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23/11/2016 Alberica Toia 1
Nuclear and Particle Physics 4aElectromagnetic Probes
Alberica ToiaGoethe University Frankfurt
GSI Helmholtzzentrum für Schwerionenforschung
Lectures and Exercise Winter Semester 2016-17
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Organization● Language: English● Lecture:
● Wednesday 09:00 (c.t.) - 11:00 ● Phys 01.402
● Marks / examination→ only if required / desired● Seminar presentation → schein ● Oral Exam → grade
● Office hours: tbd on demand
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Info: Email and Website● E-Mail:
● Website: https://web-docs.gsi.de/~alberica/lectures/KT4a_WS1617.html
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Content● 1) Introduction: Heavy Ion Physics● 2) Detectors● 3) Dielectrons: low energy● 4) Dielectrons: intermediate energy● 5) Dielectrons: high energy● *** 07.12 Dielectrons Theory (H. van Hees) ***● 6) Photons: intermediate energy● 7) Photons: high energy● 8) Dark Matter● 9) ElectroWeak Probes
OCTOBER
NOVEMBER
DECEMBER
JANUARY
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Dielectrons: intermediate energy● Motivation
● Dileptons and the QGP ● Dileptons and the Hadron Gas: Chirality, chiral symmetry
breaking and chiral symmetry restoration
● Experimental challenges● combinatorial background
● Dileptons in heavy-ion collisions experiments● Intermediate-energy
– CERES– NA60
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The little bang in the lab
• High energy nucleus-nucleus collisions: – fixed target – colliders
• 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
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Probing the QGP
penetrating beam(jets or heavy particles) absorption or scattering pattern
QGP
Rutherford experiment atom discovery of nucleus SLAC electron scatteringe proton discovery of quarks
Penetrating beams created by parton scattering before QGP is formed High transverse momentum particles jets Heavy particles open and hidden charm or bottom
Probe QGP created in Au+Au collisions Calculable in pQCD Calibrated in control experiments: p+p (QCD vacuum), p(d)+A (cold medium)
Produced hadrons lose energy by (gluon) radiation in the traversed medium QCD Energy loss → medium properties
Gluon density Transport coefficient
probe
bulk
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Electromagnetic Radiation Thermal black body radiation
Real photons Virtual photons * which appear as dileptonseeor
No strong final state interaction Leave reaction volume undisturbed and reach detector
Emitted at all stages of the space time development Information must be deconvoluted
time
hard parton scattering
AuAu
hadronization
freeze-out
formation and thermalization of quark-gluon
matter?
Space
Time Jet cc e p K
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What can we learn from dilepton emission?Emission rate of dilepton per volume
Boltzmann factortemperature
EM correlatorMedium property
ee decay
Hadronic contributionVector Meson Dominance
qq annihilation
Medium modification of mesonChiral restoration
From emission rate of dilepton, one can decode
• medium effect on the EM correlator • temperature of the medium
arXiv:0912.0244
Thermal radiation frompartonic phase (QGP)
q
q
e+
e-
e+
e-
+
-
Photonself-energy
QGP
HadronGas
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Theory predictions for dilepton emission rate
Vacuum EM correlatorHadronic Many Body theoryDropping Mass Scenarioq+q → →ee (HTL improved)(q+g → q+→qee not shown)
Theory calculation by Ralf Rapp
dMdydpp
dN
tt
ee at y=0, pT=1.025 GeV/cUsually the dilepton emission is measured and compared as dN/dpTdM
The mass spectrum at low pT is distorted by the virtual photon → ee decay factor 1/M, which causes a steep rise near M=0
qq annihilation contribution is negligible in the low mass region due to the M2 factor of the EM correlator
In the caluculation, partonic photon emission processq+g → q+ → qe+e- is not included
1/M*→ee
qq → * → e+e-
≈(M2e-E/T)×1/M
arXiv:0912.0244
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The mass of composite systems
mass given by energy stored in motion of quarks and by energy in colour gluon fields
