- Email: [email protected]
Comparison of pp and pp interactions at s = 53 GeV
Volume 112B, number 2
COMPARISON
PHYSICS
OF pp AND pp INTERACTIONS
6 May 1982
LETTERS
AT fi=
53 GeV
UAS Collaboration Bonn-Brussels-Cambridge-CERN-Stockholm K. ALPCARD e, R.E. ANSORGE c, B. ASMAN e, S. BERGLUND e, D. BERTRAND b, K. BijCKMANN a, C.N. BOOTH c, C. BUFFAM d, L. BUROW “, P. CARLSON e, J.R. CARTER c. J.-L. CHEVALLEY d, B. ECKART a, G. EKSPONG e, J.-P. FABRE d, K.A. FRENCH c, J. GAUDAEN b,l, M. GIJSEN d, K. von HOLT a, R. HOSPES “, D. JOHNSON b, K. JON-AND e, Th. KOKOTT a, L. LEISTAM d, 1~. MACKENZIE d, M.N. MAGGS c, R. MEINKE a, Th. MtiLLER a, H. MULKENS d, D.J. MUNDAY c, A. ODIAN d,2, M. ROSENBERG a, J.G. RUSHBROOKE c, II. SAARIKKO a,3, T. SAARIKKO a. S:TAVERNIER b, F. TRIANTIS dy4, Ch. WALCK d, C.P. WARD c, D.R. WARD c, A.R. WEIDBERG c, T.O. WHITE c, G. WILQUET b and N. YAMDAGNl e aPhysikalisches Instztut der Unzverszta?Bonn, Bonn, Cermany. b Inter-Unzverszty Imtttute for Hzgh Energies (r/LB- VUB), Brussels, Belgium. ’ Cavendzsh Laboratory, Ilepartment of Physzcs, bnbrzdge Unzversity, UK. d CERN, European Organrzation for Nuclear Research, Geneca, Switzerland elnstztute of Physics, Unzverszty of Stockholm, Sweden.
Recclved 29 January 1982
Results are presented from the first pp coIhding beam runs at the CERN ISR, usmg the UA5 streamer chamber detector. pp interactlons at &= 53 GeV arc compared with pp ddta taken in the same expcrunent. The results are m good agreement with cxtrapolatlons of low-energy ppdata.
Introduction. In April 198 1 collisions between proton and antiproton beams were observed for the first time at the CERN ISR. In this paper WCreport on some of the data taken in this first run, using an apparatus [l] designed for the UA5 experiment at the CERN SPS Collider, conslstmg of two streamer chambers trlggcred by forward hodoscopes. We compare pp and pp interactions at c.m. energy G= 53 GeV obtamed by the same apparatus and therefore reducmg the mfluence of systematic errors m the comparison. At low energies it is well known [2,3 1 that (pySpp) differences are dominated by pij annihilations. In
’ Also at Uruvcrutairc Instelhngen Antwcrpen, Antwerp, Belgmm. 2 Visiting scientist from SLAC, USA. 3 Vlsltmg scientist from 1Iclsmki University, Finland. 4 Visnlng smentist from Ioannma Umversny, Greece.
0 03 l-9163/82/0000-0000/$02.75
0 1982 North-Holland
experiments up to &= 14 GeV an approximate s--1/2 energy dependence for these dlffercnces is observed [3] from which WCwould expect by extrapolation differences to be at the l-3% level at &= 53 GeV. However, a recent quark model calculation [4,5] inspired by the dual topological umtariLation scheme . has dlffercnt mechanisms for particle productlon m pp and pjj non-annihilations, and so predicts much larger (pp-pp) diffcrcnccs, typlcally lo- 15% at 4 = 53 GeV. Our results are compared with these prcdictions. Apparatus and running conditions. The apparatus IS shown in fig. 1, and a detaled description may be found m ref. [I] . Two 6 m long streamer chambers were placed above and below the ISR beam pipes. The chambers were equipped with lead glass plates to convert photons over most of the rapidity range accepted.
183
Volume 112B, number 2
PHYSICS LETT[ RS SCHEMATIC
LAYOUT
OF
THE
STREAMER
CHAMBER
6 May 1982 FCRWARD HODCSCOPE 1
SYSTEM
o,o y/-J
iGER HODOSCOPE 1
LOWER CHAMBER
UPPER
903 HODOSCOPE
TRIGGER NODOSCOPE 2 ~
// //
,
j
\
FORWARD HOCOSCCP~" 2
F~--[ 0
1
~--I---2
]3
-- ~--'t.
] 5m
qJ~.
