Alberica Toia Physics Department CERN HGS-HIRe Lecture Week Manigod January 2010 High Energy Dilepton Experiments RHIC

Alberica Toia 2 HGS-HIRe Lecture Week /10 Manigod l RHIC = Relativistic Heavy Ion Collider l located at Brookhaven National Laboratory RHIC

Alberica Toia 3 HGS-HIRe Lecture Week /10 Manigod RHIC and its experiments l what’s so special about RHIC? l it’s a collider –no thick targets –detector systematics do not depend on E CM l p+p: √s ≤ 500 GeV (polarized beams) l A+A: √s NN ≤ 200 GeV (per NN pair) STAR l experiments with specific focus l BRAHMS (until Run-6) l PHOBOS (until Run-5) l multi purpose experiments l PHENIX l STAR

Alberica Toia 4 HGS-HIRe Lecture Week /10 Manigod Total baryon density p – p participants nucleons (p – p )A/Z dN( p ) / dy produced baryons (p, p, n, n ) RHIC (Au-Au) SPS (Pb-Pb) Low mass e + e - : RHIC l 2 SPS profit from high baryon density –dropping  mass –broadening of  l what to expect at RHIC? l baryon density: almost the same at SPS & RHIC (although the NET baryon density is not!)

Alberica Toia 5 HGS-HIRe Lecture Week /10 Manigod e- e+ e + e - : theoretical guidance at RHIC R. Rapp: nucl-th/ l in-medium modifications of vector mesons persists l open charm contribution becomes significant

Alberica Toia 6 HGS-HIRe Lecture Week /10 Manigod The founding fathers’ view l before 1991 l proposals for various experiments at RHIC –STAR, TALES, SPARC, OASIS, DIMUON … –except for STAR everything else is burned down l from the ashes rises PHENIX –Pioneering High Energy Nuclear Interaction eXperiment l 1991: PHENIX “conceptual design report” l philosophy –measure simultaneously as many observables relevant for QCD phase transitions as you can imagine –all but one: low-mass dielectrons l why no dielectrons? –included in first TALES proposal –considered to be “too difficult” for PHENIX l a lot of work can make impossible things happen

Alberica Toia 7 HGS-HIRe Lecture Week /10 Manigod PHENIX in practice

Alberica Toia 8 HGS-HIRe Lecture Week /10 Manigod PHENIX in principle l 3 detectors for global event characterization two forward muon spectrometers l forward spectrometers l muon measurement in range: 1.2 < |  | < 2.4 p  2 GeV/c l central spectrometers l measurement in range:  0.35 p  0.2 GeV/c two central electron/photon/hadron spectrometers

Alberica Toia 9 HGS-HIRe Lecture Week /10 Manigod Au-Au collision as seen in PHENIX

Alberica Toia 10 HGS-HIRe Lecture Week /10 Manigod PC1 PC3 DC ee e+e+   PHENIX: tracking & particle ID

Alberica Toia 11 HGS-HIRe Lecture Week /10 Manigod 11 Momentum determination Simple relation between bending and momentum  = K/p T K~200 rad GeV/c Momentum resolution is determined by the resolution of , which is determined by : l single hit resolution(SHR) and l alignment l SHR is measured to be 150mm, about 0.3 mrad, which corresponds to 0.3/200=0.1% resolution. l Affected by l global and wire alignments

Alberica Toia 12 HGS-HIRe Lecture Week /10 Manigod Electron Identification I Charged particle tracking (dm: 1%) DC, PC1, PC2, PC3 and TEC PHENIX optimized for Electron ID Cherenkov light RICH + shower EMCAL emission and measurement of Cherenkov light in the Ring Imaging Cherenkov detector → measure of min. velocity how can pions ever be mis-identified below 4.9 GeV/c? Radiation of cherenkov light (≥ 4.9 GeV/c) Production of delta electrons Random coincidence (high multiplicity) spherical mirror  parallel tracks produce rings at SAME location RICH

