High Energy Physics - Phenomenology (hep-ph)

Abstract: The precise measurement of the W boson mass is closely related to the contributions of new physics (NP), which can significantly constrain the parameter space of NP models, particularly those with an additional $U(1)$ local gauge group. The inclusion of a new $Z'$ gauge boson and gauge couplings in these models can contribute to the oblique parameters $S$, $T$, $U$ and W boson mass at tree level. In this study, we calculate and analyze the oblique parameters $S$, $T$, $U$ and W boson mass in such NP models, taking into account the effects of kinetic mixing, and it is found that the kinetic mixing effects can make significant contributions to these oblique parameters and W boson mass. Based on the obtained numerical results, the recently measured W boson mass at CDF II or ATLAS can be satisfied by choosing appropriate values of gauge coupling constants and extra $U(1)$ group charges of leptons or scalar doublets. In addition, if the leptonic Yukawa couplings are invariant under the extra $U(1)$ local gauge group, the contributions to $S$, $T$, $U$ and W boson mass can be eliminated by redefining the gauge boson fields through eliminating the neutral currents involving charged leptons.

Abstract: The strong coupling in the effective quark mass was usually taken as a constant in a quasiparticle model while it is, in fact, running with an energy scale. With a running coupling, however, the thermodynamic inconsistency problem appears in the conventional treatment. We show that the renormalization subtraction point should be taken as a function of the summation of the biquadratic chemical potentials if the quark's current masses vanish, in order to ensure full thermodynamic consistency. Taking the simplest form, we study the properties of up-down ($ud$) quark matter, and confirm that the revised quasiparticle model fulfills the quantitative criteria for thermodynamic consistency. Moreover, we find that the maximum mass of an $ud$ quark star can be larger than two times the solar mass, reaching up to $2.31M_{\odot}$, for reasonable model parameters. However, to further satisfy the upper limit of tidal deformability $\tilde{\Lambda}_{1.4}\leq 580$ observed in the event GW170817, the maximum mass of an $ud$ quark star can only be as large as $2.08M_{\odot}$, namely $M_{\text{max}}\lesssim2.08M_{\odot}$. In other words, our results indicate that the measured tidal deformability for event GW170817 places an upper bound on the maximum mass of $ud$ quark stars, but which does not rule out the possibility of the existence of quark stars composed of $ud$ quark matter, with a mass of about two times the solar mass.

Abstract: We consider the effects of a local axion dark matter background on the $g-2$ of the electron. We calculate loop corrections to the photon-electron vertex and determine analytical formulas for the spin and cyclotron frequencies when the electron is in an external magnetic field. By comparing with current measurements of these observables, we are able to place the strongest constraint on the axion-elecron coupling for axion masses below $10^{-15}\text{eV}$.

Abstract: We compute, in the framework of renormalon calculus, the ${\cal O}(\Lambda_{\rm QCD})$ corrections to the production of $t\bar{t}$ pairs in hadron collisions under the assumption that $q \bar q \to t \bar t$ is the dominant partonic channel. This assumption is not applicable to top quark pair production at the LHC but it is valid for the Tevatron where collisions of protons and anti-protons were studied. We show that the linear power correction to the total $t \bar t$ production cross section vanishes provided one uses a short-distance scheme for the top quark mass. We also derive relatively simple formulas for the power corrections to top quark kinematic distributions. Although small numerically, these power corrections exhibit interesting dependencies on top quark kinematics.

Abstract: The $ B_3 - L_2$ $ Z' $ model may explain certain features of the fermion mass spectrum as well as the $b \rightarrow s \mu^+ \mu^-$ anomalies. The $ Z' $ acquires its mass via a TeV-scale scalar field, the flavon, whose vacuum expectation value spontaneously breaks the family non-universal gauged $ U(1)_{B_3 - L_2} $ symmetry. We review the key features of the model, with an emphasis on its scalar potential and the flavon field, and use experimental data and perturbativity arguments to place bounds upon the Higgs-flavon mixing angle. Finally, we discuss flavonstrahlung as a means to discover the flavon experimentally and compute flavonstrahlung cross-sections at current and future colliders.

Abstract: Atmospheric collisions can copiously produce dark sector particles in the invisible dark photon model, leading to detectable signals in underground neutrino detectors. We consider the dark photon model with the mass mixing mechanism and use the Super-K detector to detect the electron recoil events caused by the atmospherically produced dark sector particles within the model. We find that the combined data from four Super-K runs yield new leading constraints for the invisible dark photon in the mass range of $\sim(0.5-1.4)$ GeV, surpassing the constraints from NA64, BaBar, and searches for millicharged particles.

Abstract: We analyze some aspects of the perturbative and non-perturbative interactions in the composition of heavy quarkonia, heavy and light baryons ($ccc$ and $uuu$ ones), as well as all charm tetraquarks ($cc\bar c\bar c$). Using the hyper-spherical approximation and effective radial potentials (in 6 and 9 dimensions, respectively) we derive their spectra and wave functions. In all of the cases, we focus on the splittings between the s-shell levels, which are remarkably insensitive to the quark masses, but proportional to the effective interaction potentials. We use the traditional Cornell-like potentials, and the non-perturbative instanton-induced static potentials, from correlators of two, three and four Wilson lines, and find rather satisfactory description of spectra in all cases.

