Thermal Radiation Mapping the Space-Time Evolution

Thermal radiation from hadronic collisions: An old but still hot idea!Mapping the time evolution

Experimental results on “thermal” radiation State of the Art experiment: NA60Energy frontier: PHENIX

Future perspectivesPHENIX VTX and HBD upgradesLessons learned and future opportunities

Promise to help unravel time evolution

Time evolution not understood;Hints for new physics?

Unique possibilities for future

Axel Drees, WWND 2012, Dorado del Mar, Puerto Rico

Thermal Radiation from QGP

Axel Drees2

Axel Drees

Shuryak 1978: Birth of the Quark Gluon PlasmaData from 400 GeV p-A at FNAL

e+e- for high mass PRL 37 (1976) 1374m+m- for high mass PRL 38 (1977) 1331

m GeVmm

/d nb GeVdmmm

Shuryak PLB 78B (1978) 150

J/

e e -

m m -

Drell-Yan

QGPUltimately the wrong explanation, but this paper was landmark and kicked off the search for the QGP and its radiation!

p-A 400 GeV

Key lesson: Know your backgrounds!In particular charm and bottom!

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Xe

XeDDcc

-

Thermal Radiation from Expanding SourceRadiation from longitudinally and radially expanding fire ball in “local equilibrium”

Real and virtual photon momentum spectrum at mid rapidity:

Temperature informationIntegrated over space time evolutiondue to T4 dependence sensitive to early times

Collective expansionRadial expansion results in blue and red shift Longitudinal expansion results in red shift

Virtual photon mass spectrumTemperature informationNot sensitive to collective expansion

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Mass and momentum dependence allows to disentangle flow from temperature contributions!!

Planck spectrum: yield T4 , mean T boosted by collective motion

Production process: real or virtual photons (lepton pairs)

hadron gas: photons low mass lepton pairs

QGP: photons medium mass lepton pairs

Microscopic View of Thermal Radiation

q

q

e-

e+

g

p

r

p p

p

r*

g*

e-

e+

q

qg

g

Experimentally observed yield integrated over full time evolution!

Key issues:In medium modifications of mesons

pQCD base picture requires small as

But as can not be small for dNg/dy ~ 1000 (i.e. in a strongly coupled plasma)

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Equilibrium of strong interaction! Equilibrium not a necessary condition!

Experimental Issue: Isolate Thermal Radiation

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1 10 107 log t (fm/c)

g, g* from A+A

Direct

Hadron Decays“Prompt” hard scattering

Pre-equilibrium

Quark-Gluon Plasma

Hadron Gas

ThermalNon-thermal

Sensitive to space-time evolution

Need to subtract decay and prompt

contributions

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Map Out Time Evolution with Thermal RadiationExperimental method

Measure inclusive g and ll- Subtract experimental background

e.g. combinatorial pair backgroundSubtract “cocktail” of known sources,

i.e. hadron decays after freeze outIsolate thermal radiation

Real photons cocktail: p0gg, hgg, wp0g, ...More than 90% of photon yield

Lepton pair cocktail:p0gee-, hg, wp0ee- and direct decays r,w,f ee-, J/ ee- ...Semileptonic decays of heavy flavorDrell Yan Dileptons have mass remove contribution from p0

more sensitive to thermal radiation than photons

Dileptons are more sensitive than photons7

Xe

XeDDcc

-

NA60 featuresClassic muon spectrometer Precision silicon pixel vertex tracker

tagging of heavy flavor decay muonsReduction of combinatorial background by vetoing p, K decay muons

Double dipole for large acceptance (low mass)High rate capability

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State of the Art Measurements with NA60

2.5 T dipole magnet

beam tracker

vertex tracker

MuonOther

hadron absorber

muon trigger and tracking

target

magnetic field

Next slides mostly derived from talks given by Sanja Damjanovic

NA60 can isolate “thermal” contribution

Continuum Excess Measured by NA60

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Planck-like mass spectrum, falling exponentially

(T > 200 MeV)For m>mr good agreement with three models in shape and yield

Main Sources m < 1 GeVpp- r mm-

Sensitive to medium spectral function

Main sources m > 1 GeV qq mm- p a1 mm-

(Hess/Rapp approach)

Eur. Phys. J. C 59 (2009) 607; CERN Courier 11/2009

Evidence for thermal dilepton radiation

~ 1/m exp(-m/T)

200 MeV

300 MeV

Fully acceptance corrected

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Sensitivity to Spectral Function

Models for contributions from hot medium (mostly pp from hadronic phase)

Vacuum spectral functions Dropping mass scenariosBroadening of spectral function

Broadening of spectral functions clearly favored!

pp annihilation with medium modified r

works very well at SPS energies!

