Search

We discuss the landscape of flavor physics at the Circular Electron-Positron Collider (CEPC), based on the nominal luminosity outlined in its Technical Design Report. The CEPC is designed to operate in multiple modes to address a variety of tasks. At the $Z$ pole, the expected production of 4 Tera $Z$ bosons will provide unique and highly precise measurements of $Z$ boson couplings, while the substantial number of boosted heavy-flavored quarks and leptons produced in clean $Z$ decays will facilitate investigations into their flavor physics with unprecedented precision. We investigate the prospects of measuring various physics benchmarks and discuss their implications for particle theories and phenomenological models. Our studies indicate that, with its highlighted advantages and anticipated excellent detector performance, the CEPC can explore beauty and $\tau$ physics in ways that are superior to or complementary with the Belle II and Large-Hadron-Collider-beauty experiments, potentially enabling the detection of new physics at energy scales of 10 TeV and above. This potential also extends to the observation of yet-to-be-discovered rare and exotic processes, as well as testing fundamental principles such as lepton flavor universality, lepton and baryon number conservation, etc., making the CEPC a vibrant platform for flavor physics research. The $WW$ threshold scan, Higgs-factory operation and top-pair productions of the CEPC further enhance its merits in this regard, especially for measuring the Cabibbo-Kobayashi-Maskawa matrix elements, and Flavor-Changing-Neutral-Current physics of Higgs boson and top quarks. We outline the requirements for detector performance and considerations for future development to achieve the anticipated scientific goals.

We evaluate the consistency of hadronic interaction models in the CORSIKA simulation package with publicly available fluorescence telescope data from the Pierre Auger Observatory. By comparing the first few central moments of the extended air shower depth maximum distributions, as extracted from measured events, to those predicted by the best-fit inferred compositions, we derive a statistical measure of the consistency of a given hadronic model with data. To mitigate possible systematic biases, we include all primaries up to iron, compensate for the differences between the measured and simulated energy spectra of cosmic rays and account for other known systematic effects. Additionally, we study the effects of including higher central moments in the fit and project our results to larger statistics., Comment: 9 pages, 8 figures

We present strategies for the Mu3e experiment to search for light, weakly coupled particles produced in rare muon decays, focusing on displaced $e^+e^-$ decays within the hollow target. In most scenarios the backgrounds can be fully suppressed with a suitable set of cuts. We furthermore quantify the interplay between displaced and prompt searches at Mu3e and existing constraints, showing how Mu3e has a unique opportunity to probe unexplored parameter space., Comment: 15 pages, 9 figures

Measurements of Ultra-High Energy Cosmic Rays (UHECR) suggest a complex composition with significant contributions from heavy nuclei at the highest energies. We systematically explore how the selection and number of primary nuclei included in the analysis impact the inferred UHECR mass composition. Introducing a distance measure in the space of $X_{\rm max}$ distribution moments, we demonstrate that limiting the analysis to a few primaries can introduce significant biases, particularly as observational data improves. We provide lists of primaries approximately equidistant in the new measure, which guaranty unbiased results at given statistical confidence. Additionally, we explore consistent inclusion of nuclei heavier than iron and up to plutonium, deriving first observational upper bounds on their contributions to UHECR with the Pierre Auger Open Data., Comment: 11 pages, 8 figures

We identify new room for CP violation in lepton flavor violating observables not bound by the electric dipole moment of leptons. By focusing on new physics in the muon-electron sector, we show that CP violation can make its first appearance in lepton flavor violating muon decays rather than in the electric dipole moment of the electron, further motivating the experimental program of Mu3e. We tackle this issue by performing the full one-loop running and matching from the low energy observables at the muon scale, including the T-odd asymmetry in $\mu\to3e$ decays, to the Standard Model effective field theory above the electroweak scale. We then sketch a simple UV model that can give rise to these patterns., Comment: 7 pages, 2 figures

