Search for New Physics in the Exclusive Dijet plus Missing ET Event Sample


Introduction:

Events with large missing transverse energy and one or more energetic jets can be produced in many models of new physics as well as Standard Model (SM) production from electroweak and QCD processes. The magnitude of the missing ET and the number of jets depends on the specific model of new physics, while the SM backgrounds and instrumental effects can be studied independently. In a previous analysis, we studied the "monojet" configuration consisting of one energetic jet balanced against large missing ET. Here, we describe studies of the exclusive dijet plus missing ET event signature.
We present the result of a generic search for new physics based on ~2.0 fb-1 of data collected with the missing ET trigger path (event Missing ET > 45 GeV). The base event sample is selected using kinematic requirements of HT > 125 GeV (HT is the scalar sum of the transverse energies of the two reconstructed jets) and event missing ET > 80 GeV. We also perform a separate search in the high kinematic region defined by HT > 225 GeV and Missing ET > 100 GeV. In both regions, we compare the expected SM backgrounds with observed data.
Based on the observed agreement between data and SM expectations in the two kinematic regions, it is possible to place limits on a wide range of models for new physics. Here, we use a scalar leptoquark model to illustrate the potential constraining power of the dijet plus missing ET analysis. The model considered is simple pair production of leptoquarks with leptoquark decay via a single channel (Leptoquark -> jet plus neutrino). This generic model provides coverage for a range of leptoquark models, each of which is characterized by a different set of quantum numbers. The common feature of these scalar models is an equivalent pair production cross section that is dependent on only one parameter, the mass of the leptoquark. The limits obtained are therefore 95% C.L. lower limits on the mass of the leptoquark in the context of our simple scalar model. These limits depend only slightly on leptoquark generation. The efficiency for third generation events to pass our dijet plus missing ET selection criteria is smaller due to lepton rejection criteria, and therefore the mass limits we set for third generation leptoquarks are a bit lower than those for the first and second generation.
We also interpret this analysis in terms of cross-section limits on generic minimal supersymmetric (MSSM) models. Four mass spectra are chosen in an MSSM model, with squark and gluino masses chosen which are not yet ruled out by previous Tevatron searches. Since our chosen mass spectra have gluinos which are more massive than squarks, minimal supergravity solutions (mSUGRA) are not allowed. However, we make no other assumptions on the nature of supersymmetry (SUSY) breaking. We set 95% C.L. cross-section upper limits on all four mass spectra, and compare these limits to the leading order cross sections calculated by Pythia. These limits are set both for the case of pair production of "right-handed" squarks (the superpartners of right-handed quarks), and for inclusive production of squarks and gluinos (as left-handed, right-handed, or opposite-handed pairs of squarks; pair production of gluinos; or production of a squark and a gluino.) The mass of the four squarks in the first two generations are assumed to be degenerate, while the stop and sbottom are not examined.

Kevin Burkett, Eric James
Fermilab
Pierre-Hugues Beauchemin
University of Oxford
Pier-Olivier DeVivieros, Dan MacQueen, Robert S. Orr, Pierre Savard
University of Toronto

  • Contact people : Dan, Eric, Hugo, Kevin, Pierre



  • Low Kinematic Region:
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  • Limits on Scalar Leptoquark Model:

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  • Limits on Minimal Supersymmetric Model:

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  • Last updated : July 16, 2009