Light-cone Sum Rule Analysis of Semileptonic Decays $Λ_b^0 \to Λ_c^+ \ell^- \overlineν_\ell$

Abstract

In this work, we analyze the semileptonic decay processes of Λ b → Λc in the light-cone sum rule approach. In the calculation of the form factors of Λ b baryon transition matrix element, the light-cone distribution amplitudes of Λ b which are obtained from QCD sum rule in the heavy quark effective field theory framework are used. With the calculation of the six form factors of Λ b → Λc transition matrix element, the differential decay widths of Λ 0 b → Λ + c ℓ − ν ℓ (ℓ = e, µ, τ ) and absolute branching fractions are obtained. Additionally the ratio of R(Λc) = Br(Λ b → Λceνe)/Br(Λ b → Λcτ ντ ) is obtained. These results are in accordance with the newest experimental result and other theoretical calculations and predictions. 1 Introductions Semileptonic decays of Λ b to Λ c baryons provide a good way to study and determine the CKM matrix element |V cb |. In most cases, the CKM matrix element |V cb | is extracted from the decay B to D meson semileptonic decays. From different methods and models, there is a small difference of the value of |V cb |. On the other hand, theoretical analysis and experimental measurements of the ratio R(Λ c ) = Br(Λ b → Λ c µν µ )/Br(Λ b → Λ c τ ν τ ) also give tests of QCD standard model and the lepton universely. Experimentally, LHCb report their measurement of the branching fractions of simileptonic decay Λ b to Λ c with tau lepton finnal state with the error 6.1σ recently [1]. And the detect from tau lepton decay give the branching fraction of this process Br(Λ 0 b → Λ + c τ − ν τ ) = (1.50±0.16 stat ±0.25 syst ±0.23)%, with the last uncertenty comes from the measurement of Λ 0 b → Λ + c µ − ν µ . In the same time, the experimental test of the ratio of branching fractions of semileptonic decay Λ 0 b.242 ± 0.026 ± 0.040 ± 0.050. In the early experimetal detection, the DELPHI and CDF collaborations had been reported the branching fractions of the semileptonic decay Λ 0 b → Λ + c ℓ − ν ℓ with the lepton ℓ stand for electron and muon [2,3]. With the ratio R(Λ + c ) analysed by QCD standard model, the early measurement of semileptonic decay of Λ b to Λ + c with electron and muon finnal states results gives the branching fraction of Λ 0 b → Λ + c τ − ν τ which is around two percentage. With the poor experimental test of these prcesses, the newest observation and measurement also provide the material for test the QCD standard model by the ratio R(Λ + c ). Theoretical hand, some models have been calculated and analysed the semileptonic decay processes Λ 0 b → Λ + c ℓ − ν ℓ , such as QCD sum rule [4], light-front quark model [5][6][7][8], lattice QCD [9, 10], heavy quark effective theory [4,11], relativistic quark model [12], covariant confined quark model [13] and QCD standard model [14] et al. They calculated the branching fractions at some regions, and the results are consistent with experimental values. But with the analysis of tau lepton's final semileptonic decay, some references give bigger results than the newest experiment or give a large error. Besides, the Standard Model also gives...

Heavy quark symmetry in quark models

“…Refs. [47,48] employ the light-cone sum rule (LCSR), Ref. [46] also considers the LFQM with parameters fitted from the mass spectra, and Ref.…”

“…LFQM LQCD [11,12] LHCb [10] LFQM [46] LCSR [48] 0.3154(109) 0.3457(70) 0.3328(102) 0.242(76) 0.30(9) 0.292 0.3243(126) 0.3506(46) 0.3237 (36) those of other approaches with uncertainties provided.…”

“…The branching fractions of Λ 0 b → Λ + c → ℓ − ν ℓ in units of %, where the lower and upper columns in the HBM and LFQM for each lepton pair correspond to the values with and without the hard gluon corrections, respectively. lepton pair HBM LFQM Exp[40] RQM[45] LFQM[46] LCSR[47] LCSR[48] …”

$Λ_b \to Λ_c$ Form Factors from QCD Light-Cone Sum Rules

“…Similar to B → D decays, the LCSR with Λ b -DAs is valid approximately in the momentum region 0 ≤ q 2 ≤ 8GeV 2 . In [49], the Isgur-Wise function in the Λ b → Λ c transitions have been studied using the LCSR with Λ b -DAs, and a recent study on these form factors are presented in [50]. In this paper, we will make improvement on the following aspects:…”

Scrutinizing new physics in semi-leptonic B c → J/ψτν decay

“…In this work, we try to obtain the lattice + NRQCD tensor form factors, and use them to calculate the observables for the B c → J/ψτ ν decay in the SM and the new physics scenarios/models, including the LFU ratio R(J/ψ), the longitudinal τ polarization fraction P τ (J/ψ), the longitudinal J/ψ polarization fraction F L (J/ψ) and the forward-backward asymmetry of the τ lepton A FB (J/ψ). To estimate the size of new physics effects, we perform a global fit of the Wilson coefficients to the existing b → cτν data including the LFU ratio R(Λ c ) recently measured by LHCb [43] for Λ b → Λ c τν decay [44][45][46][47][48][49][50][51]. We study both the model-independent scenarios and the leptoquark models including R 2 , S 1 and U 1 , which are expected to be able to explain the b → cτν anomalies.…”