M mi
binding energyeffect 10-8
atom 10-10 m
M » mi
nucleon 10-15 m
atomic nucleus 10-14 m
M mi
binding energyeffect 10-3
the role of chiral symmetry breaking
• chiral symmetry = fundamental symmetry of QCD for massless quarks
• chiral symmetry broken on hadron level
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Chirality
left-handed
right-handed
Chirality (from the Greek word for hand: “”)when an object differs from its mirror image
simplification of chirality: helicity(projection of a particle’s spin on its momentum direction)
massive particles P left and right handed components must exist m>0 → particle moves w/ v<c
– P looks left handed in the laboratory– P will look right handed in a rest frame
moving faster than P but in the same direction chirality is NOT a conserved quantity
in a massless word m
u = m
d = m
s = 0
– chirality is conserved
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QCD and chiral symmetry breaking the QCD Lagrangian:
free gluon field free quarks ofmass mn
explicit chiral symmetry breaking mass term mnnn in the QCD Lagrangian
chiral limit: mu = md = ms = 0 chirality would be conserved
left–handed u,d,s, quarks remain left-handed forever all states have a ‘chiral partner’
(opposite parity and equal mass)
real life: mu and m
d are so small (m
u≈ 4 MeV m
d≈ 7 MeV)
that our world should be very close to chiral limit a1 (JP=1+) is chiral partner of (JP=1-): m≈500 MeV even worse for the nucleon:
N* (½-) and N (½+): m≈600 MeV (small) current quark masses don’t explain this
chiral symmetry is also spontaneously broken spontaneously = dynamically
interaction of quarkswith gluon
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Origin of mass constituent quark mass
~95% generated by spontaneous chiral symmetry breaking (QCD mass)
current quark mass generated by spontaneous
symmetry breaking (Higgs mass) contributes ~5% to the visible
(our) mass
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Chiral symmetry restoration spontaneous symmetry breaking gives rise to a nonzero ‘order
parameter’ QCD: quark condensate <qq> ≈ -250 MeV3
many models (!): hadron mass and quark condensate are linked
numerical QCD calculations at high temperature and/or high baryon density
→ deconfinement and <qq> → 0 approximate chiral symmetry restoration (CSR)
→ constituent mass approaches current mass
Chiral Symmetry Restoration expect modification of hadron
spectral properties (mass m, width )
explicit relation between (m,) and <qq>?● QCD Lagrangian → parity doublets are degenerate in mass
● how is the degeneracy of chiral partners realized ? ● do the masses drop to zero? ● do the widths increase (melting resonances)?
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Hadron massesG. Brown & M. Rho, PRL (1991) 2720
Brown-Rho scaling
Vacuum: Vector and Axial spectral functionswell separated (ALEPH data)
At Tc: Chiral symmetry restoration
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CSR and low mass dileptons what are the best probes for CSR?
requirement: carry hadron spectral properties from (T, B) to detectors relate to hadrons in medium leave medium without final state interaction
dileptons from vector meson decays
best candidate: meson– short lived – decay (and regeneration) in medium– properties of in-medium and of medium itself not well known
meson a special probe for CSR, long lifetime but m(Ф)≈ 2 m(K) simultaneous measurement of φ → ee and φ → KK could be a
powerful tool to evidence in-medium effects
m [MeV] tot [MeV] [fm/c] BR → e+e-
770 150 1.3 4.7 x 10-5
8.6 23 7.2 x 10-5 4.4 44 3.0 x 10-4
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Dilepton Signal● What is its temperature?
→ measure thermal photons
● Does it restore chiral symmetry?→ modification of the vector mesons
● How does it affect heavy quarks?→ modification of the intermediate mass region
● All these questions can be answeredby measuring dileptons (e+e− or μ+μ−)
● no strong final state interactions:
● leave collision system unperturbed
● emitted at all stages: need todisentangle contributions
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Dilepton Signal● What is its temperature?