\ ~"
Fig. 1. View of the expertmental apparatus as used at the ISR. The two streamer chambers, placed centrally above and below the beam pipes, each have dimensaons 6 X 1.25 × 0.5 m 3. Each chamber is viewed by three cameras, a main camera at each end viewing shghtly more than half of the chamber at demagnificatlon 50, and a supplementary one viewing the whole chamber at demagnification 80. Image intensifier tubes were used with the cameras, allowing one to run with small streamers ( 5 - 1 0 mm long). The two-track resolution was found to be 2 - 3 mm in space. Mirrors were arranged so as to fold a stereo pair of views onto each camera. The geometrical acceptance in pseudorapidity 7? = - I n tan 0/2 was Inl < 3.5, 0 being the c.m.s, particle produchon angle. Two planes of scintillation counter hodoscopes at each end of the apparatus were used to trigger the chambers. A normal event trigger ("n i> 1" trigger) demanded at least one hit on each end m coincidence. This trigger accepted 89% o f inelastic events with negligxble ( < 1%) background under normal pp running conditions. This acceptance was estimated using the Monte Carlo program described below. All our pp data were taken using this trigger. However, for the 184
p~ running the luminosity was very low, in the range ( 2 - 8 ) X 1024 cm - 2 s - 1 . Under such conditions the n i> 1 trigger gave an unacceptably high ratio o f background to real events ( ~ 10:1). It was therefore necessary during pO running to employ a more restrictive trigger requiring at least two hits at each end ( " n / > 2" tugger), whereby a much better ratio o f events to background was obtained (~1 : 1). The additional loss o f events was 37% of those satisfying the n = 1 trigger condition, but these were mainly of low multiplicity. so that the fracUon o f tracks lost from inelasuc events was in fact 22%. A small amount of data was also taken with a single proton beam circulating, m order to investigate background effects. Data reduction. Before measurement the streamer chamber film was scanned to reject obvious background events such as upstream interactions. No attempt was made to distinguish primary and secondary tracks at the measurement stage, "all tracks being mea-
Volume 112B, n u m b e r 2
PHYSICS LETTERS
sured except for those whxch scattered appreciably in the chamber gas (mainly low-energy 8-rays). The tracks were geometrically reconstructed using measurements on all views on which they were visible, Le. usually on either 2 or 4 views. The overall efficiency for scanning, track measurement and reconstrucUon was determined by remeasurmg a sample of events, and found to be 98%. The results presented in this paper are based on measurements of 3600 pp events and 4000 ~p events. In addition to the tracks resulting from primary interactions, there were those originating from decays, and from ",/-conversions and secondary interactions (mainly in the beam p]pe), which had to be removed. This was done by first identifying primary and secondary vertices. The primary vertex of each event could be found from the intersection of those tracks which pointed back to the beam-crossing region (diamond). For the secondary vertices, two or more tracks would generally be found to mtersect in a known piece of material e.g. the beam pipe. Tracks assocmted with secondary vertices could then be removed, and charged primaries corrected accordingly. To calculate the effects of trigger efficiency, acceptance and vertex finding efficiency, a Monte Carlo program was used. Various event generators were employed: we principally used a longitudinal phase-space generator, Including resonances and leading baryons, adjusted to fit previous ISR data [6]. It was checked that the corrections applied were insensitive to reasonable variations in the event generator. The particles generated in the Monte Carlo simulation were tracked through the material of the apparatus, allowing for track measurement errors, multiple Coulomb scattering, ")'-conversions, bremsstrahlung, hadronic interactions and the production o f S-rays (above 1 MeV). The same algorithm used for identifying primary and secondary vertices was then applied to the generated tracks. Comparing the primary tracks found with the Monte Carlo input an overall detection efficxency e as a function o f r/was derived, which therefore incorporated the acceptance o f the apparatus (~80% in the central region), trigger efficiency, contaminat]on from strange particle decays (~6%) and uncertainties in the separation o f primaries from secondaries by the vertex finding algorithm (typically ~ 4%). The quantity e(r/) is shown in fig. 2a and is seen to be > 75% over most of the range [r/I < 3.5.
6 May 1982 10
t
0 0 0 0 0 0
06 • E(~) 04 L i
o2 r
(a)
f ZOO
(b)
Oin¢l d 1] 1 50-
+
1 25L
1 O0
0 75
0 50
0 25
~
,
~
'
NI Fig. 2. (a) Overall track detection efficiency e as a function o f c.m. pseudorapidlty r). (b) Normalized rapid]ty distribution for pp interactions at x/'s-= 53 GeV. Data from ref. [6] are shown for comparison. Errors are statistical only.
