arXiv subjects

C. S. Kim

Publications and source records attributed to C. S. Kim.

At least 19 recordsLinked to original sources

arXiv2026

Electron-muon colliders at high energies to discover heavy sterile neutrinos

We study high-energy charged-lepton-flavor-violating (cLFV) channels in $e^- \mu^+$ scattering to probe heavy sterile neutrinos, which arise naturally in minimal extensions of the Standard Model. For $\sqrt{s} \le 2M_W$, we consider the process $e^- \mu^+ \to e^+ \mu^-$, which is dominated by one-loop box diagrams. We numerically evaluate these diagrams, involving a high-energy extension of the Inami-Lim functions, and find that the amplitudes are strongly suppressed because of their quartic dependence on light-heavy mixing. Using current bounds on active-sterile neutrino mixing, we determine the maximal rates allowed by existing constraints. For $\sqrt{s} > 2M_W$, we analyze the process $e^- \mu^+ \to W^+ W^-$ and compute the corresponding cross sections in both single-sterile and minimal type-I seesaw scenarios. We find this latter process to be significantly more promising for revealing the presence of heavy sterile neutrinos at $e-\mu$ colliders.

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arXiv2025

NuSTAR as an Axion Helioscope

We present a novel approach to investigating axions and axion-like particles (ALPs) by studying their potential conversion into X-rays within the Sun's atmospheric magnetic field. Utilizing high-sensitivity data from the Nuclear Spectroscopic Telescope Array (NuSTAR) collected during the 2020 solar minimum, along with advanced solar atmospheric magnetic field models, we establish a new limit on the axion-photon coupling strength $g_{aγ}\lesssim 7.3\times 10^{-12}$~GeV$^{-1}$ at 95\% CL for axion masses $m_a\lesssim 4\times 10^{-7}$\,eV. This constraint surpasses current ground-based experimental limits, studying previously unexplored regions of the axion-photon coupling parameter space up to masses of $m_a\lesssim 3.4\times 10^{-4}$\,eV. These findings mark a significant advancement in our ability to probe axion properties and strengthen indirect searches for dark matter candidates.

astro-ph.CO↗

arXiv2025

Comments on Exploring Quantum Statistics for Dirac and Majorana Neutrinos using Spinor-Helicity technique (arXiv:2507.07180 [hep-ph])

We give our comments on Ref. [1](arXiv:2507.07180) which critiques our idea of exploring quantum statistics to distinguish between Dirac and Majorana neutrinos proposed in some of our earlier works [2-4]. The ad-hoc symmetrization of the Dirac case amplitude square advocated in Eqs. (16) and (35) of [1] has no physical basis and it leads to violation of lepton number in the the standard model for Dirac neutrinos. Therefore, this symmetrization by hand is in principle incorrect.

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arXiv2025

Invisible decays of vector Charmonia and Bottomonia to determine the Weak Mixing Angle at quarkonia scale

We compute the branching fractions of vector quarkonia ($V_Q=J/ψ, ψ', Υ(nS)$) decays into neutrino pairs, considering both Dirac and Majorana types, within the Standard Model (SM) and beyond. The vector nature of quarkonium states yields a decay width in the SM that depends upon the weak vector coupling of the heavy quark, offering the possibility to measure the weak mixing angle at the quarkonia mass scales. If neutrinos have non-standard neutral weak couplings, this could help to distinguish the nature of neutrinos in principle.

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arXiv2025

Spontaneous Symmetry Breaking and Panic Escape

Panic-induced herding in individuals often leads to social disasters, resulting in people being trapped and trampled in crowd stampedes triggered by panic. We introduce a novel approach that offers fresh insights into studying the phenomenon of asymmetrical panic-induced escape. Our approach is based on the concept of Spontaneous Symmetry Breaking (SSB), a fundamental governing mechanism in the Physical Sciences. By applying the principles of SSB, we conjecture that the onset of disastrous effects of panic can be understood as a SSB phenomenon, and we formulate the process accordingly. We highlight that this way of understanding panic escape leads to simple general measures of preventing catastrophic situations, by considering two crucial parameters: \emph{population density} and \emph{external information}. The interplay of these two parameters is responsible for either breaking or restoring the symmetry of a system. We describe how these parameters are set by design conditions as well as crowd control. Based on these parameters, we discuss strategies for preventing potential social disasters caused by asymmetrical panic escape.

