JetEtmissApproved2011PileupOffsetAndJVF

Calorimeter jet p_T as a function of number of reconstructed primary vertices (N_PV), for the central pseudo-rapidity region. Calorimeter jets reconstructed at the EM scale are associated to track jets in bins of p_T. Each point shows the most probable value of calorimeter jet p_T, for a specific p_T (track jet) range and NPV.

eps file png file

[Jet Offset in data (II)]

a p_T-independent pile-up correction based on simulation (MC11a) is applied to calorimeter jets, and is found to greatly reduce the pile-up contribution. Note that by construction, the pile-up correction is close to zero at the average number of primary vertices, N_PV = 5.4.

eps file png file

A data set with an integrated luminosity of 0.4 pb^-1 and approximately 30 interactions per bunch crossing. Reconstructed tracks originating from the chosen primary vertex were matched within &DeltaR < 0.6 to anti-k_t R = 0.6 topo-cluster jets. The sum of the p_T of the matched tracks is insensitive to pile-up and correlated with the true jet p_T. In each bin of matched track p_T, we see clearly the dependence of the calorimeter jet p_T on in-time pile-up, as described by the number of reconstructed primary vertices (N_PV). This dependence remains linear up to at least N_PV = 30, and it is in quantitative agreement with the results of similar studies performed in 2011 data with significantly less pile-up.

eps file png file

The jet vertex fraction (JVF) quantifies the fraction of track transverse momentum associated to a jet from the hard-­scattering interaction. Discrimination between jets produced in the hard-­scatter and those originating in pile-­up using the JVF in events simulated at L=2×10³³cm^-­‐2s^‐1 and 25 ns bunch spacing. Monte Carlo truth information is used to separate jets from pile-­up with p_T > 20 GeV in the central region of the detector, and the JVF discriminant is shown for each class of jets.

eps file png file

The jet vertex fraction (JVF) distribution in data and Monte Carlo simulation for jets in in Z → ll (l = e, μ) events. Events must contain at least one primary vertex consisting of 3 or more tracks and are then required to contain exactly two leptons (which are oppositely signed in the case of muons) with p_T > 20 GeV and |η| < 2.5 which form an invariant mass 76 < m_ll < 106 GeV. Jets are reconstructed using an anti‐kt algorithm with distance parameter R = 0.4 and are required to have p_T > 25 GeV and |η| < 2.5. The Monte Carlo overestimates the number of jets at low values of JVF, where the jets have a large contribution from vertices coming from additional proton-­proton interactions (“pile-up”).

eps file png file

The average number of jets, N_jet, before (black) and after (red) a cut on the jet vertex fraction, |JVF|>0.75, as a function of the number of primary vertices (containing at least 3 tracks) in data and Monte Carlo simulation for jets in Z → ll (l = e, μ) events. Events are required to contain exactly two leptons (which are oppositely signed in the case of muons) with p_T > 20 GeV and |η| < 2.5 which form an invariant mass 76 < m_ll < 106 GeV. Jets are reconstructed using an anti-­k_t algorithm with distance parameter D = 0.4 and are required to have p_T > 2 GeV and |η| < 2.5. Before the JVF cut, increases with the number of primary vertices, as expected due to the additional proton-­proton interactions (“pile-up”). This increase is faster in the Monte Carlo, suggesting the number of pile-­up jets is overestimated in the simulation. After the cut, N_jet is constant with the number of primary vertices and reasonably well described by the Monte Carlo.

eps file png file