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Direct photon production in pp and pp interactions at √s = 24.3 GeV
Physics Letters B 317 (1993) 243-249
PHYSICS LETTERS B
North-Holland
Direct photon production in at x/s = 24.3 GeV
and p p interactions
UA6 Collaboration
CERN-Lausanne-Michigan-Rockefeller G. Sozzi b, G. Ballocchi c, A. Bernasconi b, R.E. Breedon d,1, L. Camilleri a, R.L. Cool d,2, P.T. Cox d, P. Cushman d,e, L. Dick a,3, E.C. Dukes c,4, B. Gabioud b, F. Gaille b, P. Giacomelli d,a, D. Hubbard c, J.B. Jeanneret a, C. Joseph b, W. Kubisehta a, J.F. Loude b, E. Malamud b,5, C. Morel b, p. Oberson b, O.E. Overseth c, J.L. Pages b,a, j.p. Perroud b, D. Rtiegger b, R.W. Rusack d, V. Singh e, G.R. Snow c, D. Steiner b, L. Studer b, M.T. Tran b, A. Vacchi d,6, G. Valenti c,7 and M. Werlen b a CERN, CH-1211 Geneva 23, Switzerland b Universitb de Lausanne, CH-IO15 Lausanne, Switzerland c University of Michigan, Ann Arbor, MI 48109, USA d Rockefeller University, New York, N Y 10021, USA e Yale University, New Haven, CT 06511, USA
Received 30 July 1993 Editor: K. Winter
Inclusive direct photon invariant cross sections have been measured in both ~p and pp collisions at x/£ = 24.3 GeV at the CERN SPS, permitting the first measurement of the difference of the pp and pp cross sections. The direct photon cross section in ~p collisions has been found to be systematically larger than that in pp collisions, which indicates a significant contribution of the ~q annihilation term as predicted by theoretical calculations.
1. Introduction Direct photons o f high Pr are expected to be produced in hadronic collisions through three m a i n processes: gluon C o m p t o n scattering ( q g ) , a n t i q u a r k quark annihilation (~q), and to a lesser extent bremsstrahlung from a quark [ 1 ]. In p p collisions the m a i n contribution has been shown to come from q g l 2 3 4 5 6 7
Present address: KEK 1-1-OHO, Tsukuba-Shi IbarakiKen 305, Japan. Deceased. Present address: INFN, 1-20133 Milan, Italy. Present address: University of Virginia, VA 22901, USA. Present address: Fermilab, Batavia, IL 60510, USA. Present address: Universit/t di Trieste, 1-34127 Trieste, Italy. Present address: INFN, 1-40126 Bologna, Italy.
Elsevier Science Publishers B.V.
scattering [2], as expected theoretically since there are no valence antiquarks and the sea antiquarks carry only a small fraction o f the proton momentum. In order to study the annihilation contribution at the valence level, direct photons must be produced using 7z-, rt +, or ~. A thorough study has been performed by comparing production in 7t-p and 7r+p interactions [3,4], but the largest annihilation contribution is expected in ~p collisions, and measurements have been made at the C E R N ISR [5] and SPS Collider [6], and at the Fermilab Tevatron [7]. At the ISR the ~ p luminosity was too low to allow significant production o f high P r direct photons, although pp data exist [ 1 ], whereas at the ~p colliders p p collisions are not available for comparison. Furthermore in ffp colliders, although direct photons have been measured at high Pr, only the low x r 243
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PHYSICSLETTERSB
( = 2privY) range is currently explored where, in any case, due to the dominance of the gluon over the quark structure functions at low x, such photons are produced predominantly by qg scattering. This paper reports the first observation of direct photons in both ffp and pp collisions over a range of x r in which the annihilation term is expected to contribute significantly to the ffp cross section. Both data samples cover the Pr range 4.1 < p r < 6.1 GeV/c, which, at the ~ of this experiment (24.3 GeV), corresponds to an x r range from 0.34 to 0.50. The ffp data presented in this letter have already been published [8]. These data were reanalysed at the same time as the pp data using more powerful reconstruction algorithms and an improved energyscale calibration. The resulting ffp cross sections are lower than the earlier ones and supersede them.
