1 dileptons: getting to the heart of the matter thomas k hemmick stony brook university
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Dileptons: Getting to the Heart of the Matter
Thomas K HemmickStony Brook University
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COURAGE
INTENTION2
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Thomas K Hemmick
This talk is not targeted at the experts. Students should EXPECT to understand. Whenever the speaker fails to meet this
expectation:
INTERRUPT!
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For the Students!
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Thomas K Hemmick 4
What Physics do You See?
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The water droplets on the window demonstrate a principle.
Truly beautiful physics is expressed in systems whose underlying physics is QED.
Stony Brook UniversityThomas K Hemmick 5
Does QCD exhibit equally beautiful properties as a bulk medium.
ANSWER: YES!
The Focus of the School will be the QCD Phase Structure
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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!!!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
Relativity
Stars convert gravitational energy to temperature.
They “replay” and finish nucleosynthesis
~15,000,000 K in the center of our sun.
Collisions of “Large” nuclei convert beam energy to temperatures above 200 MeV or 1,500,000,000,000 K ~100,000 times higher
temperature than the center of our sun.
“Large” as compared to mean-free path of produced particles.
Reheating Matter
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Paradigm #1: Hard Probes We accelerate nuclei to high energies with the
hope and intent of utilizing the beam energy to drive a phase transition to QGP.
The created system lasts for only ~10 fm/c The collision must not only utilize the energy
effectively, but generate the signatures of the new phase for us.
I will make an artificial distinction as follows:◦ Medium: The bulk of the particles; dominantly soft
production and possibly exhibiting some phase.◦ Probe: Particles whose production is calculable,
measurable, and thermally incompatible with (distinct from) the medium.
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Nuclear Collision Terminology
Centrality and Reaction Plane determined on an Event-by-Event basis.
Npart= # of Participants◦ 2 394
Nbinary=# of Collisions
Peripheral Collision Central CollisionSemi-Central Collision
100% Centrality 0%
fReaction Plane
Fourier decompose azimuthal yield:
...2cos2cos21 21
3
vvdydpd
Nd
T
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q/g jets as probe of hot medium
hadrons
q
q
hadronsleadingparticle
leading particle
schematic view of jet production
Jets from hard scattered quarks observed via fast leading particles orazimuthal correlations between the leadingparticles
However, before they create jets, the scattered quarks radiate energy (~ GeV/fm) in the colored medium
Jet Quenching
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RAA Normalization
ddpdT
ddpNdpR
TNN
AA
TAA
TAA /
/)(
2
2
<Nbinary>/sinelp+p
nucleon-nucleon cross section
1. Compare Au+Au to nucleon-nucleon cross sections2. Compare Au+Au central/peripheral
Nuclear Modification Factor:
If no “effects”: RAA < 1 in regime of soft physics RAA = 1 at high-pT where hard scattering dominates Suppression: RAA < 1 at high-pT
AA
AA
AA
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Calibrating the Probe(s)
Measurement from elementary collisions.
“The tail that wags the dog” (M. Gyulassy)
p+p->p0 + X
Hard
Scattering
Thermally-shaped Soft Production
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Suppression Discovered in Year One
Quark-containing particles suppressed. Photons Escape! Gluon Density = dNg/dy ~ 1100
Expected
Observed
QM2001
QM2001
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Jet Tomography Jets are produced as back-to-back pairs. If one jet escapes, is the other
shadowed? Map the dynamics of Near-Side and
Away-Side jets.◦ Vary the reaction plane vs. jet orientation.◦ Study the composition of the jets◦ Reconstruct the WHOLE jet
Find “suppressed” momentum & energy.
Escaping Jet“Near Side”
Lost Jet“Far Side”
In-plane
Out-planeX-ray pictures areshadows of bones
Can Jet Absorption be Used to“Take an X-ray” of our Medium?
