Electromagnetic Probes at the LHC Ralf Rapp Cyclotron Institute + Physics Department Texas A&M University College Station, USA PANIC ‘05 Satellite Meeting “Heavy-Ion Physics at the LHC” Santa Fe, in (real + virtual) light of SPS and RHIC Results

1. Four Pillars Electromagnetic Radiation 2. EM Correlator and Chiral Symmetry 3. Space-Time Evolution of A-A 4. Low-Mass Dileptons: Vector Mesons in-Medium 5. Photons 6. Perspectives for LHC 7. Conclusions Outline

1.) Four Pillars of Thermal EM Radiation low-mass ee in-med  →ee Chiral Symmetry restoration? low-energy  hadron decays/scatt. a 1 → ,  →  Medium effects? high-energy  continuum emission HG vs. QGP, O(  s ) QGP radiation? q 0 ≈0.5GeV  T max ≈0.17GeV, q 0 ≈1.5GeV  T max =0.5GeV Thermal rate: qq int-mass ee continuum emission  a 1 → ee, qq→ee QGP radiation?

2.) EM Emission Rates and Chiral Symmetry E.M. Correlation Function: e + e - γ Im Π em (M,q) Im Π em (q 0 =q) = O(1) = O(1) = O(α s ) = O(α s ) also: e.m susceptibility (charge fluct.): χ = Π em (q 0 =0,q→0) In URHICs: source strength: dependence on T,  B,  , medium effects, … system evolution: V(  ), T(  ),  B (  ), transverse expansion, … nonthermal sources: Drell-Yan, open-charm, hadron decays, … consistency!

2.1 EM Correlator in Vacuum:  (e + e - → hadrons) e+e-e+e- h 1 h 2 … s ≥ (1.5GeV) 2 : pQCD continuum s < (1.5GeV) 2 : V-meson spectral functs. qqqq _   qq _

2.2 Low-Mass Dileptons + Chiral Symmetry Im Π em (M) dominated by  -meson → chiral partner: a 1 (1260) Vacuum At T c : Chiral Restoration pQCD cont. or:  long chiral partner of  ≡ “Vector Manifestation” [Harada+ Yamawaki ’01]

Caveat: conserve hadron ratios after chem. f.o.  chemical potentials for , K, N,…:  ,K,N > 0 for T < T chem 3.) Space-Time Evolution of A-A Collisions: Trajectories in the Phase Diagram Entropy+baryon conservation  fixes T(  B ) in the phase diagram Time scale: hydrodynamics, e.g. V FB (  )=(z 0 +v z  )  (R a ┴  2 ) 2 Thermal Dilepton Emission Spectrum  N [GeV]  [fm/c]

3.1 Electromagnetic Probes at SPS: Anno ~2004 Dileptons Medium Effects! 10% QGP [RR+ Shuryak ’99] [Turbide,RR+Gale’04] Direct Photons Bremsstrahlung  →   K→  K  [Liu+ RR’05] common thermal source!? HG: 4  →     30% QGP

> >    B *,a 1,K 1... N, ,K … Constraints: - B,M→  N,  -  N,  A,  N→  N - QCDSRs, lattice 4.) Vector Mesons in Medium D  (M,q:  B,T)=[M 2 -m  2 -   -   B -   M ] -1 (a) Hadronic Many-Body Theory Propagator: [Chanfray etal, Herrmann etal, RR etal, Koch etal, Weise etal, Post etal, Eletsky etal, Oset etal, …] (c) Vector Manifestation of Chiral Symmetry O HLS with  L ≡  (“VM”); vacuum: loop exp. O (p/  , m  /  , g) In-Med.: T-dep. m  (0), g  matched to OPE,  match <  , RG → on-shell  dropping  -mass, no vector dominance; spectral function? [Brown+Rho ’91] (b) Scale Invariance of L QCD [Harada, Yamawaki etal]

 B /  Dileptons I: News from SPS [RR+Wambach ’99]  -meson “melting” (+fireball) predictions ok, dropping mass not open issues: - absolute normalization + p t -dependence - M > 0.9GeV ? (4  →     !), in-medium  +  ? - “cocktail-  “ (+smooth signal)? vector dominance? drop. mass (norm.)  Spectral Function (many-body) NA60 Data

4.3 Vector Mesons + Dilepton Rates at LHC baryon effects important even at  B,net =0 : sensitive to  B,tot =   +  B,  more robust ↔ OZI - e + e - Emission Rates: dR ee /dM ~ f B Im  em Quark-Hadron Duality ?! in-med HG ≈ in-med QGP ! [qq→ee] [qq+O(  s )] ----

5.) Direct Photons Quark-Gluon Plasma q g q O But: other contributions in O(α s ) collinear enhanced D g =(t-m D 2 ) -1 ~1/α s [Aurenche etal ’00, Arnold,Moore+Yaffe ’01] Bremsstrahlung Pair-ann.+scatt. + ladder resummation (LPM) “Naïve” LO: q + q (g) → g (q) + γ [Kapusta,Lichard+Seibert ’91, …, Turbide,RR+Gale’04] Hot and Dense Hadron Gas    γ     a 1,   Im Π em (q 0 =q) ~ Im D vec (q 0 =q) Low energy: vector dominance High energy: meson exchange Emission Rates In-med QGP ≈ total HG ! to be understood…

5.2 Direct Photon Spectra at RHIC: PHENIX Model Predictions with initial + jet-induced + thermal quantitative agreement with new PHENIX data jet-plasma interactions outshine thermal radiation above 2GeV?! [Turbide et al. ’04, ’05 ] High-p t Low-p t

6.1 Perspectives for LHC I: Direct Photon Spectra Same sources as at RHIC [Turbide et al. ’04. ‘05] thermal QGP prevails over pQCD up to 10GeV?! thermal yield ~ N ch 1.4 jet-plasma interactions relatively less important High-p t Low-p t

hadron gas dominant, sensitive to  FB strong medium effects enrich QGP with p t e > 1GeV (or so) 6.2 Perspectives for LHC II: Thermal Dileptons Intermediate Mass Low Mass moderate sensitivity to therm. time  0 =0.28, 0.11, 0.03fm/c  0 >> R Pb /  ≈ 10 -3

6.3 Perspectives for LHC III: Dilepton Spectra open charm (bottom?) strong source at all M (energy loss?!), especially for N ch < 3000 less open charm at low mass, but more relative to QGP (IM)

7.) Conclusions Thermal EM Radiation in QCD:  em (q 0,q,  B,T) low mass:       (anti-/baryons) + IMR + thermal photons in-med HG - QGP shine equally bright? (resonances!?) NA60 precision data at SPS open the way for progress, explicit model predictions needed LHC: - QGP signal relatively more prevalent than at RHIC - reduced sensitivity to initial (thermalization) time, - hadronic signal (chiral restoration) compromised by charm - open-charm E-loss: background or signal?  LHC can address the full set of EM-Probes Physics ?!