Abstract: We investigate a local $SU(3)_F$ flavour symmetry for its viability in generating the masses for the quarks and charged leptons of the first two families through radiative corrections. Only the third-generation fermions get tree-level masses due to specific choice of the field content and their gauge charges. Unprotected by symmetry, the remaining fermions acquire non-vanishing masses through the quantum corrections induced by the gauge bosons of broken $SU(3)_F$. We show that inter-generational hierarchy between the masses of the first two families arises if the flavour symmetry is broken with an intermediate $SU(2)$ leading to a specific ordering in the masses of the gauge bosons. Based on this scheme, we construct an explicit and predictive model and show its viability in reproducing the realistic charged fermion masses and quark mixing parameters in terms of not-so-hierarchical fundamental couplings. The model leads to the strange quark mass, $m_s \approx 16$ MeV at $M_Z$, which is $\sim 2.4 \sigma$ away from its current central value. Large flavour violations are a generic prediction of the scheme which pushes the masses of the new gauge bosons to $10^3$ TeV or higher.

Abstract: The Two Higgs Doublet model extended with a complex scalar singlet (2HDMS) is a well-motivated Beyond Standard Model candidate addressing several open problems of nature. In this work, we focus on the dark matter (DM) phenomenology of the complex scalar singlet where the real part of the complex scalar obtains a vacuum expectation value. The model is characterized by an enlarged Higgs spectrum comprising six physical Higgs bosons and a pseudoscalar DM candidate. We address the impact of accommodating the 95 GeV excess on the 2HDMS parameter space and DM observables after including all theoretical and experimental constraints. Finally, we look into the prospects of this scenario at HL-LHC and future lepton colliders for a representative benchmark.

Abstract: The nucleon axial-vector coupling constant $g_A$ is studied in the presence of an external magnetic field, and in dense nuclear environments, to emulate nuclear matter in magnetars. For this purpose we use QCD finite energy sum rules for two-current and three-current correlators, the former involving nucleon-nucleon correlators and the latter involving proton-axial-neutron currents. As a result, the axial-vector coupling constant decreases both with baryon density as well as with magnetic field. The axial-vector coupling evaluated with baryon density near the nuclear density $\rho_0$ leads to $g_A^*\approx 0.92$. In the presence of magnetic fields $g_A$ decreases in general, but $g_A^*$ does not show significant changes.

Abstract: In light of recent measurements suggesting potential lepton flavor universality violations in semileptonic decays at LHCb and other collider experiments, this article offers a brief review of the theoretical basis of tree- and loop-level $B$-hadron decays, $b \to c l \nu_l$ and $b \to s l^+ l^-$, and the experimental conditions. We reassess global averages for $\mathcal{R}_{D(D^*)}$, $\mathcal{R}_{K(K^*)}$, $\mathcal{R}_{J/\psi}$, and $\mathcal{R}_{\eta_c}$ in semileptonic transitions and have also provided the results for $B_c$ decay channels within the relativistic independent quark model context. If LHC Run 2 data evaluation corroborates Run 1 measurements, the effect of statistical significance in each decay channel could reach 5\,$\sigma$. The confirmation of these measurements could soon represent the first notable observation of physics beyond the Standard Model, broadening our perspective of New Physics.

Abstract: We propose a model realizes that a semi-visible dark photon which can contribute to the anomalous magnetic moment ($g-2$) of both electron and muon. In this model, the electron $g-2$ is deviated from the Standard Model (SM) prediction by the 1-loop diagrams involving the vector-like leptons, while that of muon is deviated due to a non-vanishing gauge kinetic mixing with photons. We also argue that the $W$-boson mass can be deviated from the SM prediction due to the vector-like lepton loops, so that the value obtained by the CDF II experiment can be explained. Thus, this model simultaneously explains the recent three anomalies in $g-2$ of electron and muon as well as the $W$-boson mass. The constraints on the $\mathcal{O}(1)~\mathrm{GeV}$ dark photon can be avoided because of the semi-invisible decay of the dark photon, $A^\prime \to 2 N \to 2\nu \,2\chi \to 2\nu \,4e$, where $N$ is a SM singlet vector-like neutrino and $\chi$ is a CP-even Higgs boson of the $U(1)^\prime$ gauge symmetry.

Abstract: Extracting maximal information from experimental data requires access to the likelihood function, which however is never directly available for complex experiments like those performed at high energy colliders. Theoretical predictions are obtained in this context by Monte Carlo events, which do furnish an accurate but abstract and implicit representation of the likelihood. Strategies based on statistical learning are currently being developed to infer the likelihood function explicitly by training a continuous-output classifier on Monte Carlo events. In this paper, we investigate the usage of Monte Carlo events that incorporate the dependence on the parameters of interest by reweighting. This enables more accurate likelihood learning with less training data and a more robust learning scheme that is more suited for automation and extensive deployment. We illustrate these advantages in the context of LHC precision probes of new Effective Field Theory interactions.

