halo. Two large walls of scintillation
impulse
91%p, 7%r+
was to deflect muons in the beam halo away from the spectrometer.
5m long hadron
strip
section, in which the trajectories
modules had 5 x 5 cm2 wafers. All
and had a pitch of 50 pm. There were a total of
SSD channels.
The PWC system consisted
of four modules,
with four planes per module
[12].
Each of the four modules had one X, Y, U and V view. The wires in the U-view were tilted
at 37”, and in the V-view
at -53”,
relative
to the vertical.
The sense wires
had a pitch of 0.254 cm. The active areas of the four modules were: 1.22 m x 1.22 m, 2.03 m
x
2.03 m, 2.03 m
of the liquid
x
2.03 m, and 2.44 m x 2.44 m, and covered the accepted region
argon calorimeters.
was fed through
a current limiting
The central region high voltage of each PWC plane resistor which reduced the sensitivity 6
of that region
during
high intensity
running.
The desensitized
area varied from 2.54 cm x 2.54 cm
in the first PWC module to 5.08 cm x 5.08 cm in the last PWC module. a total of 13,400 instrumented
PWC channels.
The combined
was designed to operate at a 1 MHz interaction
There were
PWC and SSD system
rate.
C. Liquid Argon Calorimeter The next element in the experiment ter (LAC)
[13], consisting
of an electromagnetic
lengths) followed by a hadronic the calorimeters
was a 3m diameter
(HALAC)
cryostat,
supported
to the beam axis. The calorimeters
line, with the front end of the EMLAC
located
calorime-
section (z 30 radiation
section (z 8 interaction
resided in a stainless-steel
be moved transverse
(EMLAC)
liquid-argon lengths).
Together,
by a gantry that could
were centered on the beam
N 900 cm downstream
of the target.
The active area covered a polar angle range of 1.3” < .9 < 10” as seen from the target. Within
the cryostat,
through
a 40 cm diameter beam pipe, filled with helium gas, was inserted
the center of both calorimeters.
a low density
(0.07g/cm3)
liquid
In front of the electromagnetic
argon excluder.
The purpose of both the excluder
and the beam pipe was to displace argon by materials to reduce interactions
that could complicate
The electromagnetic as shown in Fig. consisting
of substantially
lower density
event reconstruction.
section of the LAC consists of four independent
4. Each quadrant
of an absorber,
and another
section was
was constructed
0.25 cm of liquid
quadrants,
of 66 layers, with
argon, a copper-clad
each layer
G-10 anode board,
0.25cm argon gap. The absorber in the first layer was aluminum
while
for all other layers it was a 0.2cm thick lead sheet.
The lead sheets were made of
98.6% lead, 0.07% calcium
and tin were added to increase
the mechanical
rigidity
and 1.3% tin; the calcium
of the lead sheets.
The G-10 anode boards were octant size, with each octant read out independently. The anode boards had alternating a radial
board.
The radial
focused (in a projective containing
boards consisted
geometry)
radial layer was = 5.5mm. 92 instrumented
an outer region containing
radial and azimuthal
strips,
azimuth
(see Fig. 4). The inner/outer
readout
board)
The width
each subtending
x/192
spatial 7
with
cut to be
of the strips on the first into an inner region,
radians
strips each subtending
separation
starting
of 254 strips,
boards were divided
188 instrumented
allows for improved
of a maximum
on the target.
The azimuthal
strip geometry
in azimuth, r/384
and
radians in
(at a radius of 40.2cm on the first
resolution
at large radii
by subdividing
the outer strips by a factor by reducing
The inner/outer
of two.
radial/azimuthal
correlation
split also aids reconstruction
ambiguities
arising
from multiple
showers
in the calorimeter. Longitudinally,
the calorimeter
sists of 22 layers (about
is read out in two sections.
10 radiation
44 layers.
This front/back
showering
particle
lengths),
The front section con-
while the back consists of the remaining
split is used to measure the direction
and to help discriminate
of incidence
between electromagnetic
of the
and hadronic
showers. D. Readout System for the Electromagnetic Data acquisition using the RABBIT
and trigger-signal (Redundant
Instrumentation
Calorimeter
(LACAMP)
amplifiers a shielded
Faraday
The outputs amount
Group at Fermilab consisted
z 20 amplifiers
of charge deposited
on their
time for signals in the EMLAC
[14]. The Liquid
converters.
the circumference
Argon
of charge integrating
The LACAMPs
were lowithin
of the top of the cryostat.
consisted of voltage signals proportional respective
was N 350ns.
channel was delayed by SOOns, giving or reject the event.
system de-
per crate; the crates were positioned
room surrounding
from the LAC amplifiers
was performed
Transfer)
of 16 channels
and eight channels of time-to-voltage
cated in 20 crates with
for the EMLAC
Analog Bus Based Information
veloped by the Particle AMPlifier
processing
Calorimeter
strips in the calorimeter. The output
the trigger
The rise
from each LAC amplifier
the time required
When an event satisfied the trigger,
to the
either to accept
each channel was sampled
twice, once before the delayed pulse and once at the peak of the delayed pulse. This voltage difference was digitized
by one of two digitizers
per crate, provided
that the
absolute value of the voltage difference was greater than a predefined
threshold
zero suppression
took place be-
window)
stored on the digitizer.
cause the voltage difference fell within
If no digitization
the zero suppression
flagged as not having useful data, and the data acquisition
window,
(the
the channel was
system then proceeded to
the next channel. Timing
information
on the LACAMPs.
