1 / 14

Bose – Einstein Correlations in DIS at HERA

This paper discusses Bose-Einstein correlations (BEC) in deep inelastic scattering (DIS), exploring the space-time structure of particle production and the effect of hard subprocesses. Results from ZEUS experiments and comparisons with other studies are presented. Measurements of the correlation function and parameterizations are discussed, with a focus on the radius of the source and the degree of incoherence. The results show consistency and no significant Q2 dependence.

glynne
Download Presentation

Bose – Einstein Correlations in DIS at HERA

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Bose – Einstein Correlations in DIS at HERA XXXIII International Symposium on Multiparticle Dynamics, Cracow, September 5 - 11, 2003 Leszek Zawiejski, Institute of Nuclear Physics, Cracow • Introduction • Correlation function measurement • One and two - dimensional BEC results from ZEUS • Conclusions Leszek Zawiejski XXXIII ISMD, September 2003

  2. Introduction 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. DIS studies of BEC may reveal changes of the size of the source with energy scale - photon virtuality Q2and sensitivity BE effect to hard subprocess To check these expectations the DIS measurements were done in the Breit frame for one and two dimensions. • This 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. Leszek Zawiejski XXXIII ISMD, September 2003

  3. Bose - Einstein correlation function measurement In theory BE effect can be expressed in terms of the two-particle correlation function (Kopylov, Podgoretskii, Cocconi, Bowler, Andersson, Hofmann) : (p1,p2) R(p1,p2)  p1,p2aretwo - particles four-momenta, where : (p1)(p2) (p1)(p2)is product of single particle probability densities (p1,p2) is two - particle probability density and R - 1is related to the space-time density distribution of emisssion sources through a Fourier transform. In experiment (p1)(p2),is replaced by0(p1,p2) no BE correlation - reference sample. In use: mixed events, unlike sign particles, MC events By choosing the appropriate variable like Q12 : Q12=  (E1 - E2)2 - (p1 - p2)2 R (Q12) can be measured as: Lorentz invariant : 4 - momentum difference of the two measured particles R(Q12) = (Q12)data 0(Q12)reference R is parametrised in terms of source radius r and incoherence (strength of effect) parameter . Fit to data allows to determine these values. Leszek Zawiejski XXXIII ISMD, September 2003

  4. Correlation function - 1 D Two parametrisations were used in analysis: R = (1 + Q12)(1 +  exp(-r2Q212)) : Well describes the BE correlations - based on assumption that the distribution of emitters is Gaussian in space - static sphere of emitters. and R = (1 + Q12)(1 +  exp(-rQ12)) : Related tocolor-string fragmentationmodel,which predicts an exponential shapeof correlation function,withrindependent of energy scale of interaction. • -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 Leszek Zawiejski XXXIII ISMD, September 2003

  5. BEC measurement Requires calculation the normalized two-particle density (Q12) pairs of charged pions (Q12) = 1/Nev dnpairs / dQ12 • for like sign pairs(, )where BECare present, • and for unlike pairs(+,–)whereno BEC are expected but short range correlations • mainly due to resonance decays will be present - reference sample Look at the ratio: This ratio can be affected by : – reconstruction efficiency – particle misidentification – momentum smearing data(Q12)= (, ) / (+,–) and remove the most of the background but no BEC using Monte Carlo without BEC : MC,no BEC . data Find as the best estimation of the measured correlation function R= MC,no BEC Detector acceptance correction, Cis calculated as : C= ((, )/(+,–))gen / ((, )/(+,–))det Leszek Zawiejski XXXIII ISMD, September 2003

  6. Results - 1D Data : 1996 -2000: 121 pb-1, 0.1 < Q2 < 8000 GeV2 Monte Carlo: ARIADNE with/without BEC, HERWIG for systematic study. An example : The fit - parameters : Values obtained for radius of source r and incoherent parameter  from Gaussian( 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 Fit to the spherical Gaussian density distribution of emitters - more convincing and was used mainly in the analysis Leszek Zawiejski XXXIII ISMD, September 2003

  7. Results - 1D BECfor different Q2 average value H1 andZEUS results on radiusr and incoherence are consistent average value no Q2 dependence is observed Leszek Zawiejski XXXIII ISMD, September 2003

  8. Results - 1D The target and current regions of the Breit frame average value Target and current fragm. - the significant difference in the underlying physics - but the similar independence r and  on the energy scale Q2. average value The global feature of hadronization phase? Leszek Zawiejski XXXIII ISMD, September 2003

  9. Results - 1D Comparison with other experiments pp and + p interactions e+ e interactions DIS filled band - ZEUS measurement for Q2  4 GeV2 Leszek Zawiejski XXXIII ISMD, September 2003

  10. Correlation function - 2 D To probe the shape of the pions (bosons) source The Longitudinally Co-Moving System (LCMS) was used. In DIS ( Breit frame), the LCMS is defined as : The physical axis was chosen as the virtual photon (quark) axis • In LCMS , for each pair of particles, the sum of two momenta p1 + p2 is perpendicular to the  * q axis, • The three momentum differenceQ = p1 - p2is decomposed in the LCMS into: • transverse QTand longitudinal componentQL = | 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) Parametrisation - in analogy to 1 D: R = (1+ TQT + LQL)(1+ exp( - r2TQ2T - r2LQ2L )) The radiirTand rLreflect the transverse and longitudinal extent of the pion source Leszek Zawiejski XXXIII ISMD, September 2003

  11. Results - 2 D An example : Two - dimensional correlation function R(Q L,QT) calculated in LCMS in analogy to 1 D analysis Curves : fit - using two-dimensional Gaussian parametrisation Projections : slices in QL and QT Fit quality : 2/ndf  1 Leszek Zawiejski XXXIII ISMD, September 2003

  12. Results - 2 D Extracted radiirL, rT and incoherence parameter  The different values for rLandrT The source is elongated in thelongitudinal direction (as reported previously by LEP experiments : DELPHI, L3, OPAL) average values The results confirm the string model predictions: the transverse correlation length showed be smaller than the longitudinal one. No significant dependence of elongation on Q2 Leszek Zawiejski XXXIII ISMD, September 2003

  13. Results - 2 D : DIS ande+e– annihilation 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) - analysis partially 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 ZEUS: rT / rL = 0.62 ± 0.18 (stat) +/- 0.07/0.06 (sys.) OPAL: rT / rL = 0.735 ± 0.014 (stat.) ( estimated from reported ratio rL/rT ) DELPHI :rT / rL = 0.62 ± 0.02 (stat) ± 0.05 (sys.) DIS results compatible with e+e– Leszek Zawiejski XXXIII ISMD, September 2003

  14. 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. Leszek Zawiejski XXXIII ISMD, September 2003

More Related