arXiv subjects

Xiao-Gang He

Publications and source records attributed to Xiao-Gang He.

At least 91 records · Page 5Linked to original sources

arXiv2018

Scalar Electroweak Multiplet Dark Matter

We revisit the theory and phenomenology of scalar electroweak multiplet thermal dark matter. We derive the most general, renormalizable scalar potential, assuming the presence of the Standard Model Higgs doublet, $H$, and an electroweak multiplet $Φ$ of arbitrary SU(2$)_L$ rank and hypercharge, $Y$. We show that, in general, the $Φ$-$H$ Higgs portal interactions depend on three, rather than two independent couplings as has been previously considered in the literature. For the phenomenologically viable case of $Y=0$ multiplets, we focus on the septuplet and quintuplet cases, and consider the interplay of relic density and spin-independent direct detection cross section. We show that both the relic density and direct detection cross sections depend on a single linear combination of Higgs portal couplings, $λ_{\rm eff}$. For $λ_{\rm eff}\sim \mathcal{O}(1)$, present direct detection exclusion limits imply that the neutral component of a scalar electroweak multiplet would comprise a subdominant fraction of the observed DM relic density.

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arXiv2018

Decay rate and asymmetries of $Σ^+\to pμ^+μ^-$

The LHCb Collaboration has recently reported evidence for the rare hyperon decay $Σ^+\to pμ^+μ^-$ that is consistent with the standard model expectation. Motivated by this new result we revisit the calculation of this mode including both long and short distance contributions. In the standard model this mode is completely dominated by long distance physics and thus subject to large uncertainties. We present a range of predictions for the rate, the dimuon invariant mass spectrum, and a muon polarization asymmetry that covers these uncertainties as well as known constraints. We study the interplay between short and long distance contributions which could result in additional asymmetries, but we find that they are negligible within the standard model. We propose a parameterization of these asymmetries in terms of a couple of constants that can arise from new physics.

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arXiv2018

Flavor Violating Higgs Couplings in Minimal Flavor Violation

Motivated by the rencent LHC data on the lepton-flavor violating (LFV) decays $h\to \ell_1 \ell_2$ and $B_{s,d}\to \ell_1 \ell_2$, we study the Higgs-mediated flavor-changing neutral current (FCNC) interactions in the effective field theory (EFT) approach without and with the minimal flavor violation (MFV) hypothesis, and concentrate on the later. After considering the $B$ and $K$ physics data, the various LFV processes, and the LHC Higgs data, severe constraints on the Higgs FCNC couplings are derived, which are dominated by the LHC Higgs data, the $B_s - \bar B_s$ mixing, and the $μ\to e γ$ decay. In the general case and the MFV framework, allowed ranges of various observables are obtained, such as $\mathcal B (B_s\to \ell_1 \ell_2)$, $\mathcal B (h\to \ell_1 \ell_2)$, $\mathcal B (h\to q_1 q_2)$, and the branching ratio of $μ\to e$ conversion in Al. Future prospects of searching for the Higgs FCNC interactions at the low-energy experiments and the LHC are discussed.

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arXiv2018

Measuring the ratio of $HWW$ and $HZZ$ couplings through $W^+W^-H$ production

For a generic Higgs boson, measuring the relative sign and magnitude of its couplings with the $W$ and $Z$ bosons is essential in determining its origin. Such a test is also indispensable for the 125-GeV Higgs boson. We propose that the ratio of the $HWW$ and $HZZ$ couplings $λ_{WZ}$ can be directly determined through the $W^+W^-H$ production, where $H$ denotes a generic Higgs boson, owing to the tree-level interference effect. While this is impractical at the LHC due to the limited sensitivity, it can be done at future $e^+e^-$ colliders, such as a 500-GeV ILC with the beam polarization $P(e^-,e^+)=(-0.8,+0.3)$ in the $jj\ell^{\pm}bb$ and $\ell^{\pm}\ell^{\pm}\ell^{\mp}jj$ channels. The discovery potential of a general ratio and the power to discriminate it from the SM value are studied in detail. Combining the cross section of $e^+e^-\to W^+ W^- H$ with the measurements of $HZZ$ coupling at the HL-LHC, one can further improve the sensitivity of $λ_{WZ}$.

