A measurement of the direct photon production cross section
Transcript
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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