Betty Abelev UI Chicago Testing the Recombination Model at RHIC using multi-strange baryon correlations for the STAR Collaboration 23 rd Winter Workshop on Nuclear Dynamics, Big Sky, MT

Betty Abelev 2 Understanding the medium  Measure probes after Thermal freeze-out  Look for probes that traverse the entire medium –Initial state hard scattering  jets p, K, , initial hard scattering

Betty Abelev 3 Medium Probes: Strangeness  Partonic system vs. hadron gas  Canonical ensemble vs. grand-canonical –Exact q.n. conservation – Overall q.n. conservation & chemical potentials E thres = 2m s ≈ 200 MeV R. Stock on strangeness enhancement: “Fading away of small volume canonical constraints” hep-ph/ Hamieh et al.: Phys. Lett. B486 (2000) 61 Enhancement of strange baryons (sss) (dss) (uds)

Betty Abelev 4 Strangeness at intermediate p T  Strange particle R CP (Central/peripheral) –Mesons suppressed more than baryons at intermediate p T  Mesons suppressed more than baryons –Baryon/meson ratio increases with centrality  /K 0 S

Betty Abelev [GeV/c] dN/p T dp T dy Coalescence/Recombination of partons  Exponential (thermal) parton spectrum qualitatively explains baryon excess at mid p T mesonsbaryons TTTTT SSSSS TSTSS STT  Recombination: –Shower (S) –Thermal (T)  S & T can mix –Particle spectra are a sum of various components 3 GeV/c hadron can be 1.Produced via fragmentation 2.If meson: coalescence of GeV/c partons 3.If baryon: 3 1 GeV/c partons (more abundant!) R.Hwa et al, Phys.Rev.C70 (024904) ReCo cartoon

Betty Abelev 6 Prediction from Rec. Model by Hwa et al R.C Hwa & C.B. Yang nucl-th/  Shower s-quarks are suppressed w.r.t u & d quarks --  Ω : sss at intermediate p T comes mostly from TTT !  Differ significantly from Λ, which includes non-strange quarks  (STAR)  (STAR)  Use Azimuthal correlations  measure fragmentation (shower-quark contribution) if  if Ω : sss at intermediate p T come mostly from TTT   no Ω correlation partners!  Use Azimuthal correlations  measure fragmentation (shower-quark contribution) if  if Ω : sss at intermediate p T come mostly from TTT   no Ω correlation partners!

Betty Abelev 7 x y Statistical Jet Measurement  Particles from same jet  closely aligned in   Use characteristic jet cone shape for a statistical jet study   High p T track 1.Find a trigger particle (p T >2 GeV/c) 2.Find an associated particle ( p T trg >p T as >1.5 GeV/c ) in the same event 3.Compute  at primary vertex for each 4.Compute  at primary vertex q q

Betty Abelev 8 Azimuthal Correlation Function  Results in a double-peaked correlation function –Normalize by N triggers  p+p and d+Au: 2 Gaussians + flat background  Au+Au: 2 Gaussians +flat background+ flow If no SSS contribution to  spectrum  same-side  -h would be flat in most central Au+Au same-sideaway-side

Betty Abelev 9  Mesons and Baryons seem to have different v 2 at same p T.  If flow is collective –Should scale with n, number of valence quarks (partons)  Scaling works! How to get V 2 for multi-strange?  Can use  v 2 for  and  J. Adams et al (STAR), Phys. Rev. Lett. 95 (2005)

Betty Abelev 10 Au+Au azimuthal correlations results  A clear same-side  -h peak is observed for GeV/c  baryon triggers!

Betty Abelev 11 Au+Au azimuthal correlation results  The magnitude of the same-side peak is independent of s-quark content! Uncertainty due to v 2 determination methods

Betty Abelev 12 Comparison to singly-strange ( Λ & K 0 S )  For same-side meson and baryon yields are similar  No dependence on strangeness content  Yields increase as a function of p T

Betty Abelev 13 A reference:  Correlations in d+Au  -h correlation is observed in d+Au. Same-side yield: ± Away-side yield: 0.04 ± p T trig >2.0 GeV/c 1.5 GeV/c <p T as <p T trig | h |<0.75 What kind of same-side signal to expect with no medium? (not enough statistics in p+p, not enough statistics to measure Ω. Look at Ξ -h in d+Au) STAR Preliminary

Betty Abelev 14 2-dimensional correlations:   Extend analysis in 2 dimensions  Same procedure as for , only use  coordinates of trigger and associated  Elongation in  under the jet peak: “the ridge” beam direction  ridge   ridge jet jet+ridge p T trigger =3-6 GeV/c, 1.5 GeV/c <p T associated < p T trigger   STAR, PRC73, (2006)\  

Betty Abelev 15 Jet and ridge as a function of centrality Jet only Jet + Ridge  Measuring jet only: divide the  space in jet+ridge (1) and ridge-only (2) regions  Subtract (2) from (1) to obtain jet-only measurement  0 0  (2) (2) (1) (1)     jet-only contribution consistent across N ch –At this p T trig & p T assoc.: 5 x increase of jet+ridge from d+Au to Au+Au  increase is all in the ridge.  Compare  d+Au result with  result in Au+Au: –The p T -integrated (2-6 GeV/c) yield in Au+Au (0.20±0.05) is 10 x the yield in d+Au (0.015±0.026)! –The ridge? Jana Bielcikova’s talk, this workshop

Betty Abelev 16  -h in  the dip  The signal is indistinguishable from the ridge (perhaps visually only)  We observe a dip in the signal in the  =0 region  This dip (though to be a detector effect) is still under investigation  Expect a significant loss of signal due to the dip 2.5<p T trig <4.5 GeV/c 1.5<p T as <p T trig STAR Preliminary

Betty Abelev 17 2-D  correlation results The point-by-point subtraction of the two regions: result consistent with 0 We don’t yet have the statistics to subtract the ridge Subtracting the fits: the result is encouraging, but is qualitative – large errors.  STAR Preliminary

Betty Abelev 18 Conclusions  Multi-strange baryon azimuthal correlations were observed in d+Au and Au+Au data   baryon same-side azimuthal correlation yield in central Au+Au is ten times that in d+Au data  Omega baryon same-side peak was observed in most central Au+Au, contrary to predictions  The ridge was observed in  -  baryon correlations in Au+Au, and the excess  Au+Au yield is likely due to the ridge  The statistics are not yet sufficient to separate jet and ridge contributions for multi-strange  Outlook: –Omega spectrum measured to a higher p T to detect onset of fragmentation –Study of the ridge: its composition and dependence on kinematic and geometrical variables –Study of the away-side