Search for W ′ boson production in the W ′ → t b̄ decay channel

We present a search for the 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 1.96 TeV. No significant excess of events above the standard model expectation is found in any region of the final state invariant mass distribution. We set upper limits on the production cross section of W ′ bosons times branching ratio to top quarks at the 95% confidence level for several different W ′ boson masses. We exclude masses between 200 and 610 GeV for a W ′ boson with standard-model-like couplings, between 200 and 630 GeV for a W ′ boson with right-handed couplings that is allowed to decay to both leptons and quarks, and between 200 and 670 GeV for a W ′ boson with right-handed couplings that is only allowed to decay to quarks. © 2006 Elsevier B.V. All rights reserved. PACS: 13.85.Rm; 14.70.Pw The top quark sector offers great potential to look for new physics related to electroweak symmetry breaking. In particu* Corresponding author. E-mail address: [email protected] (R. Schwienhorst). 1 Visitor from Lewis University, Romeoville, IL, USA. 2 On leave from IEP SAS Kosice, Slovakia. 3 Visitor from Helsinki Institute of Physics, Helsinki, Finland. lar, it is a sensitive probe for the presence of additional gauge bosons beyond those of the standard model (SM). Such new gauge bosons typically arise in extensions to the SM from the presence of additional symmetry groups [1,2]. Direct searches for the production of additional heavy gauge bosons have focused on the lepton final state of the W ′ boson decay which has good separation between the W ′ boson signal and the SM backgrounds. The W ′ boson lower mass DØ Collaboration / Physics Letters B 641 (2006) 423–431 427 limit in this decay channel is 786 GeV [3]. In these studies, the W ′ boson is allowed to have right-handed interactions with leptons and quarks, and it is assumed that the right-handed neutrino is lighter than the W ′ boson. It is also possible that such a W ′ boson does not interact with leptons and neutrinos but only with quarks. Searching in the quark decay channel avoids assumptions about the mass of a possible right-handed neutrino. Previous direct searches for W ′ bosons in the quark decay channel have excluded the mass range below 261 GeV [4] and between 300 and 420 GeV [5]. Assuming that the W ′ boson decays only to quarks and not to leptons yields a lower mass limit of 800 GeV [6]. A search has also been performed in the single top quark final state of the W ′-boson decay. Assuming the W ′ boson has only right-handed interactions and does not decay to leptons, the lower limit on the W ′ boson mass is 566 GeV [7]. The comprehensive search presented here includes all of these W ′ boson models. 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 (see Ref. [2] and references therein). The top quark was discovered in 1995 by the CDF and DØ Collaborations [8], but the production of single top quark has not yet been observed. Both collaborations have searched for single top quark production [9–13]. At the 95% confidence level, the upper limit measured by DØ on the s-channel process is 6.4 pb, and the limit measured by CDF is 13.6 pb. At the same confidence level, the limit on the t -channel production cross section is 5.0 pb from DØ and 10.1 pb from CDF. For comparison, the next-to-leading order (NLO) SM single top quark production cross sections are 0.88 pb in the s-channel and 1.98 pb in the t -channel [14]. The single top quark final state is especially sensitive to the presence of an additional heavy boson, owing to the decay chain W ′ → t b̄, where the top quark decays to a b quark and a SM W boson. This decay is kinematically allowed as long as the W ′ mass is larger than the sum of top and bottom quark masses, i.e. as long as it is above about 200 GeV. An additional heavy boson would appear as a peak in the invariant mass distribution of the t b̄ final state. Note that in this Letter, the notation t b̄ includes both final states W ′+ → t b̄ and W ′− → t̄b. The leading order Feynman diagram for W ′ boson production resulting in single top quark events is shown in Fig. 1. This diagram is identical to that for SM s-channel single top quark production where the SM W boson appears as the virtual particle [14–17]. The W ′ boson also has a t -channel exchange that leads to a single top quark final state. However, the cross section for a t -channel W ′ process is much smaller than the SM t -channel single top quark production due to the high mass of the W ′ boson. It will thus not be considered in this Letter. The SM W boson from the top quark decay then decays leptonically or hadronically. A heavy W ′ boson could also contribute to the top quark decay, but that contribution is negligible, again because of the large W ′ boson mass, and will not be considered here. We investigate three models of W ′ boson production. In each case, we set the CKM mixing matrix elements for the W ′ boson Fig. 1. Leading order Feynman diagram for single top quark production via a heavy W ′ boson. The top quark decays to a SM W boson and a b quark. Fig. 2. Histogram of the invariant mass of the top–bottom quark system at the parton level for different models of W ′ boson production. Shown are the SM s-channel distribution, the W ′ L → t b̄ boson distribution, including the interference with the SM contribution, and the W ′ R → t b̄ boson contribution, for a W ′ boson mass of 600 GeV. equal to the SM values. In the first model (W ′ L), we make the assumption that the coupling of the W ′ boson to SM fermions is identical to that of the SM W boson. Under these assumptions, there is interference between the SM s-channel single top quark process and the W ′ boson production process from Fig. 1. This interference term is small for large W ′ boson masses, but it becomes important in the invariant mass range of a few hundred GeV where the SM s-channel production cross section is largest. In our modeling of the W ′ boson production process, we take this interference into account. This is the first direct search for W ′ boson production to do so. In the second and third model (W ′ R), the W ′ boson has only right-handed interactions, hence there is no interference with the SM W boson. In the second model, the W ′ R boson is allowed to decay both to leptons and quarks, whereas in the third model it is only allowed to decay to quarks. The main difference between these two models is in the production cross section and the branching fraction to quarks, and we use the same simulated event sample for both models. Fig. 2 compares the invariant mass distribution for the W ′ models with left-handed coupling (including interference) and right-handed coupling (no interference) with the SM s-channel 428 DØ Collaboration / Physics Letters B 641 (2006) 423–431 Table 1 Production cross section at NLO for a W ′ boson × branching fraction to t b̄, for three different W ′ boson models. The production cross sections for W ′ L boson interactions also include the SM s-channel contribution as well as the interference term between the two. They have been computed at leading order and scaled to NLO according to Ref. [2]. The cross sections for W ′ R boson interactions differ depending on which decays of the W ′ boson are allowed W ′ mass [GeV] Cross section ×B(W ′ → t b̄) [pb] SM +W ′ L W ′ R (→ l or q) W ′ R (→ q only)