Search for Leptoquarks in mu nu jj at CDF with ~200pb
CDF Collaboration

Contacts Simona Rolli, Dan Ryan, Sun Hao
We report on a search for second generation scalar leptoquarks using 203pb of Run II data taken at sqrt(s) = 1960 GeV/c. 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 muon, 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 = Br ( LQ \rightarrow \mu q) = 0.5 and using the NLO theoretical estimate we reject the existence of scalar leptoquarks with mass below 170 GeV/c.

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 a muon and quark and 50% to missing energy and quark. The experimental signature is thus one isolated high-Pt muon, missing energy and 2 isolated energetic jets. The background contributions are mainly from W+2jets, top, Z + 2 jets and fakes.

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 muon. We reject events with a second tight/central or loose (stubless) muon. After making a selection of the muon, we require the following kinematic constraints:
  1. Pt muon > 25 GeV
  2. Et(jet1) > 30 GeV, Et(jet2) > 30 GeV
  3. Missing energy > 60 GeV
  4. Delta Phi (MET-jet ) > 5 degrees, Delta Phi (MET-muon ) < 175 degrees
  5. Et(jet1)+Et(jet2) > 80 GeV
  6. Transverse mass e-nu > 120 GeV
  7. mass cut around the nominal LQ mass
The kinematic acceptances are shown below:

We searched for signatures of 1st generation scalar leptoquarks in approximately 200pb of data.

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

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 170 GeV/c.

(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

Last modified: Tue Jul 20 2004