We project the reach of future lepton colliders for measuring CKM elements from direct observations of $W$ decays. We focus our attention to $|V_{cs}|$ and $|V_{cb}|$ determinations, using FCC-ee as case study. We employ state-of-the-art jet flavor taggers to obtain the projected sensitivity, and scan over tagger performances to show their effect. We conclude that future lepton collider can sizeably improve the sensitivity on $|V_{cs}|$ and $|V_{cb}|$, albeit the achievable reach will strongly depend on the level of systematic uncertainties on tagger parameters., Comment: 16 pages, 3 figures. Matches version published in JHEP

Recent advances in $b$, $c$, and $s$ quark tagging coupled with novel statistical analysis techniques will allow future high energy and high statistics electron-positron colliders, such as the FCC-ee, to place phenomenologically relevant bounds on flavor violating Higgs and $Z$ decays to quarks. We assess the FCC-ee reach for $Z/h\to bs, cu$ decays as a function of jet tagging performance. We also update the SM predictions for the corresponding branching ratios, as well as the indirect constraints on the flavor violating Higgs and $Z$ couplings to quarks. Using type III two Higgs doublet model as an example of beyond the standard model physics, we show that the searches for $h\to bs, cu$ decays at FCC-ee can probe new parameter space not excluded by indirect searches. We also reinterpret the FCC-ee reach for $Z\to bs , cu$ in terms of the constraints on models with vectorlike quarks., Comment: 22 pages, 15 figures

We explore CP-violating (CPV) effects of heavy New Physics in flavour-changing quark dipole transitions, within the framework of Standard Model Effective Field Theory (SMEFT). First, we establish the relevant dimension six operators and consider the Renormalisation Group (RG) evolution of the appropriate Wilson coefficients. We investigate RG-induced correlations between different flavour-violating processes and electric dipole moments (EDMs) within the Minimal Flavour Violating and $U(2)^3$ quark flavour models. At low energies, we set bounds on the Wilson coefficients of the dipole operators using CPV induced contributions to observables in non-leptonic and radiative $B$, $D$ and $K$ decays as well as the neutron and electron EDMs. This enables us to connect observable CPV effects at low energies and general NP appearing at high scales. We present bounds on the Wilson coefficients of the relevant SMEFT operators at the high scale $\Lambda = 5~{\rm TeV}$, and discuss most sensitive CPV observables for future experimental searches., Comment: 41 pages, 4 figures, 7 tables

We show that the maximum shower depth distributions of Ultra-High Energy Cosmic Rays (UHECRs), as measured by fluorescence telescopes, can be augmented by building a mapping to observables collected by surface detectors. The resulting statistical improvement of such augmented dataset depends in a universal way on the strength of the correlation exhibited by the mapping. Building upon the publicly available data on "golden hybrid" events from the Pierre Auger Observatory we project possible improvements in the inferred composition of UHECRs for a range of possible mappings with varying correlation strengths., Comment: 8 pages, 6 figures, method corrected

We apply statistical inference on the Pierre Auger Open Data to discern for the first time the full mass composition of cosmic rays at different energies. Working with longitudinal electromagnetic profiles of cosmic ray showers, in particular their peaking depths $X_{\rm max}$, we employ central moments of the $X_{\rm max}$ distributions as features to discriminate between different shower compositions. We find that already the first few moments entail the most relevant information to infer the primary cosmic ray mass spectrum. Our approach, based on an unbinned likelihood, allows us to consistently account for sources of statistical uncertainties due to finite datasets, both measured and simulated, as well as systematic effects. Finally, we provide a quantitative comparison of different high energy hadronic interaction models available in the atmospheric shower simulation codes., Comment: 28 pages + 16 page Appendix, 27 figures

We point out that neutrinos can have enhanced couplings to photons, if light (pseudo)scalar mediators are present, resulting in potentially measurable neutrino polarizability. We show that the expected suppression from small neutrino masses can be compensated by the light mediator mass, generating dimension 7 Rayleigh operators at low scales. We explore the rich phenomenology of such models, computing in details the constraints on the viable parameter space, spanned by the couplings of the mediator to neutrinos and photons. Finally, we build several explicit models that lead to enhanced neutrino polarizability by modifying the inverse see-saw majoron, i.e., the pseudo-Nambu-Goldstone boson of the $U(1)_L$ global lepton number responsible for generating small neutrino masses., Comment: 48 pages, 9 figures. v2: Improved star cooling discussion. Corrected bound from Xenon-nT. Added recast of monophoton searches discussion and EFT bound. Submitted to JHEP