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arXiv2018

Lepton universality violation and right-handed currents in $b \to c τν$

We consider the recent LHCb result for $B_c\to J/ψτν$ in conjunction with the existing anomalies in $R(D)$ and $R(D^\star)$ within the framework of a right-handed current with enhanced couplings to the third generation. The model predicts a linear relation between the observables and their SM values in terms of two combinations of parameters. The strong constraints from $b\to s γ$ on $W-W^\prime$ mixing effectively remove one of the combinations of parameters resulting in an approximate proportionality between all three observables and their SM values. To accommodate the current averages for $R(D)$ and $R(D^\star)$, the $W^\prime$ mass should be near 1 TeV, and possibly accessible to direct searches at the LHC. In this scenario we find that $R(J/ψ)$ is enhanced by about 20\% with respect to its SM value and about 1.5$σ$ below the central value of the LHCb measurement. The predicted $dΓ/dq^2$ distribution for $B\to D(D^\star) τν$ is in agreement with the measurement and the model satisfies the constraint from the $B_c$ lifetime.

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arXiv2018

Leptophilic dark matter in gauged $U(1)_{L_e-L_μ}$ model in light of DAMPE cosmic ray $e^+ + e^-$ excess

Motivated by the very recent cosmic-ray electron+positron excess observed by DAMPE collaboration, we investigate a Dirac fermion dark matter (DM) in the gauged $L_e - L_μ$ model. DM interacts with the electron and muon via the $U(1)_{e-μ}$ gauge boson $Z^{'}$. The model can explain the DAMPE data well. Although a non-zero DM-nucleon cross section is only generated at one loop level and there is a partial cancellation between $Z^{'}ee$ and $Z^{'}μμ$ couplings, we find that a large portion of $Z^{'}$ mass is ruled out from direct DM detection limit leaving the allowed $Z^{'}$ mass to be close to two times of the DM mass. Implications for $pp \to Z^{'} \to 2\ell$ and $pp \to 2\ell + Z^{'}$ , and muon $g-2$ anomaly are also studied.

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arXiv2018

Impacts of multi-Higgs on the $ρ$ parameter, decays of a neutral Higgs to $WW$ and $ZZ$, and a charged Higgs to $WZ$

In the standard model (SM), the $ρ$ parameter is equal to 1 and the ratio $λ_{WZ}$ of Higgs $\to ZZ$ and Higgs $\to WW$ is also equal to 1 at the tree level. When going beyond the SM with more than one types of Higgs representations these quantities may be different from the SM predictions which can provide crucial information about new physics. There may also exist a certain charged Higgs $h^+$ decays into a $W^+$ and a $Z$. Imposing a custodial symmetry can force the parameter $ρ$ to be equal to 1 with certain predictions for $λ_{WZ}$ and $h^+ \to W^+Z$. However, imposing $ρ=1$ without custodial symmetry may have different predictions. We show how differences arise and how to use experimental data to obtain information about the underlying physics in a model with the SM plus a real and a complex $SU(2)_L$ triplets.

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arXiv2018

Flavor SU(3) Topological Diagram and Irreducible Representation Amplitudes for Heavy Meson Charmless Hadronic Decays: Mismatch and Equivalence

Flavor SU(3) analysis of heavy meson ($B$ and $D$) hadronic charmless decays can be formulated in two different ways. One is to construct the SU(3) irreducible representation amplitude (IRA) by decomposing effective Hamiltonian according to the SU(3) transformation properties. The other is to use the topological diagrams (TDA). These two methods should give equivalent physical results in the SU(3) limit. Using $B \to PP$ decays as an example, we point out that previous analyses in the literature using these two methods do not match consistently in several ways, in particular a few SU(3) independent amplitudes have been overlooked in the TDA approach. Taking these new amplitudes into account, we find a consistent description in both schemes. These new amplitudes can affect direct CP asymmetries in some channels significantly. A consequence is that for any charmless hadronic decay of heavy meson, the direct CP symmetry cannot be identically zero.

