First measurement of jets and missing transverse energy with the ATLAS calorimeter

In December 2009 Atlas recorded the first soft proton-proton collisions at a centre-of-mass energy of 900 GeV. We report on the selection of jets at low transverse momenta and on first measurements of their kinematic distributions. The data are well described by a Monte Carlo simulation. In addition, we show the performance achieved for the measurement of the missing transverse momentum (Etmiss) for both randomly-triggered events and in soft proton-proton collisions. No significant tails are observed in the data, and the Etmiss resolution is compatible with that expected from the Monte Carlo simulation. The recent data collected with the Atlas detector in proton-proton collisions at a centre-of-mass energy of 900 GeV allows one to test the inputs to jet reconstruction and jet calibration. After a short overview of the main calibration schemes followed in Atlas, we report on first measurements needed to commission sophisticated jet calibration schemes based on the fine granularity of the Atlas calorimeter system. We show that the basic inputs to jet reconstruction and calibration are well described by a Monte Carlo simulation. We give a first estimate how well jets will be calibrated for the upcoming high energy LHC run. The understanding of basic jet properties such as the number of associated charged particles, or the longitudinal and radial shower topology in jets, as well as the energy flow around jets are important ingredie nts for a successful jet reconstruction. We report on first measurements of jet properties in soft proton-proton collisions at a centre-of-mass energy of 900 GeV at the LHC using the Atlas detector. We show that the measurements are well described by Monte Carlo simulations.