Search

The searches for CP violating effects in diatomic molecules, such as $\text{HfF}^+$ and ThO, are typically interpreted as a probe of the electron's electric dipole moment ($e\text{EDM}$), a new electron-nucleon interaction, and a new electron-electron interaction. However, in the case of a nonvanishing nuclear spin, a new CP violating nucleon-nucleon long range force will also affect the measurement. Here, we use the $\text{HfF}^+$ $e\text{EDM}$ search and derive a new bound on this hypothetical interaction, which is the most stringent from terrestrial experiments in the 1 eV-10 keV mass range. These multiple new physics sources motivate independent searches in different molecular species for CP violation at low energy that result in model independent bounds, which are insensitive to cancellation among them., Comment: 9 pages, 4 figures; v2: updated bound, added citation, conclusions unchanged, matched to published version

The Electron-Ion Collider~(EIC), a forthcoming powerful high-luminosity facility, represents an exciting opportunity to explore new physics. In this article, we study the potential of the EIC to probe the coupling between axion-like particles~(ALPs) and photons in coherent scattering. The ALPs can be produced via photon fusion and decay back to two photons inside the EIC detector. In a prompt-decay search, we find that the EIC can set the most stringent bound for $m_a \lesssim 20\,\GeV$ and probe the effective scales $\Lambda \lesssim 10^{5}\,$GeV. In a displaced-vertex search, which requires adopting an EM calorimeter technology that provides directionality, the EIC could probe ALPs with $m_a \lesssim 1\,\GeV$ at effective scales $\Lambda \lesssim 10^{7}\,\GeV$. Combining the two search strategies, the EIC can probe a significant portion of unexplored parameter space in the $0.2 < m_a <20\,\GeV$ mass range., Comment: 23 pages, 9 figures

Axion-like-particles are a well-motivated extension of the Standard Model that can mediate interactions between the dark matter and ordinary matter. Here we consider an axion portal between the two sectors, where the axion couples to dark matter and to QCD gluons. We establish the relevant processes of interest across the scales of dark matter and axion masses and couplings, identify the distinct mechanisms that control the dark matter relic abundance in each case, and extract the resulting experimental signatures of the gluonic axion portal to dark matter., Comment: 16 pages, two columns, 5 figures

Only two types of Standard Model particles are able to propagate the $480\,$meters separating the ATLAS interaction point and FASER: neutrinos and muons. Furthermore, muons are copiously produced in proton collisions. We propose to use FASER$\nu$ as a muon fixed target experiment in order to search for new bosonic degrees of freedom coupled predominantly to muons. These muon force carriers are particularly interesting in light of the recent measurement of the muon anomalous magnetic moment. Using a novel analysis technique, we show that even in the current LHC run, FASER$\nu$ could potentially probe previously unexplored parts of the parameter space. In the high-luminosity phase of the LHC, we find that the improved sensitivity of FASER$\nu2$ will probe unexplored parameter space and may be competitive with dedicated search proposals., Comment: 13 pages + appendices, 11 figures

While the main goal of the J-PARC KOTO experiment is to measure the rare decay $K_L \to \pi^0 \nu \bar \nu$, the unique setup of KOTO raises the possibility to search for physics beyond the Standard Model, in an attempt to probe parts of the parameter space which are not covered by other experiments. In this paper, we test the possibility of using KOTO to search for heavy QCD axions, or axion-like particles, a well-motivated extension of the Standard Model emerging in a variety of models. In particular, we estimate the sensitivity of the current KOTO setup as well as the KOTO Step-2 for various benchmark scenarios of axion coupling to the Standard Model. We find that KOTO Step-2 can probe new regions in the parameter space, while KOTO with its current form can only reaffirm the existing bounds. The obtained axion datasets are available as an update of the public code of the ALPINIST framework, including implementation of KOTO setups in the simulation, allowing for interpretation of various analyses as searches for axions in custom models., Comment: 9 pages + appendices, 6 figures, ALPINIST code available at https://github.com/jjerhot/ALPINIST/ v2: fix bug in the production from secondary mesons, main conclusions are unchanged

Dark matter (DM) in the Milky Way halo may annihilate or decay to photons, producing monochromatic gamma rays. We search for DM-induced spectral lines using 14 years of data from the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope ($\textit{Fermi}$-LAT) between $10\,\mathrm{GeV}$ and $2\,\mathrm{TeV}$ in the inner Milky Way leveraging both the spatial and spectral morphology of an expected signal. We present new constraints as strong as $\langle \sigma v \rangle \lesssim 6\times 10^{-30}\, \mathrm{cm}^3/\mathrm{s}$ for the two-to-two annihilations and $\tau \gtrsim 10^{30}\,\mathrm{s}$ for one-to-two decays, representing leading sensitivity between $10\,\mathrm{GeV}$ and $\sim$$500\,\mathrm{GeV}$. We consider the implications of our line-constraints on the Galactic Center Excess (GCE), which is a previously-observed excess of continuum $\sim$GeV gamma-rays that may be explained by DM annihilation. The Higgs portal and neutralino-like DM scenarios, which have been extensively discussed as possible origins of the GCE, are constrained by our work because of the lack of observed one-loop decays to two photons. More generally, we interpret our null results in a variety of annihilating and decaying DM models, such as neutralinos, gravitinos, and glueballs, showing that in many cases the line search is more powerful than the continuum, despite the continuum annihilation being at tree level., Comment: 29 pages, 37 figures, video abstract at https://youtu.be/MWy5zHtu18A