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Not acceptance corrected

Phys.Rev.Lett. 96 (2006) 162302

Hadronic contributions explored and exhausted

Dominance of partons for m>1GeVSchematic time evolution of collision at CERN energies

Partonic phaseearly emission: high T, low vT

Hadronic phaselate emission: low T, high vT

Experimental Data: thermal radiationMass < 1 GeV from hadronic phase

<Tth> = 130-140 MeV < Tc

Mass > 1 GeV from partonic phase

<Tth> = 200 MeV >Tc

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hadronicpp-→r→mm-

partonicqq→mm-

Dileptons for M >1 GeV dominantly of partonic origin

Eur. Phys. J. C 59 (2009) 607

Teff ~ <Tth> + M <vT>2

Phys. Rev. Lett. 100 (2008) 022302

Status: Thermal Radiation at SPS energies

State of the art dilepton experiment: NA60

Isolate thermal radiationPlanck like exponential mass spectraexponential mT spectrazero polarization general agreement with models of thermal radiation

Emission sources of thermal dileptons (from m-pT):hadronic (pp- annihilation) dominant for M<1 GeVpartonic (qq annihilation) visible M>1 GeV

In-medium r spectral function identified: no significant mass shift of the intermediate ronly broadening.

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Axel Drees

Thermal radiation at RHIC Energies: PHENIX

Photons, neutral pion p0 g g

gge-

e

Calorimeter

PC1PC2

PC3

DC

magnetic field &tracking detectors

e+e- pairsE/p and RICH

Disclaimer: ongoing analysis from STAR analysis

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No background rejection!

dileptonS/B < 1:150

HBD upgrade!

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Dilepton Continuum in p+p Collisions PHENIX Phys. Lett. B 670, 313 (2009)

Data and Cocktail of known sources represent pairs with e and e- PHENIX acceptanceData are efficiency corrected

Excellent agreement of data and hadron decay contributionswith 30% systematic

uncertainties

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Consistent with PHENIX single electron measurement

c= 567±57±193mb

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Dilepton Continuum in d+Au Collisions

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Consistent with known sourcesData will constrain known sources with high precision

In particular bottom cross section

PHENIX preliminary

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Au+Au Dilepton ContinuumExcess 150 <mee<750 MeV: 3.4 ± 0.2(stat.) ± 1.3(syst.) ± 0.7(model)

Charm from PYTHIA filtered by acceptance c= Ncoll x 567±57±193mb

Charm “thermalized” filtered by acceptancec= Ncoll x 567±57±193mb

Intermediate-mass continuum: consistent with PYTHIAsince charm is modified room for thermal radiation

hadron decay cocktail tuned to AuAu

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PHENIX VTXupgrade

PHENIX Phys. Rev. C 81 (2010) 034911

Axel Drees

Models calculations with broadening of spectral function:

Rapp & vanHees Central collisions scaled to mb+ PHENIX cocktail

Dusling & ZahedCentral collisions scaled to mb+ PHENIX cocktail

Bratkovskaya & Cassingbroadening

pp annihilation with medium modified r

insufficient to describe RHIC data!

Large low mass enhancement

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Excess not from hadronic phase!!

