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Search for Chiral Symmetry Restoration in QCD Matter

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

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  1. 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

  2. 1.) Introduction:Probing QCD Matter Big Bang Compact Stellar Objects • Bulk Properties: Equation of State, Transport Coefficients • Microscopic Properties: Degrees of Freedom, Spectral Functions • Phase Transitions: Condensate Structure

  3. 1.2 Dileptons in Heavy-Ion Collisions e+ e- qq r A + A NN coll. QGP Hadron Matter Freeze-Out Emission Sources: • Drell-Yan: NN→e+e-X • Thermal radiation - Quark-Gluon Plasma: qq → e+e- - Hadron Matter: p +p - → r → e+e-, … - • final-state decays: p0,h → ge+e-

  4. 1.3 EM Spectral Function Probing the Fireball • Thermal Dilepton Rate • unique direct access to in-medium spectral function rem(M,q;mB,T) e+e-→ hadrons r - qq rem / M2 e+ e- e+ e- • Hadrons:rem ~ ImDr,w,f - change in degrees of freedom? - restoration of chiral symmetry? • qq Continuum: rem/M2 ~ const (1+ O[T2/M2]) - temperature? M [GeV] -

  5. Outline 1.) Introduction 2.) Spontaneous Chiral Symmetry Breaking  QCD Vacuum + Excitations 3.) Axial/Vector Mesons in Medium  Vacuum + Many-Body Theory  QCD + Weinberg Sumrules  cEFT + Mechanisms of Chiral Restoration 4.) Dilepton Phenomenology  From SIS to RHIC 5.) Conclusions

  6. qR qL > > > > - - qL qR 2.1 Chiral Symmetry + QCD Vacuum 2-flavor + chiral (left/right) invariant “Higgs” Mechanism in Strong Interactions: • qqattraction  condensate fills QCD vacuum! Spontaneous Chiral Symmetry Breaking - Consequences: • effective quark mass: ↔ mass generation!? • near-massless Goldstone bosons p0,± • “chiral partners” split: DM ≈ 0.5GeV JP=0±1± 1/2±

  7. 2.2 Mass Gap + Chiral Partners Axial-/Vector Correlators Constituent Quark Mass “Data”: lattice [Bowman et al ‘02] Theory: Instanton Model [Diakonov+Petrov; Shuryak ‘85] pQCD cont. r V,A / s • Spectral shape matters for chiral symmetry breaking E.g. ● Chiral breaking:|q2| ≤ 2 GeV2

  8. Chiral Restoration 2.3 Chiral Symmetry and Dileptons Chiral Condensate [Fodor et al ’10] Vacuum rV T [MeV] rA

  9. Outline 1.) Introduction 2.) Spontaneous Chiral Symmetry Breaking  QCD Vacuum + Excitations 3.) Axial/Vector Mesons in Medium  Vacuum + Many-Body Theory  QCD + Weinberg Sumrules  cEFT + Mechanisms of Chiral Restoration 4.) Dilepton Phenomenology  From SIS to RHIC 5.) Conclusions

  10. |Fp|2 dpp 3.1 r Meson in Vacuum Introducer, a1as gauge bosons into chiral p Lagrangian p p r r propagator: • 3 parameters: mr(0), g, Lr • p EM formfactor • pp phase shift

  11. 3.2 r Meson in Hot + Dense Matter r Sp > Sp > Sp Interactions with hadrons from heat bath  In-Medium r-Propagator r Dr (M,q;mB,T) = [M2- mr2-Srpp -SrB -SrM ]-1 • In-Medium Pion Cloud Srpp = + [Chanfray et al, Herrmann et al, Urban et al, Weise et al, Oset et al, …] R=D, N(1520), a1, K1,... r • Direct r-Hadron Scattering SrB,M = [Haglin, Friman et al, RR et al, Post et al, …] h=N, p, K, … • Theoretical Control: - symmetries (gauge, chiral) - empirical constraints (decays R→r+h, scattering data gN/gA, pN→rN…)

  12. Hot Meson Matter rB/r0 0 0.1 0.7 2.6 [RR+Gale ’99] 3.2.2 r-Meson Spectral Function in Medium Hot + Dense Matter mB =330MeV [RR+Wambach ’99] • r-meson “melts” in hot/dense matter • largely driven by baryon density (rB)

  13. 3.3 QCD + Weinberg Sum Rules [Weinberg ’67, Das et al ’67] r a1 √s [GeV] [Weinberg ’67, Das et al ’67; Kapusta+Shuryak ‘94] • accurately satisfied in vacuum • In-medium input: - condensates: hadron reso. gas / lattice-QCD - in-medium r spectral function Solution for axialvector spectral function? T [GeV]

  14. 3.3.2 QCD + Weinberg Sum Rules in Medium • Quantitatively compatible (< 1%) with (approach to) chiral restoration • Chiral mass spliting burns off [Hohler +RR ‘13]

