The W and Z production cross sections and QCD effects

Nuclear Physics B (Pros . Suppl.) 16 (1990) 266-268 North-Holland

THE

O U

UA2 Collaboration

I N CROSS SECTIONS

presented by H. Plothow-Besch,

CD EFFECTS

, Heidelberg. FRG*

U ION up UA2 detectorl at the improved CERN Pp collider (4s = 630 GeV) has collected data which correspond W- a !otal integrated luminosity of 7.4 pb-1 during the 108 and 1989 running periods. In thLe paper we present preliminary results based on these data on the W --4 ev and 7-1- p+eucdon cross sections and their ratio, which a. cee well with previous Ul'a m~=asurements, and on the i1?3ive PT spectrum. Emphasis in the design of the new UA2 detector was given to detect events with the emission of non interacting particles with high transverse momentum, such as neutrinos, and to enhance the electron identification in the central region . The hermetic cal i try coverage down. to 5® with respect to the beam axis (pseudorapidity 11 = t3) provides an excellent measurement of missing transverse ntum, defined as _ -YJ E4 ® uj, where uj is a unit vector in the transverse plane po-inàng from the event vertex to the centre ofcellj with transverse energy E+ and the sum extends over all calorimeter cells. Electromagnetic calmimetä; ) extends clown to h1l < 2.5. The resolution of the central EM calorimeter (Iq I < 1) is tE/E = (14îE+1 .4)96, and for the endcap calorimeter (1 .0 < itll < 2.5) it is a,&& = (17/ÎE + 2.3)96 (E in GeV). An additional cell-to-cell variation of 1% (3.5%) in the central (endcap) calorimeter has to be taken into account. Apart from calorimetey, electron identification is based on tracking, which is achieved by means of a scintillating fibre deiectce (SFD)2 (Iql < 1.6) and ofendcap proportional tubes (ECPT)3 (1.1 1,1 < 1 .6), together with dE/dx measurements by two arrays of silicon counters4 and by a small cylindrical driftchamber (JVD). Both, the SFD and the EC contain a lead radiator in which electromagnetic particles initiate, a shower detected by additional tracking and by charge measurements. Hadrons and photons are

rejected by demanding the presence of a shower in the detector layers after the radiator in the first case and a charged track pointing to the shower fit the latter case. Background from accidental overlaps between a photonand a low momentum charged track is rejected by requiring a tight match in space between the incoming track and the shower centroid. 2. THE I PRODUCTION CROSS SECTIONS The W -+ ev events are selected from a sample of electron candidates satisfying the kinematical cuts pt > 20 (15) GeV/c and pT = > 20 (15) C°V/c for electrons in the central (endcap) region, CC (EC), with an additional cut for electrons in the EC region on the transverse mass ofthe electron-nvadno system mT = 2 " p1' " pT " (1- cos ev) > 40 GeV/c2, where $ev is the angle between the electron and the neutrino in the transverse plane . In sßdition, we reject electron candidates hielinô either the edge cells of the CC or a region near the cracks between calorimeter cells, which amounts to a loss of -35% (7%) of events in the CC (EC) region. A toted of 1266 (361) events in the CC(EC) region survive these cuts. The acceptance for electrons, qW, in the CC region iicluding the geometrical and the kinematic cuts, amounts to (38.3 i 0.8)% in the CC and (9.9 t 0.2)% in the EC region . The global electron reconstruction efficiency, EW, in the CC region is measured to be 71 t I(stat.) t 2(syst.) and 74 i 3(stat.) t 1(syst.) in the PC region, respcctiveL;These electron efficiencies have beer obtained by tightening the kinematic cuts to get a background-free sample and relaxing the electron identification cuts, and by using testbeam results extensively. The background conu-ibution to the W -+ ev sample is small and is essentially due to hadronic events from QCD processes. It is estimated to be (1 t 1)% (< 5%) in the CC (EC) region. For a total

*work supported by the genman Bundesministerium fürForschung und Technologie 0920-5632/90/$3 .50 © Elsevier Science Publishers B.V. North-Holland

H. Plothow-Besch, UA2 Collaboration/ W and Z production cross sections integrated luminosity of (7.11 0.5) pb-t after background subtraction, corrections for geometrical acceptances and electron efficiencies the W production cross section is measured to be Offip -+ W + )L)-BR(W -* ev)=630t20(stat)t50(syst) pb, in good agreempnt with the previousUA2 measurement 5. The Z sample is selected from events containing two or more EM clusters with an invariant mass above 40 GeV/c2 in the CC-CC and MEC configuration . The invariant mass, mce, for events with both legs satisfying the electron identificatian criteria shows a clear peak in the Z region around 90 GeV/c2. Above a massofmet =76GeV/c? 168 events are found, of which 86 survive the fiducial cuts with slightly looser electron identification cuts on one leg for which all efficiencies are well known. The background contribution to this sample is estimated to be - (2 t 1)9b. After background subtraction, corrections for efficiencies and geometrical acceptances and taking into account the integrated luminosity the Z production cross section is measured to be (YCpp -i~ Z + X) - BR(Z --~ e+è) = 61t7(stat) ± 5(syst) pb, again in agreement with the previous UA2 measurements. The W cross section measurement is compared to a recent6 QCD calculation up to order as, which uses three neutrino generations, theDFLM7 set ofstructure functions and MeV. While the branching ratio BR(W -+ ev) depends on the mass of the top quark, mt, the branching ratio BR(Z -+ e+è) depends on both, mt and the number of additional neutrinos. The dependence of the W cross section as a function of mt is shown in Fig . 1 (solid line). Also shown are curves based on the MRSB'8 (dotted line) and on the MRSE'8 (dashed line) set of st.:cture functions illustrating the main theoretical uncertainties which arise from the poor knowledge of the ratio between the u and d valence quarks . These theoretical uncertainties have to be further increased by -15% to take into account higher order corrections. Also shown in Fig. 1 is the UA2 measurew-uit with its statistical and s; stematic uncertainty, indicating; that for the central value a heavy top quark mass would be !°neferred by the data. While the cross seci.ons aW and qZ are affected by WgL experimental and thec;)retical uncertainties, most of the experimental and theorev cal uncertainties cancel in the ratio: RExp _ ßW - BR(W -+ ev) aZ-BR(Z-+e+e)

