Search

We propose a detector concept, LAYered CAvern Surface Tracker (LAYCAST), to be installed on the ceiling and the wall of the cavern hosting the main experiment of future electron-positron colliders such as CEPC and FCC-ee. With detailed and realistic considerations of the design of such a new experiment, the proposed detector is dedicated to extending the sensitivity reach of the main detector to various theoretical scenarios of long-lived particles (LLPs). We study carefully four such scenarios involving a light scalar boson $X$, the heavy neutral lepton $N$, the lightest neutralino $\tilde{\chi}^0_1$ in the R-parity-violating supersymmetry, and the axion-like particle $a$. Long-lived light scalar bosons are considered to be produced from the Standard-Model (SM) Higgs boson's decay ($h \to X X$) at the center-of-mass energy $\sqrt{s} =$ 240 GeV, while the other three types of LLPs are produced either from $Z$-boson decays (viz. $Z \to \nu\, N, ~\tilde{\chi}^0_1\, \tilde{\chi}^0_1 $) or direct scattering process ($ e^- e^+ \to ~\gamma\, a$) at $\sqrt{s} =$ 91.2 GeV, where $\gamma$ and $\nu$ denote the SM photon and neutrino, respectively. With Monte-Carlo simulations, we derive the sensitivities of the proposed experiment to these LLPs and the corresponding signal-event numbers. Our findings show that LAYCAST can probe large new parameter space beyond both current bounds and the expected reach of the main experiments at CEPC and FCC-ee. Comparison with existing works in similar directions is also made., Comment: 23 pages, 19 figures, 1 table

An analytic calculation is given for binary star evaporation under the tidal perturbation from randomly distributed, spatially extended dark objects. In particular, the Milky Way's wide binary star population are susceptible to such disruption from dark matter solitons of comparable and larger sizes. We identify high-probability `halo-like' wide binaries in GAIA EDR3 with separations larger than 0.1 parsec. Survival of the farthest-separated candidates will provide a novel gravitational probe to dark matter in the form of solitons. In case of dilute axion-like solitons, the observational sensitivity is shown to extend into the axion mass range $m_a \sim 10^{-17}-10^{-15}$ eV., Comment: 15 pages, 4 figures, 3 tables

Two classes of far detectors have been proposed or are under operation at the LHC. The first class is a series of neutrino detectors that are sensitive to light active neutrinos via either charged-current or neutral-current interactions; exemplary ideas are FASER$\nu$, SND@LHC, and FLArE. Another type aims primarily at looking for displaced decays of long-lived particles (LLPs) into charged final-state particles, including ANUBIS and FASER. In this work, we propose searches for probing lepton number violation associated with a Majorana active/sterile neutrino, for the first time with these experiments, which, if discovered, would be a clear signature of new physics beyond the Standard Model. With Monte-Carlo simulation, we find that while the neutrino detectors, unfortunately, are estimated to have signal-event rates orders of magnitude below $\mathcal{O}(1)$, some LLP far detectors such as ANUBIS, if upgraded, would be most promising for discovering a Majorana sterile neutrino of mass $\mathcal{O}(\text{1})$ GeV in certain so-far unexcluded parameter space. In this exploratory work, we emphasize on the importance of leveraging the LHC far detectors for purposes beyond the planned ones, such as searching for lepton number violation., Comment: v1: 17 pages + references, 4 figures, 4 tables; v2: 20 pages + refs, 4 figures, 4 tables, minor changes, accepted for publication in PRD

Because of the difficulty in detecting final state taus, the mixing parameter $|V_{\tau N}|^2$ for heavy neutrino $N$ is not well studied at current experiments, compared with other mixing parameters $|V_{e N}|^2$ and $|V_{\mu N}|^2$. In this paper, we focus on a challenging scenario where $N$ mixes with active neutrino of tau flavour only, i.e. $ |V_{\tau N}|^2 \neq 0 $ and $|V_{e N}|^2 = |V_{\mu N}|^2 = 0$. We derive current constraints on $|V_{\tau N}|^2$ from the rare $Z$-boson decay and electroweak precision data (EWPD). To forecast the future limits, we also investigate the signal $p p \to \tau^{\pm} \tau^{\pm} j j $ via a Majorana heavy neutrino at future proton-proton colliders. To suppress the background, both taus are required to decay leptonically into muons, leading to the final state containing two same sign muons, at least two jets plus moderate missing energy. The signal and relevant background processes are simulated at the HL-LHC and SppC/FCC-hh with center-of-mass energy of 14 TeV and 100 TeV. The preselection and multivariate analyses based on machine-learning are performed to reduce background. Limits on $|V_{\tau N}|^2$ are shown for heavy neutrino mass in the range 10-1000 GeV based on measurements from the rare $Z$-boson decay and EWPD, and searches at the HL-LHC and SppC/FCC-hh with integrated luminosities of 3 and 20 ab$^{-1}$., Comment: 9 figures, 4 tables. arXiv admin note: text overlap with arXiv:2210.17050