→ measure thermal photons
● Does it restore chiral symmetry?→ modification of the vector mesons
● How does it affect heavy quarks?→ modification of the intermediate mass region
● All these questions can be answeredby measuring dileptons (e+e− or μ+μ−)
● no strong final state interactions:
● leave collision system unperturbed
● emitted at all stages: need todisentangle contributions
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Dilepton Signal Dileptons characterized by 2 variables: M, pT
M: spectral functions and phase space factors pT: three contributions to pT spectra
pT - dependence of spectral function (dispersion relation)T - dependence of thermal distribution of “mother” hadron/partonM - dependent radial flow () of “mother” hadron/parton
Note I: M Lorentz-invariant, not changed by flow
Note II: final-state lepton pairs themselves only weakly coupled
dilepton pT spectra superposition of ‘hadron-like’ spectra at fixed T
early emission: high T, low T
late emission: low T, highT
final spectra from space-time folding over T- T history from Ti → Tfo
→ handle on emission region, i.e. nature of emitting source
mT
1/m
T d
N/d
mT
light
heavyT
purely thermalsource
explosivesource
T,
mT
1/m
T d
N/d
mT
light
heavy
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Dilepton signal Low Mass Region:
mee < 1.2 GeV/c2
Dalitz decays of pseudo-scalar mesons Direct decays of vector mesons In-medium decay of mesons in the hadronic gas phase
Intermediate Mass Region: 1.2 < mee < 2.9 GeV/c2
correlated semi-leptonic decays of charm quark pairs
Dileptons from the QGP
High Mass Region: mee> 2.9 GeV/c2
Dileptons from hard processes–Drell-Yan process–correlated semi-leptonic decays of heavy quark pairs
–Charmonia –Upsilons
→ HMR probe the initial stage Little contribution from thermal radiation
• LMR: mee < 1.2 GeV/c2 o LMR I (pT >> mee)
quasi-real virtual photon region. Low mass pairs produced by higher order QED correction to the real photon emission
o LMR II (pT<1GeV)Enhancement of dilepton discovered at SPS (CERES, NA60)
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HI low mass dileptons at a glance
(KEK E235) CERES
DLS
NA60
HADES
CBM
90 95 1000 0585
PHENIX
● Time scale of experiments
● Energy scale of experiments
(KEK E235)
CERES
DLS
NA60
HADES
CBM PHENIX
10 158 [A GeV]
17 [GeV]√sNN200
// // //
// // //
ALICE
[A TeV]
1
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Dilepton Analysis Challenges● Experimental Challenge
● Need to detect a very weak source of pairs ~ 10-6 /π0 ● in the presence of hundreds of charged particles in central AA
collision● and several pairs per event from trivial origin
π0 Dalitz decays ~ 10-2/π0 + γ conversions (assume 1% radiation length) 2x10-2 /π0 → huge combinatorial background (dN∝
ch/dy)2
● Analysis Challenge
● Electron pairs are emitted through the whole history of the collision (from the QGP phase, mixed phase, HG phase and after freeze-out)
– need to disentangle the different sources.
– need excellent reference pp and dA data.
– need independent information about the known sources in nuclear collisions
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Dilepton Analysis Steps● Tracking + Momentum reconstruction → Resolution
(position, momentum)● Particle Identification → Purity● Rejection close pairs → Significance,
Signal/Background
● Pairing: mee
= [2 p1p
2 (1-cosθ)]1/2
● Subtraction of Background(mixed events, like-sign)
● Efficiency Correction● Mass Spectrum
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Remarks on S/B● how is the signal obtained?
