arXiv subjects

Jusak Tandean

Publications and source records attributed to Jusak Tandean.

At least 19 recordsLinked to original sources

arXiv2026

CP violation in $Σ^+\to p\ell^+\ell^-$ within the standard model and beyond

The LHCb collaboration has recently observed the rare hyperon decay $Σ^+\to pμ^+μ^-$. It can also measure the corresponding antihyperon channel with comparable precision and is thus in a position to extract information on $CP$ violation in this mode. Interestingly, the long-distance contributions that dominate it within the standard model provide large absorptive phases that could drive substantial $CP$ violation through interference with potential new-physics contributions. Here we explore this possibility, finding that the decay rate asymmetry is currently allowed to be as high as tens of percent, which can be probed by LHCb in the near future. We additionally consider the same with regard to the dielectron mode $Σ^+\to pe^+e^-$ as well as the related radiative one $Σ^+\to pγ$.

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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

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

$B\to K{+}$invisible, dark matter, and $CP$ violation in hyperon decays

Recently the Belle II Collaboration has reported a measurement of the $B^+\to K^+ν\barν$ rate that is higher than the standard-model expectation. Since the emitted neutrinos are unobserved, the excess could be due to the $B^+$ decaying into a $K^+$ and a dark-matter pair. We entertain this possibility in a two-Higgs-doublet model supplemented with a real singlet scalar boson acting as the dark matter. This model also accommodates strangeness-changing interactions providing new sources of $CP$ violation which can affect hyperon and kaon nonleptonic transitions. We find that the resulting $CP$ violation in the hyperon sector can be significant, reaching the current empirical bounds, after taking into account constraints from kaon mixing and decay and from dark-matter relic-density data and direct searches including the Migdal effect. We demonstrate that the hyperon and kaon processes are complementary probes of this new-physics scenario. Its prediction for sizable hyperon $CP$ violation is potentially testable in ongoing experiments, such as BESIII, Belle II, and LHCb, and in next-generation ones like PANDA and at the Super Tau Charm Facility.

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arXiv2025

Large CP violation in $Λ^0_b \to pK^-π^+π^-$ and its U-spin partner decays

The LHCb Collaboration has recently found a large CP-violating rate asymmetry in the $b$-baryon decay $Λ^0_b \to pK^-π^+π^-$. This is the first observation of CP violation in baryon processes, opening a new window to test its standard model origin. Many more baryon decays are expected to exhibit observable signals of CP violation. We show that there also exists large CP violation in the U-spin partner decay mode, $Ξ^0_b \to Σ^+π^- K^+K^-$, with rate asymmetry $$A_{CP}(Ξ^0_b \to Σ^+π^- K^+ K^-) = - A_{CP}(Λ^0_b \to p K^- π^+ π^-)~ \frac{Br(Λ^0_b \to p K^- π^+ π^-)}{Br(Ξ^0_b \to Σ^+ π^- K^+ K^-)}~ \frac{τ^{Ξ_b}}{τ^{Λ_b}} $$ in the U-spin symmetry limit. By neglecting a subleading contribution in the amplitudes, we obtain $$A_{CP}(Λ^0_b \to p π^+ π^- π^-) = A_{CP}( Ξ^0_b \to Σ^+π^- K^+ K^-) = -(12 \pm 3 ) \%. $$ These predictions provide crucial tests for the standard model.

