Attività di ricerca

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  Research Activity

LHCb (standing for "Large Hadron Collider beauty") is one of seven Particle Physics detector experiments collecting data at the Large Hadron Collider accelerator at CERN. LHCb is a general purpose detector in the forward region of proton-proton collisions. It has excellent performance in measuring the properties of mesons and baryons containing b or c quarks. This is fundamental for measuring the parameters of the so-called CP violation: such studies can help in explaining the observed matter-antimatter symmetry of the Universe. The detector is also able to perform measurements of the electroweak bosons in the forward region, in order to perform unique tests of the Standard Model. Approximately 1100 people from 60 scientific institutes, representing 17 countries form the collaboration who built and operate the detector. Our research group is working on different topics, summarized in this webpage.

Beauty to open charm decays and charge conjugation parity violation at LHCb

(Alessandro Bertolin, Federica Borgato, Anna Lupato)
The study of the violation of the charge conjugation parity, CP,  discrete symmetry is one of the primary goals of the LHCb experiment. The violation of this symmetry is one of the requirements needed to explain the baryon asymmetry we observe in the Universe today. In the Standard Model (SM) CP violation is described by a single imaginary parameter in the Cabibbo Kobayashi Maskawa (CKM) unitary matrix describing the quark charged current weak interactions. It is possible to represent one of the unitary conditions of the CKM matrix  as a triangle in the complex plane. One of the angles of this unitary triangle, called gamma, can be directly measured in several tree-level decays accessible at LHCb. It is first of all important to cross check that a consistent value is obtained from different tree-level decays. Furthermore the direct measurement can be compared to indirect measurements obtained from global CKM fits. Any difference between the direct and indirect determinations would spot physics beyond the SM. With the combined Run 1 + Run 2 data set the direct LHCb measurement has an accuracy of about 4 degrees. The integrated luminosity expected from the LHCb Run 3, which will start with nominal detector conditions and luminosity in 2024, will significantly improve the accuracy and hence challenge the SM.
The interested candidate is expected to contribute to the analysis of one of the tree-level decay of interest from multiple fronts: optimization of the trigger selections, study of the reconstruction algorithms in the Run 3 data taking conditions and finally the extraction of the physically relevant quantities from the reconstructed tree-level decays.

Electroweak and Higgs Physics at LHCb

(Laura Buonincontri, Luca Giambastiani, Donatella Lucchesi, Lorenzo Sestini, Davide Zuliani)
LHCb is the only experiment at the LHC that covers the forward region of proton-proton collisions: measurements of W and Z bosons production cross sections in this phase space are unique and important tests of the Standard Model.
Thanks to the excellent detector performance, fundamental parameters of the Standard Model can be precisely measured by studying the properties of the electroweak bosons.
The Padova group is in the first line for measuring the electroweak boson production with jets in the final state. Jets are streams of particles produced by the fragmentation of quark and gluons, and all the detector subsystems are involved in their reconstruction.
W and Z bosons can decay hadronically in jets, and in order to identify them the energy scale of the jets should be precisely calibrated.
Moreover the collected W and Z boson events with the associated production jets can be used to probe the proton structure in a phase space region not accessible by other LHC experiments.
These kinds of analyses are extremely challenging, and the Padova group is developing reconstruction techniques that could also run online during the data taking.
In the near future the group aims to measure for the first time the production of two bosons in the forward region (WW, ZZ or WZ), in order to test almost unexplored sectors of the Standard Model.
Another important goal is to prepare the ground for the observation of the Higgs boson decay to b- or c-jets in the future LHCb upgrades. In this context new reconstruction and identification techniques for heavy flavor jets are being developed.

