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)