Their function
between the beginning this timing amplifier
measurement channels.
was provided
through
the time-to-voltage
converters
was to produce a voltage proportional
of a LAC pulse and the trigger was generated
by summing
signal.
the output
To avoid dead time caused by the arrival 8
(TVCs)
to the time
The signal used for of four consecutive
of a second pulse prior
to the readout
of the TVC, a second (slave) TVC circuit
was enabled while the first
was busy. E. Trigger System The trigger, the EMLAC,
which selected events depositing
high transverse
was designed to operate at lo6 interactions
was formed by weighting
the energy in the radial strips according to the distance from
four steps, which involved trigger
Figure
(pr) in
per second. The trigger pi
the center of this strip to the beam line (E sin 0). The triggering a final trigger.
momentum
defining
a beam particle,
5 shows the symbolic
process took place in
an interaction,
logic diagram
a pretrigger,
for the SINGLE
and
LOCAL
used in this analysis, whose elements are discussed below.
Beam
Definition
of the target
-
Two l/16”
x 1” x 1” scintillation
were used to define beam particles.
counters located upstream
A beam particle
was accepted if
both counters had a signal in coincidence. Interaction
Definition
the target
and covering
interaction.
The logical
beam definition in the definition
-
Four scintillation
the acceptance
counters,
located
of the spectrometer,
downstream
were used to define an
OR of the four counters was taken in coincidence
to define an interaction. of the pretrigger
An interaction
if this interaction
of
with the
strobe was generated and used candidate
satisfied the following
criteria: l
No other interaction
l
The interaction
occurred within
was not initiated
was no signal in the “halo” two beam defining
*60 ns of this candidate
by a particle
scintillation
counters.
interaction.
in the beam halo; that is, there
counter located just downstream
This 5” x 5” halo counter had a 3/S” diameter
of the hole
cut out of its center and was centered on the beam. Pretrieeer of each octant
Definition
Signals were generated for the inner and outer parts
using the sums of the weighted
signals were passed through p&rigger
-
threshold
than would normally
zero-crossing
(ZZ 1.7 GeV/c) be available
discriminators,
which
for the intrinsically a p&rigger 9
These
time (575 ns)
slow signals of the LAC.
zero-cross outputs
were used to obtain
strips.
served to define a
and t 0 set a more precise pretrigger
The logical ORs of the discriminated parts of each octant
energies in the radial
from the inner and outer
signal for that
octant.
The
following
additional
requirements
were placed on the final formation
of a p&rigger
for any octant: l
No pi deposition
in that octant greater than 1.5 GeV/c
within
z 300 ns prior to
the interaction. . Absence of a noise spike generated by the 400 Hertz power supplies for the LAC electronics. l
No incident
muon as defined by the logical AND of the signals from the two veto
XV&.
This p&rigger
signal was used to latch the LAC, PWC and SSD signals. The presence
of a p&rigger
was required
Trigger
Definition
electromagnetic
trigger.
The two main triggers
used to select events containing
showers of high pi were: (1) The SINGLE
the pi in a localized given threshold cal threshold
-
for any subsequent
region of an octant
(x 4 GeV/c),
and (2) the LOCAL*GLOBAL
was simultaneously
trigger,
and a GLOBAL
required
ment suppressed events that contained
(Z 4 GeV/c).
only low-pa photons.
two decay photons
to occupy any single LOCAL
LOCAL*GLOBAL
trigger
also recorded.
In addition,
a reduced
a TWO
local signals, one from any octant ing away-side employed
octants;
threshold
function
of pi.
GAMMA
requirements
(summed
The LOCAL
trigger
of the information
the various electronics
modules.
require-
angle between their (z
A prescaled
2.5GeV/c)
was
was formed by requiring
two
and the other from any of the three correspondtrigger. allowed
threshold
(FZ 2GeV/c)
was
The variety
of trigger
definitions
with
a measurement
of trigger
efficiencies
as a
If a LAC trigger was satisfied, a signal was sent to the online computers the recording
over
The LOCAL*GLOBAL
threshold
a reduced value for the LOCAL
for this TWO
different
GAMMA
in which the lo-
region, would trigger.
GLOBAL
in which
to be above a
threshold
trigger insured that ?y” OF 7 events, that had too large an opening with
trigger,
(16 radial strips) was required
for an octant was z 2 GeV/c,
the whole octant)
LOCAL
for the event. The trigger
Otherwise,
circuitry
z 30~s before being enabled for the next p&rigger. III. DATA ANALYSIS
10
to initiate
a reset signal was sent to
then allowed a settling
time of
During
the 1987 - 1988 Fermilab
0.5(0.8) events/pb corresponded LOCAL
was accumulated
with negative (positive)
of approximately
beam on Be; data on Cu
to about 10% of the Be sample. Only events which satisfied the SINGLE
trigger were included
off-line
fixed target run, a sensitivity
using the Fermilab
in the present analysis.
The events were reconstructed
ACP [15] sy st em. The following
the event reconstruction
procedures
losses due to various inefficiencies
and the methods
subsections
will describe
used to correct
the data for
and biases.
A. Event Reconstruction The event reconstruction track reconstruction procedures
for this experiment
and electromagnetic
in the following
Charved-Particle formed independent and downstream
procedure
included
shower reconstruction.
charged-particle We discuss these
paragraphs.
Tracking
-
The
charged-track
track segments, upstream
of the magnet
reconstruction
of the magnet using the SSD planes,
using the PWC planes.
These segments were then
linked at the center of the magnet to form the final reconstructed The first step in reconstruction
involved
algorithm
forming
straight
track.
line segments in each of
the four views (X, Y, U, V) of the PWCs, and in each of the two views (X, Y) of the SSDs. A “view-track”
was defined in the PWCs when at least three out of the four
available planes had hits associated with the track. with a track it had to lie within dimensional
coordinates,
A maximum space-track
In order for a hit to be associated
f 1.5 wire spacings of the trajectory.
termed “space-tracks”,
were formed using the view-tracks.
of 16 hits (4 planes and 4 views) could be associated had to have a total
Tracks in three
of at least 13 hits within
with a track.
