2,032 research outputs found
Soft behavior of string amplitudes with external massive states
We briefly discuss the soft behavior of scattering amplitudes both in string
and quantum field theory. In particular we show a general argument about the
validity of soft theorems for open superstring amplitudes and list some of our
recent results.Comment: 2 pages, poster presented at IFAE 201
On the soft limit of closed string amplitudes with massive states
We extend our analysis of the soft behaviour of string amplitudes with
massive insertions to closed strings at tree level (sphere). Relying on our
previous results for open strings on the disk and on KLT formulae we check
universality of the soft behaviour for gravitons to sub-leading order for
superstring amplitudes and show how this gets modified for bosonic strings. At
sub-sub-leading order we argue in favour of universality for superstrings on
the basis of OPE of the vertex operators and gauge invariance for the soft
graviton. The results are illustrated by explicit examples of 4-point
amplitudes with one massive insertion in any dimension, including D=4, where
use of the helicity spinor formalism drastically simplifies the expressions. As
a by-product of our analysis we confirm that the `single valued projection'
holds for massive amplitudes, too. We briefly comment on the soft behaviour of
the anti-symmetric tensor and on loop corrections.Comment: 18+7 pages; added some important references and corrected some typo
Bootstrapping QCD: the Lake, the Peninsula and the Kink
We consider the S-matrix bootstrap of four dimensional scattering amplitudes
with symmetry and no bound-states. We explore the allowed space of
scattering lengths which parametrize the interaction strength at threshold of
the various scattering channels. Next we consider an application of this
formalism to pion physics. A signature of pions is that they are derivatively
coupled leading to (chiral) zeros in their scattering amplitudes. In this work
we explore the multi-dimensional space of chiral zeros positions, scattering
length values and resonance mass values. Interestingly, we encounter lakes,
peninsulas and kinks depending on which sections of this intricate
multi-dimensional space we consider. We discuss the remarkable location where
QCD seems to lie in these plots, based on various experimental and theoretical
expectations.Comment: 6 pages, 7 figure
On the color structure of Yang-Mills theory with static sources in a periodic box
We present an exploratory numerical study on the lattice of the color
structure of the wave functionals of the SU(3) Yang-Mills theory in the
presence of a static pair. In a spatial box with periodic boundary
conditions we discuss the fact that all states contributing to the Feynman
propagation kernel are global color singlets. We confirm this numerically by
computing the correlations of gauge-fixed Polyakov lines with color-twisted
boundary conditions in the time direction. The values of the lowest energies in
the color singlet and octet external source sectors agree within statistical
errors, confirming that both channels contribute to the lowest (global singlet)
state of the Feynman kernel. We then study the case of homogeneous boundary
conditions in the time direction for which the gauge-fixing is not needed. In
this case the lowest energies extracted in the singlet external source sector
agree with those determined with periodic boundary conditions, while in the
octet sector the correlator is compatible with being null within our
statistical errors. Therefore consistently only the singlet external source
contribution has a non-vanishing overlap with the null-field wave functional.Comment: 9 pages, 3 figure
-potential: a numerical study
We report the results of recent lattice simulations aimed at computing the
and potential energies in the singlet and the octet (adjoint)
representation.Comment: 7 pages, 4 figures, poster presented at the 31st International
Symposium on Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013,
Mainz, German
On the exactness of soft theorems
Soft behaviours of S-matrix for massless theories reflect the underlying
symmetry principle that enforces its masslessness. As an expansion in soft
momenta, sub-leading soft theorems can arise either due to (I) unique structure
of the fundamental vertex or (II) presence of enhanced broken-symmetries. While
the former is expected to be modified by infrared or ultraviolet divergences,
the latter should remain exact to all orders in perturbation theory. Using
current algebra, we clarify such distinction for spontaneously broken (super)
Poincar\'e and (super) conformal symmetry. We compute the UV divergences of
DBI, conformal DBI, and A-V theory to verify the exactness of type (II) soft
theorems, while type (I) are shown to be broken and the soft-modifying
higher-dimensional operators are identified. As further evidence for the
exactness of type (II) soft theorems, we consider the alpha' expansion of both
super and bosonic open strings amplitudes, and verify the validity of the
translation symmetry breaking soft-theorems up to O(alpha'^6). Thus the
massless S-matrix of string theory "knows" about the presence of D-branes.Comment: 35 pages. Additional mathematica note book with the UV-divergenece of
the 6-point amplitude in AV/KS theor
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