pp data. The distribution (l/oinel) do/dr/was calculated from the measured tracks after weighting by I/e(r/) to correct for acceptance. Further corrections were made for scanning and measurement losses. In calculating r/the transformation from the laboratory system (15 ° crossing angle) to the c.m.s, system was performed assummg massless particles. To normalize the data the number o f events having > 1 primary track was taken, and the Monte Carlo was used to correct for untriggered events and for inelastic events with no primary tracks m the chambers. The resulting dlstribution is given in fig. 2b where the errors shown are statistical only. The systematic errors are estimated to be ~5%, and are dominated by the uncertainty in the trigger efficiency. Our results are compared with published pp data [6] taken at the same energy. The shapes o f the two distributions are
185
Volume 112B, number 2
PHYSICS LETTLRS
m good agreement, the shght difference m normahzation being well wittun the quoted systematic errors. pp data. As mentioned above about half the triggers used in p~ run,ung were from background, Le. Le. from b e a m - g a s interactions or from b e a m - h a l o interacting in the beam pipe and other material Such background events are expected to be highly asymmetric, mad because the proton current was ~ 1 0 0 0 times greater than the antiproton current the asymmetry would be mainly in the proton direction. The following procedures and checks were made: B e a m - h a l o events were rejected by requiring that primary vertxces lay within 2 cm of the plane o f the current. The events rejected were indeed highly asymmetric. Events with only one primary track had to be rejected, since their vertex could not be determined. The data still show an average surplus of 1.7 -+ 0.7% of tracks m the direction of the proton beam, indicating some remaining background. Smce the data taken at lowest luminosity have the highest asymmetry, this confirms that b e a m - g a s interactions are the source of the effect. We would therefore conclude from this that 2 - 3 % of the events passing our cuts could be background. - The film taken with a single proton beam was measured and analysed in the same way as the p~ data. From these data we independently estimate that 1 -+ I% o f events passing our cuts could be background. To est]mate the effect of any background contamination on our normalized distributions we removed all events wath more tracks m the proton hemisphere than the antiproton hemisphere, and counted twice those events asymmetric m the opposite sense. We thereby conclude that the effects of background on our normalized distributions are below the 1% level. -
6 May 1982
uncorrected data. Over the range Ir/I < 2.5 this ratio is cons]stent with umty though at larger rapldlties there is some ind]catlon of a fall-off. We are m agreement with another 1SR result [7] m the region [r/I <0.8. Measurement of ( p ~ - p p ) differences up to V~= 14 GeV indicate that they Ibllow a power-law behaviour, roughly as s -1/2 (as suggested [3] by Reggc theory). We have used such fits to estimate both the difference in total cross secUons and the difference in inclusive p]on-production cross sections m the central region. This gwes a p~/pp rat]o o f 1.01 in the central region (dashed curve in fig. 3a). Our value of 1.015
2 O0
-
175
+II iI i
150 OmeI dl} 125[ 1
i
dO
f
1O0 o
-
0
+
7sI 50~
02 S ~
(b) (a)
11 Results - comparison o f p # and pp. For the comparison of pp and p~ data one has to take the different triggering conditions into account. One could o f course, rely on the above Monte Carlo procedure alone, but that ]s liable to introduce different systematic errors rote the two sets of data, and so prejudice our attempt to look for differences at the ~ 2 % level. Instead the n ~> 2 trigger was applied to the pp data off-line, and direct comparisons were made between raw uncorrected pp and p~ data. In fig. 3a we show the pfi/pp ratio as a function of 7/for this raw
186
ratio
. . . . . . . . . . . . .
09~
--.
i
Fig. 3. (a) Normahsed rapidity distribution for pp interactions at x/s"-= 53 GeV. (b) Ratio of pseudoraptdaty densities from p~ and pp interaction as a function o f n . The solid curve is the prediction of the quark model of rets. [4,5]. The dashed line represents the ratio expected in the central region from an extrapolation o f low-energy data.
Volume 112B, number 2
PItYSICS LETTERS
-+ 0.012 taken over the central rapidity plateau, bvl < 2, is thus clearly compatible with lower-energy data. We also show as the full hne in fig. 3a the prediction o f a quark fragmentation model o f Capella et al. [4]. This m o d e l for the d o m i n a n t non-armihilatlon (or p o m e r o n exchange) process has been extended to include p~ anmhilatlons [ 5 ] , which we have allowed for although they contribute only ~ 2 % to the ratio. Our data are inconsistent with this m o d e l (x2/d.f. = 66/7). Having n o w shown that any difference between p~ and pp is small, it is reasonable 1o correct the p~ data for the effect o f the n ~> 2 trigger by observmg its effect off-line on the pp data, and then using the Monte Carlo to correct for the n ~> I trigger. The resulting
010
009 , ; [,, ,1
008
-,:
__
5p
....