2. Apparatus The apparatus of experiment UA6 was located in a 12 m long straight section of the CERN SPS. A H2 cluster jet [9] was used as an internal target for both the proton and the antiproton circulating beams. The 315 GeV/c beams were bunched and since the jet was 150 m from an interaction region the proton and antiproton bunches crossed the jet at different times, 907 ns apart. Thus collisions of the proton beam with the jet and those of the antiproton beam with the jet could be readily separated. Because this was a fixedtarget experiment operating in a collider, secondaries produced in pp collisions were produced forward with respect to the proton beam whereas secondaries produced in ffp collisions were produced forward with respect to the antiproton beam. The secondaries produced by the two types of collisions were therefore produced 180° apart and hence could not be studied simultaneously using a single spectrometer. In 1985 the jet was placed at one end of the 12 m straight section and the spectrometer was placed downstream with respect to the antiproton beam in order to study ffp collisions. Then in 1986 the jet was moved to the other end of the straight section and the spectrometer was positioned downstream of it with respect to the proton beam in order to study pp collisions. The pp and ffp data were therefore collected in the same spectrometer (although not simultaneously) thereby 244
4 November 1993
reducing some systematics in comparing the two types of collision. The jet target consisted of a vertical stream of pure hydrogen clusters. The dimensions of the interaction volume were defined along the beam by the length of the jet, 8 ram, vertically by the height of the SPS beam, < 1 mm, and horizontally by the overlap of the jet and the beam, each 3 mm wide. A set of solid-state detectors [ 10], placed near 90 ° in the laboratory, monitored the number of recoil protons from elastic pp or ffp scattering. From the counting rate in these detectors and the elastic cross section, obtained from an interpolation of data at other x/s values, the luminosity was determined using the Optical Theorem. The uncertainty in the overall luminosity was about ~2.5%. During the ffp data-taking the typical number of antiprotons stored was 4 x 10 l°, yielding a luminosity of 5.5 × 10 29 c m - 2 s - l . The number of protons stored during the pp data-taking was an order of magnitude higher. At the maximum jet density this gave so high an instantaneous luminosity that the probability of double interactions occurring in one bunch crossing was non-negligible. The jet density was therefore reduced, by a variable amount depending on the number of stored protons, in order to keep the luminosity at approximately 2 × l03° c m - 2 s -1. Even at the maximum jet density of 4 × 1014 protons cm -3, the density was sufficiently low that partides produced in a collision had a negligible probability of re-interacting. The spectrometer, shown in fig. 1, consisted of two arms each covering 20 to 100 mrad in polar angle and 70 ° in azimuth in the laboratory system, corresponding to 1.8 sr in the centre-of-mass (CM) system. The CM rapidity coverage was -0.2 to 1.0. Each arm consisted of five multiwire proportional chambers (MWPCs), a 2.3 T.m dipole magnet, an ionization chamber ( d E / d x ) , a Li/Xe transition-radiation detector and an electromagnetic (EM) calorimeter of the lead/proportional-tube type. A calorimeter [ 11 ] consisted of 30 lead plates, each 0.8 radiation length ()to) thick, interleaved with alternating layers of horizontal and vertical tubes of 1 cm transverse dimension. For read-out purposes each calorimeter was divided longitudinally into three identical modules of 8 Xo each. The minimum observable two-shower separation was 2.0 cm. Measurements in an electron testbeam showed no evidence of non-linearity between
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PHYSICS LETTERSB
4 November 1993
t
3 pc2 f
#~~.~"~
~
P ~
~ C
~"
•
Magnet PC3
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~-TRD
-Ca or moter SPS Beam pipe
Fig. 1. Schematic diagram of the UA6 double-arm spectrometer. 6 and 150 GeV and an energy resolution o f e ( E ) / E = 0.01 + 0 . 3 1 / ~ (E in GeV). The overall energy scale was calibrated off-line using the n°-mass peak, an adjustment which could be conveniently made for each hour of data-taking. A more complete description of the spectrometer can be found in refs. [ 12,13 ]. A hodoscope of seven horizontal scintillator counters, located between the first and second modules of each calorimeter close to shower maximum, served as a pre-trigger. A hardware processor, using the pulseheight information from the calorimeter as input, accepted just those events containing a cluster withpr > 3 GeV/c. The data described in this letter come from an integrated luminosity of 447 nb -1 in ~p and 1284 nb -l in p p .
ated module-to-module in depth by requiring that a line through the cluster centroids extrapolated to the jet position. Finally, clusters in the horizontal (H) view of energy EHi were matched with clusters in the vertical (V) view of energy E v i to obtain showers. The resolution of ambiguities in matching the two views when there were two or more dusters in either view was achieved by finding the most probable combination according to energy matching. Since the horizontal planes alternated with the vertical planes in the calorimeter, the H and V tubes provided two independent measurements, Ev and EH, of the shower energy (each with half the sampling of the whole calorimeter). This means that for a given shower the difference between Ev and EH arises mostly from the energy resolution of the calorimeter. Defining a (Ev - Eft) as the standard deviation of the distribution of Ev - En gives
3. Off-line analysis Individual EM showers were identified off-line as follows. In the horizontal and vertical views of each calorimeter module, clusters of individual tubes containing more than 30 MeV were formed around any seed tube containing more than a threshold energy of 1.3 GeV in the first module, 0.5 GeV in the second, and 0.2 GeV in the third. Clusters were then associ-
a(Ev -
EH) = 2V~V + EH.