A
?r 1.5 wire spacings of the
trajectory. After reconstruction
of the PWC space-tracks
PWC tracks and SSD tracks were projected magnet. that
After accounting
corresponded
linked
tracks.
spatially
2.5mrad.
The interaction
field, SSD and PWC tracks
at the center of the magnet were considered caused deflections
primarily
tracks in the Y view were also required
The charge and momentum vertex position
Y), and then correlated
by requiring
each of the
to the X-Y plane at the center of the
for the effects of the magnetic
Since the magnet
slopes of the linked
and SSD view-tracks,
candidate
in the X-Z plane, the to match
to within
of each linked track was then calculated.
was reconstructed
separately
in each view (X and
a match in the Z coordinate.
For each SSD view,
11
tracks that linked vertex.
Unlinked
with PWC tracks were preferentially The final interaction
of view vertices that matched Electromagnetic
independently
vertex position
Reconstruction
-
for each quadrant.
was determined
Electromagnetic
The readout
left Rand right R, containing
each octant in that quadrant,
and inner @ and outer a, containing
algorithm
operated
l
in the following
First, a search was performed,
. Third,
con-
the energies in the The reconstruction
within
each view, for energy depositions,
termed
(defined below). of these peaks were calculated
by fitting
to a
shower shape.
photons
A “group”
showers were
sequence:
Second, the energies and positions predetermined
by the pair
the energies in the radial strips of
strips for T < 40.2 cm and for T > 40.2 cm, respectively.
and “peaks”
of linked
in each quadrant
azimuthal
“groups”
number
the
best in the Z coordinate.
Shower
sisted of four “views”:
l
to reconstruct
SSD view tracks were also used if an insufficient
tracks were present.
reconstructed
employed
were reconstructed
was defined as consisting
by correlating
peaks from different
views.
of consecutive
strips, each with energy above
150 MeV, and with at least one strip with energy above 300 MeV, and a total group energy of more than 750 MeV. a group
that
had energy
A “peak”
minima
was defined by a sequence of strips within
(“valleys”)
such that
the maximum
was fitted
to the shape of an electromagnetic
simulations
on both
value was more than
2.5~~ above the valleys. shower determined
value,
Each peak
from Monte Carlo
and tuned to match showers in the data. The results of that fit include
the position
and energy of the peak.
approximately were required
Photons
were formed by correlating
the same energy in the R and @ views. (T > 40.2cm)
could only be correlated
peaks, and a peak in the @ view could only be correlated containing
of uncorrelated
- namely,
views a radial
with inner (outer)
with a radial
Q,
peak in the
that 4 value.
The correlation for complicated
peaks of
Peaks from different
to occupy the same general region of the detector
peak with T < 40.2cm octant
sides of a maximum
routine
in the electromagnetic
events containing
reconstruction
a large number of photons,
energy (not associated with any photon).
12
algorithm
leaving
may fail
a large amount
Such events were eliminated
by requiring
the total uncorrelated
energy for the triggering
quadrant
to be less than
10 GeV. A relative
interaction
time was calculated
for photons
tained from the TVC channels of the amplifiers
based on information
corresponding
to that photon’s
obenergy
deposition. B. Single-Photon Two of the largest potential from bremsstrahlung electromagnetic
and Di-photon
Identification
sources of background
to the direct-photon
signal are
of beam halo muons in the outer regions of the EMLAC
decays of r” and 7 mesons. The majority
(from beam halo) was eliminated veto wall that overlapped
by requiring
that satisfied the trigger.
of the muon background
that no hit register in the section of the
th e region of the electromagnetic
(shadowed)
and from
An off-line reconstructed
calorimeter
vertex was also required
for each
accepted event. Figure 6 displays the 7-y invariant LOCAL r”
triggers,
energy
requiring
asymmetry
mass in the ?y” and v mass ranges for SINGLE
a pi of at least 3.5 GeV/c
is defined
as A = IE,,
for the two-photon
- E,I/[E,,
pairs.
The
where ET1 and
+ E,,],
42 represent the energies of the photons. The lower points, with the dashed error bars, represent the mass spectrum for A < 0.75. For OUT study of # production, a ?y” was defined as any two-photon M,,,
in the range llOMeV/c’
combination
with
A < 0.75, and invariant
< My7 < 160MeV/c2.
Both photons
to occupy the same octant.
An 7 was defined as any two-photon
A < 0.6 (for direct-photon
background
and 450MeV/c2
< Myy
analysis was determined acquisition
period.
signals, sidebands and 7 peaks. distributions
estimates
by the trigger threshold
To account
for combinatorial
pi
within
in the same octant
candidate
to form a no or q with
be discussed in a following
the no and 7
to the trigger
13
signals.
an A < 0.75. The residual
The minimum threshold
from the
if it did not combine with another photon
using our Monte Carlo simulation
section.
data
mass range of the #
the K’ and 11mass ranges to obtain the respective
from ~“6 and I)S was calculated chosen to correspond
under
from these side bands were then subtracted
A photon was a direct-photon
in the
used during the corresponding background
with
was A < 0.75)
value employed
were selected which covered the equivalent
Distributions
were required
combination
this requirement
Th e minimum
< 650MeV/c*.
mass,
background
program,
which will
pi value in the analysis was again
set in each particular
running
period.
We defined a directionality
parameter
where Tf, Tb are the reconstructed zf and z6 are the distances calorimeters
for photons
front and back radial positions
of the front
to the target.
