High pT Suppression at Forward Rapidities

Catalin RisteaNiels Bohr Institute, Copenhagenfor the BRAHMS Collaboration

XXXV International Symposium on Multiparticle Dynamics

Kroměříž, Czech Republic

The Relativistic HeavyIon Collider

BRAHMS

BRAHMS

p+p

p+p, d+Au, Cu+Cu, Au+AusNN=200GeV

Cu+Cu, Au+AusNN=62.4GeV

Cu+CuAu+Au d+Au

The BRAHMS Experiment - global detectors - spectrometers & PID

What we measure: - RCP and RAA

Experimental resultsComparing Au and Cu

The BRAHMS Collaboration

I.Arsene11, I.G. Bearden6, D. Beavis1, S. Bekele6 , C. Besliu9, B. Budick5, H. Bøggild6 , C. Chasman1, C. H. Christensen6, P. Christiansen6, R. Clarke9,

R.Debbe1, J. J. Gaardhøje6, K. Hagel7, H. Ito1, A. Jipa9, J. I. Jordre9, F. Jundt2, E.B.

Johnson10, C.E.Jørgensen6, R. Karabowicz3, E. J. Kim4, T.M.Larsen11, J. H. Lee1, Y. K.

Lee4, S.Lindal11, G. Løvhøjden2, Z. Majka3, M. Murray10, J. Natowitz7, B.S.Nielsen6,

D. Ouerdane6, R.Planeta3, F. Rami2, C. Ristea6, O. Ristea9, D. Röhrich8, B. H. Samset11, D. Sandberg6, S. J. Sanders10, R.A.Sheetz1, P. Staszel3, T.S. Tveter11, F.Videbæk1, R. Wada7, H. Yang6, Z. Yin8, and I. S. Zgura9

1Brookhaven National Laboratory, USA, 2IReS and Université Louis Pasteur, Strasbourg, France3Jagiellonian University, Cracow, Poland,

4Johns Hopkins University, Baltimore, USA, 5New York University, USA6Niels Bohr Institute, University of Copenhagen, Denmark

7Texas A&M University, College Station. USA, 8University of Bergen, Norway 9University of Bucharest, Romania, 10University of Kansas, Lawrence,USA

11 University of Oslo Norway

48 physicists from 11 institutions

✔ excellent momentum resolution✔ unique pT coverage at high y

BBroad RaRange HHadron MMagnetic SSpectrometers

q

q

hadronsleadingparticle

leading particle

Schematic view of jet production

high pT particles are produced from the

fragmentation of jets resulting in hard scatterings

loose energy by gluon bremsstrahlung – sensitive to the properties of the traversed medium – jet quenching.

aspects to consider: gluon saturation, gluon shadowing, parton recombination, jet-quenching. Cronin enhancement.

High pT Suppression

High pT Suppression

RCP

1 N binC d 2 N C dpT

1 N binP d 2 N P dpT

RAA

d 2 N AA dpT dy

N bin d 2 N NN dpT dy

BRAHMS acceptance<->leading particle ->high pT

• RCP – systematic errors cancel out*

same data set, detector configuration pp reference not available

• RAA – more information

different collision systems no collective effects in pp

• could emphasize different effects• RAA

& RCP

ratios < 1 show medium effects wrt the ref. used

• higher η -> different medium density in longitudinal direction, reference pp softer

Nucl-th/0108056 Polleri and Yuan

BeamRapidity

[H. Weber, UrQMD]

BRAHMS Acceptance

• small solid angle: ~6 msr for MRS and 0.5 msr for FS• map out the particle phase-space by collecting data with many different spectrometer settings• average spectrum, event normalization*, corrections

Global Detectors

Beam-Beam counters:for vertex determination.

TMA & SiMA: for centrality determination.

more central collisionsSilicon Strips

Plastic Scintillator Tiles

ZDC counters: at 18 meters. Measurespectator neutrons.

0-5%

5-10

%

10-2

0%

20-3

0%

40-5

0%30

-40%

Corrections to the spectra

Tracking efficiencies, geometrical acceptances

Spectra for inclusive charged hadrons in AuAu @ 200 GeV

• inclusive pT

spectra for nonidentified h+ and h- at

η 0, 1, 2.6, 3.2

•power-law shape, where the vast majority of particles are produced in the pt region below 2 GeV/c

•steeper at higher η

RCP

in Au+Au at 200 GeV

● Not significant dependence on η

● η~3.2 h+/h- show the same behaviour

● Cronin like enhancement at 2 GeV/c

RAA

in Au+Au @ 200 GeV/c

● Suppression at all η● Cronin like enhancement in central colls at all η (p

T~2GeV/c)

● RAA

no dependent on η => pp changing the same as AuAu effects other than just medium● Already for semi-peripheral events, R

AA goes to 1.

