Dynamical Electroweak Symmetry Breaking

DYNAMICAL ELECTROWEAK SYMMETRY BREAKING AND THE TOP QUARK R. Sekhar Chivukula DepartmentofPhysics, Boston University 590 Commonwealth Ave., Boston MA 02215 E-mail: [email protected] Talk presented at the SLACTopical Workshop Stanford, July 19-21, 1995 BUHEP-95-23 & hep-ph/9509384 ABSTRACT In this talk, I discuss theories of dynamical electroweak symmetry breaking, with emphasis on the implications of a heavy top-quark on the weak-interaction parameter. 1 AN ABBREVIATED VERSION OF THIS TALK WAS PRESENTED AT THE WORKSHOP ON TOP QUARK PHYSICS,IOWASTATE UNIVERSITY, AMES, IA, MAY 25-26, 1995 AND THE YUKAWA IN- TERNATIONAL SEMINAR `95, YUKAWA INSTITUTE, KYOTO, AUG. 21-25, 1995. 1 What's Wrong with the Standard Mo del? In the standard one-doublet Higgs mo del, one intro duces a fundamental scalar doublet of SU 2 : W ! + ; 1.1 = 0 which has a p otential of the form ! 2 2 v y V = : 1.2 2 2 In the p otential, v is assumed to b e p ositive in order to favor the generation of a nonzero vacuum exp ectation value for . This vacuum exp ectation value breaks the electroweak symmetry, giving mass to the W and Z . This explanation of electroweak symmetry breaking is unsatisfactory for a numb er of reasons. For one thing, this mo del do es not give a dynamical explanation of electroweak symmetry breaking. For another, when emb edded in theories with additional dynamics at higher energy 2 scales, these theories are technically unnatural. Perhaps most unsatisfactory,however, is that theories of fundamental scalars are probably 3 \trivial," i.e., it is not p ossible to construct an interacting theory of scalars in four dimensions that is valid to arbitrarily short-distance scales. In quantum eld theories, uctuations in the vacuum screen charge|the vacuum acts as a dielectric medium. Therefore, there is an e ective coupling constant which dep ends on the energy scale at which it is measured. The variation of the coupling with scale is summarized by the {function of the theory d = : 1.3 d The only coupling in the Higgs sector of the Standard Mo del is the Higgs self-coupling .In p erturbation theory, the -function is calculated to b e 2 3 ! = : 1.4 2 2 Using this {function, one can compute the b ehavior of the coupling constant as a function of the scale. One nds that the coupling at a scale is related to the coupling at some higher Since these expressions were computed in p erturbation theory, they are only valid when is suciently 4,5 small. For large couplings, wemust rely on nonp erturbative lattice Monte Carlo studies, which show b ehavior similar to that implied by the p erturbative expressions derived here. scale by 1 3 1 = + log : 1.5 2 2 In order for the Higgs p otential to b e stable, has to b e p ositive. This implies 1 3 log : 1.6 2 2 Thus, wehave the b ound 2 2 : 1.7 3 log If this theory is to make sense to arbitrarily short distances, and hence arbitrarily high energies, we should taketo1 while holding xed at ab out 1 TeV. In this limit, we see that the b ound on go es to zero. In the continuum limit, this theory is trivial; it is free eld theory. The theory of a relatively lightweakly coupled Higgs b oson can b e self-consistent toavery high energy.For example, if the theory is to make sense up to a typical GUT scale energy, 16 6 10 GeV, then the Higgs b oson mass has to b e less than ab out 170 GeV. In this sense, although a theory with a light Higgs b oson do es not really answer any of the interesting questions e.g., it do es not explain why SU 2 U 1 breaking o ccurs, the theory do es manage to p ostp one W Y the issue up to higher energies. 2 Dynamical Electroweak Symmetry Breaking 2.1 Technicolor 7 Technicolor theories strive to explain electroweak symmetry breaking in terms of physics op er- ating at an energy scale of order a TeV. In technicolor theories, electroweak symmetry breaking is the result of chiral symmetry breaking in an asymptotically-free, strongly-interacting gauge theory with massless fermions. Unlike theories with fundamental scalars, these theories are technically natural: just as the scale arises in QCD by dimensional transmutation, so QC D to o do es the weak scale v in technicolor theories. Accordingly, it can b e exp onentially smaller than the GUT or Planck scales. Furthermore, asymptotically-free non-Ab elian gauge theories may b e fully consistent quantum eld theories. 