Physics of leptoquarks in precision experiments and at particle colliders

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

In the Standard Model (SM), the rare transitions where a bottom quark decays into a strange quark and a pair of light leptons exhibit a potential sensitivity to physics beyond the SM. In addition, the SM embeds Lepton Flavour Universality (LFU), leading to almost identical probabilities for muon and electron modes. The LHCb collaboration discovered a set of deviations from the SM expectations in decays to muons and also in ratios assessing LFU. Other experiments (Belle, ATLAS, CMS) found consistent measurements, albeit with large error bars. We perform a global fit to all available b → sl+l− data (l = e, μ) in a model-independent way allowing for different patterns of New Physics. For the first time, the NP hypothesis is preferred over the SM by 5 σ in a general case when NP can enter SM-like operators and their chirally-flipped partners. LFU violation is favoured with respect to LFU at the 3-4 σ level. We discuss the impact of LFU-violating New Physics on the observable P
 5
 ′
 from B → K∗μ+μ− and we compare our estimate for long-distance charm contributions with an empirical model recently proposed by a group of LHCb experimentalists. Finally, we discuss NP models able to describe this consistent pattern of deviations.

/pdf/patterns-of-new-physics-in-b-sl-l-transitions-in-the-light-rh65dv2zb6.pdf

Patterns of New Physics in b → sℓ+ℓ− transitions in the light of recent data

/pdf/parton-distributions-for-the-lhc-run-ii-2mw9ugeki8.pdf

Parton Distributions for the LHC Run II

The LHCb measurement of the $\mu/e$ ratio $R_{K^*}$ indicates a deficit with respect to the Standard Model prediction, supporting earlier hints of lepton universality violation observed in the $R_K$ ratio. We show that the $R_K$ and $R_{K^*}$ ratios alone constrain the chiralities of the states contributing to these anomalies, and we find deviations from the Standard Model at the $4\sigma$ level. This conclusion is further corroborated by hints in the theoretically challenging $b\to s\mu^+\mu^-$ distributions. Theoretical interpretations in terms of $Z'$, lepto-quarks, loop mediators, and composite dynamics are discussed. We highlight their distinctive features in terms of chiralities and flavour structure relevant for the observed anomalies.

Flavour anomalies after the $R_{K^*}$ measurement.

We attempt to explain recent anomalies in semileptonic B decays at LHCb via a composite Higgs model, in which both the Higgs and an SU(2)
 L
 -triplet leptoquark arise as pseudo-Goldstone bosons of the strong dynamics Fermion masses are assumed to be generated via the mechanism of partial compositeness, which largely determines the leptoquark couplings and implies non-universal lepton interactions The latter are needed to accommodate tensions in the b → sμμ dataset and to be consistent with a discrepancy measured at LHCb in the ratio of B
 + → K
 +
 μ
 +
 μ
 − to B
 + → K
 +
 e
 +
 e
 − branching ratios The data imply that the leptoquark should have a mass of around a TeV We find that the model is not in conflict with current flavour or direct production bounds, but we identify a few observables for which the new physics contributions are close to current limits and where the leptoquark is likely to show up in future measurements The leptoquark will be pair-produced at the LHC and decay predominantly to third-generation quarks and leptons, and LHC13 searches will provide further strong bounds

/pdf/composite-leptoquarks-and-anomalies-in-b-meson-decays-q2q62zpk16.pdf

Composite leptoquarks and anomalies in B -meson decays

The LHCb collaboration has recently presented their result on R
 K
 = ℬ(B
 + → K
 +
 μ
 +
 μ
 −)/ ℬ(B
 + → K
 +
 e
 +
 e
 −) for the dilepton invariant mass bin m
 2
 = 1 − 6 GeV2 (l = μ, e). The measurement shows an intriguing 2.6σ deviation from the Standard Model (SM) prediction. In view of this, we study model independent New Physics (NP) explanations of R
 K
 consistent with other measurements involving b → sl
 +
 l
 −
 transition, relaxing the assumption of lepton universality. We perform a Bayesian statistical fit to the NP Wilson Coefficients and compare the Bayes Factors of the different hypotheses in order to quantify their goodness-of-fit. We show that the data slightly favours NP in the muon sector over NP in the electron sector.

https://link.springer.com/content/pdf/10.1007%2FJHEP12%282014%29131.pdf

Hint of lepton flavour non-universality in B meson decays

We attempt to explain recent anomalies in semileptonic $B$ decays at LHCb via a composite Higgs model, in which both the Higgs and an $SU(2)_L$-triplet leptoquark arise as pseudo-Goldstone bosons of the strong dynamics. Fermion masses are assumed to be generated via the mechanism of partial compositeness, which largely determines the leptoquark couplings and implies non-universal lepton interactions. The latter are needed to accommodate tensions in the $b \to s \mu \mu$ dataset and to be consistent with a discrepancy measured at LHCb in the ratio of $B^+ \to K^+ \mu^+ \mu^-$ to $B^+ \to K^+ e^+ e^-$ branching ratios. The data imply that the leptoquark should have a mass of around a TeV. We find that the model is not in conflict with current flavour or direct production bounds, but we identify a few observables for which the new physics contributions are close to current limits and where the leptoquark is likely to show up in future measurements. The leptoquark will be pair-produced at the LHC and decay predominantly to third-generation quarks and leptons, and LHC13 searches will provide further strong bounds.

/pdf/composite-leptoquarks-and-anomalies-in-b-meson-decays-2bm8l96n6r.pdf

Composite leptoquarks and anomalies in $B$-meson decays

Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model, which interact with the known particles through higher-dimensional operators suppressed by the mass scale Λ of the new-physics sector. Starting from the most general dimension-5 interactions, we discuss in detail the evolution of the ALP couplings from the new-physics scale to energies at and below the scale of electroweak symmetry breaking. We derive the relevant anomalous dimensions at two-loop order in gauge couplings and one-loop order in Yukawa interactions, carefully considering the treatment of a redundant operator involving an ALP coupling to the Higgs current. We account for one-loop (and partially two-loop) matching contributions at the weak scale, including in particular flavor-changing effects. The relations between different equivalent forms of the effective Lagrangian are discussed in detail. We also construct the effective chiral Lagrangian for an ALP interacting with photons and light pseudoscalar mesons, pointing out important differences with the corresponding Lagrangian for the QCD axion.

/pdf/the-low-energy-effective-theory-of-axions-and-alps-3zitv9vi7f.pdf

The low-energy effective theory of axions and ALPs

We propose a mechanism that allows for sizeable flavour violation in quark-lepton currents, while suppressing flavour changing neutral currents in quark-quark and lepton-lepton sectors. The mechanism is applied to the recently proposed “4321” renormalizable model, which can accommodate the current experimental anomalies in B-meson decays, both in charged and neutral currents, while remaining consistent with all other indirect flavour and electroweak precision measurements and direct searches at high-pT. To support this claim, we present an exhaustive phenomenological survey of this fully calculable UV complete model and highlight the rich complementarity between indirect and direct searches.

/pdf/maximal-flavour-violation-a-cabibbo-mechanism-for-ytk1ak0vct.pdf

Sophie Renner

Papers

Composite leptoquarks and anomalies in B -meson decays

Hint of lepton flavour non-universality in B meson decays

Composite leptoquarks and anomalies in $B$-meson decays

The low-energy effective theory of axions and ALPs

Maximal flavour violation: a Cabibbo mechanism for leptoquarks