The coefficient 2 describes the effective energy resolution of the calorimeter. In the case of the same number, N, of clusters in the H and V views, all possible contributions ofEv, EH matchings were tried. For each combination a global Z 2 defined as 245
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Z2 = ~__.~ (Evi - EHi)2 i=1
was calculated. The combination with smallest Z 2 was then selected. F o r events in which the numbers of clusters in the H and V views were unequal, clusters in the view containing the smaller number o f d u s t e r s were allowed to be matched to sums o f clusters in the other view. A value o f 2 = 0.40, chosen to give a flat Z 2 probability distribution, was used. This value is somewhat poorer than the intrinsic energy resolution of the calorimeter ( = 0.31 ) because o f clustering and multiparticle effects, but the results are insensitive to the precise value used. Further details can be found in ref. [ 14 ]. The distribution o f the invariant mass of two showers in the same arm shows clear n ° and r/peaks, as in ref. [ 15 ], on backgrounds of approximately 5°/0 and 50% respectively. An EM shower was accepted as a direct p h o t o n candidate if (i) it had a P r larger than 4.1 GeV/c; (ii) it satisfied a software simulation o f the pretrigger and processor trigger conditions; this cut rejected EM showers that were below the processor threshold but were helped over this threshold because o f a second nearby shower. It also helped refining the hardware threshold; (iii) it was located well within the calorimeter (more than 5 cm from the horizontal edges and more than 15 cm from the vertical edges) in order to be within the magnet acceptance; (iv) it did not reconstruct as a n o candidate (0 < mrr < 200 M e V / c 2) with any other shower in the calorimeter; (v) it had an R M S width less than 1.35 cm in the first calorimeter module (to reject events in which two photons from a n o decay merged into a single shower); and (vi) it had a transverse shape with no dip near the centre (which also rejects merging showers). The background in the direct photon sample arising from n o and r / d e c a y s was estimated using a Monte Carlo program which generated these mesons according to a parametrization o f the form (1 - x)m/p 2n, with m and n tuned iteratively until the acceptancecorrected data and generator distributions matched. In the simulation, for each generated photon an EM shower of the appropriate energy and position within 246
4 November 1993
2.2
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Fig. 2. The uncorrected ~,/n° ratio for ~p interactions (open circles) and pp interactions (solid circles) as a function o f PT. The background levels predicted by the Monte Carlo from 7t° (dashed line) and r/ (dotted line) mesons are also shown.
a tube was selected from a bank of showers collected by placing one calorimeter in an electron test b e a m at the SPS. The first module of the calorimeter used in the test-beam was 1.6 Xo thinner than that o f the calorimeters used for data taking. To account for this difference and for electron/photon shower-shape differences, the longitudinal and transverse shower shapes of the electron shower banks were slightly modified to match those o f photons from reconstructed n ° candidates. Each generated event thus gave an array o f tube energies, just like a real event, and was passed through the same analysis chain as the real data. The Monte Carlo reproduced well the properties o f the measured n°'s such as the energy, asymmetry and spatial separation o f the two photons from n ° decays. The program was used to calculate the acceptances for n ° and )7 decays, and to calculate the ratios o f the number of n°'s and ~/'s that appear as single ?'s to the number correctly reconstructed. The number o f single ?'s in our sample which arise from n ° and ~/ decays was computed from these ratios and the actual numbers of n°'s and ~/'s observed in our data.
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PHYSICS LETTERS B
The uncorrected ratio of direct y candidates to reconstructed lr°'s ( = y/Tr °) is shown in fig. 2 for t h e p p and ffp data samples. The background contributions from it ° and r/decays as computed from the Monte Carlo are also shown. The largest contribution to the 7r° background (70%) is from it ° decays in which one photon is of too low energy to be reconstructed. The other sources of background are zt° decays in which one photon misses the detector and those in which the two photons coalesce. The latter source becomes important at high Pr which explains the increase in the overall background as a function o f p r . The contribution from r/'s is 15% at Pr = 4.1 GeV/c and drops with increasing pr. Other neutral hadrons and neutral mesons give a negligible contribution. The estimated background is essentially the same in ffp and pp interactions, the only differences being due to small acceptance changes between the two data-taking periods.
4. Results
4 November 1993
1.4 1.3
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1.2
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.
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,
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,
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,
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,
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I
,
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Fig. 3. The corrected y/n ° ratios for ffp interactions (open circles) and pp interactions (solid circles) as a function of PT.
The final y / n ° ratios after background subtraction and acceptance correction are given in fig. 3. The agreement with data obtained by other experiments [3,16] in pp collisions at similar CM energies is good. The invariant cross sections for direct photon production, averaged over the rapidity range - 0 . 2 to 1.0, are shown in fig. 4. The statistical errors alone are shown as solid error bars and the brackets show the total error after linear addition of a systematic error contribution. The following sources of systematic errors have been considered and found to be approximately the same in the pp and ffp data samples: Monte Carlo (+10%), the use of a singlepanicle Monte Carlo instead o f a complete event generator (+8% by comparing with a complete simulation using Pythia [19] ), the contribution from charged panicles (±1.5%), the background under the rc° mass peak (+1%), and the luminosity uncertainty (4-2.5%). The Monte Carlo contribution was derived by studying the changes in the acceptances and backgrounds resulting from variations in the input parameters compatible with their uncertainties, thus leading to: 1% from the calorimeter energy resolution, 3.5% from the generator shape, 3% from the transverse shower shape, and 9% from the longitudinal shower shape. A possible systematic error of
