An overview of the experimental results obtained with BRAHMS experimental setup

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An overview of the experimental results obtained
with BRAHMS experimental set-up

• Alexandru JIPA
• Atomic and Nuclear Physics Chair, Faculty of
  Physics,
• University of Bucharest, ROMANIA
• 3rd Winter School on RHIC, 8-11.XII.2003,
  Budapest, Hungary

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Summary

• The importance of the heavy ions collisions
• BRAHMS experimental set-up structure,
  opportunities and goals
• Global information charged particle
  multiplicities and rapidities estimation of the
  energy density
• Transverse dynamics temperatures and radial flow
• Longitudinal dynamics
• Antiparticle to particle ratios Coulomb
  momentum, chemical potentials, entropy per barion
• New aspects
• - High-pt suppression was new matter formed and
  observed?
• - Does Gluon Saturation manifest itself at RHIC
  energies?
• Final remarks
• Al.Jipa - 3rd Winter School on RHIC,
  8-11.XII.2003, Budapest

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Questions of Interest

• What has RHIC , and in particular BRAHMS done in
  its first 3 runs?
• Al.Jipa - 3rd Winter School on RHIC,
  8-11.XII.2003, Budapest

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Heavy ion collisions
???K??p??n?????????????????d,
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RHIC experiments

• Run 1 June - September 2000
• First Physics Run
• AuAu _at_ two energies
• ?SNN 56 and 130 GeV

• Run 2 July 2001- January 2002
• AuAu _at_ ?SNN 200 GeV
• (maximal design energy)
• pp (reference data)

• Run 3 December 2002- May 2003
• dAu _at_ ?SNN 200 GeV
• pp _at_ ?SNN 200 GeV

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Charged particle ? - distributions

• dN/d? resembles Bjorken boost invariant
  assumption. Due to this as well as easier to deal
  with Hydro calculations have typically been done
  with assumption.
• Part of the shape is effected by the use of ?
  rather the y.
• Most of Brahms data were collected for central
  collisions
• Energy densities seen in meson production can be
  estimated by Bjorkens formulae
• E 1.5 ltptgt/?/?R2 dN/d? 4.5 GeV/fm3
• Rapidity density uniform over -2 units of
  pseudo-rapidity.

?sNN 200 GeV
Ref PRL 88, 202301(2002) Centralities
0-10,10-20,..
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Inverse slope vs. Mass and centrality

• The dependence of the effective temperature on
  both mass and collision centrality is an
  indication of radial expansion.
• Experimental temperatures are greater than the
  temperatures obtained from simulated data with
  HIJING and UrQMD codes.

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Colective transverse flow
EXP
HIJING
UrQMD
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Hydrodynamics-based parameterizationBlast-wave
model

• Considering a hydrodynamically behaving boosted
  source, a parameterization is fitted
  simultaneously to all the particle spectra to
  determine the magnitude of the radial flow. It is
  assumed that
• all particles decouple kinematically on a
  freeze-out hypersurface at the same freeze-out
  temperature Tfo,
• the particles collectively expand with a velocity
  profile increasing linearly with the radial
  position in the source, and
• the particle density distribution is independent
  of the radial position.

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Fitting the Transverse Mass Spectra
0-10 centrality

• For 0-10 and 40-60 centrality, the first 3
  n-? contour levels are shown.
• From the peripheral to the central data, the
  single particle spectra are fit simultaneously
  for all pions, kaons, and protons.

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Fitting the Transverse Mass Spectra
40-60 centrality

• For 0-10 and 40-60 centrality, the first 3
  n-? contour levels are shown.
• From the peripheral to the central data, the
  single particle spectra are fit simultaneously
  for all pions, kaons, and protons.

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Inverse slope vs. Energy

• At RHIC energies, the collective flow velocity
  parameter is larger than that from collisions at
  AGS/SPS energies.
• The temperature parameters, compared to
  results from lower energy collisions, seem to be
  lower .