physics.soc-ph↗

arXiv2024

Searching for signatures of new physics in $B \to K \, ν\, \overlineν$ to distinguish between Dirac and Majorana neutrinos

We conduct a model-independent analysis of the distinct signatures of various generic new physics possibilities in the decay $B \to K \, ν\, \overlineν$ by analyzing the branching ratio as well as the missing mass-square distribution. Considering the final neutrinos to be of the same flavor with non-zero mass, we discuss the new physics contributions for both Dirac and Majorana neutrino possibilities. In our study, we utilize the analytical relations among form factors in semi-leptonic $B \to K$ transitions, which are consistent with current lattice QCD predictions to a very high numerical accuracy. We provide constraints on different new physics parameters, taking into account the recent measurement of $B^+ \to K^+ \, ν\, \overlineν$ branching ratio by the Belle-II collaboration. In future, if the missing mass-square distribution for $B^+ \to K^+ \, ν\, \overlineν$ decay gets reported by Belle-II with analysis of more events than their present data set, one can not only investigate possible new physics effects in these decays, but also probe the Dirac/Majorana nature of the neutrinos using quantum statistics, since a difference between the two cases is known to exist in the presence of non-standard neutrino interactions.

hep-ph↗

arXiv2024

Practical Dirac Majorana confusion theorem: Issues and Applicability

We inspect the model-independent study of practical Dirac Majorana confusion theorem (pDMCT) -- a wide spread belief that the difference between Dirac and Majorana neutrinos via any kinematical observable would be practically impossible to determine because of the difference only being proportional to the square of neutrino mass -- in context of processes that have at least a neutrino antineutrino pair in their final state. We scrutinize the domain of applicability of pDMCT and also highlight those aspects that are often misunderstood. We try to clarify some of the frequently used concepts that are used to assert pDMCT as a generic feature irrespective of the process, or observable, such as the existence of any analytic continuity between Dirac and Majorana neutrinos in the limit mass(neutrino) -> 0. In summary, we illustrate that pDMCT is not any fundamental property of neutrinos, instead, it is a phenomenological feature of neutrino non-observation, depending on models and processes.

hep-ph↗

arXiv2024

Comments on "On the Dirac-Majorana neutrinos distinction in four-body decays" (arXiv:2305.14140 [hep-ph], Phys. Rev. D 109, no.3, 033005 (2024))

In arXiv:2305.14140 [hep-ph] the authors analyze the radiative leptonic decay $\ell^- \to ν_\ell \, \overlineν_{\ell'} \, \ell^{\prime -} \, γ$ to distinguish between Dirac and Majorana nature of neutrinos. They utilize the back-to-back kinematics for this purpose, a special kinematic configuration which we first proposed in our paper arXiv:2106.11785 [hep-ph]. Here we point out how and why their analysis of the back-to-back configuration is incorrect. This makes their conclusion and comments invalid and untenable.

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arXiv2024

Comments on "Can quantum statistics help distinguish Dirac from Majorana neutrinos?" (arXiv:2402.05172 [hep-ph])

In a recent article arXiv:2402.05172 [hep-ph], the authors discuss the question "whether quantum statistics can help distinguish between Dirac and Majorana neutrinos." The paper contains, among other things, an unsubstantiated critique of the results derived in our papers arXiv:2106.11785 [hep-ph] and arXiv:2307.05654 [hep-ph]. One of the criticisms is related to our expression for differential decay rate for the back-to-back neutrino-antineutrino configuration in the decay $B^0 \to μ^- \, μ^+ \, ν_μ\, \overlineν_μ$. We show that the claim is wrong and point out how the correct result was obtained. The second criticism is related to the implementation of the anti-symmetrization as dictated by quantum statistics for Majorana neutrinos and antineutrinos (which are identical, by definition). Any direct observation of the neutrinos, as done in Ref. \cite{Akhmedov:2024}, would project the neutrinos into distinguishable helicity states, thus nullifying all observable effects of quantum statistics. They have missed the point that our procedure holds when the neutrino and antineutrino remain undetected by the detector. In the back-to-back kinematic configuration, one can infer the neutrino energies without directly detecting their identities. This smartly ensures that the quantum statistical effects are not erased. Their overriding assertion that our papers arXiv:2106.11785 [hep-ph] and arXiv:2307.05654 [hep-ph] are incorrect fails to recognize that in both arXiv:2106.11785 [hep-ph] and arXiv:2307.05654 [hep-ph] we also point out generic conditions under which the practical Dirac-Majorana confusion theorem holds. "Clearly there is no confusion over confusion theorem."

