We assess the status of past and future experiments on lepton flavor violating (LFV) muon and tau decays into a light, invisible, axion-like particle (ALP), $a$. We propose a new experimental setup for MEG II, the MEGII-fwd, with a forward calorimeter placed downstream from the muon stopping target. Searching for $μ\to e a$ decays MEGII-fwd is maximally sensitive to LFV ALPs, if these have nonzero couplings to right-handed leptons. The experimental set-up suppresses the (left-handed) Standard Model background in the forward direction by controlling the polarization purity of the muon beam. The reach of MEGII-fwd is compared with the present constraints, the reach of Mu3e and the Belle-II reach from $τ\to \ell a$ decays. We show that a dedicated experimental campaign for LFV muon decays into ALPs at MEG II and Mu3e will be able to probe the ALP parameter space in an unexplored region well beyond the existing astrophysical constraints. We study the implications of these searches for representative LFV ALP models, where the presence of a light ALP is motivated by neutrino masses, the strong CP problem and/or the SM flavor puzzle. To this extent we discuss the majoron in low-scale seesaw setups and introduce the LFV QCD axion, the LFV axiflavon and the leptonic familon, paying particular attention to the cases where the LFV ALPs constitute cold dark matter.
Axion-Like Particles (ALPs) are pseudo Nambu-Goldstone bosons associated with spontaneously broken global $U(1)$ symmetries. Such particles can have lepton-flavor-violating (LFV) couplings to the SM charged leptons. LFV ALPs provide the possibility to address some of the SM long-lasting problems. We investigate the sensitivity of a future muon collider suggested by the Muon Accelerator Program (MAP) to the production of LFV ALPs in the ALP mass range $m_a\leq1$ MeV. ALPs are assumed to be produced through the LFV decay $τ\ra\ell a$ ($\ell=e,μ$) of one of the tau leptons produced in the muon-an…
We examine the low-energy signatures of axion-like particles (ALPs) in lepton flavor violating (LFV) processes. By using a dimension-5 effective Lagrangian, we compute the most general ALP contributions to LFV decays of leptons and mesons. The provided expressions are valid for any choice of ALP mass and couplings. We explore the complementarity of different processes, identifying specific patterns to be experimentally tested. Constraints on LFV couplings are derived from existing data and prospects for forthcoming experiments are also discussed. As a by-product, we revisit the possibility of …
The impact of axion-like particles (ALPs) on the search of permanent electric dipole moments (EDMs) of molecules, atoms, nuclei and nucleons is systematically investigated. We classify first the full set of CP-violating Jarlskog invariants emerging in the ALP effective field theory (EFT) containing operators up to dimension-5. Then, we evaluate the leading short-distance effects to the EDMs up to two-loop order. The high sensitivity of EDMs to CP-violating ALP interactions is emphasised exploiting both the current and projected experimental sensitivities.
Generation of Axion-Like Couplings via Quantum Corrections in a Lorentz Violating Background
轻赝标量，或称类轴子粒子（axion like particles，ALPs），由于其与粒子物理、天体物理和宇宙学领域的潜在相关性而被广泛研究。从目前的实验研究的角度来看，最相关的耦合的ALP是光子：在这项工作中，我们研究了这种耦合的产生作为量子校正的影响，起源于一个潜在的洛伦兹违反背景。最有趣的是，我们表明，这样产生的相互作用原来是洛伦兹不变的，从而模仿标准的ALP耦合到光子，在实验中考虑。这种考虑意味着，时空对称性的破坏，作为非常高能量尺度下物理学的可能结果而被研究得很多，可能会以意想不到的方式渗透到物理学的其他领域。此外，我们推测，类似的机制也可以产生洛伦兹不变耦合涉及标量粒子和光子，发挥了可能的作用，在希格斯玻色子的现象。
Axion-like particles (ALPs) with lepton-flavor violating couplings can be probed in exotic muon and tau decays. The sensitivity of different experiments depends strongly on the ALP mass and its couplings to leptons and photons. For ALPs that can be resonantly produced, the sensitivity of three-body decays such as $μ\to 3e$ and $τ\to 3μ$ exceeds by many orders of magnitude that of radiative decays like $μ\to eγ$ and $τ\toμγ$. Searches for these two types of processes are therefore highly complementary. We discuss experimental constraints on ALPs with a single dominant lepton-flavor violating co…
Axion-like particles (ALPs) interacting with the Standard Model can be abundantly produced in proton beam fixed-target experiments. Looking for their displaced decays is therefore an effective search strategy for ALPs with a mass in the MeV to GeV range. Focusing on the benchmark models where the ALP interacts dominantly with photons or gluons, we show that the proposed DarkQuest experiment at Fermilab will be able to test parameter space which has been previously inaccessible. We pay particular attention to the self-consistency of gluon-coupled ALP production and decay calculations, which has…
