Decay Vertices Research Articles

Heavy Flavour Cross Section Measurements with the ATLAS Detector

ATLAS measurements of heavy flavour production cross sections are presented from proton-proton collisions at the LHC at a centre-of-mass energy of 7 TeV. These make use of b-hadron decays to D⁎μX, inclusive charm meson production, inclusive electron and muon production from semileptonic heavy flavour decays, and reconstruction of secondary decay vertices to tag b-jets. The results are compared with next-to-leading-order QCD calculations.

Radiation damage in the LHCb VELO

The VErtex LOcator (VELO) is a silicon strip detector designed to reconstruct particle tracks and vertices produced by proton–proton interactions near to the LHCb interaction point. The excellent track resolution and decay vertex separation provided by the VELO are essential to all LHCb analyses. For the integrated luminosity delivered by the LHC up to the end of 2011 the VELO is exposed to higher particle fluences than any other silicon detector of the four major LHC experiments. These proceedings present results from radiation damage studies carried out during the first two years of data taking at the LHC. Radiation damage has been observed in all of the 88 VELO silicon strip sensors, with many sensors showing evidence of type-inversion in the highest fluence regions. Particular attention has been given to the two n+-on-p sensors as this type of sensor is one of the leading candidates for the LHC silicon detector upgrades.

LHCb VELO tracking resolutions

The LHCb detector at the Large Hadron Collider (LHC) at CERN is designed to make precision measurements of mesons and baryons containing b and c quarks. Many analyses performed using data from LHCb examine properties of particles and anti-particles as a function of proper decay time. Precise measurements of production and decay vertices are thus essential. The Vertex Locator (VELO) subdetector of LHCb performs this task. The excellent performance of the VELO with respect to the resolutions it achieves on single hits, impact parameters, and primary vertex positions is presented, with particular attention paid to measurement of impact parameters.

The Magnetic Mass of Transverse Gluon, the B-Meson Weak Decay Vertex and the Triality Symmetry of Octonion

With an assumption that in the Yang-Mills Lagrangian, a left-handed fermion and a right-handed fermion both expressed as the quaternion make an octonion which possesses the triality symmetry, I calculate the magnetic mass of the transverse self-dual gluon from three loop diagram, in which a heavy quark pair is created and two self-dual gluons are interchanged. The magnetic mass of the transverse gluon depends on the mass of the pair created quarks, and in the case of charmed quark pair creation, the magnetic mass m mag becomes approximately equal to T c at $${T=T_c\sim 1.14\Lambda_{\overline{MS}} \sim 260}$$ MeV. A possible time-like magnetic gluon mass from two self-dual gluon exchange is derived, and corrections in the B-meson weak decay vertices from the two self-dual gluon exchange are also evaluated.

The Belle II DEPFET vertex detector: current status and future plans

An upgrade of the existing Flavour Factory KEKB (Tsukuba, Japan) is under construction, and is foreseen for commissioning by the end of 2014. This new e+e− machine ("SuperKEKB") will deliver an instantaneous luminosity of 8⋅1035 cm−2s−1, which is 40 times higher than the world record set by KEKB. In order to be able to fully exploit the increased number of events and provide high precision measurements of the decay vertex of the B meson systems in such a harsh environment, the Belle detector will be upgraded ("Belle II") and a new silicon vertex detector, based on the DEPFET technology, will be designed and constructed. The new pixel detector, close to the interaction point, will consist of two layers of DEPFET active pixel sensors. This technology combines the detection together with the in-pixel amplification by the integration, on every pixel, of a field effect transistor into a fully depleted silicon bulk. In Belle II, DEPFET sensors thinned down to 75 μm with low power consumption and low intrinsic noise will be used.

