Measurement of χc2 production in two-photon collisions

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Abstract

The production of the χc2 charmonium state in two-photon collisions has been measured with the Belle detector at the KEKB e+e collider. A clear signal for χc2γJ/ψ, J/ψ → ℓ+ is observed in a 32.6 fb−1 data sample accumulated at center-of-mass energies near 10.6 GeV, and the product of its two-photon decay width and branching fraction is determined to be Γγγ(χc2)B(χc2γJ/ψ)B(J/ψl+l)=13.5±1.3(stat.)±1.1(syst.)eV.

Introduction

The two-photon decay widths (Γγγ) of the even charge-parity charmonium states provide valuable information for testing models describing the nature of heavy quarkonia. Various theoretical calculations describing the quark–antiquark system predict the value of Γγγ(χc2) to be within the range 0.2–0.8 keV [1]. A precise experimental determination of Γγγ(χc2) will provide a strong constraint on these models. The ratio of the two-gluon decay width to the two-photon decay width Γgg(χc2)/Γγγ(χc2) has been calculated within the framework of perturbative QCD with first-order correction [2] and the result gives Γγγ(χc2) = 0.47 ± 0.13 keV.2 A comparison of Γγγ(χc2) with the two-gluon width will provide a way to study the validity of perturbative QCD corrections for quarkonium decays.

Measurements of two-photon decay widths for charmonium states are difficult because of their small production cross section and small detection efficiencies. To date, several experiments have reported the observation of two-photon production of the χc2 in the decay channel χc2γJ/ψ, J/ψ → ℓ+, where ℓ = e or μ [5]. This channel is suitable for the experimental determination of Γγγ(χc2), since the decay branching fraction is known with a relatively small error, B(χc2γJ/ψ)B(J/ψl+l)=(1.59±0.13)% for both lepton families [3]. However, the two-photon decay width results obtained from previous measurements with this process seem to be systematically larger than those from pp¯χc2γγ experiments [6]. Further studies with high statistics data samples are needed to clarify the situation.

Recently, CLEO Collaboration has reported a measurement of Γγγ(χc2) in the π+ππ+π final state [7]. Although the branching fraction of this decay mode, B(χc2π+ππ+π)=(1.2±0.5)% [3], is comparable to that of the γJ/ψ → ℓ+γ mode, its large error precludes a precise determination of Γγγ(χc2).

We have measured χc2 production in two-photon processes using the decay channel χc2γJ/ψ, J/ψ → ℓ+. The results are based on a 32.6 fb−1 data sample collected with the Belle detector.

Section snippets

Experimental data and detector system

The experiment was performed with the Belle detector [8] at the asymmetric e+e collider KEKB, where an 8.0 GeV e beam collides with a 3.5 GeV e+ beam with a crossing angle of 22 mrad. We use a 29.6 fb−1 sample of data collected at the c.m. energy corresponding to the peak of the ϒ(4S) resonance (10.58 GeV) and a 3.0 fb−1 sample collected 60 MeV below the peak.

The basic topology of events that we select is two tracks of opposite charge and a photon. The recoiling e+ and e are not tagged in

Event selection

The event selection criteria are as follows. (1) Exactly two oppositely charged tracks reconstructed by the CDC, where both tracks satisfy the following laboratory frame conditions: −0.47⩽cosθ⩽+0.82, where θ is the polar angle; pt ⩾ 0.4 GeV/c; |dr|⩽1 cm, |dz|⩽3 cm, where (|dr|,dz) are the cylindrical coordinates of the track's point of closest approach to the nominal collision point in the plane; |Δdz|⩽1 cm, where Δdz is the difference between the dz's of the two tracks; and no other well

Derivation of the number of signal events

Fig. 2 shows a scatter plot of the invariant mass of the two tracks (M+−) versus the invariant-mass difference ΔM = M+−γM+− for the selected events, where M+−γ is the invariant mass of all three particles. A clear concentration is observed around M+− = 3.097 GeV/c2 and ΔM = 0.459 GeV/c2, the signal region for χc2γJ/ψ.

The mass difference distribution is shown in Fig. 3(a) for the events falling within the J/ψ signal mass region 3.06 ⩽ M+− ⩽ 3.13 GeV/c2. After the final selection requirement

Monte Carlo calculations

We used Monte Carlo (MC) simulated e+ee+eχc2, χc2γJ/ψ, J/ψ → ℓ+ (ℓ = e or μ) events to calculate the efficiency for the signal process. The TREPS MC program [10] is used for the event generation. The effects of J/ψ radiative decays are modeled with the PHOTOS [11] simulation code, which generates photon radiation from a final-state lepton generated by TREPS with a probability determined by a QED calculation. All of the final-state particles in the MC events are processed by the full

Results and discussion

The two-photon decay width of the χc2 is related to the signal event yield as YieldLdt=20π2Lγγ(mχc2)η(c/)2mχc22Γγγ(χc2)×B(χc2γJ/ψ)B(J/ψl+l)=(0.309fb/eV)Γγγ(χc2)×B(χc2γJ/ψ)B(J/ψl+l),where Ldt is the integrated luminosity, η is the efficiency, mχc2(= 3.556 GeV/c2) is the χc2 mass and Lγγ(mχc2)(=7.75 × 10−4 GeV−1) is the two-photon luminosity function at the χc2 mass. The total width of χc2 (2.00 ± 0.18 MeV [3]) is much smaller than the present ΔM resolution (∼9 MeV), and does not

Conclusion

We have measured χc2 production from two-photon collisions with a 32.6 fb−1 data sample collected with the Belle detector at the KEKB e+e collider, using the decay mode χc2γJ/ψ, J/ψ → ℓ+. We find 136.0 ± 13.3 signal events after background subtraction. The observed polar-angle distributions of the photon and leptons are consistent with those expected from the production of χc2 in the pure helicity 2 state. The product of the two-photon decay width of χc2 and branching fractions, Γγγ(χc2)B(

Acknowledgements

We wish to thank the KEKB Accelerator Group for the excellent operation of the KEKB accelerator. We acknowledge support from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and the Japan Society for the Promotion of Science; the Australian Research Council and the Australian Department of Industry, Science and Resources; the National Science Foundation of China under contract No. 10175071; the Department of Science and Technology of India; the BK21 program of the

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1

On leave from Nova Gorica Polytechnic, Slovenia.

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