arXiv subjects

Xiao-Gang He

Publications and source records attributed to Xiao-Gang He.

At least 109 records · Page 6Linked to original sources

arXiv2017

Dark Photon Search at A Circular $e^+e^-$ Collider

One of the interesting portals linking a dark sector and the standard model (SM) is the kinetic mixing between the SM $U(1)_Y$ field with a new dark photon $A'$ from a $U(1)_{A'}$ gauge interaction. Stringent limits have been obtained for the kinetic mixing parameter $ε$ through various processes. In this work, we study the possibility of searching for a dark photon interaction at a circular $e^+e^-$ collider through the process $e^+ e^-\to γA^{\prime *} \to γμ^+μ^-$. We find that the constraint on $ε^2$ for dark photon mass in the few tens of GeV range, assuming that the $μ^+μ^-$ invariant mass can be measured to an accuracy of $0.5\%m_{A'}$, can be better than $3\times 10^{-6}$ for the proposed CEPC with a ten-year running at 3$σ$ (statistic) level, and better than $2\times 10^{-6}$ for FCC-ee with even just one-year running at $\sqrt{s} = 240$ GeV, better than the LHC and other facilities can do in a similar dark photon mass range. For FCC-ee, running at $\sqrt{s}=160$ GeV, the constraint can be even better.

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arXiv2016

New LUX and PandaX-II Results Illuminating the Simplest Higgs-Portal Dark Matter Models

Direct searches for dark matter (DM) by the LUX and PandaX-II Collaborations employing xenon-based detectors have recently come up with the most stringent limits to date on the spin-independent elastic scattering of DM off nucleons. For Higgs-portal scalar DM models, the new results have precluded any possibility of accommodating low-mass DM as suggested by the DAMA and CDMS II Si experiments utilizing other target materials, even after invoking isospin-violating DM interactions with nucleons. In the simplest model, SM+D, which is the standard model plus a real singlet scalar named darkon acting as the DM candidate, the LUX and PandaX-II limits rule out DM masses roughly from 4 to 450 GeV, except a small range around the resonance point at half of the Higgs mass where the interaction cross-section is near the neutrino-background floor. In the THDM II+D, which is the type-II two-Higgs-doublet model combined with a darkon, the region excluded in the SM+D by the direct searches can be recovered due to suppression of the DM effective interactions with nucleons at some values of the ratios of Higgs couplings to the up and down quarks, making the interactions significantly isospin-violating. However, in either model, if the 125-GeV Higgs boson is the portal between the dark and SM sectors, DM masses less than 50 GeV or so are already ruled out by the LHC constraint on the Higgs invisible decay. In the THDM II+D, if the heavier $CP$-even Higgs boson is the portal, theoretical restrictions from perturbativity, vacuum stability, and unitarity requirements turn out to be important instead and exclude much of the region below 100 GeV. For larger DM masses, the THDM II+D has plentiful parameter space that corresponds to interaction cross-sections under the neutrino-background floor and therefore is likely to be beyond the reach of future direct searches without directional sensitivity.

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arXiv2016

Consequences of R-Parity violating interactions for anomalies in $\bar B\to D^{(*)} τ\bar ν$ and $b\to s μ^+μ^-$

We investigate the possibility of explaining the enhancement in semileptonic decays of $\bar B \to D^{(*)} τ\bar ν$, the anomalies induced by $b\to sμ^+μ^-$ in $\bar B\to (K, K^*, ϕ)μ^+μ^-$ and violation of lepton universality in $R_K = Br(\bar B\to K μ^+μ^-)/Br(\bar B\to K e^+e^-)$ within the framework of R-parity violating (RPV) MSSM. Exchange of down type right-handed squark coupled to quarks and leptons yield interactions which are similar to leptoquark induced interactions that have been proposed to explain the $\bar B \to D^{(*)} τ\bar ν$ by tree level interactions and $b\to s μ^+μ^-$ anomalies by loop induced interactions, simultaneously. However, the Yukawa couplings in such theories have severe constraints from other rare processes in $B$ and $D$ decays. Although this interaction can provide a viable solution to $R(D^{(*)})$ anomaly, we show that with the severe constraint from $\bar B \to K ν\bar ν$, it is impossible to solve the anomalies in $b\to s μ^+μ^-$ process simultaneously.

