arXiv subjects

Xiao-Ping Wang

Publications and source records attributed to Xiao-Ping Wang.

At least 73 records · Page 4Linked to original sources

arXiv2019

A $\pmbν$ Solution to the Strong CP Problem

We present a new solution to the strong CP problem in which the imaginary component of the up quark mass, $\mathcal{I}[m_u]$, acquires a tiny, but non-vanishing value. This is achieved via a Dirac seesaw mechanism, which is also responsible for the generation of the small neutrino masses. Consistency with the observed value of the up quark mass is achieved via instanton contributions arising from QCD-like interactions. In this framework, the value of the neutron electric dipole moment is directly related to $\mathcal{I}[m_u]$, which, due to its common origin with the neutrino masses, implies that the neutron electric dipole moment is likely to be measured in the next round of experiments. We also present a supersymmetric extension of this Dirac seesaw model to stabilize the hierarchy among the scalar mass scales involved in this new mechanism.

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arXiv2019

A light complex scalar for the electron and muon anomalous magnetic moments

The anomalous magnetic moments of the electron and the muon are interesting observables, since they can be measured with great precision and their values can be computed with excellent accuracy within the Standard Model (SM). The current experimental measurement of this quantities show a deviation of a few standard deviations with respect to the SM prediction, which may be a hint of new physics. The fact that the electron and the muon masses differ by two orders of magnitude and the deviations have opposite signs makes it difficult to find a common origin of these anomalies. In this work we introduce a complex singlet scalar charged under a Peccei-Quinn-like (PQ) global symmetry together with the electron transforming chirally under the same symmetry. In this realization, the CP-odd scalar couples to electron only, while the CP-even part can couple to muons and electrons simultaneously. In addition, the CP-odd scalar can naturally be much lighter than the CP-even scalar, as a pseudo-Goldstone boson of the PQ-like symmetry, leading to an explanation of the suppression of the electron anomalous magnetic moment with respect to the SM prediction due to the CP-odd Higgs effect dominance, as well as an enhancement of the muon one induced by the CP-even component.

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arXiv2019

What can We Learn from Triple Top-Quark Production?

Different from other multiple top-quark productions, triple top-quark production requires the presence of both flavor violating neutral interaction and flavor conserving neutral interaction. We describe the interaction of triple top-quarks and up-quark in terms of two dimension-6 operators; one can be induced by a new heavy vector resonance, the other by a scalar resonance. Combining same-sign top-quark pair production and four top-quark production, we explore the potential of the 13 TeV LHC on searching for the triple top-quark production.

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arXiv2018

An efficient threshold dynamics method for topology optimization for fluids

We propose an efficient threshold dynamics method for topology optimization for fluids modeled with the Stokes equation. The proposed algorithm is based on minimization of an objective energy function that consists of the dissipation power in the fluid and the perimeter approximated by nonlocal energy, subject to a fluid volume constraint and the incompressibility condition. We show that the minimization problem can be solved with an iterative scheme in which the Stokes equation is approximated by a Brinkman equation. The indicator functions of the fluid-solid regions are then updated according to simple convolutions followed by a thresholding step. We demonstrate mathematically that the iterative algorithm has the total energy decaying property. The proposed algorithm is simple and easy to implement. A simple adaptive time strategy is also used to accelerate the convergence of the iteration. Extensive numerical experiments in both two and three dimensions show that the proposed iteration algorithm converges in much fewer iterations and is more efficient than many existing methods. In addition, the numerical results show that the algorithm is very robust and insensitive to the initial guess and the parameters in the model.