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Back-to-back jets
Central Au + Au
Peripheral Au + Au
Given one “jet” particle, where are it’s friends:◦ Members of the “same jet” are in nearly the same
direction.◦ Members of the “partner jet” are off by 180o
Away-side jet “gone”
STAR
In-plane
Out-plane
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px
py
y
x
Paradigm 2: Collective Flow
Origin: spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy momentum anisotropy
v2: 2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane
Almond shape overlap region in coordinate space
y2 x2 y2 x2
2cos2 v
...2cos2cos21 21
3
vvdydpd
Nd
T
x
zy
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Process is SELF-LIMITING Sensitive to the initial time
Delays in the initiation of anisotropic flow not only change the magnitude of the flow but also the centrality dependence increasing the sensitivity of the results to the initial time.
Anisotropic Flow
Liquid Li Explodes into Vacuum
Gases explode into vacuum uniformly in all directions.
Liquids flow violently along the short axis and gently along the long axis.
We can observe the RHIC medium and decide if it is more liquid-like or gas-like
Position Space anisotropy (eccentricity) is transferred to a
momentum space anisotropy visible to
experiment
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Hydrodynamic limit exhausted at RHIC for low pT particles.
Can microscopic models work as well? Flow is sensitive to thermalization time
since expanding system loses spatial asymmetry over time.
Hydro models require thermalization in less than t=1 fm/c
Large v2
Adler et al., nucl-ex/0206006
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v2 Scales with valence quarks
A flowing system would be expected to have mass-dependent v2 values
BRAHMS
STAR & PHENIX
Valence quark scaling is superb indicating that the final state hadrons may have come from recombination
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High Order Moments vn
Event Plane method yields <vn> (vodd=0). 2-particle yields SQRT(<vn
2>) (vodd>0). How to deal:
◦ PHENIX = EP method + factorization.◦ ATLAS = Rapidity OUTSIDE other Jet.◦ Everyone else = Factorization.
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arXiv:1105.3928
charged particle vn : ||<0.35reaction plane n : ||=1.0~2.8
(1) v3 is comparable to v2 at 0~10% (2) weak centrality dependence on v3
(3) v4{4} ~ 2 x v4{2}
All of these are consistent with initial fluctuation.
v2{2}, v3{3}, v4{4} at 200GeV Au+Au
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Paradigm 3: Hadrochemistry
Hadronization by random choice or recombination will follow simple statistical distributions:
12 /
2
20
33 TISBE
ii
isiBe
dppgn
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Thomas K Hemmick
Hard or Jet Probes provide useful information BECAUSE their initial production is well known.
Flow is driven by “pre-collision” spatial anisotropy.
Hadro-chemistry (and HBT) probe the final state at de-coupling time.
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Limitations of these Paradigms
PENETRATING (color-less) Probes are Transparent to the QGP medium and
directly probe the initial state
e+e- IS THE MOST DIFFICULT MEASUREMENT IN HEAVY ION PHYSICS!
History of Success: CERES, PHENIX, STAR, ALICE???
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Sources “long” after collision: p0, , h w Dalitz decays( ), r w, , f J/ , y y‘ decays
Early in collision (hard probes):Heavy flavor productionDrell Yan, direct radiation
Baseline from p-p
Thermal (blackbody) radiation in dileptons and photonstemperature evolution
Medium modifications of mesonpp r l+l-
chiral symmetry restorationMedium effects on hard probes
Heavy flavor energy loss
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Lepton-Pair Continuum Physics
known sources of lepton pairs at s = 200 GeVModifications due to QCD phase transition
Chiral symmetry restorationcontinuum enhancement modification of vector mesons
thermal radiation & modified heavy flavor
suppression (enhancement)
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Thomas K Hemmick
Particles are produced in accordance with the strength of the coupling.
Colorless particles are rare compared to those produced via strong interactions.
Requires a superb detector to distinguish electrons from other species.
