Todorova-Nová, S

Van Remortel, N

Verbeure, F

Vitale, L

CERN Document Server

DELPHI Collaboration DELPHI 2001-107 CONF 53415 May, 2001Particle correlations ine+e−→ W+W− eventswith the DELPHI detectorSˇ. Todorova-Nova´1, N. van Remortel2, F. Verbeure2, L. Vitale3AbstractA mixing method with improved sensitivity was used to investigate particle correla-tions between the decay products from different W’s in e+e− → W+W− interactionsat energies from 189–209 GeV, recorded by the DELPHI detector in 1998, ’99 and2000. These 3 years of running correspond to a collected luminosity of 153, 219, and159 pb−1, respectively. No significant evidence for inter-W correlations was found.1 CERN, European organization for Nuclear Research, CH-1211 Geneva 23, Switzerland2 Universiteit Antwerpen (UIA), Universiteitsplein 1, B-2610 Antwerpen, Belgium3 Universita` di Trieste and INFN, Area di Ricerca di Padriciano, Trieste, Italy1 IntroductionThe DELPHI analysis of Bose-Einstein correlations between pions from different W’shas been recently revised, including the new statistics taken during the 2000 run. Atotal luminosity of 531 pb−1 was collected in the energy range from 189 to 209 GeV.The particle and event selections have changed slightly both in the fully hadronic andsemileptonic decay channel. The particles used in the Bose-Einstein analysis itself weresubjected to more strict selection criteria than before [1]. The purity of the tracks withrespect to the content of primary tracks has been increased to 95.3%. The purity andefficiency of the event selection amount to 84% and 71%, respectively, for the fully hadronicchannel and to 94% and 53% for the semi leptonic channel. The understanding of theBose-Einstein effect is still far from complete, although there are many recent theoreticalreviews [2, 3, 4].2 Analysis MethodDELPHI studied the presence of two-particle correlations using two-particle densitiesdefined in terms of the four momentum transfer Q =√−(p1 − p2)2, where p1, p2 are themomenta of the two particles. The formalism described in [5] and [6] was used, withslightly modified notations. In this formalism one studies essentially the validity of thefollowing relation:ρWW (1, 2) = 2ρW (1, 2) + ρWWmix , (1)where ρWW (1, 2), ρW (1, 2) and ρWWmix represent the two-particle densities for particle pairscoming from a fully hadronic event, the hadronic part of a semi-leptonic event and pairsfrom 2 different semi-leptonic events which are combined to look like a fully hadronicevent. One can either study a deviation from zero of:∆ρ(Q) = ρWW2 (Q)− 2ρW2 (Q)− ρmix2 (Q), (2)or, alternatively, a deviation from one ofD(Q) =ρWW2 (Q)2ρW2 (Q) + ρmix(Q). (3)In order to construct the reference sample ρWWmix , pairs of semileptonic events werecombined to construct a fully hadronic event. Particles identified as lepton or belongingto the leptonic jet were removed. The remaining hadronic parts of both events werematched according to the fitted W momenta (one of the bosons was rotated oppositeto the direction of the other W boson, and both were boosted to the nominal Ecm/2with balanced momentum). Since the detector effects are not propagated through therotation and boost, for mixing only pairs of W bosons which already have well balancedmomentum were accepted:√(−→p1 +−→p2 )2 + (Ecm − E1 − E2)2 < 50 GeV (4)1and the maximal number of re-uses of a single event was limited to 10. After mixing,the fully-hadronic selection was applied to the sample of mixed events. The balancecut discarded 76% of all possible mixed combinations. The hadronic event selection cutsrejected an additional 15% from the mixed sample. The mixing procedure was checked bycomparing the relevant event shapes and single particle spectra for mixed events with thecorresponding shapes and spectra of the hadronic events using EXCALIBUR simulationwithout Bose-Einstein correlations. In all cases a good agreement was found.3 Background subtraction and modelingBackground subtraction requires special attention since one must rely on the use of amodel to correct for the residual Z0/γ background. The correct estimate of the un-certainty associated with the background subtraction is important especially in the fullyhadronic channel, where the 4-jet background events can mimic the signal for inter-W cor-relations to some extent. In the present study, background Z0/γ events were simulatedwith PYTHIA and PYBOEI model (version BE32). The model reshuﬄes the momentaof the final particles in order to reproduce the desired enhancement of the correlationfunction for pairs of close bosons. A gaussian parametrisation was used with parameters(PARJ(92) = 1.35, PARJ(93) = 0.34).Three sources of possible bias of the background Q distribution were studied:• uncertainty related to the number of background events• uncertainty related to the difference in the charged particle multiplicity betweendata and MC• uncertainty related to the shape of the Q distribution.For the estimate of the systematics originating from the number of 4-jet backgroundevents, the background content was increased by 20%. The overall difference in the chargemultiplicity between data and MC simulation, of the order of 1.5-2.5% in single particledistributions, 3-5% in 2-particle distributions, was taken into