direct photon measurements: initial conditions of heavy ion reactions at rhic
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Direct Photon Measurements: initial conditions of heavy ion reactions at RHIC. Alberica Toia for the PHENIX Collaboration Stony Brook University / CERN. IV Workshop on Particle Correlations and Femptoscopy Krakow, September 11-14 2008. Too hot for quarks to bind!!! - PowerPoint PPT PresentationTRANSCRIPT
Direct Photon Measurements:initial conditions of heavy ion
reactions at RHIC
Alberica Toiafor the PHENIX Collaboration
Stony Brook University / CERN
IV Workshop on Particle Correlations and Femptoscopy
Krakow, September 11-14 2008
Evolution of the Universe
Nucleosynthesis builds nuclei up to HeNuclear Force…Nuclear Physics
Universe too hot for electrons to bindE-M…Atomic (Plasma) Physics
E/M Plasma
Too hot for quarks to bind!!!Too hot for quarks to bind!!!Standard Model (N/P) Physics
Quark-Gluon
Plasma??
Too hot for nuclei to bindNuclear/Particle (N/P) Physics Hadron
Gas
SolidLiquidGas
Today’s Cold UniverseGravity…Newtonian/General Gravity…Newtonian/General
RelativityRelativity
The “Little Bang” in the lab• High energy nucleus-nucleus
collisions: – fixed target (SPS: √s=20GeV)– colliders
• RHIC: √s=200GeV • LHC: √s=5.5TeV
• QGP formed in a tiny region (10-14m) for very short time (10-23s)
– Existence of a mixed phase?– Later freeze-out
• Collision dynamics: different observables sensitive to different reaction stages
• 2 counter-circulating rings, 3.8 km circumference
• Top energies (each beam):– 100 GeV/nucleon Au-Au.– 250 GeV polarized p-p.– Mixed Species (e.g. d-Au)
Probing Heavy Ion Collisionstime
hard parton scattering
AuAu
hadronization
freeze-out
formation and thermalization of quark-gluonmatter?
Space
Time
expansion
Jet cc e p K
Photons and dileptons: radiation from the media– direct probes of any collision stages (no final-state
interactions)– large emission rates in hot and dense matter – according to the VMD their production is mediated in the
hadronic phase by the light neutral vector mesons (ρ, ω, and φ) which have short life-time Changes in position and width: signals of the chiral transition?
•Direct photon sources:– Compton scattering
qg q
– Annihilationqq g
– Bremsstrahlungfrom inelastic scattering of incoming or thermalized partons
Energy density in heavy ion collisions
• T.D.Lee: “In HEP we have concentrated on experiments in which we distribute a higher and higher amount of energy into a region with smaller and smaller dimensions.” [Rev. Mod. Phys. 47 (1975) 267]
• Energy density: “Bjorken estimate” (for a longitudinally expanding plasma):
Transverse Energy
central 2%
PHENIX130 GeV
int ~ 100x nucleus
~ 10x critical
The matter is so dense that it melts(?) J/ (and regenerates it ?)
sQGP @ RHIC
The matter is so opaque that even a 20 GeV 0 is stopped
The matter is so strongly coupled that even heavy quarks flow
The matter is so dense that it modifies the shape of jets
PHENIX preliminary
strongly interacting Quark-Gluon Plasma (sQGP) in HI collisions at RHIC
The matter is so dense that even heavy quarks are stopped
What does it emit?What is the temperature?
Photon Emission • Quark Gluon Plasma
– De-confined phase of quarks and gluonsshould emit thermal radiation
• Direct photons are an important probe to investigate the characteristics of evolution of the matter created by heavy ion collisions.– Penetrate the strong interacting matter– Emitted from every stage of collisions
• Hard photons (High pT)– Initial hard scattering, Pre-equilibrium
• Thermal photons (Low pT)– Thermodynamic information from QGP
and hadron gas measure temperature of the matter
– Dominant source for 1<pT<3 GeV/c– Measurement is difficut since the
expected signal is only 1/10 of photons from hadron decays
T
1nphard:
/ E Tethermal:Decay photons(0→, →, …)
S.Turbide et al PRC 69 014903
S.Turbide et al PRC 69 014903
Direct photons in p+p and d+Au
• p+pTest of QCD– direct participant in partonic
interaction– Less dependent on FF than
hadron production• Reduce uncertainty on pQCD
photons in A+A
• good agreement with NLO pQCD
• Important baseline for Au+Au
• d+Au: – initial-state nuclear effects – no final-state effects (no medium
produced)– Study initial-state effects
2
Extended in RUN5 data
Blue line: Ncoll scaled p+p cross-section
Direct Photons in Au+Au
Au-Au data consistent with pQCD calculation
scaled by Ncoll
THERMAL PHOTONS?Measurement at low pT (where an excess above the know sources may hint to thermal photon production) difficult because of detector resolution
Alternative: Virtual Photons• Any source of real can emit * with very low mass.• If the Q2 (=m2) of virtual photon is sufficiently small, the source strength should be the
same• The ratio of real photon and quasi-real photon can be calculated by QED Real photon yield can be measured from virtual photon yield, which is observed as
low mass e+e- pairs
SMM
m
M
m
dNdM
Nd
eeee
e
ee
e
ee
121
41
3
22
2
2
22
inclusive
direct
inclusive
direct
Kroll-Wada formula
S : Process dependent factor• Case of Hadrons
•
• Obviously S = 0 at Mee > Mhadron
• Case of *• If pT
2>>Mee2
• Possible to separate hadron decay components from real signal in the proper mass window.
