Ⅱ-6. Particle Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Phenomenology of Λb → Λc τ ν̄ using lattice QCD calculations†
A. Datta,∗1,∗2 S. Kamali,∗3 S. Meinel,∗3,∗4 and A. Rashed∗1,∗5

(1)
(2)

B(Λb → Λc τ ν̄).
B(Λb → Λc ℓν̄)

(3)

A precise Standard-Model prediction of R(Λc ) using
the Λb → Λc vector and axial vector form factors fromlattice QCD was given in Ref. 2).
In this work, we studied the eﬀects of several newphysics scenarios that have been proposed to explain
the excesses in R(D(∗) ) on the decay Λb → Λc τ ν̄.
Because some of these scenarios generate tensor couplings, we also determined the Λb → Λc tensor form
factors from lattice QCD.
We demonstrated that a future measurement ofR(Λc ) can tightly constrain all of the couplings gL , gR ,gS , gP , and gT in the b → cτ ν̄ eﬀective Hamiltonian.
We also analyzed six diﬀerent leptoquark models,
where we constrained the model parameters using theexperimental measurements of R(D), R(D∗ ), the Bc
lifetime τBc , and the upper limits on B(B → K (∗) ν ν̄).
As an example, Fig. 1 shows the correlations betweenRatio
the predicted values of RΛ= R(Λc )/R(Λc )SM andcRatio∗∗RD∗ = R(D )/R(D )SM for the SU (2)-singlet andSU (2)-doublet scalar leptoquarks S1 and R2 , and for
the SU (2)-singlet vector leptoquark U1 (the latter isa particularly attractive model, which can simultaneously explain hints of lepton-ﬂavor-university violation
seen in b → sℓ+ ℓ− decays3) ). Our analyses show that†∗1∗2∗3∗4∗51.151.10
1.05

Condensed from the article in JHEP 1708, 131 (2017)
Department of Physics and Astronomy, University of Mississippi
Department of Physics and Astronomy, University of Hawaii
RIKEN Nishina Center
Department of Physics, University of Arizona
Department of Physics, Ain Shams University- 123 -

1.05 1.10 1.15 1.20 1.25 1.30 1.35
RRatioΛCR2 Leptoquark

1.5
1.4
1.3

RRatioD*

where ℓ = e, µ, depend only on the lepton and hadron
masses and the hadronic form factors. The experimental measurements of R(D) and R(D∗ ) by theBabar, Belle, and LHCb collaborations exceed theStandard-Model predictions with a combined signiﬁcance of 4.1σ,1) hinting at the existence of new fundamental interactions that violate lepton-ﬂavor universality.
The underlying b → cτ ν̄ transition can also beprobed with the baryonic decay Λb → Λc τ ν̄, in particular by measuring the ratioR(Λc ) =

1.20

1.2
1.1
1.0

1.10 1.15 1.20 1.25 1.30 1.35 1.40
RRatioΛC

U1 Leptoquark

1.5
1.4
1.3

RRatioD*

B(B → Dτ ν̄)R(D) =,
B(B → Dℓν̄)
B(B → D∗ τ ν̄)R(D∗ ) =,B(B → D∗ ℓν̄)

S1 Leptoquark

1.25

RRatioD*In the Standard Model, the electroweak interactions
are lepton-ﬂavor-universal. Consequently, the ratios ofbranching fractions

1.2
1.1
1.0
1.0

1.1

1.2

1.3

RRatio
ΛC

1.4

1.5

Ratio
Fig. 1. Correlations between RΛ
= R(Λc )/R(Λc )SM andcRatio∗∗RD∗ = R(D )/R(D )SM in three diﬀerent leptoquark
scenarios. The points sample the region of couplings allowed by experimental measurements of R(D), R(D∗ ),τBc , and B(B → K (∗) ν ν̄).

a future measurement of R(Λc ) can be helpful in discriminating between the diﬀerent models.
References
1) Heavy Flavor Averaging Group, http://www.slac.
stanford.edu/xorg/hflav/semi/fpcp17/RDRDs.html
2) W. Detmold, C. Lehner, S. Meinel, Phys. Rev. D 92,034503 (2015).
3) D. Buttazzo, A. Greljo, G. Isidori, D. Marzocca, J. HighEnergy Phys. 1711, 044 (2017).