We utilize the lepton number violation signal process $p\, e^- \to \tau^+ jjj$ to search for heavy Majorana neutrinos at future proton-electron colliders. The LHeC (FCC-eh) is considered to run with an electron beam energy of 60 GeV, a proton beam energy of 7 (50) TeV and an integrated luminosity of 1 (3) ab$^{-1}$, and the electron beam is considered to be unpolarized. We apply detector configurations and simulate signal and related standard model background events for both hadronic $\tau_h$ and leptonic $\tau_\ell$ final states, $\ell$ being a muon. After preselection, multivariate analyses are performed to reject the background. The strategy to reconstruct the heavy neutrino mass is developed and distributions of reconstructed mass are presented. Discovery sensitivities on parameter $|V_{\tau N}|^2 |V_{eN}|^2 / ( |V_{\tau N}|^2 + |V_{eN}|^2 )$ for the heavy neutrino mass between 10 and 3000 GeV are predicted. At the 2-$\sigma$ significance, the best discovery sensitivity is $\sim 1.2 \times10^{-5} \,\,(5.0 \times 10^{-6})$ at the LHeC (FCC-eh) when $m_N \sim 100$ GeV for the hadronic $\tau_h$ final state. Sensitivities for the leptonic $\tau_\ell$ final state are found to be similar to those for the hadronic $\tau_h$ final state for most of the parameter space investigated. We also derive the limits on mixing parameters from electroweak precision data (EWPD) and DELPHI experiment. Assuming $|V_{\tau N}|^2 = |V_{eN}|^2 = |V_{\ell N}|^2$, sensitivity bounds from the LHeC and FCC-eh experiments are found to be stronger than those from EWPD when $m_N \lesssim 900$ GeV, and also stronger than those from DELPHI when $m_N \gtrsim 70$ GeV. Constraints are also interpreted and compared in the $|V_{\tau N}|^2$ vs. $|V_{e N}|^2$ plane., Comment: 17 figures, 6 tables. Updates: new background process B5a and analysis for the hadronic $\tau_h$ final state are updated; typos are fixed. arXiv admin note: text overlap with arXiv:2201.12997

The Circular Electron Positron Collider (CEPC) is a large-scale collider facility that can serve as a factory of the Higgs, Z, and W bosons and is upgradable to run at the ttbar threshold. This document describes the latest CEPC nominal operation scenario and particle yields and updates the corresponding physics potential. A new detector concept is also briefly described. This submission is for consideration by the Snowmass process., Comment: Updated author list

Assuming ALPs couple to gluons only, they can be produced via the $p p \to a j$ process and decay into two jets at the LHC. When the coupling parameter, $C_{\tilde{G}} / f_a$, is small, the lifetime of ALPs can be long enough leading to displaced final state jets. In this paper, we consider the signal including both the prompt and long-lived cases of ALPs by employing a specialized Delphes module to handle displaced jets. Relevant background processes are generated and simulated at the detector level, and multivariate analyses based on machine-learning are performed to discriminate signal and background events and achieve the best sensitivities. Based on the data accumulated for this study, we forecast the expected upper limits on $C_{\tilde{G}}/f_a$ for ALP mass $m_a$ in the range 5$-$2300 GeV at 2-, 3- and 5-$\sigma$ significances at the High Luminosity-LHC with 14 TeV center-of-mass energy and 3 ab$^{-1}$ integrated luminosity. Vast previously unprobed regions in the parameter space spanned by $C_{\tilde{G}}/f_a$ and $m_a$ are probed and the best upper limits on $C_{\tilde{G}}/f_a$ at 2-$\sigma$ significance is found to be around $1.0 \times 10^{-2} \,\, {\rm TeV^{-1}}$ for $m_a \sim 500$ GeV. The ALP mass is reconstructed from the kinematics of final state jets and we find that it is measurable in this method when $m_a$ is below about 1 TeV at the HL-LHC. The effects of systematic uncertainties and validation of the EFT framework are also checked and discussed., Comment: 24 pages, 10 figures, 3 tables

We develop the search strategy for a heavy Majorana neutrino via the lepton number violation signal process $p\, e^- \to \mu^+ jjj$ at future electron-proton colliders. The signal and dominant standard model background events are generated with the fast detector simulation. We apply the pre-selection criteria and perform the multi-variate analysis based on machine-learning to reject the background. Distributions of representative kinematic observables are presented for both signal and background processes and effects on final limits are compared by inputting two different set of observables when performing multi-variate analysis. The 2- and 5-$\sigma$ limits on the mixing parameter $|V_{\ell N}|^2$ are predicted for the heavy neutrino mass $m_N$ in the range of 10$-$1000 GeV. At the LHeC (FCC-eh) with an electron beam energy of 60 GeV, a proton beam energy of 7 (50) TeV and an integrated luminosity of 1 (3) ab$^{-1}$, the mixing parameter $|V_{\ell N}|^2$ can be constrained to be below $\sim 3.0~(1.0) \times 10^{-6}$ for $m_N$ around $\mathcal{O}(100)$ GeV at 2-$\sigma$ level. The limits are much stronger than the current experiment limits at the LHC for $m_N$ above 30 GeV. The positron signal final state and the effect of long-lived cases of heavy neutrinos are also checked and commented., Comment: 14 pages, 9 figures, 3 tables; contents are slightly updated and typos are fixed

