Transcript Electrons and Photons: a clock and a thermometer of

Dileptons @RHIC
a clock and a thermometer
of relativistic heavy ion collisions
Alberica Toia
for the PHENIX Collaboration
Stony Brook University / CERN
International School of Nuclear Physics
“Heavy ion collisions from the Coulomb barrier to the Quark-Gluon Plasma”
Erice, September 16-24 2008
Dileptons: radiation from the medium
HADES
CBM
DLS
NA60
(KEK E235)
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//
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158
17
• direct probes
• large emission rate in hot and dense matter
• light vector mesons: signals of chiral transition?
Time
time
gp f
g e L cc m
Jet p K p
freeze-out
hadronization
formation and
thermalization
of quark-gluon
matter?
hard parton scattering
Space
Au
Au
ALICE
[A TeV]
CERES
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10
PHENIX
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//
[A GeV]
200
√sNN [GeV]
Measurement at RHIC:
• different energy
• different initial conditions
• different hydro evolution
• hard processes have larger cross sections
• collider geometry
Dilepton Signal Extraction
arXiv: 0706.3034
Au+Au
• Single electron
– Track reconstruction
– Electron Identification (RICH +
EMCal)
• 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-toback jet
• Real signal
–
di-electron continuum
arXiv: 0802.0050
p+p
Hadronic Cocktail Calculation
• Remaining pairs after background subtraction
–
Real signal + Hadron decay components
• Parameterized PHENIX p0 data with
arXiv: 0802.0050
assumption of p0 = (p++p-)/2
d 3σ
A
E 3 
d p exp( ap T  bp T2 )  p T p 0


n
Other mesons are fit with mT scaling of π0
parameterization pT→√(pT2+mmeson2m π2)
fit the normalization constant
 All mesons mT scale!!!
• Hadronic cocktail was well tuned to
individually measured yield of mesons in
PHENIX for both p+p and Au+Au
collisions.
• 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.
Cocktail Comparison
p+p
arXiv: 0802.0050
Au+Au
• 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)
arXiv: 0706.3034
Centrality
Dependency
LOW MASS
p0 region:
• Agreement with cocktail
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
INTERMEDIATE MASS
submitted to Phys. Rev. Lett
arXiv:0706.3034
Source of the excess?
• Freeze-out Cocktail +
“random” charm +
r spectral function
Low mass
• M>0.4GeV/c2:
some calculations OK
• M<0.4GeV/c2:
data still above theory
calculations
Intermediate mass
• Random charm + thermal
partonic may work
pT-Sliced Mass Spectra
Normalized by the yield in mee < 100MeV
0 < pT < 8 GeV/c
0.7 < pT < 1.5 GeV/c
0 < pT < 0.7 GeV/c
○ Au+Au
● p+p
1.5 < pT < 8 GeV/c
PHENIX Preliminary
• The low mass enhancement decreases with higher pT
• Different shape at low and high pT
 low mass enhancement does not contribute to m<300 MeV/c2 and pT>1 GeV/c?
Low mass & High pT region
p+p
• p+p
– Good agreement between
real and cocktail
– Small excess at higher pT
Au+Au (MB)
1 < pT < 2 GeV/c
2 < pT < 3 GeV/c
3 < pT < 4 GeV/c
4 < pT < 5 GeV/c
• Au+Au
– Good agreement in Mee < 50MeV/c2
– Enhancement is clearly seen above
100MeV/c2.
Sources of e+e- pairs
Dalitz:
p0ge+ehge+ewp0e+efhe+e-
Theoretical predictions:
ppre+ethreshold at 2mp~300MeV
Direct:
Heavy flavor:
re+ecce+e- +X
we+ebbe+e- +X
fe+eJ/ye+ey’e+ecocktail
Cocktail
Hadron Gas
QGP (qqg*e+e-)
Drell-Yan:
qqe+e-
Rising at low mass (~1/m):
due to Dalitz decays of
baryonic/mesonic resonances
a1  pg
a1  pg*  pe+eN*  Ng
N*  Ng*  Ne+ewhere the underlying process is:
e+ e
Gluon Compton
q
qqbarg*e+e-
“threshold-like”
mq < 150 MeV?
Much smaller than
cocktail
g
Mass (GeV/c2)
g*
q
Quarks and gluons can be “free”
(in QGP) or “hidden” in hadrons
Direct Photons Measurement
•
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
direct
 *direct
low mass e+e- pairs
g
g
g inclusive g inclusive
Kroll-Wada formula
d 2N
2

dM ee dN g
3p
4me2
1
2
M ee
S : Process dependent factor

2me2

1  M 2
ee

 1

M S
ee

• Case of Hadrons
2
• S  F M ee
q
g
g*
q
2


M ee


1

2

M hadron 


• Obviously S = 0 at Mee > Mhadron
• Case of g*
• If pT2>>Mee2
S 1
• Possible to separate hadron decay
components from real signal in the
proper mass window.