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arXiv2018

A Light Higgs at the LHC and the B-Anomalies

After the Higgs discovery, the LHC has been looking for new resonances, decaying into pairs of Standard Model (SM) particles. Recently, the CMS experiment observed an excess in the di-photon channel, with a di-photon invariant mass of about 96~GeV. This mass range is similar to the one of an excess observed in the search for the associated production of Higgs bosons with the $Z$ neutral gauge boson at LEP, with the Higgs bosons decaying to bottom quark pairs. On the other hand, the LHCb experiment observed a discrepancy with respect to the SM expectations of the ratio of the decay of $B$-mesons to $K$-mesons and a pair of leptons, $R_{K^{(*)}} = BR(B \to K^{(*)} μ^+μ^-)/BR(B\to K^{(*)} e^+e^-)$. This observation provides a hint of the violation of lepton-flavor universality in the charged lepton sector and may be explained by the existence of a vector boson originating form a $U(1)_{L_μ- L_τ}$ symmetry and heavy quarks that mix with the left-handed down quarks. Since the coupling to heavy quarks could lead to sizable Higgs di-photon rates in the gluon fusion channel, in this article we propose a common origin of these anomalies identifying a Higgs associated with the breakdown of the $U(1)_{L_μ- L_τ}$ symmetry and at the same time responsible to the quark mixing, with the one observed at the LHC. We also discuss the constraints on the identification of the same Higgs with the one associated with the bottom quark pair excess observed at LEP.

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arXiv2018

Exposing Dark Sector with Future Z-Factories

We investigate the prospects of searching dark sector models via exotic Z-boson decay at future $e^+ e^-$ colliders with Giga Z and Tera Z options. Four general categories of dark sector models: Higgs portal dark matter, vector portal dark matter, inelastic dark matter and axion-like particles, are considered. Focusing on channels motivated by the dark sector models, we carry out a model independent study of the sensitivities of Z-factories in probing exotic decays. The limits on branching ratios of the exotic Z decay are typically $\mathcal{O} (10^{-6} - 10^{-8.5}) $ for the Giga Z and $\mathcal{O} (10^{-7.5} - 10^{-11})$ for the Tera Z, and they are compared with the projection for the high luminosity LHC. We demonstrate that future Z-factories can provide its unique and leading sensitivity, and highlight the complementarity with other experiments, including the indirect and direct dark matter search limits, and the existing collider limits. Future Z factories will play a leading role to uncover the hidden sector of the universe in the future.

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arXiv2018

Bottom-quark Forward-Backward Asymmetry, Dark Matter and the LHC

The LEP experiment at CERN provided accurate measurements of the $Z$ neutral gauge boson properties. Although all measurements agree well with the SM predictions, the forward backward asymmetry of the bottom-quark remains almost 3$σ$ away from the SM value. We proposed that this anomaly may be explained by the existence of a new $U(1)_D$ gauge boson, which couples with opposite charges to the right-handed components of the bottom and charm quarks. Cancellation of gauge anomalies demands the presence of a vector-like singlet charged lepton as well as a neutral Dirac (or Majorana) particle that provides a Dark Matter candidate. Constraints from precision measurements imply that the mass of the new gauge boson should be around $115$~GeV. We discuss the experimental constraints on this scenario, including the existence of a di-jet resonance excess at an invariant mass similar to the mass of this new gauge boson, observed in boosted topologies at the CMS experiment.

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arXiv2018

Return of the X-rays: A New Hope for Fermionic Dark Matter at the keV Scale

A long time ago (in 2014), in galaxies and galaxy clusters far, far away, several groups have reported hints for a yet unidentified line in astrophysical X-ray signals at an energy of 3.5\,keV. While it is not unlikely that this line is simply a reflection of imperfectly modeled atomic transitions, it has renewed the community's interest in models of keV-scale dark matter, whose decay would lead to such a line. The alternative possibility of dark matter annihilation into monochromatic photons is far less explored, a lapse that we strive to amend in this paper. More precisely, we introduce a novel model of fermionic dark matter $χ$ with $\mathcal{O}(\text{keV})$ mass, annihilating to a scalar state $ϕ$ which in turn decays to photons, for instance via loops of heavy vector-like quarks. The resulting photon spectrum is box-shaped, but if $χ$ and $ϕ$ are nearly degenerate in mass, it can also resemble a narrow line. We discuss dark matter production via two different mechanisms -- misalignment and freeze-in -- which both turn out to be viable in vast regions of parameter space. We constrain the model using astrophysical X-ray data, and we demonstrate that, thanks to the velocity-dependence of the annihilation cross section, it has the potential to reconcile the various observations of the 3.5\,keV line. We finally address the $ϕ$-mediated force between dark matter particles and its possible impact on structure formation.