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arXiv2024

Rare tau decays via exchange of on-shell almost degenerate Majorana neutrinos, $τ^{\mp} \to π^{\mp} N_j \to π^{\mp} μ^{\mp} π^{\pm}$ and $τ^{\mp} \to π^{\mp} N_j \to π^{\mp} μ^{\pm} π^{\mp}$

We investigate rare decays of tau leptons that occur via exchange of heavy on-shell neutrinos $N_j$ ($j=1,2$). These neutrinos can be either Dirac or Majorana, and are considered to be almost degenerate in mass. The decays can thus be either lepton number conserving (LNC), $τ^{\mp} \to π^{\mp} N_j \to π^{\mp} μ^{\mp} π^{\pm}$, or lepton number violating (LNV), $τ^{\mp} \to π^{\mp} N_j \to π^{\mp} μ^{\pm} π^{\mp}$. If neutrinos are Dirac, only LNC decays are possible. If they are Majorana, both LNC and LNV are possible. We derive the corresponding expressions for the effective decay widths $Γ_{{\rm eff},\mp}^{\rm (X)}$ (X=LNC, LNV) of these rare decays, where we account for $N_1$-$N_2$ overlap and oscillation effects and for the finite detector length effects. We then numerically evaluate these decay widths as well as the related CP violation asymmetry width $ΔΓ_{\rm CP}^{\rm (X)} = (Γ_{{\rm eff},-}^{\rm (X)} - Γ_{{\rm eff},+}^{\rm (X)})$. We conclude that for certain, presently allowed, ranges of the heavy-light neutrino mixing parameters, such decays and asymmetries could be observed in Belle II experiment.

hep-ph↗

arXiv2023

The $b$ quark fragmentation fractions at LHCb and meson decays with heavy quark spectators

We study the current estimates of $B_c\to B_s π$ to extract the fragmentation fraction $f_c/f_s$ at the LHCb. A rather robust estimate of $Br(B_c\to B_s π)$ based on factorization and lattice results for the form factor gives $f_c/f_s \sim 0.056$ with a $16\%$ error. We also revisit the extraction of $f_s/f_d$ using $B\to Dπ$ instead of the theoretical cleaner but more suppressed channel $B\to DK$. We also find a tension on the predictions of $Br(B_c\to J/ψπ)$ and $Br(B_c\to B_sπ)$ considering the measurements of these modes at LHCb, and find that, within a $23\%$ uncertainty, only the lower end of the current prediction range $Br(B_c\to J/ψ)\sim 0.4\% - 1.7\%$ would be consistent with the LHCb measurements.

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arXiv2023

Probing the non-standard neutrino interactions using quantum statistics

Using the well established principles of Lorentz invariance, CP and CPT symmetry, and quantum statistics we do a model-independent study of effects of possible non-standard couplings of (Dirac and Majorana) neutrinos. The study is sensitive to the different quantum statistical properties of the Dirac and Majorana neutrinos which, contrary to neutrino-mediated processes of lepton number violation, could lead to observable effects not suppressed by the small ratios of neutrino and heavier particle masses. For processes with a neutrino-antineutrino pair of the same flavor in the final state, we formulate the ``Dirac Majorana confusion theorem (DMCT)'' showing why it is normally very difficult to observe the different behaviour of both kinds of neutrinos in experiments if they have only the standard model (SM)-like left-handed vector couplings to gauge bosons. We discuss deviations from the confusion theorem in the presence of non-standard neutrino interactions, allowing to discover or constrain such novel couplings. We illustrate the general results with two chosen examples of neutral current processes, $Z \to ν\, \overlineν$ and $\mathcal{P}_i \to \mathcal{P}_f \, ν\, \overlineν$ (with $\mathcal{P}_{i,f}$ denoting pseudoscalar mesons, such as $B,K,π$). Our analysis shows that using 3-body decays the presence of non-standard interactions can not only be constrained but one can also distinguish between Dirac and Majorana neutrino possibilities.

hep-ph↗

arXiv2022

Inferring the nature of active neutrinos: Dirac or Majorana?