Fundamental physical constants are determined from a collection of precision measurements of elementary particles, atoms and molecules. This is usually done under the assumption of the Standard Model~(SM) of particle physics. Allowing for light new physics~(NP) beyond the SM modifies the extraction of fundamental physical constants. Consequently, setting NP bounds using these data, and at the same time assuming the CODATA recommended values for the fundamental physical constants, is not reliable. As we show in this Letter, both SM and NP parameters can be simultaneously determined in a consistent way from a global fit. For light vectors with QED-like couplings, such as the dark photon, we provide a prescription that recovers the degeneracy with the photon in the massless limit, and requires calculations only at leading order in the small new physics couplings. At present, the data show tensions partially related to the proton charge radius determination. We show that these can be alleviated by including contributions from a light scalar with flavor non-universal couplings., Comment: 8+16 pages, 3+7 figures, 5 tables

The quantum chromodynamics axion with a decay constant near the Grand Unification (GUT) scale has an ultralight mass near a neV. We show, however, that axion-like particles with masses near the keV - PeV range with GUT-scale decay constants are also well motivated in that they naturally arise from axiverse theories with dark non-abelian gauge groups. We demonstrate that the correct dark matter abundance may be achieved by the heavy axions in these models through the misalignment mechanism in combination with a period of early matter domination from the long-lived dark glueballs of the same gauge group. Heavy axion dark matter may decay to two photons, yielding mono-energetic electromagnetic signatures that may be detectable by current or next-generation space-based telescopes. We project the sensitivity of next-generation telescopes including $\textit {Athena,}$ AMEGO, and e-ASTROGAM to such decaying axion dark matter. If the dark sector contains multiple confining gauge groups, then the observed primordial baryon asymmetry may also be achieved in this scenario through spontaneous baryogenesis. We present explicit orbifold constructions where the dark gauge groups unify with the SM at the GUT scale and axions emerge as the fifth components of dark gauge fields with bulk Chern-Simons terms., Comment: 52 pages, 6 figures

In this work, we explore new spin-1 states with axial couplings to the standard model fermions. We develop a data-driven method to estimate their hadronic decay rates based on data from $\tau$ decays and using SU(3)$_{\rm flavor}$ symmetry. We derive the current and future experimental constraints for several benchmark models. Our framework is generic and can be used for models with arbitrary vectorial and axial couplings to quarks. We have made our calculations publicly available by incorporating them into the DarkCast package, see https://gitlab.com/darkcast/releases., Comment: 26 pages, 10 figures, 3 tables

We investigate the phenomenological implications of the recent $W$ mass measurement by the CDF collaboration, which exhibits tension with the standard model (SM) electroweak fit. Performing the fit to the electroweak observables within the SM effective field theory, we find that the new physics that contributes either to the determination of the electroweak vacuum expectation value, or to the oblique parameters, can improve the agreement with data. The best description is obtained from a fit where flavor universality is not required in the new physics operators, with 2 to 3 $\sigma$ indications for several nonzero Wilson coefficients. We point out that top partners with order TeV masses could lead to the observed shift in the $W$ mass., Comment: 18 pages, 4 figures, 1 Table, version matching the published one

The International Linear Collider (ILC) is on the table now as a new global energy-frontier accelerator laboratory taking data in the 2030s. The ILC addresses key questions for our current understanding of particle physics. It is based on a proven accelerator technology. Its experiments will challenge the Standard Model of particle physics and will provide a new window to look beyond it. This document brings the story of the ILC up to date, emphasizing its strong physics motivation, its readiness for construction, and the opportunity it presents to the US and the global particle physics community., Comment: 356 pages, Large pdf file (40 MB) submitted to Snowmass 2021; v2 references to Snowmass contributions added, additional authors; v3 references added, some updates, additional authors

High energy collisions at the High-Luminosity Large Hadron Collider (LHC) produce a large number of particles along the beam collision axis, outside of the acceptance of existing LHC experiments. The proposed Forward Physics Facility (FPF), to be located several hundred meters from the ATLAS interaction point and shielded by concrete and rock, will host a suite of experiments to probe Standard Model (SM) processes and search for physics beyond the Standard Model (BSM). In this report, we review the status of the civil engineering plans and the experiments to explore the diverse physics signals that can be uniquely probed in the forward region. FPF experiments will be sensitive to a broad range of BSM physics through searches for new particle scattering or decay signatures and deviations from SM expectations in high statistics analyses with TeV neutrinos in this low-background environment. High statistics neutrino detection will also provide valuable data for fundamental topics in perturbative and non-perturbative QCD and in weak interactions. Experiments at the FPF will enable synergies between forward particle production at the LHC and astroparticle physics to be exploited. We report here on these physics topics, on infrastructure, detector, and simulation studies, and on future directions to realize the FPF's physics potential., Comment: 429 pages, contribution to Snowmass 2021