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arXiv2018

$R_{K^{(*)}}$ and related $b\to s\ell\bar\ell$ anomalies in minimal flavor violation framework with $Z'$ boson

A recent LHCb measurement of the ratio $R_{K^*}$ of $B\to K^*μ\barμ$ to $B\to K^*e\bar e$ branching fractions has produced results in mild tension with the standard model (SM). This adds to the known anomalies also induced by the $b\to s\ell\bar\ell$ transitions, resulting in a confidence level now as high as 4$σ$. We analyze whether the parameter space preferred by all the $b\to s\ell\bar\ell$ anomalies is compatible with a heavy $Z'$ boson assumed to have nonuniversal couplings to SM fermions dictated by the principle of minimal flavor violation (MFV). We deal with the MFV couplings of the $Z'$ to leptons in the context of the type-I seesaw scenario for generating neutrino masses. The flavor-violating $Z'$ interactions are subject to stringent constraints from other processes, especially $B$-$\bar B$ mixing, charged lepton decays $\ell_i\to\ell_j\ell_k\bar\ell_l$ occurring at tree level, and the loop induced $μ\to eγ$. We perform scans for parameter regions allowed by various data and predict the ranges for a number of observables. Some of the predictions, such as the branching fractions of lepton-flavor violating $τ\to3μ$, $B\to K eμ$, $K_L\to eμ$, and $Z\to\ell\ell'$, are not far below their experimental bounds and therefore could be probed by searches in the near future. The viable parameter space depends strongly on the neutrino mass hierarchy, with a preference for the inverted one.

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arXiv2017

Search for a heavy dark photon at future $e^+e^-$ colliders

A coupling of a dark photon $A'$ from a $U(1)_{A'}$ with the standard model (SM) particles can be generated through kinetic mixing represented by a parameter $ε$. A non-zero $ε$ also induces a mixing between $A'$ and $Z$ if dark photon mass $m_{A'}$ is not zero. This mixing can be large when $m_{A'}$ is close to $m_Z$ even if the parameter $ε$ is small. Many efforts have been made to constrain the parameter $ε$ for a low dark photon mass $m_{A'}$ compared with the $Z$ boson mass $m_Z$. We study the search for dark photon in $e^+e^- \to γA' \to γμ^+ μ^-$ for a dark photon mass $m_{A'}$ as large as kinematically allowed at future $e^+e^-$ colliders. For large $m_{A'}$, care should be taken to properly treat possible large mixing between $A'$ and $Z$. We obtain sensitivities to the parameter $ε$ for a wide range of dark photon mass at planed $e^+\;e^-$ colliders, such as Circular Electron Positron Collider (CEPC), International Linear Collider (ILC) and Future Circular Collider (FCC-ee). For the dark photon mass $20~\text{GeV}\lesssim m_{A^{\prime}}\lesssim 330~\text{GeV}$, the $2σ$ exclusion limits on the mixing parameter are $ε\lesssim 10^{-3}-10^{-2}$. The CEPC with $\sqrt{s}=240~\text{GeV}$ and FCC-ee with $\sqrt{s}=160~\text{GeV}$ are more sensitive than the constraint from current LHCb measurement once the dark photon mass $m_{A^{\prime}}\gtrsim 50~\text{GeV}$. For $m_{A^{\prime}}\gtrsim 220~\text{GeV}$, the sensitivity at the FCC-ee with $\sqrt{s}=350~\text{GeV}$ and $1.5~\text{ab}^{-1}$ is better than that at the 13~TeV LHC with $300~\text{fb}^{-1}$, while the sensitivity at the CEPC with $\sqrt{s}=240~\text{GeV}$ and $5~\text{ab}^{-1}$ can be even better than that at 13~TeV LHC with $3~\text{ab}^{-1}$ for $m_{A^{\prime}}\gtrsim 180~\text{GeV}$.

