Early results from experiment UA1 at the CRN pp# collider Anne Kernan Citation: AIP Conference Proceedings 85, 325 (1982); View online: https://doi.org/10.1063/1.33563 View Table of Contents: http://aip.scitation.org/toc/apc/85/1 Published by the American Institute of Physics Chapter VI Early Data from CERN pp Collider 325 EARLY RESULTS FROM EXPERIMENT UAI AT THE CERN pp COLLIDER Anne Kernan University of California, Riverside, CA 92521 (Aaohen - A n n e c y (LAPP) - B i r m i n g h a m - CERN - Q u e e n M a r y College, L o n d o n - P a r i s (College de F r a n e e ) Riverside - Rome - Rutherford A p p l e t o n L a b o r a t o r y - S a c l a y (CEN) - Vienna C o l l a b o r a t i o n ) The UAI detector is a general purpose 47 apparatus for the measurement of hadron and lepton momenta at p~ collider energies. The performance of the detector and first results from 1981 running are discussed. I. INTRODUCTION The design of the UAI detector evolved from the p-~ study weeks organized by C. Rubbia at CERN in March and July of 1977. A proposal [i] was submitted in January 1978 and was approved in Summer 1978. The detector was installed in the SPS tunnel in early July 1981 and recorded proton-antiproton interactions at 540 GeV for the first time on July 17, 1981 . The results presented here are based on data taken during accelerator development operations in October and November 1981 with luminosity in the range 1025cm-2sec -I. II. ~ ~IE UAI DETECTOR The detector has three components: (i) the large angle detector covering the angular range 5~ 175 ~ with respect to the circulating beams, (ii) the forward detectors at 0.3~ ~ and (iii) the luminosity monitors at 0.2<e<2 mrad. i. The Large Angle Detector This apparatus (Fig. i) covers the rapidity range -3<Y<3 at cm 3 energy 540 GeV. It is dominated by the dipole magnet, 7.2x3.5x3.5 m , with internal magnetic volume 80 m J, and uniform horizontal field 0.7T. Each half of the magnet is composed of 8 independently movable C-shaped sections. As shown in Fig. 2 these sections are instrumented with scintillator, providing 16 samples of 5 cm iron with i cm of scintillator (5.5 interaction lengths). Endcaps of similar construction, with 23 samplings, cover the angular range 5o<0<25 ~ . The electromagnetic calorimeter inside the magnet consists of 48 units covering the region 25~ ~ Each unit (Fig. 3) is made of 26 radiation lengths of 1.2 mm lead sheets interleveaved with 1.5 E~ scintillator sheets. The endcap electromagnetic calorimeters (50<8<250) have 27 radiation lengths of lead-scintillator structure. 0094-243X/82/850325-2253.00 Copyright 1982 American Institute of Physics 326 0 o O ~.J ~0 I1) P~ 327 Fig. 2. One of the 16 "C" models comprising the magnet yoke and hadron calorimeter. 328 Each is segmented into 32 identical radial pet&Is. Fig. 3. Two units of the large angle electromagentic calorimeter. The heart of the large angle apparatus is the 6 m long x 2.4m diameter central detector (Fig. 4) which surrounds the beam pipe [3,4]. This drift chamber system has a total of 6200 sense wires, with maximum drift distance 18 cm. The "image" readout records the complete time structure of the incoming pulse, providing a bubble-chamber like picture of the event. The 2-track resolution is 3 mm and dE/dx resolution is +6%. The coordinate along the wire is obtained by current division. Fig. 5 shows the image read-out for events recorded with a "minimum bias" trigger, (section III), with 80% of the chamber volume 329 Fig. 4. The Central Detector "image" drift chambers. 330 9":"k 1 " " ~ a ' t 1/ 9, v . . ~',' ', -', ~ ~ ~ .'.. .~'t/"11.-'... \ ',. "...;fl.i/,;,t~t '~ V 99 . . . . . . ",, - \,i,B~./~!i ,. . / / - "-,... \ k~Jli " "9 "*'~"~ t ~.._...~. 9"...: ' .... ...