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 have studied three realistic benchmark geometries for a new far detector GAZELLE to search for long-lived particles at the \superkekb accelerator in Tsukuba, Japan. The new detector would be housed in the same building as Belle II and observe the same $e^+e^-$ collisions. To assess the discovery reach of GAZELLE, we have investigated three new physics models that predict long-lived particles: heavy neutral leptons produced in tau lepton decays, axion-like particles produced in $B$ meson decays, and new scalars produced in association with a dark photon, as motivated by inelastic dark matter. We do not find significant gains in the new physics discovery reach of GAZELLE compared to the Belle II projections for the same final states. The main reasons are the practical limitations on the angular acceptance and size of GAZELLE, effectively making it at most comparable to Belle II, even though backgrounds in the far detector could be reduced to low rates. A far detector for long-lived particles would be well motivated in the case of a discovery by Belle II, since decays inside GAZELLE would facilitate studies of the decay products. Depending on the placement of GAZELLE, searches for light long-lived particles produced in the forward direction or signals of a confining hidden force could also benefit from such a far detector. Our general findings could help guide the design of far detectors at future electron-positron colliders such as the ILC, FCC-ee or CEPC., Comment: Fixed a mistake in the detector size of the L-GAZELLE configuration (contribution to Snowmass)

We explore the interplay of New Physics (NP) effects in $(g-2)_\ell$ and $h \to \ell^+ \ell^-$ within the Standard Model Effective Field Theory (SMEFT) framework, including one-loop Renormalization Group (RG) evolution of the Wilson coefficients as well as matching to the observables below the electroweak symmetry breaking scale. We include both the leading dimension six chirality flipping operators including a Higgs and $SU(2)_L$ gauge bosons as well as four-fermion scalar and tensor operators, forming a closed operator set under the SMEFT RG equations. We compare present and future experimental sensitivity to different representative benchmark scenarios. We also consider two simple UV completions, a Two Higgs Doublet Model and a single scalar LeptoQuark extension of the SM, and show how tree level matching to SMEFT followed by the one-loop RG evolution down to the electroweak scale can reproduce with high accuracy the $(g-2)_\ell$ and $h \to \ell^+ \ell^-$ contributions obtained by the complete one- and even two-loop calculations in the full models., Comment: 24 pages, 9 figures; v2: corrected typos, added Rb result for electron anomalous moment

We point out that a non-relativistic $\sim 2 $ GeV dark matter (DM) which interacts with visible matter through higher dimensional Rayleigh operators could explain the excess of "electron recoil" events recently observed by the Xenon1T collaboration. A DM scattering event results in a few keV photon that on average carries most of the deposited energy, while the nuclear recoil energy is only a subleading correction. Since the Xenon1T detector does not discriminate between electrons and photons, such events would be interpreted as excess of the keV electrons. Indirect constraints from dark matter annihilation are avoided for light mediators of ${\mathcal O}(10~{\rm MeV})$ that have sizable couplings to neutrinos. One loop induced spin-independent scattering in dark matter may soon lead to a confirmation signal or already excludes regions of viable parameter space for the Rayleigh DM model, depending on what the exact values of the unknown nonperturbative nuclear matrix elements are., Comment: 5 pages, 5 figures, discussion shortened, typos corrected

This work details worked solutions to the various problems set by the lecturers during the course of the Les Houches summer school 2017 on effective field theories in particle physics and cosmology., Comment: 92 pages, 14 figures; Contribution to the Proceedings of the 2017 Les Houches Summer School on Effective Field Theory in Particle Physics and Cosmology (Les Houches, France, July 3-28, 2017)