2
3
e+
e-
Determination of g* fraction, r
Direct g*/inclusive g* is determined by fitting the following
function for each pT bin.
f data M ee   1  r  f cocktailM ee   r  f direct M ee 
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
•
–
Assuming h shape instead of
direct g* shape
•
•
•
c2/NDF=11.6/10
c2/NDF=21.1/10
Twice as much as measured h
yield
Assumption of direct g* is
favorable.
direct g*/inclusive g*
p+p
Au+Au
μ = 0.5pT
μ = 1.0pT
μ = 2.0pT
Base line
Curves : NLO pQCD
calculations with different
theoretical scales done by
W. Vogelsang.
d
NLO
g

/ dpT / d gNLO / dpT  d ghadron / dpT
p+p
• Consistent with NLO pQCD
– better agreement with small µ
Au+Au
• Clear enhancement above NLO
pQCD

Direct Photon Spectra
exp + TAA scaled pp
The virtual direct photon fraction is
converted to the direct photon yield.
*
*
g direct
g direct
g direct
 *
 g direct  *
 g inclusive
g inclusive g inclusive
g inclusive
• p+p
– First measurement in 1-4GeV/c
– Consistent with NLO pQCD and
with EmCal method
– Serves as a crucial reference
Fit to pp
NLO pQCD (W. Vogelsang)
• Au+Au
– Above binary scaled NLO pQCD
– Excess comes from thermal
photons?
A exp( pT / T )  TAA  App (1 pT2 / b)n
exponential
scaled pp
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:
–
–
–
–
–
–
D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006)
From data:
Tini > 220 MeV > TC
From models: Tini = 300 to 600 MeV
t0 = 0.15 to 0.5 fm/c
0.15 fm/c,
0.17 fm/c,
0.2 fm/c,
0.33 fm/c,
0.6 fm/c,
0.5 fm/c,
590 MeV
580 MeV
450-660 MeV
370 MeV
370 MeV
300 MeV
(d’Enterria et al.)
(Rasanen et al.)
(Srivastava et al.)
(Turbide et al.)
(Liu et al.)
(Alam et al.)
Dilepton Spectra
p+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  other source???
Summary
• First dielectron continuum measurement at RHIC
– Despite of low signal/BG
– Thanks to high statistics
– Very good understanding of background normalization
p+p
Au+Au
• Excellent agreement with hadronic decay
cocktail
• Enhancement above the cocktail expectations:
3.4±0.2(stat.) ±1.3(syst.)±0.7(model)
• Centrality dependency: increase faster than
Npart
• Enhancement concentrated at low pT
LOW MASS:
INTERMEDIATE MASS:
• Extract charm and bottom cross sections
• σcc = 544 ± 39 (stat) ± 142 (syst) ± 200
(model) μb
• σbb= 3.9 ± 2.4 (stat) +3/-2 (syst) μb
DIRECT PHOTONS
• p+p in agreement with pQCD
• HBD upgrade will reduce background
 great improvement of systematic and
statistical uncertainty (LMR)
• Silicon Vertex detector will distinguish
charm from prompt contribution (IMR)
LOW MASS:
INTERMEDIATE MASS:
• Coincidence agreement with PYTHIA
• Room for thermal radiation?
DIRECT PHOTONS:
• Dielectron mass shape for pT > 1 GeV and mee <
300MeV consistent with internal conversions of
virtual photons
• Au+Au above pQCD
• excess with inv. slope: Teff = 221 ± 23 (stat) ±
18 (syst)
• well described by hydrodynamical models with
initial coonditions of
Tinit=300–600 MeV at τ0 = 0.15–0.6 fm/c 17
Backup
sQGP @ 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 ?)
Not a new idea…
J.H.Cobb et al., PL 78B, 519 (1978)
• The idea of measuring direct
photon via low mass lepton pair
is not new one. It is as old as
the concept of direct photon.
• This method is first tried at
CERN ISR in search for direct
photon in p+p at 55GeV. They
look for e+e- pairs for
200<m<500 MeV, and they set
one of the most stringent limit
on direct photon production at
low pT
• Later, UA1 measured low mass
muon pairs and deduced the
direct photon cross section.
Credit to try it in PHENIX
goes to Y.Akiba
Previous measurements
CERES
NA60
CERES measured an excess
of dielectron pairs,
confirmed by NA60, rising
faster than linear with
centrality attributed to inmedium modification of the r
spectral function from pp
annihilation.
NA60
CERES
The enhancement
is concentrated
at low pT
Understanding the pT dependency
•
•
•
•
Comparison with
cocktail
Single exponential fit:
–
–
Low-pT: 0<mT<1 GeV
High-pT: 1<mT<2 GeV
–
–
2exponentials
mT-scaling of p0 +