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arXiv2018

Fuzzy Dark Matter and Non-Standard Neutrino Interactions

We discuss novel ways in which neutrino oscillation experiments can probe dark matter. In particular, we focus on interactions between neutrinos and ultra-light ("fuzzy") dark matter particles with masses of order $10^{-22}$ eV. It has been shown previously that such dark matter candidates are phenomenologically successful and might help ameliorate the tension between predicted and observed small scale structures in the Universe. We argue that coherent forward scattering of neutrinos on fuzzy dark matter particles can significantly alter neutrino oscillation probabilities. These effects could be observable in current and future experiments. We set new limits on fuzzy dark matter interacting with neutrinos using T2K and solar neutrino data, and we estimate the sensitivity of reactor neutrino experiments and of future long-baseline accelerator experiments. These results are based on detailed simulations in GLoBES. We allow the dark matter particle to be either a scalar or a vector boson. In the latter case, we find potentially interesting connections to models addressing various $B$ physics anomalies.

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arXiv2017

A Tale of Two Portals: Testing Light, Hidden New Physics at Future $e^+ e^-$ Colliders

We investigate the prospects for producing new, light, hidden states at a future $e^+ e^-$ collider in a Higgsed dark $U(1)_D$ model, which we call the Double Dark Portal model. The simultaneous presence of both vector and scalar portal couplings immediately modifies the Standard Model Higgsstrahlung channel, $e^+ e^- \to Zh$, at leading order in each coupling. In addition, each portal leads to complementary signals which can be probed at direct and indirect detection dark matter experiments. After accounting for current constraints from LEP and LHC, we demonstrate that a future $e^+ e^-$ Higgs factory will have unique and leading sensitivity to the two portal couplings by studying a host of new production, decay, and radiative return processes. Besides the possibility of exotic Higgs decays, we highlight the importance of direct dark vector and dark scalar production at $e^+ e^-$ machines, whose invisible decays can be tagged from the recoil mass method.

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arXiv2017

Sterile Neutrinos and Flavor Ratios in IceCube

The flavor composition of astrophysical neutrinos observed in neutrino telescopes is a powerful discriminator between different astrophysical neutrino production mechanisms and can also teach us about the particle physics properties of neutrinos. In this paper, we investigate how the possible existence of light sterile neutrinos can affect these flavor ratios. We consider two scenarios: (i) neutrino production in conventional astrophysical sources, followed by partial oscillation into sterile states; (ii) neutrinos from dark matter decay with a primary flavor composition enhanced in tau neutrinos or sterile neutrinos. Throughout the paper, we constrain the sterile neutrino mixing parameters from a full global fit to short and long baseline data. We present our results in the form of flavor triangles and, for scenario (ii), as exclusion limits on the dark matter mass and lifetime, derived from a fit to IceCube high energy starting events and through-going muons. We argue that identifying a possible flux of neutrinos from dark matter decay may require analyzing the flavor composition as a function of neutrino energy.

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arXiv2016

New Signatures of Flavor Violating Higgs Couplings

We explore several novel LHC signatures arising from quark or lepton flavor violating couplings in the Higgs sector, and we constrain such couplings using LHC data. Since the largest signals are possible in channels involving top quarks or tau leptons, we consider in particular the following flavor violating processes: (1) $pp \to thh$ (top plus di-Higgs final state) arising from a dimension six coupling of up-type quarks to three insertions of the Higgs field. We develop a search strategy for this final state and demonstrate that detection is possible at the high luminosity LHC if flavor violating top--up--Higgs couplings are not too far below the current limit. (2) $pp \to tH^0$, where $H^0$ is the heavy neutral CP-even Higgs boson in a two Higgs doublet model (2HDM). We consider the decay channels $H^0 \to tu, WW, ZZ, hh$ and use existing LHC data to constrain the first three of them. For the fourth, we adapt our search for the $thh$ final state, and we demonstrate that in large regions of the parameter space, it is superior to other searches, including searches for flavor violating top quark decays ($t \to hq$). (3) $H^0 \to τμ$, again in the context of a 2HDM. This channel is particularly well motivated by the recent CMS excess in $h \to τμ$, and we use the data from this search to constrain the properties of $H^0$.

