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The Experimental Quest for In-Medium Effects

The Experimental Quest for In-Medium Effects. Romain Holzmann GSI Helmholtzzentrum f ü r Schwerionenphysik, Darmstadt at 23 rd Indian-Summer School of Physics and 6 th HADES Summer School: Physics @ FAIR October 3-7, 2011 in Rez/Prague, Czech Republic.

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The Experimental Quest for In-Medium Effects

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  1. The Experimental Quest for In-Medium Effects Romain Holzmann GSI Helmholtzzentrum für Schwerionenphysik, Darmstadt at 23rd Indian-Summer School of Physics and 6th HADES Summer School: Physics @ FAIR October 3-7, 2011 in Rez/Prague, Czech Republic • Lecture I: Pedestrian’s approach to … • Lecture II: Experiments galore • Lecture III: HADES at GSI

  2. Lecture II:Dilepton experiments: An overview(with emphasis on HIC) Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  3. Evolution of the universe • Rafelski 2005 hadronization ρ≈ few times ρ0 T ≈ 100 MeV Such conditions can be realized in heavy-ion collisions, but treac≈ 10-23 s << 10-6 s and finite volume only ! Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  4. Typical dilepton mass spectrum Characteristic features of resulting dilepton mass distribution: Physics issues: • Low mass: • continuum enhancement ? • modification of vector mesons ? • Intermediate mass: • thermal radiation ? • charm modification • High mass: • J/ suppression ? • enhancement ? • Drell-Yan In this lecture focus is on low mass region! Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  5. atSIS100 Overview (of HI expts.) CMS ATLAS LHC s Energy RHIC SPS CERES HELIOS 3 SIS 300 SIS 100AGS SIS18 Bevalac 1990 2000 2010 2018 Time + advance in technology Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  6. H.J. Specht in Lecture Notes on Physics 221/1985 ω φ QM1985 The beginning: “anomalous” dilepton yields Compilation of results on dilepton production (from Fermilab, SLAC, ISR, KEK 1975-1985)  “anomalous” excess at low mass over known* Dalitz yields (by x3!)  Starting point of the CERN SPS dilepton expts. (HELIOS, CERES, NA38/50 and DLS at LBNL) * ”known” means known in 1985! Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  7. The early HI experiments (1990 - 2000) HELIOS/NA34 at the CERN SPS (e+e-, μ+μ-, γ) CERES/NA45 at the CERN SPS (e+e-) NA38/NA50 at the CERN SPS (μ+μ-) DLS at the Bevalac (e+e-) Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  8. The NA34 apparatus in 1989 HELIOS at the CERN SPS HELIOS data from Masera et al., NPA 590 (1995) 93 BUU calc. Cassing et al., PLB 377 (1996) 5 p + W Data agree with cocktail of free meson decays No “anomalous” yield! • HELIOS measured • μ+ μ - • e+e- • and photons S + W First observation of in-medium excess Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  9. < year 2000 > year 2000 CERES at the CERN SPS CERES CERES, a e+e- spectrometer with: • RICH-in-RICH design (f symmetry) • dE/dx in Si drift for bkgd rejection • 4% mass resolution (with TPC upgrade) Agakishiev et al., EPJC 41 (2005) 475 • HELIOS findings fully confirmed by CERES: • in p+A: cocktail of • in-vacuum decays • in A+A: large excess yield at Mee> 0.2GeV Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  10. EPJ C41 (2005) 475 PRL 91 (2003) 042301 CERES: low-mass e+e- excess in Pb+Au CERES • Central A+A collisions exhibit a strong enhancement of low-mass dilepton production • as compared to p+A reactions (HELIOS & CERES) • Vacuum properties of mesons do not suffice to describe these data, needed are: • pion annihilation (accounts for part only) • in-medium modifications of vector meson properties • e.g. broadening and/or mass shift of the rho meson Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  11. dropping  mass in-medium hadronic spectral function CERES: Comparison with theory CERES HMB = hadronic many-body models H. Hees & R. Rapp, PRL 97 (2006) 102301 e+e- pair yield after subtraction of hadronic cocktail, excluding : • Many more calculations done: • thermal models (e.g. Kämpfer et al.) • transport models (HSD, UrQMD) ► See Rez 2008 school for nice intros to • transport models (E. Bratkovskaya), • HMB theory (R. Rapp) • statistical models (PBM) In particular e+e- yield between  and  favours -broadening over dropping-mass scenario, i. e use in-medium spectral function Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  12. The DLS spectrometer at the Bevalace+e- production at 1-2 AGeV Data: R.J. Porter et al.: PRL 79(97)1229 Model: E.L. Bratkovskaya et al.: NP A634(98)168, BUU with vacuum spectral function DLS at the Bevalac (1987-1993) • Excess of e+e- yield observed over • cocktail of in-vacuum meson decays. • But, reaction is baryon dominated ► nucleon resonances (Δ, N*) ►bremsstrahlung (pp vs. pn) Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  13. Transport calc.: Frankfurt UrQMD Ernst et al. PRC 58 (1998) 447 DLS: pair excess at 1 AGeV DLS yield excess At M= 0.2 - 0.6 GeV in all A+A systems.

