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

2. 2  NA60: past precision di-muon measurements in HI at top-most SPS energy (158 AGeV) Outline  Search of the QCD critical point and onset of deconfinement: di-muon measurements from top-most SPS energy to 20 AGeV  Results for a new spectrometer for low energy dilepton (and quarkonia?) measurements at the CERN SPS

3. 3 dipole magnet hadron absorber targets beam tracker Si-pixel tracker muon trigger and tracking magnetic field >10m<1m 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 Concept working at high energy (NA60) Does it work also at low energy (20-30 GeV)?

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

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

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

7. 7 SPS 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? The QCD phase diagram

8. 8 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? Experimental search of critical point at SPS: Energy scan from ~ 20-30 GeV towards topmost SPS energy Central Pb-Pb Dileptons measurements: which energy interval to explore?

9. Phys. Rev. Lett. 91 (2003) 042301 Higher baryon density at 40 than at 158 AGeV 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) Larger enhancement and stronger  broadening in support of the decisive role of baryon interactions 9 Measuring dileptons at lower SPS energies

10. 10 NA60 precision measurement of excess yield (  -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  FB observed with identical final state hadron spectra (in terms of flow) → lifetime extension in a soft phase Nice example of complementary measurements with NA61 Low mass dileptons: constraints in fireball lifetime Eur. Phys. J. C (2009) in press

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

12. 12 partly complementary programs planned at CERN SPS BNL RHIC DUBNA NICA GSI SIS-CBM NA60’ experiment at CERN  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 Competitiveness

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

14. Pixel plane: - 400  m silicon + 1 mm carbon substrate - material budget ≈0.5% X 0 Required rapidity coverage @20 GeV starting from =1.9 ( ϑ ~ 0.3 rad) 3 T dipole field along x 40 cm x z 5 silicon pixel planes at 7<z<38 cm The vertex spectrometer

15. 15 540 cm Fe 40 cm BeO-Al 2 O 3 50 cm graphite The hadron absorber and muon wall 240 cm graphite High energy setup absorber BeO 110 cm Low energy setup absorber E loss: main factor together with detector transverse size which determines p T -y differential acceptance Compromise with signal muon energy loss: ≈ 1 GeV at most to get muons into the spectrometer with yLab~2 and p T ≥0.5 GeV 7.3 I, 14.7 X 0 : potentially not containing fully the hadronic shower Low energy muon wall: 120-160 cm graphite 14 I, 50 X 0 : contains fully hadronic showers High energy muon wall: 120 cm Fe

16. The muon spectrometer Muon Tracker 4 tracking stations (z=295, 360, 550, 650 cm) Trigger system 2 trigger stations placed after muon wall (ALICE-like) at z = 840, 890 cm No particular topological constraints introduced contrary to NA60 hodo system (muons required in different sextants) z x R=290 cm Muon wall (120 or 160 cm) Muon spectrometer field toroid magnet B 0 /r – B 0 = 0.16 Tm 380<z<530 cm; r<180 cm

17. Simulation tools Fast simulation (signal) Hadron cocktail generator derived from NA60GenGenesis Apparatus defined in setup file describing layer properties: - geometric dimensions of active and passive layers - material properties Multiple scattering generated in gaussian approximation (Geant code) Energy loss imposed and corrected for deterministically according to Bethe-Bloch Fluka (background) parametric  and K event generator (built-in decayer for  and K) Apparatus geometry defined in consistent way with fast simulation tool Hits in detector planes recorded in external file for reconstruction

18. Track reconstruction Setup parameters: - x, y resolutions of active layers - detector efficiencies: 90% for pixel, 90% muon stations Background hits sampled from multiplicity distributions (  and K) to populate vertex detector (signal embedded in background) Track reconstruction started from hits in trigger stations Kalman fit adding hits in muon stations and vertex detector Matched tracks: - Correct match: only correct hits associated to track - Fake match: one or more wrong hits associated to track

19. Single muon acceptances 5x10 3  + generated with uniform 1.5<y<4.5 and 0<p T <3 GeV and tracked through spectrometer ++ -- fast sim + rec (correct match) 85% global eff Fluka sim + rec (correct match) 81% global eff

20.     20 GeV hadron cocktail: p T vs y coverage mid y – 20 GeV = 1.9 Optimized setup with dipole+toroid Wall = 120 cm

21. y vs p T  : comparison with NA60 @ 158 GeV  Low energy setup mid y – 20 GeV = 1.9 NA60 high energy setup mid y – 158 GeV = 2.9  p T [GeV] y y

22. Signal reconstruction efficiency vs transverse momentum pixel resolution 15  m –  2 <1.5 Light absorber: broader y-p T coverage No topological constraints imposed by trigger Thick absober: narrower y-p T coverage Trigger: muons required in different hodo sextants Dead zones in toroid magnet  Low energy setup: rec eff x Acc better by more than one order of magnitude NA60 high energy setup Low energy reduced setup (dipole+toroid field, wall 120 cm )

23. Very good agreement between fast and full simulation NA60 high energy setup – full simulation NA60 high energy setup – fast simulation Rec eff in different mass bins: average of rec eff for single cocktail processes (simple average in fast simulation) Hodo trigger condition and dead zones taken into account in fast simulation Cross check: high energy setup rec  x Acc vs p T

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

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

26. Input parameters for background simulation: pions, kaons and protons Pions, kaons and protons generated according to NA49 measurements for 0-5% Pb-Pb central collisions at 20, 30 GeV Pions, kaons: - 20 GeV: dN  +K /dy(NA49) ≈ 180 - 30 GeV: dN  +K /dy(NA49) ≈ 210 Protons: - 20 GeV: dN p /dy(NA49) ≈ 80 - 30 GeV: dN p /dy(NA49) ≈ 60

27. 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 dN ch /dy 27 B S B/S=35 Assessment of B/S: choice of S

28. B/S @ 600 MeV = 90 Pix res 15  m B/S @ 600 MeV =140 Pix res 15  m 20 GeV30 GeV - Correct signal matches - Signal fakes - Correct signal matches - Signal fakes Hadronic cocktail and bkg in central Pb-Pb collisions at 20 and 30 GeV (wall 120 cm) , K, p all included in bkg estimation

29. B/S @ 600 MeV = 75 Pix res 15  m B/S @ 600 MeV = 110 Pix res 15  m 20 GeV30 GeV - Correct signal matches - Signal fakes - Correct signal matches - Signal fakes Hadronic cocktail and bkg in central Pb-Pb collisions at 20 and 30 GeV (wall 160 cm) , K, p all included in bkg estimation

30.  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 30 Tracking with 2 fields vs 1 field  Qualitative argument: concept succesfully exploited by ATLAS/CMS. Caveat: - works well for hard muons (quarkonia) - low masses (CMS): p T cut at 7 GeV  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 demonstrated by NA60 in heavy ions at 158 GeV/c

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

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

33. 1.5 m 0.75 m  Rapidity coverage: Sitting at 380<z<455 covers down to  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 Search for and existing toroid magnet Magnet: critical element to determine the cost  looking for existing magnets Chorus magnet at CERN

34. 34 Experimental aspects  High precision: - very good B/S (<100 in central collisions) - p T 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  m 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. 35 Summary  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, p T acceptance, mass resolution  Study of QCD phase diagram and critical point search: fundamental physics aspects addressed