Photons
originating
from the target parallel
have directionality
to the beam line have large
direction&ties.
Directionality
and in-time
dates in order to minimize the veto wall criteria
background
were imposed on all direct-photon
from muon bremsstrahlung vertex was found.
could not be reconstructed,
and have good directionality. out of time with
requirements
when an interaction
time or directionality
Figures
in the veto wall (in the region corresponding
from the target are frequently
to the triggering
Applying
essentially
In cases where the photon
for events with and without
signal in the veto wall produces a large concentration most of these showers, leaving
events that passed
7 and 8 show plots of the single-photon
versus directionality,
also have large directionality.
candi-
they were assumed to be in-time
Showers not originating
the interaction.
arrival time at the EMLAC
that
of the shower, and
and back sections of the electromagnetic
close to zero, while showers from muons traveling positive
as follows:
octant).
of out-of-time
Requiring
a
showers (muons)
the veto wall requirement
only in-time
a signal
showers (photons)
eliminates originating
in the target region. To eliminate nal, we required projected
possible electron
or hadronic
that accepted showers not spatially
to the LAC
(within
lcm
(Et/E,
electrons (and showering
charged hadrons)
in the front
z 8% of the candidate
cross sections.
single-photons
and that
at least
section of the electromagcut serves to reject
and is not an isolation
of direct-photon
sig-
overlap with any charged track
> 0.2). Note that th’ 1s t rack projection
ployed in collider measurements cut eliminates
to the direct-photon
of the center of the shower),
20% of each shower’s energy be contained netic calorimeter
background
cut in the sense emThis track projection
from our sample.
C. Energy Scale Due to the steep pi
dependence
the energy scale of the EMLAC section
at a specific energy.
determined
of inclusive
can produce
cross sections,
large changes in the measured
Because of this sensitivity,
with great care. One possible procedure 14
a small change in cross
the energy scale must be
is to set the energy scale such
that the reconstructed be compromised overlap
n-Omass is centered at the accepted value.
by the sensitivity
of the two photon
of the reconstructed
showers originating
the EMLAC describing
can be calibrated the different
how the momentum The tracking p+pL-.
from ?y” decay.
photons,
using the decays: Ki Ki
Before
we will describe
+ ?T+?T- and Jill,
decays were obtained
curvature,
when these tracks were interpreted
mass distribution
momenta.
events were selected by the E672 [9] dimuon
tices that had two tracks of opposite of a Ki,
charged-particle
system was established.
system was calibrated
by the muon spectrometer.
we set the
from a ?y” decay converted
applied to reconstructed
scale of the tracking
Instead,
Using these events, the energy scale in
from reconstructed
corrections
The J/$J + p+p-
identified
of the magnet.
can
mass value to the degree of
energy scale using events in which one of the photons into an e+e- pair upstream
This method
for events in the Ki
--t
trigger
and
from secondary
ver-
with an invariant
mass close to that
as pions. Figures 9 and 10 show the r+?r-
mass region,
and p+p-
mass distribution
in
the J/y5 mass region. The first energy correction account
of the difference
pedestals
determined
for showers detected
between
the pedestals
from calibration
run. A fit was performed
in the EMLAC
estimated
tasks frequently
carried
out at the start of a
reconstruction
in each channel code) to determine
the mean pedestal levels expected in the data. Special events, requiring definition
(and no interaction),
250MeV
were observed for some channels.
stable over the course of the data taking, shift corrections differences
photon
energies in each octant
kinematic
in the LAC.
Octant-to-octant
as large as
these shifts were found to be
and consequently
a single set of pedestal
This
in the energy scale caused by octant-to-
correction
was implemented
by resealing
by a factor based on the reconstructed variations
had a range of 6%.
the
no mass for
Only ?y” events in a
region where the ?y” decay photons were separated by more than 3 cm were
used to set this relative problems
However,
was for variations
octant
octant.
Pedestal differences
biases
was used for all the data.
The next correction
that
only a beam
were used for this purpose in order to minimize
caused by the presence of energy in the calorimeter.
taking
from the data and the
to the tails of the pulse height distribution
(when no peak was detected by the electromagnetic
involved
energy scale in order to minimize
the sensitivity
to possible
in energy sharing between the photons.
An additional
correction,
which had to be applied 15
separately
to single-photons
and x’s, arose from energy losses at the inner/outer from Monte
Carlo events generated
within
@ boundary.
~I~lOcrn of this boundary,
an empirical
correction
reconstructed
energy and distance to the boundary.
The Monte
for the average reconstructed
Carlo was also used to generate
showering in the material ter.
This correction
function
determined
to the reconstructed
decay in which one photon
the energy determined An empirical
E/P
=
1.0 for all energies. scaled by the relative
converted
a function
energy. The photon
has all the final corrections for the momentum
as a
via elec-
The ratio of
to the momentum
(E)
as a function
of electron
from the requirement
enthat
distribution,
E/P
correction
and
was applied
energy loss in the material
to
in front
calorimeter.
# mass for the three particle
of photon
calorimeter
A similar
to electron
of the active region of the electromagnetic The resultant
and electrons,
into an e+e- pair.
11 shows this corrected
all adjustments. photon
calorime-
showers.
to E was then calculated Figure
its average value, after making photons
for photons
system (P) was obtained
correction
of
for energy lost due to
shower energy was determined
using the electromagnetic
by the tracking
ergy.
energy loss as a function
a correction
independently
of energy, and applied to reconstructed
trons from #
and provided
in front of the active region of the electromagnetic
was determined
The final correction
This was calculated
ye+e-
system is shown in Fig. 12 as
energy used in the calculation
while the e- and e+ momenta
scale of the tracking
system.
of the mass
have the final corrections
The agreement
with
the accepted
value of the ?y” mass is better than 0.5%. A correction reconstruction EMLAC. which
was also applied
that depended on the spatial separation
These corrections
indicated
that
were determined
for energy losses in
of the two decay photons in the
from Monte Carlo studies of #
the electromagnetic
IT” energies for cases of highly uncertainty
to the ?y” energy to account
reconstruction
overlapping
showers.
algorithm
underestimated
This contributed
a systematic
in the # cross section of less than 1%.