In our previous work [Phys. Rev. D 101 (2020) 075046], we have proposed to install FAr Detectors at the Electron Positron Collider (FADEPC) to enhance the discovery potential of long-lived particles (LLPs). In this study, we consider eight designs of far detectors with different locations, volumes and geometries and investigate their potential for discovering long-lived axion-like particles (ALPs) via the process $e^-e^+ \rightarrow \gamma \,\, a,~ a \to \gamma\gamma $ at future $e^{-}e^{+}$ colliders running at a center-of-mass energy of 91.2 GeV and integrated luminosities of 16, 150, and 750 ab$^{-1}$. We estimate their sensitivities on the model parameters in terms of the effective ALP-photon-photon coupling $C_{\gamma \gamma} / \Lambda $, the effective ALP-photon-$Z$ coupling $C_{\gamma Z} / \Lambda$, and ALP mass $m_a$ for three physics scenarios: $C_{\gamma Z} = 0$; $C_{\gamma Z} = C_{\gamma \gamma}$ and both $C_{\gamma Z}$ and $C_{\gamma \gamma}$ can freely change. The results provide references for the optimization of far detectors at future electron-positron colliders., Comment: 13 pages, 7 figures, 1 table

We investigate long-lived particles (LLPs) produced in pair from neutral currents and decaying into a displaced electron plus two jets at the LHC, utilizing the proposed minimum ionizing particle timing detector at CMS. We study two benchmark models: the R-parity-violating supersymmetry with the lightest neutralinos being the lightest supersymmetric particle and two different $U(1)$ extensions of the standard model with heavy neutral leptons (HNLs). The light neutralinos are produced from the standard model $Z$-boson decays via small Higgsino components, and the HNLs arise from decays of a heavy gauge boson, $Z'$. By simulating the signal processes at the HL-LHC with the center-of-mass energy $\sqrt{s}=$ 14 TeV and integrated luminosity of 3 ab$^{-1}$, our analyses indicate that the search strategy based on a timing trigger and the final state kinematics has the potential to probe the parameter space that is complementary to other traditional LLP search strategies such as those based on the displaced vertex., Comment: v1: 13 pages, 13 figures, 1 table; v2: added notes about the history of the timing technique after publication, no change otherwise

This paper investigates the $e^-e^+\to Zh_1$ sensitivity for Higgs boson's rare decay into heavy neutrinos $h_1 \to NN$ at the proposed electron-positron collider, with the focus on multi-lepton final states that contain same-sign lepton pairs. $h_1 \to NN$ decay can derive from Higgs boson's mixing with new physics scalar(s) that is complementary to the contribution from active-sterile neutrino mixings. We consider the scenario with a singlet scalar which gives the heavy neutrino mass and has a small mixing with the SM Higgs boson. We analyze the semileptonic, fully leptonic and mixed $NN$ decay scenarios, and categorize the signal on the number of leptons in the final state: $ \ell^\pm \ell^\pm$ with at least 3 jets, $\ell^\pm \ell^\pm \ell$ with at least 2 jets, and $e^\pm e^\pm \mu^\mp \mu^\mp$ plus with at least 1 jet, each containing one or two same-sign dilepton system(s). Selection cuts are optimized for the presence of the associated $Z$-boson, which leads to additional backgrounds at the $e^-e^+$ collider. The Standard Model background channels are systematically analyzed. Sensitivity limits on $h_1 \rightarrow NN$ branching fractions are derived for signals with 2-4 final leptons assuming the heavy neutrino masses are between 10 and 60 GeV. With 240 GeV center-of-mass energy and 5.6 ab$^{-1}$ design luminosity, $h_1 \rightarrow NN$ branching fraction can be probed to $2\times 10^{-4}$ in $2\ell$ and $3\ell$ channels, and $6\times 10^{-4}$ in the $4\ell$ channel at $95\%$ confidence level. $3\ell, 4\ell$ channels expect one or fewer background event, and their sensitivities saturate the statistic limit at 5.6 ab$^{-1}$ luminosity. A same-sign trilepton ($\ell^\pm \ell^\pm \ell^\pm$) signal in the $3\ell$ channel is also discussed. Our search strategy provides an approach to discovering the singlet scalar and exploring the origin of neutrino masses at future $e^-e^+$ colliders., Comment: 10 pages, 6 figures, 3 tables, add some comments

