Measurement of the rare radiative B Meson decays B --> [phi]K[gamma] and B --> [omega]K[gamma] at the Belle experiment and contribution to software development at the LHCb experiment

The Belle experiment is one of the world's two currently running high-energy physics detectors dedicated to study B-mesons produced with an electron-positron collider. It is situated at the Japanese High Energy Research Organization KEK in Tsukuba, near Tokyo, Japan and has been taking data since 1999. So far over 700 millions of B B pairs have been collected, the largest number ever recorded by a single machine. In the analysis described by this report, 534.586±7.044 millions of these B-mesons pairs have been analyzed to measure with precision the branching fraction (BF) of the charged and neutral B → φKγ decays. In the charged φ mode we measure a BF of · 10-6 with a significance of 10.8, in the neutral mode we find a BF of · 10-6 with a significance of 5.5. In addition we claim a first evidence of the never observed B → ωKγ decays: in the charged decay mode, we find a BF of · 10-6 with a significance of 3.7 and in the neutral mode a BF of · 10-6 with a significance of 2.8. Combining the results in the neutral and charged ω modes we reach a significance of 4.5. Besides of the interest that these results represent, they are a further step in exploring the rare radiative b → sγ decays and provide the ground work for future CP violation studies. Indeed, b → sγ decays have regained interest lately in the quest for New Physics. Observing a large mixing-induced CP-violation could be a sign for New Physics – recent theoretical Standard Model (SM) calculations for the exclusive b → sγ mode B → K*γ have shown that it should be rather small. Thus in two year's time, when the Belle experiment, along with its competitor, the BaBar experiment in the United States, will come to an end, we will definitely be in the need of a newer generation experiment to either confirm the results found by then with greater precision or continue the search for New Physics in rare B decays and of course b-physics in general. By then, the new LHCb experiment situated at the large hadron collider LHC at CERN in Geneva, will have recorded its first events and hopefully there will also be a so-called Super-B factory as a follow-up experiment for BaBar and Belle, situated on an electron-positron particle accelerator.

A search for heavy long-lived staus in the LHCb detector at √s = 7 and 8 TeV

Thi Viet Nga La

The Large Hadron Collider (LHC) has been producing pp collisions at 7 and 8 TeV since 2010 and promises a new era of discoveries in particle physics. One of its experiments, the Large Hadron Collider beauty (LHCb) experiment, was constructed to study CP violation in the B meson system. In addition to B physics, new Physics beyond the Standard Model can also be searched for at this single-arm forward spectrometer. With the different sub-detectors and the high resolution of the tracking system, the LHCb detector has the ability to search for heavy, long-lived and charged particles, which are predicted by extensions of the Standard Model. One of these extensions, the minimal Gauge Mediated Supersymmetry Breaking (mGMSB), proposes such a particle, named stau (τ~) - the SUSY bosonic counterpart of the heavy lepton tau (τ). The theory proposes that the staus may be pair-produced in pp collisions or in the decays of heavier particles, and have only electromagnetic interactions with the atoms of the medium like the muons. Therefore, we expect that at the energy of the LHC these particles can be produced if they do exist and that we have a chance to discover them at LHCb, as well as at the other experiments of the LHC. This thesis is dedicated to the search for stau pairs produced in pp collisions at the centre-of-mass energies √s = 7 and 8 TeV in the LHCb detector. For this purpose, we generated the stau pairs with seven different particle masses ranging from 124 to 309 GeV/c2 and simulated their path through the LHCb detector, as well as their muon background from the decays Z0, γ∗ → μ+μ−. Based on the results from the simulation, a set of cuts are then defined to select the stau pairs. Some muon pairs at high energies will also pass the selection cuts. Thus, to separate the stau pairs from the muon pairs, the Neural Network technique has been used. A first Neural Network has been used to distinguish the stau tracks from the muon tracks using their signals left in the sub-detectors: the VELO silicon detector, the electromagnetic calorimeter, the hadron calorimeter and the RICH detectors. Then, two methods to select the stau pairs have been developed: the first one is based on the product of the two responses from the first Neural Network (NN1) for the two tracks, the second one employs a second Neural Network to separate the stau pairs from the muon pairs by using the above product of the two NN1 responses and the invariant mass of pair. Finally, a favourable region for the staus finding has been defined and the expected numbers of stau and muon pairs in this region have been evaluated. The training of the Neural Network has been achieved with the Monte Carlo variables, then the trained Neural Network has been used to classify the data. The data used in our work were collected by the LHCb experiment in 2011 and 2012 and correspond to integrated luminosities of 1 fb−1 at √s = 7 TeV and of 2 fb−1 at √s = 8 TeV. No significant excess of signal has been observed. Upper limits at 95% CL on the cross section for stau pair production in pp collisions at √s = 7 and 8 TeV have been computed by using the profile likelihood method, which is derived from the well known Feldman and Cousins method.

