High Energy Physics Phenomenology

New light particles may be produced in large numbers in the far-forward region at the LHC and then decay to dark matter, which can be detected through its scattering in far-forward experiments. We consider the example of invisibly-decaying dark photons, which decay to dark matter throughA???�χ�?. The dark matter may then be detected through its scattering off electrons?e???��?e??. We consider the discovery potential of detectors placed on the beam collision axis 480 m from the ATLAS interaction point, including an emulsion detector (FASERν2) and, for the first time, a Forward Liquid Argon Experiment (FLArE). For each of these detector technologies, we devise cuts that effectively separate the singlee??signal from the leading neutrino- and muon-induced backgrounds. We find that 10- to 100-tonne detectors may detect hundreds to thousands of dark matter events in the HL-LHC era and will sensitively probe the thermal relic region of parameter space. These results motivate the construction of far-forward emulsion and liquid argon detectors at the LHC, as well as a suitable location to accommodate them, such as the proposed Forward Physics Facility.

In this work, we propose that detecting the polarization information of?cJ?�ϕ�?(J=0,1,2)can be as good test of the role of hadronic loop effect on these decays. Our results shows that the obtained ratios of helicity amplitudes are quite stable, which are|F(0)1,1/F(0)0,0|??.359,|F(1)1,0/F(1)0,1|=1,|F(1)1,1/F(1)0,1|=0,|F(2)1,0|/|F(2)0,0|=|F(2)0,1|/|F(2)0,0|??.285, |F(2)1,??|/|F(2)0,0|=|F(2)??,1|/|F(2)0,0|??.110and|F(2)??,??|/|F(2)0,0|=|F(2)1,1|/|F(2)0,0|??.465. By adopting these predicted ratios, we further generate Monte-Carlo events of moments??tij??, which are directly related to the determination of helicity amplitudes. We suggest further experiments like BESIII and Belle II to perform an analysis on the polarizations of the?cJ?�ϕ�?process in the future, which is important to understand the underlying decay mechanism existing in?cJdecays.

We present the first determination of transverse momentum dependent (TMD) photon densities with the Parton Branching method. The photon distribution is generated perturbatively without intrinsic photon component. The input parameters for quarks and gluons are determined from fits to precision measurements of deep inelastic scattering cross sections at HERA. The TMD densities are used to predict the mass and transverse momentum spectra of very high mass lepton pairs from both Drell-Yan production and Photon-Initiated lepton processes at the LHC.

Experimental measurements in deep-inelastic scattering and lepton-pair production on deuterium targets play an important role in the flavor separation ofuandd(anti)quarks in global QCD analyses of the parton distribution functions (PDFs) of the nucleon. We investigate the impact of theoretical corrections accounting for the light-nuclear structure of the deuteron upon the fittedu,d-quark, gluon, and other PDFs in the CJ15 and CT18 families of next-to-leading order CTEQ global analyses. The investigation is done using theL2sensitivity statistical method, which provides a common metric to quantify the strength of experimental constraints on various PDFs and ratios of PDFs in the two distinct fitting frameworks. Using theL2sensitivity and other approaches, we examine the compatibility of deuteron data sets with other fitted experiments under varied implementations of the deuteron corrections. We find that freely-fitted deuteron corrections modify the PDF uncertainty at large momentum fractions and will be relevant for future PDFs affecting electroweak precision measurements.

The role of differential equations in the process of calculating Feynman integrals is shown. An example of a diagram is given, in the calculation of which the method of differential equations was introduced, the properties of the inverse-mass-expansion coefficients are shown, and modern methods based on differential equations are considered.

