Transcript Electrons and Photons: a clock and a thermometer of

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
Evolution of the Universe
Too hot for quarks to bind!!!
Standard Model (N/P) Physics
QuarkGluon
Plasma??
Too hot for nuclei to bind
Nuclear/Particle (N/P) Physics
Hadron
Gas
Nucleosynthesis builds nuclei up to He
Nuclear Force…Nuclear Physics
Universe too hot for electrons to bind
E/M
Plasma
E-M…Atomic (Plasma) Physics
Solid
Liquid
Gravity…Newtonian/General Relativity
Gas
Today’s Cold Universe
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)
Time
time
g
Probing Heavy Ion Collisions
p f Jet p K g p
e L cc m
freeze-out
hadronization
formation and
thermalization
of quark-gluon
matter?
hard parton scattering
Space
Au
Au
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  gq
– Annihilation
qq  gg
– Bremsstrahlung
from 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
PHENIX
130 GeV
central 2%
int ~ 100x nucleus
~ 10x critical
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 ?)
Photon Emission
• Quark Gluon Plasma
– De-confined phase of quarks and gluons
should emit thermal radiation
• Direct photons are an important
probe to investigate the
characteristics of evolution of the
matter created by heavy ion
collisions.
thermal: e  Eg /T
Decay photons
(p0→g+g, h→g+g, …)
1
hard: p n
T
– 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
S.Turbide et al PRC 69 014903
Direct photons in p+p and d+Au
• p+p
Test of QCD
– direct participant in partonic
interaction
– Less dependent on FF than
hadron production
Extended in
RUN5 data
• Reduce uncertainty on pQCD
photons in A+A
• good agreement with NLO
pQCD
• Important baseline for Au+Au
2
• d+Au:
– initial-state nuclear effects
– no final-state effects (no
medium produced)
– Study initial-state effects
Direct Photons in Au+Au
Blue line: Ncoll scaled p+p 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
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
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-
p+p
Signal Extraction
Au+Au
arXiv: 0802.0050
arXiv: 0706.3034
• Real signal
•
–
di-electron continuum
–
–
–
Combinatorial background
Material conversion pairs
Additional correlated background
Background sources
•
•
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
• Remaining pairs after background subtraction
– Real signal + Hadron decay components
• Estimate hadron components using hadronic cocktail
arXiv: 0802.0050
• 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 = (p++p-)/2
d 3σ
A
E 3 
d p exp( ap T  bp T2 ) + p T p 0

•

n
Hadronic cocktail was well tuned to
individually measured yield of mesons in
PHENIX for both p+p and Au+Au collisions.
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
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
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 g
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.
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
•
•
•
Mee (GeV/c2)
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
SLOPE ANALYSIS
• Single exponential fit:
– Low-pT: 0<mT<1 GeV
– High-pT: 1<mT<2 GeV
• 2-components fits
– 2exponentials
– mT-scaling of p0 +
exponential
• Low pT:
– inverse slope of
~ 120MeV
– accounts for most of the
yield
• p+p
– Agreement with cocktail
• Au+Au
– pT>1GeV/c: small excess  internal
conversion of direct photons
– pT<1GeV/c: large excess  q-q, p-p, …?
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
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 t0=0.60.15 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
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
submitted to Phys. Rev. Lett
arXiv:0706.3034
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
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
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
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
y
x
Beampipe
MVD support structures
z
Conversion pair
e+
e-
x
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%
36
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