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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● 6) Photons: intermediate energy● 7) Photons: high energy● 8) Dark Matter● 9) ElectroWeak Probes
OCTOBER
NOVEMBER
DECEMBER
JANUARY
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Dielectrons: low 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● Low-energy
– DLS– HADES
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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
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: 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
= √2Seq
/ √S
eq = √2B / S
/ S
eq = 2B / S2
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SIS/BEVALAC energy regime: 1-2 AGeV
• Final state (Freeze-out) in heavy ion
collisions
– approximately 10-15 % pions, baryon
resonances ((1232)) dominated
– up to 200 charged particles (Au+Au)
• Enhancement of baryon density in "fireball"
– Comparable to \ life times :
V=1.3\23 fm/c
15 fm/c
Dense matterFreeze-outFirst chance
collisionsAu + Au @ 1 AGeV
UrQMD:J. Phys. G: Nucl. Part. Phys. 25 (1999) 1859–
T60=80MeV
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Meson Production at SIS/BEVALAC energies
● Production of vector mesons close to the threshold : SNN< STHR=2MN + m
(Ekinthresh = 1.92 GeV)
● co-operative process :NN N, N NN/
or N, N N*() N/
● production confined to high density phase
– Low production rates: One vector meson decaying into lepton pair per 10 Million reactions !
● Investigation of NN and N collisions is prerequisite for HI !
mT=( m2 + p2 T)1/2
W. Cassing, E.L. Bratkovskaya / Phys.Rep. 308 (1999) 65Ð233Yield
(arb. units)
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DLS puzzle
Data: R.J. Porter et al.: PRL 79(97)1229
Model: E.L. Bratkovskaya et al.: NP A634(98)168, BUU, vacuum spectral function
DLS puzzle:
• Not explained even by in-medium mass shifts and broadening
• Shape of enhancement consistent with e+e- but cross section to low (TAPS)
Strong dilepton enhancement over hadronic cocktails comparable to top SPS energies (CERES)!
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TAPSTAPS – Two Arms Photon Spectrometer
Electromagnetic calorimeter : m : m =(2E =(2E11EE22(1-cos((1-cos(1122))))1/21/2
C+C @ 1 AGeV
• small acceptance (mid rapidity)
Acc~10-3
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Pi0 and eta from TAPSCross sections measured and extrapolated
to full solid assuming isotropic thermal source at rest in NN CM frame
Converted into pair yields (Cocktail):compared to data
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The DiLepton Spectrometer at LBL
• 1988-1993 at Bevalac• 2 Arm-Spectrometer
– Minimum opening angle: 40°– Each arm: 40° in Φ, 7.5° in Θ– Trigger on electron-pairs – Mass resolution– 30-40% systematical error
• pp/pd, Ca+Ca• 1993 C+C 1.04 AGeV,
– Mid-Rapidity: 0.69– Acceptance– Statistics
e+e-
Pair YieldsComb Net pairs Syst. Error
4760 1919 2841 ± 82 ±30%
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HADESSide View
START
FWFW
Acceptance: Full azimuth, polar angles 18o - 85o
Pair acceptance 0.35
Particle identification: RICH: CsI solid photo cathode, C4F10 radiator, TOF: 384 scintillator rodsTOFino: 24 scintillator paddles MUL limitation, high granularity RPC from 2008Pre-Shower: 18 pad chambers & lead converters)2' Level single leg electron trigger (Me>=1)e+e->=92%, evt. reduction : 20(pp) -3(ArKCl)
Momentum measurementMagnet: B = 0.36 Tm + MDC: 24 midi drift chambers, single-cell resolution 140 mm, 2005-6: set-up completed (MDC IV)
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Phase coverage: HADES vs DLS
mid-rapidity mid-rapidity
• Thermal π0 and η events: HADES and DLS acceptance• HADES acceptance larger but for low masses (M<0.14 GeV/c2) part of phase space covered by DLS not covered by HADES !
Red dashed lines: constant pair momenta in steps of 100 MeV/c
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Hadron ID• momentum vs velocity (momentum vs velocity (ββ) measurement: ) measurement:
181800 < <<45<4500 TOFINO TOFINO 454500 < <<85<8500 TOF TOF~450 ps~450 ps ~120 ps~120 ps
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Electron ID
hadron blind had t < lep
C4F10 : t = 18.3
pGeV/c
e+
e-0
~ 15.20
Dalitz decay
20% electrons: pi0 Dalitz
e+
e- ~ 2.20
Conversion
70% : electrons conv.
One ring:Two rings (if Ị needs high res. MDC!
