Strong signatures of radiation reaction below the radiation dominated regime

The influence of radiation reaction (RR) on multiphoton Thomson scattering by an electron colliding head-on with a strong laser beam is investigated in a new regime, in which the momentum transferred on average to the electron by the laser pulse approximately compensates the one initially prepared. This equilibrium is shown to be far more sensitive to the influence of RR than previously studied scenarios. As a consequence RR can be experimentally investigated with currently available laser systems and the underlying widely discussed theoretical equations become testable for the first time.Comment: 4 pages, 3 figure

Light diffraction by a strong standing electromagnetic wave

The nonlinear quantum interaction of a linearly polarized x-ray probe beam with a focused intense standing laser wave is studied theoretically. Because of the tight focusing of the standing laser pulse, diffraction effects arise for the probe beam as opposed to the corresponding plane wave scenario. A quantitative estimate for realistic experimental conditions of the ellipticity and the rotation of the main polarization plane acquired by the x-ray probe after the interaction shows that the implementation of such vacuum effects is feasible with future X-ray Free Electron Laser light.Comment: 5 pages, 2 figures. Published versio

Ultra-strong laser pulses: streak-camera for gamma-rays via pair production and quantum radiative reaction

We show that a strong laser pulse combined with a strong x-ray pulse can be employed in a detection scheme for characterizing high-energy $\gamma$-ray pulses down to the zeptosecond timescale. The scheme employs streak imaging technique built upon the high-energy process of electron-positron pair production in vacuum through the collision of a test pulse with intense laser pulses. The role of quantum radiation reaction in multiphoton Compton scattering process and limitations imposed by it on the detection scheme are examined

Magnetic hour-glass dispersion and its relation to high-temperature superconductivity in iron-tuned Fe1+y_{1+y}Te0.7_{0.7}Se0.3_{0.3}

High-temperature superconductivity remains arguably the largest outstanding enigma of condensed matter physics. The discovery of iron-based high-temperature superconductors has renewed the importance of understanding superconductivity in materials susceptible to magnetic order and fluctuations. Intriguingly they show magnetic fluctuations reminiscent of the superconducting (SC) cuprates, including a 'resonance' and an 'hour-glass' shaped dispersion, which provide an opportunity to new insight to the coupling between spin fluctuations and superconductivity. Here we report inelastic neutron scattering data on Fe$_{1+y}$Te$_{0.7}$Se$_{0.3}$ using excess iron concentration to tune between a SC ($y=0.02$) and a non-SC ($y=0.05$) ground states. We find incommensurate spectra in both samples but discover that in the one that becomes SC, a constriction towards a commensurate hourglass shape develop well above $T_c$. Conversely a spin-gap and concomitant spectral weight shift happen below $T_c$. Our results imply that the hourglass shaped dispersion is most likely a pre-requisite for superconductivity, whereas the spin-gap and shift of spectral weight are consequences of superconductivity. We explain this observation by pointing out that an inwards dispersion towards the commensurate wave-vector is needed for the opening of a spin gap to lower the magnetic exchange energy and hence provide the necessary condensation energy for the SC state to emerge

Ultra-high brilliance multi-MeV γ\gamma-ray beam from non-linear Thomson scattering

We report on the generation of a narrow divergence ($\theta\approx 2.5$ mrad), multi-MeV ($E_\text{MAX} = 18$ MeV) and ultra-high brilliance ($\approx 2\times10^{19}$ photons s$^{-1}$ mm$^{-2}$ mrad $^{-2}$ 0.1\% BW) $\gamma$-ray beam from the scattering of an ultra-relativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude $a_0\approx2$). The spectrum of the generated $\gamma$-ray beam is measured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear evidence of the onset of non-linear Thomson scattering. The photon source has the highest brilliance in the multi-MeV regime ever reported in the literature

Non-perturbative vacuum-polarization effects in proton-laser collisions

In the collision of a high-energy proton beam and a strong laser field, merging of the laser photons can occur due to the polarization of vacuum. The probability of photon merging is calculated by accounting exactly for the laser field and presents a highly non-perturbative dependence on the laser intensity and frequency. It is shown that the non-perturbative vacuum-polarization effects can be experimentally measured by combining the next-generation of table-top petawatt lasers with presently available proton accelerators.Comment: 5 pages, 2 figure

What is needed of a tachyon if it is to be the dark energy?

We study a dark energy scenario in the presence of a tachyon field $\phi$ with potential $V(\phi)$ and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity $\lambda=-M_pV_\phi/V^{3/2}$. If $\lambda$ is a constant, which corresponds to an inverse square potential $V(\phi) \propto \phi^{-2}$, there exists one stable critical point that gives an acceleration of the universe at late times. When $\lambda \to 0$ asymptotically, we can have a viable dark energy scenario in which the system approaches an ``instantaneous'' critical point that dynamically changes with $\lambda$. If $|\lambda|$ approaches infinity asymptotically, the universe does not exhibit an acceleration at late times. In this case, however, we find an interesting possibility that a transient acceleration occurs in a regime where $|\lambda|$ is smaller than of order unity.Comment: 11 pages and 3 figures, minor clarifications added; final version to appear in PR

Optical extinction, refractive index, and multiple scattering for suspensions of interacting colloidal particles

We provide a general microscopic theory of the scattering cross-section and of the refractive index for a system of interacting colloidal particles, exact at second order in the molecular polarizabilities. In particular: a) we show that the structural features of the suspension are encoded into the forward scattered field by multiple scattering effects, whose contribution is essential for the so-called "optical theorem" to hold in the presence of interactions; b) we investigate the role of radiation reaction on light extinction; c) we discuss our results in the framework of effective medium theories, presenting a general result for the effective refractive index valid, whatever the structural properties of the suspension, in the limit of particles much larger than the wavelength; d) by discussing strongly-interacting suspensions, we unravel subtle anomalous dispersion effects for the suspension refractive index.Comment: Submitted to Journal of Chemical Physics 37 pages, 4 figure