Search for a feebly interacting particle X in the decay K+ → π+X

Axion-like particles (ALPs) are ubiquitous in models of new physics explaining some of the most pressing puzzles of the Standard Model. However, until relatively recently, little attention has been paid to its interplay with flavour. In this work, we study in detail the phenomenology of ALPs that exclusively interact with up-type quarks at the tree-level, which arise in some well-motivated ultra-violet completions such as QCD-like dark sectors or Froggatt-Nielsen type models of flavour. Our study is performed in the low-energy effective theory to highlight the key features of these scenarios in a model independent way. We derive all the existing constraints on these models and demonstrate how upcoming experiments at fixed-target facilities and the LHC can probe regions of the parameter space which are currently not excluded by cosmological and astrophysical bounds. We also emphasize how a future measurement of the currently unavailable meson decay D → π + invisible could complement these upcoming searches.

Section: Jhep08(2021)121mentioning

confidence: 99%

Charming ALPs

Carmona

Scherb

Schwaller

2021

“…Spin 0 messengers The decay K ± → π ± s i is a smoking gun process for ALP searches at kaon factories, see e.g. [81,84]. It can be especially relevant within the context of interpreting the recent KOTO excess [215].…”

Section: Meson Interactions Of Hidden Sector Modelsmentioning

confidence: 99%

“…The PET framework is independent from additional secluded particles that do not interact directly with the SM fields. meson decays in fixed target experiments such as NA62 [78][79][80][81][82][83][84], KOTO [85], SeaQuest [86], or SHiP [63]. If the messenger particles are unstable and decay predominantly into SM particles via the suppressed portal interactions, they are long-lived and can also be searched for in dedicated long-lived particle (LLP) experiments [87], such as MATHUSLA [88], FASER [89] and CODEXb [90].…”

Section: Introductionmentioning

confidence: 99%

Portal Effective Theories. A framework for the model independent description of light hidden sector interactions

Arina

Hajer

Klose

2021

We present a framework for the construction of portal effective theory (PETs) that couple effective field theories of the Standard Model (SM) to light hidden messenger fields. Using this framework we construct electroweak and strong scale PETs that couple the SM to messengers carrying spin zero, one half, or one. The electroweak scale PETs encompass all portal operators up to dimension five, while the strong scale PETs additionally contain all portal operators of dimension six and seven that contribute at leading order to quark-flavour violating transitions. Using the strong scale PETs, we define a set of portal currents that couple hidden sectors to QCD, and construct portal chiral perturbation theory (χPTs) that relate these currents to the light pseudoscalar mesons. We estimate the coefficients of the portal χPT Lagrangian that are not fixed by SM observations using non-perturbative matching techniques and give a complete list of the resulting one- and two-meson portal interactions. From those, we compute transition amplitudes for three golden channels that are used in hidden sector searches at fixed target experiments: i) charged kaon decay into a charged pion and a spin zero messenger, ii) charged kaon decay into a charged lepton and a spin one half messenger, and iii) neutral pion decay into a photon and a spin one messenger. Finally, we compare these amplitudes to specific expressions for models featuring light scalar particles, axion-like particles, heavy neutral leptons, and dark photons.

“…Combining the above equations, one can evaluate the decay rate as a function of the EFT parameters and, importantly, of the UV cutoff Λ. This can in principle be used to place bounds on f a using the experimental constraint Br(K + → π + a) < 7.3•10 −11 from the E787 and E949 experiments at BNL [116], which NA62 expects to improve by an order of magnitude by 2025 [117,118]. In practice, the reach of the constraint depends significantly on the choice of the value of Λ.…”

Section: Jhep07(2021)059mentioning

confidence: 99%

Leading logs in QCD axion effective field theory

Alonso-Álvarez

Ertas

Jaeckel

et al. 2021

The axion is much lighter than all other degrees of freedom introduced by the Peccei-Quinn mechanism to solve the strong CP problem. It is therefore natural to use an effective field theory (EFT) to describe its interactions. Loop processes calculated in the EFT may however explicitly depend on the ultraviolet cutoff. In general, the UV cutoff is not uniquely defined, but the dimensionful couplings suggest to identify it with the Peccei-Quinn symmetry-breaking scale. An example are K+ → π+ + a decays that will soon be tested to improved precision in NA62 and KOTO and whose amplitude is dominated by the term logarithmically dependent on the cutoff. In this paper, we critically examine the adequacy of using such a naive EFT approach to study loop processes by comparing EFT calculations with ones performed in complete QCD axion models. In DFSZ models, for example, the cutoff is found to be set by additional Higgs degrees of freedom and to therefore be much closer to the electroweak scale than to the Peccei-Quinn scale. In fact, there are non-trivial requirements on axion models where the cutoff scale of loop processes is close to the Peccei-Quinn scale, such that the naive EFT result is reproduced. This suggests that the existence of a suitable UV embedding may impose restrictions on axion EFTs. We provide an explicit construction of a model with suitable fermion couplings and find promising prospects for NA62 and IAXO.