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Electromagnetic Probes at RHIC. Stefan Bathe UC Riverside. ETD-HIC07, Montreal, July 19, 2007. Em radiation: a probe of the qgp. Electromagnetic Radiation: A Probe of the QGP. EM radiation not strongly interacting Once produced, leaves collision region unscathed

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Electromagnetic Probes at RHIC

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Electromagnetic probes at rhic

Electromagnetic Probes at RHIC

Stefan BatheUC Riverside

ETD-HIC07, Montreal, July 19, 2007


Em radiation a probe of the qgp

Em radiation: a probe of the qgp

Electromagnetic Radiation:

A Probe of the QGP

  • EM radiation not strongly interacting

    • Once produced, leaves collision region unscathed

    • Carries information about early times

    • Serves as baseline for other probes

Stefan Bathe


Mass landscape of emr

g

p0

e+

g*

e-

Mass Landscape of EMRad

Mass landscape of EMR

  • p0 and h Dalitz decay

    • thermal radiation from real and virtual photons

  • light vector mesons and low-mass continuum

    • mass shifts, broadening, excess yield?

  • open heavy flavor

    • thermal radiation

    • medium modification

  • quarkonia

    • medium modification

Quarkonia: topic of its own

schematic dilepton mass distribution

Di-leptons via charged particle tracking & PID

Stefan Bathe


Pt landscape of emr

pT landscape of EMR

pT Landscape of EMRad

Dileptons and direct photons:

  • two topics

  • linked at low mass

    • physics (thermal radiation)

    • measurement (via dileptons)

  • Thermal radiation from the medium

    • Temperature of QGP

  • Medium response to jets

    • Jet photons, induced bremsstrahlung

  • Prompt photons

    • Baseline for medium modification of jets

Ch. Gale, QM05

Direct photons with EMCal

Thermal photons also by internal/external conversion!

Stefan Bathe


1 dileptons

1. Dileptons

Stefan Bathe


Chiral symmetry restoration

Chiral Symmetry Restoration

Chiral symmetry restoration

  • One of two fundamental properties of QGP (besides deconfinement)

  • Low-mass dileptons most sensitive probe

  • Primarily through r meson decays

    • t=1.3 fm/c (fireball 10 fm/c)

    • Most decays in medium

Low-mass excess at SPS:

observed by CERES (Pb+Au)

confirmed by NA60 in (In+In)

NA60

Nucl. Phys. A774, 43 (2006)

Phys. Rev. Lett. 96, 162302 (2006)

Stefan Bathe


Thermal radiation

R. Shahoyan, QM06

NA60, In+In

centralcollisions

R. Shahoyan, QM06

M (GeV/c2)

Thermal radiation

Thermal Radiation

  • Promise: initial temperature of QGP

Intermediate-mass excess at SPS:

observed by NA50 (Pb+Pb)

Shown to be prompt by NA60 (In+In)

Stefan Bathe


Raw mass distribution

Raw Mass Distribution

submitted to Phys. Rev. Lett

arXiv:0706.3034

Raw mass distribution

Converter run

  • bg 2.5x larger

  • spectra consistent

Systematic error

0.25% B/S (bg substraction)

9% S (cross pair subtraction,

below 600 MeV/c2)

Continuum, w, f, J/y visible

Au+Au

200 GeV

Small S/B

w

f

J/y

Run-4

(improved analysis compared to previous preliminary result)

Stefan Bathe


Invariant mass distribution

submitted to Phys. Rev. Lett

arXiv:0706.3034

Invariant mass distribution

Invariant Mass Distribution

  • Low-Mass Continuum

    • enhancement in 150 <mee<750 MeV

  • Intermediate-Mass

    • Data consistent with Pythia

    • PYTHIA single e- softer for p+p, but coincide for Au+Au

    • Angular correlations expected to weaken in Au+Au

    • Room for thermal contribution

Stefan Bathe


Low mass comparison with theory

submitted to Phys. Rev. Lett

arXiv:0706.3034

Low-mass: Comparison with theory

Low mass: comparison with theory

Broad-range enhancement:150 < mee < 750 MeV3.4±0.2(stat.) ±1.3(syst.)±0.7(model)

Include chiral symmetry restoration

and thermal radiation

R.Rapp, Phys.Lett. B 473 (2000)

R.Rapp, Phys.Rev.C 63 (2001)

R.Rapp, nucl-th/0204003

Stefan Bathe


Centrality dependence

submitted to Phys. Rev. Lett

arXiv:0706.3034

Centrality Dependence

Centrality dependence

  • p0 production ~ Npart

  • If in-medium enhancement from pp or qq annihilation

    yield should increase > ~ Npart

Low mass

charm follows binary scaling yield should increase ~ Ncoll

Intermediate mass

peripheral

central

Stefan Bathe


Invariant mass distribution in p p

Invariant mass distribution in p+p

Invariant Mass Distribution in p+p

Good agreement with hadronic cocktail except for intermediate-mass region

N.B.: PYTHIA known to be softer than p+p datafrom single e- analysis

Stefan Bathe


Electromagnetic probes at rhic

p+p – Au+Au comparison

First evidence for radiation from

early stage of collision!