Using all the above corrections,
the #
and q masses in the 77 decay modes
were found to be 135.26 f 0.06 MeV/c’
and 545.06 f 1.38 MeV/c’,
within
The difference
0.5% of their
accepted values.
respectively,
between the reconstructed
and 7 masses and their accepted values, combined with the uncertainty ~SSS
decays,
for the K’ (Z 0.5%), and the uncertainty
system (= 0.5%), results in a systematic
in the momentum
uncertainty 16
both ?ya
in the ye+e-
scale for the tracking
in the overall
energy scale of
N 0.9%, which contributes cross sections. provides
Figure
= 10% uncertainty
13 shows the invariant
an independent
the establishment
?y”s and single-photons.
cause r” mesons provide be properly
accounted
The contribution was determined
using electron
7/e-
This consistency
to the systematic
is particularly
important
signal, and must production.
The normalization
of the function
runs at 25,50 and 100 GeV/c.
their beam uncertainties and varying
By varying
(and thus the overall normalthe magnitude
given by the Monte Carlo, an estimate
in the cross section was obtained.
be-
in the cross section from inaccura-
as follows.
calibration
response function),
energy loss function
uncertainty
energy scale between the
to the direct-photon
uncertainty
was estimated
energies within
of the E/P
of a consistent
for to obtain the cross section for direct-photon
function
ization
~“7 mass in the w mass range which
the main background
cies in the E/P these electron
x0 and direct-photon
check on our energy scale. The value of 782.6 f 8.0 MeV/c’
for the w signal confirms reconstructed
to the absolute
of the uncertainty
The result was a contribution
in the 7 cross section of 12%, and 4% in the #
of the relative
to the systematic
cross section.
D. Monte Carlo Simulation The relatively surement
small cross section for direct-photon
particularly
sensitive
to backgrounds
production
from decays of hadrons
photons in the final state. The largest of these backgrounds losses due to the acceptance of the EMLAC, from asymmetric
x’s,
that must be estimated
geometrical
low energy photons
properties
single-photon
of the spectrometer
used the GEANT
a data base of standard
in the standard
at a level
incident
y or electron
eterized
shower simulation
[16] package shapes and
All the essential
of electromagnetic
showers are
program.
parameterization
generate complete
geometrical
that can be used to model a specific detector.
physics processes that take place in the development An empirical
background
by Monte Carlo simulation.
developed at CERN, which contains
were established
arise from:
failure to reconstruct
Such sources provide
The Monte Carlo simulation
represented
which yield
T” OI u decays; and coalescence of nearby photons from symmetric
decays of high-energy
material
makes its mea-
was developed
energy in the appropriate
to simulate strips
the deposition
of the EMLAC.
of the
A param-
was chosen because of the large CPU time required
electromagnetic
by a comparison
showers.
The parameters
of data with the full-shower 17
to
used in the simulation Monte Carlo, and were
tuned by varying teristics
the input
shower shape and parameters
(such as the zero suppression
were established
window
by the Monte
data events.
After
the simulated
events and the data, and the resultant
assess backgrounds simulation
the parameters,
to the direct-photon
Test criteria
Carlo simulation
good agreement
signal.
charac-
was achieved
Monte
with
between
Carlo was then used to
Each of these aspects of the overall
process is described below in more detail.
Simulation
of Detector
Response
shower generation
were determined
using the GEANT
full-shower
rameterized
-
and 100GeV
capability.
The lateral
obtained
simulations
during
values of Er/Et
for the front
calibration
of the detector
was also determined
(Et/E,)
response.
GEANT
simulations
to adjust the front to total energy ratio of individual
energies were modified tuations
Sian distribution
with
to incorporate
window
used during
and the presence of inactive in the simulation. of crosstalk
(
3.0 GeV/c
the momenta exception
were used as the Monte Carlo input
of all observed photons
that two-photon
as individual
photons,
combinations
Carlo equivalents
original
of all particles
then allowing
background
same production
were generated
by a random
the remainder
with
by simultaneously
For each rotating
the
angle around the beam direction,
and
from 7, 7’ and w decays, we assumed the simply replaced by particles
of the event kinematics
n”s in
of the appropri-
unchanged.
This was done
because the measured signals for states other than the r” had large backgrounds. assumption
of similar
data on relative
v/r0
production
the
were not treated
for these mesons as for n’s, so the simulated
the Monte Carlo sample were therefore ate mass, leaving
Carlo,
in their rest frames.
contributions
characteristics
in the Monte
with a mass value of 135MeV/c’.
the K’S to decay randomly
To simulate
events. For each such event,
with mass < 175 MeV/c*
but rather as r’s,
event, five Monte momenta
were included
and
properties
for the 7 and x0 is consistent
yields, which as we shall shortly
The
with our
see, do not appear to depend
on PT.
