Fouad RAMI Institut Pluridisciplinaire Hubert Curien, Strasbourg Introduction The BRAHMS...

Fouad RAMI Institut Pluridisciplinaire Hubert Curien,

Strasbourg

Introduction The BRAHMS Experiment Overview of Main Results Bulk observables High pt observables Summary & Outlook

F.Rami, IPHC Strasbourg Trento, January 9-13, 2007

Forward Rapidity Physics with the Forward Rapidity Physics with the BRAHMS ExperimentBRAHMS Experiment

Space-time evolution of a HI collision at RHIC energies

Parton scatterings takeplace during first stages Emission of hadrons

Initial State (v~c) Dense Medium


RHIC Results consistent with the existence of a dense partonic state of matter characterized by strong collective interactions: sQGP

Hints on high density gluon saturation → describe the initial state of the collision within the framework of the Color Glass Condensate: CGC

Ludlam and McLerran, Physics Today, 2003

Combines QGP and CGC

A possible scenario for Au+Au collisions at RHIC

Initial conditions of the collision provided by the CGC

The CGC matter will evolve and may eventually form a QGP (if the system thermalizes)


Initial State

Gluons inside one nucleus appear to the other nucleus as a wall made mostly of gluons travelling at high velocities (v~c)

The CGC matter is not only important for the formation of the QGP

But the study of CGC matter itself is of fundamental interest

→ Colliding nuclei in the Initial State considered as CGC matter

→ Understanding of basic properties of strong interactions

CGC: Universal form of matter

Independent of the hadrons which generated it

Can be explored in protons and in heavy nuclei using probes :

electrons to probe the structure of protons (HERA) or nuclei (e-RHIC)

protons (or deuterons) to probe nuclei (RHIC, LHC)

Advantage of nuclei Saturation can be reached at lower energies (larger x) due to the effect of their thickness


Saturation physics and the CGC → Object of intensive theoretical studies (Next Talk)

Glu

on

D

en

sit

y

x

Low energy

High energy

Gluon densityincreases

Small x

Large x

High Density Gluon Saturation

x=fraction of E transfered to the gluon

e-p scattering at HERA


At small-x,the gluon density increases very strongly → driving force toward saturation The gluon density cannot grow indefinitely (unitarity)

Gluon distribution function of the proton

Saturation at high densityQS : Saturation momentum

Nuclei → Qs2 A1/3

QS larger in A than in p

Saturation can be probed atlarger x-values in nuclei

→ RHIC, LHC

2000-2006: 6 runs

PHOBOS

PHENIX

STAR

BRAHMS

Relativistic Heavy Ion Collider @

BNL

Several systems/energies

Au+Au @ 200 GeV @ 130 GeV @ 63 GeV

Cu+Cu @ 200 GeV @ 63 GeV

d+Au @ 200 GeV (control experiment)

p+p @ 200 GeV (reference data)


Rapidity coverage Main focus → MR (y=0) (most interesting region for QGP) But also some data at forward rapidities → very promising … Results obtained from all 4 experiments

GlobalDetectors

Front Forward Spectrometer

Back ForwardSpectrometer

Two Rotatable spectrometers → Broad rapidity coverage FS → well suited for Forward Physics (up to η~4)

0<<1(MRS)


→ Centrality (Event Multiplicity)

Global Detectors & 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 (in nuclear overlap) and number of inelastic NN collisions NCOLL (from Glauber Model)F.Rami, IPHC Strasbourg Trento, January 9-

13, 2007

dNch/d - Comparison to Model Predictions

Au+Au @ SNN=200GeV

AMPTZhang et al,PRC61(2001)067901Lin et al,PRC64(2001)011902

High density QCDgluon saturationKLN modelKharzeev, Nardi & Levin,PLB523(2001)79

Similar predictions Both calculations reproduce dNch/d (shape and absolute)

Differences for peripheral Collisions but Small effect! Cannot discriminate these models

Centrality dependence is well described

BRAHMS, PRL88(2002)202301

dN

ch/d


dNch/d at Mid-Rapidity – Centrality Dependence


Saturation models reproduce also the energy dependence

KLN Model: Kharzeev, Levin and Nardi, Nucl.Phys.A730(2004)448

dNch/d - d+Au @ SNN=200GeV

BRAHMS data PHOBOS data


Good agreement except in the region of the Au fragmentation where KLN model (dotted line) fails CGC is not valid in this region (large-x)!

