Periodic Reporting for period 3 - MajorNet (Majorana neutrino discovery strategy with CMS)
Reporting period: 2022-04-01 to 2023-09-30
There is a well-established way to describe very small neutrino masses by introducing heavy sterile neutrinos - or heavy neutral leptons (HNL) - which give masses to active neutrinos through a seesaw mechanism. What has been pointed out only recently is that there is a class of models with minimal extensions to the standard model, e.g. the addition of three HNL, which automatically solves two more fundamental puzzles: The lightest HNL (with a mass of a few keV) serves as a dark matter candidate, and, if the masses of the two heavier neutrinos (a GeV or so and heavier) are comparable, a sizeable source of CP violation can be introduced - to accommodate the matter-antimatter asymmetry observed in the Universe.
This project aims to probe deeply into the parameter space of HNLs which is favored from the cosmological point of view. It will improve the existing sensitivity to the HNL parameters by several orders of magnitude with a goal of a possible new particle discovery. The main innovation behind this project which makes a difference is to search for all possible signatures of the HNL decays simultaneously, looking for short- and long-lived HNLs, and without relying on the full HNL decay chain reconstruction.
After that, we concentrated on the challenging search for HNLs which fly some distance in a detector before their decay producing so-called displaced signature. The detector design and standard particle reconstruction algorithms are not optimized for the best performance in such signatures. Therefore, we developed dedicated techniques for the best identification of displaced leptons produced in long-lived HNL decays, targeting the decays happening within the CMS tracking detector volume. With novel signatures, we needed to also develop new calibration procedures which ensure that we understand the detector performance, and to come up with new methods for standard model background estimation, as previously there was no searches performed in similar parameter space. The efforts paid off by achieving sensitivity to HNL mixing parameter which is up to two orders of magnitude better than previous state-of-the-art. In this search, we haven't observed hints of new particles yet, but we have paved the way towards new exploration with a larger dataset to be recorded in the future.