Flavor Tagging Research Articles

Real-time flavour tagging selection in ATLAS

In high-energy physics experiments, online selection is crucial to the identification of the few interesting collisions from the large data volume processed. In the overall ATLAS trigger strategy, b-jet triggers are designed to identify heavy-flavor content in real-time and, in particular, provide the only option to efficiently record events with fully hadronic final states containing b-jets. In doing so, two different, but related, challenges are faced. The physics goal is to optimise as far as possible the rejection for light jets from QCD processes, while retaining a high efficiency on selecting jets from beauty, while maintaining affordable trigger rates without raising jet energy thresholds. This poses a challenging computing task, as charged tracks and their corresponding vertices must be reconstructed and analysed for each jet above the desired threshold, regardless of the increasingly harsh pile-up conditions. The performance of b-jet triggers during the LHC Run I data-taking campaigns is presented, together with an overview of the new online b-tagging strategy and algorithms, designed to face the above mentioned challenges, which will be adopted during Run II.

An ultra-low power self-timed column-level ADC for a CMOS pixel sensor based vertex detector

The International Large Detector (ILD) is a detector concept for the future linear collider experiment. The vertex detector is the key tool to achieve high precision measurements for flavor tagging, which puts stringent requirements on the CMOS pixel sensors. Due to the cooling systems which deteriorate the material budget and increase the multiple scattering, it is important to reduce the power consumption. This paper presents an ultra-low power self-timed column-level ADC for the CMOS pixel sensors, aiming to equip the outer layers of the vertex detector. The ADC was designed to operate in two modes (active and idle) adapted to the low hit density in the outer layers. The architecture employs an enhanced sample-and-hold circuit and a self-timed technique. The total power consumption with a 3-V supply is 225μW during idle mode, which is the most frequent situation. This value rises to 425μW in the case of the active mode. It occupies an areaof 35 × 590μm2.

Flavor tagged time-dependent angular analysis of theBs0→J/ψϕdecay and extraction ofΔΓsand the weak phaseϕsin ATLAS

A measurement of the $B_s \rightarrow J/\psi \phi$ decay parameters, updated to include flavour tagging is reported using $4.9 fb^{-1}$ of integrated luminosity collected by the ATLAS detector from $\sqrt{s}= 7$TeV $pp$ collisions recorded in 2011 at the LHC. The values measured for the physical parameters are: \begin{eqnarray*} \phi_s & = & 0.12 \pm 0.25(stat.)\pm 0.05(syst.)rad \\ \Delta\Gamma & = & 0.053 \pm 0.021(stat.) 0.010(syst.)ps^{-1} \\ \Gamma_s & = & 0.677 \pm 0.007 (stat.)\pm 0.004 (syst.)ps^{-1}\\ |A_{\parallel}(0)|^2 & = & 0.220 \pm 0.008 (stat.)\pm 0.009(syst.)\\ |A_{0}(0)|^2 & = & 0.529 \pm 0.006 (stat.)\pm 0.012(syst.)\\ \delta_\perp & = & 3.89 \pm 0.47(stat.) \pm 0.11(syst.)rad \end{eqnarray*} where the parameter $\Delta\Gamma$ is constrained to be positive. The $S$-wave contribution was measured and found to be compatible with zero. Results for $\phi_s$ and $\Delta\Gamma$ are also presented as $68\%$ and $95\%$ likelihood contours, which show agreement with the Standard Model expectations.

Mixing and CP violation in theBssystem with ATLAS

The different amplitudes contributing to the decay of Bs into J /ψϕ (µ + µ − K + K − ) can be studied with a combined analysis of the decay time and angular correlations. An analysis based on the LHC data collected by the ATLAS detector in 2011 and with initial B -meson flavour tagging is presented, improving the accuracy in the CP-violating phase ϕs compared to the untagged analysis.

The calibration of an LHCb electromagnetic calorimeter by recovering the invariant mass of neutral pions

The precise calibration of an electromagnetic calorimeter is extremely important for flavor tagging and for the study of B-mesons decays and therefore is of great importance for implementing the physics program of the LHCb experiment. The purpose of the calibration process is to achieve a 2% accuracy when measuring the energies of electrons and photons. At the last step of a multi-stage procedure, the method of restoring the invariant mass of a neutral pion in its decay into two photons is used.

