Prospects to study the χ c production at CBM We give preliminary considerations to the feasibility...
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Transcript of Prospects to study the χ c production at CBM We give preliminary considerations to the feasibility...
Prospects to study theχc production at CBM
We give preliminary considerations to the feasibilityof χc 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 ZaitsevITEP, Moscow
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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Physical motivations forstudying χc in HI collisions
Additional charmonium probe and a “thermometer” for QGP
dissociation temperature: Td(χ) ~ 1.1Tc, Td(J/ψ) ~ 2Tc
[e.g. H. Satz, Nucl.Phys.A738 (2007) 249c]
χcJ → J/ψ γ, J=0,1,2 J/ψ → l+l-
Large-size excited charmonium statesensitive probe for nuclear and QGP effects
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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Physical motivations forstudying χc in HI collisions
• Account for feed-down contribution to the J/ψ suppression by the normal
nuclear matter: SJ/ψ = fJ/ψ
dirSJ/ψdir + fχSχ + fψ’Sψ’
[e.g. L. Gerland et al., Phys.Rev.Lett.81 (1998) 762]
• Additional handle to the nonperturbative MEs of NRQCD
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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The Hera-B detector
10 m
Magnet
Muon Detector4 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 Tracker7 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
RICHC4F10 radiator,
multianode PMT focal plane
ECALW/Pb-scintillator
sampling shashlik calorimeter, cluster
pretrigger
e+/e- beam
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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• 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
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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χc measurement at Hera-B
J/→e+e-
~ 108,000
σ = 64MeV/c2
/
J/ → +-
~ 177,000 σ=44MeV/c2
~3000(2S)
~ 1600 (2S)
Dilepton-trigger data sample
• N ~ 15.000 χc
(largest statistics in pA)• Large photon detection efficiency, εγ ~ 40%• Both μ+μ- and e+e- channels
Σi=1,2σ(χci)Br(χci→J/ψγ)Rχc = σ(J/ψ)
R12=σ(χc1)Br(χc1→J/ψγ)σ(χc2)Br(χc2→J/ψγ)
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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Selection criteriaJ/ψ Selection
- Track fit χ2 probability > 0.3%- Kaon RICH likelihood LK < 90%- Channel-specific PID cuts
μμ: muon likelihood Lμ > 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 χc
• Mass difference ΔM=M(J/ψγ)-M(J/ψ) within mass window |Ml+l- - 3.1| < 2σ• Ncells > 2, Ecent/Eclus > 0.8 (suppress MIPs and hadronic showers)• Eγ > 0.3 GeV (remove noise, ensure good energy resolution)• Eγ
T > 0.2 GeV (suppress soft background photons)
• √(x2/4+y2) > 22cm (mask out high occupancy region around beam pipe)
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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HERA-B χc signal spectraΔM=M(J/ψγ)-M(J/ψ)
Reliable background subtraction is crucial !!
μ+μ-channel
e+e-channel
Nχc = 9630±550 Nχc = 5250±500
N1/N2 = 1.2±0.2 N1/N2 = 0.8±0.3
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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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 (Eclus > 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/ψπ0π0 in close vicinity of the χc 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 χc vs. xF and pT • Minimizes distortions due to residual correlations• Ensures energy-momentum conservation, accounts for detector acceptance and real occupancy, etc.
Contribution to:ΔM
mix-events
for correlation between bckg. normalization and the χc peak parameters)
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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Problem:
Possible solutions:• use γ-conversions to measure the energy of the photon with the tracker (lack of statistics)• significantly increase Eγ
T cut to improve energy resolution• constrain 2-G fit parameters
χc1 - χc2 separation
background: mixed events
after background subtraction
en
trie
s/(1
MeV
/c2)
m(μ+μ-γ)-m(μ+μ-) [GeV/c2]
ET(γ) > 0.4 GeV
χc2χc1
M(χc2)-M(χc1) ~ ( )detector resolution
Necessary for R12,also important for Rχc since
εγ(χc1)≠εγ(χc2), εJ/ψ(χc1)≠εJ/ψ(χc2)
The 2-Gaussian fit parameter χc1/χc2 is most sensitive to the absolute ECALenergy calibration: δEγ ~ 2% => δM(χc1) ~ 8 MeV/c2 => δR12 ~ 35%
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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HERA-B χc Result
~(19 ± 3)%
χc
J/ψψ′
J/ψb
J/ψ
(7.0±0.4)% (0.065±0.011)%
σJ/ψdir = σJ/ψ
incl (1 - Rχc - Rψ’ - Rb )
~ 73% of J/ψ’’s are produced directly
Rχc ≈ 0.19 ± 0.03 (stat.+syst.)
R12 ≈ 1.0 ± 0.4 (stat.+syst.)(excluding polarization systematics)
Results in both dilepton channels and various target materials compatible with each other
Rχc lower than most published values
Additional systematic error due to possible polarization of J/ψ or χc
=> increase Rχc by as much as 20% and R12 by 16% at 95%C.L.
arXiv:0807.2167 [hep-ex]
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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Simulation of Xc for CBM
Xc1,2 → J/ψ + γ and J/ψ → e+e-
simulated by Pluto with thermodynamic spectrum, T = 170 MeV
Assume ECAL energy resolution σE/E ~ 4%/√E + 1%, spatial resolution σXY ~ 5mm/√E + 1mm,apply mild cut on energy of the photon Eγ > 0.2GeVECAL geometrical acceptance |X| > 84cm, |Y| > 84cm, |X| < 222cm, |Y| < 474cm
Black – generatedRed - accepted
Xc pT Xc rapidity
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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Electrons from J/ψ
Cut on pT(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(Xc) spectrum
Black – generatedRed - accepted
lepton momentum lepton pT
J/ψ mass
σ(M) ~ 43MeV
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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Photon from Xc spectra
[geometrical] εγ ~60%, the higher ET from Xc induce effective cut E > ~1GeVσ(∆M) resolution ~15MeV/c2 , fully dominated by σE/E
Black – generatedRed - accepted
Photon energy Photon ET
Y vs X ∆M
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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J/ψ + background photons
J/ψ generated by Pluto with thermodynamic spectrum, T=170MeVBackground photons taken from UrQMD central AuAu 25AGeV
Black – generatedRed - accepted E > 0.2Green – accepted E > 0.5Blue - accepted E > 1.0Cyan - accepted E > 1.5
Photon Energy Photon ET
N photons per event Combinatorial background
For further analysisselect Eγ > 1GeV
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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Extrapolation to ~106J/ψbackground simulated for 1M registered J/ψ
∆M for 200k generatedXc decays
In principle, increasing statistics the signal extraction by background subtraction becomes possible
D. Goloubkov, Yu. Zaitsev, CBM Meeting Dubna, 14 October 2008
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Conclusions
Further we plan to carry out a full-MC feasibility study of χc 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 Rχc was obtained by Hera-B in 920 GeV pA interactions. Hera-B has collected useful
experience of χc 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 Rxc measurement might be possible with CBM (by increasing statistics or in periferal or lighter ion collisions)