Long-Lived Heavy Neutral Leptons at the FCC-ee

The Future Circular electron-positron Collider (FCC-ee) is a high luminosity frontier particle accelerator for high precision measurements of the heaviest fundamental particles and possible evidence of new physics. Heavy Neutral Leptons (HNLs) are examples of new particles with discovery potential at the FCC-ee that could answer some of the most substantial questions in particle physics, such as how neutrinos oscillate between flavour eigenstates. Their estimated discovery region at the FCC-ee includes masses and mixing angles that allow the HNLs to attain long-lived properties. Therefore, long-lived signatures such as displaced vertices can be considered in this search for these particles. This thesis presents the first sensitivity analysis of these long-lived HNLs in the FCC-ee framework for the experimental run at the Z pole. The signal simulation used the Type I Seesaw mechanism with one benchmark HNL mixing with electron flavours and leptonic final states with electrons and electron neutrinos. An analysis of the signal and background is given, which results in a proposed event selection to reduce the background and increase the signal sensitivity. The final results include a sensitivity plot as a function of mass and mixing angle. This first successful implementation of a HNL analysis in future colliders can be used as a foundation for future explorations of long-lived HNLs at FCC-ee

Long-Lived Heavy Neutral Leptons at the FCC-ee

The Future Circular electron-positron Collider (FCC-ee) is a high luminosity frontier particle accelerator for high precision measurements of the heaviest fundamental particles and possible evidence of new physics. Heavy Neutral Leptons (HNLs) are examples of new particles with discovery potential at the FCC-ee that could answer some of the most substantial questions in particle physics, such as how neutrinos oscillate between flavour eigenstates. Their estimated discovery region at the FCC-ee includes masses and mixing angles that allow the HNLs to attain long-lived properties. Therefore, long-lived signatures such as displaced vertices can be considered in this search for these particles. This thesis presents the first sensitivity analysis of these long-lived HNLs in the FCC-ee framework for the experimental run at the Z pole. The signal simulation used the Type I Seesaw mechanism with one benchmark HNL mixing with electron flavours and leptonic final states with electrons and electron neutrinos. An analysis of the signal and background is given, which results in a proposed event selection to reduce the background and increase the signal sensitivity. The final results include a sensitivity plot as a function of mass and mixing angle. This first successful implementation of a HNL analysis in future colliders can be used as a foundation for future explorations of long-lived HNLs at FCC-ee

Simulations of fission fragments in VERDI : A study of the Plasma Delay Time phenomenon

The purpose of this project is to study the plasma delay time phenomenon in preparation for the construction of the VERDI spectrometer. To accomplish this, simulations of the spontaneous fissioning process of Cf-252 were created using the fission code GEF, as well as MATLAB. GEF has produced one million fission events, from which the time of flight and kinetic energy of each fission fragment have been calculated with classical mechanics, to replicate the experiment. To imitate the plasma delay time phenomenon, three different models, found in the literature, have been compared. Accounting for other realistic resolution effects and using the first model as the plasma delay time phenomenon, the absolute errors of the mass-yields reaches up to 4 u, whereas the second and third model display absolute errors up to 3 u. Furthermore, it is found that, despite the significant differences in the models' dependencies, the resulting effects are quite similar. All models are found to have a narrowing influence on the pre-neutron emission mass- yield distributions, resulting in an increased peak-to-valley ratio. In the detection of fission fragments, a higher peak-to-valley resolution is often associated with a better mass resolution. This study shows that the plasma delay time could have a misleading influence in regards to estimating an experimental mass resolution

Simulations of fission fragments in VERDI : A study of the Plasma Delay Time phenomenon

