Bose – Einstein Correlations in DIS at HERA

Bose Einstein Correlations in DIS at HERA XXXIII International Symposium on Multiparticle Dynamics, Cracow, September 5 - 11, 2003 Introduction

Correlation function measurement

One and two - dimensional BEC results from ZEUS

Conclusions Leszek Zawiejski,Institute of Nuclear Physics, Cracow

Leszek Zawiejski XXXIII ISMD, September 2003

Introduction DIS studies of BEC may reveal changes of the size of the source with energy scale - photon virtuality Q2 and sensitivity BE effect to hard subprocessThis talk : ZEUS results on: Examinations of the Q2 dependence BEC sensitive to the hard subprocesses ? Two - dimensional analysis - the shape of the production source - for the first time in DIS, Comparison with other experiments. In Bose - Einstein correlations (BEC) studies an enhancement in the number of identical bosons produced with similar energy-momenta is observed. This effect arises due to symmetrization of the two-boson wave function. BEC can be used to investigate the space-time structure of particle production in different particle interactions.To check these expectations the DIS measurements were done in the Breit frame for one and two dimensions.


BE effect can be expressed in terms of the two-particle correlation function(Kopylov, Podgoretskii, Cocconi, Bowler, Andersson, Hofmann) :(p1,p2) (p1)(p2), is replaced by 0(p1,p2) no BE correlation - reference sample. In use: mixed events, unlike sign particles, MC events Bose - Einstein correlation function measurementR(p1,p2) (p1)(p2) R is parametrised in terms of source radius r and incoherence (strength of effect) parameter . Fit to data allows to determine these values.where : p1,p2 are two - particles four-momenta,(p1,p2) is two - particle probability density (p1)(p2) is product of single particle probability densities In theoryR - 1 is related to the space-time density distribution of emisssion sources through a Fourier transform.

In experimentBy choosing the appropriate variable like Q12 : Q12 = (E1 - E2)2 - (p1 - p2)2 R (Q12) can be measured as:R(Q12) = (Q12)data 0(Q12)reference and Lorentz invariant : 4 - momentum difference of the two measured particles


Correlation function - 1 D R = (1 + Q12)(1 + exp(-r2Q212)) : - normalization factor, (1 + Q12) includes the long range correlations - slow variation of R (R) outside the interference peak radius r - an average over the spatial and temporal source dimensions, r is related to the space-time separation of the productions points - string tension in color-string model - degree of incoherence : 0 - completely coherent, 1 - total incoherent Well describes the BE correlations - based on assumption that the distribution of emitters is Gaussian in space -static sphere of emitters.R = (1 + Q12)(1 + exp(-rQ12)) :and Related to color-string fragmentation model, which predictsan exponential shape of correlation function, with r independent of energy scale of interaction.Two parametrisations were used in analysis:


BEC measurement (Q12) = 1/Nev dnpairs / dQ12 Requires calculation the normalized two-particle density (Q12) pairs of charged pions for like sign pairs (, ) where BEC are present, and for unlike pairs (+,) where no BEC are expected but short range correlations mainly due to resonance decays will be present - reference sampleLook at the ratio: data(Q12) = (, ) / (+,) and remove the most of the background but no BEC using Monte Carlo without BEC : MC,no BEC .R = dataMC,no BECThis ratio can be affected by : reconstruction efficiency particle misidentification momentum smearing Detector acceptance correction, C is calculated as :C = ((, )/(+,))gen / ((, )/(+,))det Find as the best estimation of the measured correlation function


Results - 1DValues obtained for radius of source r and incoherent parameter fromGaussian ( 2 / ndf = 148/35)r = 0.666 0.009 (stat.) +/- 0.023/0.036(syst.) = 0.475 0.007 (stat.) +/- 0.021/0.003 (syst.)