The sensitivity of future electron-proton colliders, the LHeC and FCC-eh, to weakly-produced supersymmetric particles is evaluated in this article. Supersymmetric scenarios where charginos ($\tilde{\chi}_1^{\pm}$) and neutralinos ($\tilde{\chi}_1^{0}$ and $\tilde{\chi}_2^{0}$) are nearly degenerate in mass are considered. Two sets of models, which differ in the mass of sleptons ($\tilde{\ell}$), are studied. Under the hypothesis that slepton masses are at the multi-TeV scale (``decoupled'' scenario), the production processes for charginos and neutralinos at $ep$ colliders, $p\, e^- \to j\, e^-\, \tilde{\chi} \tilde{\chi}$ with $\tilde{\chi}=\tilde{\chi}_1^{0}$, $\tilde{\chi}_1^{\pm}$ or $\tilde{\chi}_2^{0}$, are considered. For the models where slepton masses are above but close to $\tilde{\chi}_1^{\pm}$, $\tilde{\chi}_2^{0}$ masses (``compressed'' scenario), contributions from the processes $p\, e^- \to j\, \tilde{\chi}\, \tilde{e}_L^-$ and $j\, \tilde{\chi}\, \tilde{\nu}$ followed by the decays $\tilde{e}_L^- \to \tilde{\chi}_{1,2}^{0} + e^-$ and $\tilde{\nu} \to \tilde{\chi}_1^+ + e^-$ are also taken into account. These scenarios are analysed with realistic detector performance, using multivariate techniques. Effects of systematic uncertainties and electron beam polarization dependence are also discussed. The reach is found to be complementary to the one obtained at $pp$ colliders, in particular for the compressed-slepton scenario., Comment: 12 pages, 11 figures, 3 tables

At the Large Hadron Collider (LHC), several far detectors such as FASER and MATHUSLA have been proposed to target the long-lived particles (LLPs) featured with displaced vertices. Naturally one question arises as to the feasibility of installing similar far detectors at future lepton colliders like the CEPC and FCC-ee. Because of the different kinematics of final state particles and the freedom to locate both the experiment hall and the detectors, the future lepton collider with an additional far detector may play a unique role in searching for LLPs. In this study, we consider various locations and designs of far detectors at future $e^- e^+$ colliders and investigate their potentials for discovering LLPs in the physics scenarios including exotic Higgs decays, heavy neutral leptons, and the lightest neutralinos. Our analyses show that the kinematical distinctions between the lepton and hadron colliders render the optimal positions of far detectors lying at the direction perpendicular to the collider beams at future $e^- e^+$ colliders, in contrast to the LHC where a boost in the forward direction can be exploited. We also find that when searching for LLPs, such new experiments with far detectors at future lepton colliders may extend and complement the sensitivity reaches of the experiments at the future lepton colliders with usual near detectors, and the present and future experiments at the LHC. In particular, we find that, for the theory models considered in this study, a MATHUSLA-sized far detector would give a modest improvement compared to the case with a near detector only at future lepton colliders., Comment: 18 pages, 7 figures, 3 tables

The Higgs boson can mix with a singlet scalar that dynamically generates the Majorana mass of the right-handed neutrino $N_R$. We show that even a tiny mixing between the Higgs boson and a `decoupled' singlet scalar allows for Higgs-mediated pair production of $N_R$ without significant mixings between the active neutrinos and $N_R$, and thus testable at colliders via a characteristic signal of two same-sign same-flavor lepton pairs, plus missing energy. We demonstrate that this search channel is mostly background-free in $pp$-collision and can be a highly sensitive probe of the Higgs-singlet mixing at the current and future $pp$ colliders. Such channel provides a clean signal to discover the singlet scalar and explore the origin of neutrino masses., Comment: 16 pages, 7 figures, 2 tables

Future lepton colliders such as the CEPC and FCC-ee would run as high-luminosity $Z-$boson factories, which offer a unique opportunity to study long-lived particles which couple to Z-bosons. In order to exemplify this particular advantage, in this work we consider one benchmark physics scenario where the long-lived lightest neutralinos pair $(\tilde{\chi}_1^0\tilde{\chi}_1^0)$ is produced from $Z-$decays in the context of the R-parity violating supersymmetry. Our analysis indicates that when assuming BR$(Z\rightarrow \tilde{\chi}_1^0\tilde{\chi}_1^0) = 10^{-3}$ and $m_{\tilde{\chi}_1^0} \sim 40$ GeV, the model parameter $\lambda'_{112} / m^2_{\tilde{f}}$ can be discovered down to as low as $\sim 1.5 \times 10^{-14}$ ($3.9 \times 10^{-14}$) GeV$^{-2}$ at the FCC-ee (CEPC) with center-of-mass energy $\sqrt{s} = 91.2$ GeV and 150 (16) ab$^{-1}$ integrated luminosity. These limits exceed the projected sensitivity reaches of the ATLAS experiment at the HL-LHC and the proposed LHC experiments with far detectors (AL3X, CODEX-b, FASER, and MATHUSLA)., Comment: 7 pages, 2 figures, 2 tables; v2, 13 pages, 7 figures, 2 tables, accepted for publication in Phys. Rev. D

