A Future for Measuring Thermal Radiation in Heavy Ion Collisions?

Thermal radiation from hadronic collisions: An old but still hot idea!“thermal” radiation Experimental Challenges

Experimental attempts to measure thermal radiation First successful experiment: CERESState of the Art experiment: NA60Energy frontier: PHENIX

Future perspectivesLessons learned and conclusions from themDedicated experiment: Technical specifications Dedicated experiment: A strawman design

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

QGP

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

p-A 400 GeV

cc DD e X

e X

-

NA38 experiment was originally proposed to measure thermal radiation

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

3

Naming Convention: Thermal??

Axel Drees4

1 10 107 log t (fm/c)

Photons from A+A

Direct photons

Photons from hadron decays“Prompt” hard scattering

Pre-equilibrium

Quark-Gluon Plasma

Hadron gas

ThermalNon-thermal

Need to be more clear in what we mean by “thermal” and thermal equilibrium

Axel Drees

Thermal RadiationBlack body radiation

Real Photons and virtual photons (lepton pairs)

Static source back of envelope estimate:

4

~ ~

Static source at ~ 200 , for 10 / with a radius of 10 :

Wien's law: . ~ 3501250peak radiation energy: 700

Stefan-Boltzmann law:

c

peak

peakpeak

T MeV fm c fm

T const MeV fmMeVfmE MeV

Pj TA

38 1 4

3

2.4 10

~ 38

Energy radiated: 480

typical number of : N ~ ~ 700

rad

rad

peak

MeV fm c T

MeVfm c

E j t A GeVEE

-- -

Thermal photons a ~10% contribution to p0 photons Range to look few 200 MeV to 2 GeV

5

An Expanding Source in Local Equilibrium

Real and virtual photon momentum spectrum at mid rapidity:Temperature information

Integrated 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

Axel Drees6

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

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+

p

r

p p

p

r*

*

e-

e+

q

qg

Additional issue: Need to know 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)

Axel Drees7

Axel Drees

Experimental Challenge

Thermal radiation compete with “cocktail” of and ll- from hadron decays after freeze-out

Real photons: p0, h, wp0, ...More than 90% of photon yield

Virtual photons:p0ee-, h, wp0ee- and direct decays r,w,f ee-, J/ ee- ...Semileptonic decays of heavy flavor

Drell Yan Dileptons have mass remove contribution from p0

more sensitive to thermal radiation than photons

cc DD e X

e X

-

Measure hadron decay contributionsFocus on Dileptons, they are more sensitive than photons

8

Axel Drees

Dilepton Experimental Challenges (I)

Uncorrelated background: l+ and l- from different uncorrelated source

Veto as many of the pairs actively by finding partner (rejection scheme)Remove remaining background statistically

Like and unlike sign combinatorial pairs.Unlike sign background can be determined from like sign background, either measured (FG) or determined by event mixing (BG).Total number of pairs related by geometrical mean.

True for e+e- since they are produced as pairs, even for different efficiencies. For m, produced as singles, strictly true only if produced with Poisson

distribution.

For different acceptance for singles or pairs need relative acceptance correction (obtained from mixed events)

0 e e

e e

p

-

-

p m

p m

- -

2N N N- - -

( , ) 22T T

BGFG m p FG FGBG BG

-- - -

- -

9

Axel Drees

Dilepton Experimental Challenges (II)Unphysical correlated background

Limited double track/hit resolution False track match between detectorsNot equal in like and unlike; Not reproducible by mixed eventsMUST be removed from event sample

Correlated background: l+ and l- from same source but not signal“Cross” pairs “jet” pairs

Need case by case investigation with MC simulations and subtractionIf background produces same numbers of ++ and - - pair same method as for different pair acceptance works

( , ) 22T T

BGFG m p FG FGBG BG

-- - -

- -

0

e e

e e

p -

-

Xπ0

π0

e+e-

e+

e-

γ

γ

π0

e-

γ

e+

10

Axel Drees

Pioneering Dilepton Results form CERES/NA45

Discovery of low mass dilepton enhancement in 1995p-Be and p-Au well described by decay cocktailSignificant excess in S-Au (factor ~5 for m>200 MeV) Onset at ~ 2 mp suggested p-p annihilation Maximum below r meson near 400 MeV

CERES PRL 92 (95) 1272 with 466 citations

Launched massive theoretical investigation of meson properties in medium

p

p

r*

*

e-

e+

11

NA60 featuresPrecision silicon pixel vertex trackerClassic muon spectrometerDouble dipole for large acceptance (low mass)High rate capability

Axel Drees12

Precision 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

Low Mass Data Sample for 158 AGeV In-In

Experimental advanceHigh statisticsExcellent background rejectionPrecision control of decay cocktail

Example: NA60 can measure electromagnetic transition form factors for of η→μ+μ-γ and ω→μ+μ-π0

Axel Drees13

NA60 can isolate continuum excess (including r meson

from decay backgroundPhys. Rev. Lett. 96 (2006) 162302

(Phys. Lett. B 677 (2009) 260)

Intermediate Mass Data for 158 AGeV In-In

Experimental Breakthrough Separate prompt from heavy flavor muonsIsolate prompt continuum excess

Axel Drees14

Intermediate Mass Range prompt continuum excess

2.4 x Drell-Yan

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

Continuum Excess Measured by NA60

Axel Drees15

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 GeV qq mm-

p a1 mm- (Hess/Rapp approach)

Main Sources m < 1 GeVpp- r mm-

Sensitive to medium spectral function

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

Axel Drees

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!