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arXiv2025

Remarks on strong phase shifts in weak nonleptonic baryon decays

A sizable strong-interaction phase shift in weak two-body nonleptonic baryon decay would enhance the possibility of discovering charge-conjugation parity ($CP$) violation in the baryon sector, which might help in the quest for understanding the matter-antimatter asymmetry in the universe. Over the past 60 years, empirical analyses involving different types of instruments, including fixed-target experiments and $e^+e^-$ colliders, have indicated that the phase shifts in nonleptonic hyperon decays are relatively small, below order ten degrees in size. A large phase shift, however, has been observed by BESIII in the decay of a charmed baryon into a hyperon and kaon, $Λ_c^+\to Ξ^0K^+$. In various experimental and theoretical studies on hyperon, charmed-baryon, and bottomed-baryon decays, different conventions have been adopted for defining the strong phases. It is important to be aware of this situation when obtaining global averages from different measurements and applying the results to future investigations on $CP$ violation among baryons. This paper gives an overview of the conventions employed in the literature for the strong phases and suggests a unified parameterization form applicable to the different alternatives. Numerical results under the unified parameterization form are also provided, which can serve as useful inputs to further pursuits of baryon $CP$ violation.

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arXiv2025

$Σ^{+}\to p\ell^{+}\ell^{-}$ decays within the standard model and beyond

Motivated by the LHCb measurement of the hyperon decay mode $Σ^+\to pμ^+μ^-$ and prospects for improvement, we revisit the estimates for the rate and muon forward-backward asymmetry within the standard model and beyond. The standard model prediction has a fourfold ambiguity, and we suggest ways to resolve it with other measurements, including possible studies of $Σ^+\to p e^+e^-$ in the BESIII and LHCb experiments. We use the recent BESIII measurements of $Σ^+\to p γ$ and $Σ^+\to Nπ$ to reduce the uncertainty in the long-distance contribution to $Σ^+\to pμ^+μ^-$. Beyond the standard model, we consider a general effective Hamiltonian at low energy with ten operators whose Wilson coefficients parametrize the new physics. We derive expressions for the $Σ^+\to pμ^+μ^-$ rate and the associated muon forward-backward asymmetry in terms of these coefficients. Finally, we present the constraints on these Wilson coefficients that result from both kaon and hyperon decays and emphasize their complementarity.

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arXiv2023

FCNC charmed-hadron decays with invisible singlet particles in light of recent data

The flavor-changing neutral current (FCNC) decays of charmed hadrons with missing energy $(\not\!\!E)$ can serve as potentially promising hunting grounds for hints of new physics, as the standard-model backgrounds are very suppressed. A few of such processes have been searched for in recent experiments, specifically $D^0\to\,\not\!\!E$ by Belle and $D^0\toπ^0$$\not\!\!E$ and $Λ_c^+\to p\!\not\!\!E$ by BESIII, resulting in upper bounds on their branching fractions. We consider them to illuminate the possible contributions of the quark transition $c\to u\!\not\!\!E$ with a couple of invisible spinless bosons carrying away the missing energy, assuming that they are not charge conjugates of each other and hence can have unequal masses. We find that these data are complementary in that they constrain different sets of the underlying operators and do not cover the same ranges of the bosons' masses, but there are regions not yet accessible. From the allowed parameter space, we show that other $D$-meson decays, such as $D\toρ$$\not\!\!E$, and the charmed-baryon ones $Ξ_c\to(Σ,Λ)$$\not\!\!E$ can have sizable branching fractions and therefore may offer further probes of the new-physics interactions. We point out the importance of $D^0\toγ\!\not\!\!E$ which are not yet searched for but could access parts of the parameter space beyond the reach of the other modes. In addition, we look at a scenario where the invisibles are instead fermionic, namely sterile neutrinos, and a scalar leptoquark mediates $c\to u\!\not\!\!E$. We discuss the implications of the aforesaid bounds for this model. The predictions we make for the various charmed-hadron decays in the different scenarios may be testable in the near future by BESIII and Belle II.