In order to increase statistics with the original pi
> 4.0GeV/c)
tributed
at the highest-p?
event structure,
values, events were also generated
but with leading particles
boosted in pi by an additional
to the boosted samples yielded
(#,v,
1.25GeV/c.
a result consistent
. . . that had initial The background
with the original
at-
sample
in the region of overlap. To account for the systematic suppression
loss of low energy showers (a consequence of the zero
employed in the LAC readout),
low energy photons
from PYTHIA
[17]
generated # events were added to the event driven Monte Carlo in order to duplicate the particle
and energy distributions
Acceptance reconstruction described
and Reconstruction efficiencies
Efficiency
were determined
above. Except for the geometric
with large inefficiencies real events. abutted
observed in the data.
were excluded
In particular,
steel support
radius of the EMLAC,
Geometric
acceptance
using the sample of Monte acceptance
calculation,
from consideration
regions of the detector
plates),
-
regions near the central
and regions between octants 19
Carlo events
kinematic
for both Monte
near quadrant
and
regions
Carlo and
boundaries
(which
beam hole and at the outer
were not included.
The geometric rapidity.
acceptance for r’s and ys was determined
The ratio of the number
of ?y” decay, found within determined
the fiducial
as a function
for PT. To determine
of single-photons,
region to the number
single-photons
and #s in the data.
and x’s was determined
The reconstruction
in a similar
and reconstruction independent
had considerably
reconstructing
r’s
simulations
was
interpolation
larger
efficiency
for single-photons
manner using the same reconstruction
algorithm
14 shows the average weight for the combined for nos. The single-photon
corrections
and yc,,,, with
of pi
weights were
an average value of about
average weights,
from two photons,
photons had to fall within Comparison
that were generated
The inverse of this acceptance ratio was used to weight
employed to analyze the data. Figure
r’s
pairs in the case
the acceptance at any given pi and ycm, a quadratic
on this grid.
essentially
OI two-photon
of pi and
of pi and ynn, using a grid of 0.1 units for yc,,, and 0.5 GeV/c
was performed
acceptance
as a function
which
reflects the larger
and the fact that
1.25.
The
challenge
to reconstruct
a r”
of
both
the acceptance of the EMLAC.
of Data
and Monte
Carlo
agree with the data, a set of criteria
-
To verify
that
the Monte
Carlo
were used to compare the results of
the Monte Carlo with data; these are discussed below. Partially
reconstructed
or misidentified
direct-photon
background.
An indication
the two-photon
energy asymmetry
subtracted
asymmetry
asymmetry
values is attributed
corrected
account
for about
distribution.
Figure
Monte
Carlo and data.
plot for both
80% of the total
of the loss of n”s can be obtained
source of background
from
15 shows the backgroundThe fall off at large
to losses of low energy photons.
between Monte Carlo and data indicates that this dominant
r’s
The agreement
that the ?y” loss is properly to the direct-photon
simulated,
and
signal can be adequately
on the basis of the Monte Carlo calculation.
Another
important
comparison
front of the EMLAC
relative
not make a direct
contribution
the ratio of reconstructed
to the total (Ef/E,).
can influence
the reconstruction
other neutral
mesons. Figure
Carlo simulation
involves
Although,
to the direct-photon of photons,
16 shows the reconstructed
and data for different
photon
energies.
good over a wide range of energies, with a significant
the value of Ef/E,
background,
and thereby
energy in the
shifts in (Ef/E,)
the identification (Ef/E,)
does
of r’s OI
ratio from Monte
The agreement is relatively
discrepancy
only at the lowest
energies. Another
indication
of the performance
of the Monte Carlo is the comparison 20
of
x2 distributions
for fitting
photons
ference in x2 distributions in the reconstruction for reconstructed
for data and Monte Carlo could imply
suggests that there are no major
(which
This is plotted
contribute
by the comparison
in Fig.
that the Monte
Carlo properly
to the combinatorial
data that were not simulated for by a 2.6 % correction background
in the Monte
Sivnal
the background
in the plot).
photons
The small difference contributions
in the
This difference was compensated -
To determine
that satisfied the direct-photon
events (which contain no direct-photons)
-0.2
single-photon
as (+y/?~)b~k,was used to estimate
single-photons
of pi for four rapidity
as a function
< ycm < -0.2,
criteria,
the number
of
the Monte Carlo
were processed using the same analysis code
the data. The reconstructed
r” yield were then obtained ratio of the simulated
mass
are also in good agree-
to non-Gaussian
Carlo.
invariant
to the # cross section.
single-photons
ycm < 0.7, -0.7
represents
background
to the Direct-Photon
employed for analyzing
of the two-photon
18. These distributions
observed in the tails of the ?y” peak is attributed
Background
differences in reconstruction
Carlo.
measure is provided
ment, indicating
a possible difference
photons from Monte Carlo and data. Again, the agreement between
between data and Monte distribution.
dif-
efficiency between the two. Figure 17 shows the x2 distributions
the two distributions Another
to the assumed shower shapes. A systematic
and the accepted intervals:
-0.7
50%. This method a purely
Monte
of individual
for determining Carlo method,
trigger
corrections
is more appropriate
because these corrections
are based on the structure
events rather than on averages over many events. However, for unusual
decay configurations
this procedure
can occasionally
lead to large event weights.
situations
were handled by using the magnitude
correction
only when it was less than 2, but using a calculated
termined
by implementing
corrections
the trigger
that exceeded this limit.
of the individual
in the Monte
Carlo)
The average correction
this trigger for any given region of the detector, as described
in the previous
account for any variations whether
paragraph.
in the trigger
corrected
automatically
The systematic
variations
for trigger
uncertainty
in two ways. The first method trigger thresholds;
higher trigger trigger
performance
trigger
of pi and
respectively. required
for analysis of
weight was calculated,
weight is then calculated
to
of other octants by determining
the event would have satisfied the trigger in the other octants as well. This
weight accounts for threshold
different
a preliminary
An additional
average value (de-
as a function
When the pr observed in an event exceeded the minimum
Such
event based trigger
for events with
is shown in Figs. 20 and 21 f or r” and single-photons,
rapidity
than using
threshold
threshold
and dead channels in other octants.
variations
introduced involved
at fixed rapidity by the trigger
comparing
We thus
for all octants.
corrections
was estimated
no cross sections from runs with
the cross section in the turn on region for the run with the
was compared to the cross section for the run with the lower
which was nearly
100% efficient in this region.
