# Presentation: 154769

## Electromagnetic radiation from nuclear collisions @ RHIC energies

- Outline:
- Electromagnetic production processes & what they reveal
- Hadronic and partonic sectors
- Characteristics of sources
- Comparison with RHIC data (photons)
- Conclusions

## The information carried by EM radiation

- i

- f

- k

- [photons]

- [dileptons]

- [photons]

- absorption

- emission

## The information carried by EM probes

- Emission rates:

- [photons]

- [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)

## 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!

## Electromagnetic radiation from QCD 

- First approaches

- McLerran, Toimela (1986); Kajantie, Kapusta, McLerran, Mekjian (1986)
- Baier, Pire, Schiff (1988); Altherr, Ruuskanen (1992)

- Rates diverge:

- HTL
- resummation

## AMY, Arnold, Moore, and Yaffe, JHEP 12, 009 (2001); JHEP 11, 057 (2001): incorporates LPM; photon rates complete to leading order in αs

- Going to two loops:  Aurenche, Kobes, Gelis, Petitgirard (1996)
- 		          Aurenche, Gelis, Kobes, Zaraket (1998)

- Co-linear singularities:

- 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

## 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

## RHIC: jet-quenching

- 	Azimuthal correlation:
- Shows the absence of “away-side” jet.

## Jet-quenching 

- Jet-quenching

- Dominant source of energy loss: medium-induced gluon
- bremsstrahlung? However, see later...

## Quenching = Jet-Plasma interaction. Does this have an EM signature?

- The plasma mediates a jet-photon conversion

- Fries, Mueller & Srivastava, PRL **90**, 132301 (2003)

## Sources of photons:

- Hard direct photons. pQCD with shadowing
- Non-thermal

- Fragmentation photons. pQCD with shadowing
- Non-thermal

-  Radiations thermal photons
- Thermal

-  Jet in-medium bremmstrahlung
- Thermal

-  Jet-plasma photons
- Thermal

## 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

## E loss/gain: some systematics

- Includes E gain
- Evolves the whole
-   distribution function

## Time-evolution of a parton distribution

## PHENIX data

- B. Cole, QM 05

## 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.*

## But: other signature of jet-photon conversion? 

- Jet-plasma photons will come out of the
- 	hadron-blind region. “Optical” v2 < 0

- Suggestion & high pT: Turbide, Gale, Fries PRL (2006)
- Low pT: Chatterjee *et al., *PRL (2006)
- All pT: Turbide *et al., *PRC (2008) *in press*

##   0      +             -                       -           +

- Photons from primordial interactions and fragmenting jets

- All photons (NN, frag, jet-photon conv., bremss., Th.)

-   0      +             -                       -           +

- Simple dynamics:

- Turbide, Gale, Fries PRL (2006)

## Data: Results from PHENIX

- v2: small! Consistent with zero (within errors)

- T. Sakaguchi
- RHIC/AGS 07

## 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

## 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

## Results: RAA

- Results: RAA

- The discrimination between models is dependent
- on the high pT photons

- See also F. Arleo, JHEP (2007)

## Results: v2

## Results: v2 sensitivity

- Good news: high pT photon v2 sensitive to
- details of initial conditions (geometric isotropy)

- Some additional resolution with correlation analyses:

- Jet bremsstrahlung/fragmentation correlated with hadrons
- Jet-plasma & thermal, uncorrelated

## Results: dileptons

- A thermal component is expected over the purely thermal radiation
-  Caveat: correlated charm not shown
- LHC dileptons: in progress

## New: Energy loss systematics in AMY with collisional energy loss (along with radiative).
See Guang-You Qin’s poster

- 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

## 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

## RHIC photons: estimates with a thermal model

- With E loss
-  LHC also done

- Turbide, Gale, Jeon, and Moore PRC (2004)

## The current-current correlator

- A model for the hadronic electromagnetic current: VMD

- The current-field identity

- (J. J. Sakurai)

- 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

## LHC photons estimates

- Turbide, Gale, Jeon, and Moore PRC (2004)
- See also T. Sakaguchi, this conference

## How big (small) is this?

- Phenomenological
- Exploration...

- Turbide, Rapp & Gale PRC (2004)