High Energy Dilepton Experiments Introduction

M. Riordan and W. Zajc, Sci. Am., May 2006, 34-41 Outline chirality, chiral symmetry, and chiral symmetry breaking an experimentalist‘s humble approach setting the (general) stage for experiments what‘s the deal? the “soup kitchen“ basics what are we dealing with? M. Riordan and W. Zajc, Sci. Am., May 2006, 34-41 emphasis for today (very simplified) what is the objective? what are the experimental boundary conditions?

Nuclear matter as QCD laboratory “ordinary” nuclear matter 3 (light) constituent quarks quarks interact via the exchange of gluons gluons carry color charge! (→ complicated vacuum) key observations isolated quarks are NEVER observed (“confinement“) quark masses: ~1% of the nucleon mass hadron masses >> sum of quark masses related to chiral symmetry breaking properties of the strong interaction, theoretically described in QCD (Quantum Chromo Dynamics)

Chirality what is chirality? simplification of chirality: helicity origin: the greek word for hand: “” when does an object/system have “chirality”? when it differs from its mirror image L(eft) and R(ight) versions of this object/system simplification of chirality: helicity helicity = projection of a particle’s spin on its momentum direction high energy limit helicity = chirality

Chirality conservation massive particles P left and right handed components must exist m>0  particle moves with v<c if P looks left handed in the laboratory P will look right handed in a rest frame moving faster than P but in the same direction chirality is NOT a conserved quantity in a massless word chirality is conserved careful: m=0 is a sufficient but not a necessary condition right-handed left-handed

QCD and chiral symmetry the QCD Lagrangian: free gluon field interaction of quarks with gluon free quarks of mass mn formal definition of chirality in QCD chirality operators: PR, PL PR = ½ (1 + g5), PL = ½ (1 - g5) with g5 = i g0 g1 g2 g3, i.e. the product of Dirac matrices R and L projections of wave fct. u: uR,L = PR.Lu mass is problematic mu ≈ 4 MeV, md ≈ 7 MeV mnucleon ≈ 1 GeV ≈ 20 x current quark mass

Chiral symmetry breaking explicit chiral symmetry breaking mass term mnynyn in the QCD Lagrangian chiral limit: mu = md = ms = 0 chirality would be conserved  all states have a ‘chiral partner’ (opposite parity and equal mass) real life a1 (JP=1+) is chiral partner of r (JP=1-): Dm≈500 MeV even worse for the nucleon: N* (½-) and N (½+): Dm≈600 MeV  (small) current quark masses don’t explain this chiral symmetry is also spontaneously broken spontaneously = dynamically

Origin of mass current quark mass constituent quark mass generated by spontaneous symmetry breaking (Higgs mass) contributes ~5% to the visible (our) mass constituent quark mass ~95% generated by spontaneous chiral symmetry breaking (QCD mass)

Chiral symmetry restoration spontaneous symmetry breaking gives rise to a nonzero ‘order parameter’ QCD: quark condensate many models (!): hadron mass and quark condensate are linked numerical QCD calculations at high temperature and/or high baryon density  deconfinement and approximate chiral symmetry restoration (CSR)  constituent mass approaches current mass

How does CSR manifest itself? Chiral Symmetry Restoration expect modification of hadron spectral properties (mass m, width G) explicit relation between (m,G) and <qq>? QCD Lagrangian  parity doublets are degenerate in mass how is this degeneracy realized? do the masses drop to zero (or where else)? do the widths increase (melting resonances)? good questions, without (obvious) good answers

Theoretical guidance predictions for in-medium properties of the r meson one example where experiments have the potential to guide the theory mass of r width of r Pisarski 1982 Leutwyler et al 1990 (p,N) Brown/Rho 1991 ff Hatsuda/Lee 1992 Dominguez et. al1993 Pisarski 1995 Rapp 1996 ff

