Direct Photon Measurements: initial conditions of heavy ion reactions at RHIC

1
Direct Photon Measurementsinitial 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
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Evolution of the Universe
Quark-GluonPlasma??
3
The Little Bang in the lab

• High energy nucleus-nucleus collisions
• fixed target (SPS vs20GeV)
• colliders
• RHIC vs200GeV
• LHC vs5.5TeV
• 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)

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Probing Heavy Ion Collisions

• Direct photon sources
• Compton scattering
• qg ? gq
• Annihilation
• qq ? gg
• Bremsstrahlung
• from inelastic scattering of incoming or
  thermalized partons

• 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 (?, ?, and f) which have short life-time?
  Changes in position and width signals of the
  chiral transition?

5
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
PHENIX 130 GeV
central 2
?int 100x enucleus 10x ecritical
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sQGP _at_ RHIC
strongly interacting Quark-Gluon Plasma (sQGP)
in HI collisions at RHIC
The matter is so opaque that even a 20 GeV p0 is
stopped
The matter is so dense that even heavy quarks are
stopped
What does it emit? What is the temperature?
The matter is so strongly coupled that even heavy
quarks flow
PHENIX preliminary
The matter is so dense that it modifies the shape
of jets
The matter is so dense that it melts(?) J/y (and
regenerates it ?)
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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 1ltpTlt3 GeV/c
• Measurement is difficut since the expected signal
  is only 1/10 of photons from hadron decays

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Direct photons in pp and dAu

• ppTest of QCD
• direct participant in partonic interaction
• Less dependent on FF than hadron production
• Reduce uncertainty on pQCD photons in AA
• good agreement with NLO pQCD
• Important baseline for AuAu
• dAu
• initial-state nuclear effects
• no final-state effects (no medium produced)
• Study initial-state effects

2
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Direct Photons in AuAu
Blue line Ncoll scaled pp cross-section
THERMAL PHOTONS? Measurement at low pT (where an
excess above the know sources may hint to thermal
photon production) difficult because of detector
resolution
Au-Au data consistent with pQCD calculation
scaled by Ncoll
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Alternative Virtual Photons

• Any source of real g can emit g 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 ee-
  pairs

Kroll-Wada formula
S Process dependent factor

• Case of Hadrons
• Obviously S 0 at Mee gt Mhadron
• Case of g
• If pT2gtgtMee2
• Possible to separate hadron decay components
  from real signal in the proper mass window.

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Signal Extraction
AuAu
pp
arXiv 0706.3034
arXiv 0802.0050

• Real signal
• di-electron continuum
• Background sources
• Combinatorial background
• Material conversion pairs
• Additional correlated background
• Visible in pp collisions
• Cross pairs from decays with 4 electrons in the
  final state
• Pairs in same jet or back-to-back jet

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Hadronic Cocktail Calculation

• 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.
• p0, h, h, w, f, r, J/y, y
• Effects on real data are implemented.
• PHENIX acceptance, detector effect, efficiencies
  
• Parameterized PHENIX p0 data with assumption of
  p0 (pp-)/2
• Hadronic cocktail was well tuned to individually
  measured yield of mesons in PHENIX for both pp
  and AuAu collisions.

arXiv 0802.0050
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Cocktail Comparison
AuAu
pp
arXiv 0802.0050
arXiv 0706.3034

• pp
• Excellent agreement with cocktail
• AuAu
• Large enhancement in low mass region
• Integrated yield in 150MeV lt mee lt 750MeV
• data/cocktail 3.4 0.2(stat) 1.3(sys)
  0.7(model)

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pT-Sliced Mass Spectra
Normalized by the yield in mee lt 100MeV

• AuAu
• pp

• The low mass enhancement decreases with higher
  pT
• No significant indication that this low mass
  enhancement contribute to mlt300 MeV/c2 and pTgt1
  GeV/c
• We assume that excess is entirely due to internal
  conversion of direct g

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Low mass High pT region
1 lt pT lt 2 GeV/c 2 lt pT lt 3 GeV/c 3 lt pT lt 4
GeV/c 4 lt pT lt 5 GeV/c

• pp
• Good agreement between real and cocktail
• Small excess at higher pT

• AuAu
• Good agreement in Mee lt 50MeV/c2
• Enhancement is clearly seen above 100MeV/c2.

