Presentation for NFR - October 19, 2005 - Trine S.Tveter

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


The BRAHMS spectrometer


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!


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.


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.


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+


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


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


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


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


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?


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.


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?


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


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?


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!


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


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

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