Usai

Low and intermediate mass dimuons in NA60 : 

Low and intermediate mass dimuons in NA60 G. Usai – INFN and University of Cagliari (Italy)

Slide2: 

spontaneous chiral symmetry breaking  <qq> ≠ 0 General question of QCD Origin of the masses of light hadrons? Expectation: approximate chiral SU(nf)L x SU(nf)R symmetry  chiral doublets, degenerate in mass, with However, we observe

Slide3: 

‹qq› - 1.0 T/Tc cm cL L 1.0 T/Tc Lattice QCD (for mB=0 and quenched approx.) two phase transitions at the same critical temperature Tc deconfinement chiral symmetry transition restoration hadron spectral functions on the lattice only now under study explicit connection between spectral properties of hadrons (masses,widths) and the value of the chiral condensate <qq> ? Several theoretical approaches including lattice QCD still in development Use r as a probe for the restoration of chiral symmetry (Pisarski, 1982)

Slide4: 

Gtot [MeV] r (770) 150 (1.3fm/c) w(782) 8.6 (23fm/c) f(1020) 4.4 (44fm/c) In-medium radiation dominated by the  : r life time t =1.3 fm/c << tcollision > 10 fm/c continuous “regeneration” by  main difficulty: Properties of r in hot and dense matter unknown (related to the mechanism of mass generation) Properties of hot and dense medium unknow (general goal of studying nuclear collisions) Why focus (mainly) on the r?

Slide5: 

magnetic field Standard dimuon detection: NA50, PHENIX, ALICE, … Thick hadron absorber to reject hadronic background Trigger system based on fast detectors to select muon candidates (1 in 10-4 PbPb collisions at SPS energy) Muon tracks reconstructed by a spectrometer (tracking detectors+magnetic field) Extrapolate muon tracks back to the target taking into account multiple scattering and energy loss, but … - poor reconstruction of interaction vertex (sz 10 cm) - poor mass resolution (80 MeV at the f)

Slide6: 

2.5 T dipole magnet Origin of muons can be accurately determined Improved dimuon mass resolution targets beam tracker vertex tracker Measuring dimuons in heavy ion collisions – the NA60 case Matching of muon tracks

Slide7: 

DIPOLE MAGNET 2.5 T HADRON ABSORBER TARGETS ~40 cm 1 cm 12 tracking points with good acceptance 8 “small” 4-chip planes, plus 8 “big” 8-chip planes (4 tracking stations) ~ 3% X0 per plane 750 µm Si read-out chip 300 µm Si sensor ceramic hybrid 800’000 R/O channels - 96 pixel assemblies The NA60 pixel vertex detector

Slide8: 

Resolution ~ 10 - 20 m in the transverse plane z ~ 200 m along the beam direction Good vertex identification with  4 tracks X Y Extremely clean target identification (Log scale!) Vertexing

Slide9: 

The dimuon invariant mass resolution has two components Multiple scattering in the hadron absorber dominates the resolution for low momentum muons The variance ϑs of the angle distribution is proportional to 1/p At high momenta the resolution is dominated by the tracking accuracy (dp/p proportional to p) Contributions to dimuon mass resolution  at mmm ~ 3 GeV the resolution is dominated by this component Track matching not so important  at mmm ~ 1 GeV track matching is very effective to increase the momentum resolution

Slide10: 

The muon spectrometer and the pixel telescope determine the track parameters in two reference planes z1 and z2. A choice of the track parameters at each plane is Muon track matching p1,r and its covariance matrix are propagated to z2 Muon spectrometer Pixel telescope Absorber Measured points Measured points

Slide11: 

joint least square ansatz Contributions to multiple scattering between z1 and z2 are added to C’1 muon spectrometer surface z1 hadron absorber muon spectrometer track parameters propagated to pixel telescope surface pixel telescope surface z2 weigthed mean muon spectrometer track parameters with errors pixel telescope track parameters with errors M(p2,fit) distributed as a c2 with 5 dof

Slide12: 

