# NYU participation in ATLAS

The ATLAS experiment at the CERN Large Hadron Collider (LHC) is a collaboration of 3000 physicists from 174 universities and laboratories in 38 countries. The goal of the the experiment is to study the fundamental building blocks of matter by studying high energy collisions of protons and nuclei. For a complete description of ATLAS, see the ATLAS public web page http://atlas.ch. The NYU group became an official member of ATLAS in July of 2006 and we are very active in the collaboration. Below is a brief description of our major activities.

## Higgs Searches

Professor Cranmer has been active in ATLAS Higgs searches for several years, focusing primarily on low-mass Higgs searches, $H\to ZZ\to 4\ell$, and the statistical combination of all the search channels. The statistical combination allows for measurements of the Higgs couplings and other properties. Sven Kreiss recently defended his thesis "Higgs Boson Discovery and First Property Measurements with the $H\to ZZ^* \to 4\ell$ Channel and the Combination" based on this research.

Cranmer is also interested in exotic Higgs decays. Under his supervision, James Beacham recently defended his thesis "Exotic Higgs, Rare Z, and a Dark Photon: Searching for New Physics at the LHC, LEP, and Jefferson Lab"

Professor Haas has been focusing for exotic decays of the Higgs and using the Higgs as a tag for new physics. Haas leads the effort in the NMSSM Higgs searches and has advised graduate student Karthik Krishnaiyengar in an exotic Higgs decay to neutralinos and photons.

## Triggering

The NYU group is working on various aspects of the ATLAS trigger, which selects about 1 in a million proton proton interactions for furrther study. The Missing Transverse Energy (MET) part of the High Level Trigger (HLT). MET, which measures an imbalance of momentum in the plane transverse to the particle collision, is sensitive to production of particles that don't interact in the detector. It is therefore a vital part of the search for new physics, and is a natural trigger component for us to study given our group's interest in the search for supersymmetry. Some of the possible sources of fake MET, such as mismeasurement of jet energies in QCD jet events, can be many orders of magnitude more frequent than SUSY events, thus putting severe constraints on the MET trigger. Several members of our group have held leadership positions on MET, and most members of our group have contributed to the trigger in general and/or the MET trigger.

## Supersymmetry

Supersymmetry (SUSY) is one of the most well-motivated and well-developed scenarios for physics beyond the standard model. If SUSY is the solution to the hierarchy problem, then it is very likely that we will see SUSY particles at the LHC. In R-parity conserving SUSY models, decays of SUSY particles always end with the Lightest Supersymmetric Particle (LSP), which is stable and weakly interacting. Thus, the LSP provides a natural candidate for dark matter. While the LSP would escape ATLAS undetected, it would leave a signature in Missing Transverse Energy (MET). This provides a strong link between our groups interest in SUSY and our work on MET triggers.

## Statistical Methods for Particle Physics

Statistics plays an important role in science, and particularly in particle physics. Taking full advantage of the data, while properly incorporating systematic errors is a difficult task and often involves combining several sources of data in a complicated statistical model. In addition to important formal developments by our group (see links below), Kyle Cranmer is on the steering committee of the Phystat.org and an appointed member of the ATLAS statistics committee. He and Rene Brun have initiated the RooStats project -- a suite of high-level statistical tools in ROOT built on top of RooFit. It has already been demonstraited that one can digitially publish a full likelihood model, together with the relevant data, in order to statistically combine multiple measurements or perform multi-dimensional parameter scans.