Performance of b-tagging algorithms in 13 TeV pp collisions with a bunch spacing of 50ns

Abstract

At the Large Hadron Collider, the identification of jets originating from b quarks is important for searches for new physics and for measurements of standard model processes. A variety of algorithms has been developed by CMS to select b-quark jets based on variables such as the impact parameters of the charged-particle tracks, the properties of reconstructed decay vertices, and the presence or absence of a lepton, or combinations thereof. The performance of these algorithms has been measured using data from proton-proton collisions at the LHC and compared with expectations based on simulation. The data used in this study were recorded in 2015 at $\sqrt s$ = 13 TeV with a bunch spacing of 50ns and for a total integrated luminosity of around 40 pb-1. The efficiency for tagging b-quark jets and mistagging non-b quark jets has been measured in events from multijet production. Variables related to b-tagging are also shown for boosted multijet and top quark topologies.

Glossary

CSVv2: Combined Secondary Vertex version 2 algorithm, based on secondary vertex and track-based lifetime informations, it is an updated version of the CSV algorithm used in Run 1 combining the variables with a neural network instead of a likelihood ratio and the on secondary vertex information is obtained with the Inclusive Vertex Finder algorithm.

CSVv2L, CSVv2M, CSVv2T: CSVv2 algorithm at the loose, medium, tight operating points, defined as the values of the discriminator cut for which the rate for misidentifying a light jet as a b jet is 10%, 1%, and 0.1%, respectively

JP: Jet Probability algorithm, based on the likelihood of tracks to come from the primary vertex (using the impact parameter significance values)

LT: Lifetime Tagging method for the measurement of the b-tagging efficiency in multijet events, based on template fits to the JP or CSV distributions

PtRel: Method for the measurement of the b-tagging efficiency in multijet events based on the transverse momenta of muons w.r.t. the jet axis

System8: Method for the measurement of the b-tagging efficiency in multijet events with a muon, solving a system of equations

B-tagging Plots at 13 TeV, 50ns (click on plot to get the .pdf version )

For more details, see as a reference: Identification of b-quark jets with the CMS experiment, CMS Collab., CERN-PH-EP-2012-262, J. Instrum. 8 (2013) P04013.

Variables related to b-tagging in AK4 jets

Figure Caption
CSVIVF_Log.pdf The CSVv2 discriminator of ak4 jets with 60 < p^{jet}_T < 250 GeV. The filled circles correspond to the data recorded in 2015. The stacked, coloured histograms indicate the contributions of different components from simulated multijet (“QCD”) samples. Simulated events involving gluon splitting to b quarks (“b from gluon splitting”) are indicated separately from the other b production processes (“b quark”). The distributions from simulation have been normalized to match the counts in data. In each histogram, the rightmost bin includes all events from the overflow, while the underflow is added to the first bin. The operating point values for the loose, medium and tight tagging criteria are set to 0.605, 0.890, 0.970, respectively.
JP_Log.pdf The JP discriminator for ak4 jets with 60 < p^{jet}_T < 250 GeV. Symbols are the same as in Fig. 1. Overflow is added to the last bin. The small discontinuities in the JP distributions are due to the single track probabilities which are required to be greater than 0.5%. The operating point values for the loose, medium and tight tagging criteria are set to 0.275, 0.545, 0.790, respectively.
sv_multi_0_Log.pdf Number of secondary vertices associated to ak4 jets with 60 < p^{jet}_T < 250 GeV. Symbols are the same as in Fig. 1. Overflow is added to the last bin.
sv_flight3DSig_Log.pdf The flight distance significance of secondary vertices associated to ak4 jets with 60 < p^{jet}_T < 250 GeV. Symbols are the same as in Fig. 1. Overflow is added to the last bin.
trk_multi_sel_Linear.pdf Number of tracks passing quality criteria associated to ak4 jets with 60 < p^{jet}_T < 250 GeV. Symbols are the same as in Fig. 1. Overflow is added to the last bin.
track_HPix_Log.pdf Number of hits in the pixel system by tracks associated to ak4 jets with 60 < p^{jet}_T < 250 GeV. Symbols are the same as in Fig. 1.
track_IPs_Log.pdf The 3D impact parameter significance for all selected tracks associated to ak4 jets with 60 < p^{jet}_T < 250 GeV. Symbols are the same as in Fig. 1. Underflow and overflow are added to the first and last bins, respectively. The observed disagreement between data and MC is attributed to different alignment scenarios.