3
2
222 1
hadron
eeee M
MMFS
1S
q
g q
e+
e-
Signal ExtractionarXiv: 0706.3034arXiv: 0802.0050 Au+A
up+p
• Real signal– di-electron continuum
• Background sources– Combinatorial background– Material conversion pairs– Additional correlated background
• Visible in p+p collisions• Cross pairs from decays with 4
electrons in the final state• Pairs in same jet or back-to-back
jet
Hadronic Cocktail Calculation
arXiv: 0802.0050
n0T2TT
3
3
pp)bpapexp(
A
pd
σdE
• Remaining pairs after background subtraction– Real signal + Hadron decay components
• Estimate hadron components using hadronic cocktail
• Mass distributions from hadron decays are simulated by Monte Carlo.– 0, , ’, , , , J/, ’
• Effects on real data are implemented.– PHENIX acceptance, detector effect, efficiencies …
• Parameterized PHENIX 0 data with assumption of 0 = (++-)/2
• Hadronic cocktail was well tuned to individually measured yield of mesons in PHENIX for both p+p and Au+Au collisions.
Cocktail ComparisonarXiv: 0802.0050 arXiv: 0706.3034Au+A
up+p
• p+p– Excellent agreement with cocktail
• Au+Au– Large enhancement in low mass region
– Integrated yield in 150MeV < mee < 750MeV• data/cocktail = 3.4 ± 0.2(stat) ± 1.3(sys) ± 0.7(model)
0 < pT < 8 GeV/c
0 < pT < 0.7 GeV/c
0.7 < pT < 1.5 GeV/c
1.5 < pT < 8 GeV/c
PHENIX Preliminary
pT-Sliced Mass Spectra
○ Au+Au● p+p
Normalized by the yield in mee < 100MeV
• The low mass enhancement decreases with higher pT
• No significant indication that this low mass enhancement contribute to m<300 MeV/c2 and pT>1 GeV/c
• We assume that excess is entirely due to internal conversion of direct
Low mass & High pT region
p+p Au+Au (MB) 1 < pT < 2 GeV/c2 < pT < 3 GeV/c3 < pT < 4 GeV/c4 < pT < 5 GeV/c
• p+p– Good agreement between
real and cocktail
– Small excess at higher pT
• Au+Au– Good agreement in Mee <
50MeV/c2
– Enhancement is clearly seen above 100MeV/c2.
Determination of * fraction, r
r : direct */inclusive *
Direct */inclusive * is determined by fitting the following function for each pT bin.
eedirecteecocktaileedata MfrMfrMf 1
Mee (GeV/c2)
Reminder : fdirect is given by Kroll-Wada formula with S = 1.
• Fit in 80-300MeV/c2 gives– Assuming direct * mass
shape• 2/NDF=11.6/10
– Assuming shape instead of direct * shape
• 2/NDF=21.1/10• Twice as much as
measured yield
• Assumption of direct * is favorable.
direct */inclusive *
μ = 0.5pT
μ = 1.0pT
μ = 2.0pT
μ = 0.5pT
μ = 1.0pT
μ = 2.0pTBase line Curves : NLO pQCD calculations with different theoretical scales done by W. Vogelsang.
ThadronT
NLOT
NLO dpddpddpd //// p+p• Consistent with NLO pQCD
– better agreement with small µ
Au+Au• Clear enhancement above NLO pQCD
p+p Au+Au
Direct Photon Spectra
inclusiveinclusive
directdirect
inclusive
direct
inclusive
direct
The virtual direct photon fraction is converted to the direct photon yield.
