Bormio 2004

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Bormio 2004
Are there high energy phase transitions in the
QCD phase diagram?
An update on the search for novel states of
matter at the RHIC accelerator in Brookhaven,
New York.
Bjørn H. Samset University of Oslo
Norway for the BRAHMS
collaboration
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Ultrarelativistic Heavy Ion Physics

• We observe hot and dense nuclear matter
  created in heavy ion collisions

Goal To observe and understand the nature and
properties ofstrongly interactingmatter at
extreme and novel conditions.

• We try to understand matter in the very early
  universe and in the core of neutron stars.

• We see if these observations can give us an
  understanding of QCD deconfinement and the
  transition to approximate chiral symmetry

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Outline
In this talk Two novel kinds of matter
are under study.

• The Quark-gluon Plasma what is it and one
  approach to looking
  for it.
• The Color Glass Condensate a new(er) idea of a
  universal state of
  high-energy QCD matter.

• Jets a tool for searching for both of these
  states.
• Recent data from RHIC mostly from BRAHMS but
  also from other experiments.

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The Quark-Gluon Plasma?
QCD predicts that under extreme conditions a
novel phase of mattermay exist
Is this state realized in nature? If so, this
would be both a strong argument for QCD as the
theory of the strong interaction, and
interesting in that it will increase our
understanding of neutronstars, the birth of the
Universe andshe properties of the elusive
strong nuclear force.
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RHIC and BRAHMSDesigned to look for the QGP

• RHIC data
• In operation since 1999
• Has collided AuAu, pp and dAu
• Max. AuAu center-of-mass energy

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Jets a probe of novel states of matter?
There are many proposed signatures for a phase
transition in heavyion collisions. One that has
become very interesting at RHIC isthe fate of
particles of high transverse momentum - jets.
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'Jet tomography'
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What we measure Charged particle production
BRAHMS PRL 91 (2003)072305 Reference spectrum
UA1 scaled to our acceptance
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The Nuclear Modification Factor
We need some way to extract the information that
is specific to nucleus-nucleus interactions. Enter
the Nuclear Modification Factor

• Numerator The number of charged particles
  produced in an AAcollision, per unit of rapidity
  and transverse momentum.
• Denominator The same for an NN collision,
  multiplied by thenumber of such collisions in an
  AA collision.

If AA s pp, then RAA1
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SPS energies The Cronin effect

• At SPS (CERN) RAA factors from PbPb show an
  enhancement ofparticle production in for
  ptgt2GeV/c. Known as the Cronin effect,it comes
  because
• low energy particleswill get a 'kick' bythe
  surroundingparticles
• higher energy particleswill supply this kick,
  and so get slowed down
• At RHIC, however...

PHENIXPhys.Rev.Lett.88022301,2002
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RAA from AuAu at

• Suppression of high-pt particles in central
  AuAu
• No suppression for more peripheral collisions
• Suppression seems stronger at higher
  rapidities

...this fits with a picture of a hot and dense
medium formingin central AA collisions
somewhere between SPS and RHIC energies. But
could it also be just an effect of the nuclear
initial state? Let's compare with a system that
should have no extendedhot medium produced
dAu collisions at the samecenter-of-mass energy.
BRAHMS PRL 91 (2003)072305
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RdA from AuAu at
BRAHMS PRL 91 (2003)072305

• For dAu collisions, we get back the Cronin
  enhancement seen at SPS
• The high-pt suppression does not seem to be due
  to the initial state
• ...at least at midrapidity. We'll look at
  forward y in a minute.

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STAR data What happens to the far-side jets?
The far-side jets disappearin central AuAu
only,not in pp or dAu.
STAR Collaboration PRL 91,072304
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Cover of PRL twice...
The data indicate that a hot and dense mediumhas
been produced in central AuAu collisionsat RHIC
energies. Wether this medium can besaid to be a
'Quark-Gluon Plasma' remains tobe seen
theoretical interpretations and opinionsstill
differ. But what was this about dAu at forward
rapidities?
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The Color Glass Condensate
Stand still and look at a proton coming towards
you at evergreater speeds. What do you see?
Answer A certain distribuion of quarks
andgluons, as measured by e.g. the HERA
collaboration
Proton at low energies

• Q² is momentum transfer between the probe and
  the incoming nucleus
• x is the fraction of proton momentum carried
  by the probed parton

HERA gluon distribution function
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The Color Glass Condensate
At low x, high Q² the protonsaturates with
low-energygluons!
About the name Color It is a state composed of
colored partons, itinteracts strongly via the
strong force. Glass It is a dynamic state, but
it can be shown that itevolves slowly compared
to the timescale of anultrarelativistic heavy
ion collision quite like glass. Condensate The
many compressed gluons (bosons!) take on
properties akin to a Bose-Einstein Condensate.
Proton at high energies