Alberica Toia 13 HGS-HIRe Lecture Week /10 Manigod Electron Identification II production and of el.magn. shower in the Electro- Magnetic Calorimeter → measure of energy E l PbSc: sampling cal., layers of lead and scintillator l PbGl: homogeneous lead-glass volume, Cherenkov radiator Energy-Momentum All charged tracks Background Net signal Real RICH cut electron: E ≈ p hadron: E < p after RICH cuts, clear electron signal cut on E/p cleans electron sample! background photon conversions random associations (next slide) main background source: random combination of hadron track/shower with uncorrelated RICH ring “standard” subtraction technique: flip-and- slide of RICH swapped background agrees in shape with E/p distribution of identified hadrons background increases with detector occupancy (can reach ~30% in central Au+Au collisions)

Alberica Toia 14 HGS-HIRe Lecture Week /10 Manigod l first attempt from 2002 Au-Au Run l S/B ~ 1/500 (!) for minimum bias events l not enough statistics l Au-Au data taken in 2004 l ~ 100x statistics l photon conversions reduced by factor 2-3 l expect background reduction by ~ 2 PHENIX measures dielectrons Real and Mixed e + e - Distribution Real - Mixed

Alberica Toia 15 HGS-HIRe Lecture Week /10 Manigod Detailed measurement of the e+e- pair continuum in p+p and Au+Au collisions at √s NN = 200 GeV and implications for direct photon production arXiv: authors 59 institutions 56 pages 50 figures 13 tables Submitted to Physical Review C on 1 st December 2009 comprehensive results of dilepton measurements at RHIC.

Alberica Toia 16 HGS-HIRe Lecture Week /10 Manigod Background l Type I: identified on a pair-by-pair basis: l Overlapping hits in the detectors (mostly RICH) l Photon conversions l Type II: cannot be identified on pair-by-pair basis  removed statistically l Combinatorial B comb all combinations where the origin of the two electrons is totally uncorrelated l Correlated B corr –Cross pairs: Two pairs in the final state of a meson –Jet pairs: Two hadrons within the same jet or in back-to- back jets, decay into electron pairs

Alberica Toia 17 HGS-HIRe Lecture Week /10 Manigod Overlapping pairs l when a pion points to the same ring as an electron, it is associated to the same ring, therefore considered an electron This happens for a typical values of opening angle (different for like and unlike) which folded with the average momentum of the electron corresponds to a particular invariant mass (different for like and unlike)  cut: requested minimum distance between the rings (~1 ring diameter) l Cut applied as event cut Real events: discarded and never reused Mixed events: regenerated to avoid topology dependence

Alberica Toia 18 HGS-HIRe Lecture Week /10 Manigod Photon conversion rejection z y x e+e+ e-e- B Conversion pair z y x e+e+ e-e- B Dalitz decay l artifact of PHENIX tracking l assume that all tracks originate from the vertex l off vertex tracks  wrong momentum vector  conversions are reconstructed with m≠0 (m~r) l conversions “open” in a plane perpendicular to the magnetic field

Alberica Toia 19 HGS-HIRe Lecture Week /10 Manigod  Signal to Background: S/B = 1 / 250 Low-mass e + e - pairs: the problem l electrons/event in PHENIX l N e = (dN/d  )  0 * (BR+CONV) * acc * f(p T >0.2GeV) 350 ( ) 0.5* = 1.3 l combinatorial background pairs/event l B = ½ * ½N e 2 e -N = 0.1 l expected signal pairs/event (m>0.2GeV, p T >0.2 GeV) l S = 4.2*10 -4  signal/background l as small as 1/ few hundred l depends on mass l what can we do to reduce the combinatorial background? where does it come from?

Alberica Toia 20 HGS-HIRe Lecture Week /10 Manigod Conversion/Dalitz rejection? l typically only one “leg” of the pair is in the acceptance l acceptance holes l “soft” tracks curl up in the magnetic field l only (!) solution l catch electrons before they are lost l need new detector and modification of magnetic field

Alberica Toia 21 HGS-HIRe Lecture Week /10 Manigod Consequences of poor S/B comb l how is the signal obtained? l unlike-sign pairs: F l combinatorial background: B (like-sign pairs or event mixing) l  S = F – B l statistical error of S l depends on magnitude of B, not S l  S ≈ √2B (for S<<B) l “background free equivalent” signal S eq l signal with same relative error in a situation with zero background l S eq = S * S/2B l example: S = 10 4 pairs with S/B = 1/250  S eq = 20 l systematic uncertainty of S l dominated by systematic uncertainty of B l example: event mixing with 0.25% precision (fantastic!)  ~60% systematic uncertainty of S (for S/B = 1/250)

