A measurement of the direct photon production cross section

Volume 263, number 3,4
PHYSICS LETTERS B
18 July 1991
A measurement of the direct photon production cross section
at the CERN pp collider
UA2 Collaboration
Bern-Cambridge-CERN-Dortmund-Heidelberg-Melbourne-Milano-Orsay
Perugia-Pisa-Saclay (CEN)
(LAL)-Pavia-
J. Alitti a, G. Ambrosini b, R. Ansari c, D. Autiero d, p. Bareyre a, I.A. Bertram e, G. Blaylock f.l,
P. Bonamy a, M. Bonesini g, K. Borer h, M. Bourliaud a, D. Buskulic c, G. Carboni d,
D. Cavalli g, V. Cavasinni ~, P. Cenci i, J.C. Chollet c, C. Conta b, G. Costa g, F. Costantini d,
L. Cozzi g, A. Cravero g, M. Curatolo d, A. Dell'Acqua b, T. DelPrete d, R.S. DeWolf j,
L. DiLella f, Y. Ducros a, G.F. Egan e, K.F. Einsweiler f,2, B. Esposito d, L. Fayard e,
A. Federspiel h, R. Ferrari b, M. Fraternali b.3, D. Froidevaux f, G. Fumagalli b, J.M. Gaillard ~,
F. Gianotti g, O. Gildemeister f, C. G6ssling k, V.G. Goggi b, S. Grtinendahl ~, K. Hara h,4
S. Hellman f, J. Hrivnac r, H. Hufnagel k, E. Hugentobler h, K. Hultqvist f,5, E. Iacopini d,6,
J. Incandela g, K. Jakobs f, P. Jenni f, E.E. Kluge ~, N. Kurz ~, S. Lami d, p. Lariccia i,
M. Lefebvre f, L. Linssen f, M. Livan b.7, p. Lubrano f'~, C. Magneville a, L. Mandelli g,
L. Mapelli f, M. Mazzanti g, K. Meier f.8, B. Merkel c, j.p. Meyer a, M. Moniez c, R. Moning h,
M. Morganti d.9, L. Miiller h, D.J. Munday J, M. Nessi f, F. Nessi-Tedaldi f, C. Onions f, T. Pal h,
M.A. Parker J, G. Parrour c, F. Pastore b, E. Pennacchio b, J.M. Pentney f, M. Pepe f,
L. Perini g.3, C. Petridou d, p. Petroff c, H. Plothow-Besch f, G. Polesello f,b, A. Poppleton f,
K. Pretzl h, M. Primavera d.lO, M. Punturo i, J.P. Repellin c, A. Rimoldi b, M. Sacchi b,
P. Scampoli i, j. Schacher h, V. Simak f, S.L. Singh J, V. Sondermann k, S. Stapnes f,
A.V. Stirling a, C. Talarnonti i, F. Tondini i, S.N. Tovey e, E. Tsesmelis k, G. Unal ~,
M. Valdata-Nappi d, 10, V. Vercesi b, A.R. Weidberg f,l 1, P.S. Wells j,12, T.O. White J,
D.R. Wood ¢, S.A. Wotton j,12, H. Zaccone a and A. Zylberstejn a
a
b
c
d
e
f
g
h
Centre d'Etudes Nucl~aires de Saclay, F-91191 Gif-sur-Yvette Cedex, France
Dipartimento di Fisica Nucleare e Teorica, Universitil di Pavia and INFN, Sezione di Pavia, Via Bassi 6, 1-27100 Pavia, Italy
Laboratoire de l'Accdl~rateur Lindaire, Universitk de Paris-Sud, F-91405 Orsay, France
Dipartimento di Fisica dell'Universit~ di Pisa and INFN, Sezione di Pisa, Via Livornese, S. Piero a Grado, 1-56100 Pisa, Italy
SchoolofPhysics, University o f Melbourne, Parkville 3052, Australia
CERN, CH-1211 Geneva 23, Switzerland
Dipartimento di Fisica dell'Universit& di Milano and 1NFN, Sezione di Milano, 1-20133 Milan, Italy
LaboratoriumJ~r Hochenergiephysik, Universitdt Bern, Sidlerstrafle 5, CH-3012 Bern, Switzerland
Dipartimento di Fisica dell'Universidiz di Perugia and lNFN, Sezione di Perugia, via Pascoli, l-O6100 Perugia, ltaly
i Canvendish Laboratory, University of Cambridge, Cambridge CB30HE, UK
k Lehrstuhlffir Experimental Physik IV, Universitdt Dortmund, W-4600 Dortmund, FRG
lnstitut J~r Hochenergiephysik der Universitdt Heidelberg, Schr6derstraJ3e 90, W-6900 Heidelberg, FRG
Received 19 April 1991
A measurement of the inclusive cross-section for production of direct photons in lap collisions at a centre of mass energy of 630
GeV is presented as a function of the photon transverse momentum. The data correspond to a total integrated luminosity of 7.4
p b - J. The results support predictions from QCD theory.
