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 .

x Elastic events were selected by applying a cut in the collinearity plots : with the width of this cut was ± 3 standard deviation . The scattering angle l/2 6 = (6 e ) of the selected events was calculated with the values 6 obtained from 6 (d(p) d( ))/(L (p) + ) � + �v v eff 1eff and similarly for the horizontal compone+nt. p (1his procedure eliminates(p)) errors due to longitudinal or transverse displacement of the p interac tion point) . p 1he resulting t-distribution obtained with 1400 events is shown in fig. 9. It can be f itted by an exponential exp (-bt) with parameter b = 17.2 Gev-2• 1he statistical error from the fit is ab = 0.8 Gev- 2• Consistency of the 332

Two thin steel windows (0.1 mm thick , 5 cm wide and 12 cm high), in the walls of the 11pots11 correspond to the active area of the detectors .

The present data were taken with the collider operating at a beam momentum of 270 GeV/c in a mode defined as 11normal beta" with a single antiproton bunch of a few times 109 particles colliding against a proton bunch of about 5•101 0 particles. The average luminosity was of the order of 5•102 5 cm- 2 s-1• The total amount of running time was about 10 hours in the machine development period and 8 hours in the physics run .

Because of the asymme tric beam optics (see fig. the minimum detectable scattering angle is determined by the distance from thel), beam axis of the detectors on the proton side.

Th e 11pots1' on this side were displaced toward the beam to a position where the active volume of the detectors reached a distance of about 16 mm from the beam axis , corresponding to a minimum value of the scattering angle of about 0.75 mrad . At this distance from the beam the chambers could be operated safely. The maximum accepted angle, as determined by the aperture of the vacuum chamber , was about 1.6 mrad .

The elastic trigger consisted of the logical OR of the two fourfold coincidences of the trigger counters in the two corresponding telescopes, that is (up left x down right) + (down left x up right). The trigger rate was a few events per minute . Fig. 5 shows the t-range accessible at various energies . During the December runs we covered the range from t 0.06 to t 0.17.

The acceptance of the system was obtained by a Monte-Ca� rlo calculati� on taking for the longitudinal shape of the crossing region a Gaussian distribution with 7) standard deviation of 10 cm . The acceptance varies from .25 at t = 0.06 GeV/c2 to .14 at t = 0.16 GeV/c 2 mrad (fig. 6).

For track reconstruction within each teles cope it was demanded that at least six of the eight drift planes had to show a hit. A track pair in corresponding telescopes (e.g. up-left and down-right) was considered to represent a possible elastic event only if:

i) The track mu ltiplicity per telescope was 1 or 2 (i.e. we allowed for th e possibility of an accompanying track due to a 6-ray ). 333 results has been checked by analysing separately the data from the different runs. A further check was obtained by comparing the measurements of the two independent scattering telescopes (up right/down left and up left/down right). Despite the increase of the statistical errors the result of this analysis can be summarized by stating that at present systematic errors are not larger than the statistical ones .

Our preliminary result is shown in fig. 10 together with the existing data of the slope parameter measured at lower energies . 334

LIST OF AUTHORS

R. Battiston5 , M. Bozzo3 , P.L. Braccini5, F. Carbonara�, R. Carrara5, R. Castaldi5, F. Cervelli5, G. Chiefari•, A.N. Diddens1, E. Drago•, M. Haguenauer2, B. Koene 1, L. Linssen1 , G. Matthiae• , L. Merola•, M. Napolitano•, V. Palladino2, G. Sanguinetti5, G. Sciacca•, G. Sette3, R. Van Swo11, Timmermans1, Vannini2,

VelaJ. sco2 , F. Visco•c.

J.

REFERENCES 1) "The CERN Staff Proton-Antiproton Project", Phys. Lett. 107B (1981) 306. 2) "CERN-Pisa-Roma-Stony-Brook Collaboration", Phys. Lett., 62B (1976) 460 and Nucl. Phys . Bl45 (1978) 367. 3) A. Bechini et al., "A modular drift chamber vertex detector at the CERN ISR", Nucl. Instr. and Methods 156 (1978) 181. 4) F. Carbonara et al., "A system of two dimensional Drift Chambers with Printed-Board cathodes and flat solenoidal delay lines", Nucl. Instr. and Methods 171 (1980) 479. 5) U. Amaldi et al., Phys. Lett. 43B (1973) 231 and Phys. Lett. 66B (1977) 390. 6) B. Koene et al., (to be submitted to Nucl. Inst. and Methods). Private communication from UA2 . 7) 335

© u .....c � ro w 0 � � � a. © u w � w u 0 N � �

�

N � 0 0 0

Fig. l Perspective sketch of the inelastic rate detector. T1 , T2 , T3 are the trigger hodoscopes. D1 , D3f, Djg , are telescopes of modular drift chambers3 J , The chamber-modules of telescope D2 are described in ref. 4 ). One arm only is shown the other being symmetric with respect to the interaction point. 3 36

·�

Fig. 2(a)

Fig. 2(a) High mu ltiplicity event with particles crossing all six telescopes (points in the chamber and reconstructed tracks are shown ), t' �.