The purpose of this project is to study the plasma delay time phenomenon in preparation for the construction of the VERDI spectrometer. To accomplish this, simulations of the spontaneous fissioning process of Cf-252 were created using the fission code GEF, as well as MATLAB. GEF has produced one million fission events, from which the time of flight and kinetic energy of each fission fragment have been calculated with classical mechanics, to replicate the experiment. To imitate the plasma delay time phenomenon, three different models, found in the literature, have been compared. Accounting for other realistic resolution effects and using the first model as the plasma delay time phenomenon, the absolute errors of the mass-yields reaches up to 4 u, whereas the second and third model display absolute errors up to 3 u. Furthermore, it is found that, despite the significant differences in the models' dependencies, the resulting effects are quite similar. All models are found to have a narrowing influence on the pre-neutron emission mass- yield distributions, resulting in an increased peak-to-valley ratio. In the detection of fission fragments, a higher peak-to-valley resolution is often associated with a better mass resolution. This study shows that the plasma delay time could have a misleading influence in regards to estimating an experimental mass resolution

Top Secrets: Long-Lived ALPs in Top Production

We investigate the discovery potential for long-lived particles produced in association with a top-antitop quark pair at the (High-Luminosity) LHC. Compared to inclusive searches for a displaced vertex, top-associated signals offer new trigger options and an extra handle to suppress background. We design a search strategy for a displaced di-muon vertex in the tracking detectors, in association with a reconstructed top-antitop pair.For axion-like particles with masses above the di-muon threshold, we find that the (High-Luminosity) LHC can probe effective top-quark couplings as small as $|c_{tt}|/f_a = 0.03~(0.002)/$TeV and proper decay lengths as long as $10(400)$ m, assuming a cross section of $1\,$fb, with data corresponding to an integrated luminosity of 150 fb$^{-1}$ (3 ab$^{-1}$). Our predictions suggest that searches for top-associated displaced di-muons will explore new terrain in the current sensitivity gap between searches for prompt di-muons and missing energy

Searches for long-lived particles at the future FCC-ee

The electron-positron stage of the Future Circular Collider, FCC-ee, is a frontier factory for Higgs, top, electroweak, and flavour physics. It is designed to operate in a 100 km circular tunnel built at CERN, and will serve as the first step towards ≥100 TeV proton-proton collisions. In addition to an essential and unique Higgs program, it offers powerful opportunities to discover direct or indirect evidence of physics beyond the Standard Model. Direct searches for long-lived particles at FCC-ee could be particularly fertile in the high-luminosity Z run, where 5 × 1012 Z bosons are anticipated to be produced for the configuration with two interaction points. The high statistics of Higgs bosons, W bosons and top quarks in very clean experimental conditions could offer additional opportunities at other collision energies. Three physics cases producing long-lived signatures at FCC-ee are highlighted and studied in this paper: heavy neutral leptons (HNLs), axion-like particles (ALPs), and exotic decays of the Higgs boson. These searches motivate out-of-the-box optimization of experimental conditions and analysis techniques, which could lead to improvements in other physics searches

Search for a resonance decaying to a W boson and a photon in proton-proton collisions at s= \sqrt{s} = 13 TeV using leptonic W boson decays

A search for a new charged particle X with mass between 0.3 and 2.0 TeV decaying to a W boson and a photon is presented, using proton-proton collision data at a center-of-mass energy of 13 TeV, collected by the CMS experiment and corresponding to an integrated luminosity of 138 fb$^{-1}$. Particle X has electric charge $\pm$1 and is assumed to have spin 0. The search is performed using the electron and muon decays of the W boson. No significant excess above the predicted background is observed. The upper limit at 95% confidence level on the product of the production cross section of the X and its branching fraction to a W boson and a photon is found to be 94 (137) fb for a 0.3 TeV resonance and 0.75 (0.81) fb for a 2.0 TeV resonance, for an X width-to-mass ratio of 0.01% (5%). This search presents the most stringent constraints to date on the existence of such resonances across the probed mass range. A statistical combination with an earlier study based on the hadronic decay mode of the W boson is also performed, and the upper limit at 95% confidence level for a 2.0 TeV resonance is reduced to 0.50 (0.63) fb for an X width-to-mass ratio of 0.01% (5%).A search for a new charged particle X with mass between 0.3 and 2.0 TeV decaying to a W boson and a photon is presented, using proton-proton collision data at a center-of-mass energy of 13 TeV, collected by the CMS experiment and corresponding to an integrated luminosity of 138 fb$^{-1}$. Particle X has electric charge $\pm$1 and is assumed to have spin 0. The search is performed using the electron and muon decays of the W boson. No significant excess above the predicted background is observed. The upper limit at 95% confidence level on the product of the production cross section of the X and its branching fraction to a W boson and a photon is found to be 94 (137) fb for a 0.3 TeV resonance and 0.75 (0.81) fb for a 2.0 TeV resonance, for an X width-to-mass ratio of 0.01% (5%). This search presents the most stringent constraints to date on the existence of such resonances across the probed mass range. A statistical combination with an earlier study based on the hadronic decay mode of the W boson is also performed, and the upper limit at 95% confidence level for a 2.0 TeV resonance is reduced to 0.50 (0.63) fb for an X width-to-mass ratio of 0.01% (5%)