o pp~vX •
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Theoretical
curves from
P. A u r e n c h e
e t al. w i t h
~1=4.0, A = 2 3 1 . 5 10 "1 3.9
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Px [GeV/cl Fig. 4. The invariant cross sections for direct photon production averaged over the rapidity interval -0.2 < y < 1.0 as a function of Pr for ~p interactions (open circles) and pp interactions (solid circles). The theoretical predictions of ref. [ 17 ] using the ABFOW structure functions [ 18 ] are also shown. 247
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PHYSICS LETTERS B
+5% was ascribed to the analysis and reconstruction in the ffp case, and q-14% in the pp case, to reflect the sensitivity of the results to variation in the cuts and thresholds; the larger p p value was mainly due to larger backgrounds during the higher-luminosity pp data-taking, which led to greater sensitivity to lowenergy thresholds. The calorimeter electronics were affected by noise which introduced an additional uncertainty of +3% in the ~p data and +1.5% in the pp data. Finally a Pr scale uncertainty of +0.7% could also be interpreted as a -4-5% uncertainty in the photon cross sections. When added in quadrature the overall systematic uncertainty is + 15% and +20% in the ffp and pp cross sections respectively. The pp cross section is clearly larger than the pp cross section. This is in contrast to the rc° cross sections which are found to be the same in the two types of interaction. The larger ffp cross section indicates a significant contribution from the annihilation term. Also shown in fig. 4 are predictions from Aurenche et al. [17], using four quark flavours, Q2 scales obtained via the Principle of Minimal Sensitivity [20], and the ABFOW [18] set of structure functions. In ABFOW, the gluon distribution is parametrized as x G ( x , Q ~ = 2 GeV 2) = Ag(1 - x ) ~ with t/ = 4.0. The QCD scale, A, is 231.5 MeV. The agreement with the data is excellent. The first measurement of the difference a (gp --* y X ) - ~ (pp ---, y X ) is shown in fig. 5. Of the systematic errors discussed above, those relating to the Monte Carlo, event complexity, charged particles, and background under the zr°, shift both ~p and pp cross sections in the same direction, and hence contribute only once to the difference, whereas the others contribute independently. Adding in quadrature thus leads to an overall systematic error of 4-21% on the difference. The prediction of the Aurenche et al. calculation is also shown in the figure. Note that the difference is insensitive to the form of the gluon structure functions used and is therefore a good estimator of A, the strong-interaction coupling constant; this forms the subject of the companion paper. In conclusion, the direct photon cross section in ffp collisions has been found to be systematically larger than that in pp collisions. This indicates a significant contribution o f the ~q annihilation term as predicted by theoretical calculations.
248
4 November 1993
10 3
;>
o(ffp---)yX) - o(pp---)TX) 10 2
10
1 Theoretical curve from P. Aurenche et al. with A=231.5 MeV -I 10 3.9
I
4.1
4.3
4.5
4.7
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Fig. 5. The difference between the ~p and pp direct photon cross sections as a function of Pr. The theoretical prediction of ref. [ 17 ] using the ABFOW structure functions [ 18 ] is also shown.
Acknowledgement We would like to thank the CERN staff, particularly those in the SPS and CN divisions, without whom this experiment would not have been possible. The following funding agencies provided support: the Swiss National Funding Agency, the US Department of Energy, and the US National Science Foundation. We are grateful for the continued efforts of our secretarial and technical staff.
References [1 ] T. Ferbel and W.R. Molzon, Rev. Mod. Phys. 56 (1984) 181, and references therein. [2] M. Diakonou et al., Phys. Lett. B 91 (1980) 301; A.L.S. Angelis et al., Phys. Lett. B 98 (1981) 115. [3] J. Badier et al., Z. Phys. C 31 (1986) 341; C. De Marzo et al., Phys. Rev. D 36 (1987) 8. [4] M. Bonesini et al., Z. Phys. C 37 (1988) 535. [5] T. Akesson et al., Phys. Lett. B 158 (1985) 282. [6] C. Albajar et al., Phys. Lett. B 209 (1988) 397; R. Ansari et al., Z. Phys. C 41 (1988) 395; J. Alitti et al., Phys. Lett. B 288 (1992) 386. [7] F. Abe et al., Phys. Rev. Lett. 68 (1992) 2734. [8] A. Bernasconi et al., Phys. Lett. B 206 (1988) 163.
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[9] L. Dick and W. Kubischta, Physics with jet targets at the SPS ffp collider, in: Hadronic physics at intermediate energies, eds. T. Bressani and R.A. Ricci (Elsevier Science Publishers, Amsterdam, 1986) p. 209. [10] R.E. Breedon et al., Phys. Lett. B 216 (1989) 459. [ l l ] L . Camilleri et al., Nucl. Instrum. Methods A 286 (1990) 49. [12] C. Morel, Mesure de la section efficace inclusive de production du J/7' dans les collisions protonproton et antiproton-proton ~ ~ = 24.3 GeV, thesis, University of Lausanne (I 990). [13] C. Morel et al., Phys. Lett. B 252 (1990) 505. [ 14 ] G. Sozzi, Comparaison des sections efficaces inclusives de production de y directs dans les interactions ffp et pp, ~t v~ = 24.3 GeV et ~ 4 < Pr < 6 GeV/c, thesis, University of Lausanne (1992).
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[15] J. Antille et al., Phys. Lett. B 194 (1987) 568. [16] M. Bonesini et al., Z. Phys. C 38 (1988) 371; G. Alverson et al., Phys. Rev. Lett. 68 (1992) 2584. [17] P. Aurenche et al., Phys. Lett. B 140 (1984) 87; Nucl. Phys. B 286 (1987) 509; B 297 (1988) 661. [18] P. Aurenche et al., Phys. Rev. D 39 (1989) 3275. [19] H.-U. Bengtsson and T. SjiSstrand, Comput. Phys. Commun. 46 (1987) 43; T. Sj6strand, CERN-TH 6488/92. [20] P.M. Stevenson, Phys. Rev. D 23 (1981) 2916; P.M. Stevenson and H.D. Politzer, Nucl. Phys. B 277 (1986) 958.