  14. In-medium Vector Meson spectroscopy The advent of high-resolution & high-statistics experiments: NA60 at the CERN SPS: In+In →μ+μ- HADES at GSI: p+p, p+A, A+A → e+e- E325 at the KEK PS: p+Cu → e+e- CLAS at JLAB: γ+A →e+e- CB/TAPSat ELSA: γ+A →ω→π0γ (photons) LEPS at SPring-8: γ+A → →K+K- (kaons) ANKE at COSY: p+A → →K+K- (kaons) • And, of course • PHENIX& STAR at RHIC • ALICE, ATLAS & CMS at LHC Have to deal with final state interactions of VM decay products Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  15. 2.5 T dipole magnet vertex tracker beam tracker muon trigger and tracking iron wall magnetic field targets Zero degree calorimeter (centrality measurements) 17m hadron absorber Muon Spectrometer The vertex region (2 detectors): muon other The NA60 experiment at CERN • Fixed target dimuon experiment at the CERN SPS (NA50 upgrade) • Apparatus composed of 4 main detectors hadron absorber Concept of NA60: place a silicon tracking telescope in the vertex region to measure the muons before they suffer multiple scattering in the absorber and match them (in both angles andmomentum) to muon tracks measured in the spectrometer behind the absorber. • Origin of muonscan be accurately determined (less bkgd from weak decay muons) • Improved dimuon mass resolution (~20 MeV/c2 at ) • High rate because of radiation-hard Si pixel det. Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  16. NA60: Subtraction of CB 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 >1000 • resolution: factor 2-3 Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  17. NA60: hadron decay cocktail in peripheral In+In collisions log Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  18. NA60: data & cocktail vs. centrality ● data -- sum of cocktail sources including the  Clear excess of data above cocktail, rising with centrality Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  19. NA60: Excess = data - cocktail all pT No cocktail  and no DD subtracted Clear excessabove the cocktail ,centered at the nominal  poleandrising with centrality Similar behaviour in all pT bins Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  20. Two theoretical scenarios for  in-medium spectral function a.) broadening Rapp/Wambach Ad.N.P. 25 (2000) 1 b.) dropping mass Brown/Rho Phys. Rep. 363 (2002) 85 evaluated for the same fireball evolution (R. Rapp 2003 keeping the original normalization) NA60: Comparison to HMB models HMB = hadronic many-body models  dropping mass scenario fails to reproduce the NA60 data • Data not yet acounted for in mass range M > 1 GeV Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  21. Dimuon pairs with invariant mass > 1 GeV H. van Hees & R. Rapp, PRL 99 (2006)102301 J. Ruppert et al., hep-ph/0706.1934 • yield above 1 GeV attributed to • hadronic processs like 4 annihilation • (possibly augmented by chiral mixing) • yield above 1 GeV dominated by • partonic processes like qq+- Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  22. nearly linear rise of Teff with • dimuon mass for M < 1GeV • consistent with radial flow of a • hadronic source: NA60: radial flow of muon pairs • hadrons show expected flow • pattern: Teff M • however, hierarchy of freeze out: • different hadrons have different • couplings to pions: • max. for ; min. for  Teff from muon pair Pt slopes: R. Arnaldi et al., PRL 100 (2008) 022302 Thermal vs. radial expansion: What’s the deal ? • Equilibrated fireball (Maxwell-Boltzmann distribution): <Ekin> = 3/2 kT where T= same for all particles • with radial expansion (blast wave): <Ekin> = 3/2 kT + 1/2Mβ2 = 3/2 kTeff(T andβ same for all particles) ► Teff ~ M (for M>>T) LMR lMR • drop of Teff at M1 GeV • interpreted as transition to low-flow • source of e.g. partonic origin ► First observation of radial flow of thermal di-leptons; mass dependence of Teff tool to identify nature of the emitting source; mostly partonic radiation for M> 1 GeV ? Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  23. central spectrometer measurement in range: |h|  0.35 p  0.2 GeV/c • forward spectrometers muon measurement in range: 1.2 < |h| < 2.4 p  2 GeV/c two central electron/photon/hadron spectrometers two forward muon spectrometers The PHENIX detector at RHIC Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  24. p g e+ e- PHENIX: Tracking and particle id Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  25. PHENIX: Dielectrons at p + p Au+Au arXiv: 0802.0050 PRC 81 (2010) 034911 Observed dilepton yield reproduced by cocktail from known hadronic sources • signal-to-background  1/200 • combinatorial background • must be determined to 0.25% !! • ► in 2009 installed HBD, expect S/B=1/10 Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  26. PHENIX: Au+Au vs. p+p data • pp data and cocktail • normalized to Au+Au • data for mee<100 MeV • PHENIX low mass continuum (150 < mee<750 MeV): • enhancement compared to cocktail: 4.7 0.4(stat) 1.5(sys) 0.9(model) Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  27. e+ Gluon Compton g* e- q g q q q Au+Au data compared to calculations • hadronic source: • partonic sources STAR claims a much weaker effect (QM2011) : ►observed enhancement not reproduced by current calculations !!! Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  28. large excess ! The HADES experiment at GSI • f symmetry • hadron-blind RICH • 2-3% mass resolution For more see Lecture III … Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  29. The ubiquitous dilepton excess in HIC • An excess of dilepton excess yield (over a calculated cocktail of • “known” sources) has been observed for M = 0.2 – 0.6 GeV, • at all beam energies, from 1 AGeV to 100+100 AGeV ! • Its origin is thought to be: • baryon dominated at SIS energies (resonance matter!) • meson dominated at SPS energies (+ baryonic, + partonic?) • with a strong (?) partonic component at RHIC energies • A comprehensive & quantitative theoretical description is still missing • What happens at LHC ? ALICE will tell us … • At FAIR energies (2 - 25 AGeV) Nmeson≈ Nbaryon • ► HADES & CBM will survey this region and provide high-quality data Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  30. In-medium Vector Meson spectroscopy The advent of high-resolution & high-statistics experiments: NA60 at the CERN SPS: In+In →μ+μ- HADESat GSI: p+p, p+A, A+A → e+e- E325 at the KEK PS: p+Cu → e+e- CLAS at JLAB: γ+A →e+e- CB/TAPSat ELSA: γ+A →ω→π0γ (photons) LEPSat SPring-8: γ+A → →K+K- (kaons) ANKE at COSY: p+A → →K+K- (kaons) • And, of course • PHENIX& STAR at RHIC • ALICE, ATLAS & CMS at LHC Have to deal with final state interactions of VM decay products Rez 2011 - The Experimental Quest for In-Medium Effects - R. Holzmann, GSI

