News from RHIC: Expect the Unexpected

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News from RHIC Expect the Unexpected!
Barbara Jacak Stony Brook August 30, 2007
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outline

• Heat nuclei expect a plasma of quarks and gluons
• Study it experimentally
• expected QGP signatures
• useful probes of the plasma
• pp is a crucial benchmark defines the expected
• Surprise! evidence for strong coupling in QGP
• it is very opaque (jet quenching)
• it is a (perfect) thermalized fluid
• it catches even the heavy quarks
• collective response to deposited energy
• Unexpected things to expect at the LHC

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QCD predicts a phase transition

• gluons carry color charge ? gluons interact among
  themselves
• theory is non-abelian
• curious properties at large distance
• confinement of quarks in hadrons

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non-perturbative QCD lattice gauge theory
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plasma

• ionized gas which is macroscopically neutral
• exhibits collective effects
• interactions among charges of multiple particles
• spreads charge out into characteristic (Debye)
  length, lD
• multiple particles inside this length
• they screen each other
• plasma size gt lD
• weakly or strongly coupled
• depends on density (number of screeners)
• can behave like a liquid or crystal if strongly
  coupled
• normal plasmas are electromagnetic (e ions)
• quark-gluon plasma interacts via strong
  interaction
• color forces rather than EM, exchange g not g
• non-abelian plasma

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probing a heavy ion collision
we focus on mid-rapidity (y0) y1/2
ln(EpL)/(E-pL) CM of colliding system
90 in the lab at collider
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RHIC at Brookhaven National Laboratory
Collide Au Au ions for maximum volume ?s 200
GeV/nucleon pair, pp and dA to compare
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The Tools
STAR specialty large acceptance measurement of
hadrons
PHENIX specialty rare probes, leptons, and
photons
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events viewed by the 4 experiments
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Expected signatures of quark gluon plasma

• enhanced strange quark production
• if T ms, g g ? s s
• J/y suppression
• color screening breaks up cc pairs
• phase transition signatures
• if first order plateau in T vs. energy density,
  long particle emission time
• if second order critical fluctuations in
  particles
• copious thermal radiation
• chiral symmetry restoration
• jet quenching

NB These were yes/no questions, subject to
complicated S/B. The unexpected lurks in the
properties of the plasma!
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a better waylook at radiated probe
particles

• as a function of transverse momentum
• pT p sin q (with respect to beam direction)
• 90 is where the action is (max T, r)
• pL midway between the two beams!
• pT lt 1.5 GeV/c
• thermal particles
• radiated from bulk of the medium
• internal plasma probes
• pT gt 3 GeV/c
• jets (hard scattered q or g)
• heavy quarks, direct photons
• pQCD production process
• produced early?external probe

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benchmark probes in pp collisions
QCD works at RHIC can use perturbation theory
for high p transfer processes understand initial
hard interactions in AuAu scattering of q, g
inside N pion formation in jets from q,g
fragmentation (known phenomenologically)
p0
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jet fragments in AuAu vs. pp
in central collisions jets are quenched by the
plasma
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(colored) q g lose energy, photons dont
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jet quenching was expected
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Its the experimenters job to ask pesty
questions!

• So, what happens to heavy quarks traversing QGP?
• Prediction much less energy loss
• large quark mass reduces phase space for radiated
  gluons
• Measure via semi-leptonic decays

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finding c,b decays in single electron spectrum
compare data to cocktail of hadronic decays
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surprise 2 heavy quarks DO lose energy!
BUT what about e from B meson decays??!! also
collisions increase gluon u,d quark Eloss
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collective flows in heavy ion collisions
momentum space
dN/df 1 2 v2(pT) cos (2f) elliptic
flow
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v2 is large reproduced by hydrodynamics

• large pressure buildup
• anisotropy ? happens fast
• fast equilibration!

Surprise 3 must use viscosity 0 perfect
liquid (D. Teaney, PRC68, 2003)
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Elliptic flow scales with number of quarks
transverse KE
implication quarks, not hadrons, are the
relevant degrees of freedom when the pressure is
built up
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hydrodynamic flow of quarks!

• long expected, works at RHIC as long as
• viscosity per particle (h/S) vanishingly small
• initial condition thermalized system in lt 0.6
  fm/c
• use QGP equation of state for first few fm/c
• NB learning properties by constraining
  hydrodynamical calculation with data is standard
  in plasma physics!

