Contribution to the inner tracker design and penguin sensitivity studies for the measurement of sin 2ß in LHCb

LHCb is one of the four large experiments hosted at the Large Hadron Collider (LHC) at CERN in Geneva. It will start taking data in September 2008, and will then operate for several years. It consists of a single-arm forward spectrometer dedicated to precise measurements of CP violation and rare decays in the B sector, with the aim of testing the Standard Model and possibly of discovering the first signatures of New Physics. Building such a large experiment as LHCb is a challenge, and many contributions are needed. The Lausanne lab is responsible for the design and the production of the Silicon Inner Tracker (IT) of LHCb. This detector is made of Silicon sensors which need to be cooled to avoid thermal runaway. We present here a contribution to the design of this sub-detector and a description of the production steps. In particular, a study of the cooling of the Inner Tracker is described. It is shown that the cooling abilities of the IT can avoid thermal runaway. CP violation in B meson decays was first observed in the measurement of the so-called "golden channel", in which a Bd0 meson decays into a J/ψ and a Κs0 . The time-dependent CP asymmetry in Bd0 → J/ψΚs0 allows to measure the angle β of the (d, b) unitary triangle. This parameter is now known with 4% accuracy at B factories. However, this determination of sin 2β is made under the assumption that there is only a single amplitude present in this decay : this means that penguin diagrams which might be present have been neglected. In 1999, Robert Fleischer [29] proposed a theoretical method to access those penguin diagrams in the Bd0 → J/ψΚs0 decay, using the Bs0→ J/ψΚs0 channel. This method relies on U-Spin symmetry and also allows to determine the γ angle of the (d, b) unitary triangle. We have developed a selection method for the Bd0 → J/ψΚs0 channel in order to strongly suppress the background and to allow the separation of the Bs0 and Bd0 peaks. We obtained mass resolution of 8 MeV/c2 and a B/S ratio for the channel Bd0 → J/ψΚs0 estimated to belong to [0, 0.039] at 90% confidence level in a ±2σ mass window around the Bd0 mass, after the first level of trigger (L0). For the channel Bs0 → J/ψΚs0 , the B/S ratio is calculated from the result for Bd0 → J/ψΚs0 assuming known branching fractions. It lies in the interval [0, 3.33] at 90% CL. The annual yield is expected to be around 300 events for an integrated luminosity of 2 fb-1. We have simulated with fast Monte Carlo the Bs0 → J/ψΚs0 signal using the parametrization proposed in [29] and taking as input the results of selection obtained for Bd0 → J/ψΚs0 . The simulation has been repeated several times for different integrated luminosities and B/S ratios. We conclude that after 5 years of normal running, LHCb will be able to determine the penguin contribution in the Bd0 → J/ψΚs0 decay with a sensitivity of (0.172 ± 0.004) using this method based on U-spin symmetry.

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

Alignment of the LHCb tracking stations and selection of X(3872) and Z(4430)± in pp collisions at 14 TeV

Louis Nicolas

The LHCb experiment is one of the four large experiments located at the Large Hadron Collider (LHC) at CERN, close to Geneva, Switzerland. The LHCb detector is a single-arm forward spectrometer which is dedicated to precision measurements of CP violation, as well as to the study of rare b-hadron decays. Both the energy at which the proton-proton collisions will take place and the statistics of events that will be selected are unprecedented. The LHCb detector will start its measurements in November 2009 and operate for several years. Being an experiment for precision measurements, LHCb relies on excellent reconstruction and trigger efficiencies, outstanding proper-time and momentum resolutions, as well as on a reliable particle identification, both for the event selection and the flavour tagging of B-meson decays. These performances are however not possible without a precise construction and alignment of the detector. An extensive survey of the detector geometry has been performed. However these measurements, for instance for the Inner Tracker, a silicon-strip tracking device, are only precise to the order of the detector resolution. The first part of the thesis discusses the detector alignment. A software alignment method has been developed in order to improve the knowledge of the detector element position. The novelty of this method is that it uses the tracks from the standard track-fitting procedure, which is based on a Kalman filter, and not on a global track model. The advantage of this method is that it is possible to properly take the multiple Coulomb scattering, the magnetic field and the energy loss corrections into account. It also allows to correctly take into account the correlations between the hits on tracks. This document presents two realistic running scenarios, using simulated data, in which the alignment of the Inner and Outer Trackers (IT and OT) will need to be performed. A general strategy is defined for the alignment in a multi-step manner, starting from a coarse granularity and descending step-by-step to a finer detector granularity. A dedicated track selection is also developed. In particular, an evolving cut on the Χ2 of the track fit is shown to be essential in the alignment procedure. Starting from a misaligned detector, the two scenarios use tracks coming either from beam–gas collisions at an energy of 450 GeV/c without magnetic field, or from proton–proton collisions at a centre-of-mass energy of √s = 14 TeV and when the magnetic field of the experiment is turned on. It is shown that the alignment software is able to recover from misalignments of the order of 1–2mm with a precision better than 20% of the single-hit resolution for the IT and 5% for the OT, in both scenarios. The precision of this alignment is then validated by studying the mass distributions of reconstructed J/ψ and K0s mesons and by showing that the mass resolution for these particles is not degraded by more than 2% between the ideal case and the case after re-alignment of the detector. In the summer of 2008 and 2009, data were taken at LHCb during LHC synchronisation tests. The alignment of the IT using these high track-multiplicity data is presented in this document. A dedicated pattern recognition and track selection are used in order to obtain a sample of good-quality tracks. The IT is aligned down to the lowest granularity with a precision of 20 µm. This precision is obtained by studying the distributions of unbiased residuals with respect to the reconstructed tracks. It is also shown that the alignment results do not depend on the position of the detector before alignment. The second part of this thesis discusses the first studies at LHCb of the X(3872) and Z(4430)± particles. Six years ago, the Belle collaboration discovered a new state called the X(3872). Several theoretical models have been developed in order to include this state, and the many others discovered since then. On the other hand, several collaborations have repeated the observation of this resonance in several decay modes. However, the nature and the quantum numbers of this state remain uncertain. Belle also discovered a charged state called the Z(4430)±, but this discovery has not been confirmed by any other experiment yet. For both these states, LHCb is expected to play an important role. A Monte Carlo feasibility study of the selections of the X(3872) → J/ψ π+π- decay in the B± → X(3872)K± channel and of the B0 → Z(4430)±K± with Z(4430)± decaying to ψ(2S)π± are presented in this document. A yield of 1'850 reconstructed, selected and triggered B± → X(3872)K± events is obtained per nominal year (corresponding to an integrated luminosity of 2 fb-1), for a background-to-signal ratio in the interval [0.3, 3.4] at 90% CL. With such statistics, half a nominal year will be sufficient to disentangle between the two remaining spin hypotheses for the X(3872) state. For the Z(4430)±, an annual yield of 6'200 reconstructed, selected and triggered B0 → Z(4430)K± events is obtained for a B/S ratio within the interval [2.7, 5.3] at 90% CL. Hence, the confirmation or ruling out of the Belle discovery will be possible, even in the early running phase of the LHC proton collider at √s = 7–10 TeV.

EPFL2009