Search for Chiral Symmetry Restoration in QCD Matter Ralf Rapp Cyclotron Institute + Dept of Phys & Astro Texas A&M University College Station, USA HIC for FAIR Nuclear Physics Colloquium Institute for Theoretical Physics (Frankfurt, Germany) 22.10.15
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Search for Chiral Symmetry Restoration in QCD Matter Ralf Rapp Cyclotron Institute + Dept of Phys & Astro Texas A&M University College Station, USA HIC.
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Search for Chiral Symmetry Restoration
in QCD Matter
Ralf Rapp Cyclotron Institute + Dept of Phys & Astro
Texas A&M University College Station, USA
HIC for FAIR Nuclear Physics ColloquiumInstitute for Theoretical Physics (Frankfurt, Germany)
22.10.15
1.) Introduction: Probing QCD Matter
• Bulk Properties: Equation of State, Transport Coefficients• Microscopic Properties: Degrees of Freedom, Spectral Functions • Phase Transitions: Condensate Structure
See also [Dusling et al, Renk et al, Alam et al, Bratkovskaya et al, …]
4.4 Low-Mass Dileptons in Heavy-Ion Collisions
• Robust understanding across QCD phase diagram: QGP + hadronic radiation with melting resonance
<Nch>=120
• “Anomalous” low-mass enhancement [PHENIX ’08] not confirmed• Now agrees with STAR data and theoretical predictions
[PHENIX ‘15]
4.5 News from PHENIX
• tracks fireball lifetime well!• tool for critical point search?
4.6 Dilepton Excitation Functions
• unique temperature measurement• track first order transition?
Low-Mass Excess Intermediate-Mass Slope
• √s ≤ 10 GeV very promising regime for dileptons
5.) Conclusions • Dilepton radiation in HICs probes in-medium vector spectral function - fate of hadrons, chiral restoration - robust theoretical understanding of data via melting resonance
• Mechanism of chiral restoration - mounting evidence for “burning off” M:
QCD+Weinberg sum rules, EFT, lattice QCD
• Future - low-mass spec fct at B ~ 0 (RHIC/LHC)
+ B ≥ 400MeV (FAIR, SPS)
- excitation fct. of lifetime + temperature (BES-II, FAIR, SPS, NICA) - origin of photon-/dilepton-v2
JP=0± 1± 1/2±
4.2.2 Evaluation of Chiral Sum Rules in Vacuum
• vector-axialvector splitting clean observable of spontaneous chiral symmetry breaking • promising starting point to search for chiral restoration
• pion decay constants
• chiral quark condensates 00002
31
210
21
222
|q)q(|αcI|qq|mI
fIFrfI
sq
πA
M[GeV]
4.1.2 Sensitivity to Spectral Function
• avg. (T~150MeV) ~ 370 MeV (T~Tc) ≈ 600 MeV → m
• driven by (anti-) baryons
In-Medium -Meson Width
• compatible with predictions from melting meson• “universal” source around Tpc
[P. Huck et al. (STAR), QM14]
3.3 Low-Mass e+e Excitation Function: 20-200 GeV
3.2.2 Dimuon pt-Spectra + Slopes: Barometer
• slopes originally too soft • need stronger fireball acceleration, e.g. a┴ = 0.085/fm → 0.1/fm
• insensitive to Tc = 160-190 MeV
Effective Slopes Teff
3.3.1 Photon Puzzle!?
• Teffexcess = (220±25) MeV
• flow blue-shift: Teff ~ T √(1+)/(1) , ~0.3: T ~ 220/1.35 ~ 160 MeV
• small slope + large v2 suggest main emission around Tpc
• similar indications at LHC [ALICE]
Spectra Elliptic Flow
4.2 Low-Mass Dileptons: Chronometer
• first “explicit” measurement of interacting-fireball lifetime: FB ≈ (7±1) fm/c
In-In Nch>30
• Thermal rates folded over coarse-grained UrQMD medium evolution • consistent with baryon-driven medium effects at SPS+RHIC
[Endres,van Hees, Weil+Bleicher, in prep]
3.4 Low-Mass e+e at HADES (2.6 GeV)
See also [Bratkovskaya et al , Kämpfer et al, Weil et al,…]
3.2.3 Transverse-Momentum Spectra: Baro-meter
SPS
Effective Slope Parameters
• qualitative change from SPS to RHIC: flowing QGP• true temperature “shines” at large mT
• 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 B ~ 0
STAR Excess Dileptons
[STAR ‘14]
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)
• Thermal rates folded over coarse-grained UrQMD medium evolution• good description in (M,qt) • data well beyond kinematic limit (0.75GeV)!
[Endres,van Hees+Bleicher, in prep]
2.4 Low-Mass e+e at HADES (2.63 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
3.3.3 Direct Photons at LHC
• similar to RHIC results• non-perturbative photon emission rates around Tpc?
Spectra Elliptic Flow
● ALICE
[van Hees et al in prep]
5.2 Chiral Restoration Window at LHC
• low-mass spectral shape in chiral restoration window:
~60% of thermal low-mass yield in “chiral transition region” (T=125-180MeV)• enrich with (low-) pt cuts