Electromagnetic Radiation from QCD Matter: Theory Overview

0.78 MB·

by  Chun Shen

Nuclear
Physics A
NuclearPhysicsA00(2016)1–9
www.elsevier.com/locate/procedia
TheXXVthInternationalConferenceonUltrarelativisticNucleus-Nucleus
Collisions
Electromagnetic Radiation from QCD Matter: Theory
6
Overview
1
0
2
n Chun Shen
a
J DepartmentofPhysics,McGillUniversity,3600UniversityStreet,Montreal,QC,H3A2T8,Canada

1
1

]
h
t
- Abstract
l
c
Recent theory developments in electromagnetic radiation from relativistic heavy-ion collisions are re-
u
n viewed.Electromagneticobservablescanserveasathermometer,aviscometer,andtomographicprobesto
[ thecollisionsystem.Thecurrentstatusofthe“directphotonflowpuzzle”ishighlighted.

1 Keywords: Directphotons;dileptons;quark-gluonplasma;electromagnetictomography
v
3
6 1. Introduction
5
2 Relativisticheavy-ioncollisionexperimentsconductedattheRelativisticHeavy-IonCollider
0 (RHIC)andtheLargeHadronCollider(LHC)createanenvironmentatatemperatureofatril-
1. lion degrees, to study the property of dense nuclear matter. Electromagnetic probes, such as
0 direct photons and dileptons (e+e− and µ+µ− pairs) are recognized as valuable messengers in
6 suchcollisions. Becausephotonsanddileptonsinteractonlyelectromagnetically, theyareable
1
topenetratethemediumandcarryalmostundistorteddynamicalinformationandreportoncon-
:
v ditions existing at their production point. Such probes are sensitive to the early stages of the
i collision system, to thermal and transport properties of the quark-gluon plasma (QGP), and to
X
thedynamicalevolutionproceedingfromthecross-overregionstothehadronicphase. There-
r
a centdirectphotonmeasurementsshowlargeyieldsofphotonsandlargemomentumanisotropies
for p < 4GeV[1,2,3,4]. Theoreticalcalculationsstillunderestimatethesechallengingmea-
T
surements: thistensionhasbeendubbedthe“directphotonflowpuzzle”.
In this proceeding, I review recent theoretical developments which demonstrate that elec-
tromagnetic probes can provide us with important information, complementary to that carried
ChunShen/NuclearPhysicsA00(2016)1–9 2
by the majority of hadronic observables. The progress in theory towards resolving the “direct
photonflowpuzzle”isalsodiscussed.
2. Thermometer
Realandvirtualphotonsareregardedasusefultoolsforexperimentallyaccessingthetem-
peratureoftheQGPcreatedinheavy-ioncollisions[5,6]. Theslopesofthephotonanddilepton
spectraencodetemperatureinformationofthecollisionsystem.
Direct photon spectra have been measured by the PHENIX, STAR, and ALICE Collabora-
tions in heavy-ion collisions at the top RHIC and LHC energies[1, 2]. The low p part of the
T
spectracanbewellcharacterizedbytheirinverselogarithmicslopeTeff. TheP√HENIXCollabo-
rationreportedTeff =(239±25stat±7sys)MeVfor0-20%Au+Aucollisionsat sNN =200GeV
[1]andth√eALICECollaborationfoundTeff =(304±11stat±40sys)MeVin0-20%Pb+Pbcolli-
sionsat s =2.76TeV[2].Quantitivestudies[7,8]haveshownthatthermalphotonsemitted
NN
fromT <250MeVreceivedasignificantblue-shiftfromhydrodynamicflow,asillustratedinthe
leftpanelofFig.1. Asolidextractionoftheinitialtemperatureofthecollisionsystemrequires
400
Central AA Collisions
350
300
V)250
e
M200
T (150 Tpc
100 T (M=1.5-2.5 GeV)
s
50 T
i
0
10 100
1/2
s (GeV)
Fig.1. LeftPanel: Theinverseslopeofthermalphotonspectrumemittedfromhydrodynamicfluidcellsasafunction
ofthelocaltemperature. TheplotistakenfromRef.[8]. RightPanel: Temperatures(Ts)extractedfromtheslopeof
thermaldileptoninvariantmassspectrumintherangeM=1.5−2.5GeV,comparedwiththeinitialtemperatureofthe
fireballTiatthedifferentcollisionenergies.ThefigureistakenfromRef.[9].
detailedcomparisonsbetweenexperimentaldataanddynamicalmodelsimulations.