544
~
0370-2693/91/$ 03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)
Volume 263, number 3,4
PHYSICS LETTERS B
I. Introduction
The direct p r o d u c t i o n o f isolated large transverse
m o m e n t u m (PT) photons in h a d r o n - h a d r o n collisions is a convenient way to study the constituents o f
hadronic m a t t e r a n d their interactions. A measurement o f the direct photon cross-section provides a test
o f Q C D with the advantage that the p h o t o n transverse m o m e n t u m is not affected by fragmentation effects, resulting in experimental uncertainties which
are considerably smaller than those o b t a i n e d for instance in the measurement o f a jet cross-section. Nextto-leading order calculations are also available and
can be directly c o m p a r e d to the e x p e r i m e n t a l results.
The copious p r o d u c t i o n o f high transverse m o m e n t u m h a d r o n jets is, however, a large source o f background: h a d r o n jets often contain one or m o r e n o (or
rl ) mesons which decay into p h o t o n pairs that are not
resolved by the calorimeter. This b a c k g r o u n d has a
cross-section a p p r o x i m a t e l y four orders o f magnitude higher than the direct p h o t o n signal. The latter,
however, results in isolated electromagnetic clusters,
whereas the background from h a d r o n jets is accomp a n i e d by jet fragments, so that an "isolation requirem e n t " is very effective in reducing the c o n t a m i n a tion o f the signal sample. The residual contamination
from large P-c isolated n°'s (or q ' s ) is m e a s u r e d and
subtracted on a statistical basis, by considering the
Present address: University of California, Santa Cruz, CA
95064, USA.
2 Present address: Lawrence Berkeley Laboratory, Berkeley, CA
94720, USA.
3 Present address: Istituto di Fisica, Universita di Palermo, 190133 Palermo, Italy.
4 Present address: University ofTsukuba, Tsukuba, Ibaraki 305,
Japan.
5 Present address: University of Stockholm, S-113 46 Stockholm, Sweden.
6 Also at Scuola Normale Superiore, 1-56100 Pisa, Italy.
7 Present address: Dipartimento di Fisica, Universit~ di Cagliari, 1-09100 Calgliari, Italy.
s Present address: Deutsches Elektronen Synchrotron, W-2000
Hamburg, FRG.
9 Present address: Dipartimento di Fisica e INFN di Bologna,
Universit/i Bologna, 1-40126 Bologna, Italy.
~o Present address: Dipartimento di Fisica dell' Universit/l della
Calabria e gruppo INFN, Cosenza, Italy.
~' Present address: Nuclear Physics Laboratory, University of
Oxford, Oxford OX 1 3RH, UK.
~z Present address: CERN, CH-1211 Geneva 23, Switzerland.
18 July 1991
fraction o f p h o t o n s in the sample that initiate showers in a 1.5 radiation length (r.1.) thick lead converter.
The analysis, based on a sample o f data collected
during the 1988-1989 running p e r i o d at the C E R N
pp Collider ( v / s = 630 G e V ) , corresponding to an integrated luminosity 0f7.4 _+0.4 p b - 1, is p e r f o r m e d in
a central fiducial region o f the UA2 detector covering
the p s e u d o r a p i d i t y interval [qi < 0.76.