16

Not acceptance corrected

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???)

Axel Drees17

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

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

Rise and Fall of Teff of Thermal Dimuons

Mass < 1 GeVLinear increase of Teff with mSimilar trend observed in hadrons

Interpretation at SPS: Radial flow in hadronic phase!

Dileptons sense flow of in-medium r pp-→r→mm-

Mass > 1 GeVSudden drop to Teff ~ 200 MeVRemains independent of mass

Axel Drees18

Phys. Rev. Lett. 100 (2008) 022302

Teff ~ Tf + M <vT>2

Indication that source of thermal dileptons is different for low and large masses!!

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

Axel Drees19

hadronicpp-→r→mm-

partonicqq→mm-

Dileptons for M >1 GeV dominantly of partonic origin

Status: Thermal Radiation at SPS energies

History Search started in 1986First pioneering results on dileptons and photons (mostly limits) after 1995Breakthrough with precise measurements (NA60) after 2006

Current status from dilepton experiment NA60Planck like exponential mass spectra, exponential mT spectra, zero polarization and general agreement with thermal models consistent with interpretation of excess dimuons as thermal radiationEmission sources of thermal dileptons mostly hadronic (pp- annihilation) for M<1 GeV, and mostly partonic (qq annihilation) for M>1 GeVIn-medium r spectral function identified; no significant mass shift of the intermediate r, only broadening.

Axel Drees20

Axel Drees

Thermal radiation at RHIC Energies: PHENIX

Photons, neutral pion p0

e-

e

Calorimeter

PC1PC2

PC3

DC

magnetic field &tracking detectors

e+e- pairsE/p and RICH

Disclaimer: ongoing analysis from STAR and potentially at LHC, but not finalized yet

21

Axel Drees

Dilepton Continuum in p+p Collisions 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

22

Consistent with PHENIX single electron measurement

c= 567±57±193mb

Axel Drees

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

23

Axel Drees

In Medium Mesons at RHIC???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 & Cassingbroadeningbroadening and dropping

Au-Au mb

pp annihilation with medium modified r

insufficient to describe RHIC data!

with modified charm

24

Measuring direct photons via virtual photons:any process that radiates will also radiate * for m<<pT * is “almost real”extrapolate * e+e- yield to m = 0 direct yield m > mp removes 90% of hadron decay backgroundS/B improves by factor 10: 10% direct 100% direct *

Axel Drees

Contribution from Direct (pQCD) Radiation

access above cocktailfraction or direct photons:

dir dir

incl incl

r

*

*

q

qg

pQCD

Small excess at for m<< pT consistent with pQCD direct photons

1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV

hadron decay cocktail

25

Direct Real Photons from Virtual Photons

Axel Drees26

Significant direct photon excess beyond pQCD in Au+Au

Axel Drees

pQCD

* (e+e-) m=0

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

27

First thermal photon measurement: Tini > 220 MeV > TC

Calculation of Thermal Photons

D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006)

Reasonable agreement with datafactors 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.)

Correlation between T and t0

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

28 Axel Drees

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 v2Subtract hadron decay contribution from inclusive photon v2

Axel Drees29

12

.2.

2 --

RvvR

vBGinc

dir

Large v2 of low pT thermal photon

Thermal Photon v2 Model Comparison

Direct emission from hadronic phase insufficient!

Axel Drees30

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

Current models fail to describe direct photon v2R. Chatterjee and D. K. SrivastavaPRC 79, 021901(R) (2009)PRL96, 202302 (2006)

Axel Drees

Quark Scaling Behavior of v2

All hadrons flow collectivelyin common velocity fieldworks for f and D mesons toofavors a pre-hadronic originHadrons form from constituent quarks

Flow builds up in partonic phase?!

Common wisdom about space-time evolution may not be correct!

31

Summary of FindingsWe have discovered “thermal” radiation from heavy ion collisions

NA60 mm- from In-In at 158 AGeVThermal source isolated experimentallyPlanck like mass-spectrum of thermal radiationHadronic phase largest contributor (m < 1 GeV)Observe melting of r meson in medium Contribution from partonic phase (m > 1GeV) with <T> ~ 200MeV

PHENIX e+e- and from √sNN = 200 GeVLow mass excess larger than expected thermal contribution from hadron phaseThermal photons with <T> > 200 MeV (from * extrapolated to m=0)Large elliptic flow (v2) of thermal photons which exceeds expected contribution from hadron phase

More data to come in next yearsPHENIX HBD, STARExpect significant progress requires new dedicated effort!