(measured p+p reference)

p+p normalized to mee<100 MeV/c2

p+p and Au+Au normalized to p0 region

Agreement at resonances (w, f)

Au+Au enhancement for 0.2 < mee < 0.8 GeV

Agreement in intermediate mass and J/ just for ‘coincidence’(J/ happens to scale as p0 due to scaling with Ncoll + suppression)

arXiv:0706.3034)

Stefan Bathe


The future hbd

signal electron

Cherenkov

blobs

e-

partner positron

needed for rejection

e+

qpair

opening angle

~ 1 m

The future: HBD

The Future: Hadron Blind Detector

  • Dalitz & Conversion rejection via opening angle

    • Identify electrons in field free region

    • Veto signal electrons with partner

projection, 10 B evtents

PHENIX from Run-7 on

Stefan Bathe


2 direct photons

2. Direct Photons

Stefan Bathe


The promise and the peril

The promise and the peril

The promise and the peril

Promises (two of them)

  • Initial temperature, transition temperature via thermal photons

  • Jet energy scale, precise energy loss via g-jet correlations

Peril

  • Richer probe than originally thought (medium-induced photons)

  • Old idea of unfolding photon sources (starting from the clearly- understood high-pT spectra and moving down in pT) seriously challenged

Stefan Bathe


Photon sources in a a

pQCD or prompt photons

(as N+N, but modified)

Non-thermal

thermal

Hard+thermal

Pre-equili-brium photons

QGP

Hadron gas

Initial hardscattering

from medium

Interaction of hardparton with QGP

1) and 2) Medium induced photon bremsstrahlung

Photon sources in A+A

Photon Sources in A+A

Photons in A+A

Direct Photons

Decay Photons

Stefan Bathe


Electromagnetic probes at rhic

Disentangling Sources

Disentangling sources

<pT> vs. creation time

in principle: working one’s way down

from the highest pT (and excluding hadron decays)

one might be able to disentangle different sources

hard

scatt.

pT

(GeV)

jet

brems.

jet-thermal

sQGP

hadron

decays

hadron

gas

log t

1

10

107

(fm/c)

Stefan Bathe


Electromagnetic probes at rhic

Actual calculation

Actual calculations

C. Gale, NPA 785, 93c (2007)

Very difficult to disentangle sources

Stefan Bathe


Direct photon elliptic flow

Direct photon elliptic flow

Direct Photon Elliptic Flow

  • Jet-photons and induced photon bremsstrahlung: out-of-plane, negative v2

  • Jet-quenching  less fragmentation photons: in-plane, positive v2

Elliptic flow helps to disentangle various contributions

Stefan Bathe


Disentangling the contributions

3

1

2

4

Disentangling++

Disentangling the contributions

contributionsofterv2isol.

1 compton, annihi.=0yes

2 jet-thermal<0yes

3 fragmentation>0no

4 induced. brems.<0no

v2 and isolation help to

disentangle contributions

Gale, NPA 785, 93c (2007)

Stefan Bathe


P p from run 5

While data and pQCD calculation consistent, data tend to be higher than pQCD by at least 20%, increasing to low and high pT

Data

Fit

P+p from Run-5

p+p from Run-5

Stefan Bathe


Direct photon raa

Direct Photon RAA

Direct photon RAA

RAA with pQCD reference

RAA with data reference

RAA with p+p data

First direct photon RAA using p+p data as reference

Stefan Bathe


Comparison to pi0

Comparison to p0

Comparison to pi0

Direct photons

and p0 touch at

highest pT

Direct g RAA with measured

p+p reference data

g

hp0

  • Still consistent with

    • final state effect for p0 and initial state effect for direct photons

    • Modification of structure function?

      • different for p0 and direct photons

Stefan Bathe


Theory comparison i ii

Theory comparison I, II

Theory comparison I, II

  • Turbide et al.

    • Jet photons + pQCD + thermal

    • AMY formalism for jet-quenching of fragmentation photons

    • Data systematically below theory

    • Phys. Rev. C72 (2005) 014906 + private communication

  • F. Arleo

    • pQCD photons only

    • High-pT suppression due to isospin effect, shadowing, and energy loss

    • BDMPS for jet-quenching

    • JHEP 0609 (2006) 015

Stefan Bathe


Theory comparison iii

Theory comparison III

Theory comparison III

  • B.G. Zakharov,

    • Bremsstrahlung photons through bulk matter

    • JETP Lett. 80 (2004) 1

Stefan Bathe


Is the suppression real

Is the suppression real?

Can suppression be confirmed?

