Measurement of CP asymmetries in $\lambda^0_b \to pk^-$ and $\lambda^0_b \to p \pi^-$ decays at LHCb

The LHCb experiment has been designed to perform precision measurements in the flavour physics sector at the Large Hadron Collider (LHC) located at CERN. After the recent observation of CP violation in the decay of the Bs0 meson to a charged pion-kaon pair at LHCb, it is interesting to see whether the same quark-level transition in Λ0b baryon decays gives rise to large CP-violating effects. Such decay processes involve both tree and penguin Feynman diagrams and could be sensitive probes for physics beyond the Standard Model. The measurement of the CP-violating observable defined as ∆ACP = ACP(Λ0b → pK−)−ACP(Λ0b →pπ−),where ACP(Λ0b →pK−) and ACP(Λ0b →pπ−) are the direct CP asymmetries in Λ0b → pK− and Λ0b → pπ− decays, is presented for the first time using LHCb data. The procedure followed to optimize the event selection, to calibrate particle identification, to parametrise the various components of the invariant mass spectra, and to compute corrections due to the production asymmetry of the initial state and the detection asymmetries of the final states, is discussed in detail. Using the full 2011 and 2012 data sets of pp collisions collected with the LHCb detector, corresponding to an integrated luminosity of about 3 fb−1, the value ∆ACP = (0.8 ± 2.1 ± 0.2)% is obtained. The first uncertainty is statistical and the second corresponds to one of the dominant systematic effects. As the result is compatible with zero, no evidence of CP violation is found. This is the most precise measurement of CP violation in the decays of baryons containing the b quark to date. Once the analysis will be completed with an exhaustive study of systematic uncertainties, the results will be published by the LHCb Collaboration.

Search for CP violation in B-s(0) -> mu(+) D-s(-) X decays in p(p)over-bar collisions at root s=1.96 TeV

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A soft origin for CKM-type CP violation

We present a two-Higgs-doublet model, with a Z(3) symmetry, in which CP violation originates solely in a soft (dimension-2) coupling in the scalar potential, and reveals itself solely in the CKM (quark mixing) matrix. In particular, in the mass basis the Yukawa interactions of the neutral scalars are all real. The model has only eleven parameters to fit the six quark masses and the four independent CKM-matrix observables. We find regions of parameter space in which the flavour-changing neutral couplings are so suppressed that they allow the scalars to be no heavier than a few hundred GeV. (C) 2011 Elsevier B.V. All rights reserved.

Measurement of branching fractions and CP violation for charmless charged two-body B decays at LHCb

Charmless charged two-body B decays are sensitive probes of the CKM matrix, that parameterize CP violation in the Standard Model (SM), and have the potential to reveal the presence of New Physics. The framework of CP violation within the SM, the role of the CKM matrix, with its basic formalism, and the current experimental status are presented. The theoretical tools commonly used to deal with hadronic B decays and an overview of the phenomenology of charmless two-body B decays are outlined. LHCb is one of the four main experiments operating at the Large Hadron Collider (LHC), devoted to the measurement of CP violation and rare decays of charm and beauty hadrons. The LHCb detector is described, focusing on the technologies adopted for each sub-detector and summarizing their performances. The status-of-the-art of the LHCb measurements with charmless two-body B decays is then presented. Using the 37/pb of integrated luminosity collected at sqrt(s) = 7 TeV by LHCb during 2010, the direct CP asymmetries ACP(B0 -> Kpi) = −0.074 +/- 0.033 +/- 0.008 and ACP(Bs -> piK) = 0.15 +/- 0.19 +/- 0.02 are measured. Using 320/pb of integrated luminosity collected during 2011 these measurements are updated to ACP(B0 -> Kpi) = −0.088 +/- 0.011 +/- 0.008 and ACP(Bs -> piK) = 0.27 +/- 0.08 +/- 0.02. In addition, the branching ratios BR(B0 -> K+K-) = (0.13+0.06-0.05 +/- 0.07) x 10^-6 and BR(Bs -> pi+pi-) = (0.98+0.23-0.19 +/- 0.11) x 10^-6 are measured. Finally, using a sample of 370/pb of integrated luminosity collected during 2011, the relative branching ratios BR(B0 -> pi+pi-)/BR(B0 -> Kpi) = 0.262 +/- 0.009 +/- 0.017, (fs/fd)BR(Bs -> K+K-)/BR(B0 -> Kpi)=0.316 +/- 0.009 +/- 0.019, (fs/fd)BR(Bs -> piK)/BR(B0 -> Kpi) = 0.074 +/- 0.006 +/- 0.006 and BR(Lambda_b -> ppi)/BR(Lambda_b -> pK)=0.86 +/- 0.08 +/- 0.05 are determined.

Search for time-integrated cp violation in $d^0 \rightarrow k^+k^-$ and $d^0 \rightarrow \pi^+\pi^-$ decays at lhcb.

