388 research outputs found

### 750 GeV Diphotons from a D3-brane

Motivated by the recently reported diphoton excess at 750 GeV observed by
both CMS and ATLAS, we study string-based particle physics models which can
accommodate this signal. Quite remarkably, although Grand Unified Theories in
F-theory tend to impose tight restrictions on candidate extra sectors, the case
of a probe D3-brane near an E-type Yukawa point naturally leads to a class of
strongly coupled models capable of accommodating the observed signature. In
these models, the visible sector is realized by intersecting 7-branes, and the
750 GeV resonance is a scalar modulus associated with motion of the D3-brane in
the direction transverse to the Standard Model 7-branes. Integrating out heavy
3-7 string messenger states leads to dimension five operators for gluon fusion
production and diphoton decays. Due to the unified structure of interactions,
these models also predict that there should be additional decay channels to ZZ
and Z gamma. We also comment on models with distorted unification, where both
the production mechanism and decay channels can differ.Comment: v3: 12 pages, 1 figure, clarifications adde

### Particle Physics Implications of F-theory

We review recent progress in realizing Grand Unified Theories (GUTs) in a
strongly coupled formulation of type IIB string theory known as F-theory. Our
main emphasis is on the expected low-energy phenomenology of a minimal class of
F-theory GUTs. We introduce the primary ingredients in such constructions, and
then present qualitative features of GUT models in this framework such as GUT
breaking, doublet-triplet splitting, and proton decay. Next, we review
proposals for realizing flavor hierarchies in the quark and lepton sectors. We
discuss possible supersymmetry breaking scenarios, and their consequences for
experiment, as well as geometrically minimal realizations of F-theory GUTs
which incorporate most of these features.Comment: v2: 42 pages, 3 figures, submitted to Annu. Rev. Nuc. Part. Sci,
improved formatting and small clarification

### Instantons, Twistors, and Emergent Gravity

Motivated by potential applications to holography on space-times of positive
curvature, and by the successful twistor description of scattering amplitudes,
we propose a new dual matrix formulation of N = 4 gauge theory on S(4). The
matrix model is defined by taking the low energy limit of a holomorphic
Chern-Simons theory on CP(3|4), in the presence of a large instanton flux. The
theory comes with a choice of S(4) radius L and a parameter N controlling the
overall size of the matrices. The flat space variant of the 4D effective theory
arises by taking the large N scaling limit of the matrix model, with l_pl^2 ~
L^2 / N held fixed. Its massless spectrum contains both spin one and spin two
excitations, which we identify with gluons and gravitons. As shown in the
companion paper, the matrix model correlation functions of both these
excitations correctly reproduce the corresponding MHV scattering amplitudes. We
present evidence that the scaling limit defines a gravitational theory with a
finite Planck length. In particular we find that in the l_pl -> 0 limit, the
matrix model makes contact with the CSW rules for amplitudes of pure gauge
theory, which are uncontaminated by conformal supergravity. We also propose a
UV completion for the system by embedding the matrix model in the physical
superstring.Comment: v2: 64 pages, 3 figures, references added, typos correcte

### Evidence for C-theorems in 6D SCFTs

Using the recently established classification of 6D SCFTs we present evidence
for the existence of families of weak C-functions, that is, quantities which
decrease in a flow from the UV to the IR. Introducing a background R-symmetry
field strength R and a non-trivial tangent bundle T on the 6D spacetime, we
consider C-functions given by the linear combinations C = m1 alpha + m2 beta +
m3 gamma, where alpha, beta and gamma are the anomaly polynomial coefficients
for the formal characteristic classes c2(R)^2, c2(R)p1(T) and p1(T)^2. By
performing a detailed sweep over many theories, we determine the shape of the
unbounded monotonic region in "m-space" compatible with both Higgs branch flows
and tensor branch flows. We also verify that --as expected-- the Euler density
conformal anomaly falls in the admissible region.Comment: v2: 25 pages, 9 figures, typos correcte

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