An overview of measurements of Bose-Einstein correlations in W-pair events at LEP is given. The results presented are based on data collected at centre-of-mass energies between 172 and 202GeV. The review concentrates on the search for Bose-Einstein correlations between pions from different W's in e+e- -> W+W- -> qqbarqqbar events. No agreement is reached in the results of the four experiments

Valassi, A.

An overview of measurements of Bose-Einstein correlations in W-pair events at LEP is given. The results presented are based on data collected at centre-of-mass energies between 172 and 202GeV. The review concentrates on the search for Bose-Einstein correlations between pions from different W's in e+e- -> W+W- -> qqbarqqbar events. No agreement is reached in the results of the four experiments.An overview of measurements of Bose-Einstein correlations in W-pair events at LEP is given. The results presented are based on data collected at centre-of-mass energies between 172 and 202GeV. The review concentrates on the search for Bose-Einstein correlations between pions from different W's in e+e- -> W+W- -> qqbarqqbar events. No agreement is reached in the results of the four experiments

CERN Document Server

arXiv:hep-ex/0009039v1  15 Sep 2000BOSE-EINSTEIN CORRELATIONS IN W-PAIR EVENTS AT LEPA. VALASSICERN, EP Division, 1211 Gene`ve 23, SwitzerlandE-mail: Andrea.Valassi@cern.chAn overview of measurements of Bose-Einstein correlations in W-pair events at LEP is given. Theresults presented are based on data collected at centre-of-mass energies between 172 and 202 GeV.The review concentrates on the search for Bose-Einstein correlations between pions from different W’sin e+e− →W+W− → qq¯qq¯ events. No agreement is reached in the results of the four experiments.1 IntroductionThe existence of Bose-Einstein correlations(BEC) in reactions leading to hadronic finalstates is well known. BEC are observed as anenhancement of the production of multipleidentical bosons close in momentum space,first reported for pairs of charged pions in pp¯collisions1. They have also been studied fortwo or more identical bosons in hadronic Zdecays at LEP2, including π±, K0s and K±.This talk reviews recent studies ofBEC3,4,5,6 for charged pion pairs producedin e+e− →W+W− events at LEP. “Intra-W” BEC (or BEI, BEC Inside a W) be-tween pions from the same W→ qq¯ decayare observed unambiguously. As expected,these closely resemble BEC in Z→ qq¯ decays,when only udsc quark flavours are considered.Since the average separation betweenthe two W decays at LEP2 is . 0.1 fm,smaller than the hadronisation scale of ∼1 fm, “inter-W” BEC (or BEB, BEC Be-tween W’s) could also be expected betweenpions from different W’s in WW→ qq¯qq¯ (4q)events. The theoretical framework is, how-ever, still unclear. In the absence of ex-act nonperturbative QCD calculations for thesymmetrized production amplitude of multi-ple mesons from hadronic W decays, BECare described only by phenomenological mod-els. Many models exist, with contradictorypredictions7 about BEB. This note will there-fore concentrate on the experimental mea-surements of BEB in W-pair events at LEP.The question whether BEB exist is par-ticularly important as the cross-talk betweenthe two W decays could bias the W mass(mW ) measurement in 4q events. Differentmodels of BEC predict different values forthe shift7. Using MC events with full detec-tor simulation, the LEP experiments obtain8mW biases from 20 to 67 MeV, conservativelytaken as systematic errors on the individualmeasurements. In the LEP combination, acommon BEC systematic error of 25 MeVin the 4q channel is assumed9. The impactof this error is more than halved when com-bining 4q