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

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.

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

The analog readout of the LHCb vertex detector and study of the measurement of the Bs oscillation frequency

Jérémie Borel

The LHCb detector is one of the four experimental setups built to detect high-energy proton collisions to be produced by the Large Hadron Collider (LHC). Located at CERN (Geneva, Switzerland), the LHC machine and the LHCb experiment are expected to start in 2008, and will then operate for several years. Being the largest collider of its kind, the LHC will open the way to new investigations, in the very-high energies, but also in terms of statistics for the study of rare-phenomena and flavor physics. In this framework LHCb is dedicated to precise measurements of CP-violating and rare decays of beauty hadrons, in order to test (or over-constrain) the Standard Model of particle physics. From the hardware point of view, the construction of such detectors represents several challenges; one of them is the routing at a very high frequency of many signals in a harsh radiation environment. We designed to this purpose a hardware setup and a software filter which together reduce the cross-talk present in the readout of the LHCb vertex detector to a level of (1 ± 2)%, leading to an improved signal quality in the acquisition chain. From the physics point of view, many of the CP-violation measurements performed at LHCb using Bs decays, for example using Bs → Ds∓ K± decays, will require as input the Bs–Bs oscillation frequency Δms. Thus the measurement of Bs–Bs oscillations, which are best observed using the flavor-specific Bs → Ds- π+ decays, will play an important role. We have developed a complete selection of Bs → Ds- π+ and Bs → Ds∓ K± events, based on Monte Carlo simulations. Assuming 2 fb-1 of data, we expect a Bs → Ds- π+ signal yield, after the first level of trigger, of 155 k events over a background between 0.6 k and 7.8 k events at 90% confidence level. Moreover, we assess with fast Monte Carlo studies the corresponding statistical sensitivity on the Bs oscillation frequency, σ(Δms) = 0.008 ps-1, on the wrong tag fraction, σ(ω) = 0.003, as well as on other parameters related to the Bs-meson system. We also addressed an important aspect of the systematics associated with the Δms measurement, and developed a method to calibrate and assess the length scale. This calibration is performed through the reconstruction of secondary interactions occurring in the material of the vertex detector. We show that the statistical relative precision of this approach quickly matches 6 × 10-5, obtained from the survey measurements of the detector.

EPFL2008

Exclusive trigger selections and sensitivity to the Bs-Bs mixing phase at LHCb

José Luis Abelleira Fernández

The LHCb experiment is one of the main experiments that will be hosted at the Large Hadron Collider (LHC) at CERN, in the area of Geneva, and is scheduled to start after the Summer 2007. The LHCb detector is a single-arm forward spectrometer dedicated to precise measurements of CP violation and rare decays in the b sector. The primary goal is to test the Standard Model description of flavor physics, and possibly to look for New Physics beyond it. The event rate resulting from the LHC proton collisions will be tremendous, and the production of b hadrons will be copious, thus providing the required statistics for the study of flavor physics. However, not all the events are relevant to LHCb physics program, and a dedicated trigger is required. In particular, the High-Level Trigger will take the final decision whether to accept or discard an event before sending it to permanent storage, in a limited period of time. We present in this dissertation the implementation and design of the High-Level Trigger exclusive selections, and assess their performance in selecting the b and c decays of interest, based on a full Monte Carlo simulation. The physics program of LHCb is vast, with the study of a large collection of b decays offering the possibility to investigate CP violation, and any deviation from the Standard Model. In this quest, the neutral Bs–Bs system plays a predominant role. The Bs–Bs mixing phase, denoted by φs, has not yet been measured, and represents a crucial probe of New Physics. This electroweak phase may be tested through theoretically clean b → ccs quark-level transitions to CP eigenstates, by performing a time-dependent measurement of mixing-induced CP violation. Among the Bs decays to pure CP eigenstates mediated by the b → ccs transitions, the channel Bs → ηcφ yields one of the best sensitivities to φs. We present the reconstruction and selection of Bs → ηcφ events using a full Monte Carlo simulation, and we determine the characteristics relevant to a CP violation measurement with this channel. The sensitivity of LHCb to the Bs–Bs mixing parameters is studied, using a fast parameterized Monte Carlo simulation. This simulation uses the outputs of the realistic full simulation to determine the statistical precision to the mixing observables. We consider the Bs → J/ψφ decay to an admixture of CP eigenstates requiring an angular analysis to disentangle the different CP components, as well as the Bs → ηcφ, Bs → DsDs, and Bs → J/ψη decays to pure CP eigenstates. Each of these signal samples are simultaneously fitted with a flavor-specific control sample, Bs → Dsπ, allowing the extraction of the Bs–Bs mixing frequency ΔMs. We demonstrate that the sensitivity to φs is dominated by Bs → J/ψφ events, with the statistical precision required to uncover New Physics effects. In comparison, the decays to pure CP eigenstates have an order of magnitude smaller event yields, but their contribution to the determination of φs, although small compared to Bs → J/ψφ, is non negligible.

EPFL2006