Periodic Reporting for period 3 - MajorNet (Majorana neutrino discovery strategy with CMS)
Berichtszeitraum: 2022-04-01 bis 2023-09-30
Currently, our knowledge of particle physics phenomena is basically complete. With the Large Hadron Collider (LHC) at CERN, we were able to discover the last predicted particle of the standard model (SM) of particle physics, and conduct numerous precision measurements which only confirm the predictive power of the standard model. Though no indisputable evidence of new physics has been found at the LHC, there is no doubt that new physics (e.g. new particles) must exist, as the dark matter nature, the sizeable matter-antimatter asymmetry of the Universe and very small neutrino masses are not predicted by the standard model. That is why special attention has to be paid to a unique window of opportunity provided by the LHC. It not only operates at the highest energy frontier but also is about to deliver an unprecedented amount of proton-proton collisions, and hence becomes an intensity frontier machine as well. The project takes full advantage of this fact and brings the possibility to discover new particles which could solve long-standing puzzles outlined above. It is motivated by a new approach to reconcile the established standard model and these unexplained phenomena that has drawn more and more attention over the last few years.
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.
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.
First, we have explored a possibility of HNLs with rather large masses, larger than 10 GeV and up to 1000 GeV, and short lifetime, meaning that these particles would decay very close to their production point. To make this possible, we deviated from the traditional approach of looking at fully visible in a detector HNL decays, and pioneered new particle search in this mass range with a partially reconstructed final state. For this, we looked at the events with three leptons (electrons or muons), and constructed various kinematic variables to differentiate between the known standard model processes and signal we were looking for. To make sure we are able to discover new particles with such strategy, we performed a signal injection test, where we added events with expected HNL distributions to the evaluated background processes, and proved that we would be able to see their presence. With the data recorded by the CMS detector in 2016, however, we have not seen any hints of HNLs, but we have improved the boundaries on their properties for the first time since more than 20 years. A follow-up, more traditional, analysis exploiting fully visible HNL final state was able to place stricter limits on very heavy particles.
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.
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.
With these results, an LHC experiment, CMS detector, has emerged as a leader in HNL searches for the wide range of possible masses and mixing parameters, and has proven that existing facilities can significantly advance the state-of-the-art knowledge about these hypothetical particles, by improving previous constraints up to two orders of magnitude. Acquired expertise will allow to deploy new, more efficient, algorithms for data filtering during the new data-taking period which is scheduled to start in 2022. As a result, analyses of new dataset will improve not only thanks to the larger volumes of data but also due to its higher quality for HNL searches. Finally, exploration of the signatures with even larger displacement of HNLs is currently ongoing, and it will bring sensitivity to lower values of HNL couplings than those already reported.