1 / 61

First measurement of the  spectral function in high-energy nuclear collisions

First measurement of the  spectral function in high-energy nuclear collisions. Sanja Damjanovic NA60 Collaboration. Bielefeld, 13 December 2005. Outline. Motivation Experimental set-up Data analysis event selection combinatorial background

gyda
Download Presentation

First measurement of the  spectral function in high-energy nuclear collisions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. First measurement of the  spectral function in high-energy nuclear collisions Sanja Damjanovic NA60 Collaboration Bielefeld, 13 December 2005 S. Damjanovic, Bielefeld 13 December 2005

  2. Outline • Motivation • Experimental set-up • Data analysis event selection combinatorial background fake matches • Understanding the peripheral data • Isolation of an excess in the more central data • Comparison of the excess to model predictions • Conclusions S. Damjanovic, Bielefeld 13 December 2005

  3. Motivation S. Damjanovic, Bielefeld 13 December 2005

  4. Prime goal Use r as a probe for the restoration of chiral symmetry (Pisarski, 1982) Principal difficulty : properties of r in hot anddensematter unknown (related to the mechanism of mass generation) properties of hot and dense medium unknown (general goal of studying nuclear collisions)  coupled problem of two unknowns: need to learn on both S. Damjanovic, Bielefeld 13 December 2005

  5. General question of QCD Origin of the masses of light hadrons? • Expectation Mh~10-20 MeV approximate chiral SU(nf)L× SU(nf)R symmetry chiral doublets, degenerate in mass • Observed MN~1 GeV spontaneous chiral symmetry breaking <qq> ≠ 0 M ~ 0.77 GeV ≠ Ma1~ 1.2 GeV S. Damjanovic, Bielefeld 13 December 2005

  6. cL - cm ‹qq› L 1.0 T/Tc 1.0 T/Tc Many different theoretical approaches including Lattice QCD still very much under development Lattice QCD(for mB=0 andquenched 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> ? S. Damjanovic, Bielefeld 13 December 2005

  7. High Energy Nuclear Collisions Principal experimental approach: measure lepton pairs (e+e- or μ+μ-) no final state interactions; continuous emission during the whole space-time evolution of the collision system dominant component at low invariant masses: thermal radiation, mediated by the vector mesons ,(,) 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  : • life time τ=1.3 fm/c << τcollision> 10 fm/c • continuous “regeneration” by  S. Damjanovic, Bielefeld 13 December 2005

  8. Low-mass dileptons + chiral symmetry At Tc: Chiral Restoration ALEPH data: Vacuum • How is the degeneration of chiral partners realized ? • In nuclear collisions, measure vectorm+m-, but axial vector? S. Damjanovic, Bielefeld 13 December 2005

  9. In-medium changes of the r properties (relative to vacuum) Selected theoretical references very confusing, experimental data crucial S. Damjanovic, Bielefeld 13 December 2005

  10. CERES/NA45 at the CERN SPS Pioneering experiment, built 1989-1992 results on p-Be/Au, S-Au and Pb-Au first measurement of strong excess radiation above meson decays; vacuum- excluded resolution and statistical accuracy insufficient to determine the in-medium spectral properties of the  S. Damjanovic, Bielefeld 13 December 2005

  11. Experimental set-up S. Damjanovic, Bielefeld 13 December 2005

  12. muon trigger and tracking target beam hadron absorber or ? Muon Other Standard way of measuring dimuons magnetic field Energy loss Multiple scattering • Degraded dimuon mass resolution • Cannot distinguish prompt dimuons from decay muons S. Damjanovic, Bielefeld 13 December 2005

  13. muon trigger and tracking magnetic field hadron absorber or ! Measuring dimuons in NA60: concept 2.5 T dipole magnet beam tracker vertex tracker targets Matching in coordinate and momentum space • Origin of muons can be accurately determined • Improved dimuon mass resolution S. Damjanovic, Bielefeld 13 December 2005

  14. Data Analysis S. Damjanovic, Bielefeld 13 December 2005

  15. Event sample: Indium-Indium 5-week long run in Oct.–Nov. 2003 Indium beam of 158 GeV/nucleon ~ 4 × 1012 ions delivered in total ~ 230 million dimuon triggers on tape present analysis: ~1/2 of total data S. Damjanovic, Bielefeld 13 December 2005

  16. Selection of primary vertex The interaction vertex is identified with better than 20 mm accuracy in the transverse plane and 200 mm along the beam axis. (note the log scale) Beam Trackersensors windows Present analysis (very conservative): Select events with only one vertex in the target region, i.e. eliminate all events with secondary interactions S. Damjanovic, Bielefeld 13 December 2005

