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Alberica Toia Physics Department CERN HGS-HIRe Lecture Week Manigod 24-31 January 2010 High Energy Dilepton Experiments Experiments @ RHIC Slide 2 Alberica Toia 2 HGS-HIRe Lecture Week 24-31-01/10 Manigod l RHIC = Relativistic Heavy Ion Collider l located at Brookhaven National Laboratory RHIC Slide 3 Alberica Toia 3 HGS-HIRe Lecture Week 24-31-01/10 Manigod RHIC and its experiments l whats so special about RHIC? l its 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 Slide 4 Alberica Toia 4 HGS-HIRe Lecture Week 24-31-01/10 Manigod 102110 Total baryon density 8.6 21.4 33.5 85 p p participants nucleons (p p )A/Z 20.1 80.4 6.2 24.8 dN( p ) / dy produced baryons (p, p, n, n ) RHIC (Au-Au) SPS (Pb-Pb) Low mass e + e - : prospects @ RHIC l 2 scenarios @ 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!) Slide 5 Alberica Toia 5 HGS-HIRe Lecture Week 24-31-01/10 Manigod e- e+ e + e - : theoretical guidance at RHIC R. Rapp: nucl-th/0204003 l in-medium modifications of vector mesons persists l open charm contribution becomes significant Slide 6 Alberica Toia 6 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 7 Alberica Toia 7 HGS-HIRe Lecture Week 24-31-01/10 Manigod PHENIX in practice Slide 8 Alberica Toia 8 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 9 Alberica Toia 9 HGS-HIRe Lecture Week 24-31-01/10 Manigod Au-Au collision as seen in PHENIX Slide 10 Alberica Toia 10 HGS-HIRe Lecture Week 24-31-01/10 Manigod PC1 PC3 DC ee e+e+ PHENIX: tracking & particle ID Slide 11 Alberica Toia 11 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 12 Alberica Toia 12 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 13 Alberica Toia 13 HGS-HIRe Lecture Week 24-31-01/10 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) Slide 14 Alberica Toia 14 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 15 Alberica Toia 15 HGS-HIRe Lecture Week 24-31-01/10 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:0912.0244 422 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. Slide 16 Alberica Toia 16 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 17 Alberica Toia 17 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 18 Alberica Toia 18 HGS-HIRe Lecture Week 24-31-01/10 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 m0 (m~r) l conversions open in a plane perpendicular to the magnetic field Slide 19 Alberica Toia 19 HGS-HIRe Lecture Week 24-31-01/10 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 (0.012+0.02) 0.5*0.7 0.32 = 1.3 l combinatorial background pairs/event l B = * N 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? Slide 20 Alberica Toia 20 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 21 Alberica Toia 21 HGS-HIRe Lecture Week 24-31-01/10 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 0.7 GeV/c 2 Normalize mixed B + pairs to N +- = 2N ++ N -- Subtract correlated background Systematic uncertainties statistics of N ++ and N -- : 0.12% different pair cuts in like and unlike sign: 0.2 % Slide 25 Alberica Toia 25 HGS-HIRe Lecture Week 24-31-01/10 Manigod Differential Combinatorial Background Centrality DependenceTransverse Momentum Dependence --- Foreground: same evt N++ --- Background: mixed evt B++ Slide 26 Alberica Toia 26 HGS-HIRe Lecture Week 24-31-01/10 Manigod Combinatorial and Correlated Background p+p Au+Au Combinatorial Background from mixed events normalized to 2N ++ 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 Slide 27 Alberica Toia 27 HGS-HIRe Lecture Week 24-31-01/10 Manigod Uncertainty of Background Subtraction p+p Au+Au Method 1 and Method 2 Variations of the two method RMS Systematic Uncertainty Slide 28 Alberica Toia 28 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 29 Alberica Toia 29 HGS-HIRe Lecture Week 24-31-01/10 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 = 0.150.51% (consistent with the 0.25% error we assigned) Slide 30 Alberica Toia 30 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 31 Alberica Toia 31 HGS-HIRe Lecture Week 24-31-01/10 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% Slide 32 Alberica Toia 32 HGS-HIRe Lecture Week 24-31-01/10 Manigod Hadronic Cocktail Measurement arXiv: 0802.0050 Parameterization of PHENIX , 0 data 0 = ( + + - )/ 2 Other mesons:fit with m T scaling of 0 p T (p T 2 +m meson 2 -m 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 Slide 