SND@LHC is an approved experiment equipped to detect scatterings of neutrinos produced in the far-forward direction at the LHC, and aimed to measure their properties. In addition, the detector has... Show moreSND@LHC is an approved experiment equipped to detect scatterings of neutrinos produced in the far-forward direction at the LHC, and aimed to measure their properties. In addition, the detector has a potential to search for new feebly interacting particles (FIPs) that may be produced in proton-proton collisions. In this paper, we discuss signatures of new physics at SND@LHC for two classes of particles: stable FIPs that may be detected via their scattering, and unstable FIPs that decay inside the detector. We estimate the sensitivity of SND@LHC to probe scatterings of leptophobic dark matter and decays of neutrino, scalar, and vector portal particles. Finally, we also compare and qualitatively analyze the potential of SND@LHC and FASER/FASER nu experiments for these searches. Show less
The Standard Model (SM) of particle physics fails to explain several observed phenomena and is incomplete. In order to resolve this problem, one may extend the SM by adding new particles. However,... Show moreThe Standard Model (SM) of particle physics fails to explain several observed phenomena and is incomplete. In order to resolve this problem, one may extend the SM by adding new particles. However, yet they have not been observed, and currently, the scientific community tries to find a signature that manifests the existence and properties of such particles. This thesis is devoted to exploring the parameter space of Feebly Interacting new physics Particles (FIPs) in a model-independent fashion using two complementary approaches. The first one is searching for FIPs at next-generation accelerator experiments called Intensity Frontier experiments. The second one is constraining the parameter space of FIPs by considering their possible impact on the observables coming from the Early Universe - Big Bang Nucleosynthesis and Cosmic Microwave Background, which are in good agreement with the predictions of the cosmological models with SM particles. They are, therefore, very sensitive to the possible existence of FIPs in the primordial plasma. As a result of the researches constituting this thesis, novel model-independent results, as well as constraints on particular models of FIPs such as Heavy Neutral Leptons, have been obtained in both of these areas. Show less
Boiarska, I.; Boiarskyi, O.; Milulenko, O.; Ovchynnikov, M. 2021
The existence of feebly interacting massive particles (FIMPs) could have significant implications on the effective number of relativistic species N-eff in the early Universe. In this work, we... Show moreThe existence of feebly interacting massive particles (FIMPs) could have significant implications on the effective number of relativistic species N-eff in the early Universe. In this work, we investigate in detail how short-lived FIMPs that can decay into neutrinos affect N-eff and highlight the relevant effects that govern its evolution. We show that even if unstable FIMPs inject most of their energy into neutrinos, they may still decrease N-eff, and identify neutrino spectral distortions as the driving power behind this effect. As a case study, we consider heavy neutral leptons (HNLs) and indicate which regions of their parameter space increase or decrease Neff. Moreover, we derive bounds on the HNL lifetime from the cosmic microwave background and comment on the possible role that HNLs could play in alleviating the Hubble tension. Show less
Boiarskyi, O.; Ovchynnikov, M.; Ruchayskiy, O.; Syvolap, V. 2021
We constrain the lifetime of thermally produced heavy neutral leptons (HNLs) from big bang nucleosynthesis. We show that even a small fraction of mesons present in the primeval plasma leads to the... Show moreWe constrain the lifetime of thermally produced heavy neutral leptons (HNLs) from big bang nucleosynthesis. We show that even a small fraction of mesons present in the primeval plasma leads to the overproduction of primordial helium-4. This constrains the lifetime of HNLs to be tau(N) < 0.02 s for masses above the mass of the pion (as compared to 0.1 s reported previously). In combination with accelerator searches, this allows us to put a new lower bound on the HNL masses and define the "bottom line" for HNL searches at the future Intensity Frontier experiments. Show less
Bondarenko, K.; Boiarskyi, O.; Ovchynnikov, M.; Ruchayskiy, O.; Shchutska, L. 2019
Long-lived particles can manifest themselves at the LHC via “displaced vertices”—several charged tracks originating from a position separated from the proton interaction point by a macroscopic... Show moreLong-lived particles can manifest themselves at the LHC via “displaced vertices”—several charged tracks originating from a position separated from the proton interaction point by a macroscopic distance. Here we demonstrate the potential of the muon trackers at the CMS experiment for displaced vertex searches. We use heavy neutral leptons and Chern-Simons portal as two examples of long-lived particles for which the CMS muon tracker can provide essential information about their properties. Show less
Bondarenko, K.; Boyarsky, A.; Ovchynnikov, M.; Ruchayskiy, O. 2019
In recent years, a number of intensity frontier experiments have been proposed to search for feebly interacting particles with masses in the GeV range. We discuss how the characteristic shape of... Show moreIn recent years, a number of intensity frontier experiments have been proposed to search for feebly interacting particles with masses in the GeV range. We discuss how the characteristic shape of the experimental sensitivity regions — upper and lower boundaries of the probed region, the maximal mass reach — depends on the parameters of the experiments. We use the SHiP and the MATHUSLA experiments as examples. We find a good agreement of our estimates with the results of the Monte Carlo simulations. This simple approach allows to cross-check and debug Monte Carlo results, to scan quickly over the parameter space of feebly interacting particle models, and to explore how sensitivity depends on the geometry of experiments. Show less
Bondarenko, K.; Boyarsky, A.; Ovchynnikov, M.; et al 2019
Heavy Neutral Leptons (HNLs) are hypothetical particles predicted by many extensions of the Standard Model. These particles can, among other things, explain the origin of neutrino masses, generate... Show moreHeavy Neutral Leptons (HNLs) are hypothetical particles predicted by many extensions of the Standard Model. These particles can, among other things, explain the origin of neutrino masses, generate the observed matter-antimatter asymmetry in the Universe and provide a dark matter candidate. The SHiP experiment will be able to search for HNLs produced in decays of heavy mesons and travelling distances ranging between O(50 m) and tens of kilometers before decaying. We present the sensitivity of the SHiP experiment to a number of HNL's benchmark models and provide a way to calculate the SHiP's sensitivity to HNLs for arbitrary patterns of flavour mixings. The corresponding tools and data files are also made publicly available. Show less
Bondarenko, K.; Boiarskyi, O.; Ovchynnikov, M.; et al 2019
The Search for Hidden Particles (SHiP) Collaboration has shown that the CERN SPS accelerator with its 400 GeV/c proton beam offers a unique opportunity to explore the Hidden Sector {[}1-3]. The... Show moreThe Search for Hidden Particles (SHiP) Collaboration has shown that the CERN SPS accelerator with its 400 GeV/c proton beam offers a unique opportunity to explore the Hidden Sector {[}1-3]. The proposed experiment is an intensity frontier experiment which is capable of searching for hidden particles through both visible decays and through scattering signatures from recoil of electrons or nuclei. The high-intensity experimental facility developed by the SHiP Collaboration is based on a number of key features and developments which provide the possibility of probing a large part of the parameter space for a wide range of models with light long-lived super-weakly interacting particles with masses up to O(10) GeV/c(2) in an environment of extremely clean background conditions. This paper describes the proposal for the experimental facility together with the most important feasibility studies. The paper focuses on the challenging new ideas behind the beam extraction and beam delivery, the proton beam dump, and the suppression of beam-induced background.} Show less
