Phenomenology of Λ b → Λ c τ ¯ ν τ using lattice QCD calculations

: In a recent paper we studied the eﬀect of new-physics operators with diﬀerent Lorentz structures on the semileptonic Λ b → Λ c τ ¯ ν τ decay. This decay is of interest in light of the R ( D ( ∗ ) ) puzzle in the semileptonic ¯ B → D ( ∗ ) τ ¯ ν τ decays. In this work we add tensor operators to extend our previous results and consider both model-independent new physics (NP) and speciﬁc classes of models proposed to address the R ( D ( ∗ ) ) puzzle. We show that a measurement of R (Λ c ) = B [Λ b → Λ c τ ¯ ν τ ] / B [Λ b → Λ c ℓ ¯ ν ℓ ] can strongly constrain the NP parameters of models discussed for the R ( D ( ∗ ) ) puzzle. We use form factors from lattice QCD to calculate all Λ b → Λ c τ ¯ ν τ observables. The Λ b → Λ c tensor form factors had not previously been determined in lattice QCD, and we present new lattice results for these form factors here


Introduction
A major part of particle physics research is focused on searching for physics beyond the standard model (SM). In the flavor sector a key property of the SM gauge interactions is that they are lepton flavor universal. Evidence for violation of this property would be a clear sign of new physics (NP) beyond the SM. In the search for NP, the second and third generation quarks and leptons are quite special because they are comparatively heavier and are expected to be relatively more sensitive to NP. As an example, in certain versions of the two Higgs doublet models (2HDM) the couplings of the new Higgs bosons are proportional to the masses and so NP effects are more pronounced for the heavier generations. Moreover, the constraints on new physics, especially involving the third generation leptons and quarks, are somewhat weaker allowing for larger new physics effects.
This ratio of branching fractions has certain advantages over the absolute branching fraction measurement of B → D ( * ) τ ν τ decays, as this is relatively less sensitive to form factor variations and several systematic uncertainties, such as those on the experimental efficiency, as well as the dependence on the value of |V cb |, cancel in the ratio.
There are lattice QCD predictions for the ratio R(D) SM in the Standard Model [5][6][7]  These values are also in good agreement with the phenomenological prediction [8] R(D) SM = 0.305 ± 0.012, (1.3) which is based on form factors extracted from experimental data for the B → Dℓν differential decay rates using heavy-quark effective theory. See also ref. [9] for a recent analysis of B → D form factors using light-cone sum rules. A calculation of R(D * ) SM is not yet available from lattice QCD. The phenomenological prediction using form factors extracted from B → D * ℓν experimental data is [10] R(D * ) SM = 0.252 ± 0.003. (1.4) The averages of R(D) and R(D * ) measurements, evaluated by the Heavy-Flavor Averaging Group, are [11] R(D) exp = 0.397 ± 0.040 ± 0.028, (1.5) R(D * ) exp = 0.316 ± 0.016 ± 0.010, (1.6) where the first uncertainty is statistical and the second is systematic. R(D * ) and R(D) exceed the SM predictions by 3.3σ and 1.9σ, respectively. The combined analysis of R(D * ) and R(D), taking into account measurement correlations, finds that the deviation is 4σ from the SM prediction [11,12].
Since lattice QCD results are not yet available for the B → D * form factors at nonzero recoil and for the B → D tensor form factor, we use the phenomenological form factors from ref. [8] for both channels in our analysis. For B → D, we have compared the phenomenological results for f 0 and f + to the results obtained from a joint BGL z-expansion fit [13] to the FNAL/MILC lattice QCD results [6] and Babar [14] and Belle experimental data [15], and we found that the differences between both sets of form factors are below 5% across the entire kinematic range. The constraints on the new-physics couplings from the experimental measurement of R(D) obtained with both sets of form factors are practically identical.
The underlying quark level transition b → cτ −ν τ in the R(D ( * ) ) puzzle can be probed in both B and Λ b decays. Recently, the decay Λ b → Λ c τν τ was discussed in the standard model and with new physics in refs. [41][42][43][44][45][46][47]. Λ b → Λ c τν τ decays could be useful to confirm possible new physics in the R(D ( * ) ) puzzle and to point to the correct model of new physics.
In ref. [43] the following quantities were calculated within the SM and with various new physics operators: where ℓ represents µ or e. In this paper we work with the ratio R Ratio Λc , defined as We also consider the forward-backward asymmetry where θ τ is the angle between the momenta of the τ lepton and Λ c baryon in the dilepton rest frame.