and exponential (2 / ndf = 225/35)r = 0.928 0.023 (stat.) +/- 0.015/0.094 (syst.) = 0.913 0.015 (stat.) +/- 0.104/0.005 (syst.) like parametrization of R Data : 1996 -2000: 121 pb-1, 0.1 < Q2 < 8000 GeV2 Monte Carlo: ARIADNE with/without BEC, HERWIG for systematic study. The fit - parameters :Fit to the spherical Gaussian density distribution of emitters -more convincing and was used mainly in the analysis An example :


Results - 1D BEC for different Q2 no Q2 dependence is observedH1 and ZEUS results

on radius r and incoherence are consistentaverage valueaverage value


Results - 1DThe target and current regions of the Breit frame the significant differencein the underlying physics -

but the similar independence r and on the energy scale Q2.average valueThe global feature of hadronization phase?average valueTarget and current fragm. -


Results - 1DComparison with other experiments pp and + pinteractionse+ e interactionsDISfilled band -ZEUS measurementfor Q2 4 GeV2


Correlation function - 2 D In LCMS , for each pair of particles, the sum of two momenta p1 + p2 is perpendicular to the * q axis, The three momentum difference Q = p1 - p2 is decomposed in the LCMS into: transverse QT and longitudinal component QL = | pL1 - pL2 | The longitudinal direction is aligned with the direction of motion of the initial quark (in the string model LCMS - local rest frame of a string) In DIS ( Breit frame), the LCMS is defined as :Parametrisation - in analogy to 1 D:R = (1+ TQT + LQL)(1+ exp( - r2TQ2T - r2LQ2L ))The radii rT and rL reflect the transverse and longitudinal extent of the pion source To probe the shape of the pions (bosons) source The Longitudinally Co-Moving System (LCMS) was used.The physical axis was chosen as the virtual photon (quark) axis


Results - 2 DTwo - dimensional correlation function R(Q L,QT) calculated in LCMSin analogy to 1 D analysis Projections :slices in QL and QT Curves : fit An example :Fit quality :2/ndf 1- using two-dimensional Gaussian parametrisation


Results - 2 DExtracted radii rL, rT and incoherence parameter average values The different values for rL and rT The source is elongated in the longitudinal directionThe results confirm the string model predictions: the transverse correlation length showed be smaller than the longitudinal one.No significant dependence of elongation on Q2 (as reported previously by LEP experiments : DELPHI, L3, OPAL)


Results - 2 D :DIS and e+e annihilation ZEUS: rT / rL = 0.62 0.18 (stat) +/- 0.07/0.06 (sys.) DELPHI : rT / rL = 0.62 0.02 (stat) 0.05 (sys.) Can we compare DIS results ( i.e. rT / rL) with e+e ?

In e+e studies, 3D analysis and different reference samples are often used, but for OPAL and DELPHI experiments (at LEP1, Z0 hadronic decay) - analysispartially similar to ZEUS: OPAL (Eur. Phys. J, C16, 2000, 423 ) - 2 D Goldhaber like fit to correlation function in (QT,QL) variables, unlike-charge reference sample,DELPHI (Phys. Lett. B471, 2000, 460) - 2 D analysis in (QT,QL), but mixed -events as reference sample.

So try compare them with DIS results for high Q2 : 400 Q2 8000 GeV2 OPAL: rT / rL = 0.735 0.014 (stat.) ( estimated from reported ratio rL/rT )DIS results compatible with e+e


Conclusions ZEUS supplied high precision measurements on 1D and 2D Bose - Einstein correlations. The effect was measured as the function of the photon virtuality Q2, in the range 0.1 - 8000 GeV2 - in a single experiment with the same experimental procedure.

The results are comparable with e+ e experiments, but the radii are smaller than in + p and pp data.

The emitting source of identical pions has an elongated shape in LCMS consistent with the Lund model predictions.

Within the errors there is no Q2 dependence of the BEC BE effect is insensitive to hard subprocesses and is a feature of the hadronisation phase.


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Bose – Einstein Correlations in DIS at HERA