--.,-..- .-'::::i~'r i,',l.~ .'~.~.~ ~ ....-: "9 . - ~~ ~ ~11 o ,~ / ,/ ~ "l.....~T. , 3~" .........." ,r v,%. ,~ 'x.'.-.,.:,'-~ ~.~.',,'.,.. ~'~ ~.~ 0 m ,,,_~ ~176 o ...." -% .,.. d ~ ,, .~ 9 ......- ..':'.... ,,, ~'1--. "-:,,. \.. 9 I 'I~ ~ >" hl"--~','," " I I ~ "..... -..... i-:""" i .,,,i I 0 t I~OO,oo~ 0 ,~ ~l/l l !l~ ',\ .~ o .~,~ i i II il ! 331 instrumented at half density and a field of 0.28T. The exterior of the magnet and endcaps is covered down to i0 ~ by eight planes of staggered drift tubes for muon detection (Fig. 6) . These contain a total of 5200 sense wires with wire lengths up to 6 m. Fig. 6. 2. Muon chamber setup. The Forward Detector The forward detector (Fig. 7) extends to within 5 mrad of the coasting beams the UAI philosophy of complete coverage with image chambers, and hadronic/electromagnetic calorimetry. The compensator dipole in the forward arm is calorimetrized and (on the outgoing side) contains a 3 meter-long image chamber with 600 sense wires transverse to the beam direction. 3. The Luminosity Monitors For luminosity monitoring the small angle elastic scattering rate is measured by a set of 4 drift chamber telescopes which for this run were symmetrically arranged at • m from the crossing point. Fig. 8 shows the arrangement on one side. In order to access angles in the range 0.2<8<2 mrad (0.003<Iti<0.29 GeV 2) the drift chambers and triggering scintillators are placed in movable sections of the vacuum tube ("Roman pots") which can be positioned within a few mms of the coasting beams. The drift chambers (Fig. 9) have a resolution i00 ~m in the drift plane and multihit (up to 16) capability. 332 Q >-( W' 0 w bd 0 ~J W T =E n~ _ 4~ 0 0 0,~ .,N e) A (/) 0/ Z W co 0 l-W '"N 0 v v X I <.) a Z I~I OJ 333 (a) Roman pot Beam axis ~HI IFZ // ~ nP ~ ' " Beam p,pe // lator Drift chamber Fig. 8: (a) one arm of the SPS-UAI luminosity monitoring drift chamber system (b) luminosity monitor in the SPS beam line. 334 The second co-ordinate is obtained by current division with resolution about 2 mm. 176 mm ,_ 19 _, Steel Supp ii 9 . ~'~-lg-" : : ..~ : : ": ! i. :el q ~176176 ;.~176 ~ -- Sense Wire 9 Wind /Field Wire ...._ :..: i~ 44 :: ""Thin woll " BEAM Fig. 9. Luminosity drift chamber, vertical cross section (mm units). Fig. i0 shows (a) the ~ versus p angle in the drift time plane and (b) its projection for candidate elastic scattering events. The corresponding plots for the charge division plane are shown in Fig. ii. The almost complete absence of background is striking. III. THE TRIGGER The design of the trigger is governed by the bunched operation of the accelerator which is designed to give beam crossings of a few nsec duration at 3.8 ~sec intervals. At high luminosity the trigger must be quite selective. Thus at the expected maximum luminosity of L = 1029 cm-2 sec -I the interaction rate will be about 5 x 103 per sec whereas the data acquisition rate is about i0 events per second. The main trigger incorporates three separate triggers as shown in Fig. 12; the pre-trigger, (or minimum bias trigger), the calorimeter trigger and the muon trigger. The first two and the first level muon trigger given an accept-reject answer in time to clear the system before the next beam crossing occurs. 335 Fig. i0. (a) @~ versus @p in the drift time plane for candidate elastic scattering events, Fig. ii. Same for the charge division coordinate plane. 336 C~,~d I SIGNALS MUON '.HAMBER SIGNALS EM CENTRAL AND ENDCAPS FORWARD ~ VERYFORWARO (ALREADY ADDED) I ANALOGUE J ON ,AORON I ~L~_(! CH jAB RG ~AOC' TES) sJ PRETRIGGER ~UON TRIGGER IPROCESS0R |LEVEL ONE r PROCESSOR ; (EI~qGV) I PROCESSORZ I (TRANSVF~_ J\ ENERGY J t_ FINAL LEVEL LOGIC I ACCEPT1REJECT Fig. 12. The UAI main trigger logic. 