No Arabic abstract
The low-x gluon density in the proton and, in particular, in nuclei is only very poorly constrained, while a better understanding of the low-x structure is crucial for measurements at the LHC and also for the planning of experiments at future hadron colliders. In addition, deviations from linear QCD evolution are expected to appear at low x, potentially leading to gluon saturation and a universal state of hadronic matter, the color-glass condensate. However, these effects have not been unambiguously proven to date. Fortunately, data from the LHC can be used directly to provide better constraints of the parton distribution functions (PDFs). In this context, a Forward Calorimeter (FoCal) is proposed as an addition to the ALICE experiment, to be installed in the Long Shutdown 3. The main goal of the FoCal proposal is to measure forward direct photons in pp and p-Pb collisions to obtain experimental constraints on proton and nuclear PDFs in a new region of low x. Based on the current knowledge from DIS experiments and first results from LHC, we will discuss the physics case for this proposed detector. While open charm measurements do provide important constraints, a photon measurement would provide additional unique information. The direct photon measurement requires a new electromagnetic calorimeter with extremely high granularity. The corresponding innovative design principle of a high-resolution Si-W sandwich calorimeter is discussed.
Probes for the small-x parton densities and predicted effects of gluon saturation are discussed. At very low x and intermediate Q, only results on hadronic observables at the LHC are available, which do not provide unambiguous information. It is shown that the measurement of direct photons at forward rapidity at the LHC is particularly promising to provide a unique signal. We further discuss the possibilities to perform such measurements with a detector upgrade in the ALICE experiment and present the R&D activities ongoing.
We present a rigorous theoretical analysis of the ALICE measurement of low-p_T direct-photon production in central lead-lead collisions at the LHC with a centre-of-mass energy of sqrt{s_{NN}}=2.76 TeV. Using NLO QCD, we compute the relative contributions to prompt-photon production from different initial and final states and the theoretical uncertainties coming from independent variations of the renormalisation and factorisation scales, the nuclear parton densities and the fragmentation functions. Based on different fits to the unsubtracted and prompt-photon subtracted ALICE data, we consistently find T = 304 pm 58 MeV and 309 pm 64 MeV for the effective temperature of the quark-gluon plasma (or hot medium) at p_T in [0.8;2.2] GeV and p_T in [1.5;3.5] GeV as well as a power-law (p_T^{-4}) behavior for p_T > 4 GeV as predicted by QCD hard scattering.
We discuss the latest results from jet fragmentation and jet substructure measurements performed with the ALICE experiment in proton-proton and heavy-ion collisions in a wide range of jet transverse momentum. The jet production cross sections and cross section ratios for different jet resolution parameters will be shown in a wide range of $p_{textrm{T}}$. Results will be compared to next-to-leading order pQCD calculations.
Sterile neutrino ($ u_s$) conversion in meter scale baselines can be sensitively probed using mono-energetic, sub-MeV, flavor pure $ u_e$s from an artificial MCi source and the unique technology of the LENS low energy solar $ u_e$ detector. Active-sterile {em oscillations} can be directly observed in the granular LENS detector itself to critically test and extend results of short baseline accelerator and reactor experiments.
A review is given on photon-hadron and photon-photon collisions in the ALICE experiment. The physics motivation for studying such reactions is outlined, and the results obtained in proton-lead and lead-lead collisions in Run 1 of the LHC are discussed. The improvement in detector rapidity coverage due to a newly added detector system is presented. The ALICE perspectives for data taking in LHC Run II are summarised.