# Presentation: 947212

## Intermediate-mass lepton pairsin relativistic heavy-ion collisions

- Intermediate-mass lepton pairsin relativistic heavy-ion collisions

**Charles Gale**
**McGill**

## The physics of (very) hot and dense matter is best explored in the laboratory with relativistic nuclear collisions
Photons and dileptons are penetrating probes

- Photons and dileptons are penetrating probes

- Introduction and Outline

- New physics: collective, many-body effects
- Quark-gluon plasma
- In-medium modifications
- Modified spectral densities
- Chiral symmetry restoration
- Mixing effects
- Pion dispersion relation

## Outline, cont’nd

- Experiments (IM): DLS,  Helios, NA-38, -50, -60 CERES,WA98, HADES, PHENIX
- Electromagnetic radiation as a tool for hadronic tomography:
- Low mass dileptons
- Intermediate mass dileptons
- Low pT photons
- High pT photons
- High pT intermediate mass
- 	dileptons

- SPS

- RHIC
- LHC

- Sangyong Jeon’s talk

## Why? Investigation of the QCD phase diagram

- S. B. Ruster et al., PRD 72, 034004 (2005)

- F. Karsch, E. Laermann, hep-lat/0305025

## How? 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***

- McLerran, Toimela (85), Weldon (90), Gale, Kapusta (91)

## 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

## What is expected (dileptons)

- Low masses receive significant contribution from radiative decays
- High masses dominated by DY
- Intermediate mass region interesting from QGP perspective,
- 	(Shuryak (78), Shor (89))
- Photons: similar story, but featureless spectra
- Experiments: DLS, Helios, TAPS, NA38, -50, WA98, CERES, PHENIX, HADES, NA60

## Vector Meson Spectral Densities: A Sample Calculation I

- Ralf Rapp and Charles Gale, Phys. Rev. C **60**, 024003 (1999)

## The interaction is constrained by basic hadronic phenomenology

- Chiral, Massive Yang-Mills:
- O. Kaymakcalan, S. Rajeev, J. Schechter, PRD 30, 594 (1984)

- Parameters and form factors are constrained by
- hadronic phenomenology:
- Masses & strong decay widths
- Electromagnetic decay widths
- Other hadronic observables:
* **e.g.*

## Low Masses:Vector Meson Spectral Densities:Hot Meson Gas

- The spectral density is flattened
-  and broadened. Even more with
- baryons.

- Rapp, Gale (PRC, 99)

## NA60 Comparison of data to RW, BR and Vacuum 

- NA60 Comparison of data to RW, BR and Vacuum 

- pT dependence

- Sanja Damjanovic
- Quark Matter 05 (and
- this meeting)

**New!**

## Two approaches:

- H. Van Hess, R. Rapp, nucl-th/0603084

- T. Renk, J. Ruppert, hep-ph/0603110
- See J. Ruppert’s talk this aft.

- Many-body (in-medium) effects are observed!

## The intermediate mass sector: some background

- Direct connection to Hard Probes
- Off-shell effects are potentially important for effective hadronic interactions Gao & Gale, PRC **57**, 254 (1998)

- A. Shor, PLB 233, 231 (1989)

## On the dangers of extrapolations...

- (a) B. A. Li, Phys. Rev D **52**, 5165 (1995).
- (b) Gomm, Kaymakcalan, Schechter, Phys. Rev. D **30**, 2345 (1984).
- (c) Janssen, Holinde, Speth, Phys. Rev. C **49**, 2763 (1994).
- (d) Ko, Rudaz, Phys. Rev. D **50**, 6867 (1994).
- (e) Xiong, Shuryak, Brown, Phys. Rev. D **46**, 3798 (1992).

- Gao & Gale, PRC **57**, 254 (1998)

- But, a lot of data exists!...

## e+ e- Data: A Wealth of Information

- OLYA
- CMD
- DM-1(2)
- ARGUS
- M3N
-  

## I. Kvasnikova, C. Gale, and D. K. Srivastava, PRC 65, 064903 (2002)

- I. Kvasnikova, C. Gale, and D. K. Srivastava, PRC **65**, 064903 (2002)
- Z. Huang, PL **B361**, 131 (1995)

## A larger comparison

- Agreement across theoretical
-    models
- Those channels are mostly absent from the spectral densities used in comparisons with CERES and the new NA60 data.

