- 247 -
PHYSICS RESULTS FROM THE CERN PROTON-ANTIPROTON COLLIDER
J.D. Dowell
Department of Physics, University of Birmingham, England.
ABSTRACT Results from the 1981 operation of the CERN pp collider at 540 GeV obtained in experiments UA1, UA2, UA4 and UA5 are presented. The total cross section of 66±7 mb shows the expected rise from lower energies. Elastic scattering measurements are consistent with a Ins shrinkage of the forward peak. The central pseudo• rapidity density for inclusive charged particle production rises as Ins reaching a value of 3.3 particles per unit of rapidity and the multiplicity distribution exhibits KNO scaling when compared to ISR results below 63 GeV. Results for inclusive photon, strange particle and proton production are also given. The single particle pt-spectrum for pt up to 10 GeV/c is considerably flatter than at the ISR and shows a dependence on multiplicity. Evidence for jets is given. Both jet and single particle cross sections at high pt agree with QCD predictions. Negative searches for Centauro events and free quarks are reported. Finally, future prospects are outlined.
1. INTRODUCTION The proton-antiproton collider was suggested in 1976 by C. Rubbia
et al."*"' and came into operation in July 1981. The decision to build ji t was ,2) made in 1978 after the success of the Initial Cooling Experiment (ICE)' that demonstrated the feasibility of accumulating antiprotons in sufficient quantities, reducing their phase space by stochastic cooling - a technique 3) first studied by S. Van der Meer in 1968 . Construction of the Antiproton 4) Accumulator (AA) started in 1978 and five experiments to use the collider were approved in the middle of the year. The modifications to the SPS, the construction of the AA and of the experiments were all accomplished in a remarkably short time. The availability of 540 GeV in the centre of mass makes possible for + g the first time the observation of the weak intermediate bosons W and Z . The design luminosity of 1030cm 2s 1 has not yet been reached but with the 29 numbers of antiprotons already produced and accumulated a luminosity of 10 -2 -1 cm s seems in sight with a lifetime of ^20 hours. This would allow about 5 W* and 0.5 Z° decaying leptonically to be observed per day in the largest experiment UA1. During the early running in October-December 1981 the 27 -2 -1 highest luminosity achieved was 5x10 cm s . Typical running was with 26 — 2 —1 ^3x10 cm s and the integrated luminosity /Ldt for the whole period 32 -2 ^2*10 cm , too low to have seen a W. Nevertheless, the 1981 runs have allowed a first look at strong interaction physics at ten times the accelerator energy previously available and a search for interesting phenomena seen so far only with cosmic rays. Results from four of the experiments UA1, UA2, UA4 and UA5 will be discussed. - 248 -
Antiprotons have also been stored in the ISR and several groups have reported results^'. Their main impact has been to show that pp and pp interactions are very similar at this relatively high energy (/s=53 GeV) apart from a few expected differences that arise from the different charges and baryon numbers of the incident particles. The main interest therefore is to compare pp interactions at the much higher energy of 540 GeV with pp results from the ISR up to /s=63 GeV in order to study energy dependent effects and to extend the range of transverse momentum available.
2. THE UA EXPERIMENTS The UA1 detector*^ (fig. 1) was designed as a general purpose instrument with an almost 4ir solid angle coverage extending down to polar angles of a few mrad. Its central part is a 6m long, 2.4m diameter drift chamber system with 18cm drift spaces. The image readout gives space points at centimetre intervals along the tracks and records ionization information. The central track detector is surrounded along its length by 48 semicylindrical electromagnetic calorimeters and at each end by 32 similar radial sectors. All of the above are inside the coil of a dipole magnet (7m*3.5mx3.5m) which produces a highly uniform field of 0.7T. The laminated return yoke of the magnet, equipped with scintillation counters, also serves as a hadron calorimeter. The outer shell of the detector is a large-area muon detector consisting of 8 layers of drift tubes (2 separated chambers each with 4 layers). Calorimeterized compensator magnets, small angle calorimeters and further track detectors extend the detection to angles less than 5°. The measured rms accuracy of the central drift chambers is 280ym giving a transverse momentum resolution ^0.02 p^- . The electromagnetic and
hadron calorimeters have energy resolutions ^ of 0.15//Ë and 0.8//Ë respectively. An independent means of measuring the luminosity is provided by small drift chambers at ±22m to measure elastic scattering by detecting collinear particles.
