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t. Recent results from BRAHMS @ RHIC. Systems studied so far at RHIC: - Au+Au @ s NN 1/2 = 62, 130, 200 GeV - Cu+Cu @ s NN 1/2 = 62, 200 GeV (heavy-ion collisions, possible QGP formation!) - p+p @ s 1/2 = 200 GeV (elementary collisions) - d+Au @ s NN 1/2 = 200 GeV - PowerPoint PPT Presentation
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Presentation for NFR - October 19, 2005 - Trine S.Tveter
Recent results from BRAHMS @ RHIC
Systems studied so far at RHIC:
- Au+Au @ sNN1/2
= 62, 130, 200 GeV
- Cu+Cu @ sNN1/2
= 62, 200 GeV
(heavy-ion collisions, possible QGP formation!)
- p+p @ s1/2 = 200 GeV (elementary collisions)
- d+Au @ sNN1/2 = 200 GeV
(cold nuclear matter)
Stages of a heavy-ion collision at RHIC:- Initial stage with parton distributions inside Lorentz- contracted nuclei- Preequilibrium stage with hard parton-parton collisions- Phase with deconfined (but strongly interacting) partons - sQGP?- Hadronization with subsequent chemical and thermal freezeout
Presentation for NFR - October 19, 2005 - Trine S.Tveter
The BRAHMS spectrometer
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Are we creating a QGP?
Lattice QCD: Critical energy density eC ~ 0.7 GeV/fm3 for deconfinement.Bjorken's energy estimate formula:
indicates energy density e >~ 5 GeV/fm3 at RHIC !
Charged particle pseudorapidity densityin Au+Au collisions at h=0 for 3 CM energies gives:- sNN
1/2 = 200 GeV: e >~ 5.0 GeV/fm3 - sNN
1/2 = 130 GeV: e >~ 4.3 GeV/fm3
- sNN1/2 = 62 GeV: e >~ 3.7 GeV/fm3
Sufficient for QGP formation!
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Bulk hadron production
0 - 5% central Au+Au collisions at sNN
1/2 = 200 GeV:Near-Gaussian rapidity distributionsfor mesons (and antiprotons).<pT> increasing with mass.
0 - 5% central Au+Au @ sNN1/2 = 5 GeV,
Pb+Pb @ sNN1/2 = 17 GeV and
Au+Au @ sNN1/2 = 200 GeV:
From full stopping to increasing transparency.
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Antiparticle - particle ratios
Evolution of particle ratios with y and sNN1/2 :
p / p and K-/K+ at y=0 lower at sNN1/2 = 62 GeV.
They also fall off faster at higher rapidity.Behaviour of ratios well described assumingthermal / chemical equilibrium at quark level:
mq, m
S light , strange quark chemical potential.
Expectation for mS = 0: K-/ K+ = (p / p)1/3
K-/K+ and p/p correlated over large range of y and sNN
1/2 . Fit to data:
K-/K+ ~ (p / p) 0.25 -> mS ~ 0.25 mq ?Good agreement with statistical- thermalmodel assuming chemical freezeout temperature T = 170 MeV.
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Strangeness production - / K p ratios
sNN1/2 = 200 GeV
sNN1/2 = 62 GeV
K+/p+ and K-/p-: Different dependence onCM energy and on rapidity (mq- driven?)K+/p+ (y=0) peaks at lower SPS energies.RHIC energies, high y: SPS-like chemistry. Investigating / K p ratios at sNN
1/2 = 62 GeV,y ~ 3 - close to fragmentation region!
BRAHMS prelim.
K+/p+
K-/p-K-/p-
K+/p+
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Radial flow, kinetic freezeout
Exploding fireball: <pT> mass dependent.Blast wave fits with parameters Tfo and bT.Kinetic freezeout Tfo lower than forchemical freezeout (~ 120 - 140 MeV).bT higher for midrapidity, centralcollisions, higher sNN
1/2.
