\;Based on: [arXiv:1606.05636]

\; The CHY formulae are a set of remark able formulae describing the scattering amplitudes of a variety of massless theories\, as worldsheet integrals localized on the solutions to certain p olynomial equations (scattering equations). These formulae arise from a new class of holomorphic str ing theories called Ambitwistor strings that encod e exactly the classical dynamics of the supergravi ty (Yang-Mills) modes of string theory.

&nb sp\;Despite some recent progress by W. Siegel and collaborators\, it remained as an open question as to what extent this theory was connected to the f ull string theory. The most mysterious point being certainly that the localization equations of the ambitwistor string also appear in the zero tension limit of string theory (alpha&rsquo\; to infinity )\, which is the opposite limit than the supergrav ity one (alpha&rsquo\; to zero).

\;In this talk\, I&rsquo\;ll report on some work in col laboration with E. Casali and argue that the ambit wistor string is actually a tensionless string. Us ing some forgotten results on the quantization of these objects\, we explain that the quantization o f tensionless strings is ambiguous\, and can lead either to a ``higher-spin like'\;'\; theory\ , or to the ambitwistor string\, hence clarifying the previously mentioned paradox. In passing\, we see that the degenerations of the tensile worldshe et that lead to tensionless strings make connectio n with Galilean Conformal Algebras and the (3d) BM S algebra.

LOCATION:Seminar Room 1\, Newton Institute CONTACT:info@newton.ac.uk END:VEVENT END:VCALENDAR