249
PHYSICS LETTERS B
North-Holland
Direct photon production in at x/s = 24.3 GeV
and p p interactions
UA6 Collaboration
CERN-Lausanne-Michigan-Rockefeller G. Sozzi b, G. Ballocchi c, A. Bernasconi b, R.E. Breedon d,1, L. Camilleri a, R.L. Cool d,2, P.T. Cox d, P. Cushman d,e, L. Dick a,3, E.C. Dukes c,4, B. Gabioud b, F. Gaille b, P. Giacomelli d,a, D. Hubbard c, J.B. Jeanneret a, C. Joseph b, W. Kubisehta a, J.F. Loude b, E. Malamud b,5, C. Morel b, p. Oberson b, O.E. Overseth c, J.L. Pages b,a, j.p. Perroud b, D. Rtiegger b, R.W. Rusack d, V. Singh e, G.R. Snow c, D. Steiner b, L. Studer b, M.T. Tran b, A. Vacchi d,6, G. Valenti c,7 and M. Werlen b a CERN, CH-1211 Geneva 23, Switzerland b Universitb de Lausanne, CH-IO15 Lausanne, Switzerland c University of Michigan, Ann Arbor, MI 48109, USA d Rockefeller University, New York, N Y 10021, USA e Yale University, New Haven, CT 06511, USA
Received 30 July 1993 Editor: K. Winter
Inclusive direct photon invariant cross sections have been measured in both ~p and pp collisions at x/£ = 24.3 GeV at the CERN SPS, permitting the first measurement of the difference of the pp and pp cross sections. The direct photon cross section in ~p collisions has been found to be systematically larger than that in pp collisions, which indicates a significant contribution of the ~q annihilation term as predicted by theoretical calculations.
1. Introduction Direct photons o f high Pr are expected to be produced in hadronic collisions through three m a i n processes: gluon C o m p t o n scattering ( q g ) , a n t i q u a r k quark annihilation (~q), and to a lesser extent bremsstrahlung from a quark [ 1 ]. In p p collisions the m a i n contribution has been shown to come from q g l 2 3 4 5 6 7
Present address: KEK 1-1-OHO, Tsukuba-Shi IbarakiKen 305, Japan. Deceased. Present address: INFN, 1-20133 Milan, Italy. Present address: University of Virginia, VA 22901, USA. Present address: Fermilab, Batavia, IL 60510, USA. Present address: Universit/t di Trieste, 1-34127 Trieste, Italy. Present address: INFN, 1-40126 Bologna, Italy.
Elsevier Science Publishers B.V.
scattering [2], as expected theoretically since there are no valence antiquarks and the sea antiquarks carry only a small fraction o f the proton momentum. In order to study the annihilation contribution at the valence level, direct photons must be produced using 7z-, rt +, or ~. A thorough study has been performed by comparing production in 7t-p and 7r+p interactions [3,4], but the largest annihilation contribution is expected in ~p collisions, and measurements have been made at the C E R N ISR [5] and SPS Collider [6], and at the Fermilab Tevatron [7]. At the ISR the ~ p luminosity was too low to allow significant production o f high P r direct photons, although pp data exist [ 1 ], whereas at the ~p colliders p p collisions are not available for comparison. Furthermore in ffp colliders, although direct photons have been measured at high Pr, only the low x r 243
Volume 317, number 1,2
PHYSICSLETTERSB
( = 2privY) range is currently explored where, in any case, due to the dominance of the gluon over the quark structure functions at low x, such photons are produced predominantly by qg scattering. This paper reports the first observation of direct photons in both ffp and pp collisions over a range of x r in which the annihilation term is expected to contribute significantly to the ffp cross section. Both data samples cover the Pr range 4.1 < p r < 6.1 GeV/c, which, at the ~ of this experiment (24.3 GeV), corresponds to an x r range from 0.34 to 0.50. The ffp data presented in this letter have already been published [8]. These data were reanalysed at the same time as the pp data using more powerful reconstruction algorithms and an improved energyscale calibration. The resulting ffp cross sections are lower than the earlier ones and supersede them.
