Contact Persons

Documentation

  • ATLAS
    • Measurement of the b-tag Efficiency in a Sample of Jets Containing Muons with 5 fb-1 of Data from the ATLAS Detector, ATLAS-CONF-2012-043
    • Measuring the b-tag efficiency in a ttbar sample with 4.7 fb-1 of data from the ATLAS detector, ATLAS-CONF-2012-097
    • Calibration of b-tagging using dileptonic top pair events in a combinatorial likelihood approach with the ATLAS experiment, ATLAS-CONF-2014-004
    • b-jet tagging calibration on c-jets containing D* mesons, ATLAS-CONF-2012-039
    • Measurement of the Mistag Rate of b-tagging algorithms with 5 fb-1 of Data Collected by the ATLAS Detector, ATLAS-CONF-2012-040
  • CMS
    • Identification of b-quark jets with the CMS experiment, 2013 JINST 8 P04013
    • Performance of b tagging at sqrt(s)=8 TeV in multijet, ttbar and boosted topology events, CMS-PAS-BTV-13-001

Taggers in use

  • General
    • Impact parameter based algorithms
    • Reconstruction of displaced secondary vertices
    • Identification of soft leptons inside the jet cone
  • best performing algorithms for ATLAS
    • MV1: combination of the above information with a neural network to build a discriminator
    • operation points: fixed b-jet efficiencies: 60% (tight), 70% (medium) and 80% (low)
  • best performing algorithm for CMS
    • CSV: combination of secondary vertices and track-based lifetime information to build a likelihood-based discriminator
    • operation points: fixed mis-identification rate for light jets at 0.1% (tight), 1% (medium) and 10% (low)
  • both experiments are using the medium operation point (MV1 at btag eff. = 70% (ATLAS) / CSV at mistag rate = 1% (CMS))
  • most top analyses going to combination use these taggers exclusively

Calibration samples

  • data-to-MC scale factors for b-jet tagging efficiencies and for light jet mis-identification rates
    • b-tag efficiency SFs provided as a function of jet pT
    • eta dependence also studied and found to be flat for b-tag efficiency and varying for mistag rate for both experiments
    • c-jet calibration: independent analysis at ATLAS, b-jet calibration with inflated uncertainties for CMS
  • Calibration with di-jet events: jets with a soft muon
    • exploiting the high semileptonic decay branching ratio of B-hadrons
    • often looking at the away jet to enrich the sample in b-bbar content
  • Calibration with top-pair events: jets from top quark decays
    • BR(t⟶Wb) ~100%
    • isolated leptons from W decays to reduce the background contamination
    • single lepton and dilepton decay channels providing two orthogonal samples

Calibration methods

  • Di-jet based calibrations:
    • ATLAS:
      • pTrel: template fit of the muon pT w.r.t the jet axis (only for 7 TeV data)
      • System8: equations with 8 unknowns, using two samples (with different purities) and two weakly correlated taggers (tagger under study and the muon pTrel)
    • CMS:
      • pTrel and system8 method as well
      • extending pTrel method up to 800 GeV looking at muon IP3D
      • Lifetime tagger (LT) method: template fit of a reference discriminator (usually JP which is calibrated in data)
  • top-pair based calibration:
    • ATLAS:
      • tag counting: fit b-jet efficiency on tagged jets multiplicity (only for 7 TeV data)
      • Kinematic selection and tag&probe: based on sample composition estimates
      • combinatorial likelihood approach (only 8 TeV data)
      • kinematic fit: use a kinematic fit to reconstruct the final states and increase the purity of the sample
    • CMS:
      • Tag counting techniques (inclusive in pT)
      • bSample method (kinematic based, inclusive in pT)
      • LT method on dilepton events (only for 8 TeV data)
  • final usage: combination of calibrations
    • ATLAS:
      • combine pTrel, system8 and two top-pair analyses (in different channels)
      • global fit with all systematic uncertainties as nuisance parameters which can shift the mean data/MC SF
      • systematic uncertainties can be fully correlated or uncorrelated ineach single kinematic bin or across kinematic bins
    • CMS:
      • all measurements on jets with a soft muon and top based LT analysis
      • using the least squared BLUE method
      • sources of uncertainties common between two or more methods are taken as (anti-)correlated

