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Workshop on QFTHEP, Golitsyno, 08-15.September 2010

The problem of anomalous dilepton yield in relativistic heavy-ion collisions A.A. Andrianov

Sankt-Petersburg State University

in collaboration with V.Andrianov, D. Espriu and X.Planells
Some highlights in: A. Andrianov, D. Espriu, P. Giacconi and R. Soldati, JHEP 09 (2009) 057 A. Andrianov, D. Espriu, F. Mescia and A. Renau, Phys.Lett. B684 (2010) 101
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Tentative plan
1. Dilepton production in central collisions in experiments on PHENIX (RHIC) NA60 (SPS) CERES (SPS) HADES (GSI) DLS (Berkeley) The first clear signs of an excess of dileptons above the known decay sources at SPS energies were obtained by CERES for M<1 GeV, NA38/NA50 for M>1 GeV and by HELIOS-3 for both mass regions . Years 1994-1995. 2. Cocktail of hadron decays into dileptons at high temperatures + dropping mass and /or broadening of meson resonances + enhancement of eta meson production in proton-neutron scattering = failure to explain abnormal dilepton yield 3. Large fluctuation of topological charge chiral chemical potential in a fireball "axion-like" background ~ condensate OR Spontaneous parity breaking due to pion condensate = time-dependent pseudoscalar background in collider kinematics 4. Parity breaking splitting of masses for photon/vector meson polarizations appearance of giant resonances

substantial enhancement of dilepton yield

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Recent data from PHENIX RHIC

From: PHYS. REV. C 81, 034911 (2010)
(PHENIX Collaboration)

Central collisions
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Small p_T thermal sources of dileptons

Low masses = LMR, M_ee<1 GeV

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This is an old puzzle!

D. Miskowiec, Nucl. Phys. A 774, 43 (2006).

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G. Agakichiev et al., Eur. Phys. J. C 41, 475 (2005).
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Dalitz decays

2-pion fusion

G. Agakichiev et al., Eur. Phys. J. C 41, 475 (2005).

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The oldest evidence of dilepton excess

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Proton-neutron enhancement

Not sufficient to saturate anomalous e+e- yield !

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Hadron cocktail
Dilepton masses Dominant Dalitz decays Dilepton masses Dominant 2-pion fusion 300 MeV < M_ee < 900 MeV < 140 MeV, < 550 MeV

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PHENIX: LMR anomaly

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PHENIX: intermediate masses = IMR, 1
J.Manninen et al arXiv:1005.0500

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NA60 results on thermal dimuons

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Photon(vector mesons) instability in pseudoscalar background
For slowly growing/decreasing neutral pion (isovector) condensate

or large-scale isoscalar axion-like field axial chemical potential (in central heavy ion collisions)

Induced C-S term

Adiabatic approximation:

in units

time derivative of CS vector << photon frequency/vector meson energy
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P-and CP-odd condensates
Isoscalar condensate theta vacuum bubbles

D. Kharzeev, R. D. Pisarski and M. H. G. Tytgat, Phys. Rev. Lett. 81, 512 (1998) K. Buckley, T. Fugleberg, and A. Zhitnitsky, Phys. Rev. Lett. 84 (2000) 4814 D. E. Kharzeev, L. D. McLerran and H. J. Warringa, Nucl. Phys. A 803, 227 (2008)

Isotriplet neutral condensate ="pion" condensate A.A.Andrianov and D.Espriu, Phys.Lett. B 663 (2008) 450 (and refs..therein) A.A.Andrianov, V.A.Andrianov and D.Espriu, Phys.Lett. B 678 (2009) 416 (the talk by Vladimir Andrianov)
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Start with
photons of different circular polarizations. They have different dispersion relation between their frequencies and wave vectors Polarizations Mass-shell 0

(Almost) circular polarizations of distorted photons

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Polarized decay !!
Time-like CS vector Mass-shell Photon "-" is a tachyon Photon "+"decays with threshold Decay width is small But the distorted photon is not a proper Breit-Wigner resonance as its position (effective mass) moves with momentum! Threshold hierarchy! If for electrons/positrons the threshold is of order 1 MeV then for muons it is four orders of magnitude higher , i.e. 10 GeV ! No muon pairs excess in the PHENIX data!? There is an excess in NA60!
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Dilepton pair creation
QED

"Giant" resonance with variable position

|k|

"On-shell" enhancement in a range of | k | !

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Taking into account thermal distribution
Boltzmann

For transversal polarizations

Production rate is sensitive to temperatures via photon effective mass and width

RHIC temperatures

T = 150 ­ 250 MeV

Photon thermal distribution makes resonances broader

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rho meson enhancement =

Does it provide the abnormal ee excess in the range 100 ­ 700 MeV? Only partially! Zeta scale is plausibly of order of few MeV's, mesons must enter the game.
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Finite-volume suppression (qualitatively)

A typical size of nuclear fireball

Time spent by photons in nuclear medium Resonance wave function and amplitude

Breit-Wigner In the peak for Absolute enhancement

Relative enhancement

For

rather large

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Mixing with vector mesons

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Chern-Simons "mass"

For isosinglet condensate

For isotriplet condensate

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Mass splitting for transversal polarizations
Mass shell

for transversal polarizations

For massless photons and isoscalar condensate

Transversal photon are not distorted and not decaying! But for a mixture of isoscalar and isovector condensates they do decay.
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channel and VMD

Normal rho meson resonance for longitudinal polarization "Giant" resonances for transversal polarizations with variable position

|k|

production rate

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Dalitz decays

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Abnormal enhancement of dilepton yield

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Compare with PHENIX

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Signatures and searches of parity breaking

Program for

RHIC

CBM FAIR + NICA

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