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors., Comment: 108 pages, 25 figures, 9 tables, matches the published version

We make a number of comments regarding modeling degeneracies in strong lensing measurements of the Hubble parameter $H_0$. The first point concerns the impact of weak lensing associated with different segments of the line of sight. We show that external convergence terms associated with the lens-source and observer-lens segments need to be included in cosmographic modeling, in addition to the usual observer-source term, to avoid systematic bias in the inferred value of $H_0$. Specifically, we show how an incomplete account of some line of sight terms biases stellar kinematics as well as ray tracing simulation methods to alleviate the mass sheet degeneracy. The second point concerns the use of imaging data for multiple strongly-lensed sources in a given system. We show that the mass sheet degeneracy is not fully resolved by the availability of multiple sources: some degeneracy remains because of differential external convergence between the different sources. Similarly, differential external convergence also complicates the use of multiple sources in addressing the approximate mass sheet degeneracy associated with a local ("internal") core component in lens galaxies. This internal-external degeneracy is amplified by the non-monotonicity of the angular diameter distance as a function of redshift. For a rough assessment of the weak lensing effects, we provide estimates of external convergence using the nonlinear matter power spectrum, paying attention to non-equal time correlators., Comment: 16 pages, 6 figures. v3: corrected mistake in Sec.III, changing details in weak lensing bias due to kinematics and imaging combination

We propose a new mechanism to simultaneously explain the observed dark matter abundance and the baryon asymmetry of the Universe. The mechanism is based on the Filtered Dark Matter scenario, where dark matter particles acquire a large mass during a first-order phase transition. This implies that only a small fraction of them are energetic enough to enter the advancing true vacuum bubbles and survive until today, while the rest are reflected and annihilate away quickly. We supplement this scenario with a CP-violating interaction, which creates a chiral asymmetry in the population of dark matter particles. In the false vacuum phase, a portal interaction quickly converts the dark sector chiral asymmetry into a Standard Model lepton asymmetry. The lepton asymmetry is then partially converted to a baryon asymmetry by standard electroweak sphaleron processes. We discuss the dependence of the generated asymmetry on the parameters of the model for two different portal interactions and demonstrate successful baryogenesis for both. For one of the portals, it is also possible to simultaneously explain the observed dark matter abundance, over many orders of magnitude in the dark matter mass., Comment: 32 pages, 6 figures, 1 table; v2: mistake in the source term corrected, which leads to larger allowed parameter region in v2; added citations

We propose a novel way to search for feebly interacting massive particles, exploiting two properties of systems involving collisions between high energy electrons and intense laser pulses. The first property is that the electron-intense-laser collision results in a large flux of hard photons, as the laser behaves effectively as a thick medium. The second property is that the emitted photons free-stream inside the laser and thus for them the laser behaves effectively as a very thin medium. Combining these two features implies that the electron-intense-laser collision is an apparatus which can efficiently convert UV electrons to a large flux of hard, co-linear photons. We further propose to direct this unique large and hard flux of photons onto a physical dump which in turn is capable of producing feebly interacting massive particles, in a region of parameters that has never been probed before. We denote this novel apparatus as ``optical dump'' or NPOD (new physics search with optical dump). The proposed LUXE experiment at Eu.XFEL has all the required basic ingredients of the above experimental concept. We discuss how this concept can be realized in practice by adding a detector after the last physical dump of the experiment to reconstruct the two-photon decay product of a new spin-0 particle. We show that even with a relatively short dump, the search can still be background free. Remarkably, even with a 40 TW laser, which corresponds to the initial run, and definitely with a 350 TW laser, of the main run with one year of data taking, LUXE-NPOD will be able to probe uncharted territory of both models of pseudo-scalar and scalar fields, and in particular probe natural of scalar theories for masses above 100 MeV., Comment: 10 pages, 4 figures, and 6 pages of supplementary material

We develop an economical theoretical framework for combined explanations of the flavor physics anomalies involving muons: $(g-2)_\mu$, $R_{K^{(*)}}$, and $b \to s \mu^+ \mu^-$ angular distributions and branching ratios, that was first initiated by some of us in Ref. [1]. The Standard Model (SM) is supplemented with a lepton-flavored $\mathrm{U}(1)_X$ gauge group. The $\mathrm{U}(1)_X$ gauge boson with the mass of $\mathcal{O}(0.1)$ GeV resolves the $(g-2)_\mu$ tension. A TeV-scale leptoquark, charged under the $\mathrm{U}(1)_X$, carries a muon number and mediates $B$-decays without prompting charged lepton flavor violation or inducing proton decay. We explore the theory space of the chiral, anomaly-free $\mathrm{U}(1)_X$ gauge extensions featuring the above scenario, and identify many suitable charge assignments for the SM$+3\nu_R$ fermion content with the integer charges in the range $X_{F_i} \in [-10,10]$. We then carry out a comprehensive phenomenological study of the muonic force in representative benchmark models. Interestingly, we found models which can resolve the tension without conflicting the complementary constraints, and all of the viable parameter space will be tested in future muonic resonance searches. Finally, the catalog of the anomaly-free lepton-non-universal charge assignments motivated us to explore different directions in model building. We present a model in which the muon mass and the $(g-2)_\mu$ are generated radiatively from a common short-distance dynamics after the $\mathrm{U}(1)_X$ breaking. We also show how to charge a vector leptoquark under $\mathrm{U}(1)_{\mu - \tau}$ in a complete gauge model., Comment: 49 pages, 10 figures, 2 tables. Auxiliary file containing suitable charge assignments. v2: Additional neutrino bounds added, new benchmarks added, discussion extended