pp
007
006 °n Z0n 0 05 ~
004~
I [,
:
0 O3
~ J
0 O2
(a)
0()1
~p
(b)
12
ratio 1 0
6 May 1982
distribution (1/Omel) d o / d r / f o r pl3 interactions is presented in fig. 3b. In fig. 4a we compare the charged multiplicity dist n b u t i o n s in pO and pp interactions at x / s = 53. For this comparison u n c o r r e c t e d data were again used as in fig. 3a in order to m m i m i / e the effect o f systematIC errors on any small differences between pl5 and pp. The pO/pp ratio given in fig. 4b indicates that there are no significant differences. In fig. 4b we also give the result o f an extrapolation * i from lower-energy data. where a Monte Carlo calculation is used to take the acceptance o f our apparatus rote account. It is clear that only a small rise with multtphclty is to be expected, and the extrapolation from low-energy data is entirely consistent with our results. We have unfolded the true charged multiplicity dlstributton from the data o f fig. 4a following the method o f ref. [6]. The errors on individual topological cross sections are very large because our apparatus was not o p t i m i / e d for the ISR, but nevertheless we can make reliable estimates o f the mean multiplicity and various m o m e n t s of the mulUplic]ty distribution. These are presented in table 1. F r o m extrapolation o f low-energy annihilation data [3] we would expect (n) to be " 0 . 1 umts higher in pO than pp. 41 The ratio R n = [On(~p) - On(pp) ]/an(pp) is found to follow the form A :3 n at all energies [8]. We estimate values e r a and 3 at x/~ = 53 GeV as follov, s. The difference in total cross sections can be estimated from a power-law extrapolation of low-energy data. ALso, the average multiplicity from p~ is found to be ~2.5 units higher than from pp over a xwde range of energies [3], and we assume this continues to ISR energies. Then using measured values of an(pp) at x/s = 53 GeV we can determine values forA and 3. Our best estimates areA = 0.0085, 3 = 1.07. Table 1 Moments of multiplicity distributions found at x/s = 53 GeV, Dq = <0~ - (n)q) l/q for any q, and 3"2 = D2/< n2), 3"3 = 1)~/(n3).
09 08
PP
~ * ~ ;o 1'2 ;~ 1'6 1'8 2'o 2'2 2'4 2'6 2'8 ~o 3'2 ObServed
multlptlcRy,
ref. [6J
n
lag. 4. (a) Charged multiplicity dastrlbutions for pp and pp interactions, uncorrected for acceptances (see text). (b) Ratio of probabtlitms for obtaining a given multiplicity in p~and pp interactions as a function of multiplicity. The solid curve is from an extrapolation of low-energy data of the form A 3 - n (wath A =0.085,3 = 1.07). The da.~hed lines represent the uncertainties in the extrapolanon.
(n) D2
(n)/D 2
3'2 3"3
PP this expt.
11.77 ± 0 . I 0 11.55 '-0.17 6.31 +- 0.08 5.70 +- 0.12 1.857 ± 0.032 2.03 -+ 0.05 0.295 ± 0.008 0.127+_0.006
0.291 +- 0.015 0.110±0.016
this expt. 11.47 ±0.16 5.77 ± 0.11 1.99 ± 0.04 0.304 ± 0.014 0.127--0.017
187
Volume 112B, number 2
PHYSICS LETTERS
Conclusions. We have made a first study of complete events from p~ interactions in the CERN ISR at x/s-= 53 GeV. We have compared our results with pp data recorded in the same apparatus and analysed identlcall). We find that differences between p~ and pp interactxons at this energy arc very small, typically < 2%. The results are in line with power-law extrapolations from data at lower energy, but mconsxstent with a recently proposed quark model.
6 May 1982
acknowledge with thanks the financial support of the Bonn group by the Bundesmmistermm fur Wlssenschaft und Forschung, of the Cambridge group by the UK Science and Enganeering Research Councd, and of the Stockholm group by the Swedish Natural Science Researcl~ Councd. Last, but not least, we acknowledge the contribution of the engineers, scanning and measuring staff of all our laboratories.
References The contributions of many CERN staff members to this experiment are gratefully acknowledged and in partlcular of EF and ISR Dwlsions for assistance with the building, installation and operation of our detector. From the Brussels group thanks are due from D.B. and G.W. for the financial support of the Fonds National de la Recherche Scientifique and the Institut lnterunwersltaire des Sciences Nucl~mres, and from J.G. and D.J. to the Interumversltalre Instituut voor Kern Wetenschappen for financial support. We also
188
[1 ] UA5 Collab., K. Alpgard et ",d., Phys. Scr. 23 (1981) 642. [2] H. Mutrhead, Proc. I1 European NN Symp. (1974)CERN 74-I 8, p. 488. [3] J.G. Rushbrooke and B.R. Webber, Phys. Rep. 44C (1978) 1. [4] A. Capclla et al., Z. Phys. C3 (1980) 329. [5] U. Sukhatme, Phys. Rev. Lett. 45 (1980) 5. [6] W. Thom~ et al., Nucl. Phys. B129 (1977) 365. [7] The Axial Field Spectrometer Collab., T Akesson et al., Phys. Lett. 108B (1982) 58. [81 J.G. Rushbrooke et al., Phys. Lett. 59B (1975) 303.