The LHCb experiment at the LHC, by exploiting the high production cross section for $c\overline{c}$ quark pairs, offers the possibility to investigate $\mathcal{CP}$ violation in the charm sector with a very high precision.\\ In this thesis a measurement of time-integrated \(\mathcal{CP}\) violation using $D^0\rightarrow~K^+K^-$ and $D^0\rightarrow \pi^+\pi^-$ decays at LHCb is presented. The measured quantity is the difference ($\Delta$) of \(\mathcal{CP}\) asymmetry ($\mathcal{A}_{\mathcal{CP}}$) between the decay rates of $D^0$ and $\overline{D}^0$ mesons into $K^+K^–$ and $\pi^+\pi^-$ pairs.\\ The analysis is performed on 2011 data, collected at \(\sqrt{s}=7\) TeV and corresponding to an integrated luminosity of 1 fb\(^{-1}\), and 2012 data, collected at \(\sqrt{s}=8\) TeV and corresponding to an integrated luminosity of 2 fb\(^{-1}\).\\ A complete study of systematic uncertainties is beyond the aim of this thesis. However the most important systematic of the previous analysis has been studied. We find that this systematic uncertainty was due to a statistical fluctuation and then we demonstrate that it is no longer necessary to take into account.\\ By combining the 2011 and 2012 results, the final statistical precision is 0.08\%. When this analysis will be completed and published, this will be the most precise single measurement in the search for $\mathcal{CP}$ violation in the charm sector.

Measurement of CP violation with $B^0_{(s)} \to h^+ h^{\prime-}$ decays, $|V_{cb}|$ with $B^0_{s}\to D^{(*)-}_s \mu^+ \nu_\mu$ decays, and simulation and characterisation of the LHCb Upgrade II ECAL

The time-dependent CP asymmetries of the $B^0\to\pi^+\pi^-$ and $B^0_s\toK^+K^-$ decays and the time-integrated CP asymmetries of the $B^0\toK^+\pi^-$ and $B^0_s\to\pi^+K^-$ decays are measured, using the $p-p$ collision data collected with the LHCb detector and corresponding to the full Run2. The results are compatible with previous determinations of these quantities from LHCb, except for the CP-violation parameters of the $B^0_s\to K^+K^-$ decays, that show a discrepancy exceeding 3 standard deviations between different data-taking periods. The investigations being conducted to understand the discrepancy are documented. The measurement of the CKM matrix element $|V_{cb}|$ using $B^0_{s}\to D^{(*)-}_s\mu^+ \nu_\mu$ is also reported, using the $p-p$ collision data collected with the LHCb detector and corresponding to the full Run1. The measurement leads to $|V_{cb}| = (41.4\pm0.6\pm0.9\pm1.2)\times 10^{-3}$, where the first uncertainty is statistical, the second is systematic, and the third is due to external inputs. This measurement is compatible with the world averages and constitutes the first measurement of $|V_{cb}|$ at a hadron collider and the absolute first one with decays of the $B^0_s$ meson. The analysis also provides the very first measurements of the branching ratio and form factors parameters of the signal decay modes. The study of the characteristics ruling the response of an electromagnetic calorimeter (ECAL) to profitably operate in the high luminosity regime foreseen for the Upgrade2 of LHCb is reported in the final part of this Thesis. A fast and flexible simulation framework is developed to this purpose. Physics performance of different configurations of the ECAL are evaluated using samples of fully simulated $B^0\to \pi^+\pi^-\pi^0$ and $B^0\to K^{*0}e^+e^-$ decays. The results are used to guide the development of the future ECAL and are reported in the Framework Technical Design Report of the LHCb Upgrade2 detector.

Leptonic CP violation

Several topics on CP violation in the lepton sector are reviewed. A few theoretical aspects concerning neutrino masses, leptonic mixing, and CP violation will be covered, with special emphasis on seesaw models. A discussion is provided on observable effects which are manifest in the presence of CP violation, particularly, in neutrino oscillations and neutrinoless double beta decay processes, and their possible implications in collider experiments such as the LHC. The role that leptonic CP violation may have played in the generation of the baryon asymmetry of the Universe through the mechanism of leptogenesis is also discussed.

Undoubtable signs of CP-violation in Higgs decays at the LHC run 2

With the discovery of the Higgs boson at the Large Hadron Collider the high energy physics community's attention has now turned to understanding the properties of the Higgs boson, together with the hope of finding more scalars during run 2. In this work we discuss scenarios where using a combination of three decays, involving the 125 GeV Higgs boson, the Z boson and at least one more scalar, an indisputable signal of CP-violation arises. We use a complex two-Higgs doublet model as a reference model and present some benchmark points that have passed all current experimental and theoretical constraints, and that have cross sections large enough to be probed during run 2.

