Primary Vertex Research Articles

B-jet identification at High Level Trigger in CMS

The CMS experiment at the CERN LHC has been designed with a 2-level trigger system. The Level 1 Trigger (L1) is implemented on custom-designed electronics. The High Level Trigger (HLT) is a streamlined version of the CMS offline reconstruction software running on a computer farm. Being able to identify b-quark jets (b-tagging) at trigger level will play a crucial role during the Run II data taking to ensure the top quark, beyond the Standard Model and Higgs boson physics program of the experiment. It will help to significantly reduce the trigger output rate which will increase due to the higher instantaneous luminosity and higher cross sections at 13 TeV. B-tagging algorithms based on the identification of tracks displaced from the primary proton-proton collision and/or on the reconstruction of secondary vertices have been successfully used during Run I. We will present their design and performance with an emphasis on the dedicated aspects of track and primary vertex reconstruction, as well as the improvements foreseen to meet the challenges of the Run II data taking.

Diamond detector time resolution for large angle tracks

The applications which have stimulated greater interest in diamond sensors are related to detectors close to particle beams, therefore in an environment with high radiation level (beam monitor, luminosity measurement, detection of primary and secondary-interaction vertices).Our aims is to extend the studies performed so far by developing the technical advances needed to prove the competitiveness of this technology in terms of time resolution, with respect to more usual ones, which does not guarantee the required tolerance to a high level of radiation doses. In virtue of these goals, measurements of diamond detector time resolution with tracks incident at different angles are discussed. In particular, preliminary testbeam results obtained with 5GeV electrons and polycrystalline diamond strip detectors are shown.

Operational experience with the ALICE Pixel Detector

The ALICE Silicon Pixel Detector (SPD) constitutes the innermost detector of the ALICE experiment, which is the LHC experiment dedicated to the investigation of strongly interacting matter in heavy-ion collisions.The SPD consists of ∼ 10 million pixels organized in two layers at radii of 39 mm and 76 mm that cover a pseudorapidity range of | η | < 2 and | η | < 1.4, respectively. It provides the position of the primary and secondary vertices, and it has the unique feature of generating a trigger signal that contributes to the L0 trigger of the ALICE experiment. Installed in 2007, the SPD started to record data since the first LHC collisions. This contribution presents the main features of the SPD, the detector performance and the operational experience, including calibration and optimization activities, since installation in ALICE. The ongoing consolidation activities carried out to prepare the detector for the data taking during the RUN2 of LHC will be also described.

Upgrade of the ALICE Inner Tracking System

ALICE (A Large Ion Collider Experiment) is studying heavy-ion collisions at the CERN LHC, with the aim of forming, under extreme conditions of temperature and energy density, a Quark-Gluon Plasma (QGP) and studying its properties. The ALICE Collaboration is preparing a major upgrade of the experimental apparatus, planned for installation in the second long LHC shutdown in the years 2018–2019. A key element of the ALICE upgrade is the construction of a new, ultra-light, high-resolution Inner Tracking System (ITS) . The primary focus of the new ITS is on improving the performance for detection of heavy-flavour hadrons, and of thermal photons and low-mass di-electrons emitted by the QGP . With respect to the current detector, the new ITS will significantly enhance the determination of the distance of closest approach of a track to the primary vertex, the tracking efficiency at low transverse momenta, and the read-out rate capabilities. This will be achieved by seven concentric detector layers based on a 50 μm thick CMOS pixel sensor with a pixel pitch of about 30× 30 μm2. A key feature of the new ITS, which is optimized for high tracking accuracy at low transverse momenta, is the very low mass of the three innermost layers, which feature a material thickness of 0.3% X0 per layer. This contribution describes the design goals and layout of the new ALICE ITS, a summary of the R&D activities, with focus on the technical implementation of the main detector components, and the projected detector performance.

Time-based Cellular Automaton track finder for the CBM experiment

The future heavy-ion experiment CBM (FAIR/GSI, Darmstadt, Germany) will focus on the measurement of rare probes at interaction rates up to 10 MHz with data flow of up to 1 TB/s. The beam will provide free stream of particles without bunch structure. That requires full online event reconstruction and selection not only in space, but also in time, so- called 4D event building and selection. This is a task of the First-Level Event Selection (FLES) package. The FLES reconstruction and selection package consists of several modules: track finding, track fitting, short-lived particles finding, event building and event selection. The input data are distributed within the FLES farm in a form of so-called time-slices, in which time length is proportional to a compute power of a processing node. A time-slice is reconstructed in parallel between cores within a CPU, thus minimising communication between CPUs. After all tracks of the whole time-slice are found and fitted, they are collected into clusters of tracks originated from common primary vertices. After that short-lived particles are found and the full event building process is finished.