● unlike-sign pairs: F● combinatorial background: B (like-sign pairs or event mixing)
→ S = F – B
● statistical error of S● depends on magnitude of B, not S
S ≈ √2B (for S<<B)
● “background free equivalent” signal Seq
● signal with same relative error in a situation with zero backgroundSeq = S * S/2Bexample: S = 104 pairs with S/B = 1/250 → Seq = 20
● systematic uncertainty of S● dominated by systematic uncertainty of B
S/S = B/B * B/Sexample: B/B = 0.25% precision, S/B = 1/250 → S/S = ~60% systematic uncertainty of S
S2 = F2 + B2 = S2 + B2 + B2
≈ 2B2 = 2B2
B = √B
Seq
/ Seq
= √2B / S
Seq
= √Seq
/ √S
eq = √2B / S
/ S
eq = 2B / S2
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Separating Signal from Background
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SPS @ CERN SuperProtonSynchrotron (since 1976)
parameters– circumference: 6.9 km– beams for fixed target
experiments– protons up to 450 GeV/c– lead up to 158 GeV/c
past– SppS proton-antiproton
collider discovery of vector bosons W±, Z
now– injector for LHC
experiments– Switzerland: west area (WA)– France: north area (NA) dileptons speak
french!
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Experimental Setup: CERES-1
1992 setup – minimal configuration, no particle trackingOptimized for minimum response to hadrons and photons of hadronic origindouble RICH spectrometer (
thr=32), for eID and hadron rejection (hadron blind)
Magnetic field → azimuthal deflection → momentum measurement between the two radiators→ conversion pairs: double rings in RICH1, open up in RICH2
23/11/2016 Alberica Toia 29
Experimental Setup: CERES-1bis
1995 setup – tracking: doublet of SiDC – RICH1 – RICH2 - PCRadial drift Silicon Detector: high resolution vertex and tracking
RICH1
RICH2
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Target region
13
segmented target 13 Au disks (thickness: 25 m; diameter: 600 m)
Silicon drift chambers: provide vertex: z = 216 m provide event multiplicity ( = 1.0 – 3.9) powerful tool to recognize conversions at the target
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Electron Identification: RICH
main tool for electron ID use the number of hits per ring (and their analog sum)
to recognize single and double rings
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pA results dielectron mass spectra and expectation from a ‘cocktail’ of
known sources Dalitz decays of neutral mesons (→ e+e- and ’ dielectron decays of vector mesons ( → e+e-) semileptonic decays of particles carrying charm quarks
dielectron production in p+p and p+A collisions at SPS well understood in terms of known hadronic sources
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AA resultsCERES PRL 92 (95) 1272
discovery of low mass e+e- enhancement in 1995 significant excess in S-Au (factor ~5 for m>200 MeV)
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AA results dielectron excess at low and
intermediate masses in HI collisions is well established
onset at ~2 m → - annihilation?
maximum below meson near 400 MeV
→ hint for modified meson in dense matter
e-
e+
CERES Eur.Phys.Jour. C41(2005)475
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CERES-1 → CERES-2
addition of a TPC to CERES improved momentum
resolution improved mass resolution dE/dx → hadron identification
and improved electron ID particle ID and momentum
measurement are separated inhomogeneous magnetic field
→ a nightmare to calibrate
23/11/2016 Alberica Toia 36
CERES-2 results the CERES-1
results persists strong
enhancement in the low-mass region
enhancement factor (0.2 <m < 1.1 GeV/c2 ) → 3.1 ± 0.3 (stat.)
but the improvement in mass resolution isn’t outrageous
23/11/2016 Alberica Toia 37
CERES at low-energy PRL 91 (2003) 042301
data taking in 1999 and 2000
improved mass resolution improved background
rejection results remain statistics
limited
Pb-Au at 40 AGeV enhancement for
mee> 0.2 GeV/c2
– 5.9±1.5(stat)±1.2(sys)±1.8(decay)
strong enhancement at lower sor larger baryon density
vacuum Brown-Rho scalingbroadening of
23/11/2016 Alberica Toia 38
pT dependence
low mass e+e- enhancement at low pT
qualitatively in a agreement with annihilation pT distribution has little discriminative power
mee<0.2 GeV/c2 0.2<mee<0.7 GeV/c2 mee>0.7 GeV/c2
hadron cocktailBrown-Rho scalingbroadening of
23/11/2016 Alberica Toia 39
Thermal radiation from HG low mass enhancement due to annihilation?
