âfPhysics Department, California State College, San Bernardino,. California 92407, USA. TtDepartment of Physics and Astronomy, University of Hawaii, Honolulu ...
SLAC-PUB-1205 (E) March 1973 THE REACTION 3/p - 7;tn-lr+s-p AT HIGH ENERGY AND PHOTON DISSOCIATION INTO 4 PIONS*
M. Davier, I. Derado, ** D. E. C. Fries,*** F. F. Liu, 7 R. F. Mozley, A. Odian, J. Park, ** W. P. Swanson, F. Villa, and D. Yountlt Stanford Linear Accelerator Center Stanford University, Stanford, California 94305
ABSTRACT The reaction yp - l;‘n-l;‘~~-p 2-meter beam.
streamer
has been studied using the SLAC
chamber placed in an 18-GeV bremsstrahlung
We find evidence for a broad enhancement (p* ) in the 47r
mass spectrum at a mass of 1620 MeV with a width of about 300 MeV. This state is produced peripherally with being energy independent.
with a cross section consistent
I-spin = 1 (C=-1) is deduced from the
dominant pm decay mode and JfO from the observation zation phenomenon. helicity 3
Assuming quantum numbers Jp= l-,
is conserved at the y-p’
vertex.
Further
of a polaris -channel
support for the
assignment is gathered when comparing our results to data from
e+e- colliding-beam
experiments.
*Work supported in part by the U. S. Atomic Energy Commission. **Max Planck Institut fiir Physik und Astrophysik, ***Institut
fiir Experimentelle
Kernphysik,
“fPhysics Department, California California 92407, USA.
Karlsruhe,
State College,
TtDepartment of Physics and Astronomy, Hawaii 96822, USA.
Munich, Germany. Germany.
San Bernardino,
University
of Hawaii,
(Submitted to Nucl. Phys. )
Honolulu,
I
Introduction A well-known
feature of the photon interacting
Indeed, for hadronic masses up to
coupling to the neutral vector mesons. 1 GeV, the low-lying action.
However,
mesons,
p”,
with hadrons is its strong
w and Cp essentially
dominate the inter-
there is no such clear picture for higher masses.
evidence from the e+e- colliding-beam
experiments
There is
at Frascati 1) that multi-
pion final states are copiously produced in the mass range from 1.4 to 2.4 GeV. Obviously the study of the photon coupling to hadronic
states of high mass is
quite fundamental since it could reveal new vector mesons or show interactions no longer mediated by vector mesons. A direct way of studying the Jp = l- hadronic spectrum is the e+e- annihilation process through one-photon exchange (fig. la). is through diffractive
photoproduction
another approach
with real photons:
(hadrons) + p
YP-
However,
,
where the hadrons connected to the photon will appear at high energy and small momentum transfer
(fig. lb).
While the r’n-
hadronic state has already been
studied extensively 2,3) with no convincing evidence for an isovector
vector
meson of mass higher than 1 GeV, searches in the higher multiplicity have been almost nonexistent. photoproduction
In this paper we present such a search in the
of four charged pions: YP-
Preliminary
states
analyses of a limited
fn+n-7r7rp fraction
.
(1)
of our data have already been
presented 495) . The present work is based on the complete data of our experiment.
Recently new results have become available on this reaction
from the Berkeley-SLAC
bubble chamber collaboration 6) , showing good agree-
ment with our findings. -2-
Characteristics The two-meter bremsstrahlung
SLAC streamer
of the Data
chamber was exposed to an 18 GeV
beam and data were taken under a very loose trigger.
on our experimental
technique and the analysis procedures
Details
may be found
elsewhere7). Our sample for reaction
(1) amounts to 1667 events from photon energies
between 4.5 and 18 GeV, where the lower energy cutoff is imposed by the falloff of our geometrical been previously
acceptance.
General features of reaction phase-space method 8) .
studied with the longitudinal
We find that reaction
(1) have
(1) is strongly dominated by p” and A*
as seen in figs. 2a, 2b and 3a, 3b; the effect is particularly
production
clear at the highest
energies we have studied (figs. 2b and 3b). Above Ey = 10 GeV such production accounts for about 70% of the complete channel. We have fitted the fractions ++ using handdrawn background curves in order to study the energy p” and A dependence of the various subchannels.
