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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: CERES - PowerPoint PPT Presentation
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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