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arXiv2016

Hunting for Dark Matter Coannihilation by Mixing Dijet Resonances and Missing Transverse Energy

Simplified models of the dark matter (co)annihilation mechanism predict striking new collider signatures untested by current searches. These models, which were codified in the coannihilation codex, provide the basis for a dark matter (DM) discovery program at the Large Hadron Collider (LHC) driven by the measured DM relic density. In this work, we study an exemplary model featuring $s$-channel DM coannihilation through a scalar diquark mediator as a representative case study of scenarios with strongly interacting coannihilation partners. We discuss the full phenomenology of the model, ranging from low energy flavor constraints, vacuum stability requirements, and precision Higgs effects to direct detection and indirect detection prospects. Moreover, motivated by the relic density calculation, we find significant portions of parameter space are compatible with current collider constraints and can be probed by future searches, including a proposed analysis for the novel signature of a dijet resonance accompanied by missing transverse energy (MET). Our results show that the $13$ TeV LHC with $100~\mathrm{fb}^{-1}$ luminosity should be sensitive to mediators as heavy as 1 TeV and dark matter in the 400--500 GeV range. The combination of searches for single and paired dijet peaks, non-resonant jets + MET excesses, and our novel resonant dijet + MET signature have strong coverage of the motivated relic density region, reflecting the tight connections between particles determining the dark matter abundance and their experimental signatures at the LHC.

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arXiv2016

The Coannihilation Codex

We present a general classification of simplified models that lead to dark matter (DM) coannihilation processes of the form DM + X $\rightarrow$ SM$_1$ + SM$_2$, where X is a coannihilation partner for the DM particle and SM$_1$, SM$_2$ are Standard Model fields. Our classification also encompasses regular DM pair annihilation scenarios if DM and X are identical. Each coannhilation scenario motivates the introduction of a mediating particle M that can either belong to the Standard Model or be a new field, whereby the resulting interactions between the dark sector and the Standard Model are realized as tree-level and dimension-four couplings. We construct a basis of coannihilation models, classified by the $SU(3)_C\times SU(2)_L\times U(1)_Y$ quantum numbers of DM, X and M. Our main assumptions are that dark matter is an electrically neutral color singlet and that all new particles are either scalars, Dirac or Majorana fermions, or vectors. We illustrate how new scenarios arising from electroweak symmetry breaking effects can be connected to our electroweak symmetric simplified models. We offer a comprehensive discussion of the phenomenological features of our models, encompassing the physics of thermal freeze-out, direct and indirect detection constraints, and in particular searches at the Large Hadron Collider (LHC). Many novel signatures that are not covered in current LHC searches are emphasized, and new and improved LHC analyses tackling these signatures are proposed. We discuss how the coannihilation simplified models can be used to connect results from all classes of experiments in a straightforward and transparent way. This point is illustrated with a detailed discussion of the phenomenology of a particular simplified model featuring leptoquark-mediated dark matter coannihilation.

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arXiv2015

LHC diphoton excess from colorful resonances

Motivated by the possible diphoton excess around $750~\rm{GeV}$ observed by ATLAS and CMS at $13~\rm{TeV}$, we consider a coloron model from $\rm{SU}(3)_1 \times \rm{SU}(3)_2$ spontaneously breaking to the Standard Model $\rm{SU}(3)_C$. A colored massive vector boson is resonantly produced by $q \bar q $ in proton collision, followed by a colored scalar cascade decay. This process gives two photons and one jet in the final states. And the kinetic edge of the two photons can be an interpretation of the diphoton excess, while satisfying the dijet, $\rm{t}\bar{t}$, jet+photon resonance constraints. In this model, due to the large mass of vector resonance, the parton luminosity function ratio between $13~\rm{TeV}$ and $8~\rm{TeV}$ can be quite large. Therefore, the diphoton excess has not been observed at $8~\rm{TeV}$ search. On the other hand, having all the new particles color-charged around $\rm{TeV}$, this model predicts new signals at the LHC, which can be validated soon.

