thermal radiation mapping the space-time evolution
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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: NA60 Energy frontier: PHENIX Future perspectives - PowerPoint PPT PresentationTRANSCRIPT
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!
3
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
Axel Drees4
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)
Axel Drees5
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
Axel Drees
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
Axel Drees8
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
Axel Drees9
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
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!
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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.
Axel Drees12
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!
Axel Drees
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
Axel Drees21
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
Axel Drees24
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
Axel Drees25
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
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
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