decays Do + K-x+, K-w+x', and K-7rr+7r-rr+, we measure the Do lifetime to be 0.44?:*:: f 0.06 ..... Because of this, we took the range over which we varied rB.
SLAC - PUB LBL - 23321 May 1987 E
Measurement the Upgraded
of the Do Lifetime Mark
II Detector
- 4304
from at PEP*
S. R. Wagner,(l) D. A. Hinshaw,(l) R. A. Ong,t2) G. Abrams,c3) C. E. Adolphsen,c4) C. Akerlof,t5) J. P. Alexander,t2) M. Alvarez,(l)f(“) A. R. Baden,(3)y(b) J. B a 11am,t2) B. C. Barish,(‘) T. Barklow,c2) B. A. Barnett,c7) J. Bartelt,t2) D. Blockus,c8) G. Bonvicini,c5) A. Boyarski,c2) J. Boyer,c3) B. Brabson,(*) A. Breakstone,tg) J. M. Brom,c8) F. Bulos,t2) P. R. Burchat,t4) D. L. Burke,c2) F. Butler,t3) F. Calvino,(l)s(a) R. J. Cence,cg) J. Chapman,c5) D. Cords,c2) D. P. Coupal,c2) H. C. DeStaebler,c2) D. E. Dorfan,t4) J. M. Dorfan,c2) P. S. Drell,t3) G. J. Feldman,c2) E. Fernandez,(l)s(a) R. C. Field,c2) W. T. Ford,(l) C. Fordham,c2) R. Frey,c5) D. Fujino,c2) K. K. Gan,12) G. Gidal,t3) T. Glanzman,c2) G. Goldhaber,c3) A. Green,(2)*(G) P. G rosse-Wiesmann,(2) J. Haggerty,(3)l(d) G. Hanson,c2) R. Harr,t3) F. A. Harris,tg) C. M. Hawkes,c6) K. Hayes,c2) D. Herrup,c3) C. A. Heusch,c4) T. Himel,c2) M. Hoenk,(6) D. Hutchinson,(2) J. Hylen,17) W. R. Innes,t2) M. Jaffre,c3) J. A. Jaros,(2) I. Juricic,c3) J. A. Kadyk,c3) D. Karlen,c2) J. Kent,t4) S. R. Klein,c2) A. Koide,(g)j(e) W. Koska,(5) W. Kozanecki,c2) A. J. Lankford,t2) R. R. Larsen,c2) M. E. Levi,c2) Z. Li,c6) A. M. Litke,c4) V. Liith,c2) J. A. J. Matthews,t7) D. I. Meyer,t5) B. D. Milliken,(‘) K. C. Moffeit,c2) L. Miiller,(2)2(f) J. Nash,t2) M. E. Nelson,(6) D. Nitz,c5) H. Ogren,t8) K. F. O’Shaughnessy,(2) S. I. Parker,cg) C. Peck,c6) M. L. Perl,c2) A. Petersen,t2) M. Petradza,c5) F. C. Porter,(‘) P. Rankin,c2) K. Riles,c2) D. R. Rust,c8) H. F. W. Sadrozinski,(4) T. L. Schalk,c4) W. B. Schmidke,c3) A. S. Schwarz,(4) A. Seiden,t4) J. G. Smith,(l) A. Snyder,c8) E. Soderstrom,(6) D. P. Stoker,(7) R. Stroynowski,(‘) R. Thun,c5) G. H. Trilling,(3) R. Tschirhart,c5) R. Van Kooten,(2) H. Veltman,(5),(c) P. Voruganti,c2) P. Weber,(‘) A. J. Weinstein,t4) A. J. Weir,c6) S. Weisz,(4)j(g) S. L. White,(‘) E. Wicklund,(6) D. R. Wood,(3) and D. Y. Wuc6)
* This work was supported in part by Department of Energy contracts DEACOJ81ER40050 (CIT), DEAA03-76SFOOOlO (UCSC), DEAC02-86ER40253 (Colorado), DEAC03-83ER40103 (Hawaii), DEAC02-84ER40125 (Indiana), DEAC03-76SF00098 (LBL), DEAC02-84ER40125 (Michigan), and DEAC03-76SF00515 (SLAC), and by the National Science Foundation (Johns Hopkins).