M(p2,fit) distributed as a c2 with 5 degrees of freedom The pixel telescope improves drastically the angular resolution: ~10 mrad (muon spectrometer only)  ~1 mrad (adding pixel telescope) The momentum resolution is comparable in the two detectors. However, the use of the momentum information in a high multiplicity environment is fundamental to achieve the matching  the pixel telescope must be a spectrometer

Slide13: 

6500 A 4000 A dN/dMmm (Events/50 MeV) (80% of collected statistics) Opposite-sign dimuon mass distributions before quality cuts No muon track matching (two magnet settings) (100% of collected statistics) Improvement in mass resolution Vertex selection and muon track matching sM(f)  80 MeV sM(J/)  100 MeV sM(f)  20 MeV f(1020) (1020) 4000 A Drastic improvement in mass resolution: Narrow vector mesons clearly resolved But still sitting on a large unphysical background sM(J/)  70 MeV

Slide14: 

Problems with the matching: fake matches Fake match: muon matched to a wrong track in the vertex telescope Can be important in high multiplicity events (negligible in pA or peripheral AA) Simple technique: the match with the smallest c2 is retained. But is it correct or fake? Fake matches can be studied and subtracted using an overlay Monte Carlo: - Monte Carlo muons are superimposed to real events (in the vertex telescope) - Reconstructed as real events, fake matches can be tagged and the fraction relative ....to correct matched muons is then evaluated

Slide15: 

The Monte Carlo provides also the kinematic distribution (mass, pT, ...) of the fake matches Here is the example for the f meson The fake-match contribution appears localized in mass (and pT) space as a broad peak correct matches: s = 23 MeV wrong matches: fake = 110 MeV

Slide16: 

Background sources (dimuons) Main source of background: (uncorrelated) decays of p and K  the hadron absorber should be as close as possible to the interaction point If we have N pions, the average number which decays within 1m is m~10-3N  number of detected muon pairs is A++ (A--) acceptance for a like sign muon pair A+- acceptance for a opposite sign muon pair We have the probabilities ( )

Slide17: 

In NA50 acceptance was independent of charge In NA60 acceptance is different for + and – Cuts to equalize it (“image” cut in NA50) impossible  Event Mixing: Define a pool of m+ and m- tracks out of a sample of like sign events (++ and --) . Pick m+ and m- from these like sign pools corresponding to different events. The m+ and m- are picked in a fraction which reflects the probabilities to detect them in the experimental apparatus Combine them to form artificial pairs of all sign combinations. If N++(mixed) and N--(mixed) reproduce the corresponding data samples N++ and N--, then N+-(mixed) should give the combinatiorial background of the +- sample.

Slide18: 

Estimated estimated through the comparison of N++/--(mixed) to N++/--(real) In NA60 the accuracy is ~1% all over the dimuon mass range. Is that good or bad? It depends on the signal to background ratio ... Accuracy of background subtraction

Slide19: 

The signal to background ratio depends on the matching c2 cut. Tight cut: more precise matching – helps to reject tracks not precisely connected to primary vertex 1% The worst case happens in the continuum region around the w, where the bkg/signal can reach ~25 in the most central collisions ssignal/signal ~ 25% in low mass continuum region at most In more peripheral collisions ssignal/signal is much better

Slide20: 

The quest of the correct background normalization CLAS experiment: photoproduction of vector mesons off nuclei e+e- combinatorial background determined by event mixing Background normalization found directly from fit: best fit prefers r meson with mass shift (in medium effect) Background normalization following prescription for P+ P-: best fit prefers r meson with no in medium effect

Slide21: 

The final mass spectra (mmm<2GeV/c2) Fakes/CB < 10 % Red distribution: final spectrum after getting rid of fake and combinatorial background. The net data sample consists of 420000 events! ( 50% of total statistics) For the first time  and  peaks are clearly visible in dilepton channel (23 MeV/c2 mass resolution at the f)  is also visible   f

Slide22: 