The nature of a neutrino, whether it is a Dirac type or Majorana type, may be comprehensively probed using their quantum statistical properties. If the neutrino is a Majorana fermion, then by definition it is identical and indistinguishable from the corresponding antineutrino. When a Majorana neutrino and antineutrino are pair produced, the corresponding state has to obey the Pauli principle unlike in the Dirac case. We use this property to distinguish between the two cases using the process $B^0 \to μ^-\,μ^+\,ν_μ\,\barν_μ$. We show that the two cases differ dramatically in a special kinematic scenario where, in the rest frame of the parent $B$ meson, the muons fly away back-to-back (i.e. fly with 3-momenta of equal magnitudes but opposite directions), and so do the neutrino and antineutrino. Unlike any other scenario, we know the energies and magnitudes of $3$-momenta of both the neutrino and the antineutrino in this back-to-back configuration without even directly measuring them. This provides a way of avoiding the constraint imposed by the `practical Dirac-Majorana confusion theorem', as one need not fully integrate over neutrino and antineutrino in this case. As a true signature of the universal principle of quantum statistics which does not depend on the size of the mass of the particle but its spin, the difference between Dirac and Majorana cases in this special kinematic configuration does survive independent of the neutrino mass as long as neutrino mass is nonzero. The analysis presented here is applicable immediately to several other processes with the same final state as in the case of $B^0$ decay without any major change.

hep-ph↗

arXiv2021

CP violation in the rare Higgs decays via exchange of on-shell almost degenerate Majorana neutrinos, $H \to ν_k N_j \to ν_k \ell^{-} U {\bar D}$ and $H \to ν_k N_j \to ν_k \ell^{+} {\bar U} D$

We investigate rare decays of Higgs via exchange of two almost degenerate heavy on-shell Majorana neutrinos $N_j$ ($j=1,2$): $Γ_{\pm} = Γ(h \to ν_k N_j \to ν_k \ell^{\pm} π^{\mp})$, and into the open quark channels $Γ_{\pm} = Γ(h \to ν_k N_j \to ν_k \ell^{\pm} U D)$, where $U D$ are two jets of open quarks (${\bar U} D$, or $U {\bar D}$, where $U=u, c$ and $D=d, s$). The related CP violation asymmetry $A_{\rm CP} = (Γ_{-} - Γ_{+})/(Γ_{-} + Γ_{+})$ is studied in detail. We take into account the $N_1$-$N_2$ overlap and oscillation effects. We can see that for certain, presently acceptable, range of input parameters, such decays with open quark channels, and their asymmetries, could be detected in the International Linear Collider (ILC).

hep-ph↗

arXiv2020

Sensitivity bounds on heavy neutrino mixing $|U_{μN}|^2$ and $|U_{τN}|^2$ from LHCb upgrade

Decays of heavy pseudoscalar mesons $B$, $B_c$, $B_s$ and $D_s$ at LHCb upgrade are considered, which produce either two equal sign muons or taus. In addition, we consider the analogous decays with opposite sign muons or taus. All these decays are considered to be mediated by a heavy on-shell neutrino $N$. Such decays of $B$ mesons, if not detected, will give in general stringent upper bounds on the heavy-light mixing parameter $|U_{μN}|^2$ as a function of the neutrino mass $M_N \sim 1$ GeV, principally due to the large expected number of produced mesons $B$. While some of the decays of the other mentioned mesons are attractive due to a weaker CKM-suppression, the expected produced number of such mesons is significantly smaller that that of $B$'s; therefore, the sensitivity bounds from such decays are in general comparable or less restrictive. When $τ$ pairs are produced, only two types of such decays are significant: $B^{\pm}, B_{c}^{\pm} \to τ^{\pm} τ^{\pm} π^{\mp}$ (and $τ^{\pm} τ^{\mp} π^{\pm}$), giving us stringent upper bounds on $|U_{τN}|^2$; the other decays with a pair of $τ$, such as $B^0 \to D^{(*)-} τ^+ τ^+ π^-$ (and $D^{(*)-} τ^+ τ^- π^+$), are prohibited or very suppressed by kinematics.