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This paper improves upon the earlier work [43] in several ways: • We add tensor interactions in the effective Lagrangian.
• Instead of a quark model, we use form factors from lattice QCD to calculate all Λ b → Λ c τν τ observable. The vector and axial vector form factors are taken from ref. [48], and we extend the analysis of ref. [48] to obtain lattice QCD results for the tensor form factors as well.
• In addition to R(Λ c ) and B Λc (q 2 ), we also calculate the forward-backward asymmetry (1.12) in the SM and with new physics.
• We include new constraints from the B c lifetime [47,49,50] in our analysis.
• In addition to analyzing the effects of individual new physics-couplings, we study specific models that introduce multiple new-physics couplings simultaneously. We consider a 2-Higgs doublet model, models with new vector bosons, and several leptoquark models.
The paper is organized in the following manner: in section 2 we introduce the effective Lagrangian to parametrize the NP operators and give the expressions for the decay distribution in terms of helicity amplitudes. In section 3, we present the new lattice QCD results for the tensor form factors. The model-independent phenomenological analysis of individual new-physics couplings is discussed in section 4, while explicit models are considered in section 5. We conclude in section 6.

Effective Hamiltonian
In the presence of NP, the effective Hamiltonian for the quark-level transition b → cτ −ν τ can be written in the form [51,52] where G F is the Fermi constant, V cb is the Cabibbo-Kobayashi-Maskawa (CKM) matrix element, and we use σ µν = i[γ µ , γ ν ]/2. We consider that the above Hamiltonian is written at the m b energy scale. If the effective interaction is written at the cut-ff scale Λ then running down to the m b scale will generate new operators and new contributions, which have been discussed in refs. [53,54]. These new contributions can strongly constrain models but to really calculate their true impacts we have to consider specific models where there might be cancellations between various terms.
The SM effective Hamiltonian corresponds to g L = g R = g S = g P = g T = 0. In eq. (2.1), we have assumed the neutrinos to be always left chiral. In general, with NP JHEP08(2017)131 the neutrino associated with the τ lepton does not have to carry the same flavor. In the model-independent analysis of individual couplings (section 4) we will not consider this possibility. Specific models will be discussed in section 5.

Decay process
The process under consideration is The differential decay rate for this process can be represented as [23] dΓ 3) 4) and the helicity amplitude M λτ λ Λc is written as Here, (λ, λ ′ ) indicate the helicity of the virtual vector boson (see appendix A), λ Λc and λ τ are the helicities of the Λ c baryon and τ lepton, respectively, and η λ = 1 for λ = t and η λ = −1 for λ = 0, ±1. The scalar-type, vector/axial-vector-type, and tensor-type hadronic helicity amplitudes are defined as and The leptonic amplitudes are defined as Above, ǫ µ are the polarization vectors of the virtual vector boson (see appendix A). The explicit expressions for the hadronic and leptonic helicity amplitudes are presented in the following.

Hadronic helicity amplitudes
In this paper, we use the helicity-based definition of the Λ b → Λ c form factors, which was introduced in [55]. The matrix elements of the vector and axial vector currents can be written in terms of six helicity form factors F + , F ⊥ , F 0 , G + , G ⊥ , and G 0 as follows: The matrix elements of the scalar and pseudoscalar currents can be obtained from the vector and axial vector matrix elements using the equations of motion:

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In our numerical analysis, we use m b = 4.18(4) GeV, m c = 1.27(3) GeV [56]. The matrix elements of the tensor currents can be written in terms of four form factors h + , h ⊥ , h + , h ⊥ , 14) The matrix elements of the currentciσ µν γ 5 b can be obtained from the above equation by using the identity In the following, only the non-vanishing helicity amplitudes are given. The scalar and pseudo-scalar helicity amplitudes associated with the new physics scalar and pseudo-scalar interactions are The parity-related amplitudes are For the vector and axial-vector helicity amplitudes, we find

20)
We also have the relations The tensor helicity amplitudes are The other non-vanishing helicity amplitudes of tensor type are related to the above by The scalar and pseudoscalar leptonic helicity amplitudes are the vector and axial-vector amplitudes are 43) and the tensor amplitudes are Here we have the relation L λτ λ,λ ′ = −L λτ λ ′ ,λ . (2.50) The angle θ τ is defined as the angle between the momenta of the τ lepton and Λ c baryon in the dilepton rest frame.