Surprise 4 HOW can system thermalize in lt0.6
fm/c??? parton scattering insufficient (with
pQCD cross section)
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Strong coupling suggests applying methods of
string theory to QGP
slide R. Granier de Cassagnac
Maldacena
Policastro,Son, Starinets
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AdS/CFT also used to calculate energy loss

• Gubser and collaborators
• Liu, Rajagopal and Wiedemann
• Teaney and Casalderrey-Solana
• Surprise 5 the matter is so strongly coupled
  that string theorists can have fun with it

? large drag force from plasma on heavy quark!
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Does experiment indicate thermalization?

• For all distributions described by temperature T
  and (baryon) chemical potential m
• dn e -(E-m)/T d3p

Tf 175 MeV
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Strange quarks are indeed enhanced over pp
as expected for a hot equilibrated system!
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do the heavy quarks equilibrate??

• This is like putting a rock in a stream
• and watching if the stream
• can drag it along
• Rate of equilibration gives
• information on the viscosity
• of the liquid!

analogy from J. Nagle
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challenge to models of energy loss!
nucl-ex/0611018 accepted in PRL
Radiative energy loss alone fails to reproduce
v2HF. Heavy quark transport model has better
agreement with both RAA and v2HF. Small
relaxation time t or diffusion coefficient DHQ
inferred for charm.
D 1/3 ltvgt lmfp ltvgt/ 3rs D ?
collision time ? relaxation time
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transport diffusion is related to viscosity

• diffusion brownian motion of particles
• definition flux density of particles J -D
  grad n
• integrating over forward hemisphere
• D diffusivity 1/3 ltvgt l
• so D ltvgt/ 3ns
• D ? collision time, determines relaxation time
• data say it is small!

particle concentration
l mean free path
note viscosity is ability to transport
momentum h 1/3 r ltvgt l so D h/r
h/S ? measure D get h! heavy quark Langevin
model ? D few times 1/(4p) NB h/S
1/(4p) from string ?QCD correspondence
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minimum h at phase boundary?
quark gluon plasma
Csernai, Kapusta McLerran PRL97, 152303 (2006)
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Color screening? the famous J/y suppression

• J/Y (bound state of c and cbar quarks)
• Tests color screening length (lD)
• do bound c c survive the medium?
• or does QGP screening kill them?

note its not so clear what to expect other
observables strong coupling many collisions
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screening length onium spectroscopy
suppression at RHIC very similar to that at
SPS! why?? more suppressed at y? 0
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what does non-perturbative QCD say?
Hatsuda, et al.
Lattice QCD shows heavy qq correlations at T gt
Tc, also implying that interactions are not
zero Big debate ongoing whether these are
resonant states, or merely some
interactions Color screening yes! but not
fully Some J/y may emerge intact
J/y is a mystery at the moment!
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are J/ys regenerated late in the collision?

• c c coalesce at freezeout ? J/y

R. Rapp et al.PRL 92, 212301 (2004) R. Thews et
al, Eur. Phys. J C43, 97 (2005) Yan, Zhuang, Xu,
PRL97, 232301 (2006) Bratkovskaya et al., PRC
69, 054903 (2004) A. Andronic et al., NPA789,
334 (2007)
J/y is a mystery at the moment!
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so far we have seen

• medium is very opaque to light quarks and gluons
• to heavy quarks too
• system thermalizes very quickly
• collective hydrodynamic behavior
• vanishingly small viscosity
• i.e. doesnt support shear stress
• large cross sections strong coupling
• forget about perturbation theory (!)
• J/y suppression does not follow the energy density

what happens to the energy deposited in the
plasma?
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how does the medium respond?

• to study use hadron pairs
• high pT trigger to tag hard scattering
• second particle to probe the medium

collective flow in underlying event
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at high momentum, jets punch through
central collisions
Phys.Rev.Lett. 97 (2006) 162301
STAR
on away side same distribution of particles as
in pp but 5 times fewer!
X
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Surprise 7 particles at lower pT look funny
1 lt pT,a lt 2.5 lt pT,t lt4 GeV/c PHENIX
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lost energy excites a sound (density) wave?
FROM DATA PEAK LOCATION ?? /-1.231.91,4.37 ?
cs 0.33 (v0.33 in QGP, 0.2 in hadron gas)
UNEXPECTED! IS IT RIGHT? relative excitation of
sound and diffusion modes under intense
study data sound mode very dominant
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The unexpected at RHIC

• the quark gluon plasma is strongly coupled
• plasma is very opaque to fast quarks gluons
• also very opaque to heavy quarks
• viscosity is vanishingly small
• thermalization takes lt 0.6 fm/c
• can apply methods of string theory for QGP
  properties
• partons seem to leave wakes in the plasma
• MYSTERIES
• what is the fate of B quarks in the plasma?
• how can the system thermalize so fast?
• interaction mechanism for heavy quarks?
• whats going on with J/y and color screening?

