1. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Physics with  -Meson Production at STAR Xiang-Zhou Cai For China-STAR Collaboration Shanghai INstitute of Applied Physics,CAS Outline Introduction & Motivations  results from STAR experiment   Spectra shape and collision energy dependence  Ratio of particle yield (  / ,  /k,  /  )  Nuclear modification factor: R CP, R AB  Elliptic Flow v 2 of  Mesons STAR-TOF and its meanings to  Our future expectation

2. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR How about  Meson?  Phi production can be a good probe  OZI rule suppression vs strangeness enhancement  small cross section for scattering with nonstrange hadrons  Information from earlier time before chemical freezeout?  Sensitive to early flow? (Alvarez-Ruso, V. Koch)  Mass shift & width widened due to nuclear medium effect? Asakawa, Ko, Song, NPA572(1994)732, PLB388(1996)141  Production mechanism: ggg ->  ; s sbar ->  ; K + K - ->  RQMD predicted an increase of  /K - ratio (2/3 phi mesons)  whether the particle dependence of the Rcp is dividing by the particle mass or particle type? (Group:  Ks)  Elliptic flow: v 2 dependence on particle mass at low p T dependence on particle type at intermediate p T  m ~ 1019 MeV/c 2,  ~ 4.45 MeV/c 2,  ~ 45 fm/c  Vector meson  quark content : s, sbar  Studied decay channel:   K + K - (49.6%),e+e-,  +  -

3. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Data set of  analysis 1)dE/dx identify stable charged particles in a certain momentum range. 2)  meson is reconstructed from K+K- pairs by event mixing method. 62.4 GeV Au+Au collision 62.4 GeV Cu+Cu collision 130 GeV Au+Au collision 200 GeV Au+Au collision 200 GeV d+Au collision 200 GeV p+p collision 200 GeV Cu+Cu collision Can compare the  production between different collision energy & collision system  STAR TPC used to identify Kaon via dE/dx in TPC gas STAR Detector

4. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Event-Mixing and  -dip angle cut Event-Mixing technique –Select K + and K - tracks from PID by energy loss in TPC –Combine all pairs from same event  Signal+Background (same event spectra) –Combine pairs from different events  Background (mixed event spectra) –Signal = same event spectra – mixed event spectra   -dip-angle >0.04 radians  peak from photon conversion background is very effectively removed  It will reduce the reconstruction efficiency by ~12%.

5. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Invariant mass distribution  Event-mixing method used to estimate combinatorial background from uncorrelated K + K - pairs;  After back-ground subtraction, the mass peak can be seen clearly for all data-sets.  Final subtracted m inv distribution fitted with Breit-Wigner + linear (or polynomial) background function.  A residual background remains: includeing the real correlated background from decay pairs due to Coulomb interactions and particle decays

6. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Masses and widths – p T > 1 GeV/c,  mass and width are consistent with those from Monte Carlo (MC) embedding simulations – p T < 1 GeV/c, The  mass drop of ~2.5 MeV at lower p T in both real data and simulation is understood within detector effects, which is not fully corrected. – its width is larger than both PDG value and MC simulation, which is due to detector and geometry effect. – this difference limited the sensitivity to probe the possible small modifications of  meson properties in the produced medium. – Measurement though the dilepton decay channel are needed with TOF! An interesting excess on the low-mass side of the  invariant mass peak was observed in the e + +e − channel for 12 GeV p+A interactions from the KEK experiment. It indicates a vector meson mass modification at normal nuclear density. PRL98, 042501, 2007

7. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Spectra of  and fits 1)With efficiency correction (including vertex efficiency). Statistical errors only. 2)Spectra of Au+Au collisions can be described by exponential function while spectra of d+Au and p+p collisions deviate from the exponential distribution and have a power-law tail at intermediate p T 3)measurements of 200GeV Au+Au collisions at STAR for Run II and Run IV are consistent STAR Preliminary (1)Exponential shape: (2) Levy shape: The exponential function is the limit of the Levy function as n approaches infinite.

8. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Spectra of  and fits STAR Preliminary – Levy function describe well the data (exponential in central and power-law-like in peripheral) – Similar trend in Cu+Cu and Au+Au at the similar N part at same collision energy A change in the shape of spectra from p+p, d+Au and peripheral Au+Au collisions to central Au+Au collisions is clearly visible. The change of  spectra shape may indicate that different production mechanisms (hydrodynamics, coalescence/recombination, fragmentation and jet quenching mechanisms) is dominating in different kinematic regions and reaction systems.

9. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Suppression for Au+Au Comparison of  spectra shapes No suppressio n for d+Au STAR Preliminary 6) 1. Nbin normalized yield is suppressed in central Au + Au collisions at 200 GeV, while no suppression is visible for d + Au collisions at 200 GeV comparing to p+p collisions. Npart/2 normalized yield is enhanced significantly in central Au + Au collisions, which indicates the hot environment created by central Au + Au collisions is in favor of the production of soft Phi mesons. 2. The observed change of spectra shape in our measurement is more likely to be due to the change of these different production mechanisms in different kinetic regions and reaction systems. Precise measurement at (m t -m  )>3GeV/c 2 for AuAu collision are necessary!

10. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR – the N part normalized yield dN/dy increases rapidly and then saturates for mid- central Au+Au collisions. –The dN/dy and are similar for Cu+Cu and Au+Au at similar N part bin for the same collisions energy. –  yields from Au+Au and Cu+Cu collisions depend on the number of participant nucleons only, unlike Kaon and hyperons. [1] NA49 Col. Phys. Rev. Lett. 96, 052301 (2005); [2] E-802 Col. Phys. Rev. C 60, 044904 (1999). STAR Preliminary dN/dy and

11. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR T effective   of ,Λ,Ξ and Ω do not seem to follow the Vs. mass trend which increases with mass for π -, K - and pbar. It is caused by their small hadronic cross section,which may indicate  would carry more information about early state. increases slightly with N part and energy, seems to saturate at high N part. It indicates that more higher temperature can be reached in more central and energy collisions. STAR Preliminary 8) Not in same velocity!

12. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Strangeness enhancement Strangeness enhancement: yield relative to p+p  -meson enhancement: -- between K/  and Xi -- 200 GeV data > 62.4 GeV, unlike hyperons -- could not be solely due to the canonical suppression, there could be dynamics effect STAR Preliminary 62.4 GeV 200 GeV  K, 

13. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Ratios of  to π - or K - (1) φ yields increase faster than π - in higher center-of-mass beam energy  onset of strange quark degree of the freedom? AA>pp: partonic environment favors φ production? (2) No strong centrality dependence of φ/K - in Au+Au collisions; an underlying production mechanism for phi that is common to all systems. RQMD predicts increasing φ/k - vs N part, and much smaller than the data. Kaon coalescence could not be a dominant channel of φ-meson production! 9)

14. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Ratios of  to  Since  -mesons are made via coalescence of seemingly thermalized s quarks in central Au+Au collisions, the observations imply hot and dense matter with partonic collectivity has been formed at RHIC. STAR Col. Phys. Rev. Lett. 99, 112301 (2007). the mechanisms of (multi)strange particle production are predicted to be very sensitive to the early phase of nuclear collisions, the ratio of  /  is expected to reflect the partonic nature of the thermal source. With decreasing centrality, the observed  ratios seem to turn over at successively lower values of p T possibly indicating a smaller contribution from thermal quark coalescence in more peripheral collisions. This is also reflected in the smooth evolution of the p T spectra shapes from the thermal-like exponentials to Levy-like curves. More precise measurement at p T >3GeV/c are necessary!

15. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR R CP of  Mesons (Ks, K ,  ) have the similar R CP for dAu and AuAu collisions Baryons ( ,  ) have the similar R CP too, but higher than mesons. Particle production at intermediate p T region is divided by the particle’s types, not the masses Recombination/Coalescence model STAR:  behaves like mesons, despite of its large mass favor Recombination/Coalescence model dAu 200 GeV Run IV:AuAu 200 GeV For 0-5%/60-80%, it may be attributed to the shape change of spectra from exponential one at (40-60%) centrality to levy one at (60-80%) centrality, which may be due to the change of production mechanism in different environments with different degrees of strangeness equilibration.

16. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR yield suppression at intermediate P T in central Au+Au collisions weak suppression is shown in the 62.4 GeV Au+Au collisions. It becomes flat at P T >1GeV/c R CP in d+Au collisions is enhanced It is difficult to draw some definite conclusions due to the large error bars R CP of  for different collision system Low p T, R CP <1 High p T, R CP >1 STAR Preliminary Measurements of phi meson production in relativistic heavy ion collisions at RHIC B.I. AbelevB.I. Abelev et al. for STAR Collaboration, submit to PRC

17. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR STAR Preliminary Nuclear Modification factor Comparing central Au+Au and central Cu+Cu with peripheral Au+Au Collisions— 1) the yield and p T shape depend on number of participants only 2) in terms of number of binary collisons the Cu+Cu data is higher indicating given the same top 10% centrality Cu+Cu has less number of collisions per participant

18. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Elliptic Flow v 2 1.The phi meson has significantly non-zero v2 which must have been developed in the earlier partonic stage. 2.In the low p T region (0»2 GeV/c), v2 of phi sits in between K 0 S and , consistent with the expectation of a mass ordering of v2 in hydrodynamic models. 3.These observations support the hypothesis of development of partonic collectivity and possible thermalization in the early stages at RHIC 4.the elliptic flow of Φ-meson is close to Ks. It indicates that the momentum space anisotropy of constituent quarks (s and s for phi ) is developed prior to hadron formation. The heavier s quarks flow as strongly as the lighter u and d quarks. 5.v2(p T ) increases with increasing eccentricity (decreasing centrality). the centrality dependence of the of the  meson is consistent with that of the charged hadrons Error bars in high pt range (2.5GeV,5.0GeV) are still large! Measurements of  v2 from both reaction plane and invariant mass methods are consistent.

19. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR (1) Did not probe the small modifications of  mass in the produced medium. (2) The change of  spectra shape may indicate that different production mechanisms is dominating in different kinematic regions and reaction systems (3) The of ,Λ,Ξ and Ω do not seem to follow the Vs. mass trend which increases with mass for π -, K- and pbar. which may indicate  would carry more information about early state. (4) The lack of a significant centrality dependence of the  /K- effectively rules out kaon coalesence as a dominant production channel for the  at RHIC energy. (5) The phi meson has significantly non-zero v2. It indicates that the heavier s quarks flow as strongly as the lighter u and d quarks. (6) The Rcp and v2 of the  clearly behaves more like the Ks 0 meson than the Λ baryon. Therefore, processes relevant to Rcp and v2 at intermediate p T are driven by the type of particle. (7) The transverse momentum spectra in both shape and yields are similar in Cu+Cu and Au+Au for collisions with similar N part (8)  -meson enhancement shows a distinct collision centrality and energy dependence. The enhancement cannot solely due to canonical suppression of strangeness production, there could be dynamics effect. What we learned from STAR about 

20. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Performance of STAR-TOF prototypes φ background suppression STAR Preliminary TOF+TPC : one kaon from φ identified by TPC, the other by TOF TPC+TPC : the 2 kaons from φ identified using only TPC Au+Au 200GeV min-bias  TPC+TOF:  Pion: 0.-~12 GeV/c  Kaon: 0.2-~3GeV/c  Proton: 0.2-~5 GeV/c One tray will enhance the S/B ratio by a factor of 3.5 at intermediate p T. Full barrel ~ a factor of 10. Reduce the error bar Extend the measurement into high P T

21. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR What we can expect for further study of  meson with full-TOF? With TOF, We can: 1 、 Reconstruct  from di-lepton decay channel 2 、 Extend the measurement into high p T region 3 、 Increase the S/B ratio, reduce the background Focus on the relative physical topics: 1 、 Compare the production of  from both hadron and lepton decay channel with large acceptance. To answer the  puzzle (in-medium effects on  and kaons + kaon absorption and rescattering leading to reduced yield and hardened pT spectrum in hadronic channel?) 2 、 Is there any medium effect (Mass shift & width widened) at RHIC collision? 3 、 Study the centrality dependence of Ratios of N(  )/N(  ). Probe the property of thermal source? 4 、 Measure the  spectra shapes at high p T region to see the yield suppression. 5 、 Energy dependence of  production.(under STAR the energy scan plan) To see when the value of v2 of  will be small or zero.………

22. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Backup

23. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR PID TPC alone PID range: pi/k ~0.6 GeV/c, (pi,k)/p ~1.6GeV/c; TOF “alone” PID range: pi/k ~1.6GeV/c, (pi,k)/p ~ 3.0GeV/c m2m2 m2m2  TOF =110ps  k p TPC alone TOF “alone”

24. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR PID hadrons (TPC + TOF) TPC Relativistic rising of dE/dx High p T High performance of time resolution Low & intermediate p T 2.5<p T <3.0 PID 0.15 - 4 GeV/c PID up to 12 GeV/c

25. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Electron identification Combined with TOF ， electrons can be separated from pions cleanly Electorns mixed with pions if only dE/dx of TPC is used to identify particles X. Dong, Z.B. Xu, LJ Ruan, J Wu (USTC-BNL) et al Lepton decay from heavy quark

26. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR  -meson production from Au+Au STAR Col. Phys. Lett. B 612, 181 (2005), Phys. Rev. Lett. 99, 112301 (2007). 1.,  -meson decoupled early; 2. N(  )/N(K), ruled out the K-coalescence; 3. Evolution in the centrality dependence;  Cu+Cu collisions provide unique data to further probe these features.  /K -

27. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR N(  )/N(p) – A decreasing trend in N(  )/N(p) vs. p T would be an unambiguous signature for early decoupling of  mesons from the hadronic rescattering dynamics [1] – Fitting with Hirano’s model [1] yield: – p+p, ChiSquare/ndf = 6.01/8 – Au+Au w rescattering, ChiSquare/ndf = 11.42/8 – Au+Au w/o rescattering, ChiSquare/ndf = 29.14/8 – Au+Au, Tth=100 Mev, ChiSquare/ndf = 7.75/8 [1] Hydro calculation: arXiv:0710.5795v1

28. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR 1)R CP suppression of AuAu collisions at intermediate P T. 2)Particle type dependence of R CP from Au+Au collisions has been observed. Recombination/Coalescence model 3)The particle type dependence of the R CP at intermediate p T in AuAu collisions can be explained by Recombination/Coalescence model. R.C.Hwa et al. University of Oregon C.M.Ko et al. Texas A&M R. J. Fries et al. Duke/Minnesota QBXie et al., Univ. of ShanDong …… Particle-type dependence of R CP & v 2 PHENIX: PRL91, 182301(03) STAR: PRL92, 052302(04) nucl-ex/0306007 Models: Greco et al, PRC68, 034904(03) Baryon Meson 200GeV Au+Au

29. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Hydrodynamic model can describe data well at low p T by assuming a local thermal equilibrium of partons. the intermediate p T range may be described by ReC/Coalescence model Saturation at intermediate p T Baryon and meson difference -- Constituent Quark Number Scaling

30. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Compared results on the invariant mass methods Under the assumption that the background contribution to v2(minv) (the second part on right side of equation) is smooth as a function of minv, a polynomial function was used to parameterize the background v2 vs. minv by fitting v2 vs. minv distributions in each pT bin. N. Borghini and J. Y. Ollitrault, PRC70(2004) 064905

31. STAR ToF Meeting, Hangzhou, Arpril. 27~29, 2009 STAR Strange quark dynamics: Why the φ-meson ? The medium produced in HI collisions is very short- lived → we need probes which carry information from the early stage to find out about the medium constituents: The φ(s-sbar) is a clean probe from early time: K+K+ K-K- K-K- K+K+ φ φ φ K+K+ K-K- QGP –Small σ for interactions with non-strange particles –Relatively long-lived (41 fm/c) → decays outside the fireball –Previous measurements have ruled out K+K- coalescence as φ production mechanism → info not “diluted” by hadronic phase The φ can provide info on particle production mechanisms / medium constituents: –The φ is a meson but as heavy as Λ,p baryons –Differentiate between mass-type or meson/baryon- type dependencies The clean φ is a good probe to understand the strange quark properties, transverse momentum dis. or spin orientation…