2. The UA2 apparatus
The UA2 detector was substantially upgraded between 1985 and 1987 to better exploit the increased
C E R N pp Collider luminosity. An extensive description o f the apparatus can be found elsewhere [ 1 ].
Here we describe only some features relevant to this
study.
The detector provides full a z i m u t h a l coverage
a r o u n d the interaction region in a p s e u d o r a p i d i t y
range - 3 < q < 3 and consists o f a central tracking detector s u r r o u n d e d by electromagnetic and hadronic
•calorimeters [2]. The calorimeter is d i v i d e d into a
central part ( C C ) within ] ~/] < 1 and two end cap regions ( E C ) reaching ] q] = 3. The same technique
( a b s o r b e r plates with scintillator and wavelength
shifter r e a d o u t ) is used throughout. An electromagnetic c o m p a r t m e n t with lead absorber plates ( 1 7 24.4 r.1. depending on p o l a r angle) is followed by
hadronic c o m p a r t m e n t s with iron absorber plates.
G r a n u l a r i t y is p r o v i d e d by segmentation into 624
cells pointing a p p r o x i m a t e l y towards the interaction
region. Each o f the 240 CC cells subtends 10 ° in polar angle 0 and 15 ° in azimuthal angle ~. The EC cells
have a segmentation A ~ = 15 °, A q = 0 . 2 in the range
1.0 < ] q l < 2.2 while the two cells closest to the b e a m
axis (2.2 < I ql < 2.5 and 2.5 < I~/I < 3.0) cover 30 ° in
azimuth.
Clusters are reconstructed in the calorimeter by
j o i n i n g all cells with an energy greater than 400 MeV
sharing a c o m m o n edge. Clusters with a small lateral
size and a small energy leakage into the hadronic
c o m p a r t m e n t s are m a r k e d as electromagnetic clusters and are subsequently e x a m i n e d as potential photon candidates.
In a d d i t i o n to the presence o f an electromagnetic
calorimeter cluster, photons are characterized by the
absence o f an associated track. Track i n f o r m a t i o n is
545
Volume 263, number 3,4
PHYSICS LETTERSB
provided by the central detector, consisting of two arrays of silicon pad counters [ 3 ] used for tracking and
ionization measurement placed respectively at radii
of 2.9 cm and 14.8 cm around the beam. A cylindrical drift chamber with jet geometry (the jet vertex
detector JVD) [ 4 ] is located between the two silicon
arrays. The outer silicon counter is followed by a
transition radiation device TRD [ 5 ] consisting of two
sets of radiators and proportional chambers. The
outermost part of the central detector is a scintillating fibre detector SFD [6] which consists of eight
stereo triplets of scintillating fibres arranged in cylindrical layers. In the forward regions three stereo triplets of proportional tubes ECPT [7] are placed in
front of the end cap calorimeters.
The position of the event is reconstructed using the
SFD in conjuction with the two silicon arrays and the
JVD.
In both the central region and the end caps the last
elements before the calorimeters are preshower detectors used to localize the early development of the
electromagnetic shower initiated in a lead converter.
In the central region this function is provided by the
SFD where a 1.5 r.l. lead cylinder is positioned before the last two stereo triplets of fibres. In the end
cap region this is accomplished by a 2 r.1. thick ironlead converter placed before the last stereo triplet of
the ECPT.
Photons which convert in the lead are identified by
a large cluster of charge in the preshower detector in
front of an electromagnetic cluster in the calorimeter,
with no reconstructed track pointing to it.
Calorimeter clusters due to beam halo particles are
rejected using two planes of large area scintillation
counters (VETO counters) covering the back sides
of the end cap calorimeters. Particles which give an
early signal with respect to the nominal beam crossing time are rejected in the analysis.
