Contribution of heavy fermions to the mechanism of Higgs boson production
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EMENTARY PARTICLES AND FIELDS Theory
Contribution of Heavy Fermions to the Mechanism of Higgs Boson Production I. T. Dyatlov* Petersburg Nuclear Physics Institute, National Research Center Kurchatov Institute, Gatchina, 188300 Russia Received October 20, 2016
Abstract—Heavy quarks of mass much greater than the Higgs boson (H) mass have virtually no effect on the cross section for Higgs boson production at the Large Hadron Collider (LHC). A mechanism that is responsible for the appearance of the respective small factors is presented, and their meaning is clarified. It appears that they precisely correspond to a simple quantum-mechanical picture of the H-production process. DOI: 10.1134/S1063778817030061
The production of a light Higgs boson H (whose mass mH is less than the t-quark mass mt ) in proton–proton (pp) collisions at the Large Hadron Collider (LHC) [1] is determined largely by the contribution to the respective amplitude from the Standard Model (SM) Feynman diagrams featuring a transition of two gluon jets to a quark triangle (see Fig. 1). This mechanism was considered in numerous studies, and some of them were surveyed in [2]. A dominant contribution, which is quite sufficient for matching, comes from one t quark. For high quark masses such that M mH , the triangle in question is assumed to be independent of the heavy mass M [3]. Therefore, the rate of H production via this mechanism would approximately be in direct proportion to the square of the number of quarks whose masses satisfy the condition M mH . In the well-known monograph by Okun [3], the respective process is referred to as a heavy-quark counter. Indeed, the contribution of the diagram in Fig. 1 at high values of the mass M can roughly be estimated in terms of the integral M {q, k}M d4 p , (1) − η (M 2 − p2 )3
in one of the fermion-loop propagators in Fig. 1. Obviously, expression (1) is independent of M in the limit of M → ∞. Reasonable agreement between the experimental cross sections from [1] and the SM contribution of one t quark (with allowance for QCD corrections [2]) indicates that there is as if no heavier quarks in nature. On the other hand, it is stated in [4, 5] that some observed qualitative properties of the Standard Model can be explained by the involvement of very heavy (M mH , mt ) mirror fermions [6] in their formation. The properties in question are those that one includes in the Standard Model phenomenologically and whose theoretical treatment may be ambiguous. This concerns the hierarchy of the elements of weakmixing matrix for quarks, this hierarchy being associated with the hierarchy of the masses of their generations; the mixing matrix of leptons that has a different structure as if independent of their mass spectrum; the extreme smallness of the neutrino masses; and the nature and character of the neutrino spectrum. It seems that heavy mirror quarks lead to overly large contributions to H-production processes, and this is at odds with conventional SM estimates. The
¯ where h = M/η is the hψψH Yukawa coupling const
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