B-jet Identification Research Articles

Calibration of the light-flavour jet mistagging efficiency of the b-tagging algorithms with Z+jets events using 139 textrm{fb}^{-1} of ATLAS proton–proton collision data at sqrt{s} = 13 TeV

The identification of b-jets, referred to as b-tagging, is an important part of many physics analyses in the ATLAS experiment at the Large Hadron Collider and an accurate calibration of its performance is essential for high-quality physics results. This publication describes the calibration of the light-flavour jet mistagging efficiency in a data sample of proton–proton collision events at sqrt{s}=13 TeV corresponding to an integrated luminosity of 139 fb^{-1}. The calibration is performed in a sample of Z bosons produced in association with jets. Due to the low mistagging efficiency for light-flavour jets, a method which uses modified versions of the b-tagging algorithms referred to as flip taggers is used in this work. A fit to the jet-flavour-sensitive secondary-vertex mass is performed to extract a scale factor from data, to correct the light-flavour jet mistagging efficiency in Monte Carlo simulations, while simultaneously correcting the b-jet efficiency. With this procedure, uncertainties coming from the modeling of jets from heavy-flavour hadrons are considerably lower than in previous calibrations of the mistagging scale factors, where they were dominant. The scale factors obtained in this calibration are consistent with unity within uncertainties.

ATLAS flavour-tagging algorithms for the LHC Run 2 pp collision dataset

The flavour-tagging algorithms developed by the ATLAS Collaboration and used to analyse its dataset of sqrt{s} = 13 TeV pp collisions from Run 2 of the Large Hadron Collider are presented. These new tagging algorithms are based on recurrent and deep neural networks, and their performance is evaluated in simulated collision events. These developments yield considerable improvements over previous jet-flavour identification strategies. At the 77% b-jet identification efficiency operating point, light-jet (charm-jet) rejection factors of 170 (5) are achieved in a sample of simulated Standard Model tbar{t} events; similarly, at a c-jet identification efficiency of 30%, a light-jet (b-jet) rejection factor of 70 (9) is obtained.

Configuration and performance of the ATLAS b-jet triggers in Run 2

Several improvements to the ATLAS triggers used to identify jets containing b-hadrons (b-jets) were implemented for data-taking during Run 2 of the Large Hadron Collider from 2016 to 2018. These changes include reconfiguring the b-jet trigger software to improve primary-vertex finding and allow more stable running in conditions with high pile-up, and the implementation of the functionality needed to run sophisticated taggers used by the offline reconstruction in an online environment. These improvements yielded an order of magnitude better light-flavour jet rejection for the same b-jet identification efficiency compared to the performance in Run 1 (2011–2012). The efficiency to identify b-jets in the trigger, and the conditional efficiency for b-jets that satisfy offline b-tagging requirements to pass the trigger are also measured. Correction factors are derived to calibrate the b-tagging efficiency in simulation to match that observed in data. The associated systematic uncertainties are substantially smaller than in previous measurements. In addition, b-jet triggers were operated for the first time during heavy-ion data-taking, using dedicated triggers that were developed to identify semileptonic b-hadron decays by selecting events with geometrically overlapping muons and jets.

ATLAS b-jet identification performance and efficiency measurement with t{bar{t}} events in pp collisions at sqrt{s}=13\xa0TeV

The algorithms used by the ATLAS Collaboration during Run 2 of the Large Hadron Collider to identify jets containing b-hadrons are presented. The performance of the algorithms is evaluated in the simulation and the efficiency with which these algorithms identify jets containing b-hadrons is measured in collision data. The measurement uses a likelihood-based method in a sample highly enriched in t{bar{t}} events. The topology of the t rightarrow W b decays is exploited to simultaneously measure both the jet flavour composition of the sample and the efficiency in a transverse momentum range from 20 to 600 GeV. The efficiency measurement is subsequently compared with that predicted by the simulation. The data used in this measurement, corresponding to a total integrated luminosity of 80.5 hbox {fb}^{-1}, were collected in proton–proton collisions during the years 2015–2017 at a centre-of-mass energy sqrt{s}= 13 TeV. By simultaneously extracting both the efficiency and jet flavour composition, this measurement significantly improves the precision compared to previous results, with uncertainties ranging from 1 to 8% depending on the jet transverse momentum.