Phase phase coverage (mass-pT) The dimuon kinematics can be specified by (m,y,pT,cosϑ) The probability that a dimuon with certain kinematic values is detected depends on: Thickness of the muon filter, position of the target relative to the detecting elements, magnetic fields (both in the muon spectrometer and in the vertex telescope), ... Drop with no vertex magnet The dipole magnetic field in the vertex region improves significantly the acceptance for low mass and low pT opposite sign dimuons

Slide23: 

Phase phase coverage (y-pT) A fixed target experiment usually covers the forward rapidity emisphere. NA60 (and its predecessors) are optimized to cover the range 3-4 in the lab system (the target rapidity is zero, the beam rapidity is 6) corresponding to 0-1 in the CMS system Example of phase space coverage for a few processes (Monte Carlo) Dimuon rapidity coverage in the lab frame: roughly between 3.3 and 4.3 for low masses between 3 and 4 for the J/y dimuons (mid rapidity is at 2.9) mmg rmm fmm

Slide24: 

Track multiplicity of charged tracks for triggered dimuons for 4 multiplicity windows: opposite-sign pairs combinatorial background signal pairs Measuring the collision centrality The collision centrality can be measured via the charged particle multiplicity as measured by the pixel vertex telescope

Slide25: 

Which processes populate the dimuon mass spectrum below 1 GeV?

Slide26: 

m+m- Dalitz decay: m+m-p0 Anomaly in the form-factor: VMD predicts a (significantly) smaller value Vector meson dominance:

Slide27: 

m+m- Dalitz decay: m+m-g Vector meson dominance Previous data (Landsberg et al.) fitted with a pole formula:  Dalitz form factor

Slide28: 

ω and  : fix yields such as to get, after subtraction, a smooth underlying continuum  : () set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV (lower limit for excess). () use yield measured for pT > 1.4 GeV/c Isolate possible excess by subtracting cocktail (without r) from the data How to fit in the presence of an unknown source?  Try to find excess above cocktail (if it exists) without fit constraints

Slide29: 

Clear excess above the cocktail , centered at the nominal r pole and rising with centrality Excess even more pronounced at low pT data – cocktail (all pT) cocktail / =1.2 The evolution of the excess with centrality can be studied with precision with a rather fine binning in multiplicity Evolution of the excess shape with centrality

Slide30: 

Change yields of ,  and  by +10%:  enormous sensitivity, on the level of 1-2%, to mistakes in the particle yields. The difference spectrum is robust to mistakes even on the 10% level, since the consequences of such mistakes are highly localized. Sensitivity of the difference procedure

Slide31: 

The systematic errors of continuum 0.4<M<0.6 and 0.8<M<1GeV are 25% (at most) in the most central collisions Illustration of sensitivity  to correct subtraction of combinatorial background and fake matches;  to variation of the  yield The structure in  region looks rather robust Systematics The largest source of systematic error comes from the subtraction of combinatorial and fake matches background. In principle there are other uncertainty sources as the form factors, but these are negligible compared to the background.

Slide32: 

Evolution of the excess shape as a function of centrality Quantify the peak and the broad symmetric continuum with a mass interval C around the peak (0.64 <M<0.84 GeV) and two equal side bins L, U peak/r continuum/r peak/continuum continuum = 3/2(L+U) peak = C-1/2(L+U) Peak/cocktail r drops by a factor 2 from peripheral to central: the peak seen is not the cocktail r nontrivial changes of all three variables at dNch/dy>100 ? Fine analysis in 12 centrality bins

Slide33: 

Free pions Lagrangian p-r, and electromagnetic interactions in vacuum Free r Lagrangian (vector meson) self interactions are neglected p-r and e.m. interactions introduced via gauge couplings g = grp = pion-r coupling constant Direct g-r coupling The r couples only to conserved currents, so that

Slide34: 

If g = grg  electromagnetic field equations The hadronic part of the electromagnetic current is then proportional to the r meson field  The r meson is the only hadronic source of the electromagnetic field What does it mean? Hadron matter couples to a qqbar pair which propagates as a vector meson which then materializes as a photon All QCD complexity, gluon self interactions and confinement are incorporated in the physical vector meson which forms the intermediate state r Hadron medium g r m+ m-

