2 Search Results

As the planning stages for a high energy muon collider enter a more concrete era, an important question arises as to what new physics could be uncovered. A TeV-scale muon collider is also a vector boson fusion (VBF) factory with a very clean background, and as such it is a promising environment to look for new physics that couples to the electroweak (EW) sector. In this paper, we explore the ability of a future TeV-scale muon collider to search for Majorana and Dirac heavy neutral leptons (HNLs) produced via EW bosons. Employing a model-independent, conservative approach, we present an estimation of the production and decay rate of HNLs over a mass range between 200 GeV and 9.5 TeV in two benchmark collider proposals with $\sqrt{s}=3$ , 10 TeV, as well as an estimation of the dominant Standard Model (SM) background. We find that exclusion limits for the mixing between the HNLs and SM neutrinos can be as low as $\mathcal{O}\left({10}^{-6}\right)$ . Additionally, we demonstrate that a TeV-scale muon collider allows for the ability to discriminate between Majorana and Dirac type HNLs for a large range of mixing values. Published by the American Physical Society 2024

As one of the hypothetical principles in the Standard Model (SM), lepton flavor universality (LFU) should be tested with a precision as high as possible such that the physics violating this principle can be fully examined. The run of a $Z$ factory at a future $e^{+}{e}^{-}$ collider, such as the Circular Electron-Positron Collider or Future Circular Collider (electron/positron), provides a great opportunity to perform this task because of the large statistics and high reconstruction efficiencies for $b$ hadrons at the $Z$ pole. In this paper, we present a systematic study on the LFU test in future $Z$ factories. The goal is threefold. First, we study the sensitivities of measuring the LFU-violating observables of $b\to c\tau \nu$ , i.e., $R_{J/\psi }$ , $R_{D_s}$ , $R_{D_s^{*}}$ , and $R_{{\mathrm{\Lambda }}_c}$ , where $\tau$ decays muonically. For this purpose, we develop the strategies for event reconstruction, based on the track information significantly. Second, we explore the sensitivity robustness against detector performance and its potential improvement with the message of event shape or beyond the $b$ -hadron decays. A picture is drawn on the variation of analysis sensitivities with the detector tracking resolution and soft photon detectability, and the impact of Fox-Wolfram moments is studied on the measurement of relevant flavor events. Finally, we interpret the projected sensitivities in the SM effective field theory, by combining the LFU tests of $b\to c\tau \nu$ and the measurements of $b\to s{\tau }^{+}{\tau }^{-}$ and $b\to s\overline{\nu }\nu$ . We show that the limits on the LFU-violating energy scale can be pushed up to $\sim \mathcal{O}\left(10\right)\mathrm{TeV}$ for $\lesssim \mathcal{O}\left(1\right)$ Wilson coefficients at Tera- $Z$ . Published by the American Physical Society 2024