Which property of the electron is KEY to distinguishing electrons in your detector?◦ charge, mass, spin, flavor, colorless, weak
interaction,
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Problem #1: Electrons are Rare
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Thomas K Hemmick Stony Brook University 25
2
2ln
14 2
222
2222
I
cm
A
ZzcmrN
dx
dE eeeABethe-Bloch Formula
e-
qk ,
0,mv
d
dE
dNE
dx
dE
0
How do particles lose energy in matter?
density effect
ionization constant
22ln dx
dE“relativistic rise” “minimum ionizing particles” 3-4
2
1
dx
dE
“kinematic term”
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Thomas K Hemmick Stony Brook University 26
Pb-Scint Calorimeter
Electromagnetic Calorimeter modules use the “Shish-Kebab” geometry and achieve excellent energy resolution (7%/sqrt(E)) and timing (< 200 psec).
Bremsstrahlung
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PID via Cherenkov
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Ring Radius determines qc
High Index distinguisheshadrons
No Ring Unless1/b > n
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Thomas K Hemmick Stony Brook University 28
• 2560 PMTs, 107 gain.• Ethane or CO2 radiator.• 104 charge pion rejection.• 1 degree ring res.• N0= 118.• 12-18 p.e./ring• C-fiber/Rohacell mirror 0.3% X0
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Thomas K Hemmick Stony Brook University 29
Optics of the RICH
• Two circular mirrors each of which reflects light onto arrays of PMT’s.• PMT’s are located so that the central magnet poles shield the array.• Offset focal plane leads to ellipse rather than circle images.
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Thomas K Hemmick Stony Brook University 30
STAR
p
K p
e
Particle Identification by dE/dx
2
21
~2
)(transferEnergy
e
ey
m
p
dE/dx: The 1/ b2 survives
integration over impact parameters
Measure average energy loss to find b
Looks tough for e-sep. How possible???
2
2ln
14 2
222
2222
I
cm
A
ZzcmrN
dx
dE eeeA
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Thomas K Hemmick
Charged Particles may radiate X-ray photons upon “transition”.
LARGE ionization at small angle & short distance.
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Transition Radiation
ALICE
123
2 hEloss
Small angles
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Challenge: Pair Background
No background rejection Signal/Background 1/100 in Au-Au Unphysical correlated background
◦ Track overlaps in detectors◦ Not reproducible by mixed events: removed from event sample (pair cut)
Combinatorial background: e+ and e- from different uncorrelated source
◦ Need event mixing because of acceptance differences for e+ and e-
◦ Use like sign pairs to check event mixing
Correlated background: e+ and e- from same source but not “signal”
◦ “Cross” pairs “jet” pairs
◦ Use Monte Carlo simulation and like sign data to estimate and subtract background
0 e e e e
0
e e
e e
Xπ0
π0e+
e-
e+
e-γ
γ
π0
e-γ
e+
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Normalization Mathematics Combinatorial Mathematics
◦ Leptons are ALWAYS produced as pairs. Within a collection of events, any single event produced N pairs
governed by some probability distribution P(N). For a single event with N primary lepton pairs, there is a BINOMIAL
distribution for the number of these pairs that are fully reconstructed:
Additionally, this same event will contain partially reconstructed events (+ only; - only; neither) that are governed by a MULTINOMIAL
◦ We will 1count the number of pairs (true and combinatorial) for a single np, 2sum over all np, 3sum over all N, and 4demonstrate the relationship for combinatorial rates:
Thomas K Hemmick
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Thomas K Hemmick
Useful Relations In General:
Binomial:
Poisson:
Multinomial:
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Thomas K Hemmick
Three combinations:
fully-fully, fully-partial, partial-partial:
Cov(M)
For a given np, unlike sign pairs are:
algebra
algebra
<n>=eN
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Thomas K Hemmick
Sum these over np: Total Reconstructed Pairs <N+->:
<n2>=<n>2+s2
algebra
algebra
Substitute <n+_>
This result contains epN true pairs and combinatorial.
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Thomas K Hemmick
Same thing for like-sign: Like pairs as 2n++ = n+(n+-1):
Similarly for -- pairs
algebra
algebra
<n2>=<n>2+s2
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Thomas K Hemmick
Rate of like-sign pairs Summing over np:
algebra
algebra
<n2>=<n>2+s2;s2(B) =eN(1-e)
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Thomas K Hemmick
Lastly, sum over all N Unlike sign
Like sign
By inspection:
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Thomas K Hemmick
Side Notes For independent production:
However, experimental determination of <FG+> is limited by the in ability to select collisions of identical centrality.◦ Such effects must be modeled using a Glauber Monte Carlo of
your experiment’s centrality measure and generates some systematic error in the determination of the absolute combinatorial background.