Alberica Toia 22 HGS-HIRe Lecture Week /10 Manigod Type II background METHOD 1 l Combinatorial background: event mixing l Like and Unlike-sign pairs taking electons from different events l Normalize like-sign background to like-sign foreground in a region in (m,p T )where they agree l Normalize unlike-sign background to 2√N ++ N -- l Correlated background: simulations l Cross pairs: EXODUS l Jet pairs: PYTHIA l Normalize like-sign background to like-sign foreground l Normalize unlike-sign background in the same way ADVANTAGE l Great statistics (much larger than foreground) DISADVANTAGE l Assume simulation shape l Need independent normalization

Alberica Toia 23 HGS-HIRe Lecture Week /10 Manigod Type II background METHOD 2 l If dN like = dN unlike  S +- = N √N ++ N — l In PHENIX dN like  dN unlike l But unlike-sign background B +- = 2√N ++ N -- can be corrected by acceptance difference ADVANTAGE l This method measures ALL type II background simultaneously l only assumptions needed: – dN like measures only background – Background symmetric in like and unlike DISADVANTAGE l Poor statistics (similar to foreground)

Alberica Toia 24 HGS-HIRe Lecture Week /10 Manigod Combinatorial Background shape --- Foreground: same evt N Background: mixed evt B++ Shape determined with event mixing Excellent agreements for like- sign pairs Normalization of mixed pairs Small correlated background at low masses normalize B ++ and B − − to N ++ and N − − for m ee > 0.7 GeV/c 2 Normalize mixed B + − pairs to N +- = 2√N ++ N -- Subtract correlated background Systematic uncertainties statistics of N ++ and N -- : 0.12% different pair cuts in like and unlike sign: 0.2 %

Alberica Toia 25 HGS-HIRe Lecture Week /10 Manigod Differential Combinatorial Background Centrality DependenceTransverse Momentum Dependence --- Foreground: same evt N Background: mixed evt B++

Alberica Toia 26 HGS-HIRe Lecture Week /10 Manigod Combinatorial and Correlated Background p+p Au+Au Combinatorial Background from mixed events normalized to 2√N ++ N -- Cross pairs simulated with decay generator EXODUS Jet pairs simulated with PYTHIA normalized to like sign data and use same normalization for unlike-sign

Alberica Toia 27 HGS-HIRe Lecture Week /10 Manigod Uncertainty of Background Subtraction p+p Au+Au Method 1 and Method 2 Variations of the two method RMS  Systematic Uncertainty

Alberica Toia 28 HGS-HIRe Lecture Week /10 Manigod Cross check Converter Method We know precise radiation length (X 0 ) of each detector material The photonic electron yield can be measured by increase of additional material (photon converter was installed) The non-photonic electron yield does not increase Photonic single electron: x 2.3 Inclusive single electron :x 1.6 Combinatorial pairs :x 2.5 Photon Converter (Brass: 1.7% X 0 ) N e Electron yield Material amounts:  0 0.4%1.7% Dalitz : 0.8% X 0 equivalent radiation length 0 With converter W/O converter 0.8% Non-photonic Photonic converter

Alberica Toia 29 HGS-HIRe Lecture Week /10 Manigod The raw subtracted spectrum Same analysis on data sample with additional conversion material  Combinatorial background increased by 2.5 Good agreement within statistical error  signal /signal =  BG /BG * BG/signal large!!! 0.25% From the agreement converter/non-converter and the decreased S/B ratio scale error = 0.15±0.51% (consistent with the 0.25% error we assigned)

Alberica Toia 30 HGS-HIRe Lecture Week /10 Manigod Efficiency Correction p+p Au+Au Trigger Efficiency (p+p) Efficiency Correction: Derived from single electron efficiency Include detector dead areas Include pair cuts Same shape for p+p and Au+Au p+p further corrected for trigger efficiency

