arXiv subjects

Zong-guo Si

Publications and source records attributed to Zong-guo Si.

9 recordsLinked to original sources

arXiv2025

Exploring multi-step electroweak phase transitions in the 2HDM+$\boldsymbol{a}$

Multiple electroweak phase transitions occurring sequentially in the early universe can give rise to intriguing phenomenology, compared to the typical single-step electroweak phase transition. In this work, we investigate this scenario within the framework of the two-Higgs-doublet model with a pseudoscalar, utilizing the complete one-loop finite-temperature effective potential. After considering relevant experimental and theoretical constraints, we identify four distinct types of phase transitions. In the first case, only the configuration of the CP-even Higgs acquires a non-zero value via a first-order or a cross-over electroweak phase transition, leading to electroweak symmetry breaking. In the remaining three cases, the pseudoscalar fields can obtain vacuum expectation values at different phases of the multi-step phase transition process, leading to spontaneous breaking of the CP symmetry. As the temperature decreases, the phase shifts to the vacuum observed today via first-order electroweak phase transition, at this point, the vacuum expectation value of the pseudoscalar field returns to zero, restoring the CP symmetry. Finally, we compare the transition strength and the stochastic gravitational wave background generated in the four situations along with the projected detection limits.

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arXiv2020

Top quark as a probe of heavy Majorana neutrino at the LHC and future collider

Right-handed (RH) Majorana neutrinos play a crucial role in understanding the origin of neutrino mass, the nature of dark matter and the mechanism of matter-antimatter asymmetry. In this work, we investigate the observability of heavy Majorana neutrino through the top quark neutrinoless double beta decay process $t \to b \ell^+ \ell^+ j j$ at hadron colliders. By performing detector level simulation, we demonstrate that our method can give stronger limits on the light-heavy neutrino mixing parameters $|V_{eN, μN}|$ in the mass range of 15 GeV $< m_N <$ 80 GeV than other existing collider bounds.

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arXiv2018

Study Standard Model and Majorana Neutrino Contributions to $B^{+} \to K^{(*)\pm}μ^+μ^{\mp}$

Lepton number violation processes can be induced by the Majorana neutrino exchange, which provide evidence for the Majorana nature of neutrinos. In addition to the natural explanation of the small neutrino masses, Type-I seesaw mechanism predicts the existence of Majorana neutrinos. The aim of this work is to study the B meson rare decays $B^{+} \to K^{(*)+}μ^+μ^-$ in the standard model and its extensions, and then to investigate the same-sign decay processes $B^{+}\to K^{(*)-}μ^{+}μ^+$. The corresponding dilepton invariant mass distributions are predicted. It is found that the dilepton angular distributions illustrate the properties of new interactions induced by the Majorana neutrinos.

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arXiv2018

Central-edge asymmetry as a probe of Higgs-top coupling in $t\bar{t}h$ production at LHC

The Higgs-top coupling plays a central role in the hierarchy problem and the vacuum stability of the Standard Model (SM). We propose a central-edge asymmetry ($A_{CE}$) to probe the CP violating Higgs-top coupling in dileptonic channel of $t\bar{t}h(\to b\bar{b})$ production at the LHC. We demonstrate that the CP-violating Higgs-top coupling can affect the central-edge asymmetry through distorting $Δy_{\ell^+\ell^-}$ distribution because of the contribution of new top charge asymmetric term. Since $Δy_{\ell^+\ell^-}$ distribution is frame-independent and has a good discrimination even in boosted regime, we use the jet substructure technique to enhance the observability of the dileptonic channel of $t\bar{t}h$ production. We find that (1) the significance of dileptonic channel of $t\bar{t}h$ production can reach $5σ$ for CP phase $ξ=0,π/4,π/2$ when the luminosity ${\cal L}=795,993,1276$ fb$^{-1}$ at 14 TeV LHC. (2) the central-edge asymmetry $A_{CE}$ show a good discrimination power of CP phase of $t\bar{t}h$ interaction, which are -40.26\%, -26.60\%, -9.47\% for $ξ=0$, $π/4$, $ π/2$ respectively and are hardly affected by the event selections. Besides, by performing the binned-$χ^2$ analysis of $Δy_{\ell^+\ell^-}$ distribution, we find that the scalar and pseudo-scalar interactions can be distinguished at 95\% C.L. level at 14 TeV HL-LHC.

