Enhanced nonperturbative photon-pair conversion in small-angle laser collisions

We show a new scheme of nonperturbative pair production by high energy photons ($\omega\gtrsim m$) in a strong external field is achievable at the next high intensity laser experiments. The pair momentum is boosted and for $\omega\gtrsim 1.2m$ the pair yield is increased when the external field is formed by two laser pulses converging at a small angle. These characteristics are nonperturbative in origin and related to the presence of magnetic field in addition to electric field. By enhancing the signal over perturbative backgrounds, these features allow the employment of above-threshold photons $\omega>2m$, which further increases the pair yield. We note the close relation of this photon-pair conversion mechanism to spontaneous pair creation, recommending it as an accessible stepping stone experiment using state-of-the-art or soon-to-be laser technology.Comment: 5 pages, 2 figures, revtex forma

Higgs two-gluon decay and the top-quark chromomagnetic moment

We study the top quark chromomagnetic factor $\kappa_t$-dependence of the effective interaction ${\cal L}_{hgg}$ and apply the result to the case of Higgs two gluon decay rate $\Gamma_{h\to gg}$. Perturbative standard model provides $\kappa_t< 2$, and for the standard model value $\Gamma_{h\to gg}(\kappa_t)$ is suppressed by 9% as compared to $\kappa_t= 2$. We further show that in the leading order $\Gamma_{h\to gg}$ for $\kappa_t\simeq \pm 1.2$ can be very small due to complete cancelation between the top and bottom quark fluctuations. We discuss the extension of our results to $\kappa_t>2$ and consider potential paths to obtain experimental information for the $h\to gg$ rate.Comment: 4 pages, 3 figures, elsarticle format, v2 added section "Gluon Fusion into Higgs

Pair Production from Asymmetric Head-on Laser Collisions

We evaluate particle production in highly asymmetric head-on collisions of lasers pulses due to non-perturbative coherent action of many photons. We obtain the yield of electron-positron pairs, which is controlled by the photon content of the weaker pulse, and show that the wavelength of the weaker pulse and the momentum asymmetry determine laboratory energy of the produced particles.Comment: 4 pages, 1 figur

Effective Field Theory of Broken Spatial Diffeomorphisms

We study the low energy effective theory describing gravity with broken spatial diffeomorphism invariance. In the unitary gauge, the Goldstone bosons associated with broken diffeomorphisms are eaten and the graviton becomes a massive spin-2 particle with 5 well-behaved degrees of freedom. In this gauge, the most general theory is built with the lowest dimension operators invariant under only temporal diffeomorphisms. Imposing the additional shift and SO(3) internal symmetries, we analyze the perturbations on a FRW background. At linear perturbation level, the observables of this theory are characterized by five parameters, including the usual cosmological parameters and one additional coupling constant for the symmetry-breaking scalars. In the de Sitter and Minkowski limit, the three Goldstone bosons are supermassive and can be integrated out, leaving two massive tensor modes as the only propagating degrees of freedom. We discuss several examples relevant to theories of massive gravity.Comment: 26 pages, V2 more references, several remarks and a new subsection are added, V3 a major revision, with two new subsections added, as well as several new discussions on the construction of our EF

Critical Acceleration and Quantum Vacuum

Little is known about the physics frontier of strong acceleration; both classical and quantum physics need further development in order to be able to address this newly accessible area of physics. In this lecture we discuss what strong acceleration means and possible experiments using electron-laser collisions and, data available from ultra-relativistic heavy ion collisions. We review the foundations of the current understanding of charged particle dynamics in presence of critical forces and discuss the radiation reaction inconsistency in electromagnetic theory and the apparent relation with quantum physics and strong field particle production phenomena. The role of the quantum vacuum as an inertial reference frame is emphasized, as well as the absence of such a `Machian' reference frame in the conventional classical limit of quantum field theory.Comment: 12 pages, 1 figure, for the proceedings of the First LeCosPA Symposium, February 201

Acceleration and vacuum temperature

The quantum fluctuations of an "accelerated" vacuum state, that is vacuum fluctuations in the presence of a constant electromagnetic field, can be described by the temperature \TEH. Considering \TEH for the gyromagnetic factor $g=1$ we show that \TEH(g=1)=\THU, where \THU is the Unruh temperature experienced by an accelerated observer. We conjecture that both particle production and nonlinear field effects inherent in the Unruh accelerated observer case are described by the case $g=1$ QED of strong fields. We present rates of particle production for $g=0,1,2$ and show that the case $g=1$ is experimentally distinguishable from $g=0,2$. Therefore, either accelerated observers are distinguishable from accelerated vacuum or there is unexpected modification of the theoretical framework.Comment: 4 pages, 1 figure; expanded discussion of experimental observables, added references, version appearing in Phys Rev