Thermal radiation from HG spectral shape dominated meson
vacuum vacuum values of width and mass
in-medium Brown-Rho scaling
– dropping masses as chiral symmetry is restoredconjecture that links hadron masses to the quark condensate.Effective QCD Lagrangian, quarks are the relevant d.o.f.
Rapp-Wambach melting resonances– ρ-meson scatters off particles in
the high density medium→ collision broadening of spectral function
– Pure hadronic model, only indirectly related to CSR
medium modifications driven by baryon density
model space-time evolution of collision
e-
e+
0
1/3
0
ρ
ρ
ρ
ρ
ρ0.161
m
m
23/11/2016 Alberica Toia 40
Theory comparison attempt to attribute the
observed excess to vacuum meson ( )
– inconsistent with data– overshoot in region– undershoots @ low mass
modification meson – needed to describe data– data do not distinguish
between– broadening or melting of
-meson (Rapp-Wambach)– dropping masses (Brown-
Rho)
indication for medium modifications, but data are not accurate enough to distinguish models
largest discrimination between and → need mass resolution!
23/11/2016 Alberica Toia 41
Theory comparison● Interpretation invoke+- → → * → e+e-Thermal radiation from hadron gas● Vacuum not enough to reproduce the data● In medium-modifications of :
● Broadening spectral fc. (Rapp-Wambach)● Dropping mass (Brown-Rho)● Thermal radiation: e+e- yield
from qq annihilation in pQCD (Kaempfer)
Data favor broadening scenarioUncertainties are large for a firm conclusion
23/11/2016 Alberica Toia 42
Centrality dependence
naïve expectation: quadratic multiplicity dependence medium radiation particle density squared
more realistic: smaller than quadratic increase density profile in transverse plane life time of reaction volume
F=
yiel
d/c
ock
tail
mee<0.2 GeV/c2 0.2<mee<0.6GeV/c2 mee>0.6 GeV/c2
CERESpT > 200 MeV/c
1995/962000
Nch
strong centrality dependence→ challenge for theory !
23/11/2016 Alberica Toia 43
Summary of CERES first systematic study of e+e- production in elementary
and HI collisions at SPS energies pp and pA collisions are consistent with the expectation
from known hadronic sources a strong low-mass low-pT enhancement is observed in
HI collisions
→ consistent with in-medium modification of the meson→ data can’t distinguish between two scenarios dropping mass as direct consequence of CSR collisional broadening of in dense medium
WHAT IS NEEDED FOR PROGRESS? STATISTICS MASS RESOLUTION
23/11/2016 Alberica Toia 44
How to overcome these limitations more statistics
run forever → not an option higher interaction rate
– higher beam intensity– thicker target
needed to tolerate this– extremely selective hardware trigger– reduced sensitivity to secondary interactions, e.g. in target
→ can’t be done with dielectrons as a probe, but dimuons are just fine!
better mass resolution stronger magnetic field detectors with better position resolution → silicon tracker embedded in strong magnetic field!
23/11/2016 Alberica Toia 45
The NA60 Experiment a huge hadron absorber
and muon spectrometer (and trigger!)
