High Energy Physics - Phenomenology (hep-ph)

Abstract: With the rapid development of high-power petawatt class lasers worldwide, exploring physics in the strong field QED regime will become one of the frontiers for laser-plasma interactions research. Particle-in-cell codes, including quantum emission processes, are powerful tools for predicting and analyzing future experiments where the physics of relativistic plasma is strongly affected by strong-field QED processes. The spin/polarization dependence of these quantum processes has been of recent interest. In this article, we perform a parametric study of the interaction of two laser pulses with an ultrarelativistic electron beam. The first pulse is optimized to generate high-energy photons by nonlinear Compton scattering and efficiently decelerate the electron beam through quantum radiation reaction. The second pulse is optimized to generate electron-positron pairs by nonlinear Breit-Wheeler decay of the photons with the maximum polarization dependence. This may be experimentally realized as a verification of the strong field QED framework, including the spin/polarization rates.

Abstract: $R_{D^{(*)}}$ is the ratio of branching ratio $\overline{B} \rightarrow D^{(*)}\tau\overline{\nu}_{\tau}$ to $\overline{B} \rightarrow D^{(*)}l\overline{\nu}_{l}$. There is a gap of $2\sigma_{exp}$ or more between its experimental value and the prediction under the standard model(SM). People extend the MSSM with the local gauge group $U(1)_X$ to obtain the $U(1)_X$SSM. Compared with MSSM, $U(1)_X$SSM has more superfields and effects. In $U(1)_X$SSM, we research the decays $\overline{B} \rightarrow D^{(*)}l\overline{\nu}_{l}$ and calculate $R_{D^{(*)}}$. The obtained numerical results of $R_{D^{(*)}}$ are further corrected under $U(1)_X$SSM, which is much better than the SM predictions. After correction, the theoretical value of $R_{D^{(*)}}$ can reach in one $\sigma_{exp}$ range of the averaged experiment central value.

Abstract: For the study of the structure of baryons it is necessary the investigate the production of a baryon pair in $e^+e^-$ annihilation. The baryon-antibaryon pair production at the electron-positron linear collider makes it possible to investigate in detail the basic structure of the Standard Model. The creation of baryon-antibaryon pairs in electron-positron annihilation provides an increasingly powerful tool at higher center-of-mass energies. We present phenomenological results for $\Sigma^0 \bar {\Sigma}^0$ production in $e^+e^-$ interaction at the BESIII and BABAR Colliders. In the present work, we investigate a hyperon pair produced in the reaction $e^+e^- \to \Sigma^0 \bar{\Sigma}^0$. We calculate the total cross section of the process $e^+e^- \to \Sigma^0 \bar {\Sigma}^0$ taking into account the contributions of the $D$-meson loop and three gluon loops as well as the interference of all diagrams to the Born approximation. For these contributions large relative phases are generated with respect to the pure electromagnetic mechanism. For the large momentum transferred region we obtain as a by product a fit of the electromagnetic form factor of the $\Sigma$ hyperon. The obtained results are in satisfactory agreement with experimental data.

Abstract: We present the first data-driven result for $a_\mu^{\rm win,lqc}$, the isospin-limit light-quark connected component of the intermediate-window Hadronic-Vacuum-Polarization contribution to the muon anomalous magnetic moment. Our result, $(198.8\pm 1.1)\times 10^{-10}$, is in significant tension with eight recent mutually compatible high-precision lattice-QCD determinations, and provides enhanced evidence for a puzzling discrepancy between lattice and data-driven determinations of the intermediate window quantity, one driven largely by a difference in the light-quark connected component.

Abstract: The self-interacting dark matter (SIDM) paradigm provides a potential solution to the challenge faced by the cold dark matter model in explaining small-scale structure problems. This paradigm incorporates self-interactions among DM particles, typically mediated by a particle with a mass around MeV. The recent evidences of nano-Hertz gravitational waves from NANOGrav, EPTA, PPTA, and CPTA collaborations indicate a first-order phase transition (FOPT) occurring at a temperature of the MeV scale. Considering the close proximity between these two scales, we postulate that the mediator mass in the SIDM model originates from the spontaneous breaking of a $U(1)'$ symmetry, which is driven by the FOPT indicated by pulsar time array data. Consequently, the alignment of these two scales is believed to be deeply connected by the same underlying physics. Through a comprehensive survey of the parameter space, we identify the viable region favored by SIDM and simultaneously provide an explanation for the pulsar timing array data.

