Missing p_T Reconstruction at ATLAS

The missing transverse momentum (E^miss_T) in particle collider experiments is defined as the momentum imbalance in the plane transverse to the beam axis: the resultant of the negative vectorial sum of the momenta of all the particles that are involved in the pp collision of interest. A precise measurement of the E^miss_T is essential for many physics studies at the LHC, such as Higgs boson searches and measurements, as well as searches beyond the Standard Model. The E^miss_T measurement is constructed from the reconstructed and calibrated energy deposits inside the calorimeters, a method that has historically served experiments well, but one which is sensitive to fluctuations from noise and, in particular, additional unrelated collisions within the same event - an effect that is becoming more critical with the increasing luminosity of the LHC. A complementary method for measuring the missing transverse momentum is presented, in which track momenta are used in place of the calorimeter energy measurements, allowing the calculation to be made from particles originating solely from the collision vertex of interest. The reconstruction of this track-based missing transverse momentum, p^miss_T, and its performance in W and Z boson events, is described here. (paper)