Ontheotherhand,thermaldileptoninvariant-massspectraarefreefromtheblue-shiftcon-
tamination. The authors in Ref.[9] demonstrated that the slope of dilepton spectrum in the in-
termediatemassregion(IMR),1.5GeV< M < 2.5GeV,couldprovidecleantemperatureinfor-
mation about the collision system. The extracted averaged temperature was about 30% lower
thanthecorrespondinginitialoneatthetopRHICenergy(seerightpanelofFig.1). Thisdiffer-
enceshrankasthecollisionenergydecreased. Thermaldileptoninvariantmassspectraprovide
model-independent information about the collision system. In addition, Refs.[9, 10, 11, 12]
showed that dilepton invariant mass spectra were valuable tools to probe the properties of the
baryon-richfireballintheRHICBeamEnergyScan(BES)program[13,14]. Finally, dilepton
invariant mass spectrum can help us to study the in-medium modification of the vector meson
(ρ-meson)spectralfunction,whichhasadirectconnectiontochiralrestorationduringthephase
transition[15].
ChunShen/NuclearPhysicsA00(2016)1–9 3
3. Viscometer
Inrelativisticheavy-ioncollisions,themediumdoesnotstayinthermalequilibriumduringits
evolution.Out-of-equilibriumdynamicscanthereforeinfluencetheelectromagneticobservables.
Effects on thermal photon emission owing to locally anisotropic particle distributions were
firstinvestigatedinRef.[16,17]. Morecompletecalculationsoftheshearviscouscorrectionsto
thephotonproductionrateswereperformedinRef.[18,19,20],andtherelatedphenomenologi-
calimpactswerestudiedinRef.[18,21,22]. Theviscoushydrodynamicevolutionbyitselfwas
foundtoincreasethenetphotonellipticflow[22],becausetheinitialtemperatureofthesystem
is lower with non-zero specific shear viscosity in order to compensate for the entropy produc-
tion during the evolution. This reduced early stage QGP photon emission at high p , which
T
then increased the relative weight of photons from later stages which carry a large momentum
anisotropy. Similar effects were also found in Ref.[23]. The further inclusion of the viscous
correctionstothephotonemissionratesreducedthethermalphotonv [18,21].
n
Effects from bulk viscosity were recently studied in Ref.[24, 25]. The inclusion of a non-
vanishingbulkviscositynearthephasetransitionwasrecentlyfoundtobeessentialtoprovidea
gooddescriptionofidentifiedhadronmean-p measurements[24].Theextraentropyproduction
T
from bulk viscosity increases the space-time volume in the late hadronic phase by about 50%,
which allows more thermal photon radiation. Another consequence of the inclusion of bulk
viscosityistoreducethehydrodynamicalradialflowby∼10%atthelatestageoftheevolution:
this transport coefficient slows down the fireball expansion and weakens the blue shift of the
thermalphotonspectrum. Botheffectstogetherincreasethethermalphotonyieldsinthelow p
T
regionsandshiftthepeakofthedirectphotonv (p )towardsthelow p regions[25].
2 T T
Finally,virtualphotons,measuredasleptonpairs,havealsobeenshowntobeacleanandsen-
sitiveprobeoftheout-of-equilibriumdynamicsofthesystem. Recentstudieshavedemonstrated
that, compared to hadronic observables, the thermal dilepton spectrum and its flow anisotropy
show a larger sensitivity to the early time dynamics, to the system’s shear stress tensor πµν, to
thetemperaturedependenceofshearviscosityη/s(T),andeventothechoiceofthesecondorder
transportcoefficientτ [26,27].
π
4. Statusofresolvingthe“directphotonflowpuzzle”
EversincetheunexpectedlylargedirectphotonellipticflowreportedbythePHENIXCol-
laboration in Au+Au collisions at RHIC[28], the tension with the experimental measurements
hasgeneratedconsiderabletheoreticalefforttoresolvethe“directphotonflowpuzzle”.
