arXiv subjects

Ivan Nišandžić

Publications and source records attributed to Ivan Nišandžić.

13 recordsLinked to original sources

arXiv2026

New Predictions for the Lifetimes of Doubly Heavy Baryons and the $B_c$ Meson

We present updated predictions for the lifetimes of all weakly decaying doubly heavy baryons, including $bb$, $cc$, and $bc$ baryons, as well as for the $B_c$ meson. The analysis includes NNLO corrections to the leading dimension-three contribution, NLO corrections to the chromomagnetic term, and the complete set of currently known NLO corrections to the dimension-six heavy-light quark spectator contributions, including penguin terms. We also compare the results in the $\overline{\rm MS}$, kinetic, and, where applicable, $Υ$ mass schemes. For the $bc$ baryons, we present predictions for both possible ground-state diquark-spin assignments, $S_{bc}=0$ and $S_{bc}=1$. In the kinetic scheme we obtain the lifetime hierarchies $τ(Ξ^0_{bb})<τ(Ξ^-_{bb})\simeqτ(Ω^-_{bb})$, $τ(Ξ^+_{cc})<τ(Ω^+_{cc})<τ(Ξ^{++}_{cc})$, $τ(Ξ^0_{bc})\lesssimτ(Ω^0_{bc})<τ(Ξ^+_{bc})$ for $S_{bc}=0$, and $τ(Ξ^{\prime\, 0}_{bc})<τ(Ω^{\prime\, 0}_{bc})<τ(Ξ^{\prime\, +}_{bc})$ for $S_{bc}=1$. We also revisit the $B_c$ lifetime and discuss the impact of the newly included Darwin term.

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arXiv2026

Revisiting lifetimes of doubly charmed baryons

We present updated predictions for lifetimes of doubly charmed baryons, within the heavy quark expansion, including available NLO $α_s$ contributions and newly-computed terms in the $1/m_c$ series. Our improved results confirm the expected hierarchy $$τ(Ξ_{cc}^{+}) < τ(Ω_{cc}^{+}) < τ(Ξ_{cc}^{++}) \,, $$ while the predicted lifetime $τ(Ξ_{cc}^{++}) = 0.32 \pm 0.05 ^{+0.08}_{-0.07} \,\textrm{ps} $ is consistent with the recent LHCb determination. We provide predictions for the lifetime ratios of the $Ξ_{cc}^{+}$ and $Ω_{cc}^+$ baryons relative to the $Ξ_{cc}^{++}$ baryon, namely $τ(Ξ_{cc}^{+})/τ(Ξ_{cc}^{++})=0.22\pm 0.05\pm 0.04$ and $τ(Ω_{cc}^{+})/τ(Ξ_{cc}^{++})=0.52\pm 0.13^{+0.03}_{-0.02}$.

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arXiv2025

Nonperturbative Parameters of Inclusive $Λ_b$ Decays from Small Velocity Sum Rules

We investigate the nonperturbative parameters entering the heavy quark expansion for the inclusive decay rates of the $Λ_b$ baryon, focusing on the kinetic term $\hatμ_π^2$ and the Darwin term $\hatρ_D^3$. Recent evaluations have yielded an unexpectedly large Wilson coefficient associated with the dimension-six Darwin operator, which motivates a more detailed examination of the associated hadronic matrix element. We constrain $\hatμ_π^2$ and $\hatρ_D^3$ using Small Velocity Sum Rules for the inclusive semileptonic decay $Λ_b \to X_c e^- \barν_e$. These sum rules relate moments of hadronic structure functions, computed via the Operator Product Expansion, to hadronic matrix elements of relevant exclusive transitions. Truncating the hadronic side to include the lowest-lying charmed baryon states with spin-parity $ J^P = 1/2^+, 1/2^-, 3/2^- $, we employ the corresponding lattice QCD form factors near zero recoil as inputs. By analysing zeroth and higher moments, we derive inequalities that define an allowed region in the $(\hatμ_π^2, \hatρ_D^3)$ plane. The derived constraints are consistent, within uncertainties, with estimates from spectroscopic relations and the nonrelativistic constituent quark model. This work refines the determination of key HQE parameters for $Λ_b$, with implications for precision predictions of heavy baryon lifetimes and inclusive decays of heavy baryons in general.

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arXiv2024

Revisiting the Higgs Boson Decay into a Photon and a Neutrino Pair

We present new calculations of the leading-order one-loop amplitude and the decay rate for $H\to ν\barν γ$ within the standard model, employing reduction to a set of basic Passarino-Veltman functions resulting in more concise expressions compared to prior works. Our result for the total decay rate is $Γ(H\to ν\barν γ)=1.33\,\text{keV}$, in close agreement with the narrow width approximation (NWA) for the intermediate Z boson. A comparison with previous results available in the literature is also provided. We evaluate the differential decay rate with respect to the energy of the photon and find that the non-resonant contributions from box diagrams beyond the NWA notably affect the kinematic region associated with high-energy photons, where the intermediate Z boson is offshell. This suggests potential implications for searches of dark sector mediators, particularly in future lepton collider experiments.

