Prospects to study the χ c production at CBM We give preliminary considerations to the feasibility...

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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

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)