Quark Level Transition Research Articles

Exploring the lepton flavor violating decay modes b → sμ±τ∓ in SMEFT approach

We perform an analysis of the consequences of various new physics operators on the lepton flavor violating (LFV) decay modes mediated through b→sℓ1ℓ2 transitions. We scrutinize the imprints of the (pseudo)scalar and axial (vector) operators on the exclusive LFV decay channels B(s)→(ϕ,K⁎,K2⁎)ℓ1ℓ2 and Λb→Λℓ1ℓ2, where ℓ1,ℓ2 represent μ or τ. The new physics parameters are constrained by using the upper limits of the branching fractions of the Bs→τμ and B→Kτμ processes, assuming the new physics couplings to be real. We then explore the key observables such as the branching fractions, the forward-backward asymmetries, and the longitudinal polarization fractions of the B→(K⁎,ϕ,K2⁎)τ±μ∓ decays. In addition, we also investigate the impact of the new physics couplings on the baryonic Λb→Λτ±μ∓ decay channels mediated by the b→s quark level transition. With the experimental prospects at LHCb upgrade and Belle II, we also predict the upper limits of the above-discussed observables, which could intrigue the new physics search in these channels.

Imprints of new physics operators in the semileptonic [formula omitted] process in SMEFT approach

At present, there are several measurements of B decays that exhibit discrepancies with the predictions of the Standard Model, and suggest the presence of new physics in b→s and b→c(u) quark level transitions. Motivated by the prospects of the ongoing high-luminosity B factories, we study the exclusive B→a1(1260)ℓ−ν¯ℓ process within the Standard Model Effective Field Theory (SMEFT) formalism, to understand the sensitivity of various new physics operators. The new physics parameters are constrained by using the experimental branching fractions of the (semi)leptonic B→ℓν¯ and B→(π,ρ,ω)ℓν¯ processes (where ℓ=e,μ,τ) which undergo b→uℓν¯ quark level transitions. We then perform a comprehensive angular analysis of the exclusive B→a1(1260)ℓ−ν¯ℓ process in the Standard Model and in the presence of various new physics operators. We also provide the predictions and comment on various observables, such as branching ratio, forward-backward asymmetry, and the test of lepton flavor non universality of the B→a1(1260)ℓ−ν¯ℓ channel.

Delving into new physics in semileptonic b→cτν̄ transitions

In recent times, there have been several observations suggesting deviations from lepton universality in semileptonic decays of [Formula: see text] mesons. These deviations have been noticed both in the neutral-current transitions ([Formula: see text]) and charged-current transitions ([Formula: see text]). Motivated by these intriguing findings, we investigate the semileptonic decays involving the quark-level transitions of [Formula: see text]. To explore the presence of new physics, we adopt a model-independent approach and analyze the processes [Formula: see text], [Formula: see text] and [Formula: see text], where [Formula: see text] represent the four lightest excited charm mesons. Considering a comprehensive effective Hamiltonian, we perform a global fit using various sets of new coefficients and incorporate measurements of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], as well as the upper limit on the branching ratio of [Formula: see text]. Subsequently, we examine the impact of constrained new couplings on the branching ratios, forward–backward asymmetry parameters, lepton nonuniversality (LNU) ratios, as well as lepton and hadron polarization asymmetries in these decay modes.

Analysis of some [formula omitted] decay modes beyond the standard model

The experimental results from BaBar, Belle, and LHCb direct us to look for various new physics scenarios beyond the standard model. The observed b-physics anomalies in the b→cτ−ν¯τ transitions motivate the study of decays with similar quark-level transitions. In this work, we focus on these transitions, particularly examining the semileptonic b-baryon Ωb→Ωc(⁎)τ−ν¯τ and Ξb′→Ξc′(⁎)τ−ν¯τ decay modes. We take into account the general effective Hamiltonian for b→cℓν¯ℓ transitions which incorporates new physics contributions into the standard model. We explore the q2-dependency of different observables, such as the differential branching fraction, ratio of branching fractions, forward-backward asymmetry, convexity parameter, and longitudinal polarization of the charged lepton, related to these decay processes in the standard model and beyond.