Associated production of a Higgs boson decaying into bottom quarks and a weak vector boson decaying leptonically at NNLO in QCD

We present the calculation of next-to-next-to-leading order (NNLO) corrections in perturbative QCD for the production of a Higgs boson decaying into a pair of bottom quarks in association with a leptonically decaying weak vector boson: mathrm{pp}to Vmathrm{H}+Xto mathrm{ell}overline{mathrm{ell}}mathrm{b}overline{mathrm{b}}+X. We consider the corrections to both the production and decay sub-processes, retaining a fully differential description of the final state including off-shell propagators of the Higgs and vector boson. The calculation is carried out using the antenna subtraction formalism and is implemented in the NNLOjet framework. Clustering and identification of b-jets is performed with the flavour-kt algorithm and results for fiducial cross sections and distributions are presented for the LHC at sqrt{s} = 13 TeV. We assess the residual theory uncertainty by varying the production and decay scales independently and provide scale uncertainty bands in our results, yielding percent-level accurate predictions for observables in this Higgs production mode computed at NNLO. Confronting a na¨ıve perturbative expansion of the cross section against the customary re-scaling procedure to a fixed branching ratio reveals that starting from NNLO, the latter could be inadequate in estimating missing higher-order effects through scale variations.

Search for supersymmetry in final states with two same-sign or three leptons and jets using 36 fb\u22121 of sqrt{s}=13 TeV pp collision data with the ATLAS detector

A search for strongly produced supersymmetric particles using signatures involving multiple energetic jets and either two isolated same-sign leptons (e or μ), or at least three isolated leptons, is presented. The analysis relies on the identification of b-jets and high missing transverse momentum to achieve good sensitivity. A data sample of proton-proton collisions at sqrt{s}=13 TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to a total integrated luminosity of 36.1 fb−1, is used for the search. No significant excess over the Standard Model prediction is observed. The results are interpreted in several simplified supersymmetric models featuring R-parity conservation or R-parity violation, extending the exclusion limits from previous searches. In models considering gluino pair production, gluino masses are excluded up to 1.87 TeV at 95% confidence level. When bottom squarks are pair-produced and decay to a chargino and a top quark, models with bottom squark masses below 700 GeV and light neutralinos are excluded at 95% confidence level. In addition, model-independent limits are set on a possible contribution of new phenomena to the signal region yields.

Tracking at High Level Trigger in CMS

The trigger systems of the LHC detectors play a crucial role in determining the physics capabilities of experiments. A reduction of several orders of magnitude of the event rate is needed to reach values compatible with detector readout, offline storage and analysis capability. The CMS experiment has been designed with a two-level trigger system: the Level-1 Trigger (L1T), implemented on custom-designed electronics, and the High Level Trigger (HLT), a streamlined version of the CMS offline reconstruction software running on a computer farm. A software trigger system requires a trade-off between the complexity of the algorithms, the sustainable output rate, and the selection efficiency. With the computing power available during the 2012 data taking the maximum reconstruction time at HLT was about 200 ms per event, at the nominal L1T rate of 100 kHz. Track reconstruction algorithms are widely used in the HLT, for the reconstruction of the physics objects as well as in the identification of b-jets and lepton isolation. Reconstructed tracks are also used to distinguish the primary vertex, which identifies the hard interaction process, from the pileup ones. This task is particularly important in the LHC environment given the large number of interactions per bunch crossing: on average 25 in 2012, and expected to be around 40 in Run II. We will present the performance of HLT tracking algorithms, discussing its impact on CMS physics program, as well as new developments done towards the next data taking in 2015.

Performance of b-jet identification in the ATLAS experiment

The identification of jets containing b hadrons is important for the physics programme of the ATLAS experiment at the Large Hadron Collider. Several algorithms to identify jets containing b hadrons are described, ranging from those based on the reconstruction of an inclusive secondary vertex or the presence of tracks with large impact parameters to combined tagging algorithms making use of multi-variate discriminants. An independent b-tagging algorithm based on the reconstruction of muons inside jets as well as the b-tagging algorithm used in the online trigger are also presented. The b-jet tagging efficiency, the c-jet tagging efficiency and the mistag rate for light flavour jets in data have been measured with a number of complementary methods. The calibration results are presented as scale factors defined as the ratio of the efficiency (or mistag rate) in data to that in simulation. In the case of b jets, where more than one calibration method exists, the results from the various analyses have been combined taking into account the statistical correlation as well as the correlation of the sources of systematic uncertainty.