  15. 3.4 Massive Yang-Mills Approach in Vacuum • Gauge r + a1 into chiral pion lagrangian: • problems with vacuum phenomenology → global gauge? • Improvement - full rpropagator in a1 selfenergy - vertex corrections to preserve PCAC: [Urban et al ‘02, Rischke et al ‘10] [Hohler +RR ‘14] • enables fit to t-decay data • local-gauge approach viable • starting point for evaluating chiral restoration in medium

  16. 3.4.2 Massive Yang-Mills in Hot Pion Gas Temperature progression of vector + axialvector spectral functions • supports “burning” of chiral-mass splitting as mechanism for chiral restoration [as in sum rule analysis] [Hohler+RR ‘15]

  17. 3.5 Lattice-QCD Results for N(940)-N*(1535) Euclidean Correlator Ratios Exponential Mass Extraction “N*(1535)” “Nucleon” • also indicates MN*(T) → MN (T) ≈ MNvac [Aarts et al ‘15]

  18. Outline 1.) Introduction 2.) Spontaneous Chiral Symmetry Breaking  QCD Vacuum + Excitations 3.) Axial/Vector Mesons in Medium  Vacuum + Many-Body Theory  QCD + Weinberg Sumrules  cEFT + Mechanisms of Chiral Restoration 4.) Dilepton Phenomenology  From SIS to RHIC 5.) Conclusions

  19. 4.1 Dilepton Rates: Hadronic vs. Partonic rem(M,q;mB,T) - [qq→ee] • resonance melting hadronic rate approaches QGP rate • suggestive for deconfinement and chiral restoration • robust modeling in heavy-ion collisions

  20. e+ e- r 4.2 EM Spectra in Heavy-Ion Collisions • Evolve rates over fireball: • Space-time evolution: - lattice EoS - require fit to final hadron spectra Au-Au (200GeV) - qq [M.He et al ’12]

  21. 4.3 Precision Dileptons at SPS (17.3 GeV) Invariant-Mass Excess Spectrum <Nch>=120 [van Hees+RR ’13] • Low mass: radiation from T ~ Tpcc ~ 150MeV -spectrometer • Intermediate mass: T ~ 200MeV - thermometer • Total yield: fireball lifetime tFB =7 ± 1fm/c - chronometer See also [Dusling et al, Renk et al, Alam et al, Bratkovskaya et al, …]

  22. 4.4 Low-Mass Dileptons in Heavy-Ion Collisions <Nch>=120 • Robust understanding across QCD phase diagram: QGP + hadronic radiation with meltingr resonance

  23. 4.5 News from PHENIX [PHENIX ‘15] • “Anomalous” low-mass enhancement [PHENIX ’08] not confirmed • Now agrees with STAR data and theoretical predictions

  24. 4.6 Dilepton Excitation Functions Low-Mass Excess Intermediate-Mass Slope • unique temperature measurement • track first order transition? • tracks fireball lifetime well! • tool for critical point search? • √s ≤ 10 GeVvery promising regime for dileptons

  25. 5.) Conclusions • Dilepton radiation in HICs probes in-medium vector spectral function - fate of hadrons, chiral restoration - robust theoretical understanding of data via melting r resonance • Mechanism of chiral restoration - mounting evidence for “burning off” DMc: QCD+Weinberg sum rules, cEFT, lattice QCD • Future - low-mass spec fct at mB ~0(RHIC/LHC) + mB ≥400MeV (FAIR, SPS) - excitation fct. of lifetime + temperature(BES-II, FAIR, SPS, NICA) - origin of photon-/dilepton-v2 JP=0±1± 1/2±

  26. 4.2.2 Evaluation of Chiral Sum Rules in Vacuum • pion decay constants • chiral quark condensates • vector-axialvector splitting clean observable of spontaneous chiral symmetry breaking • promising starting point to search for chiral restoration

  27. 4.1.2 Sensitivity to Spectral Function In-Medium r-Meson Width • avg. Gr(T~150MeV)~370MeVGr (T~Tc) ≈ 600 MeV → mr • driven by (anti-) baryons Mmm [GeV]

  28. 3.3 Low-Mass e+e- Excitation Function: 20-200 GeV [P. Huck et al. (STAR), QM14] • compatible with predictions from melting r meson • “universal” source around Tpc

  29. 3.2.2 Dimuon pt-Spectra + Slopes: Barometer Effective Slopes Teff • slopes originally too soft • need stronger fireball acceleration, e.g. a┴ = 0.085/fm → 0.1/fm • insensitive to Tc = 160-190 MeV

  30. 3.3.1 Photon Puzzle!? Spectra Elliptic Flow • Teffexcess = (220±25) MeV • flow blue-shift: Teff ~ T √(1+b)/(1-b) , b~0.3: T ~ 220/1.35 ~160 MeV • small slope + large v2 suggest main emission around Tpc • similar indications at LHC [ALICE]