0 .7 0 .6

3b

0 .5 0 .4

Fig. l : W produedinn crass section as a The ratio REV depends si ' y , fcr a mass of the top below 45 GeV/c2 decay W -+ t6 and Z -+ tt are allowed ; while for a mass in the range 45 < mt < 75 GeV/c2 only the decay allowed. UA2 measures the ratio R RERP =10.35 +-11.-05 (stat.) t 0.3 (syst), which gives 9.1 < RExp < 12.3 at the level. This preliminary evaluat on o, R is compatible with NV = 3 and a heavy top quark. Part oô in zz-' ' the RExp can be reduced by increasing n ofZ events used to compute the Z production cross section. W is in progress to evaluate efficiencies and systematic effects when looser cuts for the electron identificati~ are used in the selection ofthe Z ale. 3. THE MCLUS `T° SPELT While at leading order, the in i to vector boson (IVB) production can be described by the Drill-Yan qq annihilation process, QCD corrections to the Drell-Yan mechanism result in a sizable IVB verse ntum. The W and Z transverse momentum diszributions provide therefore a good test of . In addition, any excess of events at high PITTB may indicate new physics. A detailed understanding of the PT is also essential, because it influences substantially the measurement ofthe massofthe W, mW. And last, ymunting the n ofjets produced in association with the W boson provides a nt of as at the scale QZ = may . There are NO relevant Mgcs In P T : the first, at erate values ofpT, dornhiated by multiple radiation of soft gluons; the second, in the region of large pT, where contr?butions from processes like q°q-+(IVB)g and qg -+ (IVB)q leading to IVB + jets in the final states are expected. Multiple radiation. of soft gluonv cannot. be

H. Plothow-Besch, UA2 Collaboration/ Wand Z production cross sections and sitgluon resummation

n in as have . At higher can used, calculations up to 10

at all o

us.; a sample of 1 W --+ ev electron d in the CC region,

10-1 A

u

UA2

21

10-a

after releasing the fiducial cuts but with an additional transverse mass cut ( 40 Gev/cZ).

mT >

pT

tum of the W is measured as

°

(pT -a T p) mann of all

_ - pT , where gT is tha transverse ding

.A

'son between

experimental distributions and theory is perform by a generates onte o simulation which. events according to theoretical predictions and then applies the effect of the

pT

detector response. Ile resolution is derived from a study of the - resolution and from the observed relation between the pT and the total transverse energy, ET. Fig. 2a shows the experimental distribution below

pT

30 GeV/c, together with the theoretical prediction (solid iine)9, where D structure functions with AM3 = 2 eV and 1~1 = have n used, convoluted with an average detects response. Also shown is tine range `f uncertainties in the simulation die to de effects (dotted and dashed curves). All curves are normalised to the toud number of observed events. For PT > 20 GeV/c, 83 events are observed Fig. 2b ws the observed 'on of events as a function of PT . to solid line represents die tical prediction of 2 , . 0 with scale 1W= , AS =160 MeV and D Other structure functions. ranges of Args

20 Fig. 2 :

a

transverse momentum distribution with

EFE ENCES 1. UA2 Co-Ha

5.

6. 7. 8.

'on, C. Booth, Proc. 6th Topical Workshop on Proton:®AiSpav,1o tbllider Physics, ARchen 1986. World Scientific, Singapore (1987) 381. R.E . Ansorge et ai., N.icl. Inst. Meth . 265A(1988)33 . K. Borer et al ., Nucl. Inst. Meth . 253A (1987) 548. R. Ansari et al ., Nucl. Inst. Meth. 279A (1989) 388. UA2 Collaboration, R. Ansari et al., Phys. Lett . 186B (1987)440-, UA2 Collaboration, R. Ansari et al., Phys. Lett. 194B (19F7) 158. P. Nason et al., Nucl. Phys. B303 (1988) 607. M. Diemoz et al., Z. Phys. C39 (1988) 21 .

A.D. Martin et al ., preprint DPT/88/52, RAL-88-113 . G. Altarelli et al., Nucl . Phys . B308 (1988) 724. !I0. P.B . Arnold mid M. Hall Reno, preprint FERhULABPUB-88/168-T. 11 . A.C. Bawa and W.I. Stirling, Durham preprint

9.

®

4

8

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ow (GeV,'c)

20

24

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Wtransverse momentum distribution with PT below 30 GeV/c.

100

is very similarto the pT spectrum .

3. 4.

160

p (GeV/c)

80

agreement between data and theory is good, giving no evidence for new physics. _ The transverse momentum distribution of the Z, 1fT , which suffers from statistics,

2.

20C

60

PT above20 GeV/c. (AMS = 360 M and other values for the scale (Q_2 p~) are barely distiguishable from the solid line. 11c upper solid line is a calculation of Ref. 11 with a somewhat careme scale (t = 25 *) showing that the theoretical and the experimental uncertainties are of the same order. The

z80 240

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DPT/87/42 (1987).