B-jet identification algorithms and performance in the ATLAS experiment

For many physics analyses at the LHC, the ability to identify jets containing B-hadrons (b-tagging) is one of the most crucial ingredients. After a brief review of the b-tagging algorithms developed in the ATLAS experiment, we present the refined b-jet tagging efficiency measurement with a combinatorial likelihood approach, a novel c-jet tagging efficiency measurement using W boson production in association with a c-quark, and the updates of c- and light-jet tagging efficiency measurements with increased statistics.

Performance of Tracking, b-tagging and Jet/MET reconstruction at the CMS High Level Trigger

The trigger systems of the LHC detectors play a crucial role in determining the physics capabilities of experiments. In 2015, the center-of-mass energy of proton-proton collisions will reach 13 TeV up to an unprecedented luminosity of 1 × 1034 cm−2s−1. A reduction of several orders of magnitude of the event rate is needed to reach values compatible with detector readout, offline storage and analysis capabilities. The CMS experiment has been designed with a two-level trigger system: the Level-1 Trigger (L1T), implemented on custom-designed electronics, and the High Level Trigger (HLT), a streamlined version of the offline reconstruction software running on a computer farm. A software trigger system requires a trade-off between the complexity of the algorithms, the sustainable output rate, and the selection efficiency. With the computing power available during the 2012 data taking the maximum reconstruction time at HLT was about 200 ms per event, at the nominal L1T rate of 100 kHz. Tracking algorithms are widely used in the HLT in the object reconstruction through particle-flow techniques as well as in the identification of b-jets and lepton isolation. Reconstructed tracks are also used to distinguish the primary vertex, which identifies the hard interaction process, from the pileup ones. This task is particularly important in the LHC environment given the large number of interactions per bunch crossing: on average 25 in 2012, and expected to be around 40 in Run II with a large contribution from out-of-time particles. In order to cope with tougher conditions the tracking and vertexing techniques used in 2012 have been largely improved in terms of timing and efficiency in order to keep the physics reach at the level of Run I conditions. We will present the performance of these newly developed algorithms, discussing their impact on the b-tagging performances as well as on the jet and missing transverse energy reconstruction.

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.

Top antitop quarks production in tau lepton + b-jets final states

Results obtained by the ATLAS and CMS experiments, using data collected in 2011, on the tt¯ production cross section in channels containing a tau lepton in the final state, are presented. The cross sections in the lepton-plus-tau and in the hadronic tau-plus-jets final states have been measured by both collaborations. The technique for the tau lepton reconstruction as well as for the b-jet identification, used by ATLAS and CMS, are described. The trigger and the offline strategies for these measurements are detailed. The results obtained by ATLAS and CMS are compatible with each other and in agreement with the Standard Model (SM) expectation. No hints of new physics are observed.

B-jet Identification in PbPb Collisions with CMS

The flavor dependence of jet quenching is a powerful handle to discriminate between models of parton energy loss in heavy-ion collisions. We demonstrate the capacity of CMS to identify jets initiated by bottom quarks using displaced vertices reconstructed in the silicon tracking system. The b-jet to inclusive jet ratio is measured in PbPb collisions and compared to pp collisions and simulations at the same center-of-mass energy.

Introduction to HOBIT, a b-jet identification tagger at the CDF experiment optimized for light Higgs boson searches

We present the development and validation of the Higgs Optimized b Identification Tagger (HOBIT), a multivariate b-jet identification algorithm optimized for Higgs boson searches at the CDF experiment at the Fermilab Tevatron. At collider experiments, b taggers allow one to distinguish particle jets containing B hadrons from other jets; these algorithms have been used for many years with great success at CDF. HOBIT has been designed specifically for use in searches for light Higgs bosons decaying via H→bb¯. This fact combined with the extent to which HOBIT synthesizes and extends the best ideas of previous taggers makes HOBIT unique among CDF b-tagging algorithms. Employing feed-forward neural network architectures, HOBIT provides an output value ranging from approximately −1 (“light-jet like”) to 1 (“b-jet like”); this continuous output value has been tuned to provide maximum sensitivity in light Higgs boson search analyses. When tuned to the equivalent light jet rejection rate, HOBIT tags 54% of b jets in Monte Carlo simulated Higgs boson events (mH=120GeV/c2) compared to 39% for SecVtx, the most commonly used b tagger at CDF. We present features of the tagger as well as its characterization in the form of b-jet finding efficiencies and false (light-jet) tag rates.