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arXiv2017

Broken $S^{}_{3{\rm L}} \times S^{}_{3{\rm R}}$ Flavor Symmetry and Leptonic CP Violation

In the framework of canonical seesaw model, we present a simple but viable scenario to explicitly break an $S^{}_{3{\rm L}} \times S^{}_{3{\rm R}}$ flavor symmetry in the leptonic sector. It turns out that the leptonic flavor mixing matrix is completely determined by the mass ratios of charged leptons (i.e., $m^{}_e/m^{}_μ$ and $m^{}_μ/m^{}_τ$) and those of light neutrinos (i.e., $m^{}_1/m^{}_2$ and $m^{}_2/m^{}_3$). The latest global-fit results of three neutrino mixing angles $\{θ^{}_{12}, θ^{}_{13}, θ^{}_{23}\}$ and two neutrino mass-squared differences $\{Δm^2_{21}, Δm^2_{31}\}$ at the $3σ$ level are used to constrain the parameter space of $\{m^{}_1/m^{}_2, m^{}_2/m^{}_3\}$. The predictions for the mass spectrum and flavor mixing are highlighted: (1) The neutrino mass spectrum shows a hierarchical pattern and a normal ordering, e.g., $m^{}_1 \approx 2.2~{\rm meV}$, $m^{}_2 \approx 8.8~{\rm meV}$ and $m^{}_3 \approx 52.7~{\rm meV}$; (2) Only the first octant of $θ^{}_{23}$ is allowed, namely, $41.8^\circ \lesssim θ^{}_{23} \lesssim 43.3^\circ$; (3) The Dirac CP-violating phase $δ\approx -22^\circ$ deviates significantly from the maximal value $-90^\circ$. All these predictions are ready to be tested in the ongoing and forthcoming neutrino oscillation experiments. Moreover, we demonstrate that the cosmological matter-antimatter asymmetry can be explained via resonant leptogenesis, including the individual lepton-flavor effects. In our scenario, the leptonic CP violation at low- and high-energy scales are closely connected.

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arXiv2013

Lepton Number Violation and W' Chiral Couplings at the LHC

We study the observability for a heavy Majorana neutrino N along with a new charged gauge boson W' at the LHC. We emphasize the complementarity of these two particles in their production and decay to unambiguously determine their properties. We show that the Majorana nature of N can be verified by the lepton-number violating like-sign dilepton process, and by polar and azimuthal angular distributions. The chirality of the W' coupling to leptons and to quarks can be determined by a polar angle distribution in the reconstructed frame and an azimuthal angle distribution.

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arXiv2010

Chiral Couplings of W' and Top Quark Polarization at the LHC

If a TeV-scale charged gauge boson (W') is discovered at the Large Hadron Collider (LHC), it will become imperative to determine its chiral couplings to standard model (SM) fermions in order to learn about the underlying theory containing the W'. We describe the reconstruction of the t, b decay mode of the W' at the LHC, and identify various kinematic observables such as the angular distributions of the top quark and the lepton resulting from top decay that can be used to disentangle the chiral couplings of the W' to SM fermions. We demonstrate by presenting analytical expressions, numerical simulations, as well as intuitive illustrations for these observables at the LHC that among the SM fermions, the polarized top quark can most directly probe the chirality of such couplings.

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arXiv2009

Testing the Realistic Seesaw Model with Two Heavy Majorana Neutrinos at the CERN Large Hadron Collider

In the conventional type-(I+II) seesaw model, the effective mass matrix of three known light neutrinos is given by M_nu = M_L - M_D M^{-1}_R M^T_D in the leading-order approximation. We propose an intriguing scenario, in which the structural cancellation condition M_D M^{-1}_R M^T_D = 0 is guaranteed by the A_4 x Z_2 flavor symmetry. As a consequence, neutrino masses are mainly generated by the Higgs triplet M_nu = M_L, while the neutrino mixing matrix is non-unitary and takes on the nearly tri-bimaximal pattern. A discriminating feature of this scenario from the pure type-II seesaw model is that the lepton-number-violating signatures induced by the heavy Majorana neutrinos can be discovered at the CERN Large Hadron Collider. We calculate the total cross section of the same-sign dilepton events pp to l^+(-)_alpha N_i to l^+(-)_alpha l^+(-)_beta jj (for i=1, 2 and alpha, beta = e, mu, tau), and emphasize the significant interference of the contributions from two different heavy Majorana neutrinos. The background from the standard model and the kinematic cuts used to reduce it have been considered. The possible way to distinguish between the signals from heavy Majorana neutrinos and those from doubly-charged Higgs bosons is briefly discussed.

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arXiv1999

$e^{+}e^{-}\to Z^0Z^0\to b\bar{b}c\bar{c}$ events as model independent probe of colour reconnection effects

According to the basic properties of QCD, colour reconnection effects can occur in hadronic processes at high energies. The comparison of $e^{+}e^{-}\to Z^0Z^0\to q\bar{q}q'\bar{q'}$ events with the superposition of $Z^0\to q\bar{q}$ and $Z^0\to q'\bar{q'}$ events from LEP1 would provide an unambiguous model-independent probe. We show that at LEP2 energy, the background processes are negligible if we select only $e^{+}e^{-}\to Z^0Z^0\to b \bar{b}c\bar{c}$ events, and limit the measurements in the phase space of on-shell $Z^0Z^0$ events.

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