dN/dη = Npart dNpp/dη (solid line) → agreement in the Au fragmentation region


Good description of particle production at RHIC Several features observed in the data are nicely reproduced - Rapidity dependence - Centrality dependence - Energy dependence - System dependence - Limiting Fragmentation phenomenon

Particle Production at RHIC vs. Saturation Models

6% centralAu+Au

dN

ch/d

/<

Np

art>

/2

PHOBOS PRL 91 (2003)

Limiting Fragmentation


BRAHMS PRL 88 (2002)

When shifted by ybeam (’ ybeam) → No Energy Dependence

Limiting behavior (LF) in the forward rapidity region (’ ~ 0)

Also observed in pp, pp, p-emulsion, π-emulsion, A-A at SPS(Alner et al, Z.Phys.C33(1986)1, Deines-Jones et al, PRC(2000)4903)

_

Similar effect observed for v2 (PHOBOS)

Can be explained within the CGC (Jalilian-Marian, nucl-th/0212018)

Limiting Fragmentation in the CGC approach


_

Reasonable agreement (Fragmentation region)

Good agreement also for pp data (UA5)

F.Gelis, A.M.Stasto and R. VenugopalanEur. Phys. J. C48 (2006) 489

Au+Au▲19.6 GeV (PHOBOS)

■ 130 GeV (PHOBOS)

● 200 GeV (PHOBOS)

□ 130 GeV (BRAHMS)

○ 200 GeV (BRAHMS)

Good description of particle production at RHIC Several features observed in the data are nicely reproduced - Rapidity dependence - Centrality dependence - Energy dependence - System dependence - Fragmentation phenomenon

Particle Production at RHIC vs. Saturation Models


Saturation effects seem to play an important role in particle production and dynamics at the early stages of A-A collisions at RHIC energies But other models can also reproduce most of the data! → Need for more “direct” evidence (experimental signatures)! CGC theorists suggested to investigate the high pt region of hadron spectra If saturation effects are present at RHIC energies → should be seen as a suppression at high pt (relative to N-N reference)

d+Au

Forward Rapidities Most appropriate conditions

Forward measurements in d+Au collisions

Qs2 A1/3 (Thickness effect)

Saturation momentum in Au larger than in p (saturation can be probed at larger x)

BRAHMS measures in this side(d-fragmentation region)

d Au

MRS

FSxAu = mt/S e-y

Forward measurements → Access to small x in the gluon distribution of the Au nucleus

From y=0 to y=4 x values lower by ~10-2


No final state effects in d+Au If suppression → Only due to the Initial State

Parton Distributions Functions

xAu = mt/S e-y

Mostly valence quarks


xd = mt/S e+y

In the d-fragmentation region

xd rangexAu rangeMainly gluons

(Saturated wave function?)

High pt suppression in d+Au collisions at forward rapidities

Probing the CGC matter at RHIC


Nuclear Modification Factor

BRAHMS, PRL91(2003)072305

RAA =Yield(AA)

NCOLL(AA) Yield(pp)

Scaled N+N reference


R<1 Suppression relative to scaled NN reference

RCP =Yield(Cent)/NCOLL(Cent)

Yield(Periph)/NCOLL(Periph)

Central/Peripheral


=0BRAHMS

Decisive test (control experiment) → Interpretation of Au+Au in terms of Energy Loss in dense partonic matter (Jet Quenching)

d+Au shows very different behavior as compared to Au+Au Au+Au → suppression d+Au → Enhancement (Cronin effects) Observed in all 4 experiments


Absence of suppression in d+Au data at MR

Not necessarily inconsistent with CGC

No sensitivity to low-x at MR Important to go forward (smaller x)

Data: Nuclear Modification Factor at MR

BRAHMS

Data: Going to Forward Rapidities (RdAu)

MB collisions


BRAHMS, PRL 93 (2004) 242303

Gradual transition from Cronin enhancement to suppression

Occurrence of suppression (relative to p+p collisions) at large rapidities

Consistent with the expected behavior for saturation effects

For pt=2 GeV/cx ~ 10-2 x ~ 510-4

θ=90° θ=12°θ=40° θ=4°

MRS MRS FSFS

BRAHMS

Data: Going to Forward Rapidities (RCP)

