Florian Jonas

This report presents the technical design of the ALICE Forward Calorimeter (FoCal). FoCal is an upgrade of the ALICE experiment at the LHC, to be installed during Long Shutdown 3 for data-taking in the period 2029-2032. FoCal consists of a highly granular Si+W electromagnetic calorimeter combined with a Cu+scintillating-fiber hadronic calorimeter, covering pseudo-rapidity 3.2 < η < 5.8.FoCal has unique capabilities to measure direct photon production at forward rapidity, which probes the gluon distribution in protons and nuclei at small-x, and is theoretically calculable at high precision. FoCal will carry out inclusive and correlation measurements of photons, neutral mesons, and jets in hadronic pp and p-Pb collisions, as well as J/ψ production in ultra-peripheral p-Pb and Pb-Pb collisions. FoCal extends significantly the scope of the ALICE physics program to explore the dynamics of hadronic matter and the nature of QCD evolution at small x, down to x 10-6.

Recent data on the nuclear modification of W and Z boson production measured by the ATLAS collaboration in PbPb collisions at \(}sqrt{s_{}rm nn}}=5.02\)TeV show an enhancement in peripheral collisions, seemingly contradicting predictions of the Glauber model. The data were previously explained by arguing that the nucleon-nucleon cross section may be shadowed in nucleus-nucleus collisions, and hence suppressed compared to the proton-proton cross section at the same collision energy. This interpretation has quite significant consequences for the understanding of heavy-ion data, in particular in the context of the Glauber model. Instead, we provide an alternative explanation of the data by assuming that there is a mild bias present in the centrality determination of the measurement; on the size of the related systematic uncertainty. Using this assumption, we show that the data is in agreement with theoretical calculations using nuclear parton distribution functions. Finally, we speculate that the centrality dependence of the W^-/W^{+} ratio may point to the relevance of a larger skin thickness of the Pb nucleus, which, if present, would result in a few percent larger PbPb cross section than currently accounted for in the Glauber model and may hence be the root of the centrality bias.

The performance of the electromagnetic calorimeter of the ALICE experiment during operation in 2010-2018 at the Large Hadron Collider is presented. After a short introduction into the design, readout, and trigger capabilities of the detector, the procedures for data taking, reconstruction, and validation are explained. The methods used for the calibration and various derived corrections are presented in detail. Subsequently, the capabilities of the calorimeter to reconstruct and measure photons, light mesons, electrons and jets are discussed. The performance of the calorimeter is illustrated mainly with data obtained with test beams at the Proton Synchrotron and Super Proton Synchrotron or in proton-proton collisions at }sqrt{s}=13 TeV, and compared to simulations.