23
The second method
involved
shifting
to determine paring
the trigger
the sensitivity
runs with
different
pi corresponding atic uncertainty
of the cross section to the threshold trigger
to a trigger
thresholds
efficiency
was parameterized
the pi dependence corresponding
to a trigger
using a function
efficiency
an uncertainty
Com-
of 2.5% at a of this system-
whose shape was determined
corrections,
normalized
of 80%. Although,
of these systematic
the first procedure
indicated
measurement.
of 80%. The pi dependence
of the average trigger
much lower estimates through
turn on curve for a given run by one standard-deviation
uncertainties,
by
to 2.5% at the pi
the second technique the uncertainties
yielded
estimated
were used as upper limits to the actual uncertainties
from
the trigger. Due to inefficiencies
in the pretrigger,
17% for the backward
rapidity
an additional
pi independent
region and 2% for the central
regions was applied to each cross section, which contributed uncertainty
correction
and forward
an additional
of
rapidity
systematic
of 2%. F. Other Corrections to the Cross Sections
In addition invariant l
to the corrections
cross sections also include
Absorption material
of beam particles between
Vertex reconstruction examination
corrections - This
the beam counters
The average correction l
discussed in the preceding for the following
correction
effects:
was calculated
and the interaction
point
based on the in the target.
was 6%. inefficiency
- This correction
was determined
of events that did not have a reconstructed
was found to be linear in vertex position, end of the target,
sections, the reported
vertex.
from a visual The correction
decreasing from 1.09 at the upstream
to 1.04 at the downstream
end, with an average correction
of
6.5%. . Online
veto due to backscatter
which particles thereby
vetoing
from the interaction the event.
target configuration. which
did not include
in which
- Corrections
were emitted
The magnitude
The correction veto-wall
were made for losses of events in backward
of this correction
was determined
depended
using minimum-bias
signals in the trigger.
the veto wall had a signal in time was studied
24
into the veto walls,
The fraction
on the events
of events
as a function
of the
number
of interaction
correction
l
counters
that had signals.
for the number of accidentally
direct-photon
and rr” triggers is comparable
Losses due to the off-line criteria
This correction
to the triggering
of the EMLAC.
Losses from
with good events were corrected by determining
the number
quadrant
by the application
averaged 8.5%. The same correction
Losses due to the uncorrelated above, was used to estimate small uncorrelated
of the veto-wall
was used for direct-photons.
energy requirement
- A similar
procedure
the loss of events from the requirement
energy in the trigger
quadrant.
This
to the
of having
correction
averaged
Losses due to 7 conversions
in material
downstream
of the interaction
- The correction
conversions
downstream
of the vertex
for photon
by tracking
individual
tion lengths
of material
Asymmetry
criteria.
0.6%.
corresponded l
events.
on signals in the veto wall - In order to eliminate
of good R’ events that were eliminated
l
for
by a muon, an event was rejected if either of the veto walls had
random coincidences
approximately
to that for minimum-bias
a
which
to losses of x 3.5%. We assume that the correction
typically
a signal corresponding
l
vetoed events due to backscatter,
corresponded
events triggered
This was used to generate
photon
trajectories
the photon
traveled
to determine through.
the number
vertex
was found of radia-
The average correction
to 7% per photon. cuts applied to rr” and r~ candidates
were applied
to two-photon
pairs in the ““(7)
samples of data for the ?y” and 7 cross sections.
- Asymmetry
cuts of 0.75 (0.6)
mass regions The isotropy
to obtain
clean
of r” and 7 decay
was used to account for these losses. l
Losses due to restrictions and the restriction were applied photon
signal.
l
directionality,
photon
arrival
time, Ef/E,,
that charged tracks not overlap with showers - These criteria
to minimize
background
Only a small number
which a correction from #
on photon
was estimated
decays. The correction
Losses of direct-photons
from muons and hadrons
in the direct-
of photons were removed by these cuts, for
by gauging their effect on photons
originating
was 5.5% for direct-photons.
due to random combinations
with other photons result-
ing in a pair in the x0 or 7 mass band - This loss was estimated 25
from the Monte
Carlo by replacing mentum.
?y”s or 7s in an event with
The events were then reconstructed,
that were lost was determined
as a function
to correct the observed direct-photon
single-photons
of the same mo-
and the fraction of pi.
of single-photons
This dependence
signal; the correction
was used
factor was typically
10%. Table I summarizes both beam polarities, corrections
the corrections
discussed above, for x0 and y cross sections, for
on Be. Except for the overall trigger and acceptance corrections,
to the 7 cross section are similar
The net systematic
uncertainty
to those for x0 production.
in the overall normalization
due to the corrections
listed above, as well as from several smaller effects (at the 1% level) that have not been described,
is N 10%. G. Summary of Systematic Uncertainties
The primary sources: (E/P)
contributions
uncertainty function,
in parameters (including
in the determination
uncertainty
in the magnitude
related to background These systematic
These systematic
are summarized net uncertainty
tables.
in the direct-photon
energy
corrections,
response),
uncertainty cross section
and the overall
have been discussed in the preceding
for the direct-photon in quadrature,
Combining
as given by the
for the direct-photon
of the detector
uncertainties
uncertainties
arise from the following
of the trigger
subtraction
in Table II and, combined
sections in the appropriate
uncertainty
of the EMLAC
such issues as proper modeling
normalization. sections.
to the systematic
and rr” cross sections
are quoted with the cross
these systematic
uncertainties
cross section of N 25% at a pi of 4 GeV/c,
decreases to x 16% at a pr of 8GeV/c;
yields a which
for the x0 cross section, the uncertainty
is
x 11% for all pi values. Based on the analysis
of the final distribution
regions, and results from ye+e-
of 77 pairs in the rr” and n mass
studies, we estimate an upper limit on the uncertainty
in the energy scale of 0.9%. This represents
an uncertainty
contributes
to the cross sections, which has not been
included
an additional
N 10% uncertainty
in the energy scale, and
in the tables.