In this article we investigate the prospects of searching for sterile neutrinos in lowscale seesaw scenarios via the lepton flavour violating (but lepton number conserving) dilepton dijet signature. In our study, we focus on the final state $e^\pm \mu^\mp jj$ at the HL-LHC and the FCC-hh (or the SppC). We perform a multivariate analysis at the detector level including the dominant SM backgrounds from di-top, di-boson, and tri-boson. Under the assumption of the active-sterile neutrino mixings $|V_{ l N}|^2=|\theta_e|^2=|\theta_\mu|^2$ and $|V_{ \tau N}|^2 = |\theta_\tau|^2=0$, the sensitivities on the signal production cross section times branching ratio $\sigma(p p \to l^\pm N)\times {\rm BR} (N \to l^{ \mp} jj)$ and on $|V_{ l N}|^2$ for sterile neutrino mass $M_N$ between 200 and 1000 GeV are derived. For the benchmark $M_N=500$ GeV, when ignoring systematic uncertainties at the HL-LHC (FCC-hh/SppC) with 3 (20) ${\rm ab}^{-1}$ luminosity, the resulting 2-$\sigma$ limits on $|V_{ l N}|^2$ are $4.9\times 10^{-3}$ ($7.0\times 10^{-5}$), while the 2 -$\sigma$ limit on $\sigma \times {\rm BR}$ are $4.4\times10^{-2}$ ($1.6\times10^{-2}$) fb, respectively. The effect of the systematic uncertainty is also studied and found to be important for sterile neutrinos with smaller masses. We also comment on searches with $\tau^\pm \mu^\mp jj$ and $\tau^\pm e^\mp jj$ final states., Comment: 20 pages, 9 figures

We search for a five-plet singly charged Higgs $H_5^\pm$ in the Georgi-Machacek model at the ep colliders. The charged Higgs bosons are produced via the $ZW^{\pm}$ fusion process $p\, e^- \to j\, e^-\, H_5^\pm$, and decay as $H_5^\pm \to Z\, W^{\pm} \to (l^+ l^-)\, (jj)$. With a detector-level simulation at the FCC-eh and LHeC, a multi-variate analysis is performed to yield limits on the production cross section times branching ratio $\sigma (p\, e^- \to j e^- H_5^\pm) \times {\rm BR}(H_5^\pm \to Z\, W^\pm)$ and on the model parameter $\sin\theta_H$ for charged Higgs masses between 200 and 1000 GeV. The effects of electron beam polarization are also investigated., Comment: 7 pages, 6 figures

Precision study of electroweak symmetry breaking strongly motivates the construction of a lepton collider with center-of-mass energy of at least 240 GeV. Besides Higgsstrahlung ($e^+e^- \to hZ$), such a collider would measure weak boson pair production ($e^+e^- \to WW$) with an astonishing precision. The weak-boson-fusion production process ($e^+e^- \to \nu \bar{\nu} h$) provides an increasingly powerful handle at higher center-of-mass energies. High energies also benefit the associated top-Higgs production ($e^+e^-\to t\bar th$) that is crucial to constrain directly the top Yukawa coupling. The impact and complementarity of differential measurements, at different center-of-mass energies and for several beam polarization configurations, are studied in a global effective-field-theory framework. We define a "global determinant parameter" (GDP) which characterizes the overall strengthening of constraints independently of the choice of operator basis. The reach of the CEPC, CLIC, FCC-ee, and ILC designs is assessed., Comment: 55 pages, lots of figures, v2: references added, minor corrections, extended discussions on quadratic EFT contributions and beam polarization effects, matches published version in JHEP

Heavy sterile neutrinos with masses below $M_W$ can induce trilepton events at the 14 TeV LHC through purely leptonic $W$ decays of $W^\pm \to e^\pm e^\pm \mu^\mp \nu$ and $\mu^\pm \mu^\pm e^\mp \nu$ where the heavy neutrino will be in an intermediate state on its mass shell. Discovery and exclusion limits for the heavy neutrinos are found using both Cut-and-Count (CC) and a Multi-Variate Analysis (MVA) methods in this study. We also show that it is possible to discriminate between a Dirac and a Majorana heavy neutrino, even when lepton number conservation cannot be directly tested due to unobservability of the final state neutrino. This discrimination is done by exploiting a combined set of kinematic observables that differ between the Majorana vs. Dirac cases. We find that the MVA method can greatly enhance the discovering and discrimination limits in comparison with the CC method. For a 14-TeV $pp$ collider with integrated luminosity of 3000 ${\rm fb}^{-1}$, sterile neutrinos can be found with 5$\sigma$ significance if heavy-to-light neutrino mixings $|U_{Ne}|^2 \sim |U_{N\mu}|^2 \sim 10^{-6}$, while the Majorana vs. Dirac type can be distinguished if $|U_{Ne}|^2 \sim |U_{N\mu}|^2 \sim 10^{-5}$ or even $|U_{N\ell}|^2\sim 10^{-6}$ if one of the mixing elements is at least an order of magnitude smaller than the other., Comment: 10 pages, 12 figures