Suppression mechanism depends on centrality → Larger effect in Central Collisions Consistent with saturation


BRAHMS, PRL 93 (2004) 242303

Same behavior as for RdAu

Onset of suppression: 1<η<2

Centrality dependence: different behavior from η=0 → large η’s

Comparison to CGC calculations (RCP)


Kharzeev, Kovchegov and Tuchin, Phys. Lett. B599 (2004) 23

Good agreement also for RdAu

○ 30-50%/60-80%

0-20%/60-80%

●

Good agreement with data

→ Transition from Cronin to suppression

→ Centrality dependence

STAR, nucl-ex/0602011

STAR Results

→ Clear suppression at large η

Calculations that do not include saturation effects cannot reproduce data


→ Good agreement with BRAHMS

for charged hadrons

Suppression of the back- to-back peak in d+Au

Back-to-back Correlations in d+AuSTAR, nucl-ex/0602011

Kharzeev, Levin, and McLerran, Nucl. Phys. A748 (2005) 627


Azimuthal correlation between forward π0 mesons (η=4) and Leading Charged Particles (LCP) detected at MR with pt>0.5GeV/c

Qualitatively consistent with the CGC picture

Additional argument in favor of saturation at RHICImportance of correlation measurements and the need for quantitative understanding

Saturation effects provide an explanation to the high pt suppression observed in d+Au at forward y’s

Saturation models provide a good description of particle production

dNch/dη, Energy and Centrality dependences well reproduced for both Au+Au and d+Au collisions

RHIC results suggest the formation of CGC matter in the initial state of the collision

Summary & Outlook


Limiting Fragmentation also described

Supported by quantitative CGC model calculations Transition from Cronin enhancement to suppression and Centrality Dependence

Confirmation of CGC requires further experimental tests Open charm, dileptons, photons

Azimuthal correlations in the forward direction …

Main challenges in the future Upgrades of RHIC experiments (including forward detectors) LHC much higher energies (smaller x)

BRAHMS CollaborationI. C. Arsene12, I. G. Bearden7, D. Beavis1, S. Bekele12, C. Besliu10, B. Budick6,

H. Bøggild7, C. Chasman1, C. H. Christensen7, P. Christiansen7, H.Dahlsgaard7, R. Debbe1, J. J. Gaardhøje7, K. Hagel8, H. Ito1, A. Jipa10, E.B.Johnson11, J. I. Jørdre9,

C. E. Jørgensen7, R. Karabowicz5, N. Katrynska5 ,E. J. Kim11, T. M. Larsen7, J. H. Lee1, Y. K. Lee4,S. Lindahl12, G. Løvhøiden12, Z. Majka5, M. J. Murray11,J. Natowitz8, C.Nygaard7

B. S. Nielsen8, D. Ouerdane8, D.Pal12, F. Rami3, C. Ristea8, O. Ristea11, D. Röhrich9, B. H. Samset12, S. J. Sanders11, R. A. Scheetz1, P. Staszel5,

T. S. Tveter12, F. Videbæk1, R. Wada8, H. Yang9, Z. Yin9, I. S. Zgura2

1. Brookhaven National Laboratory, Upton, New York, USA2. Institute of Space Science, Bucharest - Magurele, Romania3. Institut Pluridisciplinaire Hubert Curien et Université Louis Pasteur, Strasbourg, France4. Johns Hopkins University, Baltimore, USA5. M. Smoluchkowski Institute of Physics, Jagiellonian University, Krakow, Poland6. New York University, New York, USA7. Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark8. Texas A&M University, College Station, Texas, USA9. University of Bergen, Department of Physics and Technology, Bergen, Norway10. University of Bucharest, Romania11. University of Kansas, Lawrence, Kansas, USA12. University of Oslo, Department of Physics, Oslo, Norway


Color Glass Condensate: Why? Color : Composed of colored particles

Glass : In the gluon wall, gluons do not change their position rapidly because of Lorentz time dilatation Will evolve on long time scale relative to their natural time scale

Similar property as in glasses

Condensate : High density Coherent multi-gluon system (gluon condensate) If the phase space is filled with gluons gluons from different nucleons will start to overlap (saturation effect)

