Higgs radiation off top-antitop pairs at future Linear Colliders: a background study

The process ee → Htt̄ can be exploited at future Linear Colliders (LCs) [1]–[2] to measure the Higgs-top Yukawa coupling. In this note, we estimate the size of the irreducible backgrounds in the channel Htt̄ → bb̄bb̄WW → bb̄bb̄l±νl qq̄, for the case of a Standard Model (SM) Higgs boson with mass between 100 and 140 GeV. 1. Interplay of signal and backgrounds Higgs-strahlung off top-antitop pairs [3] is the most promising channel to measure the Yukawa coupling of the top quark to a Higgs boson [4]. In the SM, the production rate of this final state at future LCs is sizable, if the energy of the latter is of the order √ s >∼ 2mt +MH . Furthermore, the one-loop QCD corrections to e e → Htt̄ are under control [5]. Thus, in the context of simulation studies, the next thing to be assessed is the viability of the signal above the backgrounds. The current experimental limit from LEP2 on MH is about 95 GeV [6]. In addition, electroweak fits to precision data prefer a rather light Higgs boson, say, below 150 GeV within the SM [7]. Thus, a LC with centre-of-mass (CM) energy √ s = 500 GeV (the default of our numerical studies) represents an ideal laboratory to study Htt̄ final states (as mt = 175 GeV). In the mass range 90 GeV < ∼ MH < ∼ 150 GeV, the dominant Higgs decay mode is H → bb̄, this being overtaken by the off-shell decay into two W’s, i.e., H → WW, only forMH >∼ 140 GeV [8]. Besides, the experimentally preferred channel in searching for tt̄ events is the semi-leptonic channel, i.e., tt̄ → bb̄WW → bb̄l±νl qq̄, where l and ν represent a lepton at high transverse momentum (used for triggering purposes) and its companion neutrino and qq̄ refers to all possible combinations of light quarks. These are the signatures of Higgs and top-antitop pairs that we will concentrate on here. A study of ‘reducible’ backgrounds in the above decay channels was preliminarly presented in [9]: there, they were found to be under control after appropriate selection cuts were implemented. In this note we consider the effects of the dominant ‘irreducible’ backgrounds, i.e., those of the type [10]–[12]: ee → Htt̄ → Hbb̄WW → bb̄bb̄l±νlqq̄′, (1) Talk given at the 2nd ECFA/DESY Study on Physics and Detectors for a Linear Electron-Positron Collider, Oxford, UK, 20-23 March 1999. ee → Ztt̄ → Zbb̄WW → bb̄bb̄l±νlqq̄′, (2) ee → gtt̄ → gbb̄WW → bb̄bb̄l±νlqq̄′, (3) whose corresponding Feynman graphs can be found in Figs. 1–3 of Ref. [13] (the latter to be further supplemented with the H,Z and g decay currents) as well as those of all other subleading reactions at the same perturbative orders, ee → bb̄WW → bb̄bb̄l±νlqq̄′, (4) ee → Zbb̄WW → bb̄bb̄l±νlqq̄′, (5) ee → gbb̄WW → bb̄bb̄l±νlqq̄′. (6) In total, one has to consider at tree-level 350 Feynman graphs for process (4), 546 for reaction (5) and 152 for case (6), each number including the graphs associated to (1)– (3). Their computation has been accomplished by using helicity amplitude techniques to evaluate the complete 2 → 8 body reactions, without any factorisation of production and decay processes [13]. 2. Cross sections If one does assume the mentioned Higgs and top-antitop decay modes, then signal events can be searched for in data samples made up by four b quark jets, two light quark jets, a lepton and a neutrino. In other terms, a ‘4b+2 jets + l±+Emiss’ signal, with l = e, μ, τ . Fig. 1 presents the production cross sections for the leading subprocesses (1)–(3) as well as those for the complete ones (4)–(6). From there, two aspects emerge. On the one hand, the QCD processes are always dominant, whereas the interplay between the other two depends upon the Higgs mass value. On the other hand, the bulk of the cross sections of processes (4)–(6) comes from (1)–(3) with the only exception of the QCD case. The increase of the QCD rates seen in the figure is mainly due to the large amount of gluon radiation off b (anti)quarks produced in ‘radiative’ tt̄ decays [12]: i.e., via tt̄ → bb̄WW → gbb̄WW, with the gluon eventually yielding bb̄ pairs: see Fig. 2. However, it is instructive to further explore the background effects in the case of Hbb̄WW events. This is done in Fig. 3, where we have distinguished between subprocess (1) and four sizable components of (4), i.e., the cases proceeding via intermediate states of the following forms: (4a)Htb̄W+ c.c., (4b) HZWW, (4c) HγWW, (4d) HHWW, (4e) all remaining interferences. A notable aspect in Fig. 3 is the taking over of the ‘single-top’ production rates (4a) respect to the ‘double-top’ ones (1) for large Higgs masses, because of the strong phase space suppression on the latter when MH approaches √ s − 2mt. Besides, (4a) events carry the same Yukawa dependence as (1), see Fig. 4, thus they should rather be regarded as an additional contribution to the top-Higgs Yukawa signal, these two mechanisms being somehow complementary with respect to the MH dependence. We include τ ’s to enhance the signal rate, assuming that they are distinguishable from quark jets.