3. Photon identification
The trigger requirements for photon candidates are
implemented in a three-level trigger system [ 8 ] based
mainly on information from the calorimeters. The
first level uses analog sums of the signals from the
photomultipliers of the calorimeter cell compartments. At the second level, electromagnetic and had546
18 July 1991
ronic clusters are reconstructed in a special-purpose
processor using information from a fast digitization
of the calorimeter cell signals. A complete calorimeter reconstruction is performed in the third level processors using the final digitization and the full set of
calibration constants. Information on the shower radius (transverse size) and the fraction of shower energy detected in hadronic compartments (longitudinal depth) are used to reject the jet background.
The present analysis is performed only in the central region of the UA2 apparatus. Each photon candidate is required to have an electromagnetic cluster
in the central calorimeter. To ensure that the cluster
is fully contained in the central region it is required
that the centroid of the cluster have pseudorapidity
[q] < 0.76 and that the displacement of the event vertex from the centre of the detector along the beam
axis be less than 250 ram.
Only events with a single reconstructed f~p interaction vertex are considered. In addition the events
must contain at least one cluster having the characteristics expected for an isolated single photon:
(a) The lateral and longitudinal profiles of the
cluster are required to be consistent with that expected for a single isolated electron or photon.
(b) The absence of any charged track in front of
the calorimeters as ensured by pulse height requirements imposed on any silicon pad or pad cluster
present in either silicon counter within a window of
Aq<0.2 and Aq~< 15 ° about the cluster axis as defined by the line joining the interaction vertex to the
cluster centroid.
(c) At most one preshower signal in a cone
x/A~2 + Aq2 < 0.265 about the cluster axis.
A total of 26 086 photon candidates with p v > 15
GeV are found in the central region satisfying these
selection criteria. The global efficiency for detecting
a photon candidate is estimated to be ec=0.443
_+0.009. This value does not include effects associated with photon conversions in the preshower
detector.
4. Conversion probability and background
calculations
Isolation criteria reject a large fraction of n°'s and
rfs while retaining direct photons. The residual back-
Volume 263, number 3,4
PHYSICS LETTERSB
ground originating from n o and q decays is measured
by considering the fraction ot of events in which the
photon has begun showering in the convener of the
preshower detector.
In order to compute a, all the efficiencies which
have a different value for converted and unconverted
photon candidates must be taken into account. The
definition of a photon candidate as converted or unconverted relies on the detection of charge clusters in
the preshower detector. A photon conversion is defined by the observation of a signal in the preshower
detector exceeding that expected for three minimum
ionizing panicle ( M I P ) equivalents associated with
the calorimeter cluster. Preshower clusters not related to real photons originate mainly from two
sources: "ghost" preshower signals due to SFD pattern recognition ambiguities and real preshower signals generated by the underlying event of by panicles
from the jet (s) in the event. A fake preshower cluster
can move events from the unconverted to the convened class or can cause the rejection of a convened
photon by simulating a non-isolated preshower. We
define e~ to be the probability of having one and only
one preshower signal in association with a convened
photon; and e8 and e~ to be the probabilities of finding respectively zero and one preshower signal in the
case of an unconverted photon.
Another correction factor to be applied only to the
unconverted photons comes from the efficiency of the
method used to find a good unconverted photon candidate without knowing its direction precisely (e u ).
The efficiencies e~ and e~ are estimated using a
sample of identified electrons from W decays [9 ],
while e~ and e~ are measured by inspecting the uncorrelated regions at + 90 ° in azimuth with respect
to an electromagnetic high energy cluster.
The numbers of observed converted (Nc~een) and
unconverted (N~een ) photons are then related to the
true numbers (Nct r u e , Ntr ue ) of photons by the following relationships:
N S c e e n = ~ I Nc
et r u e + e l N uu
Nue. . .=. e o e s
g utrue .
true ,
The conversion probability ot in the sample of photon candidates is defined as
Sic rue
o(=
true
Nc
+ N ut r u e
"
18 July 1991
The conversion probability ev on an incident single
photon is evaluated as a function of the photon energy using the EGS shower simulation programme
[ 10 ]. The simulation has been tuned to describe correctly the response to test beam electrons of 10 and
40 GeV and electrons from W decays in the detector.
The total systematic error in er is estimated to range
from 2.4% to 2.0% with increasing energy.
