Experimental%20evidence%20for%20color-neutral%20pre-hadronic%20states%20above%20the%20critical%20temperature%20at%20RHIC%20and%20LHC

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Experimental evidence for color-neutral
pre-hadronic states above the critical
temperature at RHIC and LHC

• Rene Bellwied (University of Houston)
• Wroclaw, Poland, May 19-21, 2011

for more detail please see RB and C.Markert (PLB
691, 208 (2010))
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The fundamental questions

• How do hadrons form ?
• Parton fragmentation or string fragmentation or
  recombination
• An early color neutral object (pre-hadron) or a
  long-lived colored object (quasi-particle or
  constituent quark)
• When do hadrons form ?
• Inside the deconfined medium or in the vacuum ?

Not addressed by HEP because it is
non-perturbative and can not be
calculated Phenomenological approach Fragmentati
on, Factorization
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The formation time of hadrons

• Can a hadron form inside the deconfined medium
  above Tc ?
• Three Scenarios
• Is the energy loss in medium affected by the
  formation of the hadronic state ?
• Are the properties of the hadronic state affected
  by the formation in medium ?
• Signatures any early probe which is sensitive to
  the medium (e.g. energy loss or v2)

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The treatment of formation time

• Formation time is largely ignored in heavy ion
  collisions.
• Based on the simple Lorentz boost argument, which
  is insufficient for in-medium fragmentation, it
  was concluded early on that only colored partons
  will traverse the system and only fragment
  outside the medium i.e. in vacuum.
• All energy loss models (ASW, AMY, GLV) are
  based on purely partonic energy loss, either
  collisional or radiative energy loss.
• Greiner, Gallmeister, Cassing (Phys. Rev. C67,
  044905 (2003)) suggested early hadronization and
  hadronic energy loss.

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The principle a question of time

• There is per-se no reason to believe that, in
  heavy ion collisions, a process such as
  fragmentation, which does not thermalize with the
  surrounding medium, would take more or less time
  than in vacuum.
• One could use in-vacuum formation time.
• BUT there are two aspects to consider in-medium
• Lorentz boost the higher the energy the longer
  the formation time
• (based on Heisenbergs uncertainty principle,
  true for string fragmentation)
• Energy conservation the higher the fractional
  momentum the shorter the formation time, since
  partons lose energy through bremsstrahlung in
  medium (true for parton fragmentation in medium).

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Schematic Modeling of hadronization
e.g. Lund String Model breaking the color string
from the struck parton to the target remnant
(constituent length)
Energy conservation Lorentz boost
Eq struck quark energy kstr string tension
Bjorken (1976) The higher the energy and the
lighter the final state, the later the hadron
will form (inside-outside cascade) Kopeliovich
(1979) A high z particle has to form early
otherwise the initial parton loses too much
energy (outside-inside cascade).
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Combining inside-out and outside-in in light
cone variables
Inside-out cascade (boost) to 1 fm/c proper
formation time in hadron rest frame E energy of
hadron m mass of hadron E/m g

• high energy particles are produced later
• heavy mass particles are produced earlier

C. Markert, RB, I. Vitev (PLB 669, 92 (2008))
Outside-in Cascade (pre-hadron formation) large
z (ph / pq) leading particle ? shortens
formation time
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Evolution of a RHIC heavy ion collision(as a
function of temperature and time)
Model lQCD SHM
Blastwave Effect hadronization chemical
f.o. kinetic f.o. Freeze-out
surface Tcrit Tch Tkin(X,W)
Tkin(p,k,p,L) Temperature (MeV) 190 165
160 80 Expansion velocity
(c) b0.45 b0.6
Hydro condition ?
Tinit 370 MeV
References Lattice QCD hep-lat/0608013 arXiv090
3.4155 Statistical Hadronization hep-ph/0511094
nucl-th/0511071 Blastwave nucl-ex/0307024 arXiv
0808.2041
partons
hadrons
?0 ?QGP
Experiment time 5 fm/c 4 fm/c
(STAR, PRL 97132301,(2006))
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Analytic approach to proper times

• What is the proper t0 ?
• t0 requires thermalization which is an open issue
  at RHIC and LHC.
• Simple collision time tc 2RA/g is definitely
  too short.
• General approach t0 1/ltpTgt
• Leads to t0(RHIC)0.44 fm/c and t0(LHC)0.23 fm/c
  
• (with ltpTgt 450 and 850 MeV/c respectively)
• What is the proper QGP lifetime ?
• Upper limit based on longitudinal Bjorken
  expansion
• tQGP t0 (T0/Tc)3 with
• T0(t0,RHIC) 435 MeV and T0(t0, LHC) 713 MeV
  and Tc 180 MeV
• (see pre-print for more detail)
• tQGP (RHIC) 6.2 fm/c , tQGP (LHC) 14 fm/c
• RHIC result slightly higher than data driven
  partonic lifetime estimate based
• on HBT and resonances (tQGP (RHIC) 5 fm/c)

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Formation Time of Hadrons in RHIC / LHC QGP(C.
Markert, RB, I. Vitev, PLB 669, 92 (2008))
RHIC LHC
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What did we really calculate ?

• Brodsky Mueller (PLB 206, 685 (1988))
• First time distinction between tp (production
  time) and tf (formation time). Production time
  determines production of co-moving constituent
  quarks (coherence length).
• Since all quark configurations are possible the
  order parameter becomes the mass difference
  between all possible states of the same quark
  configuration.
• Kopeliovich (e.g. arXiv1009.1162) approximate
  the order parameter by the mass difference
  between the ground state and the first excited
  state (resonant state), e.g. for the pion this
  time is considerably shorter than the time when
  taking the final hadron mass.
• RB CM taking the final hadron mass can
  approximate the formation time of the final
  hadron wavefunction.

