A measurement of the ratio of branching fractions of the decays 
B
+
→
K
+
μ
+
μ
−
 and 
B
+
→
K
+
e
+
e
−
is presented. The proton-proton collision data used correspond to an integrated luminosity of 
5.0
 
 
fb
−
1
recorded with the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV. For the dilepton mass-squared range 
1.1
<
q
2
<
6.0
 
 
GeV
2
/
c
4
 the ratio of branching fractions is measured to be 
R
K
=
0.84
6
+
0.060
−
0.054
+
0.016
−
0.014
, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of 
R
K
 to date and is compatible with the standard model at the level of 2.5 standard deviations.

/pdf/search-for-lepton-universality-violation-in-b-k-l-l-decays-2srktesra7.pdf

Search for Lepton-Universality Violation in B + →K + ℓ+ ℓ- Decays

The recent LHCb angular analysis of the exclusive decay B → K
 *
 μ
 +
 μ
 −
 has indicated significant deviations from the Standard Model expectations. Accurate predictions can be achieved at large K
 *-meson recoil for an optimised set of observables designed to have no sensitivity to hadronic input in the heavy-quark limit at leading order in α
 s
 . However, hadronic uncertainties reappear through non-perturbative ΛQCD
 /m
 b
 power corrections, which must be assessed precisely. In the framework of QCD factorisation we present a systematic method to include factorisable power corrections and point out that their impact on angular observables depends on the scheme chosen to define the soft form factors. Associated uncertainties are found to be under control, contrary to earlier claims in the literature. We also discuss the impact of possible non-factorisable power corrections, including an estimate of charm-loop effects. We provide results for angular observables at large recoil for two different sets of inputs for the form factors, spelling out the different sources of theoretical uncertainties. Finally, we comment on a recent proposal to explain the anomaly in B → K
 *
 μ
 +
 μ
 −
 observables through charm-resonance effects, and we propose strategies to test this proposal identifying observables and kinematic regions where either the charm-loop model can be disentangled from New Physics effects or the two options leave different imprints.

/pdf/on-the-impact-of-power-corrections-in-the-prediction-of-b-k-1hgxj7se07.pdf

On the impact of power corrections in the prediction of B → K*μ+μ− observables

We provide results for the full set of form factors describing semileptonic B-meson transitions to pseudoscalar mesons π, K, $$ \overline{D} $$
 and vector mesons ρ, K∗, $$ {\overline{D}}^{\ast } $$
 . Our results are obtained within the framework of QCD Light-Cone Sum Rules with B-meson distribution amplitudes. We recalculate and confirm the results for the leading-twist two-particle contributions in the literature. Furthermore, we calculate and provide new expressions for the two-particle contributions up to twist four. Following new developments for the three-particle distribution amplitudes, we calculate and provide new results for the complete set of three-particle contributions up to twist four. The form factors are computed numerically at several phase space points using up-to-date input parameters, including correlations across phase space points and form factors. We use a model ansatz for all contributing B-meson distribution amplitudes that is self-consistent up to twist-four accuracy. We find that the higher-twist two-particle contributions have a substantial impact on the results, and dominate over the three-particle contributions. Our numerical results, including correlations, are provided as machine-readable files in the supplementary material. We discuss the qualitative phenomenological impact of our results on the present b anomalies.

/pdf/b-p-and-b-v-form-factors-from-b-meson-light-cone-sum-rules-4i34vrkc47.pdf

B →P and B →V form factors from B-meson light-cone sum rules beyond leading twist

The three-site Pati-Salam gauge model provides a consistent description for the hints of lepton-flavor non-universality observed in B decays, connecting the present pattern of “anomalies” to the origin of the Standard Model Yukawa couplings. We present here a detailed analysis of the model predictions for a series of low-energy observables, mainly in B and τ physics. The model is in good agreement with present data and predicts a well-defined pattern of non-standard effects in low-energy observables that could allow us to test it in the near future. Particularly interesting are the predictions of large τ → μ Lepton Flavor Violating processes, such as τ → μγ, τ → 3μ, B → Kτ μ, and Bs → τ μ. Also μ → 3e, μ → eγ, and KL → μe decays could be not far from the present exclusion bounds, although this conclusion is more model dependent.

/pdf/low-energy-signatures-of-the-ps3-model-from-b-physics-3umfocr2tb.pdf

Low-energy signatures of the PS3 model: from B-physics anomalies to LFV

Motivated by the success of the flavour physics programme carried out over the last decade at the Large Hadron Collider (LHC), we characterize in detail the physics potential of its High-Luminosity and High-Energy upgrades in this domain of physics. We document the extraordinary breadth of the HL/HE-LHC programme enabled by a putative Upgrade II of the dedicated flavour physics experiment LHCb and the evolution of the established flavour physics role of the ATLAS and CMS general purpose experiments. We connect the dedicated flavour physics programme to studies of the top quark, Higgs boson, and direct high-$p_T$ searches for new particles and force carriers. We discuss the complementarity of their discovery potential for physics beyond the Standard Model, affirming the necessity to fully exploit the LHC's flavour physics potential throughout its upgrade eras.