  • By going to smaller collision energy (200 GeV->62 GeV), modification of structure function can be tested at smaller pT (24 GeV->~7.5 GeV)

  • Advantage: independent detector systematics (no cluster merging)

  • Caveat: other direct photons sources may come into play

(Run5) p+p data” to NLO pQCD

Au+Au, 62.4 GeV

minimum bias

  • Result still suffers from large uncertainties

  • Needs higher-statistics run and p+p measurement at same energy

  • Ideally structure function measurement in p+A

  • Other tests

    • PbGl (PbGl better spatial resolution than PbSc)

    • RxNP (shadowing should be RxNP independent)

Stefan Bathe


Towards thermal photons conversion measurements

Towards thermal photons:

conversion measurements

Towards thermal photons: conversion measurements

Confirmation from external conversion

Internal conversion in Au+Au

Needs p+p reference measurement

Almost there (see dilepton result)

Internal conversion in d+Au

Still too large uncertainties to draw a conclusion

Stefan Bathe


Photon tagged jets

Photon-tagged jets

Photon-tagged Jets

Trigger particle

Because of energy loss, di-jet measurement suffers from surface bias

No trigger surface bias for direct photon tagged jets

Direct photon is measure of jet energy

di-jet

Hadrons

Associated particle

g

g-jet

More on this in Saskia’s talk

Hadrons

Stefan Bathe


Summary and outlook

Summary and Outlook

  • Electromagnetic radiation in HI collisions comprises two subfields

    • Dileptons

    • Direct photons

  • Coupled at low mass by physics and measurement techniques

  • PHENIX dielectron measurement in Au+Au (combined w/ baseline p+p) provides first evidence for early-time radiation at RHIC

  • Interpretation of direct photon measurements challanged by previously non-thought-of sources (medium modifications)

  • New measurements (conversion, v2, isolation) combined with future high-luminosity runs and detector upgrades will help to solve the puzzle

  • STAR has entered the game, healthy competition for PHENIX

Stefan Bathe


Extra slides

Extra Slides

Stefan Bathe


Gg hbt

gg HBT

D1

h/R

Dp

2

d

R

1

D2

L

Dp

The Hanbury-Brown-Twiss method of photon interferometry works from stars to nuclei!

3/2

1+f 2/2

f

1

Stefan Bathe

Dp


Two photon correlations

Two-photon correlations

e+

g2

g2rec

g1

g1

e-

External conversion:

  • No cluster interference

  • No hadron contamination

C2 calculated from EMCal only and

conversion+EMCal agree =>

detector effects under control

Stefan Bathe


G jet correlations

Inclusive g-h

Decay g-h contribution

(via p0-hadron)

Direct g-h !

g-Jet Correlations

p+p collisions at 200 GeV

Stefan Bathe


Comparison to pythia

Comparison to Pythia

Stefan Bathe


G jet correlations in auau

g-Jet Correlations in AuAu

Stefan Bathe


Electromagnetic probes at rhic

PHENIX Cu+Cu – jet E-scale

systematic from Rg

systematic from subtraction method

Stefan Bathe


Electromagnetic probes at rhic

The Spectrum

Compare to NLO pQCD

  • L.E.Gordon and W. Vogelsang

  • Phys. Rev. D48, 3136 (1993)

  • above (questionable) pQCD

Compare to thermal model

  • D. d’Enterria, D. Perresounko

  • nucl-th/0503054

2+1 hydro

T0ave=360 MeV(T0max=570 MeV)

t0=0.15 fm/c

  • data above thermal at high pT

Compare to thermal + pQCD

  • data consistent with thermal + pQCD

  • Needs confirmation from p+p measurement

Stefan Bathe


Internal conversion d au

Internal conversion: d+Au

Fit measured mass spectrum with function:

a - absolute normalization

b – fraction of direct photons:

Stefan Bathe


Electromagnetic probes at rhic

p+p–Au+Au comparison

both normalized to mee<100 MeV/c2

p+p multiplied by Npart/2

p+p multiplied by Ncoll

p+p and Au+Au normalized to p0 region

Agreement at resonances (w, f)

Au+Au enhancement for 0.2- mee < 0.8 GeV

Agreement in intermediate mass and J/ just for ‘coincidence’(J/ happens to scale as p0 due to scaling with Ncoll + suppression)

arXiv:0706.3034)

Stefan Bathe


Direct photon r aa

RAA with pQCD reference

RAA with p+p data

Direct Photon RAA

First direct photon RAA using p+p data as reference

Stefan Bathe


First g jet results

First g-jet results

Ettrigger from 8.5GeV/c to 10.5 GeV

  • Both STAR and PHENIX have first g-jet results

  • So far more proof-of-principle than precise measurements

  • Need higher-statistics run

Stefan Bathe


Measurement

Measurement

measurement

g

  • 3: Isolation

    • Can be applied in p+p and very-high-pT Au+Au

  • Large background from decay photons

  • Three methods

    • 1: Statistical

      • Measure inclusive photons

      • Measure main sources of decay photons (p0, h)

      • Calculate spectrum of decay photons from measurement

      • Subtract decay photons from inclusive photons

      • applied at all pT

    • 2: Conversion

      • similar to dilepton measurement

      • Low pT only

p0

g

Leading Particle

Hadrons

frag.

g

q

Direct g

Stefan Bathe


Electromagnetic probes at rhic

Current v2 results

Future improvements: more statistics, RxNP detector, internal conversion

Stefan Bathe


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