We present a search strategy for both Dirac and Majorana sterile neutrinos from the purely leptonic decays of $W^\pm \to e^\pm e^\pm \mu^\mp \nu$ and $\mu^\pm \mu^\pm e^\mp \nu$ at the 14 TeV LHC. The discovery and exclusion limits for sterile neutrinos are shown using both the Cut-and-Count (CC) and Multi-Variate Analysis (MVA) methods. We also discriminate between Dirac and Majorana sterile neutrinos by exploiting a set of kinematic observables which differ between the Dirac and Majorana cases. We find that the MVA method, compared to the more common CC method, can greatly enhance the discovery and discrimination limits. Two benchmark points with sterile neutrino mass $m_N = 20$ GeV and 50 GeV are tested. For an integrated luminosity of 3000 ${\rm fb}^{-1}$, sterile neutrinos can be found with $5 \sigma$ significance if heavy-to-light neutrino mixings $|U_{Ne}|^2 \sim |U_{N\mu}|^2\sim 10^{-6}$, while Majorana vs. Dirac discrimination can be reached if at least one of the mixings is of order $10^{-5}$., Comment: 4 pages, 6 figures. arXiv admin note: substantial text overlap with arXiv:1703.01934

We study the purely leptonic decays of $W^\pm \to e^\pm e^\pm \mu^\mp \nu$ and $\mu^\pm \mu^\pm e^\mp \nu$ produced at the LHC, induced by sterile neutrinos with mass $m_N$ below $M_W$ in the intermediate state. Since the final state neutrino escapes detection, one cannot tell whether this process violates lepton number, what would indicate a Majorana character for the intermediate sterile neutrino. Our study shows that when the sterile neutrino mixings with electrons and muons are different enough, one can still discriminate between the Dirac and Majorana character of this intermediate neutrino by simply counting and comparing the above decay rates. After performing collider simulations and statistical analysis, we find that at the $14~\text{TeV}$ LHC with an integrated luminosity of $3000~\text{fb}^{-1}$, for two benchmark scenarios $m_N$ = 20 GeV and 50 GeV, at least a $3\sigma$ level of exclusion on the Dirac case can be achieved for disparities as mild as e.g. $|U_{Ne}|^2 < 0.7~ |U_{N\mu}|^2$ or $|U_{N\mu}|^2 < 0.7~ |U_{N e}|^2$, provided that $|U_{Ne}|^2$, $|U_{N\mu}|^2$ are both above $\sim 2\times 10^{-6}$., Comment: 18 pages, 7 figures

We use the Higgs coupling and the muon anomalous magnetic moment measurements to constrain the parameter space of the natural supersymmetry (SUSY) in the Generalized Minimal Supergravity (GmSUGRA) model. We scan the parameter space of the GmSUGRA model with small electroweak fine-tuning measure ($\Delta_{\rm EW} \leq 100$). The parameter space after applying various sparticle mass bounds, Higgs mass bounds, B-physics bounds, the muon magnetic moment constraint, and the Higgs coupling constraint from measurements at HL-LHC, ILC, and CEPC, is shown in the planes of various interesting model parameters and sparticle masses. Our study indicates that the Higgs coupling and muon anomalous magnetic moment measurements can constrain the parameter space effectively. It is shown that $\Delta_{\rm EW}\sim$ 30, consistence with all constraints, and having supersymmetric contributions to the muon anomalous magnetic moment within 1$\sigma$ can be achieved. The precision of $k_b$ and $k_{\tau}$ measurements at CEPC can bound $m_A$ to be above 1.2 TeV and 1.1 TeV respectively. The combination of the Higgs coupling measurement and muon anomalous magnetic moment measurement constrain $\tilde{e}_R$ mass to be in the range from 0.6 TeV to 2 TeV. The range of both $\tilde{e}_L$ and $\tilde{\nu}_e$ masses is 0.4 TeV $\sim$ 1.2 TeV. In all cases, the $\tilde{\chi}_1^0$ mass needs to be small (mostly $\leq$ 400 GeV). The comparison of bounds in the $\tan\beta - m_A$ plane shows that the Higgs coupling measurement is complementary to the direct collider searches for heavy Higgs when constraining the natural SUSY. A few mass spectra in the typical region of parameter space after applying all constraints are shown as well., Comment: 30 pages, 7 figures, 1 Table