7) ± o The UA2 detector (fig. 2), well matched to the W , Z search, is composed of finely segmented calorimeter cells, 2 40 in the central region and 240 in the forward and backward cones. They are arranged in tower structures pointing at the intersection region. An inner detector, using drift and proportional chambers, determines the vertex position. The forward and backward detectors include magnetic spectrometers (toroidal magnets and drift chambers) to measure the asymmetry of the electrons from VT decays. For the 1981 and 1982 runs a 60° azimuthal wedge of the central calorimeter is replaced by a magnetic spectrometer covering ±0.7 units of rapidity which uses drift chambers, time of flight counters and a lead-glass array, outside the main apparatus, that can be used to identify charged particles and neutral pions.
8 ) Experiment UA3 shares the same intersection region as UA1 and is a search for magnetic monopoles by looking for their expected high ionization - 249 - in 125um thick kapton foils. These are placed both inside and outside the vacuum chamber near the intersection point and around the outside of the UA1 central track detector. The foils have been scanned but no evidence for monopoles has yet been observed.
9) Experiment UA4 coexists with UA2. It measures elastic scattering and (using the optical theorem) the pp total cross section. Small drift and proportional chambers can be placed very near to the beams inside "Roman pots" situated 40m on either side of the intersection point. Smaller angles of scattering can be detected than in UA1. The main part of the UA5 detector"*"0' consists of two 6m long streamer chambers above and below the intersection region incorporating lead-glass plates to allow photon detection. There is no magnetic field and the experiment is an alternative to UA2. It is designed to study simple characteristics of particle production.(See fig. 3.)
As data acquisition rates are limited some form of triggering is needed to select events of interest. Up to now, two types of trigger have been used to study inelastic events: a) simple hodoscopes at each end of the detector (UA1, UA2, UA5) giving a 'minimum bias' trigger and
b) transverse energy triggers (UA1, UA2) where ET=EEiSin9i is summed over calorimeter cells and a threshold is imposed to select high p^- processes.
ELASTIC SCATTERING AND TOTAL CROSS SECTION pp elastic scattering has been measured by UA411' and UA112' in two 2 2 adjacent range of -t centred at 0.1 (GeV/c) and 0.2 (GeV/c) giving -2 different exponential slope parameters b of 17.2±1.0 (GeV/c) and _ 2
13.3±1.5 (GeV/c) respectively where dNel/dt « exp (bt) (see fig. 4). Both slopes agree with extrapolations from ISR energies and are consistent with a logarithmic (Ins) shrinkage with increasing energy of the width of the forward elastic differential cross section peak as expected from Pomeron exchange with a non-zero Regge-trajectory slope13'. The total cross section is obtained from a simultaneous measurement of low-t elastic scattering and the total inelastic rate. Using the optical theorem one can write the differential elastic rate as
dN CT (1+p2 el = L t > exp (bt) dt 16ir(ftc)2 where p is the ratio of the real to imaginary parts of the forward scattering amplitude and L is the luminosity. Furthermore
N , + N. . = L a. el xnel t
where N n and N. , are the total elastic and inelastic rates. Combining el inel the two expressions and extrapolating the elastic scattering to t=0 allows - 250 -
the luminosity to be eliminated giving
2 16TT (ftc) N o*. = — 2 1+p N , +N. n ell dtineJ t=l 0 14) Figure 5 shows the UA4 result of 66±7mb compared to lower energy measurements. The rise of the total cross section agrees with dispersion relation predictions''"5' (see curves) .