Slide35: 

The free rm field describes a “bare” meson which we can interpret as the qqbar component of the physical r meson. The bare r propagator is given by The r self-energy However, r is strong coupled to pions  the physical r meson appears as a broad resonance. properties accounted for by the second order self-energy diagrams

Slide36: 

Without loss of generality The r dressed propagator where The full (dressed) propagator comes from an infinite sum of diagrams with self-energy insertions This infinite series can be easily summed ... r “polarization scalar” 1PI 1PI 1PI The r field is always coupled to conserved currents (qmJm = 0) and so the terms proportional to qmqn can be dropped

Slide37: 

According to the optical theorem general expression of the decay width In this specific case the final state is (dominantly) pp. Thus we come to the result mass dependent width 1PI r r = scattering rr The imaginary part of Pr

Slide38: 

The real part of Pr Determines the mass shift due to the self-energy: Needed to keep the photon massless Fixes c1 m0r can be fixed from the comparison to the measured p+p-  p+p- elastic cross section The mass shift induced by the rpp self-energy is small Regularization. Cut-off or dispersion relations

Slide39: 

vacuum spectral function  is dressed with free pions (like ALEPH data V(t→ 2pnt ))

Slide40: 

Dilepton Rate in a strongly interacting medium dileptons produced by annihilation of thermally excited particles: +- in hadronic phase qq in QGP phase photon selfenergy at SPS energies +  - →*→μ+μ- dominant Vector-Dominance Model hadron basis spectral function

Slide41: 

Study the properties of the r spectral function Im Dr in a hot and dense medium Physics objective in heavy ion collisions

Slide42: 

Hadronic many-body approach Rapp/Wambach et al., Weise et al. hot and baryon-rich matter hot matter  is dressed with: hot pions Prpp , baryons Pr B (N,D ..) mesons Pr M (K,a1..) “melts” in hot and dense matter - pole position roughly unchanged - broadening mostly through baryon interactions r spectral function in hot and dense hadronic matter

Slide43: 

Dropping mass scenario Brown/Rho et al., Hatsuda/Lee universal scaling law explicit connection between hadron masses and chiral condensate continuous evolution of pole mass with T and r ; broadening at fixed T,r ignored r spectral function in hot and dense hadronic matter

Slide44: 

integration of rate equation over space-time and momenta required continuous emission of thermal radiation during life time of expanding fireball example: broadening scenario Final mass spectrum

Slide45: 

Thus, the spectral function accessible through rate equation, integrated over space-time and momenta Limitation: Continuously varying values of temperature T and baryon density rB,

Slide46: 

Comparison of predictions to data Two possibilities, in principle: 1) Use the prediction for Generate Monte Carlo events of g* decays into muon pairs Propagate through the acceptance filter and compare to uncorrected data Done presently for invariant mass (work in progress for acceptance correction) 2) Correct the data for acceptance in 3-dim space M-pT-y and compare them directly to predictions Done for pT distributions

Slide47: 

Output: spectral shape much distorted relative to input, but somehow reminiscent of the spectral function underlying the input; by chance? Input (example): thermal radiation based on RW spectral function Acceptance filtering of theoretical prediction in NA60 all pT

Slide48: 

Predictions for In-In by Rapp et al (2003) for dNch/d = 140, covering all scenarios Theoretical yields normalized to data in mass interval < 0.9 GeV Only broadening of  (RW) observed, no mass shift (BR) Rapp-Wambach: hadronic model predicting strong broadening/no mass shift Brown/Rho scaling: dropping mass due to dropping of chiral condensate After acceptance filtering, data and predictions display spectral functions, averaged over space-time and momenta Comparison to the main models that appeared in the 90s

Slide49: 

Without baryons: Not enough broadening Lack of strength below the r peak Improved model: Fireball dynamics 4 p processes spectrum described in absolute terms Comparison to the main models that appeared in the 90s

Slide50: 