Semileptonic decays at LHCb

(Federica Borgato, Anna Lupato, Gabriele Simi)
The semileptonic decays play a prominent role in heavy quark physics since they are a rich source of observables sensitive to the Standard Model and New Physics parameters. The semileptonic decays are used to measure the values of CKM matrix elements V_ub and V_cd, to study the properties of b-hadrons (production fractions, form factors and lifetimes), the neutral meson mixing and in order to test the lepton flavor universality (LFU).
Despite the efforts from both experimental and theoretical sides, the measurements of the CKM parameter from inclusive and exclusive decays, remain in significant tension. Moreover, the measurements of the LFU gained in the last decade a lot of interest because of the intriguing discrepancies with respect to their precise SM expectations. Even if the tension with the SM seems to have reduced recently, more precise and complementary measurements are necessary. In particular, the measurements in the baryonic sector provide complementary constraints on a potential lepton flavor universality violation because of the half-integer spin of the initial state.
The LHCb Padova is strongly involved in LHCb semileptonic measurement, a lot of possible measurements can be performed and tools developed. The group is working on the test of LFU using the baryonic sector and also on the measurement of the form factors of the decay Λb0→ Λc*.
Furthermore the group is involved on the search for Λb0→ Λc*Ds(*) decays measurement. This decay mode of the Λb0 has never been observed before and is a significant source of background for the semi-tauonic decay of the  Λb0→ Λc*τ-ντ. A preliminary study shows that it is possible to select O(100) events over a small background. This analysis is expected to yield the first observation of this decay and the first measurement of its branching fraction.

Quantum algorithms for High Energy Physics

(Alessio Gianelle, Donatella Lucchesi, Lorenzo Sestini, Davide Zuliani)
Machine Learning algorithms have played an important role in High Energy Physics with a wide range of applications, from classification problems to simulation. The large variety of models applied to Large Hadron Collider data has demonstrated that there is still room for improvement. In this context Quantum Machine Learning is a new and almost unexplored methodology, where the intrinsic properties of quantum computation could be used. These kinds of algorithms could be run on available quantum computers with a limited number of qubits, but a major breakthrough is expected in the near future with new many-qubits systems.
The LHCb-Padova group has developed one of the first applications of Quantum Machine Learning for High Energy Physics: the identification of hadronic jets produced by b- or b-bar quarks.
Newer applications are being developed, as an example the classification of jets produced from the Higgs decay to charm quarks. Moreover studies on how the entanglement entropy could be used to improve the quantum circuit performance have been conducted and they are currently on-going.
Other on-going studies in this field, done in the LHCb-Padova group, are related to the simulation of LHCb data with quantum generative algorithms.

Developing of a calorimeter system for LHCb upgrades and future experiments

(Paolo Andreetto, Alessio Gianelle, Donatella Lucchesi, Lorenzo Sestini, Davide Zuliani)
A major upgrade of the LHCb experiment is planned during the high luminosity era of the LHC: of about 50 proton-proton collisions per bunch crossing are expected. In this environment a new radiation-hard and high-granularity calorimeter will be necessary to measure the high multiplicity of produced particles. This calorimeter could be also designed for enhancing the electroweak physics capabilities of LHCb.
The Padova group is involved in simulation studies for the design of the new calorimeter, in particular in the assessment of electron reconstruction performance with different configurations. The group is also participating in test beams at CERN and other laboratories where different calorimeter technologies are tested.
Moreover the group is working on the development of new systems based on programmable chips (FPGAs) for the online reconstruction of calorimeter clusters, by employing groundbreaking algorithms.
These studies are done in synergy with the development of detectors for experiment in future facilities, like muon colliders.

First time resolved RICH detector for LHCb and future experiments

(Federica Borgato, Anna, Lupato, Gabriele Simi)
The LHCb RICH detectors are built for particle identification. Lying on either side of LHCb’s magnet, the detectors are positioned to intercept particles flying at different angles. RICH detectors work by measuring the Cherenkov light emitted when a charged particle passes through a certain medium (in this case, a dense gas). As it travels, the particle emits a cone of light, which the RICH detectors reflect onto an array of sensors using mirrors. The shape of the cone of light depends on the particle’s velocity, enabling the detector to determine its speed. This information can be combined with the particle trajectory to deduce its identity. The two RICH detectors are responsible for identifying a range of different particles that result from the decay of B mesons, including pions, kaons and protons. The LHCb Padova group has been involved in the construction of the new RICH detectors for the upgrade of the experiment in 2021. The group was responsible for the characterization of the Multi-Anode PMTs (MaPMTs), the design and construction of the mechanical support structure of the MaPMTs and the integrated cooling system for the electronics. In addition, Padova is taking part to the commissioning of the RICH detector in order to defining the optimal working point at which the MaPMTs will operate during Run3 data-taking.