Low Mass Dilepton Puzzle

PHENIX Phys. Rev. C 81 (2010) 034911

Soft Low Mass Dilepton Puzzle

mT spectrum of excess dileptonsSubtract cocktail Correct for pair acceptanceFit two exponentials in mT –m0

Eludes any theoretical interpretation

Hint also in NA60 dataInsufficient date for more detailed information

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92 11 9 MeV

258 37 10 MeV

300 < m < 750 MeV

Soft component below mT ~ 500 MeV: Teff ~ 100MeV independent of mass

more than 50% of yield

Excess has 2 components : (1) Thermal radiation (next slides); (2) Soft “exotic” source, red shift, glasma, color B-field

acceptance corrected

PHENIX Phys. Rev. C 81 (2010) 034911

Axel Drees

pQCD

g* (m0)

g

T ~ 220 MeV

First Measurement of Thermal Radiation at RHIC

Direct photons from real photons:Measure inclusive photonsSubtract p0 and h decay photons at S/B < 1:10 for pT<3 GeV

Direct photons from virtual photons:Measure e+e- pairs at mp < m << pT

Subtract h decays at S/B ~ 1:1 Extrapolate to mass 0

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First thermal photon measurement: Tini > 220 MeV > TC

g* (m→0) = g ; m << pT

Need to consider radial flow!

PHENIX Phys. Rev. C 81 (2010) 034911

Calculation of Thermal Photon YieldReasonable agreement with data

factors of two to be worked on ..

Initial temperatures and times from theoretical model fits to data:

0.15 fm/c, 590 MeV (d’Enterria et al.)0.2 fm/c, 450-660 MeV (Srivastava et al.)0.5 fm/c, 300 MeV (Alam et al.)0.17 fm/c, 580 MeV (Rasanen et al.)0.33 fm/c, 370 MeV (Turbide et al.)

Observations comparing models:Correlation between T and t0Yield typically to lowYield not correlated with Tini

Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c

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Low yield earlier emission (yield T4) increase factor 2 with <20% change in T

PHENIX Phys. Rev. C 81 (2010) 034911

Thermal Photons also FlowHow to determine elliptic flow of thermal photons?

Establish fraction of thermal photons in inclusive photon yieldPredict hadron decay photon v2 from pion v2 Subtract hadron decay contribution from inclusive photon v2

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12

.2.

2 --

=g

g

RvvR

vBGinc

dir

Large v2 of low pT thermal photon

PHENIX arXiv:1105.4126

Thermal Photon Puzzle

Large flow requires late emission! Apparent contradiction with yield,

which points towards early emission!Axel Drees22

Hees/Gale/Rapp Phys.Rev.C84:054906,2011.

R. Chatterjee and D. K. SrivastavaPRC 79, 021901(R) (2009)PRL96, 202302 (2006)

Models fail to describe simultaneously photon yield, T and v2! Tini ~ 325MeV

Status: Thermal Radiation at RHIC energies

PHENIX e+e- and g from √sNN = 200 GeV

Soft low mass dilepton puzzlelarger excess beyond contribution from hadronic phase with medium modified r-meson properties … not from hadronic phasesoft momentum distribution … not from hot partonic phase

Thermal photon puzzleLarge thermal yield with T > 220 MeV (20% of decay photons)

… suggests early emission Large elliptic flow (v2) … suggests late emission

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PHENIX data on E&M probes seems INCONSISTENT with standard hydro space-time evolution! And exhibits UNKOWN additional sources!

Speculation: look between impact (t=0) and t0

10% central, 62 GeV:

efficiency 60%Rejection 90%

Near Future: PHENIX HBD upgrade

HBD fully operational:Single electron ~ 20 P.E.Conversion rejection ~ 90% Dalitz rejection ~ 80%Improvement of S/B factor 5 to published results

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Window less CF4 Cherenkov detectorGEM/CSI photo cathode readoutOperated in B-field free region

p+p data in 2008/9Au+Au data in 2009/10

Au+Au background subtraction needs to be finalized, results at QM

p+p with HBDuncorrected

Improve S/B by rejecting combinatorial background

Near Future: PHENIX VTX upgrade

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VTX in 2010/11

FVTX in 2011/12

Tracking with 4 layers of silicon vertex detector

Online display of Au+Au collision

49.6mm 24.8mm

(cm)

Vertex resolution in Y

29.2mm(sim)

300mm

DCA resolution

DCA ~ 80 mm

Promise to tag e+e- pairs from ccbarOpens opportunity to measure thermal radiation above M = 1 GeV

Drawback added material, increased backgroundNot compatible with HBD, no rejection

Impact on dilepton measurement unclear

Summary of Findings

We have discovered “thermal” radiation from heavy ion collisionsDileptons allow to disentangle space-time evolution of collision

NA60 established method with mm- from In-In at 158 AGeV

PHENIX e+e- and g from √sNN = 200 GeVSoft low mass dilepton puzzleThermal photon puzzleData inconsistent with “standard hydrodynamic space-time evolution

Next steps towards state of the art experiments (at RHIC)

PHENIX HBD & VTX data STAR with full detector upgrades

Significant progress will requires a new experiment at RHIC dedicated to thermal radiation measurements!

