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SUMMARY:The  birth of string theory or how to build a theory without knowi
 ng the Lagrangian
DTSTART:20260515T120000Z
DTEND:20260515T140000Z
DTSTAMP:20260615T045100Z
UID:indico-event-52242@agenda.infn.it
DESCRIPTION:Speakers: Paolo Di Vecchia (Nordita)\n\nI will start describin
 g how the Veneziano 4-point amplitude and its generalisation to N-point\, 
 called Dual Resonance Model (DRM)\,   were constructed using the basic pr
 operties of S-matrix theory. This procedure is opposite to the usual one w
 here one constructs the S-matrix starting from a Lagrangian. In this case 
  the S-matrix was constructed without knowing the corresponding Lagrangia
 n.  Only later it was shown that the S-matrix followed from the string La
 grangian.\nThen I will discuss  how the scattering amplitude was rewritte
 n in terms of an infinite set of harmonic oscillators that allowed to dete
 rmine the spectrum of physical states and their scattering amplitudes. Abs
 ence of negative norm states\, required by unitarity\, restricts  the int
 ercept of the Regge trajectory to  be  equal to 1. \nAlready in 1970 Na
 mbu\, Nielsen and Susskind proposed that the underlying theory was a strin
 g theory\, but it took few more years\, by quantising the Nambu-Goto actio
 n in the light-cone gauge\, to show that the spectrum   of physical state
 s coincided with the spectrum extracted from the DRM with $\\alpha_0=1$. 
   \nLater it was shown that also  the scattering amplitudes at tree and
  multiloop  level were the same. \nI will conclude discussing the attemp
 t of  writing the scattering amplitude for pions that brought instead to 
  superstring. A string theory for pions is still absent. In modern langua
 ge this will require to construct a string extension of the non-linear sig
 ma-model that is the low-energy effective Lagrangian of QCD\, but all atte
 mpts in this direction generated scattering amplitudes with negative norm 
 states violating unitarity.  \nThe only theories that allow for a string
  extension are the gauge theories and gravity and no string theory exists 
 with one of the two without the other."\n\nhttps://agenda.infn.it/event/52
 242/
LOCATION:Aula Conversi (Dipartimento di Fisica-Ed.G.Marconi)
URL:https://agenda.infn.it/event/52242/
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