• p+p– First measurement in 1-4GeV/c– Consistent with NLO pQCD and
with EmCal method– Serves as a crucial reference
• Au+Au– Above binary scaled NLO pQCD– Excess comes from thermal
photons?NLO pQCD (W. Vogelsang)
Fit to pp
exp + TAA scaled pp
nTppAAT bpATTpA )/1()/exp( 2
scaled ppexponential
1st measurement of Thermal Radiation• Au+Au = pQCD + exp.
T = 221 23 (stat) 18 (sys)
• Initial temperatures and times from theoretical model fits to data:
– 0.15 fm/c, 590 MeV (d’Enterria et al.)– 0.17 fm/c, 580 MeV (Rasanen et al.)– 0.2 fm/c, 450-660 MeV (Srivastava et al.)– 0.33 fm/c, 370 MeV (Turbide et al.)– 0.6 fm/c, 370 MeV (Liu et al.)– 0.5 fm/c, 300 MeV (Alam et al.)
From data: Tini > 220 MeV > TC From models: Tini = 300 to 600 MeV = 0.15 to 0.5 fm/c
D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006)
Dilepton Spectrap+p Au+Au
• p+p– Agreement with cocktail
• Au+Au– pT>1GeV/c: small excess internal
conversion of direct photons
– pT<1GeV/c: large excess q-q, -, …?
SLOPE ANALYSIS• Single exponential fit:
– Low-pT: 0<mT<1 GeV– High-pT: 1<mT<2 GeV
• 2-components fits– 2exponentials– mT-scaling of 0 +
exponential• Low pT:
– inverse slope of ~ 120MeV
– accounts for most of the yield
Previous measurements
The enhancement is concentrated at low pT
CERES measured an excess of dielectron pairs, confirmed by NA60, rising faster than linear with centrality attributed to in-medium modification of the spectral function from annihilation.
CERES
CERES
NA60
NA60
Summary• We have measured e+e- pairs in p+p and Au+Au
collisions at √sNN=200 GeV– Large excess above hadronic background is observed
• For m<300MeV/c2 and 1<pT<5 GeV/c– Excess is much greater in Au+Au than in p+p
• Treating the excess as internal conversion of direct photons, the yield of direct photon is deduced.
• Direct photon yield in p+p is consistent with a NLO pQCD• Direct photon yield in Au+Au is much larger.
– Spectrum shape above TAA scaled pp is exponential, with inverse slope T=221 ±23(stat)±18(sys) MeV
• Hydrodynamical models with Tinit=300-600MeV at 0=0.6-0.15 fm/c are in qualitative agreement with the data.
• Additional excess in Au+Au at pT<1GeV/c– Inverse slope T~120 MeV Additional source of virtual g around Tcrit, responsible of most of
the inclusive dilepton yield, so far not explained by theories…
Backup
Centrality Dependency
submitted to Phys. Rev. Lett
arXiv:0706.3034
0 region:•Agreement with cocktail
LOW MASS
Low Mass:•yield increases faster than
proportional to Npart enhancement from binary annihilation (ππ or qq) ?
Intermediate Mass:•yield increase proportional to
Ncoll charm follows binary scaling
Understanding the pT dependency
• Comparison with cocktail
• Single exponential fit:– Low-pT: 0<mT<1 GeV– High-pT: 1<mT<2 GeV
• 2-components fits– 2exponentials– mT-scaling of 0 +
exponential• Low pT:
– inverse slope of ~ 120MeV
– accounts for most of the yield
Theory Comparison II• Freeze-out Cocktail + “random”
charm + spectral functionLow mass• M>0.4GeV/c2:
some calculations OK• M<0.4GeV/c2:
not reproducedIntermediate mass• Random charm + thermal partonic
may work
Low-pT slope not reproduced
Gluon Compton
q
g q
e+
e-q
q
HADRONIC
PARTONIC
- annihilationq-q annihilation
R.Rapp + H.vanHeesK.Dusling + I.ZahedE.Bratkovskaja + W.Cassing
Extract 2 components2 EXPONENTIALS HAGEDORN + EXPONENTIAL
•We fit the sum of 2 exponentials (a*exponential1 + b*exponential2) •We fit Hagedorn to Mee<100MeV (0-dominated)
•Then we fit (a*mT-scaling + exponential) to the other mass bins•Because of their different curvature, mT-scaling and the exponential account for more or less of the yield in the low-pT region.