COMPARISON
PHYSICS
OF pp AND pp INTERACTIONS
6 May 1982
LETTERS
AT fi=
53 GeV
UAS Collaboration Bonn-Brussels-Cambridge-CERN-Stockholm K. ALPCARD e, R.E. ANSORGE c, B. ASMAN e, S. BERGLUND e, D. BERTRAND b, K. BijCKMANN a, C.N. BOOTH c, C. BUFFAM d, L. BUROW “, P. CARLSON e, J.R. CARTER c. J.-L. CHEVALLEY d, B. ECKART a, G. EKSPONG e, J.-P. FABRE d, K.A. FRENCH c, J. GAUDAEN b,l, M. GIJSEN d, K. von HOLT a, R. HOSPES “, D. JOHNSON b, K. JON-AND e, Th. KOKOTT a, L. LEISTAM d, 1~. MACKENZIE d, M.N. MAGGS c, R. MEINKE a, Th. MtiLLER a, H. MULKENS d, D.J. MUNDAY c, A. ODIAN d,2, M. ROSENBERG a, J.G. RUSHBROOKE c, II. SAARIKKO a,3, T. SAARIKKO a. S:TAVERNIER b, F. TRIANTIS dy4, Ch. WALCK d, C.P. WARD c, D.R. WARD c, A.R. WEIDBERG c, T.O. WHITE c, G. WILQUET b and N. YAMDAGNl e aPhysikalisches Instztut der Unzverszta?Bonn, Bonn, Cermany. b Inter-Unzverszty Imtttute for Hzgh Energies (r/LB- VUB), Brussels, Belgium. ’ Cavendzsh Laboratory, Ilepartment of Physzcs, bnbrzdge Unzversity, UK. d CERN, European Organrzation for Nuclear Research, Geneca, Switzerland elnstztute of Physics, Unzverszty of Stockholm, Sweden.
Recclved 29 January 1982
Results are presented from the first pp coIhding beam runs at the CERN ISR, usmg the UA5 streamer chamber detector. pp interactlons at &= 53 GeV arc compared with pp ddta taken in the same expcrunent. The results are m good agreement with cxtrapolatlons of low-energy ppdata.
Introduction. In April 198 1 collisions between proton and antiproton beams were observed for the first time at the CERN ISR. In this paper WCreport on some of the data taken in this first run, using an apparatus [l] designed for the UA5 experiment at the CERN SPS Collider, conslstmg of two streamer chambers trlggcred by forward hodoscopes. We compare pp and pp interactions at c.m. energy G= 53 GeV obtamed by the same apparatus and therefore reducmg the mfluence of systematic errors m the comparison. At low energies it is well known [2,3 1 that (pySpp) differences are dominated by pij annihilations. In
’ Also at Uruvcrutairc Instelhngen Antwcrpen, Antwerp, Belgmm. 2 Visiting scientist from SLAC, USA. 3 Vlsltmg scientist from 1Iclsmki University, Finland. 4 Visnlng smentist from Ioannma Umversny, Greece.
0 03 l-9163/82/0000-0000/$02.75
0 1982 North-Holland
experiments up to &= 14 GeV an approximate s--1/2 energy dependence for these dlffercnces is observed [3] from which WCwould expect by extrapolation differences to be at the l-3% level at &= 53 GeV. However, a recent quark model calculation [4,5] inspired by the dual topological umtariLation scheme . has dlffercnt mechanisms for particle productlon m pp and pjj non-annihilations, and so predicts much larger (pp-pp) diffcrcnccs, typlcally lo- 15% at 4 = 53 GeV. Our results are compared with these prcdictions. Apparatus and running conditions. The apparatus IS shown in fig. 1, and a detaled description may be found m ref. [I] . Two 6 m long streamer chambers were placed above and below the ISR beam pipes. The chambers were equipped with lead glass plates to convert photons over most of the rapidity range accepted.
183
Volume 112B, number 2
PHYSICS LETT[ RS SCHEMATIC
LAYOUT
OF
THE
STREAMER
CHAMBER
6 May 1982 FCRWARD HODCSCOPE 1
SYSTEM
o,o y/-J
iGER HODOSCOPE 1
LOWER CHAMBER
UPPER
903 HODOSCOPE
TRIGGER NODOSCOPE 2 ~
// //
,
j
\
FORWARD HOCOSCCP~" 2
F~--[ 0
1
~--I---2
]3
-- ~--'t.