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arXiv2023

$ΔS=2$ nonleptonic hyperon decays as probes of new physics

Hyperon nonleptonic decays that change strangeness by two units, such as $Ξ\to Nπ$ and $Ω^-\to nK^-,Λπ^-,Σ^{(*)}π$, are highly suppressed in the standard model. Only a few of them have been searched for to date, leading to experimental upper bounds which are many orders of magnitude above the expectations of the standard model. This leaves ample opportunity to look for indications of new physics in these processes. At the same time, most, but not all, $ΔS=2$ interactions beyond the standard model are severely constrained by kaon-mixing data. We present two scenarios where new physics satisfying the kaon-mixing constraints can enhance the hyperon decay rates to levels that can be probed in future quests by BESIII and LHCb and at the proposed Super Tau-Charm Factory. Both scenarios require significant fine-tuning.

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arXiv2022

Pursuit of $CP$ violation in hyperon decays at $e^+e^-$ colliders

We present a concise overview on $CP$ violation in hyperon decays, including past, present, and future efforts to search for it. We highlight in particular the most recent results of the BESIII experiment using a quantum-entangled pair of hyperon and antihyperon produced by a charmonium resonance formed in $e^+e^-$ annihilation. The subsequent nonleptonic decays of these hyperon pairs allow for the simultaneous determination of various parameters pertaining to $CP$ violation in the decays. We compare the reported data with the corresponding current predictions for $CP$ asymmetries within the standard model of particle physics. Moreover, we touch on how much these asymmetries might be enhanced by possible new physics beyond the standard model. We also comment briefly on the importance of the proposed super tau-charm factories in future quests for hyperon $CP$-violation.

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arXiv2022

Study of CP violation in hyperon decays at Super Charm-Tau Factories with a polarized electron beam

Non-leptonic two-body weak decays of baryons are an important tool to probe the combined charge-conjugation--parity symmetry (CP) violation. We explain why the decays of strange baryons provide complementary information to the decays of kaons. A model-independent parameterization of the non-leptonic decays of the $Λ$- and $Ξ$-baryons is reviewed, and the amplitudes are updated according to the latest experimental input. We demonstrate the potential of performing precision tests in strange baryon decays at the next generation electron-positron $J/ψ$ factories with luminosity of $10^{35}$ cm$^{-2}$s$^{-1}$. The copious production of spin-entangled hyperon-antihyperon pairs via the $J/ψ$ resonance allows for a direct comparison of the baryon and antibaryon decay properties. Using analytic approximations and numerical calculations, we study the quantitative impact of spin correlations and polarization in such CP tests. We show that by using a longitudinally-polarized electron beam, the statistical precision of the CP tests can be significantly improved compared to the experiments without polarized beams. Furthermore, we map out further directions for possible improvements, like analysis of incompletely reconstructed events or a combination of the isospin related processes. Altogether, these methods are promising for the observation of a statistically significant CP-violation signal with a strength corresponding to the standard model predictions. Our conclusions should encourage more detailed feasibility studies, including optimisation of the measurement methods and studies of systematic effects. Finally, our results call for an update of the theory predictions with increased precision.

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arXiv2021

Exploring charm decays with missing energy in leptoquark models

We investigate the possibility that scalar leptoquarks generate consequential effects on the flavor-changing neutral-current decays of charmed hadrons into final states with missing energy ($\not\!\!E$) carried away by either standard model or sterile neutrinos. We focus on scenarios involving the $R_2$, $\tilde R_2$, and $\bar S_1$ leptoquarks and take into account various pertinent constraints, learning that meson-mixing ones and those inferred from collider searches can be of significance. We find in particular that the branching fractions of charmed meson decays $D\to M\!\not\!\!E$, $M=π,ρ$, and $D_s\to K^{(*)}\!\not\!\!E$ and singly charmed baryon decays $Λ_c^+\to p\!\not\!\!E$ and $Ξ_c\toΣ\!\not\!\!E$ are presently allowed to attain the $10^{-7}$-$10^{-6}$ levels if induced by $R_2$ and that the impact of $\tilde R_2$ is comparatively much less. In contrast, the contributions of $\bar S_1$, which couples to right-handed up-type quarks and the sterile neutrinos, could lead to branching fractions as high as order $10^{-3}$. This suggests that these charmed hadron decays might be within reach of the BESIII and Belle II experiments or future super charm-tau factories and could serve as potentially promising probes of leptoquark interactions with sterile neutrinos.

