CHARGED CURRENT ANOMALIES WITH BARYONS D.BECIREVIC F. JAFFREDO IJCLab P ole Th eorie B at. 210 CNRSIN2P3 et Universit e Paris-Saclay 91405 Orsay France

2025-04-30 0 0 410.63KB 6 页 10玖币

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CHARGED CURRENT ANOMALIES WITH BARYONS

D.BECIREVIC, F. JAFFREDO

IJCLab, Pˆole Th´eorie (Bˆat. 210), CNRS/IN2P3 et Universit´e Paris-Saclay, 91405 Orsay, France

Motivated by the ﬁrst observation of Λb→Λcτ¯νby LHCb, we discuss various observables that

can be derived from angular distribution of this decay which, if measured, could be essential

for explanation of the b→cτ ¯νanomalies. Furthermore, we show that the measurement of

RΛc=B(Λb→Λcτ¯ν)/B(Λb→Λcµ¯ν) does not change the current global picture arising

from RDand RD∗being larger than predicted in the Standard Model. Finally, we show the

example of a few angular observables, with and without a secondary decay, that could have a

strong discriminating power on New Physics.

1 Introduction

Hints of lepton ﬂavor universality violation (LFUV) have been observed in Bmeson decay by

measuring the universality ratios RD(∗)and RK(∗)

1,2,3,4. Being the simplest in terms of valence

quark content, mesons are hadrons of choice for the precision ﬂavor physics observables both

in experimental studies, due to their abundance in the clean environment of B-factories, and

because of the relatively small theoretical uncertainties associated with the relevant hadronic

matrix elements.

In what follows, we discuss the possibility of observing other hints of LFUV by looking at

the baryon decays. Those decays involve the same b→c`¯νsemileptonic transition and are

produced in large quantity at LHCb

2 Theory prediction

To describe the charged current mediated semileptonic decay, in addition to the Standard Model

(SM) Lagrangian, we also consider the following New Physics contributions (NP):

LNP ⊃√2GFVijgV(uiγµdj)(`LγµνL) + gA(uiγµγ5dj)(`LγµνL) (1)

+gS(uidj)(`RνL) + gP(uiγ5dj)(`RνL) + gT(uiσµν (1 −γ5)dj)(`Rσµν νL)+ h.c.

The resulting L=LSM +LNP can describe various ﬂavor transitions; including the semilep-

tonic decays of baryons such as Λb→Λc`ν, Λb→Λ∗

c`ν, Λc→Λ`ν, Λ →p`ν. The main obstacles

to the precision computation of decay rates are the hadronic matrix element of the quark cur-

rents sandwiched by the two hadronic states. Such matrix elements have been computed to a

remarkable accuracy for many processes by means of lattice QCD 6,7,8. For baryons, the results

are expressed in term of 10 form factors, denoted F0, F+, F⊥, G0, G+, G⊥, h+, h⊥,e

h+,e

h⊥(resp.

3 vector, 3 axial, and 4 tensor form factors). The tensor form factors are not needed for the

SM prediction but they are required in some scenarios of NP. The fact that all of these 10 form

arXiv:2210.12071v1 [hep-ph] 21 Oct 2022

factors are available from lattice QCD for the Λb→Λc`ν decay in the full phase space is a

unique situation that makes this study particularly interesting.

By focusing on Λb→Λc`ν, we can express the decay distribution as:

d2Γ(Λb→Λλc

c`λlν)

dq2d cos θ=aλl

λc(q2) + bλl

λc(q2)cos θ+cλl

λc(q2)cos2θ, (2)

where λcand λlare the polarization states of the outgoing Λcand `.θis the angle between the

lepton and the Λcbaryon in the dilepton rest-frame. The angular coeﬃcients a,b, and care

functions of q2= (pΛb−pΛc)2= (p`+pν)2, the NP couplings gi,(i∈V, A, S, P, T ), form factors

and the polarizations of the outgoing particles. Explicit formulas can be found in Refs.

8,9,10,11,12,

as well as in Ref.

13, in which we discuss this decay in more detail.

From the explicit formulas, we can see that

b−

±=±(a−

±+c−

±).(3)

Contrary to the case of the pseudoscalar to pseudoscalar meson decays where we only have a

single spin-1/2 particle in the ﬁnal state yielding at most 4 linearly independent observables

14,

here we expect 12 observables (4 for each of the a, b, c). Eq. (3) reduces this number to 10.

Setting all gi= 0, we can compute the SM prediction for Λb→Λcτν and Λb→Λcµν, and

combine them to

RΛc=B(Λb→Λcτν)

B(Λb→Λcµν),(4)

in which a bulk of uncertainties on the hadronic matrix elements cancel. Using the lattice QCD

form factors

6,8, we get

RSM

Λc= 0.333(13).(5)

3 New LHCb measurement

LHCb recently reported on the ﬁrst observation of the semileptonic decay Λb→Λcτν.

5They

combine it with the old LEP result for B(Λb→Λcµν)

15, and obtain

RLHCb

Λc= 0.242(76).(6)

The error on this measurement is dominated by the systematics coming from the external

branching fractions: (i) B(Λb→Λcµν)

15 (15%), and (ii) from the normalization channel

B(Λb→Λc3π)

16,17 (23%).

This result is compatible with the SM prediction, merely ∼1σbelow. Therefore the anomaly

observed with mesons (RExp

D(∗)> RSM

D(∗)) is not yet observed with baryons, which might change

once the more accurate LHCb data become available. We remind the reader that the observed

value of RDand RD∗are respectively 1.5σand 3.5σabove their SM predictions 3,4,18. It can

be shown that due to the relatively large uncertainty, RΛchas a small eﬀect on the overall

constraints on NP.

To illustrate the impact of this new measurement, we consider several scenarios in which

NP only involves a single NP coupling (or a combination of two couplings). In particular, we

consider scenarios involving gVL= (gV−gA)/2 and gSL= (gS−gP)/2, since they can be

observed in several popular extensions of the SM. We also show the results for gTonly and for

the combination gSL=±4gT, which is usually evoked in the scenarios involving Low Energy

Scalar Leptoquarks 18. Since gSLand gTare renormalization scale dependant, after running

from the matching scale µ'1 TeV down to µ=mb, the above relations become gSL=−8.5gT

and gSL= 8.1gT19. In the latter scenario we consider only the imaginary values of gSLsince

it has been shown that a real-valued gSLis not compatible with the observed values of RDand

RD∗

18.

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CHARGEDCURRENTANOMALIESWITHBARYONSD.BECIREVIC,F.JAFFREDOIJCLab,P^oleTheorie(B^at.210),CNRS/IN2P3etUniversiteParis-Saclay,91405Orsay,FranceMotivatedbytherstobservationofb!cbyLHCb,wediscussvariousobservablesthatcanbederivedfromangulardistributionofthisdecaywhich,ifmeasured,couldbeessentialfore...

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CHARGED CURRENT ANOMALIES WITH BARYONS D.BECIREVIC F. JAFFREDO IJCLab P ole Th eorie B at. 210 CNRSIN2P3 et Universit e Paris-Saclay 91405 Orsay France

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