6,059 research outputs found
Cosmology from String Theory
We explore the cosmological content of Salam-Sezgin six dimensional
supergravity, and find a solution to the field equations in qualitative
agreement with observation of distant supernovae, primordial nucleosynthesis
abundances, and recent measurements of the cosmic microwave background. The
carrier of the acceleration in the present de Sitter epoch is a quintessence
field slowly rolling down its exponential potential. Intrinsic to this model is
a second modulus which is automatically stabilized and acts as a source of cold
dark matter with a mass proportional to an exponential function of the
quintessence field (hence realizing VAMP models within a String context).
However, any attempt to saturate the present cold dark matter component in this
manner leads to unacceptable deviations from cosmological data -- a numerical
study reveals that this source can account for up to about 7% of the total cold
dark matter budget. We also show that (1) the model will support a de Sitter
energy in agreement with observation at the expense of a miniscule breaking of
supersymmetry in the compact space; (2) variations in the fine structure
constant are controlled by the stabilized modulus and are negligible; (3)
``fifth''forces are carried by the stabilized modulus and are short range; (4)
the long time behavior of the model in four dimensions is that of a
Robertson-Walker universe with a constant expansion rate (w = -1/3). Finally,
we present a String theory background by lifting our six dimensional
cosmological solution to ten dimensions.Comment: Version to be published in Physical Review
Minimal left-right symmetric intersecting D-brane model
We investigate left-right symmetric extensions of the standard model based on
open strings ending on D-branes, with gauge bosons due to strings attached to
stacks of D-branes and chiral matter due to strings stretching between
intersecting D-branes. The left-handed and right-handed fermions transform as
doublets under Sp(1)_L and Sp(1)_R, and so their masses must be generated by
the introduction of Higgs fields in a bi-fundamental (2,2) representation under
the two Sp(1) gauge groups. For such D-brane configurations the left-right
symmetry must be broken by Higgs fields in the doublet representation of
Sp(1)_R and therefore Majorana mass terms are suppressed by some higher physics
scale. The left-handed and right-handed neutrinos pair up to form Dirac
fermions which control the decay widths of the right-handed W' boson to yield
comparable branching fractions into dilepton and dijets channels. Using the
most recent searches at LHC13 Run II with 2016 data we constrain the (g_R,
m_{W'}) parameter space. Our analysis indicates that independent of the
coupling strength g_R, gauge bosons with masses m_{W'} \agt 3.5~{\rm TeV} are
not ruled out. As the LHC is just beginning to probe the TeV-scale, significant
room for W' discovery remains.Comment: To be published in PR
Update on 750 GeV diphotons from closed string states
Motivated by the recent update on LHC searches for narrow and broad
resonances decaying into diphotons we reconsider the possibility that the
observed peak in the invariant mass spectrum at M_{\gamma \gamma} = 750 GeV
originates from a closed string (possibly axionic) excitation \varphi
(associated with low mass scale string theory) that has a coupling with gauge
kinetic terms. We reevaluate the production of \varphi by photon fusion to
accommodate recent developments on additional contributions to relativistic
light-light scattering. We also study the production of \varphi via gluon
fusion. We show that for both a narrow and a broad resonance these two initial
topologies can accommodate the excess of events, spanning a wide range of
string mass scales 7 \alt M_s/TeV \alt 30 that are consistent with the
experimental lower bound: M_s > 7 TeV, at 95% CL. We demonstrate that for the
two production processes the LHC13 data is compatible with the lack of a
diphoton excess in LHC8 data within \sim 1\sigma. We also show that if the
resonance production is dominated by gluon fusion the null results on dijet
searches at LHC8 further constrain the coupling strengths of \varphi, but
without altering the range of possible string mass scales.Comment: 11 pages revtex. arXiv admin note: text overlap with arXiv:1512.0850
Stringy origin of diboson and dijet excesses at the LHC
Very recently, the ATLAS and CMS collaborations reported diboson and dijet
excesses above standard model expectations in the invariant mass region of 1.8
-2.0 TeV. Interpreting the diboson excess of events in a model independent
fashion suggests that the vector boson pair production searches are best
described by WZ or ZZ topologies, because states decaying into W^+W^- pairs are
strongly constrained by semileptonic searches. Under the assumption of a low
string scale, we show that both the diboson and dijet excesses can be steered
by an anomalous U(1) field with very small coupling to leptons. The Drell-Yan
bounds are then readily avoided because of the leptophobic nature of the
massive Z' gauge boson. The non-negligible decay into ZZ required to
accommodate the data is a characteristic footprint of intersecting D-brane
models, wherein the Landau-Yang theorem can be evaded by anomaly-induced
operators involving a longitudinal Z. The model presented herein can be viewed
purely field-theoretically, although it is particularly well motivated from
string theory. Should the excesses become statistically significant at the
LHC13, the associated Z\gamma{} topology would become a signature consistent
only with a stringy origin.Comment: References added. To be published in PL
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