Differential decay rate and forward-backward asymmetry
From the twofold decay distribution (2.2), we obtain the following expression for the differential decay rate by integrating over cos θ τ :

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where are the (axial-)vector non-spin-flip and spin-flip terms respectively, A SP 3 and A T 4 are the pure (pseudo-)scalar and tensor terms respectively; and are interference terms. The scalar-tensor interference term is proportional to cos θ τ and vanishes after integration over cos θ τ .
For the forward-backward asymmetry (1.12) we have There is no contribution from pure (pseudo-)scalar operators to the forward-backward asymmetry, but all possible interference terms are present.

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3 Λ b → Λ c tensor form factors from lattice QCD This work uses Λ b → Λ c form factors computed in lattice QCD. The vector and axial vector form factors defined in eqs. (2.10) and (2.11) are taken from ref. [48]. For the purposes of the present work, one of us (SM) extended the analysis of ref. [48] to include the tensor form factors defined in eq. (2.14). The tensor form factors were extracted from the lattice QCD correlation functions using ratios defined as in ref. [57]. The lattice parameters are identical to those in ref. [48], except that for the tensor form factors the "residual matching factors" ρ T µν and the O(a)-improvement coefficients were set to their tree-level values, with appropriately increased estimates for the resulting systematic uncertainties as detailed further below. Following ref. [48], two separate fits were performed to the lattice QCD data using BCL z-expansions [58] augmented with additional terms to describe the dependence on the lattice spacing and quark masses. The "nominal fit" is used to evaluate the central values and statistical uncertainties of the form factors (and of any observables depending on the form factors), while the "higher-order fit" is used in conjunction with the nominal fit to evaluate the combined systematic uncertainty associated with the continuum extrapolation, chiral extrapolation, z expansion, renormalization, scale setting, b-quark parameter tuning, finite volume, and missing isospin symmetry breaking/QED. The procedure for evaluating the systematic uncertainties is given in eqs. (82)-(84) of ref. [48]. The renormalization uncertainty in the tensor form factors is dominated by the use of the tree-level values, ρ T µν = 1, for the residual matching factors in the mostly nonperturbative renormalization procedure. We estimate the systematic uncertainty in ρ T µν to be 2 times the maximum value of |ρ V µ − 1|, |ρ A µ − 1|, which is equal to 0.0404 [48]. Note that the tensor form factors are scale-dependent, and our results and estimates of systematic uncertainties should be interpreted as corresponding to µ = m b in the MS scheme. To account for the renormalization uncertainty in the higher-order fit, we introduced nuisance parameters multiplying the form factors, with Gaussian priors equal to 1 ± 0.0404. In the physical limit (zero lattice spacing and physical quark masses), the nominal fit function for a form factor f reduces to the form while the higher-order fit function is given by The values of the pole masses are given in table 1, and the kinematic variables z f are defined as 3.0851 ± 7.9037 Table 2. Results for the z-expansion parameters describing the Λ b → Λ c tensor form factors in the physical limit (in the MS scheme at the renormalization scale µ = m b ). Files containing the values and covariances of the parameters of all ten Λ b → Λ c form factors are provided as supplemental material.
As in ref. [48], in the fits to the lattice data we evaluated the pole masses as am f pole = am Bc are the lattice QCD results for the pseudoscalar B c mass on each individual data set, and the splittings ∆ f are fixed to their physical values ∆ h + ,h ⊥ = 56 MeV and ∆ h + , h ⊥ = 492 MeV. The form factor results are very insensitive to the choices of ∆ f (as expected for poles far above q 2 max ). When varying ∆ f by ±10%, the z-expansion parameters returned from the fit are found to change in such a way that the changes in the form factors themselves are below 0.2% in the entire semileptonic region.
Plots of the lattice QCD data for the tensor form factors, along with the nominal fit functions in the physical limit, are shown in figure 1. The same fit functions are plotted in the entire kinematic range in figure 2, where also the total (statistical plus systematic) uncertainties are shown. The form factor h + has larger uncertainties than the other form factors because of larger excited-state contributions in the lattice QCD correlation functions.
The values of the nominal and higher-order fit parameters for the tensor form factors are given in table 2. Because of the kinematic constraint tensor form factors in the high q 2 -region: lattice results and extrapolation to the physical limit (nominal fit). The bands indicate the statistical uncertainty. The lattice QCD data sets are labeled as in ref. [48].
which is at the point z = 0, the form factors h ⊥ and h + share the common parameters  factor parameters. The full covariance matrices of the nominal and higher-order parameters of all ten Λ b → Λ c form factors (vector, axial vector, and tensor) are provided as supplemental files.