Opportunities in Flavour Physics at the HL-LHC and HE-LHC

We present a precise calculation of the dilepton invariant-mass spectrum and the decay rate for $B^\pm \to \pi^\pm \ell^+ \ell^-$ ($\ell^\pm = e^\pm, \mu^\pm $) in the Standard Model (SM) based on the effective Hamiltonian approach for the $b \to d \ell^+ \ell^-$ transitions With the Wilson coefficients already known in the next-to-next-to-leading logarithmic (NNLL) accuracy, the remaining theoretical uncertainty in the short-distance contribution resides in the form factors $f_+ (q^2)$, $f_0 (q^2)$ and $f_T (q^2)$ Of these, $f_+ (q^2)$ is well measured in the charged-current semileptonic decays $B \to \pi \ell \nu_\ell$ and we use the $B$-factory data to parametrize it The corresponding form factors for the $B \to K$ transitions have been calculated in the Lattice-QCD approach for large-$q^2$ and extrapolated to the entire $q^2$-region using the so-called $z$-expansion Using an $SU(3)_F$-breaking Ansatz, we calculate the $B \to \pi$ tensor form factor, which is consistent with the recently reported lattice $B \to \pi$ analysis obtained at large~$q^2$ The prediction for the total branching fraction ${\cal B} (B^\pm \to \pi^\pm \mu^+ \mu^-) = (188 ^{+032}_{-021}) \times 10^{-8}$ is in good agreement with the experimental value obtained by the LHCb Collaboration In the low $q^2$-region, heavy-quark symmetry (HQS) relates the three form factors with each other Accounting for the leading-order symmetry-breaking effects, and using data from the charged-current process $B \to \pi \ell \nu_\ell$ to determine $f_+ (q^2)$, we calculate the dilepton invariant-mass distribution in the low $q^2$-region in the $B^\pm \to \pi^\pm \ell^+ \ell^-$ decay This provides a model-independent and precise calculation of the partial branching ratio for this decay

/pdf/precise-calculation-of-the-dilepton-invariant-mass-spectrum-4k5azsaab5.pdf

Precise Calculation of the Dilepton Invariant-Mass Spectrum and the Decay Rate in $B^\pm \to \pi^\pm \mu^+ \mu^-$ in the SM

We provide hadronic input for the B-meson semileptonic transitions to a light pseudoscalar meson at large recoil. The B
 s
 → K form factor calculated from QCD light-cone sum rule is updated, to be used for a |V
 ub
 | determination from the B
 s
 → Klν width. Furthermore, we calculate the hadronic input for the binned observables of B → πl
 +
 l
 − and B → Kl
 +
 l
 −. In addition to the form factors, the nonlocal hadronic matrix elements are obtained, combining QCD factorization and light-cone sum rules with hadronic dispersion relations. We emphasize that, due to nonlocal effects, the ratio of branching fractions of these decays is not sufficient for an accurate extraction of the |V
 td
 /V
 ts
 | ratio. Instead, we suggest to determine the Wolfenstein parameters A, ρ, η of the CKM matrix, combining the branching fractions of B → Kl
 +
 l
 − and B → πl
 +
 l
 − with the direct CP -asymmetry in the latter decay. We also obtain the hadronic matrix elements for a yet unexplored channel B
 s
 → Kl
 +
 l
 −.

/pdf/b-s-kln-l-and-b-s-p-k-l-l-decays-at-large-recoil-and-ckm-38tl1dk9pz.pdf

B s → Kℓν ℓ and B (s) → π(K)ℓ + ℓ − decays at large recoil and CKM matrix elements

In light of the recent LHCb observation of CP violation in the charm sector, we review standard model (SM) predictions in the charm sector and in particular for ΔACP. We get as an upper bound in the SM |ΔA
 CP
 SM
 | ≤ 3.6 × 10−4, which can be compared to the measurement of ΔA
 CP
 LHCb2019
 = (−15.4 ± 2.9) × 10−4. We discuss resolving this tension within an extension of the SM that includes a flavour violating Z′ that couples only to $$ \overline{s}s $$
 and $$ \overline{c}u $$
 . We show that for masses below 80 GeV and flavour violating coupling of the order of 10−4, this model can successfully resolve the tension and avoid constraints from dijet searches, D0 − $$ {\overline{D}}^0 $$
 mixing and measurements of the Z width.

/pdf/dacp-within-the-standard-model-and-beyond-13457wt2rh.pdf

ΔACP within the Standard Model and beyond

In light of the recent LHCb observation of CP violation in the charm sector, we review standard model (SM) predictions in the charm sector and in particular for $\Delta A_{CP}$. We get as an upper bound in the SM $| \Delta A_{CP} ^{\rm SM}| \leq 3.6 \times 10^{-4}$, which can be compared to the measurement of $\Delta A_{CP} ^{\rm LHCb2019} = (-15.4 \pm 2.9) \times 10^{-4}$. We discuss resolving this tension within an extension of the SM that includes a flavour violating $Z'$ that couples only to $\bar{s}s$ and $\bar{c}u$. We show that for masses below 80 GeV and flavour violating coupling of the order of $10^{-4}$, this model can successfully resolve the tension and avoid constraints from dijet searches, $D^0-\overline{D}^0$ mixing and measurements of the $Z$ width.

/pdf/delta-a-cp-within-the-standard-model-and-beyond-80nkhhr5zk.pdf

Aleksey V. Rusov

Papers

Precise Calculation of the Dilepton Invariant-Mass Spectrum and the Decay Rate in $B^\pm \to \pi^\pm \mu^+ \mu^-$ in the SM

B s → Kℓν ℓ and B (s) → π(K)ℓ + ℓ − decays at large recoil and CKM matrix elements

ΔACP within the Standard Model and beyond

$\Delta A_{CP}$ within the Standard Model and beyond.

Inclusive semitauonic B decays to order O(ΛQCD3/mb3)