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THE UA2 APPARATUS AT THE CERN COLLIDER pp

The UA2 Collaboration (presented by B. MANSOULIE) DPhPE , CEN-saclay , Gif-sur-Yvette Cedex, France . 91191

ABSTRACT

The UA2 detector has been taking data since November This talk describes the apparatus from the point of view of ll.i year operating experience81. and results . some details are given about calorimeter response and calibration. 610

1) General Features ' J

The UA2 detector was designed mainly for the search of W± and z0 intermediate bosons . The very low production rates expected led to a large angular coverage : polar angle (9) from 20° to 140°, azimuthal angle (I/>) from to 21T radians . Electron decay modes were o chosen because excellent space and energy resolution can be achieved in compact calorimeters . As is well known , the w+ and w- created are expected to be fully polarized along the beam direction. At a given polar angle , this leads to an asymmetry between e+ and e- rates . The optimal polar angular range for the detection of this asymmetry lies between 20° and 40° (it. is Oat 90° ). Therefore , the detector is different in the central region , which covers the polar angle range from 40° to 140° , and in the forward and backward regions (F/B ), which cover the ranges 20° to 40° and 140° to 160° (Fig. 1).

To achieve electron identification and measurement, t:1e whole acceptance is covered by early shower detectors (located behind radiation 1Pngtt1 converters ), followed by 1.5 good granularity calorimeters. In the central region , the E'lectromagnetic calorimeter is followed by a highly segmented calorimeter . T:1is makes the whole central calorimeter a powerful tool for studies . In the F/B region , a toroidal magnetic field and a spectrometer allow for momentum and charge measurements.

pp experiment UA2

FO RWARD-3ACKWARD CALORIMETER

Fig. 1 Longitudinal cross-section of thE' apparatus . 611

coils

Drift chambers

_J Lead Glass p wall

Central calorimeter ( return yoke)

Iron plate /,,,

Fig. 2 Perspective view showing the central calorimeter and the wedge detector .

Until the end of 82 , the part of the central calorimeter between -30° and 300 in q, was not installed, to leave space for a magnet . A neutral and charged particle detector was facing it (so called 'wedge ' detector , Fig. 2).

2) The vertex detector ' l

The determination of the interaction vertex and of the tracks corning from it is done by a cylindrical detector surrounding the beam pipe (Fig. 3). This detector is composed of two 'JADE ' type drift chambers (VJl , VJ2 ) equipped with charge division on the wires and multi-hit capability , of five cathode strip readout proportional chambers (VS1-VS2 ), and of a barrel hodoscope (VH) . Each 'strip' chamber consists of an internal and external layer of strips and of proportional wires parallel to the cylinder axis. The strips are helicoidal and at an angle of about ± 45° (Fig.4). The outer strip chamber (VS5 ) is located behind a 1.5 r.l. tungsten converter and is equipped with charge division on the wires .

The vertex finding efficiency in data events is better than 97 %. The striFJ chambers allow for longitudinal determination of the vertex to ± 1.5 Resolutions achieved in mm. 'JADE' chambers are 300 µ.in transverse view .( drift time), and 1 % of wire length in lon­ gitudinal view (charge division ). 612

Tungsten r.l. converter 1.5

TRAMSVERSE VIEW OF VERTEX DETECTOR

Fig. Small arrows indicate the 3 path of electrons issued from an ionizing track and drifting to the wires in 'JADE ' chambers .

VS1 VH V VS3 VJ2 VS4 VSS vs2 J1 w

10 cm

Fig. 4 Schematic view of a strip chamber . Only one wire out of ten and one strip out of ten are drawn for simplicity . The arrowed line is a track coming from the beam line. Hit strips and wire are in thick . 613

CENTRAL CALORIMETER

Fig. 5 Transverse view of an "overlap" background event .

VS5 , early shower detector , has been extensively used in the W search analysis>> , for electron signature and 'overlap' background rejection (a low energy particle near ahard "0 , simulating an electron ). Fig. 5 shows a typical 'overlap background' event . Note that most of the pulse heights read in VS5 do not face the charged track. For single charged tracks , the matching between the coordinate given by VS5 and the track has a R.M.S. of 3.5 mm both in transverse and in longitudinal view.

3) The Wedge detector• >

The wedge detector was in place until December Jt is now removed and the complete 82 . � symmetry of the central calorimeter is now restored. This detector included (starting from the beam ) :

- a) a spectrometer consisting of a magnet giving a field integral of 1.1 T.m. and 12 drift chamber planes with vertical wires, with wires at 7° with respect to vertical ) (6 6 equipped with doublet sens-wires3 >. 300 resolution was achieved in the chambers giving µ a momentum resolution l:.P/P 0.5 x P. = %

- two scintillator hodoscopes with a cm iron plate in between , with time measurement on 2 the front one . 614

- an array of 280 lead-glass blocks , (15 15) cm' each , 14 .'.i r.l. thick , giving an energy x / resolution for photons and electron a/E i (3.4)2 + (5.7)2/E = %.