and a tiny, high resolution, radiation hard vertex spectrometer
23/11/2016 Alberica Toia 46
Standard detection: NA50MuonOther
hadron absorber
target
beammagnetic field
thick hadron absorber to reject hadronic background trigger system based on fast detectors to select muon
candidates (1 in 104 PbPb collisions at SPS energy) muon tracks reconstructed by a spectrometer (tracking
detectors+magnetic field) extrapolate muon tracks back to the target taking into account
multiple scattering and energy loss, but … poor reconstruction of interaction vertex (z ~10 cm) poor mass resolution (80 MeV at the )
muon trigger and tracking
23/11/2016 Alberica Toia 47
The NA60 experimentBased on the NA50 spectrometer with the addition of a Si tracker
Additional bend by the dipole fieldTrack matching in coordinate and momentum space• Improved dimuon mass resolution• Distinguish prompt from decay dimuonsDimuon coverage extended to low pT
23/11/2016 Alberica Toia 48
The NA60 pixel vertex detectorDIPOLE MAGNET2.5 T
HADRON ABSORBER
TARGETS
~40 cm
1 cm
12 tracking points with good acceptance 8 small 4-chip planes 8 large 8-chip planes in 4 tracking stations
~3% X0 per plane 750 m Si readout chip 300 m Si sensor ceramic hybrid
800000 readout channels in 96 pixel assemblies
23/11/2016 Alberica Toia 49
Vertexing in NA60
Beam Trackersensors
windows
z ~ 200 m along the beam directionGood vertex identification with 4 tracks
X
Extremely clean target identification (Log scale!)
Resolution ~ 10 - 20 m in the transverse plane
23/11/2016 Alberica Toia 50
Contribution to mass resolution two components
multiple scattering in the hadron absorber– dominant at low momentum
tracking accuracy– dominant at high momentum
high mass dimuons (~3 GeV/c2) absorber doesn’t matter
low mass dimuons (~1 GeV/c2) absorber is crucial momentum measurement before
the absorber promises huge improvement in mass resolution
→ track matching is critical for high resolution low mass dimuon measurements!
23/11/2016 Alberica Toia 51
Muon Track Matching
track matching has to be done in position space momentum space
to be most effective the pixel telescope has to be a spectrometer!
Muon spectrometer Pixel telescope
1p )1(
2p
2p
1z 2z
Absorber
Measured points Measured points
23/11/2016 Alberica Toia 52
In+In: LMR Min. Bias unlike sign dimuon mass
distribution before quality cuts and without muon track matching
S/B ~ 1/7 , and even peak clearly
visible Mass resolution: 23 MeV at the
peak
drastic improvement in mass resolution
still a large unphysical background
BR = 5.8x10-6!
23/11/2016 Alberica Toia 53
Fake matches
hadron absorber
muon trigger and tracking
targetfake
correct
Hadron absorber
Muon spectrometer
fake match: matched to wrong track in pixel telescope important in high multiplicity events
how to deal with fake matches keep track with best 2 (but is it right?) embedding of muon tracks into other event identify fake matches and determine the fraction of these relative to correct
matches as function of– centrality– transverse momentum
23/11/2016 Alberica Toia 54
Event mixing: like-sign pairs compare measured and mixed like-sign pairs
accuracy in NA60: ~1% over the full mass range
23/11/2016 Alberica Toia 55
In+In: LMR peripheral
Eur.Phys.J.C 49 (2007) 235
Well described by meson decay ‘cocktail’: η, η’, ρ, ω, f and DD contributions(Genesis generator developed within CERES and adapted for dimuons by NA60).
Similar cocktail describes NA60 data: p-Be, p-Pb, In-In peripheral
Eur.Phys.J.C 43 (2005) 407
23/11/2016 Alberica Toia 56
Digression: EM transition form factor
56
In-In, peripheral
hep-ph/0902.2547
Acceptance-corrected data (after subtraction of , and peaks) fitted by three contributions:
2/1
2
2
2
23
2
2
22
41
211
)(
3
2)(
m
m
m
m
m
m
mdm
d 22 )( mF
TM
eMTMmM
M
m
M
m
M
m
m
Md
Rd
2/3
22222
2
22/1
2
22/3
2
2
4
42
2
21
41
41
)2(3
)(
22 )( mF
2/3
22
222
22
22/1
2
2
2
2
2
0
2
0
00
41
41
21
)(
3
)(
mm
mm
mm
m
m
m
m
m
mdm
d
Confirmed anomaly ofF wrt VDM prediction.
Improved errors wrt the Lepton-G results.
