[Mean jet p_{T} vs number of vertices] Average reconstructed jet p_{T} at the EM scale versus the number of reconstructed primary vertices (N_{PV}), for matched truthjet p_{T} between 20 and 25 GeV. Requiring that all matched tracks originate from the hard scatter (JVF = 1) reduces the pileup induced p_{T} offset by roughly 40%, indicating that tracks effectively describe only 40% of the impact of pileup on reconstructed jets.

[Response vs the true jet p_{T}] Calorimeter response to central jets at the LCW scale, as a function of the MC truth jet p_{T}. The p_{T} offset induced by a mean of 40 pileup interactions (⟨μ⟩ = 40) causes a shift in the reconstructed jet p_{T} of approximately 25 GeV. Application of an average offset correction, parametrized by the number of reconstructed primary vertices (N_{PV}), recovers the response observed in the absence of pileup (⟨μ⟩ = 0).

[Subtraction of intime pileup using the “jet areas” method, central] Average MC truthbased jet p_{T} offset at the LCW scale versus the number of reconstructed primary vertices (N_{PV}). Outoftime pileup 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 intime pileup on the jet p_{T}, though a small overcorrection is apparent.

[Subtraction of intime pileup using the “jet areas” method, forward] Average MC truthbased jet p_{T} offset at the LCW scale versus the number of reconstructed primary vertices (N_{PV}). Outoftime pileup 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 intime pileup on the jet p_{T}, though a small overcorrection is apparent.

[Subtraction of outoftime pileup using the “jet areas” method, central] Average MC truthbased jet p_{T} offset at the LCW scale versus the average number of interactions per bunch crossing (⟨μ⟩), which is sensitive to outoftime pileup. Intime pileup is constrained by using only events with N_{PV} = 6. In the central region (0.0 < η < 0.3), the jet areas correction overcorrects slightly for the impact of outoftime pileup.

[Subtraction of outoftime pileup using the “jet areas” method, forward region] Average MC truthbased jet p_{T} offset at the LCW scale versus the average number of interactions per bunch crossing (⟨μ⟩), which is sensitive to outoftime pileup. Intime pileup is constrained by using only events with N_{PV} = 6. In the forward region (3.6 < η < 4.5), sensitivity to outoftime pileup 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 outoftime pileup on forward jets

[Jet multiplicity vs number of vertices] Average multiplicity of pileup jets with LCWscale p_{T} > 20 GeV versus the number of reconstructed primary vertices (N_{PV}) before and after correcting for the p_{T} offset induced by pileup, in simulated dijet events with a mean of 20 overlaid pileup interactions. The average offset correction, parametrized by N_{PV}, shifts up to 90% of pileup jets below the p_{T} threshold. By estimating the pileup offset event by event using lowp_{T} calorimeter deposits, the jet areas correction rejects a substantial fraction of the remaining 10%.

[Jet multiplicity of pileup jets vs number of vertices] Average multiplicity of pileup jets with LCWscale p_{T} > 20 GeV versus the number of reconstructed primary vertices (N_{PV}) after correcting for the p_{T} offset induced by pileup, in simulated dijet events with a mean of 20 overlaid pileup interactions (⟨μ⟩). There is a significant residual number of pileup jets, even after the jet areas correction. To reject the remaining jets, we can use tracking information. Pileup jets are virtually eliminated by requiring at least a quarter of the matched track p_{T} to come from the hard scatter vertex (JVF > 0.25).

[Jet resolution for various pileup subtraction methods] The width of the MC truthbased jet p_{T} offset at the LCW scale, in a sample with zero pileup (⟨μ⟩ = 0), compared to a mean of 40 pileup interactions, before and after correcting for the p_{T} offset induced by pileup. Pileup 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 3040% of the degradation. The remaining 6070% is due to localized fluctuations in pileup, which may be reduced by using information from tracks matched to jets.

[Trackbased pileup activity vs calorimeter based] Trackbased local versus global estimates of pileup activity in a jet, in simulated events with a mean of 40 pileup interactions. For a given amount of global pileup activity, there are significant fluctuations in the activity that is directly affecting a specific jet.

Major updates:
 TancrediCarli  19Jul2012
Responsible: TancrediCarli
Subject: public
I  Attachment  History  Action  Size  Date  Who  Comment 

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