BRAHMS preliminary results for 10 most central
events in comparison with the results from other
experiments at lower energies (AGS, SPS).1 The
BRAHMS extracted Tfo and beta have statistical
errors only.
1. N. Xu, M. Kaneta Nucl. Phys. A698 (2002)
306c
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Coulomb interaction study
Coulomb interaction is investigated through
the produced charged pions ratio in Au-Au
collisions obtained with BRAHMS experimental
set-up.
Coulomb momentum (kick) is
The pion ratio can be described by the
relationship
Where
Freeze-out radius is
geometrical (initial) radius of the
fireball transverse flow velocity
freeze-out time
Results obtained at lower energies (AGS si SPS)
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0-10, 40-60
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Coulomb momentum at BRAHMS

• The Coulomb effects in pion spectra are
  sensitive to the degree of stopping and the
  distribution of positive charge, as well as at
  the flow velocity of the participant region.
• The values reflect a reduced Coulomb effect
  because of higher flow velocities of the nuclear
  matter from participant region.

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Chemical potential vs. Energy
The energy dependence of the chemical potential
was shown to be parametrized as
P. Braun-Munzingen, K. Redlich, J. Stachel -
nucl-th/0304013
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Chemical freeze-out temperature vs. energy
The energy dependence of the chemical
temperature can be parametrized as
The chemical freeze-out temperature seems to
saturate close to the critical temperature of 170
MeV extracted from lattice QCD calculation.
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Baryonic chemical potential
The chemical potential increases from
midrapidity to forward rapidities, because at
y0, the net-baryon density is much reduced than
what was observed at forward rapidities.
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Strange chemical potential
The small value obtained for 200 GeV may suggest
that the we are close to the full chemical
equilibrium for strange particles.
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Charged particle ? - distributions
?sNN 200 GeV
d-Au
Charged particle multiplicities for the
centrality ranges of 0-30 and 30-60. The
square points and circular points from SiMA and
TMA detectors, respectively, while the triangles
are from the BBC detectors.
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Charged particle multiplicities for the
centrality range 0-30 and 30-60. The shaded
regions indicate the total (statistical and
systematic) uncertainties. The dotted and dashed
curves are the results of HIJING and Saturation
Model predictions. Model calculations based on
perturbative QCD (shadowing and jet-quenching
mechanisms) lead to excellent agreement with
experimental results. Model calculations based
on the saturation picture of non-perturbative QCD
do not reproduce the centrality or pseudorapidity
dependence of the measurements.
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Rapidity dependent ratios

• At y0 (20 central)
• pbar/p 0.75 0.04
• K-/K 0.95 0.05
• p-/p 1.01 0.04
• Highest pbar/p ratio indicating a nearly
  transparent system with very few net baryons.
• Ratios identical over -1 unit around
  mid-rapidity.
• Only weak centrality and pT dependence (not shown
  here)
• No Hyperon feed down correction applied less
  then 5 correction to ratios.
• Dynamical (cascade, string) models do NOT
  describes rapidity dependent ratios and yields
  correctly

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Thermal Interpretation

• The baryon chemical potential is given by
  p-bar/p exp(- 2?B/T)
• By simple quark counting
• in quark recombination
• K-/K
• exp(2ms/T)exp(-2mq/T)
• exp(2ms/T)(pbar/p)1/3
• (pbar/p)1/3
• by assuming local (in y)
• strangeness conservation
  
• K-/K(p-bar/p)a
• a 0.240.02 for BRAHMS
• a 0.200.01 for SPS
• Good agreement with the statistical-thermal model
  prediction by Becattini et. al. (PRC64 2001)
  Based on SPS results and assuming T170 MeV

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Is there a commonTemperature ifall particle
are considered?

