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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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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