  17. Muon track matching Matching between the muons in the Muon Spectrometer (MS) and the tracks in the Vertex Telescope (VT) is done using the weighted distance (2) in slopes and inverse momenta. For each candidate a global fit through the MS and VT is performed, to improve kinematics. A certain fraction of muons is matched to closest non-muon tracks (fakes). Only events with 2< 3 are selected. Fake matches are subtracted by a mixed-events technique (CB) and an overlay MC method (only for signal pairs, see below) S. Damjanovic, Bielefeld 13 December 2005

  18. Determination of Combinatorial Background Basic method: Event mixing • takes account of • charge asymmetry • correlations between the two muons, induced by magnetic field sextant subdivision trigger conditions S. Damjanovic, Bielefeld 13 December 2005

  19. Combinatorial Background from ,K→ decays Agreement of data and mixed CB over several orders of magnitude Accuracy of agreement ~1% S. Damjanovic, Bielefeld 13 December 2005

  20. Fake Matches Fake matches of the combinatorial background are automatically subtracted as part of the mixed-events technique for the combinatorial background Fake matches of thesignal pairs (<10% of CB) are obtained in two different ways: • Overlay MC : Superimpose MC signal dimuons onto real events. Reconstruct and flag fake matches. Choose MC input such as to reproduce the data. • Event mixing : More complicated, but less sensitive to systematics S. Damjanovic, Bielefeld 13 December 2005

  21. Fake-match background example from overlay MC: the ffake-match contribution localized in mass (and pT) space:  = 23 MeV, fake = 110 MeV; fake prob. 22% complete fake-match mass spectrum agreement between overlay MC and event mixing, in absolute level and in shape, to within <5% S. Damjanovic, Bielefeld 13 December 2005

  22. Subtraction of combinatorial background and fakes Net data sample: 360 000 events Fakes / CB < 10 % For the first time,  and  peaks clearly visible in dilepton channel ; even μμ seen w f h Mass resolution:23 MeV at the  position Progress over CERES: statistics: factor >1000resolution: factor 2-3 S. Damjanovic, Bielefeld 13 December 2005

  23. Associated track multiplicity distribution Track multiplicity from VT tracks for triggered dimuons for opposite-sign pairscombinatorial backgroundsignal pairs 4 multiplicity windows: S. Damjanovic, Bielefeld 13 December 2005

  24. Signal and background in 4 multiplicity windows S/B Decrease of S/B with centrality, as expected S. Damjanovic, Bielefeld 13 December 2005

  25. Phase space coverage in mass-pT plane Final data after subtraction of combinatorial background and fake matches MC The acceptance of NA60 extends (in contrast to NA38/50) all the way down to small mass and small pT S. Damjanovic, Bielefeld 13 December 2005

  26. Phase space coverage in y-pT plane Examples from MC simulations Optimal acceptance: at high mass, high pT <y> = 3.5 at low mass, low pT <y> = 3.8 Shift of acceptance away from midrapidity not much different from CERES S. Damjanovic, Bielefeld 13 December 2005

  27. Results S. Damjanovic, Bielefeld 13 December 2005

  28. Understanding the Peripheral data Fit hadron decay cocktail and DD to the data 5 free parameters to be fit: h/w, r/w, f/w, DD, overall normalization (h/h = 0.12, fixed) Fit range: up to 1.4 GeV S. Damjanovic, Bielefeld 13 December 2005

  29. all pT log Very good fit quality Comparison of hadron decay cocktail to data S. Damjanovic, Bielefeld 13 December 2005

  30. Comparison of hadron decay cocktail to data pT < 0.5 GeV The  region (small M, small pT) is remarkably well described → the (lower) acceptance of NA60 in this region is well under control S. Damjanovic, Bielefeld 13 December 2005

  31. Comparison of hadron decay cocktail to data 0.5 < pT < 1 GeV pT > 1 GeV Again good agreement between cocktail and data S. Damjanovic, Bielefeld 13 December 2005

  32. Particle ratios from the cocktail fits • h/w and f/w nearly • independent of pT; • 10% variation due to • the w • increase of r/w • at low pT (due to • ππ annihilation, • see later) • General conclusion: • peripheral bin very well described in terms of known sources • low M and low pT acceptance of NA60 under control S. Damjanovic, Bielefeld 13 December 2005

  33. Isolation of an excess in the more central data S. Damjanovic, Bielefeld 13 December 2005

  34. Understanding the cocktailfor the more central data Need to fix the contributions from the hadron decay cocktail Cocktail parameters from peripheral data? How to fit in the presence of an unknown source?  Nearly understood from high pT data, but not yet used Goal of the present analysis: Find excess above cocktail (if it exists) without fits S. Damjanovic, Bielefeld 13 December 2005