Operational experience, improvements, and performance of the CDF Run II silicon vertex detector

The Collider Detector at Fermilab (CDF) pursues a broad physics program at Fermilab's Tevatron collider. Between Run II commissioning in early 2001 and the end of operations in September 2011, the Tevatron delivered 12fb−1 of integrated luminosity of pp¯ collisions at s=1.96TeV. The physics at CDF includes precise measurements of the masses of the top quark and W boson, measurement of CP violation and Bs mixing, and searches for Higgs bosons and new physics signatures, all of which require heavy flavor tagging with large charged particle tracking acceptance. To realize these goals, in 2001 CDF installed eight layers of silicon microstrip detectors around its interaction region. These detectors were designed for 2–5 years of operation, radiation doses up to 2Mrad (0.02Gy), and were expected to be replaced in 2004. The sensors were not replaced, and the Tevatron run was extended for several years beyond its design, exposing the sensors and electronics to much higher radiation doses than anticipated. In this paper we describe the operational challenges encountered over the past 10 years of running the CDF silicon detectors, the preventive measures undertaken, and the improvements made along the way to ensure their optimal performance for collecting high quality physics data. In addition, we describe the quantities and methods used to monitor radiation damage in the sensors for optimal performance and summarize the detector performance quantities important to CDF's physics program, including vertex resolution, heavy flavor tagging, and silicon vertex trigger performance.

General analysis ofB¯→K¯(*)ℓ+ℓ−decays at low recoil

We analyze the angular distributions of $\overline{B}\ensuremath{\rightarrow}{\overline{K}}^{*}(\ensuremath{\rightarrow}\overline{K}\ensuremath{\pi}){\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ and $\overline{B}\ensuremath{\rightarrow}\overline{K}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ decays in the region of low hadronic recoil in a model-independent way by taking into account the complete set of dimension-six operators $[\overline{s}\ensuremath{\Gamma}b][\overline{\ensuremath{\ell}}{\ensuremath{\Gamma}}^{\ensuremath{'}}\ensuremath{\ell}]$. We obtain several novel low-recoil observables with high sensitivity to nonstandard-model Dirac structures, including $CP$ asymmetries, which do not require flavor tagging. The transversity observables ${H}_{T}^{(1,3,4,5)}$ are found to be insensitive to hadronic matrix elements and their uncertainties even when considering the complete set of operators. In the most general scenario we show that the low recoil operator product expansion can be probed at the few-percent level using the angular observable ${J}_{7}$. Higher sensitivities are possible assuming no tensor contributions, specifically by testing the low-recoil relation $|{H}_{T}^{(1)}|=1$. We explicitly demonstrate the gain in reach of the low-recoil observables in accessing the ratio $|{\mathcal{C}}_{9}/{\mathcal{C}}_{10}|$ compared to the forward-backward asymmetry, and probing $CP$-violating right-handed currents $\mathrm{Im}{\mathcal{C}}_{{10}^{\ensuremath{'}}}$. We give updated Standard Model predictions for key observables in $\overline{B}\ensuremath{\rightarrow}{\overline{K}}^{(*)}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ decays.

LHCb detector status and upgrade

The LHCb experiment is a specialized experiment for B and D physics at the CERN LHC. The layout of the experiment and the various components are described, with an overview of their performance. Global running conditions and performance are also shown. The motivation for the foreseen detector upgrade is presented, together with the proposed new detectors, to be installed during Long Shutdown 2, in 2018. 1 General overview The LHCb detector is intended to study beauty and charm physics. At LHC, the production is mainly in the forward and backward directions. Both b particles are going in the same direction, thus a single arm spectrometer allows measuring the interesting b decay together with tracks from the other b decay, for flavour tagging. The overall detector layout is shown in figure 1. The vertex locator measures all tracks near the interaction point, a dipole magnet and tracking stations after are providing accurate momentum measurement, two RICH detectors, calorimeters and a muon system provide full particle identification.

Time reversal violation from the entangled B 0 $ \overline B $ 0 system

We discuss the concepts and methodology to implement an experiment probing directly Time Reversal (T ) non-invariance, without any experimental connection to CP violation, by the exchange of in and out states. The idea relies on the B 0 $ \overline B $ 0 entanglement and decay time information available at B factories. The flavor or CP tag of the state of the still living neutral meson by the first decay of its orthogonal partner overcomes the problem of irreversibility for unstable systems, which prevents direct tests of T with incoherent particle states. T violation in the time evolution between the two decays means experimentally a difference between the rates for the time-ordered (l+ X, J/ψK S ) and (J/ψK L, l− X) decays, and three other independent asymmetries. The proposed strategy has been applied to simulated data samples of similar size and features to those currently available, from which we estimate the significance of the expected discovery to reach many standard deviations.

Dijet signals of the little Higgs model with T-parity

The Littest Higgs model with T-parity (LHT), apart from offering a viable solution to the naturalness problem of the Standard Model, also predicts a set of new fermions as well as a candidate for dark matter. We explore the possibility of discovering the heavy T-odd quark Q H at the LHC in a final state comprising two hard jets with a large missing transverse momentum. Also discussed is the role of heavy flavor tagging.

