arXiv subjects

Xiao-Dong Ma

Publications and source records attributed to Xiao-Dong Ma.

At least 19 recordsLinked to original sources

arXiv2026

Chiral matching of dimension-7 baryon-number-violating operators and application to $ΔI=3/2$ nucleon decays

We study chiral matching of dimension-7 (dim-7) baryon-number-violating operators in low-energy effective field theory. We show that all dim-7 operators can be classified into two distinct Lorentz structures and their chirality-flipped counterparts, and we derive their leading-order hadronic realizations in chiral perturbation theory. As an application, we systematically investigate $ΔI=3/2$ nucleon decays, including $n\to \ell^-π^+$, $p\to \ell^- π^+π^+$, and $n\to \ell^- π^+ (π^0, η)$, with $\ell=e,~μ$. Using existing experimental bounds on $n\to \ell^-π^+$, we set stringent constraints on the relevant Wilson coefficients and derive improved lower limits on the partial lifetimes of the six three-body modes from their correlations with the two-body modes. We further present a concrete ultraviolet model that generates the dim-7 operators and the associated $ΔI=3/2$ nucleon decays at leading order. Our results pave the way for exploring dim-7 contributions to nucleon decays and motivate future searches for these exotic modes.

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arXiv2026

Systematic study of lepton-flavor-violating dark matter interactions via indirect detection in effective field theories

Lepton-flavor-violating (LFV) interactions involving dark matter (DM) particles remain a largely unexplored area. In this study, we systematically investigate LFV DM interactions within the framework of effective field theories by analyzing astrophysical photons and positrons produced from DM annihilation. Employing the astrophysical photon and positron data collected by Fermi-LAT, INTEGRAL, XMM-Newton, and AMS-02, we place meaningful constraints on all leading-order effective operators involving a DM pair and a flavor-violating charged lepton pair. Our analysis covers the three well-known DM candidates: a scalar, a fermion, and a vector particle. For the photon flux, we consider contributions from final-state radiation, radiative decay, and inverse Compton scattering and examine their respective sensitivity regions across different DM masses and photon energies. We find that, for DM masses below $\mathcal{O}(20\,\rm GeV)$, INTEGRAL provides the most stringent constraints on annihilation cross sections and effective operators in all three LFV channels, whereas AMS-02 offers the strongest constraints above $\mathcal{O}(20~\rm GeV)$.

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arXiv2026

Baryon-number-violating nucleon decays into a dark photon particle

Baryon-number-violating (BNV) nucleon decays into a light new particle represent an exciting yet experimentally unexplored frontier. In this work, we systematically study nucleon decays into a dark photon using a low-energy effective field theory extended with a dark photon $X$, referred to as $X$LEFT. We first construct a complete set of leading-order BNV $X$LEFT operators and then perform a systematic matching onto the chiral perturbation theory for operators involving light $u,d,s$ quarks that dominantly contribute to nucleon decays. Within the chiral framework, we derive general expressions for the decay widths of both two- and three-body nucleon decays and analyze the momentum distributions in the latter. Finally, we thoroughly reinterpret the existing experimental data on conventional two-body modes (into a lepton and a meson) to set lower bounds on partial lifetimes of the corresponding three-body modes involving an additional dark photon. These bounds allow us to further set stringent constraints on the $X$LEFT operators and other correlated decay modes. Our results provide a toolkit for future experimental and theoretical studies of these exotic nucleon decays.

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arXiv2026

Renormalization-group-improved constraints on dimension-7 baryon-number-violating operators

We study constraints on dimension-7 SMEFT baryon-number-violating operators from nucleon decays by incorporating full renormalization group (RG) running effects. At high new physics scales, we demonstrate that RG running effects help set stringent bounds on all 297 Wilson coefficients compared to the tree-level analysis in which only coefficients involving the first and second fermion generations could be constrained. Our findings highlight that the RG running effects through Yukawa mixings are particularly important for indirectly probing operators involving the second and third generation fermions.