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arXiv2020

Seeking massless dark photons in the decays of charmed hadrons

A massless dark photon could affect standard-model particles only via higher-dimensional operators and would therefore have eluded recent searches for its massive counterpart, which were based on the assumption that the latter had renormalizable interactions with known fermions due to gauge kinetic mixing. In this study we entertain the possibility that the massless dark photon has nonnegligible flavor-changing dipole-type couplings with the $u$ and $c$ quarks, giving rise to the decays of charmed hadrons into a lighter hadron plus missing energy carried away by the dark photon. We propose to investigate decays of this kind, especially those in which the parents are the charmed pseudoscalar-mesons $D^+$, $D^0$, and $D_s^+$ and singly charmed baryons $Λ_c^+$, $Ξ_c^+$, and $Ξ_c^0$. Employing a simplified new-physics model satisfying the relevant constraints, we find that the branching fractions of these processes could be as large as several times $10^{-5}$. This suggests that one or more of them might in the near future fall within reach of the ongoing Belle II and BESIII experiments. Since the same underlying operators are responsible for all of these transitions, detecting one of them automatically implies particular predictions for the others, allowing for additional experimental checks on the massless-dark-photon scenario.

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arXiv2020

Probing new physics with the kaon decays $K\toππ\!\not\!\!E$

The latest search for the rare kaon decay $K^+\toπ^+ν\barν$ by the NA62 experiment has produced evidence for it with a branching fraction consistent with the prediction of the standard model. The new result implies that in this decay, with the $ν\barν$ pair appearing as missing energy ($\not\!\!E$), the room for possible new physics is no longer sizable and that therefore its contributions to underlying four-particle $s\to d\!\not\!\!E$ operators with parity-even $ds$ quark bilinears have become significantly constrained. Nevertheless, we point out that appreciable manifestations from beyond the standard model induced by the corresponding operators with mainly parity-odd $ds$ quark bilinears could still occur in $K\toππ\!\not\!\!E$ modes, on which there are only minimal empirical details at present. We find in particular that new physics of this kind may enhance the branching fraction of $K_L\toπ^0π^0\!\not\!\!E$ to values reaching its current experimental upper limit and the branching fractions of $K^+\toπ^+π^0\!\not\!\!E$ and $K_L\toπ^+π^-\!\not\!\!E$ to the levels of $10^{-7}$ and $10^{-6}$, respectively. Thus, quests for these decays in existing kaon facilities such as KOTO and NA62 or future ones could provide valuable information complementary to that gained from $K\toπ\!\not\!\!E$.

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arXiv2020

Exploring leptoquark effects in hyperon and kaon decays with missing energy

We entertain the possibility that scalar leptoquarks (LQs) generate consequential effects on the strangeness-changing decays of hyperons and kaons involving missing energy carried away by a pair of invisible fermions. Although such processes have suppressed rates in the standard model (SM), they could get significant enhancement in the presence of the LQs. In order to respect the available data on the kaon modes $K\toπν\barν$ and increase the rates of the hyperon decays substantially at the same time, two different scalar LQs are needed. If the LQs have Yukawa couplings solely to SM fermions, we find that the hyperon rates cannot attain values within the reach of ongoing or near-future experiments because of the combined constraints from the measurements on kaon mixing and lepton-flavor-violating processes. However, if we include light right-handed neutrinos in the LQ interactions, their contributions can evade the leading restrictions and translate into hyperon rates which may be big enough to be probed by upcoming searches. Thus, these hyperon modes could provide a new avenue for seeking sterile neutrinos.

