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Gianluca Usai – University of Cagliari and INFN

The QCD phase diagram and search of critical point: dilepton measurements in the range 20-158 AGeV at the CERN-SPS. Gianluca Usai – University of Cagliari and INFN. Electromagnetic Probes of Strongly interacting Matter in ECT* Trento - 23/05/2013. Outline .

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Gianluca Usai – University of Cagliari and INFN

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  1. The QCD phase diagram and search of critical point: dilepton measurements in the range 20-158 AGeV at the CERN-SPS Gianluca Usai – University of Cagliari and INFN ElectromagneticProbes of StronglyinteractingMatter in ECT* Trento - 23/05/2013

  2. Outline • NA60: pastprecision di-muonmeasurements in HI at top-most • SPS energy (158 AGeV) • Search of the QCD criticalpoint and onset of deconfinement: • di-muonmeasurements from top-most SPS energy to 20 AGeV • Results for a new spectrometer for lowenergydilepton (and • quarkonia?) measurementsat the CERN SPS

  3. dipole magnet muon trigger and tracking beam tracker Si-pixel tracker magnetic field targets hadron absorber • Track matching in coordinate and momentum space Improved dimuon mass resolution Distinguish prompt from decay dimuons • Radiation-hard silicon pixel detectors (LHC developments) High luminosity of dimuon experiments maintained High precision muon measurements in HI collisions <1m >10m • Concept working at high energy (NA60) Does it work also at low energy (20-30 GeV)? 3

  4. NA60: In-In at 158 AGeV Measurement of muon offsets Dm:distance between interaction vertex and track impact point Phys. Rev. Lett. 96 (2006) 162302 Eur. Phys. J. C 59 (2009) 607 • 440000 events below 1 GeV (3 weeks run) • 23 MeV mass resolution at the w • Charm not enhanced • Excess above 1 GeV prompt

  5. Inclusive excess mass spectrum [Eur. Phys. J. C 59 (2009) 607-623]CERN Courier 11/ 2009, 31-35 Chiral 2010 , AIP Conf.Proc. 1322 (2010) 1-10 All known sources subtracted M<1 GeV integrated over pT Fully corrected for acceptance ρ dominates, ‘melts’ close to Tc absolutely normalized to dNch/dη best described by H/R model M>1 GeV ~ exponential fall-off ‘Planck-like’ fit to range 1.1-2.0 GeV: T=205±12 MeV 1.1-2.4 GeV: T=230±10 MeV T>Tc: partons dominate only described by R/R and D/Z models

  6. Dilepton measurements in the range 20-158 AGeV at the CERN-SPS

  7. The QCD phase diagram QCD phase diagram poorly known in the region of highest baryon densities and moderate temperatures Fundamental question of strong interaction theory: is there a critical point? SPS

  8. Dileptonsmeasurements: which energy interval to explore? Steepening of the increase of the pion yield with collision energy and sharp maximum in the energy dependence of the strangeness to pion ratio. Onset of deconfinement? Central Pb-Pb Experimental search of critical point at SPS: Energy scan from ~ 20-30 GeV towards topmost SPS energy

  9. Measuring dileptons at lower SPS energies Phys. Rev. Lett. 91 (2003) 042301 Enhancement factor: 5.9±1.5(stat.)±1.2(syst.) (published ±1.8 (syst. cocktail) removed due to the new NA60 results on the η and ω FFs) Higher baryon density at 40 than at 158 AGeV Larger enhancement and stronger r broadening in support of the decisive role of baryon interactions

  10. Low mass dileptons: constraints in fireball lifetime Eur. Phys. J. C (2009) in press NA60 precision measurement of excess yield (r-clock): provided the most precise constraint in the fireball lifetime (6.5±0.5 fm/c) in heavy ion collisions to date! Crucial in corroborating extended lifetime due to soft mixed phase around CP: if increased tFB observed with identical final state hadron spectra (in terms of flow) →lifetime extension in a soft phase Nice example of complementary measurements with NA61

  11. Investigation of IMR at lower energies • Direct extraction of source temperature: measurement of T vs beam energy from mass distribution (Lorentz invariant) • How will the drop decrease or disappear (partonic radiation significant at 158 AGeV)? Where? 11

  12. Competitiveness NA60’ experiment at CERN partly complementary programs planned at CERN SPS BNL RHIC DUBNA NICA GSI SIS-CBM • Availability of ion beams at CERN: dilepton experiment covering a large energy interval - crucial for QCD phase diagram • Energy interval not covered by any other experiment: unique experiment • Experiment focused on dileptons: highly optimized for precision measurements • Complementary to NA61