Kinetic freezeout temperature Tfo
Kinetic freezeout temperature Tfo
Expansion velocity bT
Expansion velocity bT
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Elliptic flow
Elliptic flow v2 in non - central collisions:
f is azimuthal angle rel. to reaction plane.v2 (pT) scales with number of valence quarks.Small decrease with rapidity.v2 magnitude close to hydrodynamical limit.Requires thermalization early in partonic phase -> strongly interacting QGP!
p++p- 10- 20%central
Au+Au @ 200 GeVCharged hadrons10 - 30% central
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Hard scattering and jets
Hard scattering in vacuum
Deconfined medium: Jet energy loss from gluon bremsstrahlung
STAR data: p+p, d+Au, peripheral Au+Au: BTB jet correlationsCentral Au+Au: Extended medium, far-side jet quenched
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Nuclear modification of high- pT hadrons
Nuclear modification factor from comparisonto p+p reference spectra:
Should be 1 for superposition of NN collisions.
- Strong suppression in central Au+Au collisions, both at y ~ 0 and forward y- Suppression strongly increasing with centrality
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Suppression - initial- or final- state effect?
Compare suppression in hot (Au+Au) and cold (d+Au) nuclear matter. d+Au @ sNN
1/2 = 200 GeV at = 0h :
Cronin-like enhancement, increasing with centrality.Indicates suppression in Au+Au to befinal-state effect -> QGP?
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Nuclear modification - dependence on energy and system size
Au+Au Nuclear modification forcharged hadrons at = 0, = 1: h h
- Au+Au: Same centralitydependence but less suppression at sNN
1/2 = 62 GeV.Enhancement in semi-peripheral collisions.
- Cu+Cu at sNN1/2 = 62 GeV:
Enhancement already incentral collisions.
General trend:Suppression increases withsystem size, centrality andenergy.
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Nuclear modification -dependence on particle species
Pions and other mesons clearly suppressed.Baryons not suppressed, even enhanced!p/p ratios strongly enhanced rel. to p+p collisions (effect peaks at pT ~ 2 - 3 GeV/c).p/p enhancement scales with Npart.Parton recombination involved in hadronformation?
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Nuclear modification -Dependence on rapidity
No rapidity dependence of RAA observedin central collisions, regardless ofparticle species.Centrality dependence of RAA strongerat forward y than at midrapidity.Dense partonic medium at y ~ 0 ->surface emission?More diluted medium at forward y?
BRAHMS preliminary - y = 0, 1, 2 for 0 - 10% central
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Initial state effects - dAu collisions
Initial- state gluondistributions d+Au collisions: Probing initial parton
distribution functions in Au nuclei. High energy: Gluon density grows at low x, limited by shadowing / saturation?At rapidity y one probes xAu values of: xAu ~ pT
/ sNN1/2 exp(- y)
Experimental RdA (h): Enhancement at h ~ 0 turns into suppression at forward h!
Gluon saturation - > Color Glass Condensate?
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Initial state effects - dAu collisions
h ~ 0: Increasing enhancement with centralityh >1: Increasing suppression with centralityData qualitatively described by "extreme" CGC model, "normal" shadowing, ++Particle species dependence at h ~ 3.2: Mesons suppressed, baryons enhanced.Not well understood for cold nuclear matter.Experimental situation needs clarification!
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Summary and plans
Heavy-ion collisions at RHIC - observations so far:Bulk properties and suppression / enhancement pattern of high- pT particles reveal new form of matter:- Very high energy density- Strongly interacting, most likely partonic (sQGP?)- Behaviour governed by complex interplay between initial- and final- state effects and by various competing processes
Further investigations (-> 2008):- Analysis of high-statistics data sets from Au+Au and Cu+Cu @ sNN
1/2 = 200 GeV: Detailed mapping of RAA as function of y, pT,
system size, for identified hadrons- Run 2006: p+p @ s1/2 = 200 (+ hopefully, 62) GeV - improve elementary reference spectra- Run 2007: Possible d+Au run
Presentation for NFR - October 19, 2005 - Trine S.Tveter
Norwegian BRAHMS participation
Present BRAHMS contingent in Norway:
- Dieter Röhrich (professor, UiB)- Gunnar Løvhøiden (professor, UiO)- Trine S. Tveter (professor, UiO)
- Ionut C. Arsene (Ph.D. Student, UiO)- Bjørn H. Samset (Ph.D. Student, UiO)- Hongyan Yang (Ph.D. Student, UiB)
- Svein Lindal (master student, UiO)- Atle Qviller (master student, UiO)
Collaborating institutions in Denmark, France, Poland, Romania,New York, Kansas, Texas. ~ 50 participants.
Presentation for NFR - October 19, 2005 - Trine S.Tveter