We show that a thermal relic which decouples from the standard model (SM) plasma while relativistic can be a viable dark matter (DM) candidate, if the decoupling is followed by a period of entropy dilution that heats up the SM, but not the dark sector. Such diluted hot relics can be as light as a keV, while accounting for the entirety of the DM, and not conflicting with cosmological and astrophysical measurements. The requisite dilution can be achieved via decays of a heavy state that dominates the energy budget of the universe in the early matter dominated era. The heavy state decays into the SM particles, heats up the SM plasma, and dilutes the hidden sector. The interaction required to equilibrate the two sectors in the early universe places a bound on the maximum possible dilution as a function of the decoupling temperature. As an example of diluted hot relic DM we consider a light Dirac fermion with a heavy dark photon mediator. We present constraints on the model from terrestrial experiments (current and future), astrophysics, and cosmology., Comment: 38 pages, 8 figures, to be published in JHEP, v2: ref added, substantial revisions in free-streaming constraints, minor revisions elsewhere

We introduce two anomaly free versions of Froggatt-Nielsen (FN) models, based on either $G_{\rm FN}=U(1)^3$ or $G_{\rm FN}=U(1)$ horizontal symmetries, that generate the SM quark and lepton flavor structures. The structure of these "inverted" FN models is motivated by the clockwork mechanism: the chiral fields, singlets under $G_{\rm FN}$, are supplemented by chains of vector-like fermions charged under $G_{\rm FN}$. Unlike the traditional FN models the hierarchy of quark and lepton masses is obtained as an expansion in $M/\langle \phi\rangle$, where $M$ is the typical vector-like fermion mass, and $\langle \phi\rangle$ the flavon vacuum expectation value. The models can be searched for through deviations in flavor observables such as $K-\bar K$ mixing, $\mu\to e$ conversion, etc., where the present bounds restrict the masses of vector-like fermions to be above ${\mathcal O}(10^7~{\rm GeV})$. If $G_{\rm FN}$ is gauged, the models can also be probed by searching for the flavorful $Z'$ gauge bosons. In principle, the $Z'$s can be very light, and can be searched for using precision flavor, astrophysics, and beam dump experiments., Comment: 50 pages, 20 figures, discussion of rare meson decay bounds amended, Figs. 13, 14 and 18 updated

This report summarizes the present status of neutrino non-standard interactions (NSI). After a brief overview, several aspects of NSIs are discussed, including connection to neutrino mass models, model-building and phenomenology of large NSI with both light and heavy mediators, NSI phenomenology in both short- and long-baseline neutrino oscillation experiments, neutrino cross-sections, complementarity of NSI with other low- and high-energy experiments, fits with neutrino oscillation and scattering data, DUNE sensitivity to NSI, effective field theory of NSI, as well as the relevance of NSI to dark matter and cosmology. We also discuss the open questions and interesting future directions that can be pursued by the community at large. This report is based on talks and discussions during the Neutrino Theory Network NSI workshop held at Washington University in St. Louis from May 29-31, 2019 (https://indico.cern.ch/event/812851/), Comment: 104 pages; minor revision

We formulate an Effective Field Theory (EFT) for Non Standard neutrino Interactions (NSI) in elastic scattering with light quarks, leptons, gluons and photons, including all possible operators of dimension 5, 6 and 7. We provide the expressions for the cross sections in coherent neutrino-nucleus scattering and in deep inelastic scattering. Assuming single operator dominance we constrain the respective Wilson coefficient using the measurements by the COHERENT and CHARM collaborations. We also point out the constraining power of future elastic neutrino-nucleus scattering experiments. Finally, we explore the implications of the bounds for SMEFT operators above the electroweak breaking scale., Comment: 42 pages + appendices, 13 figures

We present the full basis of effective operators relevant for dark matter direct detection, up to and including operators of mass dimension seven. We treat the cases where dark matter is either a Dirac fermion, a Majorana fermion, a complex scalar, or a real scalar, allowing for dark matter to furnish a general representation of the electroweak gauge group. We describe the algorithmic procedure used to obtain the minimal set of effective operators and provide the tree-level matching conditions onto the effective theory valid below the electroweak scale., Comment: 33 pages, 1 figure. Updated operator basis