Removes FF ambiguity in the ‘cocktail’
pole approximation:
22222 /1)( mmF
0
23/11/2016 Alberica Toia 57
Cocktail subtraction (w/o )
ω and : fix yields such as to get, after subtraction, a smooth underlying continuum
:
- set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV (lower limit for excess).- use yield measured for pT > 1.4 GeV/c
how to nail down an unknown source? → try to find excess above cocktail without fit constraints
23/11/2016 Alberica Toia 58
Excess vs centralityData – cocktail (all p
T)
● No cocktail and DD subtracted● Clear excess above cocktail , - centered at the nominal pole - rising with centrality● Excess even more pronounced at low p
T
Confirm CERES data!
23/11/2016 Alberica Toia 59
Excess shape vs centralityQuantify the peak and the broad symmetric continuum with a mass interval C around the peak (0.64 <M<0.84 GeV) and two equal side bins L, U
continuum = 3/2(L+U) peak = C-1/2(L+U)
Peak/cocktail drops by a factor 2 from peripheral to central:
the peak seen is not the cocktail
nontrivial changes of all three variables at dNch/dy>100 ?
peak/
continuum/
peak/continuum
Fine analysis in 12 centrality bins
23/11/2016 Alberica Toia 60
Theory comparison
data consistent with
broadening of (RW),mass shift (BR) not needed
Rapp & Wambach hadronic model with strong broadening but no mass shift
Brown & Rho dropping mass due to dropping chiral condensate
calculations for all scenarios in In-In for dNch/d = 140 (Rapp et al.)
spectral functions after acceptance filtering, averaged over space-time and momenta
Keeping original normalization
23/11/2016 Alberica Toia 61
Acceptance corrected spectrum
Mass spectrumcorrected for acceptancein M-p
T
23/11/2016 Alberica Toia 62
In+In: IMR hadron-parton duality
Rapp / van Hees Ruppert / Renk
dominant at high M hadronic processes 4
dominant at high M partonic processes mainly qqbar annihilation
23/11/2016 Alberica Toia 63
IMR: the NA50 measurement
centralcollisions
M (GeV/c2)
NA50: excess observed in IMR in central Pb-Pb collisions
charm enhancement? thermal radiation?
answering this question was one of the main motivations for building NA60
Drell-Yan and Open Charm are the main contributions in the IMR• p-A is well described by the sum of these two contributions (obtained from Pythia)• The yield observed in heavy-ion collisions exceeds the sum of DY and OC decays,extrapolated from the p-A data.• The excess has mass and pT shapes similar to the contribution of the Open Charm (DY + 3.6xC nicely reproduces the data).
23/11/2016 Alberica Toia 64
IMR: disentangling the sources
D0
K-
+
e
D0
100m
charm quark-antiquark pairs are mainly produced in hard scattering processes in the earliest phase of the collisions
c c
0DK
l
0D
l
K+
-
charmed hadrons are “long” lived → identify the typical offset (“displaced vertex”) of D-meson decays (~100 m)
need superb vertexing accuracy (20-30 m in the transverse plane) → NA60
23/11/2016 Alberica Toia 65
IMR: disentangling the sources measure for vertex displacement
primary vertex resolution momentum dependence of secondary vertex resolutions → “dimuon weighted offset”
charm decays (D mesons) → displaced J/→prompt
vertex tracking is well under control!
