– Machine Learning and Measurements of Higgs boson properties
The discovery of the Higgs boson in 2012 was a major step towards improving the understanding of the mechanism of electroweak symmetry breaking (EWSB). With the value of the mass of the H boson now experimentally measured, the structure of the Higgs scalar field potential and the intensity of the Higgs boson self-couplings are precisely predicted in the SM. There are, however, compelling reasons to believe the SM is not complete. In particular, the LIP/CMS group is engaged in the study of SM and BSM processes to fully exploit the opportunities of the unparalleled energy of the LHC collisions. While measured properties are so far consistent with the expectation from the SM predictions, measuring the Higgs boson self-couplings provides an independent test of the SM and allows a direct measurements of the scalar sector properties. The self-coupling of the Higgs boson can be extracted from the measurement of the Higgs boson pair production cross section. A large amount of data of approximately 150/fb have been collected in Run1+Run2 and are available to study this process. With Run3, additional 300/fb of data may become available in the next few years and there will be excellent opportunities for major discoveries in this domain.
– Probing the Standard Model with Forward Proton Tagging at the LHC
Central exclusive production (CEP) in high-energy proton-proton collisions provides a unique method to access a variety of physics topics, such as new physics via anomalous production of W and Z boson pairs, high transverse momentum (pT) jet production, top quark pairs, and possibly the production of new resonances. These studies can be carried out in particularly clean experimental conditions thanks to the absence of proton remnants. CEP of an object X may occur in the process pp → p+X+p, where ”+” indicates the “rapidity gaps” adjacent to the state X. Rapidity gaps are regions without primary particle production. In CEP processes, the mass of the state X can be reconstructed from the fractional momentum losses ξ1 and ξ2 of the scattered protons. At the LHC, the mass reach of the system X, MX, is significantly larger than at previous colliders because of the larger collision energy. The scattered protons can be observed mainly thanks to their momentum loss, due to the horizontal deviation from the beam trajectory. For the first time, proton-proton collisions at the LHC provide the conditions to study particle production with masses at the electroweak scale through photon-photon fusion. At the LHC energies in Run2 and in Run3, values of MX above 300 GeV can be probed. CEP processes at these masses have small cross sections, typically of the order of a few fb, and thus can be studied in normal high-luminosity fills.
– Top quark physics and search for physics beyond the Standard Model at the Large Hadron Collider
Top quarks are abundantly produced in pairs at a hadron collider, and they constitute the main background to searches for New Physics. In the framework of the Standard Model (SM), each top quark decays into a W and a b quark. A good understanding of the top quark events will allow a more sensitive reach into the realm of searches for Beyond SM (BSM) processes. Recent checks of lepton flavour universality violation sparked a renewed interest towards measurements involving tau leptons, owing to a potential disagreement with SM predictions. The work will focus on studying the properties of the top quark dilepton final state and measure the tau and heavy flavour contents of top quark events. Studies of final states, including 3rd generation leptons and quarks such as tau leptons and b-jets, produced in association with top quark pair events may provide first hints for New Physics processes and shed light on the anomalies of Lepton Flavor Universality measurements. An anomalous flavor production is directly “visible” in this study. Deviations from SM predictions will indicate evidence for New Physics.
– Vector Boson Scattering processes at the Large Hadron Collider
The high-energy scattering of massive electroweak bosons, known as vector boson scattering (VBS), is a sensitive probe of new physics. VBS signatures will be thoroughly and systematically investigated with the large data samples available and those that will be collected in the near future at the LHC. Searches for deviations from Standard Model (SM) expectations in VBS will be performed with the goal of testing the Electroweak Symmetry Breaking (EWSB) mechanism. Current state-of-the-art tools and theory developments, together with the latest experimental results and the studies foreseen for the near future will be studied, implemented, and integrated in the research program. New data analysis strategies to understand the interplay between models and the effective field theory paradigm for interpreting experimental results will be developed with the goal of probing existing Beyond the SM (BSM) models.