Effects of NMDA and GABA-A Receptor Antagonism on Auditory Steady-State Synchronization in Awake Behaving Rats.

Background:The auditory steady-state response, which measures the ability of neural ensembles to entrain to rhythmic auditory stimuli, has been used in human electroencephalogram studies to assess sensory processing and electrical oscillatory deficits. Patients with schizophrenia show a deficit in auditory steady-state response at 40 Hz, and therefore this may be a useful biomarker to study this disorder.Methods:We used auditory steady-state response recordings from the primary auditory cortex, hippocampus, and vertex electroencephalogram sites in awake behaving rats to determine whether pharmacological impairment of excitatory or inhibitory neurotransmission mimics auditory steady-state response abnormalities in schizophrenia.Results:We found the most robust response to auditory stimuli in the primary auditory cortex, in line with previous studies suggesting this region is the primary generator of the auditory steady-state response in humans. Acute MK-801 (0.1mg/kg i.p.) increased primary auditory cortex intertrial coherence during auditory steady-state response at 20 and 40 Hz. Chronic MK-801 (21-day exposure at this daily dose) had no significant effect on 40-Hz auditory steady-state response. Furthermore, we found no effect of acute or chronic picrotoxin (a GABA-A antagonist) on intertrial coherence.Conclusions:Our data indicate that acute N-methyl-d-aspartate receptor antagonism increases synchronous activity in the primary auditory cortex in a frequency-specific manner, supporting the widely held view that acute N-methyl-d-aspartate antagonism augments gamma oscillations. Thus, rodent auditory steady-state response could be a valuable method to study the cortical ability to support synchronous activity at specific frequencies.

Description and performance of track and primary-vertex reconstruction with the CMS tracker

A description is provided of the software algorithms developed for the CMS tracker both for reconstructing charged-particle trajectories in proton-proton interactions and for using the resulting tracks to estimate the positions of the LHC luminous region and individual primary-interaction vertices. Despite the very hostile environment at the LHC, the performance obtained with these algorithms is found to be excellent. For tt̄ events under typical 2011 pileup conditions, the average track-reconstruction efficiency for promptly-produced charged particles with transverse momenta of pT > 0.9GeV is 94% for pseudorapidities of |η| < 0.9 and 85% for 0.9 < |η| < 2.5. The inefficiency is caused mainly by hadrons that undergo nuclear interactions in the tracker material. For isolated muons, the corresponding efficiencies are essentially 100%. For isolated muons of pT = 100GeV emitted at |η| < 1.4, the resolutions are approximately 2.8% in pT, and respectively, 10μm and 30μm in the transverse and longitudinal impact parameters. The position resolution achieved for reconstructed primary vertices that correspond to interesting pp collisions is 10–12μm in each of the three spatial dimensions. The tracking and vertexing software is fast and flexible, and easily adaptable to other functions, such as fast tracking for the trigger, or dedicated tracking for electrons that takes into account bremsstrahlung.

Performance of the LHCb Vertex Locator

The Vertex Locator (VELO) is a silicon microstrip detector that surrounds the proton-proton interaction region in the LHCb experiment. The performance of the detector duringthe first years of its physics operation is reviewed. The system isoperated in vacuum, uses a bi-phase CO2 cooling system, and thesensors are moved to 7 mm from the LHC beam for physics data taking. The performance and stability of these characteristic features of the detector are described, and details of the material budget are given. The calibration of the timing and the data processing algorithms that are implemented in FPGAs are described.The system performance is fully characterised. The sensors have asignal to noise ratio of approximately 20 and a best hit resolution of4 μm is achieved at the optimal track angle. The typical detectoroccupancy for minimum bias events in standard operating conditions in2011 is around 0.5%, and the detector has less than 1% of faultystrips. The proximity of the detector to the beam means that the innerregions of the n+-on-n sensors have undergone space-charge signinversion due to radiation damage. The VELO performance parametersthat drive the experiment's physics sensitivity are also given. Thetrack finding efficiency of the VELO is typically above 98% and themodules have been aligned to a precision of 1 μm for translations inthe plane transverse to the beam. A primary vertex resolution of 13 μm in thetransverse plane and 71 μm along the beam axis is achieved forvertices with 25 tracks. An impact parameter resolution ofless than 35 μm is achieved for particles with transverse momentumgreater than 1 GeV/c.

