Construction of the Inner Tracker and Sensitivity to the B°s -> [mu][mu] Decay at LHCb

LHCb is one of the four main experiments hosted at the Large Hadron Collider (LHC) at CERN. The LHC first started in September 2008 and, after a one-year hiccough, restarted in November 2009. In the course of three weeks, the HEP community witnessed the first LHC proton-proton collisions and a new record of the most energetic particle beam. The ease shown by the operators of the complex LHC machine augurs very well for the extended period of data-taking scheduled to start at the end of February 2010. LHCb is the LHC experiment primarily dedicated to the b realm, through the study of CP violation and rare decays. Its physics goals are ambitious: it aims at the indirect search of New Physics and at the precise measurements of CP violation parameters. The LHCb detector was designed as a single-arm forward spectrometer. The branching fraction of the yet-unobserved Bs0 → μ+ μ- decay is currently considered as one of the most stringent tests for the existence of physics beyond the Standard Model (SM) of particle physics. There exists indeed a large unexplored range between the current experimental upper limit and the SM prediction about this branching fraction, making a measurement incompatible with the SM possible. Furthermore, there exist predictions for this branching fraction in the frame of numerous theoretical models that aim at a more accurate description of matter than that offered by the Standard Model. Some of those predictions significantly differ from the SM one, opening the possibility of an indirect discovery of New Physics. In this thesis, I present the study of the sensitivity of the LHCb experiment to this branching fraction with Monte Carlo simulations. I show that LHCb will compete with the Tevatron for the exclusion limit of the Bs0 → μ+ μ- branching fraction already in 2010 and that approximately 3fb-1 at √s = 14TeV are enough for a 3σ evidence of a SM signal. In a particle physics experiment, the precise measurement of the charged particle trajectories is essential. Indeed tracking gives experimental access to their momentum and allows the reconstruction of the physical properties of the short-lived particle of which they are the decay products. The Inner Tracker is the detector that provides tracking information for the particles ying in the innermost part of LHCb. Because of its central position, the Inner Tracker calls for the use of very light-weighted material which compete with the precision and rigidity required by such a detector. During my thesis work, I contributed to the construction of the Inner Tracker through the set up of an assembly procedure for the detector boxes, uncovering several design aws and contributed to solve them pragmatically. I defined quality requirements at key steps of the assembly and implemented tests to assess them. I conducted the assembly of the twelve Inner Tracker detectors boxes that were installed in the LHCb cavern in Summer 2008. After software alignment, the overall precision of the Inner Tracker modules position is on average 19 µm along the relevant direction. The careful box assembly and the quality tests along the procedure allowed to keep the dead strips fraction below 1 %. Finally, tracks from the LHC collisions have been seen!

Model-independent measurement of charm mixing parameters in D0 -> K0S pi+ pi- decays

Surapat Ek-In

This thesis presents a measurement of charm mixing and CP-violation parameters using D0 -> K0S pi+ pi- decays. These mixing parameters include the mass and decay-width differences of the mass eigenstates of the D0-D0bar system. A search for Charge-Parity violation is performed through measurements of D0-D0b mixing and of the interference between mixing and decay amplitudes. The dataset consists of events reconstructed in proton-proton collisions collected by the LHCb experiment during theoperation of the Large Hadron Collider (LHC) from 2016 to 2018. This sample corresponds to an integrated luminosity of 5.4 fb^-1. The D0 decays are selected in semileptonically decays of b hadrons. The results are expressed in terms of the CP-averaged mixing parameters and CP-violating differences asx_cp= (+4.29 +/- 1.48 (stat) +/- 0.26 (syst)) x 10^{-3}y_cp= (+12.61 +/- 3.12 (stat) +/- 0.83 (syst)) x 10^{-3}delta x= (-0.77 +/- 0.93 (stat) +/- 0.28 (syst)) x 10^{-3}delta y= (+3.01 +/- 1.92 (stat) +/- 0.26 (syst)) x 10^{-3}where the first uncertainty is statistical and the second is systematic. They correspond to the mixing and CP-violation parameters x = 0.46^{+0.15}{-0.16} x 10^{-2} y = 1.24^{+0.32}{-0.33} x 10^{-2} |q/p| = 1.21^{+0.21}{-0.15} \phi = -0.132^{+0.088}{-0.120} rad.The value of x deviates from zero with a significance of almost 3 sigma.These results are complementary to and consistent with previous measurements and with the current world-average values.They represent an independent and complementary knowledge of the charm-mixing parameters.A combination with the recent LHCb analysis of Dstar -> D0 (->K0S pi+ pi-) pi decays is also reported.

EPFL2022

First indication of the Lambda-b baryon decay to a charmless baryonic final state

Vladimir Macko

The Standard Model of particle physics provides predictions of many fundamental processes. One of them is the existence of charmless processes involving only baryons, namely charmless purely baryonic decays of baryons. Yet none of these decay modes has been experimentally observed so far. Following theoretically predicted branching fractions, the LHCb experiment is currently the only one capable of measurements concerning these rare decays. The work presented in this thesis describes the measurement techniques and the results of the study of Lambda-b baryon decays to a charmless baryonic final state using the LHCb detector involving the rare Lambda-b -> Lambda p pbar and Xib0 -> Lambda p pbar decays. The significance of existence of the Lambda-b -> Lambda p pbar signal channel is found to be 4.82 sigma, constituting the first evidence of existence of this channel. In the case of the Xib0 -> Lambda p pbar channel the found significance is 2.26 sigma, which is compatible with the non observation of this channel with the present level of statistics. Preliminary measurements of branching fraction of Lambda-b -> Lambda p pbar are reported. In addition, the production of Scintillating Fibre (SciFi) Tracker, an essential part of the upgrade of the LHCb detector, is also discussed.

EPFL2021

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