Charles GaleMcGill
Electromagnetic radiation from nuclear Electromagnetic radiation from nuclear collisions @ RHIC energiescollisions @ RHIC energies
Outline:•Electromagnetic production processes & what they reveal
•Hadronic and partonic sectors•Characteristics of sources•Comparison with RHIC data (photons)•Conclusions
Charles GaleMcGill
The information carried by EM radiation
i
fk 2| |fi
fi
SR
V
4 ˆ| ( ) ( ) |fiS f d x J x A x i [photons]
4 4 ˆ| ( ) ( ) ( ) |efiS f d xd yJ x D x y J y i
[dileptons]
4(2 ) ( ) ( )2
ˆ ˆ| | | |
fi i f i f
gR P k P P k P
V
f J i i J f
[photons]
absorption emission
Charles GaleMcGill
The information carried by EM probes
3
3 3
1Im ( )
(2 ) 1Rd R g
kd k e
Emission rates:
[photons]
6 2
3 3 6 4
2 1 1Im ( )
(2 ) 1Rd R e
E E L kd p d p k e
[dileptons]
•The electromagnetic spectra will be direct probes of the in-medium photon self-energy•They are hard probes:
•EM signals as probes for hadronic tomography•Need a model for the dynamics of the HI collision
McLerran, Toimela (85), Weldon (90), Gale, Kapusta (91)
em 3%s
Charles GaleMcGill
Caution: not all dynamical models are the same…
• Microscopic transport models (UrQMD, HSD…)• Hydrodynamic models• Thermal fireball models
• Those differ in details (symmetry assumptions, chemical potentials, freezeout conditions, cross sections…)
• Need to be constrained by hadronic observables!
Charles GaleMcGill
Electromagnetic radiation from QCD
First approaches
McLerran, Toimela (1986); Kajantie, Kapusta, McLerran, Mekjian (1986)Baier, Pire, Schiff (1988); Altherr, Ruuskanen (1992)
Rates diverge: 2~ ln( / 0)s T q
HTLresummation
Charles GaleMcGill
Going to two loops: Aurenche, Kobes, Gelis, Petitgirard (1996) Aurenche, Gelis, Kobes, Zaraket (1998)
Co-linear singularities:2
22
~s sth
T
m
AMY, Arnold, Moore, and Yaffe, JHEP 12, 009 (2001); JHEP 11, 057 (2001): incorporates LPM; photon rates complete to leading order in αs
Can be expressed in terms of the solution to a linear integral equation
Charles GaleMcGill
Electromagnetic radiation (photons) from hadrons
•Details in Turbide, Rapp, Gale, PRC (2004)
•Same spectral densities as used for dileptons
•Low momentum radiation from thermal sources
Charles GaleMcGill
RHIC: jet-quenching
Azimuthal correlation:– Shows the absence of “away-side” jet.
Pedestal&flow subtracted
Charles GaleMcGill
hadrons
q
q
hadrons
leadingparticle
leading particle
Jet-quenching
hadrons
q
q
hadrons
leadingparticle suppressed
leading particle suppressed
Dominant source of energy loss: medium-induced gluonbremsstrahlung? However, see later…
Charles GaleMcGill
Quenching = Jet-Plasma interaction. Does this have an EM signature?
qg q
The plasma mediates a jet-photon conversion
Fries, Mueller & Srivastava, PRL 90, 132301 (2003)
Charles GaleMcGill
Sources of photons:
Hard direct photons. pQCD with shadowingNon-thermal
Fragmentation photons. pQCD with shadowingNon-thermal
Radiations thermal photonsThermal
Jet in-medium bremmstrahlungThermal
Jet-plasma photons Thermal
Charles GaleMcGill
A theoretical connection between jet energy loss and the electromagnetic emissivity
Use again the approach of Arnold, Moore, and Yaffe
JHEP 12, 009 (2001); JHEP 11, 057 (2001)
• Incorporates LPM
• Complete leading order in S
• Inclusive treatment of collinear enhancement, photon and gluon
emission
Can be expressed in terms of the solution to a linear integral equation
Charles GaleMcGill
E loss/gain: some systematics
( , )qqg k p k
( ) ( , ) ( , )( ) ( )
( , )2 ( )
q qq qg qg
q qk
gqq
g
dP p d p k k d p kP p k P p
dt dk dt dk dt
d p k kP p k
dk dt
( ) ( , ) ( , )( ) ( )
( , )( ) (2 )
q gg qg gg
q gk
g gqq gg
g
dP p d p k p d p k kP p k P p k
dt dk dt dk dt
d p k dP p k p
dk dt dk dt
•Includes E gain•Evolves the whole distribution function
Charles GaleMcGill
Time-evolution of a parton distribution
The entiredistribution isevolved by the collision Kernel(s)of the FP equation
Turbide, Gale, Jeon, and Moore (2004)
Charles GaleMcGill
PHENIX data2
2
/( )( )
( ) ( / ) /
A AT
AA T pp p pcoll coll inelastic
d N dp dYield per collisionR p
N Yield per pp collision N d d
B. Cole, QM 05
Charles GaleMcGill
Photons: establishing a baseline
QCD @ NLO, Aurenche et al., NPB 286, 553 (1987)See also Gordon & Vogelsang
Turbide, Gale, Frodermann, Heinz, PRC (2008) in press.