The Data Handling Processor for the Belle II Pixel Vertex Detector: efficiency optimization

A major upgrade of the current Japanese B-Factory (KEK-B) is planned for 2015. Together with this new machine (SuperKEK-B) also a new detector, Belle-II, will be operated to fully exploit the planned luminosity of 8 × 1035 s−1cm−2 (40 times larger than previously). One of the major changes in the new experiment will be the introduction of a new sub-detector, close to the interaction point, to allow a precise reconstruction of the decay vertices of the B meson systems. This pixel detector, based on the DEPFET technology, will consist of 20 ladder modules arranged in two cylindrical layers around the beam pipe. Each of the modules will be read-out independently by a combination of analog and digital ASICs placed at both ends of each sensor. The digital chip, the Data Handling Processor (DHP), is designed to control the readout chain and to pre-process and to compress the data. The chip structure and the latest results about its performance optimization will be presented.

Performance of the LHCb Vertex Locator

LHCb is a dedicated flavour physics experiment at the Large Hadron Collider (LHC) at CERN. The Vertex Locator (VELO) is an essential part of the LHCb detector, permitting precision measurements of the production and decay vertices of beauty and charm particles. The VELO consists of a series of silicon micro-strip detectors, arranged in two retractable halves. Positioned only 7 mm from the beam during normal operations, it must withstand very high levels of radiation. The performance of the LHCb VELO during the first year of LHC physics running is presented.

Performance of the LHCb Vertex Locator

The Vertex Locator is a silicon microstrip detector which provides the LHCb experiment with high precision measurements of tracks and decay vertices. The VELO sensors are exposed to a radiation dose of (2.5 — 6.5) × 1013 neq/cm2 per fb−1 in the area which is most irradiated. A best hit resolution of 4μm; is obtained for angled tracks, in agreement with expectations. The VELO has a vertex position resolution down to 11μm; in the transverse direction and an excellent momentum dependent performance.

Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at $\sqrt{s}=7\mbox{~TeV}$ with the ATLAS detector

The inclusive and dijet production cross-sections have been measured for jets containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass energy of sqrt(s) = 7 TeV, using the ATLAS detector at the LHC. The measurements use data corresponding to an integrated luminosity of 34 pb^-1. The b-jets are identified using either a lifetime-based method, where secondary decay vertices of b-hadrons in jets are reconstructed using information from the tracking detectors, or a muon-based method where the presence of a muon is used to identify semileptonic decays of b-hadrons inside jets. The inclusive b-jet cross-section is measured as a function of transverse momentum in the range 20 < pT < 400 GeV and rapidity in the range |y| < 2.1. The bbbar-dijet cross-section is measured as a function of the dijet invariant mass in the range 110 < m_jj < 760 GeV, the azimuthal angle difference between the two jets and the angular variable chi in two dijet mass regions. The results are compared with next-to-leading-order QCD predictions. Good agreement is observed between the measured cross-sections and the predictions obtained using POWHEG + Pythia. MC@NLO + Herwig shows good agreement with the measured bbbar-dijet cross-section. However, it does not reproduce the measured inclusive cross-section well, particularly for central b-jets with large transverse momenta.

Heavy-flavour production in Pb–Pb collisions at the LHC, measured with the ALICE detector

We present the first results from the ALICE experiment on the nuclear modification factors for heavy-flavour hadron production in Pb–Pb collisions at TeV. Using proton–proton and lead–lead collision samples at TeV and TeV, respectively, the nuclear modification factors RAA(pt) were measured for D mesons at central rapidity (via displaced decay vertex reconstruction), and for electrons and muons, at central and forward rapidity, respectively.

Measurement of $ {\text{B}}\overline {\text{B}} $ angular correlations based on secondary vertex reconstruction at $ \sqrt {s} = 7\,{\text{TeV}} $

A measurement of the angular correlations between beauty and anti-beauty hadrons (B B-bar) produced in pp collisions at a centre-of-mass energy of 7 TeV at the CERN LHC is presented, probing for the first time the region of small angular separation. The B hadrons are identified by the presence of displaced secondary vertices from their decays. The B hadron angular separation is reconstructed from the decay vertices and the primary-interaction vertex. The differential B B-bar production cross section, measured from a data sample collected by CMS and corresponding to an integrated luminosity of 3.1 inverse picobarns, shows that a sizable fraction of the B B-bar pairs are produced with small opening angles. These studies provide a test of QCD and further insight into the dynamics of b b-bar production.