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Electron ID• Spatial correlations
– RICH rings ↔ MDC tracks– MDC tracks ↔ TOF and PreShower
hits• PID : e+, e-
– β vs momentum correlation– PreShower condition
DATA
C + C @ 2AGeV
• hadron admixture < 3% at 1000 MeV/c
e-
e+
velocity vs. momentum
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Combinatorial BackgroundCombinatorial background:
M < 150 MeV/c2 - sLS
sLs = 2
(checked with MC for HADES )
M > 150 MeV/c2 - mixedOpposite Sign (mOS)
MMeeee > 150MeV/c > 150MeV/c22
MMeeee > 150MeV/c > 150MeV/c22
eeee NN
CC 2AGeV
• Normalization done between 150-550 MeV/c2 Mee
• sLS and mOS background show same behavior for Mee > 150 MeV/c2
• For Mee < 150 MeV/c2
deviations due to correlated background eeX
Pair cut: opening angle > 9º
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C+C @ 2 AGeV
~ 2300023000 signal pairs for full Mee range
~ 30003000 signal pairs for full Mee > 150 MeV/c2
(M(Meeee) ~9 % @ M) ~9 % @ Meeee~0.8 GeV/c~0.8 GeV/c22
2002 set-up with 2 inner MDC only
•Spectra before efficiency correctionnormalizated to ½(+ + -) yield
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C+C pion production
mmt t spectraspectra
C +C @ 2AGeVC +C @ 2AGeVanisotropiesanisotropies )(cos1
)(cos2
2 cmCM
Ad
dN
2 AGeV 1 AGeV
HADESN< -+>/Apart = 0.137 ± 0.015
Apart =8.4 <b>=3.2 fm
N< -+>/Apart = 0.069 ± 0.008
Apart =8.6 <b>=3.0 fm
Yields extrapolated (LVL1) to 4Yields extrapolated (LVL1) to 4::
SYS error =11%
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Comparison to Physics GeneratorsExperimentExperiment
RawRaw DataData
HADESHADES
AnalysisAnalysis
PhysicsPhysics
PairPair SpectraSpectra
Event GeneratorEvent Generator
EfficiencyEfficiency CorrectionCorrection
dN/dMdN/dM
AcceptanceAcceptance FilterFilter
time consuming but done to cross-time consuming but done to cross-check eff.corrections and acc. filterscheck eff.corrections and acc. filters
final comparison: only final comparison: only in HADES acceptancein HADES acceptance
comparison incl. comparison incl. efficiency factors efficiency factors
AccAcc±± (p, (p, , , ΦΦ))
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Acceptance and Efficiency matrices
acc
rec
N
NpEff ),,(
• pair production and decay is described by 6 dof (3 production and 3 decay)p (0 – 2 GeV/c), Φ (0o – 60o), Θ (0o – 90o)
and similarlyand similarly
all
acc
N
NpAcc ),,(
acc
rec
N
NpEff ),,(
CC @ 2 AGeVCC @ 2 AGeV
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Coctail A (long lived mesonic components)
0 thermal source, anisotropic angular distribution according to measured +-
isotropic mT scaling
18 %18 % 21 %21 %
systematic errors:systematic errors:15 % - efficiency correction 10 % - combinatorial background11 % - 0 normalization
• Cocktail A: 0 + η + ω
• Cocktail B: Cocktail A + Δ(Ne+e-) + ρ
short lived component
A. Agakichiev Phys.Rev. Lett 98(2007) 052302
C+C @ 2 AGeV
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C+C @ 1 AGeV
Cocktail A: 0 + η + ω
= “long-lived” components only
Large excess yield
•Good agreement in π0 region•Underestimates the data for Mee > 0.15 GeV/c2
Cocktail B: Cocktail A + Δ + ρ
Contribution from short lived resonances (ρ, Δ, N*)
•Reduced discrepancy for Mee>0.15 GeV/c2
•Cocktail B underestimates dataPR
ELIMINARY
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HADES vs DLS direct comparison
Fit:
Mee < 0.15 GeV/c² 0.15 GeV/c² ≤ Mee ≤ 0.55 GeV/c²
PRELIMINARY
mapping of the measured HADES pair yield onto the DLS acceptance, defined inthe 3-d space spanned by the pair variables Mee, Pee and the rapidity Yee⊥HADES coverage is larger and almost fully contains the DLS acceptance
1) projecting out 2d slices (p vs.y) of the efficiency-and ⊥acceptance-corrected pair yield for different mass bins2) fitting a reasonable 2d function 3) using the resulting fits to extrapolate in 3d phase space, mass slice by mass slice, to small p and large y ⊥
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HADES vs DLS direct comparison
DLS: Porter et al., PRL 79 (1997) 1229HADES: Agakishiev et al., PLB 663 (2008) 43
HADES fully confirmed highly controversialDLS findings in C+C !
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Measuring the reference
●shape of the mass spectra changes dramaticallywhen going from p+p to n+p interactions
● 0.15 <m<0.35 GeV/c2: n+p yield ~10x p+p yield one would expect 2x if were the only relevant source
● the tail at high invariant mass extends much further pn / pp ~ 100 at 0.5 GeV/c2.
●OBE effective models reproduce p+p but not (yet) p+n
HADES, PLB 690 (2010)
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Measuring the reference
+
Mass distribution (-subtracted) similar at 1 and 2 AGeV (except only high mass)
½ (pp + pn) = C+C
DLS puzzle: has its origin in a insufficient treatment of radiation from elementary N+N collisions.No compelling evidence for in-medium effects in C+C
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Putting the puzzle togetherRecent transport calculations:enhanced NN bremsstrahlung , in line with recent OBE calculationsHSD: Bratkovskaya et al. NPA 807214 (2008)
The DLS puzzle seems to be reduced to an understanting of the elementarycontributions to NN reactions.
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Heavier Systems
HADES, Nuclear Physics A 830 (2009)
Both for Ar+KCl and Au+Au:● A+A > ½ (pp+pn) = C+C● Strong excess of dielectron pairs● No mere superposition of elementary pp and np collisions● Almost exponential spectrum up tovector meson region
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Comparison with transport modelCoarse-graining UrQMD1) Average over many UrQMD transport events2) Determine local temperature & density in a grid of space-time cells3) Use Hadronic Many Body Theory (HMBT) & spectral functions to compute EM emission rates4) Sum up all cells → thermal dilepton radiation5) Add freeze-out contributions → non-thermal part(Endres, van Hees, Weil & Bleicher, PRC 92 (2015) 014911)