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jet shapes. Lily Asquith (ANL) on behalf of ATLAS Boost 2012, Valencia. Outline. What are jet shapes, and why are we measuring them? Experimental challenges. The measurements . arxiv:1206.5369 What’s new?. What are jet shapes?. - PowerPoint PPT Presentation
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Plots for approval- jet shapes as a function of NPV
Lily Asquith (ANL) on behalf of ATLASBoost 2012, Valencia
jet shapes0OutlineWhat are jet shapes, and why are we measuring them?
Experimental challenges.
The measurements. arxiv:1206.5369
Whats new?
1What are jet shapes?All of these observables are constructed using the angular separation and energy of the jet constituents. e.g. mass:A jet.A constituent.
Traditionally jet shapes are differential and integrated. arxiv:1101.0070, arxiv:1204.3170These shapes are different measures of energy flow: mass, width, planar flow, eccentricity and angularity.
2Core-heavy jet: width0
Width3WidthBroad jet: width1
4Quark/ gluon?Quark/gluon jets: width (or girth); gluon jets are broader than quark jets, with more tracks.
arXiv:1106.3076v25Planar flowTwo-body jet: Linear energy deposition: Planar flow0
6Planar flowThree-body jet: Planar energy deposition: Planar flow1
7EccentricityIsotropic energy deposition: eccentricity0
8EccentricityElongated energy deposition: eccentricity1
9Two- and higher-body decaysPlanar flow can distinguish between three-body (top) jets and two-body (light quark/ gluon) jets.
arXiv:0807.023410Angularity -2Asymmetric energy deposition: -2maximum
11Angularity -2Symmetric energy deposition: -20
12Different two-body decaysAngularities can distinguish between two-body (W/Z/H) jets with different polarisation and two-body (light quark/gluon) jets.
arXiv:0807.0234
Longitudinal Z/ QCDTransverse Z/ QCDz=m/pT Longitudinal Z jetsQCD (light quark, gluon) jets13Correlations between observables
High pT, central, Pythia6 dijets.
Mass and width are strongly correlated.
Planar flow and eccentricity are strongly anti-correlated.14Correlations between observables
At high mass, the correlations change. These are for QCD.Mass > 100 GeVNo mass cut15The experimental challenges:aka Pileup
16Why pileup is such a problem for jet shapes and substructure
1: These jets are big. These sorts of observables generally change under pileup like R2 or more17Why pileup is such a problem for jet shapes and substructure2: We want to be able to distinguish A from BAB18Why pileup is such a problem for jet shapes and substructure2: We want to be able to distinguish A from BAB in these conditions.19Pileup
2010: ~2 (28% of events NPV=1) special datasetThe Number of reconstructed Primary Vertices - NPV can tell us how much additional radiation we are dealing with.
2011: ~ 10 20122012*: ~ 25+. 2020Controlling pileup
Complementary cone technique (CDF) looks in region transverse (in azimuth) to the jet.
Energy deposits in this region are attributed to pileup and underlying event (UE): soft radiation that is always present.arxiv:1101.3002, 1106.5952v2
21Controlling pileupSingle vertex events contain only the UE contribution characterise pileup by comparing events with single and multiple vertices.
expectedmeasured
arxiv:1206.5369 Can then find the scaling of e.g. M with R obtain subtractions for R=1 jets.
22Controlling pileupComplementary cone technique restores distributions to shape seen in single vertex events.
23The measurements24DetailsEvents are selected based on run conditions, data quality and detector conditions.The anti-kT algorithm is used with locally calibrated topological clusters as input.The highest pT jet in each event is measured, must have pT>300 GeV.ObservableRMass rangePileup correctionMass M0.6,1.0AllWidth W0.6,1.0AllPlanar flow P1.0130-210NPV=1Eccentricity 0.6,1.0>100Angularity -20.6100-130Not needed25Jet massPYTHIA8, PYTHIA6HERWIG++ 2.4.2, 2.5.1POWHEG, PYTHIA6R=0.6R=1.0
26Jet mass
Herwig++ 2.5.1 jet mass prediction is greatly improved w.r.t 2.4.227Jet mass
Eikonal approx of QCD for gluons and quarks is compatible with our expectation that the data is a mixture of quark and gluon initiated jets.28Jet mass
Dominant contributions to the systematic uncertainty are the cluster energy scale and Monte Carlo predictions.These show C on the y-axis: C is the correction factor in bin i when going from detector-level to particle-level jets in the baseline Pythia6 (AMBT1) MC sample.C is the difference when we vary the sample w.r.t this baseline.Shading is statistical uncertainty. 29Jet widthWidth is well-modeled by all MCs beyond the first bin.
30DetailsObservableRMass rangePileup correctionMass M0.6,1.0AllWidth W0.6,1.0AllPlanar flow P1.0130-210NPV=1Eccentricity 0.6,1.0>100Angularity -20.6100-130Not neededPlanar flow is measured for jets with mass in a window around the top mass.Not many R=0.6 jets have such a high mass:Only measure P for R=1.0 jets.Only measure P in pileup-free (NPV=1) events.31Planar flow
Again we see Herwig++ 2.5.1 providing a superior description of the energy flow wrt 2.4.2.Note: this is not the same mass range as the eccentricity measurements.32DetailsEccentricity is measured in the general region of interest for boosted particle searches: M>100 GeV.
ObservableRMass rangePileup correctionMass M0.6,1.0AllWidth W0.6,1.0AllPlanar flow P1.0130-210NPV=1Eccentricity 0.6,1.0>100Angularity -20.6100-130Not needed33Eccentricity
Eccentricity is a magnifying glass for differences in the distributions of constituents on the local angular scale:
34Eccentricity
Eccentricity is a magnifying glass for differences in the distributions of constituents on the local angular scale:
This piece varies significantly between MCs, but (mostly) washes away with energy weight (soft particles).Highly anti-correlated with planar flow (-90% for jets in same high mass range)35DetailsQCD small-angle approximation gives a prediction for the peak and maximum values of the -2 distribution:Valid for fixed high mass and pT (we choose 1000.3, then we chuck it. Then we rebuild the jet from remaining clusters.Trimmed: The jet constituents are reclustered with a small distance parameter R=0.3 into subjets. Any subjet with pT