The detector perfectly fulfilled its task in provicling the spectrum (from 7T0 Tr reconstructed "0 's), the charged particle spectrum , and (by time-of-flight measurement ), K/1T ratio up to 1.1 GeV /c and p/11 ratio up to 1.4 GeV /c 'l

4) The central calorimeter 7 l

The central calorimeter is divided into 24 'orange slices ' in Each of these slices q,. is in turn divided in 10 cells along Each cell (Fig. is a tower , segmented in 3 B. 6) compartments in depth : the inner one (electromagnetic part ) is a 17 r.l. lead-NE104 scintillator sandwich , the two others ( hadronic parts ) are :cron-scintillator sandwiches . The total calorimeter thickness is 4.5 absorption lengths . Each compartment read out is on two opposite sides by two lucite light guides doped with wave length shifter on the BBQ part facing it .

radiation lengths e -m calorimeter : 17

total calorimeter absorption lengths · 4 5

Fig. Central calorimeter cell. 6 615

- Response to electrons .

The response to electrons has been studied in P.S. and S.P.S. beams from to 1 70 GeV/c . Fig. a) and b) show the deviation from linearity and the resolution observed. 7

The data were taken with a tungsten converter in a position equivalent to that in UA2 . Energy leakage in hadron first compartment is added at GeV). Beam momentum spread ( 4 % 40 is not unfolded. (:. 1 %)

1,, •.,,,, lll<'dlll a) b)

• ,, •-- - - - ,-.·L - L - . '7 • . ! � - . '1 - - ·- - .- ; ; . - .- - f -.- �-���__L--L .!t'.�(•·VI N

Fig. 7 Central calorimeter response to electrons a) Deviation from linearity b) Resolution

RM" (r) b) . '..:(\ a) + • + + • "' • + + +

1 .H1 + 1w,un + E,b<>am 1c;.,VJ (GeV) ll1 40 1 4 6 "1\1 iO 4 h ... '" /\ "r

Fig. 8 Central calorimeter response to . : a) Deviation from linearity [r (E measured / E beam) 1) = - b) Resolution 616

The impact point influence has been carefully studied

in beams . In one cell , the ratio of left and right x .. light guides outputs allow for x determination of the

impact point to better than at GeV and better 6 mm 10 BB right than 3 at GeV . Due' to thin light guides ( 2 , BBQ l e mm 40 mm) � \ the gap between cells is small (its influence is even IIone� cell (FrontII Face) reduced by the p vertex spread cm ) . "' 10 p

In all cases , the energy response can be corrected to better than the resolution 1 % , being essentially unaffected.

- Response to hadrons.

Since a jet contains particles of very different momenta, good linearity down to

small momenta is important in jet energy measurement . Fig 8 a and b show the deviation .. from linearity and the resolution for hadrons from to GeV/c . The measured energy is 1 70 obtained by adding the weighted energies of the three compartments. The weights are

ajusted at GeV to give the best resolution . Note that , due to escape , the resolution 10 goes rather like E- 11• than E- 112• Data have also been taken with a polyethylene target

located before the calorimeter, and asking for an interaction in the target . These

multiparticle data are shown on Fig. together with results obtained from a jet 9, Monte-car lo program by adding single particle responses .

l!HS(rl 1.0 0.2 + 0.9 0.1 ?-� 0.8

80 10 20 "° 10 EBE:�

Fig. Central calorimeter response to jets : dots are multiparticle 9 data , crosses are Monte car10, the line is a fit through single hadron data [r (E measured / E l�eam ) - = l] 617

IJMI • 14- Sc • 4• Pltl

Field wire pitch 5 •• CeU width !Stm

Fig . 10 Forward-Backward detector : exploded wiew of one sector .

5) The Forward - Backward detector (Fig. 10)

The forward and backward regions are each instrumented with twelve toroidal-magnet sectors (0.38 T.m. ). Following each sector , drift chamber planes (3 with wires along 9 the field direction , 3 at + 7° and 3 at - with respect to it ) allow for a momentum 7° measurement on charged tracks. The resolution is 300 and momentum determination µ, accuracy is l::.P/P 1% x P. After these chambers , a 1.5 r.l. iron and lead plate initiates = electromagnetic showers which are localized by four layers of 20 diameter proportional mm brass tubes , before energy measurement in calorimeter . This F/B calorimeter contains a 240 cells for the whole . Each cell is segmented in depth in 2 compartments (24 r.l. plus 6 r hadron veto), each compartment is read out by two BBQ doped light guides. Linearity .1. and resolution (a/E 14 %/ ) are similar to that of the central calorimeter . = vE