Fig.2capturesthecurrentsituationofmodel-datacomparisonsfordirectphotonspectraand
theiranisotropicflowcoefficients.1 Withrespecttosomeearliercalculations[33,34],thetension
betweenexperimentalmeasurementsandtheoryisnowconsiderablyreduced. Calculationswith
different hydrodynamic evolution [25, 29] give similar results for direct photon spectrum and
ellipticflowcoefficientfor p >1GeV.Themajorimprovementscommontothesecalculations
T
were the inclusion of a more complete set of hadronic emission channels, namely the contri-
butions from ρ-spectral function[35, 36], ππ bremsstrahlung[37, 38], πρω channels[39], and
short-lived resonances feed-down[29, 34]. The remaining difference below 1 GeV originates
1Please see Ref.[32] for a recent detailed review about the theoretical efforts to resolve the “direct photon flow
puzzle”.
ChunShen/NuclearPhysicsA00(2016)1–9 4
102 (a) 0.2 ccoanlovreimrseiotenr (b) 1102..00 (d) 0-20% Pb+aPrXbi v@:1 520.97.066 T7e38V
0.15 ALICE data
101 0-20% Au+Au @ RHIC 8.0
v2 0.1 γRAA 6.0
2eV)−100 0.05 4.0
(Gπdypdp)TT10-1 00..01 (c) 028...000 (e) 20-40% Pb+PbA L@IC E2 d.7a6ta TeV
N/(210-2 0.08 6.0
d v30.06 γRAA
10-3 PHSD 0.04 4.0
TAMU (with ideal hydro)
McGill (IP-Glasma shear+bulk) 0.02 2.0
PHENIX data
10-4 0.0 0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0
pT (GeV) pT (GeV) pT (GeV)
Fig.2. Panels(a)-(c): Theoreticalcalculationsofdirectphotonspectra√andtheirflowanisotropyv2(pT)andv3(pT)
comparedwiththePHENIXmeasurementsin0-20%Au+Aucollisionsat sNN=200GeV[1,3].Theuncertaintyband
intheTAMUresultspresentedtwochoicesofthepromptsources[29].ThePHSDresultsweretakenfromRef.[30].The
errorbandisdominatedbytheuncertaintyinthemodelingofthecrosssections. Statisticalerrorswereindicatedasa
bandintheMcGillresults[25].Thelegendin(a)appliestoPanels(b)and(c).Panels(d),(e):Thenuclearmodification
√
factorofdirectphotonRγAAin0-20%and20-40%Pb+Pbcollisionsat sNN=2.76TeV[2,31].
from different extrapolations procedures for the prompt photon source. The McGill group ex-
trapolatedthepQCDpromptphotonsourcetolow p usingresultsfromdifferentfragmentation
T
scales[25], while the TAMU group suppressed soft prompt photon production with formation
timeslongerthanthethermalizationtimeofthecollisionsystem[29,36]. Byconsideringevent-
by-event fluctuations as well as both shear and bulk viscous effects on the photon production
mechanisms,theresultspresentedinRef.[25]achievegoodagreementwithexperimentaldata,
andalsoanmakestatementsonthetriangularflowofdirectphotons. Finally,theoreticaluncer-
taintiesinthephotonemissionrelatedtononperturbativephysicswereinvestigatedinRef.[25],
byconsideringtheSemi-QGPscenario[40,41,42]andanalternativehadronicemissionrate[43]
(alsoseediscussionsinRef.[32]). Adoptingthose,thevariationinthefinalresultissmallerthan
currentdatasystematicerrors. Moreaccuratemeasurementscouldhoweverdistinguishbetween
thosedifferenttechniquesandthereforesetconstraintsonphotonemissionrates.
