AtlasCanada Web>PostDocResearchTopics>AbsoluteJetEnergyScaleWithMPF (2009-05-13, TeresaSpreitzer)

In-situ absolute jet energy scale with Missing Et Projection Fraction

The missing transverse energy projection fraction (MPF) method of determining the in situ jet energy calibration is a useful tool in determining the response to jets in the ATLAS calorimeters. The method provides a jet energy scale measurement totally based on observed data, no simulated events are needed. Another utility of the MPF technique is its insensitivity to the jet algorithm.

Introduction

The energy of jets observed in the calorimeter is calibrated to the jet energy at particle level by the jet energy scale. The calibration accounts for energy not measured in the calorimeter due to non-compensation of the calorimeter, dead material and other detector effects. Other biases also exist which are specific to the jet algorithm such as out-of-cone losses. In order to determine the jet energy at parton level, corrections based on the model and physics are needed. Some physics goals of the ATLAS experiment will require that the jet energy scale be known to within 1%. To acheive this, further study of the jet energy scale is required. The jet energy scale can be found in-situ for the ATLAS calorimeter by balancing the momentum between an electromagnetic particle and a jet, in this case we study gamma+jet events. Using the momentum balance in the transverse plane, an energy calibration for the jet can be derived using the missing transverse energy projection fraction method (MPF).

Jet Response

The jet response is a measure of the calorimeter response to the hadrons inside jets, and is the leading contribution to the jet energy scale calibration. Other contributions, such as jet algorithm corrections, must also be applied after the jet response correction.

A detailed derivation of the MPF can be found in ATL-PHYS-INT-2007-011. The response of a jet is defined as

j(E_{jet}) = 1 + \frac{ {E^{\gamma}_{T}} \cdot { \slashed{E}_{T} } } { (E^{\gamma}_{T})^{2} }

Release 14 Results

The $Z/\gamma$+jet data used are the MC08 datasets PhotonJet (108001 - 108004) e344_s456_r545. The partons were generated with Pythia, and subsequently simulated in Geant4 using version 14.1.0.3, and then processed with reconstruction version 14.2.20.3. The detector simulated the calorimeter with a sector of the HEC off. DPDs were then created with the same event format as the official AODs, except for the exclusion of some event information, such as muon data blocks, to facilitate local storage.

With these events we find the response using the MPF technique. The fits to the response curves are listed here.

jet configuration	energy scale	b0	b1	b2
Cone 4 Tower	EM	0.741 \pm 0.002	0.037 \pm 0.004	-0.007 \pm 0.002
Cone 4 Topo	EM	0.739 \pm 0.002	0.041 \pm 0.004	-0.003 \pm 0.002
Cone 7 Tower	EM	0.729 \pm 0.002	0.042 \pm 0.003	-0.006 \pm 0.002
Cone4 Tower	H1	0.942 \pm 0.002	0.014 \pm 0.003	-0.010 \pm 0.002
Cone 4 Topo	H1	0.939 \pm 0.002	0.017 \pm 0.003	-0.007 \pm 0.003
Cone 7 Tower	H1	0.941 \pm 0.002	0.019 \pm 0.002	-0.012 \pm 0.003
Cone 7 Topo	LC	0.928 \pm 0.002	-0.011 \pm 0.002	-0.005 \pm 0.003
Kt 4 Topo	LC	0.927 \pm 0.002	-0.010 \pm 0.003	-0.001 \pm 0.003
Kt 6 Topo	LC	0.923 \pm 0.002	-0.010 \pm 0.003	-0.002 \pm 0.003

-- TeresaSpreitzer - 11 May 2009

Topic revision: r2 - 2009-05-13 - TeresaSpreitzer

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In-situ absolute jet energy scale with Missing Et Projection Fraction

Introduction

Jet Response

Release 14 Results

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