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Mitra, Prahar

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Mitra, Prahar

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2014 - 20152

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Author's Publications: search.filters.isAuthorOfPublication.80f42ef8-1939-457b-b6a2-341b62879a06

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    BMS supertranslations and Weinberg’s soft graviton theorem
    (Springer Science + Business Media, 2015) He, Temple Mu; Lysov, Vyacheslav; Mitra, Prahar; Strominger, Andrew
    Recently it was conjectured that a certain infinite-dimensional “diagonal” subgroup of BMS supertranslations acting on past and future null infinity (I − and I +) is an exact symmetry of the quantum gravity S-matrix, and an associated Ward identity was derived. In this paper we show that this supertranslation Ward identity is precisely equivalent to Weinberg’s soft graviton theorem. Along the way we construct the canonical generators of supertranslations at I ±, including the relevant soft graviton contributions. Boundary conditions at the past and future of I ± and a correspondingly modified Dirac bracket are required. The soft gravitons enter as boundary modes and are manifestly the Goldstone bosons of spontaneously broken supertranslation invariance.
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    New symmetries of massless QED
    (Springer Science + Business Media, 2014) He, Temple Mu; Mitra, Prahar; Porfyriadis, Achilleas; Strominger, Andrew
    An infinite number of physically nontrivial symmetries are found for abelian gauge theories with massless charged particles. They are generated by large U(1) gauge transformations that asymptotically approach an arbitrary function ε(z,z¯) on the conformal sphere at future null infinity (ℐ+) but are independent of the retarded time. The value of ε at past null infinity (ℐ−) is determined from that on ℐ+ by the condition that it take the same value at either end of any light ray crossing Minkowski space. The ε≠ constant symmetries are spontaneously broken in the usual vacuum. The associated Goldstone modes are zero-momentum photons and comprise a U(1) boson living on the conformal sphere. The Ward identity associated with this asymptotic symmetry is shown to be the abelian soft photon theorem.