RICH upgrades: first time resolved RICH

In the high luminosity LHC (HL-LHC) phase the collider will operate at 1.5x10$^{34}$/cm/s  and this poses stringent requirements on the capabilities of subdetectors due to the  increased particle multiplicity and hit occupancy. The Upgrade II LHCb RICH subsystem, in  particular, will require improvements in spatial and time resolution to maintain high particle  identification performance in the HL-LHC environment.
To address these requirements, a readout electronics enhancement is planned during the  Long Shutdown 3 (LS3) phase. The goal is to provide hit timestamps with an accuracy on  the order of O(100) ps from Run4 onwards, with further research and development on  novel sensors aiming for sub-100 ps resolution by Run5. The LS3 enhancements will  involve the use of the FastIC 65-nm CMOS chip, developed by ICCUB-CERN. In order to evaluate the time resolution of the prototype photo-detection chain equipped  with the FastIC chip, two beam test campaigns were conducted at CERN SPS with 1-inch and 2-inch Multi Anode Photomultipliers (MaPMTs) currently used in the LHCb RICH, as well as a Silicon  Photomultiplier matrix. We are using the test-beam data to estimate the estimated Single Photon Time Resolution (SPTR) of the 1-inch  and 2-inch MaPMTs, this involves development of specific algorithms to extract the time resolution from the test-beam data and from dedicated laser lab setup measurements in order to demonstrate that the time resolution is dominated by the detector transit time resolution. Available Ph. D. thesis: simulation the detector behavior by means of GEANT4, study and optimization of the performance of the time resolved rich, development of reconstruction algorithms to make use of the time information.

Next generation of ultrafast and radiation hard SiPM

Next generation RICH detectors for experiments as LHCb will need photon detectors capable of withstanding doses up to 3e13 neutron equivalent/cm2, time resolution better than 100ps and granularity of 1mm2.
In collaboration with FBK, we plan to develop and characterize Back Side Illuminated SiPM engineered to reduce the effects of radiation damage, improve the temperature coefficient and the fill factor, while maintaining excellent time resolution.
These detectors are one of the candidates considered as a replacement for the current photon detectors of the LHCb RICH during the LS4 of the LHC. This will allow to maintain the same particle identification performance of the detectors as in run1 and run2.
Available Ph.D. thesis: study of the prototypes of SiPM detectors in order to optimize their capabilities to resist radiation damage, their temperature coefficient, their geometric efficiency and their time resolution, simulation of the detector performance after upgrade, development of reconstruction algorithms using time stamped detector signals.

Development of Metamaterials for improved PID in RICH detectors

Nanostructured materials offer new possibilities to engineer the properties of Cherenkov radiators for RICH detectors. Among the many possibilities we plan to investigate negative index materials which emit backward cherenkov radiation allowing new arrangements of photodetectors, and anisotropic metamaterials with enhanced sensitivity for particle identification at high momentum. Available Ph D. thesis : fabrication of metamaterials in collaboration with the condensed matter group, study their characteristics trough tests with a beam of particles, simulation of the application to a detector for particle identification

Next generation 4D tracking

(Federica Borgato, Gabriele Simi)
High Luminosities planned at colliders of the next decades pose very severe requirements on vertex detector systems in terms of space resolution (tens of  μm), radiation hardness (some 1016 1 MeV neq cm-2 and some Grad) and data throughput (Tbit/s). Expected event pile-up (of the order of 100) introduces the need to add high resolution time measurements (100 ps) already at the single pixel level. This demand pushes towards a new concept of vertex detector system, where all these features must operate at the same time. The Padova INFN Section has been involved in the TIMESPOT collaboration (INFN Call Gr V), which has the purpose to finalize existing technologies in the direction of such an innovative tracking apparatus. In particular, the Padova team worked on the silicon sensors characterization, the validation of the functional parameters (charge collection efficiency, intrinsic time resolution, radiation resistance) and on the detailed analysis of the time resolution achieved in tests with a beam of pions before and after irradiation. We also studied  timing and charge collection performances as a function of the particle hit position using the AN-Microbeam facility, Legnaro National Laboratories - I.N.F.N. A tracker demonstrator is in preparation and will be tested and characterized with a beam of pions at CERN.  Available Ph.D. thesis: the Ph.D. candidate will take part in the tests with beam at CERN, the data analysis, the development of the reconstruction algorithms, the characterization of the front-end readout ASIC.