CSR and low mass dileptons what are the best probes for CSR? requirement: carry hadron spectral properties from (T, rB) to detectors relate to hadrons in medium leave medium without final state interaction dileptons from vector meson decays best candidate: r meson short lived decay (and regeneration) in medium properties of in-medium r and of medium itself not well known f meson (mf≈2xmK)  ee/KK branching ratio! m [MeV] Gtot [MeV]  [fm/c] BRe+e- r 770 150 1.3 4.7 x 10-5 w 782 8.6 23 7.2 x 10-5 f 1020 4.4 44 3.0 x 10-4

One experimentalists dream a fundamental question! how are hadron masses generated? theoretical guidance but no crisp answer! availability of a sensitive probe (dilepton decays of low-mass vector mesons)! strong variation of <qq> with T (above critical TC) and rB (continuous)  feasibility of systematic studies!

Another ones nightmare a lot of ‘horrible’ experimental difficulties prepare a system with (T,rB)≠(0,r0) and control (or determine) these parameters lepton measurements are difficult ‘needle in the haystack’ compared to hadrons many lepton sources beyond vector meson decays lepton pair measurements suffer from combinatorial background, i.e. pairs not originating from the same parent interpretation is difficult due to ‘other’ medium effects

Stage is set for a first class drama what is the real theory of CSR?  Volker what have experiments observed at and close to the nuclear ground state?  Piotr what have experiments observed far away from the nuclear ground state (in particular along temperature axis)?  Ralf

Probing strongly interacting matter nuclear matter close to the ground state electromagnetic probes (photon or electron beams) hadronic probes (pion or proton beams) excited strongly interacting matter relativistic nuclear collisions accessible regions high temperature at low net baryon density  colliders moderate temperature at high net baryon density  fixed-target machines

High energy heavy-ion accelerators fixed-target machines ~1 AGeV beam energy  √sNN ~ 2 GeV Bevalac@LBNL, SIS@GSI ~10 AGeV beam energy  √sNN ~ 5 GeV AGS@BNL (no dileptons) ~160 AGeV beam energy  √sNN ~ 17 GeV SPS@CERN future: ~30 AGeV beam energy  √sNN ~ 8 GeV SIS300@FAIR colliders ~100 AGeV beam energy  √sNN = 200 GeV RHIC@BNL future: 2.25 AGeV beam energy  √sNN = 5.5 TeV LHC@CERN

Not all collisions are the same Participants Spectators impact parameter b small impact parameter (b~0) high energy density large volume large number of produced particles measured as: fraction of cross section “centrality” number of participants number of nucleon-nucleon collisions from a “Glauber” MonteCarlo calculation

Experimental determination of geometry Paddle signal (a.u.) Paddles/BBC ZDC Au Central Multiplicity Detectors 5% Central STAR

The experimental challenge (RHIC) STAR ONE central Au+Au collision at max. energy PHENIX production of MANY secondary particles

Anatomy of a Au+Au collision time g e m p K L Jet cc g f Time freeze-out expansion hadronization formation and thermalization of quark-gluon matter? hard parton scattering Space Au