Something is missing at high pT. What? Semicentral collisions: low vs high pT Rapp-Hees Rapp-Hees

Slide51: 

The vacuum r (and other) contributions At high pT there is an important contribution from the “vacuum r”: r decays at kinetic freeze-out Additional contribution: Primordial r (Rapp-Hees) Rapp-Hees Ruppert-Renk

Slide52: 

In addition, because of the pion “heat bath”, it is possible also to have processes in which an axial vector particle interacts with a pion, as pa1m+m-. This effectively introduces a mixing between vector and axial-vector states (at the correlator level). This mixing depends on the “amount” of chiral symmetry restoration The mass region above 1 GeV: vector-axial vector mixing Above 1 GeV we can have contributions from 4p processes. The spectral shape can be found for instance from e+e-4p or studying (ALEPH) t(2np)ν 3p, 5p… 2p, 4p, 6p …

Slide53: 

Mass region above 1 GeV described dominantly in terms of hadronic processes, 4 p …  Hadron-parton duality The mass region above 1 GeV: models vs data Rapp/Hees Ruppert / Renk, Phys.Rev.C (2005) Mass region above 1 GeV described dominantly in terms of partonic processes, dominated by qqbar annihilation

s(e+e-→hadrons) in vacuum: 

s(e+e-→hadrons) in vacuum e+ e- h1 h2 … s ≥ sdual~(1.5GeV)2 : pQCD continuum s < sdual : Vector-Meson Dominance q q _ r +w +f r I =1 qq _ pp 2p + 4p +... KK e+ e- p - p + r

Disentangling the signal sources in the IMR: 

Disentangling the signal sources in the IMR The dileptons from charm decay can be identified by tagging their production point with respect to the primary interaction vertex Identify the typical offset of D-meson decay (~100 µm) Need a very good vertexing accuracy (~20-30 µm, in the transverse plane)

Measuring the muon offset: 

Offsets: δX, δY between the vertex and the track impact point in the transverse plane at Zvertex Resolution depends on track momentum: use offset weighted by the covariance matrices of the vertex and of the muon track: Measuring the muon offset For dimuons

Slide57: 

dN/dΔ Procedure: Fix the prompt contribution to the expected DY yield and see if the offset distribution can be described with enhanced Charm Answer: No, Charm can’t fill the small offset region…  more prompts are needed New alignment Is the excess enhanced charm?

Slide58: 

Procedure: Leave both contributions free and see if we can describe the offset distribution for 1.2 < Mµµ < 2.7 Answer: The best fit requires 2.6 times more prompts than the expected Drell-Yan yield dN/dΔ New alignment How many prompts are needed?

Slide59: 

Transverse momentum spectra

Slide60: 

Spectra from a static fireball In a static fireball at temperature T the differential particle momentum distribution is Lorentz invariant phase space element Assume a thermal Boltzmann shape  transverse mass spectra (integrated over rapidity) mT scaling: all particle spectra have the same T “slope”

Slide61: 

An expanding fireball Thermalized matter starts to expand because of the pressure gradient with respect to the surrounding vacuum. A collective motion (flow) develops. Flow velocity of a volume element of thermalized matter in a spacetime point x Sum all the particles 3-momenta and energies The ratio gives the collective velocity for completely random thermal motions

Slide62: 

Fluid 4-velocity = radial flow field Superimposed tranverse expansion  Bjorken scaling: At very high energies the physics of secondary particle production should be the same as described in different frames moving along the z axis. longitudinal flow field transverse flow

Slide63: 

m- Excess dimuons: continuum emission during all the fireball lifetime (4-dim volume) – we see not only the emission at freeze-out! Since the r is strongly coupled to the pions, the thermal pT is boosted by flow in the lab frame In fluid local rest frame thermal pT Dimuon emission g

Slide64: 

Example of hydrodynamic evolution (specific for In-In – Dusling et al.) Monotonic decrease of T from: early times to late times medium center to edge Monotonic increase of vT from: early times to late times medium center to edge  emission of dileptons sensing - Large T and small vT at early times - Large VT and small T at later time T– vT anticorrelation Potentially could permits to distinguish between hadronic and partonic nature

