High Energy Physics Phenomenology

A classically scale-invariant scalar singlet can be a MeV-scale dark matter, with a feeble Higgs portal coupling atO(10??0). Besides, anO(0.1)self-interaction coupling could further serve to alleviate the small-scale problems in the Universe. We show that, such a dark matter candidate can naturally arise in the warped extra dimension, with the huge span of parameter space predicted well withinO(1)fundamental parameters.

We present a novel method for extracting the proton radius from elastic electron-proton (ep) scattering data. The approach is based on interpolation via continued fractions augmented by statistical sampling and avoids any assumptions on the form of function used for the representation of data and subsequent extrapolation ontoQ2??. Applying the method to extant modernepdata sets, we find that all results are mutually consistent and, combining them, arrive atrp=0.847(8)fm. This result compares favourably with values obtained from contemporary measurements of the Lamb shift in muonic hydrogen, transitions in electronic hydrogen, and muonic deuterium spectroscopy.

We present fully differential predictions for the production cross section of a Higgs boson via the gluon fusion mechanism at next-to-next-to-next-to leading order (N3LO) in QCD perturbation theory. To perform our calculation we apply the Projection-to-Born method for the first time to the calculation of the non-factorising production of a colorless final state at the LHC at N3LO. We predict differential distributions for the two photon final state produced by the decay of the Higgs boson and apply fiducial cuts on the photon rapidities and momenta. The N3LO corrections to these differential distributions have complex features and are in part larger than the inclusive N3LO corrections to the production cross section. Overall, we observe that the inclusion of the N3LO QCD corrections significantly reduces the perturbative uncertainties and leads to a stabilisation of the perturbative expansion.

We present the package for the simulation of DM (Dark Matter) particles in fixed target experiments. The most convenient way of this simulation (and the only possible way in the case of beam-dump) is to simulate it in the framework of the program for tracing particles in the experimental setup. One of the most popular such programs is Geant4.Specifically, the package includes the processes of DM particles production via electron and muon bremsstrahlung off nuclei, resonant in-flight positron annihilation on atomic electrons and gamma to ALP (axion-like particles) conversion on nuclei. Four types of DM mediator particles are considered: vector, scalar, pseudoscalar and axial vector.In particular, for bremsstrahlung the total cross sections are calculated at exact tree level (ETL). The code handles both the case of invisible DM mediator decay and of visible decay intoe+e??(or intoγγin the case of ALP).The software consists of a collection of different classes, inheriting from the Geant4 framework classes, thus the expected use of this package is to include it in a Geant4-based code for the simulation of particles propagation and interaction in the detector.As an example of its usage, we discuss the results obtained from the simulation of a typical active beam-dump experiment, considering5?1012100 GeV electrons impinging on a lead/plastic scintillator active thick target, showing the expected sensitivity for the four types of DM mediator particles mentioned above.

We present a comprehensive study of the quark sector of2+1flavour QCD, based on a self-consistent treatment of the coupled system of Schwinger-Dyson equations for the quark propagator and the full quark-gluon vertex. The individual form factors of the quark-gluon vertex are expressed in a special tensor basis obtained from a set of gauge-invariant operators. The sole external ingredient used as input to our equations is the Landau gauge gluon propagator with2+1dynamical quark flavours, obtained from studies with Schwinger-Dyson equations, the functional renormalisation group approach, and large volume lattice simulations. The appropriate renormalisation procedure required in order to self-consistently accommodate external inputs stemming from other functional approaches or the lattice is discussed in detail, and the value of the gauge coupling is accurately determined at two vastly separated renormalisation group scales.Our analysis establishes a clear hierarchy among the vertex form factors. We identify only three dominant ones, in agreement with previous results. The components of the quark propagator obtained from our approach are in excellent agreement with the results from Schwinger-Dyson equations, the functional renormalisation group, and lattice QCD simulation, a simple benchmark observable being the chiral condensate in the chiral limit, which is computed as(245MeV)3. The present approach has a wide range of applications, including the self-consistent computation of bound-state properties and finite temperature and density physics, which are briefly discussed.

We reconsider a scenario in which photons and other gauge fields appear as the composite vector bosons made of the fermion pairs that may happen with or without spontaneous violation of Lorentz invariance. The class of composite models for emergent gauge fields is proposed, where these fields are required to be restricted by by the nonlinear covariant constraint of typeA2μ=M2. Such a constraint may only appear if the corresponding fermion currents in the prototype model, being invariant under some global internal symmetryG, are properly constrained as well. In contrast to the conventional approach, the composite bosons emerged in this way appear naturally massless, the global symmetryGin the model turns into the local symmetryGloc, while the vector field constraint reveals itself as the gauge fixing condition. Finally, we consider the case when the constituent fermions generating emergent gauge bosons could be at the same time the preons composing the known quark-lepton species in the Standard Model and Grand Unified Theories.

We revisit the problem of the gauge invariance in the Coleman-Weinberg model in which aU(1)gauge symmetry is driven spontaneously broken by radiative corrections. It was noticed in previous work that masses in this model are not gauge invariant at one-loop order. In our analysis, we use the dressed propagators of scalars which include a resummation of the one-loop self-energy correction to the tree-level propagator. We calculate the one-loop self-energy correction to the vector meson using these dressed propagators. We find that the pole mass of the vector meson calculated using the dressed propagator is gauge invariant at the vacuum determined using the effective potential calculated with a resummation of daisy diagrams.

High energy photons can decay to electron-positron pairs via the nonlinear Breit-Wheeler process when colliding with an intense laser pulse. The energy spectrum of the produced particles is broadened because of the variation of their effective mass in the course of the laser pulse. Applying a suitable chirp to the laser pulse can narrow the energy distribution of the generated electrons and positrons. We present a scenario where a high-energy electron beam is collided with a chirped laser pulse to generate a beam of quasi-monoenergeticγ-photons, which then decay in a second chirped, UV pulse to produce a quasi-monoenergetic source of high-energy electrons and positrons.

This is a brief summary of the latest searches for virtual effects of new physics in the top sector. In the framework of the Standard Model Effective Field Theory (SMEFT), I show how to resolve the structure of effective couplings by combining observables at the LHC and at flavor experiments in a global fit. With this approach we start exploring the features of a UV theory at energies beyond current colliders.

We perform a perturbative QCD analysis of the gluonic gravitational form factors (GFFs) of the proton and pion at large momentum transfer. We derive the explicit factorization formula of the GFFs in terms of the distribution amplitudes of hadrons. At the leading power, we find thatA?g(t)=C?g(t)??/(?�t)for pion,Apg(t)??/(?�t)2andCpg(t)??ln2(?�t/?2)/(?�t)3for proton, respectively, wheretis the momentum transfer and?a non-perturbative scale to regulate the endpoint singularity inCpgcalculation. Our results provide a unique perspective of the momentum dependence of the GFFs and will help to improve our understanding of the internal pressure distributions of hadrons.