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arXiv2026

Comprehensive investigation of nucleon decays into one lepton plus two mesons

We systematically investigate baryon number violating (BNV) nucleon decays into one lepton ($e,μ,ν/\barν$) and two pseudoscalar mesons ($ππ,πη,πK$) within the low-energy effective field theory (LEFT) framework. By employing chiral perturbation theory, we obtain general expressions for the decay widths of these three-body nucleon decay modes induced by dimension-6 LEFT BNV operators and express them in terms of the associated Wilson coefficients. Since the same set of LEFT operators contribute to the experimentally well-constrained two-body nucleon decays, we then utilize the experimental bounds on them to constrain the relevant Wilson coefficients. From the obtained constraints, we derive improved limits on the occurrence of 22 three-body modes involving a charged lepton and 9 modes containing a neutrino or an antineutrino, with the new partial lifetime bounds being orders of magnitude stronger than the existing experimental limits. Our framework and derived bounds will facilitate future experimental searches for these nucleon decays.

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arXiv2026

Nucleon decays into one lepton plus two non-strange mesons

Nucleon decays into a lepton and two pseudoscalar mesons represent key channels for probing baryon number violation, complementing conventional two-body modes. In this Letter, we model-independently correlate two- and three-body processes within the framework of low-energy effective field theory, performing a global analysis that avoids single-operator-dominance assumption. We derive significantly improved bounds on 15 three-body modes with a lepton ($e^+,\,μ^+,\hatν=ν/\barν$) and two non-strange mesons ($π,η$). For charged-lepton modes, our lower limits on partial lifetimes ($Γ^{-1}$) surpass current Particle Data Group (PDG) values by more than three orders of magnitude. For 5 (anti)neutrino modes, we establish for the first time $Γ^{-1}\gtrsim 10^{34}\,\rm yr$. Additionally, our analysis improves constraints on two-body processes $n\to e^+π^-$, $n\to μ^+π^-$, and $p\to \hatνπ^+$ by approximately a factor of 2 compared to the PDG limits. These results highlight the importance of leveraging correlations among different processes to better probe new physics, enabling more stringent constraints on experimentally challenging processes from well-measured ones.

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arXiv2026

Nucleon decays into three leptons: contact contributions

Baryon number violating (BNV) nucleon decays into three leptons provide a unique probe of BNV interactions beyond the conventional two-body modes involving a single lepton and a light meson. In a previous work [Nucleon decays into three leptons: noncontact contributions, arXiv:2512.02692.], two of us analyzed the noncontact contributions to these decays arising from dimension-6 (dim-6) operators within the low-energy effective field theory (LEFT), and found that they are severely suppressed due to stringent constraints on these dim-6 operators. In this work, we continue this endeavor by systematically investigating the contact contributions originating from dim-9 LEFT operators. We construct a complete basis of dim-9 operators relevant to these processes, and subsequently match them onto chiral perturbation theory to calculate their decay widths. By employing existing experimental data, we derive stringent constraints on the relevant operators. In addition, we present the analysis of an ultraviolet-complete model to demonstrate its connection with our theoretical framework, thereby facilitating further studies of these exotic nucleon decays in upcoming neutrino experiments with large fiducial masses.

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arXiv2026

Nucleon decays into three leptons: Noncontact contributions

We investigate baryon number violating (BNV) nucleon decays into three leptons from noncontact contributions that are induced by dimension-6 (dim-6) BNV operators in low-energy effective field theory (LEFT) with an exchange of a baryon, meson, lepton, or photon field. We systematically classify all these processes that change lepton flavor by one unit and formulate their decay widths in terms of the dim-6 LEFT Wilson coefficients. By applying constraints on these Wilson coefficients derived from current experimental limits on BNV two-body nucleon decays, we obtain stringent bounds on the rates of these triple-lepton modes. These bounds vary significantly from one dim-6 operator to another under consideration. Our results for the $Δ(B-L)=0$ modes differ by several orders of magnitude from previous phase-space estimates in the literature, thereby providing a more reliable assessment of their potential occurrence. In addition, we provide improved bounds on $Δ(B+L)=0$ modes compared to the existing experimental limits.