Method for reconstruction of direction of gamma quanta registered by CsI calorimeter in E391 experiment

This study presents a new method for the reconstruction of the direction of γ quanta registered by a CsI calorimeter in the E391 experiment. A neural network in the form of a generalized basis function is used as the mathematical tool for reconstruction. The effective mass of the registered π0 mesons and z coordinate of the decay vertex was first reconstructed in E391 using the measured direction of the registered γ quanta. The reconstruction of these quantities increases the sensitivity of the E391 experiment by at least 35%.

The Belle-II DEPFET pixel detector: A step forward in vertexing in the superKEKB flavour factory

Abstract An upgrade of the successful asymmetric e+e− collider in KEK (Tsukuba, Japan) is foreseen by the fall of 2013. This new Super Flavor Factory will deliver an increased instantaneous luminosity of up to L = 8 × 10 35 cm − 2 s − 1 , 40 times larger than the current KEKB machine. To exploit these new conditions and provide high precision measurements of the decay vertex of the B meson systems, a new silicon vertex detector will be operated in Belle. This new detector will consist of two layers of DEPFET Active Pixel Sensors as close as possible to the interaction point. DEPFET is a field effect transistor, with an additional deep implant underneath the channel's gate, integrated on a completely depleted bulk. This technology offers detection and an in-pixel amplification stage, while keeping low the power consumption. Under these conditions, thin sensors with small pixel size and low intrinsic noise are possible. In this article, an overview of the full system will be described, including the sensor, the front-end electronics and both the mechanical and thermal proposed solutions as well as the expected performance.

A MAPS based vertex detector for the STAR experiment at RHIC

The STAR experiment at RHIC is in the process of upgrading the inner detector region of the experiment to improve the vertex resolution. We describe the current design of a MAPS based vertex detector, which is the innermost and highest resolution detector of the set of three planned upgrade detectors. This detector will enable the identification of decay vertices displaced from the interaction vertex by 100–150 μm and extend the capabilities of the STAR detector in the heavy flavor domain. We present selected detector design characteristics and prototyping results, which help to validate the design in preparation for the construction of the detector.

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.

Prospects of measuring the CKM matrix element [formula omitted] at the LHC

We study the prospects of measuring the CKM matrix element |Vts| at the LHC with the top quarks produced in the processes pp→tt¯X and pp→t/t¯X, and the subsequent decays t→W+s and t¯→W−s¯. To reduce the jet activity in top quark decays, we insist on tagging the W± leptonically, W±→ℓ±νℓ (ℓ=e,μ,τ), and analyse the anticipated jet profiles in the signal process t→Ws and the dominant background from the decay t→Wb. To that end, we analyse the V0 (K0 and Λ) distributions in the s- and b-quark jets concentrating on the energy and transverse momentum distributions of these particles. The V0s emanating from the t→Wb branch have displaced decay vertexes from the interaction point due to the weak decays b→c→s and the b-quark jets are rich in charged leptons. Hence, the absence of secondary vertexes and of the energetic charged leptons in the jet provide additional (b-jet vs. s-jet) discrimination in top quark decays. These distributions are used to train a boosted decision tree (BDT), a technique used successfully in measuring the CKM matrix element |Vtb| in single top production at the Tevatron. Using the BDT classifier, and a variant of it called BDTD, which makes use of decorrelated variables, we calculate the BDT(D)-response functions corresponding to the signal (t→Ws) and background (t→Wb). Detailed simulations undertaken by us with the Monte Carlo generator PYTHIA are used to estimate the background rejection versus signal efficiency for three representative LHC energies s=7 TeV, 10 TeV and 14 TeV, of which only the analysis for the s=14 TeV case is shown in detail. We argue that a benchmark with 10% accuracy for the signal (t→Ws) at a background (t→Wb) rejection by a factor 103 (required due to the anticipated value of the ratio |Vts|2/|Vtb|2≃1.6×10−3) can be achieved at the LHC@14 TeV with an integrated luminosity of 10 fb−1.