Plots related to the performance measurements in AK4 jets

Figure Caption
tagger_CSVv2PFCut_40.pdf Signed b-jet tagging CSVv2 discriminator in data (dots) and simulation for light-parton jets (blue histogram, with a lighter colour for the negative discriminators), c jets (green histogram), and b jets (red histogram). A jet-trigger pT threshold of 40 GeV/c is required for both data and simulation, and at least one Jet with pT higher than 50 GeV/c is required. The simulation is normalized to the number of entries in the data. Underflow and overflow entries are added to the first and last bins, respectively.
tagger_CSVv2PFCut_320.pdf Signed b-jet tagging CSVv2 discriminator in data (dots) and simulation for light-parton jets (blue histogram, with a lighter colour for the negative discriminators), c jets (green histogram), and b jets (red histogram). A jet-trigger pT threshold of 320 GeV/c is required for both data and simulation, and at least one Jet with pT higher than 360 GeV/c is required. The simulation is normalized to the number of entries in the data. Underflow and overflow entries are added to the first and last bins, respectively.
plot33New_2_CSVv2M_0.0_2.4.pdf For the CSVv2M tagger as a function of the jet pT : (top) misidentification (mistag) rate in data (bullet) and simulation (open circles); (bottom) data-to-MC scale factor of the mistag rate. The SFs are found to be close to unity (solid line) over the whole jet pt spectrum investigated. The uncertainty is conservatively estimated as 20% (dashed line), driven mainly by the limited statistics available in the control samples.
“PtRelFit_CSVv2M_Pt70100.pdf" PtRelFit_AntiCSVv2M_Pt70100.pdf Fits of the summed b and non-b templates, for simulated muon jets, to the muon pTrel distributions from data. The top plot shows the result for muon-jets that pass (tagged) the b-jet tagging criteria of the CSVv2M method. The bottom plot shows the result for muon-jets that fail (vetoed) the b-jet tagging criteria of the CSVv2M method. The muon-jet pt is between 70 and 100 GeV/c.
pt200t300_result.pdf Results for the template fit in the LT method for selected jets with pT from 200 GeV to 300 GeV before applying b-tagging requirement. JP discriminant is used as a fitted variable. Three templates derived from MC are used: b-jets, c-jets and udsg-jets, to fit the data. The hatched area shows the total uncertainty on the measured MC templates.
pt200t300_result_tag.pdf Results for the template fit in the LT method for selected jets with pT from 200 GeV to 300 GeV after applying CSVv2M b-tagging requirement. JP discriminant is used as a fitted variable. Three templates derived from MC are used: b-jets, c-jets and udsg-jets, to fit the data. The hatched area shows the total uncertainty on the measured MC templates.
SFb_Run2015B_CSVv2L.pdf Individual and combined measurements of the ratio of the b-jet tagging efficiencies of the data to that in simulation for the CSVv2L tagger. The top panel show the individual measurements from the muon pTrel (“PtRel”), System8 (“System8”) and lifetime tagger method on muon-jet events (“LT”). The inner and outer error bars indicate the statistical and the combined uncertainties, respectively. The grey hatched areas represent the combined measurements at $\sqrt s$ = 13 TeV. In the lower panel, the combined measurements have been parameterized by functions of the form SFb(pT) = $\alpha$ (1 + $\beta$ pT) / (1 + $\gamma$ pT). The error bars attached to the function have the same size as the uncertainties from the combined measurement in each bin.
SFb_Run2015B_CSVv2M.pdf Same as the previous figure for the CSVv2M tagger. Combined scale factors are fitted to a constant value.
SFb_Run2015B_CSVv2T.pdf Same as the previous figure for the CSVv2T tagger. Combined scale factors are fitted to a constant value.