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arXiv2017

Electrophilic dark matter with dark photon: from DAMPE to direct detection

The electron-positron excess reported by the DAMPE collaboration recently may be explained by an electrophilic dark matter (DM). A standard model singlet fermion may play the role of such a DM when it is stablized by some symmetries, such as a dark $U(1)_X^{}$ gauge symmetry, and dominantly annihilates into the electron-positron pairs through the exchange of a scalar mediator. The model, with appropriate Yukawa couplings, can well interpret the DAMPE excess. Naively one expects that in this type of models the DM-nucleon cross section should be small since there is no tree-level DM-quark interactions. We however find that at one-loop level, a testable DM-nucleon cross section can be induced for providing ways to test the electrophilic model. We also find that a $U(1)$ kinetic mixing can generate a sizable DM-nucleon cross section although the $U(1)_X^{}$ dark photon only has a negligible contribution to the DM annihilation. Depending on the signs of the mixing parameter, the dark photon can enhance/reduce the one-loop induced DM-nucleon cross section.

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arXiv2017

Exploring Spin-3/2 Dark Matter with Effective Higgs Couplings

We study an economical model of weakly-interacting massive particle dark matter (DM) which has spin 3/2 and interacts with the 125-GeV Higgs boson via effective scalar and pseudoscalar operators. We apply constraints on the model from the relic density data, LHC measurements of the Higgs boson, and direct and indirect searches for DM, taking into account the effective nature of the DM-Higgs couplings. We show that this DM is currently viable in most of the mass region from about 58 GeV to 2.3 TeV and will be probed more stringently by ongoing and upcoming experiments. Nevertheless, the presence of the DM-Higgs pseudoscalar coupling could make parts of the model parameter space elusive from future tests. We find that important aspects of this scenario are quite similar to those of its more popular spin-1/2 counterpart.

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arXiv2017

Are the B-anomalies evidence for heavy neutrinos?

The existing anomalies appearing in decays of the form $b\to s \ell^+ \ell^-$ constitute a possible hint for new physics. We point out that modifications to the SM results due to heavy neutrinos could account for the observed deviations while satisfying existing constraints from lepton flavor violating processes. The required mixing angle, however, is an order of magnitude larger than suggested by recent global fits to lepton flavor conserving processes. We frame our discussion in terms of a Type-I seesaw model, but it can be made more general.

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arXiv2017

Constraints and Implications on Higgs FCNC Couplings from Precision Measurement of $B_s \to μ^+μ^-$ Decay

We study constraints and implications of the recent LHCb measurement of ${\cal B}(B_s \to μ^+μ^-)$ for tree-level Higgs-mediated flavor-changing neutral current (FCNC) interactions. Combined with experimental data on $B_s$ mass difference $Δm_s$, the $h \to μτ$, and the $h \to τ^+τ^-$ decay branching ratios from the LHC, we find that the Higgs FCNC couplings are severely constrained. The allowed regions for $B_s \to μτ$, $ττ$ and $h \to sb$ decays are obtained. Current data allow large CP violation in the $h \to τ^+ τ^-$ decay. Consequences of the Cheng-Sher ansatz for the Higgs Yukawa couplings are discussed in some detail.

hep-ph↗

arXiv2017

Tests for CPT sum rule and U-spin violation in Time-dependent CP violation of $B^0_s \to K^+ K^-$ and $B^0_d \to π^+ π^-$

