ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 1 Raluca Muresan NBI Copenhagen Bose-Einstein Correlation Studies at HERA-B

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 2 Outline HERA-B experiment BEC – introduction Why BEC at HERA-B Main ingredients for BEC analysis Preliminary results  in pA with A=C, Ti, W On-going studies Summary

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 3 HERA-B experiment Located at the proton-electron collider HERA at DESY. Fixed target experiment: 920 GeV proton collisions on various targets. Large acceptance at mid-rapidity x  (15, 220) mrad, y  (15-160) mrad

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 4 BEC  Symmetric wave functions of bosons, no exclusion principle;  Enhanced probability for the identical bosons to be emitted with small relative momenta;  Quantum statistical correlations between pairs of identical particles. Presuming that only particles emitted from the same or very close sources exhibit this behaviour  from studies of BEC one can obtain information about the size, shape and space–time development of the particle emitting source.

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 5 BEC For two identical bosons the Bose-Einstein correlation is defined as: probability density of two particles to be produced with 4-momentum p 1 and p 2 ; probability densities for a single particle to be produced with 4-momentum p 1 or p 2, difficult to build in practice  reference sample

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 6 BEC function parametrisation invariant four-momentum difference, related to the fraction of identical bosons which do interfere, interpreted as the geometrical radius of the presumably spherical boson emitting source (just an approximation), overall normalization, linear background.

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 7 Why BEC at HERA-B ? Opportunity to measure BEC parameters at sqrt(s)=41,6 GeV, intermediary value between SPS and RHIC; Possibility of studying the BEC parameters dependence on the target material (C, Ti, W). BEC can be studied for both pion and kaon pairs (RICH selection). Large minimum bias sample(single wire runs) 103 million pC events, 74 million pW events, 28 million pTi events. large enough for differential studies (directional dependence, multiplicity, transverse mass dependence ). This talk is presenting only a feasibility study, made on a small sample (few million events).

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 8 Reference sample Mixed sample – tracks from different events, some other kind of correlations are also disappearing (long range correlations, energy-momentum...) To correct for the effects introduced by mixing double ratio.

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 9 Remove from the analysed sample the pairs of tracks too close in space : abs(tx1-tx2)<0.0008, tx=px/pz; abs(ty1-ty2)<0.0008, ty=py/pz; and momentum abs(p1-p2)<0.5 GeV p=abs( ) to be resolved. The C 2 (Q) distributions for pions corrected for Coulomb interaction (Gamow), only small variation in the parameter values occured. Event, track, pair selection Pion and kaon tracks - RICH likelihood.

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 10 MC studies -pC BEC JETSET: MSTJ(51)=2 the shape of correlation function – Gaussian MSTJ(52)=9 BEC for , K,  PARJ(92)=1 meaning that particles that can be subject to BEC are subject to BEC. PARJ(93)= 2 fm =R. PARJ(91)= GeV minimum particle width above which the particle decays are assumed to take place before the stage when BE effects are introduced. Particles with broader width than are assumed to have time to decay before BE effects are to be considered Not all the pions are correlated and PARJ(92) and the measured lambda (obtained by fitting) are different

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 11 MC study -Influence of the non-belonging pairs- pC Q(GeV) C 2 (Q) Non-belonging pairs = pairs in which, at least one particle is not a pion (based on MC truth). Is a junk, a kaon, a proton etc... The junks and the possible pion contamination do not affect our result. P1=N, P2=  P3=R, P4= 

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 12 MC studies – pC C2 (Q) Q(GeV) P1=N, P2=  P3=R, P4=  C input Radius 2 fm All range  R(fm) 0.294± ±0.161 To test the procedure Studies of systematic errors are necessary but so far it seems that the procedure gives the correct results. The radius used as MC example is bigger than the radius we measure.

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 13 C 2 (Q) - Data C C2 (Q) Q(GeV) P1=N, P2=  P3=R, P4=  C2 (Q) Q(GeV) W  ± R = 1.057±0.056 (fm)  0.293± R = 1.307±0.076 (fm)

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 14 C 2 (Q) - data Ti Q(GeV) C2 (Q) P1=N, P2=  P3=R, P4=   0.253± R = 1.298±0.010 (fm)

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 15 Fit results- Dependence on the fit range Q<1.2 GeVQ<0.72GeVQ<0.36 GeV C 12 R(fm) χ ± ± / ± ± / ± ± /25 Ti 48 R(fm) χ 2  0.207± ± / ± ± / ± ± /25 W 184 R(fm) χ 2  0.305± ± / ± ± / ± ± /25 About events for each sample Weak dependence, if any, on the target material ( same as Na44- S on S, Ag, Pb). Radius value smaller than the Na44 one, pPb R=2.89±0.30, but p t larger at HERA-B, source size decreases….

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 16 LCMS The spherical source shape just an approximation, difficult to interpret. LCMS - the spatial dimension of the source couples to all components - the temporal component couples only to Q t,out. beam

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 17 Multidimensional correlation function 2-dim 3-dim

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 18 Even more preliminary results 2-dim  0.261±0.002 R T (fm)0.885±0.034 R L (fm)1.053±0.040 About 5.5 million pC events

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 19 On-going studies - Kaons First results (about 18 million pC events) indicate, as it was expected that the BEC radius for kaons is smaller than for pions (about one half).

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 20 Plans - Multiplicity dependence Code ready, results on the way

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 21 Plans - m t dependence of the 3-dim BEC parameters Code ready, results on the way

ISMD 2004, Sonoma State University R. Muresan NBI BEC studies 22 Summary  HERA-B is the place to do interesting studies of BEC correlations  More results are expected very soon It still a lot to be done in terms of running over all the data set, careful study of systematic errors, undestanding the results and comparing them with other experimental results. BEC correlations were observed both for  and KK pairs. Preliminary results, , were presented for both 1-dim BEC parameters (pA - A=C, Ti, W) and two-dim BEC (pC). Studies on-going:  3-dim, multiplicity and mt dependence; KK.