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Ultra-peripheral collisions of heavy ions and protons are the energy frontier for electromagnetic interactions. Both photonuclear and two-photon collisions are studied, at collision energies that are far higher than are available elsewhere. In this review, we will discuss physics topics that can be addressed with UPCs, including nuclear shadowing and nuclear structure and searches for beyond-standard-model physics.
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Calibration of the absolute energy scale at high-energy photon (gamma-gamma, gamma-electron) colliders is discussed. The luminosity spectrum at photon colliders is broad and has a rather sharp high-energy edge, which can be used, for example, to measure the mass of the Higgs boson in the process gamma-gamma to H or masses of charged scalars by observing the cross-section threshold. In addition to the precise knowledge of the edge energy of the luminosity spectrum, it is even more important to have a way to calibrate the absolute energy scale of the detector. At first sight, Compton scattering itself provides a unique way to determine the beam energies and produce particles of known energies that could be used for detector calibration. The energy scale is given by the electron mass m_e and laser photon energy omega_0. However, this does not work at realistic photon colliders due to large nonlinear effects in Compton scattering at the conversion region (xi^2 sim 0.3). It is argued that the process gamma-electron to eZ_0 provides the best way to calibrate the energy scale of the detector, where the energy scale is given by m_Z.
Two-photon events at the LHC are characterized by the protons scattered at very small angles and the particles centrally produced via the photon-photon fusion. To select these events from the huge samples of generic pp interactions a detection of the scattered protons, or tagging two-photon interactions is necessary. It requires installation of the high-resolution position-sensitive detectors close to the proton beam and far from the interaction point. Efficient measurement of the forward-scattered protons will open a new field of studying high-energy photon-photon interactions at remarkable luminosity, reaching 1% of that in pp collisions. In this paper a few aspects of tagging two-photon interactions as well as several most exciting topics in the high-energy two-photon physics at the LHC are presented.
Experimental prospects for studying high-energy photon-photon and photon-proton interactions at the CERN Large Hadron Collider (LHC) are discussed. Cross sections are calculated for many electroweak and beyond the Standard Model processes. Selection strategies based on photon interaction tagging techniques are studied. Assuming a typical LHC multipurpose detector, various signals and their irreducible backgrounds are presented after applying acceptance cuts. Prospects are discussed for the Higgs boson search, detection of supersymmetric particles and of anomalous quartic gauge couplings, as well as for the top quark physics.
We report new precision measurements of the elastic electron-proton scattering cross section for momentum transfer squared (Q$^2$) up to 15.75~gevsq. These data allow for improved extraction of the proton magnetic form factor at high Q$^2$ and nearly double the Q$^2$ range of direct longitudinal/transverse separated cross sections. A comparison of our results to polarization measurements establishes the presence of hard two-photon exchange in the $e$-$p$ elastic scattering cross section at greater than 95% confidence level for Q$^2$ up to 8 (GeV/c)$^2$.
A summary is presented of the workshop top physics at linear colliders that was held at IFIC Valencia from the 30th of June to the 3rd July 2015. We present an up-to-date status report of studies into the potential for top quark physics of lepton colliders with an energy reach that exceeds the top quark pair production threshold, with a focus on the linear collider projects ILC and CLIC. This summary shows that such projects can offer very competitive determinations of top quark properties (mass, width) and its interactions with other Standard Model particles, in particular electroweak gauge bosons and the Higgs boson. In both areas the prospects exceed the LHC potential significantly - often by an order of magnitude.
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