String theory in target spaceThey are effectively generalizing the Britto-Cachazo-Feng-Witten (BCFW) techniques from the case of quantum field theory (gauge theory) to the case of perturbative string theory.
Slightly off-topic. Congratulations to Joe Polchinski who will be 60 in the spring but who already has the Joefest now; see Clifford Johnson and Matt Strassler. If you need a higer-res version of the picture, let me know.
One may say – and they probably say – that they're returning to the mode of reasoning that existed shortly after the birth of string theory (the publication of the Veneziano amplitude) before it was shown that Veneziano's amplitude followed from a theory of strings.
Once this "constructive" derivation of the amplitudes was found by the co-fathers of string theory such as Nambu, Susskind, and Nielsen, string theorists largely abandoned the string theory's roots in the "S-matrix program". This program – whose original emergence traces back to Werner Heisenberg – became unfashionable once both the QCD and string theory's amplitudes were derived from a very particular set of degrees of freedom (quarks and gluons; strings) and a particular Lagrangian.
Against the spirit of the S-matrix program and the bootstrap paradigm, the internal consistency ceased to be "fundamental"; it could have been derived from the defining conditions.
However, these two researchers in Germany show that many still unknown or overlooked "not constructive" or "S-matrix-consistency-related" insights about string theory may be waiting – a whole machinery to reinterpret all the calculations in which strings and their 2D world sheets don't look central at all. If true, these are the stringy counterparts of the S-matrix flavor that was revived by the recent twistor minirevolution in the context of gauge theories.
Their main technical foci of interest are roots (zeroes) of the scattering amplitudes, effectively coming from the poles of the Gamma function in the amplitudes' denominators. Note that normally we only know where the poles of the amplitudes, i.e. poles of the Gamma functions in the numerator, come from. They come from virtual particles in the processes.
But they use some Plahte's 1970 insights about the monodromies of the amplitudes under the permutations of the external particles, see e.g. a 2010 paper by Boels et al., to write down the amplitudes differently.
They show the unitarity of the S-matrix in a new light. Unitarity relates the loop amplitudes to the tree-level amplitudes, among other things. It's possible to prove it (and the no-ghost theorem about the positivity of residues etc.) using the world sheet construction – a strategy of the proof that isn't really useful for a direct, alternative calculation of the loop diagrams. But they effectively achieve the same outcome without any world sheets, using BCFW-like tricks extended from field theory to string theory. And they reduce the proofs of some mathematical claims about the positivity of some sums that don't seem to be "manifestly stringy" in any sense.
There are many recursion relations that one may write down for the stringy scattering amplitudes. They're not quite independent and they argue that the old-fashioned string duality (currently known as the "world-sheet duality") relates these different recursion relations with each other. Closed string amplitudes are mostly obtained by the KLT (closed equals open squared) relations in this paper.
I don't quite understand how it works so far but I do believe that it may be a good idea for experts among the TRF readers to spend some time with this preprint and its references. It seems to me that in the future, people will understand the power of internal consistency much more intimately than today. They will know that whole S-matrices or their subsets (by the number of external particles or loops or other labels) follow from consistency and a few assumptions. Ideally, sometime in the future, people should be able to rigorously, mathematically prove that a "consistent theory of quantum gravity" and "string/M-theory" are really two descriptions of the same thing. I view S-matrix-program-based advances such as this paper to be steps in this direction.
eXciting dark matter
Just two more sentences. Dark matter top experts Douglas Finkbeiner and Neal Weiner released An X-Ray Line from eXciting Dark Matter where they try to explain the 3.5 keV line not by a 7 keV sterile neutrino but by a decay \(\chi^*\to \chi+\gamma\) between two nearly degenerate states \(\chi,\chi^*\) of the dark matter, while the decay is mediated by the field \(\phi\).
The particle may also be a 7-keV string-theoretical modulus, i.e. scalar field describing the shape of extra dimensions etc., according to a paper released tomorrow.
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