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We look at simple BPS systems involving more than one field. We discuss the conditions that have to be imposed on various terms in Lagrangians involving many fields to produce BPS systems and then look in more detail at the simplest of such cases. We analyse in detail BPS systems involving 2 interacting Sine-Gordon like fields, both when one of them has a kink solution and the second one either a kink or an antikink solution. We take their solitonic static solutions and use them as initial conditions for their evolution in Lorentz covaria
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We discuss various aspects of models with long-lived or stable colored particles. In particular we focus on an ideal Quirk model with electroweak neutral heavy (O(TeV)) particles which carry ordinary color and another $ SU(3)$ color with a very low scale $Lambda$. We show that contrary to what one might think, such a model is cosmologically consistent and evades many Pitfalls even for very low O(10 eV) $Lambda$ and without assuming a low reheat temperature. We also show that the expected production of Quirks by cosmic rays which are incorporated in heavy Isotopes in Ocean water is consistent with the highly stringent bounds on the latter. This evades a real threat to the Quirk model which would have excluded it regardless of Cosmology. Finally we briefly comment on possible LHC signatures.
We give a simple discussion of ghosts, unitarity violation, negative norm states and quantum vs classical behavior in the simplest model with four derivative action - the Pais-Uhlenbeck oscillator. We also point out that the normalizable vacuum state (in the sense defined below) of this model can be understood as spontaneous breaking of the emergent conformal symmetry. We provide an example of an interacting system that couples the particle and ghost degrees of freedom and nevertheless remains unitary on both classical and quantum level.
We discuss aspects of magnetically charged black holes in the Standard Model. For a range of charges, we argue that the electroweak symmetry is restored in the near horizon region. The extent of this phase can be macroscopic. If $Q$ is the integer magnetic charge, the fermions lead to order $Q$ massless two dimensional fermions moving along the magnetic field lines. These greatly enhance Hawking radiation effects.
We revisit the idea that the quantum dynamics of open strings ending on $N$ D3-branes in the large $N$ limit can be described at large `t Hooft coupling by classical closed string theory in the background created by the D3-branes in asymptotically flat spacetime. We study the resulting thermodynamics and compute the Hagedorn temperature and other properties of the D3-brane worldvolume theory in this regime. We also consider the theory in which the D3-branes are replaced by negative branes and show that its thermodynamics is well behaved. We comment on the idea that this theory can be thought of as an irrelevant deformation of $mathcal{N}=4$ SYM, and on its relation to $Tbar T$ deformed $CFT_2$.
In AdS/CFT partition functions of decoupled copies of the CFT factorize. In bulk computations of such quantities contributions from spacetime wormholes which link separate asymptotic boundaries threaten to spoil this property, leading to a factorization puzzle. Certain simple models like JT gravity have wormholes, but bulk computations in them correspond to averages over an ensemble of boundary systems. These averages need not factorize. We can formulate a toy version of the factorization puzzle in such models by focusing on a specific member of the ensemble where partition functions will again factorize. As Coleman and Giddings-Strominger pointed out in the 1980s, fixed members of ensembles are described in the bulk by $alpha$-states in a many-universe Hilbert space. In this paper we analyze in detail the bulk mechanism for factorization in such $alpha$-states in the topological model introduced by Marolf and Maxfield (the MM model) and in JT gravity. In these models geometric calculations in $alpha$ states are poorly controlled. We circumvent this complication by working in $textit{approximate}$ $alpha$ states where bulk calculations just involve the simplest topologies: disks and cylinders. One of our main results is an effective description of the factorization mechanism. In this effective description the many-universe contributions from the full $alpha$ state are replaced by a small number of effective boundaries. Our motivation in constructing this effective description, and more generally in studying these simple ensemble models, is that the lessons learned might have wider applicability. In fact the effective description lines up with a recent discussion of the SYK model with fixed couplings arXiv:2103.16754. We conclude with some discussion about the possible applicability of this effective model in more general contexts.
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