Recent LHCb data for time-dependent CP violation in $B_d^0 \to π^+π^-$ and $B^0_s\to K^+K^-$ show deviations from theoretical predictions. Besides their central values for $\mathcal C_{K^+K^-}$, $\mathcal S_{K^+K^-}$ and $\mathcal A^{ΔΓ}_{K^+K^-}$ violate quantum mechanic CPT invariant sum rule (CPT sum rule) prediction of $|\mathcal C_{K^+K^-}|^2 + |\mathcal S_{K^+K^-}| ^2 + |\mathcal A^{ΔΓ}_{K^+K^-}|^2 = 1$ (LHCb data imply the sum to be $0.67\pm 0.20$.), their values for $\mathcal C_{K^+ K^-}= 0.24\pm 0.06\pm {0.02}$ and $\mathcal C_{π^+ π^-} = - 0.24\pm 0.07\pm 0.01$ also show large violation of SU(3) or its U-spin sub-group symmetry (SU(3)/U) relation $\mathcal C_{K^+ K^-} /\mathcal C_{π^- π^+} = - \mathcal B(B_d^0 \to π^- π^+)τ_{B^0_s}/\mathcal B(B^0_s \to K^+ K^-)τ_{B_d^0}$ (LHCb data imply the ratio of left-side to right-side to be $4.67\pm 1.88$.) . The LHCb results need to be further confirmed to be taken seriously. We suggest to use time-dependent CP violation in $B_s\to K^0\bar K^0, π^+π^-, π^0π^0$ to further test the CPT sum rule. Assuming that the sum rule holds, we propose that violation of the SU(3)/U relation may indicate a large FSI phase difference in the $π^+π^-$ and $K^+K^-$ re-scattering. We suggest several other U-spin pairs of $B\to PP$ decays to further test SU(3)/U relations.

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arXiv2017

Dark Gauge Bosons: LHC Signatures of Non-Abelian Kinetic Mixing

We consider non-abelian kinetic mixing between the Standard Model SU(2$)_L$ and a dark sector U(1$)^\prime$ gauge group associated with the presence of a scalar SU(2$)_L$ triplet. The magnitude of the resulting dark photon coupling $ε$ is determined by the ratio of the triplet vacuum expectation value, constrained to by $\lsim 4$ GeV by electroweak precision tests, to the scale $Λ$ of the effective theory. The corresponding effective operator Wilson coefficient can be $\mathcal{O}(1)$ while accommodating null results for dark photon searches, allowing for a distinctive LHC dark photon phenomenology. After outlining the possible LHC signatures, we illustrate by recasting current ATLAS dark photon results into the non-abelian mixing context.

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arXiv2017

Two-Higgs-Doublet-Portal Dark-Matter Models in Light of Direct Search and LHC Data

We explore simple Higgs-portal models of dark matter (DM) with spin 1/2, 3/2, and 1, respectively, applying to them constraints from the LUX and PandaX-II direct detection experiments and from LHC measurements on the 125-GeV Higgs boson. With only one Higgs doublet, we find that the spin-1/2 DM having a purely scalar effective coupling to the doublet is viable only in a narrow range of mass near the Higgs pole, whereas the vector DM is still allowed if its mass is also close to the Higgs pole or exceeds 1.4 TeV, both in line with earlier analyses. Moreover, the spin-3/2 DM is in a roughly similar situation to the spin-1/2 DM, but has surviving parameter space which is even more restricted. We also consider the two-Higgs-doublet extension of each of the preceding models, assuming that the expanded Yukawa sector is that of the two-Higgs-doublet model of type II. We show that in these two-Higgs-doublet-portal models significant portions of the DM mass regions excluded in the simplest scenarios by direct search bounds can be reclaimed due to suppression of the effective DM interactions with nucleons at some ratios of the $CP$-even Higgs bosons' couplings to the up and down quarks. The regained parameter space contains areas which can yield a DM-nucleon scattering cross-section that is far less than its current experimental limit or even goes below the neutrino-background floor.

hep-ph↗

arXiv2017

Relativistic dipole interaction and the topological nature for induced HMW and AC phases

In this work we give, for the first time, the full relativistic Lagrangian density describing the motion of induced electric dipoles in the electric fields which induce the dipole, and the magnetic fields which generate the HMW topological phase. We then use this relativistic Lagrangian density to derive the complete set of conditions for producing topological phases with induced dipoles. We also give the relativistic Lagrangian density describing the motion of induced magnetic dipoles in the magnetic fields which induce the dipole, and the electric fields which generate the AC topological phase, and derive the conditions for this AC phase to be topological. These conditions have been incompletely discussed in previous studies. We note that, in both the AC and HMW cases, the topological phases are generated by the cross product of electric and magnetic fields in the form $\bm{B} \times \bm{E}$ which reinforces the dual nature of these two topological phases.

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