Variables related to b-tagging in boosted topologies: multijet

Figure Caption
SoftDropSubJet_CSVIVFv2_Log.pdf The combined secondary vertex b-tagging discriminator distribution of soft drop subjets of AK8 jets in a multijet sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated multijet samples. The distributions from the simulation have been scaled to match the number of observed events. The last bins of the histograms contain all entries above the histogram range.
SoftDropSubJet_sv_multi_0_Log.pdf Number secondary vertices associated to soft drop subjets of AK8 jets in a multijet sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated multijet samples. The distributions from the simulation have been scaled to match the number of observed events. The last bins of the histograms contain all entries above the histogram range.
SoftDropSubJet_sv_flight3DSig_Log.pdf The flight distance significance of secondary vertices associated to soft drop subjets of AK8 jets in a multijet sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated multijet samples. The distributions from the simulation have been scaled to match the number of observed events. The last bins of the histograms contain all entries above the histogram range.
SoftDropSubJet_trk_multi_sel_Linear.pdf Number of tracks passing quality criteria associated to soft drop subjets of AK8 jets in a multijet sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated multijet samples. The distributions from the simulation have been scaled to match the number of observed events. The last bins of the histograms contain all entries above the histogram range.
SoftDropSubJet_track_HPix_Log.pdf Number of hits in the pixel system by tracks associated to soft drop subjets of AK8 jets in a multijet sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated multijet samples. The distributions from the simulation have been scaled to match the number of observed events. The last bins of the histograms contain all entries above the histogram range.
SoftDropSubJet_track_IPs_Log.pdf The 3D impact parameter significance of tracks associated to soft drop subjets of AK8 jets in a multijet sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated multijet samples. The distributions from the simulation have been scaled to match the number of observed events. The first and last bins of the histograms contain all entries below and above the histogram range, respectively.

Variables related to b-tagging in boosted topologies: top quark events

Figure Caption
zsubCSV_Canvas.pdf The combined secondary vertex b-tagging discriminator distribution of HepTopTagger subjets of CA15 jets in a boosted top quark sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated top quark events and Standard Model backgrounds. The last bins of the histograms contain all entries above the histogram range.
zsubJetNSecondaryVertices_Canvas.pdf Number secondary vertices associated to HepTopTagger subjets of CA15 jets in a boosted top quark sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated top quark events and Standard Model backgrounds. The last bins of the histograms contain all entries above the histogram range.
zsubFlightDistance3dSig_Canvas.pdf The flight distance significance of secondary vertices associated to HepTopTagger subjets of CA15 jets in a boosted top quark sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated top quark events and Standard Model backgrounds. The last bins of the histograms contain all entries above the histogram range.
zsubTrackMultiplicity_Canvas.pdf Number of tracks passing quality criteria associated to HepTopTagger subjets of CA15 jets in a boosted top quark sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated top quark events and Standard Model backgrounds. The last bins of the histograms contain all entries above the histogram range.
zsubTrackNPixelHits_Canvas.pdf Number of hits in the pixel system by tracks associated to HepTopTagger subjets of CA15 jets in a boosted top quark sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated top quark events and Standard Model backgrounds. The last bins of the histograms contain all entries above the histogram range.
zsubTrackSip3dSig_Canvas.pdf The 3D impact parameter significance of tracks associated to HepTopTagger subjets of CA15 jets in a boosted top quark sample. The filled circles correspond to the data. The stacked, coloured histograms indicate the contributions of different types of jets from simulated top quark events and Standard Model backgrounds.

Event display

%TABLE{ datavalign="left" }

Figure Caption
2-black-zoom.png Reconstructed di-jet event in 50 ns data collected at 13 TeV with one displaced muon track. Both jets are tagged by the CSVv2T b-tagging algorithm. Jet containing muon: pT(j) = 34.3 GeV, eta(j) = -1.77, phi(j) = -2.06. Muon: pT(mu) = 16.7 GeV, eta(mu) = -1.54, phi(mu) = -2.07. Tracks with pT > 2 GeV are shown. Dimensions are in cm.
2-white-zoom.png Reconstructed di-jet event in 50 ns data collected at 13 TeV with one displaced muon track. Both jets are tagged by the CSVv2T b-tagging algorithm. Jet containing muon: pT(j) = 34.3 GeV, eta(j) = -1.77, phi(j) = -2.06. Muon: pT(mu) = 16.7 GeV, eta(mu) = -1.54, phi(mu) = -2.07. Tracks with pT > 2 GeV are shown. Dimensions are in cm.

-- LucaScodellaro - 2015-09-12

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