MINIMUM BIAS PHYSICS 4.1 Pseudo-rapidity density distributions Charged particle multiplicities as a function of pseudo-rapidity 16 17) 18) n=-ln(tan0/2) have been studied by UA5 ' and UA1 using zero-field data. Excellent agreement is found. In addition UA5 has studied photon multiplicities by detecting Y~conversions in the beam vacuum pipe and the lead-glass plates. Figure 6 shows the UA5 results and includes ISR measurements19' at /s = 53 GeV for comparison. Simple Feynman scaling20' says that the pseudo-rapidity density dn _ 1 da should be constant,
dn1 a. , dn1 inel independent of s. Because the range of rapidity available increases as Ins the average multiplicity of the event would then increase as Ins. It 21) is already known from the ISR that the average central rapidity density increases by 40% from /s = 23 GeV to 63 GeV violating Feynman LdnJ n=o scaling. A continued Ins dependence is observed in moving to collider energy /s = 540 GeV (Fig. 7). As a consequence the average total charged
< 17) 2 multiplicity increase to nch> = 26.5±1 requires an (Ins) term to account for its energy dependence. However the width of the pseudo-rapidity density distribution has grown by only 2 units from ISR to collider compared to the 4.6 units available kinematically. Furthermore, a
cylindrical phase space description with
with the collider results for
ISR measurements, but for pfc > 1.5 GeV/c. The UA5 measurements show that
4.2 Multiplicity distributions
If particles were produced randomly and independently their _
have been normalised to the full inclusive cross section at Pt=0. Even at rather low p values the spectrum becomes flatter with increasing 26) multiplicity, a result already suggested by cosmic ray experiments The average transverse momentum
< > effect. The points without error bars give Pt averaged over all multiplicities at FNAL, ISR and the collider. The fact that the lower
< > energy points tend to lie on the same curve suggests that Pt may be related to multiplicity in some general way. 27) ± The UA2 group has measured the pfc spectrum of pions, TI being
identified by time of flight (pt<1.5 GeV/c) and TT° by reconstruction from their decay. Good agreement is found (fig. 12) between the two. For
Pt<1.2 GeV/c charged kaons and protons have been identified and the ratios K/ir and p/ir are independent of the transverse mass (fig. 13) .
UA5 have identified K° and A° from their decays into charged particles. Their p^_ distributions (fig. 14) are less steep than for pions with mean transverse momenta of 0.70±0.12 and 0.67±0.20 GeV/c for kaons and lambdas respectively. The multiplicity of the neutral kaons (K0+K^) is 2.0±0.4 per inelastic event while for (A° + A73) it is 0.35+0.10. The ratio of o o ± (K +K )/ir is 11±2% showing a rise from lower energies (fig. 15) possibly due to increasing heavy flavour production. The apparently higher K/IT
ratio in figure 13 is a consequence of the different pfc distributions for - 252 -
TTs and Ks. These results also indicate a rise from the ISR energy range.
4.4 Transverse energy distributions 29 ) The central calorimeters of the UA1 experiment have been used to study the transverse energy distribution for |n | <3.0. As all the particles
deposit their energy in the calorimeters the total transverse energy ET should be simply related to the multiplicity of the event and the p^_ carried
by each particle within the selected n-region. it is found that ET rises almost linearly with observed multiplicity (fig. 16(a) ) showing that selecting high E preferentially selects large multiplicity. Similar 30)
observations were made in the NA5 experiment at CERN . A high ET trigger
is the natural way to search for high pfc jets in an unbiassed way so these high multiplicity events are a background at least at the triggering level. Figure 16(b) shows the average E^ per charged particle. The curve is a Monte Carlo prediction, allowing for neutrals, and using the observed
multiplicity distribution, but taking the single particle pfc distribution to be independent of multiplicity. The fact that the points rise above the
< > curve for increasing multiplicity confirms that Pt is a function of multiplicity. LARGE TRANSVERSE MOMENTUM SCATTERING AND JETS 5.1 Single particle p^_ spectra Particles produced by "soft" processes have mainly been discussed so 2 2
far. They are expected to have a Gaussian pfc dependence dN/dpt <* exp(-bpt). "Hard" processes, on the other hand, in which point-like partons scatter 2 -4
are naively expected to behave as dN/dpt <* pfc , in analogy with Rutherford scattering. Such a power-law behaviour should dominate the scattering at
sufficiently high pfc because of its less steep pfc dependence. Parton scattering gives rise to jets in the final state (fragmentation) but the individual particles within a jet should reflect the parents power-law behaviour. QCD and parton structure effects modify this simple argument but the basic point remains true.
The invariant cross section for unidentified charged hadrons as a 25 ) function of transverse momentum is given in figure 17 for |y| < 2.5. 31) The distribution is considerably flatter than that at the ISR at /s = 6 3 GeV exceeding it by three orders of magnitude at p = 10 GeV/c. 32 ) The QCD calculations of Odorico , which include the effect of gluon radiation of the initial and final state partons, are shown in the figure and agree well with the data. The p.-dependence for the collider data is — 8
close to pt , very different from the naive prediction but nevertheless expected. It is a consequence of the sharply peaked parton distributions at low x in the colliding hadrons and scaling violations in the structure and fragmentation functions. - 253 -
33) Measurements from the UA1 end-cap electromagnetic calorimeters for 1.6 < |y| < 2.5 are included in figure 17 (a further point at 12 GeV/c is not shown) . These include ir , n and y and agree well in spite of the different rapidity range and particle composition.
