Search for lepton flavour violation in B meson decays at LHCb

The current limitations of the standard model of elementary particle physics drive the search for phenomenons forbidden within that framework with the goal to unveil a more universal theory. This thesis presents the search for two lepton flavour violating decays forbidden in the standard model: $B^{+} \rightarrow K^{+} \tau^{-} \mu^{+}$ and $B^{+} \rightarrow K^{+} \tau^{+} \mu^{-}$ . The search is performed by studying proton-proton collision data collected by the detector of the LHCb collaboration corresponding to $9.1\text{fb}^{-1}$ of integrated luminosity. No significant signal is observed and as such the following upper limits at $90\%$ confidence level are set: $\mathcal{B}(B^{+} \rightarrow K^{+} \tau^{-} \mu^{+})

Study of the radiative penguin B0s [to] [phi] [gamma] and B0s [to] [gamma] [gamma] decays and the resonant B+ [to] K+ [gamma] [gamma] decays at Belle

Jean Wicht

We present the study of the radiative penguin Bs0 → ϕγ and Bs0 → γγ decays and the resonant B+ → K+h → K+γγ decays. We use data samples recorded at the Υ(4S) and Υ(5S) resonances with the Belle detector at KEKB, an electron-positron collider located in Tsukuba, Japan. The Υ(4S) sample corresponds to an integrated luminosity of 492 fb-1 and contains 535 million BB pairs. At the Υ(5S) resonance, we use a sample of 23.6 fb-1 containing 2.8 million Bs0 mesons. Penguin decays are loop-induced diagrams involving the heaviest particles of the Standard Model, the model describing, successfully so far, all particles and interactions, except the gravitation. Penguin decays are sensitive to physics beyond the Standard Model: particles foreseen by theories extending the Standard Model, whatever their masses are, can enter the loop and modify physics observables away from their Standard Model expectations. We report the first observation of a radiative penguin decay of the Bs0 meson in the Bs0 → ϕγ mode with a significance of 5.5 standard deviations. We measure Β(Bs0 → ϕγ) = (57 -15+18 (stat) -11+12 (syst)) × 10-6 in agreement with the expectation of the Standard Model. We do not observe any significant Bs0 → γγ signal and we compute an upper limit at the 90% confidence level on its branching fraction of Β(Bs0 → γγ) < 8.7 × 10-6. This limit is about six times more stringent than the previously published one. However, it is still about one order of magnitude larger than the expectation of the Standard Model and still above expectations of theories beyond the Standard Model. For the resonant B+ → K+h → K+γγ decays, we search for decays where the h particle can be a η, η', ηc, ηc(2S), χc0, χc2 or a J/ψ meson, or the X(3872) particle discovered in 2003 by the Belle collaboration. We observe the modes with h = η and η'. We obtain an evidence of the mode with h = ηc; this is the first time that a B+ → K+ηc signal is seen in the K+ γγ final state. We measure Β(B+ → K+η → K+γγ) = (0.87 -0.15+0.16 (stat) -0.07+0.10 (syst)) × 10-6, Β(B+ → K+η' → K+γγ) = (1.40 -0.15+0.16 (stat) -0.12+0.15 (syst)) × 10-6, Β(B+ → K+ηc → K+γγ) = (0.22 -0.07+0.09 (stat) -0.02+0.04 (syst)) × 10-6, with significances of 7.3, 13.8 and 4.1, respectively. For the other modes, we obtain limits on their branching fractions. We also measure or set limits on the branching fractions of the h → γγ decays for the modes where Β(B+ → K+h) has been measured elsewhere. We set for the first time an upper limit at the 90% confidence level on the branching fraction of the decay of the X(3872) particle into two photons of Β(X(3872) → γγ) < 1.1%.

EPFL2008

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

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

Aurélie Perrin

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.

EPFL2008

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

Jérémie Borel

EPFL2008

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

Aurélie Perrin

EPFL2008

Study of the radiative penguin B0s [to] [phi] [gamma] and B0s [to] [gamma] [gamma] decays and the resonant B+ [to] K+ [gamma] [gamma] decays at Belle

Jean Wicht

EPFL2008