Saturation is characterized by a saturation scale below which recombination occurs QS Density of gluons in the transverse plane Increases with s (1/x) and A

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

Wang and Gyulassy, PRD44(91)3501

Hard processes leading to minijet production are calculated using pQCD (PYTHIA) pt p0=2GeV/c

Soft processes are calculated using the Lund String Model Hadronization in Strings

Shadowing Modification of parton structure functions in the medium

Jet quenching Energy loss of partons traversing dense matter

Parton cascade calculations where partons are treated as free particles and their evolution is studied taking into account QCD interactions and assuming that the initial distributions in phase space are given by the structure function of the nuclei. provide detailed description at the partonic level of the early stages of nucleus-nucleus collisions

Two Component Model

Includes Nuclear effects

dNch/d = (1-x)Npart xNcoll

x=fraction of hard processes

HIJING: Heavy Ion Jet Interaction Event Generator

AMPT modelLin et al, PRC64(2001)011902Zhang et al, PRC61(2001)067901

Hybrid model:

- It uses HIJING to generate the initial phase space of partons.

- It takes into account hadronic interactions in the final state (hadron rescattering) using a Relativistic Transport Model (ART).

1

2

3

HIJING – Jet quenching

HIJING – No Jet quenching

EKRT (Gluon Saturation)

Wang & Gyulassy, PRL86(2001)3496

BRAHMS

1

2

3

|

|

|

|

Both models HIJING and EKRT reproduce the measured multiplicities

Au+Au data much larger than pp Not a simple superposition of pp Evidence for collective behavior

dNch/d at Mid-Rapidity - Energy Dependence

Good agreement between all 4 RHIC experiments

=0

Small difference in the predictionsof these models at RHIC energies


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)

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

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

Origin of high–pt

suppression

Saturation of gluon densities in the colliding nuclei (Initial State effect)

Jets do not lose energy but they are produced in a smaller number (due to saturation effects)

Jet Quenching effect (Final State effect)

Parton energy loss in the traversed dense medium suppression in jet production (high pt hadrons)

High pt Suppression clearly observed in central Au+Au collisions by all 4 RHIC experiments (Run1&2)

PHOBOS Results

PRC 70 (2004) 061901(R)

PHENIX Results

PRL 94 (2005) 082302

Comparison to CGC calculations (RdAu)


Kharzeev, Kovchegov and Tuchin, Phys. Lett. B599 (2004) 23

CGC calculations (different assumptions)

CGC calculations: Predictions for LHC

LHC, =0

RHIC, =3.2

Predictions for LHC

Stronger suppression at LHC (smaller x)

p-A collisions

High pt Suppression in Au+Au

No clear Rapidity Dependence

Au+Au @ SNN=200 GeV BRAHMS, PRL91(2003)072305

Central

Peripheral

Central/Peripheral

Confirmed by more recent results

at η = 1 and 3.2 and also in Cu+Cu (preliminary data)

Dense medium extends to high rapidity

Gluon saturation (larger contribution at Forward Rapidities)

Rapidity Dependence in Au+Au

Rapidity Dependence of high pt spectra (Polleti and Yuan (nucl-th/0108056))

Variation of the amount of energy loss (dE/dx) with the density of the traversed medium.

(a)

(b)

Larger suppression ( small R) at y=0 than at higher rapidities Reflects changes in the density of the traversed medium

y=0

y=3

y=2

R = Yield(AA) / <Nbinary> Yield (pp)

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

RAA =Yield(AA)

NCOLL(AA) Yield(pp)

Scaled pp reference


Tra

nsv

ers

e m

om

en

tum

[G

eV

/c]

Rapidity

BRAHMS Acceptance

Large rapidity coverage→ Forward region covered by the FS


Particle Identification

Particle Identification (BRAHMS RICH)

Ring radius vs momentum gives PID

/ K separation 25 GeV/cProton ID up to 35 GeV/c

MR spectrometer Forward spectrometer

/ K separation 2.5 GeV/cProton ID up to 4 GeV/c


MRS=0

Fouad RAMI Institut Pluridisciplinaire Hubert Curien, Strasbourg Introduction The BRAHMS...

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Transcript of Fouad RAMI Institut Pluridisciplinaire Hubert Curien, Strasbourg Introduction The BRAHMS...