events to WW→ ℓνqq¯ (2q) events.While presently the BEC error is not the lim-iting factor to the LEP measurement of mW ,a better understanding of BEC in W decaysis important in view of the future reductionof the other, currently larger, systematics.2 Overview of analysis methodsBEC in identical boson pairs are often stud-ied by the two-particle correlation functionR(Q) =ρ(Q)ρ0(Q). (1)Here, ρ is the two-boson density in the pres-ence of BEC, while the reference ρ0 shouldideally describe pair-production if there wereno BEC. Since BEC are largest at small four-momentum difference Q =√−(p1 − p2)2, ina simplified approach the effect can be stud-ied in terms of this variable alone10.In the studies reviewed in this note, Bose-Einstein (BE) enhancements of productionrates are parametrised using functions likeR(Q) ∼ (1 + λe−r2Q2) , (2)To appear in Proceedings of XXXth ICHEP, Osaka (2000) 1which describe source distributions of radiusr and BE “strength” λ. Fit results for r andλ will not be compared here, since the variousanalyses differ in detector acceptance, choiceof fit function and definition of R.The choice of the reference ρ0 and thedefinition of R are distinctive features of eachBEC analysis. The simplest is to derive ρ0from MC events with no BEC (“standardMC”). Data can also be used: for instance,the density for unlike-sign pion pairs, ρ+−,is often taken as a reference to study BECin like-sign pairs, ρ++,−−. Since unlike-signpion pairs are not free from correlations otherthan BEC, such as those due to resonancedecays, the correlation function is frequentlycomputed as a double ratio of the formRd.r.(Q) =(ρ++,−−/ρ+−) data(ρ++,−−/ρ+−) standard MC, (3)where resonance correlations in the MC can-cel those in data. Additional corrections maybe introduced to describe final-state Coulombinteractions11, different in like- and unlike-sign pairs and usually absent in the MC.In the study of BEB, instead of compar-ing 4q data to a reference with no BEC atall, one can finally define12 a function likeRmix(Q) =(ρ++,−−) 4q(ρ++,−−) “mixed” 2q, (4)where a reference with BEI and no BEB isobtained by “mixing” hadronically decayingW’s from pairs of different 2q data events.Different mixing procedures exist; in general,the two W’s, chosen of opposite charges, areboosted to be approximately back-to-back inthe lab frame, as in 4q events. A double-ratioversion of Eq. (4) can also be used.3 Experimental results at LEP2The analyses presented use data collectedby the four experiments at 172–202 GeV(Tab. 1). Signal efficiency and backgroundcontamination are 70–90% and 20% (mainlyqq¯) for 4q, 50–75% and 6% for 2q events. Upto ∼7000 selected WW events per experimentare used, including both channels.Exper.√s [GeV] L [pb−1] StatusALEPH3 172–202 479 Prel.DELPHI4 183–202 437 Prel.L35 189 177 Publ.OPAL6 172–189 250 Prel.Table 1. Data samples used in the analyses pre-sented. The L3 analysis is submitted for publication,those from the other experiments are preliminary.All analyses are performed on pairs oflike-sign charged pions, with generally loosepion selection criteria. In each event, everypion enters several pairs, which introducesbin-to-bin correlations in some distributions.Where necessary, these are corrected for byall experiments, using various techniques.The four experiments simulate BEC us-ing different MC models, differently tuned.WW MC events with BEC are needed formodel-dependent analyses or cross-checks ofmodel-independent assumptions. BEC alsoexist in the qq¯ background, although theymay differ in the four experiments dependingon the anti-b tagging criteria used in the se-lection. The simulation of BEC in qq¯ eventsselected as 4q is especially important, as3they