Status of Quark-Gluon Plasma and saturation effects at RHIC
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Status of Quark-Gluon Plasma and saturation Status of Quark-Gluon Plasma and saturation effects at RHICeffects at RHIC
Fouad RAMI Institut de Recherches Subatomiques, Strasbourg
Introduction Status of QGP at RHIC Particle multiplicities Elliptic flow High pt suppression & jet quenching High density gluon saturation (CGC) d-Au data Forward rapidities Summary & perspectives
F.Rami, IReS Strasbourg Sinaia2005
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Phase Diagram of Nuclear Matter
En
erg
y
Den
sit
y/T
4
Temperature
Lattice QCD
F.Karsch, hep-lat/0106019
TC 160MeV (B = 0)
• Study QCD matter at high densities
• Explore and characterize the QGP
Main Goals in RHIC experiments
F.Rami, IReS Strasbourg Sinaia2005
Large experimental program
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First Physics Run: June 2000
2000-2005: 5 runs
PHOBOS
PHENIX
STAR
BRAHMS
Relativistic Heavy Ion Collider @ BNL
p+p 500 1032 Au+Au 200 1026
SNN (GeV) L(cm-2 s-1)
RHIC accelerates all species from p to Au
Two independent rings ~3.8 km in circumference
Several systems/energies Au+Au @ 200 GeV @ 130 GeV @ 62.4 GeV Cu+Cu @ 200 GeV @ 63 GeV d+Au @ 200 GeV 62.4 GeV p+p @ 200 GeV (reference data)
F.Rami, IReS Strasbourg Sinaia2005
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Particle multiplicities at RHIC
Central Au+Au event measured by STAR/TPC, @ 130 GeV
F.Rami, IReS Strasbourg Sinaia2005
Very large number of charged particles per event
dNch/d|=0 ~ 650 (at 200GeV) much higher than at SPS
Number of charged particles per unit of rapidity at =0
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Au+Au data much larger than pp Not a simple superposition Medium effects important role in AA collisions
Large increase from SPS to RHIC (almost a factor of 2) Higher energy densities
Wang & Gyulassy, PRL86(2001)3496
|
|
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=0
BRAHMS
RHIC (average)
F.Rami, IReS Strasbourg Sinaia2005
dNch/d at Mid-Rapidity
Energy Dependence
An estimate of ε ~ 5 GeV/fm3 at 200GeV (Bjorken model) PHENIX, PRL87(2001)052301 Well above the critical density (~ 1 GeV/fm3)
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Semi-central Collisions
x
yz
Collective Flow Collective expansion of Nuclear Matter following the compression phase
Response to early pressure
Elliptic Flow Pressure converts spatial anisotropy into p-space anisotropy
Reactionplane
Elliptic Flow
Fourier decomposition of the azimuthal distributions
dN/d = F0 (1 + 2vicos(i))v2 : 2
nd harmonic Fourier coefficient
Measure of Elliptic Flow
STAR, PRL90(2003)032301
MR
F.Rami, IReS Strasbourg Sinaia2005
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STAR, PRL86(2001)402
Au+Au @ 130GeV
Hydro limit
Pb+Pb at SPS
CentralPeripheral
Elliptic Flow at RHIC
Much larger elliptic flow at RHIC high degree of thermalization (multiple interactions of produced particles) Supported by the good agreement with hydrodynamical model
F.Rami, IReS Strasbourg Sinaia2005
Large set of v2 data available at RHIC (STAR, PHENIX) for ≠ particle species All can be reproduced by hydro including the mass dependence
Hydro good agreement for soft particles:• pions up to pt ~ 1.5GeV/c• protons up to pt ~ 2.5GeV/c
More than 95% of the emitted particles
The bulk of the fireball behaves hydrodynamically
To reproduce the large v2 values Hydro evolution must start very early Fast thermalization
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q
q
hadronsleadingparticle
leading particle
Schematic view of jet production Particles with high pt’s (above ~2GeV/c) are primarly produced in hard scattering processes early in the collision Probe of the dense and hot stage
Experimentally Suppression in the high pt regionof hadron spectra (relative to p+p)
p+p experiments Hard scattered partons fragment into jets of hadrons
In A-A, partons traverse the medium
If QGP partons will lose a large part of their energy (induced gluon radiation) Suppression of jet production Jet Quenching
High pt Suppression & Jet Quenching
F.Rami, IReS Strasbourg Sinaia2005
RAA =Yield(AA)
NCOLL(AA) Yield(pp)
Scaled pp reference
Nuclear Modification Factor
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At RHIC Significant suppression
Suppression consistent with partonic energy loss (Quenching)
But, it might be also due to saturation of gluon densities (initial state effect) Jets do not lose energy but they are produced in a smaller number
Compare A+A and d+A(Run3, Control experiment)
(h++h-)/20
PHENIX, PRL88(2002)022301
RHIC
High pt Suppression at RHIC
New phenomenon at RHIC
Not observed at lower energies SPS(Pb+Pb) Enhancement due to initial state multiple scattering (Cronin effect) well known in p+A collisions
Gluon sat. Suppression in dAuQuenching No suppression in dAuF.Rami, IReS Strasbourg
Sinaia2005
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Initial or Final State effect ?
