Charmonium and e + e − pair photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76\ \mbox{TeV}$

Abstract

The ALICE Collaboration at the LHC has measured the J/ψ and ψ′ photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at .The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 μb−1. The cross section for coherent and incoherent J/ψ production in the rapidity interval −0.9<y<0.9, are and , respectively. The results are compared to theoretical models for J/ψ production and the coherent cross section is found to be in good agreement with those models incorporating moderate nuclear gluon shadowing at Bjorken-x around 10−3, such as EPS09 parametrization. In addition the cross section for the process γγ→e+e− has been measured and found to be in agreement with models implementing QED at leading order.

Searching for Axionlike Particles with Ultraperipheral Heavy-Ion Collisions

“…An important reducible background could come from e + e − pair production [37], where both the electron and positron are misidentified as photons. The leading order, fiducial cross section for this process (right-hand panel in Fig.…”

Photoproduction of J/ψin Pb–Pb and p–Pb collisions at the LHC with the ALICE detector

“…The ALICE Collaboration has recently published two papers on the photoproduction of J/ψ in UPC of Pb-Pb using data taken during the 2011 run [9,10]. The first paper covers the measurement performed with the muon spectrometer and the second the measurement performed with the central barrel.…”

“…Figure 2b [10] shows the comparison between the measurement of ALICE and the three predictions. The predictions differ in almost an order of magnitude while the measurement has an uncertainty of the order of 20%.…”

“…Figure 3 [10] shows the ALICE measurement in two ranges of the invariant mass of the electron-positron pair: [2.2,2.6] GeV/c 2 and [3.7,10] GeV/c 2 . Note that the tail from bremsstrahlung is negligible in the first mass range.…”

WG2 Highlights - Small x, Diffraction and Vector Mesons

“…The result was interpreted in terms of Anomalous Quartic Gauge Couplings, with the obtained limits exceeding those of LEP by a factor of ∼100 with a form factor of Λ cutoff =500 GeV. Finally, J/ψ photoproduction in ultraperipheral heavy ion collisions (UPC) was studied by ALICE [10]. Large signals were observed in the J/ψ → µ µ and J/ψ → ee channels, allowing a differential measurement of cross sections vs. rapidity dσ /dy (Fig.…”