We show that muonium spectroscopy in the coming years can reach a precision high enough to determine the anomalous magnetic moment of the muon below one part per million (ppm). Such an independent determination of muon g-2, which is not limited by hadronic uncertainties, would certainly shed light on the 2ppm difference currently observed between spin-precession measurements and (R-ratio based) Standard Model predictions. The magnetic dipole interaction between electrons and (anti)muons bound in muonium gives rise to a hyperfine splitting (HFS) of the ground state which is sensitive to the muon anomalous magnetic moment. A direct comparison of the muonium frequency measurements of the HFS at J-PARC and the 1S-2S transition at PSI with theory predictions will allow to extract muon g-2 with high precision. Improving the accuracy of QED calculations of these transitions by about one order of magnitude is also required. Moreover, the good agreement between theory and experiment for the electron g-2 indicates that new physics interactions are unlikely to affect muonium spectroscopy down to the envisaged precision., Comment: matches published version, full supplemental material

The aim of the proposed CERN Gamma Factory is to produce $\sim 10^{17}$ photons per second with energies up to 400 MeV. The photon beam intensity is expected to be a factor of $\mathcal{O}(10^7)$ larger than that of the presently available photon beams in the MeV energy range. In this work, we explore its potential to probe physics beyond the Standard Model. In particular, we discuss searches for axion like particles (ALPs) with dominant couplings to photons and consider various production scenarios - fixed target, photon-photon collision, and conversion by a magnetic field and detection schemes - via decay to photons or back-conversion. We find that the Gamma Factory in a fixed target mode can probe ALPs with mass $m_a \lesssim \mathcal{O}(100\,\mathrm{MeV})$ and decay constants larger than $10^7$ GeV, improving by an order of magnitude the discovery potential of previous beam dump experiments., Comment: 12 pages, 4 figures

In this paper, we describe the potential of the LHCb experiment to detect Stealth physics. This refers to dynamics beyond the Standard Model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or together with overwhelming backgrounds. We will discuss why LHCb is equipped to discover this kind of physics at the Large Hadron Collider and provide examples of well-motivated theoretical models that can be probed with great detail at the experiment., Comment: 82 pages, 31 figures. This is the version of the article before peer review or editing, as submitted by an author to Reports on Progress in Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at doi:10.1088/1361-6633/ac4649

We investigate the sensitivity of the FASER$\nu$ detector to new physics in the form of non-standard neutrino interactions. FASER$\nu$, which has recently been installed 480 m downstream of the ATLAS interaction point, will for the first time study interactions of multi-TeV neutrinos from a controlled source. Our formalism -- which is applicable to any current and future neutrino experiment -- is based on the Standard Model Effective Theory~(SMEFT) and its counterpart, Weak Effective Field Theory~(WEFT), below the electroweak scale. Starting from the WEFT Lagrangian, we compute the coefficients that modify neutrino production in meson decays and detection via deep-inelastic scattering, and we express the new physics effects in terms of modified flavor transition probabilities. For some coupling structures, we find that FASER$\nu$ will be able to constrain interactions that are two to three orders of magnitude weaker than Standard Model weak interactions, implying that the experiment will be indirectly probing new physics at the multi-TeV scale. In some cases, FASER$\nu$ constraints will become comparable to existing limits - some of them derived for the first time in this paper - already with $150~$fb${}^{-1}$ of data., Comment: 44 pages, 9 figures, 4 tables, 2 babies

We discuss the recent results on the muon anomalous magnetic moment in the context of new physics models with light scalars. We propose a model in which the one-loop contributions to g-2 of a scalar and a pseudoscalar naturally cancel in the massless limit due to the symmetry structure of the model. This model allows to interpolate between two possible interpretations. In the first interpretation, the results provide a strong evidence of the existence of new physics, dominated by the positive contribution of a CP-even scalar. In the second one, supported by the recent lattice result, the data provides a strong upper bound on new physics, specifically in the case of (negative) pseudoscalar contributions. We emphasize that tree-level signatures of the new degrees of freedom of the model are enhanced relative to conventional explanations of the discrepancy. As a result, this model can be tested in the near future with accelerator-based experiments and possibly also at the precision frontier., Comment: 13 pages, 2 figures