  31. Cold vs. hotnuclear matter A+ A p () + A e+ r/ F e- No FSI ! FSI !

  32. 2. Measurement of the transparency ratio: in A ► in-medium width (note that TA is similar to the RAA of HICs) Experimental approaches using elementary probes Measurement of the in-medium meson line shape: ► in-medium mass shift? broadening? structures? 3. Measurement of the excitation function:(study threshold behaviour) ► in-medium mass shift, broadening

  33. J.G.Messchendorp et al., Eur. Phys. J. A 11 (2001) 95 p     A  + X 0    Re-analysis: M. Nanova • advantage: • 0large branching ratio • (8.3 ·10-2 ) • no  contribution • (0 : 7 10-4) No significant difference between lineshapes from Nb and LH2 !!! Sensitivityto in-medium modifications ? disadvantage: • 0 rescattering, reduced by removing low energy0 (T> 150 MeV) CBELSA/TAPS: lineshape on Nb and LH2

  34. in the medium: additional inelasticchannelsremovemesons, e.g. N N ► shorteningoflifetime; increase in width in-mediuminelastic width proportional toabsorption:(,|p|) vabs M. Kaskulov, E. Hernandez, E. Oset EPJ A 31 (2007) 245 P. Mühlich and U. Mosel NPA 773 (2006) 156 transparency ratio: normalized to C! Comparison to transport model calculations: Results in (0,<|p|>1.1 GeV/c) 130-150 MeV is broadened in the medium by a factor16!! in-medium ωwidth comparable to freeρwidth !! Data: M. Kotulla et al., PRL 100 (2008) 192302 ωin-medium width & collisional broadening

  35. Role of regeneration in TA :  , ,’ CBELSA/TAPS E (1.2-2.2) 0 A M. Nanova: Hadron2011 •  absorption NOT affected by secondary reactions • „fast”  are absorbed similarly to  , • slow  are enhanced by secondary production NN , NN(1535) • weaker absorption of ’ (’- N coupling) : ’ in-medium : 25-30 MeV

  36. ANKE:  Transparency from p +A A. Polyanskiy : Hadron2011 ANKE : p @ 2.83 GeV K+ K- Preliminary Phys. Lett. B 695, 74-77 (2011). 0 < < 8 0 0.6 <p  < 1.6 GeV/c lab • p + A : in-medium ~ 33 -50 MeV @ <p > =1.1 GeV/c and 0:+A (Spring-8, JLab) : in-medium ~ 23-100 MeV @ <p > 1.7 GeV/c •   28 MeV from predictions (Valencia/Giessen) - kaon loop modification

  37. e+e- pairs from p+A: HADES Preliminary Large acceptance for pe+e- < 1GeV/c : not measured by CLAS, KEK-E325 CLAS,KEK 0  N*/  ? VM cocktail : HADES + PYTHIA

  38. HADES: / region p>0.8 GeV/c • HADES: 3.5 GeVp + Nb vs. p + p • for pe+e- <0.8 GeV/c excess over pp: secondary  from N(1520),  (1700),… • ►Slow pairs show strong in-medium effect •  absorption compatible with CBTAPS: • GiBuu gives in-medium  150 MeV p<0.8 GeV/c To be compared with theory …

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