Categorization of systematics

The systematics are divided in four main categories

Detector Description Physics Modeling Method specific Detector and physics modeling
ATLAS JER, JRE, Lep, fake lep. ISR/FSR, top+W gen., top+W norm. PS, jet-pt, c/l ratio flavour comp. JES
CMS PU, track mult. mu-pt, GS, b-frag, DR(mu,j), c/l ratio, top mod. LT JES
correlation not correlated correlated, must be considered not correlated correlated, but small

  • explanation of variables
    • JER: Jet Energy Resolution, JRE: Jet Reconstruction Efficiency, JES: Jet Energy scale
    • PU: Pile Up
    • track. mult.: Track multiplicity description
    • Lep: lepton (e/mu) energy resolution, trigger and scale
    • fake: fake leptons
    • ISR/FSR: inital and final state radiation
    • GS: gluon splitting
    • top+W gen / norm.: top and W MC generator and cross section normalization
    • PS: parton shower
    • c/l ration: charm/light quart ratio
    • top mod: top-quark modeling
    • flavour comp: flavour composition (top events)
    • LT: life time tagger (CMS)

Correlation between the sysemaics

Systematics can be correlated between the two experiments and/or with other parts of the analyses within one experiment:

  uncorr. ATLAS/CMS corr. ATLAS/CMS
uncorr. with Analysis method spec. specific phys. modeling
corr. with Analysis detector descr. general phys. modeling

This must be taken into account for the categorization of the systematics treating the potential correlated ones.

Treatment of uncertainties

  • systematics uncorrelated between experiments and analyses
    • quote just on combined total uncertainty
  • systematics correlated between experiments and analyses
    • check the individual relevance
    • consider correlation only for significant contributions for both, analysis and b-tagging
  • systematics correlated within experiments but not with analyses
    • sizes and signs for each significant systematic uncertainty needed
    • one by one these systematics must be analyzed to decide how to treat them in the combination
    • give only the dominant ones for the combination
  • a detailed study of the uncertainties is given at the last ATLAS+CMS+b-tagging Meeting:

Systematics and further Studies

  • several sources of correlated uncertainties between ATLAS and CMS:
    • slightly different procedures to evaluate the uncertainty
    • general agreement between the sizes and signs
    • no anti-correlated effects observed
  • different values of the systematics:
    • different methods employed to compute the efficiencies
    • different choices on variation size (only for l/c)
  • main common correlated systematics correlated among the experiments but not in common to analyses (see table below):
    • Gluon Splitting (GS), di-jet calibration
    • muon pT (mupT), di-jet calibration
    • charm to light ratio (clr), di-jet calibration
    • b-fragmentation (b-frag), di-jet calibration
    • Parton Shower (PS), top-pair calibration
    • ISR/FSR (IFSR), top-pair calibration
  • additional studies done
    • reference modeling and gluon splitting difference in CMS (Pythia6) and ATLAS (Pythia8)
      • the gluon splitting fraction is reasonable consistent between Pythia6 and Pytia8
      • the small difference is covered by the 50% systematic variation
    • check if the muon spectrum modeling is an additional systematic source not counted somewhere else
      • possiblity to see if the effect observe in ATLAS and CMS is consistent between the two experiment
      • good agreement within the uncertainties observed
      • plot see below
    • check, if muon-pt is an independent systematics
      • studied by varying the other physics modeling systematics (for instance the GS) and observing the effect on the data/MC agreement of the muon pt spectrum.
      • plot see below
    • check if gluon splitting is a potential source of the mis-modeling of the muon pT spectra
      • variations done of +/-50% gluon splitting in b/c-pairs
      • reasonable agreement within the variations
      • plot see below

Proposed Treatment:

  • uncorrelated systematics among the experiments (or very small ones)
    • make one category
  • systematics correlated between the experiments but uncorrelated to analyses
    • GS, mupT, clr, b-frag, PS, IFSR
    • they contribute all on the same level of 0.2 - 1.3%
    • they are about equal in size
    • behavior in pT is comparable
    • give them all individually as separate category
    • charm-to-light systematics is equally small for both experiments (up to 0.2%)
      • its the only one remaining "small" uncertainty
      • do not consider separately, add to the main "uncorrelated" systematics
  • uncertainties which are correlated with flavor tagging SF and the analyses but not correlated between the experiments
    • add to the uncorrelated systematics
    • the experiments must this clarify internally
  • experiments should provide the categories accordingly
    • hand over one number per category (in pT bins)
    • do the systematic variations of the analyses accordingly
  • agreed splitting should be used for the upcoming 8 TeV analyses
    • the ongoing analyses should not be affected

Summary table:

source size at ATLAS size at CMS
b/c prod. low pT: 0.1% - 0.2%, high ph: 1.2% - 2.0% low pT: 0.1% - 0.3%, high ph: 0.5% - 1.3%
muon pT low pT: 0.8% - 2.7%, high ph: 0.2% - 0.8% low pT: 0.1% - 1.1%, high ph: 0.1 - 0.9%
c/l ratio <0.1% - 0.2% <0.1% - 0.2%
b-frag. 0.2% - 2.7% 0.2% - 0.8%
PS 0.1% - 4.1% 0.3% - 0.6%
IFSR 01.% - 4.1% 0.3% - 0.6%
Please consider, that the ATLAS uncertainties are from the not combined calibration results, where as the numbers from CMS are form the final, combined calibration results. Accordingly, it is expected that the numbers of ATLAS will decrease and be then in better agreement with CMS.

Table with the full pt dependent uncertainties can be found below. Please consider, that the numbers of CMS are related to the combined calibration measurement, whereas the numbers from ATLAS correspond to individual analyses. This will be updated as soon as possible.

CMS
pT bin 30-40 40-50 50-60 60-70 70-80 80-100 100-120 120-160 160-210 210-260 260-320 320-400 >400
Uncor. 1.8 1.7 1.9 1.7 1.7 2.1 2.0 2.1 1.5 2.1 2.2 4.6 4.6
b/c prod. 0.1 <0.1 0.2 0.3 0.1 0.4 0.5 0.6 1.0 0.7 1.3 0.9 1.0
muon pT 0.1 0.1 0.7 0.6 0.4 1.0 0.8 0.5 0.1 0.1 0.1 0.8 1.0
b-frag. 0.7 0.7 0.4 0.6 0.5 0.5 0.3 0.2 0.2 0.3 0.2 0.4 0.4
PS 0.4 0.4 0.5 0.4 0.5 0.6 0.5 0.4 0.3 0.3 0.3 0.3 0.3
IFSR 0.4 0.4 0.5 0.4 0.5 0.6 0.5 0.4 0.3 0.3 0.3 0.3 0.3

ALTAS
pT bin 25-30 30-40 40-50 50-60 60-75 75-90 90-110 110-140 140-200 200-300
GS   0.2 <0.1 <0.1 <0.1 <0.1 0.1 1.2 1.2 2.0
mu pT   2.4 0.3 0.3 0.5 0.5 0.6 0.8 0.9 0.2
c2l   0.2 <0.1 <0.1 0.1 <0.1 <0.1 <0.1 0.1 <0.1
b-Frag   2.7 0.3 0.7 1.3 0.8 0.7 0.8 0.2 0.2
PS 1.4 3.0 4.1 2.7 1.7 1.7 0.2 0.1 0.8 4.1
IFSR 2.0 1.5 1.7 0.7 1.1 1.3 1.4 1.6 1.9 5.9

Meegings

2012

Initial Meeting 10.12.2012

2013

20.06.2013, 17.10.2013, 19.11.2013

2014

04.03.2014, 04.04.2014


Major updates:
-- MartinZurNedden - 02 Apr 2014
  • PtRel_DepOnMuPtWeight_CSVM_KinWeights_PUWeighting_SystMoreLowPt.png:
    PtRel_DepOnMuPtWeight_CSVM_KinWeights_PUWeighting_SystMoreLowPt.png

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Topic revision: r8 - 2015-10-08 - MartijnMulders
 
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