] 5m
qJ~.
\ ~"
Fig. 1. View of the expertmental apparatus as used at the ISR. The two streamer chambers, placed centrally above and below the beam pipes, each have dimensaons 6 X 1.25 × 0.5 m 3. Each chamber is viewed by three cameras, a main camera at each end viewing shghtly more than half of the chamber at demagnificatlon 50, and a supplementary one viewing the whole chamber at demagnification 80. Image intensifier tubes were used with the cameras, allowing one to run with small streamers ( 5 - 1 0 mm long). The two-track resolution was found to be 2 - 3 mm in space. Mirrors were arranged so as to fold a stereo pair of views onto each camera. The geometrical acceptance in pseudorapidity 7? = - I n tan 0/2 was Inl < 3.5, 0 being the c.m.s, particle produchon angle. Two planes of scintillation counter hodoscopes at each end of the apparatus were used to trigger the chambers. A normal event trigger ("n i> 1" trigger) demanded at least one hit on each end m coincidence. This trigger accepted 89% o f inelastic events with negligxble ( < 1%) background under normal pp running conditions. This acceptance was estimated using the Monte Carlo program described below. All our pp data were taken using this trigger. However, for the 184
p~ running the luminosity was very low, in the range ( 2 - 8 ) X 1024 cm - 2 s - 1 . Under such conditions the n i> 1 trigger gave an unacceptably high ratio o f background to real events ( ~ 10:1). It was therefore necessary during pO running to employ a more restrictive trigger requiring at least two hits at each end ( " n / > 2" tugger), whereby a much better ratio o f events to background was obtained (~1 : 1). The additional loss o f events was 37% of those satisfying the n = 1 trigger condition, but these were mainly of low multiplicity. so that the fracUon o f tracks lost from inelasuc events was in fact 22%. A small amount of data was also taken with a single proton beam circulating, m order to investigate background effects. Data reduction. Before measurement the streamer chamber film was scanned to reject obvious background events such as upstream interactions. No attempt was made to distinguish primary and secondary tracks at the measurement stage, "all tracks being mea-
Volume 112B, n u m b e r 2
PHYSICS LETTERS
sured except for those whxch scattered appreciably in the chamber gas (mainly low-energy 8-rays). The tracks were geometrically reconstructed using measurements on all views on which they were visible, Le. usually on either 2 or 4 views. The overall efficiency for scanning, track measurement and reconstrucUon was determined by remeasurmg a sample of events, and found to be 98%. The results presented in this paper are based on measurements of 3600 pp events and 4000 ~p events. In addition to the tracks resulting from primary interactions, there were those originating from decays, and from ",/-conversions and secondary interactions (mainly in the beam p]pe), which had to be removed. This was done by first identifying primary and secondary vertices. The primary vertex of each event could be found from the intersection of those tracks which pointed back to the beam-crossing region (diamond). For the secondary vertices, two or more tracks would generally be found to mtersect in a known piece of material e.g. the beam pipe. Tracks assocmted with secondary vertices could then be removed, and charged primaries corrected accordingly. To calculate the effects of trigger efficiency, acceptance and vertex finding efficiency, a Monte Carlo program was used. Various event generators were employed: we principally used a longitudinal phase-space generator, Including resonances and leading baryons, adjusted to fit previous ISR data [6]. It was checked that the corrections applied were insensitive to reasonable variations in the event generator. The particles generated in the Monte Carlo simulation were tracked through the material of the apparatus, allowing for track measurement errors, multiple Coulomb scattering, ")'-conversions, bremsstrahlung, hadronic interactions and the production o f S-rays (above 1 MeV). The same algorithm used for identifying primary and secondary vertices was then applied to the generated tracks. Comparing the primary tracks found with the Monte Carlo input an overall detection efficxency e as a function o f r/was derived, which therefore incorporated the acceptance o f the apparatus (~80% in the central region), trigger efficiency, contaminat]on from strange particle decays (~6%) and uncertainties in the separation o f primaries from secondaries by the vertex finding algorithm (typically ~ 4%). The quantity e(r/) is shown in fig. 2a and is seen to be > 75% over most of the range [r/I < 3.5.