Abstract: We report on the computation of NLO QCD corrections to top-quark pair production in association with two photons at the LHC. Higher-order effects and photon bremsstrahlung are taken into account in the production and decays of the top-quark pair. Top-quark and $W$-boson decays are treated in the Narrow Width Approximation conserving spin correlations up to NLO in QCD. This is the first time that the complete set of NLO QCD corrections to the $pp \to t\bar{t}\gamma\gamma$ process including top-quark decays is calculated. We present results at the integrated and differential cross-section level in the di-lepton and lepton $+$ jet channel. In addition, we investigate the effect of photon bremsstrahlung in $t\bar{t}$ production and top-quark decays, as well as the mixed contribution. The latter contribution, in which two photons occur simultaneously in the production and decay of the $t\bar{t}$ pair, proved to be significant at both the integrated and differential cross-section level.

Abstract: In one of our recent papers, a second Higgs-like boson $h'$ with the mass near 0.5~TeV was predicted from a dual holographic model (borrowed from the AdS/QCD approach) for a hypothetical strongly-coupled BSM sector. In the present work, we reproduce this prediction within the framework of more traditional phenomenological approaches to effective description of a strongly-coupled field theory -- the Nambu--Jona-Lasinio model and spectral sum rules. A good quantitative agreement between these three drastically different methods is obtained. Interestingly, the existence of the $h'$ with the predicted mass turns out to be a minimal possibility to cancel the quadratic divergence in the vacuum energy density. We argue also that the dominant channel for the production of $h'$ should be $h\rightarrow h'h$, where $h$ is the standard Higgs boson. Hence, the $H(650)$ resonance observed recently at the LHC can be naturally interpreted as an enhancement due to the $hh'$-threshold. Within the considered scenario, the $h$ and $h'$ bosons in a conjectured BSM theory play the role of the $\pi$ and $\sigma$ mesons in low-energy descriptions of QCD. Our result can be understood as a quantitative consequence of universality of strong coupling regime in both theories, i.e., the prediction represents a BSM analogue of the numerical QCD relation $m_\sigma\simeq 4m_\pi$. We argue that the obtained result is a model-independent consequence of the assumed universality.

Abstract: We present the second part of a paper series devoted to the study of the multi-loop effective potential evolution in $\varphi^4$-theory using the conformal symmetry. In this paper, we demonstrate that the conformal symmetry can still be useful for the effective potential approach even at the presence of the mass parameter. To this goal, it is necessary to introduce the special treatment of the mass terms as sorts of interaction in an asymptotical expansion of the generating functional. The introduced vacuum $V_{z,x}$-operation is one the main tool to the algebraic scheme of anomalous dimension calculations. It is shown that the vacuum $V_{z,x}$-operation transforms the given Green functions to the corresponding vacuum integrations which generate the effective potential.

Abstract: Very recently Pulsar Timing Array (PTA) collaborations have independently reported the evidence for a stochastic gravitational-wave background (SGWB), which can unveil the formation of primordial seeds of inhomogeneities in the early universe. With the SGWB parameters inferred from PTAs data, we can make a prediction of the Primordial Black Hole (PBH) clusters from the domain walls of axion-like particles (ALPs). These primordial seeds can naturally provide a solution to the early Active Galactic Nuclei (AGN) formation indicated by James Webb Space Telescope (JWST). Besides, the mass of ALP is also constrained, $m_a \sim 10^{-15}-10^{-14}$ eV, within the reach of upcoming cavity experiments.

Abstract: For the typical forbidden dark matter (DM), the correct relic density is determined exclusively by kinetically forbidden DM annihilations which vanish at zero temperature. We present a model that contains the DM and a heavier but unstable scalar mediator in the hidden sector. When the temperature drops below $\sim m_{\rm DM}$, this hidden sector, thermally decoupled from the visible sector, enters a cannibal phase (with zero chemical potential), during which the DM density is depleted with the out-of-equilibrium decay of the scalar mediator. As such, the freeze-out process, described by forbidden DM annihilations to mediators, evolves with a temperature different from the SM bath. The DM candidate of having a mass in the range of tens of MeV can result in the correct relic density and sizable 2-to-2 self-interactions, which fit small structure problems. The future sensitivity of the NA62 beam dump experiment can probe the parameter space of the related scalar mediator.

Abstract: We investigate the potential of the warped-extradimension framework as an explanation for the recently observed stochastic gravitational background at nHz frequencies in pulsar timing arrays (PTA). Our analysis reveals that the PTA data can be effectively accommodated by a first-order phase transition triggered by a radion at the MeV-GeV scale feebly coupled to the Standard Model. Remarkably, this outcome remains robust irrespective of the specific details of the warped extradimension embedding, providing a foundation for future investigations aiming to develop concrete extradimension descriptions of Nature. We also demonstrate that many existing embeddings are not viable, as their radion and graviton phenomenology clash with a MeV-GeV scale radion. As a possible way-out, we sketch a promising solution involving multiple branes, wherein the light radion, graviton, and ensuing light resonances remain consistent with collider bounds and gravity tests.