Detection of a second [formula omitted] event

OPERA is a neutrino oscillation experiment aiming to prove the tau neutrino appearance for the first time. A tau decay is identified by the detection of its characteristic decay topologies in nuclear emulsion films. OPERA reported the detection of a first tau neutrino event in 2010. We recently detected a second tau neutrino event in the 2011 run sample. The primary vertex has two charged particles of which one exhibits a three-prong vertex in the plastic base of a film after a flight length of 1.54 mm. It satisfies all the selection criteria to be a tau neutrino interaction. Especially the azimuthal angle in the transverse plane between the tau candidate track and the primary track is almost 180 degrees. The number of tau to 3π signal events expected in the analyzed sample is 0.18, the number of background events is 0.05. Considering all modes, the signal expectation is 1.91 events and the background 0.18 events.

Technical Design Report for the Upgrade of the ALICE Inner Tracking System

ALICE (A Large Ion Collider Experiment) is studying the physics of strongly interacting matter, and in particular the properties of the Quark–Gluon Plasma (QGP), using proton–proton, proton–nucleus and nucleus–nucleus collisions at the CERN LHC (Large Hadron Collider). The ALICE Collaboration is preparing a major upgrade of the experimental apparatus, planned for installation in the second long LHC shutdown in the years 2018–2019. A key element of the ALICE upgrade is the construction of a new, ultra-light, high-resolution Inner Tracking System (ITS) based on monolithic CMOS pixel detectors. The primary focus of the ITS upgrade is on improving the performance for detection of heavy-flavour hadrons, and of thermal photons and low-mass di-electrons emitted by the QGP. With respect to the current detector, the new Inner Tracking System will significantly enhance the determination of the distance of closest approach to the primary vertex, the tracking efficiency at low transverse momenta, and the read-out rate capabilities. This will be obtained by seven concentric detector layers based on a 50 μm thick CMOS pixel sensor with a pixel pitch of about 30×30 μm2. This document, submitted to the LHCC (LHC experiments Committee) in September 2013, presents the design goals, a summary of the R&D activities, with focus on the technical implementation of the main detector components, and the projected detector and physics performance.

GPU Enhancement of the Trigger to Extend Physics Reach at the LHC

At the Large Hadron Collider (LHC), the trigger systems for the detectors must be able to process a very large amount of data in a very limited amount of time, so that the nominal collision rate of 40 MHz can be reduced to a data rate that can be stored and processed in a reasonable amount of time. This need for high performance places very stringent requirements on the complexity of the algorithms that can be used for identifying events of interest in the trigger system, which potentially limits the ability to trigger on signatures of various new physics models. In this paper, we present an alternative tracking algorithm, based on the Hough transform, which avoids many of the problems associated with the standard combinatorial track finding currently used. The Hough transform is also well-adapted for Graphics Processing Unit (GPU)-based computing, and such GPU-based systems could be easily integrated into the existing High-Level Trigger (HLT). This algorithm offers the ability to trigger on topological signatures of new physics currently not practical to reconstruct, such as events with jets or black holes significantly displaced from the primary vertex. This paper presents, for the first time, an implementation and preliminary performance results using NVIDIA Tesla C2075 and K20c GPUs.

The STAR Vertex Position Detector

The 2×3 channel pseudo Vertex Position Detector (pVPD) in the STAR experiment at RHIC has been upgraded to a 2×19 channel detector in the same acceptance, called the Vertex Position Detector (VPD). This detector is fully integrated into the STAR trigger system and provides the primary input to the minimum-bias trigger in Au+Au collisions. The information from the detector is used both in the STAR Level-0 trigger and offline to measure the location of the primary collision vertex along the beam pipe and the event “start time” needed by other fast-timing detectors in STAR. The offline timing resolution of single detector channels in full-energy Au+Au collisions is ~100ps, resulting in a start time resolution of a few tens of picoseconds and a resolution on the primary vertex location of ~1cm.