◦ This technique, however, has proved viable in the analysis of background in 2-particle correlations and obviates the need for a “ZYAM” normalization.
Muons are not as well behaved as electrons:◦ Many muons come from hadron decays that don’t
produce pairs.◦ These are “in-between” statistics and require ◦ When using an absorber muons from K+ decays are enhanced as
compared to other sources.
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Proper Statistics depend upon fundamental production stats:◦ If particles are always produced in pairs:
Nearly correct for e+e- (violated in double-semi-leptonic decays).
Normalization is absolutely independent of the singles efficiency.
NOT (necessarily) the same as: N++ + N—
Independent of “pooling” Normalization IS sensitive to correlated particle losses:
Fv, Double-Track Resolution , Ring Sharing, EMC Sharing. Handling the intrinsic pair cuts is the toughest part.
◦ If particles are produced independently Nearly correct for hadron correlations (Jets). Sensitive to width of pools: requires x correction.
◦ If you use the “wrong” statistics: z correction is necessary:
Mathematical Summary:
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Thomas K Hemmick
Unphysical Background Rejection: “Pair Cut”
Pions identified as electrons in presents of electron◦ RICH measures angle only and not position!!!◦ Pion can be misidentified as electron◦ Leads to correlated but unphysical pairs ◦ Not reproduced by mixed events◦ Different probability and kinematics for like and unlike sign pairs
Remove by rejecting events with parallel tracks in RICH
e-
p+
UNLIKE
mass
pT
Effectively promotes pions into electrons,
RUINS statistics
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Thomas K Hemmick
Mixing Without Pair Cut
Large unphysical background!
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Thomas K Hemmick
Combinatorial Background: Like Sign Pairs
--- Foreground: same evt N++--- Background: mixed evt B++
Shape from mixed events Excellent agreements for like
sign pairs
Normalization of mixed pairs Small correlated background at
low masses from double conversion or Dalitz+conversion
normalize B++ and B- - to N++ and N- - for m > 0.7 GeV
Normalize mixed + - pairs to
Subtract correlated BG
Systematic uncertainties statistics of N++ and N--: 0.12 % kappa: 0.2 %TOTAL SYSTEMATIC
ERROR = 0.25%
Au-Au
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Thomas K Hemmick
e+ e-
Unlike: data - mixedLike: data - mixedMonte Carlo:Cross LikeCross Unlike
p0 g g*
X
unlikecross likecross unlike4-body
yield in 4p
yield in acceptance
Signals in the like sign! -- “Cross” Pairs
e+ e-
Include also h decay
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Thomas K Hemmick
Background Description of Function of pT
Good agreement
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Continuum in p+p and AuAu Phys. Lett. B 670, 313 (2009)
Data and Cocktail of known sourcesExcellent Agreement
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arXiv:0912.0244
Data and Cocktail of known sources
Striking Enhancement at and below the w mass.
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STAR has Excellent Results:
Sees enhancement, but smaller than PHENIX. PHENIX aperture with minimum acc at pT=m, is a significant factor. Need detailed calculations to show whether data agree.
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STAR Enhancement in Central collisions
49
The fit of charm cocktail to the data gives:0.62 ± 0.14 mb
PYTHIA setting for charm:V6.416MSEL=1, PARP(91)=1.0 (kt), PARP(67)=1 (parton shower level)
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Thomas K Hemmick 50
Centrality Dependence Enhancement in
low mass region is a strong function of centrality.
Statistics are also sufficient to analyze pT dependence.
Need methodical approach to the spectra.
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Thomas K Hemmick
IMR in cocktail is dominated by correlated open charm.
LMR-I wherein mee<<pT
LMR-II where the above condition does not apply.
