ISSN:
1749-6632
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Natural Sciences in General
Notes:
The Standard Model (SM) gives no clue as to the number of particle generations. Since three generations are known to exist, one may wonder whether the fourth is next to be discovered. One may seek the fourth generation directly by means of new fermion production at accelerators, or indirectly through radiative corrections to tree-level processes. The virtual existence of the new fermions in loop amplitudes contribute to the corrections. The direct method has resulted in lower bounds of 60 to 70 GeV for the top quark,1 40 GeV for the fourth charged lepton2L4, and 34 GeV for the fourth bottom quark3b4. Each of these bounds assumes a significant branching fraction into a final state with a charged lepton. The loop correction method4 has resulted in an upper bound of 200 GeV for the top quark, and in fact an upper bound of 200 GeV (350 GeV) for the mass splitting of any quark (lepton) weak isodoublet. The loop correction method has the advantage that it is sensitive to virtual fermions, and so may probe the existence of new fermions with masses exceeding present accelerator energies. We will call fermions with mass exceeding the W mass “ultraheavy.”
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1749-6632.1989.tb50619.x
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