Final State effects are dominant in central Au+Au at RHIC as expected from the formation of a hot and dense medium of partonic matter
Same conclusion
F.Rami, IReS Strasbourg Sinaia2005
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Summary of the main experimental observations for central Au+Au collisions
All of these results are consistent with the existence of a dense partonic state of matter characterized by strong collective interactions
F.Rami, IReS Strasbourg Sinaia2005
Large particle multiplicities
Presence of a dense partonic medium
High energy densities (well above critical)
High degree of collectivity and early thermalization
Main conclusion of the 4 RHIC White Papers (to be published in Nucl.Phys.A)
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High Density Gluon Saturation at RHIC
McLerran, hep-ph/0402137
Glu
on
D
en
sit
y
xAs x becomes smaller and smaller,the gluon density increases faster driving force toward saturation
Several global features of Au+Au and d+Au collisions at RHIC can be reproduced by the Color Glass Condensate model (high density gluon saturation in the initial state)
=0
No apparent sign of saturation in high pt hadron spectra for d+Au
Those data are for MR particles More forward rapidities
(smaller x values)
BRAHMS
F.Rami, IReS Strasbourg Sinaia2005
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Forward measurements in d+Au collisions Sensitivity to smaller-x values
BRAHMS spectrometers measure in the d-fragmentation region
d Au
MRS
FS
D.Kharzeev et al, hep-ph/0307037
xAu = mt/S e-y
To reach small x in the gluon distribution of the Au nucleus
Go very forward
F.Rami, IReS Strasbourg Sinaia2005
From y=0 to y=4 x values lower by ~10-2
One could hope to see the occurrence of a suppression effect
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What do we expect?
CGC at y=0
D. Kharzeev et al, hep-ph/0307037
Very high energy
As y grows
At RHIC energies Cronin effects predominant at mid-rapidity
RpA : Nuclear Modification Factor
At more forward y’s Transition from Cronin enhancement to a suppression effect
This is what one would expect if there is an effect of gluon density saturation in the initial state
F.Rami, IReS Strasbourg Sinaia2005
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BRAHMS, PRL 93 (2004) 242303
x ~ 10-2
η=0, (h++h-)/2 η=3.2, h-
For pt=2 GeV/c
x ~ 510-4
Transition from Cronin enhancement to suppression Qualitatively consistent with the expected behavior for CGC
(θ=4deg)
F.Rami, IReS Strasbourg Sinaia2005
What do we see in the data?
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Suppression increases with rapidity as expected for saturation effects
x ~ 10-2
(for pt=2GeV/c)
η=0, (h++h-)/2 η=1, (h++h-)/2
η=2.2, h-
η=3.2, h-
(Min bias)
BRAHMS, PRL 93 (2004) 242303
F.Rami, IReS Strasbourg Sinaia2005
Rapidity Dependence
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Results from other RHIC experiments
F.Rami, IReS Strasbourg Sinaia2005
Central [0-20%]
BRAHMS PHENIX (hadrons)
d-sideAu-side
nucl-ex/0411054
Good agreement between BRAHMS and PHENIX PHOBOS consistent results (limited y-range)
RCP =Yield(0-20%)/NCOLL(0-20%)Yield(60-80%)/NCOLL(60-80%) Reference from
peripheral collisions
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Quantitative CGC calculations for d+Au @ SNN=200 GeV
Comparison to CGC calculations
F.Rami, IReS Strasbourg Sinaia2005
Overall good agreement
Calculations predict also a transition from Cronin enhancement at MR to suppression at larger y’s
So far No alternative explanations within realistic model calculations
D. Kharzeev at al. hep-ph/0405045
=0 =1
=2.2 =3.2
Nucl
ear
Modifi
cati
on F
act
or
Nucl
ear
Modifi
cati
on F
act
or
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Summary & Perspectives