  13. From the high energy to a low energy apparatus layout Muon spectrometer Toroid field (R=160 cm) 5 m beam Compress the spectrometer reducing the absorber and enlarge transverse dimensions dimuons@160 GeV (NA60) rapidity coverage 2.9<y<4.5 Muon spectrometer Toroid field (R=180 cm) 3m toroid 2.4 m 9 m Vertex spectrometer Dipole field beam dimuons@20 GeV rapidity coverage 1.9<y< 4 Longitudinally scalable setup for running at different energies

  14. The vertex spectrometer 3 T dipolefieldalong x 40 cm • Required rapidity coverage @20 GeV starting from <y>=1.9 (ϑ~0.3 rad) x • 5 silicon pixel planesat 7<z<38 cm • Pixel plane: • - 400 mm silicon + 1 mm carbon substrate • - material budget ≈0.5% X0 z

  15. The hadron absorber and muon wall High energy setup absorber BeO-Al2O3 50 cm • 14 lI , 50 X0: containsfullyhadronicshowers • High energymuonwall: 120 cm Fe 540 cm graphite Fe 40 cm BeO 110 cm • Low energy setup absorber • E loss: main factor together with detector transverse size which determines pT-y differential acceptance • Compromise with signal muon energy loss: ≈ 1 GeV at most to get muons into the spectrometer with yLab~2 and pT≥0.5 GeV • 7.3 lI , 14.7 X0: potentially not containing fully the hadronic shower • Low energy muon wall: 120-160 cm graphite 240 cm graphite

  16. The muon spectrometer Muonwall (120 or 160 cm) • MuonTracker • 4 trackingstations (z=295, 360, 550, 650 cm) R=290 cm x z • Muon spectrometer field • toroid magnet B0/r – B0 = 0.16 Tm • 380<z<530 cm; r<180 cm • Trigger system • 2 trigger stationsplacedaftermuonwall (ALICE-like) atz = 840, 890 cm • No particulartopologicalconstraintsintroducedcontrary to NA60 hodosystem (muonsrequired in differentsextants)

  17. Simulation tools • Fast simulation (signal) • Hadron cocktail generator derived from NA60GenGenesis • Apparatusdefined in setup file describinglayerproperties: • - geometricdimensions of active and passive layers • - materialproperties • Multiple scatteringgenerated in gaussianapproximation (Geant code) • Energy lossimposed and corrected for deterministicallyaccording to Bethe-Bloch • Fluka (background) • parametricp and K event generator (built-in decayer for p and K) • Apparatusgeometrydefined in consistent way with fast simulationtool • Hits in detector planesrecorded in external file for reconstruction

  18. Track reconstruction • Setup parameters: • - x, y resolutions of activelayers • - detector efficiencies: 90% for pixel, 90% muonstations • Background hitssampled from multiplicitydistributions (p and K) to populatevertex detector (signalembedded in background) • Trackreconstructionstarted from hits in trigger stations • Kalmanfitaddinghits in muonstations and vertex detector • Matchedtracks: • - Correctmatch: onlycorrecthitsassociated to track • - Fakematch: one or more wronghitsassociated to track

  19. Single muon acceptances 5x103m+generated with uniform 1.5<y<4.5 and 0<pT<3 GeV and trackedthroughspectrometer Flukasim + rec (correct match) 81% global eff fast sim + rec (correct match) 85% global eff m+ m-

  20. 20 GeV hadron cocktail: pTvs y coverage mid y – 20 GeV = 1.9 mid y – 20 GeV = 1.9 mmg wmmg Optimized setup with dipole+toroid Wall = 120 cm wmm rmm

  21. y vspTr: comparison with NA60 @ 158 GeV Low energy setup mid y – 20 GeV = 1.9 NA60 high energy setup mid y – 158 GeV = 2.9 rmm rmm pT [GeV] y y

  22. Signal reconstruction efficiency vs transverse momentum Lowenergyreduced setup (dipole+toroidfield, wall 120 cm ) NA60 high energy setup pixel resolution 15 mm – c2<1.5 • Thickabsober: narrower y-pTcoverage • Trigger: muonsrequired in differenthodosextants • Dead zones in toroidmagnet • Light absorber: broader y-pTcoverage • No topologicalconstraintsimposed by trigger Lowenergy setup: receff x Accbetter by more thanoneorder of magnitude

  23. Cross check: high energy setup rec e x AccvspT NA60 high energy setup – fast simulation NA60 high energy setup – full simulation • Receff in different mass bins: average of receff for single cocktail processes (simpleaverage in fast simulation) • Hodo trigger condition and dead zonestakeninto account in fast simulation • Verygoodagreementbetween fast and full simulation

  24. Mass resolution NA60 high energy setup Lowenergysetup (dipole+ toroidfield) M [GeV] M [GeV] NA60 lowenergysetup: mass resolution 8 MeV NA60 high energy setup: Mass resolution 23 MeV  New setup: mass resolution can be improved at least by a factor 2-3 …..with respect to the old high energy setup