1 / J 337 i. The Pre-Trigger The pre-trigger aims to select relatively unbiased beam-beam interactions while rejecting background such as beam-gas collisions. For the data reported here the pre-trigger was implemented by a • nsec coincidence, centered around the crossing time, between hodoscopes at • m on t h e p r o t o n and antiproton arms. These hodoscopes covered the angular range 0.8<e<3.4, (3.5<n<5.0); the minimum angle of 0.8 ~ (t = 13 GeV2), excludes single diffraction dissociation events. 2. Calorimeter Trigger The calorimeter trigger is based on the pattern of energy deposition in the various calorimeters. Two custom-built digital processors convert the ADC imputs to energy and transverse energy (E sinS) respectively. In November, December 1981 data was recorded with a range of E T thresholds up to 40 GeV. 3. Muon Trigger Prompt high energy muons are identified by the fact that their tracks point back to the crossing region. The first level muon trigger uses the pattern of wire hits, in combination with a signal in the corresponding hadron calorimeter block, to select muon candidates which point within i00 mrad of the interaction vertex. The second level trigger uses microprocessors to reconstruct muon chamber tracks with i0 mrad accuracy (i00 ~sec per track). IV. PHYSICS The data presented here was obtained with the pre-trigger (section III). This is a "minimum bias" trigger except that single diffraction dissociation events are excluded by the minimum angle (0.8 ~ ) of the triggering hodoscopes. All this data comes from the largeangle detector (section I.l) which covers the em angular range 5~ 175 ~ with respect to the circulating beams. All energy measurements have been made with calorimeters. i. General Features of pp Interactions at 540 GeV Figs. 13 through 16 survey the global properties of the interactions. Fig. 13 is the distribution in total transverse energy ~ E sin8 measured by the electromagnetic and hadronic calorimeters for the angular range 5~ ~ (The energy scale has not been corrected for the lower response of the EM calorimeter to hadrons as compared to photons.) In agreement with the results obtained by the BariKrakow-Liverpool - MPI Munich - Nijmegen collaboration in pp and ~-p interactions at 300 GeV/C , we observe a high probability for the occurrence of events with large total E T. Our multiplicity measurements (section (IV.2) suggest that high E T events are primarily due to "soft" collisions of high multiplicity rather than to hard scattering of constituents. 338 I000 9 lO 9 u n i I i i i 5"<8< O9 o J75 ~ 11042 events Oo 00 I00 "E o E Z I0 | 0 Fig. 13. I 20 I I I I i I 40 60 80 @ansverse energy (GeV) Transverse energy distribution for minimum bias events. Fig. 14 plots the number of segments (maximum 64) hit in the endcap Em calorimeters (5~ ~ versus the number of hits (maximum 48) in the central EM calorimeter (25~ ~ A hit may be due to a photon from n ~ decay or to a charged hadron. We note a strong correlation between the multiplicities recorded in each calorimeter. Fig. 15 shows the number of charged tracks versus the number of EM calorimeter hits for the angular range 25~ ~ The line indicates the approximate correlation for equal production of ~+, ~-, n ~ wit~ each ~= giving rise to two calorimeter hits. 339 p~ 0 H" 0 fD r? i-~. R m i~. Ln oO A ~ 0 E ) i~. t-h ^~ Lne ~ ~ e R R i~. v 340 Fig. 15 has some relevance to the possible existence of "Centauro" events in the collider energy range [ 7 ] . These events which have been reported at cm energies ~ i000 GeV in cosmic ray studies are characterized by an anomalously low T ~ component. In this plot such events would appear close to a line of unit slope passing through the origin. Because of the saturation of the calorimeter at 48 