Search for Neutral Heavy Leptons in a High-Energy Neutrino Beam

Abstract

A search for neutral heavy leptons (NHLs) has been performed using an instrumented decay channel at the NuTeV (E-815) experiment at Fermilab. The decay channel was composed of helium bags interspersed with drift chambers, and was used in conjunction with the NuTeV neutrino detector to search for NHL decays. The data were examined for NHLs decaying into muonic final states (µµν, µeν, µπ, and µρ); no evidence has been found for NHLs in the 0.25 -2.0 GeV mass range. This analysis places limits on the mixing of NHLs with standard light neutrinos at a level up to an order of magnitude more restrictive than previous search limits in this mass range.PACS numbers: 13.15.+g,13.35.Hb,14.60.Pq 1 Various extensions [1,2] to the Standard Model predict neutral heavy leptons (NHLs) which can mix with the standard light neutrinos. In these extensions, the NHLs are weak isosinglets that do not couple directly to the Z and W bosons, but can decay via mixing with the Standard Model neutrinos. Figure 1 shows one possible set of tree-level diagrams for NHL production and decay. The NuTeV (E815) neutrino experiment at Fermilab has made a sensitive search for these NHLs by combining the capabilities of a high intensity neutrino source with an instrumented decay region.In these extended models [1], the NHL lifetime depends on the mixing parameter |U | 2 and the mass of the NHL. They are expected to decay (e.g. Fig. 1b) into a neutrino and two charged leptons, into a lepton and two quarks, or into three neutrinos.NHLs may be created in the NuTeV beamline by the decays of secondary mesons produced by the Tevatron proton beam. During the 1996-1997 fixed-target run at Fermilab, NuTeV received 2.54 × 10 18 800 GeV protons on a beryllium oxide production target with the detector configured for this search. A sign-selected quadrupole train focused secondary π and K mesons down a beamline 7.8 mrad from the primary proton beam direction. 1.13 × 10 18 protons were received with the magnets set to focus positive mesons, and 1.41 × 10 18 protons with negative meson focusing. The mesons could decay into NHLs as shown in Fig. 1a. The production of secondary pions and kaons was simulated using the parameterization in [3]; the Decay Turtle program [4] simulated the propagation of charged particles through the beamline.NHLs may also be produced by prompt decays of charmed mesons produced by incident protons on the BeO target and proton dumps. These processes were simulated using a Monte Carlo program based on measured production cross sections [5]. The effects of decay phase space, NHL polarization, and helicity suppression [6] were included in the simulation of the production and decays. For NHLs of mass 1.45 GeV from D meson decay, the average momentum was ∼100 GeV; for the 0.35 GeV NHLs coming mainly from K decay, the average momentum was ∼140 GeV.This analysis reports the results of a search for NHLs with masses between 0.25 to 2.0 GeV, which decay with a muon in the final state. The primary NHL decay modes of this type are µeν, µµν, µπ, and µρ. In...

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