33 Alberica Toia 33 HGS-HIRe Lecture Week 24-31-01/10 Manigod Cocktail Comparison p+p Light hadron contributions subtracted Heavy Quark Cross Sections: Charm: integration after cocktail subtraction cc = 544 39 stat 142 syst 200 model b Simultaneous fit of charm and bottom: cc = 518 47 stat 135 syst 190 model b bb = 3.9 2.4 stat +3/-2 syst b Charm cross section from single electron measurement [PRL97, 252002 (2006)] : cc = 567 57 193 b 2.25pb -1 of triggered p+p data Data absolutely normalized Excellent agreement with Cocktail Filtered in PHENIX acceptance PLB 670,313(2009) arXiv:0912.0244 PLB 670,313(2009) Slide 34 Alberica Toia 34 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 35 Alberica Toia 35 HGS-HIRe Lecture Week 24-31-01/10 Manigod Cocktail Comparison Au+Au Low Mass Region: large enhancement 150 Alberica Toia 39 HGS-HIRe Lecture Week 24-31-01/10 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- Slide 40 Alberica Toia 40 HGS-HIRe Lecture Week 24-31-01/10 Manigod Determination of * fraction, r r = direct * /inclusive * determined by fitting the following function for each p T bin. arXiv:0804.4168 arXiv:0912.0244 f direct is given by Kroll- Wada formula with S = 1. f cocktail is given by cocktail components Normalized to the data for m Slide 41 Alberica Toia 41 HGS-HIRe Lecture Week 24-31-01/10 Manigod Direct measurement of S(m ee, p T ) Au+Au 200 GeV Vaccuum HMBT @ 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:0912.0244 Slide 42 Alberica Toia 42 HGS-HIRe Lecture Week 24-31-01/10 Manigod direct * /inclusive * = 0.5p T = 1.0p T = 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 Au+Au Clear enhancement above NLO pQCD p+pAu+Au arXiv:0804.4168 arXiv:0912.0244 Slide 43 Alberica Toia 43 HGS-HIRe Lecture Week 24-31-01/10 Manigod 1 st measurement of Thermal Radiation Direct photon real (p T >4GeV) virtual (1 Alberica Toia 44 HGS-HIRe Lecture Week 24-31-01/10 Manigod From data: Au+Au = pQCD + exp: T ave = 221 19 stat 19 syst Comparison to hydrodynamical models: p T220 MeV > T C From models:T ini = 300 to 600 MeV 0 = 0.15 to 0.5 fm/c Slide 45 Alberica Toia 45 HGS-HIRe Lecture Week 24-31-01/10 Manigod Consistent with flat S(m ee ) const =0.1770.032 Consistent with higher p T values Large and broad enhancement S(m ee ) no longer const LMR II arXiv:0912.0244 Slide 46 Alberica Toia 46 HGS-HIRe Lecture Week 24-31-01/10 Manigod Extrapolate the spectrum of direct photons l For 0.8 = 0.1770.032 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 0.4 0.17 The enhancement in the low p T region is larger than that expected from internal conversion of direct photons. Slide 47 Alberica Toia 47 HGS-HIRe Lecture Week 24-31-01/10 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 TSlide 48 Alberica Toia 48 HGS-HIRe Lecture Week 24-31-01/10 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 = 258.4 37.3 stat 9.6 syst MeV 2 /NDF= 4.00/9 T 1 = 86.5 12.7 stat +11.0 -28.4syst MeV T = 221 19 stat 19 syst MeV 2 /NDF= 16.6/11 or low mass enhancement has inverse slope of ~100 MeV. arXiv:0912.0244 Slide 49 Alberica Toia 49 HGS-HIRe Lecture Week 24-31-01/10 Manigod Theory comparison annihilation + modified spectral function l Broadening l Mass shifting l Both l Insufficient to explain data arXiv:0912.0244 Slide 50 Alberica Toia 50 HGS-HIRe Lecture Week 24-31-01/10 Manigod Theory comparison II arXiv:0912.0244 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+ * Slide 51 Alberica Toia 51 HGS-HIRe Lecture Week 24-31-01/10 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:0912.0244 Slide 52 Alberica Toia 52 HGS-HIRe Lecture Week 24-31-01/10 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 4.70.4 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 Slide 53 Alberica Toia 53 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 54 Alberica Toia 54 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 55 Alberica Toia 55 HGS-HIRe Lecture Week 24-31-01/10 Manigod EM Probes at LHC At higher dN/dy thermal radiation from hadron gas dominant for m1GeV 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 Slide 56 Alberica Toia 56 HGS-HIRe Lecture Week 24-31-01/10 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 Slide 57 Alberica Toia 57 HGS-HIRe Lecture Week 24-31-01/10 Manigod Projections for RHIC: low energy l collision rates decrease with decreasing beam energy l ~40 Hz @ 8.6 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!!!