Figure 4 and table 1 give the energy
dependence of the cross sections for reaction YP -
o+PTTP
YP-
+---IirnnA
o--HYP --pnA Since we are interested
(1) and the following reactions:
,
(2)
, .
at high energy, where we primarily
phenomenon, the A* Therefore
(4)
in the photon coupling to the 47r system we wish to
remove events produced by exchange processes, Fortunately
(3)
such as reaction
look for a diffractive
peak is very clear with a relatively
small background.
we do not bias the 4n system too severely by removing
a pi? mass combination
in the A*
band (1.16 - 1.32 GeV) . -3-
(3).
events with
of
The 4-Pion Mass Spectrum Figure 5a shows the 47r mass distribution
for all events above Er = 6 GeV
where some peaking is apparent around 1.6 GeV. When events with A* removed (fig. 5b) the effect appears as a more prominent
are
broad peak centered
at about 1.6 GeV. Since about half of the events with no A* to look for a correlation
contain a p”,
between the 2n and 47r mass distributions.
done in fig. 6 where it is seen that a large fraction to the 4n bump.
Conversely,
7~+7r-combination
in the p” band (0.68 - 0.84 GeV).
p07r+n- mass distribution
it is interesting This is
of the produced p” contribute
almost all the events in the 4n peak have one Figure 5c then shows the
where the peak at 1.6 GeV is now very clear.
To study the behavior of our results as a function of incident energy, we divide our sample into two energy bins, Ey = 6 - 12 GeV and 12 - 18 GeV.
We
see the enhancement in both cases, but the effect is more apparent in the highenergy sample since more phase-space is available in this case (figs. 7a and 7c).
On the other hand, if we require
that no p” be produced there is no evi-
dence for an excess of events, thus confirming
the strong correlation
(figs. 7b
and 7d). To ascertain whether we are seeing a genuine effect produced by the p” cut, we have generated “peripheral”
not artificially phase-space Monte
Carlo events with exponential falloff in pion transverse-momenta and in momentum transfer
to the proton.
squared 9)
These events have been subjected to
the same cuts we have applied to the real data.
We are unable to reproduce
the 4n peak at 1.6 GeV whereas the higher mass part of the 4n mass distribution is well reproduced distribution
(solid line in figs. 7a, 7~).
with no p” is well described -4-
Furthermore,
the 47~mass
(solid lines in figs. 7b, 7d). We take
this as evidence that the enhancement in the p”n+7r- system is not a kinematical reflection
of other gross features of data, nor is it an artifact
generated by the
cuts. Using the Monte Carlo generated events to determine background,
the shape of the
we have fitted the 4n mass spectrum to a nonrelativistic
Wigner form in order to determine
the parameters
of the peak.
Breit-
The values
we have found for the mass and width are shown in table 2 for the two E Y selections considered: the values agree well between the two bins, although the width is rather poorly determined
in both.
To compute cross sections we
have fitted the distributions simultaneously
in the ranges E = 6 - 12 GeV and 12 - 18 GeV Y using the same mass and width. The results are
a)
apparent mass M = (1620 f 30) MeV and width I’ = (310 f 70) MeV
b)
a cross section roughly energy independent* (0.85*0.3)pbforEy=6-12GeV (0.95 zt 0.3) pb for Ey= 12-18 GeV.
*The cross section values, extrapolating
corrected by using the observed t-dependence and
at t=O - averaging over M(47r) and E - are Y (1.23 5 0.43) pb for 6 < Ey < 12 GeV (1.07 rt 0.34) pb
for
12 < Ey < 18 GeV
These values are used in the last section for the discussion on coupling constants. Although the fitted values of “mass” eterization
and “width”
do provide an adequate param-
of our data, they depend strongly on assumptions made for the actual
shapes of the resonance and background curves.
This situation is aggravated
by the observed enhancement being broad and lying close to the pr7r threshold -5-
where the shape of the phase space is crucial 10) . Using reasonable estimates for this phase space, we have obtained fitted mass values as low as 1.2 GeV. Therefore
the values quoted should be taken with some reservation.
The pnn events in the peak have a peripheral bution (fig. 8) which can be parameterized
in the form
distri-
e (5.8 * 0-4)t’ where we
In contrast to this the p” in this reaction are produced with
define t’ = t-tmin. a much flatter
momentum-transfer
t distribution
e
(1.4+0.6)t’
which is indicative
of their secondary
origin. We have looked for possible two-body structure
in the 47~decay.