Footprints of New Physics in the angular distribution of [formula omitted] decays

We investigate the angular decay distribution of the four-fold Bc→Ds⁎(→Dsγ)μ+μ−, and Bc→Ds⁎(→Dsπ)μ+μ− decays that proceed through b→sμ+μ− quark level transition. We use the model independent effective Hamiltonian with vector and axial vector new physics operators to formulate the angular observables and study the implications of different latest new physics scenarios, taken from the global fits to all the b→s data, on these observables. We also give Standard Model and new physics predictions of several observables such as differential branching ratios, forward backward asymmetry, longitudinal polarization fraction of Ds⁎, and the unpolarized and polarized lepton flavor universality violating ratios. Future measurements of the predicted angular observables, both at current and future high energy colliders, will add to the useful complementary data required to clarify the structure of new physics in b→sℓℓ neutral current decays.

Impact of b → c measurements on [formula omitted] decay in U1 leptoquark model

The measurements of several lepton flavor universality (LFU) violating observables in the decays induced by the quark level transition b→cτν¯ provide an inkling of plausible physics beyond the standard model of electroweak interactions. Such new physics would also impact other sectors. In this work, we estimate the leverage of new physics in b→cτν¯ on Λb→pτν¯ decay in the context of U1 leptoquark model. In this model, the new physics couplings in b→uτν¯ transition can be written in terms of b→cτν¯ couplings and hence the extent of allowed new physics in Λb→pτν¯ would be determined by b→cτν¯ transition. Using the new physics parameter space obtained by performing a fit to all b→cτν¯ data, we obtain predictions of several Λb→pτν¯ observables. We find that the current b→cτν¯ data allows two times of magnitude enhancement in the branching ratio as well as in the LFU ratio. The other observables such as convexity parameter, lepton forward-backward asymmetry, longitudinal polarization of final state baryon and tau lepton are consistent with the SM value.

Consequences of b→sμ+μ− anomalies on B→K(*)νν¯ , Bs→(η,η′)νν¯ and Bs→ϕνν¯ decay observables

The long persistent discrepancies in $b\,\to\,s\, \ell\, \ell$ quark level transitions continue to be the ideal platform for an indirect search of new physics that lies beyond the SM. The measurements of $R_K$, $R_{K^*}$, $P_5^{\prime}$, $\mathcal{B}(B_s\, \to\, \phi\, \mu^+\,\mu^-)$ and $\mathcal{B}(B_s\, \to\, \mu^+\,\mu^-)$ persistently deviate from the standard model expectations. Similarly, the new tests of lepton flavor universality performed using the isospin partners such as $B^0 \, \to \, K_S^{0} \, \ell\,\ell$ and $B^+ \, \to \, K^{+*} \, \ell\,\ell$ exhibit the same pattern of deviation in $R_{K_S^0}$ and $R_{K^{*+}}$ with the existing results. Motivated by these anomalies we search for the patterns of new physics in the family of flavor changing neutral decays with neutral leptons in the final state undergoing $b \, \to \,s\, \nu \, \bar{\nu}$ quark level transitions. There are close relations between $b\,\to\,s\, \ell\, \ell$ and $b \, \to \,s\, \nu \, \bar{\nu}$ transitions not only in standard model but also in various beyond the standard model scenarios. For beyond the standard model physics under the $\rm SU_L(2)$ gauge symmetry one can relate the left handed charged leptons to the neutrinos. Moreover, there are several advantages of studying $b \, \to \,s\, \nu \, \bar{\nu}$ transitions over $b\,\to\,s\, \ell\, \ell$ as they are free from various hadronic uncertainties beyond the form factors such as the non-factorizable corrections and photonic penguin contributions. Hence, we explore the consequences of $b\, \to\, s\, \mu^+\, \mu^-$ anomalies on $B \, \to \, K^{(*)} \,\nu \, \bar{\nu}$, $B_s \to\, (\eta,\eta') \, \nu\,\bar{\nu}$ and $B_s \, \to \, \phi \, \nu \, \bar{\nu}$ decay observables in SMEFT platform within various 1D and 2D new physics scenarios.