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Backup Slides

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Short Detour on Cosmic Background RadiationDiscovered by chance in 1962

Perfect Black Body spectrum with T=2.37 K in 1992 (COBE)

WMAP power spectrum 2006

First data from Planck Satellite search for finger print of Inflation probing early evolution at t < 3 10-12 fm

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Homogeneity of background radiation Requires inflation phase!

STAR p+p Dilepton Data

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STAR arXiv:1204.1890

STAR charm cross section = 920 mb

PHENIX cocktail in STAR acceptanceMC@NLO for heavy flavor

resolution not tuned for STAR

Lesson learned to Pursue Thermal Radiation

Build dedicated thermal radiation experiment

Map thermal radiation in phase spaceDeconvolve temperature and flowMap time evolution of system

Focus on Dileptons e+e- preferred for collider and y=0g in coincidence is a must to tag backgroundmm- good at forward rapidity might be nice addition at y=0

Measure heavy flavor simultaneouslyOpen and closed heavy flavor and much more as by product

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Strong Physics ProgramLarge Discovery Potential

Comment on RHIC vs SPS vs LHCRHIC is at a sweet spot

System is well in partonic phaseProof of principle to measure thermal radiation existsMany unsolved puzzle – which are not small!large unknown source, large partonic contribution, rapid thermalization, time evolution?

SPS at to low energy Dominated by hadronic phaseLittle to learn about early phase Program at its end (or already beyond)

LHC at to high energySystem created at very similar condition compared to RHIC temperatureDilepton continuum inaccessible due to background

Charm cross-section so high that irreducible background (both physics and random) becomes prohibitive for precision measures

Thermal photons may be possibly via low mass high pT virtual photons?Detectors not setup for dilepton measurements

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Strong physics program at RHIC with little competition from LHC

Thermal Radiation Experiment

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Design requirement (educated guess)

Large acceptance (2p ; Dy=2)For high statistics and better systematics

Charged trackingGood electron id (1:1000 p rejection)Excellent momentum resolution (dp/p < 0.2% p)

Combinatorial background rejection Passive: minimize material budget (in particular before first layer)Active: solid Dalitz rejection scheme

Heavy flavor detectionLow mass precision vertex tracker (<10-20mm DCA)

Photon measurementSufficient energy resolution (<10%/√E; small constant term)

High DAQ rate (all min bias you can get ~ 40 kHz)

Do not compromise on requirements!

Transverse Mass Distributions of Excess Dimuon

All mT spectra exponential for mT-m > 0.1 GeV

Fit with exponential in 1/mT dN/mT ~ exp(-mT/Teff)

Soft component for <0.1 GeV ??Only in dileptons not in hadrons (speculate red shift???)

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transverse mass: mT = (pT2 + m2)1/2

Phys. Rev. Lett. 100 (2008) 022302 Eur. Phys. J. C 59 (2009) 607

Intermediate Mass Data for 158 AGeV In-In

Experimental Breakthrough Separate prompt from heavy flavor muonsIsolate prompt continuum excess

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Intermediate Mass Range prompt continuum excess

2.4 x Drell-Yan

Eur.Phys.J. C 59 (2009) 607

Axel Drees

Interpretation as Direct PhotonRelation between real and virtual photons:

0for MdMdN

dMdNM ee g

Extrapolate real g yield from dileptons:dydp

dML

MdydpdMd

TT

ee2222 )(1

3g

pa

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Virtual Photon excessAt small mass and high pT

Can be interpreted asreal photon excess

no change in shapecan be extrapolated to m=0

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Search for Thermal Photons via Real Photons

PHENIX has developed different methods: Subtraction or tagging of photons detected by calorimeterTagging photons detected by conversions, i.e. e+e- pairs

Results consistent with internal conversion method

The internal conversion method should also work

at LHC!

internal conversions

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