6 May 1982 10
t
0 0 0 0 0 0
06 • E(~) 04 L i
o2 r
(a)
f ZOO
(b)
Oin¢l d 1] 1 50-
+
1 25L
1 O0
0 75
0 50
0 25
~
,
~
'
NI Fig. 2. (a) Overall track detection efficiency e as a function o f c.m. pseudorapidlty r). (b) Normalized rapid]ty distribution for pp interactions at x/'s-= 53 GeV. Data from ref. [6] are shown for comparison. Errors are statistical only.
pp data. The distribution (l/oinel) do/dr/was calculated from the measured tracks after weighting by I/e(r/) to correct for acceptance. Further corrections were made for scanning and measurement losses. In calculating r/the transformation from the laboratory system (15 ° crossing angle) to the c.m.s, system was performed assummg massless particles. To normalize the data the number o f events having > 1 primary track was taken, and the Monte Carlo was used to correct for untriggered events and for inelastic events with no primary tracks m the chambers. The resulting dlstribution is given in fig. 2b where the errors shown are statistical only. The systematic errors are estimated to be ~5%, and are dominated by the uncertainty in the trigger efficiency. Our results are compared with published pp data [6] taken at the same energy. The shapes o f the two distributions are
185
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PHYSICS LETTLRS
m good agreement, the shght difference m normahzation being well wittun the quoted systematic errors. pp data. As mentioned above about half the triggers used in p~ run,ung were from background, Le. Le. from b e a m - g a s interactions or from b e a m - h a l o interacting in the beam pipe and other material Such background events are expected to be highly asymmetric, mad because the proton current was ~ 1 0 0 0 times greater than the antiproton current the asymmetry would be mainly in the proton direction. The following procedures and checks were made: B e a m - h a l o events were rejected by requiring that primary vertxces lay within 2 cm of the plane o f the current. The events rejected were indeed highly asymmetric. Events with only one primary track had to be rejected, since their vertex could not be determined. The data still show an average surplus of 1.7 -+ 0.7% of tracks m the direction of the proton beam, indicating some remaining background. Smce the data taken at lowest luminosity have the highest asymmetry, this confirms that b e a m - g a s interactions are the source of the effect. We would therefore conclude from this that 2 - 3 % of the events passing our cuts could be background. - The film taken with a single proton beam was measured and analysed in the same way as the p~ data. From these data we independently estimate that 1 -+ I% o f events passing our cuts could be background. To est]mate the effect of any background contamination on our normalized distributions we removed all events wath more tracks m the proton hemisphere than the antiproton hemisphere, and counted twice those events asymmetric m the opposite sense. We thereby conclude that the effects of background on our normalized distributions are below the 1% level. -
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uncorrected data. Over the range Ir/I < 2.5 this ratio is cons]stent with umty though at larger rapldlties there is some ind]catlon of a fall-off. We are m agreement with another 1SR result [7] m the region [r/I <0.8. Measurement of ( p ~ - p p ) differences up to V~= 14 GeV indicate that they Ibllow a power-law behaviour, roughly as s -1/2 (as suggested [3] by Reggc theory). We have used such fits to estimate both the difference in total cross secUons and the difference in inclusive p]on-production cross sections m the central region. This gwes a p~/pp rat]o o f 1.01 in the central region (dashed curve in fig. 3a). Our value of 1.015
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11 Results - comparison o f p # and pp. For the comparison of pp and p~ data one has to take the different triggering conditions into account. One could o f course, rely on the above Monte Carlo procedure alone, but that ]s liable to introduce different systematic errors rote the two sets of data, and so prejudice our attempt to look for differences at the ~ 2 % level. Instead the n ~> 2 trigger was applied to the pp data off-line, and direct comparisons were made between raw uncorrected pp and p~ data. In fig. 3a we show the pfi/pp ratio as a function of 7/for this raw
186
ratio
. . . . . . . . . . . . .
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--.
i
Fig. 3. (a) Normahsed rapidity distribution for pp interactions at x/s"-= 53 GeV. (b) Ratio of pseudoraptdaty densities from p~ and pp interaction as a function o f n . The solid curve is the prediction of the quark model of rets. [4,5]. The dashed line represents the ratio expected in the central region from an extrapolation o f low-energy data.
