Measurements of CKM angle γ

(A. Bertolin)

Understanding the origin of the baryon asymmetry of the Universe is one of the key issues of modern physics. Sakharov showed that such an asymmetry can arise if three conditions are fulfilled, one of which is the requirement that both charge (C) and charge-parity (CP) symmetries are broken. The latter phenomenon arises in the Standard Model of particle physics through the complex phase of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix, although the effect in the SM is not large enough to account for the observed baryon asymmetry in the Universe. Violation of CP symmetry can be studied by measuring the angles of the CKM unitarity triangle. The least precisely known of these angles, γ ≡ arg[−Vud Vub*/ Vcd Vcb*], can be measured using only tree-level processes; a method that, assuming new physics is not present in tree-level decays, has negligible theoretical uncertainty. Disagreement between such direct measurements of γ and the value inferred from global CKM fits, assuming the validity of the SM, would indicate new physics beyond the SM. The value of γ can be determined by exploiting the interference between favored b →cW (Vcb) and suppressed b → uW (Vub) transition amplitudes. The B0s → Ds∓ K± decay channel is one of the “golden modes” for this measurement. The LHCb Padova group collaborated to the extraction of γ using this decay channel. Recently the results of this analysis have been published using all the LHCb Run 1 data set, corresponding to an integrated luminosity of 3 pb-1. Additionally, the LHCb Padova group is coordinating the efforts to pursue a similar analysis using the B0s → Ds*∓ K± decay channel. This later mode has a smaller candidates yield with respect to the former, mostly due to the difficulty to reconstruct the decay  Ds*∓ → Ds∓ γ in the LHC environment. However, an analysis performed using an independent mode offers the unique opportunity to cross check the result of the “golden mode”, B0s → Ds∓ K±.


Study of jets generated by bottom and charm quarks

(D. Lucchesi, L. Sestini, A. Gianelle)
LHCb is mainly designed for the study of bottom quark (b) and charm quark (c) Physics. These two flavored quarks are considered the key to access and observe new Physics phenomena, like new particles that decay in one b-quark and one anti-b-quark. The free quarks produced at the LHC are bounded in composite particles almost instantaneously, producing particle showers called jets, that are detected by the LHCb tracking and calorimeter systems. The LHCb-Padova group is on the front line in the reconstruction and identification of the b- and c-jets. First of all we are studying the kinematical properties of the b b-bar and c c-bar production in the forward region of p-p collision: this is important to understand the perturbative Quantum Chromo Dynamics (QCD) theory in a phase space region complementary to the other LHC experiments, where the theoretical uncertainties are still large. Then we are searching for Higgs-like scalar particles in the decay H→bb or H→cc . These searches are extremely challenging due to the overwhelming QCD multi-jet background. We have already probed the power of our analysis techniques by measuring and reconstructing the well known Z→bb resonance. Moreover we are developing new and more efficient jet identification algorithms that exploit the jet substructure to identify the flavour of the quark that has generated the shower. To do this we are using Deep Neural Networks techniques, that represent the latest development in machine learning techniques and artificial intelligence.


Lepton Flavour Universality Tests

(G. Simi, A. Lupato)
Lepton Flavour Universality (LFU) implies the equality of the coupling between the gauge bosons and the three families of leptons. It entails that the branching fractions of decays involving leptons do not depend on lepton flavour but differ only by phase space and helicity-suppressed contributions. Semileptonic and rare decays of heavy hadrons are an optimal laboratory to test LFU, since they allow to access all three generations of leptons. LFU is a Standard Model (SM) property and therefore, any violation would be a clear sign of New Physics. Over the years, LFU has been tested and has proven to be accurate in several systems providing very strong limits. A large class of models extending the SM contains additional interactions involving enhanced couplings to the third lepton generation that could violate LFU. An extended Higgs Sector could have a large effect on semitauonic decay rates through the coupling of new charged Higgs scalars. Therefore, semileptonic decays of b-hadrons to the third generation provide a sensitive probe for such effects. Ratios of branching fractions represent a powerful test of LFU since theoretical uncertainties cancel in the predictions and experimental systematics are much reduced. LHCb, using LHCb run I data, has already performed the measurement in the rare decay sector which resulted in a 2.6σ compatibility with the SM predictions. In the semileptonic decay sector, LHCb performed the measurements of R(D*) = B(B→ D*µν)/B(B→ D*τν), where the muonic and hadronic decays of tau are evaluated. The average (HFLAV) of these results with the R(D) measurements performed at B-factories is 4.1σ from the SM expectations. The LHCb Padova group is performing the measurement of  R(Λc*)=B(Λb→ Λc*µν)/B(Λb→ Λc*τν), where τ→µνν.This measurement is of great interest in order to confirm or disprove the discrepancies with respect to SM predictions found in analogue measurement performed on mesonic decays. Moreover it is the first measurement focused on baryonic decays. The LHCb Padova group is also collaborating to the measurement of Λb→ Λc* form factors and to the B(Λb→ Λc*Ds), ancillaries to the R(Λc*) measurement.


Ring-imaging Cherenkov detector (RICH)

(G. Simi, S. Gallorini, A. Lupato)
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 responisible for identifying a range of different particles that result from the decay of B mesons, including pions, kaons and protons. The LHCb Padova group is involved in the construction of the new RICH detectors for the upgrade of the experiment in 2021. The group is working on the test and quality assurance of the Multi-Anode PMTs (MaPMTs) and in the construction of the cold-bars where the MaPMTs will be mounted. In addition, Padova is taking part to the test-beams at CERN for the commissioning of the RICH acquisition system (DAQ) and in the analyis of the acquired data. In particular, the Padova group is currently working on defining the procedure for setting the optimal working point at which the MaPMTs will operate during Run3 data-taking.



(G. Collazuol, G. 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 is 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 will work on the silicon sensors characterization and in the validation for functional parameters (charge collection efficiency, intrinsic time resolution, radiation resistance). 3D silicon sensors are one of the two solutions under investigation for the TIMESPOT project. With the present proposal we plan to use the AN-Microbeam facility, Legnaro National Laboratories - I.N.F.N., in order to investigate the 3D sensors in terms of timing and charge collection performances as a function of the particle hit position, as well as to study the effect of a localized bulk damage in the silicon substrate. 


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