Session X12: Flavor Physics In and Beyond the Standard Model

Live

The KOTO experiment recently presented an excess of events in their search for the rare Standard Model (SM) process $K_L \to \pi^0 \nu\bar{\nu}$, well above the combined SM signal and background prediction. In this talk, I will show how an excess of events may be explained by weakly-coupled scalar particles produced in Kaon decays that escape KOTO undetected. I will review two concrete realizations: the minimal Higgs portal and a hadrophilic scalar model. Both have regions of parameter space that can explain the observed events while satisfying bounds from other flavor and beam-dump experiments. Hadronic beam-dump experiments provide particularly interesting constraints on light scalar particles, and I will discuss in detail the often underestimated uncertainties associated with these bounds. The simplicity of the models which can explain the excess, and their possible connections to other well known theories of beyond the Standard Model physics, provides strong theoretical motivation for a new physics interpretation of the KOTO data.   

Live

One of the most notable downfalls of the standard model (SM) and some of its theoretical extensions is the inability to provide a Dark Matter (DM) particle candidate with characteristics such that $DM + DM \leftrightarrow SM+SM $ interactions in the early universe produce the correct DM relic density measured from astronomy probes. Here we focus on a theoretical extension that predicts a new scalar or vector boson, referred to as a leptoquark (LQ) in the literature. The LQ enables quark-lepton interactions at high energy scales, thus carries both lepton and baryon number. LQ models can produce the correct DM relic density via a coannihilation (CA) mechanism, where CA refers to DM interactions with other particles, mediated by a LQ, and that result in the production of SM particles. LQ models are also a potential explanation for the recently observed anomalies in B-meson decays, where a $ \sim 4 \sigma$ deviation from SM predictions is observed. We explore some of the properties of LQs as well as their discovery potential at the CERN large hadron collider (LHC) using vector boson fusion processes, with focus on regions with small mass differences $\Delta m$ between the LQ, DM, and its CA partner (known as compressed mass spectra regions).   

Live

Studying the semileptonic decays of b hadrons can be a powerful way to probe for possible new physics effects that violate Lepton Flavor Universality (LFU). LFU enshrined in the Standard Model requires reactions involving the three charged leptons to differ only in the lepton mass. I will present the status and outlook of LHCb measurements to test LFU in the semileptonic $b \to c \ell \nu$ decays with $\tau$/$\mu$ lepton.   

Live

We investigate the prospects for discovering a top quark decaying into one light Higgs boson ($h^0$) along with a charm quark in top quark pair production at the CERN Large Hadron Collider (LHC) and future hadron colliders. A general two Higgs doublet model is adopted to study the signature of flavor changing neutral Higgs (FCNH) interactions with $t \to c h^0$, followed by $h^0 \to \tau^+ \tau^-$. We study the discovery potential for the FCNH signal and physics background from dominant processes with realistic acceptance cuts and tagging efficiencies. Promising results are found for the LHC running at 13 or 14 TeV collision energy as well as a future pp collider at 27 TeV.   

Live

Muon accelerators offer unique potential for high energy physics applications. Muon storage rings can provide intense, pure, and precisely measured neutrino beams for neutrino oscillation studies. TeV-class muon beams offer a potential route to an energy frontier collider operating at several TeV with luminosities approaching 10$^{\mathrm{35~}}$cm$^{\mathrm{{\-}2~}}$s$^{\mathrm{{\-}1}}$. The ability to accelerate muons with multi-pass acceleration systems offers significant benefits for the energy efficiency of a collider utilizing these beams. Interest in continued R{\&}D on these machines has recently re-kindled and current studies of the potential physics performance of a detector operating at such a collider appear very promising. An overview of the status of key feasibility R{\&}D for such a collider, based on a proton-driver source, is presented.   

Live

PardIn the framework of the European Strategy Update on Particle Physics, the working group appointed to review Muon Colliders has become the de facto seed of an on-going international effort.  A muon collider, if demonstrated to be feasible, is a unique discovery machine and the best tool to fully study the Higgs potential: point-like particles collide at very high energies, significantly surpassing the energy reach of other lepton colliders. It can even match the discovery potential of a proton collider with much higher energy, since the muon collision energy is fully available at the constituent level. The need for high luminosity faces technical challenges which arise from the short muon lifetime at rest and the difficulty of producing large numbers of muons in bunches with small emittance. The development of innovative concepts and demanding technologies is mandatory, exploiting synergies with other future projects. A detailed plan to launch the studies for a vigorous and conclusive R{\&}D program is presently under discussion. A well-focused international community will be required to exploit existing key competencies and to develop such a novel and promising project for the future of High Energy Physics.   

Not Participating

The KOTO experiment at the J-PARC research facility in Tokai, Japan aims to observe and measure the rare decay of the neutral kaon, $K_L^0 \rightarrow \pi^0 \nu \bar{\nu}$. The signal signature is two photons from the neutral pion decay and the two neutrinos are not seen by the KOTO detectors. The photons are captured by a Cesium Iodide (CsI) electromagnetic calorimeter and the position and energy of the hits are recorded. Due to kinematic constraints, the origin of the photons cannot be known for certain. As a result, photons that come from off-axis particle interactions and decays become a background. This work uses machine learning neural network techniques to obtain the incident angles of the photons based on the transverse shower profiles in the calorimeter. This talk will focus on the details and results of the photon angle reconstruction with neural networks.   

Not Participating

A large variety of data from the Large Hadron Collider (LHC) points to new physics beyond the Standard Model (SM), with a Higgs sector containing more than one state. In these extended Higgs sectors, it is possible that the mass difference between two (or more) of the Higgs bosons is comparable to the sum of their decay widths, which would result in quantum mechanical interference between their propagators. A particularly interesting possibility is that of two Higgs bosons having masses so close to 125 GeV each that they have hitherto appeared as a single resonance at the LHC, due to insufficient mass resolution in any of the decay channels. We analyzed the interference effects for such Higgs bosons, produced in gluon fusion at the 14 TeV LHC and decaying into a pair of photons, in the theoretical framework of the Next-to-Minimal Supersymmetric Standard Model. We also investigated the alternative scenario, that may arise in this model, of a strong mass-degeneracy between the two additional scalars, much heavier than 125 GeV, decaying into pairs of tau leptons or of the 125 GeV SM-like Higgs states. For all these cases, we have found sizeable interference effects, which invalidates the narrow-width approximation commonly employed to estimate production rates.