7 In the simplest theory, one intro duces the doublet of new massless fermions 1 0 U A @ U ;D 2.8 = R R L D L which are N 's of a technicolor gauge group SU N . In the absence of electroweak interac- TC tions, the Lagrangian for this theory may b e written L = U iDU/ + U iDU/ + 2.9 L L R R D iDD/ + D iDD/ 2.10 L L R R and thus has an SU 2 SU 2 chiral symmetry. In analogy with QCD, we exp ect that L R when technicolor b ecomes strong, hU U i = hD D i6=0; 2.11 L R L R which breaks the global chiral symmetry group down to SU 2 , the vector subgroup anal- L+R ogous to isospin in QCD. If weweakly gauge SU 2 U 1, with the left-handed technifermions forming a weak doublet, and identify hyp ercharge with a symmetry generated by a linear combination of the T in SU 2 and technifermion numb er, then chiral symmetry breaking will result in the 3 R electroweak gauge group's breaking down to electromagnetism. The Higgs mechanism then pro duces the appropriate masses for the W and Z b osons if the F -constant of the technicolor theory the analog of f in QCD is approximately 246 GeV. The residual SU 2 symmetry L+R ensures that, to lowest-order, M = M cos and the weak interaction -parameter equals W Z W 8 one at tree-level. 2.2 Top-Mo de and Strong-ETC Mo dels There is also a class of theories in which the scale M of the dynamics resp onsible for all or part of electroweak symmetry breaking can, in principle, takeanyvalue of order a TeV or greater. 9 These mo dels, inspired by the Nambu-Jona-Lasinio NJL mo del of chiral symmetry breaking in QCD, involve a strong, but spontaneously broken, noncon ning gauge interaction. Examples 10{14 include top quark condensate and related mo dels, as well as mo dels with strong extended 15 technicolor interactions. When the strength of the e ective four-fermion interaction describing the broken gauge interactions|i.e. the strength of the extended technicolor interactions in strong ETC mo dels or the strength of other gauge interactions in top-condensate mo dels|is adjusted close to the critical value for chiral symmetry breaking, the high-energy dynamics may play a role in electroweak symmetry breaking without driving the electroweak scale to a value of order M . The high-energy dynamics must have the appropriate prop erties in order for it to play a role 16 in electroweak symmetry breaking: If the coupling constants of the high-energy theory are small, only low-energy dynamics such as technicolor can contribute to electroweak symmetry breaking. If the coupling constants of the high-energy theory are large and the interactions are attractive in the appropriate channels, chiral symmetry will b e broken by the high-energy interactions and the scale of electroweak symmetry breaking will b e of order M . If the transition between these two extremes is continuous, i.e., if the chiral symmetry breaking phase transition is second order in the high-energy couplings, then it is p ossible to adjust the high-energy parameters so that the dynamics at scale M can contribute to electroweak symmetry breaking. The adjustment of the high-energy couplings is a re ection of the ne-tuning required to create a hierarchy of scales. What is crucial is that the transition b e at least approximately second order in the high- energy couplings. If the transition is rst order, then as one adjusts the high-energy couplings, the scale of chiral symmetry breaking will jump discontinuously from approximately zero at weak coupling to approximately M at strong coupling. Therefore, if the transition is rst order, it will generally not b e p ossible to maintain any hierarchybetween the scale of electroweak symmetry breaking and the scale of the high-energy dynamics. If the transition is second order and if there is a large hierarchy of scales M 1TeV, then close to the transition the theory may b e describ ed in terms of a low-energy e ective Lagrangian with comp osite \Higgs" scalars|the Ginsburg-Landau theory of the chiral phase transition. However, if there is a large hierarchy, the arguments of triviality given in the rst section apply to the e ectivelow-energy Ginsburg-Landau theory describing the comp osite scalars: the e ectivelow-energy theory would b e one which describ es a weakly coupled theory of almost fundamental scalars, despite the fact that the \fundamental" interactions are strongly self-coupled! 3 m in Mo dels of Dynamical EWSB t In technicolor mo dels, the masses of the ordinary fermions are due to their coupling to the technifermions, whose chiral-symmetry breaking is resp onsible for electroweak symmetry breaking.

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