Different probes tell different stories investigate evolution of a system that “lives” for ~10-22 s (~100 fm/c) in a volume ~10-42 m3 (~1000 fm3) with energy ~6 x 10-6 J (~40 TeV) p K cc J/ p q hadrons: p, K, p, … abundant, final state yields, spectra → energy density, thermalization, hadrochemistry correlations, fluctuations, azimuthal asymmetries → collective behavior e- e+ g electromagnetic radiation: g, e+e-, m+m- rare, no strong interaction probe all time scales in-medium properties of light vector mesons  probe for chiral symmetry restoration effects dies ist eine schematische darstellung einer SI Kollision. Zwei kerne terffen sich mit einem stossparameter b. Je kleiner b ist um so mehr nucleonen nehmen and er kollision teil, um so mehr kollsisionen zwischen nucleonen finden statt und um so mehr teilchen werden erzeugt. In centralen kollisionen, d.h. mit sehr kleinem Stossparameter b, werden viele tausend teilchen erzeugt - die meisten werden senkrecht zur strahlachse erzeugt. Dies bezeichent man als mid rapidity oder rapiditaet null. die meisten teilen sind hadronen pi,k,p. Sie werden bei der hadronsynthese produziert und tragen information vor allem von der letzten phase der kollision von kurz bevor die teilechen aufhoeren strak zu ww. Aus den eigenschaften der hadronen koennen wir auf die anfangs energie dichte zurueckschliessen. Testen ob im reactions volumen thermishen und chemischen gleichgewicht herscht. Collectives Verhalten - expansion unter druck. Eine besondere Bedeutung kommt Teilechen mit strange quarks zu. eine erhoete production dieser teilchen ist eine der schluessel vorhersagen fuer die erzeugung von qm. Um Information ueber die heisse und dichte phase zu erhalten braucht man ander observabelen. Die sensitive sind auf fruehe phase der kollision. Hier unterscheiden man zwei gruppen von observablen. Electromagnetische Strahlung - photonen oder virtuelle photonen, lepton paare. Werden zu jeder zeit abgestrahlt ww dann aber nicht mehr stark und tragen daher information von der fruehen phase zum detector. Zum einen kann man sich das als schwarzkoerperstrahlung vorstellen die dann die anfangs temperatur wiederspiegelt. Dies war auch eine der schluesselvorhersagen. Zum anderen hat man ueber em strachlung aber auch zugriff auf die eigenschaften von hadronen in dichter heisser materie dies ist vermutlich der einziege zugriff auf chirale symmtry wiederherstellung die man hat. Die ander klasse von observablen sind sogenannte harte oder durchdringenede proben. Wie charmonium oder bottonium J/psi und upsilon sowie jets. diese sonden werden in den ersten collisionen noch for der formation von qm erzeugt. Sie durchdringen die heisse materie und sind deshalb sensitive auf deren eigenschaften. Hier gibt es zwei schluesselvorhersagen (i) abschirmung der farbladungen im plasma fuehrt zur underdrueckung aller J/psi resonancen (ii) starker energieverlust in farbiger materie fuehrt zum verschwinden von jets Alle experimentellen beobachtungen sind consistent mit diesen erwartungen. Das moechte ich ihnen nun im folgenden Zeigen. zunaecht fuer Ergebnisse vom SPS und dann fuer erste Ergebnisse vom RHIC. “hard” probes: jets, heavy quarks, direct g rare, produced initially (before quark-gluon matter forms!) probe hot and dense matter

Final state hadrochemistry do the huge yields of various hadron species in the final state reflect a THERMAL distribution? abundances in hadrochemical equilibrium spin isospin degeneracy temperature at chemical freezeout baryochemical potential one particle ratio (e.g. p/p) determines mB/T a second ratio (e.g. p/p) then determines T predict all other hadron abundances and ratios final state: hadron gas close to phase boundary

How close to the phase boundary? final state at RHIC (and elsewhere!): hadronic black body consistent with chemical equilibrium very close to phase boundary between hadronic and quark-gluon matter mB → 0 means B/B → 1 (early universe) T = 177 MeV provides lower limit for initial temperature

Particle counting  kinematics 4-vector of particle: More practical variables: transverse momentum Lorentz invariant related transverse mass rapidity Lorentz transformation: related pseudo rapidity h mass m momentum p polar angle q azimuth f beam axis measure: p and q are not Lorentz invariant!!

