PHENIX Perspectives for the RHIC Energy Scan Ralf Averbeck, GSI Helmholtzzentrum für Schwerionenforschung GmbH for the Collaboration Symposium on "The Physics of Dense Baryonic Matter" GSI, Darmstadt, March 9-10, 2009 � Introduction � Search for the QCD Critical Point � Search for the Onset of sQGP Production � Summary and Outlook
QCD phase diagram � goal of high energy heavy-ion physics � identify phases of matter and their properties � locate transitions and their properties � vanishing µ B � sQGP at top RHIC energy � evolution to hadron gas through a continuous rapid crossover transition � larger µ B � possibility of a 1 st order phase transition � critical point? � phase coexistence line? � energy scan at RHIC , 03/10/2009 R. Averbeck, 2
Questions for an energy scan � search for the critical point � where should we look? � guidance from lattice QCD – critical point in reach at » FAIR » SPS » RHIC � what (T, µ B ) for given √ s? � constraints from experiment � what to measure? � evolution of the sQGP � how do the medium properties evolve with √ s? � where do individual sQGP signatures "turn off"? � experimental boundary conditions � performance of RHIC at lower √ s? � what are the constraints in PHENIX? , 03/10/2009 R. Averbeck, 3
Where are we in (T, µ B )? � important prerequisite � initial thermalization in partonic world Initial Thermalization – some idea of T initial ? Freezeout � evolution into hadronic world – determine (T, µ B ) at freezeout from particle species ratios , 03/10/2009 R. Averbeck, 4
Initial T from thermal photons � enhanced emission of arXiv:0804.4168v1 1 < p T < 2 GeV 2 < p T < 3 GeV "soft" low-mass virtual 3 < p T < 4 GeV 4 < p T < 5 GeV photons in Au+Au compared to pp � consistent with hydrodynamic model calculation assuming 300 MeV < T initial < 600 MeV � difficulties at low √ s � signal/background � interaction rate at RHIC � feasible at higher end of RHIC arXiv:0804.4168v1 energy scan , 03/10/2009 R. Averbeck, 5
Finding the critical point Nonaka & Asakawa, PRC 71(2005)044904 � hydro prediction � critical point "attracts" isentropic trajectories in the (T, µ B ) plane � focusing causes a broadening of the signal region in (T, µ B ) � not necessary to exactly "hit" the critical point in an energy scan! , 03/10/2009 R. Averbeck, 6
Stationary state variables � properties � divergence of stationary state variables at critical point – compressibility − γ ⎛ ⎞ − T T ⎜ ⎟ ∝ C k ⎜ ⎟ T ⎝ ⎠ T C – heat capacity − α ⎛ ⎞ − T T ⎜ ⎟ ∝ C C ⎜ ⎟ V ⎝ ⎠ T C � related to event-by-event fluctuations of observables σ – multiplicity fluctuations 2 ( ) T = k T / V k µ B 2 1 ∑ = – <p T > fluctuations 4 pT C � strategy V � study fluctuations as function of µ B ( √ s) � search for anomalies, i.e. large critical fluctuations , 03/10/2009 R. Averbeck, 7
Fluctuations � PHENIX measures � limits and caveats fluctuations � fluctuations σ and correlation length ξ (Stephanov, Rajagopal, Shuryak: PRD 60(1999)114028) σ ∝ ξ 2 – finite system size – finite evolution time � divergence of ξ (and σ ) limited � system slows down � no compelling evidence for near critical point critical fluctuations yet � fluctuations damped (Berdnikov and Rajagopal: PRD 61(2000)105017) critical point search needs � do critical fluctuations further observables survive hadronization? , 03/10/2009 R. Averbeck, 8
Antiproton-to-proton ratio � back to hydro � critical point deforms ("attracts") isentropic trajectories in the � PHENIX measures (T, µ B ) plane identified hadron � antiproton-to- spectra proton ratio p ( ) − µ ~ exp 2 B / T p � prediction (Asakawa et al., arXiv:0803.2449) – antiproton spectra are steeper than proton spectra at high p T – more robust than fluctuation observables , 03/10/2009 R. Averbeck, 9
Dynamic variables � again: correlation length ξ is important � relation between diffusion constant D and ξ − ξ (Son & Stephanov) 1 D ~ � large ξ near critical point � small diffusion constant D � small shear viscosity to entropy density ratio η /s − η ξ 0 . 05 0 . 06 � bulk viscosity is different ~ � again � limited system size � no extreme effects � expectation close to the critical point � minimum in shear viscosity to entropy ratio η /s � bulk viscosity only somewhat sensitive , 03/10/2009 R. Averbeck, 10
