Starting the Energy Scan - First Results from 62 Starting the Energy Scan - First Results from 62.4 GeV Au+Au Collisions Introduction. High pT. Bulk matter observations. Collective motion. Summary

The story so far. We have discovered a strongly interacting medium with extremely high energy density which cannot be described in terms of simple hadronic degrees of freedom. Phobos

Nuclear modification factor at 200 GeV Charged Hadrons d2N/dpTd (Au+Au) RAA = NColld2N/dpTd (p+p) High pT suppression and loss of back-to-back signal in Au+Au but not in d+Au showed that effect due to Jet Quenching in final state NOT initial state parton saturation (GCG). BRAHMS Can we turn this effect off? Observed by all 4 experiments

Need for an energy scan Varying the beam energy changes: Initial state: sNN, Nbin/Npart, Qs(?) System: , mB, Nch Partonic: xT, dE/dx Varying the beam energy changes: Provide constraints on jet quenching models. Study the excitation function of baryon transport. Constrain models for hadron production at intermediate pT. Why 62.4 GeV? Located (on log scale) mid-way between SPS and RHIC top energies. Many reference data from ISR.

At 62.4 GeV Ncoll very different to 200 GeV Ncoll definition Glauber Monte Carlo Au+Au b ~ 10.5 fm Npart “Participants” Npart/2 ~ A L~A1/3 Ncoll= # of NN collisions: ~A4/3 At 62.4 GeV Ncoll very different to 200 GeV “Collisions”

First immediate result… Paddle Counter Signal top 50% of cross-section 200 GeV PHOBOS Preliminary 62.4 GeV Significant decrease in maximal multiplicity

Charged hadron spectra and yields PHOBOS TO BE SEEN Expect 62.4 GeV fit into √s systematics

Jet quenching predictions at 62.4 GeV I. Vitev nucl-th/0404052 Adil & Gyulassy nucl-th/0405036 RAA (p0) ~ 0.5 - 0.3 at pT = 4 GeV

RAA charged particles Maximum significantly higher than at 200 GeV. same results from all 4 experiments BRAHMS Preliminary Maximum significantly higher than at 200 GeV. There is a suppression for most central data Similar shape evolution with centrality

Universal centrality evolution? PHOBOS 62.4 GeV 200 GeV But, varying the beam energy changes: Initial state: sNN, Ncoll/Npart System : , mB, Nch, Partonic : xT, dE/dx Energy-independent Weak function of Npart, pT . Use central A+A as denominator Scale with 1/<Npart> Allows comparison between different experiments nucl-ex/0405003

PID spectra BRAHMS Phenix preliminary p0 p STAR Preliminary W- X-

RAA for p0 for central data Predictions 0.3 - 0.5 at 4 GeV/c Again maximum > 200 GeV STAR Preliminary Charged ISR reference used for p0 Still discrepancy between charged and p0 Large baryon contribution up to at least 4 GeV/c

Rcp of baryons and mesons Stat. Errors Only STAR preliminary STAR Preliminary (stat. errors only) The RCP(baryon) > RCP(meson) at intermediate pT. Problem with limited statistics.

Baryon/meson ratios Again, large proton contribution at intermediate pT Small difference as function of centrality (not very peripheral 30-60%) STAR Preliminary Lessp in central collisions at 62.4 GeV Ratios factor 2-3 higher than in p+p at pT= 2-4 GeV

Ratios at mid-rapidity Clear systematic trend with collision energy Minbias (0-80%) 62.4 GeV STAR Preliminary Stat. Errors Only STAR Preliminary L/L -/ +: 1.017±0.002 K-/K+: 0.835±0.006 p/p: 0.458±0.005 confirm varying y same effect as varying √s. Ratios flat as function of pT

Statistical model results STAR preliminary Au+Au at √sNN=200GeV and 62 GeV TLQCD~160-170MeV TLQCD~160-170MeV Close to chem. equilibrium ! Close to net-baryon free Tch flat with centrality ● p, K,p ● p, K,p, L, X ● p, K,p ● p, K,p, L, X Energy dependence but small Nch dependence…

but in such a way as to ensure yields stay ~constant (In)dependence of mid-rapidity yields Preliminary Preliminary T, µB, and V all vary with energy, but in such a way as to ensure yields stay ~constant Preliminary

