Pushing the boundaries of the LHCb rare decays program: search for the Λ0b→ Λγ decay

[eng] The main goal of this thesis is to test the Standard Model (SM) of particle physics through the experimental analysis of rare processes, those that are suppressed in the SM. In particular, radiative b and rare strange decays are very sensible to potential contributions from beyond the SM (BSM) physics. The data recorded by LHCb in proton-proton (pp) collisions during the Run 1 (2010-2012) and Run 2 (2015-2018) of the LHC is exploited with this purpose. The LHCb detector is a single-arm forward spectrometer characterised by a precise momentum and impact parameter resolution, a good separation between particles of different species and a very flexible trigger system. A trigger selection dedicated to radiative decays of b-hadrons has been developed for Run 2, based on multivariate techniques exploiting the common features of these decay modes. Two complementary configurations select two- and three-body plus photon candidates, respectively. The results obtained on different Monte-Carlo (MC) signal samples show an improvement with respect to the Run 1 selection of at least 20% in absolute efficiency for all the modes, while for some special cases, as B0 K+π−γ and B+ K∗+γ, larger gains of 70% and 60%, respectively, are achieved. Moreover, the rate is reduced to a half with respect to the Run 1 configuration, which allows to dedicate the remaining budget to exclusive selections for decays with special topologies, expanding even more the reach of the radiative decays physics program. Of special interest among rare strange decays are the K0 f+f−f+f− modes, where f can be either an electron or a muon. As a first milestone towards the study of these channels, the feasibility of observing the topologically similar K0 π+π−e+e− at LHCb has been assessed. Selection efficiencies have been extracted from MC samples, obtaining an expected signal yield in Run 1 conditions of 120+280/ fb−1. No −100 signal peak has been observed in the invariant mass distribution of selected candidates in the 2012 dataset and the possibility of observing this mode with these data has been evaluated with pseudoexperiments, concluding that an observation if feasible using a selection with a separation power similar to that usually achieved by multivariate algorithms. For Run 2, tow complementary trigger selections have been developed to improve its efficiency, reaching an expected yield of 620+290/ fb−1, which guarantees an 0−120 + − + − observation of this mode in Run 2 and allows the first studies of the KS → f f f f modes. In the area of radiative b-decays, the most interesting observable is the polarisation of the emitted photon, which is predicted to be almost 100% left-handed in the SM, while right-handed currents can contribute in BSM models. This observable is accessible through the previously unobserved Λ0 Λγ mode so a search for this decay has been performed using the dataset recorded by LHCb in pp collisions during 2016. Due to its particular topology, a dedicated reconstruction has been developed and used both in the trigger and offline. The candidate selection is based on multivariate methods, which have been trained using MC as a proxy for the signal and data as background. The requirement on the output distribution has been optimised by maximising the probability of achieving an observation of the signal, obtaining a 33% signal efficiency with a 99.8% background rejection. The B0 K∗0γ mode has been used as normalisation channel, which allows to measure the signal branching ratio from the ratio of yields. A simultaneous maximum likelihood fit of the invariant mass distributions has been performed resulting in a Λ0 → Λγ yield of (64 ± 13) events and a measurement of the branching fraction of B(Λ0 → Λγ) = (2.59 ± 0.52 (estad.) ± 0.28 (sist.) ± 0.33 (fΛ0 /fB0 )) × 10−6. The statistical significance of the signal has been evaluated by means of a likelihood profile scan, obtaining a 5.7σ significance, which represents the first observation of the Λ0 → Λγ decay and opens the doors to the study of the photon polarisation in this mode.[cat] En aquesta tesi s’han estudiat modes de desintegració radiatius d’hadrons b i rars de mesons estranys, amb l’objectiu de testejar el Model Estàndard (MS). S’han utilitzat dades enregistrades pel detector LHCb en col lisions protó-protó (pp) durant el Run 1 (2010-2012) i el Run 2 (2015-2018) de l’LHC. LHCb ´es un espectòmetre d’un sol brac¸ caracteritzat per una alta precisió en la reconstrucció del moment i el paràmetre d’impacte respecte del punt d’interacció, una gran capacitat per separar partícules de diferents espècies i un sistema activador (trigger) molt flexiu. Per al Run 2 s’ha desenvolupat una selecció de trigger dedicada a processos radiatius basada en algoritmes multivariats, explotant-ne les característiques comunes. S’ha obtingut un augment absolut en l'eficiència entorn del 20%. Per a casos amb topologia especial, com B0 K+π−γ i B+ K∗+γ, el guany arriba al 70% i 60%, respectivament. Pel que fa als modes rars de desintegració d’hadrons estranys, s’ha avaluat la viabilitat d’observar el canal K0 π+π−e+e− a LHCb. Utilitzant mostres de Monte-Carlo (MC), s’ha obtingut una senyal esperada de 120+280/ fb−1 en condicions del Run 1. En les dades del 2012 no s’ha observat la senyal −100 degut a la presència d’un gran fons i per mitjà de pseudo-experiments s’ha arribat a la conclusió que l’observació és possible utilitzant una selecció multivariada. Per al Run 2 s’han desenvolupat seleccions de trigger millorant la senyal esperada fins a 620+290/ fb−1. −120 0 Finalment, utilitzant les dades del 2016 s’ha realitzat una cerca del mode de desintegració Λb Λγ basada en el desenvolupament d’una reconstrucció especial i d’una selecció multivariada, que aconsegueix una eficiència del 33% rebutjant al mateix temps un 99.8% del fons. El nu´mero d’esdeveniments de senyal s’ha extret d’un ajust simultani de màxima versemblança de la distribució de massa invariant, obtenint-ne (64 ± 13), amb una significació estadística de 5.7σ, que constitueix la primera observació d’aquest mode de desintegració. Utilitzant el canal B0 → K∗0γ com a normalització, s’ha mesurat també la fracció d’embrancament B(Λ0 → Λγ) = (2.59 ± 0.52 (estad.) ± 0.28 (sist.) ± 0.33 (fΛ0 /fB0 )) × 10−6