## Intermediate mass data

- A. L. S. Angelis *et al. (*Helios 3)*, *Eur. Phys. J. (1998)

- R. Rapp & E. Shuryak, PLB (2000)

## NA50 Data (cont’nd)

- I. Kvasnikova, C. Gale, and
-  D. K. Srivastava, PRC 2002

- In agreement with multiplicity dependence
- Includes detector acceptance & efficiency
- (O. Drapier, NA50)

## NA60 IMR analysis: weighted offset fits (A. David’s talk)

- 

- or

- Fix Charm contribution to “world average” value

- Fix Charm contribution to NA50 p-A expected value

- Fit always requires ~2 times more Prompts

- Extract prompts by fixing Open Charm contribution

## Low and Intermediate masses: partial summary

- Thermal sources shine in the LMR and IMR. No great sensitivity to the QGP. Intermediate-mass excess is **not** charm enhancement!
- The new data is precise enough to consider a differentiation of space-time models
- DY? At low M, medium-enhanced multiple parton scatterings might be large (Qiu, Zhang (02), Fries, Schaefer, Stein, Mueller (00). pA measurement.)

## A recent analysis: (van Hees & Rapp, hep-ph/0603084)

- Sensitive to the space-time modeling:
- Still sensitive to the low-mass spectral densities

- Analyses should be redone with those

## Homework

- Unite (standardize?) space-time modeling [nD hydro, fireballs, transport approaches...].
- The power of the data is fully realized if a general-purpose acceptance filter exists. (On the way!...)
- Chiral symmetry? Still not obvious... An independent access to a1 spectral density is missing. More work on the theory side too. (Urban, Buballa, Hochsch. &  Wambach, PRL, 88, 042002 (2002))

## Jet-quenching 

- Jet-quenching

- Source of energy loss: medium-induced gluon
- Bremsstrahlung (+ elastic scattering?)

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

- The plasma mediates a jet-photon conversion

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

## Photon sources

- Hard direct photons
- Fragmentation
- Thermal photons from hot medium
- Jet-photon conversion
- Jet in-medium bremsstrahlung

## Dilepton sources

- Drell-Yan dileptons
- Thermal dileptons
- Jet-virtual photon conversion

## Energy loss in the jet-photon conversion? Jet bremsstrahlung?

- Use 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

- S. Jeon’s talk

## Jet characteristics:

## Time-evolution of quark distribution

## Jet-plasma dileptons?

- Same basic idea as photons, details are slightly different:
- High pT cut helps with background subtraction
- Go beyond LO, do a HTL analysis

## The thermal “baseline” calculation agrees with that of 

- The thermal “baseline” calculation agrees with that of
- M. Thoma, C. T. Traxler, Phys. Rev. D **56**, 198 (1997)

## Dileptons from jet-thermal interactions

- Jet-thermal as large as DY/heavy quark decay (RHIC)
- Jet-thermal still as large as DY. S/B could improve with harder pT cut.
- At RHIC, there is a  contribution to the IM region
- In-medium jet bremsstrahlung will also add to the signal
- No heavy quark energy loss
- Signal as large as it is for photons

- Turbide, Gale, Srivastava, Fries, PRC (2006)

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

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

- Turbide, Gale, Fries (PRL 06)

- Photons from primordial interactions and fragmenting jets

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

-   0      +             -                       -           +

- v2 from “Isolated photon cut” is negative
- Photons associated with a “crowded photon cut” show v2 with changing sign
- Same game can be played with dileptons (in progress)
- See talk by U. Heinz

- Similar story for the LHC

## Jet-plasma interactions: measurable EM signatures! These are fairly robust with respect to changing temperature & dynamics

- RHIC & LHC:
- Jet-plasma interactions: dilepton channel: signal competes with Drell-Yan (NLO)
- Towards a consistent treatment of jets & EM radiation

-  Intermediate-mass dileptons
- New data will put further stringent tests on models, both rates & space-time modeling

- Conclusions, part I

## Conclusions, part II

- Low and mass dileptons: NA60 data can distinguish between models. In-medium effects!
- RHIC/LHC: There are measurable electromagnetic signatures of jet-plasma interaction: those constitute complementary observables to signal the existence of conditions suitable for jet-quenching
- Photon v2: a revealing probe
- EM radiation and hard probes: the start of a beautiful friendship...
- EM radiation: change of paradigm

## Interested in the techniques discussed here?
Want to know more?

- Want to know more?

- Found where fine books are sold
- (July 2006)

- Chatterjee, Frodermann, Heinz, Srivastava, nucl-th/0511079

## Jet-plasma photons:

- S. Jeon’s talk

## New (preliminary) PHENIX Data

- New (preliminary) PHENIX Data

## Vector Meson Spectral Densities, II(adding baryons)

- R. Rapp & J.Wambach, 1999

## Soft photons @ RHIC:

- There is a window

- Turbide, Rapp & Gale PRC (2004)

## Same spectral densities:Low mass dileptons and real photons

- S. Turbide, R. Rapp, and C. Gale, PRC (2004)

- Same spectral densities as low mass dileptons
- Same dynamical model; same boundary conditions
- Cronin contribution estimated from pA data (E629, NA3)
- QGP: small

- Turbide, Rapp & Gale
- PRC 2004