A tale of invisibility: constraints on new physics in b → sνν

The Belle II experiment will measure the rare decays B → Kνν and B → K∗νν with increased sensitivity which can hence be expected to serve as a very efficient probe of new physics. We calculate the relevant branching ratios in low-energy effective field theory (LEFT) including an arbitrary number of massive sterile neutrinos and discuss the expected sensitivity to the different operators. We also take into account the longitudinal polarisation fraction FL and the inclusive decay rate B → Xsνν. In our investigation we consider new physics dominantly contributing to one and two operators both for massless and massive (sterile) neutrinos. Our results show a powerful interplay of the exclusive decay rates B → Kνν and B → K∗νν, and a surprisingly large sensitivity of the inclusive decay mode to vector operators even under conservative assumptions about its uncertainty. Furthermore, the sensitivity of FL is competitive with the branching ratio of B → K∗νν in the search for new physics contributing to scalar operators and thus also complementary to B → Kνν and B → Xsνν.

The Energy-Energy Correlation in the back-to-back limit at N3LO and N3LL′

We present the analytic formula for the Energy-Energy Correlation (EEC) in electron-positron annihilation computed in perturbative QCD to next-to-next-to-next-to-leading order (N3LO) in the back-to-back limit. In particular, we consider the EEC arising from the annihilation of an electron-positron pair into a virtual photon as well as a Higgs boson and their subsequent inclusive decay into hadrons. Our computation is based on a factorization theorem of the EEC formulated within Soft-Collinear Effective Theory (SCET) for the back-to-back limit. We obtain the last missing ingredient for our computation — the jet function — from a recent calculation of the transverse-momentum dependent fragmentation function (TMDFF) at N3LO. We combine the newly obtained N3LO jet function with the well known hard and soft function to predict the EEC in the back-to-back limit. The leading transcendental contribution of our analytic formula agrees with previously obtained results in $$ \mathcal{N} $$ N = 4 supersymmetric Yang-Mills theory. We obtain the N = 2 Mellin moment of the bulk region of the EEC using momentum sum rules. Finally, we obtain the first resummation of the EEC in the back-to-back limit at N3LL′ accuracy, resulting in a factor of ∼ 4 reduction of uncertainties in the peak region compared to N3LL predictions.

Magnetic-dipole corrections to RK and RK* in the Standard Model and dark photon scenarios

In this work we evaluate the long-distance QED contributions, induced by the magnetic-dipole corrections to the final charged leptons, on the B meson decay widths B → (K, K*)ℓ+ℓ− and ratios RK,K* = Γ(B → (K, K*)μ+μ−)/Γ(B → (K, K*)e+e−), as well as on $$ {R}_{K,K\ast}^{\tau } $$ R K , K ∗ τ (with μ replaced by the τ lepton). QED long-distance contributions induced by the Coulomb potential corrections (Fermi-Sommerfeld factors) were also included. Corresponding corrections to the inclusive decay widths of B → Xsℓ+ℓ−, with ℓ = e, μ, τ, are also analyzed for completeness. The magnetic-dipole corrections, which are manifestly Lepton Flavor Universality violating and gauge-invariant, are expected to be particularly enhanced in RK* for the dilepton mass region close to the threshold. However, we find that the largest contribution of all these corrections to the RK,K* observables do not exceed a few per mille effect, thus reinforcing the validity of previous estimates about the leading QED corrections to RK,K*. Finally, viable new physics contributions to RK,K* induced by the exchange of a massless dark-photon via magnetic-dipole interactions, which provide the leading contribution to the corresponding B → (K, K*)ℓ+ℓ− amplitudes in this scenario, are analyzed in light of the present RK,K* anomalies.

Measurement of the absolute branching fraction of the inclusive decay $$\Lambda _c^+ \rightarrow K_S^0X$$

We report the first measurement of the absolute branching fraction of the inclusive decay $$\Lambda _c^+ \rightarrow K_S^0X$$ Λ c + → K S 0 X . The analysis is performed using an $$e^+e^-$$ e + e - collision data sample corresponding to an integrated luminosity of 567 $$\hbox {pb}^{-1}$$ pb - 1 taken at $$\sqrt{s}$$ s = 4.6 GeV with the BESIII detector. Using eleven Cabibbo-favored $${\bar{\Lambda }}_c^-$$ Λ ¯ c - decay modes and the double-tag technique, this absolute branching fraction is measured to be $${\mathcal {B}}(\Lambda _c^+ \rightarrow K_S^0X)=(9.9\pm 0.6\pm 0.4)\%$$ B ( Λ c + → K S 0 X ) = ( 9.9 ± 0.6 ± 0.4 ) % , where the first uncertainty is statistical and the second systematic. The relative deviation between the branching fractions for the inclusive decay and the observed exclusive decays is $$(18.7\pm 8.3)\%$$ ( 18.7 ± 8.3 ) % , which indicates that there may be some unobserved decay modes with a neutron or excited baryons in the final state.

Phenomenology of inclusive $$ \overline{B}\to {X}_s{\mathrm{\ell}}^{+}{\mathrm{\ell}}^{-} $$ for the Belle II era

With the first data being recorded at Belle II, we are at the brink of a new era in quark flavour physics. The many exciting new opportunities for Belle II include a full angular analysis of inclusive $$ \overline{B}\to {X}_s{\mathrm{\ell}}^{+}{\mathrm{\ell}}^{-} $$ B ¯ → X s ℓ + ℓ − which has the potential to reveal new physics, in particular by its interplay with the exclusive b → sℓ+ℓ− counterparts studied extensively at LHCb. In this paper, we present fully updated Standard Model predictions for all angular observables necessary for this endeavour. These predictions are tailored to Belle II and include an elaborate study of the treatment of collinear photons which become crucial when aiming for the highest precision. In addition, we present a phenomenological study of the potential for Belle II to reveal possible new physics in the inclusive decay channel, both in an independent manner and in combination with exclusive modes.

Λc Physics at BESIII

In 2014 BESIII collected a data sample of 567 [Formula: see text] at [Formula: see text] = 4.6 GeV, which is just above the [Formula: see text] pair production threshold. By analyzing this data sample, we have measured the absolute branching fractions for many decays of [Formula: see text] for the first time. These decays include the semileptonic decays of [Formula: see text], [Formula: see text], the hadronic decays of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and inclusive decay [Formula: see text] + anything. The decays of [Formula: see text], [Formula: see text] and [Formula: see text] are observed for the first time and the others are measured with significantly improved precision. These results are important to benefit the development of the related theories, and provide important inputs for both charmed baryons and [Formula: see text] physics.

Rare B-meson decays at the crossroads

Experimental era of rare [Formula: see text]-decays started with the measurement of [Formula: see text] by CLEO in 1993, followed by the measurement of the inclusive decay [Formula: see text] in 1995, which serves as the standard candle in this field. The frontier has moved in the meanwhile to the experiments at the LHC, in particular, LHCb, with the decay [Formula: see text] at about 1 part in 10[Formula: see text] being the smallest branching fraction measured so far. Experimental precision achieved in this area has put the standard model to unprecedented stringent tests and more are in the offing in the near future. I review some key measurements in radiative, semileptonic and leptonic rare [Formula: see text]-decays, contrast them with their estimates in the SM, and focus on several mismatches reported recently. They are too numerous to be ignored, yet, standing alone, none of them is significant enough to warrant the breakdown of the SM. Rare [Formula: see text]-decays find themselves at the crossroads, possibly pointing to new horizons, but quite likely requiring an improved theoretical description in the context of the SM. An independent precision experiment such as Belle II may help greatly in clearing some of the current experimental issues.