Results obtained so far at RHIC for central Au+Au collisions are consistent with the formation of a dense partonic state of matter characterized by strong collective interactions
F.Rami, IReS Strasbourg Sinaia2005
Strong hints of saturation effects at RHIC (from d+Au data) CGC might provide the initial conditions for A-A collisions at RHIC
Confirmation of the Color Glass Condensate
RHIC (upgrades improved physics capabilities) LHC much higher energies (smaller x)
Characterize the properties of this dense partonic state of matter
The task now
will require further experimental tests (more sensitive probes)
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Backup slides
F.Rami, IReS Strasbourg Sinaia2005
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Future experimental progran at RHIC
F.Rami, IReS Strasbourg Sinaia2005
Next 5 years Significant detector upgrades
Improved vertexing for charm measurements Better particle id (TOF) Low-mass dilepton measurements Expanded forward coverage ( low-x physics)
Longer term Significant upgrade of the machine (RHIC II) based on electron cooling ( higher luminosities)
Additional upgrades Proposal for a new detector to exploit the increased luminosity Extend jet-related measurements to much higher pt’s (into the perturbative regime)
Physics program of RHIC II still under discussion
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• Study QCD matter at high densities
Phase Diagram of Nuclear Matter
En
erg
y
Den
sit
y/T
4
Temperature
Lattice QCD
F.Karsch, hep-lat/0106019
TC 160MeV (B = 0)
• Explore and characterize the QGP
Main Goals in RHIC experiments
F.Rami, IReS Strasbourg Sinaia2005
B = F/NB
F = Free energy NB = Baryonic Number
(baryon – anti-baryon) Large experimental program
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Parton interactions takeplace during first stages
Emission of hadrons(t 20fm/c)
There are several stages in the collision
Space-time evolution of a heavy-ion collision at collider energies
Initial State (v~c) Dense MediumCGC ?F.Rami, IReS Strasbourg
Sinaia2005
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STAR Solenoidal field
Large Solid Angle TPCSi-Vertex detectorRICH, EM Cal, TOF
Measurements of hadronic observables with a large acceptance and Pid
Event-by-event analyses
PHENIXAxial Field
High Resolution & Rates2 Central Arms, 2 Forward Arms TEC, RICH, EM Cal, Si, TOF, -ID
Designed to measure simultaneously Leptons, Photons, and Hadrons in selected solid angles
Rare signals such as J/ψ decaying into muons and electrons, direct photons
Two “Large” Detectors at RHIC
F.Rami, IReS Strasbourg Sinaia2005
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BRAHMS2 spectrometers (movable)
Magnets, Tracking Chambers, TOF, RICH
Detailed measurements of momentum spectra and yields of charged hadrons over a wide range of rapidities (including the forward kinematical region)
PHOBOSNearly 4 coverage with Si-detectors
2 Arm Spectrometers (also based on Si)
Total charged particle multiplicity in 4 & Global properties (elliptic flow) Charged hadron spectra (small acceptance)
Ring Counters
Paddle Trigger Counter
Spectrometer
TOF
Octagon+Vertex
Two “Small” Detectors at RHIC
F.Rami, IReS Strasbourg Sinaia2005
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B = 46±5 MeV
T=174±7 MeV
Similar analysis at SNN=200GeV
B = 29±8 MeV
T=177±7 MeV
Statistical Model AnalysisAnalysis of particle ratios measured at RHIC in a grand canonical ensemble with baryon number, strangeness and charge conservation
SNN=130GeV
P.Braun-Munzinger et al, PLB518(2001)41
Thermal model parametersat chemical freeze-out
Agreement -> indicates a high degree of chemical equilibrationFlow -> hydro (thermalisation ..)