(l) University of Colorado, Boulder, Colorado 80309 (2)Stanford Linear Accelerator Center, Stanford Uniuersity, Stanford, California 94.905 t3) Lawrence Berkeley Laboratory and Department of Physics, University of California, Berkeley, California 94 720 c4)University of California, Santa Cruz, California 95064 c5) University of Michigan, Ann Arbor, Michigan 48109 c6)California Institute of Technology, Pasadena, California 91125 c7)Johns Hopkins University, Baltimore, Maryland 21218 t8) Indiana University, Bloomington, Indiana 47405 cg)University of Hawaii, Honolulu, Hawaii 96822
ABSTRACT We present a new measurement of the Do meson lifetime based on 31 pb-’ of e+e- interaction
data taken at fi
= 29 GeV. The data were recorded with
the upgraded Mark II detector, which allowed the identification
of a clean Do
signal from this small data set. Using a sample of 53 Do candidates based on the decays Do + K-x+,
K-w+x’,
and K-7rr+7r-rr+,
we measure the Do lifetime to
be 0.44?:*:: f 0.06 picoseconds. Submitted
(a) Present address: Universidad
Autonoma
(b) Present address: Harvard University, (c) Present address: University (d) Present address: Brookhaven
to Physical Review D
de Barcelona, Bellaterra,
Cambridge,
of California, National
Spain
MA 02138
Berkeley, CA 94720
Laboratory,
Upton, NY 11973
(e) Present address: Vista Research Inc., 3600 Bayshore Road, Palo Alto, CA 94303 (f) Present address: Lab. fiir Hochenergie Physik Bern, CH-3012 Bern, Switzerland (g) Present address: CERN, CH-1211, Genkve 23, Switzerland 2
In this paper we report a new measurement of the Do meson lifetime, formed with the upgraded Mark II detector[” 31 pb-’ of e+e- interaction
per-
at PEP. This analysis is based on
data taken at ,/Z=
29 GeV. The Do mesons were
tagged by the spectator pion from the rr+Do decay of the D*+.
The same de-
cay length method used to measure the Do lifetime in a previous publication the Mark II collaboration”’
is employed here, but the components
of
of the de-
tector crucial to this analysis have changed completely, and this measurement is statistically
independent
of the previous one.
The original Mark II detector”’
was upgraded for its planned use at the SLAC
Linear Collider (SLC), w h ere the average particle momentum the hadronic jet environment
will be higher and
denser than at PEP. The PEP run, from which the
data presented in this paper is taken, was used to check out the upgraded detector and to take advantage of the many improvements
to the detector in the study of
physics at PEP energies. The new components used in this analysis are a 72 layer cylindrical
main drift chamber”’
(DC), situated in a solenoidal magnet with a
nominal field strength of 4.5 kG, and a six layer cylindrical (TC),
located b e t ween the DC and the beam pipe.
grouped together
in twelve “superlayers”
trigger drift chamber[”
The layers of the DC are
each consisting of six layers of wires.
The wires in the odd numbered superlayers are parallel to the cylindrical and the wires in alternating
axis
even numbered superlayers are pitched at +66 mrad
and -66 mrad with respect to the cylindrical
axis to provide stereo information.
The DC has an active length of 2.30 m; the number of sense wires per layer ranges from 26 at the inner radius of 25.0 cm to 136 at the outer radius of 144.4 cm.
The TC consists of six layers of axial sense wires in 4 mm radius
aluminized
mylar tubes.
The inner layer of this chamber has 72 wires at a
radius of 9.5 cm, and the outer layer has 112 wires at a radius of 14.8 cm. The average point measurement
resolution
for the TC. The non-vertex-constrained (GeV/c)-l,
is 175 pm for the DC and 90 pm
momentum
resolution is oP/p2 = 0.0034
measured with 14.5 GeV/c electron tracks which traverse all layers
of both chambers (I cos 81 < 0.62) in Bhabha scattering events. 3
The position
resolution
of a track extrapolated
back to the beam position,
in the plane transverse to the beam axis, can be measured from the distribution of separation
distances of collinear
Bhabha tracks.
This resolution,
which is
the standard deviation of the separation distance distribution
divided by 4,
oezt = 78 pm for our detector.
to this resolution
due to multiple
scattering
We calculate the contribution
in the beam pipe and TC chamber inner wall to be
0~s = 152 pm/p for tracks perpendicular The average beam interaction events for luminosity the vertical
(horizontal)
beam-beam
overlap.
to the beam axis.
point was derived from Bhabha-scattering
subsamples of -
0.5 pb-l.
