arXiv : Probing flavoured Axions in the Tail of $B_q \to \mu^+\mu^-$

Albrecht, Johannes; Ziegler, Robert; Zwicky, Roman; Stamou, Emmanuel

doi:10.1007/JHEP09(2021)139

Article
Report number	arXiv:1911.05018 ; CP3-Origins-2019-38 DNRF90 ; TTP19-036
Title	Probing flavoured Axions in the Tail of $B_q \to \mu^+\mu^-$
Author(s)	Albrecht, Johannes (Dortmund U.) ; Stamou, Emmanuel (Chicago U., EFI ; ITPP, Lausanne) ; Ziegler, Robert (CERN ; KIT, Karlsruhe, TTP) ; Zwicky, Roman (U. Edinburgh, Higgs Ctr. Theor. Phys.)
Publication	2021-09-22
Imprint	2019-11-12
Number of pages	37
Note	37 pages, 6 figures, version to appear in JHEP, added multiple subtracted dispersion relation to extend in resonance region including ancillary Mathematica notebook with Form Factors
In:	JHEP 2021 (2020) 139
DOI	10.1007/JHEP09(2021)139 (publication)
Subject category	hep-ph ; Particle Physics - Phenomenology
Abstract	We discuss how LHC di-muon data collected to study $B_q \to \mu\mu$ 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 $B_q \to \mu \mu a$ and the SM background process $B_q \to \mu \mu \gamma$ near the kinematic endpoint. These rates depend on non-perturbative $B_q \to \gamma^{(*)}$ 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. With these results, we analyse available LHCb data to obtain the sensitivity on $B_q \to \mu \mu a$ at present and future runs. We find that the full LHCb dataset alone will allow to probe axion-coupling scales of the order of $10^6$ GeV for both $b\to d$ and $b \to s$ transitions.
Copyright/License	publication: © 2021-2025 The Authors (License: CC BY 4.0), sponsored by SCOAP³ preprint: (Licenses: arXiv nonexclusive-distrib 1.0, CC BY 4.0)

$The diagram to the left is the main axion process $B_q \to \ell \ell a$ whereas the two diagrams in the centre and the right belong to the $B_q \to \ell \ell \ga$ background. The single and double lines stand for the $q$ and $b$-quark, respectively. The left and central diagrams depend on off-shell form factors in the sense that the photon that emits the two leptons is off-shell. Diagrams in which the photon couples to b-quarks are not shown, but are analogous. Also diagrams with $Q_{9,10}$-operator insertions are not shown, and resemble the diagram on the right and are proportional to $C_9 \FV$ and $C_{10} \FA$.$ $Comparison of the axionic decay mode $B_q\to \mu\mu a$ (red solid lines) and the radiative $B_q\to\mu\mu\gamma$ modes (black lines). The left panel shows the $B_s$ case while the right the $B_d$ case. For the axion predictions $F_{bq}^A=10^6$~GeV is assumed as a reference value. The different black lines are the photon predictions with different form factor treatments (see legend and main text). In green are bins of the two-body $B_q\to\mu\mu$ rate including radiation from final-state muons. To better compare the $B_s$ and $B_d$ cases, all rates are normalised to their respective two-body decay $B_q\to\mu\mu$, which is why the $B_d\to \mu\mu a$ line appears enhanced with respect to the $B_s\to \mu\mu a$ one.$ $Projected sensitivity of LHCb to probe the flavour-violating axion couplings $F^A_{bs}$ (filled red region) and $F^A_{bd}$ (hatched region) as a function of the total integrated luminosity. Shown are the $90\%$ CL exclusion limits assuming that the observed number of events will be the same as predicted in the SM hypothesis.$ Show more plots

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