  25. Sources of combinatorial background vertex offset Keep this distance as small as possible ~40 cm Muon wall (not to scale) dipole field µ prompt muon primary hadron punch-through Seconday hadron punch-through , K µ decay muons from primary hadrons Beam Tracker decay muons from secondary hadrons Vertex Detector Hadron absorber (not to scale) • Fluka: • - Full hadronicshowerdevelopment in absorber • - Punch-through of primary and secondaryhadrons (p, K, p) • - Muons from secondaryhadrons

  26. Input parameters for background simulation: pions, kaons and protons • Pions, kaonsand protonsgeneratedaccording to NA49 measurements for 0-5% Pb-Pb centralcollisionsat 20, 30 GeV • Pions, kaons: - 20 GeV: dNp+K/dy(NA49) ≈ 180 - 30 GeV: dNp+K/dy(NA49) ≈ 210 • Protons: - 20 GeV: dNp/dy(NA49) ≈ 80 - 30 GeV: dNp/dy(NA49) ≈ 60

  27. Assessment of B/S: choice of S B B/S=35 S choose hadron cocktail in mass window 0.5-0.6 GeV for S - free from prejudices on any excess; no ‘bootstrap’; most sensitive region - unambiguous scaling between experiments; B/S dNch/dy

  28. Hadronic cocktail and bkg in central Pb-Pb collisions at 20 and 30 GeV (wall 120 cm) B/S @ 600 MeV =140 Pix res 15 mm B/S @ 600 MeV = 90 Pix res 15 mm 20 GeV 30 GeV - Correctsignalmatches - Signalfakes - Correctsignalmatches - Signalfakes • p, K, p all included in bkg estimation

  29. Hadronic cocktail and bkg in central Pb-Pb collisions at 20 and 30 GeV (wall 160 cm) B/S @ 600 MeV = 110 Pix res 15 mm B/S @ 600 MeV = 75 Pix res 15 mm 20 GeV 30 GeV - Correctsignalmatches - Signalfakes - Correctsignalmatches - Signalfakes • p, K, p all included in bkg estimation

  30. Tracking with 2 fields vs 1 field • CBM: vertex spectrometer + active absorber • Advantages: • - very compact • Potential disadvantages • - Fe absorber: not optimized for MS • - matching not exploiting momentum • - tracking in very high multiplicity • NA60’ setup (vertex + muon spectrometers) • Advantages: • - tracking in low multiplicity environment • - matching exploits momentum info • - optimized for MS • Potential disadvantages • - larger apparatus • Qualitative argument: concept succesfully exploited by ATLAS/CMS. Caveat: - works well for hard muons(quarkonia) • - low masses (CMS): pT cut at 7 GeV • Qualitative argument: concept succesfully demonstrated by NA60 in heavy ions at 158 GeV/c

  31. Comparison to CBM CBM 25 GeV Au-Au central B/S=600 NA60’-B/S = 110 30 GeV Pb-Pb × New Much setup and selection cuts CBM 35 GeV Au-Au central B/S=200 • B/S: factor ≈ 2 difference • Reconstruction efficiency: factor ≈ 10 difference tracking problem in active absorber?

  32. NA60’ setup: tracking switching off toroid field • Strong increase of signal fakes and, in particular, combinatorial background (factor ≈ 3) • The tracking after the absorber requires a second field B/S @ 600 MeV = 300 Pix res 15 mm 20 GeV - Correctsignalmatches - Signalfakes

  33. Search for and existing toroid magnet Magnet: critical element to determine the cost  looking for existing magnets Chorus magnet at CERN 0.75 m • Rapidity coverage: • Sitting at 380<z<455 covers down to h≈1.8 • Issues: • - Maximum quoted B=0.12 T bending power 1/2 what considered in simulations • - Magnet operated in pulsed mode with a neutrino beam: cooling aspect to be considered 1.5 m

  34. Experimental aspects • High precision: • - very good B/S (<100 in central collisions) • - pT acceptance (a factor >10 better than high energy setup) • - 8 MeV mass resolution (a factor 2-3 better than high energy setup) • Silicon detector: use of existing hybrid technologies possible • - 50 mm cell, ≈0.5% material budget/plane • Muon tracking chambers: large area but conventional detectors • Muon spectrometer magnet: • - toroid magnet (R≈1.5-2 m) required with relatively low field strength • Further work for optimizing the setup required

  35. Summary • Study of QCD phase diagram and critical point search: • fundamental physics aspects addressed • Ion beams exists and will be available for experiments at the SPS in the incoming years (presently scehduled up to 2021) • A Dilepton experiment at CERN can provide crucial information on the QCD phase diagram: • Covering a large energy interval crucial for QCD phase diagram • Energy interval together with high luminosity not covered by any other experiment : unique experiment • High precision: very good B/S, pT acceptance, mass resolution

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