95 research outputs found
An R^4 non-renormalisation theorem in N=4 supergravity
We consider the four-graviton amplitudes in CHL constructions providing
four-dimensional N=4 models with various numbers of vector multiplets. We show
that in these models the two-loop amplitude has a prefactor of d^2R^4. This
implies a non-renormalisation theorem for the R^4 term, which forbids the
appearance of a three-loop ultraviolet divergence in four dimensions in the
four-graviton amplitude. We connect the special nature of the R^4 term to the
U(1) anomaly of pure N=4 supergravity.Comment: v2: added comments about one-loop UV divergences. Assorted stylistic
corrections. Added references. v3: Eq. III.21 corrected and assorted minor
corrections and clarifications. Version to be published. v4: minor
corrections. 18 pages. one figur
Basics of Generalized Unitarity
We review generalized unitarity as a means for obtaining loop amplitudes from
on-shell tree amplitudes. The method is generally applicable to both
supersymmetric and non-supersymmetric amplitudes, including non-planar
contributions. Here we focus mainly on N=4 Yang-Mills theory, in the context of
on-shell superspaces. Given the need for regularization at loop level, we also
review a six-dimensional helicity-based superspace formalism and its
application to dimensional and massive regularizations. An important feature of
the unitarity method is that it offers a means for carrying over any identified
tree-level property of on-shell amplitudes to loop level, though sometimes in a
modified form. We illustrate this with examples of dual conformal symmetry and
a recently discovered duality between color and kinematics.Comment: 37 pages, 10 figures. Invited review for a special issue of Journal
of Physics A devoted to "Scattering Amplitudes in Gauge Theories", R.
Roiban(ed), M. Spradlin(ed), A. Volovich(ed
Dualities for Loop Amplitudes of N=6 Chern-Simons Matter Theory
In this paper we study the one- and two-loop corrections to the four-point
amplitude of N=6 Chern-Simons matter theory. Using generalized unitarity
methods we express the one- and two-loop amplitudes in terms of dual-conformal
integrals. Explicit integration by using dimensional reduction gives vanishing
one-loop result as expected, while the two-loop result is non-vanishing and
matches with the Wilson loop computation. Furthermore, the two-loop correction
takes the same form as the one-loop correction to the four-point amplitude of
N=4 super Yang-Mills. We discuss possible higher loop extensions of this
correspondence between the two theories. As a side result, we extend the method
of dimensional reduction for three dimensions to five dimensions where dual
conformal symmetry is most manifest, demonstrating significant simplification
to the computation of integrals.Comment: 32 pages and 6 figures. v2: minus sign corrections, ref updated v3:
Published versio
Unfitting, uncomfortable, unacademic: a sociological reading of an interactive mobile phone app in university lectures
Abstract Scholarly literature on education technology uptake has been dominated by technological determinist readings of students’ technology use. However, in recent years there has been a move by sociologists of education to highlight how the contexts in which educational technologies are introduced are not tabula rasa but socially and culturally complex. This study approaches technology as a social construct, arguing that students construct discursive meaning of, rather than simply respond to, technologies for learning. The study explores students’ constructions of a mobile learning app that was introduced into lectures during a year-long university course. Students largely rejected the app, constructing it as unfitting for the context, a socially uncomfortable experience and an unacademic way of learning. The paper highlights the limitations of technological determinism and closes by arguing for readings of educational technologies that pay close attention to students’ voices
Examining the impact of digital technologies on students’ Higher Education outcomes:The case of the Virtual Learning Environment and social media
Digital natives is a term used to describe current higher education (HE) students, whose lives are proliferated by digital technologies. To cater to the needs of this new generation of students, HE institutions increasingly adopt digital tools such as virtual learning environments (VLE) and social media (SM). Little is known, however, about the impact of these digital technologies on students’ HE outcomes. Drawing from service productivity theories, this study aims to address this gap. Through exploratory sequential mixed research methods, we identify five HE outcomes and reveal that Learning-Oriented Outcomes are the most important in HE even when digital technologies are not used; and these outcomes are further enhanced when students use VLE. Learning-Oriented Outcomes, however, are the least important when SM is used in HE; students tend to prioritise outcomes related to Knowledge Transfer instead. Our research findings derive theoretical and practical contributions and open up avenues for future research
Integrand Oxidation and One-Loop Colour-Dual Numerators in N=4 Gauge Theory
We present a systematic method to determine BCJ numerators for one-loop
amplitudes that explores the global constraints on the loop momentum
dependence. We apply this method to amplitudes in N=4 gauge theory, working out
detailed examples up to seven points in both the MHV and the NMHV sectors. The
structure of Jacobi identities between BCJ numerators is seen to be closely
connected to that of algebraic integrand reductions. We discuss the
consequences for one-loop N=8 supergravity amplitudes obtained through the
double copy prescription. Moreover, in the MHV sector, we show how to obtain
simple BCJ box numerators using a conjectured relationship to amplitudes in
self-dual gauge theory. We also introduce simpler trace-type formulas for
integrand reductions.Comment: minor corrections, published versio
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