In this paper we study CP violation in photon self-interactions at low energy. These interactions, mediated by the effective operator $FFF\tilde{F}$, where ($\tilde F$) $F$ is the (dual) electromagnetic field strength, have yet to be directly probed experimentally. Possible sources for such interactions are weakly coupled light scalars with both scalar and pseudoscalar couplings to photons (for instance, complex Higgs-portal scalars or the relaxion), or new light fermions coupled to photons via dipole operators. We propose a method to isolate the CP-violating contribution to the photon self-interactions using Superconducting Radio-Frequency cavities and vacuum birefringence experiments. In addition, we consider several theoretical and experimental indirect bounds on the scale of new physics associated with the above effective operator, and present projections for the sensitivity of the proposed experiments to this scale. We also discuss the implications of these bounds on the CP-violating couplings of new light particles coupled to photons., Comment: 28 pages, 6 figures

With the establishment and maturation of the experimental programs searching for new physics with sizeable couplings at the LHC, there is an increasing interest in the broader particle and astrophysics community for exploring the physics of light and feebly-interacting particles as a paradigm complementary to a New Physics sector at the TeV scale and beyond. FIPs 2020 has been the first workshop fully dedicated to the physics of feebly-interacting particles and was held virtually from 31 August to 4 September 2020. The workshop has gathered together experts from collider, beam dump, fixed target experiments, as well as from astrophysics, axions/ALPs searches, current/future neutrino experiments, and dark matter direct detection communities to discuss progress in experimental searches and underlying theory models for FIPs physics, and to enhance the cross-fertilisation across different fields. FIPs 2020 has been complemented by the topical workshop "Physics Beyond Colliders meets theory", held at CERN from 7 June to 9 June 2020. This document presents the summary of the talks presented at the workshops and the outcome of the subsequent discussions held immediately after. It aims to provide a clear picture of this blooming field and proposes a few recommendations for the next round of experimental results., Comment: 240 pages, 71 figures (version published on EPJC)

This Conceptual Design Report describes LUXE (Laser Und XFEL Experiment), an experimental campaign that aims to combine the high-quality and high-energy electron beam of the European XFEL with a powerful laser to explore the uncharted terrain of quantum electrodynamics characterised by both high energy and high intensity. We will reach this hitherto inaccessible regime of quantum physics by analysing high-energy electron-photon and photon-photon interactions in the extreme environment provided by an intense laser focus. The physics background and its relevance are presented in the science case which in turn leads to, and justifies, the ensuing plan for all aspects of the experiment: Our choice of experimental parameters allows (i) effective field strengths to be probed at and beyond the Schwinger limit and (ii) a precision to be achieved that permits a detailed comparison of the measured data with calculations. In addition, the high photon flux predicted will enable a sensitive search for new physics beyond the Standard Model. The initial phase of the experiment will employ an existing 40 TW laser, whereas the second phase will utilise an upgraded laser power of 350 TW. All expectations regarding the performance of the experimental set-up as well as the expected physics results are based on detailed numerical simulations throughout.

We consider models of decaying spin-1 dark matter whose dominant coupling to the standard model sector is through a dark-Higgs Yukawa portal connecting a TeV-scale vector-like lepton to the standard model (right-handed) electron. Below the electron-positron threshold, dark matter has very slow, loop-suppressed decays to photons and (electron) neutrinos, and is stable on cosmological time-scale for sufficiently small gauge coupling values. Its relic abundance is set by in-equilibrium dark lepton decays, through the freeze-in mechanism. We show that this model accommodates the observed dark matter abundance for natural values of its parameters and a dark matter mass in the ~5keV to 1MeV range, while evading constraints from direct detection, indirect detection, stellar cooling and cosmology. We also consider the possibility of a nonzero gauge kinetic mixing with the standard model hypercharge field, which is found to yield a mild impact on the model's phenomenology., Comment: 21p, 3 figures; v2 - added discussion on resonant photon conversion, conclusion unchanged, to be published in JHEP

We study the recent \XeT excess in context of solar scalar, specifically in the framework of Higgs-portal and the relaxion model. We show that $m_\phi = 1.9\,\keV$ and $g_{\phi e}=2.4\times 10^{-14}$ can explain the observed excess in science run 1 (SR1) analysis in the 1-7 keV range. When translated into the scalar-Higgs mixing angle, the corresponding mixing angle $\sin\theta = 10^{-8}$ is intriguingly close to the maximum value of mixing angle for the technical naturalness of the scalar mass. Unlike the solar axion model, the excess favors a massive scalar field because of its softer spectrum. In the minimal scenarios we consider, the best fit parameters are in tension with stellar cooling bounds. We discuss a possibility that a large density of red giant stars may trigger a phase transition, resulting in a local scalar mass increase suppressing the stellar cooling. For the particular case of minimal relaxion scenarios, we find that such type of chameleon effects is automatically present but they can not ease the cooling bounds. They are however capable of triggering a catastrophic phase transition in the entire universe. Following this observation we derive a new set of bounds on the relaxed-relaxion parameter space., Comment: 8 pages, 5 figures; new bounds on relaxion parameter space are added

