Abstract
We have performed the first measurement of the coherent ψ(2S) photo-production cross section in ultra-peripheral PbPb collisions at the LHC. This charmonium excited state is reconstructed via the ψ(2S)→l+l− and ψ(2S)→J/ψπ+π− decays, where the J/ψ decays into two leptons. The analysis is based on an event sample corresponding to an integrated luminosity of about 22 μb−1. The cross section for coherent ψ(2S) production in the rapidity interval −0.9<y<0.9 is dσψ(2S)coh/dy=0.83±0.19(stat+syst) mb. The ψ(2S) to J/ψ coherent cross section ratio is 0.34−0.07+0.08(stat+syst). The obtained results are compared to predictions from theoretical models.
Highlights
Two-photon and photo-nuclear interactions at unprecedented energies can be studied in heavy-ion Ultra-Peripheral Collisions (UPC) at the LHC
We published the first results on the coherent photo-production of J/ψ in UPC Pb–Pb collisions at the LHC [13] in the rapidity region −3.6 < y < −2.6, which constrain the nuclear gluon distribution at Bjorken-x 10−2
The event sample considered for the present analysis was collected during the 2011 Pb–Pb run, using a dedicated Barrel UltraPeripheral Collision trigger (BUPC), selecting events with the following characteristics: (i) at least two hits in the Silicon Pixel Detector (SPD) detector; (ii) a number of fired pad-OR (Non) in the TOF detector [25] in the range 2 ≤ Non ≤ 6, with at least two of them with a difference in azimuth, φ, in the range 150◦ ≤ φ ≤ 180◦; (iii) no hits in the V0-A and no hits in the V0-C detectors
Summary
Two-photon and photo-nuclear interactions at unprecedented energies can be studied in heavy-ion Ultra-Peripheral Collisions (UPC) at the LHC. Incoherent production is characterized by a somewhat higher transverse momentum of the vector mesons (pT 500 MeV/c) In this case the nucleus interacting with the photon breaks up but, apart from single nucleons or nuclear fragments in the very forward region, no other particles are produced besides the vector meson. We published the first results on the coherent photo-production of J/ψ in UPC Pb–Pb collisions at the LHC [13] in the rapidity region −3.6 < y < −2.6, which constrain the nuclear gluon distribution at Bjorken-x 10−2. ALICE published a second paper measuring both the coherent and the incoherent J/ψ vector meson cross section at mid-rapidity [14], allowing the nuclear gluon distribution at Bjorken-x 10−3 to be explored. S√) smNNes=on2s.76atTemVida-rreappirdeistyenitnedu. lTtrhae-pmereiapshuerreadl coherent ψ(2S) cross section and the ψ(2S)/J/ψ cross section ratio are compared to model predictions [17,18,19,20,21,22]
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