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RHIC Low Energy Scan

RHIC Low Energy Scan. APS Division of Nuclear Physics 2007 Long Range Plan: Phases of QCD Matter. Paul Sorensen . outline. why is a low energy scan interesting explore the phase diagram of nuclear matter and discover the critical point: a landmark study

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RHIC Low Energy Scan

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  1. RHIC Low Energy Scan APS Division of Nuclear Physics 2007 Long Range Plan: Phases of QCD Matter Paul Sorensen

  2. outline • why is a low energy scan interesting • explore the phase diagram of nuclear matter and • discover the critical point: a landmark study • turn off signatures of deconfinement • why at RHIC • large energy range accessible • collider geometry provides great advantages • RHIC detectors: commissioned and well suited for the search • what indications for a critical point do we have • what can RHIC experiments accomplish • what does the CBM detector at FAIR add • conclusions

  3. physics motivation

  4. physics motivation

  5. *why does a critical point exist? pro con/maybe existence depends on curvature of the critical surface: “critical endpoint is extremely quark mass sensitive” (O. Philipsen) Lattice: P. de Forcrand and O. Philipsen hep-lat/0607017 B=0 transition is a crossover at TC=155-205 MeV Lattice: F.R. Brown et al., Phys. Rev. Lett. 65 (1990) 2491 for T=0, the B transition seems to be first order Model calculations: J. Berges and K. Rajagopalhep-ph/9804233; M. Halasz, A. Jackson … hep-ph/9804290; O. Scavenius, Á. Mocsy … nucl-th/0007030; N. Antoniou, A. Kapoyannis hep-ph/0211392; “fluctuations on the crossover line increase with increasing B, strongly suggesting the existence of a critical point” (F. Karsch) Lattice: Bielefeld-Swansea, Phys. Rev. D68 (2003) 014507

  6. why at RHIC? RHIC collisions cover a broad region of interest

  7. √sNN 6.27 GeV √sNN 17.3 GeV advantages of a collider acceptance track density for fixed target geometry: detector acceptance changes with energy  not nice for energy scans track density at midrapidity increases rapidly with √sNN  changes in hit-sharing and track merging  changes in dE/dx and pT resolution for a collider: acceptance does not change and track density only varies slowly point-to-point systematic errors will be better under control

  8. BBC BBC RHIC detectors STAR PHENIX commissioned, proven performers: 1) 2 coverage at  2) P.I.D. across a broad pT range

  9. some of the key measurements • yields and particle ratios  T and B •elliptic flow v2  and  v2 (deconfinement?) quark number scaling (deconfinement?) collapse of proton flow? (phase trans?) •v2 fluctuations  enhancement near critical point •k/, p/, pT fluctuations  enhancement near critical point •D mesons, di-leptons  chiral phase transition look for non-monotonic behavior as correlation lengths increase near the  N.B. finite system size and finite lifetime: correlation lengths are limited ~2 fm hydrodynamic focusing can spread the signals over a broad √sNN range we can’t necessarily count on sharp signatures

  10. what we already know at lower √sNN

  11. particle ratios and fluctuations the horn  non-monotonic signature, but… • dynamical fluctuations: •  no clear signature seen at energy where k/ peaks •  hadron/string model matches the proton but not the kaon data • what do we make of the energy dependence? evidence still inconclusive  energy scans at FAIR, SPS, and RHIC under consideration

  12. NA49 PRC 40A GeV proton v2 proton v2 √sNN=8.77 GeV • collapse of proton v2: signature of phase transition (H. Stöcker, E. Shuryak) • but resultdepends on analysis technique:uncertain and inconclusive • difference between v2{4} and v2{2} depends on non-flow andfluctuations • is itnon-flow or fluctuations?A signature for a phase transition? • measurement needs to be repeated:uncertainty can be removed by measuring v2 fluctuations

  13. what can RHIC detectors do event rates without electron cooling: ~5 Hz at 4.6 GeV no. days to record 106 events 6 days at 4.6 GeV 1.5 days at 8.0 GeV 0.25 days at 16 GeV electron cooling will improve rates by > an order of magnitude can we trigger on events at such low energies?!  simulations indicate no problems 2 tracking and particle identification  full barrel TOF expected in 2009 elliptic flow measurements  good reaction-plane resolution multi-strange hadron v2 within reach fluctuation measurements v2 fluctuations: an important new capability at RHIC particle ratio fluctuations (k/ fluctuations aren’t trivial at RHIC: kaon decays reduce efficiency and purity is poor without a TOF pT fluctuations

  14. triggering at low √sNN BBC Inner: 3.3 to 5.0 BBC Outer: 2.1 to 3.3 Number of particles striking Beam-Beam Counters (UrQMD Simulations). (scintillator tiles) • simulations indicate BBCs will be adequate for triggering • expected no. of particles is larger than what is used for p+p collisions what will the background rates be?