look more similar to BEC for 4q eventsin BEI+BEB MC than in BEI-only MC.In ALEPH3, a double ratio R∗ of like-and unlike-sign pion pairs for data over stan-dard MC is used, similar to that of Eq. (3).The analysis of BEB is based on the com-parison of R∗ for data to that expected fromMC with BEI-only or BEI+BEB, simulatedaccording to a specific model of BEC tunedon Z data. Background is added to the WWsignal in computing R∗ for the MC. Fits tothe R∗(Q) distributions are performed usinga function similar to Eq. (2). The resultsare compared in terms of the integral of theBE signal, I =∫∞0λe−r2Q2dQ =√pi2λr. Thevalue for I in the BEI-only MC is compati-ble with that in data, while I in data and inthe MC with BEI+BEB are inconsistent atthe level of 2.2σ. This includes systematics,dominated by the tuning of BEC MC param-eters using Z data. It is concluded that dataTo appear in Proceedings of XXXth ICHEP, Osaka (2000) 2support the BEI model considered, while theBEB model is disfavoured. An alternativeanalysis, using a double ratio Rm of 4q andmixed 2q events in data over standard MC,yields qualitatively similar results (Fig. 1).0.90.9511.051.11.151.20 0.5 1 1.5 2 2.5 3Q (GeV)Rm(Q)Rm(Q)=(p ++,-- 4j/ p ++,-- 2j,mixed)/(MC standard)ALEPH preliminaryData 183 - 202 GeVMC full BEMC inside BEFigure 1. ALEPH3 distributions of Rm for data,BEI-only MC (“inside”) and BEI+BEB MC (“full”).In the DELPHI analysis4, also presentedin another talk13, correlation functions forlike-sign pion pairs R4q and Rmix4q are builtfor both 4q events and mixed 2q events, assingle ratios over standard MC (Fig. 2a).Unlike-sign pairs are also used for qualita-tive comparisons (Fig. 2b). Expected back-grounds are subtracted from data in comput-ing R4q and Rmix4q . By construction, R4q(Q)and Rmix4q (Q) are thus expected to be equal(different) in the absence (presence) of BEB.Signal MC events with BEC (BEI-only, orBEI+BEB) are used to verify these hypothe-ses and for systematic studies. A simultane-ous fit to R4q(Q) and Rmix4q (Q) determines acommon BE source radius and two differentBE strengths, which are expected to be equalin the absence of BEB. The fit to their differ-ence ∆λmix = λ4q − λmix4q yields∆λmix = 0.062± 0.025± 0.021 . (5)It is concluded that data support BEB at thelevel of ∼ 2σ. The largest systematic errorscome from the check that ∆λmix = 0 in BEI-only MC, and from bin-to-bin correlations.In L35, the correlation functions for like-sign charged pions are built taking mixed 2qDELPHI (preliminary)0.80.911.11.21.31.41.50 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5Q (GeV/c2)R(Q)DATAWW  4  jetWW 4 jet constructed for no BE between Ws(mixing)0.80.911.11.21.31.41.50 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5Q (GeV/c2)R(Q)DATAWW  4  jetWW 4 jet constructed for no BE between Ws(mixing)Figure 2. DELPHI4 distributions of R4q (black cir-cles) and Rmix4q (hollow circles) for a) like-sign and b)unlike-sign pion pairs, as a function of Q.events as a reference. A single ratio D isbuilt as in Eq. (4). The main analysis usesa double ratio D′, where D for data is di-vided by that for BEI-only MC events. Back-grounds are subtracted from data in the cal-culation of D′. By construction, it is thusexpected that D′(Q) = 1 in the absence ofBEB. Signal MC with BEI-only is used toverify this hypothesis (Fig. 3a). Unlike-signpairs are also used for qualitative comparison(Fig. 3b). The D′(Q) distribution is fitted forthe BEB strength Λ, expecting Λ = 0 in theabsence of BEB. The fit yieldsΛ = 0.001± 0.026± 0.015 , (6)indicating that data are compatible with theabsence of BEB. Systematics are dominatedby the selection of charged tracks and the in-clusion of the low-purity τνqq¯ channel. Thefit is repeated for MC events with a model ofBEB, yielding Λ = 0.127± 0.007, where onlystatistical errors are given. The BEB modelconsidered is disfavoured by more than 4σ.In OPAL6, WW→ ℓνqq¯, WW→ qq¯qq¯and non-radiative high-energy (Z/γ)∗ → qq¯decays are analysed. Correlation functionsfor these samples, built from double ratios asTo appear in Proceedings of XXXth ICHEP, Osaka (2000) 30.80.911.11.21.31.40 0.5 1Q [GeV]Q [GeV](a) (b)D(±,±)' D(+,-)'datainter-Wno inter-WL30.80.911.11.21.31.40 0.5 1Figure 3. L35 distributions of D′ for like-sign (a) andunlike-sign (b) pion pairs. Data are compared to BEI-only (“intra-W”) and BEI+BEB (“inter-W”) MC.in Eq. (3) and adjusted for resonance mul-tiplicities observed in data, are unfolded assums of contributions from three categories:pion pairs coming from the same W, from dif-ferent W’s, or from hadronic Z decays. BECparameters for each category are determinedin a simultaneous fit, correcting for the dif-ferences in BEC between qq¯ events selectedas (Z/γ)∗ and those selected as WW. TheBEC strength for the second category, λdiff ,directly measures BEB and is expected to bezero in their absence. MC events with BECare only used for qualitative comparison tothe data (Fig. 4). Fits are performed withdifferent assumptions about BEC source sizesin the three categories. The main fit yieldsλdiff = 0.05± 0.67± 0.35 , (7)where systematics mainly come from trackingfor low-Q like-sign pairs and the HERWIGsimulation of resonances. While the result iscompatible with the absence of BEB, OPALconclude that, at the current precision, theyare unable to determine whether BEB exist.OPAL preliminaryCdiff (Q)different WsQ(GeV/c2)0.511.522.50 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2Figure 4. Unfolded correlation for pions from differ-ent W’s in OPAL6. Data (crosses) are compared toMC with BEI-only (hatched), or BEI+BEB (open).4 ConclusionsBose-Einstein correlations between like-signcharged pion pairs produced in W-pair eventsat LEP are studied by the four experimentsusing different techniques. The existence ofBEC for pions coming from the same Wis firmly established, while no agreement isreached in the results of the four experimentsabout the possible existence of BEC betweenpions coming from different W’s. The exis-tence of inter-W BEC is supported by DEL-PHI at the 2σ level, whereas models of inter-W BEC are disfavoured by L3 and ALEPH atthe 4σ and 2.2σ level, respectively. No con-clusion is reached by OPAL at the presentlevel of statistical accuracy.References1. G.Goldhaber et al., Phys. Rev. 120(1960) 300.2. ALEPH, Phys.Rep. 294 (1998) 1; DEL-PHI, Phys. Lett. B 471 (2000) 460; L3,Phys. Lett. B 458 (1999) 517; OPAL,CERN-EP/2000-004; and ref. therein.3. ALEPH 2000-039 CONF 2000-059, andref.[1,2] therein.4. DELPHI 2000-115 CONF 414, andref.[16] therein.5. L3, CERN-EP/2000-084.6. OPAL PN393 (1999), and ref.[9] therein.7. L.Lo¨nnblad, T.Sjo¨strand, Eur. Phys. J.C 2 (1998) 165, and ref. therein.8. ALEPH 2000-018 CONF 2000-015;DELPHI 2000-149 CONF 446; L3 Note2575; OPAL PN422; and ref. therein.9. The LEP WW Working Group, NoteLEPWWG/MW/00-01 in preparation.http://lepewwg.web.cern.ch/LEPEWWG.10. M.G.Bowler, Z. Phys. C 29 (1985) 617,and ref. therein.11. M.Gyulassi et al., Phys. Rev. C 20(1979) 2267.12. S.V.Chekanov, E.A. De Wolf and W.Kittel, Eur. Phys. J. C 6 (1999) 403.13. N.Pukhaeva, these proceedings.To appear in Proceedings of XXXth ICHEP, Osaka (2000) 4

Bose-Einstein Correlations in W-pair events at LEP

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.341.4604

Bose-Einstein Correlations in W-pair events at LEP

Abstract

Similar works

Full text

Available Versions

CERN Document Server