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arXiv2020

Kaon decays shedding light on massless dark photons

We explore kaon decays with missing energy carried away by a massless dark photon, $\barγ$, assumed to have flavor-changing dipole-type couplings to the $d$ and $s$ quarks. We consider in particular the neutral-kaon modes $K_L\toγ\barγ$ and $K_L\toπ^0γ\barγ$ and their $K_S$ counterparts, as well as the charged-kaon channel $K^+\toπ^+γ\barγ$, each of which also has an ordinary photon, $γ$, in the final state. In addition, we look at $K_{L,S}\toπ^+π^-\barγ$ and $K^+\toπ^+π^0\barγ$. Interestingly, the same $ds\barγ$ interactions give rise to the flavor-changing two-body decays of hyperons with missing energy and are subject to model-independent constraints that can be inferred from the existing hyperon data. Taking this into account, we obtain branching fractions ${\cal B}(K_L\toγ\barγ)$ and ${\cal B}(K_L\toπ^0γ\barγ)$ which can be as high as $10^{-3}$ and $10^{-6}$, respectively, one or both of which may be within the sensitivity reach of the KOTO experiment. Furthermore, we find that ${\cal B}(K^+\toπ^+γ\barγ)$ and ${\cal B}(K^+\toπ^+π^0\barγ)$ are allowed to be maximally of order $10^{-6}$ as well, which may be probed by NA62. Complementarily, the hyperon modes can have rates which are potentially accessible by BESIII. Thus, these ongoing experiments could soon be able to offer significant tests on the existence of the massless dark photon.

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arXiv2020

Evading the Grossman-Nir bound with $ΔI=3/2$ new physics

Rare kaon decays with missing energy, $K\toπ$+$E_{\rm miss}$, have received considerable attention because their rates can be calculated quite precisely within the standard model (SM), where the missing energy is carried away by an undetected neutrino-antineutrino pair. Beyond the SM, clean theoretical predictions can also be made regarding these processes. One such prediction is the so-called Grossman-Nir (GN) bound, which states that the branching fractions of the $K_L$ and $K^+$ modes must satisfy the relation $\mathcal{B}(K_L\toπ^0$+$E_{\rm miss})\lesssim4.3\,\mathcal{B}(K^+\toπ^+$+$E_{\rm miss})$ and applies within and beyond the SM, as long as the hadronic transitions change isospin by $ΔI=1/2$. In this paper we extend the study of these modes to include new-physics scenarios where the missing energy is due to unobserved lepton-number-violating neutrino pairs, invisible light new scalars, or pairs of such scalars. The new interactions are assumed to arise above the electroweak scale and described by an effective field theory. We explore the possibility of violating the GN bound through $ΔI=3/2$ contributions to the $K\toπ$ transitions within these scenarios and find that large violations are only possible in the case where the missing energy is due to an invisible light new scalar.

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arXiv2020

Breaking the Grossman-Nir Bound in Kaon Decays

The ratio $\mathcal{B}(K_L\toπ^0ν\barν)/\mathcal{B}(K^+\toπ^+ν\barν)$ of the branching fractions of kaon decays $K_L\toπ^0ν\barν$ and $K^+\toπ^+ν\barν$ has a maximum of about 4.3 under the assumption that the underlying interactions change isospin by $ΔI=1/2$. This is referred to as the Grossman-Nir (GN) bound, which is respected by the standard model (SM) and by many scenarios beyond it. Recent preliminary results of the KOTO and NA62 Collaborations searching for these kaon modes seem to imply a violation of this bound. The KOTO findings also suggest that $\mathcal{B}(K_L\toπ^0ν\barν)$ could be much larger, by nearly two orders of magnitude, than that predicted in the SM. In this work we study the possibility of violating the GN bound in an effective field theory approach with only SM fields. We show that the bound holds, in addition to the original GN scenarios, whether or not the kaon decays conserve lepton number. We demonstrate that the inclusion of $ΔI=3/2$ operators can lead to a violation of the GN bound and illustrate with an example of how the KOTO numbers may be reached with a new physics scale of order tens of GeV.

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