Alberica Toia 31 HGS-HIRe Lecture Week /10 Manigod Acceptance Correction Acceptance Correction: Derived from single electron acceptance Compare Hadron decays (full cocktail) Flat distribution in different mass regions as function of p T Difference within ~10%

Alberica Toia 32 HGS-HIRe Lecture Week /10 Manigod Hadronic Cocktail Measurement arXiv: Parameterization of PHENIX  ±,  0 data  0 = (  + +  - )/ 2 Other mesons:fit with m T scaling of π 0 p T →√(p T 2 +m meson 2 -m π 2 ) fit the normalization constant  All mesons m T scale!!! Hadronic cocktail was well tuned to individually measured yield of mesons in PHENIX for both p+p and Au+Au collisions. Mass distributions from hadron decays are simulated by Monte Carlo.  0, ,  ’, , , , J/ ,  ’ Effects on real data are implemented

Alberica Toia 33 HGS-HIRe Lecture Week /10 Manigod Cocktail Comparison p+p Light hadron contributions subtracted Heavy Quark Cross Sections: Charm: integration after cocktail subtraction σ cc = 544 ± 39 stat ±142 syst ± 200 model μb Simultaneous fit of charm and bottom: –σ cc = 518 ± 47 stat ± 135 syst ± 190 model μb –σ bb = 3.9 ± 2.4 stat +3/-2 syst μb Charm cross section from single electron measurement [PRL97, (2006)] : –σ cc = 567 ± 57 ± 193 μb 2.25pb -1 of triggered p+p data Data absolutely normalized Excellent agreement with Cocktail Filtered in PHENIX acceptance PLB 670,313(2009) arXiv: PLB 670,313(2009)

Alberica Toia 34 HGS-HIRe Lecture Week /10 Manigod Charm and bottom cross sections CHARMBOTTOM Dilepton measurement in agreement with single electron, single muon, and with FONLL (upper end) Dilepton measurement in agreement with measurement from e-h correlation and with FONLL (upper end) First measurements of bottom cross section at RHIC energies! PLB670,313(2009) PRL103,082002

Alberica Toia 35 HGS-HIRe Lecture Week /10 Manigod Cocktail Comparison Au+Au Low Mass Region: large enhancement 150 <m ee <750 MeV 4.7±0.4 stat ±1.5 syst ±0.9 model Intermediate Mass Region: dominated by charm (N coll x  cc ) PYTHIA Random cc correlation Single electron measurement High p T suppression Flow  Expected modifications in the pair invariant mass  random cc correlation?  Room for thermal contribution? 800M min.Bias Au+Au events Data absolutely normalized Light hadrons cocktail Charm normalized N coll x  pp Filtered in PHENIX acceptance arXiv:

Alberica Toia 36 HGS-HIRe Lecture Week /10 Manigod  0 region: consistent with cocktail Low Mass Region: yield increases faster than proportional to N part  enhancement from binary annihilation (ππ or qq) ? Centrality Dependence LMR arXiv:

Alberica Toia 37 HGS-HIRe Lecture Week /10 Manigod Centrality Dependence IMR arXiv: l charm is a hard probe l total yield follows binary scaling (known from single e ± ) l intermediate mass yield shows the same scaling

Alberica Toia 38 HGS-HIRe Lecture Week /10 Manigod Momentum Dependence p+p in agreement with cocktail Au+Au low mass enhancement concentrated at low p T p+p Au+Au arXiv:

Alberica Toia 39 HGS-HIRe Lecture Week /10 Manigod LMR I: Virtual Photons Any source of real  can emit  * with very low mass. l If the Q 2 (=m 2 ) of virtual photon is sufficiently small, the source strength should be the same l The ratio of real photon and quasi-real photon can be calculated by QED  Real photon yield can be measured from virtual photon yield, which is observed as low mass e + e - pairs Kroll-Wada formula S : Process dependent factor Case of Hadrons Obviously S = 0 at M ee > M hadron Case of  * If p T 2 >>M ee 2 Possible to separate hadron decay components from real signal in the proper mass window. q  g q e+e+ e-e-

Alberica Toia 40 HGS-HIRe Lecture Week /10 Manigod Determination of  * fraction, r r = direct  * /inclusive  * determined by fitting the following function for each p T bin. arXiv: arXiv: f direct is given by Kroll- Wada formula with S = 1. f cocktail is given by cocktail components Normalized to the data for m<30 MeV/c 2 Fit in MeV/c 2 (insensitive to  0 yield) – Assuming direct  * mass shape:  2 /NDF=12.2/6