5.2 Evidence for clustering Both UA1 and UA2 have observed clusters of particles with high p 34) indicating jet production. In UA1 a trigger particle measured in the
central drift chambers was defined as that having the largest pt and at least 4 GeV/c. The distributions of the rapidity difference, Ay, and azimuthal angle difference, àfy, of all other particles relative to the trigger particle are plotted in figure IS. The Ay distributions are plotted separately for the hemispheres centred around the trigger particle ("towards") and opposite to it ("away"). A correlation in y which becomes sharper with
increasing secondary particle pfc is seen on the "towards" side but not on the "away" side. Simultaneously, there is a peaking for Aefj=0° and A(j)=180°, i.e. opposite azimuths. These features are expected for jets produced by colliding partons. The trigger particle defines the jet-axis reasonably well and the large p secondaries cluster around it because of their limited transverse momenta with respect to the parton direction. The away-side jet is expected to be coplanar to balance p^_, explaining the correlation, but cannot be predicted in y because of the lack of knowledge of the colliding parton momenta in the incident hadrons.
Both UA1 and UA2 have used their calorimeters to look for localised depositions of energy. Clusters of adjacent cells are defined using suitable algorithms and it is found that the fraction of the total transverse energy that is deposited into one or more clusters increases as £ E is raised reaching levels of 50% in a single cluster for £ E_%100 GeV 35) and In | < 1.0 in UA2 . Comparable- figures are found in UA1 showing that for such high E E^s the fraction of jet-like events is indeed large. Many examples of spectacular events have been observed. Figure 19 shows a 2 2-jet event from UA2 having a jet-jet mass of - 140 GeV/c and Figure 20 2 a 3-jet event from UA1 with a mass of ^90 GeV/c . 5.3 Jet cross sections UA1 and UA2 have determined jet cross sections and are broadly in 35) agreement. Figure 21 shows the inclusive jet production cross section 36 ) The solid curve is a QCD calculation of Horgan and Jacob using A=0.5 GeV. The dashed line is for A=0.15 GeV. Given the present level of study the agreement is excellent. At these values of the jet E gluon-gluon 36 ) scattering dominates . Consequently the collider offers the possibility of studying gluon fragmentation functions and initial indications favour 2 a gluon shape Ml-Z) /Z where Z is the fraction of the jet momentum carried 37) by a particle . In contrast, at the ISR quark jets are more important because of the relatively larger x-values needed for the partons to produce - 254 -
high p. collisions. Figure 21 shows results for jet production at the 38) ISR . They are well described by the same QCD calculation (curve not shown). SEARCH FOR CENTAUROS AND FREE QUARKS 39) Centauro events, seen in cosmic rays , are half a dozen high energy events characterised by a high multiplicity of hadrons and a multiplicity of electromagnetically showering particles consistent with zero. The
< > hadrons have an abnormally high Pt (^lGeV/c). UA1 has used the energy deposited in the first four radiation lengths of the electromagnetic calorimeters as a measure of electromagnetic energy 40) and compared this with the hadronic energy to look for abnormal sharing No evidence for any clustering of events with unusual properties is seen in a sample of 48,000 even after selecting those with high p and multiplicity. 17) UA5 has seen no candidates in a sample of 1860 events . However the collider has only 1/3 of the centre of mass energy at which Centauros have 39) been reported 22) UA2 has searched for fractionally charged particles using their ionization in a set of scintillation counters. For light quarks (m<\)0) -4 they quote a 90% confidence level limit of <2.2xio , relative to singly charged particles, for either Q = 1/3 of Q = 2/3. FUTURE PROSPECTS 28 —2 —1 With an expected luminosity of ^3*10 cm s in late 1982 there is 34 -2 the prospect of an /Ldt of a few xio cm sufficient to produce about + o 20W and 2 Z , detected by their leptonic decays in UA1, and sizeable yields of jets out to ^80 GeV/c. Backgrounds for the W-search are 41) expected to be dominated by leptons from heavy quark decay (fig. 22) Now that agreement with QCD is being observed for hadronic scattering one has increased confidence that backgrounds from decay, punchthrough and misidentification of normal hadrons should not be higher than those shown in the figure. Furthermore, background triggering rates have been measured in UA1 and UA2 for the W-search and are satisfactory. We can look forward with excitement to the next round of collider physics. ACKNOWLEDGEMENTS I would like to thank the members of the various UA collaborations who have supplied me with information. I have benefitted from a recent manuscript by K. Eggert to whom I am grateful. - 255 -
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Figure 1. View of the central part of the UA1 detector
Figure 2. Plan view of the UA2 experiment
Figure 3. The UA5 experiment - 258 -
Figure 4. Elastic scattering results (UA1, UA4)
ELAB (GeV)
T 1 1 I ^ Photons from A Plote • Beam pipe
351I I I 111 I I I I I I 111 1 I I I I 1111 I I I i i I 5 10 ^50 100 500 1000 yi ( GeV ) 0 I 2 3 4 5
Figure 5. The total cross section for pp Figure 6. Pseudo-rapidity density and pp. The lines are a dispersion distributions for charged hadrons relation fit 15)(UA4) and photons (UA5) - 259 -
Ei„b.
