Recent results on ultra-peripheral Pb-Pb and p-Pb collisions at the ALICE experiment

In this paper, we will discuss results on coherent and incoherent J/ψ photoproduction in ultra-peripheral Pb-Pb collisions at √ sNN = 2.76, as well as preliminary results on exclusive J/ψ and two-photon production in ultra-peripheral p-Pb collisions at √ sNN = 5.02 TeV. Results on four-pion production in ultra-peripheral Pb-Pb collisions will also be given. Finally, the prospects for other UPC measurements in ALICE will be briefly mentioned. Introduction The LHC is not only the most powerful collider for proton-proton and heavy-ion collisions, but also for photon-photon and photon-hadron (γp and γPb) interactions, offering a unique opportunity to study fundamental aspects of QED and QCD via photon-induced processes. In ultra-peripheral collisions (UPC), two ions (or protons) pass by each other with an impact parameter at least two times larger than the nuclear radii, and here the hadronic interactions are strongly suppressed. The number of photons scales like Z2 for a single source, so that in heavy-ion collisions exclusive particle production is dominated by electromagnetic processes. Typical examples are γγ → l+l− (where l+l− = e+e−, μ+μ−, τ+τ−) or exclusive γ + p → J/ψ + p, which is modeled in pQCD by the exchange of two-gluons with no net-color transfer. The experimental challenge for such measurements consists in having dedicated UPC triggers that are often orthogonal to the general trigger strategy of the experiments. Moreover, validating an exclusive analysis requires a good understanding of the trigger efficiency for the exclusivity conditions imposed at the online and offline levels, for which control triggers are usually required. 1. The ALICE experiment The ALICE experiment [1] is well suited to study UPC vector meson production in a systematic way. The UPC triggers used the VZERO forward detector for vetoing, and the muon spectrometer, Time-of-Flight and Silicon Pixel Detector as triggers. The muon spectrometer covers the range −4.0 < y < −2.5, while a central barrel placed inside a large solenoid magnet (0.5 T) covers the pseudorapidity range |η| < 0.9. The pseudorapidity range 2.8 < η < 5.1 is covered by VZERO-A and −3.7 < η < −1.7 by VZERO-C, which are scintillator tile arrays with a time resolution better than 1 ns allowing us to distinguish between beam-beam and beam-gas collisions. For the J/ψ analysis at mid-rapidity the TPC provides particle ID information. This is complemented with information provided by other detectors at the offline level, including the 14th International Conference on Strangeness in Quark Matter (SQM2013) IOP Publishing Journal of Physics: Conference Series 509 (2014) 012107 doi:10.1088/1742-6596/509/1/012107 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 ZDCs, which are located at about 116 m at both sides from the IP, and allow us to study both neutrons and protons emitted in the very forward region. Additional details on the way in which these detectors were used for these analyses are given in [2, 3] 2. Coherent and incoherent J/ψ production in UPC Pb-Pb The first LHC measurements on exclusive photoproduction of J/ψ vector mesons produced in Pb-Pb collisions at √ sNN = 2.76 TeV were carried out by ALICE [2, 3]. This measurement is particularly interesting as the UPC J/ψ cross section depends on the nuclear gluon distribution squared [5], making it a very good probe to study nuclear shadowing at low Bjorken-x values. At mid-rapidity J/ψs have a Bjorken-x value of about 10−3. One of ALICE’s advantage is that J/ψs can be measured down to zero pT , which is important here as UPC J/ψs are characterized by their very low pT . Coherent J/ψ production, where the photon couples coherently to all nucleons, is characterized by an average J/ψ pT of about 60 MeV/c. For these events the target nucleus usually stays intact. For the incoherent case, the photon couples to a single nucleon, representing the quasi-elastic scattering off a single nucleon and the J/ψ has a somewhat larger average pT of about 500 MeV/c. The pT distributions for both coherent-enhanced and incoherent-enhanced data samples are well described by fitting together the expected signals and backgrounds using templates built from MC and data. This was used to extract the measured yield of coherent and incoherent J/ψ candidates. The measured cross sections are dσ J/ψ Coh/dy = 1.00 ± 0.18 (stat) +0.24 −0.26 (syst) mb at −3.6 < y < −2.6 and dσ Coh/dy = 2.38 +0.34 −0.24 (stat+syst) mb at −0.9 < y < 0.9. These results were compared to the available model calculations as described in [2, 3]. Best agreement is found with models that include nuclear gluon shadowing, consistent with EPS09 (see Figure 1). Recent model calculations provide ways on how these measurements can be used to extract information on the nuclear gluon density [4]. Furthermore, the incoherent J/ψ cross section was also recently published [3] and provides additional constraints to model calculations. y -4 -2 0 2 4 /d y ( m b )