Physics Letters B 728 (2014) 288–295
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Physics Letters B
www.elsevier.com/locate/physletb
On the scattering over the GKP vacuum
Davide Fioravanti
a,∗
, Simone Piscaglia
a,b
, Marco Rossi
c
a
Sezione INFN di Bologna, Dipartimento di Fisica e Astronomia, Università di Bologna, Via Irnerio 46, Bologna, Italy
b
Centro de Física do Porto and Departamento de Física e Astronomia, Universidade do Porto, Rua do Campo Alegre 687, Porto, Portugal
c
Dipartimento di Fisica dell’Università della Calabria and INFN, Gruppo Collegato di Cosenza, Arcavacata di Rende, Cosenza, Italy
article info abstract
Article history:
Received 2 October 2013
Received in revised form 22 November 2013
Accepted 1 December 2013
Available online 4 December 2013
Editor: L. Alvarez-Gaumé
By converting the asymptotic Bethe Ansatz (ABA) of N = 4 SYM into non-linear integral equations, we
find 2D scattering amplitudes of excitations on top of the GKP vacuum. We prove that this is a suitable
and powerful set-up for the understanding and computation of the whole S-matrix. We show that all the
amplitudes depend on the fundamental scalar–scalar one.
© 2013 The Authors. Published by Elsevier B.V.
1. Introduction
In integrable system and condensed matter theories the study
of the scattering of excitations over the antiferromagnetic vacuum
is at least as much important as that over the ferromagnetic one
(cf. one of the pioneering papers [1] and its references). Often the
excitations over the ferromagnetic vacuum are called magnons, as
well as those over the antiferromagnetic one kinks or solitons or
spinons. Also, two dimensional (lattice) field theories (like, for in-
stance, Sine-Gordon) are often examples with an antiferromagnetic
vacuum [2]. If we wish to parallel this reasoning in the framework
of the Beisert–Staudacher asymptotic (i.e. large s )BetheAnsatz
(ABA) for
N = 4SYM[3], we are tempted to choose, as antifer-
romagnetic vacuum, the GKP long (i.e. fast spinning) AdS
5
string
solution [4]. According to the AdS/CFT correspondence [5],the
quantum GKP string state is dual to a single trace twist two op-
erator of
N = 4 (at high spin); thus let us consider two complex
scalars, Z, at the two ends of a long series of s (light-cone) covari-
ant derivatives, D
+
. Then, excitations of the GKP string correspond
to insertions of other operators over this Fermi sea, thus generat-
ing higher twist operators. More precisely, operators associated to
one-particle
ϕ states are built as
O
=Tr ZD
s−s
+
ϕD
s
+
Z +···. (1.1)
The set of lower twist (twist three) excitations includes ϕ = Z ,one
of the three complex scalars;
ϕ = F
+⊥
,
¯
F
+⊥
, the two components
*
Corresponding author.
E-mail addresses: fioravanti@bo.infn.it (D. Fioravanti), piscagli@bo.infn.it
(S. Piscaglia), rossi@cs.infn.it (M. Rossi).
of the gluon field; ϕ = Ψ
+
,
¯
Ψ
+
,the4+4 fermions, respectively. All
these fields are the highest weight of a precise representation of
the residual so
(6) su(4) symmetry of the GKP vacuum: the scalar
of the vector 6, the fermions of the 4 and
¯
4, respectively, and the
gluons of the 1 representation. All these features and the exact dis-
persion relations for these excitations in the different regimes have
been studied recently in deep and interesting detail by Basso [6].
Now, as for the scattering, we shall consider at least two particles
states (twist-4), namely
O
=Tr ZD
s−s
1
−s
2
+
ϕ
1
D
s
1
+
ϕ
2
D
s
2
+
Z +···. (1.2)
This situation was already analysed in partial generality in the case
when both the excitations are identical
ϕ
1
= ϕ
2
= Z [7].Impor-
tantly, an impressive recent paper [8] proposed a non-perturbative
approach to 4D scattering amplitudes in
N = 4SYMbyusingas
building blocks the 2D scattering amplitudes we wish to com-
pute here: we will see some non-trivial checks of their conjec-
tures.
Computing the scattering matrix has a long history (see, for
ins
tance, [9] and references therein). From this wide literature
we can argue that an efficient method of computation rests on
the non-linear integral equation (for excited states) [10].Infact,
the same idea of counting function gives a quantisation condition
which can be interpreted as (asymptotic) Bethe Ansatz, defining
the scattering matrix (elements). In this note we will use this
strategy to provide general formulæ for the scattering amplitudes
between the aforementioned excitations. This should give the non-
trivial part (in front) of the scattering matrices, the so-called scalar
factor, being the matrix structure fixed by the aforementioned
residual symmetry representations. Remarkably, all the scattering
phases (eigenvalues) are expressible in terms of the scalar–scalar
one. Moreover, we evaluate one loop and strong coupling limits
of (scalar–scalar and) gluon–gluon scattering amplitudes and find
confirmation of the conjecture of [8].
0370-2693 © 2013 The Authors. Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.physletb.2013.12.003
Open access under CC BY license.
Open access under CC BY license.