Atomic spectral lines for different isotopes are shifted, revealing a change in the properties of the nucleus. For spinless nuclei such isotope shifts for two distinct transitions are expected to be linearly related, at least at leading order in a change of the nuclear mass and charge distribution. Looking for a breaking of linearity in so-called King plots was proposed as a novel method to search for physics beyond the standard model. In the light of the recent experimental progress in isotope shift spectroscopy, the sensitivity of these searches will become limited by the determination of the isotope masses and/or by nuclear effects which may induce nonlinearities at an observable level. In this work, we propose two possible generalizations of the traditional King plot that overcome these limitations by including additional isotope shift measurements, thus significantly extending the new physics reach of King plots in a purely spectroscopy-driven approach.

We report on the status of efforts to improve the reinterpretation of searches and measurements at the LHC in terms of models for new physics, in the context of the LHC Reinterpretation Forum. We detail current experimental offerings in direct searches for new particles, measurements, technical implementations and Open Data, and provide a set of recommendations for further improving the presentation of LHC results in order to better enable reinterpretation in the future. We also provide a brief description of existing software reinterpretation frameworks and recent global analyses of new physics that make use of the current data., Comment: 58 pages, minor revision following comments from SciPost referees

The KOTO experiment recently reported four candidate events in the signal region of $K_L\to \pi^0 \nu\bar\nu$ search, where the standard model only expects $0.10\pm 0.02$ events. If confirmed, this requires physics beyond the standard model to enhance the signal. We examine various new physics interpretations of the result including these: (1) heavy new physics boosting the standard model signal, (2) reinterpretation of "$\nu\bar{\nu}$" as a new light long-lived particle, or (3) reinterpretation of the whole signal as the production of a new light long-lived particle at the fixed target. We study the above explanations in the context of a generalized new physics Grossman-Nir bound coming from the $K^+ \to \pi^+\nu\bar{\nu}$ decay, bounded by data from the E949 and the NA62 experiments., Comment: 7 pages, 3 figures; v2: references added, simulation refined; v3: matches version published in PRL

The LHC copiously produces muons via different processes, and the muon sample will be large at the high-luminosity LHC (HL-LHC). In this work we propose to leverage this large muon sample and utilize the HL-LHC as a muon fixed-target experiment, with the ATLAS calorimeter as the target. We consider a novel analysis for the ATLAS detector, which takes advantage of the two independent muon momentum measurements by the inner detector and the muon system. We show that a comparison of the two measurements, before and after the calorimeters, can probe new force carriers that are coupled to muons and escape detection. The proposed analysis, based on muon samples from $W$ and $Z$ decays only, has a comparable reach to other proposals. In particular, it can explore the part of parameter-space that could explain the muon $g-2$ anomaly., Comment: 9 Pages, 4 figures

We study the potential of the LHCb experiment to discover, for the first time, the $\mu^+\mu^-$ true muonium bound state. We propose a search for the vector $1^3S_1$ state, $\mathcal{T\!M}$, which kinetically mixes with the photon and dominantly decays to $e^+e^-$. We demonstrate that a search for $\eta \to \gamma \mathcal{T\!M}$, $\mathcal{T\!M}\to e^+e^-$ in a displaced vertex can exceed a significance of 5 standard deviations assuming statistical uncertainties. We present two possible searches: an inclusive search for the $e^+e^-$ vertex, and an exclusive search which requires an additional photon and a reconstruction of the $\eta$ mass., Comment: 12 pages, 5 figures; v2: edits from comments and review, matched to PRD version

We explore the sensitivity of photon-beam experiments to axion-like particles (ALPs) with QCD-scale masses whose dominant coupling to the Standard Model is either to photons or gluons. We introduce a novel data-driven method that eliminates the need for knowledge of nuclear form factors or the photon-beam flux when considering coherent Primakoff production off a nuclear target, and show that data collected by the PrimEx experiment could substantially improve the sensitivity to ALPs with $0.03 \lesssim m_a \lesssim 0.3$ GeV. Furthermore, we explore the potential sensitivity of running the GlueX experiment with a nuclear target and its planned PrimEx-like calorimeter. For the case where the dominant coupling is to gluons, we study photoproduction for the first time, and predict the future sensitivity of the GlueX experiment using its nominal proton target. Finally, we set world-leading limits for both the ALP-gluon coupling and the ALP-photon coupling based on public mass plots., Comment: 17 pages, 7 figures; v3 corrected PrimEx results for luminosity error; v2 added missing factor when drawing the GlueX limits on the ALP-gluon coupling, fixed convention discrepancy in the SeaQuest ALP-photon limits, other minor edits