Track reconstruction and b-jet identification for the ATLAS trigger system

A sophisticated trigger system, capable of real-time track reconstruction, is used in the ATLAS experiment to select interesting events in the proton-proton collisions at the Large Hadron Collider at CERN. A set of b-jet triggers was activated in ATLAS for the entire 2011 data-taking campaign and successfully selected events enriched in jets arising from heavy-flavour quarks. Such triggers were demonstrated to be crucial for the selection of events with no lepton signature and a large jet multiplicity. An overview of the track reconstruction and online b-jet selection with performance estimates from data is presented in these proceedings.

Inclusive b-jet production in pp collisions at $\sqrt {s} = 7TeV$

Abstract The inclusive b-jet production cross section in pp collisions at a center-of mass energy of 7 TeV is measured using data collected by the CMS experiment at the LHC. The cross section is presented as a function of the jet transverse momentum in the range 18 < p T < 200 GeV for several rapidity intervals. The results are also given as the ratio of the b-jet production cross section to the inclusive jet production cross section. The measurement is performed with two different analyses, which differ in their trigger selection and b-jet identification: a jet analysis that selects events with a b jet using a sample corresponding to an integrated luminosity of 34 pb−1, and a muon analysis requiring abjetwithamuonbasedonanintegratedluminosityof3pb−1. Inbothapproachestheb jets are identified by requiring a secondary vertex. The results from the two methods are in agreement with each other and with next-to-leading order calculations, as well as with predictions based on the pythia event generator.

Measurement of thett¯production cross section with anin situcalibration ofb-jet identification efficiency

A measurement of the top-quark pair-production cross section in ppbar collisions at sqrt{s}=1.96 TeV using data corresponding to an integrated luminosity of 1.12/fb collected with the Collider Detector at Fermilab is presented. Decays of top-quark pairs into the final states e nu + jets and mu nu + jets are selected, and the cross section and the b-jet identification efficiency are determined using a new measurement technique which requires that the measured cross sections with exactly one and multiple identified b-quarks from the top-quark decays agree. Assuming a top-quark mass of 175 GeV/c^2, a cross section of 8.5+/-0.6(stat.)+/-0.7(syst.) pb is measured.

Tracking and vertexing capabilities of the CMS tracking detector with the first LHC data

Abstract The CMS experiment relies on a tracking detector entirely based on silicon sensors for the reconstruction of charged particles in the hostile radiation environment of proton–proton collisions at the CERN LHC. From the recent data collected by CMS at a center-of-mass energy of 7 TeV, the tracking, vertexing and b-jet identification performance have been measured. These measurements include the muon tracking efficiency, track transverse momentum resolution, track impact parameter resolution, primary vertex efficiency and resolution, and b-jet identification efficiency. The tracking performance relies, in particular, on the excellent pixel detector tracking capabilities, which are exploited in many contexts such as seeding, b-jet identification, and beam-spot monitor. The pixel stand-alone tracking and vertexing capabilities are shown for a particular application: the beam-spot monitor.

Beauty Production and Identification at CMS

At the LHC, b-hadrons will be produced in a very high quantity at a yet unreached center-of-mass energy, enabling precision measurements to improve our understanding of the physics of b-quarks. The capability to measure the inclusive b-hadron production cross section is discussed on the basis of realistic detector simulations. Effects which are limiting the accessible range of differential cross sections are outlined. The identification of b-jets is a crucial tool for a large number of topics, like top quark physics, and potential discoveries of the Higgs Boson and Supersymmetry. The applied methods of identifying b-jets are addressed and a discussion of the expected performance in terms of efficiencies and misidentification probabilities under realistic conditions is given. Approaches to measure these efficiencies with real data are also presented.

Measurement of the [formula omitted] production cross section in [formula omitted] collisions at [formula omitted] using lepton + jets events with lifetime b-tagging

We present a measurement of the top quark pair (tt¯) production cross section (σtt¯) in pp¯ collisions at s=1.96 TeV using 230 pb−1 of data collected by the DØ experiment at the Fermilab Tevatron Collider. We select events with one charged lepton (electron or muon), missing transverse energy, and jets in the final state. We employ lifetime-based b-jet identification techniques to further enhance the tt¯ purity of the selected sample. For a top quark mass of 175 GeV, we measure σtt¯=8.6−1.5+1.6(stat.+syst.)±0.6(lumi.) pb, in agreement with the standard model expectation.