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Pre-hadron formation time (?p)

• A. Accardi, arXiv0808.0656 A quark q created in
  a hard collision turns into a
• colored pre-hadron, which subsequently
  neutralizes its color and collapses
• on the wave function of the observed hadron h
• B) RB CM Using the final hadron mass will
  increase the formation time
• since the final mass is smaller than the
  difference between the ground and
• excited states.

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Quasi-particle or pre-hadron ? Is their a
difference ?

• A quasi-particle is a colored object, i.e. a
  dressed up quark which has attained a thermal
  mass that can potentially exceed the final state
  hadron mass and then decay into the hadronic
  state. (e.g. Cassing et al. (DQPM model) or Ratti
  et al. (lattice QCD))
• A pre-hadron is a color neutral object that
  approaches the final hadronic wave function
  during its evolution, i.e. quark content fixed
  but not all hadron properties fixed (e.g.
  Kopeliovich or Accardi)
• A colored object will continue to interact and
  not develop a hadronic wave function early on
  (constituent quark or quasi-particle)
• A color-neutral object will have a reduced size
  and interaction cross section (color
  transparency) and develop wave function
  properties early
• Only a color neutral state can exhibit hadronic
  features (e.g. can pre-resonance decay prior to
  pion hadronization ?)

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A hybrid model (Cassing Bratkovskaya, PRC 78,
034919 (2008))

• Partons dress up throughout the partonic phase
  quasi-particles (PHSD parton-hadron string
  dynamics)
• At hadronization very massive pre-hadronic
  resonances form through recombination of dressed
  (constituent) quarks color neutral states
• (DQPM dynamic quasi-particle model)
• Resonances subsequently decay into ground state
  mesons and octet baryons
• Color neutrality is achieved LATE !

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Lattice QCD inspired quasi-particle mass
calculations
Levai Heinz, PRC 57, 1879 (1998)
Ratti, Greco et al., arXiv1103.5611
Slight difference in temperature dependence of
the quasi-particle masses for two lattice QCD
actions
Important is the general trend just above Tc.
Quasi-particle masses increase and exceed the
constituent quark masses.
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Does this make sense near the QCD phase
transition ?A re-interpretation of the Polyakov
Loop calculation in lattice QCD
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A key concept for experimental signatures Color
transparency (P. Jain et al., Phys. Rep. 271, 67
(1996))

• Color transparency reduces (or eliminates) the
  interaction probability between color-neutral and
  colored objects.
• In the strictest sense only applicable to
  point-like configurations, i.e. directly produced
  color-neutral states from higher twist diagrams
  (Brodsky, Sickles).
• Kopeliovich showed that early produced
  color-neutral states (i.e. pre-hadrons) also have
  reduced interaction cross section.

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Experimental Signatures

• Reduction in pT broadening of final state due to
  color transparency in medium (verified in HERMES
  results)
• Medium modification of early produced resonances
  due to chiral restoration in medium (project at
  LHC, see next talk)
• Reduction of v2 at high pT due to early formation
  and color transparency (masked by loss of
  collectivity at high pT)
• Reduction in energy loss due to color
  transparency of color-neutral pre-hadrons in
  medium (evidence at RHIC and LHC)

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B/M ratio in AA can be attributed to
recombination or to color transparency (Sickles
Brodsky, PLB 668 (2008))
A directly (or early) produced proton
(color-neutral) will undergo almost no
rescattering, thus its high pT yield is enhanced
relative to later formed mesons.
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Nuclear suppression patterns in HI collisions
become more complex
Surprising particle dependence in RAA
(hadro-chemistry or flavor change) ? This is not
simple partonic energy loss. Early
hadronization or enhanced species dependent
gluon-splitting factors (Sapeta
Wiedemann) SW use a parameter in their
splitting probability that depends on the final
hadron mass (ad-hoc parametrization)
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Predictions for energy loss RB C. Markert (PLB
691, 208, 2010))
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Comparison to preliminary STAR data
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The latest evidence RAA at the LHC
ALICE, PLB 696, 30 (2011)
Shadowing quenching (P.Levai, arXiv1104.4162)
RAA is not constant at high pT. Extreme shadowing
and eloss pathlength dependence ?
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An explanation based on color-neutral states and
color transparencyKopeliovich et al.,
PRC83,021901(2011)

• The color-neutral state (in this case
• labeled color dipole) will form the
• earlier the higher the fractional
• momentum.
• No distinction between pre-hadrons
• of different flavor.
• All charged hadrons are based on
• the same color dipoles which traverse
• the medium and exhibit color
• transparency.

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Summary

• Hadronization in QCD is highly relevant to
  understand the evolution of the initial
  deconfined, chirally symmetric QCD phase.
• Studies of pT, width, mass broadening, nuclear
  suppression, yields and ratios of identified
  particles and resonances in the
  fragmentation/recombination region of their
  spectrum gives us a unique tool to answer these
  many decade old questions
• Is there local parton-hadron duality ?
• Is hadronization due to recombination or
  fragmentation ?
• Do color neutral objects form early and are they
  less likely to interact with the colored medium ?
• When does the hadronic wave function (or mass)
  form ?
• There is evidence at high pT that energy loss in
  medium is not featureless even for light quark
  particles.

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Measure signatures inside outside of jet cones

• at sufficiently high fractional momentum (pT
  3-10 GeV/c)
• baryon / meson ratios
• rare particle species
• (s,c,b)
• resonances
• pT broadening
• energy loss

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