High Energy Physics - Phenomenology (hep-ph)

Abstract: In this paper, we study the feeble sterile neutrino portal dark matter under the $Z_3$ symmetry. The dark sector consists of one fermion singlet $\chi$ and one scalar singlet $\chi$, which transforms as $\chi\to e^{i2\pi/3}\chi, \phi\to e^{i2\pi/3}\phi$ under the $Z_3$ symmetry. Regarding fermion singlet $\chi$ as the dark matter candidate, the new interaction terms $y_\chi \phi \bar{\chi^c}\chi$ and $\mu\phi^3/2$ could induce various new production channels. For instance, when $m_\phi>2m_\chi$, the pair decay $\phi\to\chi\chi$ could be the dominant channel, rather than the delayed decay $\phi\to\chi\nu$. Another appealing scenario is when the dark sector is initially produced through the scattering process as $NN\to\chi\chi, NN\to\phi\phi,h\nu\to\chi\phi$, then the semi-production processes $N \chi\to\phi\phi, N\phi\to\phi\chi, N\chi\to\chi\chi$ could lead to the exponential growth of dark sector abundances. The phenomenology of sterile neutrino and the cosmological impact of the dark scalar are also considered in the $Z_3$ symmetric model.

Abstract: We investigate fermionic dark matter interactions with standard model particles from an additional $\mathrm{U}(1)_\mathrm{X}$ gauge symmetry, assuming kinetic mixing between the $\mathrm{U}(1)_\mathrm{X}$ and $\mathrm{U}(1)_\mathrm{Y}$ gauge fields as well as a nonzero $\mathrm{U}(1)_\mathrm{X}$ charge of the Higgs doublet. For ensuring gauge-invariant Yukawa interactions and the cancellation of gauge anomalies, the standard model fermions are assigned $Y$-sequential $\mathrm{U}(1)_\mathrm{X}$ charges proportional to the Higgs charge. Although the Higgs charge should be small due to collider constraints, it is useful to decrease the effective cross section of dark matter scattering off nucleons by two orders of magnitude and easier evade from direct detection bounds. After some numerical scans performed in the parameter space, we find that the introduction of the Higgs charge can also enhance the dark matter relic density by at least two orders of magnitude. When the observed relic density and the direct detection constraints are tangled, at the case where the resonance effect is important for dark matter freeze-out, the Higgs charge can expand physical windows to some extent by relieving the tension between the relic density and the direct detection.

Abstract: We investigate the properties of the gravitational waves (GWs) generated during a strongly first order electroweak phase transition (EWPT) in models with the classical scale invariance (CSI). Here, we distinguish two parameter space regions that correspond to the cases of (1) light dilaton and (2) purely radiative Higgs mass (PRHM). In the CSI models, the dilaton mass, or the Higgs mass in the PRHM case, in addition to some triple scalar couplings are fully triggered by the radiative corrections (RCs). In order to probe the RCs effects on the EWPT strength and on the GW spectrum, we extend the standard model by a real singlet to assist the electroweak symmetry breaking and an additional scalar field $Q$ with multiplicity $N_Q$ and mass $m_Q$. After imposing all theoretical and experimental constraints, we show that a strongly first order EWPT with detectable GW spectra can be realized for the two cases of light dilaton and PRHM. We also show the corresponding values of the relative enhancement of the cross section for the di-Higgs production process, which is related to the triple Higgs boson coupling. We obtain the region in which the GW spectrum can be observed by different future experiments such as LISA and DECIGO. We also show that the scenarios (1) and (2) can be discriminated by future GW observations and measurements of the di-Higgs productions at future colliders.

Abstract: We estimate the coupling constants and decay widths of the $SU(3)$ partners of the $\Omega(2012)$ hyperon, as discovered by the BELLE Collaboration, using the light cone sum rules method. Our study includes a comparison of the obtained results for relevant decay widths with those derived within the framework of the flavor $SU(3)$ analysis. We observe a good agreement between the predictions of both approaches. The results we obtain for the branching ratio can provide helpful insights for determining the nature of the $SU(3)$ partners of the $\Omega(2012)$ baryon.