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For plasma at the LHC

• Initial temperature is higher
• RHIC 2Tc
• LHC should be twice that at RHIC
• Coupling likely to still be strong!
• but plasma lifetime longer
• probe S/B differs (? need systematic
  measurements)
• System will cool through same temperature as
  RHIC!
• observables could be the same or different
• Must go measure and find out!
• dont quite know what to expect

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How hot is the medium, anyway?

• at RHIC Tinit 1.5 2 Tc ( 300 MeV)
• indirect! flow, energy loss constrain initial
  conditions
• will measure T via radiation of g g ? ee-

real photons inclusive/hadron decays
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di-electrons in AuAu and pp
pp normalized to meelt100 MeV/c2

• pp and AuAu normalized to p0 region
• Agreement at resonances (w, f)
• AuAu enhancement for 0.2 lt mee lt 0.8 GeV
• Agreement in intermediate mass and J/? just for
  coincidence(J/? happens to scale as p0 due to
  scaling with Ncoll suppression)

arXiv0706.3034)
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Low-mass Comparison with theory
A. Toia, S. Bathe
Broad-range enhancement150 lt mee lt 750
MeV3.40.2(stat.) 1.3(syst.)0.7(model)
Includes chiral symmetry restoration
calculations for min bias, QGP thermal radiation
included
R.Rapp, Phys.Lett. B 473 (2000) R.Rapp,
Phys.Rev.C 63 (2001) R.Rapp, nucl/th/0204003
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low mass dilepton excess at RHIC
yield excess grows faster than Npart excess gt r
modification
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Spin of the proton the surprises continue!
arXiv 0704.3599 (accepted for publication)
?
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must examine lower x
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baryon puzzle
baryons enhanced for 1.5 lt pT lt 5 GeV/c
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excess continues to 5 GeV/c
STAR
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formation of baryons coalescence of quarks
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• backup slides

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Are back-to-back jets there in dAu?
Yes! no medium ? no jet quenching
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At CERN (vs 17 GeV)

• NA50 and NA60 show suppression in PbPb InIn
• suppression follows system size
• Normal nuclear absorption from pA data ?
  4.180.35 mb

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Is the energy density high enough?
PRL87, 052301 (2001)
Colliding system expands

• e ? 5.5 GeV/fm3 (200 GeV AuAu)
• well above predicted transition!

value is lower limit longitudinal expansion
rate, formation time overestimated
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Locate RHIC on phase diagram
From fit of yields vs. mass (grand canonical
ensemble) Tch 176 MeV mB 41 MeV These are
the conditions when hadrons stop interacting
T
Observed particles freeze out at/near the
deconfinement boundary!
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p0 suppressed to high pT, direct g at very high
pT?
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The acid test for quark scaling
PHENIX
f meson (bound state of ssbar) mass mproton
but flows like the mesons
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plasma basics Debye screening

• distance over which the influence of an
  individual charged particle is felt by the other
  particles in the plasma
• charged particles arrange themselves so as to
  effectively shield any electrostatic fields
  within a distance of order lD
• lD e0kT
• -------
• nee2
• Debye sphere sphere with radius lD
• number electrons inside Debye sphere is large
• ND N/VD rVD VD 4/3 p lD3

1/2
ne number density e charge
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plasma frequency and oscillations

• instantaneous disturbance of a plasma ?
  collective motions
• plasma wants to restore the original charge
  neutrality
• electrons oscillate collectively around the
  (heavy) ions
• characterized by natural oscillation frequency
• plasma frequency
• its typically high
• restoring force
• ion-electron coulomb attraction
• damping happens via collisions
• if e-ion collision frequency lt electron plasma
  frequency wpe/2p
• then oscillations are only slightly damped
• a plasma condition electron collision time large
  vs. oscillation