• Apparently
• Assume all distributions described by one
  temperature T and one ( baryon) chemical
  potential ??
• One ratio (e.g., ?p / p ) determines ? / T
• A second ratio (e.g., K / ? ) provides T ? ?
• Then predict all other hadronic yields and ratios

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• This exercise in hadro-chemistry
• Applies to final-state (ordinary) hadrons at end
  of reaction.
• Does not (necessarily)indicate
• QGP formation
• Deconfinement
• New state of matter
• The exploration of the freeze-out phase diagram
  shows a smooth continuation with RHIC results
  and of trends seen
• at lower energies
• in p-p, even ee-

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Longitudinal Bulk Properties
BRAHMS Preliminary
Pion Power law fit A(pt/p01)-n
Kaon mT single exponential fit
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Longitudinal Meson Distributions
No wide plateau observed in rapidity for
identified mesons. Close to a Gaussian shape
(?(?) 2.35 ?(k) 2.39) Total yield in
agreement with published dN/d? measurements from
multiplicity sub-system. The RMS of ?
distributions from low energy to RHIC is
strickingly close to prediction of Landau Hydro
model ?2 0.5
ln(s/(4m2)) P.Carrruthers and M.Duong-van
PRD8,859(1973)
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Net-Baryon Densities
Earlier saw that p-bar/p 0.75 near
mid-rapidity. The system has very few
net-baryons ie. baryon number that must be
conserved in the reaction transported to
mid-rapidity.
The shape of the net-proton distribution measured
at RHIC is different rofm what is observed at
lower energies. At RHIC the mid-rapidity region
is almost net-proton free. Pair baryon production
dominates at RHIC. The net-baryons at y0 is
10-2, compared with produced pions of 900. The
rapidity loss ?y ? (yb-y) dN/dy / ? dN/dy
Represents the energy transfer from incident
beam.
Preliminary
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Energy systematic of Rapidity lossand Net-Proton

• These data showing the increase in ?y for AA,
  while pp is approximately constant.
• The estimated value at RHIC is consistent with a
  continuous increase of ?y.
• E/Einitial e- ?y
• This implies that 85 of the initial energy is
  stopped and emerges as internal energy, produced
  particles and at end of reactions in longitudinal
  and transverse momentum distributions.
• Net-protons at y0 continuously decrease with
  energy.

? y
pp
Net protons at y0
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High pt Suppression Jet Quenching

• Particles with high pts (above 2GeV/c)
• are primarily produced in hard scattering
• processes early in the collision
• ? Probe of the dense and hot stage

• pp 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

Experimentally ? depletion of the high pt region
in hadron spectra
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Systematizing Our Expectations
no effect ?
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Charged Hadron Spectra

• Reference spectrum
• ? ppbar spectra (UA1)

• Data do not show suppression
• Enhancement (RAAgt1)
• due to initial state multiple
• scattering (Cronin effect)
• Known in pA collisions

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High pt Suppression in AuAu
At Mid-Rapidity (?0)

• Central Collisions
• RAA lt 1 at high pt

• Clear suppression effect
• Consistent with Jet Quenching

• Peripheral Collisions
• RAA 1 at high pt

• No suppression (as expected)

• Consistent with observations
• by PHENIX and STAR

• BRAHMS can also measure at
• more forward rapidities

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Suppression at large ?
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Is this a new Result ?

• Yes- all previous nucleus-nucleus measurements
  see enhancement, not suppression.
• Effect at RHIC is qualitatively new physics
  made accessible by RHICs ability to produce
• (copious) perturbative probes
• New states of matter?

SPS 17 GeV
ISR 31 GeV
Au-Au 200
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Is this Unique to Heavy Ion ?

• at RHIC energies -- YES!
• a crucial control measurement via d-Au
  collisions

• Enhancement in dAu

Typical behaviour of Cronin effect

• Absence of suppression in d-Au

? Supports the Jet Quenching interpretation
for central AuAu collisions ? Excludes
alternative interpretation in terms of
initial state parton saturation effects
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High pt Suppression Hydro-Jet Model Calculations

• Use full 3-D hydro simulations
• to study the density effects
• on parton energy loss

• Hydro ? description of the soft
• Part of the produced matter

• Hard part ? use a pQDC model
• (PYTHIA)