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arXiv2026

Light fermionic dark matter window in the scotogenic inverse seesaw model

The origin of neutrino mass and the nature of dark matter (DM) remain unresolved puzzles in particle physics, and an appealing possibility is to address both in a unified picture. This paper explores a light fermionic DM candidate within the scotogenic inverse seesaw model, which can simultaneously provide a mechanism for neutrino mass generation. By incorporating constraints from neutrino oscillation data, charged lepton flavor violating processes, invisible decays of the Higgs and $Z$ bosons, DM relic density, and direct detection of DM, we uncover a light fermionic DM window in the mass range $58\,{\rm GeV} \lesssim m_{\tt DM} \lesssim 63\,{\rm GeV}$ that can satisfy all of the aforementioned constraints. We find that this window can be jointly tested by next-generation ton-scale DM direct detection experiments including PandaX-xT and XENONnT, Higgs invisible decays, and future lepton colliders such as ILC.

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arXiv2026

Charged lepton flavor violating decays with a pair of light dark matter and muonium invisible decay

In this paper, we initiate the study of lepton flavor violating (LFV) dark matter (DM) interactions, expanding our focus beyond the flavor-conserving DM interactions typically considered in conventional direct and indirect detections. We work in an effective field theory (EFT) framework, focusing on the leading-order local operators of the form, $\bar \ell_j Γ\ell_i\,{\tt DM}^2$, where $(ij)=(eμ, eτ, μτ)$ and the DM includes the three well-known scenarios: a scalar, a fermion, and a vector. We derive the invariant-mass distribution for the three-body decay $\ell_i \to \ell_j +{\tt DM+DM}$ and demonstrate that it can be used to distinguish between different operator structures and to determine the DM mass. By utilizing current experimental bounds on the charged muon LFV decay involving neutrinos and the ratio of tau leptonic decay widths, we establish stringent limits on the effective scale associated with each operator. Additionally, for the $eμ$ flavor combination, we investigate the muon four-body radiative decay ($μ\to e +{\tt DM+DM}+γ$) to complement our probe of such interactions. Finally, we examine muonium invisible decays based on the derived bounds on the effective operators and find that the branching ratios can be significantly enhanced compared to the predictions of the standard model. In particular, any future observation of the para-muonium invisible decay serves as a compelling signature for these flavored DM interactions.

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arXiv2026

Baryon-number-violating nucleon decays in SMEFT extended with a light scalar

New light particles have received considerable attention in recent years. Baryon-number-violating (BNV) nucleon decays involving such light particles are able to provide stringent constraints. They exhibit distinctive experimental signatures that merit thorough investigation. We systematically investigate BNV nucleon decay with a light scalar in an effective field theory framework. Within this framework, we set stringent bounds on BNV operators using available experimental data and predict the occurrence of several BNV three-body nucleon decays. We further study contributions to dinucleon to dilepton transitions in a nucleus mediated by the scalar, which complements single nucleon decay. Finally, we provide three ultraviolet-complete models that can generate different subsets of BNV operators in leading order. Our theoretical framework will facilitate experimental searches for those exotic nucleon decays.

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arXiv2026

Comprehensive investigation on baryon number violating nucleon decays involving an axion-like particle

In this study, we systematically investigate baryon number violating (BNV) nucleon decays into an axion-like particle (ALP), within a low energy effective field theory extended with an ALP, which is referred to as aLEFT. Unlike previous studies in the literature, we consider contributions to nucleon decays from a complete set of dimension-eight BNV aLEFT operators involving light $u,\,d,$ and $s$ quarks. We perform the chiral irreducible representation (irrep) decomposition of these interactions under the QCD chiral group $\rm SU(3)_{\tt L}\times SU(3)_{\tt R}$, and match them onto the recently developed chiral framework to obtain nucleon-level effective interactions among the ALP, octet baryons, and octet pseudoscalar mesons. Within this framework, we derive general expressions for the decay widths of nucleon two- and three-body decays involving an ALP. Subsequently, we analyze momentum distributions for three-body modes and find that operators belonging to the newly identified chiral irreps $\pmb{6}_{\tt L(R)}\times \pmb{3}_{\tt R(L)}$ exhibit markedly different behavior compared to that in the usual irreps $\pmb{8}_{\tt L(R)}\times \pmb{1}_{\tt R(L)}$ and $\pmb{3}_{\tt L(R)}\times \bar{\pmb{3}}_{\tt R(L)}$. In addition, we reanalyze experimental data collected by Super-Kamiokande and establish bounds on the inverse decay widths of these new modes by properly accounting for experimental efficiencies and Cherenkov threshold effects because of the lack of direct constraints on those exotic decay modes. Our recasting constraints are several orders of magnitude more stringent than inclusive bounds used in the literature. Based on these improved bounds, we set conservative limits on associated effective scales across a broad range of ALP mass and predict stringent bounds on certain neutron and hyperon decays involving an ALP.