exponential
2-components fits
Low pT:
–
–
inverse slope of
~ 120MeV
accounts for most of
the yield
Extract 2 components
2 EXPONENTIALS
HAGEDORN + EXPONENTIAL
•We fit the sum of 2 exponentials (a*exponential1 + b*exponential2)
•We fit Hagedorn to Mee<100MeV (p0-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
SLOPES
YIELDS
Low-pT yield
2expo fit
mT-scaling +expo fit
Total yield (DATA)
•Intermediate pT: inverse slope increase with mass,
consistent with radial flow
•Low pT:
•inverse slope of ~ 120MeV
•accounts for most of the yield
R.Rapp + H.vanHees
K.Dusling + I.Zahed
E.Bratkovskaja + W.Cassing
Theory Comparison II
•
Freeze-out Cocktail + “random” charm
+ r spectral function
Low mass
•
•
M>0.4GeV/c2:
some calculations OK
M<0.4GeV/c2:
not reproduced
Intermediate mass
•
Random charm + thermal partonic may
work
Low-pT slope not reproduced
PARTONIC
HADRONIC
p-p annihilation
q
q
q
e+ e
Gluon Compton
g
q-q annihilation
g*
q
Theory Comparison II
Calculations from
R.Rapp & H.vanHees
K.Dusling & I.Zahed
E.Bratovskaja & W.Cassing (in 4p)
RHIC
Questions
1. Enhancement at M<2Mp
If pions are massless can pp 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 temperature
Different system evolution
Do we miss something in the system evolution
which may have different relevance at
SPS and at RHIC?
SPS
PHENIX
(Pioneering High Energy Nuclear Interaction eXperiment)
designed to measure rare probes:
Au-Au & p-p spin
•
•
•
+ high rate capability & granularity
+ good mass resolution and particle ID
- limited acceptance
2 central arms:
electrons, photons, hadrons
– charmonium J/y, y’ > ee
– vector meson r, w, f > ee
– high pT
po, p, p
– direct photons
– open charm
– hadron physics
2 muon arms:
muons
– “onium” J/y, y’,  > mm
– vector meson f > mm
– open charm
p
g
e
PC3
combined central and muon arms:
charm production
DD > em
e+
DC
PC1
magnetic field &
tracking detectors
•
global detectors
forward energy and multiplicity
– event characterization
Photon conversion rejection
g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
Conversion removed with
orientation angle of the pair in
the magnetic field
Photon conversion
B
r ~ mee
z
Dalitz decay
e-
B
y
x
Beampipe
MVD support structures
z
Conversion pair
e+
e-
x
y
e+
Inclusive
Removed by phiV cut
After phiV cut
Photon conversion cut
No cut
M<30 MeV & fV<0.25 &
M<600 MeV & fV<0.04
M<600 MeV & fV<0.06
M<600 MeV & fV<0.08
M<600 MeV & fV<0.10
M<600 MeV & fV<0.12
M<600 MeV & fV<0.14
M<600 MeV & fV<0.20
M<600 MeV & fV<0.40
Physical background
Background is charge-independent
Calculate the shape with MC
Normalize to the like-sign spectra
 Good description of the data
Semi-correlated Background
• p0g g*
e+eX
e+e-
• “jets”
e+
e+
e-
arXiv: 0802.0050
e-γ
π0
π0
π0
γ
γ
e+
e-
Photon conversion
Conversion pair
z
B
x
y
e+
e-
x
Dalitz decay
z
-
e
B
y
e+
ge+e- at r≠0 have m≠0
(artifact of PHENIX tracking)
Conversion removed with orientation
angle of the pair in the magnetic field
Combinatorial Background
LIKE SIGN SPECTRA
Use same event topology
(centrality, vertex, reaction plane)
Remove every unphysical correlation
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 (> 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-- = 1.004
Systematic error (conservative): 0.2%
TOTAL
SYSTEMATIC
ERROR = 0.25%
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%
33
Acceptance
q0
charge/pT
• Define acceptance filter (from real data)
• Correct only for efficiency IN the acceptance
• “Correct” theory predictions IN the acceptance
pT
z vertex
f0
Single electron pT > 200 MeV
Pair mT > 400 MeV
Not an analysis cut, but a constrain from
the magnetic field
mass
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 yield
does not increase
Photonic single electron: x 2.3
Ne Electron yield
Inclusive single electron :x 1.6
converter
0.8% 0.4%
1.7%
Combinatorial pairs :x 2.5
With converter
Photon Converter (Brass: 1.7% X0)
Photonic
W/O converter
Dalitz : 0.8% X0 equivalent
radiation length
Non-photonic
0
Material amounts: 0