2. Apparatus The apparatus of experiment UA6 was located in a 12 m long straight section of the CERN SPS. A H2 cluster jet [9] was used as an internal target for both the proton and the antiproton circulating beams. The 315 GeV/c beams were bunched and since the jet was 150 m from an interaction region the proton and antiproton bunches crossed the jet at different times, 907 ns apart. Thus collisions of the proton beam with the jet and those of the antiproton beam with the jet could be readily separated. Because this was a fixedtarget experiment operating in a collider, secondaries produced in pp collisions were produced forward with respect to the proton beam whereas secondaries produced in ffp collisions were produced forward with respect to the antiproton beam. The secondaries produced by the two types of collisions were therefore produced 180° apart and hence could not be studied simultaneously using a single spectrometer. In 1985 the jet was placed at one end of the 12 m straight section and the spectrometer was placed downstream with respect to the antiproton beam in order to study ffp collisions. Then in 1986 the jet was moved to the other end of the straight section and the spectrometer was positioned downstream of it with respect to the proton beam in order to study pp collisions. The pp and ffp data were therefore collected in the same spectrometer (although not simultaneously) thereby 244
4 November 1993
reducing some systematics in comparing the two types of collision. The jet target consisted of a vertical stream of pure hydrogen clusters. The dimensions of the interaction volume were defined along the beam by the length of the jet, 8 ram, vertically by the height of the SPS beam, < 1 mm, and horizontally by the overlap of the jet and the beam, each 3 mm wide. A set of solid-state detectors [ 10], placed near 90 ° in the laboratory, monitored the number of recoil protons from elastic pp or ffp scattering. From the counting rate in these detectors and the elastic cross section, obtained from an interpolation of data at other x/s values, the luminosity was determined using the Optical Theorem. The uncertainty in the overall luminosity was about ~2.5%. During the ffp data-taking the typical number of antiprotons stored was 4 x 10 l°, yielding a luminosity of 5.5 × 10 29 c m - 2 s - l . The number of protons stored during the pp data-taking was an order of magnitude higher. At the maximum jet density this gave so high an instantaneous luminosity that the probability of double interactions occurring in one bunch crossing was non-negligible. The jet density was therefore reduced, by a variable amount depending on the number of stored protons, in order to keep the luminosity at approximately 2 × l03° c m - 2 s -1. Even at the maximum jet density of 4 × 1014 protons cm -3, the density was sufficiently low that partides produced in a collision had a negligible probability of re-interacting. The spectrometer, shown in fig. 1, consisted of two arms each covering 20 to 100 mrad in polar angle and 70 ° in azimuth in the laboratory system, corresponding to 1.8 sr in the centre-of-mass (CM) system. The CM rapidity coverage was -0.2 to 1.0. Each arm consisted of five multiwire proportional chambers (MWPCs), a 2.3 T.m dipole magnet, an ionization chamber ( d E / d x ) , a Li/Xe transition-radiation detector and an electromagnetic (EM) calorimeter of the lead/proportional-tube type. A calorimeter [ 11 ] consisted of 30 lead plates, each 0.8 radiation length ()to) thick, interleaved with alternating layers of horizontal and vertical tubes of 1 cm transverse dimension. For read-out purposes each calorimeter was divided longitudinally into three identical modules of 8 Xo each. The minimum observable two-shower separation was 2.0 cm. Measurements in an electron testbeam showed no evidence of non-linearity between
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t
3 pc2 f
#~~.~"~
~
P ~
~ C
~"
•
Magnet PC3
J
~-TRD
-Ca or moter SPS Beam pipe
Fig. 1. Schematic diagram of the UA6 double-arm spectrometer. 6 and 150 GeV and an energy resolution o f e ( E ) / E = 0.01 + 0 . 3 1 / ~ (E in GeV). The overall energy scale was calibrated off-line using the n°-mass peak, an adjustment which could be conveniently made for each hour of data-taking. A more complete description of the spectrometer can be found in refs. [ 12,13 ]. A hodoscope of seven horizontal scintillator counters, located between the first and second modules of each calorimeter close to shower maximum, served as a pre-trigger. A hardware processor, using the pulseheight information from the calorimeter as input, accepted just those events containing a cluster withpr > 3 GeV/c. The data described in this letter come from an integrated luminosity of 447 nb -1 in ~p and 1284 nb -l in p p .
ated module-to-module in depth by requiring that a line through the cluster centroids extrapolated to the jet position. Finally, clusters in the horizontal (H) view of energy EHi were matched with clusters in the vertical (V) view of energy E v i to obtain showers. The resolution of ambiguities in matching the two views when there were two or more dusters in either view was achieved by finding the most probable combination according to energy matching. Since the horizontal planes alternated with the vertical planes in the calorimeter, the H and V tubes provided two independent measurements, Ev and EH, of the shower energy (each with half the sampling of the whole calorimeter). This means that for a given shower the difference between Ev and EH arises mostly from the energy resolution of the calorimeter. Defining a (Ev - Eft) as the standard deviation of the distribution of Ev - En gives
3. Off-line analysis Individual EM showers were identified off-line as follows. In the horizontal and vertical views of each calorimeter module, clusters of individual tubes containing more than 30 MeV were formed around any seed tube containing more than a threshold energy of 1.3 GeV in the first module, 0.5 GeV in the second, and 0.2 GeV in the third. Clusters were then associ-
a(Ev -
EH) = 2V~V + EH.
The coefficient 2 describes the effective energy resolution of the calorimeter. In the case of the same number, N, of clusters in the H and V views, all possible contributions ofEv, EH matchings were tried. For each combination a global Z 2 defined as 245
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Z2 = ~__.~ (Evi - EHi)2 i=1
was calculated. The combination with smallest Z 2 was then selected. F o r events in which the numbers of clusters in the H and V views were unequal, clusters in the view containing the smaller number o f d u s t e r s were allowed to be matched to sums o f clusters in the other view. A value o f 2 = 0.40, chosen to give a flat Z 2 probability distribution, was used. This value is somewhat poorer than the intrinsic energy resolution of the calorimeter ( = 0.31 ) because o f clustering and multiparticle effects, but the results are insensitive to the precise value used. Further details can be found in ref. [ 14 ]. The distribution o f the invariant mass of two showers in the same arm shows clear n ° and r/peaks, as in ref. [ 15 ], on backgrounds of approximately 5°/0 and 50% respectively. An EM shower was accepted as a direct p h o t o n candidate if (i) it had a P r larger than 4.1 GeV/c; (ii) it satisfied a software simulation o f the pretrigger and processor trigger conditions; this cut rejected EM showers that were below the processor threshold but were helped over this threshold because o f a second nearby shower. It also helped refining the hardware threshold; (iii) it was located well within the calorimeter (more than 5 cm from the horizontal edges and more than 15 cm from the vertical edges) in order to be within the magnet acceptance; (iv) it did not reconstruct as a n o candidate (0 < mrr < 200 M e V / c 2) with any other shower in the calorimeter; (v) it had an R M S width less than 1.35 cm in the first calorimeter module (to reject events in which two photons from a n o decay merged into a single shower); and (vi) it had a transverse shape with no dip near the centre (which also rejects merging showers). The background in the direct photon sample arising from n o and r / d e c a y s was estimated using a Monte Carlo program which generated these mesons according to a parametrization o f the form (1 - x)m/p 2n, with m and n tuned iteratively until the acceptancecorrected data and generator distributions matched. In the simulation, for each generated photon an EM shower of the appropriate energy and position within 246
4 November 1993
2.2
2
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•
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PT [ G e V / c l
Fig. 2. The uncorrected ~,/n° ratio for ~p interactions (open circles) and pp interactions (solid circles) as a function o f PT. The background levels predicted by the Monte Carlo from 7t° (dashed line) and r/ (dotted line) mesons are also shown.
