327 "FIRST RESULTS FROM THE UA4 EXPERIMENT AT THE CERN COLLIDER" UA4 Collaboration Amsterdam1-CERN2-Genova3-Napoli"-Pisa5 * Presented by M. Bozzo( ) ABSTRACT The UA4 experiment on the measurement of elastic scattering and of the total cross section at the CERN pp Collider is described. The experimental method is discussed , and first results on elastic scattering at low momentum transfer are presented. ( *) The full list of authors is given at the end of the contribution. 328 INTRODUCTION The aim of the experiment is to measure elastic scattering and the total 1) cross section at the CERN antiproton-proton Co llider . The total cross 2) section is obtained by means of a method previously used at the ISR and is based on the simultaneous measurement of low-t elastic scattering and of the total inelastic rate. By applying the optical theorem one finds the following relation : d 1 16 (tlc)' t 0 = (1) tot 1 + ;� If ( ) a p 2 where dN /dt is the differential elastic rate, N and N are the e1 el inel elastic and inelastic rates respectively , and is the ratio of the real to the imaginary part of the forward elastic amplitup de. Th is method does not require an independent determination of the machine luminosity. In the following we discuss the measurements of elastic and inelastic interactions which were performed in the first collider run , November-December 1981 , at centre-of-mass energy of 540 GeV . INELASTIC INTERACTIONS Multiparticle inelastic intercctions are observed in a vertex detector having a large angular coverage . As shown in fig. 1 it consists of a system of six drift chamber telescopes o , o and o placed symmetrically on the 1 2 3 left and right side of the crossing region covering angles from about 8° down to about 0.44°. Coverage in the central region for angles above 20°� is ensured by the vertex detector of the UA2 experiment. The range in pseudo-rapidity covered by the D telescopes is from 2.6 to 5.6 (the beam i rapidity is 6.3). Each of the Di telescopes is composed of six drift chamber planes followed by a plane of trigger counters T . The coordinate along the drift i wire is measured by detecting the signal induced in a delay line read at both ) ends , which is close to the drift wire itself3 • 4 . A hit in a chamber plane allows to determine the coordinates of a space point directly from three time measurements. Moreover, the constraint due to the known length of the delay line allows easy handling of mu ltiple hits in the same cell . This arrangemen t leads to a considerable simplification of the procedure of pattern recognition. 329 The r.m.s. values of the spatial accuracy for the drift and delay line measurements were found to be of about 0.4 mm and 4 mm respectively including the uncertainties of the survey. Two different triggers were used in theI experiment: i) a left-right arm coincidence; ii) a single arm trigger on one side only . Various comb inations of the T1 , T2 and T3 counters were used . The trigger (T +T ) (T +T ) which excludes background events due to beam halo 2 3 L • 2 3 R corresponds to a large fraction of the inelastic cross-section. The single arm trigger (T1+T2+T3)L or R allows to pick up diffractive events which may easily escape a left-right trigger. A true diffractive trigger , defined as the coincidence of one 11elastic arm11 and the corresponding opposite inelastic arm, was also tested during part of the runs. Fig. 2 shows two events representative of the two above mentioned triggers. In the electronic logics, the trigger was tuned with respect to a gate signal synchronous with the bunches. During data taking elastic and inelastic triggers were taken simultaneously in order to ensure that the integrated luminosity be the same for the two types of events. In the analysis program a track in a telescope was defined by the presence of at least four out of six aligned hits. For each event with at least two reconstructed tracks , the program attempts to find a vertex. The transverse and longitudinal distributions of the vertices found are shown in fig. 3 . ELASTIC SCATTERING At collider energies the typical value of the scattering angle in the region of the forward diffraction peak is of the order of one milliradian. The detection of elastic events at such small angles requires the use of a technique similar to that already employed at the ISR5 l . A side view of the experimental layout is shown schematically in fig. 4. Elastic scattering events are detected in the vertical plane by means of a system of four telescopes placed symmetrically above and