Towards a [formula omitted], thin and high resolution pixelated CMOS sensor system for future vertex detectors

The physics goals of many high energy experiments require a precise determination of decay vertices, imposing severe constraints on vertex detectors (readout speed, granularity, material budget,…). The IPHC-IRFU collaboration developed a sensor architecture to comply with these requirements. The first full scale CMOS sensor was realised and equips the reference planes of the EUDET beam telescope. Its architecture is being adapted to the needs of the STAR (RHIC) and CBM (FAIR) experiments. It is a promising candidate for the ILC experiments and the ALICE detector upgrade (LHC). A substantial improvement to the CMOS sensor performances, especially in terms of radiation hardness, should come from a new fabrication technology with depleted sensitive volume. A prototype sensor was fabricated to explore the benefits of the technology. The crucial system integration issue is also currently being addressed. In 2009 the PLUME collaboration was set up to investigate the feasibility and performances of a light double sided ladder equipped with CMOS sensors, aimed primarily for the ILC vertex detector but also of interest for other applications such as the CBM vertex detector.

The Belle-II pixel vertex tracker at the SuperKEKB Flavour Factory

Building on the success of the current KEKB Flavour Factory and the Belle experiment, which helped to firmly establish the CKM picture of quark mixing and CP violation in the Standard Model, a luminosity upgrade of the asymmetric e +e − machine is planned. This new Super Flavour Factory, SuperKEKB, will deliver a luminosity of 8×10 35 cm −2 s −1, a factor 40 of increase over the present luminosity world record. It is foreseen to begin its operation in 2013. With these increased statistics, a new silicon vertex detector is needed in order to obtain precise decay vertex resolution measurements; these new precise measurements in the flavour sector can probe new physics well beyond the scales accessible to direct observations.

Results from the commissioning of the ATLAS Pixel Detector with cosmic ray data

The ATLAS Pixel Detector is one of the largest silicon pixel hybrid detectors in the world. It has a total active area of 1.7 m 2 of silicon read out every 25 ns by approximately 80 million electronic channels. It is the innermost tracking detector of the ATLAS experiment at the Large Hadron Collider (LHC) at CERN, designed to measure particle tracks and decay vertices with a very high precision and efficiency. Since August 2008, after more than 10 years of development and construction, the whole detector has been operated together. After tuning, calibration and timing-in the detector has demonstrated excellent noise occupancy of 10 −10 and a tracking hit efficiency greater than 99.7%. The paper will describe the detector performance and discuss the studies performed with cosmic ray data, such as alignment and the Lorentz angle measurement.

LHCb vertex locator commissioning and results from the first LHC beam reconstructed tracks

LHCb is a dedicated experiment at the LHC to study CP violation and rare b-quark decays. The vertex locator (VELO) is a silicon strip detector designed to measure precisely the production and the decay vertices of B-mesons. The VELO consists of two retractable detector halves with 21 silicon micro-strip tracking modules each. The full system has been operated since June 2008 and its commissioning experience is reported. During the LHC synchronization tests in 2008, LHCb detectors measured particles produced by the interaction of the LHC primary beam on an absorber. About 2200 tracks were reconstructed in the VELO and they were used for the first evaluation of the alignment. A single hit resolution of 10μm was obtained at the smallest pitch for tracks of perpendicular incidence. The design and the main components of the detector system are introduced, focusing on the commissioning of the detector and on the results of the measurements of the first tracks in the VELO.