Thermal model parameters from particle ratios
F.Rami, IReS Strasbourg Sinaia2005
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Saturation models also reproduce the measured multiplicities
2
3
HIJING – Jet quenching
EKRT (Gluon Saturation)
Wang & Gyulassy, PRL86(2001)3496
1
2
3
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=0
BRAHMS
RHIC (average)
HIJING – No Jet quenching
1
F.Rami, IReS Strasbourg Sinaia2005
dNch/d at Mid-Rapidity
Energy Dependence
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BRAHMS, PLB523(2001)227
0-5% 30-40%
BRAHMS, PRL88(2002)202301
3860 300 4630 370
SNN=130GeV SNN=200GeV
Nch(-4.7<<4.7)
Very high charged hadron multiplicities
dNch/d|=0 = 553 36 dNch/d|=0 = 625 55
F.Rami, IReS Strasbourg Sinaia2005
Number of charged particles per unit of rapidity in the MR (at =0)
Much higher multiplicities than at CERN-SPS (Pb+Pb)
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dy
dE
RT
Bj0
2
11
BJ ~ 4.6 GeV/fm3
BJ ~ 3.2 GeV/fm3 for Pb+Pb at SPS (NA49, PRL75(1995)3814)
Well above the value expected for the
Critical Energy Density (crit ~ 1 GeV/fm3)
Very high energy densities
Transverse energy distributions measured by PHENIX (calorimetry)
PHENIX, PRL87(2001)052301
Au+Au @ SNN=130GeV
MBCentral(top 5%)
Central events
dET/d|=0 ~ 500 GeV ( SPS)
F.Rami, IReS Strasbourg Sinaia2005
Using Bjorken estimate for the energy density (J.D.Bjorken, PRD27(83)140)
At 200GeV) BJ ~ 5 GeV/fm3
factor of ~1.6 larger than at SPS
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R2
dydz 0
Bjorken formula for thermalized energy density
time to thermalize the system (0 ~ 1 fm/c)
~6.5 fm
dy
dE
RT
Bj0
2
11
J.D.BjorkenPRD27(83)140
Longitudinal expansion of thermalized system
Energy Density from Transverse Energy Measurements
F.Rami, IReS Strasbourg Sinaia2005
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EVENT CHARACTERIZATION COLLISION CENTRALITY
Au+Au @ SNN=130GeV
Measured with Multiplicity Detectors (TMA and SiMA)
Central Peripheral
Define Event Centrality Classes Slices corresponding to different fractions of the cross section
Central b=0
Peripheral b large
For each Centrality Cut Evaluate the corresponding number of participants Npart and
number of inelastic NN collisions NCOLL (from Glauber Model)F.Rami, IReS Strasbourg Sinaia2005
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Kolb & Heinz, nucl-th/035084
Elliptic Flow: pt dependence
Good agreement for central and mid-central events But overpredicts v2 for peripheral events (b 10fm) incomplete thermalization
F.Rami, IReS Strasbourg Sinaia2005
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Elliptic Flow from Parton Cascade Transport Calculations
Molnar & Gyulassy, NPA 697 (2002) 495
Calculations with ≠ transport opacities ζ
Agreement with data if ζ is very large large number of interactions among the fireball constituents (partons)
ζ very large hydro limit (pt < 1.5GeV/c)
Parton cascade predicts saturation at high pt’s (observed in the data)
High pt particles escape the fireball before having suffered a sufficient number of rescattering to thermalize their momenta.
F.Rami, IReS Strasbourg Sinaia2005
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Flow is Sensitive to Early Stages
Elliptic flow builds up in the first instants of the collision (before hadronization) and then stays constant
v2 is proportional to the parton-parton scattering
cross section used in the calculations
Rescattering converts the initial space anisotropy of the overlap region to the momentum anisotropy of elliptic flow
v2 is sensitive to the number of interactions and can be considered as a measure of thedegree of thermalization at early time.
Au+Au at 200GeV
time(fm/c)
v2
Parton Cascade Model (AMPT,Zhang et al, PLB455(1999)45)
F.Rami, IReS Strasbourg Sinaia2005
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Elliptic Flow: Sensititivity to the EoS
Hydro calculations: Huovinen, Kolb & HeinzNPA698 (2002) 475
EOS/Q quark gluon plasma EOS (hard)EOS/H pure hadron resonance gas (soft)
Elliptic flow builds up and saturates early in the collision sensitivity to high density EOS
Hydrodynamical mass splitiing (observed in the data) underpredicted by EOS/H
F.Rami, IReS Strasbourg Sinaia2005
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RAA =Yield(AA)
NCOLL(AA) Yield(pp)
Scaled pp reference
Nuclear Modification Factor
RAA<1 Suppression relative to scaled pp reference
PHENIX, PRL91(2003)072301
Nuclear Modification Factor RAA
Scaled pp reference
F.Rami, IReS Strasbourg Sinaia2005
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Nuclear Modification Factor RAA
For peripheral collisions NCOLL scaling works well
Nucl-ex/0410003 (PHENIX White paper) NCOLL scaling of hard processes has been also checked using directphotons, which are produced via hard scattering processes but do not loose energy in the medium since they have no color chargeF.Rami, IReS Strasbourg
Sinaia2005
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Evaluation of Npart and NCOLL