• The CGC is a state that
• is calculable with classical field theory
• is universal and independ of the hadron(s) which
  generates it
• should make up the initial state in an AA
  collision at high energy

(for a description of both QGP and the CGC see
hep-ph/0311028)
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CGC Predictions for RHIC
The CGC is a nice initial state, because it is
calculable. But does it exist? In what part of
phase space should we look for it at RHIC?
Answer (from QCD, PDFs and phenomenology...)
In the range 0.5GeV/c lt pt lt 4GeV/c at
y3 Here BRAHMS is unique at RHIC.
D. Kharzeev et al. Phys.Rev.D68094013,2003
y0
y1
y2
y3
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BRAHMS spectra from dAu and pp
dAu
pp

• Spectra represent 50 of BRAHMS dataset from
  2001 and 2003
• More rapidities (i.e. spectrometer angles) will
  be analyzed
• pp data are well reproduced by PYTHIA/HIJING
• dAu data are well reproduced by HIJING

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RdA factors vs. rapidity
RdA
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Centrality dependence - RCP
RCP data from centralcollisions dividedby
peripheral
From a CGC we expect more suppression in central
collisions, since the condensate will be denser.
This effect should become apparentas we go
higher in rapidity towards the saturation regime.
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Centrality dependence - RCP
RCP data from centralcollisions dividedby
peripheral

• The BRAHMS data are consistent with a Color
  GlassCondensate interpretations, and good
  predictions weremade based on this
  hypothesis.However
• other possible explanations are being developed
  (e.g. valence quark fragmentation)
• the CGC model is an extrapolation of a
  theoretical idea quite a long extrapolation
• So The CGC is currently a hot but dense topic in
  the heavy ion community. We've shown the data
  now weneed cool-headed interpretations.

The expected dependence on centrality is seen at
high rapidity, but at lower rapidities this
dependence is reversed. This has, as far as I
know, not been predicted by any theory yet. (Not
really been looked at either...)
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Summary what are we cooking at RHIC?
The behaviour of high-pt particles at RHIC
indicates that

• The collision initial state can be seen as two
  slabs of strongly interacting colored glass
  colliding at vc. Now calculable?
• A hot and dense deconfined medium forms,
  which then maybe thermalizes into a
  quark-gluon plasma.
• This state hadronizes according to
  statistical laws, either because it was a
  state in equilibrium or because hadronization
  is by nature statistical.

For MUCH more on this, see
http//qm2004.lbl.gov/
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The BRAHMS collaboration
I. Arsene10, I. G. Bearden7, D. Beavis1, C.
Besliu10, B. Budick6, H. Bøggild7, C. Chasman1,
C. H. Christensen7, P. Christiansen7, J. Cibor4,
R. Debbe1, E. Enger12, J. J. Gaardhøje7, M.
Germinario7, K. Hagel8, O. Hansen7, H. Ito1,
11, A. Jipa10, F. Jundt2, J. I. Jørdre9, C. E.
Jørgensen7, R. Karabowicz3, E. J. Kim5, T.
Kozik3, T. M. Larsen12, J. H. Lee1, Y. K. Lee5,
S. Lindal12, R. Lystad9, G. Løvhøiden2, Z.
Majka3, A. Makeev8, B. McBreen1, M. Mikelsen12,
M. Murray8, 11, J. Natowitz8, B. Neumann11, B.
S. Nielsen7, J. S. Norris11, D. Ouerdane7, R.
Planeta4, F. Rami2, C. Ristea10, O. Ristea10, D.
Röhrich9, B. H. Samset12, D. Sandberg7, S. J.
Sanders11, R. A. Scheetz1, P. Staszel7, T. S.
Tveter12, F. Videbæk1, R. Wada8, Z. Yin9, I. S.
Zgura10 1Brookhaven National Laboratory,
USA 2IReS and Université Louis Pasteur,
Strasbourg, France 3Jagiellonian University,
Krakow, Poland 4Institute of Nuclear Physics,
Cracow, Poland 5Johns Hopkins University,
Baltimore, USA 6New York University, USA 7Niels
Bohr Institute, University of Copenhagen,
Denmark 8Texas AM University, College Station,
USA 9University of Bergen, Norway 10University
of Bucharest, Romania 11University of Kansas,
Lawrence, USA 12 University of Oslo Norway

• RHIC is currently in its fourth running
  period.
• More data is being taken, AuAu at full energy
  and maybe also at a lower energy.
• BRAHMS is upgraded to take even better data
  at forward rapidities.

Stay tuned for more on the QGP and the CGC!