High Energy Physics - Phenomenology (hep-ph)

Abstract: We study the $W$ boson associated single top quark production at the Large Hadron Collider (LHC) to probe anomalous chromo-magnetic and chromo-electric moments of the top quark with the help of polarization and spin correlation observables besides the cross-section in the leptonic final state. We reconstruct the two neutrinos in the final state using the $M_{T2}$ assisted on-shell (MAOS) reconstruction method to measure the polarization and spin correlation asymmetries of the top quark and the $W$ boson. We estimate the limits on the anomalous moments in a detector-level simulation considering possible backgrounds for a few sets of integrated luminosities and examined the effect of systematic uncertainties.

Abstract: In this paper, we employ gauge/gravity duality to investigate the string breaking and melting of doubly-heavy tetraquark that includes two heavy quarks and two light antiquarks in a holographic model at finite temperature. Firstly, we investigate four configurations of $\rm{QQ\bar{q}\bar{q}}$ in the confined phase and consider different separation distances of the heavy quarks at varying temperatures. At high temperature, $\rm{QQ\bar{q}\bar{q}}$ melts at certain distances and the confined quarks are released. As the temperature continues to increase, some configurations of doubly-heavy tetraquark can not exist. Furthermore, we investigate three decay modes of $\rm{QQ\bar{q}\bar{q}}$ and compare the potential energy of $\rm{QQ\bar{q}\bar{q}}$ with that of $\rm{QQq}$ at finite temperature .

Abstract: Axions provide a solution to the strong CP problem and are excellent dark matter candidates. The presence of additional sources of CP violation, for example to account for the matter/antimatter asymmetry of the universe, can lead to CP-violating interactions between axions and Standard Model fields. In case axions form a coherent dark matter background, this leads to time-oscillating fundamental constants such as the fine-structure constant and particle masses. In this work we compare the sensitivity of various searches for CP-odd axion interactions. These include fifth-force experiments, searches for time-oscillating constants induced by axion dark matter, and direct limits from electric dipole moment experiments. We show that searches for oscillating constants can outperform fifth-force experiments in the regime of small axion masses, but, in general, do not reach the sensitivity of electric dipole moment experiments.

Abstract: This is a written version of lectures delivered at TASI 2022 ``Ten Years After the Higgs Discovery: Particle Physics Now and Future''. Mechanisms and symmetries beyond the Standard Model (BSM) are presented capable of elegantly and robustly generating the striking hierarchies we observe in particle physics. They are shown to be among the central archetypes of quantum effective field theory and to strongly resonate with the tight structure and phenomenology of the Standard Model itself, allowing one to motivate, develop and test a worthy successor. The (Little) Hiearchy Problem is discussed within this context. The lectures culminate in specific BSM case-studies, gaugino-mediated (dynamical) supersymmetry breaking to generate the weak/Planck hierarchy, and (in less detail) extra-dimensional wavefunction overlaps to generate flavor hierarchies.

Abstract: A prominent effective description of particles interacting with the quantum properties of gravity is through modifications of the general relativistic dispersion relation. Such modified dispersion relations lead to modifications in the relativistic time dilation. A perfect probe for this effect, which goes with the particle energy cubed $E^3$ over the quantum gravity scale $E_{\text{QG}}$ and the square of the particle mass $M^2$ would be a very light unstable particle for which one can detect the lifetime in the laboratory as a function of its energy to very high precision. In this article we conjecture that a muon collider or accelerator would be a perfect tool to investigate the existence of an anomalous time dilation, and with it the fundamental structure of spacetime at the Planck scale.

Abstract: The electron shell of the daughter atoms often appears excited in the double-$\beta$ decays, which causes a change in the energy taken away by $\beta$-electrons. The average value and variance of the excitation energy of the electron shell of the daughter atom are calculated for the double-$\beta$ decay of germanium $_{32}^{76}\mathrm{Ge} \rightarrow _{34}^{76}\mathrm{Se}^*+2\beta^-(+~2\bar{\nu_e})$ in both the Thomas--Fermi model and the relativistic Dirac--Hartree--Fock theory. Using the results obtained, a two-parameter model of the energy spectrum of $\beta$-electrons in the neutrinoless mode is constructed, taking into account reaction energy redistribution in the decay channels. The shift in total energy of $\beta$-electrons is found to be under 50 eV at a confidence level of 90%. The average excitation energy, on the other hand, is an order of magnitude higher and equal to $\sim 400$ eV, while the square root of the variance is equal to $\sim 2900$ eV, which is presumably explained by the contribution of the core electrons to the energy characteristics of the process. The probability is nearly saturated with excitations with a small amount of released energy, which is common for the outermost electrons. The distortion of the peak shape of the neutrinoless double-$\beta$ decay should be taken into consideration when analyzing data from detectors with a resolution of $\sim 100$ eV or higher.

Abstract: Stochastic backgrounds of gravitational waves (GWs) from the pre-BBN era offer a unique opportunity to probe the universe beyond what has already been achieved with the Cosmic Microwave Background (CMB). If the source is short in duration, the low frequency tail of the resulting GW spectrum follows a universal frequency scaling dependent on the equation of state of the universe when modes enter the horizon. We demonstrate that the distortion of the equation of state due to massive particles becoming non-relativistic can lead to an observable dip in the GW spectrum. To illustrate this effect, we consider a first order chiral symmetry breaking phase transition in the weak-confined Standard Model (WCSM). The model features a large number of pions and mostly elementary fermions with masses just below the critical temperature for the phase transition. These states lead to a 20$\%$ dip in the GW power. We find potential sensitivity to the distortions in the spectrum to future GW detectors such as LISA, DECIGO, BBO, and $\mu$Ares.