Consideringalternativestohydrodynamicapproaches,directphotonswerestudiedusingmi-
croscopic transport simulations[30]. The PHSD model provides a level of agreement with the
experimentalmeasurementin0-20%Au+AucollisionsshowninFig.2a. Thecentralitydepen-
dence of direct photon observables in semi-peripheral collisions were better reproduced in the
PHSDmodelowingtolargerhadronicbremsstrahlungcontributions[30]. Achannel-by-channel
comparisonbetweenthetransportandhydrodynamicapproacheswaspresentedinRef.[32].One
foundasimilarQGPphotonemissionfromthetwoapproaches,butaratherdifferentcontribution
fromhadronicbremsstrahlungprocesses. ThePHSDmodeladditionallyincludesmeson-baryon
bremsstrahlung,usinganimprovedsoftphotonapproximation[30]. Forππbremsstrahlung,the
PHSD model produced ∼4 times more photons than in hydrodynamic calculations[32]. This
might suggestadditional sourcesfrom thedilute hadronicphase, whichare not currentlymod-
elled in the hydrodynamic framework. A closer look at the space-time structure of hadronic
photon emission will elucidate the origin(s) of the difference between the two models. Com-
parisonswithrecentphotoncalculationsusingacoarse-grainedUrQMDmedium[44,45]inthe
ChunShen/NuclearPhysicsA00(2016)1–9 5
hadronicphasewillalsoshedlightonthisissue.
5. Event-by-eventtomography
Thermalphotonscanalsoserveasatomographicprobewhichreportsonthedynamicsand
property of the interior of the fireball, in analogy with the now common medical CT scan. A
space-timeanalysisofthermalphotonproductionin0-20%Au+AucollisionsisshowninFig.3a.
A two-wave structure of thermal photon emission was found [21]. The first wave is from high
thermal photons thermal photons thermal photons
before weighting before weighting after weighting
by their yields by their yields by their yields
(a)
(b) (b) (c)
Fig.3.ColorcontourplotsforthermalphotonyieldPanel(a)anditsellipticflowcoefficientPanels(b)and(c)withand
withoutmultiplicityweightsasfunctionoflocaltemperatureandpropertimeofthefluidcells.
temperature regions during the early stage of the evolution and the second wave sits near the
cross-overregionandisconnectedwithalargespace-timevolume[21,29]. Thedevelopmentof
thermalphotonellipticflowineveryfluidcellisillustratedinFig.3b. Thermalphotonsemitted
atthelatestageofthecollisionscarrylargeellipticflow. Afterweightingwiththephotonyield
ineveryfluidcell,weobservethatthemostoftheellipticflowsignaliscomingfromthecross-
over region between τ = 3−8 fm/c shown in Fig.3c. In contrast to charged hadrons, whose
momentum distributions only freeze-out below T = 120MeV, thermal photons indeed carry
dec
directdynamicalinformationfromthehighertemperatureregionsandfromearliertime.
Furthermore, color tomographic probes, such as energetic quarks, will lose some of their
energy and radiate soft photons when penetrating the QGP medium. Photons from these jet-
medium interactions were shown as an important source in the direct photon spectra for 2 <
p < 4GeV[46]. Recently,anewphotonproductionprocessduringthejet-mediuminteraction
T
wascomputedinRef.[47].Theinvestigationofitsimportanceinphenomenologicalstudieswith
realistic event-by-event hydrodynamic medium is an active and ongoing research subject. Ad-
ditionally,photonproductionduringthehadronizationstageisproposedinRef.[48,49]. These
processesprobedifferentspace-timeregionsoftheevolvingmedium. Theirrelativeimportance
in the final direct photon signal requires a quantitative calculation of their absolute yield, and
a comparison with the dominant thermal sources. The current state-of-the-art hydrodynamic
framework[25,50]willprovideanexcellenttestgroundforthesenewideas.
Finally,arecentwork[51]studiedthedirectphotonproductioninsmallcollisionsystemsat
theRHICandLHCenergies. Fig.4(a-d)presentstheorypredictionsofthedirectphotonnuclear
modificationfactorRγ forthesecases. A∼50%thermalenhancementoverthepromptpQCD
AA
contribution in the direct photon yield was found for p < 3GeV in minimum bias collisions.
T
Eventhoughtheoverallstrengthofthesignalissmallwhencomparedtothethermalenhance-
ment in Pb+Pb collisions shown in Figs.2d and 2e, such a measurement can serve to support
ChunShen/NuclearPhysicsA00(2016)1–9 6
2.0 pQCD 2.0 PHENIX
Pb1.5 min. bias Au1.5
Rp1.0 Rd1.0
0.5 (a) p+Pb @ 5.02 TeV 0.5 (c) d+Au @ 200 GeV
0.0 0.0
2.0 2.0
RpAu11..05 3HeAu11..05
R
0.5 (b) p+Au @ 200 GeV 0.5 (d) 3He+Au @ 200 GeV
0.0 0.0
1.0 2.0 3.0 4.0 5.0 6.0 1.0 2.0 3.0 4.0 5.0 6.0
p (GeV) p (GeV)
T T
Fig.4.ThenuclearmodificationfactorRγ ofdirectphotonsinsmallcollisionsystems[51].Minimumbiascalculation
AA
ind+Aucollisionsat200GeViscomparedwiththePHENIXmeasurement[52].
