The Tevatron has inspired new interest in the subject of magnetic monopoles. First there was the 1998 D0 limit on the virtual production of monopoles, based on the theory of Ginzburg and collaborators. In 2000 and 2004 results from an experiment (Fermilab E882) searching for real magnetically charged particles bound to elements from the CDF and D0 detectors were reported. The strongest direct experimental limits, from the CDF collaboration, have been reported in 2005. Less strong, but complementary, limits from the H1 collaboration at HERA were reported in the same year. Interpretation of these experiments also require new developments in theory. Earlier experimental and observational constraints on point-like (Dirac) and non-Abelian monopoles were given from the 1970s through the 1990s, with occasional short-lived positive evidence for such exotic particles reported. The status of the experimental limits on monopole masses will be reported, as well as the limitation of the theory of magnetic charge at present.

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Theoretical and experimental status of magnetic monopoles

Recent theoretical results have demonstrated that noncommutative geometries naturally appear within the context of string or M theory. One consequence of this possibility is that QED takes on a non-Abelian nature due to the introduction of 3- and 4-point functions. In addition, each QED vertex acquires a momentum dependent phase factor. We parametrize the effects of noncommutative space-time coordinates and show that they lead to observable signatures in several $\stackrel{\ensuremath{\rightarrow}}{2}2$ QED processes in ${e}^{+}{e}^{\ensuremath{-}}$ collisions. In particular, we examine pair annihilation, Moller and Bhabha scattering, as well as $\ensuremath{\gamma}\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}\ensuremath{\gamma}\ensuremath{\gamma}$ scattering and show that noncommutative scales of order a TeV can be probed at high energy linear colliders.

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Signals for noncommutative interactions at linear colliders

Theoretical and Experimental Status of Magnetic Monopoles

We discuss the discovery potential of light-by-light scattering at the Large Hadron Collider (LHC), induced by the Standard Model (SM) and by new exotic charged particles. Our simulation relies on intact proton detection in the planned forward detectors of CMS and ATLAS. The full four-photon amplitudes generated by any electrically charged particles of spins $1/2$ and $1$, including the SM processes involving loops of leptons, quarks and $W$ bosons are implemented in the Forward Physics Monte Carlo generator. Our method provides model-independent bounds on massive charged particles, only parametrized by the spin, mass and "effective charge" $Q_{\rm eff}$ of the new particle. We find that a new charged vector (fermion) with $Q_{\rm eff}=4$ can be discovered up to $m=640~\rm GeV$ ($m=300~\rm GeV$) with an integrated luminosity of $300~\rm fb^{-1}$ at the LHC. We also discuss the sensitivities to neutral particles such as a strongly-interacting heavy dilaton and warped Kaluza-Klein gravitons, whose effects could be discovered for masses in the multi-TeV range.

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Light-by-light scattering with intact protons at the LHC: from Standard Model to New Physics

We calculate cross sections for diphoton production in (semi)exclusive $\mathrm{PbPb}$ collisions, relevant for the CERN Large Hadron Collider (LHC). The calculation is based on the equivalent photon approximation in the impact parameter space. The cross sections for the elementary $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ subprocess are calculated including two different mechanisms. We take into account box diagrams with leptons and quarks in the loops. In addition, we consider a vector-meson dominance (VDM-Regge) contribution with virtual intermediate hadronic (vector-like) excitations of the photons. We get measurable cross sections in $\mathrm{PbPb}$ collisions. This opens a possibility to study the $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$ (quasi)elastic scattering at the LHC. We present many interesting differential distributions which could be measured by the ALICE, CMS, or ATLAS Collaborations at the LHC. We study whether a separation or identification of different components (boxes, VDM-Regge) is possible. We find that the cross section for elastic $\ensuremath{\gamma}\ensuremath{\gamma}$ scattering could be measured in the heavy-ion collisions for subprocess energies smaller than ${W}_{\ensuremath{\gamma}\ensuremath{\gamma}}\ensuremath{\approx}15\text{--}20$ GeV.

Light-by-light scattering in ultraperipheral Pb-Pb collisions at energies available at the CERN Large Hadron Collider

Photon-photon scattering at the photon linear collider

Photon-photon scattering at the Photon Linear Collider is considered. Explicit formulas for helicity amplitudes due to $W$ boson loops are presented. It is shown that photon-photon scattering should be easily observable at PLC and separation of the $W$ loop contribution (which dominates at high energies) will be possible at $e^+e^-$ c.m. energy of 500~GeV or higher.

G. Jikia

Papers

Photon-photon scattering at the photon linear collider

Photon-Photon Scattering at the Photon Linear Collider