hits, and the need for a detailed understanding of relative calibrations for charged and neutral particles in the calorimeter, no conclusions can be drawn at this time. Fig. 16. Hadronic versus electromagnetic energy in the angular range 25~ ~ . The inset indicates how these energies are measured. Fig. 16 also shows how the UAI detector may be used to search for Centauro events. "Hadronic" energy is plotted versus "Electromagnetic" energy for the angular range 25~ ~ Attempting a correspondence between UAI and the cosmic ray detectors we define electromagnetic energy as the energy deposited in the first l0 radiation lengths of the electromagnetic calorimeter; the remaining energy deposited in the rear EM calorimeter and the hadronic calorimeter is taken as hadronic. 341 2. Charged Particle Multiplicity in the Central Region  At the ISR it was found that the height of the rapidity plateau increased by 40% over the energy range ~ss = 24-63 GeV . The degree of violation of Feynman scaling between 63 GeV and the 540 GeV energy of the p~ collider is therefore of considerable interest. We have measured the charged particle multiplicity n• in the angular range 30~176 the corresponding range in pseudorapidity q = -log tan 0/2 is lqI<l.3. The data comes from a run without magnetic field in October, November 1981 with the pre-trigger described in section III.l. For this run approximately 50% of the central detector drift chambers were instrumented at half density. About 90% of the triggers are p~ collisions, the remaining 10% being due to beam gas interactions. A total of 789 events were scanned by physicists on a Megatek display (Fig. 5). To obtain the mean charged particle multiplicity, corrections were applied for electrons from Dalitz decay and conversion in the beam pipe and surrounding material (-6% • 2%) and nuclear interactions in the same material (1.5% • 1%). No correction was applied for K~ or A ~ charged decays which would normally be counted as two charged tracks. We obtain a mean charged particle multiplicity of 3.9 • 0.3 per unit of q at lql<l.3. The quoted error includes an allowance for the uncertainties in the correction terms. In order to compare our data with those from Thom~ et al. [9, Fig. 17] we have included only events having at least one track in our fiducial region. If events with zero central tracks are included the quoted numbers is reduced by ~ 6% to 3.6 • 0.3. Eiob. (GeV) i0 3 ~A +1 tO 4 10 5 4 2 e~ o~--e ~ ~ ~ ~ et ~-~-~176 al xTasoka et el oSoto el el ISR pp(9) pp (ll) pp (12) IIThis experiment p~ I I I l I 30 50 I00 300 500 V-~ ( GeV ) Fig. 17. The mean charged particle multiplicity per unit of q for Inl<l.5 as a function of centre-of-mass energy for events with at least one charged particle in the fiducial region. The line is the linear fit of Thome et al. to their data. 342 Because thegminimum angle of the triggering hodoscopes is = 14 mrad (t = 13 GeV-) single diffraction dissociation events are excluded from the sample. At the ISR these constitute ~ 14% of all inelastic events [i0]. However these events should negligibly populate the region of JnJ < 1.3, and their loss should not affect the estimate of n• for events with at least one track in this interval. Fig. 17 shows our result together with data from the ISR and results from two cosmic ray experiments [11,12]. These cosmic ray results are from balloon experiments with nuclear emulsions. 