New physics footprints in the angular distribution of Bs→Ds*(→Dsγ, Dsπ)τν decays

Hints of lepton flavor universality violation observed in various flavor ratios such as $R_D$, $R_{D^*}$, $R_{J/\psi}$, $P_\tau^{D^*}$ and $F_L^{D^*}$ in $B\to D^{(*)}\,\ell \, \nu$ and $B_c\to J/\psi \, \ell\, \nu$ charge current decays have opened new avenues to search for indirect evidences of beyond the standard model physics. Motivated by these anomalies, we perform a detailed angular analysis of $B_s\to D_s^*(\to D_s\gamma,\,D_s\pi)\, \ell\, \nu$ decays that proceed via similar $b \to c\, \ell\, \nu$ quark level transition. We use the most general effective Hamiltonian for $b\to \,c\, l\nu$ process and give predictions of several $q^2$ and $\cos\theta$ dependent observables for the $B_s\to D_s^*(\to D_s\gamma,\,D_s\pi)\, \ell\, \nu$ decays in the standard model and in the presence of various real and complex new physics couplings. The results pertaining to this decay are competent to address the anomalies in the charge current sector.

Impact of b rightarrow s ell ell anomalies on rare charm decays in non-universal Z' models

In this work, we study the impact of b rightarrow s ell ell , B_s - bar{B_s} mixing and neutrino trident measurements on observables in decays induced by c rightarrow u transition in the context of a non-universal Z' model which generates C^{mathrm{NP}}_{9} <0 and C^{mathrm{NP}}_9 = - ,C^{mathrm{NP}}_{10} new physics scenarios at the tree level. We inspect the effects on D^0 rightarrow pi ^0 nu {bar{nu }}, D^+ rightarrow pi ^+ nu {bar{nu }} and B_c rightarrow B^+ nu {bar{nu }} decays which are induced by the quark level transition c rightarrow u nu {bar{nu }}. The fact that the branching ratios of these decays are negligible in the standard model (SM) and the long distance effects are relatively smaller in comparison to their charged dileptons counterparts, they are considered to provide genuine null-tests of SM. Therefore the observation of these modes at the level of current as well as planned experimental sensitivities would imply unambiguous signature of new physics. Using the constraints on Z' couplings coming from a combined fit to b rightarrow s ell ell , varDelta M_s and neutrino trident data, we find that any meaningful enhancement over the SM value is ruled out in the considered framework. The same is true for D - {bar{D}} mixing observable varDelta M_D along with D^0 rightarrow mu ^+ mu ^- and D^+ rightarrow pi ^+ mu ^+ mu ^- decay modes which are induced through c rightarrow u mu ^+ mu ^- transition.

Implications of new physics in B → K 1 μ + μ − decay processes

Recently, several discrepancies at the level of (2–3)σ have been observed in the decay processes mediated by flavour changing neutral current (FCNC) transitions b → sℓ + ℓ −, which may be considered as the smoking-gun signal of new physics (NP). These intriguing hints of NP have attracted a lot of attention and many attempts are being made to find the possible NP signature in other related processes, which are mediated through the same quark-level transitions. In this work, we perform a comprehensive analysis of the FCNC decays of the B meson to axial vector mesons K 1(1270) and K 1(1400), which are an admixture of the 13 P 1 and 11 P 1 states K 1A and K 1B , in a model independent framework. Using the B → K 1 form factors evaluated in the light cone sum rule approach, we investigate the rare exclusive semileptonic decays B → K 1(1270)μ + μ − and B → K 1(1400)μ + μ −. Considering all the possible relevant operators for b → sℓ + ℓ − transitions, we study their effects on various observables such as branching fractions, the lepton flavor universality violating ratio , forward–backward asymmetries, and lepton polarization asymmetries of these processes. These results will not only enhance the theoretical understanding of the mixing angle but also serve as a good tool for probing NP.

Scrutinizing new physics of B d → ϕ(η (′), π, ω) decay modes

We inspect the exclusive hadronic decay modes B d → ϕ(η (′), π, ω), induced by quark-level transitions such as b → d (ΔS = 0) in the vector-like down quark model. These decay modes exhibit high levels of suppression followed by the predicted branching fraction which leads us to scrutinize physics beyond the standard model. We constrain the new parameter space inferred from experimental limits to the leptonic B d → ℓℓ(ℓ = e, μ, τ) and nonleptonic decay modes B d → η′π 0 and B u → ρ − η′. We then check that the new physics contributions have a significant impact on the clearly observed so-called branching ratio of the B d → ϕ(η (′), π, ω) processes.