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arXiv2023

Charmed hadron lifetimes

We provide updated predictions of the lifetimes of singly charmed baryons and mesons within the heavy quark expansion, with all known corrections included. A special attention is devoted to the choice of the charm mass and wavefunctions of heavy baryons. Our results accommodate the experimentally-favoured hierarchy of singly charmed baryon lifetimes \begin{eqnarray*} τ(Ξ_c^0) < τ(Λ_c^+)< τ(Ω_c^0) < τ(Ξ_c^+)\, \end{eqnarray*} in contrast to earlier theoretical findings. Predictions for charmed meson lifetimes and semileptonic branching ratios are also in agreement, within uncertainties, with a recent comprehensive study and with experimental results.

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arXiv2023

Quark-hadron duality at work: lifetimes of bottom baryons

In the 1990s, very low experimental values for the lifetime ratio $τ(Λ_b)/τ(B_d)$ triggered a considerable amount of doubt in the applicability of the heavy quark expansion (HQE), which is based on the assumption of quark-hadron duality (QHD) for inclusive total decay rates. However, these low values turned out to be the result of purely experimental problems, and the current HFLAV average reads $τ(Λ_b)/τ(B_d) = 0.969(6)$. In this work, we present the Standard Model predictions for the $b$-baryon lifetimes within the framework of the HQE. In particular, we include for the first time the contribution of the Darwin term and we update the estimates for the matrix elements of the dimension-six four-quark operators. Within experimental and theoretical uncertainties, we find excellent agreement between the data and the HQE predictions, and thus no indication for any visible violation of QHD. Our numerical results can be summarised by the ratios $τ(Λ_b)/τ(B_d) = 0.955(14)$, $τ(Ω_b^-)/τ(B_d) = 1.081(42)$, and $τ(Ξ_b^0)/τ(Ξ_b^-) = 0.929(28)$.

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arXiv2022

Lifetimes of singly charmed hadrons

We provide an extensive study of the lifetimes of singly charmed baryons and mesons, within the heavy quark expansion with all known corrections included. A special attention is devoted to the choice of the charm mass and wavefunctions of heavy baryons. We give our predictions for lifetimes, lifetime ratios, and semileptonic branching ratios of singly charmed baryons. Our results accommodate the experimentally-favoured hierarchy of singly charmed baryon lifetimes \begin{eqnarray*} τ\left(Ξ_c^{0}\right) < τ\left(Λ_c^{+}\right)< τ\left(Ω_c^{0}\right) < τ\left(Ξ_c^{+}\right)\, \end{eqnarray*} in contrast to earlier theoretical findings. Predictions for charmed meson lifetimes and semileptonic decay rates are in agreement with a recent comprehensive study and experimental results within uncertainties.

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arXiv2022

Higgs decay into a lepton pair and a photon: a roadmap to $\mathbf{H\to Zγ}$ discovery and probes of new physics

The decay $H\to \ell^+\ell^- γ$, $\ell=e,μ$, receives contributions from $H\to Z[\to \ell^+\ell^-] γ$ and a non-resonant contribution, both of which are loop-induced. We describe how one can separate these sub-processes in a gauge-independent way, define the decay rate $Γ(H\to Zγ)$, and extract the latter from differential $H\to \ell^+\ell^- γ$ branching ratios. For $\ell=μ$ also the tree decay rate, which is driven by the muon Yukawa coupling, is important. We propose kinematic cuts optimized to separate the three contributions, paving the way to the milestones (i) discovery of $H\to Z γ$, (ii) discovery of $H\to μ^+μ^- \left. γ\right|_{\rm tree}$, and (iii) quantification of new physics in both the effective $H$-$Z$-$γ$ and non-resonant $H$-$\ell^+$-$\ell^-$-$γ$ couplings.

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arXiv2021

Flavorful leptoquarks at the LHC and beyond: Spin 1

Evidence for electron-muon universality violation that has been revealed in $b\to s \ell\ell$ transitions in the observables $R_{K,K^*}$ by the LHCb Collaboration can be explained with spin-1 leptoquarks in $SU(2)_L$ singlet $V_1$ or triplet $V_3$ representations in the ${\cal{O}}(1-10)$ TeV range. We explore the sensitivity of the high luminosity LHC (HL-LHC) and future proton-proton colliders to $V_1$ and $V_3$ in the parameter space connected to $R_{K,K^*}$-data. We consider pair production and single production in association with muons in different flavor benchmarks. Reinterpreting a recent ATLAS search for scalar leptoquarks decaying to $b μ$ and $j μ$, we extract improved limits for the leptoquark masses: For gauge boson-type leptoquarks ($κ=1$) we obtain $M_{V_1}> 1.9$ TeV, $M_{V_1}> 1.9$ TeV, and $M_{V_1}> 1.7$ TeV for leptoquarks decaying predominantly according to hierarchical, flipped and democratic quark flavor structure, respectively. Future sensitivity projections based on extrapolations of existing ATLAS and CMS searches are worked out. We find that for $κ=1$ the mass reach for pair (single) production of $V_1$ can be up to 3 TeV (2.1 TeV) at the HL-LHC and up to 15 TeV (19.9 TeV) at the FCC-hh with $\sqrt{s}=100$ TeV and $20 \, \mbox{ab}^{-1}$. The mass limits and reach for the triplet $V_3$ are similar or higher, depending on flavor. While there is the exciting possibility that leptoquarks addressing the $R_{K,K^*}$-anomalies are observed at the LHC, to fully cover the parameter space $pp$-collisions beyond the LHC-energies are needed.