Opposite-side flavour tagging of B mesons at the LHCb experiment.

The calibration and performance of the opposite-side flavour tagging algorithms used for the measurements of time-dependent asymmetries at the LHCb experiment are described. The algorithms have been developed using simulated events and optimized and calibrated with B+→J/ψK+, B0→J/ψK∗0 and B0→D∗−μ+νμ decay modes with 0.37 fb−1 of data collected in pp collisions at sqrt{s} = 7mbox{~TeV} during the 2011 physics run. The opposite-side tagging power is determined in the B+→J/ψK+ channel to be (2.10±0.08±0.24) %, where the first uncertainty is statistical and the second is systematic.

New measurement of the B0s mixing phase and observation of suppressed B0s decays at CDF

Recent CDF measurements of B0s decay parameters are presented. The CP violating phase βs has been measured in B0s → J/ψϕ decays using 5.2 fb−1 integrated luminosity of CDF data. This updated βs measurement includes the contribution of B0s → J/ψK+K− or B0s → J/ψf0 events to the signal sample, where the f0 and non-resonant K+K− are S-wave states. Additional improvements for this update include more than doubling the signal sample, improved selection and particle ID, and fully calibrated flavour tagging for the complete dataset. This measurement shows good agreement with the Standard Model expectation for βs, with a significance of < 1σ for deviation from the expected value. In a related analysis, two suppressed B0s decay channels, B0s → J/ψK*0 and B0s → J/ψKs, have been observed for the first time, in 5.9 fb−1 CDF data. Their branching fractions and relative branching ratios are reported. These newly observed channels have the potential for extraction of important CKM parameters including the phase βs.

Triple-product asymmetries inK,D(s), andB(s)decays

One distinguishes between "true" CP violating triple product (TP) asymmetries which require no strong phases and "fake" asymmetries which are due to strong phases but require no CP violation. So far a single true TP asymmetry has been measured in $K_L\to \pi^+\pi^- e^+e^-$. A general discussion is presented for T-odd TP asymmetries in four-body decays. It is shown that TP asymmetries vanish for two identical and kinematically indistinguishable particles in the final state. Two examples are $D^0\to K^-\pi^+\pi^-\pi^+$ and $D^+\to K^-\pi^+\pi^+\pi^0$. A non-zero TP asymmetry can be expected when non-trivial kinematic correlations exist, as in the decay $K_L \to e^+ e^- e^+ e^-$. Triple product asymmetries measured in charmed particle decays indicate an interesting pattern of final-state interactions. We reiterate a discussion of TP asymmetries in $B$ meson decays to two vector mesons each decaying to a pseudoscalar pair, extending results to decays where one vector meson decays into a lepton pair. We derive expressions for time-dependent TP asymmetries for neutral B decays to flavorless states in terms of the neutral $B$ mass difference $\Delta m$ and the width-difference $\Delta\Gamma$. Time-integrated true CP violating asymmetries, measurable for untagged $B_s$ decays, are shown to be suppressed by neither $\Gamma_s/\Delta m_s$ nor $\Delta\Gamma_s/\Gamma_s$ if transversity amplitudes for CP-even and CP-odd states involve different weak phases. In contrast, fake asymmetries require flavor tagging and are suppressed by the former ratio when time-integrated. We apply our results to $B\to K^*\phi$ and $B_s\to\phi\phi$ data and suggest an application for $B_s\to J/\psi\phi$.

Search for top quark FCNC couplings in Z′ models at the LHC and CLIC

The top quark is the heaviest particle to date discovered, with a mass close to the electroweak symmetry breaking scale. It is expected that the top quark would be sensitive to the new physics at the TeV scale. One of the most important aspects of the top quark physics can be the investigation of the possible anomalous couplings. Here, we study the top quark flavor changing neutral current (FCNC) couplings via the extra gauge boson Z' at the Large Hadron Collider (LHC) and the Compact Linear Collider (CLIC) energies. We calculate the total cross sections for the signal and the corresponding Standard Model (SM) background processes. For an FCNC mixing parameter x=0.2 and the sequential Z' mass of 1 TeV, we find the single top quark FCNC production cross sections 0.38(1.76) fb at the LHC with sqrt{s_{pp}}=7(14) TeV, respectively. For the resonance production of sequential Z' boson and decays to single top quark at the Compact Linear Collider (CLIC) energies, including the initial state radiation and beamstrahlung effects, we find the cross section 27.96(0.91) fb at sqrt{s_{e^{+}e^{-}}}=1(3) TeV, respectively. We make the analysis to investigate the parameter space (mixing-mass) through various Z' models. It is shown that the results benefit from the flavor tagging.

