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

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.

B°s --> J/[psi][eta]' decay and sensitivity to the Bs mixing phase at LHCb

Sergio Jiménez-Otero

The LHCb experiment is one of four main experiments to be setup at IP8 point of the proton-proton Large Hadron Collider (LHC) at CERN, Geneva. The data taking is scheduled to start end 2007. The LHCb detector is a forward single-arm spectrometer conceived to study CP violation and rare phenomena in the b-quark sector with very high precision. LHCb should provide a profound understanding of flavour physics in the framework of Standard Model (SM), so that the SM will be over-constrained, hopefully, it will exhibit some inconsistencies which may reveal a sign of New Physics beyond the SM. The physics studies being considered by the LHCb collaboration offer the possibility of furthering in the SM description of CP violation. The SM provides the theoretical framework for the violation of the CP symmetry in neutral B mesons as well as in neutral K mesons. The neutral Bs0 - Bs0 system stays at a privileged position in the quest for CP violation evidences. The Bs0 - Bs0 mixing phase, φs, has never been measured. This mixing phase has its origins in the interference between the decay and the mixing of those neutral mesons. b → ccs quark-level transitions decaying to CP eigenstates, may directly probe this electroweak phase by performing a time-dependent measurement of mixing-induced CP violation. Among those quark level transitions, we have the golden-plated channel to perform such a measurement that is Bs0 → J/ψφ and that requires an angular analysis to disentangle its CP eigenstates components. On the other hand, b → ccs transitions to pure CP eigenstates provide another probe to the φs not requiring such angular studies. In this dissertation we will be consider the Bs0 → J/ψ(μ+μ-) η' (π+π-η(γγ)) decays which are b → ccs transitions to pure CP even eigenstates. The reconstruction and selection of those decays will be presented here using full Monte Carlo simulations. An annual event yield of ∼ 2000 is assured with a background-over-signal level close to unity. The mass and proper time resolution will be also presented and some methods searching to improve those performances. In the last chapter of the dissertation, we will also present a study of the statistical sensitivity of Bs0 → J/ψ(μ+μ-) η' (π+π-η(γγ)) to the Bs0 - Bs0 mixing parameters, using a fast parameterised Monte Carlo simulation. Those simulations take into account in a realistic manner the outputs from the full Monte Carlo simulations. Specifically, we will be simulate the background-over-signal levels, the time dependent acceptance efficiency and tagging and reconstruction performances for that decay mode. It will also take as inputs the event-by-event proper time errors obtained from the full MC. The likelihood fit performed to extract the physics parameters requires a flavour-specific control sample, Bs0 → Ds π, which allows for the extraction of the Bs0 - Bs0 mixing frequency, ΔMs. The sensitivity achieved by our channel decay will be then combined to other b → ccs quark level transitions to pure CP eigenstates and finally also to the Bs0 → J/ψφ decay to admixture of CP eigenstates. The sensitivity obtained from the pure CP modes is σ(φs) = 0.056 rad whereas if we combine all decay channels into a single sensitivity measurement we achieve σ(φs) = 0.021 rad. The contribution from the pure CP modes is non-negligible, ∼ 14%, so that it is expected that these channels will certainly help in the measurement of φs.

EPFL2007

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

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