Neutrino production of opposite sign dimuons in the NOMAD experiment

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Abstract

The NOMAD Collaboration presents a study of opposite sign dimuon events in the framework of Leading Order QCD. A total of 2714 neutrino- and 115 antineutrino-induced opposite sign dimuon events with Eμ1,Eμ2>4.5 GeV, 15<Eν<300 GeV and Q2>1(GeV/c)2 are observed in the Front-Calorimeter of NOMAD during the 1995 and 1996 runs. The analysis yields a value for the charm quark mass of mc=1.3+0.3+0.3−0.3−0.3GeV/c2 and for the average semileptonic branching ratio of Bc=0.095+0.007+0.014−0.007−0.013. The ratio of the strange to non-strange sea in the nucleon is measured to be κ=0.48+0.09+0.17−0.07−0.12. The measured rate of charm-induced dimuon relative to single muon, as a function of neutrino energy, is consistent with the slow rescaling hypothesis of heavy quark production.

Introduction

A charged current muon-neutrino deep inelastic scattering event produces a single muon at the leptonic vertex and changes the flavour of the initial quark at the hadronic vertex. If the initial quark is a strange (down) quark, a charm quark may be produced via a Cabibbo enhanced (suppressed) transition. The charm quark then fragments into a charmed hadron which may decay semileptonically, producing either a second muon or an electron with its electric charge opposite to that of the muon from the leptonic vertex. This type of event is referred to as an opposite sign dilepton event. If the second lepton is a muon the event is usually labelled as a dimuon event. This process is a powerful and clean probe of both the strange component of the nucleon sea and the kinematics of heavy quark production.

The phenomenon of charm production by neutrinos has been investigated by a number of experiments. Dimuons have been studied in counter experiments: CDHS [1], CHARM II [4], CCFR [3] and FMMF [5]; in bubble chamber experiments such as Col-BNL [6], BEBC [7] and E362 [8] at Fermilab; and in the Fermilab emulsion experiments, E531 [9] and E564 [10]. Bubble chamber experiments generally have too few events to study the parameters of charm production with any precision. It is the counter experiments, and the E531 experiment, which have provided much of our knowledge of neutrino charm production.

This paper presents the results of an analysis of opposite sign dimuon events carried out by the NOMAD collaboration. The distributions of various kinematic variables are compared with a theoretical model of dimuon production, constructed within the framework of leading order QCD, to yield a determination of the charm quark mass, mc, the strange quark fraction of the nucleon sea, κ, and the average semileptonic branching ratio, Bc. The paper is organised as follows: Section 2 outlines the theoretical description of opposite sign dimuon production within the framework of leading order QCD and Section 3 gives a brief description of the neutrino beam. The detector is described in Section 4 along with a description of the simulation programs used in the analysis (Section 4.2), a description of the data selection process and a discussion of the background determination. Section 5 presents details of the analysis and Section 6 contains a discussion and a comparison with the results of other experiments. Finally, Section 7 summarizes the results.

Section snippets

Theory

In the Standard Model, an opposite sign dimuon event is produced when a neutrino interacts, via a charged current, with a strange (s) or down (d) quark, producing a charm (c) quark. The charm quark fragments into a charmed hadron (most frequently a D meson) which can then decay semileptonically resulting in a final state containing two oppositely charged muons: the primary muon which comes from the leptonic vertex and the secondary muon which arises from the decay of the charmed hadron.

The

Neutrino beam

The CERN-SPS wide band beam is produced by 450 GeV/c protons incident on a beryllium target. The secondary pions and kaons pass through a large angle aluminium collimator and are focussed by a system of magnetic lenses, which focus(defocus) positive(negative) mesons. The particles decay in a 290 m long evacuated decay tunnel and the decay products then pass through an earth and iron shield which filters out all but the neutrinos and some muons. Monte Carlo predictions of the relative beam

Apparatus

The NOMAD detector, designed to search for a neutrino oscillation signal in the CERN SPS wide band neutrino beam, is described in detail in Ref. [14]. A side view of the detector is shown in Fig. 1. It consists of a number of subdetectors, most of which are located inside a 0.4 T dipole magnet with a volume of 7.5×3.5×3.5m3. The relevant features for the present study will be briefly mentioned.

An iron-scintillator hadronic calorimeter, denoted the Front Calorimeter (FCAL), located upstream of

Description and results

The production of opposite sign dimuon events is governed by four parameters: the charm quark mass, mc, the proportion of strange to nonstrange quarks in the nucleon sea, usually determined by the parameter κ=2S/(U+D) (where S=01xs(x)dx, U=01xu(x)dx and D=01xd(x)dx), the CKM matrix element, Vcd1, and Bc, the average semileptonic branching ratio of charmed hadrons. Determinations of these parameters were made

Discussion

A comparison of these results with those reported by the CDHS [1], CHARM II [4], CCFR [3] and FMMF [5] experiments is shown in Table 7. All parameters measured in this study are compatible with previous experiments.

Within the framework of leading order QCD, an SU(3) symmetric sea would yield a value of κ=1. The smaller value observed supports the conclusion that the size of the strange sea is suppressed with respect to the size of the non-strange sea. The strange sea content of the nucleon may

Summary

Dimuon production by neutrinos in the NOMAD detector has been studied in the context of Leading Order QCD. The data are consistent with the hypothesis of charm production via the slow rescaling model. Within this framework, the charm quark mass, mc=1.3+0.3+0.3−0.3−0.3 (GeV/c2), and the strange content of the nucleon, ηs=0.071+0.011+0.020−0.009−0.015, have been determined and shown to be compatible with the results of previous leading order QCD analyses. The strange quark content has been shown

Acknowledgements

We thank the management and staff of CERN and of all participating institutes for their support. Particular thanks are due to the CERN accelerator and beam-line staff for the magnificent performance of the neutrino beam. The following funding agencies have contributed to this experiment: Australian Research Council (ARC) and Department of Industry, Science, and Resources (DISR), Australia; Institut National de Physique Nucléaire et Physique des Particules (IN2P3), Commissariat à l'Energie

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