Generation of momentum spectrum jets
? Good agreement with BRAHMS data at both ?0
and 2.2 ? Similar effect at ?0 and 2.2. Due
to comparable time evolution of the parton
density at ?0 and 2.2 in hydro.
Indirect evidence of the presence of hot
thermalized matter in the region -2.2 lt ?
lt 2.2
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Information from high-pt quenchingan bulk
properties

• Both
• Au-Au suppression (I. Vitev and M. Gyulassy,
  hep-ph/0208108)
• d-Au enhancement (I. Vitev, nucl-th/0302002 )
• understood in an approach that combines multiple
  scattering with absorption in a dense partonic
  medium
• Our high pT probes have been calibrated dNg/dy
  1100 e gt 100 e0

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Gluon Saturation
Results just obtained from d-Au measurement near
y0 have shown that final state effects are
dominant. New regimes of partonic physics are
expected to appear as x-gt0. Gluon structure
functions are rising in d-A the gluons with be
very large and the effect from the Parton
Distribution Functions will saturate. To reach
small x regions one needs high energies. Physics
near the fragmentation region of the nucleon in
p-A collisions offer similar window go as
forward as possible and use the highest A you can
work with. Higher rapidities are equivalent to
higher energies.
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BRAHMS can reach very small values of x in the Au
gluon distributions
A is d and B is Au. Energy and momentum
conservation xL xa - xb (MT/vs)sinh y
ka kb k
xaxb MT2/s A solution to this system is
xa (MT/vs) ey xb (MT/vs) e-y where y is
the rapidity of the (xL,, k) system
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Two extreme Model predictions
I. Vitev nucl-th/0302002 v2
D. Kharzeev hep-ph/0307037
CGC at y0
Y0
As y grows
Y3
Y-3
Very high energy
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p-p d-Au distributions . 2.9 lt?lt3.3
This distribution was obtaine from different
magnetic field settings. Geometric acceptance and
tracking efficiency corrections have been
applied Pythia describes the pp data well.
BRAHMS preliminary
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d-Au Nuclear Modification factor at ? 3.2
RdAu compares the yield of negative particles
produced in dAu to the scaled number of
particles with same sign in p-p For d-Au min.bias
data Ncoll7.2 Error is systematic.
PRL 91 072305 (2003)
BRAHMS preliminary
The high rapidity d-Au do also show a significant
suppression. This is consistent with the
schematic prediction of gluon saturation, albeit
it does not prove it. It is certainly significant
below the pQCD calculation of Vitev including
the Cronin effect.
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Final Remarks

• RHIC has obtained a wealth of new and detailed
  information on relativistic heavy ion reactions.
• From these experimental data we now know that the
  stage is set to explore and quantify very hot and
  dense matter.
• The Net-baryon density is very small dN/dy10,
  and the corresponding baryon chemical potential
  ?B 29 MeV.
• The system exhibit a large transverse and
  longitudinal expansion with the azimuthally
  asymmetries being large, reflecting the initial
  partonic distributions. The system has reached a
  hydro dynamical limit, which can be used to
  explore the Equation of State of the hot dense
  matter.
• Suppression of high-pt particles relative to
  elementary pp collisions is observed in central
  Au-Au collisions, but neither in peripheral, nor
  in the control d-Au experiment.

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Conclusions and Outlook

• The heavy ion data from RHIC are consistent with
  formation of a hot dense system that
• exhibits hydrodynamic behaviour with rapid
  transverse and longitudinal expansion.
• Absorbs high-pt probes corresponding to a large
  gluon density in the initially formed system
• Is an almost Baryon-free system

• Much remains to be done before one can claim the
  discovery and characterization of Quark Gluon
  Plasma done. Examples
• suppression pattern of J/Ys sensitive to the
  screening in de -confined phase.
• Properties of thermal photons from the initial
  hot phase.
• In addition it may be that we can also start
  probing the gluon saturation at forward
  rapidities.

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The BRAHMS Collaboration
Other romanian physicists participating in
BRAHMS Dr. Dan Argintaru, Dr. Florin Constantin,
Dr. Daniel Felea, Ciprian Mitu, Mihai Potlog,
Silvia Ochesanu, Costin Caramarcu