The PHENIX Forward Silicon Vertex Detector

A new silicon detector has been developed to provide the PHENIX experiment with precise charged particle tracking at forward and backward rapidity. The Forward Silicon Vertex Tracker (FVTX) was installed in PHENIX prior to the 2012 run period of the Relativistic Heavy Ion Collider (RHIC). The FVTX is composed of two annular endcaps, each with four stations of silicon mini-strip sensors, covering a rapidity range of 1.2<|η|<2.2 that closely matches the two existing PHENIX muon arms. Each station consists of 48 individual silicon sensors, each of which contains two columns of mini-strips with 75μm pitch in the radial direction and lengths in the ϕ direction varying from 3.4mm at the inner radius to 11.5mm at the outer radius. The FVTX has approximately 0.54million strips in each endcap. These are read out with FPHX chips, developed in collaboration with Fermilab, which are wire bonded directly to the mini-strips. The maximum strip occupancy reached in central Au–Au collisions is approximately 2.8%. The precision tracking provided by this device makes the identification of muons from secondary vertices away from the primary event vertex possible. The expected distance of closest approach (DCA) resolution of 200μm or better for particles with a transverse momentum of 5 GeV/c will allow identification of muons from relatively long-lived particles, such as D and B mesons, through their broader DCA distributions.

Implementation of a configurable FE-I4 trigger plane for the AIDA telescope

Tracking detectors in particle physics experiments allow a precise reconstruction of particle tracks close to the interaction point and the identification of primary and secondary decay vertices. In order to evaluate the performance of detector prototypes for future particle physics experiments — at hadron colliders (e.g. HL-LHC), flavour factories (e.g. SuperKEKB) or a future lepton collider (e.g. ILC) — under realistic conditions, a fast readout reference device, a telescope, with excellent resolution and modular configuration, is required. The successful development of a telescope with these characteristics was part of the EU-project EUDET, which is continued within the framework of the AIDA activity.One key addition to the telescope within AIDA is the implementation of at least one new reference plane dedicated to provide an adjustable geometry. This plane is realized with a hybrid pixel detector consisting of a sensor bump bonded to the ATLAS pixel readout chip FE-I4. Its masking capability allows the definition of a user-defined region-of-interest (ROI) trigger area, that can be tuned to match the area defined by the device under test (DUT). Such a flexible setup is presented and the operation is demonstrated using a DEPFET pixel sensor as DUT in between the two telescope arms. The data acquisition (DAQ) for this small area DUT is improved in terms of trigger efficiency by more than a factor of three with respect to the standard telescope configuration. The integration of the DUT into the telescope and the online results obtained with it using the ROI trigger will be shown.

Search for the Standard Model Higgs boson at CMS in the 4-lepton channel

One of the main targets of the CMS experiment is to search for the Standard Model Higgs boson. The 4-lepton channel (from the decay H → ZZ → 4l, l = e ,μ ) is one of the most promising. The analysis is based on the identification of two opposite-sign, same-flavor lepton pairs: leptons are required to be isolated and to come from the same primary vertex. The Higgs would be statistically revealed by the presence of a resonance peak in the 4-lepton invariant mass distribution. The Higgs mass is a free parameter of the Standard Model, and the 4-lepton channel search is sensitive almost in the entire mass range. With data collected in 2010 and 2011 (4.7 fb −1 at √ s = 7 TeV) the Higgs boson has been excluded in a wide region of mass at 95% CL.

Studies on implementation of pellet tracking in hadron physics experiments

A system for optical tracking of frozen hydrogen microsphere targets (pellets) has been designed. It is intended for the upcoming hadron physics experiment PANDA at FAIR, Darmstadt, Germany. With such a tracking system one can reconstruct the positions of the individual pellets at the time of a hadronic interaction in the offline event analysis. This gives information on the position of the primary interaction vertex with an accuracy of a few 100 μm, which is very useful e.g. for reconstruction of charged particle tracks and secondary vertices and for background suppression. A study has been done at the WASA detector setup (Forschungszentrum Julich, Germany) to check the possibility of classification of hadronic events as originating in pellets or in background. The study has been done based on the instantaneous rate a Long Range TDC which was used to determine if a pellet was present in the accelerator beam region. It was clearly shown that it is possible to distinguish the two event classes. Also, an experience was gained with operation of two synchronized systems operating in different time scales, as it will also be the case with the optical pellet tracking.