Model-independent analysis of individual new-physics couplings
In this section we consider one new-physics coupling at a time. We first compute the constraints from the existing measurements with mesons, and then study the impact of a future measurement of R(Λ c ).

Constraints from the existing measurements of R(D), R(D * ), and τ Bc
We require the NP couplings to reproduce the measurements (1.7) and (1. within the 3σ range. The coupling g S (g P ) only contributes to R Ratio D (R Ratio D * ) while the other couplings contribute to both channels. If only g L is nonzero, the SM contribution gets rescaled by an overall factor |1 + g L | 2 , so that [31] R Ratio

Im[gs]
Only gs present

Im[gP]
Only gP present

Im[gL]
Only gL present

Im[gR]
Only gR present  which is consistent with the present measurements (1.7) and (1.8). Note that in the g L -only scenario the forward-backward asymmetry (1.12) is unmodified, A FB = A SM FB . There is also a measurement of the τ polarization by Belle [59] with the result P τ = −0.44 ± 0.47 +0. 20 −0.17 . The uncertainties of this measurement are presently too large to provide a significant additional constraint and we therefore do not include P τ in our analysis.
It was recently pointed out [47,49,50] that the measured lifetime of the B c meson, τ Bc = 0.507(9) ps [56], provides an upper bound on the B c → τ −ν τ decay rate, which yields a strong constraint on the g P coupling. According to SM calculations using an operator product expansion [60], only about 5% (for the central value) of the total width of the B c , Γ Bc = 1/τ Bc , can be attributed to purely tauonic and semi-tauonic modes. This can be relaxed as the parameters in the calculations are varied. In our analysis, we use an upper limit of B(B c → τ −ν τ ) ≤ 30% to put constraints on the new-physics couplings. We use f Bc = 0.434(15) GeV from lattice QCD [61].
In figure 3, we present the constraints on the new-physics couplings coming from the measurements of R Ratio D , R Ratio D * , and τ Bc . We see that τ Bc puts a strong constraint on g P , and weak constraints on g L and g R . The tensor coupling g T is strongly constrained by R Ratio D * , and only weakly constrained by R Ratio JHEP08 (2017) Example values of the ratios R(Λ c ) and R Ratio Λc = R(Λ c )/R(Λ c ) SM for representative allowed values of the NP couplings are given in table 3. The standard-model prediction for R(Λ c ) is 0.333 ± 0.010 [48]. We find that large deviations from this value are possible with the present mesonic constraints. In table 4, we present the maximum and minimum allowed values of R Ratio Λc = R(Λ c )/R(Λ c ) SM in the presence of each individual new-physics coupling, and the corresponding values of the coupling at which these occur. Figure 4 shows the effect of representative values of the individual NP couplings on the Λ b → Λ c τν τ differential decay rate (evaluated assuming |V cb | = 0.041) as well as B Λc (q 2 ) [defined in eq. (1.10)] and A FB (q 2 ). In all cases, except for the strongly constrained pure g P coupling, substantial deviations from the SM predictions are allowed. We notice that A FB is typically above the SM prediction in the presence of g R or g T , while it is typically below the SM prediction in the presence of g S . Hence, it is possible to use A FB to distinguish between the different couplings.