We interpret the recent ATLAS multi-jet search results using 20.3 fb-1 data at sqrt(s)=8 TeV in the context of searching for excited quarks. Within the effective field theory framework, using the null results of that search, our analysis shows that the excited quark mass below 5 TeV can be excluded at the 95% confidence level. Our analysis also indicates that when the validation of effective field theory is considered, the limit can be largely compromised., Comment: 4 pages, 6 figures

We study angular observables in the $e^+e^-\to Z H\to \ell^+ \ell^-\,b\bar{b}$ channel at future circular $e^+ e^-$ colliders such as CEPC and FCC-ee. Taking into account the impact of realistic cut acceptance and detector effects, we forecast the precision of six angular asymmetries at CEPC (FCC-ee) with center-of-mass energy $\sqrt{s} =$ 240 GeV and 5 (30) ${\rm ab}^{-1}$ integrated luminosity. We then determine the projected sensitivity to a range of operators relevant for the Higgs-strahlung process in the dimension-6 Higgs EFT. Our results show that angular observables provide complementary sensitivity to rate measurements when constraining various tensor structures arising from new physics. We further find that angular asymmetries provide a novel means of both probing BSM corrections to the $H Z \gamma$ coupling and constraining the "blind spot" in indirect limits on supersymmetric scalar top partners., Comment: 28 pages, 9 figures. v2: references added, matches published version in JHEP

We study the interplay of natural supersymmetry and a supersymmetric solution to the discrepancy observed in measurements of the muon magnetic moment. The strongest constraints on the parameter space currently come from chargino searches in the trilepton channel and slepton searches in the dilepton channel at the LHC, and vast regions are currently allowed, especially at large $\tan{\beta}$. With light top squarks in the spectrum, as required from naturalness arguments, the situation changes dramatically; stop-assisted chargino and neutralino production via $\tilde{t} \rightarrow b \tilde{\chi}_{1}^{\pm}$ and $\tilde{t} \rightarrow t \tilde{\chi}_{1}^0$ are already probing the entire parameter space compatible with the muon magnetic moment at $\tan{\beta} \sim \mathcal{O}(10)$, while upcoming stop searches will probe most of the parameter space at larger $\tan{\beta} \sim 40$. Direct Higgsino searches as well as compressed slepton and stop searches are crucial to close out corners of parameter space. We consider one such example: in the presence of light sleptons and charginos as required to obtain appreciable contributions to the muon magnetic moment, compressed stops can dominantly undergo the following decay $\tilde{t} \, \rightarrow \, b \tilde{\ell} \nu (\tilde{\nu} \ell) \, \rightarrow \, b \ell \nu \tilde{\chi}_{1}^0$, facilitated by off-shell charginos. We find that the enhanced branching to leptons leads to a $5 \sigma$ mass reach (with $3000$ fb$^{-1}$ of data at LHC14) of $m_{\tilde{t}} \, \sim \, 350$ GeV, with the mass difference between stops and the lightest neutralino being $\sim 80$ GeV. This will further close out a significant part of the parameter space compatible with naturalness and the muon magnetic moment., Comment: 13 pages, 14 figures. Matches published version

The vector boson fusion (VBF) topology at the Large Hadron Collider at 14 TeV provides an opportunity to search for new physics. A feasibility study for the search of sleptons in a compressed mass spectra scenario is presented in the final state of two jets, one or two low $p_{T}$ non-resonant leptons, and missing energy. The presence of the VBF tagged jets and missing energy are effective in reducing Standard Model backgrounds. Using smuon production with a mass difference between $\tilde{l}_{L}$ and $\tilde{\chi}_{1}^0$ of 5-15 GeV, the significance of observing the signal events is found to be $\sim$ 3-6$\sigma$ for $m_{\tilde{l}}$=115-135 GeV, considering an integrated luminosity of 3000 fb$^{-1}$., Comment: 6 pages, 1 figure, 3 tables; v3: Journal matched version

A feasibility study is presented for the search of the lightest top squark in a compressed scenario, where its mass is approximately equal to the sum of the masses of the top quark and the lightest neutralino. The study is performed in the final state of two b-jets, one lepton, large missing energy, and two high-$E_{\rm T}$ jets with large separation in pseudo-rapidity, in opposite hemispheres, and with large dijet mass. The LHC could discover compressed top squarks with mass up to approximately 340 GeV (390 GeV) with an integrated luminosity of 1000 ifb (3000 ifb)., Comment: Version updated with major changes: (a) 3-body stop decay (to b+W+n1) analyzed for first time (b) systematics calculation and discussion significantly upgraded (c) new kinematic and mass reach plots for the 3-body decay scenario added (d) discussions clarified throughout