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arXiv2016

Realistic model for a fifth force explaining anomaly in ${^8Be^*} \to {^8Be} \;{e^+e^-}$ Decay

A $6.8\,σ$ anomaly has been reported in the opening angle and invariant mass distributions of $e^+e^-$ pairs produced in ${^8Be}$ nuclear transitions. It has been shown that a protophobic fifth force mediated by a $17\,\textrm{MeV}$ gauge boson $X$ with pure vector current interactions can explain the data through the decay of an excited state to the ground state, ${^8Be^*} \to {^8Be}\, X$, and then the followed saturating decay $X \to e^+e^-$. In this work we propose a renormalizable model to realize this fifth force. Although axial-vector current interactions also exist in our model, their contributions cancel out in the iso-scalar interaction for ${^8Be^*} \to {^8Be} \,X$. Within the allowed parameter space, this model can alleviate the $(g-2)_μ$ anomaly problem and can be probed by the LHCb experiment. Several other implications are discussed.

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arXiv2016

Yukawa sector for LFV in $h\to μτ$ and CP violation in $h\to ττ$

The Higgs boson discovered at the LHC opened a new chapter for particle physics. Its properties need to be studied in detail to distinguish a purely standard model (SM) Higgs boson from one of many scalars in an enlarged Higgs sector. The CMS collaboration has reported a possible lepton flavor violating (LFV) signal $h\toμτ$, which if confirmed, implies that the Higgs sector is larger than in the SM. New physics responsible for this type of decay may, in general, also introduce other observable effects such as charge-parity (CP) violation in $h\to ττ$. We study two types of models that single out the third generation and can induce large $h \to μτ$ rates with different consequences for CP violation in $h \to ττ$. Predictions for the size of the CP violating couplings require knowledge of the lepton Yukawa matrices and we discuss this in the context of two different textures considering all existing constraints.

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arXiv2016

Production of Charmed Tetraquarks from $B_c$ and $B$ decays

Hadronic states composed of multi-quark flavors may exist in reality since they are not prohibited by QCD. Compact four quark systems of color singlet are classified as tetraquarks. To understand the properties of these states, more theoretical and experimental efforts are needed. In this work, we study charmed tetraquarks with three light flavors using flavor $SU(3)$ symmetry. States with three different light quarks must be in a ${\bf \bar 6 }$ or a ${\bf 15}$ multiplet. We investigate the production of charmed tetraquarks $X_c$ in $B\to X_c (\overline {X}_c) P$ and $B_c \to X_c P$ decays. Whether the states with three light quarks belong to ${\bf \bar 6}$ or ${\bf 15}$ can be determined by studying various tetraquark $B$ and $B_c$ decays. We demonstrate that the decay amplitudes for these decays can be parametrized by a few irreducible SU(3) invariant amplitudes. We then derive relations for decay widths and CP violating rate difference which can be examined experimentally. Although no experimental measurement is available yet, they might be accessed at the ongoing and forthcoming experiments like the LHCb and Belle-II. Measurements of these observables can not only provide useful information for the study of exotics spectroscopy but are also valuable information towards a better understanding of some non-perturbative aspects of QCD.

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arXiv2016

CP violation in $h\to ττ$ and LFV $h\to μτ$

The CMS collaboration has reported a possible lepton flavour violating (LFV) signal $h\toμτ$. Whereas this does not happen in the standard model (SM), we point out that new physics responsible for this type of decay would, in general, also produce charge-parity (CP) violation in $h\to ττ$. We estimate the size of this effect in a model independent manner and find that a large asymmetry, of order 25\%, is allowed by current constraints.

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arXiv2016

Flavor $SU(3)$ properties of beauty tetraquark states with three different light quarks

Beauty tetraquark states $X(\bar b q'q'' \bar q )$ composed of $ \bar b s u \bar d$, $\bar b d s \bar u$, and $\bar b u d \bar s$, are unique that all the four valence quarks are different. Although the claim of existence of the first two states by D0 was not confirmed by data from LHCb, the possibility of such states still generated a lot of interests and should be pursued further. Non-observation of $X(\bar b q'q'' \bar q )$ states by LHCb may be just due to a still lower production rate than the limit of LHCb or at some different mass ranges. In this work we use light quark $SU(3)$ flavor symmetry as guideline to classify symmetry properties of beauty tetraquark states. The multiplets which contain states with three different light quarks must be one of ${\bf \bar 6}$ or ${\bf 15}$ of $SU(3)$ representations. We study possible decays of such a tetraquark state into a $B$ meson and a light pesudoscalar octet meson by constructing a leading order chiral Lagrangian, and also provide search strategies to determine whether a given tetraquark state of this type belongs to ${\bf \bar 6}$ or ${\bf 15}$. If $X(\bar b q'q''\bar q )$ belongs to ${\bf 15}$, there are new doubly charged tetraquark states $\bar b u u \bar d$ and $\bar b u u \bar s$.