Relations between b \u2192 c\u03c4\u03bd decay modes in scalar models

As a consequence of the Ward identity for hadronic matrix elements, we find relations between the differential decay rates of semileptonic decay modes with the underlying quark-level transition b → cτν, which are valid in scalar models. The decay-mode dependent scalar form factor is the only necessary theoretical ingredient for the relations. Otherwise, they combine measurable decay rates as a function of the invariant mass-squared of the lepton pair q2 in such a way that a universal decay-mode independent function is found for decays to vector and pseudoscalar mesons, respectively. This can be applied to the decays Bto {D}^{ast}tau v,{B}_sto {D}_s^{ast}tau v,{B}_cto J/psi tau v and B → Dτv, Bs → Dsτv, Bc → ηcτv, with implications for R(D(*)), Rleft({D}_s^{left(ast right)}right) , R(J/ψ), R(ηc), and ℬ(Bc → τv). The slope and curvature of the characteristic q2-dependence is proportional to scalar new physics parameters, facilitating their straight forward extraction, complementary to global fits.

Implication of flavor anomalies on decay observables

The experimental predictions of R D , , R J/ψ , and in B decays mediated via quark level transition deviate significantly from the standard model expectations. The current world average of the ratio of branching ratios R D and in (l ∈ e, τ) show 1.4σ and 2.5σ deviation from the SM expectations. Similarly, the τ polarization fraction and the longitudinal polarization fraction of the D* meson in are found to deviate from the standard model expectations at 1.6σ and 1.5σ level, respectively. In addition, the ratio of branching ratio R J/ψ in deviates from the standard model prediction at 1.7σ level. In this regard, we study the implication of R D , , R J/ψ , and anomalies on decay observables in a model independent effective field theory formalism. We give predictions of several physical observables in the standard model and in the presence of various 1D and 2D new physics scenarios.

Inspection of new physics in Bs→K+K− decay mode

We scrutinize the penguin dominated $ B_s \to K^+ K^-$ decay mode involving $b \to s$ quark level transition in family non-universal $Z^ \prime$ and vector-like down quark model. There is discrepancy in the standard model branching ratio value of this mode with the experimental results reported by Belle, CDF and LHCb Collaborations. Additionally, the measured values of CP-violating asymmetries $ C_{K^+K^-}$ and $ S_{K^+K^-}$ deviate from the SM results. We constrain the new parameter space by using the existing experimental limits on leptonic $ B_s\to \ell \ell$ ($\ell = e, \mu, \tau$) processes. We then check the effects of new physics on the branching ratio and CP violating parameters of the $ B_s \to K^+ K^-$ process.

Implication of Z-mediated FCNC on semileptonic decays Bs→φl+l–and B+→K+l+l–

Rare B meson decays mediated by flavour changing neutral current (FCNC) transition play interesting role to probe the flavour sector of the standard model (SM). Generally at the tree level, FCNC processes are not allowed in the SM but occurs at the loop levels. This gives an excellent hunting ground for new physics (NP). From various experimental studies it is found that the FCNC processes having quark level transition $b \to s$ are challenging. Here, we investigate different kinematic observables like forward-backward asymmetry, differential branching ratio and lepton polarization asymmetry for semileptonic rare B decay modes $B_s \to \phi l^+ l^-$ and $B^+ \to K^+ l^+ l^-$ ($l= \mu,\tau$) considering the contribution of Z-mediated FCNC. A noticeable deviation of the observables for these decay channels from the SM value is found because of non-universal $Z$-$bs$ coupling.

Anatomy of a six-parameter fit to the brightarrow s ell ^+ell ^- anomalies

Discrepancies between measurements of decay modes with an underlying quark level transition brightarrow s ell ^+ell ^- and standard model (SM) predictions have persisted for several years, particularly for the muon channels. The inadequacy of the SM becomes more compelling in a global fit. For example, Capdevila et al. (JHEP 01, 093. arXiv:1704.05340, 2018) described 175 observables by six parameters encoding new physics and quantified the disagreement with the SM at about the 5sigma level. While certain one and two parameter fits have previously been considered in detail, we establish a framework for the detailed discussion of the full 6d fit. We visualize and quantify the 6d 1sigma region around the best fit point and define fit uncertainties for both current and future observables. We then define metrics quantifying the deviations between measurements and both SM and best fit predictions. These metrics relate observables to directions in parameter space, revealing their precise role in the fit, thus providing guidance for future theoretical and experimental work. Some metrics further quantify the role of correlated uncertainties, which turns out to be significant. For example the relevance of angular observables such as P_5^prime is reduced in this context. Finally, studying the space of observables allows us to discuss the internal tensions in the fit.