The effective conversion probability e~ of the background originating from unresolved multiphotons
from n o and 11 decays is calculated using ev for each
photon. The ratio of the number of q to the number
o f n ° has been measured to be 0.6 and is PT independent [ 11 ].
The effect of multi-n ° states in the background has
been estimated by comparing the conversion probability measured for a background data sample selected by requiring a large lateral cluster profile, after
subtracting the residual single photon component,
with the value of e~ calculated from single n°'s and
n's. The two values are found to agree. This suggests
that despite the fact that the conversion probability
of a multi-n ° system is close to unity, such a background component is highly suppressed by the preshower isolation requirement. The uncertainty in the
two-photon angular resolving power (20 + 7 mrad in
the SFD preshower detector) is the main source of
systematic error for e~. This error is slightly energy
dependent and ranges from 2.7% to 1.5% with increasing n o energy.
The calculated values ofe~ and ev are shown in fig.
1 as a function of the photon energy Ev. The measured conversion probabilities, a, lie between the two
theoretical curves and tend towards ~ as Ev increases.
The only significant background in this analysis is
due to multiphoton contamination. Its fraction in the
sample is computed from the values of or, ev, e~:
b(PT)= ~ - - ~
The fraction of multiphoton background is shown
in fig. 2, where a wider binning than for the crosssection calculation is used for display purposes. As it
is seen in the figure it decreases with increasing PT.
The remaining background caused by beam halo
particles has been estimated to be less than 1% of the
photon candidate sample and has been neglected.
547
Volume 263, number 3,4
PHYSICSLETTERSB
+
0.9
UA2
(1988-1989)
0.8
O
O
o_
03
d,o"
E d3p
f
~
0.7
~). 6
g
0
O.5
0.4
03
- - ~ ,
, I
J
10
102
E, (a+v)
Fig. 1. Conversion fraction for photon candidates. The curves
labeled ~ and e~are the conversionprobabilities for single photons and multiphoton background, respectively.
0.5
UA2 (,1988-1989)
+°+
+
"(3
C
Cv
0.2
g
m
0.3
0
~
o
1
,
, .i
,
ao
i
40
h
i
,
+o
,
,
,
i
so
+
,
i
~oo
p, (CeV)
Fig. 2. The fractional multiphotonbackgroundcontaminationin
the sample of photon candidate events.
W o e v decays are expected to contribute at most
30 events in the PT interval between 20 and 45 GeV
as a result of inefficiencies in the electron track reconstruction. This contribution corresponds to 0.2%
of the photon candidate events in the same PT range.
5. Inclusive cross-section
The invariant inclusive cross-section for direct
photon production is evaluated from
548
18 July 1991
N.,(,oT)[ 1 - b(P-r) 1
27tpwAp.r 2fecA (PT)
where Nv(pv) is the number of photon candidates in
a px-bin of width Apx, b (PT) is the background fraction in that bin, L f = 7 . 4 + 0 . 4 pb -t is the integrated
luminosity corresponding to the data sample [ 12 ], ec
is the efficiency of the selection criteria for detecting
direct photon events and A(Px) is the geometrical
acceptance computed by a Monte Carlo simulation.
The results are consistent, within statistical and
systematic errors, with similar measurements at the
same center of mass energy [ 13,14 ]. The cross-section values together with the statistical and the Pv dependent systematic errors are listed in table 1.
The systematic uncertainty on the normalization
of the cross-section has a component that depends on
the PT of the photon and a component which is independent on it.
The overall PT independent systematic error is 9%.
It includes contributions from a 1% uncertainty on
the energy scale (6.4%), a 5.4% error on the luminosity measurement, a 2% uncertainty on the evaluation
of the photon identification cut efficiency excluding
the preshower isolation efficiency and a 1% error on
the acceptance determination.