Abstract: The inclusion of an additional $U(1)$ gauge symmetry is a common feature in many extensions of the Standard Model, revealing the intricate connections between particle physics and cosmology. The $L_{\mu} - L_{\tau}$ model stands as a prominent member of this distinguished family, characterized by its anomaly-free nature and resilience in the face of collider constraints. This framework provides a unique vantage point for investigating both the intriguing mystery of the muon $(g-2)$ anomaly and the puzzling issue of the Hubble tension. However, due to the presence of kinetic mixing between the photon and $Z'$ in this model, the neutrinos have the potential to acquire minuscule electric charges, often referred to as millicharges ($q_{\nu}$) which is directly related to the strength of the new gauge couplings. A crucial question emerges: how does the model's inclusion of millicharges, while adhering to the stringent constraints imposed by experimental observations, influence its inherent ability to address the muon $(g-2)$ anomaly and the Hubble tension? We find the current upper bounds on $q_{\nu}$ derived from experiments such as the beam dump, XENONnT and LUX-ZEPLIN experiments can impose strong constraints on the $U(1)_{L_{\mu} - L_{\tau}}$ coupling. Consequently, these constraints may limit the ability of the model to fully accommodate the current measurement of $(g-2)_{\mu}$ while having a relatively minor impact on the resolution of the Hubble tension.

Abstract: Slepton coannihilation is one of the most promising scenarios that can bring the predicted Dark Matter (DM) abundance in the the Minimal Supersymmetric Standard Model (MSSM) into agreement with the experimental observation. In this scenario, the lightest supersymmetric particle (LSP), usually assumed to be the lightest neutralino, can serve as a Dark Matter (DM) candidate while the sleptons as the next-to-LSPs (NLSPs) lie close in mass. In our previous studies analyzing the electroweak (EW) sector of MSSM, a degeneracy between the three generations of sleptons was assumed for the sake of simplicity. In case of slepton coannihilation this directly links the smuons involved in the explanation for $(g-2)_\mu$ to the coannihilating NLSPs required to explain the DM content of the universe. On the other hand, in Grand Unified Theories such degeneracy do not hold, and often the lighter stau turns out to be the NLSP at the EW scale, with the smuons (and selectrons) somewhat heavier. In this paper we analyze a non-universal slepton mass scenario at the EW scale where the first two generations of sleptons are taken to be mass-degenerate and heavier than the staus, enforcing stau coannihilation. We analyze the parameter space of the MSSM in the light of a variety of experimental data namely, the DM relic density and direct detection (DD) limits, LHC data and especially, the discrepancy between the experimental result for $(g-2)_\mu$, and its Standard Model (SM) prediction. We find an upper limit on the LSP and NLSP masses of about ~ 550 GeV. In contrast to the scenario with full degeneracy among the three families of sleptons, the upper limit on the light smuon/selectron mass moves up by ~ 200 GeV. We analyze the DD prospects as well as the physics potential of the HL-LHC and a future high-energy $e^+ e^-$ collider to investigate this scenario further.

Abstract: We compute a theoretically driven prediction for the hadronic contribution to the electromagnetic running coupling at the $Z$ scale using lattice QCD and state-of-the-art perturbative QCD. We obtain$$\Delta\alpha^{(5)}(M^2_Z)=\left[279.5\pm0.9\pm0.59\right]\times10^{-4}\quad\quad\,\,\,\,\,\,(\mathrm{Mainz \,\,\,Collaboration})$$$$\Delta\alpha^{(5)}(M^2_Z)=\left[278.42\pm0.22\pm0.59\right]\times10^{-4}\,\,\,\,\,\,\,\,\quad(\mathrm{ BMW \,\,\,Collaboration}),$$ where the first error is the quoted lattice uncertainty. The second is due to perturbative QCD, and is dominated by the parametric uncertainty on $\hat{\alpha}_s$, which is based on a rather conservative error. Using instead the PDG average, we find a total error on $\Delta\alpha^{(5)}(M^2_Z)$ of $0.4\times10^{-4}$. Furthermore, with a particular emphasis on the charm quark contributions, we also update $\Delta\alpha^{(5)}(M^2_Z)$ when low-energy cross-section data is used as an input, obtaining $\Delta\alpha^{(5)}(M^2_Z) = \left[276.29 \pm 0.38 \pm 0.62\right] \times 10^{-4}$. The difference between lattice QCD and cross-section-driven results reflects the known tension between both methods in the computation of the anomalous magnetic moment of the muon. Our results are expressed in a way that will allow straightforward modifications and an easy implementation in electroweak global fits.

Abstract: We revisit the graphic table of QCD signatures in our 1996 Annual Reviews article "The Search for the Quark-Gluon Plasma" and assess the progress that has been made since its publication towards providing quantitative evidence for the formation of a quark-gluon plasma in relativistic heavy-ion collisions and its characteristic properties.