Abstract: We explore the possibility that a confining first-order phase transition of a nearly-conformal dark sector generates the reported NANOGrav signal of a stochastic gravitational wave background. The visible Standard Model (SM) sector and the dark sector are initially thermally decoupled so that their temperatures are different. The nearly conformal phase transition is described by the shallow potential of a dilaton (or a radion in the 5D holographic perspective) generated by a new dark Yang-Mills field coupled to the conformal sector. For a dark sector only gravitationally connected with the visible sector, the NANOGrav signal is explained by the phase transition without contradicting the $\Delta N_{\rm eff}$ constraint, together with a contribution from supermassive black hole binaries. While the dilaton and dark glueballs can be produced after the phase transition, they immediately decay into dark radiation, which can help ameliorate the Hubble tension and be tested by the future CMB-S4 experiment. Alternatively, for a dark conformal sector decaying into the visible sector after the phase transition, the $\Delta N_{\rm eff}$ constraint is not applied and the phase transition can solely explain the NANOGrav signal.

Abstract: Recently, the Hellings Downs correlation has been observed by different pulsar timing array (PTA) collaborations, such as NANOGrav, European PTA, Parkes PTA, and Chinese PTA. These experimental studies through PTA of the most precise pulsars within the Milky Way show the first robust evidence for the stochastic gravitational wave background of our Universe. We study the ultralight axion interpretation of the new discovery by investigating the gravitational wave from the energy-level transition of the gravitational atoms, which is composed of cosmic populated Kerr black holes and their surrounding axion clouds from the superradiance process. We demonstrate that this new observation admits an axion interpretation for the ultralight axion mass in the range $10^{-21}\sim 10^{-20}$~eV.

Abstract: We examine the couplings of quarkonium hybrids to heavy-light meson pairs in the Born-Oppenheimer approximation for QCD. The lowest hybrid multiplets consist of bound states of the $\Pi_u$ and $\Sigma_u^-$ potentials. We find that the $\Sigma_u^-$ potential can couple to pairs of $S$-wave mesons through string breaking, while the $\Pi_u$ potential cannot. From this observation, we derive model-independent selection rules that contradict previous expectations that quarkonium hybrids are forbidden to decay into pairs of $S$-wave mesons. These Born-Oppenheimer selection rules are consistent with the partial decay widths of the lowest charmonium hybrid with exotic quantum numbers $J^{PC}=1^{-+}$ recently calculated in lattice QCD.

Abstract: We show that the recently reported NANOGrav, EPTA, PPTA, and CPTA data suggesting the existence of stochastic gravitational waves in the nanohertz region can be explained by axion domain walls coupled to QCD. In this scenario, the non-perturbative effects of QCD generate a temperature-dependent bias for the domain wall around the QCD phase transition, leading to an immediate collapse of the domain walls. We perform dedicated lattice simulations of the axion domain walls, taking into account the temperature dependence of the bias, to estimate the gravitational waves emitted during the domain wall annihilation process. We also discuss the future prospects for accelerator-based searches for the axion and the potential for the formation and detection of primordial black holes.

Abstract: For a discrete symmetry that is anomalous under QCD, the domain walls produced in the early universe from its spontaneous breaking can naturally annihilate due to QCD instanton effects. The gravitational waves generated from wall annihilation have their amplitude and frequency determined by both the discrete symmetry breaking scale and the QCD scale. The evidence of stochastic gravitational waves at nanohertz observed by pulsar timing array experiments suggests that the discrete-symmetry-breaking scale is around 100 TeV, assuming the domain-wall explanation. The annihilation temperature is about 100 MeV, which could naturally be below the QCD phase transition temperature. We point out that the QCD phase transition within some domains with an effective large QCD $\theta$ angle could be a first-order one. To derive the phase diagram in $\theta$ and temperature, we adopt a phenomenological linear sigma model with three quark flavors. The domain-wall explanation for the NANOGrav, EPTA, PPTA and CPTA results hints at a first-order QCD phase transition, which predicts additional gravitational waves at higher frequencies. If the initial formation of domain walls is also a first-order process, this class of domain-wall models predicts an interesting gravitational wave spectroscopy with frequencies spanning more than ten orders of magnitude, from nanohertz to 100 Hz.