Measurement of lifetimes for heavy flavour mesons using semileptonic B decays at LHCb

This thesis work presents lifetime measurements of heavy-flavour mesons made with semileptonic $B^0$ and $B^0_s$ decays based on $3~{\rm fb}^{-1}$ of data collected with the LHCb detector in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV. The study of meson lifetimes is important to constrain phenomenological models for hadronic interactions based on the Standard Model of particle physics. Better understanding of hadronic interactions is essential for making precise predictions, which can then be confronted to experimental data in order to look for signs of physics beyond the Standard Model.

We measure the differences between the decay widths of the $B^0_s$ and $B^0$ mesons, $\Delta(B)$ , and between that of the $D_s^-$ and $D^-$ mesons, $\Delta(D)$ , by analysing approximately 410000 $B^0_s \to D_{s}^{(*)-}\mu^{+}\nu_{\mu}$ and 110000 $B^0 \to D^{(*)-}\mu^{+}\nu_{\mu}$ decays, which are partially reconstructed in the same $K^+K^-\pi^-\mu^+$ final state. We measure $\Delta(B) = -0.0115 \pm 0.0053~{\rm(stat)} \pm 0.0041~{\rm(syst)}~{\rm ps}^{-1}$ and $\Delta(D) = 1.0131 \pm 0.0117~{\rm(stat)} \pm 0.0065~{\rm(syst)}~{\rm ps}^{-1}.$ Using the obtained values of $\Delta(D)$ and $\Delta(B)$ and the $B^0$ and $D^-$ lifetimes as external inputs, we obtain a measurement of the flavour-specific $B^0_s$ lifetime, $\tau_s^{\rm fs} = 1.547 \pm 0.013~{\rm(stat)} \pm 0.010~{\rm (syst)} \pm 0.004~{\rm(\tau_{B^0})}~{\rm ps},$ and of the $D_s^-$ lifetime, $\tau_{D_s^-} = 0.5064 \pm 0.0030~{\rm(stat)} \pm 0.0017~{\rm (syst)} \pm 0.0017~{\rm(\tau_{D^-})}~{\rm ps},$ where the last uncertainties originate from the limited knowledge of the $B^0$ and $D^-$ lifetimes, respectively. Both results are compatible with, and improve upon, previous determinations.

A feasibility study of a $D^0$ lifetime measurement is performed, by measuring the difference between the decay widths of the $D^0$ and $D^-$ mesons, $\Delta(D)'$ . We reconstruct approximately $2.2\times 10^6$ $B^0\to D^{*-}(\to \bar{D}^0 (\to K^+\pi^-)\pi^-)\mu^+\nu_{\mu}$ and $1.6\times 10^6$ $B^0\to D^{(*)-}(\to K^+\pi^-\pi^- (X))\mu^+\nu_{\mu}$ decays. We measure $\Delta(D)' = 1.4644 \pm 0.0043~{\rm(stat)} \pm 0.0132~{\rm(syst)}~{\rm ps}^{-1}$ and with the $D^-$ lifetime as external input, we get an estimate of the $D^0$ lifetime, $\tau_{D^0} = 0.4122 \pm 0.0007~{\rm(stat)} \pm 0.0022~{\rm (syst)} \pm 0.0011~{\rm(\tau_{D^-})}~{\rm ps}.$ This result is compatible with, but less precise than, current precision and thus validates the method. We discuss possible improvements with larger simulation samples and data sets.

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

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

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