-·- - - - - � p..i�mr-QS 1 t- 3 llw �-- - (1 J. f ------."�_:;4� l:radd � • =-.::;,-==

I- L Fig. 2(b) � r-·

'

Fig . 2 (b) Event taken with ...... the_,_LL�__,_,_,__,_ diffractive..._,_-1...1__,_...... _ trigger_J� (one :__._.elas-�.tic arm ,"1 and ' the ' '-� L��_,_corresponding oppos ite inelastic arm hit) . One good track found in the elastic arm present in the trigger. 337

0 0 0 0

...,._ If) If) .i r--. r--. 0 0 If) If) If) If) N N

0 0 If) If) I N N I 0 I '• . If) 0 I If) If) I r--. If) �- r--. 6 0 0 6 0 N I I 0 0 0 0 0 N .q- C{)

x 0 w 0 I- .q- O:'. � If)

0 LL 0 N 0 z If) 0 ,,;, I F . .; 0 ::::> .., 0 al I CY If) 0 tJi N I I 0 0 x N 0 I 0 0 0 0 ol .q- N 0 If) 0 I[) I N

0 0 0 0 0 0 0 0 0 0 0 .q- r') N N N x If) If) w I- 0:: 0 0 w > LL If) If) 0 z 0 0 0 I F ::::> If) If) rn I CY 0 0 � I I 0 If) If) >- I I I x 0 0 N N Ill o Ill o Ill oIll 01 0 0 0 0 ol t'-- tO N 0 t'-- tO N N .....-- ..--- ..--- .....-- N Fig. Scatter plot and projections of vertex as reconstructed fromI inelastic event for the x, y and planes. 3 z, x 338

ELASTIC SCATTERING LAYOUT

Quadrupoles Vacuum chamber

/.Crossing point � � 16 m

Fig . 4(a)

Trigger .----- counter

I I / / / / - / / - - / / - - - / --;;> - - ·- - � 1. :: � nger v :: counters � :: +- I- I, - Fi - <- - - - - ,.� - Wires nal >------::.,, plane � '- b-"i,;. proportio

I--blL !";._ Sense wires I/ planes ,, I_;,[/ � drift Fig. 4(b)

Fig. 4(a ) Sketch of the elastic scattering set-up. A detail of the detector inside the 11pot11 is shown in a perspective view (4b). 339

RANGE TH E EXPERIMENT - t OF

coulomb interference 10 region

e (mrad) I I I 1.6 --i---- 1 0.75

Fig. 5 range of the experiment at various energies . t- -t (GeV 2)

- 03 UA4 PP RUNS DEC 81 BETA ' NORMAL'

w u � 0.2 ti: w u u <(

0. 1

0 005 010 - 0. 15 020 t (GeV/c2) Fig. 6 Acceptance curve for "normal runs of December 1981. a11 w.... 0

Fig. 7(b) Collinearity plot for the events of fig. 7(a). PRUTON AN"Tl-PROTON ANALYSIS YS0085

"TAPE RUN 2089

80

VertCT �0.ical047

mrad O- 3 70 >t l centroid at 2. u 1 1.8

CL' 60 � iE 1.6 . · . :- · . -:..:..�: :::..· . 50 1.• .. . . . ·' '

. . 1.2 . fi� ji 40 . . 'f:!:· :::rjf 1. "'·' ·- · · . -� � - · t. .�;-. .. Jlt JO · : � 0.8 . • f . ::-:�· .: 7(b) 0.6 Fi3. 20 0.6 0.8 1. 1.4 1.6 1.8 Z.10- "THE"TA(PBAR) (mrad) 10 1.2 3

7(a) Fig.

Fig. 7(a) Scatter plot of the vertical component of the scattering angles for events which passed tests on track mu ltiplicity and track origin. 341

Fig. B(b) Co llinearity plot for the events of fig. (Ba).

' 0

�----, ·�

� g '--'

·{ \5.·�· .

0 ' . ·.

0

'

' 0 0 0 � ·1 ' ' § � L (PO.Jw) (d)VlJHl Fig. B(a) Scatter plot of the horizontal components of the scatteri_Jng angles for the events of fig. 7 (a). 342

UA4- P P (DEC'81 ) PRELIMINARY . fi = 540 GeV

b=l7.2-t!0 8 (006 :":: :":: 018)

u ' z u t

005 010-t 2 (GeV/c

Fig. 9 The measured t-distribution of elastic scattering resulting from 1400 events. .., - ,.. ':" 20 1 . ,... t� O. I GeV/c ;,,_c 0 ,., "' 2 PRELIMINARY "' UA4 0 ,... ,....? s " 0 ,...,., 16 t --C> .... t o ro ..., <:� " 0 0 " . "' ,.,ro ...,... 14 ¢ " OQ" 4' ,., + �� "0. ..., 12 t " + + "'0 b "' x * 'f� * 0 o *"*. " $� PP t? + " t· ? 10 0 Ayres(77) Ambotspp l74 ) "' "''0. FNAL Bortenev(73 ) B1rnboum(69) 111 FoJardo(81 ) ,... • >< Antipov(73) " + {° + ro"' Amoldi (71 ,77) ,., 8 ISR v • Ayres (77 ) Holder, Borbiellini, Boksoy FNAL F 0 ,. o ordo l81) (71) (72) (78) J ...."' lo - 1 10, 1• " 6 ,., 0,., " ,.. . ro"

" 2 OQ,., 10 105 GeV 4 10 1 2 0 10 3 4 1 "" w w