Tracks within
a measurement distributions
f52
mrad of
plane were used to measure the size oZ (or,) of the PEP The distributions
of impact parameters
plane, relative to the derived average interaction true beam-beam
is
in the transverse
point, are convolutions
overlap, the movement of the average interaction period, and the track extrapolation
error.
point within
For our data, these
are well described by Gaussians centered at zero.
beam size measured from these distributions,
of the
The effective
with the track extrapolation
error
unfolded, is cr, = 403 f 14 pm and cry = 52 f 11 pm. An event was included in the multihadron
data sample used in this analysis if
it had at least five charged tracks, a reconstructed
primary vertex with a position
less than 4 cm in radius and 10 cm in z (distance along the beam axis) from the beam intersection
point, more than 25% of the beam energy in charged tracks,
and more than 50% of the beam energy in charged tracks and neutral clusters in the calorimeters which surround the DC.““’
The charged tracks used to meet the
above cuts were each required to have at least 0.1 GeV/c of momentum transverse to the beam, an angle 8 with respect to the beam such that 1cos 81 < 0.8, and a distance of closest approach to the beam intersection radius and 10 cm in z. In addition,
point of less than 6 cm in
if a charged track had a momentum
than 12.5% of the beam energy, its contribution to this amount. 4
greater
to the energy sums was limited
The method of tagging D*+ + ?r+D” decays”’ pion has been in use for many years,“] divided each multihadron
with the spectator charged
and is well established.
In our analysis, we
event into two hemispheres using a plane perpendicular
to the thrust axis determined from charged tracks, and tried every charged track with p > 0.4 GeV/ c as a spectator pion in association with the Do candidates in the same thrust hemisphere. track combination two-track
with a K-z+
combination
combination
A Do candidate was chosen as either: I) a twomass between 1810 and 1920 MeV/c2,
with 1580 < mK-*+
with 1810 < mK-*+r-r+
< 1700 MeV/c2,
< 1920 MeV/c2.
II) a
or III) a four-track
No particle identification
We tried all possible K and 7~ mass assignments,
was used in our analysis.
consistent with the requirement
that the kaon has a charge opposite that of the
spectator pion, and that the total charge of the Do candidate be zero. Cases I and III correspond to fully reconstructed
Do decays, and Case II corresponds to
the satellite resonance of the decay Do -+ K-r+?rO, is reconstructed. of the K-T+
where only the K-T+
Due to the dynamics of the Do -+ K-r+n”
invariant
mass distribution
mass
decay, a large part
is in the mass region of Case II.“’
We
required each track from the Do candidate to have p > 1.2 GeV/c for Case II and p > 0.8 GeV/c for Case III. For Case I we required that the helicity angle of the K-
in the Do rest frame satisfy -0.8
equivalent to a 1.0 GeV/c minimum Do decay. In addition,
< cos0H < +0.8, which is nearly
momentum
cut on the K-
we required that x, the combined energy of the spectator
pion and the Do candidate
divided by the beam energy, be greater than 0.5.
This was done because the D*+ signal has less combinatorial smaller contribution
and r+ from the
from the decay B --) D*+X
from B meson decay complicate negligible B meson lifetime. Do candidates in a thrust
and a
at x > 0.5.“’ The D*+ mesons
the Do lifetime measurement
due to the non-
For Case III there were occasionally more than one hemisphere which satisfied all the above cuts. Since
these duplicate combinations cases we chose the K-n+rlr-r+
background
generally shared many of the same tracks, in these combination
with mass closest to the Do mass as
the only Do candidate in that thrust hemisphere. 5
We then calculated the difference in mass (Am) between the spectator pionDo candidate pair and Do candidate. I and III)
are histogrammed
together
The fully reconstructed in Fig.
Do decays (Cases
la, and show a narrow signal,
consistent with our expected resolution, centered near the world average valuetl” of Am = mg+ produced
-
mD0
= 145.45 MeV/c2.
satellite decay
a broad Am enhancement between 140 and 152 MeV/c2
The degraded Am resolution momentum
(Fig.
in Case II is caused by the uncertainty
lb).
in the Do
due to the missing TO. We took our D*+ signal for Cases I and III
to be all spectator MeV/c2
The Do ---) K-rr+ro
pion-Do candidate combinations
and, for Case II, those combinations
with 143.5 < Am < 147.5
with Am < 152 MeV/c2.