23/11/2016 Alberica Toia 66
IMR excess is prompt approach
Dix Drell-Yan (within 10%)
Fix prompt component Charm can't describe the small
offset region
Fix charm component Good description of offset
→ IMR excess is a prompt component
Fit range
DD
DY
1.120.17
DataPrompt: 2.290.08Charm: 1.160.16Fit 2/NDF: 0.6
DD
Prompt
Eur.Phys.J. C59 (2009) 607
DD
Prompt
~50m ~1mm
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Analysis of mT
decomposition of low mass region
contributions of mesons (,,) continuum plus meson extraction of vacuum
hadron mT spectra for ,, vacuum
dilepton mT spectra for low mass excess intermediate mass excess
Phys. Rev. Lett. 96 (2006) 162302
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Interpretation of Teff
interpretation of Teff from fitting to exp(-mT/Teff) static source: Teff interpreted as the source temperature radially expanding source:
– Teff reflects temperature and flow velocity
– Teff depends on the mT range
– large pT limit: common to all hadrons
– low pT limit: mass ordering of hadrons
final spectra: space-time history Ti→Tfo & emission time hadrons
– interact strongly– freeze out at different times depending on cross section with pions– Teff → temperature and flow velocity at thermal freeze out
dileptons– do not interact strongly– decouple from medium after emission– Teff → temperature and velocity evolution averaged over emission time
mpTT TT
Tfeff
v1
v1
mpmTT TTfeff v2
1 2
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Mass ordering of hadronic slopes
158 AGeV Central collisions
Pb-Pb
In-In
Si-Si
C-C
pp
separation of thermal and collective motion reminder
blast wave fit to all hadrons simultaneously
simplest approach
slope of <Teff> vs. m isrelated to radial expansion
baseline is related tothermal motion
works (at least qualitatively) at SPS
mpmTT TTfeff v2
1 2
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Example of hydrodynamic evolution
vT =
0.1
vT =
0.4
(specific for In-In: Dusling et al.)
hadronphase
partonphase
monotonic decrease of T from
early times to late times
medium center to edge
monotonic increase of vT from
early times to late times
medium center to edge
dileptons may allow to disentangle emission times
early emission (parton phase)– large T, small vT
late emission (hadron phase)– small T, large vT
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mT distributions
Fit with: effT Tm
TT
edmm
dN /
Extract Teff
from each mass slice
Extract -peakSame-side window:continuum = 3/2(L+U) peak = C-1/2(L+U)
IMR
LMR
-region
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Dilepton Teff
systematics hadrons ()
Teff depends on mass Teff smaller for
→ decouples early Teff large for
→ decouples late
low mass excess clear flow effect visible follows trend set by hadrons possible late emission
intermediate mass excess no mass dependence indication for early emission Close to T
c, critical
temperature where phase transition occursEur.Phys.J. C (2009), in press, nucl-ex/0812.3053
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Digression: Polarization
73
NA60 also measured the polarization (in the Collins-Soper frame) for m≤ m
Lack of any polarization in excess (and in hadrons) supports emission from thermalized source.
2cossin2
cos2sincos11 22
dd
Submitted to PRL, nucl-ex/0812.3100
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Digression: in medium effects?
Flattening of the pT distributions at low pT, developing very fast with centrality.
Low-pT ω’s have more chances to decay inside the fireball ?
Appearance of that yield elsewhere in the spectrum, due to ω mass shift and/or broadening, unmeasurable due to masking by the much stronger contribution.
Disappearance of yield out of narrow ω peak in nominal pole position
Can only measure disappearance
Eur.Phys.J. C (2009), in press, nucl-ex/0812.3053
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Digression: in medium effects?Determine suppression vs pT with respect to (extrapolated from pT>1GeV/c)
Account for difference in flow effects using the results of the Blast Wave analysis
effTT TmdmdN exp~/ 2
Reference line: ω/Npart = 0.131 f.ph.s.
Strong centrality-dependent suppression at pT<0.8 GeV/c ,
beyond flow effects
Eur.Phys.J. C (2009),nucl-ex/0812.3053
Reference line: /Npart = 0.0284 f.ph.s. (central
coll.)
Consistent with radial flow effects
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Summary NA60 high statistics & high precision dimuon spectra decomposition of mass spectra into “sources” LMR:
– access to in-medium spectral function
– data consistent with broadening of the – data do not require mass shift of the
IMR:– large prompt component at intermediate masses
dimuon mT spectra promise to separate time scales low mass dimuons shows clear flow contribution indicating late
emission intermediate mass dimuons show no flow contribution hinting
toward early emission