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K / π Fluctuations and the Balance Function

K / π Fluctuations and the Balance Function. Gary Westfall Michigan State University For the STAR Collaboration INT Workshop on the QCD Critical Point August 12, 2008. Fluctuations in the K / π Ratio. Event-by-event fluctuations in K / π may give insight into the QCD critical point

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K / π Fluctuations and the Balance Function

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  1. K/π Fluctuations and the Balance Function • Gary Westfall • Michigan State University • For the STAR Collaboration • INT Workshop on the QCD Critical Point • August 12, 2008

  2. Fluctuations in the K/π Ratio • Event-by-event fluctuations in K/π may give insight into the QCD critical point • NA49 measured the fluctuations of K/π as a function of incident energy for central Pb+Pb collisions fromsNN1/2 = 6 to 17 GeV using the observable σdyn • measure the K/π ratio event-by-event • K = K+ + K- • π = π+ + π- • produce histogram of the K/π ratio • extract the width of K/π histogram to get σdata • do the same for mixed event to get σmixed

  3. K/π Fluctuations at the SPS • Define the dynamical fluctuations in terms of σdyn • Divide by the mean and multiplity by 100 to get % C. Blume (NA49), hep-ph/0505137

  4. K/π Fluctuations in STAR • Study Au+Au collisions at sNN1/2 = 20, 62, 130 and 200 GeV • Extract the number of K+ + K- and π+ + π- event-by-event using energy loss and curvature in the STAR TPC • Take kaons and pions with 0.2 < pt < 0.6 GeV/c and |η| < 1.0 • kaons: Nσ,K < 2, Nσ,π > 2 • pions: Nσ,π < 2, Nσ,K > 2 • electrons are suppressed with Nσ,e > 1

  5. K and π Identification in STAR

  6. K/π Histograms for Au+Au Collisions • Mixed events are created by taking one track from different events to produce new events that have no correlations • Mixed events are produced using 10 bins in centrality and 5 bins in vertex position • The K/ distributions are wider for real events than for mixed events

  7. Excitation Function for σdyn • Compare STAR results for central Au+Au collisions with SPS results for central Pb+Pb collisions

  8. Excitation Function for σdyn (2) • STAR results for σdyn are similar to those at the top SPS energies • The statistical hadronization model (SH) of Torrieri [nucl-th/0702062 (2007)] for the light quark phase space density γq = 1 (equilibrium) under-predicts σdyn at all energies • The statistical hadronization model for a fitted γq (non-equilibrium) explains the STAR results but under-predicts the SPS measurements

  9. Different Fluctuations Measure • The use of σdyn is problematic for low multiplicities • A better measure is νdyn,Kπ • dyn was introduced to study net charge fluctuations (PRC 68, 044905 [2003]) • dyn,K is insensitive to efficiency • dyn,K deals well with low multiplicities and does not require mixed events

  10. Are σdyn and νdyn,Kπ Different? 200 GeV Au+Au Sergei Voloshin, J. Phys. Conf. 50, 111 (2006)

  11. νdyn,Kπ for Au+Au at 62 and 200 GeV Au+Au • Centrality dependence of K/π fluctuations • Inverse multiplicity dependence • Relatively poor fit versus 1/Npart • NA49 results are all central Pb+Pb collisions • Similar Npart

  12. Compare with NA49 using dN/d • We can compare the results for the centrality dependence of νdyn,Kπ to the incident energy dependence of σdyn in central collisions using the following method • Use PHOBOS systematics fordN/dη versus sNN1/2 • Use the identity σdyn2 = νdyn,Kπ

  13. PHOBOS Systematics for dN/dηin Central Collisions B. Back et al. (PHOBOS Collaboration) Phys. Rev. C 74, 021902(R) (2006)

  14. νdyn,Kπ Plotted versus dN/dη Au+Au Better fit with 1/(dN/dη)than with 1/Npart

  15. Excitation Function for σdyn • Scale centrality dependence of νdyn,Kπ to compare with excitation function of σdyn in central collisions

  16. Addition of TOF to STAR P. Sorensen Charged pions and kaons 0.2 < pt < 0.6 GeV/c STAR will add TOF for Run 10 The TOF will provide excellent particle identification for π, K, and p for a large fraction of the measured particles event-by-event Improved K/π fluctuation measurements Improved balance functions with identified π, K, and p

  17. Look at Charges Separately 200 GeV Au+Au

  18. HIJING Predictions - Separated Charges 200 GeV Au+Au

  19. Scale with dN/dη andCompare with HIJING • Average νdyn,K+/π+ and νdyn,K-/π- to get Same • Average νdyn,K+/π- and νdyn,K-/π+ to get Opposite

  20. Scale with dN/dη andCompare with AMPT • AMPT (version 1.21, hard scattering) for summed charges is better than HIJING, but centrality dependence is not correct • No difference between same and opposite

