Studies of j et s hapes and s ubstructure with atlas adam davison university college london
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Studies of J et S hapes and S ubstructure with ATLAS Adam Davison University College London. ATLAS. Recorded ~5 fb -1 of 7 TeV pp collisions from the LHC Highly granular calorimetry (down to 0.025x0.025 in η-φ ). Jets at ATLAS. At the LHC jets are everywhere

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Studies of J et S hapes and S ubstructure with ATLAS Adam Davison University College London

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Studies of j et s hapes and s ubstructure with atlas adam davison university college london

Studies of Jet Shapes and Substructure with ATLASAdam DavisonUniversity College London


Atlas

ATLAS

  • Recorded ~5 fb-1 of 7 TeVpp collisions from the LHC

  • Highly granular calorimetry (down to 0.025x0.025 in η-φ)


Jets at atlas

Jets at ATLAS

  • At the LHC jets are everywhere

  • ATLAS primarily uses anti-kt jets with R = 0.4 and 0.6

    • Other algorithms available for specialised needs

  • Rich series of jet physics publications from ATLAS already

    • Inclusive jets, dijets, multijets, b-jets, azimuthal correlations etc…


Inside a jet

Inside a Jet

  • Jets can contain a lot of complex physics

    • Non-perturbative effects like fragmentation and hadronisation

    • Hard process, colour connections, UE, pile-up

    • Massive particle production (b, c, W, Z, top etc…)

  • Measurements of the internal structure of jets can help us test models and tunes for all of these

  • Good models of all of these a necessity for BSM searches


Jet shapes

Jet Shapes

  • Jet shapes are the classic test of the internals of jets

  • Measure the fraction of a jet’s pt which is within an annulus

  • Formally, measure the quantity:

pT in annulus

pT of jet


Jet shapes1

Jet Shapes

2.4.2

  • Results released 2010, now published:

  • Phys.Rev. D83 (2011) 052003

  • More recently, newer tunes tested

  • ATL-PHYS-PUB-2011-010


Fragmentation

Fragmentation

  • Can also study the distribution of charged particles

the jet (jet)

a track (ch)

  • Distribution of energy parallel or transverse to jet axis:


Fragmentation1

Eur. Phys. J. C 71 (2011) 1795

Fragmentation

  • Distributions are reasonably well modelled


Jet mass and substructure

Jet Mass and Substructure

  • Jets are 4-vectors, for example (E/pT, η, φ, m)

  • Of these, mass is probably the least studied

  • At the LHC, lots of interest in jet mass and substructure

    • Especially large radius jets

    • Jets can contain boosted W, Z, top …

    • … and potentially new physics

  • Measurements targetted at this

ATLAS-CONF-2011-103


Jet mass and substructure1

New Results!

Jet Mass and Substructure

  • Looking at Cambridge-Aachen R=1.2 and anti-kt R=1.0

  • Jet mass measured for jet pt of 200 – 600 GeV

  • Systematics determined in-situ using inner-detector track jets

  • Herwig++didn’t describe this terribly well in version 2.4.2

http://arxiv.org/abs/1203.4606


Jet mass and substructure2

Jet Mass and Substructure

  • Also look at application of a substructure procedure:

Splitting and filtering a la

PRL 100, 242001 (2008)

Less sensitive to soft physics, observe better agreement than in plain jet mass

http://arxiv.org/abs/1203.4606


Jet mass and pile up

Jet Mass and Pile-up

  • Looking at mean mass vs. pile-up

  • Splitting/filtering reduces dependence significantly

http://arxiv.org/abs/1203.4606


Jet mass and substructure3

Jet Mass and Substructure

  • Also look at kT splitting scales (aka y-scales, d12)

  • Measure of scale at which a jet breaks into two subjets


Jet mass and substructure4

Jet Mass and Substructure

More like 3 subjets than 2

  • n-subjettiness, new variable (arXiv:1011.2268 (2011) Thaler, Tilburg)

Sum distances from constituents to 2 or 3

ktsubjet axes

Take ratio of sums

http://arxiv.org/abs/1203.4606


Conclusions

Conclusions

  • ATLAS has made many measurements of internals of jets

    • Jet shapes (Phys.Rev. D83 (2011) 052003)

    • Jet fragmentation (Eur. Phys. J. C 71 (2011) 1795)

    • Jet mass and substructure variables (arxiv:1203.4606)

  • All available in HepData(in a week-ish for jet mass)

  • Variables designed for new physics searches appear to be well modelled generally by Monte Carlo

  • More to come with 8 and 14 TeV


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