a tube was selected from a bank of showers collected by placing one calorimeter in an electron test b e a m at the SPS. The first module of the calorimeter used in the test-beam was 1.6 Xo thinner than that o f the calorimeters used for data taking. To account for this difference and for electron/photon shower-shape differences, the longitudinal and transverse shower shapes of the electron shower banks were slightly modified to match those o f photons from reconstructed n ° candidates. Each generated event thus gave an array o f tube energies, just like a real event, and was passed through the same analysis chain as the real data. The Monte Carlo reproduced well the properties o f the measured n°'s such as the energy, asymmetry and spatial separation o f the two photons from n ° decays. The program was used to calculate the acceptances for n ° and )7 decays, and to calculate the ratios o f the number of n°'s and ~/'s that appear as single ?'s to the number correctly reconstructed. The number o f single ?'s in our sample which arise from n ° and ~/ decays was computed from these ratios and the actual numbers of n°'s and ~/'s observed in our data.
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The uncorrected ratio of direct y candidates to reconstructed lr°'s ( = y/Tr °) is shown in fig. 2 for t h e p p and ffp data samples. The background contributions from it ° and r/decays as computed from the Monte Carlo are also shown. The largest contribution to the 7r° background (70%) is from it ° decays in which one photon is of too low energy to be reconstructed. The other sources of background are zt° decays in which one photon misses the detector and those in which the two photons coalesce. The latter source becomes important at high Pr which explains the increase in the overall background as a function o f p r . The contribution from r/'s is 15% at Pr = 4.1 GeV/c and drops with increasing pr. Other neutral hadrons and neutral mesons give a negligible contribution. The estimated background is essentially the same in ffp and pp interactions, the only differences being due to small acceptance changes between the two data-taking periods.
4. Results
4 November 1993
1.4 1.3
o(-¢)/o('n °)
1.2
o ~p
1.1 !
•
pp
J 4.3
~
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
.
0
,
+ +
4.1
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,
I 4.7
,
r 4.9
,
I 5.1
I
,
5.3
I
,
5,5
I
,
5.7
I
,
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P'r [ G e V / c ]
Fig. 3. The corrected y/n ° ratios for ffp interactions (open circles) and pp interactions (solid circles) as a function of PT.
The final y / n ° ratios after background subtraction and acceptance correction are given in fig. 3. The agreement with data obtained by other experiments [3,16] in pp collisions at similar CM energies is good. The invariant cross sections for direct photon production, averaged over the rapidity range - 0 . 2 to 1.0, are shown in fig. 4. The statistical errors alone are shown as solid error bars and the brackets show the total error after linear addition of a systematic error contribution. The following sources of systematic errors have been considered and found to be approximately the same in the pp and ffp data samples: Monte Carlo (+10%), the use of a singlepanicle Monte Carlo instead o f a complete event generator (+8% by comparing with a complete simulation using Pythia [19] ), the contribution from charged panicles (±1.5%), the background under the rc° mass peak (+1%), and the luminosity uncertainty (4-2.5%). The Monte Carlo contribution was derived by studying the changes in the acceptances and backgrounds resulting from variations in the input parameters compatible with their uncertainties, thus leading to: 1% from the calorimeter energy resolution, 3.5% from the generator shape, 3% from the transverse shower shape, and 9% from the longitudinal shower shape. A possible systematic error of
o pp~vX •
e~ 10 2
pp--->TX
~t-~
I0
Theoretical
curves from
P. A u r e n c h e
e t al. w i t h
~1=4.0, A = 2 3 1 . 5 10 "1 3.9
,
I
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,
I
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I
4.7
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r 5.1
,
I
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~
I 5.5
,
,
I
5.7
I
5.9
, 6.1
Px [GeV/cl Fig. 4. The invariant cross sections for direct photon production averaged over the rapidity interval -0.2 < y < 1.0 as a function of Pr for ~p interactions (open circles) and pp interactions (solid circles). The theoretical predictions of ref. [ 17 ] using the ABFOW structure functions [ 18 ] are also shown. 247