– Probing the primordial quark gluon plasma with heavy flavour
At the LHC we recreate droplets of the primordial medium that permeated the universe in its first microseconds. This hot, dense, coloured medium, the quark-gluon plasma (QGP), is produced in ultra-relativistic heavy-ion collisions. The highest energies attained at the LHC and its state-of-the-art detectors are facilitating tremendous advancements in our understanding of the strong interaction, and of QCD matter at extreme conditions. Such data-driven advances also highlight unexpected behaviour, and the study of the QGP medium is fostered by novel probes facilitated by the large datasets being collected. One such probe is provided by heavy quarks. The bottom quarks are particularly interesting probes, as they are produced early in the collision and thus experience the full evolution of the hot medium. The aim of the Thesis project is the detection and study of b-quark hadrons in heavy-ion collisions (with particular the focus on the still-to-be-detected B0 meson, the rarer Bc meson, and possible exotic hadrons). These novel probes facilitate unique information on the flavour and mass dependence of energy loss mechanisms, as particles traverse the medium, and on underlying quark-recombination mechanisms yet to be observed at these scales. The Thesis project aims at making unique contributions to further our understanding of the primordial medium, by exploring novel probes and unprecedented energies at the LHC.
– LHC Anomalies and New Physics
The thesis explores the b->sll quark-level transition that is at the core of the current flavour anomalies. This is a flavor changing neutral current (FCNC) process that is highly sensitive to NP particles (that contribute to the decay process amplitude via quantum loops). Currently, the combined departure from the SM predictions, estimated from all of the available data, lie at about 7 standard deviations. The CMS experiment has the capability of facilitating precise measurements of this process with independent systematics that are crucial for clarifying the anomalies and establish the presence of NP at the LHC. The LIP group currently coordinates the exploration of such rare decays with the CMS experiment at the LHC. Members of the group have played leading roles in the exploration of b -> s mu mu processes, including the golden decay channels Bs->mumu and B0->K*mumu. The latter decay facilitates a set of NP sensitive angular observables. One of these, the so-called P5′, is a major contributor driving the anomalies. We are leading these measurements at CMS. We are currenly finalizing the analysis of the data collected in the previous LHC Run(2). The successful candidate will play the leading role in the exploration of the data collected by CMS in the current LHC Run(3).
– Tau lepton studies at the CMS experiment and at the future Muon Collider
This work will extensively investigate the tau reconstruction by means of new ML methods in two different environments at the CMS experiment at the LHC and at the new proposed Muon Collider. In the first case, data will be used to characterize tau lepton reconstruction. In the second case and due to the presence of the beam-induced background, a study of the tracking detector is also foreseen in order to maximize the exploitation of the time information in the silicon detector. Several options are possible, either measuring the time of each hit or having a dedicated layer in a strategic configuration. The algorithms will be studied on simulation and compared to CMS data. Using the new developed algorithms, the dedicated Higgs physics process will be reconstructed at CMS improving the current performance. They will be used to reconstruct the Higgs processes at the Muon Collider at the center-of-mass (CoM) energy of 3 TeV where the results will be compared to the those of the linear collider (CLIC). A CoM energy of 10 TeV is completely new and it has never been studied, and a brand new algorithm may be needed. Also at this energy the precision on Higgs boson couplings to tau leptons will be determined.
– High Precision Timing Detectors for Future Experiments
New challenges in current and future accelerator facilities have set stricter requirements on the timing and rate capabilities of particle detectors. The PICOSEC Micromegas detector has proven to time the arrival of Minimum Ionizing Particles (MPIs) with a sub-25 ps precision. Model predictions and laser beam tests demonstrated that an optimized PICOSEC design can time single photons with an accuracy of 45 ps which indicates an improved resolution in timing MIPs of the order of 15 ps. We propose the implementation of the PICOSEC detector for timing the arrival of EM showers with very high precision as well as for Time-of-Flight measurements for particle identification applications.
– High-performance timing detectors for the CMS upgrade at the High-Luminosity LHC and searches for new physics processes
One of the major challenges of operating such detectors at the HL-LHC is the “pileup”, or additional collisions occurring in the same proton bunch crossing as the collision of interest. By precisely measuring their time-of-flight, forward protons produced in these collisions can be correctly associated to the correct collision vertex, enabling rare photon-photon interactions to be reconstructed even at the maximum pileup foreseen for the HL-LHC. In order to fully resolve all collisions with forward protons, timing precisions of ~20ps or less will be of paramount importance, and enhance the sensitivity to these processes. During Run 2 of the LHC, proton fast timing detectors were already operated as a proof of principle, achieving resolutions of ~100ps with high efficiency. While these detectors were very resistant to radiation, they were limited, particularly by the TDC and related electronics, to timing resolutions of ~30-40ps per plane. For the HL-LHC, new technologies will be required to cope with the pileup, radiation, and event rates, both in terms of sensors and readout electronics. This project will help to address that challenge by exploring the latest developments in timing detectors and electronics, with the smallest possible segmentation and under extreme radiation conditions.