51
Methodical Spectral Analysis
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Thomas K Hemmick
Hot Objects produce thermal spectrum of EM radiation.
Red clothes are NOT red hot, reflected light is not thermal.
52
Remote Temperature Sensing
Red Hot
Not Red Hot!
White Hot
Photon measurements must distinguish thermal radiation from other sources:
HADRONS!!!
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Thomas K Hemmick
ginclusive/ghadronic (1st plot)
exceeds 1 at high pT indicating presence of non-hadronic photons.
RAA equals 1 for these same pT indicating that high pT yields are similar to pp: initial state hard scattering.
Measurement difficult at low pT w/ real photons.
53
Non-Thermal Real Photon Sources
ddpdT
ddpNdpR
TNN
AA
TAA
TAA /
/)(
2
2
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Thomas K Hemmick 54
Direct Virtual Photons Any process that radiates real photons
with E > 2me will also radiate virtual
photons: g*e+e-.
Background remains principally hadronic (Dalitz decay) w/ one decay photon being virtual: p0ge+e-
Differences in mass distribution between signal and background sources improve measurement:
for m<<pT *g is “almost real”
extrapolate * g e+e- yield to m = 0 direct g yield
m > mp removes 90% of hadron decay
background
S/B improves by factor 10: 10% direct g 100% direct g*
Phys. Lett. B 670, 313 (2009)
Data and Cocktail of known sourcesExcellent Agreement
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Thomas K Hemmick 55
LRM divided into pT Slices
pp shows excess growing with pT.
pp excess slopes downward.
AuAu shows excess at all pT
AuAu excess similarly shaped to pp in higher pT region
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56
Measuring direct photons via virtual photons:any process that radiates g will also radiate * gfor m<<pT *g is “almost real”extrapolate * g e+e- yield to m = 0 direct gyield m > mp removes 90% of hadron decay backgroundS/B improves by factor 10: 10% direct g 100% direct g*
Direct (pQCD) Radiation
arXiv:0804.4168
access above cocktail
fraction or direct photons:
dir dir
incl incl
r
q
qg
g
pQCD
Small excess for m<< pT consistent with pQCD direct photons
1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV
hadron decay cocktail
56
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Fit Mass Distribution to Extract the Direct Yield: Example: one pT bin for Au+Au collisions
and
normalized to da
( )
ta
(
f
)
or 30
dir eec
e
e
e
ef
m
m
V
f m
Me
Direct * yield fitted in range 120 to 300 MeVInsensitive to 0 yield
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Interpretation as Direct PhotonRelation between real and virtual photons:
0for MdM
dN
dM
dNM ee
Extrapolate real g yield from dileptons:
dydp
dML
MdydpdM
d
TT
ee2222
)(1
3
Virtual Photon excessAt small mass and high pT
Can be interpreted asreal photon excess
no change in shapecan be extrapolated to m=0
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Search for Thermal Photons via Real Photons
PHENIX has developed different methods: ◦ Subtraction or tagging of photons detected by calorimeter◦ Tagging photons detected by conversions, i.e. e+e- pairs
Results consistent with internal conversion method
internal conversions
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pQCD
g* (e+e-) m=0
g
T ~ 220 MeV
Thermal Radiation at RHIC
Direct photons from real photons:◦ Measure inclusive photons◦ Subtract p0 and h decay photons at
S/B < 1:10 for pT<3 GeV
Direct photons from virtual photons:◦ Measure e+e- pairs at mp < m << pT
◦ Subtract h decays at S/B ~ 1:1 ◦ Extrapolate to mass 0
First thermal photon measurement: Tini > 220 MeV > TC
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Initial temperatures and times from theoretical model fits to data: ◦ 0.15 fm/c, 590 MeV (d’Enterria et
al.)◦ 0.2 fm/c, 450-660 MeV (Srivastava et
al.)◦ 0.5 fm/c, 300 MeV (Alam et al.)◦ 0.17 fm/c, 580 MeV (Rasanen et
al.)◦ 0.33 fm/c, 370 MeV (Turbide et al.