Impact of a future R(Λ c ) measurement
In this subsection we present the effect of possible future measurements of R(Λ c ) on the NP couplings constraints. We consider two cases, one in which the measured value is near the SM prediction and one with measured value far from SM. For the first case we take

Im[gL]
Only gL present

Im[gR]
Only gR present

Im[gT]
Only gT present  ). Note that we take the 1σ uncertainty as 0.05. Figures 5 and 6 show the allowed regions of the parameter space for the first and second case, respectively. We observe the following when adding the R Ratio Λc constraints to the mesonic constraints: • For R(Λ c ) near the SM (figure 5), the allowed regions for (g L , g R , g T ) are reduced significantly, the allowed region for g S shrinks only slightly, and the allowed region for g P remains the same (as it is dominantly constrained by τ Bc ).
• For R(Λ c ) far from the SM (figure 6), most of the previously allowed region for g S becomes excluded by R(Λ c ). Even more importantly, the g P -only scenario becomes ruled out. In this case, R(Λ c ) also provides strong constraints on (g L , g R , g T ), but these constraints still overlap with the mesonic constraints.

Im[gL]
Only gL present

Im[gR]
Only gR present

Im[gT]
Only gT present

Explicit models
In this section we will discuss explicit models that can generate the couplings in the effective Hamiltonian (2.1). We will consider three categories: two-Higgs-doublet models which generate (g S , g P ), SU(2) models which generate g L , and leptoquark models which generate (g S , g P , g L , g T ). We do not consider models that generate g R , as in the standard-modeleffective-theory picture it is difficult to have a g R coupling that leads to lepton universality violation effects [62].

Two-Higgs-doublet models
The simplest scalar extensions of the SM are the two-Higgs-doublet models (2HDM). The 2HDM of type II is disfavored by experiment [1]. We will consider the Aligned Two-Higgs-Doublet Model (A2HDM) from ref. [21]. The Lagrangian of the model is

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where u, d, and l denote all three generations of up-type quarks, down-type quarks, and charged leptons, M u and M d are the quark mass matrices, and V is the CKM matrix.
Above, ξ f (f = u, d, l) are the proportionality parameters in the so-called "Higgs basis", in which only one scalar doublet acquires a nonzero vacuum expectation value. The cases ξ d = ξ l = −1/ξ u = − tan β and ξ u = ξ d = ξ l = cot β correspond to the Type-II and Type-I models, respectively. The general effective couplings in eq. (2.1) read The scenario in which the ξ u,d,l parameters are universal for all three generations is ruled out [21]. We therefore assume that eq. (5.3) only gives the couplings for processes involving the b quark, while the couplings for the first two generations are considered independently.
In this model we find significant deviation from the standard model contribution to the decay Λ b → Λ c τν τ , but for a more complete analysis RGE evolution should be considered. The RGE evolution of the couplings of the A2HDM has been discussed in ref. [63]. The alignment condition, which guarantees the absence of tree-level FCNC processes, is preserved by the RGE only in the case of the standard type-I, II, X, and Y models which are discussed in [64]. However, our framework requires non-universal flavor dependent couplings and the RGE evolution has not been worked out and is not included in the analysis. Keeping in mind that RGE effects could change the phenomenology of the model, the discussion of the full numerical analysis of the model is not included in this work.

SU(2) and Leptoquark models
The analysis of the R(D ( * ) ) and R K anomalies could favor the left-handed operator g L . In ref. [31], it was pointed out that, assuming that the scale of NP is much higher than the weak scale, the g L operator should be invariant under the full SU(3) C × SU(2) L × U(1) Y gauge group. There are two possibilities: where G 1 and G 2 are both O(1), and the σ I are the Pauli matrices. Here The key point is that O NP 2 contains both neutral-current (NC) and charged-current (CC) interactions. The NC and CC pieces can be used to respectively explain the R K and R(D ( * ) ) puzzles. In the following, we briefly review the literature on models of this type. In ref. [36], UV completions that can give rise to O NP 1,2 [eq. (5.4)], were discussed. One among the four possibilities for the underlying NP model is a vector boson (VB) that transforms as (1, 3, 0) under SU(3) C × SU(2) L × U(1) Y , as in the SM.