Strong constraints on the b → sγ photon polarisation from B 0 → K *0 e + e − decays

An angular analysis of the B0 → K∗0 e +e − decay is performed using a data sample corresponding to an integrated luminosity of 9 fb−1 of pp collisions collected with the LHCb experiment. The analysis is conducted in the very low dielectron mass squared (q 2 ) interval between 0.0008 and 0.257 GeV2 , where the rate is dominated by the B0 → K∗0γ transition with a virtual photon. The fraction of longitudinal polarisation of the K∗0 meson, FL, is measured to be FL = (4.4 ± 2.6 ± 1.4)%, where the first uncertainty is statistical and the second systematic. The ARe T observable, which is related to the lepton forward-backward asymmetry, is measured to be ARe T = −0.06 ± 0.08 ± 0.02. The A (2) T and AIm T transverse asymmetries, which are sensitive to the virtual photon polarisation, are found to be A (2) T = 0.11 ± 0.10 ± 0.02 and AIm T = 0.02 ± 0.10 ± 0.01. The results are consistent with Standard Model predictions and provide the world's best constraint on the b → sγ photon polarisation

Constraints on the unitarity triangle angle γ from Dalitz plot analysis of B0 →dK+π- decays

The first study is presented of CP violation with an amplitude analysis of the Dalitz plot of $B^0 \to D K^+ \pi^-$ decays, with $D \to K^+ \pi^-$, $K^+ K^-$ and $\pi^+ \pi^-$. The analysis is based on a data sample corresponding to $3.0\,{\rm fb}^{-1}$ of $pp$ collisions collected with the LHCb detector. No significant CP violation effect is seen, and constraints are placed on the angle $\gamma$ of the unitarity triangle formed from elements of the Cabibbo-Kobayashi-Maskawa quark mixing matrix. Hadronic parameters associated with the $B^0 \to D K^*(892)^0$ decay are determined for the first time. These measurements can be used to improve the sensitivity to $\gamma$ of existing and future studies of the $B^0 \to D K^*(892)^0$ decay

Study of η − η′ mixing from measurement of B (s)0 → J/ψη(′) decay rates

A study of B and B0 s meson decays into J/ψη and J/ψη0 final states is performed using a data set of proton-proton collisions at centre-of-mass energies of 7 and 8 TeV, collected by the LCHb experiment and corresponding to 3.0 fb−1 of integrated luminosity. The decay B0 → J/ψη0 is observed for the first time. The following ratios of branching fractions are measured: B(B0 → J/ψη0 ) B(B0 s → J/ψη0) = (2.28 ± 0.65 (stat) ± 0.10 (syst) ± 0.13 (fs/fd)) × 10−2 , B(B0 → J/ψη) B(B0 s → J/ψη) = (1.85 ± 0.61 (stat) ± 0.09 (syst) ± 0.11 (fs/fd)) × 10−2 , where the third uncertainty is related to the present knowledge of fs/fd, the ratio between the probabilities for a b quark to form a B0 s or a B0 meson. The branching fraction ratios are used to determine the parameters of η−η 0 meson mixing. In addition, the first evidence for the decay B0 s → ψ(2S)η 0 is reported, and the relative branching fraction is measured, B(B0 s → ψ(2S)η 0 ) B(B0 s → J/ψη0) = (38.7 ± 9.0 (stat) ± 1.3 (syst) ± 0.9(B)) × 10−2 , where the third uncertainty is due to the limited knowledge of the branching fractions of J/ψ and ψ(2S) mesons