Slide65: 

Dilepton transverse momentum spectra Obtained integrating dR/dq4 over fireball space-time history Superposition of spectra at different T weighted by - Thermal factor exp(-E/T) (pmum = dilepton energy in local fluid rest frame) - Invariant mass shape of spectral function - Volume increase In addition, resonance decays determine an overpopulation of pions  Non zero chemical potential mp(T)  Fugacity exp(mp(T)/T)

Slide66: 

Hadron pT spectra When the temperature of a fluid element drops below a certain value Tf , the mean free path exceeds the dimesions of the system Thermal equilibrium is broken and particles stream out free to the detectors The isotherm T(r,t)=Tf defines a 3-dim hypersurface S in the space-time  last-scattering surface Total number of particles crossing S  sum over d3s 3-dim hypersurface  divide S in infinitesimal elements d3s outward-pointing 4-vector perpendicular to S(x) number of particles passing through d3s current of particles through x

Slide67: 

Transverse flow-field Integrated over f Once the mass is fixed (the particle is specified), the function has only three parameters: vT, Tf and a normalization With some mathematics one can show that Cooper-Frye formula

Slide68: 

Common flow velocity in p,K,p and their anti-particles is seen at SPS and AGS energies NA49/SPS results: Common flow velocity seen for very wide particle species (Nucl.Phys A 715 61) Pion and deuteron are taken out from fit procedure (many pions come from resonance decays - deuterons are most likely produced with proton-neutron coalescence) However, spectra described are very well described with the thermal parameter extracted with other particles Common flow velocities are seen also in RHIC Au-Au data (PHENIX and STAR)

Slide69: 

NA57 158 GeV Centrality classes: 0  40 to 53 % most central 1  23 to 40 % most central 2  11 to 23 % most central 3  4.5 to 11 % most central 4  4.5 % most central Tf – vT,f anticorrelation as a function of centrality Peripheral collisions: shorter fireball lifetime  less time to develop flow (smaller vT) – earlier decoupling at higher Tf Central collisions: bigger fireball lifetime  more time to develop flow (larger vT) – later decoupling at smaller Tf Extracted with a two parameter fit to experimental distributions: Evaluate c2 for fixed vT and Tf  Create a c2 map as a function of vT and Tf Tf and vT,f are strongly anticorrelated

Slide70: 

Stable hadrons reflect the kinetic freeze-out conditions.  Fitting with exp(-mT/T) gives a T dependent on the momentum range  T from exponential fit (call Tslope) is not anymore the source temperature Tf. At high pT the spectra are still exponential with a common slope which reflects a freeze-out temperature blue-shifted by the flow transverse velocity vT: At low pT, the pT spectra appear flattened and mT scaling is broken. The T slope becomes mass dependent (mT scaling is broken) In principle allows to separate the thermal from the collective motion Effect of radial flow on hadron pT spectra

Slide71: 

Notice that for mi  0 we should see Ti,slope  Tf However, Ti,slope  170 MeV, while we know that Tf ~ 110-120 MeV for central Pb-Pb collisions  the linear approximation fails for mi  0 Mass ordering of hadronic slopes  Flattening of spectra at low pT resulting in higher Teff Pions: softening at very low pT because of resonance decays

Slide72: 

f pT spectra are corrected for acceptance after background and side-window subtraction T slope extracted fitting f transverse momentum spectra

Slide73: 

The In-In measurement of NA60 follows the NA49 systematics NA60 (pT fit range 0-2.6 GeV) NA50 and NA49 differerences (f puzzle): Decay channel (mm vs KK) pT fit range (high vs low) NA49 (pT fit range 0-1.6 GeV) NA50 (pT fit range 1.2-2.6 GeV) NA60 Preliminary T slope as a function of centrality Fit with exp(-mT/Tslope) vs centrality: increase of Tslope (indication of radial flow)

Slide74: 