Abstract: In this letter, we perform a chiral perturbative analysis by singular values $m_{Di}$ of the Dirac mass matrix $m_{D}$ for the type-I seesaw mechanism. In the basis where $m_{D} = V m_{D}^{\rm diag} U^{\dagger}$ is diagonal, the mass matrix of right-handed neutrinos $M_{R}$ is written by $M_{R} = m_{D}^{\rm diag} m^{-1} m_{D}^{\rm diag}$. If the mass matrix of light neutrinos $m$ has an inverse matrix and the singular values $m_{Di}$ are hierarchical ($m_{D1} \ll m_{D2} \ll m_{D3}$), the singular values $M_{i}$ and diagonalization matrix $U$ of $M_{R}$ are obtained perturbatively. By treating $m_{Di}$ and $V$ as input parameters, $m_{D}$ is represented in the basis where $M_{R}$ is diagonal, and we perturbatively derive the orthogonal matrix $R$ in Casas--Ibarra parameterization. As a result, $R$ is independent of $m_{Di}$ in the leading order, and it is reconstructed as an orthonormal basis $R_{i1} \simeq \pm \sqrt{m_{i} / m_{11} } (U_{\rm MNS}^{T} V^{*})_{i1} \, , R_{i2} \simeq \pm \epsilon_{ijk} R_{j3} R_{k1} \, , R_{i3} \simeq \pm {(U_{\rm MNS}^{\dagger} V)_{i3} / \sqrt {m_{i} (m^{-1})_{33}} } $. Here, $m_{i}$ is the masses of light neutrinos and $\pm$ denotes the independent degree of freedom for each column vector.

Abstract: We study the phenomenology of leptophilic $Z'$ gauge bosons at the future high-energy $e^+e^-$ or $\mu^+\mu^-$ colliders, as well as at the gravitational wave observatories. The leptophilic $Z'$ model, although well-motivated, remains largely unconstrained from current low-energy and collider searches for $Z'$ masses above ${\cal O}(100~{\rm GeV})$, thus providing a unique opportunity for future lepton colliders. Taking leptophilic $U(1)_{L_\alpha-L_\beta}~(\alpha,\beta=e,\mu,\tau)$ models as concrete examples, we show that future $e^+e^-$ and $\mu^+\mu^-$ colliders with multi-TeV center-of-mass energies provide unprecedented sensitivity to heavy $Z'$ bosons. Moreover, if these $U(1)$ models are classically scale-invariant, the phase transition at the $U(1)$ symmetry-breaking scale tends to be strongly first-order with ultra-supercooling, and leads to observable stochastic gravitational wave signatures. We find that the future sensitivity of gravitational wave observatories, such as advanced LIGO-VIRGO and Cosmic Explorer, can be complementary to the collider experiments, probing higher $Z'$ masses up to ${\cal O}(10^4~{\rm TeV})$.

Abstract: Soft and collinear radiation in collider processes can be described in a universal way, that is independent of the underlying process. Recent years have seen a number of approaches for probing whether radiation beyond the leading soft approximation can also be systematically classified. In this paper, we study a formula that captures the leading next-to-soft QCD radiation affecting processes with both final- and initial-state partons, by shifting the momenta in the non-radiative squared amplitude. We first examine W+jet production, and show that a previously derived formula of this type indeed holds in the case in which massive colour singlet particles are present in the final state. Next, we develop a physical understanding of the momentum shifts, showing precisely how they disrupt the well-known angular ordering property of leading soft radiation.

Abstract: The work presented considers a class of minimally constructed Yukawa unified SUSY GUTs - NUHM2 - and explore their implications when their soft supersymmetry breaking Lagrangian is generalized by the non-holomorphic terms which provide extra contributions to the Higgsino mass and couple the supersymmetric scalar fields to the wrong Higgs doublets. With such a simple extension, it can be found several regions with interesting implications which cannot be realized in the usual restricted models. It is observed that the Yukawa unification solutions can be compatible with relatively light mass spectrum and acceptable low fine-tuning measurements. In the restricted models such effects can directly be addressed to the non-holomorphic terms. They can provide a slight improvement in the SM-like Higgs boson mass without altering the mass spectrum too much, and they can accommodate relatively lighter sbottom and stau masses, while they do not change the stop sector much. The dark matter can be Higgsino-like or Bino-like, but the experimental relic density measurements favor the Higgsino-like dark matter, while the Bino-like dark matter is predicted with a quite large relic density. Also several coannihilation scenarios are identified in the Higgsino-like dark matter regions, while the Bino-like dark matter do not allow any of such coannihilation processes. The presence of the non-holomorphic terms can weaken the impact from the phenomenological or indirect constraints such as low fine-tuning, Yukawa unification and rare decays of $B-$meson, the direct and model independent constraints still yield a strong strike on the solutions. Such constraints are discussed in regard of the current collider analyses on $\tau\tau$ events and direct detection of dark matter experiments.