Alberica Toia 41 HGS-HIRe Lecture Week /10 Manigod Direct measurement of S(m ee, p T ) Au+Au 200 GeV Vaccuum pt=1.025 GeV/c Drop mass qq No indication of strong modification of EM correlator at this high p T region (presumably the virtual photon emission is dominated by hadronic scattering process like  +    +  * or q+g  q+  *) Extrapolation to M=0 should give the real photon emission rate arXiv:

Alberica Toia 42 HGS-HIRe Lecture Week /10 Manigod direct  * /inclusive  * μ = 0.5p T μ = 1.0p T μ = 2.0p T Base line Curves : NLO pQCD calculations with different theoretical scales done by W. Vogelsang. p+p Consistent with NLO pQCD – better agreement with small µ Au+Au Clear enhancement above NLO pQCD p+pAu+Au arXiv: arXiv:

Alberica Toia 43 HGS-HIRe Lecture Week /10 Manigod 1 st measurement of Thermal Radiation Direct photon –real (p T >4GeV) –virtual (1<p T <4GeV & m ee <300MeV) pQCD consistent with p+p down to p T =1GeV/c Au+Au above N coll x p+p for p T < 2.5 GeV/c Au+Au = pQCD + exp: T ave = 221  19 stat  19 syst exp + T AA scaled pp NLO pQCD (W. Vogelsang) Fit to pp arXiv: arXiv:

Alberica Toia 44 HGS-HIRe Lecture Week /10 Manigod From data: Au+Au = pQCD + exp: T ave = 221  19 stat  19 syst Comparison to hydrodynamical models: p T <3 GeV/c thermal contribution dominates over pQCD. Assume formation of a hot QGP with 300 MeV < T init < 600 MeV 0.6 fm/c <  0 < 0.15 fm/c Models reproduce the data within a factor of two. Comparison to Hydro models T C from Lattice QCD arXiv: From data:T ini > 220 MeV > T C From models:T ini = 300 to 600 MeV  0 = 0.15 to 0.5 fm/c

Alberica Toia 45 HGS-HIRe Lecture Week /10 Manigod Consistent with flat  S(m ee ) const =0.177±0.032 Consistent with higher p T values Large and broad enhancement  S(m ee ) no longer const LMR II arXiv:

Alberica Toia 46 HGS-HIRe Lecture Week /10 Manigod Extrapolate the spectrum of direct photons l For 0.8 = 0.177±0.032 consistent with higher p T Decay photons spectrum steeper than direct  spectrum  At lower p T, the expected direct photon fraction r = direct  / inclusive  = direct  / (direct + decay)  ≤ 0.17 l For  The enhancement in the low p T region is larger than that expected from internal conversion of direct photons.

Alberica Toia 47 HGS-HIRe Lecture Week /10 Manigod p+pAu+Au p+p –Agreement with cocktail + internal conversion of direct photons Au+Au –p T >1GeV/c: small excess  internal conversion of direct photons –p T <1GeV/c: large excess for 0.3<m ee <1 GeV  Low temperature component with strong modification of EM correlator? Dilepton Spectra Acceptance- corrected arXiv:

Alberica Toia 48 HGS-HIRe Lecture Week /10 Manigod Average Temperature of the sources l m T – m 0 spectrum of Excess = Data – (cocktail+charm) l Fit: T 2 consistent with T  Direct  T 1 = 92.0 ± 11.4 stat ± 8.4 syst MeV T 2 = ± 37.3 stat ± 9.6 syst MeV  2 /NDF= 4.00/9 T 1 = 86.5 ± 12.7 stat syst MeV T  = 221 ± 19 stat ± 19 syst MeV  2 /NDF= 16.6/11 or low mass enhancement has inverse slope of ~100 MeV. arXiv:

Alberica Toia 49 HGS-HIRe Lecture Week /10 Manigod Theory comparison  annihilation + modified  spectral function l Broadening l Mass shifting l Both l Insufficient to explain data arXiv:

Alberica Toia 50 HGS-HIRe Lecture Week /10 Manigod Theory comparison II arXiv: Even when looking differentially in various p T bins the theoretical calculations are insufficient to explain the data High p T region: here we isolated a contribution arising from  +    +  * (typically included) or q+g  q+  * (not included so far) Low p T region: where the enhancement becomes large and its shape seems incompatible with unmodified q+g  q+  *

Alberica Toia 51 HGS-HIRe Lecture Week /10 Manigod Theory comparison III l The theoretical calculations are insufficient to explain the data l High p T : they are too soft (except for HSD which does not include partonic contribution) l Low p T : they are too hard to explain the enhancement (T~100 MeV) what is missing ? arXiv:

Alberica Toia 52 HGS-HIRe Lecture Week /10 Manigod Summary l EM probes ideal “penetrating probes” of dense partonic matter created at RHIC l Double differential measurement of dilepton emission rates can provide l Temperature of the matter l Medium modification of EM spectral function l PHENIX measured dilepton continuum in p+p and Au+Au p+p Low Mass Region Excellent agreement with cocktail Au+Au Low Mass Region Enhancement above the cocktail 4.7±0.4 stat ±1.5 syst ±0.9 model Intermediate Mass Region Extract charm and bottom cross section LMR I deduce photon emission in agreement with pQCD Intermediate Mass Region Agreement with PYTHIA: coincidence? LMR II Excellent agreement with cocktail LMR II Centrality dependency: increase faster than N part p T dependency: enhancement concentrated at low p T, T ~ 100 MeV LMR I deduce photon emission exponential above pQCD, T>200 MeV

Alberica Toia 53 HGS-HIRe Lecture Week /10 Manigod Near-Future Measurements at RHIC l Improve measurement in the LMR  reduce combinatorial background  Hadron Blind Detector: Dalitz rejection via opening angle l identify e ± in field free region l veto signal e ± with partner l HBD concept l windowless CF4 Cherenkov detector l 50 cm radiator length l CsI reflective photocathode l triple GEM with pad readout l HBD time scale l Proof of principle in 2007 l Successful data taking with p+p 2009 l Ready for Au+Au in 2010 l Improve measurement in the IMR  disentangle charm and thermal contribution  Silicon Vertex Detector signal electron Cherenkov blobs partner positron needed for rejection e+e+ e-e-  pair opening angle ~ 1 m

Alberica Toia 54 HGS-HIRe Lecture Week /10 Manigod Future l dielectron measurements in high energy HI collisions l go to even higher energy, i.e. maximum temperature  LHC l go back to lower energy, i.e. maximum baryon density  FAIR l stay at RHIC –HBD (and silicon vertex upgrades) for improved experiments at maximum RHIC energy –“low energy” program, i.e. use RHIC as a storage ring instead of an accelerator

Alberica Toia 55 HGS-HIRe Lecture Week /10 Manigod EM Probes at LHC At higher dN/dy thermal radiation from hadron gas dominant for m<1GeV For m>1GeV relatively stronger QGP radiation: comparable to DD but energy loss??? DILEPTONS PHOTONS Low p T Thermal/bulk photons (QGP + hadronic phase) Photons from jet-medium interactions –Jet-photon conversion, Induced photon bremsstrahlung –Cross sections forward/backward peaked –Yields approximately proportional to the jet distributions  Sensitivity to early time jet distributions –Longer path lads to increased production  Negative v2 High p T Prompt photons from initial hard processes –No final state effects at all. Fragmentation/vacuum bremsstrahlung –Sensitivity to medium effects in the final state H.Van Hees and R.Rapp S.Turbide

Alberica Toia 56 HGS-HIRe Lecture Week /10 Manigod Projections for RHIC: high energy l impact of the HBD & modified B field at top energy l recorded collisions l 10 9 l 10 10

Alberica Toia 57 HGS-HIRe Lecture Week /10 Manigod Projections for RHIC: low energy l collision rates decrease with decreasing beam energy l ~ GeV/u l 2 weeks run time gives ~50M events l HBD ‘eliminates’ sys. uncertainty l electron cooling in RHIC can increase the collision rate by a factor 10  ~500M events in 2 weeks  very promising!!!