Abstract
We discuss how LHC di-muon data collected to study Bq → μμ can be used to constrain light particles with flavour-violating couplings to b-quarks. Focussing on the case of a flavoured QCD axion, a, we compute the decay rates for Bq → μμa and the SM background process Bq → μμγ near the kinematic endpoint. These rates depend on non-perturbative Bq → γ(*) form factors with on- or off-shell photons. The off-shell form factors — relevant for generic searches for beyond-the-SM particles — are discussed in full generality and computed with QCD sum rules for the first time. This includes an extension to the low-lying resonance region using a multiple subtracted dispersion relation. With these results, we analyse available LHCb data to obtain the sensitivity on Bq → μμa at present and future runs. We find that the full LHCb dataset alone will allow to probe axion-coupling scales of the order of 106 GeV for both b → d and b → s transitions. As a spin-off application of the off-shell form factors we further analyse the case of light, Beyond the Standard Model, vectors.
Highlights
Introduction and motivationOpen questions in particle physics and cosmology may well be addressed by very light particles that interact only feebly with the Standard Model (SM)
Focussing on the case of a flavoured QCD axion, a, we compute the decay rates for Bq → μμa and the SM background process Bq → μμγ near the kinematic endpoint. These rates depend on non-perturbative Bq → γ(∗) form factors with on- or off-shell photons
We describe Bq(pB) → γ∗(k) transitions with off-shell photons by a set of form factors with two arguments F ∗(q2, k2) ≡ F B→γ∗(q2, k2)
Summary
Open questions in particle physics and cosmology may well be addressed by very light particles that interact only feebly with the Standard Model (SM). We point out that the same datasets can be used to constrain the decays Bq → μμX, where X is a neutral, beyond-the-SM (BSM) particle with a mass that is sufficiently small to be kinematically allowed at the tail of Bq → μμ, i.e., mX mBq − 5 GeV ≈ 300 MeV In this respect, the radiative decay Bq → μμγ merely constitutes a SM background, which we take into account in our analysis. In the following we focus on the case of the invisible QCD axion, a, but our analysis can be readily extended to other particles appearing in the final state, as long as they are not vetoed in the event In particular these could be heavy axions decaying within the detector, i.e., axion-like particles (ALPs). In appendix B we provide the differential rate for B → γ∗(→ + −)V and apply the proposed analysis
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