We study dimuon events in 2.11/fb of 7 TeV pp collisions, using CMS Open Data, and search for a narrow dimuon resonance with moderate mass (14-66 GeV) and substantial transverse momentum (pT). Applying dimuon pT cuts of 25 GeV and 60 GeV, we explore two overlapping samples: one with isolated muons, and one with prompt muons without an isolation requirement. Using the latter sample requires information about detector effects and QCD backgrounds, which we obtain directly from the CMS Open Data. We present model-independent limits on the product of cross section, branching fraction, acceptance, and efficiencies. These limits are stronger, relative to a corresponding inclusive search without a pT cut, by factors of as much as nine. Our "pT-enhanced" dimuon search strategy provides improved sensitivity to models in which a new particle is produced mainly in the decay of something heavier, as could occur, for example, in decays of the Higgs boson or of a TeV-scale top partner. An implementation of this method with the current 13 TeV data should improve the sensitivity to such signals further by roughly an order of magnitude., Comment: 21 pages, 8 figures, 4 tables; v2: approximate version to appear in PRD, with references added and some discussions expanded

Axion-like particles (ALPs) with couplings to electromagnetism have long been postulated as extensions to the Standard Model. String theory predicts an "axiverse" of many light axions, some of which may make up the dark matter in the universe and/or solve the strong CP problem. We propose a new experiment using superconducting radiofrequency (SRF) cavities which is sensitive to light ALPs independent of their contribution to the cosmic dark matter density. Off-shell ALPs will source cubic nonlinearities in Maxwell's equations, such that if a SRF cavity is pumped at frequencies $\omega_1$ and $\omega_2$, in the presence of ALPs there will be power in modes with frequencies $2\omega_1 \pm \omega_2$. Our setup is similar in spirit to light-shining-through-walls (LSW) experiments, but because the pump field itself effectively converts the ALP back to photons inside a single cavity, our sensitivity scales differently with the strength of the external fields, allowing for superior reach as compared to experiments like OSQAR while utilizing current technology. Furthermore, a well-defined program of increasing sensitivity has a guaranteed physics result: the first observation of the Euler-Heisenberg term of low-energy QED at energies below the electron mass. We discuss how the ALP contribution may be separated from the QED contribution by a suitable choice of pump modes and cavity geometry, and conclude by describing the ultimate sensitivity of our proposed program of experiments to ALPs., Comment: v2: 6+4 pages, 2+1 figures. Typos fixed, references added, matches PRL version. v1: 7+2 pages, 2+1 figures

We present a novel data-driven method for determining the hadronic interaction strengths of axion-like particles (ALPs) with QCD-scale masses. Using our method, it is possible to calculate the hadronic production and decay rates of ALPs, along with many of the largest ALP decay rate to exclusive final states. To illustrate the impact on QCD-scale ALP phenomenology, we consider the scenario where the ALP-gluon coupling is dominant over the ALP coupling to photons, electroweak bosons, and all fermions for $m_{\pi} \lesssim m_a \lesssim 3$ GeV. We emphasize, however, that our method can easily be generalized to any set of ALP couplings to SM particles. Finally, using the approach developed here, we provide calculations for the branching fractions of $\eta_c \to VV$ decays, i.e. $\eta_c$ decays into two vector mesons, which are consistent with the known experimental values., Comment: 19 pages, 7 figures; v3 Fig 4 updated to account for a small change in the limit taken from [1903.03586]

We explore the possibility that bound states involving dark matter particles could be detected by resonance searches at the LHC, and the generic implications of such scenarios for indirect and direct detection. We demonstrate that resonance searches are complementary to mono-jet searches and can probe dark matter masses above 1 TeV with current LHC data. We argue that this parameter regime, where the bound-state resonance channel is the most sensitive probe of the dark sector, arises most naturally in the context of non-trivial dark sectors with large couplings, nearly-degenerate dark-matter-like states, and multiple force carriers. The presence of bound states detectable by the LHC implies a minimal Sommerfeld enhancement that is appreciable, and potentially also radiative bound state formation in the Galactic halo, leading to large signals in indirect searches. We calculate these complementary constraints, which favor either models where the bound-state-forming dark matter constitutes a small fraction of the total density, or models where the late-time annihilation is suppressed at low velocities or late times. We present concrete examples of models that satisfy all these constraints and where the LHC resonance search is the most sensitive probe of the dark sector., Comment: 22 pages plus appendices, 10 figures, comments welcome

Searches for dark photons provide serendipitous discovery potential for other types of vector particles. We develop a framework for recasting dark photon searches to obtain constraints on more general theories, which includes a data-driven method for determining hadronic decay rates. We demonstrate our approach by deriving constraints on a vector that couples to the $B\!-\!L$ current, a leptophobic $B$ boson that couples directly to baryon number and to leptons via $B$-$\gamma$ kinetic mixing, and on a vector that mediates a protophobic force. Our approach can easily be generalized to any massive gauge boson with vector couplings to the Standard Model fermions, and software to perform any such recasting is provided at https://gitlab.com/philten/darkcast ., Comment: 33 pages, 11 figures