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-+ 0.012 taken over the central rapidity plateau, bvl < 2, is thus clearly compatible with lower-energy data. We also show as the full hne in fig. 3a the prediction o f a quark fragmentation model o f Capella et al. [4]. This m o d e l for the d o m i n a n t non-armihilatlon (or p o m e r o n exchange) process has been extended to include p~ anmhilatlons [ 5 ] , which we have allowed for although they contribute only ~ 2 % to the ratio. Our data are inconsistent with this m o d e l (x2/d.f. = 66/7). Having n o w shown that any difference between p~ and pp is small, it is reasonable 1o correct the p~ data for the effect o f the n ~> 2 trigger by observmg its effect off-line on the pp data, and then using the Monte Carlo to correct for the n ~> I trigger. The resulting
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distribution (1/Omel) d o / d r / f o r pl3 interactions is presented in fig. 3b. In fig. 4a we compare the charged multiplicity dist n b u t i o n s in pO and pp interactions at x / s = 53. For this comparison u n c o r r e c t e d data were again used as in fig. 3a in order to m m i m i / e the effect o f systematIC errors on any small differences between pl5 and pp. The pO/pp ratio given in fig. 4b indicates that there are no significant differences. In fig. 4b we also give the result o f an extrapolation * i from lower-energy data. where a Monte Carlo calculation is used to take the acceptance o f our apparatus rote account. It is clear that only a small rise with multtphclty is to be expected, and the extrapolation from low-energy data is entirely consistent with our results. We have unfolded the true charged multiplicity dlstributton from the data o f fig. 4a following the method o f ref. [6]. The errors on individual topological cross sections are very large because our apparatus was not o p t i m i / e d for the ISR, but nevertheless we can make reliable estimates o f the mean multiplicity and various m o m e n t s of the mulUplic]ty distribution. These are presented in table 1. F r o m extrapolation o f low-energy annihilation data [3] we would expect (n) to be " 0 . 1 umts higher in pO than pp. 41 The ratio R n = [On(~p) - On(pp) ]/an(pp) is found to follow the form A :3 n at all energies [8]. We estimate values e r a and 3 at x/~ = 53 GeV as follov, s. The difference in total cross sections can be estimated from a power-law extrapolation of low-energy data. ALso, the average multiplicity from p~ is found to be ~2.5 units higher than from pp over a xwde range of energies [3], and we assume this continues to ISR energies. Then using measured values of an(pp) at x/s = 53 GeV we can determine values forA and 3. Our best estimates areA = 0.0085, 3 = 1.07. Table 1 Moments of multiplicity distributions found at x/s = 53 GeV, Dq = <0~ - (n)q) l/q for any q, and 3"2 = D2/< n2), 3"3 = 1)~/(n3).
09 08
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lag. 4. (a) Charged multiplicity dastrlbutions for pp and pp interactions, uncorrected for acceptances (see text). (b) Ratio of probabtlitms for obtaining a given multiplicity in p~and pp interactions as a function of multiplicity. The solid curve is from an extrapolation of low-energy data of the form A 3 - n (wath A =0.085,3 = 1.07). The da.~hed lines represent the uncertainties in the extrapolanon.
(n) D2
(n)/D 2
3'2 3"3
PP this expt.
11.77 ± 0 . I 0 11.55 '-0.17 6.31 +- 0.08 5.70 +- 0.12 1.857 ± 0.032 2.03 -+ 0.05 0.295 ± 0.008 0.127+_0.006
0.291 +- 0.015 0.110±0.016
this expt. 11.47 ±0.16 5.77 ± 0.11 1.99 ± 0.04 0.304 ± 0.014 0.127--0.017
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Conclusions. We have made a first study of complete events from p~ interactions in the CERN ISR at x/s-= 53 GeV. We have compared our results with pp data recorded in the same apparatus and analysed identlcall). We find that differences between p~ and pp interactxons at this energy arc very small, typically < 2%. The results are in line with power-law extrapolations from data at lower energy, but mconsxstent with a recently proposed quark model.
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acknowledge with thanks the financial support of the Bonn group by the Bundesmmistermm fur Wlssenschaft und Forschung, of the Cambridge group by the UK Science and Enganeering Research Councd, and of the Stockholm group by the Swedish Natural Science Researcl~ Councd. Last, but not least, we acknowledge the contribution of the engineers, scanning and measuring staff of all our laboratories.
References The contributions of many CERN staff members to this experiment are gratefully acknowledged and in partlcular of EF and ISR Dwlsions for assistance with the building, installation and operation of our detector. From the Brussels group thanks are due from D.B. and G.W. for the financial support of the Fonds National de la Recherche Scientifique and the Institut lnterunwersltaire des Sciences Nucl~mres, and from J.G. and D.J. to the Interumversltalre Instituut voor Kern Wetenschappen for financial support. We also
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[1 ] UA5 Collab., K. Alpgard et ",d., Phys. Scr. 23 (1981) 642. [2] H. Mutrhead, Proc. I1 European NN Symp. (1974)CERN 74-I 8, p. 488. [3] J.G. Rushbrooke and B.R. Webber, Phys. Rep. 44C (1978) 1. [4] A. Capclla et al., Z. Phys. C3 (1980) 329. [5] U. Sukhatme, Phys. Rev. Lett. 45 (1980) 5. [6] W. Thom~ et al., Nucl. Phys. B129 (1977) 365. [7] The Axial Field Spectrometer Collab., T Akesson et al., Phys. Lett. 108B (1982) 58. [81 J.G. Rushbrooke et al., Phys. Lett. 59B (1975) 303.