Charles GaleMcGill
But: other signature of jet-photon conversion?
• Jet-plasma photons will come out of the
hadron-blind region. “Optical” v2 < 0
1 2 cos2 n
nT T T T
dN dNv n
p dp d p dp
Suggestion & high pT: Turbide, Gale, Fries PRL (2006)Low pT: Chatterjee et al., PRL (2006)All pT: Turbide et al., PRC (2008) in press
Charles GaleMcGill
Photons from primordial interactions and fragmenting jets
All photons (NN, frag, jet-photon conv., bremss., Th.) 0 + - - +
Simple dynamics:Turbide, Gale, Fries PRL (2006)
Charles GaleMcGill
Data: Results from PHENIX
v2: small! Consistent with zero (within errors)
T. SakaguchiRHIC/AGS 07
Charles GaleMcGill
AZHYDRO (Heinz & Kolb)
(c.f. Quark Gluon Plasma III)
•Tc=164 MeV, =0.2 fm/c, Tfo=130 MeV•Good modeling of bulk dynamics•Small values of momentum anisotropies•Geometric anisotropy shrinks rapidly
Charles GaleMcGill
Results: Spectra
•Window for thermal effects at low to intermediate pT
•Same dynamical model as hadronic data
•NO sdditional parameters in the EM fits, over the hadronic fits
•The preliminary experimental data is being finalized
Charles GaleMcGill
Results: RAA
The discrimination between models is dependent on the high pT photonsSee also F. Arleo, JHEP (2007)
Charles GaleMcGill
Results: v2
Charles GaleMcGill
Results: v2 sensitivity
Good news: high pT photon v2 sensitive todetails of initial conditions (geometric isotropy)
Some additional resolution with correlation analyses:
•Jet bremsstrahlung/fragmentation correlated with hadrons•Jet-plasma & thermal, uncorrelated
Charles GaleMcGill
Results: dileptons
• A thermal component is expected over the purely thermal radiation
• Caveat: correlated charm not shown
• LHC dileptons: in progress
Charles GaleMcGill
New: Energy loss systematics in AMY with collisional energy loss (along with radiative).
See Guang-You Qin’s poster
What next?
G. Qin, J. Ruppert, C. Gale, S. Jeon, G. D. Moore, M. G. Mustafa, PRL (2008) in press.arXiv:0710.0605
There is (some) room to re-examine the effect on EM emission
Charles GaleMcGill
Electromagnetic signals @ RHIC: great results
• Important progress towards an inclusive treatment of EM radiation and hadronic observables (more work to do)
• Important progress towards an inclusive treatment of jet energy loss and EM emissivities (more work to do)
• Spectra and elliptic flow: compatible with data– v2: a sensitive probe– Hope of making more progress with (anti-)isolation cuts
• LHC: – Jet-plasma photon signal is also important
Charles GaleMcGill
Charles GaleMcGill
RHIC photons: estimates with a thermal model
• With E loss
• LHC also done
Turbide, Gale, Jeon, and Moore PRC (2004)
Charles GaleMcGill
The current-current correlatorA model for the hadronic electromagnetic current: VMD
2 2 2e e eJ m m m
g g g
The current-field identity(J. J. Sakurai)
Im Im ImVMD
TT TJ J D Spectral density
The photon/dilepton signal can tell us about the in-medium
spectral densities of vector mesons. Rates need to be integrated
over the space-time history, with some dynamical model
3 3( , ) Im ( , )RdR
E E C p p k Td p d p
D
2 2 2 2 2 2( , )
L TP P k kk T
k m F k m G m k
D
Charles GaleMcGill
LHC photons estimates
Turbide, Gale, Jeon, and Moore PRC (2004)See also T. Sakaguchi, this conference
Charles GaleMcGill
How big (small) is this?
PhenomenologicalExploration…
Turbide, Rapp & Gale PRC (2004)