Search for New Physics in the Exclusive Dijet plus Missing E_{T} 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 E_{T}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 E_{T}. Here, we describe studies of the exclusive dijet plus missing E_{T}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 E_{T}trigger path (event Missing E_{T}> 45 GeV). The base event sample is selected using kinematic requirements of H_{T}> 125 GeV (H_{T}is the scalar sum of the transverse energies of the two reconstructed jets) and event missing E_{T}> 80 GeV. We also perform a separate search in the high kinematic region defined by H_{T}> 225 GeV and Missing E_{T}> 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 E_{T}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 E_{T}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

FermilabPierre-Hugues Beauchemin

University of OxfordPier-Olivier DeVivieros, Dan MacQueen, Robert S. Orr, Pierre Savard

University of Toronto

- 2 Jets with E
_{T}> 30 GeV - No 3rd Jet with E
_{T}> 15 GeV - Scalar Jet H
_{T}> 125 GeV - Event Missing E
_{T}> 80 GeV

- Background Predictions:

Background |
Number of Events |

Z -> ν ν |
888 +/- 54 |

W -> τ ν |
669 +/- 42 |

W -> μ ν |
399 +/- 25 |

W -> e ν |
256 +/- 16 |

Z -> l l |
29 +/- 4 |

Top Production |
74 +/- 9 |

Diboson Production |
90 +/- 7 |

QCD |
49 +/- 30 |

Gamma plus Jet |
75 +/- 11 |

Non-Collision |
4 +/- 4 |

Total Predicted |
2533 +/- 151 |

Data Observed |
2506 |

- Comparison of Standard Model Background Estimate with events observed in data
as a function of event missing E
_{T}and jet scalar H_{T}(linear scale): - Comparison of Standard Model Background Estimate with events observed in data
as a function of event missing E
_{T}and jet scalar H_{T}(log scale) showing the stacked contributions of the various SM background processes:

- 2 Jets with E
_{T}> 30 GeV - No 3rd Jet with E
_{T}> 15 GeV - Scalar Jet H
_{T}> 225 GeV - Event Missing E
_{T}> 100 GeV

- Background Predictions:

Background |
Number of Events |

Z -> ν ν |
86.4 +/- 12.7 |

W -> τ ν |
50.6 +/- 8.0 |

W -> μ ν |
32.9 +/- 5.2 |

W -> e ν |
14.0 +/- 2.2 |

Z -> l l |
1.7 +/- 0.2 |

Top Production |
10.8 +/- 1.7 |

Diboson Production |
4.9 +/- 0.4 |

QCD |
9.0 +/- 9.0 |

Gamma plus Jet |
4.8 +/- 1.1 |

Non-Collision |
1.0 +/- 1.0 |

Total Predicted |
216.1 +/- 29.8 |

Data Observed |
186 |

- Comparison of Standard Model Background Estimate with events observed in data
as a function of event missing E
_{T}and jet scalar H_{T}(linear scale):

- 95% C.L. Lower Mass Limits and Upper Cross-section Limits:

Leptoquark Generation |
Lower Mass Limit (GeV/c^{2}) |
Cross-section Limit (pb) |

1st or 2nd |
190 |
0.290 |

3rd |
178 |
0.442 |

- Comparison of Standard Model Background Estimate with events observed in data
in the high kinematic region as a function of event missing E
_{T}and jet scalar H_{T}(linear scale). The additional potential signal contribution from a 180 GeV/c^{2}scalar leptoquark -- just above the limit we set -- is also shown. Note that the high kinematic region is the more sensitive of the two for the mass region in the neighborhood of our limit: - Graphical representation of expected and observed 95% C.L. lower limits on
scalar leptoquark mass for first and second generations. Note that the
low kinematic region is a priori more sensitive for mass points of 140 GeV/c
^{2}and below, while the high kinematic region is a priori more sensitive for mass points above 140 GeV/c^{2}:

- MSSM spectra:
- 95% Cross-section upper limits compared to leading order calculations, right-handed squark production:
- 95% Cross-section upper limits compared to leading order calculations, inclusive squark+gluino production:
- Since the 95% C.L. cross-section upper limits for spectra S2 and S3 are less than that obtained from the Pythia LO calculation, these two spectra are ruled out at leading order. For the other two spectra, the ratio of the observed limit to the Pythia cross-section is a minimal k-factor which would have to be applied to the result of the LO Pythia calculation in order for these spectra to be ruled out. Note that the high kinematic region is the more sensitive region for the S1, S2, and S3 spectra, while the low kinematic region is more sensitive for the S4 mass spectrum.

MSSM spectrum |
Squark Mass (GeV/c^{2}) |
Gluino Mass (GeV/c^{2}) |
Lightest Neutralino Mass (GeV/c^{2}) |

S1 |
320 |
390 |
60 |

S2 |
250 |
450 |
72 |

S3 |
220 |
520 |
85 |

S4 |
120 |
550 |
89 |

MSSM spectrum |
Observed Limit (pb) |
Pythia LO Calculation (pb) |
Ratio (Observed/Pythia) |

S1 |
0.10 |
0.05 |
2.28 |

S2 |
0.18 |
0.28 |
0.64 |

S3 |
0.35 |
0.61 |
0.58 |

S4 |
81.0 |
18.0 |
4.49 |

MSSM spectrum |
Observed Limit (pb) |
Pythia LO Calculation (pb) |
Ratio (Observed/Pythia) |

S1 |
0.37 |
0.36 |
1.03 |

S2 |
0.62 |
1.73 |
0.36 |

S3 |
1.33 |
3.21 |
0.41 |

S4 |
73.8 |
57.4 |
1.29 |

- Comparison of Standard Model Background Estimate with events observed in data
in the high kinematic region as a function of event missing E
_{T}and jet scalar H_{T}(linear scale). The additional potential signal contribution from MSSM spectrum S2 -- one of the two spectra ruled out at leading order -- is also shown. Note that the high kinematic region is the more sensitive of the two for this particular mass spectrum. - Comparison of Standard Model Background Estimate with events observed in data
in the high kinematic region as a function of event missing E
_{T}and jet scalar H_{T}(linear scale). The additional potential signal contributions from a 180 GeV/c^{2}scalar leptoquark and MSSM spectrum S2 are both shown.

Last updated : July 16, 2009