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+5% was ascribed to the analysis and reconstruction in the ffp case, and q-14% in the pp case, to reflect the sensitivity of the results to variation in the cuts and thresholds; the larger p p value was mainly due to larger backgrounds during the higher-luminosity pp data-taking, which led to greater sensitivity to lowenergy thresholds. The calorimeter electronics were affected by noise which introduced an additional uncertainty of +3% in the ~p data and +1.5% in the pp data. Finally a Pr scale uncertainty of +0.7% could also be interpreted as a -4-5% uncertainty in the photon cross sections. When added in quadrature the overall systematic uncertainty is + 15% and +20% in the ffp and pp cross sections respectively. The pp cross section is clearly larger than the pp cross section. This is in contrast to the rc° cross sections which are found to be the same in the two types of interaction. The larger ffp cross section indicates a significant contribution from the annihilation term. Also shown in fig. 4 are predictions from Aurenche et al. [17], using four quark flavours, Q2 scales obtained via the Principle of Minimal Sensitivity [20], and the ABFOW [18] set of structure functions. In ABFOW, the gluon distribution is parametrized as x G ( x , Q ~ = 2 GeV 2) = Ag(1 - x ) ~ with t/ = 4.0. The QCD scale, A, is 231.5 MeV. The agreement with the data is excellent. The first measurement of the difference a (gp --* y X ) - ~ (pp ---, y X ) is shown in fig. 5. Of the systematic errors discussed above, those relating to the Monte Carlo, event complexity, charged particles, and background under the zr°, shift both ~p and pp cross sections in the same direction, and hence contribute only once to the difference, whereas the others contribute independently. Adding in quadrature thus leads to an overall systematic error of 4-21% on the difference. The prediction of the Aurenche et al. calculation is also shown in the figure. Note that the difference is insensitive to the form of the gluon structure functions used and is therefore a good estimator of A, the strong-interaction coupling constant; this forms the subject of the companion paper. In conclusion, the direct photon cross section in ffp collisions has been found to be systematically larger than that in pp collisions. This indicates a significant contribution o f the ~q annihilation term as predicted by theoretical calculations.
248
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10 3
;>
o(ffp---)yX) - o(pp---)TX) 10 2
10
1 Theoretical curve from P. Aurenche et al. with A=231.5 MeV -I 10 3.9
I
4.1
4.3
4.5
4.7
4.9
i
i
5.1
I
5.3
i
I
5.5
i
I
i
I
i
5.7 5.9 6.1 P'r [GeV/c]
Fig. 5. The difference between the ~p and pp direct photon cross sections as a function of Pr. The theoretical prediction of ref. [ 17 ] using the ABFOW structure functions [ 18 ] is also shown.
Acknowledgement We would like to thank the CERN staff, particularly those in the SPS and CN divisions, without whom this experiment would not have been possible. The following funding agencies provided support: the Swiss National Funding Agency, the US Department of Energy, and the US National Science Foundation. We are grateful for the continued efforts of our secretarial and technical staff.
References [1 ] T. Ferbel and W.R. Molzon, Rev. Mod. Phys. 56 (1984) 181, and references therein. [2] M. Diakonou et al., Phys. Lett. B 91 (1980) 301; A.L.S. Angelis et al., Phys. Lett. B 98 (1981) 115. [3] J. Badier et al., Z. Phys. C 31 (1986) 341; C. De Marzo et al., Phys. Rev. D 36 (1987) 8. [4] M. Bonesini et al., Z. Phys. C 37 (1988) 535. [5] T. Akesson et al., Phys. Lett. B 158 (1985) 282. [6] C. Albajar et al., Phys. Lett. B 209 (1988) 397; R. Ansari et al., Z. Phys. C 41 (1988) 395; J. Alitti et al., Phys. Lett. B 288 (1992) 386. [7] F. Abe et al., Phys. Rev. Lett. 68 (1992) 2734. [8] A. Bernasconi et al., Phys. Lett. B 206 (1988) 163.
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[9] L. Dick and W. Kubischta, Physics with jet targets at the SPS ffp collider, in: Hadronic physics at intermediate energies, eds. T. Bressani and R.A. Ricci (Elsevier Science Publishers, Amsterdam, 1986) p. 209. [10] R.E. Breedon et al., Phys. Lett. B 216 (1989) 459. [ l l ] L . Camilleri et al., Nucl. Instrum. Methods A 286 (1990) 49. [12] C. Morel, Mesure de la section efficace inclusive de production du J/7' dans les collisions protonproton et antiproton-proton ~ ~ = 24.3 GeV, thesis, University of Lausanne (I 990). [13] C. Morel et al., Phys. Lett. B 252 (1990) 505. [ 14 ] G. Sozzi, Comparaison des sections efficaces inclusives de production de y directs dans les interactions ffp et pp, ~t v~ = 24.3 GeV et ~ 4 < Pr < 6 GeV/c, thesis, University of Lausanne (1992).
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[15] J. Antille et al., Phys. Lett. B 194 (1987) 568. [16] M. Bonesini et al., Z. Phys. C 38 (1988) 371; G. Alverson et al., Phys. Rev. Lett. 68 (1992) 2584. [17] P. Aurenche et al., Phys. Lett. B 140 (1984) 87; Nucl. Phys. B 286 (1987) 509; B 297 (1988) 661. [18] P. Aurenche et al., Phys. Rev. D 39 (1989) 3275. [19] H.-U. Bengtsson and T. SjiSstrand, Comput. Phys. Commun. 46 (1987) 43; T. Sj6strand, CERN-TH 6488/92. [20] P.M. Stevenson, Phys. Rev. D 23 (1981) 2916; P.M. Stevenson and H.D. Politzer, Nucl. Phys. B 277 (1986) 958.
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