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.

• 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 -> &nu &nu	777 +/- 49
W -> &tau &nu	669 +/- 42
W -> &mu &nu	399 +/- 25
W -> e &nu	256 +/- 16
Z -> l l	29 +/- 4
Top Production	74 +/- 9
QCD	49 +/- 30
Gamma plus Jet	55 +/- 13
Non-Collision	4 +/- 4
Total Predicted	2312 +/- 140
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):



(eps version)

(eps version)



• 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:



(eps version)

(eps version)

• 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 -> &nu &nu	71 +/- 12
W -> &tau &nu	50 +/- 8
W -> &mu &nu	33 +/- 5
W -> e &nu	14 +/- 2
Z -> l l	2 +/- 0
Top Production	11 +/- 2
QCD	9 +/- 9
Gamma plus Jet	5 +/- 3
Non-Collision	1 +/- 1
Total Predicted	196 +/- 29
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):



(eps version)

(eps version)

• 95% C.L. Lower Mass Limits:



Leptoquark Generation	Lower Mass Limit (GeV/c2)
1st or 2nd	177
3rd	167

• 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/c² scalar leptoquark 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:



(eps version)

(eps version)



• 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/c² and below, while the high kinematic region is a priori more sensitive
  for mass points above 140 GeV/c²:



(eps version)

Last updated : April 25, 2008