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Extended hypercolor theories have been plagued with inherent flavor-changing effects occurring above experimental levels. The problem may be alleviated in a scenario in which hypercolor interactions are not asymptotically free. Then the scale of broken gauged family symmetry may be higher than before. The masses of the pseudo-Goldstone bosons of hypercolor may also be increased.

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Abstract

This summary of the Workshop "Beyond the 3-generation SM in the LHC era" presents a brief discussion of the following four statements about the fourth generation: 1) It is not excluded by EW precision data; 2) It addresses some of the currently open questions; 3) It can accommodate emerging possible hints of new physics; 4) LHC has the potential to discover or fully exclude it.

PACS codes: 12.60.-i; 14.60. Hi; 14.80.-j

Holdom Replies: The derivation of p functions requires the consideration of all relevant operators. In the continuum limit of quenched QED, four-fermion interactions are relevant operators. Their role in the ladder approximation has been recently discussed by Bardeen, Leung, and Love (see section 4 of their paper 1 ). It is found that physical quantities besides the dynamical mass m may be held fixed as the ultraviolet cutoff A varies if the fourfermion coupling is allowed to run. Their result is that a continuum limit defined by fixing both m and the renormalized fermion condensate requires a running fourfermion coupling and a nonrunning gauge coupling. This supports my observations regarding the dependence of physical quantities on the gauge coupling. The Miransky procedure of ignoring the relevant four-fermion interaction and adjusting the gauge coupling to keep m fixed as A varies is not likely to represent a true renormalization-group flow. 2 On the other hand, the Miransky procedure does give a sequence of cutoff theories which approaches the continuum theory in the limit. This is a useful procedure and it is basically adopted by the recent Monte Carlo calculations. The relation between m/A and the gauge coupling in a cutoff theory is also of interest in technicolor models.

We study the dynamics of branes in configurations where ͑1͒ the brane is the edge of a single anti-de Sitter ͑AdS͒ space and ͑2͒ the brane is the surface of a vacuum bubble expanding into a Schwarzschild or AdS-Schwarzschild bulk. In both cases we find solutions that resemble the standard Robertson-Walker cosmologies, although, in the latter, the evolution can be controlled by a mass parameter in the bulk metric. We also include a term in the brane action for the scalar curvature. This term adds a contribution to the low-energy theory of gravity which does not need to affect the cosmology, but which is necessary for the surface of the vacuum bubble to recover four-dimensional gravity.

There has been a striking realization that physics resolving the black hole information paradox could imply postmerger gravitational wave echoes. We here report on evidence for echoes from the LIGO compact binary merger events, GW151226, GW170104, GW170608, GW170814, as well as the neutron star merger GW170817. There is a signal for each event with a p-value of order 1% or sometimes significantly less. Our study begins with the comparison of echoes from a variety of horizonless exotic compact objects. Next we investigate the effects of spin. The identification of the more generic features of echoes then leads to the development of relatively simple windowing methods, in both time and frequency space, to extract a signal from noise. The time delay between echoes is inversely related to the spacing between the spectral resonances, and it is advantageous to look directly for this resonance structure. We find time delays for the first four events that are consistent with a simple model that accounts for mass and spin of the final object, while for the neutron star merger the final mass and spin are constrained.