  31. 4.2 Low-Mass Dileptons: Chronometer In-In Nch>30 • first “explicit” measurement of interacting-fireball lifetime: tFB≈ (7±1) fm/c

  32. 3.4 Low-Mass e+e- at HADES (2.6 GeV) [Endres,vanHees, Weil+Bleicher, in prep] • Thermal rates folded over coarse-grained UrQMD medium evolution • consistent with baryon-driven medium effects at SPS+RHIC See also [Bratkovskaya et al , Kämpfer et al, Weil et al,…]

  33. 3.2.3 Transverse-Momentum Spectra: Baro-meter Effective Slope Parameters RHIC SPS QGP HG [Deng,Wang, Xu+Zhuang ‘11] • qualitative change from SPS to RHIC: flowing QGP • true temperature “shines” at large mT

  34. 2.2 Chiral Condensate + r-Meson Broadening > Sp effective hadronic theory > - Sp • h = mq h|qq|h > 0 contains quark core + pion cloud = Shcore + Shcloud ~ ++ • matches spectral medium effects: resonances + pion cloud • resonances + chiral mixing drive r-SF toward chiral restoration r - - qq / qq0

  35. 3.2 Vector Correlator in Thermal Lattice QCD • Analyticity: Euclidean Correlator Ratio Spectral Function [Ding et al ‘10] [RR ‘02] • correlator enhancement comparable to lattice QCD • indicates transition from hadronic to partonic degrees of freedom

  36. 4.1 Prospects I: Spectral Shape at mB ~ 0 STAR Excess Dileptons [STAR ‘14] • rather different spectral shapes compatible with data • QGP contribution?

  37. 2.2 Transverse-Momentum Dependence pT -Sliced Mass Spectra mT -Slopes x100 • spectral shape as function of pair-pT • entangled with transverse flow (barometer)

  38. 2.4 Low-Mass e+e- at HADES (2.63 GeV) [Endres,vanHees+Bleicher, in prep] • Thermal rates folded over coarse-grained UrQMD medium evolution • good description in (M,qt) • data well beyond kinematic limit (0.75GeV)!

  39. 3.3.2 Effective Slopes of Thermal Photons Thermal Fireball Viscous Hydro [van Hees,Gale+RR ’11] [S.Chen et al ‘13] • thermal slope can only arise from T ≤ Tc(constrained by • closely confirmed by hydro hadron data) • exotic mechanisms: glasma BE? Magnetic fields+ UA(1)? [Liao at al ’12, Skokov et al ’12, F. Liu ’13,…]

  40. 3.3.3 Direct Photons at LHC Spectra Elliptic Flow ● ALICE [van Hees et al in prep] • similar to RHIC results • non-perturbative photon emission rates around Tpc?

  41. 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

  42. 4.4 Elliptic Flow of Dileptons at RHIC • maximum structure due to late r decays [He et al ‘12] [Chatterjee et al ‘07, Zhuang et al ‘09]

  43. 3.3.2 Fireball vs. Viscous Hydro Evolution [van Hees, Gale+RR ’11] [S.Chen et al ‘13] • very similar!

  44. 2.3 Dilepton Rates vs. Exp.: NA60 “Spectrometer” • Evolve rates over fireball expansion: Acc.-correctedm+m- Excess Spectra In-In(17.3GeV) [NA60 ‘09] [van Hees+RR ’08] Mmm [GeV] • invariant-mass spectrum directly reflects thermal emission rate!

  45. 4.2 Low-Mass e+e- at RHIC: PHENIX vs. STAR • PHENIX enhancement (central!) not accounted for by theory • STAR data ok with theory (charm?!)

  46. 4.3.2 Revisit Ingredients Emission Rates Fireball Evolution • multi-strange hadrons at “Tc” • v2bulkfully built up at hadronization • chemical potentials for p, K, … • Hadron - QGP continuity! • conservative estimates… [Turbide et al ’04] [van Hees et al ’11]

  47. 4.7.2 Light Vector Mesons at RHIC + LHC • baryon effects important even at rB,tot= 0 : sensitive to rBtot= rB + rB (r-N and r-N interactions identical) • w also melts, f more robust ↔ OZI - -

  48. 4.1 Nuclear Photoproduction: rMeson in Cold Matter g + A → e+e- X • extracted “in-med” r-width Gr≈ 220 MeV e+ e- Eg≈1.5-3 GeV g r [CLAS+GiBUU ‘08] • Microscopic Approach: + in-med. r spectral fct. product. amplitude full calculation fix density 0.4r0 Fe-Ti r g N [Riek et al ’08, ‘10] M[GeV] • r-broadening reduced at high 3-momentum; need low momentum cut!

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