CMS Collaboration, JHEP 03 (2024) 105 Search for long-lived heavy neutral leptons with lepton flavour conserving or violating decays to a jet and a charged lepton

The LHC collision data has been searched for signs of exotic long-lived particles, so-called heavy neutral leptons. Their existence could answer various observed phenomena: Why are the neutrino masses so tiny? What is the nature of dark matter? Why is there much more matter than antimatter in the universe?

Unfortunately, no evidence for such particles have been found, yet the search significantly narrowed their possible parameters (eg. the particle's lifetime \(c\tau_{0}\) as shown in the plot) helping future analyses to focus their efforts.

CMS Collaboration, Mach. Learn.: Sci. Technol. 1 (2020) 035012 A deep neural network to search for new long-lived particles decaying to jets

An advanced deep neural network (DNN) has been developed, designed to tag jets, sprays of collimated particles, that originate from 0.1mm and up to 1m away from the proton-proton collision point in the CMS detector's center. This novel classifier significantly enhances our sensitivity in searching for exotic long-lived particles (LLPs), which can travel macroscopic distances before decaying, as illustrated in the accompanying plot.

A key innovation is the use of domain adaptation. This technique ensures that the performance of the classifier, which is initially trained on simulated data, matches its performance on real data. This on-the-fly calibration ensures that the DNN predictions remain accurate and reliable in practical applications.

CMS Collaboration, Eur. Phys. J. C 80 (2020) 370 Measurement of differential cross sections and charge ratios for \(t\)-channel single top quark production in proton-proton collisions at \(\sqrt{s}\) =13 TeV

The elusive production of single top quarks was first observed in 2009, at the Tevatron collider. Now, for the first time, the LHC enables comprehensive studies of single top quark production. This paper includes also a first measurement of the production ratio of top quark (\(t\)) and its antiparticle (\(\bar{t}\)), providing insights into the quark composition of the proton.

The measured ratio of about \(\frac{2}{3}\), as shown in the figure, confirms that the proton consists of two up quarks and one down quark. This is because interacting up quarks can lead to the production of top quarks, whereas interacting down quarks yield their antiparticles.

CMS Collaboration, JHEP 04 (2016) 073 Measurement of top quark polarisation in \(t\)-channel single top quark production

A distinctive feature of single top quarks is their near 100% polarization. Simply put, this means their spin is almost always aligned with their direction of flight. The accompanying figure illustrates an approximation of the cosine of the polarization angle. The degree of polarization can be derived from the slope of the data points.

The polarization provides valuable insights into the properties and interactions of single top quarks. Such results can also indirectly exclude certain new theories, that postulate new interactions and/or particles. If these would cause a depolarization of top quarks that contradicts the observed data, they can be ruled out.

CMS Collaboration, CMS-PAS-TOP-16-004 Measurement of the differential cross section for \(t\)-channel single-top-quark production at \(\sqrt{s}\) =13 TeV

Previously, the LHC operated at around half its maximum energy, specifically at 7 and 8 TeV. In 2015, it achieved a significant milestone by increasing its energy to 13 TeV for the first time. This paper presents the initial measurement of single top quark production using the very first data collected at this unprecedented energy level.

The figure displays the output score of a boosted decision tree (BDT), which effectively separates single top quark events (shown in red and labeled "\(t\)-channel") from other processes.