Femtoscopy with STAR Zbigniew Chajęcki for the Collaboration Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Outline • Probing space and time in Heavy Ion Collisions with Femtoscopy • Evolution and emission time • Size and shape of the source • Do we understand the transverse mass dependence of HBT radii? • Underlying physics in AA: dynamically-driven geometric substructure • AuAu relative to pp (and dAu) • Problem with baseline of correlation function for p+p and d+Au collisions • Decomposition of correlation function in terms of spherical harmonics –new approach of studying particle correlations • Summary Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
The sketch of Heavy Ion Collision Femtoscopy probes the space and time in HIC dN/dt time hadronic phase and freeze-out QGP and hydrodynamic expansion hadronization initial state pre-equilibrium Emission time “temperature” Evolution time Chemical freeze out Thermal freeze out Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Hydro predictions: Evolution and Emission time Hydrodynamic predictions Heinz & Kolb, hep-ph/0204061 • Long lifetime of the source • as signal of the QGP formation • Ro- Rs→Emission time • Rl→Evolution time • Hydrodynamic calculations reproduce momentum observables but fail to reproduce HBT results → RHIC HBT puzzle Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Origin of HBT puzzle Evolution time Emission time Au+Au@200GeV Evolution time changes from ~9 fm/c for central collisions to ~5 fm/c for peripheral colisions 1.2 Blast Wave fit to spectra, v2 and HBT Hydro prediction:~ 15 fm/c Heinz & Kolb, hep-ph/0204061 Nparticipants 1.0 Nparticipants STAR - AuAu@200GeV, PRC71 (2005) 044906 Modified Sinyukov fit to RL M. Herrmann and G.F. Bertsch, PRC51 (1995) 328 * in the absence of transverse flow Short emission time (~2fm/c) consistent with RO/RS ratio Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Properties of the source at freeze-out STAR, PRL93 012301 (2004) STAR, PRL93 012301 (2004) system changes the shape system expands eccentricity • estimate initial from Glauber • εfinalfrom asHBT: • Size of homogeneity region scales with initial source size (increase with increase of centrality of the collision -b→0 ) • Oscillation phases: out-of-plane extended source PRC66 (2002) 044903 Source remains out-of-plane at freeze out Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Why do the HBT radii fall with increasing pT ? Au+Au@200GeV 0. 0. 0. .2 0.2 0.3 0.4 0.5 0.2 0.3 0.4 0.5 0.6 • Decrease of HBT radii with pT • usually attributed to collective flow • RL → longitudinal flow • RT→ transverse flow • Non-flow possibilities • termally (not collectively) expanding source • hot core surrounded by cool shell • important ingredient of Buda-Lund hydro picture (Csorgo et al.) t Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
STAR, Au+Au@200GeV STAR preliminary 2 Ratio of (AuAu, CuCu, dAu) HBT radii by pp p+p+X STAR preliminary Rlong • All pT(mT) dependences of HBT radii observed by STAR scale with pp although it’s expected that different origins drive these dependence • Is it only a coincidence or something deeper? Rout 1 Rside 0.25 0.5 pT HBT radii from p+p collisions fall with pT – as observed in the past, and usually attributed to string fragmentation Similar pT (mT) dependence as in p+p observed in d+Au collisions – do they have the same origin? 0.2 0.3 0.4 0.5 0.2 0.3 0.4 0.5 0.6 In Au+Au pT (mT) dependence attributed to collective expansion of the source mT mT HBT in Au+Au vs p+p (d+Au) Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
p+p like a little Au+Au? • flow not expected in such a small system as pp • Shuryak: a hydro approach cannot describe data in pp at RHIC – hep-ph/0405066 • Csorgo: mT dependence of HBT radii in pp is not generated by the transverse flow, but by the transverse temperature inhomegeneities of hadron-hadron collisions due to the freezing scale - hep-ph/0406042 Au+Au like superposition of p+p? • AuAu = N*(strings in pp) ? • AuAu = N*(„blast-waves”) ? NO! – that would generate local space-momentum correlations but in AuAu we see global correlations – although both scenarios would lead to the identical p-space observables Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Initial vs Final Size : pp(dAu) vs AuAu STAR preliminary • AuAu mostly “understood” in terms of collective flow • pp (dAu) show apparent globalspace momentum correlations, but no (or little) expansion Model-dependent approach of veryfication of expansion scenario • RMS of initial size of the source calculated with Glauber model vs. experimental RSIDE for <kT>=0.2 GeV/c Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Intriguing results ....Let’s take a closer look Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Projection of 3D CF in d+Au STAR preliminary • Gaussian source vs non-Gaussianess of CF • It looks like fit describes the shape of correlation function d+Au central, kt: [0.25-0.35] GeV/c Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Worst-case picture of d+Au STAR preliminary • Not great fit • One imagines one could do a better job, but fit must work in 3D; we see tiny portion of space • What is going on in the normalization region?(independent of Gaussian ansatz) • Problem with projections • cannot see systematics in 3D • experiments w/same CF will have different projections d+Au peripheral, kt: [0.15-0.60] GeV/c Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Decomposition of CF onto spherical harmonics QLONG Q QOUT QSIDE Z.Ch., Gutierrez, Lisa, Lopez-Noriega, nucl-ex/0505009 • Cartesian-space (out-side-long) naturally encodes physics, but is poor/inefficient representation • Recognize symmetries of Q-space -- decompose by spherical harmonics! • Direct connection to source shapes [Danielewicz,Pratt: nucl-th/0501003] – decomposition of CF on cartesian harmonics • ~immune to acceptance • full information content at a glance[thanks to symmetries] : [0,2p] : [0,p] Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Some properties of Alm coefficients • Alm = 0 for l or m odd – identical particle correlations (for non-id particles, odd l encodes shift information) • A00(Q) ≈ one-dimensional “CF(Qinv)” (bump ~ 1/R) • Alm(Q) = l,0 where correlations vanish • Al≠0,m(Q) ≠ 0 anisotropy in Q space • Im[Alm] = 0 Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Simulations with Gaussian model RL < RT ~acceptance free RL > RT RO < RS Simple, Gaussian source calculations RO > RS Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Decomposition of CF for centrald+Au collision Decomposition of CF for peripherald+Au collision Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Baseline in p+p, d+Au and Au+Au • Some coefficients don’t vanish for large Q • Bias from zero is larger for more peripheral collisions • This problem is getting more significant for small systems (pp & dAu) • It can be due to • an experimental artifact • long range correlations (e.g. Jets) • problem with momentum conservation ? • or ? Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
Summary • Femtoscopy indicates system evolution due to dynamic bulk behaviour • mT dependence of HBT radii for AuAu, dAu and pp seems to be the same although it’s expected (?) to have different origins • a new puzzle? • Looking in more details • baseline problem • Spherical harmonics allow efficent diagnostic of the correlation function • Use symmetry of the q-space • All information at a glance • Pratt and Danielewicz [nucl-th/0501003]: connection of one-to-one CF and spatial anisotropies • Investigation into the source of non-femtoscopic correlations under way Z. Chajęcki - Workshop on Femtoscopy at RHIC - BNL - June 21, 2005