The Do
mesons used for our lifetime measurement were the decay products of these D*+ mesons. In addition to the cuts described above, we required that at least two tracks from the Do decay contain four or more TC position measurements (hits), and we used only these tracks in the secondary vertex fit to determine the Do decay point and its associated error matrix.
For the Do + K-?r+?r-?r+
decay, this meant
that not all tracks from the Do decay were required in the vertex fit, though the full information
from the Do decay was used in all other stages of the analysis.
This was done because it was possible for some tracks from the Do decay not to have at least four good TC hits because of track overlap, but by the time they traversed the DC, to be cleanly separated and reconstructed, angular resolution
and multi-hit
capability
due to the superior
of the DC.
There were 53 Do candidates which passed the above requirements a vertex fit probability candidates
greater than 0.01. We estimate that 17 f 7% of these
are from random combinatorial
B ---) D*+X
and had
background,
and 6 f 3% are from
decays. We scanned the events to make sure that none of the tracks
included in the Do vertex fit had erroneous or ambiguous TC hits associated with them, and to inspect the rest of the event for obvious event selection or track reconstruction
errors which would affect the lifetime measurement.
Do candidates failed these scanning criteria. 6
None of the
The formulae for calculating the most probable path length (in the transverse plane) of the Do, from its production
point to its decay vertex, and the associated
error, are given in Ref. 2. These formulae are functions of the vertex position, the beam position,
the associated error matrices, and the reconstructed
vector of the Do.
The path length was converted into a lifetime
momentum by dividing
it by the measured ~Pc sineD, where 00 is the angle between the Do direction and the beam axis. KK-r+
For the Do +
K-vr+r”
and rIT+ tracks are reconstructed, combination
to be a negligible
satellite
the momentum
decay, where only the vector and mass of the
were used. Monte Carlo simulations show this approximation source of error.
A histogram
of the distribution
of lifetime
measurements, which has a mean of 0.48 f 0.11 psec, is shown in Figure 2. We performed
a maximum
likelihood
fit to the lifetime measurements.
The
fitting function, which is also described in detail in Ref. 2, was the sum of a decay exponential
convoluted with a Gaussian resolution function for the directly pro-
duced Do mesons in the sample, the same function convoluted with another decay exponential to simulate the B + D”X
background, and a Gaussian with its mean
offset from zero lifetime to describe the background from random combinations of tracks in multihadron
events which pass all of the Do selection requirements.
We used a B meson lifetime of 1.16 psec in the background function.[111 The offset of the Gaussian used for the combinatorial weighted-mean
background was taken from the
lifetime of a side-band sample. This sample consisted of all two-
and four-charged-track
combinations
which passed the same cuts as were applied
to tracks in the Do signal region, except that we required invariant
mass to be between 2000 and 2400 MeV/c2.
the combination’s
To increase the statistics
in this sample, we did not require the total charge of a combination and did not require a spectator pion with which the combination Am cut.
This sample should have approximately
the random combinatorial of the distribution 0.17f0.01
background
to be zero
would pass the
the same mix of lifetimes as
in the D*+ sample. The weighted-mean
of side-band lifetimes is 0.19 f 0.01 psec for the data, and is
psec for a Monte Carlo data sample generated with the Lund JETSET 7
6.3 program1121 and passed though our detector simulation. The fit to the data gave a Do lifetime only). The contributions
of 0.44+::$
psec (statistical
error
to the estimate of the systematic error from the various
parameters used in the fit are shown in Table 1. As in Ref. 2, we included a scale factor in our fits to account for systematic mis-assignment
of track errors. The
scale factor was set to 1.2, based on a comparison of the track error matrices and the measured separation
distance distribution
events. Other checks of track resolution scale factor as a free parameter within
the amount (f0.2)
for tracks in Bhabha scattering
in multihadron
events, and leaving the
in the fit, gave values for the scale factor well
by which we varied it to estimate its contribution
to
the systematic error. The fraction
of random combinations
taking the shape of the Am distribution studies and normalizing la and lb.
