1. Prospects to study the χ c production at CBM We give preliminary considerations to the feasibility of χ c production study at CBM involving experience of an analogous measurement performed in proton-nucleus collisions at 920 GeV with the Hera-B detector Dmitry Goloubkov, Yuri Zaitsev ITEP, Moscow

2. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 2 Physical motivations for studying χ c in HI collisions  Additional charmonium probe and a “thermometer” for QGP dissociation temperature: T d (χ) ~ 1.1T c, T d (J/ψ) ~ 2T c [e.g. H. Satz, Nucl.Phys.A738 (2007) 249c] χ cJ → J/ψ γ, J=0,1,2 J/ψ → l + l - Large-size excited charmonium state sensitive probe for nuclear and QGP effects

3. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 3 Physical motivations for studying χ c in HI collisions Account for feed-down contribution to the J/ψ suppression by the normal nuclear matter: S J/ψ = f J/ψ dir S J/ψ dir + f χ S χ + f ψ’ S ψ’ [e.g. L. Gerland et al., Phys.Rev.Lett.81 (1998) 762] Additional handle to the nonperturbative MEs of NRQCD

4. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 4 The Hera-B detector 10 m Magnet Muon Detector 4 layers of gas pixel chambers (inner) and MWPC with cathode pad readout (outer), pad-coincidence pretrigger 0 m20 m Top View 920 GeV/c Protons Inner / Outer Tracker 7 layers of GEM-MSGCs (inner) and honeycomb drift chambers (outer) Wire Target 2 stations, 4 wires each (C, Ti, W) Silicon Vertex Detector 8 double-layers of double-sided Si-microstrips RICH C 4 F 10 radiator, multianode PMT focal plane ECAL W/Pb-scintillator sampling shashlik calorimeter, cluster pretrigger e + /e - beam

5. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 5 kjhkjhkjhkjh HERA-B detector: data is read out and buffered for 12  s (proton bunches cross every 96 ns, 0.5 interactions/BX) PreTriggers: ECAL cluster or muon hit coincidence as trigger seed (custom hardware) 5 MHz First Level Trigger (FLT): Track trigger in hardware using tracking detectors, seeding by pretriggers 3 MHz Second Level Trigger (SLT): FLT tracking confirmed, extrapolation to vertex detector, 2 track vertex fit (PC farm) 20 kHz 100 Hz Fourth Level Trigger (4LT): online reconstruction (and filtering) on PC farm, ca. 1500 rec. J/ψ per h 100 Hz The Hera-B dilepton trigger

6. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 6 χ c measurement at Hera-B J/  → e + e - ~ 108,000 σ = 64MeV/c 2 //  J/  →  +  - ~ 177,000 σ=44MeV/c 2 ~3000  (2S) ~ 1600  (2S) Dilepton-trigger data sample N ~ 15.000 χ c (largest statistics in pA) Large photon detection efficiency, ε γ ~ 40% Both μ + μ - and e + e - channels Σ i=1,2 σ( χ ci )Br(χ ci →J/ ψγ ) R χc = σ(J/ψ) R 12 = σ( χ c1 )Br(χ c1 →J/ ψγ ) σ( χ c2 )Br(χ c2 →J/ ψγ )

7. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 7 Selection criteria J/ψ Selection - Track fit χ 2 probability > 0.3% - Kaon RICH likelihood L K < 90% - Channel-specific PID cuts  μ μ : muon likelihood L μ > 5%  ee (values depend on the presence of bremsstrahlung radiated before the magnet): - geometrical matching of the track and the ECAL cluster, - E/p ratio - Dilepton vertex prob. > 1% Reconstruction of χ c Mass difference ΔM=M(J/ψγ)-M(J/ψ) within mass window |M l+l- - 3.1| < 2σ N cells > 2, E cent /E clus > 0.8 (suppress MIPs and hadronic showers) E γ > 0.3 GeV (remove noise, ensure good energy resolution) E γ T > 0.2 GeV (suppress soft background photons) √(x 2 /4+y 2 ) > 22cm (mask out high occupancy region around beam pipe)

8. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 8 HERA-B χ c signal spectra ΔM=M(J/ψγ)-M(J/ψ) Reliable background subtraction is crucial !! μ+μ- channel e+e- channel N χc = 9630±550N χc = 5250±500 N 1 /N 2 = 1.2±0.2 N 1 /N 2 = 0.8±0.3

9. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 9 Background description  Main component of the background has combinatorial origin and is simulated by a mix-event method: Mix J/ψ’s and γ’s from events with same high energetic (E clus > 5 GeV) neutral cluster multiplicity, and similar projections on the XY plane of the total vector momentum of all clusters observed by the ECAL  Background from ψ(2S) → J/ψπ 0 π 0 in close vicinity of the χ c signal  Bckg. scale factor incorporated in the fit of the ΔM spectrum (accounts Procedure extensively tested with MC and real data Applicable for differential studies of χ c vs. x F and p T Minimizes distortions due to residual correlations Ensures energy-momentum conservation, accounts for detector acceptance and real occupancy, etc. Contribution to: ΔMΔM mix-events for correlation between bckg. normalization and the χ c peak parameters)

10. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 10 Problem: Possible solutions: use γ-conversions to measure the energy of the photon with the tracker (lack of statistics) significantly increase E γ T cut to improve energy resolution constrain 2-G fit parameters χ c1 - χ c2 separation M(χ c2 )-M(χ c1 ) ~ ( ) detector resolution Necessary for R 12, also important for R χc since ε γ (χ c1 )≠ε γ (χ c2 ), ε J/ψ (χ c1 )≠ε J/ψ (χ c2 ) The 2-Gaussian fit parameter χ c1 /χ c2 is most sensitive to the absolute ECAL energy calibration: δE γ ~ 2% => δM(χ c1 ) ~ 8 MeV/c 2 => δR 12 ~ 35%

11. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 11 HERA-B χ c Result ~(19 ± 3)% χcχc J/ψ ψ′ψ′ b (7.0±0.4)% (0.065±0.011)% σ J/ψ dir = σ J/ψ incl (1 - R χc - R ψ’ - R b ) ~ 73% of J/ψ’’s are produced directly R χc ≈ 0.19 ± 0.03 (stat.+syst.) R 12 ≈ 1.0 ± 0.4 (stat.+syst.) (excluding polarization systematics)  Results in both dilepton channels and various target materials compatible with each other  R χc lower than most published values  Additional systematic error due to possible polarization of J/ψ or χ c => increase R χc by as much as 20% and R 12 by 16% at 95%C.L. arXiv:0807.2167 [hep-ex]

12. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 12 Simulation of X c for CBM X c1,2 → J/ψ + γ and J/ψ → e + e - simulated by Pluto with thermodynamic spectrum, T = 170 MeV Assume ECAL energy resolution σ E /E ~ 4%/√E + 1%, spatial resolution σ XY ~ 5mm/√E + 1mm, apply mild cut on energy of the photon E γ > 0.2GeV ECAL geometrical acceptance |X| > 84cm, |Y| > 84cm, |X| < 222cm, |Y| < 474cm Black – generated Red - accepted X c p T X c rapidity

13. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 13 Electrons from J/ψ Cut on p T (lepton) > 1GeV/c Momentum resolution dp/p ~ 2% neglect any background under J/ψ Aim: estimate the combinatorial (J/ψ + background photons) contribution to the ∆M(X c ) spectrum Black – generated Red - accepted lepton momentum lepton p T J/ψ mass σ(M) ~ 43MeV

14. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 14 Photon from X c spectra [geometrical] ε γ ~60%, the higher E T from X c induce effective cut E > ~1GeV σ(∆M) resolution ~15MeV/c 2, fully dominated by σ E /E Black – generated Red - accepted Photon energy Photon E T Y vs X ∆M∆M

15. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 15 J/ψ + background photons J/ψ generated by Pluto with thermodynamic spectrum, T=170MeV Background photons taken from UrQMD central AuAu 25AGeV Black – generated Red - accepted E > 0.2 Green – accepted E > 0.5 Blue - accepted E > 1.0 Cyan - accepted E > 1.5 Photon Energy Photon E T N photons per event Combinatorial background For further analysis select E γ > 1GeV

16. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 16 Extrapolation to ~10 6 J/ψ background simulated for 1M registered J/ψ ∆M for 200k generated Xc decays In principle, increasing statistics the signal extraction by background subtraction becomes possible

17. D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008 17 Conclusions Further we plan to carry out a full-MC feasibility study of χ c production measurement at CBM Production of excited charmonium states in HI collisions provide information complementary to J/ψ, important both in the QGP search and QCD studies of the standard nuclear effects The newest precise measurement of R χc was obtained by Hera-B in 920 GeV pA interactions. Hera-B has collected useful experience of χ c measurement at relatively high combinatorial background. Existing theoretical calculations of J/ψ suppression in HI collisions have to rely on the knowledge of fractions of fed-down J/ψ measured at different conditions in proton- and pion-induced interactions A generator MC study tells R xc measurement might be possible with CBM (by increasing statistics or in periferal or lighter ion collisions)