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Concrete VB models were discussed in refs. [37,38] and the simplest VB model was considered in ref. [39]. We refer to the VBs as V = W ′ , Z ′ . In the gauge basis, the Lagrangian describing the couplings of the VBs to left-handed third-generation fermions is where σ I (I = 1, 2, 3) are the Pauli matrices. Once the heavy VB is integrated out, one obtains the following effective Lagrangian, relevant for b → sℓ + ℓ − , b → cτ −ν and b → sνν decays: One can study the phenomenology of the model with an ansatz for the mixing matrices.
The assumption of refs. [36,39] is that the transformations D and L involve only the second and third generations. The key observation in ref. [39] is the Z ′ interaction also contributes to B s mixing and the model becomes highly constrained. If fact only a few percent deviation from the SM is allowed in the R(D ( * ) ) observables. For this reason, we do not present a detailed numerical analysis of the SU(2) models for the Λ b → Λ c τν τ decay. We next move to leptoquark models. In ref. [65], several leptoquark models are considered that generate scalar, vector, and tensor operators. The SU(3)×SU(2)×U(1) quantum numbers of these models are summarized in table 5. We can group the leptoquarks as vector or scalar leptoquarks. These leptoquarks can in turn be SU(2) singlets, doublets, or triplets.
The Lagrangians for the various leptoquarks are

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where h ij and g ij are dimensionless couplings, S 1 , S 3 , and R 2 are the scalar leptoquark bosons, U µ 1 , U µ 3 , and V µ 2 are the vector leptoquark bosons, and the index i (j) indicates the generation of quarks (leptons).
The leptoquark Lagrangian generates the following couplings in eq. (2.1): where the Wilson coefficients in the leptoquark models are given by (5.20) These Wilson coefficients are defined at the energy scale µ = M X , where X represents a leptoquark. Above, V k3 denotes the relevant CKM matrix element, where the 3 corresponds to the bottom quark. In the following, we neglect the CKM-suppressed contributions from k = 1 and k = 2 in the sums. Because the neutrino is not observed, we have l = 1, 2, 3. Note that there is a Standard-Model contribution for l = 3 but not for l = 1, 2; hence, the constraints for different l will be different. The renormalization-group running of the scalar and tensor Wilson coefficients from µ = M X to µ = µ b , where µ b is the mass scale of the bottom quark, is given by

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where α s (µ) is the QCD coupling at scale µ. Because the anomalous dimensions of the vector and axial-vector currents are zero, the Wilson coefficients for V 1,2 are scale-independent. The different leptoquarks produce different effective operators as summarized below: • The S 1 leptoquark with nonzero (g 1L , g * 1R ) generates C l V 1 , C l S 2 , and C l T , with the relation C l S 2 = −4C l T .
• The R 2 leptoquark with (h 2L , h * 2R ) generates C l S 2 and C l T with the relation C l S 2 = 4C l T .
• The V 2 leptoquark generates C l S 1 and is tightly constrained, so we do not consider this model.
• The U 1 leptoquark with nonzero (g 2L , g * 2R ) generates C l S 1 and C l V 1 .
The leptoquark couplings can also be constrained using b → sνν decays. As pointed out in ref. [39], the exclusive decaysB → Kνν andB → K * νν provide more stringent bounds than the inclusive mode B → X s νν. The U 1 and R 2 leptoquarks do not contribute to b → sνν, while the left-handed couplings of S 1 , S 3 , and U 3 do. (The V 2 leptoquark also contributes to b → sνν, but we do not consider this model.) The BaBar and Belle Collaborations give the following 90% C.L. upper limits [66,67]: Taking into account the theoretical uncertainties [68], the 90% C.L. upper bounds on the NP contributions are Following ref. [8], the b → sν jνi process can be described by the effective Hamiltonian where the left-handed and right-handed operators are defined as

