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JetEtmissJetPerformancePileup

Jet performance under pile-up

[Mean jet pT vs number of vertices]

Average reconstructed jet pT at the EM scale versus the number of reconstructed primary vertices (NPV), for matched truth-jet pT between 20 and 25 GeV. Requiring that all matched tracks originate from the hard scatter (JVF = 1) reduces the pile-up induced pT offset by roughly 40%, indicating that tracks effectively describe only 40% of the impact of pile-up on reconstructed jets.

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[Response vs the true jet pT]

Calorimeter response to central jets at the LCW scale, as a function of the MC truth jet pT. The pT offset induced by a mean of 40 pile-up interactions (⟨μ⟩ = 40) causes a shift in the reconstructed jet pT of approximately 25 GeV. Application of an average offset correction, parametrized by the number of reconstructed primary vertices (NPV), recovers the response observed in the absence of pile-up (⟨μ⟩ = 0).

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[Subtraction of in-time pile-up using the “jet areas” method, central]

Average MC truth-based jet pT offset at the LCW scale versus the number of reconstructed primary vertices (NPV). Out-of-time pile-up is constrained by using only events with 5 < ⟨μ⟩ < 6. In the central region (0.0 < |η| < 0.3) the jet areas correction removes the impact of in-time pile-up on the jet pT, though a small overcorrection is apparent.

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[Subtraction of in-time pile-up using the “jet areas” method, forward]

Average MC truth-based jet pT offset at the LCW scale versus the number of reconstructed primary vertices (NPV). Out-of-time pile-up is constrained by using only events with 5 < ⟨μ⟩ < 6. In the forward region (3.6 < |η| < 4.5), the jet areas correction removes the impact of in-time pile-up on the jet pT, though a small overcorrection is apparent.

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[Subtraction of out-of-time pile-up using the “jet areas” method, central]

Average MC truth-based jet pT offset at the LCW scale versus the average number of interactions per bunch crossing (⟨μ⟩), which is sensitive to out-of-time pile-up. In-time pile-up is constrained by using only events with NPV = 6. In the central region (0.0 < |η| < 0.3), the jet areas correction over-corrects slightly for the impact of out-of-time pile-up.

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[Subtraction of out-of-time pile-up using the “jet areas” method, forward region]

Average MC truth-based jet pT offset at the LCW scale versus the average number of interactions per bunch crossing (⟨μ⟩), which is sensitive to out-of-time pile-up. In-time pile-up is constrained by using only events with NPV = 6. In the forward region (3.6 < |η| < 4.5), sensitivity to out-of-time pile-up is substantially different then in the central region. Since we calculate rho in the central region, the jet areas correction does not account for the impact of out-of-time pile-up on forward jets

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[Jet multiplicity vs number of vertices]

Average multiplicity of pile-up jets with LCW-scale pT > 20 GeV versus the number of reconstructed primary vertices (NPV) before and after correcting for the pT offset induced by pile-up, in simulated dijet events with a mean of 20 overlaid pile-up interactions. The average offset correction, parametrized by NPV, shifts up to 90% of pile-up jets below the pT threshold. By estimating the pile-up offset event by event using low-pT calorimeter deposits, the jet areas correction rejects a substantial fraction of the remaining 10%.

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[Jet multiplicity of pile-up jets vs number of vertices]

Average multiplicity of pile-up jets with LCW-scale pT > 20 GeV versus the number of reconstructed primary vertices (NPV) after correcting for the pT offset induced by pile-up, in simulated dijet events with a mean of 20 overlaid pile-up interactions (⟨μ⟩). There is a significant residual number of pile-up jets, even after the jet areas correction. To reject the remaining jets, we can use tracking information. Pile-up jets are virtually eliminated by requiring at least a quarter of the matched track pT to come from the hard scatter vertex (JVF > 0.25).

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[Jet resolution for various pile-up subtraction methods]

The width of the MC truth-based jet pT offset at the LCW scale, in a sample with zero pile-up (⟨μ⟩ = 0), compared to a mean of 40 pile-up interactions, before and after correcting for the pT offset induced by pile-up. Pile-up increases the width by nearly a factor of 3, and the average offset correction does not help to recover this degradation in resolution. However, the jet areas correction recovers 30-40% of the degradation. The remaining 60-70% is due to localized fluctuations in pile-up, which may be reduced by using information from tracks matched to jets.

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[Track-based pile-up activity vs calorimeter based]

Track-based local versus global estimates of pile-up activity in a jet, in simulated events with a mean of 40 pile-up interactions. For a given amount of global pile-up activity, there are significant fluctuations in the activity that is directly affecting a specific jet.

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Major updates:
-- TancrediCarli - 19-Jul-2012

Responsible: TancrediCarli
Subject: public

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Topic revision: r2 - 2012-07-20 - JohnBackusMayes
 
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