61
Calculation of Thermal Photons
D.d’Enterria, D.Peressounko, Eur.Phys.J.C 46 (2006)
Tini = 300 to 600 MeV t0 = 0.15 to 0.5 fm/c
61
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Local Slopes – Cold Component
6262
Soft component below mT ~ 500 MeV:
Teff < 120MeV independent of mass
more than 50% of yield
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Thomas K Hemmick
pp well described by Cocktail + gamma.AuAu not well described:
Additional excess at low pT 63
pT Spectra
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Future of the Continuum in PHENIX
HBD is fully operational ◦ Proof of principle in 2007◦ Taking data right now with p+p◦ Hope for large Au+Au data set
in 2010
6464
Need tools to reject photon conversions and Dalitz decays
and to identify open charm
Open experimental issuesLarge combinatorial background prohibits precision measurements in low mass region!Disentangle charm and thermal contribution in intermediate mass region!
signal electron
Cherenkov blobs
partner positronneeded for rejection
e+
e-
qpair opening
angle
~ 1 m
HBD
False combinations dominated by region where yield is largest
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65
HBD Used Run-11 “Standard” CERN Cu GEM foils in HBD 2nd HBD installed in PHENIX
CSI photocathods on GEM foils
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66
e.g.0.75 < pt < 1.00 GeV/c
HBD charge distribution forreconstructed electrons
0.75<pt<1.00GeV/celectronsingleN
electronmergeN
reference distributions
electronscintiN
3 contributions
Fractions of single e clusters, merged clusters, scintillation hits extracted via fit.
w/o HBD clustercharge cut
HBD: Pion Rejection in pp:
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Results:
Direct photonic result consistent w/ traditional. Au+Au single electron via HBD almost preliminary. Pair Analysis ongoing…
67
non-photonic e cross section(e- + e+)/2
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Thomas K Hemmick
Di-electron measurements are the most challenging among all RHI measurements.
Superb and varied detector technology is necessary.
Data handling requires precision. IT IS WORTH IT!
◦ Penetrating probes provide a glimpse of QGP matter unlike any other measurement.
CONGRATULATIONS on being a student in this field during these exciting times.
68
Summary
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Backups…
69
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70
Au+Au Dilepton Continuum
Excess 150 <mee<750 MeV:3.4 ± 0.2(stat.) ± 1.3(syst.) ± 0.7(model)
Intermediate-mass continuum: consistent with PYTHIA if charm is modified room for thermal radiation
70
Yield / (Npart/2) in mass windows p0 region: production scales
approximately with Npart
Excess region: expect contribution from hot matter in-medium production from pp
or qq annihilation yield should scale faster than Npart
Excess region: 150 < m < 750 MeV
p0 region: m < 100 MeV
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71
pT < 1 GeV Enhancement
Low mass excess in Au-Au concentrated at low pT!
71
Poorly described as g*
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Open Charm (and bottom) states decay with significant branching ratios (~10%) semi-leptonically.
Parent quark mass makes these the dominant source at high pT
Cocktail (or convertor) subtraction yields spectrum of heavy flavor lepton decays.
72
Heavy Flavor from Single Leptons
pp results presented both as inclusive heavy flavor and “open” heavy flavor.
Good agreement with pQCD
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Heavy Flavor shows suppression similar to p0 at fill RHIC Energy.
Heavy Flavor even flows. These results are the principal ones that define h/s. Similar conclusion for muons from CuCu:
suppression similar to p
73
Heavy Flavor Leptons in AuAu
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Using low mass pairs, one can select a sample with large opening angle (isolated) or small opening angle (overlapping)
The responses are 20 p.e. & 40 p.e. respectively. (WOW!)
74
Single and Double Response
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75
Compared to Run 4 Results
Effective statistics increased at least by factor 32 errors reduced by factor 5.6 – 8.5
Improvement of effective Signal vs <Npe> for same
length run.