Signatures of CP violation in the presence of multiple b-pair production at hadron colliders

We calculate the production of two b-quark pairs in hadron collisions. Sources of multiple pairs are multiple interactions and higher order perturbative QCD mechanisms. We subsequently investigate the competing effects of multiple b-pair production on measurements of CP violation: (i) the increase in event rate with multiple b-pair cross sections which may reach values of the order of 1 b in the presence of multiple interactions and (ii) the dilution of b versus b tagging efficiency because of the presence of events with four B mesons. The impact of multiple B-meson production is small unless the cross section for producing a single pair exceeds 1 mb. We show that even for larger values of the cross section the competing effects (i) and (ii) roughly compensate so that there is no loss in the precision with which CP-violating CKM angles can be determined.

Linking natural supersymmetry to flavour physics

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Search for direct CP violation in charm neutral meson decays at LHCb

A search for time-integrated violation of the CP symmetry, ACP(K−K+), in the Cabibbo-suppressed D0 → K−K+ decays is performed at the LHCb detector using proton- proton collisions recorded from 2015 to 2018 at the centre of mass energy of 13 TeV. The data used corresponds to an integrated luminosity of 5.7 fb−1. The flavour of the charm mesons is defined from the charge of the pion in D∗+ → D0π+ and D∗− → D0π− decays. Nuisance asymmetries are constrained from D∗+ → D0(→ K−π+)π+, D+ → KS0π+, D+ → K−π+π+, Ds+ → KS0K+ and Ds+ → φπ+ decays. The ACP(K−K+) asymme- try is measured to be ACP (K−K+) = (6.8 ± 5.4 (stat) ± 1.6 (syst)) · 10−4, in agreement with the previous LHCb results and the current world average. This represents the world’s most precise measurement of this quantity to date. Combining ACP (K−K+) with the time-integrated CP asymmetry difference, ∆ACP = ACP (K−K+)− ACP (π−π+), and the time-dependent CP asymmetry, ∆Y , measured with D0 → K−K+ and D0 → π−π+ decays, the direct CP asymmetries in D0 → K−K+ and D0 → π−π+ decays, adKK and adππ, result to be adKK =(7.7±5.7)·10−4, adππ =(23.2±6.1)·10−4, where the errors include systematic and statistical uncertainties and the correlation be- tween the two values is ρ(adKK,adππ) = 0.88. The values differ from zero for 1.4 and 3.8 standard deviations, respectively. In particular, adππ shows an evidence for direct CP violation in D0 → π−π+ decays.

A new bridge between leptonic CP violation and leptogenesis

Flavour effects due to lepton interactions in the early Universe may have played an important role in the generation of the cosmological baryon asymmetry through leptogenesis. If the only source of high-energy CP violation comes from the left-handed leptonic sector, then it is possible to establish a bridge between flavoured leptogenesis and low-energy leptonic CP violation. We explore this connection taking into account our present knowledge about low-energy neutrino parameters and the matter-antimatter asymmetry observed in the Universe. In this framework, we find that leptogenesis favours a hierarchical light neutrino mass spectrum, while for quasi-degenerate and inverted hierarchical neutrino masses there is a very narrow allowed window. The absolute neutrino mass scale turns out to be m less than or similar to 0.1 eV. (c) 2007 Elsevier B.V. All rights reserved.

Leptogenesis, CP violation and neutrino data: what can we learn?

A detailed analytic and numerical study of baryogenesis through leptogenesis is performed in the framework of the standard model of electroweak interactions extended by the addition of three right-handed neutrinos, leading to the seesaw mechanism. We analyze the connection between GUT-motivated relations for the quark and lepton mass matrices and the possibility of obtaining a viable leptogenesis scenario. In particular, we analyze whether the constraints imposed by SO(10) GUTs can be compatible with all the available solar, atmospheric and reactor neutrino data and, simultaneously, be capable of producing the required baryon asymmetry via the leptogenesis mechanism. It is found that the Just-So(2) and SMA solar solutions lead to a viable leptogenesis even for the simplest SO(10) GUT, while the LMA, LOW and VO solar solutions would require a different hierarchy for the Dirac neutrino masses in order to generate the observed baryon asymmetry. Some implications on CP violation at low energies and on neutrinoless double beta decay are also considered. (C) 2002 Elsevier Science B.V. All rights reserved.

Soft superweak CP violation in a 3-3-1 model

We show that it is possible to implement soft superweak CP violation in the context of a 3-3-1 model with only three triplets. All CP violation effects come from the exchange of singly and doubly charged scalars. We consider the implication of this mechanism in the quark and lepton sectors. In particular it is shown that, in this model, as in most of those which incorporate scalar mediated CP violation, it is possible to have large electric dipole moments for the muon and the tau lepton while keeping small those of the electron and neutron. The CKM mixing matrix is real at the tree level but gets a phase at the 1-up loop level. ©1999 The American Physical Society.