Jets with high transverse momenta produced in proton–proton collisions must be distinguished from background jet candi-dates not originating from hard-scattering events. A first strat-egy to select jet from collisions and to suppress background is presented in Ref. [3].
The main backgrounds to jets coming from collision events are the following:
1. Beam–gas events, where one proton of the beam collides with the residual gas within the beam pipe.
2. Beam-halo events, for example caused by interactions in the tertiary collimators in the beam-line far away from the ATLAS detector.
3. Cosmic-ray muons overlapping in-time with collision ev-ents.
4. Calorimeter noise.
Dag Gillberg, Carleton Jet calibration schemes 2014-05-13 6
Calorimeter jets (EM or LCW scale)
Pile-up offset
correction Origin correction Energy & η calibration
Residual in situ calibration
Calorimeter jets (EM+JES or LCW+JES scale)
Jet calibration
Changes the jet direction to point to the primary vertex.
Does not affect the energy.
Calibrates the jet energy and pseudorapidity to the particle jet scale.
Derived from MC.
Residual calibration derived using in situ measurements.
Derived in data and MC.
Applied only to data.
Corrects for the energy offset introduced by pile-up.
Depends on µ and NPV.
Jet finding Calorimeter jets (LCW scale)
Jet finding Calorimeter jets
(EM scale)
Tracks Track jets
Simulated
particles Truth jets
Calibrates clusters based on cluster properties related to shower development
Jet finding Jet finding
Fig. 2: Overview of the ATLAS jet reconstruction. After the jet finding, the jet four momentum is defined as the four momentum sum of its constituents.
Dag Gillberg, Carleton Jet calibration schemes 2012-09-05 6
Calorimeter jets
(EM or LCW scale) Pile-up offset
correction Origin correction Energy & !
calibration Residual in situ calibration
Calorimeter jets (EM+JES or LCW+JES scale)
Jet calibration
Changes the jet direction to point to the primary vertex.
Does not affect the energy.
Calibrates the jet energy and pseudorapidity to the particle jet scale.
Derived from MC.
Residual calibration derived using in situ measurements.
Derived in data and MC.
Applied only to data.
Corrects for the energy offset introduced by pile-up.
Depends on µ and NPV.
Jet finding Calorimeter jets (LCW scale)
Jet finding Calorimeter jets
(EM scale)
Tracks Track jets
Simulated
particles Particle jets
(aka truth jets)
Calibrates clusters based on cluster properties related to shower development
Jet finding Jet finding
Fig. 3: Overview of the ATLAS jet calibration scheme used for the 2011 dataset. The pile-up, absolute JES and the residual in situ corrections calibrate the scale of the jet, while the origin and the η corrections affect the direction of the jet.
det| η Jet |
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Jet response at EM scale
0.3
Jet response at LCW scale
0.5
Fig. 4: Average response of simulated jets formed from topo-clusters, calculated as defined in Eq. (1) and shown in(a)for the EM scale (REM) and in(b)for the LCW scale (RLCW). The response is shown separately for various truth-jet energies as function of the uncorrected (detector) jet pseudorapidity ηdet. Also indicated are the different calorimeter regions. The inverse of REM (RLCW) corresponds to the average jet energy scale correction for EM (LCW) in each ηdetbin. The results shown are based on the baseline PYTHIAinclusive jet sample.
Jet selection efficiency
Fig. 5: Jet quality selection efficiency for anti-ktjets with R = 0.4 measured with a tag-and-probe technique as a function of pjetT in various η ranges, for the four sets of selection criteria. Only statistical uncertainties are shown. Differences between data and MC simulations are also shown.
The jet quality selection criteria should efficiently reject jets from these background processes while maintaining high effi-ciency for selecting jets produced in proton–proton collisions.
Since the level and composition of background depend on the event topology and the jet kinematics, four sets of criteria called LOOSER, LOOSE, MEDIUMand TIGHTare introduced in Ref.
[74]. They correspond to different levels of fake-jet rejection and jet selection efficiency, with the LOOSER criterion being the one with the highest jet selection efficiency while the TIGHT
criterion is the one with the best rejection. The discrimination between jets coming from the collisions and background jet candidates is based on several pieces of experimental informa-tion, including the quality of the energy reconstruction at the cell level, jet energy deposits in the direction of the shower de-velopment, and reconstructed tracks matched to the jets.
The efficiencies of the jet selection criteria are measured using the tag-and-probe method described in Ref. [3]. The re-sulting efficiencies for anti-kt jets with R = 0.4 for all selec-tion criteria are shown in Fig.5. The jet selection efficiency of the LOOSERselection is greater than 99.8% over all calibrated transverse jet momenta pjetT and η bins. A slightly lower effi-ciency of about 1-2% is measured for the LOOSEselection, in particular at low pjetT and for 2.5 < |η| < 3.6. The MEDIUMand TIGHT selections have lower jet selection efficiencies mainly due to cuts on the jet charged fraction, which is the ratio of the scalar sum of the pT of all reconstructed tracks matching the jet, and the jet pTitself, see Ref. [74] for more details. For jets with pjetT ≈ 25 GeV, the MEDIUM and TIGHTselections have inefficiencies of 4% and 15%, respectively. For pjetT > 50 GeV, the MEDIUM and TIGHT selections have efficiencies greater than 99% and 98%, respectively.
The event selection is based on the azimuthal distance be-tween the probe and tag jet ∆ φ (tag, probe) and the significance of the missing transverse momentum ETmiss[75] reconstructed for the event, which is measured by the ratio ETmiss/√
Σ ET. Here Σ ETis the scalar transverse momentum sum of all parti-cles, jets, and soft signals in the event. The angle ∆ φ (tag, probe), ETmiss/√
Σ ET, and the TIGHTselection of the reference (tag) jet are varied to study the systematic uncertainties. For the LOOSE
and LOOSER selections, the jet selection efficiency is almost unchanged by varying the selection cuts, with variations of less than 0.05%. Slightly larger changes are observed for the two other selections, but they are not larger than 0.1% for the MEDIUMand 0.5% for the TIGHTselection.
The jet selection efficiency is also measured using a MC simulation sample. A very good agreement between data and simulation is observed for the LOOSERand LOOSEselections.
Differences not larger than 0.2% and 1% are observed for the MEDIUMand TIGHTselections, respectively, for pjetT > 40 GeV.
Larger differences are observed at lower pjetT, but they do not exceed 1%(2%) for the MEDIUM(TIGHT) selection.