Model-independent investigation of the RJ/ψ,ηc and ratios of decay widths of semileptonic Bc decays into a P-wave charmonium

Abstract

Experimental measurements of decay branching fractions of semitaunic and semimuonic Bc into J/ψ has challenged the lepton flavor universality in standard model with about two standard deviations. In this paper, we first investigate the unitary constraint on form factors of Bc meson into S-wave and P -wave charmonium. Such constraint leads to the exploration of the R J/ψ and other ratios Rη c , R hc , and Rχ cJ in a model-independent way. These results together with future experimental measurements can be used to explore the lepton flavor universality in a more systematic way. In addition, we point out that the helicity-dependent ratios R L J/ψ and R ⊥ J/ψ can also provide complementary information. I. INTRODUCTIONWeak decays of heavy mesons play an important role in testing the standard model (SM) precisely and probing new physics effects beyond SM. In SM, the lepton interactions are universal for all three generations. Semileptonic decays of B mesons induced by the b → c transition have revealed hints lepton flavor non-university [1], which may indicate the presence of possible new physics (NP). To probe the NP effects in a more systematic way, more accurate and solid predictions of branching fractions are highly demanded. Ratios of semitaunic and semimuonicB decays intoD or D * , i.e. R D and R D * have been measured by many experiments such as BABAR [2], Belle [3], and LHCb [4]. These measurements indicate the data of R D and R D * deviate from the SM predictions by about 3σ. Recently, the LHCb collaboration have reported the measurement of the ratio of the semitaunic and semimuonic B + c decays into J/ψ, i.e. R J/ψ , using a sample of proton-proton collision data corresponding to 3.0f b −1 of integrated luminosity [5]. A deviation, about 2σ, from SM is found in this measurement by LHCb. On the theoretical side, a few schemes are used to study the B c decays into a charmonium, such as the perturbative QCD (PQCD) approach [6-14], QCD sum rules (QCD SR) [15-18], light-cone sum rules (LCSR) [19, 20], quark model (QM) [21-27], light-front quark model (LFQM) [28-30], nonrelativistic QCD (NRQCD) [31-41] and others [42]. Uncertainties in some of these schemes are hard to control especially in the nonperturbative region and power corrections are expected important [43]. At zero recoil, many of these theoretical investigations become less solid and the extrapolations must be used in order to access the full momentum dependence of form factors. On the lattice, HPQCD collaboration have calculated form factors for decays B c into the S-wave charmonium in the entire q 2 range, using a highly improved lattice quark action [44,45]. These works make the analysis of semileptonic B c decays sitting on a firm ground.In this paper, we will adopt a model-independent way to study the form factors of B c into the S-wave and P-wave charmonium in the entire phase space. Using the dispersion relation and crossing symmetry, we will give a new parametrization form expanded in a series of polynomials. The coefficients in the expansion can...

Probing new physics in semileptonic Σb and Ωb decays

“…Apart from R D and R D Ã measurements, the ratio R J=ψ has also been measured by LHCb [13] R J=ψ ¼ BðB c → J=ψτν τ Þ BðB c → J=ψlν l Þ ¼ 0.71 AE 0.17 AE 0.18; ð4Þ which central value prediction of the SM is in the range 0.25 ∼ 0.28 and the experimental result has about 2σ tension with its SM prediction [14,15]. The uncertainties arise from the choice of the approach for the B c → J=ψ from factors [15][16][17][18]. These deviations between the experimental measurements and their SM predictions are perhaps from the uncertainties of hadronic transition form factors.…”

$${b\rightarrow c\tau \nu }$$ b → c τ ν transitions in the standard model effective field theory

“…Using the model-dependent calculations of B c → (J/ψ, η c ) transition form factors [106][107][108][109][110][111][112][113][114][115][116][117][118] [120][121][122] and R(η c ) [121][122][123] are also obtained by constraining the transition form factors through a combination of dispersive relations, heavy-quark relations at zero-recoil, and the limited existing form-factor determinations from lattice QCD [124,125], resulting in 0.20 ≤ R(J/ψ) ≤ 0.39 [120] and 0.24 ≤ R(η c ) ≤ 0.34 [123], both of which agree with the weighted averages of previous model predictions.…”

New physics and the tau polarization vector in b → $$ \mathrm{c}\tau {\overline{\nu}}_{\tau } $$ decays

“…The ratio R J/ψ has been recently measured by the LHCb collaboration [19] and it shows a 1.8 σ discrepancy with SM results, which are in the range R SM J/ψ ∼ 0.25 − 0.28 [20][21][22][23][24][25][26][27][28][29][30][31][32].B c decays induced by the c → s, d transition at the quark level are also being investigated as a possible source of information on NP [33], taking advantage of the recent results of ref. [34].…”