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arXiv2016

A $γγ$ Collider for the 750 GeV Resonant State

Recent data collected by ATLAS and CMS at 13 TeV collision energy of the LHC indicate the existence of a new resonant state $ϕ$ with a mass of 750 GeV decaying into two photons $γγ$. The properties of $ϕ$ should be studied further at the LHC and also future colliders. Since only $ϕ\to γγ$ decay channel has been measured, one of the best ways to extract more information about $ϕ$ is to use a $γγ$ collider to produce $ϕ$ at the resonant energy. In this work we show how a $γγ$ collider helps to verify the existence of $ϕ$ and to provide some of the most important information about the properties of $ϕ$, such as branching fractions of $ϕ\to V_1V_2$. Here $V_i$ can be $γ$, $Z$, or $W^\pm$. We also show that by studying angular distributions of the final $γ$'s in $γγ\to ϕ\to γγ$, one can obtain crucial information about whether this state is a spin-0 or a spin-2 state.

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arXiv2016

The Higgs Portal and Cosmology

Higgs portal interactions provide a simple mechanism for addressing two open problems in cosmology: dark matter and the baryon asymmetry. In the latter instance, Higgs portal interactions may contain the ingredients for a strong first order electroweak phase transition as well as new CP-violating interactions as needed for electroweak baryogenesis. These interactions may also allow for a viable dark matter candidate. We survey the opportunities for probing the Higgs portal as it relates to these questions in cosmology at the LHC and possible future colliders.

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arXiv2016

A $Z^\prime$ Model for $b\to s \ell\bar \ell$ Flavour Anomalies

We study the implications of flavour-changing neutral currents (FCNC's) in a model with the $SU(2)_l\times SU(2)_h\times U(1)_Y$ electroweak gauge symmetry for several anomalies appearing in $b\to s \ell\bar \ell$ induced $B$ decays in LHCb data. In this model, $SU(2)_l$ and $SU(2)_h$ govern the left-handed fermions in the first two generations and the third generation, respectively. The physical $Z$ and $Z'$ generate the $b\to s$ transition at tree level, leading to additional contributions to the $b \to s$ semileptonic operators ${\cal O}_{9,10}$. We find that although $B_s$-$\bar B_s$ mixing constrains the parameters severely, the model can produce values of ${\cal C}^{\rm NP}_{9,10}$ in the range determined by Descotes-Genon {\it et. al.} in Ref.~\cite{Descotes-Genon:2015uva} for this scenario to improve the global fit of observables in decays induced by the $b\to s μ\bar μ$ transition. The $Z'$ boson in this model also generates tree-level FCNC's for the leptonic interactions that can accommodate the experimental central value of $R_K = {\cal B}(B\to K μ\bar μ)/{\cal B}(B\to K e\bar e)=0.75$. In this case, the model predicts sizeable branching ratios for $B\to K e \bar τ$, $B\to K τ\bar e$, and an enhancement of $B\to K τ\bar τ$ with respect to its SM value.

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arXiv2016

A global $SU(3)/U(3)$ flavor symmetry analysis for $B\to PP$ with $η-η'$ Mixing

A large number of new experimental data on $B$ decay into two light pesudoscalar ($P$) mesons have been collected by the LHCb collaboration. Besides confirming information on $B_{u,d} \to PP$ decays obtained earlier by B-factories at KEK and SLAC, new information on $B_s\to PP$ and also more decay modes with $P$ being $η$ or $η'$ have been obtained. Using these new data, we perform a global fit for $B \to PP$ to determine decay amplitudes in the framework of $SU(3)/U(3)$ flavor symmetry. We find that $SU(3)$ flavor symmetry can explain data well. The annihilation amplitudes are found to be small as expected. Several CP violating relations predicted by $SU(3)$ flavor symmetry are in good agreement with data. Current available data can give constraints on the amplitudes which induce $P = η,\;η'$ decays in the framework of $U(3)$ flavor symmetry, and can also determine the $η-η'$ mixing angle $θ$ with $θ= (-18.4\pm1.2)^\circ$. Several $B \to PP$ decay modes which have not been measured are predicted with branching ratios accessible at the LHCb. These decays can provide further tests for the framework of $SU(3)/U(3)$ flavor symmetry for $B$ decays.

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arXiv2015

CP violation in neutrino mixing with $δ= -π/2$ in $A_4$ Type-II seesaw model

We study a class of models for neutrino mass matrix in Type-II seesaw with $A_4$ family symmetry. The resulting neutrino mass matrix can be naturally made to respect a $μ-τ$ exchange plus CP conjugate symmetry (GLS) with the CP violating phase $δ$ and the mixing angle $θ_{23}$ predicted to be $\pmπ/2$ and $π/4$, respectively. When GLS is explicitly broken by complex Yukawa couplings, the model predictions for $δ$ and $θ_{23}$ can be significantly modified. Should future experiments will indeed determine $θ_{23}$ and $δ_{CP}$ away from the GLS limit values, one then had to consider models with broken GLS. We study several simple scenarios to show how the modifications arise when GLS is broken and how future experiments can test this class of models.

