Characterization of new solid state particle detectors and measurement of the central exclusive production of tt \u304 pairs at CMS

The CERN Large Hadron Collider (LHC) allows particle physics to explore unprecedented regimes and make a huge step forward in the understanding of fundamental interactions. The discovery of the Higgs boson by the ATLAS [1] and CMS [2] collaborations provided an excellent test of the standard model (SM) of particle physics. On the other hand, it is long known that the SM can only be an approximation at low energies of a more general theory, yet to be discovered: therefore, in addition to precision measurements within the SM, searches for signatures of new physics (NP) models beyond the SM represent a key goal of the physics programs of the LHC experiments. Searches are performed analysing a range of experimental signatures as wide as possible. So far, no evidence for deviations from the SM has been reported. The top quark, owing to its large mass, close to the electroweak (EWK) symmetry breaking scale, has long been seen as a window on NP. Several NP models indeed foresee a privileged role for the top quark sector. The LHC experiments have a huge program of measurements of top quark properties. At the LHC, in proton-proton collisions, the top quark is predominantly produced via quantum chromodynamics (QCD) diagrams that yield top quark-antiquark pairs, or via EWK diagrams in the so called \u201dsingle top\u201d production. Recently, at the CMS collaboration, the possibility arose to explore a new production mechanism of the top quark, the central exclusive production via gamma-gamma fusion. In fact, beam protons can often interact without disintegrating themselves, rather losing a small fraction of their energy and momentum, by exchanging photons, for example, and continuing their path: the lost 4-momentum can yield a variety of particles, referred to as the X system in the following, giving rise to events of the form pp \u2192 pXp; the X system can be top quark-antiquark pairs. CMS installed a new detector, the proton precision spectrometer (PPS), positioned at around 210m, along the beam line, on either side of the beam inter- action point: PPS allows to reconstruct those protons that interacted without disintegrating themselves. The measurement of the proton lost momentum, together with the reconstruction of the decay products of the X system by the central CMS detector, allows the study of events of the form pp \u2192 pttp. The cross section for this process has never been measured before. From a theoretical point of view, in the context of the standard model, the cross section is foreseen to be very small, generally below 1fb: calculations using the Monte Carlo generator FPMC [3] combined with MadGraph5 [4] yield a value of around 0.3fb. However NP scenarios can enhance it to values that can be tested with the data already collected by the LHC. In my PhD thesis, I participated in the ongoing efforts to measure the cross section of the pp \u2192 pttp process, selecting the so-called semi-leptonic channel, that is events where one of the two top quarks decays to a fully hadronic final state and the other to a final state containing a charged lepton-neutrino pair. While in the first run of its operation PPS comprised silicon-strip tracking detectors, a new silicon-pixel-based detector has been designed and built: during my PhD, I participated in all phases of the construction, commissioning and installation of the new pixel detectors. The system is made of several layers of sensitive material arranged in a mechanical structure, called \u201droman pot\u201d, that allows a positioning very close to the beam line: in fact, protons undergo only a tiny deviation after the interaction and, exploiting the LHC optics system, at 210mathrmm from the interaction point, they are still very close to the beam line. For these reasons, PPS silicon detectors operate under extreme conditions, in a very high radiation environment. Characterising and optimising the behaviour of the new detectors under various levels of radiation exposure played a role of paramount importance during the commissioning phase. The LHC is in operation since 2008. With the aging of some parts and the evolution of the operating conditions, the experiments have constantly updated and improved all systems along the years, taking advantage of the continuous advancements of the technologies for particle detectors. In 2020, the LHC and the experiments were in a shutdown phase for repairs and upgrades. In 2022, the operations will resume with a higher proton center-of-mass energy and with a larger luminosity. Very high luminosities yield extreme pile-up conditions, that is a large number of multiple interactions during the same beam bunch crossing, a phenomenon that can make event reconstruction problematic for the experiments. To cope with such extreme operating conditions, and at the same time maintain excellent performances, efforts have been devoted to design new generation timing detectors: in addition to spatial information, a precise tim- ing information in fact can help correctly assigning the reconstructed tracks to the interaction that produced them. In my PhD, I joined the TimeSpot collaboration, a team aimed at conceiving new solid-state timing detectors that implement novel configurations of p-n junctions to achieve unprecedented reso- lutions on the timing measurements