in the Do sample was estimated for this background
by
from Monte Carlo
it to the Am regions above the D*+ signals in Figures
The significant
positive lifetime
that there are some non-zero lifetime decays in this background,
of the side-band sample confirms
decay products
from charm and beauty
so we assign a large range of possible values to the
Gaussian offset when estimating
its contribution
to the error. For another check,
we generated Monte Carlo samples with the Do lifetime set to 0.1, 0.4, and 0.8 psec. We analyzed these samples, which had complete detector simulation,
with
the same programs as were used for the real data. The measured lifetimes of the Do candidates in these samples were consistent with the input lifetimes to within ho.03 psec, which we took as the systematic error contribution
from any overall
lifetime measurement offset. While the average B meson lifetime is now reasonably well determined, possible that the B” and Bsignificantly
different.
lifetimes and their branching
ratios to D*+X
it is are
Because of this, we took the range over which we varied rB
in our systematic error estimate to f0.4
psec. The Monte Carlo estimate of the
fraction of Do mesons from B decay is likewise dependent on the fragmentation 8
functions used for c and b quarks, the dynamics of the B + D*+X
decay, and the
above branching ratios. We have adjusted the Monte Carlo parameters to give the good agreement with measured fragmentation and the D*+ momentum uncertainty
functions,
B decay multiplicities,
spectrum from B decays, and estimate the systematic
in the fraction
of Do mesons from B decays in our sample to be
f0.03. We have investigated rotation
other possible sources of systematic
error, such as a
of the TC with respect to the DC, and a difference between the real
beam spot size and the one used ,in the decay length calculation, these to be negligible.
When added in quadrature,
error from the contributions
in Table 1 is f0.06
but we find
the total estimated systematic psec.
In conclusion, we have measured the lifetime of the Do meson to be 0.44fi:$f 0.06 psec. This result is consistent with our previous measurement lifetime and with the current world average,“” measurement
from the Fermilab
0.435 f 0.015 f 0.010 psec.[ls’
which is dominated
Tagged Photon Spectrometer
of the Do by a recent
collaboration
of
REFERENCES
1. Proposal for the Mark II at SLC, CALT-68-1015
(1983).
2. L. Gladney et al, Phys. Rev. D34 (1986) 2601. 3. R.H. Schindler et al, Phys. Rev. D24 (1981) 78. 4. G. Hanson, Nucl. Instr. Meth. A252
(1986) 343.
5. W.T. Ford et al, Nucl. Instr. Meth. A255 6. The charge-conjugate
(1987) 486.
reactions are implicitly
included for all reactions de-
scribed in this paper. 7. G.J. Feldman et al, Phys. Rev. Lett. 38 (1977) 1313. 8. G. Goldhaber,
Proceedings
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Plagne (1983). 9. See, for example, S. Bethke, Z. Phys. C29 (1985) 175. 10. Particle Data Group, Phys. Lett.
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11. M. G. D. Gilchriese, Proceedings of the XXIII
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M.
Source of
Estimated
Contribution
Systematic Error
Range
Resolution scale factor
1.2 f 0.2
f0.03
psec
Fraction of signal from
0.17 f 0.07
f0.02
psec
f0.19
f0.01
psec
f0.03
psec
f0.03
psec
random combinations Offset of Gaussian for random combinations
f0.10
psec
Overall measurement offset Fraction of signal
0.06 f 0.03
from B + D”X 1.16
Average B lifetime for B + D”X Table
1. Contributions
f0.40
ho.01 psec
psec
to estimated systematic error.
11
FIGURE 1. The Am distribution
CAPTIONS
for all Do candidate-spectator
pion combinations,
as
described in the text, where (a) the Do candidate is a fully reconstructed K-K+
or K-rIT+7r-rIT+ decay, and (b) the Do candidate is in the satellite re-
gion, corresponding to the high mass K-n+
reflection of the Do + K-d?r”
decay, 2. The distribution
of measured Do lifetimes.
the signal and background functions, to the fit of random combinatorial contribution
from B --+ D”X
The solid curve is the full fit of
the dotted curve is the contribution
background,
background.
12
and the dashed curve is the
I
50
(a)
40 30 20 IO 0 12 -
(b)
1
8 --
4 --
0 100 4-87
120
140 Am
I
160
(MeV/c2)
Figure
1.
5766A2
V
IO 8
-
6
-
4
-
2 0 -4 4-87
-2
2
0 Lifetime
Figure
(psec)
2.
4 5766A
1