There
is no evidence for a pop0 channel which would of course violate charge conjugation invariance
if we are really
observing a diffractive
also looked for possible structure are substantially .
We have
in the p’.lr* spectra (fig. 9): although they
enhanced at low mass - as expected,
itself peaks at low mass -we .
process.
since the p 71~spectrum
are unable to identify known structures.
Assuming that only L=O, 1, or 2 waves can be present in the pm system, even I-spin states are excluded as they require
either no p ’ or double p”
production.
C=-1.
This leaves only I=1 and therefore
Angular Analysis of the Decay For an (I=l,
Jp=l-)
state decaying into p”~‘r-,
the following
decay matrix
element can be written MD= corresponding
4 g.A,=li;.xqiBW(mi) i=l -
,
to the lowest angular momentum state, i. e. , an s-wave dipion
pair and an s-wave between the p” and the dipion system. zation vector,
2 is the l- polari-
zi the momentum vector in the ith 77+7r-center-of-mass
mi the corresponding
7r’n- invariant
mass and SW(m) the p” Breit-Wigner -6-
frame, form.
For not too high a 4n mass it can be shown 1696) that a simple approximation for the vector .& may be used: 4-9’=gl+~, where
g;?,are the momentum vectors of the two 7r+in the 4n rest frame.
pl,
The quality of the analyzer increases
,
A,’ is expected to become worse as the 4n mass
and useful analysis using g
the 47r peak.
must be restricted
Figure 10 shows the distribution
(defined by the direction
of &
to the lower part of
in the helicity
of flight of the 4n in the overall
center-of-mass):
M(47r) < 1.6 GeV one observes a rather pure sin2 BH distribution characteristic
of s-channel helicity
we know that this is a property
conservation
for a F=
of p” photoproduction,
tion that the 47~peak has the quantum numbers P= 1-. this polarization
phenomenon conclusively
Conversely,
we can use the property
for
which is
l- object.
Since
it gives strong indicaThe observation
of
excludes a state of Jp= Of. of s-channel helicity
enhance the 4n peak without selecting on p". momentum transfer
system
conservation
to
This is shown in fig. 11 for low
events (It I < 0.3 GeV2) and excluding events with A *:
the
4n mass spectrum weighted with sin2 8H is displayed together with the raw spectrum.
It is however difficult
to estimate the mass and width of the enhance-
ment in this way since the projection distorts
the original
using the approximate
analyzer
A’
peak. Discussion of the Results
Since our preliminary
results have been presented, 495) a four-pion
enhancement with quite similar
parameters
(M - 1.6 GeV
r - 0.35 GeV) has
been found in e+e- collisions 11) . It is then quite tempting to assume that the two experiments
are indeed
seeing a new vector meson pt; if so, the coupling -7-
constant to the photon, as derived from the two experiments, Unfortunately,
in the case of photoproduction,
should agree.
this is not directly
accessible:
assuming !fgeneralized’T vector meson dominance of the electromagnetic rent, one is led to consider two possibly dominant diagrams If we assume that the amplitude corresponding compared with that corresponding elastic scattering
(Fig. 12a and 12b).
to diagram (b) is negligible
to (a) and that the amplitudes
are comparable,
cur-
for pp and p’p
we can derive a coupling constant f
YP’
from the relationship: 2
y)’
NYP ---DP) = + yp, c (YP ‘P’P) ( ) Analogously,
.
J3P’ - 47?)
the peak cross sections for ese- production
cross sections for p (from ref.
Using the measured photoproduction for p’ (from this experiment),
are related by:
and the p colliding-beam
12) ) ,
cross section13),
eqs. (5) and (6) would predict: Gpeak
(e+e-
-
pt --L 47r*) = (17 f 6) nb
,
consistent with the measured value of (28 f 12) nb of ref. From our experiment constant f
YP’
11) .
alone we cannot say anything about the coupling
since we do not know the branching ratio for pt - 47r*.
However,
other decay modes can be considered: (i)
the 7;‘~~ decay mode seems to be quite suppressed,
experiments 3) have shown some indication
-8
-
for a structure
although two
in M(T’T-)
near
1.5 GeV.
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