Photons and Dileptons at LHC Rainer Fries Texas A&M University & RIKEN BNL Heavy Ion Collisions at the LHC: Last Call for Predictions CERN, June 1, 2007

Photons and Dileptons at LHC

Rainer FriesTexas A&M University & RIKEN BNL

Heavy Ion Collisions at the LHC: Last Call for Predictions CERN, June 1, 2007

LHC Predictions: Photons & Dileptons 2 Rainer Fries

Outline

Electromagnetic Probes

Direct Photons & Dileptons

Jet-Medium Photons; Elliptic Flow

Calculations for RHIC and LHC

Work* done by S. Turbide, C. Gale, D. K. Srivastava and R. J. Fries

*based on earlier work involving B. Muller, G. Moore, S. Jeon



PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

Evolution of High Energy Nuclear Collisions

Measuring bulk hadrons: Information about the fireball at chemical/thermal freeze-out Extrapolate back to much earlier times. Biased by models?

Measuring jets/hadrons from jets: Information about the first 4-6 fm/c. Information about dNg/dy, cs (?), maybe more Information indirect = multiple integrations involved.

QGP



PCM & clust. hadronization

NFD

NFD & hadronic TM

PCM & hadronic TM

CYM & LGT

string & hadronic TM

Evolution of High Energy Nuclear Collisions

Photons and dileptons (virtual photons) Penetrating probes, some from very early times. Direct + background

Sensitive to temperature (or temperature history) Approximate hierarchy: larger PT or M ~ larger

sensitivity to early times


Photons at high PT

Prompt photons from initial hard processes e.g. initial Compton q + g q + No final state effects at all.

Fragmentation/vacuum bremsstrahlung Possible large contribution from q, g + X Sensitivity to medium effects in the final state ~ similar to

hadrons

For both: sensitivity to shadowing + initial state nuclear effects Needs to be studied in detail in p+A; still not done at RHIC. Definition of RAA ?


Photons at Low and Intermediate PT

Thermal/bulk photons (QGP + hadronic phase) Measure temperature T Difficult measurement, need to subtract background at

low PT

Photons from jet-medium interactions [RJF, Müller, Srivastava; Zakharov]

Jet-photon conversion

Induced photon bremsstrahlung


Photons from Jet-Medium Interactions

Mechanism is novel but very natural RHIC: jets do interact with the medium strongly

But: no unambiguous signature for these photons so far. YAPS: yet another photon source

Rates sensitive to fireball physics Temperature measurement with GeV Photons? Sensitivity to energy loss: different integral over fireball

history Complementary information, but how feasible are

measurements?


Jet-Photon Conversion

Annihilation and Compton scattering in the fireball

Cross sections forward/backward peaked: “Conversion”: very effective process.

Yields approximately proportional to the jet distributions Sensitivity to early time jet distributions E.g. annihilation:

2

2

8

9s qed u t

dt s t u

2

23s qed u s

dt s s u

andq qp p p p

( )

4 3 61

23 ( )

16

2(2 )

( 4 )(

(

)[1 ( )] ( ) ( )2

)f

q

NA

q

Agq

EdNEd xd p

s s md p

f p

f p f p s q qE E


Jet-Photon Conversion

How bright are these new sources? First estimate:

As important as other direct photon sources for PT ~ 4-6 GeV at RHIC energies, even more important at LHC.

PT slope steeper than initial hard photons.

FMS PRL 90 (2003)


Elliptic Flow

Jet-medium photons show azimuthal anisotropy Hadrons: longer path leads to more suppression Conversion photons: longer path leads to increased production!

Prediction: conversion photons have negative v2!

Other photon sources: Initial hard processes: v2 = 0

Fragmentation: v2 > 0 (like hadrons from jets)

Thermal photons: v2 > 0 ( U. Heinz talk)

Hadrons

Conversion Photons


Elliptic Flow

Bad news: No clear signal at RHIC Strong dependence on details of the calculation

Good news: sensitive to initial conditions ??

PHENIX

Turbide, Gale, Fries


Dileptons

Very similar for dileptons

Same sources: Thermal emission

Jet-induced emission

Initial hard scattering (Drell-Yan)

Background from charm & bottom


Techniques

LO prompt photon and jets with K = 1.5 at LHC

NLO Drell-Yan

EKS shadowing included.

Jet-photon conversion Define fjet x fthermal


Energy Loss: AMY

AMY formalism [Arnold, Moore, Yaffe]

Rate equation for jet energy loss

Induced photon emission included.

One parameter: coupling s

s = 0.3 describes the 0 and photons at RHIC


Modeling & Parameters

Fireball: 1-dimensional Bjorken expansion & cooling

Tc = 160 MeV

Initial conditions: RHIC: temperature Ti = 370 MeV at i = 0.26 fm/c.

LHC: temperature Ti = 845 MeV at i = 0.088 fm/c.

Little dependence on i for i Ti3 ~ dN/dy held const.


Predictions


Photons

Photon spectra and RAA in central collisions.

S. Turbide, PRELIMINARY


Dileptons

Dilepton PT spectrum at small mass, central collisions.

[Turbide,Gale, Srivastava, RJF]


Dileptons

Dilepton mass spectrum at high PT, central collisions.

[Turbide,Gale, Srivastava, RJF]


Photon – Jet Correlations

Jet-medium photons change jet-photon correlations!

Away-side spectra of photons / jets triggered by 20 GeV jet / 15 GeV photons

Toy model c

alculation!


Summary

Electromagnetic probes at large and intermediate PT: new and complementary information about the fireball.

Jet-photon conversion photons/induced photon bremsstrahlung from the medium: important effect at LHC, has to be established.

Negative v2 for conversion, maybe observable

Jet-photon conversion might be important for photon-tagged jets.

Documents

Photons and Dileptons at LHC Rainer Fries Texas A&M University & RIKEN BNL Heavy Ion Collisions at the LHC: Last Call for Predictions CERN, June 1, 2007