The PT dependent part of the systematic error is
due to the uncertainties in the preshower isolation efficiency (sys. 1 ) and in the Monte Carlo evaluation
ofe v and e~ (sys. 2). In addition, an uncertainty arises
from the difference in the energy reconstruction for
converted photons for which the point of impact with
the calorimeter is defined by the preshower location,
and for unconverted photon for which it is determined from calorimeter information alone (sys. 3 ).
As a cross-check of the estimate of the systematic
error, the analysis has been repeated using a different
set of isolation criteria based on calorimeter and track
information. The two cross-section measurements
agree within statistical and systematic errors.
The results are compared to next-to-leading order
QCD calculations [ 15 ]. Different sets of structure
functions [16] and different choices of the Q2 scale
[ 17 ] have been used for the comparison. In addition,
the isolation cut used in the selection of the data suppresses the contribution of bremsstrahlung from final state quarks so that this effect is not included in
the calculation of the QCD expectation. The PT dis-
Volume 263, number 3,4
PHYSICS LETTERS B
18 July 1991
Table 1
Inclusive direct photon cross-section at I r/I = 0. The quoted errors A do not include the overall systematic scale uncertainty of _+9%. See
text for definition of the systematic errors sys. 1, sys. 2, sys. 3. They have been added in quadrature to give the total Pr dependent
systematic error sys. tot.
PT
E da/ d3p
(GeV)
(pb GeV - z )
A(Eda/d3p) (pb GeV -2)
stat.
15.9
17.9
19.9
21.9
23.9
25.9
27.9
7.26
3.81
1.74
sys. 1
6.04X 10 -~
3.60X 10 -1
2.05X 10 -~
4.01 X
2.46X
1.53X
1.01 X
7.24X
5.23X
3.83X
29,9
1.72X 10 - l
2.98X 10 -2
33,5
40.4
48,8
6.88X 10 -2
1.83X 10 -2
7.31 X 10 -3
1.06X 10 -2
8 . 7 8 X 10 - t
56,1
1.66X 10 -3
65.7
81.4
6.21 X 10 -4
1.97X 10 -4
10 - t
10 -~
10 - l
10 - l
10 -2
10 -2
10 -2
1.38X
6.10X
2.61X
1.32X
sys. 2
1.37
6.44X
2.91X
1.40X
7.79X
4.43X
2.50X
8.08X 10 -3
2.03X 10 -2
8.05 X 10 -3
1.55X 10 -3
6 . 8 8 X 10 - 4
1.86X 10 -2
7 . 0 2 X 10 -3
4 . 2 9 X 10 -3
2 . 2 4 X 10 -3
1.06X 10 -3
1.83X 10 -4
5.85X 10 -5
1.70X 10 -3
6.51 X 10 - 4
3.92X 10 -3
1.83X 10 -4
4.46X 10 -4
1.66X 10 -5
1.41 X 10 -4
2.07X 10 -4
7 . 8 9 X 10 -4
2.51X 10 -4
5.33X
4.35X 10 -6
1.58X 10 -6
5.03X 10 -5
1.52X 10 -5
5.40X 10 -3
1.10X 10 - s
7.39X 10 -5
1.87X 10 -5
1.50X 10 -4
10 -~
10 - l
10 - l
10 -2
10 -2
10 -2
•
UA2
UA2 (1988- lg89)
1.72X 10 -3
%
(1988-1989)
104
X
__
NLOABFOWOPT
- - - NLO DO1 OPT
NLO 0010~= ~
1° 3
,/s = 6 3 0 OeV
ee
~=0
1~
10 2
•
2
10
b
7D
LJJ
~.
-o
10
-o
1
b
%
1{3 I
16 4
~p--~ jet + X
6] PP--~7 +X
I{31
ez
e
•
¢-
oo
4,-
_._
*tT- -,
-2
10
-5
~
10
0
J
,
20
40
,
,
,
I
,
,
60
,
,
I
,
,
,
80
-3
10
,
~-~-~_,,
o
1 O0
I,,,,
25
50
p, (OeV)
of the data together with the QCD
uncertainties
I,,,,
I,,,,
I ....
i ....
i,,,
~00 ~25 150 175 200
Fig. 4. The differential cross-sections for direct photon productions and jet production [ 18 ] are compared at ~/= 0.