Temperature dependence of the response of ultra fast silicon detectors

The Ultra Fast Silicon Detectors (UFSD) are a novel concept of silicon detectors based on the Low Gain Avalanche Diode (LGAD) technology, which are able to obtain time resolution of the order of a few tens of picoseconds. First prototypes with different geometries (pads/pixels/strips), thickness (300 and 50μm), and gain (between 5 and 20) have been recently designed and manufactured by CNM (Centro Nacional de Microelectrónica, Barcelona) and FBK (Fondazione Bruno Kessler, Trento). Several measurements on these devices have been performed in the laboratory and in beam test and dependence of the gain on the temperature has been observed. Some of the first measurements will be shown (leakage current, breakdown voltage, gain, and time resolution on the 300μm from FBK and gain on the 50μm-thick sensor from CNM) and a comparison with the theoretically predicted trend will be discussed

Fabrication and Characterisation of 3D Diamond Pixel Detectors With Timing Capabilities

Diamond sensors provide a promising radiation hard solution to the challenges posed by the future experiments at hadron machines. A 3D geometry with thin columnar resistive electrodes orthogonal to the diamond surface, obtained by laser nanofabrication, is expected to provide significantly better time resolution with respect to the extensively studied planar diamond sensors. We report on the development, production, and characterisation of innovative 3D diamond sensors achieving 30% improvement in both space and time resolution with respect to sensors from the previous generation. This is the first complete characterisation of the time resolution of 3D diamond sensors and combines results from tests with laser, beta rays and high energy particle beams. Plans and strategies for further improvement in the fabrication technology and readout systems are also discussed

Intrinsic time resolution of 3D-trench silicon pixels for charged particle detection

In the last years, high-resolution time tagging has emerged as the tool to tackle the problem of high-track density in the detectors of the next generation of experiments at particle colliders. Time resolutions below 50ps and event average repetition rates of tens of MHz on sensor pixels having a pitch of 50$\mu$m are typical minimum requirements. This poses an important scientific and technological challenge on the development of particle sensors and processing electronics. The TIMESPOT initiative (which stands for TIME and SPace real-time Operating Tracker) aims at the development of a full prototype detection system suitable for the particle trackers of the next-to-come particle physics experiments. This paper describes the results obtained on the first batch of TIMESPOT silicon sensors, based on a novel 3D MEMS (micro electro-mechanical systems) design. Following this approach, the performance of other ongoing silicon sensor developments has been matched and overcome, while using a technology which is known to be robust against radiation degradation. A time resolution of the order of 20ps has been measured at room temperature suggesting also possible improvements after further optimisations of the front-end electronics processing stage.Comment: This version was accepted to be published on JINST on 21/07/202

Species diversity and distribution of amphibians and reptiles in Sardinia, Italy

Although distribution databases are a dynamic tool, continuously updated, it is important to take "snapshots" of the species distribution over time to promptly identify potential conservation issues. With this work, we provide an update of the distribution of amphibians and reptiles in Sardinia and satellite islands. Data derive from both direct field observations (carried out since 2005 until July 2022) and literature, accounting for over 7000 records: 1416 records of 11 species of amphibians and 5600 records of 18 species of reptiles. Distribution maps (on 10 × 10 km UTM grid) of 29 species are provided in supplementary materials as well as the updated list of the amphibians and reptiles occurring in the circum-Sardinian islands. Most of the meshes were characterized by the presence of 1-3 amphibian species (73%) and 6-8 or 9-11 reptile species (32% with 6-8 species, 30% with 9-11 species). Species abundance was favoured by environmental heterogeneity, and mostly varied in relation to elevation range and edge density

An embedding technique to determine ττ backgrounds in proton-proton collision data