Particle spectra (basics) indication for chemical equilibrium  good chance for kinetic equilibrium as well first guess: a thermal Boltzmann source: but we are dealing with a system of interacting particles expanding into vacuum  flow natural ordering of particles occurs with the highest velocity being present at the system’s ‘outer edge’ particle spectra represent a convolution of thermal motion radial expansion of the source, i.e. radial flow

Particle spectra (radial flow) blast wave model if a thermal source is boosted radially with a velocity bboost and evaluated at y=0 with simple assumption: uniform sphere of radius R and boost velocity varies linearly with r:

What does this mean? different spectral shapes for particles of different mass  strong collective radial flow purely thermal source mT 1/mT dN/dmT light heavy T reasonable agreement with hydrodynamic prediction at RHIC Tfo ~ 100 MeV <br> ~ 0.55 c explosive source light T,b 1/mT dN/dmT heavy mT mT = (pT2 + m2)½

Kinetic freeze-out systematics increases continuously Tfo saturates around AGS energy strong collective radial expansion at RHIC high pressure high rescattering rate thermalization likely Slightly model dependent here: blast wave model

Elliptic flow → early thermalization initial state of non-central Au+Au collision spatial asymmetry asymmetric pressure gradients x z Non-central Collisions in-plane out-of-plane y Au nucleus translates into momentum anisotropy in final state Fourier expansion elliptic flow strength shape “washes out” during expansion, i.e. elliptic flow is “self quenching” v2 reflects early interactions and pressure gradients

Hadron v2 and hydrodynamics observations at RHIC v2 is large and for soft hadrons in reasonable agreement with ideal hydrodynamics (not true at lower energies) mesons baryons PHENIX: nucl-ex/0608033 indication for thermalization at a time when quark degrees of freedom are important!

One view behind the curtain tomography - one way to establish properties of a system calibrated probe (e±, X-rays with known beam energy & direction) calibrated interaction (known interaction mechanism!) suppression/absorption pattern reveals details about the interior “hard probe” tomography at RHIC probe has to be “auto generated” initial state hard parton (quark, gluon) scattering  jets calibration: p+p collisions g medium

Nuclear modification of jets “shooting” hard probes through the medium nuclear modification factor: direct photons “shine” through the medium (no strong interaction) hadrons (from jet fragmentation) don’t very large density of strongly interacting scattering centers!

High energy heavy-ion collisions central HI collisions @ RHIC hot and dense phase of strongly interacting matter! central HI collisions @ SPS quantitatively and qualitatively different conditions major in-medium effects due to the onset of chiral symmetry restoration expected @ SPS & RHIC experiments are promising

Lepton-pair physics: topics known sources of lepton pairs emitted over the full evolution of the collision reach detectors undistorted from strong FSI chiral symmetry restoration continuum enhancement modification of vector mesons thermal radiation suppression (enhancement) modifications expected due to the QCD phase transition(s) lepton pairs are rich in physics experimentally challenging

The double challenge the experimental challenge the analysis challenge example: central Au+Au collisions @ √sNN = 200 GeV need to detect a weak e+e- source hadron decays (mee > 200 MeV/c2; pT > 200 MeV/c) ~4x10-6/p0 in the presence of many charged particles dNch/dy ~ 700 and several pairs/event from trivial origin Dalitz decay of p0 ~10-2/p0 conversion of g (assuming 1% radiation length) ~2x10-2/p0  huge combinatorial background  (dNch/dy)2 the analysis challenge e+e- emitted through the whole history of the collision need to disentangle the different sources need excellent p+p and p(d)+A reference data

HI low-mass dileptons at a glance time scale of experiments (KEK E235) CERES DLS NA60 HADES CBM 90 95 10 00 05 85 PHENIX ALICE = period of data taking

HI low-mass dileptons at a glance energy scale of experiments (KEK E235) CERES DLS NA60 HADES CBM PHENIX 10 158 [A GeV] 17 [GeV] √sNN 200 // ALICE [A TeV]

Summary and Outlook chiral symmetry restoration (CSR) a topic of fundamental interest a topic that can be studied via (challenging) dilepton experiments high energy HI collisions produce strongly interacting matter in which major CSR effects are expected dilepton experiments have been done and have produced results 2nd lecture: SPS experiments 3rd lecture: RHIC experiments & future