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arXiv2015

A Model of Neutrino Mass Matrix With $δ= -π/2$ and $θ_{23} = π/4$

Experimental data have provided stringent constraints on neutrino mixing parameters. In the standard parameterization the mixing angle $θ_{23}$ is close to $π/4$. There are also evidences show that the CP violating phase is close to $-π/2$. We study neutrino mass matrix reconstructed using this information and find several interesting properties. We show that a theoretical model based on the $A_4$ symmetry naturally predicts $δ= -π/2$ and $θ_{23} = π/4$ when the Yukawa couplings and scalar vacuum expectation values are real reaching a $μ-τ$ exchange and CP conjugate symmetry limit. In this case CP violation solely comes from the complex group theoretical Clebsh-Gordan coefficients. The model also predicts $|V_{e2}|=1/\sqrt{3}$ consistent with data. With complex Yukawa couplings the values for $δ$ and $θ_{23}$ can be significantly deviate away from the symmetry values $-π/2$ and $π/4$, respectively. But $|V_{e2}|= 1/\sqrt{3}$ is not altered. This matrix is an excellent lowest order approximation for theoretical model buildings of neutrino mass matrix.

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arXiv2015

Leptogenesis parametrized by lepton mass matrices

The conventional seesaw-leptogenesis can simultaneously explain the suppression of neutrino masses and the generation of cosmic baryon asymmetry, but usually cannot predict an unambiguous relation between these two sectors. In this work we shall demonstrate a novel left-right symmetric scenario, motivated to solve the strong CP problem by parity symmetry, where the present baryon asymmetry is determined by three charged lepton masses and a seesaw-suppressed hermitian Dirac neutrino mass matrix up to an overall scale factor. To produce the observed baryon asymmetry, this scenario requires that the neutrinos must have a normal hierarchical mass spectrum and their mixing matrix must contain a sizable Dirac CP phase. Our model can be tested in neutrino oscillation and neutrinoless double beta decay experiments.

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arXiv2015

Some Predictions of Diquark Model for Hidden Charm Pentaquark Discovered at the LHCb

The LHCb has discovered two new states with preferred $J^P$ quantum numbers $3/2^-$ and $5/2^+$ from $Λ_b$ decays. These new states can be interpreted as hidden charm pentaquarks. It has been argued that the main features of these pentaquarks can be described by diquark model. The diquark model predicts that the $3/2^-$ and $5/2^+$ are in two separate octet multiplets of flavor $SU(3)$ and there is also an additional decuplet pentaquark multiplet. Finding the states in these multiplets can provide crucial evidence for this model. The weak decays of b-baryon to a light meson and a pentaquark can have Cabibbo allowed and suppressed decay channels. We find that in the $SU(3)$ limit, for $U$-spin related decay modes the ratio of the decay rates of Cabibbo suppressed to Cabibbo allowed decay channels is given by $|V_{cd}|^2/|V_{cs}|^2$. There are also other testable relations for b-baryon weak decays into a pentaquark and a light pseudoscalar. These relations can be used as tests for the diquark model for pentaquark.

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arXiv2015

Higgs Decay $h\toμτ$ with Minimal Flavor Violation

We consider the tentative indication of flavor-violating Higgs boson decay $h\toμτ$ recently reported in the CMS experiment within the framework of minimal flavor violation. Specifically, we adopt the standard model extended with the seesaw mechanism involving right-handed neutrinos plus effective dimension-six operators satisfying the minimal flavor violation principle in the lepton sector. We find that it is possible to accommodate the CMS $h\toμτ$ signal interpretation provided that the right-handed neutrinos couple to the Higgs boson in some nontrivial way. We take into account empirical constraints from other lepton-flavor-violating processes and discuss how future searches for the $μ\to eγ$ decay and $μ\to e$ conversion in nuclei may further probe the lepton-flavor-violating Higgs couplings.

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arXiv2015

CP Violating Polarization Asymmetry in Charmless Two-Body Decays of Beauty Baryons

Several baryons containing a heavy b-quark, the b-baryons, have been discovered. The charmless two-body decays of b-baryons can provide a new platform for CP violating studies in a similar way as charmless two-body decays of B-meson. In b-baryon decays there are new CP violating observable related to baryon polarization. We show that in the flavor $SU(3)$ limit there exist relations involve different combinations of the decay amplitudes compared with those in CP violating rate asymmetry. These new relations therefore provide interesting tests for the mechanism of CP in the standard model (SM) and flavor $SU(3)$ symmetry. Future data from LHCb can test these relations.

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