  35. Conservative approach Useparticle yields so as to set a lowerlimit to a possible excess S. Damjanovic, Bielefeld 13 December 2005

  36. Comparison of data to “conservative” cocktail all pT Cocktail definition: see next slide / fixed to 1.2 ● data -- sum of cocktail sources including the  Clear excess of data above cocktail, rising with centrality But: how to recognize the spectral shape of the excess? S. Damjanovic, Bielefeld 13 December 2005

  37. Isolate possible excess by subtracting cocktail (without ) from the data  :set upper limit, defined by “saturating” the measured yield in the mass region close to 0.2 GeV  leads to a lower limit for the excess at very low mass  andf : fix yields such as to get, after subtraction, a smooth underlying continuum difference spectrumrobust tomistakes even at the 10% level;consequences highly localized S. Damjanovic, Bielefeld 13 December 2005

  38. Sensitivity of the difference procedure 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. S. Damjanovic, Bielefeld 13 December 2005

  39. Excess spectra from difference: data - cocktail all pT No cocktail and no DD subtracted Clear excess above the cocktail , centered at the nominal  pole andrising with centrality Similar behaviour in the other pT bins S. Damjanovic, Bielefeld 13 December 2005

  40. Excess spectra from difference data-cocktail pT < 0.5 GeV No cocktail and no DD subtracted Clear excess above the cocktail , centered at the nominal  pole andrising with centrality Similar behaviour in the other pT bins S. Damjanovic, Bielefeld 13 December 2005

  41. Systematics Illustration of sensitivity to correct subtraction of combinatorial background and fake matches;to variation of the  yield Systematic errors of continuum 0.4<M<0.6 and 0.8<M<1GeV 25% Structure in  region completely robust S. Damjanovic, Bielefeld 13 December 2005

  42. Comparison of excess to model predictions S. Damjanovic, Bielefeld 13 December 2005

  43. g*(q) Dilepton Rate in a strongly interacting medium μ+ μ- (T,mB) dileptons produced by annihilation of thermally excited particles: +- in hadronic phase qq in QGP phase at SPS energies +  -→*→μ+μ- dominant hadron basis photon selfenergy spectral function Vector-Dominance Model S. Damjanovic, Bielefeld 13 December 2005

  44. Physics objective Goal: Study properties of the rho spectral function Im Dr in a hot and dense medium Procedure:Spectral function accessible through rate equation, integrated over space-time and momenta Limitation:Continuously varying values of temperature T and baryon density rB, (some control via multiplicity dependences) S. Damjanovic, Bielefeld 13 December 2005

  45. p p r  spectral function in vacuum Introduce r as gauge boson into free p+r Lagrangian  is dressed with free pions vacuum spectral function (like ALEPH data V(t→ 2pnt )) S. Damjanovic, Bielefeld 13 December 2005

  46. r spectral function in hot and dense hadronic matter (I) Dropping mass scenario Brown/Rho et al., Hatsuda/Lee explicit connection between hadron masses and chiral condensate universal scaling law continuous evolution of pole mass with T and r ; broadening atfixed T,r ignored S. Damjanovic, Bielefeld 13 December 2005

  47. rB /r0 0 0.1 0.7 2.6 r spectral function in hot and dense hadronic matter (II) Hadronic many-body approachRapp/Wambach et al., Weise et al. hot matter hot and baryon-rich matter Dr (M,q;mB,T)=[M2-mr2-Sr pp-Sr B-Sr M ]-1  is dressed with: hot pions Srpp , baryons Sr B (N,D ..) mesons Sr M (K,a1..) • “melts” in hot and dense matter • - pole position roughly unchanged - broadening mostly through baryon interactions S. Damjanovic, Bielefeld 13 December 2005

  48. rB /r0 0 0.1 0.7 2.6 Final mass spectrum continuous emission of thermal radiation during life time of expanding fireball integration of rate equation over space-time and momenta required example: broadening scenario S. Damjanovic, Bielefeld 13 December 2005

  49. How to compare data to predictions? • correct data for acceptance in 3-dim. space M-pT-y and compare directly to predictions at the input (to be done in the future) • 2) use predictions in the form • decay the virtual photons g* into m+m- pairs, propagate these through the NA60 acceptance filter and compare results to uncorrected data at the output (done presently)conclusions as to agreement or disagreement of data and predictions are independent of whether comparison is done at input oroutput S. Damjanovic, Bielefeld 13 December 2005

  50. Acceptance filtering of theoretical prediction all pT Input (example): thermal radiation based on RW spectral function Output:spectral shape much distorted relative to input, but somehow reminiscent of the spectral function underlying the input; by chance? S. Damjanovic, Bielefeld 13 December 2005

More Related