below the SPS vacuum 330 chamber at a distance of about 40 m from the crossing point. A telescope is composed of two detectors that are 6 m apart . Each detector , consisting of a wire chamber and of a scintillation counters hodoscope , is placed in a movable section of the vacuum chamber ("pot") which is connected to the main body of the accelerator pipe by a bellow. Once stable beam conditions are reached , the "pots11 are displaced vertically toward the beam. Particles leaving the crossing region after interaction traverse the quadrupoles of the machine lattice Q in the left arm outgoing) and Q in the right arm (p outgoing). L R In the vertical plane(p the quadrupoles QL and QR act as a defocusing and lens respective ly. The p and trajectories for 1 rnrad scattering angle are also shown in fig. 1. The vertip cal displacement of the particle trajectories at the detectors is proportional to the vertical component e f the scattering v o angle. If the scattering takes place at the centre of the crossing region the vertical displacement d at a position mid way between the two detectors of a telescope can be written at d L e where the effective distance eff v L is equal to m and 22.8 m for the left and right arms respective ly. eff = 57.4 The wire chamber6) in each pot contains four independent drift planes for measurement of the vertical coordinate of the particle trajectories. Each plane is in the vertical direction subdivided into three drift cells of maximum drift length. The horizontal sense wires are 6.2 cm long , suitably21 staggered nun a s shown in fig. The drift-time resolution, expressed as the r.rn.s. scatter of the drift coordinate4. in an individual plane around the reconstructed track , was about mm during actual data taking . The drift planes are followed by a proport0.ional13 plane with high-resistivity anode wires . Charge-division readout of these wires provides the horizontal coordinates of the trajectories with a r.m.s. accuracy of 0.4 The mechanical frame of the chambers is U-shaped , and the side closer themm. bea m is made with a vetronite plate 0.8 mm thick. A pattern of field-restoring potential strips on this plate ensures good detection ) efficiency down to a few tenths of a millimeter from the plate6 . For each chamber , the distance of the horizontal plane of reference to the nominal machine plane is known from the measured displacement of the "pots" and from the overall survey of the experiment with an accuracy of about 0.1 •• mm As shown in fig. 4 inside each "pot" the wire chamber is followed first by a plane of eight vertical scintillators ( each 5.5 wide and 105 high) and then by a trigger counter. The vertical stintillatorsmm proved verymm usef ul also in monitoring the calibration of the amplification factors and the pedestals' values in the charge-division electronics. 331 ii) At least one track in each of the two telescopes extrapolated backwa rds was pointing to the crossing region. iii) No tracks were seen by the other pair of telescopes. About 40% of the triggers did not satisfy these criteria. Almost all of these events had a large number of tracks in several tele scopes and are attributed to accidental coincidences of beam-gas or beam-wall interactions . For the elastic candidates the values of 6 v of the tracks in the two oppos ite telescopes were calculated from the measured vertical displacements and from the known effective distances. In cases were two tracks were recorded in a telescope , the track pair with the best match of the values of 6v was chosen. 1he scatter plot of 6 (p) versus 6 ( ) in fig. 7(a ) clearly v v shows the ridge of elastic events , we ll identifip ed by their collinearity requirement. The distribution of the quantity 6 (p) - 6 ( ) as given in v v fig. 7 (b) has a standard deviation of about 0.05 mrad corresp ponding to a transverse momentum unbalance of less than 15 MeV/c . The intrinsic vertical angular spread of each primary beam is about 0.015 mrad . The finite size of the crossing region, the alignment inaccuracies and the experimental resolution of the detectors further contribute to the width of the collinearity plot . The horizontal components of the scattering angle, eH was obtained in a similar way from the horizontal displacements measured by charge division. The scatter plot of 6H (p) versus 6H ( ) is shown in fig. 8(a). 1he distribution of (p) p the difference 6H - 6H in fig. 8(b) has a r.m.s. width of about 0.065 mrad . The angular spread(p) of the proton and of the antiproton beams contributes about 12 0.02 mrad .