The RICH detectors for the upgraded LHCb experiment

The LHCb experiment is designed to operate at an LHC luminosity of 2×10 32 cm −2 s −1, or typically one collision per bunch crossing. After about five years it will have recorded a data sample of 10 fb −1. At this time LHCb plans an upgrade to operate the detector at a significantly increased luminosity that will greatly extend its potential for discovery and study of new phenomena. Studies performed to optimise the design of the LHCb upgrade are presented. The RICH detector will require new photon detectors as the current HPDs have encapsulated electronics which only support a readout rate of up to 1 MHz. Multi-anode and flat-panel photo multiplier tubes (PMTs) are evaluated as photon detector candidates and their properties including pulse height, shape and cross-talk are measured. The particle identification performance is studied as a function of luminosities ranging up to ten times the design as foreseen for the LHCb upgrade. Finally, the performance of flavour tagging using kaons, which strongly relies on RICH particle identification is also presented.

Higgs boson hadronic branching ratios at the ILC

We present a study of the Higgs boson decay branching ratios to $b\overline{b}$, $c\overline{c}$, and gluons, one of the cornerstones of the physics program at the International Linear Collider. A standard model Higgs boson of 120 GeV mass, produced in the Higgs-strahlung process at $\sqrt{s}=250\text{ }\text{ }\mathrm{GeV}$, was investigated using the full detector simulation and reconstruction procedures. The analysis was performed in the framework of the Silicon Detector concept with full account of inclusive standard model backgrounds. The selected decay modes contained two heavy flavor jets in the final state and required excellent flavor tagging through precise reconstruction of interaction and decay vertices in the detector. A new signal discrimination technique using correlations of neural network outputs was used to determine the branching ratios and estimate their uncertainties, 4.8%, 8.4%, and 12.2% for $b\overline{b}$, $c\overline{c}$, and gluons, respectively.

Discovering bottom squark coannihilation at the ILC

We study the potential of International Linear Collider (ILC) at sqrt(s) = 500 GeV to probe new dark matter motivated scenario where the bottom squark (sbottom) is the next to lightest supersymmetric particle. For this scenario, which is virtually impossible for the LHC to test, the ILC has a potential to cover a large fraction of the parameter space. The challenge is due to a very low energy of jets, below 20-30 GeV, which pushes the jet clustering and flavour tagging algorithms to their limits. The process of sbottom pair production was studied within the SiD detector concept. We demonstrate that ILC offers a unique opportunity to test the SUSY parameter space motivated by the sbottom neutralino co-annihilation scenario in cases when the sbottom production is kinematically accessible. The study was done with the full SiD simulation and reconstruction chain including all Standard Model and beam backgrounds.

The LCFIVertex package: Vertexing, flavour tagging and vertex charge reconstruction with an ILC vertex detector

The precision measurements envisaged at the International Linear Collider (ILC) depend on excellent instrumentation and reconstruction software. The correct identification of heavy flavour jets, placing unprecedented requirements on the quality of the vertex detector, will be central for the ILC programme. This paper describes the LCFIVertex software, which provides tools for vertex finding and for identification of the flavour and charge of the leading hadron in heavy flavour jets. These tools are essential for the ongoing optimisation of the vertex detector design for linear colliders such as the ILC. The paper describes the algorithms implemented in the LCFIVertex package as well as the scope of the code and its performance for a typical vertex detector design.

Flavour tagging performance in LHCb

To do precise CP violation measurements, the best possible determination of the flavour of the B-meson is necessary. This report summarizes the flavour tagging performances for the LHCb experiment. The flavour tagging is obtained through a combination of several methods, based on different signatures. The use of control channels, which are decays to flavour-specific final states, will allow to determine the wrong tag fraction ω (the probability of a tag to be wrong), which can be used as an input for the determination of CKM unitarity triangle angles.

CMOS pixel sensor development: a fast read-out architecture with integrated zero suppression

CMOS Monolithic Active Pixel Sensors (MAPS) have demonstrated their strong potential for tracking devices, particularly for flavour tagging. They are foreseen to equip several vertex detectors and beam telescopes. Most applications require high read-out speed, which imposes sensors to feature digital output with integrated zero suppression. The most recent development of MAPS at IPHC and IRFU addressing this issue will be reviewed. The design architecture, combining pixel array, column-level discriminators and zero suppression circuits, will be presented. Each pixel features a preamplifier and a correlated double sampling (CDS) micro-circuit reducing the temporal and fixed pattern noises. The sensor is fully programmable and can be monitored. It will equip experimental apparatus starting data taking in 2009/2010.