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

Contacts Simona Rolli, Dan Ryan, Sun Hao
Abstract
We report on a search for second generation scalar leptoquarks using 198pb¯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 two energetic muons 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) = 1 and using the NLO theoretical estimate we reject the existence of scalar leptoquarks with mass below 224 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 2nd generation scalar leptoquarks decaying 100% to a muon and quark. The experimental signature is thus 2 isolated high-Pt muons and 2 isolated energetic jets. The background contributions are mainly from DY+2jets, top 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 that of the the 2 muons selected, at least one contains a muon stub (CMUP or CMX). The second muon may (CMUP or CMX) or may not (TRK) contain a muon stub. After making a selection of the muons, we require the following kinematic constraints:
1. Pt(muon1 and muon2) > 25 GeV
2. Et(jet1) > 30 GeV, Et(jet2) > 15 GeV
3. Reject Mµµ < 15 GeV and 75 < Mµµ < 105
4. Et(jet1)+Et(jet2) > 80 GeV and Pt(muon1)+Pt(muon2) > 80 GeV
5. Sqrt((Et(jet1)+Et(jet2))² + (Pt(muon1)+Pt(muon2))²) > 200 GeV
The kinematic acceptances are shown below:

The last two cuts discriminate the leptoquark signal from background as illustrated below for a leptoquark mass of 220 GeV/c²:

(eps)

Searches
We searched for signatures of 2nd 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
160 53.02 42.24
18033.12 26.57
20019.0115.38
22010.72 8.66
2406.21 5.01
260 3.56 2.85
280 1.95 1.55
300 1.12 0.89
320 0.63 0.50

The number of expected background events from DY+2jets, Top and Fakes is 1.87 +/- 1.0 events. We observe 2 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 224 GeV/c².

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 Second Generation Scalar Leptoquarks", CDF Public Note