New infra-red enhancements in 4-derivative gravity

4-derivative gravity provides a renormalizable theory of quantum gravity at the price of introducing a physical ghost, which could admit a sensible positive-energy quantization. To understand its physics, we compute ghost-mediated scatterings among matter particles at tree-level, finding a new power-like infra-red enhancement typical of 4-derivative theories, that we dub '$ghostrahlung$'. Super-Planckian scatterings get downgraded to Planckian by radiating hard gravitons and ghosts, which are weakly coupled and carry away the energy.Comment: 20 pages, 3 figure

Anomalous Higgs-boson coupling effects in HWW production at the LHC

We study the LHC associated production of a Higgs boson and a W^+W^- vector-boson pair at 14 TeV, in the Standard Model and beyond. We consider different signatures corresponding to the cleanest H and W decay channels, and discuss the potential of the high-luminosity phase of the LHC. In particular, we investigate the sensitivity of the HWW production to possible anomalous Higgs couplings to vector bosons and fermions. Since the b-quark initiated partonic channel contributes significantly to this process, we find a moderate sensitivity to both the size and sign of an anomalous top-quark Yukawa coupling, because perturbative unitarity in the standard model implies a destructive interference in the b b-bar subprocess. We show that a combination of various signatures can reach a ~9 standard-deviation sensitivity in the presently allowed negative region of the top-Higgs coupling, if not previously excluded.Comment: 13 pages, 3 figure

Collisionless shocks in self-interacting dark matter

Self-interacting dark matter (DM) has been proposed as a solution to small scale problems in cosmological structure formation, and hints of DM self scattering have been observed in mergers of galaxy clusters. One of the simplest models for self-interacting DM is a particle that is charged under dark electromagnetism, a new gauge interaction analogous to the usual electromagnetic force, but operating on the DM particle instead of the visible particles. In this case, the collisional behaviour of DM is primarily due to the formation of collisionless shocks, that should affect the distribution of DM in merging galaxy clusters. We evaluate the time and length scales of shock formation in cluster mergers, and discuss the implications for modelling charged DM in cosmological simulations.Peer reviewe

The recent gravitational wave observation by pulsar timing arrays and primordial black holes: the importance of non-gaussianities

The recent data releases by multiple pulsar timing array experiments (NANOGrav, EPTA, PPTA and CPTA) show evidence for Hellings-Downs angular correlations indicating that the observed stochastic common spectrum can be interpreted as a stochastic gravitational wave background. In this letter, we study whether the signal may originate from gravitational waves induced by high-amplitude primordial curvature perturbations. Such large perturbations may be accompanied by the generation of a sizeable primordial black hole (PBH) abundance. We improve existing analyses of the PBH abundance by including non-Gaussianities typical of several scenarios such as curvaton and inflection-point models. We show that Gaussian scenarios for scalar-induced gravitational waves are disfavoured by more than 2{\sigma} as the sole explanation of the most constraining NANOGrav 15-year data by the overproduction of PBHs. This excludes most explanations relying on single-field inflation by more than 3{\sigma}. This tension, however, can be alleviated in models in which non-Gaussianites suppress the PBH abundance, for instance, in curvaton models with a large rdec or models with a negative fNL. On the flip side, the current NANOGrav data does not constrain the abundance of PBHs in the stellar mass range.Comment: 6 pages and 3 figures. Supplementary materials availabl

Tachyonic preheating in plateau inflation

Plateau inflation is an experimentally consistent framework in which the scale of inflation can be kept relatively low. Close to the edge of the plateau, scalar perturbations are subject to a strong tachyonic instability. Tachyonic preheating is realized when, after inflation, the oscillating inflaton repeatedly re-enters the plateau. We develop the analytic theory of this process and expand the linear approach by including backreaction between the coherent background and growing perturbations. For a family of plateau models, the analytic predictions are confronted with numerical estimates. Our analysis shows that the inflaton fragments in a fraction of an e-fold in all examples supporting tachyonic preheating, generalizing the results of previous similar studies. In these scenarios, the scalar-to-tensor ratio is tiny, r < 10-7