EPFL2013

B°s --> J/[psi][eta]' decay and sensitivity to the Bs mixing phase at LHCb

Sergio Jiménez-Otero

The LHCb experiment is one of four main experiments to be setup at IP8 point of the proton-proton Large Hadron Collider (LHC) at CERN, Geneva. The data taking is scheduled to start end 2007. The LHCb detector is a forward single-arm spectrometer conceived to study CP violation and rare phenomena in the b-quark sector with very high precision. LHCb should provide a profound understanding of flavour physics in the framework of Standard Model (SM), so that the SM will be over-constrained, hopefully, it will exhibit some inconsistencies which may reveal a sign of New Physics beyond the SM. The physics studies being considered by the LHCb collaboration offer the possibility of furthering in the SM description of CP violation. The SM provides the theoretical framework for the violation of the CP symmetry in neutral B mesons as well as in neutral K mesons. The neutral Bs0 - Bs0 system stays at a privileged position in the quest for CP violation evidences. The Bs0 - Bs0 mixing phase, φs, has never been measured. This mixing phase has its origins in the interference between the decay and the mixing of those neutral mesons. b → ccs quark-level transitions decaying to CP eigenstates, may directly probe this electroweak phase by performing a time-dependent measurement of mixing-induced CP violation. Among those quark level transitions, we have the golden-plated channel to perform such a measurement that is Bs0 → J/ψφ and that requires an angular analysis to disentangle its CP eigenstates components. On the other hand, b → ccs transitions to pure CP eigenstates provide another probe to the φs not requiring such angular studies. In this dissertation we will be consider the Bs0 → J/ψ(μ+μ-) η' (π+π-η(γγ)) decays which are b → ccs transitions to pure CP even eigenstates. The reconstruction and selection of those decays will be presented here using full Monte Carlo simulations. An annual event yield of ∼ 2000 is assured with a background-over-signal level close to unity. The mass and proper time resolution will be also presented and some methods searching to improve those performances. In the last chapter of the dissertation, we will also present a study of the statistical sensitivity of Bs0 → J/ψ(μ+μ-) η' (π+π-η(γγ)) to the Bs0 - Bs0 mixing parameters, using a fast parameterised Monte Carlo simulation. Those simulations take into account in a realistic manner the outputs from the full Monte Carlo simulations. Specifically, we will be simulate the background-over-signal levels, the time dependent acceptance efficiency and tagging and reconstruction performances for that decay mode. It will also take as inputs the event-by-event proper time errors obtained from the full MC. The likelihood fit performed to extract the physics parameters requires a flavour-specific control sample, Bs0 → Ds π, which allows for the extraction of the Bs0 - Bs0 mixing frequency, ΔMs. The sensitivity achieved by our channel decay will be then combined to other b → ccs quark level transitions to pure CP eigenstates and finally also to the Bs0 → J/ψφ decay to admixture of CP eigenstates. The sensitivity obtained from the pure CP modes is σ(φs) = 0.056 rad whereas if we combine all decay channels into a single sensitivity measurement we achieve σ(φs) = 0.021 rad. The contribution from the pure CP modes is non-negligible, ∼ 14%, so that it is expected that these channels will certainly help in the measurement of φs.

EPFL2007

Measurement of time-dependent CP violation in B0->Dpi decays and optimisation of flavour tagging algorithms at LHCb

Vincenzo Battista

This thesis presents the results of a time-dependent analysis of

B^0\to D^{\mp}\pi^{\pm}

decays using

3~\rm fb^{-1}

of proton-proton collision data collected with the LHCb detector at CERN's Large Hadron Collider during Run 1 with a centre-of-mass energy of

7

(2011) and

8

(2012) TeV. The LHCb experiment is dedicated to the study of the properties of

b

-flavoured hadrons, in particular

CP

violation in the

B

meson system. The Standard Model of Particle Physics describes very precisely the mechanism and the amount of

CP

violation expected in the Universe. However, the observed matter-antimatter asymmetry is larger by several order of magnitude compared to the predictions. This could be explained by the existence of a new source of

CP

violation, originating in New Physics beyond the Standard Model. The time-dependent analysis of

B^0\to D^{\mp}\pi^{\pm}

decays provides constraints on the angle

\gamma

of the Unitarity Triangle, one of the fundamental parameters of the Standard Model related to

CP

violation. Since no sizeable high-order Standard Model processes are expected to contribute, any deviation from the predictions would be an unambiguous signature of New Physics. The current experimental precision on

\gamma

is significantly lower than that of theoretical predictions. This motivates the effort for new experimental determinations of

\gamma

in order to reduce its uncertainty. The analysis of

\Bz\to\Dmp\pipm

decays, although not as sensitive as that obtained from decays of charged

B

mesons into

D^{(*)0}K^{(*)+}

final states, represents an independent and uncorrelated estimation of

\gamma

that contributes to the global combination of all

\gamma

measurements. The result obtained in this thesis is more precise than previous determinations from other experiments (BaBar, Belle) using

B^0\to D^{\mp}\pi^{\pm}

decays. Although based on a very large sample of about half a million signal events, it is still dominated by statistical uncertainties, indicating good prospects for future improvements in precision with more data from Run 2 and beyond. In addition to the

B^0\to D^{\mp}\pi^{\pm}

analysis, this thesis also summarizes the studies to improve the performances of the flavour tagging algorithms used by the LHCb collaboration to infer the flavour of neutral

B

mesons in time-dependent analyses. The performance of these algorithms, being correlated with the kinematics of the reconstructed particles as well as the complexity of the event (number of tracks and primary vertices), showed a significant decrease on Run 2 data (2015--2018), which were collected at a centre-of-mass energy of

13~\rm TeV

. Thanks to new implementations, these algorithms now have a performance similar to that obtained with Run 1 data.

EPFL2018