A Summary of BRAHMS Results Presented at QM2006

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A Summary of BRAHMS Results Presented at QM2006
R. Debbe BNL
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Outline of the summary

• Rapidity dependence of Nuclear Modification
  Factors.
• An investigation on the contribution of energy
  conservation to particle production at high
  rapidity.
• Elliptic flow at high rapidity.

• Baryon transport.
• Strangeness production at 62 GeV.
• High pT and high rapidity particle production in
  pp collisions.
• Proton and neutron coalescence at different
  rapidities.

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• We have studied the identified changed particle
  production in different systems in wide rapidity
  range and intermediate pT
• pp at 62 and 200 GeV
• dAu at 200 GeV
• AuAu CuCu 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 AA systems.

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Rapidity Dependent High-pT Measurement
y0

• 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
high-y
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Rapidity Dependence of Intermediate pT
Suppression in AuAu at 200 GeV
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RAA 62 GeV mid-rapidity. Top, CuCu bottom
AuAu
Preliminary
Competing effects suppression by energy loss and
Cronin enhancement.
Systematic difference in one unit of rapidity for
the smaller CuCu system
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Things that do change with rapidity
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.
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The proton/? ratio at forward rapidities
Dramatic difference between y0 and 3. There are
2.5 more protons than pions at 2.5 GeV/c
Preliminary
Forward production of protons is favored in
recombination because of scarcity of anti-quarks
R. Hwa et al. nucl_th/0605037
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BRAHMS Data/Acceptance pT vs xF at vsNN 200
and 62 GeV
FS
MRS
vsNN 200 GeV
vsNN 62 GeV

• 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)

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• 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)

MRS, FS
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Differential Elliptical Flow measured with BRAHMS
spectrometers
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Elliptic flow of identified particles at
different rapidities compared to 3D Hydro. and a
cascade model.
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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.
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Stopping 62 GeV AuAu
The curve is a fit to data with two gaussians in
pz constrained by the number of participant
protons.
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AGS
An extrapolation to AA collisions at LHC
dN/dy
SPS
RHIC 62
(BRAHMS preliminary)
netproton
RHIC 200
LHC 5500
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We have a good data sample of pp collisions at
200 GeV and are in the process of studying baryon
transport.
Red squares are the integrals of extrapolated
distributions down to pT0
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Net-proton distribution in rapidity for three
centrality samples in dAu collisions at 200
GeV. The dAu system is asymmetric and the most
central collisions show a hint of a strong
source of protons on the Au side.
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Strangeness production in AuAu collisions at 62
GeV
The plot below suggests that high rapidity in low
energy collisions can be equivalent to SPS
systems that show a horn in the k/? ratio.
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The system formed at high rapidity in AuAu at 62
GeV falls on the high side of the horn and
corroborates the trend measured at SPS.
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Particle production at high rapidity in pp
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.
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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.
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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.
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Coalescence pn d

• Characterize source via B2
• Assume B2V-1
• Details
• C. Nygaard

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B2 as Function of Rapidity
Within errors the system size at freeze-out
doesnt change with rapidity.
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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 AA system dynamics
including its strong longitudinal evolution as
well as its initial state probed from high
rapidity.
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Backup slides
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RAA CuCu,AuAu 62 GeV, ?3.1
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