Hydro + jet model

i Hydro description of the soft part of the produced matter

ii Hard part use a pQDC model (PYTHIA)

i+ii – generation of jets is evolving medium

iv – place for more refined initial conditions

Reasonable descriptionof data at =0 and =2.2 and =3.2

Strong energy absorption model from a static 2D matter source. (Insprired by A.Dainese (Eur.Phys.J C33,495) and A.Dainese , C.Loizides and G.Paic (hep-ph/0406201) )

• Parton spectrum using pp reference spectrum• Parton energy loss E ~ q.L**2• q adjusted to give observed RAA at ~1.

The change in dN/d will result in slowly rising RAA .

The modification of reference pp spectrum causes the RAA to be approximately constant as function of .

Can corona effect mask the lower parton density at =3.2 ?

Au+Au @ 200 && 62.4 GeV

62.4 GeV pp reference is based on ISR collider data62.4 GeV pp reference is based on ISR collider data

200 GeV pp reference from BRAHMS data – PRL 93, 242303 (2004)200 GeV pp reference from BRAHMS data – PRL 93, 242303 (2004)

200 GeV

62.4 GeV

Au+Au, Cu+Cu @ 62.4 GeVR

aa @

AuA

u

Raa

@ C

uCu

T.M. Larsen, QM2005PreliminaryCu+Cu

Au+Au

RCP

@ 200, 62.4 GeVR

cp @

62.

4 R

cp @

200

Little Little dependence at 200 dependence at 200 GeV…GeV…

……but greater but greater suppression suppression approaching approaching beam rapidity at beam rapidity at 62.4 GeV.62.4 GeV.

Preliminary

kinematiclimit

Rcp

@ C

uCu

……CuCu @ 62.4 CuCu @ 62.4 GeV ...GeV ...no suppression! no suppression!

Peripheral Au+Au is not pp!

PID - Time-of-flight

m2 p2 c2 TOF2

L2 1TIME-OF-FLIGHT

TOF2TOF1

TOFW

TOFW TOF1 TOF2

/ K

2 cut

K / p

2.0 GeV/c 3.0 GeV/c 4.5 GeV/c

3.5 GeV/c 5.5 GeV/c 7.5 GeV/c

TOFW2

2.5 GeV/c

4.0 GeV/c

MRS FS

PID - Cherenkov

RICHC1

C4

RICH: Cherenkov light focusedon spherical mirror ring on image plane

Ring radius vs momentum gives PID / K separation 25 GeV/cProton ID up to 35 GeV/c

CHERENKOV

(2 settings)

C4 Threshold (MRS): / K separation 9 GeV/c

RAA

for identified particles

No p suppression!

Preliminary

RAA versus centrality for identified hadronsR. Karabowicz, QM2005

RCP

for p, π, K at y ~ 3

Rcp for Identified particles at y~3

Suppression for all particles

Rcp(p) > Rcp(K) > Rcp()

Cent bins using multiplicity in ||<2Glauber Model

<Ncoll> for 0-10% ~880

<Ncoll> for 40-60% ~ 78

± K±

p,pbar

RAA at y=0 and y=3.1

RAA for pions at forward

rapidities agrees

with RAA at midrapidit

y

R. Karabowicz, QM2005

d+Au system...

D. Kharzeev hep-ph/030737

initially used to emphasize the Au+Au suppression at η ~ 0CGC - high enery, forward y, large nuclei, saturationCGC inspired much teoretical and experimental work

Charged hadron spectra

In all

p+p

d+Au

Au+Au

systems, the distributions get softer at higher rapidities

... dN/dη

... <pT>

d+Au SystemCroninCronin like enhancement at like enhancement at =0.=0.

Clear suppression as Clear suppression as changes up changes up to 3.2to 3.2

Same ratio made with dn/dSame ratio made with dn/d follows follows the low pthe low pTT R RdAudAu

RdA d2Nd+Au/dpTd

<Ncoll> d2Nppinel/dpTd

where < < Ncoll> = 7.2> = 7.2±0.3

At At =0 the central events have the =0 the central events have the ratio systematically above that of ratio systematically above that of semi-central events. We see a semi-central events. We see a reversal of behavior as we study reversal of behavior as we study events at events at =3.2=3.2

D. Kharzeev, Y.V. Kovchegov, K. Tuchin, hep-ph/0405054 (2004)

• CGC model describes RdAu and RCP

• Suppression comes in at y > 0.6

CGC saturation model

Recombination...

important contributions from thermal/shower partons

Parton recombination (up to moderate pT)

Variety of processes can result in suppression

Quality of data is insufficient for ruling out models

Hwa, Yang and FriesPhys.Rev.C71:024902,2005

Summary

RAA show suppression @ 200 GeV for h+/h-

- no significant diff between h+/h-

RAA moderate/no supp @ 63 GeV (AuAu, CuCu)

RCP – not clear what's the dependency on different systems – few effects competting at intermediate pt (eg. v2)

centrality dependence of nucl. mod. factors && slow/NO dependency with η => consistency with jet surface emission (and hydro+jet model)

different mechanism of suppression for baryons and mesons – recombination?

RAA for dA consistent with CGC predictions, though quality of data is insufficient to rule out models