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

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.

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

LHCb Detector Momentum Calibration with the First Year Data and a Performance Study of CP Violation Measurement with the B°s -> J/[Psi][Phi] Decays

Géraldine Conti

A new area in particle physics has begun with the start of the Large Hadron Collider (LHC) at CERN at the end of 2009, which collides protons at energies never reached before. To detect the products of the collisions, four main experiments are located around the LHC, among which the LHCb experiment. LHCb has been designed as a single-arm forward spectrometer and is dedicated to measurements of CP violation and rare decays of B hadrons. These searches could also potentially lead to the discovery of phenomena that cannot be described by the model used to date, called the Standard Model. The physics beyond the Standard Model is referred to as New Physics. A measurement of the phase of the Bs0–Bs0 oscillation amplitude with respect to that of the b → c+W- tree decay amplitude, called , is one of the key goals of the LHCb experiment with first data. In the Standard Model, this phase equals –2βs, with βs the smallest angle of the unitary triangle of the CKM matrix relevant to Bs0. The phase is hence predicted to be small, rad. However, possible contributions of New Physics to the Bs0–Bs0 box diagram, such as new particles entering into it, could modify the value from its Standard Model expectation. Due to its very small theoretical uncertainty in the Standard Model, is therefore a very sensitive probe to detect the presence of New Physics. The phase will be measured from a time-dependent angular analysis to Bs0 → J/ψφ events with tagging the initial flavor of the Bs0 mesons. Due to the pseudo-scalar to vector-vector particle nature of the decay, an angular analysis is required to disentangle statistically the CP-even and CP-odd components present in the final state. Already with 2 fb-1 of data taken at the nominal luminosity ℒ = 2·1032 cm2s-1, corresponding to ∼ 117,000 Bs0 → J/ψφ signal events, the LHCb experiment is expected to achieve a statistical uncertainty σ() ≃ 0.03 rad, similar to the value predicted by the Standard Model. On the way to this measurement, we present prospects for the time-dependent angular analysis to Bs0 → J/ψφ events without tagging the initial flavor of the Bs0 mesons. This analysis is less sensitive to , but it has the advantages of being independent of the tagging calibration and less stringent about the proper time resolution, as it does not have to resolve the fast Bs0–Bs0 oscillations. Consequently, it can be applied on first data. Sensitivity results for , but also for the other parameters entering in the analysis are given. In particular, the amplitudes of the CP-even and CP-odd components and the Bs0 meson lifetime can already be measured with a better precision than the latest CDF results (June 2010) with only 0.2 fb-1 of data at nominal luminosity with the untagged analysis. To perform this analysis, the trajectories through the spectrometer of the particles coming from the Bs0 → J/ψ(µ+µ-)φ(Κ +Κ-) decay, the muons and kaons, need to be reconstructed very precisely. Indeed, from the curvature of their trajectory in the magnetic field, the momentum of the particles is obtained, which is then used in the calculation of the angles forming the basis for the angular analysis to Bs0 → J/ψφ events. For particles flying in the innermost part of LHCb, the Inner Tracker is the detector that provides information about the tracks. It consists of twelve detector boxes fixed on three tracking stations located downstream of the magnet in a cross-shaped configuration around the beam pipe. They are each filled with four layers of silicon microstrip sensors. We have assembled the twelve Inner Tracker detector boxes in a clean environment at CERN. By summer 2008, they were all assembled and installed in the LHCb cavern. To reconstruct track trajectories very accurately, two ingredients are essential: a good alignment description of the detectors used for tracking and an accurate description of the magnetic field. Before closing the Inner Tracker detector boxes, we organized with the survey team at CERN measurements of the silicon sensor positions. Once installed in the LHCb cavern, the Inner Tracker detector boxes were aligned as close to their nominal position as possible on the tracking stations previously adjusted. From the final measurements, we provided to the LHCb software a first realistic geometry description of the Inner Tracker, which was validated using particles coming from LHC injection tests of September 2008 and June 2009. The magnetic field map used initially in the LHCb software had been generated with a model based on finite element calculation. To obtain a better magnetic field map description, we developed a method for parameterizing magnetic field measurements that were recorded in December 2005 in the LHCb cavern using Hall probes. This new map replaced the former field map in the LHCb software. However, the magnetic field measurements showed some different features compared to the simulated magnetic field values, which needed to be validated with real data. As an example, an asymmetry of the magnetic field between the lower and upper parts of the magnet is observed in the measurements, while simulated values do not have this by design of the dipole magnet. Using reconstructed masses of Κs → π+π-, Λ → p+π- and Λ → p-π+ decays coming from 2009 and early 2010 real data, we validated the magnetic field map based on the measurements against the one based on simulated values according to several criteria, one of which being the confirmation of the up-down asymmetry in the field. From the same studies, we calculated a factor to correct globally for the momentum scale. As this factor should be universal, it can be calculated for one resonance and validated using the results for other resonances.

EPFL2010