Use Glauber Model Nucl.Phys.B21(1970)135
Npart : Nucleons that interact inelastically in the overlap region between the two interacting nuclei
NCOLL : Number of binary nucleon-nucleon collisions (one nucleon can interact successively with several nucleons if they are in its path)
Main assumption : Independent collisions of part. nucleonsNucleons suffer several collisions along their incident trajectory (straight-line) without deflection and without energy loss
Nucleons inside nuclei distributed according to a Woods-Saxon density profile Interaction probability between 2 nucleons is given by the pp cross section Calculate the overlap integral at a given impact parameter
p+p : Np=1 and NCOLL=1p+A : Np=1 and NCOLL>1F.Rami, IReS Strasbourg
Sinaia2005
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High pt suppression: Theoretical calculations
pQCD based calculations incorporating parton energy loss via medium induced gluon radiation are able to reproduce the data
Energy dependence can be explained by the competition between quenching, nuclear shadowing and Cronin effect
F.Rami, IReS Strasbourg Sinaia2005
Vitev & Gyulassy, hep-ph/0209161
Realistic hadronic calculations (Cassing, Gallmesiter and Greiner, hep-ph/0311358) unable to reproduce the observed effect
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STAR, PRL91(2003)072304
Strong experimental evidence for Jet Quenching in Au+Au
d+Au
Another evidence in favor of Jet Quenching Azimuthal Correlations
Azimuthal correlations between a high pt particle (trigger) with 4 pt 6 GeV/c and all other particles with pt above 2 GeV/c Indirect way to identify the formation of jets
Jets are deflected in the medium destroys the coplanarity of the 2 jets
p+p Clear two-jet signal (back to back correlation)
d+Au The signal survives
Central Au+Au The signal disappears
F.Rami, IReS Strasbourg Sinaia2005
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Glu
on
D
en
sit
y
x
Low energy
High energy
Gluon densityincreases
Small x
Large x
x = Econstituant/Ehadron
High Density Gluon Saturation
Gluon density in a proton increases strongly from large x to small x (x=fraction of E transfered to the gluon)
e-p scattering at HERA
Saturation at high densityQS : Saturation scale
F.Rami, IReS Strasbourg Sinaia2005
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McLerran, hep-ph/0402137
F.Rami, IReS Strasbourg Sinaia2005
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Forward measurements in d+Au collisions Sensitivity to smaller-x values
BRAHMS spectrometers measure in the d-fragmentation region
d Au
MRS
FS
D.Kharzeev et al, hep-ph/0307037
xAu = mt/S e-y
To reach small x in the gluon distribution of the Au nucleus
Go very forward
Qs2 A1/3 (Thickeness effect)
Larger saturation scale QS : Qs2(x) = Q0
2 (x0/x)λ
Saturation scale in Au larger than in p (saturation can be probed at lower x)
F.Rami, IReS Strasbourg Sinaia2005
From y=0 to y=4 x values lower by ~10-2
One could hope to see the occurrence of a suppression effect
No final state effects in d+Au
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A is p and B is Au
Energy and momentum conservation
xL = xA - xB =(2MT/√s)sinh y
kA + kB = k
xAxB = MT2/s
A solution to this system is:
xA = (MT/√s) ey
xB = (MT/√s) e-y
xAxB
rapidity
Kinematics of p-Au
y is the rapidity of the detected particle (xL,k)xL is its logitudinal momentum fraction
xA (xB) is the longitudinal momentum fractionof the projectile (target) parton
F.Rami, IReS Strasbourg Sinaia2005
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Highest density of gluons in central collisions Largest suppression
=3.2More suppression in central events
Also consistent with CGC matter
Centrality Dependence
F.Rami, IReS Strasbourg Sinaia2005
RCP =Yield(0-20%)/NCOLL(0-20%)
Yield(60-80%)/NCOLL(60-80%)
Reference fromperipheral collisions
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CGC calculations: Predictions for LHC
F.Rami, IReS Strasbourg Sinaia2005
LHC, =0
RHIC, =3.2
Predictions for LHC
Stronger suppression at LHC (smaller x)
p-A collisions
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MQQ : Invariant mass of the QQ pair produced in the hard scatteringyQQ : Rapidity of the pair
--
-
A.Dainese, nucl-ex/0311004
X2 = [MQQ/SNN] exp(- yQQ)--X1 = [MQQ/SNN] exp(+ yQQ)--
F.Rami, IReS Strasbourg Sinaia2005
Accessible x range at RHIC and LHC
LHC higher energies, higher rapidities (smaller x) p-A (and A+A) deeply inside the saturation regime Possibility to probe saturation also in p+p
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F.Rami, IReS Strasbourg Sinaia2005
Nuclear modification factor for h- and h+
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F.Rami, IReS Strasbourg Sinaia2005
Nuclear modification factor for mesons and baryons
anti-proton datanot corrected foranti-lambda feed down
- Difference between baryons and mesons- Related to parton recombination? (Hwa et al, PRC71(2005)024902