Axel Drees32

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

Axel Drees33

Much to learn from thermal radiation beyond temperature!

Lesson learned: Build a Dedicated Experiment

Build dedicated thermal radiation experiment

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

Focus on Dileptons e+e- preferred for collider and y=0 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

Axel Drees34

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 energyDominated 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

Axel Drees35

Strong physics program at RHIC with little competition from LHC

Thermal Radiation Experiment

Axel Drees36

Design requirement (educated guess)

Large acceptance (2p ; y=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: 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!

Strawman Design

Axel Drees37

active beam pipe

~30 cm

vacuum pipe 2 cm6 cm

beam axis

MAPS active pixel xy < 20 mm X/X0 < 1% DCA ~ 15 mm

Solenoid with ~2 Ty = 2

Silicon strip with f ~100 mmdp/p < 0.1%p

1m

active beam pipe

GEM tracker, med. resolution vector with dE/dx, or RICH/HBD

0.7 m

0.1 m

EMCal longitudinal segmented few 100ps resolution

Rejection scheme: full track + tracklet mass cut tracklet: active beam pipe + inner GEM eID via dE/dx ~ 1/10 rejection few % p-resolution

Electron ID: GEM tracker dE/dx EMCal TOF/ shower shape E/p

My Personal Conclusion

Axel Drees38

Heavy ion physics at RHIC beyond PHENIX and STAR (>2015) should focus on “thermal” radiation

Backup Slides

Axel Drees39

Axel Drees

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

40

Axel Drees

Combinatorial Background: Like Sign Pairs

--- Foreground: same evt N++--- Background: mixed evt B++

Shape from mixed events Excellent agreements for like

sign pairs also with centrality and pT

Normalization of mixed pairs Small correlated background at

low masses from double conversion or Dalitz+conversion

normalize B++ and B- - to N++ and N- - for m > 0.7 GeV

Normalize mixed - pairs to

Subtract correlated BG

Systematic uncertainties statistics of N++ and N--: 0.12 % different pair cuts in like and

unlike sign: 0.2 %Normalization of mixed events:systematic uncertainty = 0.25%

2N N N- - -

Au-Au

41

Axel Drees

Au-Au Raw Unlike-Sign Mass Spectrum

Mixed unlike sign pairs normalized to:

2N N N- - -

Unlike sign pairs data

signal/signal = BG/BG * BG/signal

as large as 200!! 0.25%

Systematic errors from background subtraction:

up to 50% near 500 MeV

arXiv: 0706.3034

Run with addedPhoton converter

2.5 x background

Excellent agreement within errors!

42

Axel Drees

p-p Raw Data: Correlated Background

( , ) 22T T

BGFG m p FG FGBG BG

-- - -

- -

Mixed events

Correlated Signal = Data-Mix

Cross pairsSimulate cross pairs with decay generator Normalize to like sign data for small mass

Jet pairsSimulate with PYTHIANormalize to like sign data

2N N N- - -

Like Sign Data

Unlike Sign DataUnlike sign pairs

same simulationsnormalization from like sign pairs

Alternative methodeCorrect like sign

correlated background with mixed pairs

Signal: S/B 1

43

Axel Drees44

Centrality Dependence of Background Subtraction

For all centrality binsmixed event background and like sign data agree withinquoted systematic errors!!

Evaluation in 0.2 to 1 GeV range

Compare like sign data and mixed background

Similar results for background evaluation as function pT

Axel Drees

Background Description of Function of pT

Good agreement

45

Axel Drees

Fit Mass Distribution to Extract the Direct Yield: Example: one pT bin for Au+Au collisions

and normalized to da

( )ta

(

f

)

or 30

dir eec

e

e

e

ef

m

m

V

f m

Me<

Direct * yield fitted in range 120 to 300 MeVInsensitive to p0 yield

dydpd

MLMdydpdM

d

TT

ee2222 )(1

3

pa

1/m

46

Axel Drees

Interpretation as Direct PhotonRelation between real and virtual photons:

0for MdMdN

dMdNM ee

Extrapolate real yield from dileptons:dydp

dML

MdydpdMd

TT

ee2222 )(1

3

pa

47

Virtual Photon excessAt small mass and high pT

Can be interpreted asreal photon excess

no change in shapecan be extrapolated to m=0

Axel Drees

Interpretation as Direct PhotonRelation between real and virtual photons:

0for MdMdN

dMdNM ee

Extrapolate real yield from dileptons:

dydpd

MLMdydpdM

dT

T

ee

2222 )(1

3

pa

Exc

ess*

M (A

.U).

Example for one pT range:

48

Virtual Photon excessAt small mass and high pT

Can be interpreted asreal photon excess

no change in shapecan be extrapolated to m=0