Search for new heavy boson W' production in the top quark decay channel at DØ in Run II

Conference Note
 
R. Schwienhorst, M. Perfilov, S. Bunichev, E. Boos, L. Dudko

E-mail the first authors: Reinhard Schwienhorst, Maxim Perfilov, S. Bunichev, E. Boos, L. Dudko
E-mail the DØ single top group conveners: Arán García-Bellido, Ann Heinson.

 
Abstract

We present a search for production of a new heavy gauge boson W' that decays to a top quark and a bottom quark. We have analyzed 230 pb-1 of data collected with the DØ detector at the Fermilab Tevatron collider at a center-of-mass energy of sqrt{s} = 1.96 TeV. No significant excess of events is found in any region of the final state invariant mass distribution. We set upper limits on the production cross section of W' bosons at the 95% confidence level. For a W' boson mass of 600 GeV (700 GeV, 800 GeV) the upper limit is 1.8 pb (1.4 pb, 2.2 pb). We exclude the mass range between 200 GeV and 650 GeV for a W' boson with Standard Model-like couplings.

 
Please scroll down the page for: analysis description, cross section and mass limits.
Please read the 9 page conference note for a more detailed description. This is DØ conference note 5024.

 
Introduction

The top quark sector offers a sensitive probe to the presence of additional gauge bosons beyond the W boson of the Standard Model (SM). Such additional gauge bosons W' and Z' typically arise in extensions to the SM from the presence of additional symmetry groups.
Direct searches for the production of additional heavy gauge bosons W' have focused on the lepton final state which has good separation between the $W'$~boson signal and the SM backgrounds. The W' boson lower mass limit in this decay channel is 786~GeV (from the Tevatron in Run I). However, it is also possible that such a W' boson does not interact with leptons and neutrinos but only with quarks. Previous direct searches for the presence of W' bosons in the quark decay channel have excluded the mass range below 261~GeV (from UA1) and between 300 GeV and 420 GeV (from CDF in Run I at the Tevatron). Assuming that the W' boson decays only to quarks and not to leptons yields a lower mass limit of 800~GeV (from DØ in RunI). Indirect searches for evidence of a $W'$~boson depend on exactly how it interferes with the SM $W$~boson and the results are thus highly model specific. An extra heavy boson would appear in the single top search as a peak in the invariant mass distribution of the final state objects, see the figure below. The leading order Feynman diagram for W' boson production resulting in single top quark events is shown on the right. This diagram is identical to that for s-channel SM single top quark production where the SM W boson appears as the virtual particle.
In this analysis, we assume the W' boson has SM-like couplings. We properly include interference terms between the SM W boson and the W' boson.
 
Analysis Description

The dataset, trigger selection, object reconstruction, event selection, etc are based on the DØ neural network analysis with 230pb-1.
We select events with exactly two or exactly three jets to keep the background from top pair production small. The figure on the right shows the invariant mass distribution of the reconstructed W' boson, comparing the signal for different W' boson masses to the backgrounds from W+jets and top pair production. This is the invariant mass of the reconstructed final state objects. The two figures below show the invariant mass distribution of the final state objects, comparing the background sum to the observed events.

 
Result

The W' boson signal dominantes in the region of high invariant mass. We thus limit the search to the region with sqrt(s)>400GeV. We set limits independently in four different channels, separated by electron and muon final states and by single b-tags and double b-tags. The figure below shows the 95% confidence level limit at three different mass points, together with the theoretical prediction. The individual points are connected by straight lines. The intersection between the limit line and the theory line defines the limit on the W' mass.
We set a limit of 650 GeV on the W' boson mass. This is the first limit on W' boson production in the top quark decay channel.

 
Material for Talks

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Last modified: March, 2006
Send comments to Reinhard Schwienhorst.