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arXiv2020

Higgs decay into a lepton pair and a photon revisited

We present new calculations of the differential decay rates for $H\to \ell^+\ell^- γ$ with $\ell=e$ or $μ$ in the Standard Model. The branching fractions and forward-backward asymmetries, defined in terms of the flight direction of the photon relative to the lepton momenta, depend on the cuts on energies and invariant masses of the final state particles. For typical choices of these cuts we find the branching ratios $B(H\to e \bar e γ)=5.8\cdot 10^{-5}$ and $B(H\to μ\bar μγ)=6.4\cdot 10^{-5}$ and the forward-backward asymmetries $\mathcal{A}^{(e)}_{\text{FB}}=0.343$ and $\mathcal{A}^{(μ)}_{\text{FB}}=0.255$. We provide compact analytic expressions for the differential decay rates for the use in experimental analyses.

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arXiv2020

Higgs portal to dark matter and $B\to K^{(*)}$ decays

We consider a Higgs portal model in which the 125-GeV Higgs boson mixes with a light singlet mediator $h_2$ coupling to particles of a Dark Sector and study potential $b\to s h_2$ decays in the Belle II experiment. Multiplying the gauge-dependent off-shell Standard-Model $b$-$s$-Higgs vertex with the sine of the Higgs mixing angle does not give the correct $b$-$s$-$h_2$ vertex. We clarify this issue by calculating the $b$-$s$-$h_2$ vertex in an arbitrary $R_ξ$ gauge and demonstrate how the $ξ$ dependence cancels from physical decay rates involving an on-shell or off-shell $h_2$. Then we revisit the $b\to s h_2$ phenomenology and point out that a simultaneous study of $B\to K^* h_2$ and $B\to K h_2$ helps to discriminate between the Higgs portal and alternative models of the Dark Sector. We further advocate for the use of the $h_2$ lifetime information contained in displaced-vertex data with $h_2$ decaying back to Standard-Model particles to better constrain the $h_2$ mass or to reveal additional $h_2$ decay modes into long-lived particles.

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arXiv2019

Addendum to "Impact of polarization observables and $B_c\to τν$ on new physics explanations of the $b\to c τν$ anomaly"

In this addendum to arXiv:1811.09603 we update our results including the recent measurement of ${\cal R}(D)$ and ${\cal R}(D^*)$ by the Belle collaboration: ${\cal R}(D)_{\rm Belle} = 0.307\pm0.037\pm0.016$ and ${\cal R}(D^*)_{\rm Belle}=0.283\pm0.018\pm0.014$, resulting in the new HFLAV fit result ${\cal R}(D) = {0.340\pm0.027 \pm 0.013}$, ${\cal R}(D^*) = {0.295\pm0.011 \pm 0.008 }$, exhibiting a $3.1\,σ$ tension with the Standard Model. We present the new fit results and update all figures, including the relevant new collider constraints. The updated prediction for ${\cal R}(Λ_c)$ from our sum rule reads ${\cal R}(Λ_c)= \mathcal{R}_{\rm SM}(Λ_c) \left( 1.15 \pm 0.04 \right) = 0.38 \pm 0.01 \pm 0.01$. We also comment on theoretical predictions for the fragmentation function $f_c$ of $b\to B_c$ and their implication on the constraint from $B_{u/c}\toτν$ data.

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arXiv2019

Impact of polarization observables and $ B_c\to τν$ on new physics explanations of the $b\to c τν$ anomaly

The combined analysis of the BaBar, Belle, and LHCb data on $B\to Dτν$, $B\to D^*τν$ and $B_c\to J/Ψτν$ decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one- and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of $F_L(D^*)$ and of the $τ$ polarizations, and especially on the constraint from the branching fraction ${\rm BR}(B_c\toτν)$. We critically review this constraint and do not support the aggressive limit of ${\rm BR}(B_c\toτν)<10\%$ used in some analyses. While the impact of $F_L(D^*)$ is currently still limited, the ${\rm BR}(B_c\toτν)$ constraint has a significant impact: depending on whether one uses a limit of $60\%$, $30\%$ or $10\%$, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative $60\%$ limit a scenario with scalar operators gives the best fit to data, while for an aggressive $10\%$ limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of $B\to Dτν$, $B\to D^*τν$ and $Λ_b\to Λ_cτν$ which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of ${\rm BR}(Λ_b\to Λ_cτν)$ over its SM prediction by $(24\pm 6)\%$ for the current $\mathcal{R}(D^{(*)})$ data.

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