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The SM Wilson coefficient C (SM) L receives contributions from box and Z-penguin diagrams, which yield C where the loop function X(x t ) can be found e.g. in ref. [69]. The leptoquarks that we consider produce contributions to C ij L which, to leading order, are equal to [8] We obtain common coefficients for b → cτν l and b → sν τνl processes, Hence, for l = 3 we obtain while for l = 1, 2 we have Since the S 3 and U 3 leptoquarks produce only the vector coupling g L , all ratios get rescaled by the common factor of |1 + g L | 2 . The S 3 and U 3 models are tighly constrained and only small effects are allowed. The other leptoquark models can produce substantial effects in R Ratio Λc , with varying degrees of correlation between the mesonic and baryonic observables.
The values of R(Λ c ) and R Ratio Λc for two typical allowed combinations of the couplings in each model are given in table 6. In figure 13, we present plots of the observables (dΓ/dq 2 , B Λc , A FB ) for the same values of the couplings.

Conclusions
The baryonic decay Λ b → Λ c τν τ has the potential to shed new light on the R(D ( * ) ) puzzle. Here, we studied the phenomenology of Λ b → Λ c τν τ in the presence of newphysics couplings with all relevant Dirac structures. In contrast to the mesonic decays, JHEP08(2017)131  In the first part of our phenomenological analysis, we considered individual new-physics couplings in the effective Hamiltonian in a model-independent way. After constraining these couplings using the R(D ( * ) ) measurements and the B c lifetime, we calculated the effects of the NP couplings in Λ b → Λ c τν τ decays, focusing on the observables R(Λ c ), B Λc (q 2 ), and A FB (q 2 ). Measurements of these observables can help in distinguishing among the  different NP operators. For instance, the forward-backward asymmetry A FB (q 2 ) tends to be mostly above the SM value in the presence of right-handed (g R ) or tensor (g T ) couplings, but is lower than the SM value for most allowed values of the scalar (g S ) coupling. To illustrate the impact of a future R(Λ c ) measurement, we presented the constraints on all couplings resulting from two possible ranges of R(Λ c ). The baryonic decay can tightly constrain all of the couplings g L , g R , g S , g P , and g T . For example, we have shown that if R Ratio Λc = R(Λ c )/R(Λ c ) SM is observed to have a value around 1.3, the scenario with only g P becomes ruled out by the combined constraints from R(Λ c ) and τ Bc .
In the second part of our phenomenological analysis, we considered explicit models in which multiple NP operators are present. For the two-Higgs-doublet model we found significant contribution to Λ b → Λ c τν τ . However, the full numerical analysis was not included in this work as we did not consider RGE evolution which could impact the phenomenology of the model. Models with SU(2) gauge symmetry generally cannot produce large effects in b → cτν τ transitions without violating bounds from other observables such as B s mix-  ing, and we therefore did not present their effects on Λ b → Λ c τν τ . On the other hand, we have demonstrated that some of the leptoquark models can produce large effects in the Λ b → Λ c τν τ observables, in particular through scalar and tensor couplings. We have presented correlation plots of R Ratio     Table 6. The values of the R(Λ c ) and R Ratio Λc ratios for two representative cases of the couplings of the different leptoquark models. Above, the index i = 1, 2 denotes the electron and muon neutrinos. The Standard-model value of the ratio is R(Λ c ) = 0.333 ± 0.010 [48]. The uncertainties given are due to the Λ b → Λ c form factor uncertainties. JHEP08(2017)131 Figure 13. The effects of the different leptoquark models on the Λ b → Λ c τν τ differential decay rate (left), the ratio of the Λ b → Λ c τν τ and Λ b → Λ c ℓν ℓ differential decay rates (middle), and the Λ b → Λ c τν τ forward-backward asymmetry (right), for two representative choices of the couplings. The red and blue curves correspond to the couplings from Cases 1 and 2 in table 6, respectively, while the green curves correspond to the Standard Model. Because the S 3 and U 3 leptoquarks produce only the vector coupling g L , the forward-backward asymmetry remains equal to the Standard Model in those cases. The bands indicate the 1σ uncertainties originating from the Λ b → Λ c form factors.

A.2 Dilepton rest frame
In the calculation of the lepton helicity amplitudes, we work in the rest frame of the virtual vector boson boson, which is equal to the rest frame of the τν τ dilepton system. We define the angle θ τ as the angle between the three-momenta of the τ and the Λ c in this frame.

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Open Access. This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.