Search for Leptoquarks in e nujj at CDF with ~200pb¯¹ CDF Collaboration

Contacts Simona Rolli
Abstract
We report on a search for first generation scalar leptoquarks using 203pb¯¹ of Run II data taken at sqrt(s) = 1960 GeV/c². Leptoquarks are assumed to be pair produced and to decay into a lepton and a quark of the same generation. We focus on the signature represented by one energetic electron, missing energy and two jets and set an upper limit at 95% CL on the production cross section as a function of the mass of the leptoquark. By assuming ß = Br ( LQ \rightarrow \mu q) = 0.5 and using the NLO theoretical estimate we reject the existence of scalar leptoquarks with mass below 176 GeV/c².

Introduction
Leptoquarks are theoretical particles that couple to leptons and quarks. They are predicted by extensions of and models beyond the Standard Model, e.g. GUT's, Technicolor, SUSY, etc, however the basic premise of leptoquarks is the same regardless of the different theories that predict them. The fact that there are is a symmetry of 3 generations of both quarks and leptons suggests that at higher energies leptons and quarks could be coupled. Leptoquarks would be the mediators of the forces between quarks and leptons.

The properties of leptoquarks vary depending on the theory that predicts them. In general, they are color-triplet scalar (spin=0) or vector (spin=1) bosons which carry both baryon and lepton quantum numbers. Their weak isospin and fractional electric charge are model dependent. They can be produced both singley and in pairs. However, due to experimental non-observation of flavor changing neutral currents (FCNC) and helicity-supress decays, it is assumed that leptoquarks couple uniquely among the 3 generations of leptons and quarks--hence the term, 1st generation leptoquark, 2nd generation leptoquark, etc.

This analysis is a search for pair produced 1st generation scalar leptoquarks decaying 50% to an electron and quark and 50% to missing energy and quark. The experimental signature is thus one isolated high-Pt electron, missing energy and 2 isolated energetic jets. The background contributions are mainly from W+2jets, top, Z + 2 jets and fakes.

Production
At the Tevatron, leptoquark production is dominated by quark-antiquark annihilation and would be produced mainly in pairs. For scalar leptoquarks, their production cross-section is essentially independent of any parameters other than their mass due to gauge symmetries. In contrast, vector leptoquarks are characterized by 2 anomolous couplings which are real numbers and are related to the anomolous magnetic and electric moments.

The scalar leptoquark production cross section has been calculated to NLO(1):

Event Selection
We require to have events with one tight central electron. We reject events with a second tight/central or plug electron. After making a selection of the electron, we require the following kinematic constraints:
1. Et electron > 25 GeV
2. Et(jet1) > 30 GeV, Et(jet2) > 30 GeV
3. Missing energy > 60 GeV
4. Delta Phi (MET-jet ) > 10 degrees
5. Et(jet1)+Et(jet2) > 80 GeV
6. Transverse mass e-nu > 120 GeV
7. 3sigma mass cut around the nominal LQ mass
The kinematic acceptances are shown below:

Searches
We searched for signatures of 1st generation scalar leptoquarks in approximately 200pb¯¹ of data. The predicted number of events are shown in the below table for two different choices of Q².

sigma_Theory CTEQ4M (nev)
Mass (GeV/c²)Q² = (M/2)²Q² = (2M)²
100 35 26.5
120 27 21
140 21 16
160 13.5 10.5
180 8.5 7
200 5.2 4.2
220 3.2 2.6
240 1.65 1.5
260 1.1 0.9
280 0.65 0.5

The number of expected background events from W+2jets, Top, Z + 2 jets is reported in the table below. QCD background is estimated to be negligible.

We observe a number of events surviving our selection which is consistent with the calculated background and compare the 95% CL cross-section lower limit with the theoretical estimate. We reject the existence of a scalar leptoquark with a mass lower than 176 GeV/c².

( data in blue )

Final transverse mass distribution nu-jet for the surviving events, compared to the expected signal distribution for production of a scalar LQ of mass 200 GeV.

( data in blue )

Final invariant mass distribution electron-jet for the surviving events, compared to the expected signal distribution for production of a scalar LQ of mass 200 GeV.

References
(1) Kramar, M. et al, "Pair Production of Scalar Leptoquarks at the TeVatron", Phys. Rev. Lett. 79 (1997), 341
(2 ) Simona Rolli , "Searches for First Generation Scalar Leptoquarks", CDF Public Note