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arXiv2015

Boosted Dark Matter in IceCube and at the Galactic Center

We show that the event excess observed by the IceCube collaboration at TeV--PeV energies, usually interpreted as evidence for astrophysical neutrinos, can be explained alternatively by the scattering of highly boosted dark matter particles. Specifically, we consider a scenario where a $\sim 4$ PeV scalar dark matter particle $ϕ$ can decay to a much lighter dark fermion $χ$, which in turn scatters off nuclei in the IceCube detector. Besides these events, which are exclusively shower-like, the model also predicts a secondary population of events at $\mathcal{O}(100 \text{TeV})$ originating from the 3-body decay $ϕ\to χ\barχa$, where $a$ is a pseudoscalar which mediates dark matter--Standard Model interactions and whose decay products include neutrinos. This secondary population also includes track-like events, and both populations together provide an excellent fit to the IceCube data. We then argue that a relic abundance of light Dark Matter particles $χ$, which may constitute a subdominant component of the Dark Matter in the Universe, can have exactly the right properties to explain the observed excess in GeV gamma rays from the galactic center region. Our boosted Dark Matter scenario also predicts fluxes of $\mathcal{O}(10)$ TeV positrons and $\mathcal{O}(100 \text{TeV})$ photons from 3-body cascade decays of the heavy Dark Matter particle $ϕ$, and we show how these can be used to constrain parts of the viable parameter space of the model. Direct detection limits are weak due to the pseudoscalar couplings of $χ$. Accelerator constraints on the pseudoscalar mediator $a$ lead to the conclusion that the preferred mass of $a$ is $\gtrsim 10$ GeV and that large coupling to $b$ quarks but suppressed or vanishing coupling to leptons are preferred.

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arXiv2015

The leptophilic dark matter with $Z'$ interaction: from indirect searches to future $e^+ e^-$ collider searches

We investigate the scenario where the dark matter only interacts with the charged leptons in the standard model via a neutral vector mediator $Z'$. Such a scenario with a 430 GeV dark matter can fit the recent positron fluxes observed by the AMS-02 Collaborations, with the reasonable boost factors. We study the possibility of searching such leptophilic $Z'$ via its lepton final states and invisible decay modes at the future electron-positron colliders, such as the International Linear Collider (ILC) and the Compact Linear Collider (CLIC). We find that for the benchmark models with $Z'$ mass from 1.0 TeV to 1.5 TeV, the searches for the invisible decays of $Z'\to \bar χχ$ is easily achieved at the CLIC 1.5 TeV runs via the mono-photon process. However, lighter $Z'$ with mass from 0.5 TeV to 0.8 TeV are challenging to see. The di-lepton plus single photon channel can reveal the $Z'$ mass at the ILC and CLIC with moderate luminosities.

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arXiv2013

Discovering extra Higgs boson via pair production of the SM-like Higgs bosons

In the standard model (SM), pair production rate of Higgs boson at the Large Hadron Collider (LHC) is quite low. One usually think that it is extremely important for the measurement of triple Higgs coupling at the high luminosity LHC. In this paper, we propose to search for the extra Higgs boson (denoted as $S$) utilizing pair production of the SM-like Higgs boson ($H$) which was discovered in July, 2012.The pair production of $H$ can be huge due to the resonant production of heavy scalar $S$, namely $PP \rightarrow S \rightarrow HH$. The couplings of $H$ with weak gauge boson are similar to ones in the SM and it implies that the couplings between $S$ and gauge bosons are likely suppressed. Provided that $S$ is heavy enough, the decay into weak gauge bosons may not be the dominant modes. Instead $S$ can decay into a pair of $H$ and offer the promising channel to discover it. In this paper, we studied the 5 promising decay modes of $H$, i.e. $b\bar{b}$, $WW^*$, $ZZ^*$, $γγ$ and $τ^+τ^-$, and simulated the signals and backgrounds for the 15 combination modes for $HH$ at the LHC with $\sqrt{s}= 14$ TeV and integrated luminosity $\mathcal{L}=1000fb^{-1}$. We found that with the help of suitable selection rules, very good signal to background ratio $S/B$ can be archived in many decay channels, for example $b\bar b+ (WW^*, ZZ^*, γγ, τ^+τ^-)$, $WW^*+(WW^*, ZZ^*, γγ, τ^+τ^-)$ and $τ^+τ^- +(γγ, τ^+τ^-)$. For the detailed results please refer to Table \ref{tab:discovery} in the text. On the contrary, $b\bar b b\bar b$ mode is less important due to the huge QCD background. However if one has excellent control on light jet mis-tagging, the $b\bar b b\bar b$ mode can be promising to discover the extra Higgs boson due to its the largest branching ratio compared to other modes.

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