The central values of the beam momenta were uncertain an uncertainty
(FZ 10%) in th e c al cu 1a t’ion of the theoretical
26
to N 2%. This introduces expectations.
IV. CROSS SECTIONS A. Neutral Mesons Tables III,
IV and V show the invariant
duction
in 500 GeV/c
rapidity
as well as the full rapidity
comprehensive
x-Be
cross sections per nucleon
and pBe interactions,
respectively,
range, as a function
than those given in our previous
of pi.
for r” pro-
for three regions of These results are more
abbreviated
publication
[8]. Figures
22 and 23 show the cross sections for the same data sets averaged over the rapidity range -0.7
< ycrn < 0.7.
The curves in Figs.
22 and 23 represent
by Aversa et al. [18], using parton
based upon code provided for the pion and nucleon are illustrated
next-to-leading-log
from Aurenche
et
al. [19].
for two choices of the scale (Q’
between data and theory x- and proton
QCD calculations distribution
The calculated
functions
cross sections
= p$ and Q* = &/4).
(for the smalIer choice of scale) is reasonable
Agreement for both the
data over the full pi range shown. r
The analogous over the rapidity
results for ?y” production
in K-Cu
and pCu collisions,
averaged
< gc,,, < 0.7, are given in Table VI, as a function
range -0.7
of
PT.
A Dependence
nucleon.
-
However,
The cross sections for rr” and 7 production
effects such as nuclear
nucleus can result in deviations nucleon number.
in the
of the cross sections from linear dependence
on the
The form A”, with a being a parameter
both beams are consistent
is often used to parameterize
on Cu and Be targets.
with one another,
are consistent
with
‘The calculations used preliminary al. via private communication. fits.
in question,
that can depend on rapidity,
Table VII shows the measured values of a determined
for T- and proton induced collisions The results
or secondary
previous
and indicate measurements
x0 fragmentation
E706 x0 production
function,
The average values of a for a small deviation
27
from unity.
by the Chicago-Princeton
functions provided by J. P. Guillet et
data were included in the fragmentation
while the quark fragmentation
by results from e+e- data.
the A-
for r” production
However, data from UA2 and the ISIt dominated the determination
fragmentation
per
scattering
pi, and on the nature of the particle dependence.
shadowing
are reported
function
of the gluon
was constrained primarily
group [20], as well as recent measurements similar
from Fermilab
experiment
E605 [21], for
regions of PT. The curves shown in the Figs. 22 and 23 have been calculated
using a value of cx equal to 1.08. The value of (Y was not well determined inadequate
statistics
direct-photon
for direct-photon
value of a for direct-photon x0 Production
targets,
and are expected to yield a
production.
Ratio
-
The corrections
using methods similar
necessary to measure 7 production
to those already described for nos. The mea-
sured ratio of T/T’
production
is shown in Fig. 24; within
ties no significant
dependence
on pT or on incident
value for the 7/x0
production
ratio is 0.44 & 0.05 f 0.05 for r-Be
0.44 !c 0.06 f 0.05 for pBe interactions. of the same magnitude uncertainties
Systematic
as the statistical
in the trigger corrections
the rather large uncertain-
beam is observed. uncertainties
uncertainties, measurements
actions
26 displays
as a function
and
on these results are reflect residual
1221. Although
our pBe re-
all the data are compatible
uncertainties. B. Direct-Photon
Figure
interactions
and primarily
sult is somewhat lower than those of the ISR experiments, their statistical
The average
and in the energy scale. Figure 25 summarizes
our results and those of selected previous within
due to
for data on Cu. Recent runs of E706 have a far larger sample of
events from Be, Cu and hydrogen
were determined
production
the observed 7/rr” of pT. Photons
regions (with energy asymmetries sample; however, no isolation
Cross Sections production
contributing
ratio for x-Be
and pBe inter-
to 77 pairs in the rr” or 7 mass
less than 0.75) are excluded from the direct-photon
cuts are imposed.
The bands shown in the figure repre-
sent Monte Carlo estimates of the residual contributions
to these ratios from photons
originating
is due to ?y” decays and the
from meson decays; 80% of this background
rest is dominantly uncertainties
due to 7 mesons. The width
on the Monte Carlo calculation.
The background sponding
estimates
shown in Fig.
26 are lower than those in the corre-
figures in our earlier paper [8]. Although
a shift downward
of the midpoint
est values of pi, and essentially moreover
of the bands represent the systematic
the direct
photon
the data points in Fig.
the difference is relatively
of the band by about 30% of its width
small
at the low-
no change at high pi - the effect is systematic,
signal, which is proportional
26 and the estimated 28
background,
to the difference is sensitive
and
between
to even such
small effects. This change, which results from an overestimation from r” decays in our earlier analysis, has been incorporated sections presented below, which as a result are slightly pi
(by 60% of the estimated
systematic
of the contribution
into the inclusive
cross
higher at the lowest values of
uncertainties)
than those presented
in our
earlier paper. Tables VIII, photon
IX and X show the invariant
production
a function
in r-Be
of PT.
Figures
over the full rapidity to-leading-log functions