A comparison
between the inclusive cross-sections
for direct photons and jets at q=0
[ 1 8 ] is s h o w n i n
fig. 4.
expec-
tations for various choices of structure functions are
s h o w n i n fig. 3. W i t h i n
I,,,,
75
PT (OeV)
Fig. 3. The invariant differential cross-section for direct photon
production is compared with the Q C D calculation of ref. [ 15]
with two different sets of structure functions [ 16 ], namely D u k e Owens set 1 ( D O I ) and Aurenche et al. (ABFOW) with an optimized Q2 scale ( O P T ) and Q2=p.].. The errors shown include
statistical and PT dependent systematic errors added in
quadrature.
tribution
10 -~
10 - l
10 -1
10 -2
10 -2
10 -2
10 5
10
O_
2.76X 10 -~
1.45X 10 - j
1.29X 10 -~
7.46X 10 -2
1.39X 10 -z
1.01 X 10 -2
7 . 5 8 x 10 -3
7 . 2 5 X 10 -3
4 . 3 2 X 10 -3
2 . 4 6 X 10 -3
10 2
(-9
sys. tot.
1.34
6.25X
2.59X
1.18X
7.61 X
4.28X
2.38X
10 -4
10 -~
10 -2
10 -2
10 -z
sys. 3
Acknowledgement
the data agree
well with the QCD predictions but do not distinguish
among the different structure function sets.
We gratefully
contributions
acknowledge
and guidance
P. D a r r i u l a t
during
for his
the design and
549
Volume 263, number 3,4
PHYSICS LETTERS B
c o n s t r u c t i o n o f the U A 2 u p g r a d e project. We especially t h a n k P. A u r e n c h e , R. Baier, M. F o n t a n n a z a n d
D. S c h i f f for p r o v i d i n g the t h e o r e t i c a l p r e d i c t i o n s
s u b j e c t e d to the e x p e r i m e n t a l c o n d i t i o n s for the present study. T h e t e c h n i c a l staff o f the Institutes collaborating in U A 2 h a v e c o n t r i b u t e d substantially to the
c o n s t r u c t i o n a n d o p e r a t i o n o f the e x p e r i m e n t . We
t h a n k t h e m deeply for t h e i r c o n t i n u o u s support. T h e
e x p e r i m e n t w o u l d not h a v e been possible w i t h o u t the
v e r y successful o p e r a t i o n o f the i m p r o v e d C E R N pp
Collider, w h o s e staff a n d c o o r d i n a t o r s we sincerely
t h a n k for t h e i r c o l l e c t i v e effort. F i n a n c i a l s u p p o r t
f r o m the S c h w e i z e r i s c h e n N a t i o n a l f o n d s zur F 6 r d e rung d e r W i s s e n s c h a f t l i c h e n F o r s c h u n g to the B e r n
group, f r o m the U K Science a n d E n g i n e e r i n g Research C o u n c i l to the C a m b r i d g e group, f r o m the
B u n d e s m i n i s t e r i u m ftir F o r s c h u n g u n d T e c h n o l o g i e
to the D o r t m u n d a n d H e i d e l b e r g groups, f r o m the
A u s t r a l i a n R e s e a r c h C o u n c i l , the C R A Pry Ltd, a n d
the V i c t o r i a n E d u c a t i o n F o u n d a t i o n to the Melb o u r n e group, f r o m the I n s t i t u t N a t i o n a l de Physique N u c l 6 a i r e et de P h y s i q u e des P a r t i c u l e s to the
Orsay group, f r o m the Istituto N a z i o n a l e di Fisica
N u c l e a r e to the M i l a n o , P a v i a , Perugia a n d Pisa
groups and f r o m the Institut de R e c h e r c h e F o n d a mentale
(CEA)
to
the
Saclay
group
are
acknowledged.
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