Thermal EM Radiation in Heavy-Ion Collisions Ralf Rapp Cyclotron Institute + Dept of Phys & Astro Texas A&M University College Station, USA Symposium on “Jet and Electromagnetic Tomography of Dense Matter” McGill University (Montreal, Canada), 26.+27.06.15
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Thermal EM Radiation in Heavy-Ion Collisions
Ralf Rapp Cyclotron Institute + Dept of Phys & Astro
Texas A&M University College Station, USA
Symposium on “Jet and Electromagnetic Tomography of Dense Matter”
McGill University (Montreal, Canada), 26.+27.06.15
)T,q(fMqdxd
dN Bee023
2em
44 π
α−= Im Πem(M,q;µB,T)
Im Π
em(M
) / M
2
e+e- → hadrons
1.) Intro: EM Spectral Function to Probe Fireball
e+ e-
ρ
• Hadronic Resonances - change in degrees of freedom? - restoration of chiral symmetry?
• Continuum - temperature?
M [GeV]
• Thermal Dilepton Rate
e+ e-
• Total yields: fireball lifetime?
1.2 30 Years of Dileptons in Heavy-Ion Collisions
• Robust understanding across QCD phase diagram: QGP + hadronic radiation with melting ρ resonance
<Nch>=120
1.) Introduction 2.) Dileptons as Phenomenological Tool • Excitation Function: Fireball Lifetime + Temperature • Fireball in pA?
3.) Dileptons as Theoretical Tool • Chiral Restoration: - QCD +Weinberg Sum Rules - Massive Yang-Mills Revisited
• Sensitive to flow profile (contrary to dilepton invariant-mass spectra) • 3 challenges from experiment: suggestive for: spectral yield - large large rates, large radial flow spectral slope - “small” “later” emission elliptic flow - large later emission, rapid build-up
• Hadronic emission rate close to QGP-AMY • semi-QGP much more suppressed [Pisarski et al ‘14]
4.3 Comparison to Data: RHIC
[van Hees et al, ‘11, ’14]
• same rates + intial flow ⇒ similar results from various evolution models [Paquet et al ’15]
Fireball Ideal Hydro Viscous Hydro
5.) Conclusions • Dilepton radiation as a precision tool to measure - fireball lifetime (low mass) - early temperature (intermed. mass; no blue-shift)
• Progress in understanding mechanisms of chiral restoration - evaporation of chiral mass ρ-a1 splitting (sum rules, MYM)
• Direct photons - identify key components of space-time evolution + rates (e.g., initial flow, ``realistic” QGP + hadronic rates) - mild discrepancies between data and theory
4.3.2 Photon Puzzle!?
• Tslopeexcess ~240 MeV
• blue-shift: Tslope ~ T √(1+β)/(1-β) ⇒ T ~ 240/1.4 ~ 170 MeV
• first “explicit” measurement of interacting-fireball lifetime: τFB ≈ (7±1) fm/c
In-In Nch>30
3.2 Vector Correlator in Thermal Lattice QCD
]T/q[)]T/(q[)T;q,q(dq)T;q,( i ii i 2s inh
21cosh2 0
00
em
0
0em −= ∫∞
τρ
πτΠ• Analyticity:
)T,(G)T,(G
V
Vτ
τfree
Spectral Function Euclidean Correlator Ratio
• correlator enhancement comparable to lattice QCD • indicates transition from hadronic to partonic degrees of freedom
[Ding et al ‘10]
[RR ‘02]
• rather different spectral shapes compatible with data • QGP contribution?
4.1 Prospects I: Spectral Shape at µB ~ 0
STAR Excess Dileptons [STAR ‘14]
4.5 QGP Barometer: Blue Shift vs. Temperature
• QGP-flow driven increase of Teff ~ T + M (βflow)2 at RHIC • high pt: high T wins over high-flow ρ’s → minimum (opposite to SPS!) • saturates at “true” early temperature T0 (no flow)
SPS RHIC
• compatible with predictions from melting ρ meson • “universal” source around Tpc
P. Huck et al. [STAR], QM14
2.3 Low-Mass e+e- Excitation Function: 20-200 GeV
3.3.2 Effective Slopes of Thermal Photons
• thermal slope can only arise from T ≤ Tc (constrained by • closely confirmed by hydro hadron data) • exotic mechanisms: glasma BE? Magnetic fields+ UA(1)?
[van Hees,Gale+RR ’11]
[Liao at al ’12, Skokov et al ’12, F. Liu ’13,…]
[S.Chen et al ‘13]
Thermal Fireball Viscous Hydro
2.2 Transverse-Momentum Dependence pT -Sliced Mass Spectra
mT -Slopes
x 100
• spectral shape as function of pair-pT • entangled with transverse flow (barometer)
3.1.2 Transverse-Momentum Spectra: Baro-meter
SPS Effective Slope Parameters
• qualitative change from SPS to RHIC: flowing QGP • true temperature “shines” at large mT