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arXiv2026

A systematic investigation on vector dark matter-nucleus scattering in effective field theories

In this paper, we systematically investigate the general spin-one dark matter-nucleus interactions within the framework of effective field theories (EFT). We consider both the nonrelativistic (NR) and the relativistic EFT descriptions of the DM interactions with nucleons and quarks. In the NREFT framework, we present a complete list of NR operators for spin-one DM coupling to nucleons and compute their contributions to the DM response functions. Next, we consider all possible leading-order relativistic EFT operators between DM and light quarks and the photon, and perform NR reductions to match them onto the NREFT. We then derive the nuclear scattering rate from these interactions, and employ recent DM direct detection data (from both the nuclear recoil and the Migdal effect) to constrain all these EFT operators and DM electromagnetic properties. We find the elastic nuclear recoil data (from PandaX-4T, XENONnT, LZ, and DarkSide-50) set stringent bounds on the EFT coefficients for a DM mass above a few GeV while the Migdal effect datasets (from PandaX-4T, XENONnT, and DarkSide-50) can probe the DM mass region as small as 20 MeV. Lastly, we construct a UV complete model that can provide a complex spin-one DM candidate, and at the same time generate DM-quark/photon operators discussed in this work.

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arXiv2026

Probing lepton flavor violating dark matter scenarios via astrophysical photons and positrons

In this Letter we explore, for the first time, the constraints on lepton flavor violating (LFV) dark matter (DM) scenarios via the astrophysical photons and positrons, including both the annihilation and decay modes, ${\tt DM(+DM)}\to e^\pm μ^\mp, e^\pm τ^\mp, μ^\pm τ^\mp$. Given the presence of LFV interactions in various DM models and the challenge of probing such interactions at terrestrial facilities, such as DM direct detection and collider experiments, indirect detection offers a unique approach to investigating them. We utilize the currently available photon datasets from the XMM-Newton, INTEGRAL, and Fermi-LAT telescopes, along with the positron datasets from the AMS-02 satellite, to establish stringent bounds on the relevant annihilation cross sections or decay widths. In particular, we include contributions to the photon spectrum from final state radiation, radiative decays, and inverse Compton scattering. We find that the INTEGRAL (AMS-02) provides the most stringent bound on the annihilation cross sections and decay widths for DM mass below (above) approximately 20 GeV, which are comparable to those of their lepton flavor conserving counterparts.

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arXiv2026

CP violation in the hyperon decays $Σ\to Nπ$

The study of $CP$ violation in hyperon transitions has a long history. In the early 2000s the HyperCP experiment made a major effort to seek $CP$-odd signals in the decay sequence $Ξ^-\toΛπ^-$ and $Λ\to pπ^-$, which motivated more searches. Most recently the BESIII and LHCb Collaborations have acquired or improved the upper bounds on $CP$ violation in a variety of hyperon nonleptonic processes, including $Σ^+\to nπ^+$ and $Σ^+\to pπ^0$. These measurements have not reached the standard-model level yet, but have stimulated a renewed interest in $CP$-violating new physics in strange-quark decay beyond what is constrained by the parameters $\varepsilon$ and $\varepsilon^\prime$ from the kaon sector. In this paper, after updating the standard-model expectations for $CP$-odd observables in the modes $Σ^\pm\to Nπ$, we revisit new-physics scenarios that could enhance the corresponding quantities in $Λ\to Nπ$ and $Ξ\toΛπ$ and apply them to the $Σ^\pm$ modes. We find that the $CP$ asymmetries in the latter can be significantly increased over the standard-model expectations, at levels which may be tested in the ongoing BESIII experiment and in future endeavors such as PANDA and the Super Tau Charm Facility.