An embedding technique is presented to estimate standard model tau tau backgrounds from data with minimal simulation input. In the data, the muons are removed from reconstructed mu mu events and replaced with simulated tau leptons with the same kinematic properties. In this way, a set of hybrid events is obtained that does not rely on simulation except for the decay of the tau leptons. The challenges in describing the underlying event or the production of associated jets in the simulation are avoided. The technique described in this paper was developed for CMS. Its validation and the inherent uncertainties are also discussed. The demonstration of the performance of the technique is based on a sample of proton-proton collisions collected by CMS in 2017 at root s = 13 TeV corresponding to an integrated luminosity of 41.5 fb(-1).Peer reviewe

Measurement of t(t)over-bar normalised multi-differential cross sections in pp collisions at root s=13 TeV, and simultaneous determination of the strong coupling strength, top quark pole mass, and parton distribution functions

Peer reviewe

Measurement of the top quark forward-backward production asymmetry and the anomalous chromoelectric and chromomagnetic moments in pp collisions at √s = 13 TeV

Abstract The parton-level top quark (t) forward-backward asymmetry and the anomalous chromoelectric (d̂ t) and chromomagnetic (μ̂ t) moments have been measured using LHC pp collisions at a center-of-mass energy of 13 TeV, collected in the CMS detector in a data sample corresponding to an integrated luminosity of 35.9 fb−1. The linearized variable AFB(1) is used to approximate the asymmetry. Candidate t t ¯ events decaying to a muon or electron and jets in final states with low and high Lorentz boosts are selected and reconstructed using a fit of the kinematic distributions of the decay products to those expected for t t ¯ final states. The values found for the parameters are AFB(1)=0.048−0.087+0.095(stat)−0.029+0.020(syst),μ̂t=−0.024−0.009+0.013(stat)−0.011+0.016(syst), and a limit is placed on the magnitude of | d̂ t| < 0.03 at 95% confidence level. [Figure not available: see fulltext.

Measurement of b jet shapes in proton-proton collisions at root s=5.02 TeV

We present the first study of charged-hadron production associated with jets originating from b quarks in proton-proton collisions at a center-of-mass energy of 5.02 TeV. The data sample used in this study was collected with the CMS detector at the CERN LHC and corresponds to an integrated luminosity of 27.4 pb(-1). To characterize the jet substructure, the differential jet shapes, defined as the normalized transverse momentum distribution of charged hadrons as a function of angular distance from the jet axis, are measured for b jets. In addition to the jet shapes, the per-jet yields of charged particles associated with b jets are also quantified, again as a function of the angular distance with respect to the jet axis. Extracted jet shape and particle yield distributions for b jets are compared with results for inclusive jets, as well as with the predictions from the pythia and herwig++ event generators.Peer reviewe

Search for dark matter in events with a leptoquark and missing transverse momentum in proton-proton collisions at 13 TeV

A search is presented for dark matter in proton-proton collisions at a center-of-mass energy of root s= 13 TeV using events with at least one high transverse momentum (p(T)) muon, at least one high-p(T) jet, and large missing transverse momentum. The data were collected with the CMS detector at the CERN LHC in 2016 and 2017, and correspond to an integrated luminosity of 77.4 fb(-1). In the examined scenario, a pair of scalar leptoquarks is assumed to be produced. One leptoquark decays to a muon and a jet while the other decays to dark matter and low-p(T) standard model particles. The signature for signal events would be significant missing transverse momentum from the dark matter in conjunction with a peak at the leptoquark mass in the invariant mass distribution of the highest p(T) muon and jet. The data are observed to be consistent with the background predicted by the standard model. For the first benchmark scenario considered, dark matter masses up to 500 GeV are excluded for leptoquark masses m(LQ) approximate to 1400 GeV, and up to 300 GeV for m(LQ) approximate to 1500 GeV. For the second benchmark scenario, dark matter masses up to 600 GeV are excluded for m(LQ) approximate to 1400 GeV. (C) 2019 The Author(s). Published by Elsevier B.V.Peer reviewe