Most embeddings of the Standard Model into a more unified theory, in particular those based on supergravity or superstrings, predict the existence of a hidden sector of particles that have only very weak interactions with visible-sector Standard Model particles. Some of these exotic particle candidates [for instance, axions, axion-like particles, and hidden U(1) gauge bosons] may be very light, with masses in the subelectronvolt range, and may have very weak interactions with photons. Correspondingly, these very weakly interacting subelectronvolt particles (WISPs) may lead to observable effects in experiments (as well as in astrophysical and cosmological observations) searching for light shining through a wall, for changes in laser polarization, for nonlinear processes in large electromagnetic fields, and for deviations from Coulomb's law. We present the physics case and a status report of this emerging low-energy frontier of fundamental physics.

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The Low-Energy Frontier of Particle Physics

Strong-field effects in laboratory and astrophysical plasmas and high intensity laser and cavity systems are considered, related to quantum electrodynamical (QED) photon-photon scattering. Current state-of-the-art laser facilities are close to reaching energy scales at which laboratory astrophysics will become possible. In such high energy density laboratory astrophysical systems, quantum electrodynamics will play a crucial role in the dynamics of plasmas and indeed the vacuum itself. Developments such as the free-electron laser may also give a means for exploring remote violent events such as supernovae in a laboratory environment. At the same time, superconducting cavities have steadily increased their quality factors, and quantum nondemolition measurements are capable of retrieving information from systems consisting of a few photons. Thus, not only will QED effects such as elastic photon-photon scattering be important in laboratory experiments, it may also be directly measurable in cavity experiments. Here implications of collective interactions between photons and photon-plasma systems are described. An overview of strong field vacuum effects is given, as formulated through the Heisenberg-Euler Lagrangian. Based on the dispersion relation for a single test photon traveling in a slowly varying background electromagnetic field, a set of equations describing the nonlinear propagation of an electromagnetic pulse on a radiation plasma is derived. The stability of the governing equations is discussed, and it is shown using numerical methods that electromagnetic pulses may collapse and split into pulse trains, as well as be trapped in a relativistic electron hole. Effects, such as the generation of novel electromagnetic modes, introduced by QED in pair plasmas is described. Applications to laser-plasma systems and astrophysical environments are also discussed.

Nonlinear collective effects in photon-photon and photon-plasma interactions

The Two photon particle production mechanism. Physical problems. Applications. Equivalent photon approximation

A complete calculation of the order αs2 correction to the Drell-Yan K-factor

New experimental approaches in the search for axion-like particles

Effects of nearly massless, spin-zero particles on light propagation in a magnetic field

Muon pairs in the mass range $1l{m}_{\ensuremath{\mu}\ensuremath{\mu}}l67 \frac{\mathrm{Gev}}{{c}^{2}}$ have been observed in collisions of high-energy protons with uranium nuclei At an incident energy of 29 GeV, the cross section varies smoothly as $\frac{d\ensuremath{\sigma}}{d{m}_{\ensuremath{\mu}\ensuremath{\mu}}}\ensuremath{\approx}\frac{{10}^{\ensuremath{-}32}}{m_{\ensuremath{\mu}\ensuremath{\mu}}^{}{}_{}{}^{5}} {\mathrm{cm}}^{2} {(\frac{\mathrm{GeV}}{c})}^{\ensuremath{-}2}$ and exhibits no resonant structure The total cross section increases by a factor of 5 as the proton energy rises from 22 to 295 GeV

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E. Zavattini

Papers

Effects of nearly massless, spin-zero particles on light propagation in a magnetic field

Observation of Massive Muon Pairs in Hadron Collisions

Observation of πo mesons with large transverse momentum in high-energy proton-proton collisions

Experimental method to detect the vacuum birefringence induced by a magnetic field

Measurement of the ortho-para transition rate in the pμp molecule and deduction of the pseudoscalar coupling constant gpμ