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Determination of g fraction, r
Direct g/inclusive g is determined by fitting
the following function for each pT bin.
Reminder fdirect is given by Kroll-Wada formula
with S 1.
r direct g/inclusive g

• Fit in 80-300MeV/c2 gives
• Assuming direct g mass shape
• c2/NDF11.6/10
• Assuming h shape instead of direct g shape
• c2/NDF21.1/10
• Twice as much as measured h yield
• Assumption of direct g is favorable.

Mee (GeV/c2)
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direct g/inclusive g
pp
AuAu
µ 0.5pT µ 1.0pT µ 2.0pT
Base line Curves NLO pQCD calculations with
different theoretical scales done by W. Vogelsang.

• pp
• Consistent with NLO pQCD
• better agreement with small µ

• AuAu
• Clear enhancement above NLO pQCD

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Direct Photon Spectra
exp TAA scaled pp
The virtual direct photon fraction is converted
to the direct photon yield.

• pp
• First measurement in 1-4GeV/c
• Consistent with NLO pQCD and with EmCal method
• Serves as a crucial reference
• AuAu
• Above binary scaled NLO pQCD
• Excess comes from thermal photons?

Fit to pp
NLO pQCD (W. Vogelsang)
scaled pp
exponential
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1st measurement of Thermal Radiation

• AuAu pQCD exp.
• ? T 221 ? 23 (stat) ? 18 (sys)
• Initial temperatures and times from theoretical
  model fits to data
• 0.15 fm/c, 590 MeV (dEnterria 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.)

D.dEnterria, D.Peressounko, Eur.Phys.J.C 46
(2006)
From data Tini gt 220 MeV gt TC From
models Tini 300 to 600 MeV
t0 0.15 to 0.5 fm/c
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Dilepton Spectra

• SLOPE ANALYSIS
• Single exponential fit
• Low-pT 0ltmTlt1 GeV
• High-pT 1ltmTlt2 GeV
• 2-components fits
• 2exponentials
• mT-scaling of p0 exponential
• Low pT
• inverse slope of 120MeV
• accounts for most of the yield

pp
AuAu

• pp
• Agreement with cocktail
• AuAu
• pTgt1GeV/c small excess ? internal conversion of
  direct photons
• pTlt1GeV/c large excess ? q-q, p-p, ?

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Previous measurements
NA60
CERES
CERES measured an excess of dielectron pairs,
confirmed by NA60, rising faster than linear with
centrality attributed to in-medium modification
of the r spectral function from pp annihilation.
NA60
CERES
The enhancement is concentrated at low pT
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Summary

• We have measured ee- pairs in pp and AuAu
  collisions at vsNN200 GeV
• Large excess above hadronic background is
  observed
• For mlt300MeV/c2 and 1ltpTlt5 GeV/c
• Excess is much greater in AuAu than in pp
• Treating the excess as internal conversion of
  direct photons, the yield of direct photon is
  deduced.
• Direct photon yield in pp is consistent with a
  NLO pQCD
• Direct photon yield in AuAu is much larger.
• Spectrum shape above TAA scaled pp is
  exponential, with inverse slope T221
  23(stat)18(sys) MeV
• Hydrodynamical models with Tinit300-600MeV at
  t00.6-0.15 fm/c are in qualitative agreement
  with the data.
• Additional excess in AuAu at pTlt1GeV/c
• Inverse slope T120 MeV
• ? Additional source of virtual g around Tcrit,
  responsible of most of the inclusive dilepton
  yield, so far not explained by theories

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Backup
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Centrality Dependency
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Understanding the pT dependency

• Comparison with cocktail
• Single exponential fit
• Low-pT 0ltmTlt1 GeV
• High-pT 1ltmTlt2 GeV
• 2-components fits
• 2exponentials
• mT-scaling of p0 exponential
• Low pT
• inverse slope of 120MeV
• accounts for most of the yield

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Theory Comparison II
R.Rapp H.vanHees K.Dusling I.Zahed E.Bratkovsk
aja W.Cassing

• Freeze-out Cocktail random charm r spectral
  function
• Low mass
• Mgt0.4GeV/c2 some calculations OK
• Mlt0.4GeV/c2 not reproduced
• Intermediate mass
• Random charm thermal partonic may work
• Low-pT slope not reproduced

PARTONIC
HADRONIC
p-p annihilation
q-q annihilation
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Extract 2 components
2 EXPONENTIALS
HAGEDORN EXPONENTIAL

• We fit the sum of 2 exponentials (aexponential1
  bexponential2)
• We fit Hagedorn to Meelt100MeV (p0-dominated)
• Then we fit (amT-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.