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arXiv2025

New chiral structures for baryon number violating nucleon decays

We examine the most general nucleon decay interactions that involve three light quarks without being acted upon by a derivative. We identify four generic operator structures that correspond to the irreducible representations in the chiral group ${\rm SU(3)}_{\tt L}\otimes {\rm SU(3)}_{\tt R}$ of QCD, \{$\pmb{8}_{\tt L}\otimes \pmb{1}_{\tt R}$, $\bar{\pmb{3}}_{\tt L}\otimes \pmb{3}_{\tt R}$, $\pmb{6}_{\tt L}\otimes \pmb{3}_{\tt R}$, $\pmb{10}_{\tt L}\otimes \pmb{1}_{\tt R}$\}, plus their chirality partners under the interchange of chiralities ${\tt L}\leftrightarrow {\tt R}$. While half of them have been extensively discussed in the literature, the other half, $\pmb{6}_{\tt L(R)}\otimes \pmb{3}_{\tt R(L)}$ and $\bar{\pmb{10}}_{\tt L(R)}\otimes \pmb{1}_{\tt R(L)}$, are identified for the first time. We perform chiral matching for these interactions at the leading chiral order and find that each has a unique chiral realization in terms of the octet baryons and pseudoscalars. Notably, the chiral interaction in the $\pmb{6}_{\tt L(R)}\otimes \pmb{3}_{\tt R(L)}$ representation appears at the same chiral order as those of the known ones, while the one in the $\pmb{10}_{\tt L(R)}\otimes \pmb{1}_{\tt R(L)}$ representation appears at a higher chiral order. These new structures are prevalent in effective field theories and ultraviolet models, and they offer novel experimental avenues to search for baryon number violating nucleon decays.

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arXiv2025

Light dark-matter window constrained by \boldmath$K^+\toπ^+$$+$$\not{\!\!E}$

We explore the constraints on new physics from the recent NA62 observation of the kaon decay $K^+\toπ^+$+$\not{\!\!E}$ with missing energy $\not{\!\!E}$ in the context of a dark-matter (DM) scenario recently used to accommodate the Belle II finding of an enhanced rate of the $b$-meson decay $B^+\to K^+$+$\not{\!\!E}$ compared to the standard-model expectation. Specifically, assuming that a light real scalar boson $ϕ$ plays the role of DM and working in an effective field-theory framework, we study model independently the impact of operators involving $ϕ$ and ordinary quarks on the aforementioned transitions over the kaon mode's kinematical mass region of $m_ϕ< (m_K - m_π)/2 = 177$ MeV. Such a DM particle is subject to significant restrictions from the observed relic abundance and from DM direct-detection experiments incorporating the Migdal effect, as well as from indirect searches in cosmic microwave background data and collider experiments, except when its mass is between 110 and 146 MeV. We demonstrate that $K^+\toπ^+ϕϕ$ can saturate the new-physics window in the NA62 result if $m_ϕ$ lies in the 110-130 MeV portion of the range left by the DM constraints, thus providing a complementary constraint on this scenario. Improved data from future Belle II and NA62 measurements and DM quests can test it more stringently. In particular, expanding the NA62 signal window into the region that is now removed due to three-body decay background modes could further explore the remaining mass window for this type of invisible particle, $130 < m_ϕ< 177$ MeV.

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arXiv2025

Chiral perturbation theory for baryon-number-violating nucleon decay into a vector meson

In a recent work [New chiral structures for baryon number violating nucleon decays, arXiv:2504.14855], we identified generic baryon-number-violating (BNV) structures containing triple light quarks and achieved their leading-order chiral realizations involving octet pseudoscalars and baryons. Although many two-body nucleon decays into a vector meson have been experimentally searched for and stringently constrained, a consistent theoretical framework for their calculation is still lacking. In this Letter, we fill the gap by implementing chiral matching of all these triple-quark interactions onto hadronic interactions involving octet vector mesons, baryons, and pseudoscalars. This paves the way for a consistent and comprehensive study of all relevant BNV processes. As an illustration of application, we show how degeneracy in the parameter space of Wilson coefficients can be broken by synthesizing experimental constraints on nucleon decays into a vector or pseudoscalar meson when relevant hadronic low-energy constants can be reasonably determined.

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