High Energy Physics - Phenomenology (hep-ph)

Abstract: The near-threshold exotic hadrons such as $T_{cc}$ and $X(3872)$ are naively considered as the hadronic molecular state from the viewpoint of the low-energy universality. However, it is also known that the elementary dominant state is not completely excluded as the internal structure of the near-threshold states. Furthermore, the dominance of molecules is expected to be modified by the decay or coupled channels. We discuss these features of the near-threshold bound states by calculating the compositeness with the effective field theory.

Abstract: Modular symmetries offer a dynamic approach to understanding the flavour structure of leptonic mixing. Using the modular $\mathcal{A}_4$ flavour symmetry integrated in a type-II seesaw, we propose a simple and minimalistic model that restricts the neutrino oscillation parameter space and, most importantly, introduces a sum rule in the physical neutrino masses. When combined with the mass squared differences observed in neutrino oscillations, this sum rule determines the absolute neutrino mass scale. This has significant implications for cosmology, neutrinoless double beta decay experiments and direct neutrino mass measurements. In particular, the model predicts $\sum_i m_i \approx 0.1$ eV for both normal and inverted ordering, and thus can be fully probed by the current generation of cosmological probes in the upcoming years.

Abstract: In this letter we analyze the associated production of a vector meson with the bound - free $e^+e^-$ process in ultraperipheral $PbPb$ collisions through the double scattering mechanism for the energy of the Large Hadron Collider (LHC). Such process is characterized by the presence of a meson and a positron in the final state and by a forward hydrogen - like ion with a distinct electric charge. We present our predictions for the total cross sections and rapidity distributions considering the rapidity ranges covered by the ALICE and LHCb detectors, which indicate that a future experimental analysis of the $\phi + e^+$ and $J/\Psi + e^+$ final states is feasible.

Abstract: A realistic model of $SU(5) \times U(1)_{\chi}$, embedded in $SO(10)$ supersymmetric grand unified theory, is investigated for the emergence of a metastable cosmic string network. This network eventually decays via the Schwinger production of monopole-antimonopole pairs, subsequently generating a stochastic gravitational wave background that is compatible with the NANOGrav 15-year data. In order to avoid the monopole problem in the breaking of both $SO(10)$ and $SU(5)$, a non-minimal Higgs inflation scenario is incorporated. The radiative breaking of the $U(1)_{\chi}$ symmetry at a slightly lower scale plays a pivotal role in aligning the string tension parameter with the observable range. The resultant gravitational wave spectrum not only accounts for the signal observed in the most recent pulsar timing array (PTA) experiments but is also accessible to both current and future ground-based and space-based experiments.

Abstract: In the 1970s, an unexpected transverse $\Lambda$ polarization in unpolarized proton-Beryllium collisions was discovered, which initiated extensive studies on spin phenomena in high-energy physics. Over the past five decades, similar transverse $\Lambda$ polarization has been observed across various collision systems, including lepton-hadron deep inelastic scattering, hadron-hadron collisions, and electron-positron collisions. Despite numerous promising theoretical models, the fundamental mechanism underlying this polarization phenomenon remains inconclusive to this day. However, in both longitudinally and transversely polarized lepton-hadron and hadron-hadron collisions, it is found that the $\Lambda$ hyperon is not polarized with respect to the initial parton spin direction. How the $\Lambda$ hyperon acquires its spin has become one of the most crucial questions to address in order to resolve this puzzle. In this Letter, I propose to use an exclusive process that can be measured at the Electron-Ion Collider, the Deep Exclusive Meson Production, to explicitly test the mechanism of $\Lambda$ polarization. The outcomes of this experimental measurement are anticipated to unveil the dominant mechanism by which $\Lambda$ obtains its spin, eliminating many of the ambiguities that have been encountered in previous studies. Finally, experimental challenges and requirements will be discussed.

Abstract: We consider the possibility that neutrino masses arise from the exchange of dark matter states. We examine in detail the phenomenology of fermionic dark matter in the singlet-triplet scotogenic reference model. We explore the case of singlet-like fermionic dark matter, examining in detail all co-annihilation effects relevant for determining its relic abundance, including fermion-fermion as well as scalar-fermion co-annihilation, which are properly taken into account. Although this in principle allows for dark matter below 60 GeV, the latter is in conflict with charged lepton flavour violation (cLFV) and/or collider physics constraints. We examine the prospects for direct dark matter detection in upcoming experiments up to 10 TeV. Fermion-scalar coannihilation is needed to obtain viable fermionic dark matter in the 60-100 GeV mass range. Fermion-fermion and fermion-scalar coannihilation play complementary roles in different parameter regions above 100 GeV.

Abstract: We show that the conditions allowing for a spontaneous breaking of the $U(1)_Y$ hypercharge gauge symmetry of the Standard Model (SM) in the early Universe are generically present in extensions of the SM addressing the generation of light neutrino masses via radiative contributions. In such scenarios, the breaking of (hyper)charge at high-temperatures yields new possibilities for explaining the observed matter-antimatter asymmetry of the Universe. Considering for concreteness the Zee-Babu radiative neutrino mass generation model, we show that a period of hypercharge breaking prior to the electroweak phase transition could allow for successful baryogenesis via a non-conventional leptogenesis mechanism, based on the presence of charge-breaking masses for the SM leptons in the early Universe.