Vector boson fusion (VBF) processes at the Large Hadron Collider (LHC) provide a unique opportunity to search for new physics with electroweak couplings. Two studies are presented: (i) A search of supersymmetric dark matter in the final state of two VBF jets and large missing transverse energy is presented at 14 TeV. Prospects for determining the dark matter relic density are studied for the cases of Wino and Bino-Higgsino dark matter. The LHC could probe Wino dark matter with mass up to approximately 600 GeV with a luminosity of 1000 fb$^{-1}$. (ii) A search for the chargino/neutralino system in the final state of two VBF jets, missing transverse energy and two $\tau$'s (light stau case) and light lepton $e$ and $\mu$ (light slepton case). The $5 \sigma$ mass reach at 300 fb$^{-1}$ (1000 fb$^{-1}$) of LHC14 for inclusive and opposite-sign $\tau$ pairs are 250 GeV (300 GeV) and 200 GeV (250 GeV), respectively, for $\Delta M \, = \, m_{\tilde{\tau}_1} - m_{\neu{1}} \, = \, 30$ GeV. For $\Delta M \, = \, 15$ GeV, the $3 \sigma$ mass reach at 300 fb$^{-1}$ (1000 fb$^{-1}$) of LHC14 for inclusive $\tau$ pairs is 180 GeV. The $5 \sigma$ mass reach at 300 fb$^{-1}$ (1000 fb$^{-1}$) of LHC14 for inclusive and opposite-sign $\mu$ pairs are approximately 350 GeV (400 GeV) and 300 GeV (350 GeV), respectively. The mass reaches in the same-sign final state cases are similar to those in the opposite-sign cases., Comment: 8 pages, 6 figures

Vector boson fusion (VBF) processes at the Large Hadron Collider (LHC) provide a unique opportunity to search for new physics with electroweak couplings. A feasibility study for the search of supersymmetric dark matter in the final state of two VBF jets and large missing transverse energy is presented at 14 TeV. Prospects for determining the dark matter relic density are studied for the cases of Wino and Bino-Higgsino dark matter. The LHC could probe Wino dark matter with mass up to approximately 600 GeV with a luminosity of 1000 fb$^{-1}$., Comment: 4 pages, 5 figures

Pair production of light top squarks at the 8-TeV LHC can be used to probe the gaugino-Higgsino sector of the Minimal Supersymmetric Standard Model. The case where the lightest neutralino is a mixture of Bino and Higgsino, satisfying the thermal dark matter relic density, is investigated. In such a scenario, the lightest top squark decays mostly into $(i)$ a top quark plus the second or third lightest neutralino, and $(ii)$ a bottom quark plus the lightest chargino, instead of a decay scenario of the lightest top squark into a top quark and the lightest neutralino. Final states with $\geq 2$ jets, dileptons, and missing energy are expected in a subsequent decay of the second or third lightest neutralinos into the lightest neutralino via an intermediate slepton ("light sleptons" case) or $Z$ boson ("heavy sleptons" case). The opposite-sign same flavor dilepton mass distribution after subtracting the opposite-sign different flavor distribution shows a clear edge in the case of light sleptons. The significance for discovering such a scenario is calculated with optimized cuts in both light and heavy sleptons cases., Comment: 6 pages, 3 figures. Version accepted in Phys. Rev. D

We pursue a scenario where the lighter top squark (stop) mass is accessible for the Large Hadron Collider (LHC) in the near future, while gluinos and first two generation squarks are heavier. At $\sqrt{s} = 8$ TeV, we investigate the identification of stops which decay predominantly into a top quark and the stable lightest supersymmetric particle. We use a simple kinematical variable, $M3$, to reconstruct two top quarks which are pair-produced from the stops, in the fully hadronic channel. The dominant Standard Model (SM) background for this signal stems from $t\bar t$ plus jets, with one top quark decaying into $ bl\nu$, where the lepton is undetected and the $\nu$ produces missing transverse momentum. The lepton identification efficiency is thus crucial in order to estimate the background correctly. We identify kinematical variables to reduce the SM background. We find that it is possible to achieve signal and background cross-section at similar levels for stop masses around $350 - 500$ GeV for a neutralino mass of 100 GeV., Comment: 7 pages, 6 figures

If supersymmetry is discovered at the LHC, the next question will be the determination of the underlying model. While this may be challenging or even intractable, a more optimistic question is whether we can understand the main contours of any particular paradigm of the mediation of supersymmetry breaking. The determination of superpartner masses through endpoint measurements of kinematic observables arising from cascade decays is a powerful diagnostic tool. In particular, the determination of the gaugino sector has the potential to discriminate between certain mediation schemes (not all schemes, and not between different UV realizations of a given scheme). We reconstruct gaugino masses, choosing a model where anomaly contributions to supersymmetry breaking are important (KKLT compactification), and find the gaugino unification scale. Moreover, reconstruction of other superpartner masses allows us to solve for the parameters defining the UV model. The analysis is performed in the stop and stau coannihilation regions where the lightest neutralinos are mainly gauginos, to additionally satisfy dark matter constraints. We thus develop observables to determine stau and stop masses to verify that the coannihilation mechanism is indeed operational, and solve for the relic density., Comment: 16 pages, 14 figures. Figure 7 updated to show top peak. Minor changes to other figures