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Yields and Slopes
SLOPES

• Intermediate pT inverse slope increase with
  mass, consistent with radial flow
• Low pT
• inverse slope of 120MeV
• accounts for most of the yield

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Theory Comparison II
Calculations from R.Rapp H.vanHees K.Dusling
I.Zahed E.Bratovskaja W.Cassing (in 4p)
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Questions
SPS
RHIC
1. Enhancement at Mlt2Mp If pions are massless can
pp annihilation produce ee with Mlt300MeV? 2.
Enhancement at low pT, with T120 MeV and now
flow Is the same low-pT enhancement seen at SPS
never reproduced by theory? Different initial
temperature Different system evolution Do we miss
something in the system evolution which may have
different relevance at SPS and at RHIC?
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PHENIX (Pioneering High Energy Nuclear
Interaction eXperiment)
designed to measure rare probes high rate
capability granularity good mass
resolution and particle ID - limited
acceptance
Au-Au p-p spin

• 2 central arms
• electrons, photons, hadrons
• charmonium J/?, ? -gt ee-
• vector meson r, w, ? -gt ee-
• high pT po, p, p-
• direct photons
• open charm
• hadron physics
• 2 muon arms muons
• onium J/?, ?, ? -gt mm-
• vector meson ? -gt mm-
• open charm
• combined central and muon arms
• charm production DD -gt em

• global detectors
• forward energy and multiplicity
• event characterization

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Photon conversion rejection

• g?ee- at r?0 have m?0(artifact of PHENIX
  tracking
• no tracking before the field)
• effect low mass region
• have to be removed

Conversion removed with orientation angle of the
pair in the magnetic field
Photon conversion
r mee
Inclusive Removed by phiV cut After phiV cut
Beampipe
MVD support structures
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Photon conversion cut
No cut Mlt30 MeV fVlt0.25 Mlt600 MeV fVlt0.04
Mlt600 MeV fVlt0.06 Mlt600 MeV fVlt0.08
Mlt600 MeV fVlt0.10 Mlt600 MeV fVlt0.12
Mlt600 MeV fVlt0.14 Mlt600 MeV fVlt0.20
Mlt600 MeV fVlt0.40
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Physical background
Semi-correlated Background
Background is charge-independent Calculate the
shape with MC Normalize to the like-sign
spectra ? Good description of the data

• p0?g g
• ee-
• ee-
• jets

X
arXiv 0802.0050
Photon conversion
g?ee- at r?0 have m?0(artifact of PHENIX
tracking) Conversion removed with orientation
angle of the pair in the magnetic field
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Combinatorial Background

• PHENIX 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 mass
• N and N- estimated from the mixed events like
  sign B and B-- normalized at high mass (gt 700
  MeV)
• Normalization 2vN N--
• Uncertainty due to statistics of N and N--
  0.12
• Correction for asymmetry of pair cut
• Kk-/vk k-- 1.004Systematic error
  (conservative) 0.2

LIKE SIGN SPECTRA
TOTAL SYSTEMATIC ERROR 0.25
Use same event topology (centrality, vertex,
reaction plane) Remove every unphysical
correlation
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Comparison of BG subtraction Methods
Monte Carlo method Like sign method (with some
variations) give consistent results over the full
invariant mass range

• to determine syst. uncertainty
• spread of two extreme cases (blue orange) 5-10

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Acceptance
q0

• Define acceptance filter (from real data)
• Correct only for efficiency IN the acceptance
• Correct theory predictions IN the acceptance

charge/pT
z vertex
pT
f0

• Single electron pT gt 200 MeV
• Pair mT gt 400 MeV
• Not an analysis cut, but a constrain from the
  magnetic field

mass
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Cross check Converter Method
We 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 yielddoes not increase Photonic single
electron x 2.3 Inclusive single electron x
1.6 Combinatorial pairs x 2.5
Photon Converter (Brass 1.7 X0)