theexistenceofathermalizedQGPinsmallsystemsatRHICandtheLHC,thereforeactingto
complementtheevidencecarriedbyhadronicobservables. Theexistingdataind+Aucollisions
atthetopRHICenergystillcarrylargeuncertaintiesandcanthisnotbeusedtodrawdecisive
conclusionontheexistenceofathermalcomponent[52]. Futureelectromagneticmeasurements
with improved accuracy and for different collision systems (p+Au, 3He+Au, and p+Pb) will
guideourunderstandingofthedynamicsandofthepropertiesofsmallQGPdroplets.
6. Conclusionandoutlook
Modeling electromagnetic observables is a double-edged sword, as it is sensitive to every
aspectoftherelativisticheavy-ioncollisions: out-of-equilibriumdynamicsofthecollisionsys-
tems;thermalandtransportpropertiesoftheQGPmatter;andnon-perturbativeaspectsnearthe
cross-over region. For the very same reasons, a well-calibrated calculation can turn the elec-
tromagneticprobesintovaluablemessengersofthefleetingmedium. Thermaldileptonspectra
have the potential to offer a direct access to the thermal property of the fireball. Direct photon
observablesareintimatelyconnectedwiththedynamicalevolutionofthebulkmedium,andcan
beutilizedasaviscometerandasatomographicprobe. Electromagneticprobesshouldcontinue
toplayanactiveroleinthefutureRHIC(BES)programphaseII,andinexperimentsatFAIR.
Yesterday’spuzzlestimulatedtoday’seffortsandwillbecometomorrow’sbackground. The
recenttheoreticalworkhasincreasedtherelativeweightoflatehadronicphotonemissioninthe
directphotonsignal. Thisproducesalargersoftphotonemission,accompaniedbylargerdirect
photon anisotropic flow coefficients. The net effect is to greatly reduced the tension between
theoryandtheexperimentalmeasurements. Indeed,themodel-datacomparisonshavetransited
fromaqualitativedescriptiontoamorequantitativeextractionofthephotonemissionratesand
andamoreprecisedeterminationofthemediumproperties.
Currenttheoreticalcalculationsarelimitedbyuncertaintieslinkedtonon-perturbativephysics;
theinputfromexperimentscanguideandinformfuturecalculations. Firstly,theoryneedsmea-
surementsofthelowmomentum(p < 1GeV)photonspectrainppcollisionsatthetopRHIC
T
and LHC energies. They can reduce the (considerable) theoretical uncertainties in extrapolat-
ing the pQCD prompt source to low p , and provide a reliable baseline for measurements in
T
alllargercollisionsystems. Secondly,areductionofthesystematicuncertaintiesinthecurrent
direct photon measurements can set stronger constraints on our current knowledge of the pho-
ton emission rates and on the dynamical evolution of the collision systems. Finally, it will be
usefulandevenimportanttohavedirectphotonanddileptonmeasurementsinvariouscollisions
ChunShen/NuclearPhysicsA00(2016)1–9 7
systems,e.g. (p,d,3He,Al,Cu)+Au,Au+AuandPb+Pbcollisionsatdifferentenergies: every
collisionsystemhasitsuniquespace-timeevolutionstructure. Acollectionofcollisionsystems
willprovidetheimportantandcomplementarydiagnosticmeasurementsnecessarytoprobehot
anddensenuclearmatterinthedifferentregionsoftheQCDphasediagram.
Acknowledgement: IthankGabrielDenicol,CharlesGale,SangyongJeon,andJean-Franc¸ois
Paquetfortheircollaboration,andElenaBratkovskaya,WolfgangCassing,OlenaLinnyk,Hen-
drikvanHees,MinHe,andRalfRappforprovidingthetheoreticalcalculationsinFig.2,andfor
usefuldiscussions. ThisworkwassupportedbytheNaturalSciencesandEngineeringResearch
CouncilofCanada.
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