Probing new physics in semileptonic Λb decays

In recent times, several hints of lepton non-universality have been observed in semileptonic $B$ meson decays, both in the charged-current ($b \to c l \bar \nu_l$) and neutral-current ($b \to s ll $) transitions. Motivated by these intriguing results, we perform a model independent analysis of the semileptonic $\Lambda_b$ decays involving the quark level transitions $b \to (u,c) l \nu_l$, in order to scrutinize the nature of new physics. We constrain the new parameter space by using the measured branching ratios of $B_{c, u}^+ \to \tau^+ \nu_\tau$, $B \to \pi \tau \nu_\tau$ processes and the existing experimental results on $R_{D^{(*)}}$, $R_{J/\psi}$ and $R_{\pi}^l$ parameters. Using the constrained parameters, we estimate the branching ratios, forward backward asymmetries, hadron and lepton polarization asymmetries of the $\Lambda_b \to (\Lambda_c, p) l \nu_l$ processes. Moreover, we also examine whether there could be any lepton universality violation in these decay modes.

Scalar Leptoquark Effects in the Lepton Flavor Violating Exclusive b → s ℓ i − ℓ j + $b \to s {\ell }_{i}^{-} {\ell }_{j}^{+}$ Decays

Leptoquarks have been suggested to solve a variety of discrepancies between the expected and observed phenomenon. In this paper, we investigate the effect of scalar leptoquarks on the lepton flavor violating B meson rare decays which involve the quark level transition $\bar{b} \to \bar{s} \ell^-_i \ell^+_j(i \neq j)$. The leptoquark parameter space is constrained by using the recently measured upper limits on the $\mathcal{B}(B_s^0 \to \ell^-_i \ell^+_j)$ and $\mathcal{B}(B \to K^{(*)}\ell^-_i \ell^+_j)$. Using such constrained leptoquark parameter space, some relevant physical quantities are also predicted and we find that the constrained New Physics parameters in the leptoquark model have very obvious effects on the relevant physical quantities in this paper. With future measurements of observable in $B \to K^{(*)}\ell^-_i \ell^+_j$ decays at the LHCb, more and more differentiated from the other NP explanations could be tested.

Weak decays of doubly heavy baryons: multi-body decay channels

The newly-discovered Xi _{cc}^{++} decays into the Lambda _{c}^+ K^-pi ^+pi ^+, but the experimental data has indicated that this decay is not saturated by any two-body intermediate state. In this work, we analyze the multi-body weak decays of doubly heavy baryons Xi _{cc}, Omega _{cc}, Xi _{bc}, Omega _{bc}, Xi _{bb} and Omega _{bb}, in particular the three-body nonleptonic decays and four-body semileptonic decays. We classify various decay modes according to the quark-level transitions and present an estimate of the typical branching fractions for a few golden decay channels. Decay amplitudes are then parametrized in terms of a few SU(3) irreducible amplitudes. With these amplitudes, we find a number of relations for decay widths, which can be examined in future.

Model-independent investigation of rare semileptonic b→ulν¯l decay processes

Motivated by the recent observation of lepton universality violation in the flavour changing charged current transitions $b \to c l \bar{\nu}_l$, we intend to scrutinize the lepton non-universality effects in rare semileptonic $B$ meson decays involving the quark level transitions $b \to u l \bar{\nu}_l$. In this regard, we envisage the model-independent approach and consider the generalized effective Lagrangian in the presence of new physics and constrain the new parameters by using the experimental branching fractions of $B_u^+ \to l^+ \nu_l$ and $B^- \to \pi^0 \mu^- \bar \nu_\mu$ processes, where $l=e, \mu, \tau$. We then estimate the branching ratios and forward-backward asymmetries of $B_{(s)} \to P (V)l \bar{\nu}_l$ processes, where $P(=K, \pi, \eta^{(\prime)})$ denotes the pseudoscalar meson and $V(=K^*, \rho)$ is the vector meson. We also find out various lepton non-universality parameters in these processes in the presence of new physics.