We study the diffusion properties of the strongly interacting quark-gluon plasma (sQGP) and evaluate the diffusion coefficient matrix for the baryon (B), strange (S) and electric (Q) charges -κqq??(q,q??=B,S,Q) and show their dependence on temperatureTand baryon chemical potentialμB. The non-perturbative nature of the sQGP is evaluated within the Dynamical Quasi-Particle Model (DQPM) which is matched to reproduce the equation of state of the partonic matter above the deconfinement temperatureTcfrom lattice QCD. The calculation of diffusion coefficients is based on two methods: i) the Chapman-Enskog method for the linearized Boltzmann equation, which allows to explore non-equilibrium corrections for the phase-space distribution function in leading order of the Knudsen numbers as well as ii) the relaxation time approximation (RTA). In this work we explore the differences between the two methods. We find a good agreement with the available lattice QCD data in case of the electric charge diffusion coefficient (or electric conductivity) at vanishing baryon chemical potential as well as a qualitative agreement with the recent predictions from the holographic approach for all diagonal components of the diffusion coefficient matrix. The knowledge of the diffusion coefficient matrix is also of special interest for more accurate hydrodynamic simulations.

Using a mechanism which allows naturally small Dirac neutrino masses and its linkage to a dark gaugeU(1)Dsymmetry, a realistic Dirac neutrino mass matrix is derived fromS3. The dark sector naturally contains a fermion singlet having a small seesaw mass. It is thus a good candidate for freeze-in dark matter from the decay of theU(1)DHiggs boson.

In perturbative QCD approach, based on the first order of isospin symmetry breaking, we study the directCPviolation in the decay ofB¯0s?��??)K?????+???K??. An interesting mechanism is applied to enlarge theCPviolating asymmetry involving the charge symmetry breaking between?and?. We find that theCPviolation is large by the??��?mixing mechanism when the invariant masses of the?+???pairs is in the vicinity of the?resonance. For the decay process ofB¯0s?��??)K?????+???K??, the maximumCPviolation can reach??9.12%. Furthermore, taking??��?mixing into account, we calculate the branching ratio forB¯0s?��??)K??. We also discuss the possibility of observing the predictedCPviolation asymmetry at the LHC.

Single phonon excitations are sensitive probes of light dark matter in the keV-GeV mass window. For anisotropic target materials, the signal depends on the direction of the incoming dark matter wind and exhibits a daily modulation. We discuss in detail the various sources of anisotropy, and carry out a comparative study of 26 crystal targets, focused on sub-MeV dark matter benchmarks. We compute the modulation reach for the most promising targets, corresponding to the cross section where the daily modulation can be observed for a given exposure, which allows us to combine the strength of DM-phonon couplings and the amplitude of daily modulation. We highlight Al2O3(sapphire), CaWO4and h-BN (hexagonal boron nitride) as the best polar materials for recovering a daily modulation signal, which featureO(1??00)%variations of detection rates throughout the day, depending on the dark matter mass and interaction. The directional nature of single phonon excitations offers a useful handle to mitigate backgrounds, which is crucial for fully realizing the discovery potential of near future experiments.

Although the 125 GeV Higgs boson discovered at the LHC is often heralded as the origin of mass, it may not in fact be the origin of Yukawa couplings. In alternative models, Yukawa couplings may instead arise from a seesaw type mechanism involving the mixing of Standard Model (SM) chiral fermions with new vector-like fermions, controlled by the vacuum expectation value (VEV) of a new complex Higgs singlet field?�Φ⟩. For example, the largest third family(t,b)quark Yukawa couplings may be forbidden by aU(1)??gauge or global symmetry, broken by?�Φ⟩, and generated effectively via mixing with a vector-like fourth family quark doublet(T,B). Such theories predict a new physical Higgs singlet?, which we refer to as the Yukon, resulting from?�Φ⟩, in the same way that the Higgs bosonh0results from?�H??. In a simplified model we discuss the prospects for discovering the Yukon?in gluon-gluon fusion production, with(t,b)and(T,B)quarks in the loops, and decaying in the channels??�γ�?Zγand??�tT?�tth0,ttZ. The potential for discovery of the Yukon?is studied at present or future hadron colliders such as the LHC (Run 3), HL-LHC, HE-LHC and/or FCC. For example, we find that a 300-350 GeV Yukon?could be accessed at LHC Run 3 in the di-photon channel in the global model, providing a smoking gun signature of the origin of Yukawa couplings. Thetth0,ttZchannels are more involved and warrant a more sophisticated analysis.