Thursday, April 25, 2019

Four interesting papers

On hep-ph, I choose a paper by Benedetti, Li, Maxin, Nanopoulos about a natural D-braneworld (supersymmetric) inspired model. The word "inspired" means that they believe that similar models (effective QFTs) arise from a precise string theory analysis of some compactifications but as phenomenologists, they don't want to do the string stuff precisely. ;-) It belongs to these authors' favorite flipped \(SU(5)\) or \({\mathcal F}\)-\(SU(5)\) class of models and involves new fields, flippons, near \(1\TeV\). The LSP is Higgsino-like and the viable parameter space is rather compact and nice. The model seems natural – one may get fine-tuning below \(\Delta_{EW}\lt 100\).

It's an example showing that with some cleverness, natural models are still viable – and Nature is almost certainly more clever than the humans.

On hep-th, I start with Shirai-Yamazaki who point out some interesting tension between two types of ideas both of which arise from Cumrun Vafa's swampland program. One of them is a scalar-force generalization of our weak gravity conjecture; the other is a more recent Vafa-et-al. de Sitter swampland conjecture.

The tension has a simple origin: the (scalar-force-type) weak gravity conjectures tend to prohibit light scalars like quintessence because those would yield "weaker-than-gravity forces mediated by scalars". On the other hand, as you know, the Swampland de Sitter conjecture wants to outlaw inflation and replace it with quintessence, so it needs quintessence. Whether there is a real contradiction depends on the precise formulation of both conjectures, especially the scalar-force weak gravity conjecture, they conclude.

It's very interesting. We're apparently still not certain about the precise form of these inequality-like principles. There are contexts in which I am even uncertain about the direction of the inequalities. So the vagueness may be as bad as saying that "something new happens when some ratio of parameters approaches some value from either side".

Finally, a cool research of the scattering theory involving new or understudied asymptotic states. There are two new hep-th papers, by Pate, Raclariu, and Strominger; and by Nandan, Schreiber, Volovich, Zlotnikov. They have some overlap – the authors know each other (Volovich was a student of Strominger's; yes, Strominger's current two co-authors are nice young ladies, too) so they probably knew about their shared interests in advance.

Normally, we study scattering with asymptotic states which are particles with nicely well-defined momentum vectors \(p^\mu\). Those are eigenstates under spacetime translations. However, they study "celestial" scattering amplitudes of asymptotic states that are eigenstates of the boosts, basically of the Lorentz group \(SO(3,1)\). These asymptotic states should be eigenstates \((h,\bar h)\) of the \(SL(2)\times SL(2)\) complexified version of the Lorentz algebra. OK, you want eigenstates of \(j_L^2,j_R^2,j_{L,3},j_{R,3}\) in the complexified \(SL(2,\CC)\times SL(2,\CC)\).

Either one or two papers study new kind of soft theorems – designed for these "celestial" states instead of the momentum eigenstates. Pate et al. say these new theorems cannot be derived from low-energy effective action. It seems rather incredible or extraordinary because they seem to work just with another "differently singular" basis of the states – all their laws should be just some rearrangement of the usual ones. OK, but we are told that they are not. These new claims about soft theorems etc. spiritually agree with Strominger-and-collaborators' recent claims that the information about the infalling matter may be stored in rather "classical" properties (or hair) of the black hole, and similar stuff.

Nandan et al. analyze the "celestial" scattering and try to reproduce the counterparts of the insights we normally do with the momentum-based scattering: new, "celestial" partial wave decomposition, crossing symmetry, and optical theorem. They also study some soft limits.

The momentum scattering states are the gold standard and I believe they will remain dominant in the literature in coming years. But these "celestial" states and symmetries – and all the laws in the new bases – are arguably "comparably" fundamental. To focus on the momentum eigenstates and not the "celestial", Lorentz group eigenstates means to be biased in a certain way and the authors of both of these papers are trying to remove the bias from the literature and fill the holes.

The claim that these new states – with some singular support near the light-cone or high-energy particles – allow us to derive something that was previously unknown is extremely provocative or exciting. It should be possible according to some intuition, like mine, but the intuition isn't backed by any real evidence. Similar patterns obtained from the "celestial" viewpoint could be relevant for deep questions in physics as well, including the clarification of the twistor/amplituhedron forms of some amplitudes, the information loss problem, and naturalness, I vaguely think.

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