The electroweak interactions of the top quark as a door to new physics at the LHC

Since the start of the Large Hadron Collider (LHC) at 13 TeV, the CMS experiment recorded a data set of proton-proton collisions amounting to 80/fb. The heaviest known elementary particle of the standard model, the top quark, has been observed again and first measurements of its properties enter the precision regime. Another salient feature of the data is the absence of any signal of physics beyond the standard model, resulting in strong limits on models with supersymmetry, large extra dimensions or on other theories that each attempt to solve the various shortcomings of the standard model. Can we thus be sure that nature is well described by the standard model, even at the highest achievable energies? We challenge this view, because most of these searches focus on high energetic events that clearly stand out against the standard model backgrounds, neglecting the possibility that fainter signals are concealed in more subtle properties of the decaying particles, such as their momentum spectra. Such spectra have hardly been considered when searching for physics beyond the standard model. There are, however, many theories predicting such modifications, in particular the coupling of the electroweak gauge bosons to the top quark is of central interest. We confront these predictions with measurements of events where top quarks are produced in conjunction with gauge bosons, in particular the Z boson and the photon. Because the full CMS Run-II data set of 150/fb will provide a sufficient amount of events for precision measurements, it proposes a unique opportunity to directly measure fundamental top quark properties for the first time. We aim to characterize the electroweak interactions of the Top quark and perform the first direct measurement of the top quark electroweak dipole moments, and to obtain the most precise LHC result on the neutral current interactions. With carefully tuned reconstruction techniques and by a combination of results from final states with a Z boson or a photon, the top quark opens new experimental opportunities that we employ for a novel and profound view at physics beyond the standard model.

Electromagnetic interactions govern everyday life: You see an object when particles of light deposit energy on the retinas in your eyes. Our sense of touch results from the exchange of the same particles, the photons, between the atoms in our skin and the objects we touch. A unified understanding of these seemingly unrelated phenomena accounts for the fascination with physics. This project studied the interaction of photons with the top quark, the heaviest known fundamental particle in data from the CMS experiment at CERN's Large Hadron collider. Among the more than 100 million collision events that produced top-quark pairs from 2016 to 2018, the new result focuses on a tiny fraction: the one in 3000 collisions where we find a photon and top quarks at the same time. We probe the top quark's electromagnetic interaction in that case: it shines highly energetic photons at the detector, and this allows physicists to 'see' the top quark in a different light. At 10 billion times the energy of visible light, such photons cannot be seen directly. Instead, the CMS electromagnetic calorimeter indirectly provides eyesight. When photons impact the detector material, their energy showers into large numbers of electrons and more photons. The CMS electromagnetic calorimeter, located inside the large CMS solenoid magnet, uses crystals to record these showers. It measures the energy and individually identifies the incident particles. The main active elements are 75848 scintillating crystals made of lead tungstate, which is heavier than iron but transparent as glass. The crystals are arranged in a barrel section and two endcaps, thus allowing accurate measurements of the photon's energy. And the result of the measurement? We know that the top quark has a bizarre quantum mechanical property, the "spin": although the top quark has no dimensions, it has angular momentum as if it were rotating. The measured cross section allows us to determine the spin axis' reaction to an electric or magnetic field. We found small negative values for the electromagnetic dipole moments of the top quark, describing such effects. This result is in slight tension with the prediction from the standard model of particle physics. The top quarks' energetic light is visible, but the "color" seems slightly stained. Future analysis at the high-luminosity LHC will make the conclusion definite.