Radial flow Same flow as at 200 GeV Blastwave fit Shape of the mT spectrum depends on mass: Tch Radial flow! p K STAR Preliminary STAR Preliminary STAR Preliminary Same flow as at 200 GeV

Kaon Slopes Top 5% central collisions

 interferometry HBT probes space-time evolution of system and system size at freeze-out. Studies at √s=130, 200 GeV yielded similar HBT radii to SPS energies (“HBT puzzle”). Severe challenge to hydrodynamic calculations. At an intermediate energy, a larger expansion time might point to a long-lived mixed phase. Systematics of central 0-5% Fully consistent Coulomb treatment in kT dependence PRELIMINARY same results from PHOBOS

HBT from SPS to RHIC No sign of qualitatively different expansion dynamics at 62 GeV. Continues to be a severe challenge submitted to Phys. Rev. C Rapid Communications Same results from STAR

Charged particle correlations PHENIX PRELIMINARY PHENIX PRELIMINARY pT Substantial signals attributable to elliptic flow (v2 = <cos(2f)>) v2 apparently saturates and is the same as at 200 GeV Jets are going to be a challenge

Longitudinal elliptic flow h’=|h|-ybeam Longitudinal scaling of v2 nucl-ex/0406021

Identified Elliptic Flow v2 pT [GeV/c] PHENIX preliminary sNN = 62.4 GeV Au+Au centrality : 0-84% stat. error only sys. error <20% v2 pT [GeV/c] Charged p,K,p : PRL91, 182301 (2003) p0 : work in progress sNN = 200 GeV Au+Au centrality : 0-92% stat. error only sys. error <15% 00:03:40 (60s) This is elliptic flow of identified hadron in 62.4 GeV gold on gold collisions. When we compare the results with 200GeV, we can see the pT dependence is similar. The data is limited, therefore once we have much more statistics, we can do centrality dependence and of more particle species. Although statistics not great again resembles 200 GeV

Coalescence at intermediate pT leads to: Quark coalescence? Coalescence at intermediate pT leads to: stat. error only sys. error <20% (62GeV) 15% (200GeV) 62.4 GeV Au+Au: PHENIX preliminary 200 GeV Au+Au, charged p,K,p : from PRL p0 : work in progress The scaling works as for 200 GeV Au+Au v2 /nquark Seems to be slightly lower than 200 GeV for pT/nquark<1 GeV/c 00:04:40 (60s) One of interest results is a scaling of number of quarks in v2. in approximation, both v2 and pT is scaled to number of constituent quark. The plot shows results from 200 GeV gold on gold. 62.4GeV result is like this. We can see the scaling looks working in 62GeV. Also it seems to be slightly lower than 200 GeV. pT /nquark [GeV/c]

Summary Many of the results indicate environment similar at 62.4 GeV to 200 GeV but it’s not identical Initial energy density lower Evidence of jet quenching Cronin effect is stronger HBT radii at ~6 fm Chemical freeze-out conditions similar to 200 GeV (fn Nch) Baryon chemical potential much higher Radial flow as strong as at 200 GeV (fn Nch) Elliptic flow as strong as at 200 GeV (fn centrality) Consistent with Nquark scaling Elliptic flow appears universal at forward rapidities Jet contribution much weaker than at 200 GeV As yet un-answered questions: Are the gluon densities the same in both systems? Do they spend the same amount of time in each stage? i.e. do they reach the same freeze-out conditions in the same manner?

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Why an energy scan? Varying the beam energy changes: Initial state: sNN, Ncoll/Npart, Qs(?) System: , mB, Nch Partonic: xT, dE/dx Varying the beam energy changes: Varying the geometry (A,b): jet quenching vary overall path length vary asymmetry Elliptic (and directed) flow Study A dependence at fixed eccentricity. Npart eccentricity

p+p references p0 reference p0 reference charged reference +/-25% uncertainty p0 reference p0 reference charged reference