Stochastic Cooling at RHIC
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VTX, FVTX and NCC add key measurements to RHIC program: Heavy quark characteristics in dense medium Charmonium spectroscopy (J/, ’ , c and ) Light qurak/gluon energy loss through -jet Gluon spin structure (DG/G) through -jet and c,b quarks A-, pT-, x-dependence of the parton structure of nuclei
76
PHENIX Upgrades @ Vertex
Focal
VTXSi Barrel
FVTXSi Endcaps
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77
RAA(ce) and RAA(be) with VTX
Decisive measurement of RAA for both c and b
PHENIX VXT ~2 nb-1
RHIC II increases statistics by factor >10
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78
v2(be) and v2(ce) with VTX
Decisive measurement of v2 for both c and b
PHENIX VXT ~2 nb-1
RHIC II increases statistics by factor >10
PRELIMINARY
Run-4
Run-7
Rapp & van Hees, PRC 71, 034907 (2005)
minimum-bias
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Thomas K Hemmick
Immovable Object – Irresistable Force Problem. I’m again rooting for the immovable object!
79
Importance of c/b Separation
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Thomas K Hemmick 80
Future Looks Bright!
Focal
VTXSi Barrel
FVTXSi Endcaps
signal electron
Cherenkov blobs
partner positronneeded for rejection
e+e-
qpair opening
angle~ 1 m
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81
Timeline of PHENIX Upgrades2008 2012
RHIC
2010
Stochastic cooling “RHIC II”
2014
Construction
VTX
Large acceptance tracking |Dh|<1.2
Displaced vertex at mid rapidity
FVTX Displaced vertex at forward y
Physics
NCC
AuAu dileptons HBD
m TriggerW - physics
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Critical Point and the Onset of Deconfinement studies necessarily involve lowering the beam energy in the machine.
Luminosity scales as the square of beam energy. Furthermore, heavy quarks suffer in production rate at lower
energies. The product of these factors limits all present RHIC experiment
capabilities, but will be offset by future efforts:◦ Stochastic Cooling for high energy running.◦ E-beam cooling (3-6 X) for below 10.7 GeV running.
82
Examining these signatures at finite mB
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With the inclusion of the HBD, PHENIX could get a marginal measurement for energies as low as 17.2 GeV w/ 50 M-evts
However(!!!), the rate of collisions at this low energy makes the collection time for 50 million evts prohibitively long.◦ Practical di-electron measurements are at 62.4 & ~39 GeV.◦ Marginal measurements available at 27 GeV.◦ Impractical due to running time at lower energy.
83
Dielectron Capabilities at low Energy
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84
Dilepton Excess at High pT – Small Mass
0<pT<0.7 GeV/c
0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c
0<pT<8.0 GeV/c
p+pAu+Au
arXiv: 0802.0050arXiv: 0706.3034
1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV
hadron decay cocktail
84
Significant direct photon excess beyond pQCD in Au+Au
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85
Interpretation as Direct PhotonRelation between real and virtual photons:
0for MdM
dN
dM
dNM ee
Extrapolate real g yield from dileptons:
dydp
dML
MdydpdM
d
TT
ee2222
)(1
3
Exc
ess*
M (
A.U
).