The upgrade of the ALICE Inner Tracking System

ALICE is a general purpose experiment dedicated to the study of nucleus–nucleus collisions at LHC. After more than 3 years of successful operation, an upgrade of the apparatus during the second long shutdown of LHC (LS2) in 2017/18 is in preparation. One of the major goals of the proposed upgrade is to extend the physics reach for rare probes at low transverse momentum. The reconstruction of the rare probes requires a precise determination of the primary and secondary vertices that is performed in ALICE by the Inner Tracking System (ITS). The present ITS made of 6 layers of three technologies of silicon devices allows, for example, to reconstruct D mesons with the transverse momentum down to ∼2GeV/c.Further extension of this range towards lower pT requires the installation of the new ITS consisting of 7 layers of silicon detectors with significantly better single point resolution and reduced material budget. It is expected that the new system will allow to improve the impact parameter resolution by a factor of ∼3. Moreover, the data rate capability of the upgraded ITS should be significantly improved in order to exploit the full expected LHC lead–lead interaction rate of 50kHz, almost two orders of magnitude above the present readout rate.The present contribution describes first the requirements for the new ITS followed by the conceptual design of the system and its expected performance. Secondly, an overview of the different R&D activities from the concept towards the final detector is given.

Sharing Rewards in Cooperative Connectivity Games

We consider how selfish agents are likely to share revenues derived from maintaining connectivity between important network servers. We model a network where a failure of one node may disrupt communication between other nodes as a cooperative game called the vertex Connectivity Game (CG). In this game, each agent owns a vertex, and controls all the edges going to and from that vertex. A coalition of agents wins if it fully connects a certain subset of vertices in the graph, called the primary vertices. Power indices measure an agent's ability to affect the outcome of the game. We show that in our domain, such indices can be used to both determine the fair share of the revenues an agent is entitled to, and identify significant possible points of failure affecting the reliability of communication in the network. We show that in general graphs, calculating the Shapley and Banzhaf power indices is #P-complete, but suggest a polynomial algorithm for calculating them in trees. We also investigate finding stable payoff divisions of the revenues in CGs, captured by the game theoretic solution of the core, and its relaxations, the epsilon-core and least core. We show a polynomial algorithm for computing the core of a CG, but show that testing whether an imputation is in the epsilon-core is coNP-complete. Finally, we show that for trees, it is possible to test for epsilon-core imputations in polynomial time.

Observability of “cascade mixing” inBo→J/ψKo

In high statistics observations of ${B}^{o}\ensuremath{\rightarrow}J/\ensuremath{\psi}{K}^{o}$ originating from the process $\ensuremath{\Upsilon}(4S)\ensuremath{\rightarrow}{B}^{o}{\overline{B}}^{o}$ it should be possible to observe ``cascade mixing,'' where one mixing particle, the ${B}^{o}$, turns into another, the ${K}^{o}$. This is possible despite the difficulty that the length of the beam crossing region makes a precise definition of the primary vertex impossible. This difficulty is circumvented by using an ``away side'' tag to specify the initial time. We review the formalism for describing such processes, and first apply it to simple ${B}^{o}$ mixing, noting that it gives a transparent description for $CP$ and $T$ asymmetries. In particular, we show that three different asymmetries of the $CP$ and $T$ type, with neglect of direct $CP$ violation, are given by the same expression. For cascade mixing we present predictions for processes of the type ${B}_{i}\ensuremath{\rightarrow}{K}_{j}$ via $J/\ensuremath{\psi}$, where in the limit of no direct $CP$ violation each state $i$ or $j$ is determined by a simple tag. There are 16 such simple measurable processes, involving 10 functions of the two time intervals involved. The coefficients of the functions are different for each of the processes and are given in terms of the mass splitting and the $CP$, $T$ violating parameter of the ${B}^{o}$ mass matrix ${m}_{2}$. The results presented here are just consequences of the quantum mechanics of particle mixing and do not involve any particular model of $CP$ violation.

Suppression of open bottom at high pT via non-prompt J/ψ decays in PbPb collisions at 2.76 TeV with CMS

Abstract Measurements of the nuclear modification factor of mesons with open heavy flavor content in PbPb collisions at s N N = 2.76 TeV from the CMS experiment will be presented. These modification factors provide stringent constraints on the theoretical models of heavy quark energy loss. Until recently only indirect measurements of this effect existed, through single electrons from semileptonic open heavy-flavor decays. The importance of an unambiguous measurement of open bottom flavor is driven by the lack of knowledge regarding key features of the dynamics of parton energy loss in the QGP, such as its color-charge and parton-mass dependencies and the relative role of radiative and collisional energy loss. CMS measures the nuclear modification factor of b hadrons, identified via their decays into J / ψ displaced from the primary collision vertex. First results have shown that b hadrons are strongly suppressed in PbPb collisions at a level comparable to open charm. New results on the centrality dependence of non-prompt J / ψ R A A are presented, based on the full 2011 PbPb data sample corresponding to an integrated luminosity of 150 μ b − 1 .