<
• Thome et al ISR pp 12) 1 .-+-• K Tosoka et al pp ( 5 ) o Sato et ol pp 16) • This experiment pp _l 30 50 100 300 500 Vs ( GeV )
Figure 7. Average multiplicity per unit of rapidity at n=0 as a function of energy (UA1)
0.75
(GeV .ra o 23.6 ^ Figure 8. » 30.8 Multiplicity distributions of 0.50 • 45.2 PP \T\\< 1.5 for restricted n plotted f . 53.2 in KNO variables (UA1) 5 • 62.8 > 540.0 pp I77M.3
-5- 0.25
1.0 2.0 3.0 4.0 5.0
Z = nt /
Figure 9. Multiplicity distribution for the full plateau region compared to the independent cluster model of de Groot24) (UA1) - 260 -
2 EdV/d'p [cmVGev- ]
+ '-L rr I
• -A.
Global Averages:
A
2.5 5. 7.5 10. 12.5 15. 17.5 20.
PT GeV/c Number of Charged Particles / Unit of Rapidity
Figure 10. p distributions for Figure 11.
15 o P/rr \
1 -
Of + K/TT
«5 t •i—«Ai 39X 4- ± /SR Z8« Ui V ù « ú w I Ji ~ H H Mr(6te//c*;
I 1. 3.0 -«.0 P,10eV/cl
Figure 12. p -distribution for Figure 13. K/ir and p/ir ratios as a identified pions (UA2) function of transverse mass 2 n>T = ¿p\ + m (UA2) 100 a) UA5 1 1 • pp — Ksx i r o pp— A/AX- (a)
80 > î Î 60 > -Ib
UJ V 40 -
0,01
0.5 1.0 1.5 2.0 PT , in GeV/c 20 Figure 14. p. distributions for K and A° (UA5)
i r 0.15 o K" I TT î (b)
UA5| tît, > tu cd 0,10
0.05
(b) _L J I 10 20 30 40 50 60 10 100 1000 Observed Mult •/s , in GeV
Figure 15. The K/ir ratio as a function Figure 16. a) Total transverse energy for of energy (UA5). pin j <3.0 versus multiplicity b) Average E_ per charged particle versus multiplicity (see text) (UAl) Figure 17. The invariant cross sections as a Figure 18. Rapidity differences between function of transverse momentum at ISR energies secondaries and a trigger particle of p.>4 GeV/c and the collider /s=540 GeV (UA1) in a) the towards and b) the away hemispheres; c) azimuthal difference. The three rows are for secondary p^>0, 1 and 2 GeV/c (UAl) - 263 -
(a) (b)
Figure 19. Configuration of an event in the UA2 apparatus showing a) charged tracks (inside) and calorimeter cell energies, b) the cell energy distributions as a function of polar angle 6 and azimuth (J>. - 264 -
10-* I i_ I I I I I J 0 10 20 30 40 50 60 E, (GeV)
Figure 21. Inclusive jet production cross sections at the collider and ISR. The solid line is a QCD calculation of Horgan and Jacob (ref. 36) with A=0.5 GeV. The dashed line is for A=0.15 GeV
20 30 40 50
PT (GeV)
Figure 22. Lepton spectrum at 90 from W-decay. The solid (dashed) curve is the leading (first) order QCD calculation. The band shows the estimated background from c and b quark decays (ref. 41).