account in the subtractionby a correction factor 0.96 applied on the subtracted terms in expression 2. The bias of themodeling of the 2-particle distributions was studied in Z0 4-jet events at the peak energyand in a high energy sample dominated by Z0/γ events. The discrepancies observed inthe shape of two-particle densities do not exceed 10%. The correction factors fitting thedata/MC ratio in the peak sample were used to estimate the systematic error associatedwith the background subtraction. The study of the systematic uncertainty due to thebackground and other sources is still ongoing but the actual status is summarised inTable 1.4 ResultsAll the terms of Equation 1 were computed using the combined statistics of 3 years ofdata taking. From these terms the variables D(Q) and ∆ρ(Q) were computed. Theexpected background content was subtracted from the selection of fully hadronic eventsusing PYBOEI(BE32) model with parameters (PARJ(92) = 1.35,PARJ(93) = 0.34) as2Source Systematic error on measured variablesΛ δmixing procedure 0.02 0.003background subtraction (hadronic sel.) 0.05 0.003background subtraction (mixed sample) 0.003 0.0003reconstruction of close pairs 0.015 0.0003Total 0.055 0.004Table 1: The main sources of systematic uncertainty and the associated systematic errorsdescribed in the text.discussed in Section 3. The correlations between bins as well as bin errors were computedfrom the data using the covariance matrix. The distributions of the variables D(Q) and∆ρ(Q) for like-sign pairs are shown in Fig.1.The data were compared with model predictions from the PYTHIA WW sample usingthe BE32 model with the same parameters as mentioned above, for two scenarios: onewith intra-W correlations only and one with both inter- and intra-W correlations. Ascan be seen from Fig. 1, the scenario where both intra- and inter-W correlations arepresent shows an enhancement at low Q values for the variable D(Q) (∆ρ(Q)), whileboth data and the model without inter-W correlations agree between themselves andwith the straight line equal 1 (0). Note that the model predictions are plotted just forillustration, and that our measurement is absolute and model independent except for thebackground subtraction. In order to quantify any possible inter-W correlations, the D(Q)distribution for like sign pairs was fitted with an exponential parametrisation:D(Q) = N(1 + δQ)(1 + Λe−RQ). (5)First, the prediction of the model with the full correlation scenario was fitted withresults:Λ(full BE) = 0.24± 0.03R(full BE) = 1.01± 0.14 fmδ(full BE) = 0.003± 0.004N = 0.997± 0.006with the χ2/ndf = 41.1/46.As the data agree very well with a straight line, a multi-parameter exponential fit withfree R and Λ is extremely unstable. For this reason we take the dampening parameter Rfrom the model including all correlations (R = 1.01 fm) and the fit was repeated for thedata and the model without inter-W correlations.The fit of the D(Q) for the data yielded:Λ(data) = −0.037± 0.055 (stat)± 0.055 (syst)R = 1.01 fm(fixed)δ(data) = −0.009± 0.007 (stat)± 0.004 (syst)N(data) = 1.017± 0.0123distribution data BE32 inside BE32 full∫∆ρ 0.012± 0.015 0.007± 0.006 0.052± 0.006Table 2: The integral of the distribution in the region 0 – 1 GeV/c2 to be compared withthe absolute prediction for the absence of inter-W correlations,∫∆ρ = 0. Errors arestatistical only.with χ2/ndf= 44.5/47, and the model without inter-W correlations gaveΛ(BEin) = −0.021± 0.021R = 1.01 fm(fixed)δ(BEin) = 0.002± 0.003N(BEin) = 1.005± 0.004with χ2/ndf= 29.8/47.In order to give a further quantification of possible inter-W correlations, the integralof the ∆ρ(Q) distributions for like-sign pairs was calculated in the Q region between 0and 1 GeV/c2. The results for the data and both models are shown in Table 2.5 ConclusionsThe presence of inter-W correlations has been investigated using the data collected byDELPHI for 3 years of data taking, amounting in a total used luminosity of 530 pb−1.No significant indication of the presence of the inter-W correlations was found.References[1] DELPHI COLL., Correlations between particles in e+e− →W+W− events , DELPHI00 – 115 OSAKA CONF 414.[2] B. Andersson and M. Ringne´r, Nucl. Phys. B 513, 627 (1998).[3] B. Andersson, Coherence and Incoherence in Bose-Einstein Correlations Proc.XXXth Int. Symp. on Multiparticle Dynamics, edts. T.Cso¨rgo˝ et al. (World Sci-entific, Singapore, 2001) p.364 (to be publ.)[4] K. Zalewski, Physics from Bose-Einstein correlations in high energy multiparticleproduction, hep-ph/0009122.[5] S.V. Chekanov, E.A. De Wolf, W. Kittel, Eur. Phys. J. C 6, 403 (1999).[6] E.A. De Wolf, Correlations in e+e− →W+W− hadronic decays, hep-ph/0101243.4data 98-2000 (++,--)inter+intra BECintra BECdata 98-2000 (++,--)inter+intra BECintra BECFigure 1: Upper plot: The D(Q) variable for like-sign combinations. Lower plot: The∆ρ(Q) variable for like-sign combinations5

Particle correlations in $e^{+}e^{-} to W^{+}W^{-}$ events with the DELPHI detector

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Particle correlations in e+e−toW+W−e^{+}e^{-} to W^{+}W^{-}e+e−toW+W− events with the DELPHI detector

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Particle correlations in $e^{+}e^{-} to W^{+}W^{-}$ events with the DELPHI detector