Observation of charmonium pairs produced exclusively in pp collisions

The CP-violating asymmetry a s sl is studied using semileptonic decays of B0 s and B0 s mesons produced in pp collisions at a centre-of-mass energy of 7 TeV at the LHC, exploiting a data sample corresponding to an integrated luminosity of 1.0 fb−1 . The reconstructed final states are D± s µ ∓, with the D± s particle decaying in the φπ± mode. The D± s µ ∓ yields are summed over B0 s and B0 s initial states, and integrated with respect to decay time. Data-driven methods are used to measure efficiency ratios. We obtain a s sl = (−0.06 ± 0.50 ± 0.36)%, where the first uncertainty is statistical and the second systematic

Model-independent evidence for J /ψp contributions to Λb0 →j /ψpK- decays

The data sample of Λ0b→J/ψpK− decays acquired with the LHCb detector from 7 and 8 TeV pp collisions, corresponding to an integrated luminosity of 3  fb−1, is inspected for the presence of J/ψp or J/ψK− contributions with minimal assumptions about K−p contributions. It is demonstrated at more than nine standard deviations that Λ0b→J/ψpK− decays cannot be described with K−p contributions alone, and that J/ψp contributions play a dominant role in this incompatibility. These model-independent results support the previously obtained model-dependent evidence for P+c→J/ψp charmonium-pentaquark states in the same data sample

First observation of D0-D-0 oscillations in D0 →k+π-π+π- decays and measurement of the associated coherence parameters

Charm meson oscillations are observed in a time-dependent analysis of the ratio of D0→K+π−π+π− to D0→K−π+π−π+ decay rates, using data corresponding to an integrated luminosity of 3.0  fb−1 recorded by the LHCb experiment. The measurements presented are sensitive to the phase-space averaged ratio of doubly Cabibbo-suppressed to Cabibbo-favored amplitudes rK3πD and the product of the coherence factor RK3πD and a charm mixing parameter y′K3π. The constraints measured are rK3πD=(5.67±0.12)×10−2, which is the most precise determination to date, and RK3πDy′K3π=(0.3±1.8)×10−3, which provides useful input for determinations of the CP-violating phase γ in B±→DK±, D→K∓π±π∓π± decays. The analysis also gives the most precise measurement of the D0→K+π−π+π− branching fraction, and the first observation of D0-¯D0 oscillations in this decay mode, with a significance of 8.2 standard deviations

Search for the rare decay Λc+ →pμ+μ-

The flavor-changing neutral-current (FCNC) decay Λþ c → pμþμ− (inclusion of the charge-conjugate processes is implied throughout) is expected to be heavily suppressed in the Standard Model (SM) by the Glashow-IliopoulosMaiani mechanism [1]. The branching fractions for shortdistance c → ulþl− contributions to the transition are expected to be of Oð10−9Þ in the SM but can be enhanced by effects beyond the SM. However, long-distance contributions proceeding via a tree-level amplitude, with an intermediate meson resonance decaying into a dimuon pair [2,3], can increase the branching fraction up to Oð10−6Þ [4]. The short-distance and hadronic contributions can be separated by splitting the data set into relevant regions of dimuon mass. The Λþ c → pμþμ− decay has been previously searched for by the BABAR Collaboration [5], yielding 11.1 5.0 2.5 events and an upper limit on the branching fraction of 4.4 × 10−5 at 90% C.L. Similar FCNC transitions for the b-quark system (b → slþl−) exhibit a pattern of consistent deviations from the current SM predictions both in branching fractions [6] and angular observables [7], with the combined significance reaching 4 to 5 standard deviations [8,9]. Processes involving c → ulþl− transitions are far less explored at both the experimental and theoretical levels, which makes such measurements desirable. Similar analyses of the D system have reported evidence for the longdistance contribution [10]; however, the short-distance contributions have not been established [11]

Angular analysis and differential branching fraction of the decay B {/s 0} → ϕμ + μ -

An angular analysis and a measurement of the differential branching fraction of the decay B0 s → φµ+µ − are presented, using data corresponding to an integrated luminosity of 3.0 fb−1 of pp collisions recorded by the LHCb experiment at √ s = 7 and 8 TeV. Measurements are reported as a function of q 2 , the square of the dimuon invariant mass and results of the angular analysis are found to be consistent with the Standard Model. In the range 1 < q2 < 6 GeV2/c4 , where precise theoretical calculations are available, the differential branching fraction is found to be more than 3 σ below the Standard Model predictions