We propose a search for long lived axion-like particles (ALPs) in exotic top decays. Flavour-violating ALPs appear as low energy effective theories for various new physics scenarios such as t-channel dark sectors or Froggatt-Nielsen models. In this case the top quark may decay to an ALP and an up- or charm-quark. For masses in the few GeV range, the ALP is long lived across most of the viable parameter space, suggesting a dedicated search. We propose to search for these long lived ALPs in $t\bar{t}$ events, using one top quark as a trigger. We focus on ALPs decaying in the hadronic calorimeter…
Probing CP and flavor violation in neutral kaon decays with ALPs
University of California Santa Cruz（加州大学圣克鲁兹分校）；Santa Cruz Institute for Particle Physics（圣克鲁兹粒子物理研究所）；University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）
A prominent aspect of the hunt for cold dark matter is looking for light scalar candidates such as axion-like particles (ALPs). The coupling between ALPs and photons allows for the spontaneous decay of ALPs into pairs of photons. It has been previously shown that stimulated ALP decay rates can become significant on cosmic time scales. Furthermore, it has been claimed, in several recent works, that ALPs can gravitationally thermalize and form macroscopic condensates. Consequently, the photon occupation number of ambient populations (like the cosmic microwave background) can receive Bose enhance…
We determine limits from SN 1987A on massive axion-like particles (ALPs) with masses in the 10 keV - 100 MeV range and purely coupled to two photons. ALPs produced in the core collapse escape from the star and decay into photons that can be observed as a delayed and diffuse burst. We discuss the time and angular distribution of such a signal. Looking into the future we also estimate the possible improvements if the red supergiant Betelgeuse explodes in a supernova event.
The origin of the matter-antimatter asymmetry apparently obligates the laws of physics to include some mechanism of baryon number ($\mathcal{B}$) violation. Searches for interactions violating $\mathcal{B}$ and baryon-minus-lepton number $\mathcal{(B-L)}$ represent a rich and underutilized opportunity. These are complementary to the existing, broad program of searches for $\mathcal{L}$-violating modes such as neutrinoless double $β$-decay which could provide deeper understandings of the plausibility of leptogenesis, or $\mathcal{B}$-violating, $\mathcal{(B-L)}$-conserving processes such as pro…
ALP production in Lepton Flavour Violating meson, tau and gauge boson decays
Instituto de Física Corpuscular, Universidad de Valencia and CSIC（瓦伦西亚大学与西班牙国家研究委员会联合粒子物理研究所）；School of Physics, Nankai University（南开大学物理学院）；PRISMA ++ Cluster of Excellence & Mainz Institute for Theoretical Physics Johannes Gutenberg University（约翰内斯·古腾堡美因茨大学美因茨理论物理研究所）；Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati（意大利国家核物理研究院弗拉斯卡蒂国家实验室）
Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model (SM). We investigate the phenomenology of an ALP with flavor-changing couplings, and present a comprehensive analysis of quark and lepton flavor-changing observables within a general ALP effective field theory. Observables studied include rare meson decays, flavor oscillations of neutral mesons, rare lepton decays, and dipole moments. We derive bounds on the general ALP couplings as a function of its mass, consistently taking into account the ALP lifetime and branching ra…
We present a comprehensive discussion of the phenomenology of flavourful axions, including both standard Peccei-Quinn (PQ) axions, associated with the solution to the strong $CP$ problem, and non-standard axion-like particles (ALPs). We give the flavourful axion-fermion and axion-photon couplings and calculate the branching ratios of heavy meson ($K$, $D$, $B$) decays involving a flavourful axion. We also calculate the mixing between axions and heavy mesons $ K^0 $, $ D^0 $, $ B^0 $ and $ B_s^0 $, which affects the meson oscillation probability and mass difference. Mixing also contributes to m…
Experiments looking for a lepton flavor-violating decay $μ^{+}\!\!\rightarrow \!e^{+} X^{0}$ are reviewed in light of present-day germanium detector technology, with an eye on scenarios where a long-lived, slow-moving massive boson $X^{0}$ might have a cosmological impact. A broad swath of interesting, unexplored parameter space very close to the kinematic limit of the decay is found to be within the reach of a new proposed search. A number of possible roles for $X^{0}$ in past and present epochs can be investigated.
In this report we discuss the main theories to understand the origin of baryon and lepton number violation in physics beyond the Standard Model. We present the theoretical predictions for rare processes such as neutrinoless double beta decay, proton decay, and neutron-antineutron oscillation, and overview the prospects to discover these rare processes in the near future. The possibility to observe baryon and lepton violating signatures at current and future colliders and through precision studies of other rare processes, and the testability of different baryogenesis mechanisms is discussed in …