7) calibration

All cells of all calorimeters (central , F/B , wedge lead-glass) , have been calibrated in 10 GeV electrons and beams at the CERN PS. Long term tracking of the calibration is done using a ••co source in current measurement . Short term fluctuations monitoring 1 is ensured by a Xe flasher system (2 per orange slice in central , 1 per sector in F/B) giving light to light guides and through optical fibers. Electronics gains are monitored by calibrated pulses. 618

A decrease with time of the light output is observed in source measurement (confirmed in test beam). The average slope is for the central calorimeter , for the F/B. -6.4 % -13 % This decrease is probably due to the BBQ (test beam data s11ow for central calorimeter , a small decrease in scintillator attenuation length, but tttis only accounts for %). -1 .7 However , the calorimeters gave much light at the start 8000 photo-electrons for a 40 (� GeV electron).

- Calibration checks of orange slices in a P.S. beam

Three of the four slices built to cover the wedge were calibrated with all others and had their calibration followed like those of the detector . They have been put in a beam in Sept . 82 and Dec . 82 (Set 1). Four of the 20 slices instal:ced in UA2 had to be removed to close the wedge (Set 2). Advantage was taken of this to put them in a beam in Dec . 82 .

This allows for direct comparison with the calibration in the UA2 detector (Fig. 11 ). This shows that the calibration is under control to better than 2.0 (Even note that any % energy is in fact measured by two channels ).

{ hd!l!H'/-.,1] ,

l\ Set 2 Set 1

[\\

·" I.I · " I.I

Fig. Distribution of the ratio of measured energy (with usual 11 UA2 calibration ) to beam energy , for Set 2 (80 channels ) and Set (60 channels ). Syst. error1 (beam momentum Knowledge ) is �o.8 %. 619

calibration check in the F/B calorimeter

Fig. 12 shows a 77 mass spectrum obtained in the F/B calorimeter . The emergence of a clear 11° peak also gives confidence in the calibration procedure .

IWJ

Fig. 12 mass spectrum in F/B calorimeters. Background11 is indicated by the dots, obtained by randomly rotating the directions of 7's.

8) Trigger and Read out

The following triggers are (or were ) available

- minimum bias trigger made by coincidence of UA4 (total cross-section experiment ) counters located around the vacuum pipe , it sees 98 of the non-diffractive cross section. %

- Wedge charged trigger (until Dec . 82 ) was a coincidence of the two wedge hodoscopes .

- Total energy triggers E.M. total : electromagnetic part of central + F/B energies exceeds threshold Hadronic total : total central calorimeter energy exceeds threshold.

- Electromagnetic clusters logic 2 x 2 cells clusters are made , logic may be added W trigger : one cluster energy exceeds threshold Tl trigger : two clusters with energy above threshold T2 , and minimal

- Read out is done through CERN Romulus/Remus autonomous read out system. Data taking and monitoring , as well as calibrations , are done by a DEC VAY. 11/780 computer .

Monitoring (spy mode ) or testing is also possible from 4 Mini-Computers for specific parts of the apparatus .

The data taking efficiency for Nov.-Dec. 82 run was of 20 nb-1 taken , out of 27 nb-1 given by the machine .

REFERENCES

[1] The UA2 Collab ., a detector to study pp interactions at the SPS Collider , poster contribution at the Intern. Conf . on Experimentation at LEP (Uppsala, June 1980).

The UA2 Collab. , First results from the UA2 experiment at the SPS pp Collider , presented at the Ith Intern. Symp . XII on Multiparticle Dynamics (Volendam , The Netherlands , 6-11 June 1982).

The UA2 Collab. , Status and first results from the UA2 experiment , presented at the 2nd Intern. Conf . on Physics in Collisions (Stockholm, June 1982), to be published in Phys . Ser .

[2] N. Prevot These de 3eme Cycle , Universite de Paris-Sud , Centre d'Orsay (May 1982), (LAL 82-13 ).

R. Battiston These de 3eme Cycle , Universite de Paris-Sud, Centre d' Orsay (July 1982 ), (LAL 82-15 ).

[3] M. Banner et al ., Phys . Lett . 122B (1982 ) 476.

[4] P. Perez et al ., These de 3eme Cycle , Universite de Paris-Sud, Centre d'Orsay , (1978), unpublished.

[5] M. Banner et al ., Phys . Lett . 115B (1982 ) 59 .

[6] M. Banner et al ., Phys. Lett . 122B (1982 ) 322 .

[7] A.G. Clark , Proc . Intern. Conf. on Instrumentation for Colliding beam physics , SLAC Report 250 (1982 )