Search for CP violation in D0→KS0KS0 \mathrm{D^0}\to\mathrm{K^0_S}\mathrm{K^0_S} decays in proton-proton collisions at s= \sqrt{s} = 13 TeV

A search is reported for charge-parity CP violation in $\mathrm{D^0}\to\mathrm{K^0_S}\mathrm{K^0_S}$ decays, using data collected in proton-proton collisions at $\sqrt{s} =$ 13 TeV recorded by the CMS experiment in 2018. The analysis uses a dedicated data set that corresponds to an integrated luminosity of 41.6 fb$^{-1}$, which consists of about 10 billion events containing a pair of b hadrons, nearly all of which decay to charm hadrons. The flavor of the neutral $\mathrm{D}$ meson is determined by the pion charge in the reconstructed decays $\mathrm{D}^{*+}\to\mathrm{D^0}\pi^{+}$ and $\mathrm{D}^{*-}\to\overline{\mathrm{D}}^{0}\pi^{-}$. The CP asymmetry in $\mathrm{D^0}\to\mathrm{K^0_S}\mathrm{K^0_S}$ is measured to be $A_{CP}(\mathrm{K^0_S}\mathrm{K^0_S}) =$ (6.2 $\pm$ 3.0 $\pm$ 0.2 $\pm$ 0.8)%, where the three uncertainties represent the statistical uncertainty, the systematic uncertainty, and the uncertainty in the measurement of the CP asymmetry in the $\mathrm{D^0}\to\mathrm{K^0_S}\pi^{+}\pi^{-}$ decay. This is the first CP asymmetry measurement by CMS in the charm sector as well as the first to utilize a fully hadronic final state.A search is reported for charge-parity D$^0$$\to$ K$^0_\mathrm{S}$K$^0_\mathrm{S}$$CP$ violation in D$^0$$\to$ K$^0_\mathrm{S}$K$^0_\mathrm{S}$ decays, using data collected in proton-proton collisions at $\sqrt{s}$ = 13 TeV recorded by the CMS experiment in 2018. The analysis uses a dedicated data set that corresponds to an integrated luminosity of 41.6 fb$^{-1}$, which consists of about 10 billion events containing a pair of ẖadrons, nearly all of which decay to charm hadrons. The flavor of the neutral D meson is determined by the pion charge in the reconstructed decays D$^{*+}$$\to$ D$^0\pi^+$ and D$^{*-}$$\to$ D$^0\pi^-$. The D$^0$$\to$ K$^0_\mathrm{S}$K$^0_\mathrm{S}$$CP$ asymmetry in D$^0$$\to$ K$^0_\mathrm{S}$K$^0_\mathrm{S}$ is measured to be $A_{CP}$( K$^0_\mathrm{S}$K$^0_\mathrm{S}$) = (6.2 $\pm$ 3.0 $\pm$ 0.2 $\pm$ 0.8)%, where the three uncertainties represent the statistical uncertainty, the systematic uncertainty, and the uncertainty in the measurement of the D$^0$ $\to$ K$^0_\mathrm{S}$K$^0_\mathrm{S}$ $CP$ asymmetry in the D$^0$ $\to$ K$^0_\mathrm{S}\pi^+\pi^-$ decay. This is the first D$^0$ $\to$ K$^0_\mathrm{S}$K$^0_\mathrm{S}$ $CP$ asymmetry measurement by CMS in the charm sector as well as the first to utilize a fully hadronic final state