85
Virtual Photon excessAt small mass and high pT
Can be interpreted asreal photon excess
no change in shapecan be extrapolated to m=0
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Direct photons from real photons:Measure inclusive photonsSubtract p0 and h decay photons at S/B < 1:10 for pT<3 GeV
Direct photons from virtual photons:
Measure e+e- pairs at mp < m << pT
Subtract h decays at S/B ~ 1:1 Extrapolate to mass 0
86
First Measurement of Thermal Radiation at RHIC
pQCD
g* (e+e-) m=0
g
T ~ 220 MeV
86
First thermal photon measurement: Tini > 220 MeV > TC
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Sarah Campbell WWND 2010 Jan 7, 2010 9:50 am
False Pair Rejection
Conversion Pairs◦ Opening angle in the
plane perp. to B field Charges ordered by B
field◦ Mass of the pair is
roughly proportional to the radius of the conversion point
Overlapping Pairs◦ RICH ring overlap◦ Require pairs are
separated by twice the nominal ring size
z
y
x
e+e-
B Conversion pairz
y
x
e+
e-
B
Dalitz decay
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Sarah Campbell WWND 2010Jan 7, 2010 9:50 am
Combinatorial Background Largest background
in heavy ions ◦ Large multiplicities
Shape determined by event mixing
Normalization determined using the like-sign pairs in regions where combinatorial dominates
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Sarah Campbell WWND 2010 Jan 7, 2010 9:50 am
Correlated Background Jet Background
◦ Pions in jets dalitz decay into electrons Produced electron pairs
are correlated by the jet Like-sign and unlike-sign
pairs produced at same rate
◦ Simulated with Pythia
“Cross” pairs◦ Decays that produce
multiple lepton pairs Double dalitz, double
conversion, dalitz + conversion
Like-sign and unlike-sign pairs produced at same rate
◦ Simulated with Exodus Pions, etas only sizable
source
π0
π0
e+
e-
e+
e-
γ
γ
π0
e-
γ
e+
e-
0 e+
e-
e+
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90
Estimate of Expected Sources
Hadron decays: Fit p0 and p± data p+p or Au+Au
For other mesons h, , , , w r fJ/y etc. replace pT mT and fit normalization to existing data where available
Heavy flavor production: sc= Ncoll x 567±57±193mb from single electron measurement
Hadron data follows “mT scaling”
n0T2TT
3
3
pp)bpapexp(
A
pd
σdE
Predict cocktail of known pair sources
90
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Thomas K Hemmick
Combinatorial Background: Event Mixing
Event with e+ (p)and/or e-(e)
Centrality iVertex j
pool (j,j)
e1e2e3e4..
en
p1p2p3p4..
pn
e0example 1 track
eventcut
no pass
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Thomas K Hemmick
Combinatorial Background: Event Mixing
Event with e+ (p)and/or e-(e)
Centrality iVertex j
pool (j,j)
e1e2e3e4..
en
p1p2p3p4..
pn
e0example 1 track
mix with poolStore like sign pairsStore unlike sign pairs
eventcut
no pass
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Thomas K Hemmick
Combinatorial Background: Event Mixing
Event with e+ (p)and/or e-(e)
Centrality iVertex j
pool (j,j)
e0e1e2e3..
en-1
p1p2p3p4..
pn
e0example 1 track
mix with poolStore like sign pairsStore unlike sign pairs
eventcut
update pool
no pass
DO NOT USE THIS FOR
NORMALIZATION!
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Thomas K Hemmick 94
IMR Region (f J/y)
Charm: after cocktail subtraction sc=544 ± 39 (stat) ± 142 (sys) ± 200 (model)
mbSimultaneous fit of charm and bottom:
sc=518 ± 47 (stat) ± 135 (sys) ± 190 (model) mb
sb= 3.9 ± 2.4 (stat) +3/-2 (sys) mb
Subtract hadron decay contribution and fit difference:
Surprise!• AuAu matches cocktail in MB.• Slightly higher in peripheral• Dashed line is result of max.
smearing of charm pairs.
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Cu+Cu Au+Au comparisonSpectral modification should lower yield.
• Charm singles are well known to be strongly modified by the medium.
• These effects should lower the IMR yield most at the most central bin.
Prompt yields were observed by NA60 in this regime.
• Prompt yields might rise with centrality.
• Competing or compensating effects?
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Because of large errors, the IMR of AuAu is still consistent with unmodified scaled pp or Pythia.
Additional sources may also be present since “suppression” due to charm spectral modification is not observed in the pair data.
AuAu IMR yield vs Centrality.
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Sarah Campbell WWND 2010Jan 7, 2010 9:50 am
Full Background Removal
In Cu+Cu and Au+Au jet awayside component (d > 90) altered to account for jet modification in HI systems
0-10% CuCu All like-sign pairsCombinatorial BGCorrelated PairsCross PairsJet Pairs
0-10% CuCu All unlike-sign pairsCombinatorial BGCorrelated PairsCross PairsJet Pairs