100, /, \ J , 744 + 45 I ' o-prong events /~ ,t,* r. : 6.54 10 @ :J 0 z 011 0 L 5 I 10 CHARGED Fig. 18. a 15 I 20 I 25 30 MULTIPLICITY Charged particle multiplicity distributions for Jnl< 1.3. Fig. 18 shows that the charged particle multiplicity distribution for JNJ<I.3 is significantly braoder than Poissonian. In Fig. 19 the normalized charged particle multiplicity is plotted in terms of the KNO scaling variable z = n• The distribution argues well with those measured over the ISR energy range for approximately the same rapidity range, indicating that KNO scaling holds over the cm energy range f~s = 50-500 GeV. 343 <n>+ o O ~o 0 0 I I I I I Pn I 0 0 I I I I I I I I i I i I I I I I 0 --4>--- --41-- N II k~. i+ A ,+v O ~ OJ O ~ rt'~ 01+ ~:~'I-IZ0 O po0.10"l -~ -~ A A O 04 I I --- 0 l> ~ - ~ '8 I> O -I! m o-' N i'~ m N n 0 ~ ~ I ol 0 I I l ! i , i I i I I I ! 344 The long tail in Figs. 18, 19 is very likely correlated with the realtively large cross section observed for events with large E T (Fig. 13). It has been suggested by Lattes et al.  that the C-jet events of the Chacaltaya experiment, with an average incident energy of ~130 TeV, show a significant difference from the extrapolation of accelerator events both in their multiplicity and their transverse energy. They propose a correlation between E T and multiplicity, suggesting that events fall into different classes. One such class has a low value of average E T per secondary particle (gamma rays in their case) and a low average number of gamma rays per unit of rapidity. Another class has a high average value of ET per secondary and also a high multiplicity. The SPS collider at ~s = 540 GeV has a laboratory equivalent energy of ~155 TeV for fixed target collisions and so it is meaningful to examine the data for these effects. ! | ! L~ v O) (I) - '15 I0- o) I/) +§ t- .l,=- § ++ I0 5 -5 G) . m (/) 0 r O) O 0 Observed Fig. 20. I I i 5 I0 15 charged track 0 multiplicity Calorimeter transverse energy as a fonction of observed charged particle multiplicity. Fig. 20 shows the calorimeter transverse energy as a function of observed charged track multiplicity. The left-hand vertical scale shows the visible energy measured in the electromagnetic and hadron calorimeters. The right-hand scale has been corrected by a factor 1.35 to take account of the lower response to hadrons of the electromagnetic calorimeter. An absolute scale uncertainty of • still 345 remains, as a precise application of this factor requires knowledge of the momentum of the incident hadrons. The average value of the transverse energy per event divided by the charged particle multiplicity seen in the central detector does not appear to depend on the multiplicity for these events. Assuming a ratio 2 for charged/neutral particle production Fig. 20 implies an average E T per secondary particle of 0.50 • 0.i0 GeV. ACKNOWLEDGEMENTS I wish to thank my colleagues in UAI for asking me to represent them at this conference. This work was supported in part by the United States Department of Energy. 3~.6 RE FERENCE S i. Aachen-Annecy (LAPP)-Birmingham-CERN-London (Queen Mary College)Paris (Coll~ge de France)-Riverside-Rutherford-Sacley (CEN)Vienna Collab., A 4 solid-angle detector for the SPS used as a proton-antlproton collider at a centre-of-mass energy of 540 GeV, Proposal CERN/SPSC/78-O6/P92 (1978). 2. C. Rubbia, Proceedings of the EPS Conference, Lisbon, July 1981. 3. M. Barranco Luque et al., Nucl. Inst. 176 (1980) 175. 4. M. Calvetti et al., Nucl. Instr. 176 (1980) 255. 5. K. Eggert, et al., Nucl. Inst. 176 (1980) 217. 6. K. Pretzl, this conference. 7. C . M . G . Lattes, Y. Fujimoto and S. Hasegawa, Phys. Rep. 65 (1980) 151, and references therein. 8. G. Arnison et al., Phys. Lett. I07B (1981) 320. 9. W. Thom~ et al., Nucl. Phys. B129 (1977) 365. i0. M. G. Albrow et al., Nucl. Phys. BI08 (1976) i. ii. S. Tasaka et al., Proc. 17th Intern. Cosmic-ray Conf. (Paris, 1981) (CEN, Saclay, 1981) Vol. 5, p. 126. 12. Y. Sato et al., J. Phys. Soc. Japan 41 (1976) 1821. 13. Z. Koba, H. B. Nielsen and P. Olesen, Nucl. Phys. B40 (1972) 317.