η /s measurements � need observables that are sensitive to shear stress � damping ~ η /s R. Lacey et al.: PRL 98:092301, 2007 η = ± ± π S. Gavin and M. Abdel-Aziz: / s ( 1 . 1 0 . 2 1 . 2 ) / 4 � flow PRL 97:162302, 2006 p T fluctuations STAR � fluctuations � heavy quark motion η = − π / ( 1 . 0 3 . 8 ) / 4 s v 2 PHENIX A. Adare et al.: PRL 98:092301, 2007 & STAR � top RHIC energy � η /s close to η = − π / s ( 1 . 3 2 . 0 ) / 4 conjectured minimum 1/4 π , 03/10/2009 R. Averbeck, 11
η /s near the critical point � η /s goes through a Lacey et al., arXiv:0708.3512 minimum near the critical point � estimate from Lacey et al. (based on v 2 systematics) – T ~ 165-170 MeV – µ B ~ 120-150 MeV critical point search in the region 20 GeV ≤ √ s ≤ 62 GeV , 03/10/2009 R. Averbeck, 12
Flow systematics out-of-plane � initial state of non-central collision y � large asymmetric pressure gradients Au nucleus � hydrodynamic flow of partons � control parameters: ε 0 , η , c s in-plane � translates into x Au nucleus � final state momentum anisotropy z [ ] ( ) ∞ 3 3 ( ) d N d N ∑ = ϕ − Ψ ( ) = ϕ − Ψ v cos 2 n E 2 v cos n ϕ n R 2 n R 3 d p p d dp dy = n 0 T T � hydrodynamic flow exhibits scaling properties which can be validated (or invalidated), e.g.: 2 ≈ 0.9 � v 4 /v 2 , 03/10/2009 R. Averbeck, 13
Flow at RHIC � flow shows KE T and quark number scaling at top RHIC energy � flow is dominantly pre-hadronic baryons mesons � at what collision energy does scaling set in? , 03/10/2009 R. Averbeck, 14
Jet quenching at RHIC � energy loss of partons from hard scattering through re-scattering in the hot & dense medium � nuclear modification factor � access medium properties R AA << 1 at high p T through statistical analysis – example: transport coefficient in PQM model (A. Dainese et al.) A. Adare et al., PRC 77(2008)064907 PQM � huge opacity of the medium + = 2 . 1 2 ˆ q 13 . 2 GeV / fm � − 3 . 2 , 03/10/2009 R. Averbeck, 15
Light quark opacity � at what collision energy does the onset of light quark opacity occur? A. Adare et al., PRL 101(2008)162301 � PHENIX R AA measurements in Cu+Cu collisions – onset for 22.4 GeV ≤ √ s NN ≤ 62.4 GeV � needs p+p and d+A samples in addition to A+A � feasible only for SPS energies or higher , 03/10/2009 R. Averbeck, 16
Heavy quark opacity � where is the onset of heavy quark opacity? R AA for Au+Au @ √ s NN = 200 GeV R AA for Au+Au @ √ s NN = 62.4 GeV � R AA consistent with unity PRL 98, 172301 (2007) � poor statistics � p+p reference missing � interesting energies for heavy quark observables are above SPS energies, not below , 03/10/2009 R. Averbeck, 17
Low-mass dileptons at RHIC � dielectrons from PHENIX in p+p and Au+Au collisions at √ s NN = 200 GeV A. Adare et al., PLB 76(2009)313 S. Afanasiev et al., arXiv:0706.3034 � agreement with expected e + e - sources in p+p � enhancement observed in Au+Au collisions � can PHENIX measure e + e - in an energy scan? , 03/10/2009 R. Averbeck, 18
e + e - at low RHIC energies � dielectron cocktail calculation for Au+Au at √ s = 17.2 GeV � assumptions – meson yields and phase space distributions as measured at SPS – no low-mass enhancement or any other medium effects � key ingredients – electron ID beyond PHENIX baseline is a must � Hadron Blind Detector (HBD) – increased luminosity (electron cooling) could have a huge impact 50M events w HBD 50M events w/o HBD 500M events w HBD � e + e - measurements are possible with "CERES quality" (or better) at low RHIC energies! , 03/10/2009 R. Averbeck, 19
RHIC boundary conditions � life becomes difficult towards low energies � key issues � luminosity limited by intra-beam scattering – below injection: γ 3 scaling – decent event rates above injection – difficult below injection energy – � improvement: electron cooling � lifetime only few minutes (below injection energy) – "continuous" injection? – � improvement: electron cooling � large "diamond" length spread of collision – vertices along beam axis � improvement: electron cooling , 03/10/2009 R. Averbeck, 20
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