Alexandre Deur

In quantum field theory, the vacuum is popularly considered to be a complex medium populated with virtual particle + antiparticle pairs. To an observer experiencing uniform acceleration, it is generally held that these virtual particles become real, appearing as a gas at a temperature that grows with the acceleration. This is the Unruh effect. However, it has been shown that vacuum complexity is an artifact produced by treating quantum field theory in a manner that does not manifestly enforce causality. Choosing a quantization approach that patently enforces causality, the quantum field theory vacuum is barren, bereft even of virtual particles. We show that acceleration has no effect on a trivial vacuum; hence, there is no Unruh effect in such a treatment of quantum field theory. Since the standard calculations suggesting an Unruh effect are formally consistent, insofar as they have been completed, there must be a canceling contribution that is omitted in the usual analyses. We argue that it is the dynamical action of conventional Lorentz transformations on the structure of an Unruh detector.

The experiment at Jefferson Lab has observed p p ¯ and, for the first time, Λ Λ ¯ and p Λ ¯ photoproduction from a proton target at photon energies up to 11.6 GeV. The angular distributions are forward peaked for all produced pairs, consistent with Regge-like t -channel exchange. Asymmetric wide-angle antibaryon distributions show the presence of additional processes. In a phenomenological model, we find consistency with a double- t -channel exchange process where antibaryons are created only at the middle vertex. The model matches all observed distributions with a small number of free parameters. In the hyperon channels, we observe a clear distinction between photoproduction of the Λ Λ ¯ and p Λ ¯ systems but general similarity to the p p ¯ system. We report both total cross sections and cross sections differential with respect to momentum transfer and the invariant masses of the created particle pairs. No narrow resonant structures were found in these reaction channels. The suppression of s s ¯ quark pairs relative to d d ¯ quark pairs is similar to what has been seen in other reactions.

We measure the spin-density matrix elements (SDMEs) for the photoproduction of ϕ ( 1020 ) off of the proton in its decay to K S 0 K L 0 , using 105 pb − 1 of data collected with a linearly polarized photon beam using the GlueX experiment. The SDMEs are measured in nine bins of the squared four-momentum transfer t in the range − t = 0.15 − 1.0 GeV 2 , providing the first measurement of their t dependence for photon beam energies of E γ = 8.2 − 8.8 GeV. We confirm the dominance of Pomeron exchange in this region and put constraints on the contribution of other Regge exchanges. We also find that helicity amplitudes where the helicity of the photon and ϕ ( 1020 ) differ by two units are negligible.

We examine the claimed observations of a gravitational external field effect (EFE) reported by Chae et al. We show that observations suggestive of the EFE can be interpreted without violating Einstein’s equivalence principle, namely from known correlations between the morphology, the environment, and dynamics of galaxies. While Chae et al.’s analysis provides a valuable attempt at a clear test of modified Newtonian dynamics, an evidently important topic, a re-analysis of the observational data does not permit us to confidently assess the presence of an EFE or to distinguish this interpretation from that proposed in this article.

We report on the first measurement of J/ψ photoproduction from nuclei in the photon energy range of 7 to 10.8 GeV, extending above and below the photoproduction threshold in the free proton of ∼8.2 GeV. The experiment used a tagged photon beam incident on deuterium, helium, and carbon, and the GlueX detector at Jefferson Lab to measure the semi-inclusive A(γ,e^{+}e^{-}p) reaction with a dilepton invariant mass M(e^{+}e^{-})∼m_{J/ψ}=3.1 GeV. The incoherent J/ψ photoproduction cross sections in the measured nuclei are extracted as a function of the incident photon energy, momentum transfer, and proton-reconstructed missing light-cone momentum fraction. Comparisons with theoretical predictions assuming a dipole form factor allow for extracting a gluonic radius for bound protons of sqrt[⟨r^{2}⟩]=0.85±0.14 fm. The data also suggest an excess of the measured cross section for subthreshold production and for interactions with high missing light-cone momentum fraction protons. The measured enhancement can be explained by modified gluon structure for high-virtuality bound protons.