Dimuon excess pT spectra Divide the pT interval 0-2 GeV/c in 200 MeV bins For each pT bin consider the mass projection and determine the excess yield with the local subtraction procedure  pT spectrum of the excess  Make this for 3 different mass windows

Slide75: 

 reduce 3-dimensional acceptance correction in M-pT-y to 2-dimensional correction in M-pT, using measured y distribution as an input  use slices of m = 0.1 GeV and pT = 0.2 GeV  resum to three extended mass windows 0.4<M<0.6 GeV 0.6<M<0.9 GeV 1.0<M<1.4 GeV Strategy of acceptance correction subtract charm from the data before acceptance correction (based on IMR results – we pospone this discussion)

Slide76: 

hardly any centrality dependence  integrate over centrality (spectra arbitrarily normalized) Dimuon excess pT spectra for three centrality bins

Slide77: 

significant mass dependence (also vs. mT, see below ) possible origin: different physics sources radial flow  p-dependence of in-medium spectral function (arbitrarily normalized at pT=1GeV) Centrality integrated excess pT spectra

Slide78: 

f: mT spectrum nearly pure exponential – Teff nearly independent of fit range with some hint of radial flow Excess: spectra show an increase (not flattening) at very low mT reminiscent of pions Why? Centrality integrated mT spectra physics differences are better visible in mT- than in pT

Slide79: 

at high pT, rho like region hardest, high-mass region softest ! differential fits to pT spectra, assuming locally 1-parameter mT scaling and using gliding windows of pT=0.8 GeV  local slope Teff Mass dependence of pT/mT spectra

Slide80: 

pT spectrum of f at low pT much flatter (higher Teff) acceptance of f in between that of the two mass windows  enhanced yield at low pT not due to incorrect acceptance Systematics: acceptance correction

Slide81: 

mT spectrum of f nearly pure exponential Teff of f nearly independent of fit range  Different behaviour of excess not due to incorrect acceptance

Slide82: 

peripheral 1% semiperipheral 0.8% semicentral 0.6% central 0.8% corresponding fraction of CB for the four centrality bins Uncertainty in combinatorial bkg subtraction Estimated estimated through the comparison of N++/--(mixed) to N++/--(real)

Slide83: 

enhanced yield at low-pT seen at all centralities, including the peripheral bin estimate of errors at low pT, due to subtraction of combinatorial background: peripheral 1% semiperipheral 10% semicentral 20% central 25%

Slide84: 

evolution of Teff vs mmm: , ,  Linear rise – the f seems to flow less f   Fit the spectra in the range 0.4-1.8 GeV/c

Slide85: 

evolution of excess Teff vs mmm across the low and intermediate mass Linear rise also for excess quite reminiscent of radial flow of a hadronic source! But excess Teff higher than hadron Teff. Why? Fit the spectra in the range 0.4-1.8 GeV/c

Slide86: 

Evolution of the excess shape as a function of centrality Peak sitting on the continuum: freeze-out r without in-medium effects continuum = 3/2(L+U) peak = C-1/2(L+U) The peak and the continuum can be disentangled in the mass window 0.6-0.9 GeV with a simple shape analysis by using side-windows Teff of continuum and peak can be measured separately

Slide87: 

evolution of excess Teff vs mmm across the low and intermediate mass Mass window 0.6-0.9: The peak Teff gets to 300 MeV! The continuum Teff drops to ~ 230 MeV

Slide88: 

evolution of excess Teff vs mmm across the low and intermediate mass Sudden drop at ~ 1 GeV For M>1 GeV Teff is roughly constant  Seemingly non flow?

Slide89: 

evolution of excess Teff vs mmm across the low and intermediate mass Summary: In the region where 2p processes are dominant (up to 1 GeV) there is strong evidence for radial flow of dileptons. What is the explanation for the drop? If the rise is truly due to flow: - the lack of flow above 1 GeV could be naturally related to emission in an early stage  partonic processes - If the region above 1 GeV is dominated by hadronic sources, shouldn’t Teff keep rising? How is the drop explained in that case?