hep-ph↗

arXiv2020

Exploring CP-violation, via heavy neutrino oscillations, in rare B meson decays at Belle II

In this article we study the rare B-meson decay via two on-shell almost-degenerate Majorana Heavy Neutrinos, into two charged leptons and two pseudoscalar mesons ($B^{\pm} \to D^0 \ell^{\pm}_1 \ell^{\pm}_2 π^{\mp}$). We consider the scenario where the heavy neutrino masses are $\sim 2$ GeV and the heavy-light mixing coefficients are $|B_{\ell N}|^2 \sim 10^{-5}$, and evaluate the possibility to measure the CP-asymmetry at Belle II. We present some realistic conditions under which the asymmetry could be detected.

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arXiv2020

Probing sterile neutrino in $B$ ($D$) meson decays at Belle II (BESIII)

We present, how a systematic study of $B \to D\ell N$ ($D \to K \ell N$) decays with $\ell=μ,τ$, at Belle II (BESIII) can provide unambiguous signature of a heavy neutrino $N$ and/or constrain its mixing with active neutrinos $ν_\ell$, which is parameterized by $|U_{\ell N}|^2$. Our constraint on $|U_{μN}|^2$ that can be achieved from the full Belle II data is comparable with what can be obtained from the much larger data set of the upgraded LHCb. Additionally, our method offers better constraint on $|U_{μN}|^2$ for mass of sterile neutrino $m_N < 2$ GeV. We can also probe the Dirac and Majorana nature of $N$ by observing the sequential decay of $N$, including suppression from observation of a displaced vertex as well as helicity flip, for Majorana $N$.

hep-ph↗

arXiv2020

Sensitivity limits on heavy-light mixing $|U_{μN}|^2$ from lepton number violating $B$ meson decays

We consider the lepton number violating decays $B \to μ^{\pm} μ^{\pm} π^{\mp}$ and $B \to D^{(*)} μ^{\pm} μ^{\pm} π^{\mp}$ which may be detected at LHCb and Belle-II experiments; and $B \to μ^{\pm} μ^{\pm} e^{\mp} ν$ and $B \to D^{(*)} μ^{\pm} μ^{\pm} e^{\mp} ν$ decays which may be detected at Belle-II experiment. The projected total number of produced $B$ mesons is $4.8 \times 10^{12}$ at LHCb upgrade and $5 \times 10^{10}$ at Belle-II. For the case that the above decays are not detected, we deduce the new upper bounds (sensitivity limits) for the mixing parameter $|U_{μN}|^2$ of heavy sterile neutrino with sub-eV light neutrino, as a function of the sterile neutrino mass in the interval $1.75 \ {\rm GeV} < M_N < 5.0 \ {\rm GeV}$. We take into account the probability of decay of the sterile neutrino $N$ within the detector, taking as the effective detector length $L=2.3 \ m$ at LCHb upgrade and $L=1 \ m$ at Belle-II. In the interval $1.75 \ {\rm GeV} < M_N < 3 \ {\rm GeV}$, the most stringent bounds can be obtained with the decays $B \to μ^{\pm} μ^{\pm} π^{\mp}$ at LHCb upgrade. The sensitivity limits are expected to be in general more stringent at LHCb upgrade than at Belle-II, principally because the number of produced $B$ mesons in LHCb upgrade is expected to be by about two orders of magnitude larger than at Belle-II. We conclude that the LHCb upgrade and Belle-II experiments have the potential to either find a new heavy Majorana neutrino $N$, or to improve significantly the sensitivity limits (upper bounds) on the heavy-light mixing parameter $|U_{μN}|^2$, particularly in the mass range $1.75 \ {\rm GeV} < M_N < 3 \ {\rm GeV}$. This work is a continuation and refinement of our previous work [1] on the subject.

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