David D'Enterria

The physics of ultraperipheral collisions at the LHC

Abstract We discuss the physics of large impact parameter interactions at the LHC: ultraperipheral collisions (UPCs). The dominant processes in UPCs are photon–nucleon (nucleus) interactions. The current LHC detector configurations can explore hard phenomena at small x with nuclei and nucleons at photon–nucleon center-of-mass energies above 1 TeV, extending the x range of HERA by a factor of ten. In particular, it will be possible to probe diffractive and inclusive parton densities in nuclei using several processes. The interaction of small dipoles with protons and nuclei can be investigated in elastic and quasi-elastic J / ψ and Υ production as well as in high t ρ 0 production accompanied by a rapidity gap. Several of these phenomena provide clean signatures of the onset of the new high gluon density QCD regime. The LHC is in the kinematic range where nonlinear effects are several times larger than those at HERA. Two-photon processes in UPCs are also studied. In addition, while UPCs play a role in limiting the maximum beam luminosity, they can also be used as a luminosity monitor by measuring mutual electromagnetic dissociation of the beam nuclei. We also review similar studies at HERA and RHIC as well as describe the potential use of the LHC detectors for UPC measurements.

Quantitative constraints on the gluon distribution function in the proton from collider isolated-photon data

The impact of isolated-photon data from proton-(anti)proton collisions at RHIC, SppS, Tevatron and LHC energies, on the parton distribution functions of the proton is studied using a recently developed Bayesian reweighting method. The impact on the gluon density of the 35 existing isolated- measurements is quantified using next-to-leading order (NLO) perturbative QCD cal- culations complemented with the NNPDF2.1 parton densities. The NLO predictions are found to describe well most of the datasets from 200 GeV up to 7 TeV centre-of-mass energies. The isolated-photon spectra recently measured at the LHC are precise enough to constrain the gluon distribution and lead to a moderate reduction (up to 20%) of its uncertainties around fractional momenta x ≈ 0.02. As a particular case, we show that the improved gluon density reduces the PDF uncertainty for the Higgs boson production cross section in the gluon-fusion channel by more than 20% at the LHC. We conclude that present and future isolated-photon measurements constitute an interesting addition to coming global PDF analyses.

Observing light-by-light scattering at the large hadron collider

Elastic light-by-light scattering (γγ→γγ) is open to study at the Large Hadron Collider thanks to the large quasireal photon fluxes available in electromagnetic interactions of protons (p) and lead (Pb) ions. The γγ→γγ cross sections for diphoton masses m(γγ)>5 GeV amount to 12 fb, 26 pb, and 35 nb in p-p, p-Pb, and Pb-Pb collisions at nucleon-nucleon center-of-mass energies √(s(NN))=14, 8.8, and 5.5 TeV, respectively. Such a measurement has no substantial background in Pb-Pb collisions where one expects about 20 signal events per run, after typical detector acceptance and reconstruction efficiency selections.

Physics at a 100 TeV pp collider: Standard Model processes

This chapter documents the production rates and typical distributions for a number of benchmark Standard Model processes, and discusses new dynamical phenomena arising at the highest energies available at this collider. We discuss the intrinsic physics interest in the measurement of these Standard Model processes, as well as their role as backgrounds for New Physics searches.This chapter documents the production rates and typical distributions for a number of benchmark Standard Model processes, and discusses new dynamical phenomena arising at the highest energies available at this collider. We discuss the intrinsic physics interest in the measurement of these Standard Model processes, as well as their role as backgrounds for New Physics searches.This chapter documents the production rates and typical distributions for a number of benchmark Standard Model processes, and discusses new dynamical phenomena arising at the highest energies available at this collider. We discuss the intrinsic physics interest in the measurement of these Standard Model processes, as well as their role as backgrounds for New Physics searches.

Indications of suppressed high pT hadron production in nucleus-nucleus collisions at CERN-SPS

Abstract Inclusive pion production at high transverse momenta ( p T ≳ 2 GeV / c ) in nucleus–nucleus ( A + A ) collisions at CERN-SPS ( s NN ≈ 20 GeV ) is revisited and systematically compared to all existing proton–proton data in the same range of center-of-mass energies. The ratio of A + A to p + p pion cross-sections (nuclear modification factor) for central Pb + Pb, Pb + Au and S + Au reactions does not show a strong enhancement as a function of p T as previously found, but is consistent with scaling with the number of nucleon–nucleon (NN) collisions. Neutral pion yields per NN collision in head-on Pb + Pb reactions are suppressed, whereas peripheral yields are enhanced. These results together indicate that some amount of “jet quenching” may already be present in central heavy-ion reactions at s NN ≈ 20 GeV .

Confronting current NLO parton fragmentation functions with inclusive charged-particle spectra at hadron colliders

Abstract The inclusive spectra of charged particles measured at high transverse momenta ( p T ≳ 2 GeV / c ) in proton–proton and proton–antiproton collisions in the range of center-of-mass energies s = 200 – 7000 GeV are compared with next-to-leading order perturbative QCD calculations using seven recent sets of parton-to-hadron fragmentation functions (FFs). Accounting for the uncertainties in the scale choices and in the parton distribution functions, we find that most of the theoretical predictions tend to overpredict the measured LHC and Tevatron cross sections by up to a factor of two. We identify the currently too-hard gluon-to-hadron FFs as the probable source of the problem, and justify the need to refit the FFs using the available LHC and Tevatron data in a region of transverse momenta, p T ≳ 10 GeV / c , which is supposedly free from additional non-perturbative contributions and where the scale uncertainty is only modest.

Parton Propagation and Fragmentation in QCD Matter

We review recent progress in the study of parton propagation, interaction and fragmentation in both cold and hot strongly interacting matter. Experimental highlights on high-energy hadron production in deep inelastic lepton-nucleus scattering, proton-nucleus and heavy-ion collisions, as well as Drell-Yan processes in hadron-nucleus collisions are presented. The existing theoretical frameworks for describing the in-medium interaction of energetic partons and the space-time evolution of their fragmentation into hadrons are discussed and confronted to experimental data. We conclude with a list of theoretical and experimental open issues, and a brief description of future relevant experiments and facilities.

Quark–gluon matter

A concise review of the experimental and phenomenological progress in high-energy heavy-ion physics over the past few years is presented. Emphasis is put on measurements at BNL-RHIC and CERN-SPS which provide information on fundamental properties of QCD matter at extreme values of temperature, density and low-x. The new opportunities accessible at the LHC, which may help clarify some of the current open issues, are also outlined.

High \(p_{\rm T}\) leading hadron suppression in nuclear collisions at \(\sqrt{s_{NN}}\approx\) 20-200 GeV: data versus parton energy loss models

Abstract.Experimental results on high transverse momentum (leading) hadron spectra in nucleus-nucleus collisions in the range

Heavy ions at the Future Circular Collider

\sqrt{s_{NN}}\approx