  15. event-plane resolution • better resolution means smaller errors than NA49 • (given the same number of events) • NA49 flow PRC used less than 500k events per energy • a big improvement on v2 measurements • Quark-number scaling and  v2(deconfinement) with several million events • (several days at √sNN = 9 GeV) • Estimates made using: • v2 from NA49 measurements • estimate the dN/dy using 1.5*Npart/2 • use tracks with |y|<0.5 (should be able to do better) • simulate events STAR NA49 1/

  16. v2/v2 critical region √sNN, B v2 fluctuations v2 fluctuations at the crit. point: new potential for discovery analysis relies on central limit theorem: needs multiplicity and full acceptance also reduces uncertainty on mean v2 *critical point signal size still to be investigated • a new technique possible at RHIC to improve all v2 measurements • an additional robust critical point signature

  17. Simulations Counts (K++K-)/(++-) K/ fluct. error estimate √sNN=8.77 GeV • 100k central 40 AGeV Au+Au events: statistical errors only • with ToF 5% (relative) without 11% (relative) • but systematic errors may be dominant • particle mis-identification changes the width of the distribution • 0.5% K swapping: width 5% and the signal is only 4%! TOF is important

  18. In-plane Out-of-plane pT fluctuations STAR Preliminary collision overlap zone • acceptance is important: elliptic flow can enhance apparent pT fluctuations in measurements without 2 coverage • differential analyses are often essential for correct interpretation:full acceptance matters • RHIC has the tools needed to best understand pT fluctuations

  19. CBM detector @FAIR: rare probes + RHIC scan can sweep a broad energy and B range in upcoming runs + large acceptance commissioned detector already available + at a collider: acceptance won’t change with √sNN but rare probes may be out of reach: lower luminosity CBM@FAIR will study chiral symmetry restoration and hadrons in medium 1) low-mass di-leptons (feasibility at RHIC is under study) 2) open charm and deconfinement using 3) multi-strange hadrons (also accessible at RHIC) 4) charmonium suppression

  20. conclusions • compelling physics motivation: • mapping the phase diagram • locating the critical point • turning off signatures of deconfinement • current SPS data are suggestive but inconclusive • RHIC detectors are proven and important upgrades are under way: • large acceptance available • stable acceptance with √sNN smaller systematic errors • full STAR TOF ~2009 • accelerator capabilities have been studied down to √sNN = 4.5 GeV: • no “show stoppers” • complementary international efforts being pursued • good potential for discovery andwithin reach

  21. Thanks

  22. z = ln{dE/dx} - ln{Bethe-Bloch} proton protons pion kaons efficiency kaon z for kaons pions electrons momentum p (GeV/c) transverse momentum pT (GeV/c) K/ fluct: challenges at RHIC STAR acceptance and efficiency mis-identification K K/ (K+1)/(-1) or (K-1)/(+1) K/ fluctuations can be distorted electron contamination pions leptons that look like kaons mixed events can’t compensate kaon decays:K+ + (c=3.7 m) tracking efficiency < 50% for colliders PID cuts reduce efficiency another 50% kaon decays reduce efficiency at a collider p.i.d. purity without TOF will help be limited

  23. what to expect chiral and confinement critical points may be different  experimental searches for chiral symmetry restoration and deconfinement are complimentary heavy-ion collisions may not probe the critical region  T0 could drop below TC before we hit critical B  still interesting to search for disappearance of QGP can we turn it off? heavy ion collisions won’t provide sharp signatures  limited correlation lengths (~1-2 fm)  focusing may broaden √sNN range of signatures a RHIC energy scan may yield 1) critical point signatures in a wide √sNN range 2) and/or disappearance of QGP signatures

  24. N.B. some expected limitations C. Nonaka Focusing by the hydro evolution could cause many initial conditions to cross the critical point region: broadening the signal region Correlation lengths expected to reach at most 2 fm  pT<0.5 GeV/c (Berdnikov, Rajagopal and Asakawa, Nonaka): reduces signal amplitude We can’t count on sharp discontinuities

  25. pT fluctuations

  26. pT fluctuations fluctuations correlations variance excess scale = full acceptance • acceptance is important: elliptic flow can enhance apparent pT fluctuations in measurements without 2 coverage • differential analyses are often essential for correct interpretation:full acceptance and statistics matter • RHIC has the tools needed to better understand pT fluctuations

  27. v2 motivation slide S. Voloshin Hydrodynamic interpretation still evolving as analyses progress Energy dependence plays an important role in our interpretations

  28. Gavai, Gupta 2005 Taylor Expansion location of the critical point

  29. BBC BBC STAR Detector • Designed for these kinds of measurements • “The Solenoidal Tracker at RHIC (STAR) will search for signatures of quark-gluon plasma (QGP) formation and investigate the behavior of strongly interacting matter at high energy density. The emphasis will be on the correlation of many observables on an event-by-event basis… This requires a flexible detection system that can simultaneously measure many experimental observables.” • STAR Conceptual Design Report (July 1992)

  30. log10(dE/dx) log10(p) particle identification PID capabilities at RHIC over a broad pT range: TPC dE/dx, ToF, Aerogel, Topology, EMC, etc. no anticipated obstacles to measuring particle spectra and ratios (T and B) fluctuation analyses prefer track-by-track I.D.

  31. detector capabilities Star: TOF, full acceptance, HFT Phenix: low-mass dileptons? HBD CBM: rare probes Jpsi, Dmesons, low mass dileptons RHIC (STAR PHENIX): Wide energy range, collider configuration, makes v2 and fluctuation measurements easier, critical point location FAIR (CBM): chiral symmetry restoration, rare probes, studies of first order phase transition? TOF+dE/dx+rdE/dx ( ,p) 0.3~12 GeV/c M. Shao et al., NIMA 558, (419) 2006

  32. PRL 92 (2004) 052302; PRL 91 (2003) 182301 v2 and deconfinement large  and  v2 and quark number scaling  deconfined valence quark stage? can we turn these signatures off ? can we prove they are not from a hadronic stage ? these are questions addressed with a low energy scan

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