Searching for Axionlike Particles in Flavor-Changing Neutral Current Processes

Abstract

We propose new searches for axion-like particles (ALPs) produced in flavor-changing neutral current (FCNC) processes. This proposal exploits the often-overlooked coupling of ALPs to W ± bosons, leading to FCNC production of ALPs even in the absence of a direct coupling to fermions. Our proposed searches for resonant ALP production in decays such as B → K ( * ) a, a → γγ and K → πa, a → γγ could greatly improve upon the current sensitivity to ALP couplings to Standard Model particles. We also determine analogous constraints and discovery prospects for invisibly decaying ALPs.Introduction: Axion-like particles (ALPs) are among the best-motivated candidates for particle extensions of the Standard Model (SM). ALPs arise as the Goldstone bosons of any theory with a Peccei-Quinn symmetry (PQ) [1][2][3], which is a spontaneously broken global symmetry that is anomalous with respect to the SM gauge interactions. PQ symmetries, and hence ALPs, are ubiquitous in theories beyond the SM such as string theory [4][5][6][7] and supersymmetry [8][9][10]. ALPs were originally motivated by dynamical solutions to the strong-CP problem [1-3], although recent proposals suggest that ALP dynamics could also resolve the hierarchy problem [11].Because ALPs are pseudo-Goldstone bosons, their properties are more constrained than arbitrary scalar fields. In particular, the shift invariance associated with the anomalous symmetry protects ALP masses from radiative corrections and so their masses can naturally take on any value. Furthermore, their interactions with SM fields arise through higher-dimensional (or otherwise suppressed) operators, and these feeble couplings naturally situate ALPs within models of hidden sectors, either as the dark matter candidate itself [12][13][14] or as a mediator through the "axion portal" [15]. This has driven a comprehensive laboratory search program ranging from high-energy collider physics to condensed matter systems (see for example Refs. ).In the simplest models, ALPs couple to the SM gauge boson field strengths according to the anomaly of the corresponding PQ symmetry,

Majoron at two loops

“…They only decouple as log(M R )/f , whereas all other couplings decouple at least as 1/f . It was noted before that Goldstone bosons with effective diagonal couplings Jm qq iγ 5 q yield flavorchanging quark couplings at one loop that depend logarithmically on the UV scale [63,64], whereas an initial coupling JW µνW µν does not have such a dependence [58]. In our case JW µνW µν gives only a seesawsuppressed contribution to Jqq , and the log(M R ) terms originate from an effective coupling to Goldstone bosons, JG + G − .…”

“…At the two-loop level we find off-diagonal Majoron couplings to quarks, which can lead, for example, to s → dJ or K → πJ at hadron level. Such flavor-changing couplings have long been advocated to search for light bosons and axions [56,57] and have enjoyed increased attention in recent years [58][59][60][61], partly because of an improved reach at existing and upcoming experiments such as NA62 and Belle II.…”

“…[61], based on Refs. [58,79,80]). These decays probe the quantity tr(M D log(M R /m W )M † D ), but to simplify comparison with the other limits we set log(M R /m W ) = 1 1 to obtain a limit on tr(K).…”

The minimal flavour violating axion

“…In other analyses [149,150,155], meson-to-axion decays, where the latter subsequently decays into two photons, are also considered: these observables are however inconclusive for the MFV axion model because the constraints on aγγ coupling are so strong that prevent any present or future signal at experiments for this class of observables. The two PQ charge assignment scenarios lead to distinct results when considering these observables.…”

“…As the axion does not couple to up-type quarks (c au = 0), the two decays K + → π + + a and B + → K + + a can only occur at 1-loop level with the axion arising from the interaction with the internal W propagator, as for the S1 scenario. The constraints that can be deduced on g aW W read as [149]:…”