Yields and Slopes
•Intermediate pT: inverse slope increase with mass, consistent with radial flow•Low pT:
•inverse slope of ~ 120MeV •accounts for most of the yield
SLOPES YIELDS
Total yield (DATA)
2expo fitmT-scaling +expo
fit
Low-pT yield
Theory Comparison II
Calculations fromR.Rapp & H.vanHeesK.Dusling & I.ZahedE.Bratovskaja & W.Cassing (in 4)
Questions
1. Enhancement at M<2M
If pions are massless can annihilation produce ee with M<300MeV?
2. Enhancement at low pT, with T~120 MeV and now flow
Is the same low-pT enhancement seen at SPS never reproduced by theory?
Different initial temperatureDifferent system evolutionDo we miss something in the system
evolution which may have different relevance at SPS and at RHIC?
RHIC SPS
PHENIX (Pioneering High Energy Nuclear Interaction eXperiment)
• 2 central arms: electrons, photons, hadrons– charmonium J/, ’ ee
– vector meson ee – high pT
– direct photons– open charm – hadron physics
• 2 muon arms: muons– “onium” J/, ’, – vector meson – open charm
• combined central and muon arms: charm production DD e
• global detectors forward energy and multiplicity– event characterization
Au-Au & p-p spin
PC1
PC3
DC
magnetic field &tracking detectors
e+e
designed to measure rare probes: + high rate capability & granularity+ good mass resolution and particle ID- limited acceptance
Photon conversion rejectionConversion removed with orientation angle of the pair in the magnetic field
Photon conversion
e+e- at r≠0 have m≠0(artifact of PHENIX tracking: no tracking before the field)• effect low mass region• have to be removed
Beampipe
MVD support structures
r ~ mee
InclusiveRemoved by phiV cutAfter phiV cut
z
y
x e+e-
BConversion pair
z
y
xe+
e-
B
Dalitz decay
Photon conversion cutNo cutM<30 MeV & V<0.25 &
M<600 MeV & V<0.04 M<600 MeV & V<0.06 M<600 MeV & V<0.08 M<600 MeV & V<0.10 M<600 MeV & V<0.12 M<600 MeV & V<0.14 M<600 MeV & V<0.20 M<600 MeV & V<0.40
Physical backgroundSemi-correlated Background
π0
π0
e+
e-
e+
e-
γ
γ
π0
e-
γ
e+
X
• 0*
e+e- e+e-
• “jets”
z
y
x e+ e-
B
Conversion pairz
y
x e+
e-
B
Dalitz decay
Photon conversion
e+e- at r≠0 have m≠0(artifact of PHENIX tracking)Conversion removed with orientation angle of the pair in the magnetic field
Background is charge-independentCalculate the shape with MCNormalize to the like-sign spectra Good description of the data
arXiv: 0802.0050
Combinatorial BackgroundPHENIX 2 arm spectrometer acceptance:
dNlike/dm ≠ dNunlike/dm different shape need event mixing(like/unlike differences preserved)Use Like sign as a cross check for the shape and to determine normalization Small signal in like sign at low massN++ and N–- estimated from the mixed events like sign B++ and B-- normalized at high mass (> 700 MeV)Normalization: 2√N++ N-- Uncertainty due to statistics of N++ and N--: 0.12%Correction for asymmetry of pair cut K=k+-/√k++ k-- = 1.004Systematic error (conservative): 0.2%
TOTAL SYSTEMATIC ERROR = 0.25%
LIKE SIGN SPECTRA
Use same event topology (centrality, vertex, reaction plane)Remove every unphysical correlation
Comparison of BG subtraction Methods
36
to determine syst. uncertainty:spread of two extreme cases (blue & orange): 5-10%
Monte Carlo methodLike sign method (with some variations)give consistent results over the full invariant mass range
Acceptancech
arg
e/p
T
0
0
z vertex
• Define acceptance filter (from real data)• Correct only for efficiency IN the acceptance• “Correct” theory predictions IN the acceptance
Single electron pT > 200 MeVPair mT > 400 MeVNot an analysis cut, but a constrain from the magnetic field
mass
pT
Cross check Converter MethodWe know precise radiation length (X0) of each detector material
The photonic electron yield can be measured by increase of additional material (photon converter was installed)The non-photonic electron yield
does not increasePhotonic single electron: x 2.3Inclusive single electron :x 1.6Combinatorial pairs :x 2.5
Photon Converter (Brass: 1.7% X0)
Ne Electron yield
Material amounts:
0
0.4% 1.7%
Dalitz : 0.8% X0 equivalent radiation length
0
With converter
W/O converter
0.8%
Non-photonic
Photonic
converter