93 research outputs found
Evolving Planck Mass in Classically Scale-Invariant Theories
We consider classically scale-invariant theories with non-minimally coupled
scalar fields, where the Planck mass and the hierarchy of physical scales are
dynamically generated. The classical theories possess a fixed point, where
scale invariance is spontaneously broken. In these theories, however, the
Planck mass becomes unstable in the presence of explicit sources of scale
invariance breaking, such as non-relativistic matter and cosmological constant
terms. We quantify the constraints on such classical models from Big Bang
Nucleosynthesis that lead to an upper bound on the non-minimal coupling and
require trans-Planckian field values. We show that quantum corrections to the
scalar potential can stabilise the fixed point close to the minimum of the
Coleman-Weinberg potential. The time-averaged motion of the evolving fixed
point is strongly suppressed, thus the limits on the evolving gravitational
constant from Big Bang Nucleosynthesis and other measurements do not presently
constrain this class of theories. Field oscillations around the fixed point, if
not damped, contribute to the dark matter density of the Universe.Comment: 28 pages, 2 figures, version published in JHE
Cavity-assisted measurement and coherent control of collective atomic spin oscillators
We demonstrate continuous measurement and coherent control of the collective
spin of an atomic ensemble undergoing Larmor precession in a high-finesse
optical cavity. The coupling of the precessing spin to the cavity field yields
phenomena similar to those observed in cavity optomechanics, including cavity
amplification, damping, and optical spring shifts. These effects arise from
autonomous optical feedback onto the atomic spin dynamics, conditioned by the
cavity spectrum. We use this feedback to stabilize the spin in either its high-
or low-energy state, where, in equilibrium with measurement back-action
heating, it achieves a steady-state temperature, indicated by an asymmetry
between the Stokes and anti-Stokes scattering rates. For sufficiently large
Larmor frequency, such feedback stabilizes the spin ensemble in a nearly pure
quantum state, in spite of continuous measurement by the cavity field.Comment: 5 pages, 4 figures, and supplemental materia
A SUSY Inspired Simplified Model for the 750 GeV Diphoton Excess
The evidence for a new singlet scalar particle from the 750 GeV diphoton
excess, and the absence of any other signal of new physics at the LHC so far,
suggest the existence of new coloured scalars. To study this possibility, we
propose a supersymmetry inspired simplified model, extending the Standard Model
with a singlet scalar and with heavy scalar fields carrying both colour and
electric charges -- the `squarks'. To allow the latter to decay, and to
generate the dark matter of the Universe, we also add a neutral fermion to the
particle content. We show that this model provides a two-parameter fit to the
observed diphoton excess consistently with cosmology, while the allowed
parameter space is bounded by the consistency of the model. In the context of
our simplified model this implies the existence of other supersymmetric
particles accessible at the LHC, rendering this scenario falsifiable. If this
excess persists, it will imply a paradigm shift in assessing supersymmetry
breaking and the role of scalars in low scale physics.Comment: 7 pages, 2 figures, SUSY incarnat
Magnetic ordering tendencies in hexagonal-boron-nitride–bilayer-graphene moiré structures
When hexagonal boron nitride (hBN) and graphene are aligned at zero or a small twist angle, a moiré structure is formed due to the small lattice constant mismatch between the two structures. In this paper, we analyze magnetic ordering tendencies, driven by on-site Coulomb interactions, of encapsulated bilayer graphene (BG) forming a moiré structure with one (hBN-BG) or both hBN layers (hBN-BG-hBN), using the random phase approximation. The calculations are performed in a fully atomistic Hubbard model that takes into account all π electrons of the carbon atoms in one moiré unit cell. We analyze the charge neutral case and find that the dominant magnetic ordering instability is uniformly antiferromagnetic. Furthermore, at low temperatures, the critical Hubbard interaction Uc required to induce magnetic order is slightly larger in those systems where the moiré structure has caused a band gap opening in the noninteracting picture, although the difference is less than 6%. Mean-field calculations are employed to estimate how such an interaction-induced magnetic order may change the observable single-particle gap sizes
Towards a transportable aluminium ion quantum logic optical clock
With the advent of optical clocks featuring fractional frequency uncertainties on the order of 10-17 and below, new applications such as chronometric leveling with few-centimeter height resolution emerge. We are developing a transportable optical clock based on a single trapped aluminum ion, which is interrogated via quantum logic spectroscopy. We employ singly charged calcium as the logic ion for sympathetic cooling, state preparation, and readout. Here, we present a simple and compact physics and laser package for manipulation of 40Ca+. Important features are a segmented multilayer trap with separate loading and probing zones, a compact titanium vacuum chamber, a near-diffraction-limited imaging system with high numerical aperture based on a single biaspheric lens, and an all-in-fiber 40Ca+ repump laser system. We present preliminary estimates of the trap-induced frequency shifts on 27Al+, derived from measurements with a single calcium ion. The micromotion-induced second-order Doppler shift for 27Al+ has been determined to be δνEMMν=-0.4-0.3 +0.4×10-18 and the black-body radiation shift is δνBBR/ν = (-4.0 ± 0.4) × 10-18. Moreover, heating rates of 30 (7) quanta per second at trap frequencies of ωrad,Ca+ ≈ 2π × 2.5 MHz (ωax,Ca+ ≈ 2π × 1.5 MHz) in radial (axial) direction have been measured, enabling interrogation times of a few hundreds of milliseconds
Two Simple W' Models for the Early LHC
W' gauge bosons are good candidates for early LHC discovery. We define two
reference models, one containing a W'_R and one containing a W'_L, which may
serve as ``simplified models'' for presenting experimental results of W'
searches at the LHC. We present the Tevatron bounds on each model and compute
the constraints from precision electroweak observables. We find that indirect
low-energy constraints on the W'_L are quite strong. However, for a W'_R
coupling to right-handed fermions there exists a sizeable region in parameter
space beyond the bounds from the Tevatron and low-energy precision measurements
where even 50 inverse picobarns of integrated LHC luminosity are sufficient to
discover the W'_R. The most promising final states are two leptons and two
jets, or one lepton recoiling against a ``neutrino jet''. A neutrino jet is a
collimated object consisting of a hard lepton and two jets arising from the
decay of a highly boosted massive neutrino.Comment: 20 pages, 8 figures. v2: references adde
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