The very high precision of current measurements and theory predictions of spectral lines in few-electron atoms allows to efficiently probe the existence of exotic forces between electrons, neutrons and protons. We investigate the sensitivity to new spin-independent interactions in transition frequencies (and their isotopic shifts) of hydrogen, helium and some helium-like ions. We find that present data probe new regions of the force-carrier couplings to electrons and neutrons around the MeV mass range. We also find that, below few keV, the sensitivity to the electron coupling in precision spectroscopy of helium and positronium is comparable to that of the anomalous magnetic moment of the electron. Finally, we interpret our results in the dark-photon model where a new gauge boson is kinetically mixed with the photon. There, we show that helium transitions, combined with the anomalous magnetic moment of the electron, provide the strongest indirect bound from laboratory experiments above 100keV., Comment: 16 pages, 7 figures

We show that new physics models without new flavor violating interactions can explain the recent anomalies in the $b\to s\ell^+\ell^-$ transitions. The $b\to s\ell^+\ell^-$ arises from a $Z'$ penguin which automatically predicts the $V-A$ structure for the quark currents in the effective operators. This framework can be realized either in a renormalizable $U(1)'$ setup or be due to new strongly interacting dynamics. The di-muon resonance searches at the LHC are becoming sensitive to this scenario since the $Z'$ is relatively light, and could well be discovered in future searches by ATLAS and CMS., Comment: 5 pages, 3 figures, correction of a mistake in eq. (16), Fig. 2 updated, conclusions unchanged

In this Letter we explore the potential of probing new light force-carriers, with spin-independent couplings to the electron and the neutron, using precision isotope shift spectroscopy. We develop a formalism to interpret linear King plots as bounds on new physics with minimal theory inputs. We focus only on bounding the new physics contributions that can be calculated independently of the Standard Model nuclear effects. We apply our method to existing Ca+ data and project its sensitivity to possibly existing new bosons using narrow transitions in other atoms and ions (specifically, Sr and Yb). Future measurements are expected to improve the relative precision by five orders of magnitude, and can potentially lead to an unprecedented sensitivity for bosons within the 10 keV to 10 MeV mass range., Comment: 6 pages, 1 figure

In scenarios that stabilize the electroweak scale, the top quark is typically accompanied by partner particles. In this work, we demonstrate how extended stabilizing symmetries can yield scalar or fermionic top partners that transform as ordinary color triplets but carry exotic electric charges. We refer to these scenarios as "hypertwisted" since they involve modifications to hypercharge in the top sector. As proofs of principle, we construct two hypertwisted scenarios: a supersymmetric construction with spin-0 top partners, and a composite Higgs construction with spin-1/2 top partners. In both cases, the top partners are still phenomenologically compatible with the mass range motivated by weak-scale naturalness. The phenomenology of hypertwisted scenarios is diverse, since the lifetimes and decay modes of the top partners are model dependent. The novel coupling structure opens up search channels that do not typically arise in top-partner scenarios, such as pair production of top-plus-jet resonances. Furthermore, hypertwisted top partners are typically sufficiently long lived to form "top-partnerium" bound states that decay predominantly via annihilation, motivating searches for rare narrow resonances with diboson decay modes., Comment: 39 pages, 6 figures; v2: added minor clarifications and references; published version

Utilizing the Fermi measurement of the gamma-ray spectrum toward the Galactic Center, we derive some of the strongest constraints to date on the dark matter (DM) lifetime in the mass range from hundreds of MeV to above an EeV. Our profile-likelihood based analysis relies on 413 weeks of Fermi Pass 8 data from 200 MeV to 2 TeV, along with up-to-date models for diffuse gamma-ray emission within the Milky Way. We model Galactic and extragalactic DM decay and include contributions to the DM-induced gamma-ray flux resulting from both primary emission and inverse-Compton scattering of primary electrons and positrons. For the extragalactic flux, we also calculate the spectrum associated with cascades of high-energy gamma-rays scattering off of the cosmic background radiation. We argue that a decaying DM interpretation for the 10 TeV-1 PeV neutrino flux observed by IceCube is disfavored by our constraints. Our results also challenge a decaying DM explanation of the AMS-02 positron flux. We interpret the results in terms of individual final states and in the context of simplified scenarios such as a hidden-sector glueball model., Comment: Updated to correct an issue in Fig. S12, 9+19 pages, 2+14 figures, Supplementary Data available at https://dspace.mit.edu/handle/1721.1/105550

This report, based on the Dark Sectors workshop at SLAC in April 2016, summarizes the scientific importance of searches for dark sector dark matter and forces at masses beneath the weak-scale, the status of this broad international field, the important milestones motivating future exploration, and promising experimental opportunities to reach these milestones over the next 5-10 years., Comment: 66 pages, 15 figures, 3 tables. Workshop website and agenda: http://www-conf.slac.stanford.edu/darksectors2016/ https://indico.cern.ch/event/507783/ Editors: J. Alexander, M. Battaglieri, B. Echenard, R. Essig, M. Graham, E. Izaguirre, J. Jaros, G. Krnjaic, J. Mardon, D. Morrissey, T. Nelson, M. Perelstein, M. Pyle, A. Ritz, P. Schuster, B. Shuve, N. Toro, R. Van De Water