A Summary of BRAHMS Results Presented at QM2006

QM2006 Summary 28-NOV-2006 1

A Summary of BRAHMS Results Presented at QM2006

R. Debbe BNL


Outline of the summaryo Rapidity dependence of Nuclear Modification Factors.

oAn investigation on the contribution of energy conservation to particle production at high rapidity.

oElliptic flow at high rapidity.

oBaryon transport.

oStrangeness production at 62 GeV.

oHigh pT and high rapidity particle production in p+p collisions.

o Proton and neutron coalescence at different rapidities.


We have studied the identified changed particle production in different systems in wide rapidity range and intermediate pT:

p+p at 62 and 200 GeV

d+Au at 200 GeV

Au+Au Cu+Cu at 62 and 200 GeV

We have found a puzzling absence or weak rapidity dependence of intermediate pT suppression, elliptic flow as well as “source size”.

We need an overarching phenomenological description of these results, we have to include the longitudinal evolution of the formed system and translate the description of the initial state as a Color Glass Condensate and its appearance at high rapidity to A+A systems.


Partons are expected to loose energy in the dense matter

Rapidity dependent high-pT suppression factors: provide information on dynamical medium effect

RAA may be a complicated quantity as it “carries” too many parameters that remain unknown or poorly controlled.

at fixed time

y~0

high-y

Rapidity Dependent High-pT Measurement

BRAHMS

NA49

AGS


y=0 y=1 y=3.1

pions

kaons

protons

Rapidity Dependence of Intermediate pT Suppression in Au+Au at 200 GeV


RAA 62 GeV mid-rapidity. Top, Cu+Cu; bottom Au+Au

Competing effects: suppression by energy loss and Cronin enhancement.

Systematic difference in one unit of

rapidity for the smaller Cu+Cu system

Preliminary


Rapidity dependence of the pbar/ - ratio

Contrary to the absence of rapidity dependence of the RAuAu factor, the pbar/ - ratio does show a marked reduction as the rapidity increases.

Things that do change with rapidity


“Forward production of protons is favored in recombination because of scarcity of anti-quarks” R. Hwa et al. nucl_th/0605037

The proton/+ ratio at forward rapidities

Dramatic difference between y=0 and 3. There are ~2.5 more protons than pions at ~ 2.5 GeV/c

Preliminary


BRAHMS Data/Acceptance: pT vs xF at √sNN = 200 and 62 GeV

FSMRSMRS

• Strong pT-xF correlation due to limited spectrometer solid angle acceptance• Measurements from BRAHMS Mid-Rapidity Spectrometer (MRS) and Forward Spectrometer (FS)• “Dynamic” xF binning in pT 0.2 GeV/c• RCP for centrality dependence in pT-xF: RCP(0-20/40-70%)

√sNN = 200 GeV √sNN = 62 GeV


MRS, FS

•More suppression as xF increases at fixed pT (0.3 ~ 2.2 GeV/c)

•For the soft Rcp increase and maximize at pT ~ 1 GeV/c

•Statistical errors only shown•Systematic Uncertainties: 10% (p-to-p) + 10% (normalization)


Differential Elliptical Flow measured with BRAHMS spectrometers


Elliptic flow of identified particles at different rapidities compared to 3D Hydro. and a “cascade” model.


Baryon Transport in Different Systems.BRAHMS results at RHIC have shown that mid-rapidity is not completely baryon free. The transport of baryon number thru ~5 units of rapidity could be related to baryon junctions that have a sizable small-x components in their wave function, but the subject is still open and BRAHMS can bring new insight with further analysis.


Stopping 62 GeV Au+Au

The curve is a fit to data with two gaussians in pz constrained by the number of participant protons.


“net”

pro

ton

AGS

SPS

RHIC 62

RHIC 200

LHC 5500

dN

/dy

(BRAHMS preliminary)

An extrapolation to A+A collisions at LHC


p+p 200GeVWe have a good data sample of p+p collisions at 200 GeV and are in the process of studying baryon transport.

Red squares are the integrals of extrapolated distributions down to pT=0


d+Au 200GeV

Net-proton distribution in rapidity for three centrality samples in d+Au collisions at 200 GeV.

The d+Au system is asymmetric and the most central collisions show a hint of a strong “source” of protons on the Au side.


Strangeness production in Au+Au collisions at 62 GeVThe plot below suggests that high rapidity in low energy collisions can be equivalent to SPS systems that show a “horn” in the k/ ratio.


The system formed at high rapidity in Au+Au at 62 GeV “falls” on the high side of the “horn” and corroborates the trend measured at SPS.


Particle production at high rapidity in p+p collisions at 200 GeV

The success of NLO pQCD calculations at describing pion production at different rapidities make us confident that parton degrees of freedom are the correct description for the physics at RHIC.


Calculations by W. Vogelsang

The KKP set of fragmentation functions have been modified to get fragmentation into charged pions and kaon.

As it was the case for mid-rapidity, the favored FF are the KKP set implying a strong contribution from gluon driven partonic processes.


Red: p/+ Blue: p-bar/ -

e+e-: p+p-bar/++ - (ALEPH)

BRAHMS Preliminary

Expected higher abundance of + at high pT and rapidity (isospin effect).

Small values measured at high rapidity rule out the use of AKK FF at these rap.

High p/+ values at high pT and rapidity may shed new light on baryon transport.


Coalescence: p+n d

• Characterize source via B2

• Assume B2≈V-1

• Details: • C. Nygaard


B2 as Function of Rapidity

Within errors the system size at freeze-out doesn’t change with rapidity.


Outlook

Now that all the data has been collected and is practically all analyzed, we will proceed to work on several publications that will report systematic studies of identified particle production.

We are reaching out to our colleagues with to develop a phenomenological description of the complete A+A system dynamics including its strong longitudinal evolution as well as its initial state probed from high rapidity.


Backup slides


RAA Cu+Cu,Au+Au 62 GeV, =3.1

A Summary of BRAHMS Results Presented at QM2006

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