B-meson mixing bounds on technicolor theories

Recent work by Burdman, Lane, and Rador has shown that BB̄ mixing places stringent lower bounds on the masses of topgluons and Z ′ bosons in classic topcolorassisted technicolor (TC2) models. This paper finds analogous limits on the Z ′ bosons of flavor-universal TC2 and non-commuting extended technicolor models, and compares the limits with those from precision electroweak measurements. A discussion of the flavor structure of these models (contrasted with that of classic TC2) shows that Bmeson mixing is a less reliable probe of these models than of classic TC2. ∗e-mail address: [email protected] The flavor problem remains a challenge for dynamical models of mass generation. While technicolor [1] can provide appropriate masses for the electroweak gauge bosons, explaining the masses and mixings of the quarks and leptons is more difficult. Extended technicolor models [2] postulate an enlarged gauge group coupling the quarks and leptons to the technifermion condensate, enabling them to acquire mass. However, the simplest models of this type tend to produce large flavor-changing neutral currents. Given the large value of the top quark’s mass and the sizable splitting between the masses of the top and bottom quarks, it is natural to wonder whether mt has a different origin than the masses of the other quarks and leptons. A variety of dynamical models that exploit this idea have been proposed, including topcolor-assisted technicolor (TC2)[3], flavor-universal TC2 [4], and non-commuting extended technicolor (NCETC) [5]. A common feature of these models is that they extend one or more of the standard model SU(N) gauge groups to an SU(N) × SU(N) structure at energies well above the weak scale. The TC2 models have extended color and hypercharge groups; NCETC has an extended weak gauge group. Spontaneous breaking of the extended groups to their diagonal (standard model) subgroups produces extra massive gauge bosons: a color-octet of topgluons plus a Z ′ boson in TC2, a color-octet of colorons and a Z ′ boson in flavor-universal TC2, and a trio of of W ′ and Z ′ bosons in NCETC. In order to use their enlarged gauge groups to help explain the heavy top quark mass, all of these models propose that some of the gauge groups should be flavor non-universal, treating the third generation fermions differently than those in the first and second generations. Accordingly, the topgluons and Z ′ of classic TC2, the Z ′ of flavor-universal TC2, and the extra weak bosons of NCETC all have flavor non-universal couplings – meaning that these gauge bosons can cause tree-level flavor-changing neutral currents. Recently, Burdman et al. [6] pointed out that BdB̄d mixing provides strong lower bounds on the masses of the topgluon and Z ′ bosons of classic TC2 models. They briefly mentioned that their work should constrain flavor-universal TC2 models, but did not present specific results for those models. This paper explicitly extends the analysis to study both flavoruniversal TC2 and non-commuting ETC. We find that the limits on the gauge bosons of these models are numerically much weaker than those for classic TC2 models and are comparable to limits from precision electroweak observables. In addition, we show that the flavor structure of these models differs from that of classic TC2 in ways that render limits from B-meson mixing less generally applicable. As discussed in ref. [6], the dominant contributions of the topgluon and Z ′ bosons in TC2 models to the B LB 0 S mass difference is given [7] by ∆MBd = 2|M12|TC2 2(M12)TC2 = 4π 3 ηBMBdf 2 Bd BBd(D ∗ LbbDLbd) 2RTC2 (1)