  21. Relation of K/π Fluctuations to Resonance Re-interaction • Model of Torrieri, Jeon and Rafelski • Predict K/π fluctuations and resonance production using statistical hadronization model • www.gsi.de/documents/DOC-2007-Jul-101-1.pdf • Jeon and Koch, PRL 83, 5435 (1999) (π+/π-) • Relate νdyn,K+/π- and νdyn,K-/π- to K*(892)/K ratio • (3/4)<Nπ>(νdyn,K+/π- - νdyn,K-/π-) ∼K*/K

  22. Resonance Reinteraction Compared with Thermal Model of Torrieri Au+Au T = 170 MeV

  23. Full Acceptance Dependence on Acceptance Au+Au 200 GeV

  24. Balance Function • Balance function represents charge balance of charge/anti-charge pairs • Balance functions can be expressed in terms of , y, qinv, qout, qside, qlong, and  Bass, Danielewicz, Pratt PRL 85 2689 (2000)

  25. Balance Function 200 GeV Au+Au Data Shuffled Mixed

  26. Balance Function Widths - All Charged Particles 200 GeV

  27. Balance Function Widths - Pions and Kaons 200 GeV

  28. B(qinv) for Pions 200 GeV Au+Au Data Shuffled

  29. B(qinv) for Pions Charged pion pairs 0.2 < pt < 0.6 GeV/c

  30. B(qinv) for Kaons Charged kaon pairs 0.2 < pt < 0.6 GeV/c

  31. B(qinv) for Pions and Kaonsfrom p+p at 200 GeV Kaons • B(qinv) for pions shows K0 and 0 • The 0 peak is shifted down as previously observed • B(qinv) for kaons shows  Pions

  32. Balance Function Widths from B(qinv) Pions Kaons

  33. Balance Function - Excitation Function NA49 Phys. Rev. C 76, 024914 2007 Balance functions for Pb+Pb at sNN½ = 6.3 to 17.3 GeV STAR, QM 02, QM 04 Balance functions for Au+Au at sNN½ = 20 to 200 GeV

  34. Balance Function Widths -Excitation Function NA49 Phys. Rev. C 76, 024914 2007 NA49 Large W means narrow balance function UrQMD predicts wide balance function with no centrality dependence

  35. RHIC Low Energy Scan • For central Au+Au and Pb+Pb collisions, dyn for K/ fluctuations may show a deviation from the fluctuations predicted by a statistical hadronization model as a function of incident energy • The width of the balance function in central Au+Au and Pb+Pb collisions decreases as the energy is increased until around 20 GeV, where it seems to stay constant • These two observations hint at some kind of phase transition occurring between 7 and 20 GeV • A comprehensive energy scan from 7 to 60 GeV with STAR and the new TOF will answer many questions

  36. Conclusions - K/ • Dynamical fluctuations in the K/π ratio in central Au+Au collisions represented by σdyn show little incident energy dependence at RHIC energies within errors and compare reasonably with SPS results at the top energies • νdyn,Kπ seems to scale with dN/dη • (dN/dη)νdyn,Kπ increases slightly with centrality • νdyn,Kπ for same sign particles is close to zero • νdyn,Kπ for opposite sign particles is negative • HIJING overpredicts (dN/dη)νdyn,Kπ while AMPT seems to get the correct magnitude but not the centrality dependence • Fluctuations of same and opposite sign particles may give us information about the re-interaction of kaons and pions

  37. Conclusions - Balance Function • Balance function B() for all charged particles narrows in central Au+Au collisions • HIJING shows no centrality dependence • AMPT narrows in central collisions, but not as much as the data • Balance function B(y) widths for pions and kaons are different • Balance function B(qinv) widths for pions and kaons are the same • Central Au+Au widths scaled with shuffled events (W) are the same at 20, 62, 130, and 200 GeV • Balance function B(qinv) for pions shows the K0, but not the 0 • Widths extracted from B(qinv) for pions scale with dN/d

  38. The End

  39. Extra Slides

  40. Balance Function with Identified Pions - Excitation Function Charged pion pairs 0.2 < pt < 0.6 GeV/c

  41. B(qinv) Widths using Identified Pions - Excitation Function Charged pion pairs 0.2 < pt < 0.6 GeV/c

  42. Kinetic Temperatures as a Reference

  43. The QCD Phase Diagram

  44. The Search for the QCD Phase Transition • The production of strangeness may be related to the onset of deconfinement • Excitation function of<K+>/<π+> shows “horn” aroundsNN1/2 = 7 GeV • The excitation function of <K->/<π-> is smooth C. Blume (NA49), hep-ph/0505137

  45. HBT-Coulomb Effects for B(qinv) • Expanded scale in qinv • Compare correlation function to B(qinv)

  46. RHIC Energy Scan • Energies as low as sNN1/2 = 4.5 GeV (10 AGeV fixed target) T. Satogota, RHIC

  47. NA 49/61 Future Program M. Gazdzicki

  48. Proposed Energy and Mass Scans

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