An exploratory study for a new method to search for heavy neutrinos with data at the Large Hadron Collider (LHC)

• Main Author: Tan, Cherlene Shu Ning
• Other Authors: Leek Meng Lee
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Science::Physics
• Online Access: https://hdl.handle.net/10356/144837
• Institution: Nanyang Technological University

The Standard Model of Particle Physics classifies all known elementary particles. All fermions of the elementary particles in the standard model have a known left and right chirality- except the neutrino. Till this date, all neutrinos observed are left-handed, rotating clockwise with respect to the direction of motion while the corresponding antineutrinos are right-handed, spinning counterclockwise. However, the fact that neutrinos have mass and spin implies that right handed neutrinos and their corresponding left-handed antineutrinos may exist and have differing properties from their chiral counterparts. We introduce a mechanism called the seesaw mechanism where we introduce right handed neutrinos to fill these gaps so that the picture is more complete. Through the seesaw mechanism, we found the right handed neutrino can possess a light mass eigenstate and a heavy mass eigenstate of which the latter is the main focus of this report. It is also known as the Heavy Neutral Lepton (HNL) or heavy neutrino. If the mass scale of HNLs are sufficiently low, current and future research facilities would be able to detect HNLs. So far, neutrino appearance oscillation experiments, specifically the LSND and MiniBooNE hints the existence of HNLs. However, disappearance experiments had invalidated such results. Nevertheless to this date, there are no conclusive evidence proving or disproving the evidence of HNLs in laboratory tests and the search for HNLs is still ongoing at several current research facilities. However, there still is much potential mass of the HNL $M_N$ combined with its mixing probability $|V_{\ell N}|$ unexplored, especially at the lower limit of couplings. This report is an initial and simplified study of the CMS detector with the ultimate goal of finding out if mesons produced by the CMS could extend the search range of the HNL. Simulating using a Monte Carlo approach based on the data collected at the LHC so far, the sensitivity reach for these possible new particles is probed. The study will evaluate a new method for the LHC using B and D mesons as well as pions and kaons produced in proton-proton collisions at a centre of mass energy $\sqrt{s}= 13 TeV$, with an integrated luminosity of 150 fb$^{−1}$ at the Large Hadron Collider, collected by the LHC experiment. This report would focus on low energy HNLs that are in range of 100 MeV up to a maximum of 5 GeV. Using PYTHIA and ROOT, histograms of B and D mesons as well as pions and kaons of its transverse momentum (p$_T$), energy (E) as well as the 2D and 3D decay of pions and kaons were studied in order to give a better understanding on the characteristics of the mesons that produces the various flavoured neutrinos. The number of neutrinos produced in 500000 events was calculated, where the total number of kaons and pions producing neutrinos were 48187.28\% more than the number of B and D mesons produced, which increase the chance of detecting a HNL. However, unlike the B and D mesons, pions and kaons do not decay instantly and hence is a limiting factor to detect the products of HNL in the CMS detector that have a radius of 1m. In addition, the pions and kaons do not produce tau neutrinos making the study of HNL mixing with tau flavoured neutrinos only applicable to those produced by B and D mesons. In the next part, PYTHIA and PYTHON was used to calculate the number of HNL produced by the various meson types producing the various flavours of neutrinos that decay before 50cm $N_{HNLTOT}$. There are a significant number of HNL produced from the various mesons, where the largest $M_N$ $|V_{\ell N}|$ range of HNL can be probed by B Mesons due to it having the largest mass. Consequently, the B meson is able to extend the search of HNLs to the largest extend as compared to current experimental facilities around the world. If the HNL search focus is placed near the seesaw limit, B and D mesons as well as kaons are viable options for producing competitive HNL mass measurements. Searching for HNLs corresponds of trying to address some of the most fundamental questions in particle physics and the understanding of the Universe as we know it. The existence of HNLs could explain the matter-antimatter asymmetry of our Universe and it could explain what the nature of dark matter is. If the discovery potential of the HNL is significant, this can directly affect the data collection strategy of the CMS experiment to make such a search, and new on line data selection techniques can be defined and deployed for future data taken. A study on the already collected data can be initiated.