Purdue University, April 28, 2011 Purdue University, April 28, 2011 Itzhak Tserruya Dileptons as a probe of the Quark Gluon Plasma Itzhak Tserruya Purdue University, April 28, Outline Introduction SPS energy Low-mass region Intermediate mass region Low energies: DLS and HADES RHIC energy first results from PHENIX Thermal radiation at RHIC Summary The Quark Gluon Plasma created in relativistic heavy ion collisions is characterized by two fundamental properties: Deconfinement Chiral Symmetry Restoration Electromagnetic probes (real or virtual photons) are sensitive probes of both properties and in particular lepton pairs are unique probes of CSR. Thermal radiation emitted in the form of dileptons (virtual photons) provides a direct fingerprint of the matter formed (QGP and HG) and a measurement of its temperature. What have we learned in almost 20 years of dilepton measurements? Introduction Itzhak Tserruya Purdue University, April 28, Origin of mass X Origin of (our) mass: ~5% of the (visible) mass is due to the Higgs field. 95% of the (visible) mass is due to the spontaneous breaking of the chiral symmetry. Current quark masses generated by spontaneous symmetry breaking (Higgs field) Constituent quark masses generated by spontaneous chiral symmetry breaking current quark masses: m u 4 MeV m d 7 MeV proton = uud neutron = udd m Nucleon 1 GeV Itzhak Tserruya Purdue University, April 28, Mass [MeV] Itzhak Tserruya PHENIX Focus, BNL, April 4, Chirality What is chirality? Comes from the greek word meaning hand An object or a system has chirality if it differs from its mirror image. Such objects then come in two forms, L and R, which are mirror images of each other. Simple definition: the chirality of a particle is determined by the projection of its spin along its momentum direction (this is in fact the definition of helicity. In the high energy limit chirality helicity)) Two fundamental properties of the QGP: Deconfinement Chiral symmetry restoration Itzhak Tserruya PHENIX Focus, BNL, April 4, Chiral Symmetry In a massless world chirality is conserved (sufficient but not necessary condition) If a particle has mass both right- and left-handed components must exist. The reason is that massive particles travel slower than the speed of light and a particle that appears left-handed in a particular reference frame will look right-handed from a reference frame moving faster than the particle chirality is not conserved Left-handed Right-handed Explicit and Spontaneous Chiral Symmetry Breaking (I) The mass term m n n n explicitly breaks the chiral symmetry of the QCD Lagrangian all states have a chiral partner with opposite parity and equal mass QCD, the theory of the strong interaction, is encoded in a one line Lagrangian: Free g field q interaction with g field Free q of mass m n at rest Chiral limit: m u = m d = m s = 0 In this idealized world, the interactions quark-gluon conserve the quark chirality. (lefthanded u,d,s, quarks remain left-handed forever) and as a consequence m u and m d are so small that our world should be very close to the chiral limit Explicit and Spontaneous Chiral Symmetry Breaking (II) In reality: (J P = 1 - ) m=770 MeV chiral partner a 1 (1 + ) m=1250 MeV 500 MeV For the nucleons the splitting is even larger: N (1/2 + ) m=940 MeV chiral partner N * (1/2 - ) m=1535 MeV 600 MeV The differences are too large to be explained by the small current quark masses Chiral symmetry is spontaneously ( dynamically) broken Itzhak Tserruya Purdue University, April 28, Chiral Symmetry Restoration Spontaneous breaking of a symmetry is marked by: * a non-zero order parameter, the quark condensate in the case of QCD: At high temperatures (T>T C ) or high baryon densities ( > C ), numerical QCD calculations on the lattice predict that the quark condensate vanishes: Many models link the hadron masses to the quark condensate. constituent mass current mass chiral symmetry (approximately) restored Itzhak Tserruya Purdue University, April 28, How does CSR manifest itself ? What happens when chiral symmetry is restored? Meson properties (m, ) expected to be modified but how? Is there an explicit connection between the spectral properties of hadrons (masses,widths) and the value of the chiral condensate ? From the QCD Lagrangian, the only requirement is that parity doublets should be degenerate in mass. how is the degeneracy of chiral partners realized ? do the masses drop to zero? do the widths increase (melting resonances)? All very good questions with no good answer The theoretical picture is confusing mass of mass of width of width of Pisarski 1982 Leutwyler et al 1990 ( ,N) Brown/Rho 1991 ff Hatsuda/Lee 1992 Dominguez et. al1993 Pisarski 1995 Rapp 1996 ff Theoretical predictions for the in-medium modification of the -meson properties Lattice QCD coming to the rescue: hadron spectral functions are being studied on the lattice. Another example where experiment has the potential to guide the theory. The Double Challenge Need to detect a very weak source of e + e - pairs hadron decays (m>200 MeV/c 2 p T > 200 MeV/c) ~ / o in the presence of several pairs per event from trivial origin o Dalitz decays ~ / o conversions (assume 1% radiation length) / o and hundreds of charged particles per event in central Au+Au collision at RHIC dN ch / dy 700 huge combinatorial background (dN ch / dy ) 2 (pairing of tracks originating from unrecognized o Dalitz decays and conversions Experimental challenge Electron pairs are emitted through the entire history of the collision: need to disentangle the different sources. need reference pp, dA data and precise data on each source separately. Analysis challenge Energy Scale DLS HADES 10158[A GeV] 17[GeV]s NN 200 // CBM CERES NA60 PHENIXMPD Dileptons in A+A at a Glance: Itzhak Tserruya CERES DLS NA60 HADES CBM PHENIX Time Scale MPD = Period of data taking 13 Purdue University, April 28, 2011 PHENIX + HBD STAR SPS SPS Low-masses Low-masses (m 1GeV/c 2 ) Itzhak Tserruya14 Purdue University, April 28, 2011 CERES Pioneering Results (I) Strong enhancement of low-mass e + e - pairs (wrt to expected yield from known sources) Enhancement factor (0.2 1 GeV/c 2 : Purdue University, April 28, 2011 hadronic processes, 4 partonic processes, qq annihilation Quark-Hadron duality? p T distributions Low-mass region Intermediate mass region Fit in 0.5