79 research outputs found
Dimensionless cosmology
Although it is well known that any consideration of the variations of
fundamental constants should be restricted to their dimensionless combinations,
the literature on variations of the gravitational constant is entirely
dimensionful. To illustrate applications of this to cosmology, we explicitly
give a dimensionless version of the parameters of the standard cosmological
model, and describe the physics of Big Bang Neucleosynthesis and recombination
in a dimensionless manner. The issue that appears to have been missed in many
studies is that in cosmology the strength of gravity is bound up in the
cosmological equations, and the epoch at which we live is a crucial part of the
model. We argue that it is useful to consider the hypothetical situation of
communicating with another civilization (with entirely different units),
comparing only dimensionless constants, in order to decide if we live in a
Universe governed by precisely the same physical laws. In this thought
experiment, we would also have to compare epochs, which can be defined by
giving the value of any {\it one} of the evolving cosmological parameters. By
setting things up carefully in this way one can avoid inconsistent results when
considering variable constants, caused by effectively fixing more than one
parameter today. We show examples of this effect by considering microwave
background anisotropies, being careful to maintain dimensionlessness
throughout. We present Fisher matrix calculations to estimate how well the fine
structure constants for electromagnetism and gravity can be determined with
future microwave background experiments. We highlight how one can be misled by
simply adding to the usual cosmological parameter set
Tests of light-lepton universality in angular asymmetries of decays
We present the first comprehensive tests of light-lepton universality in the
angular distributions of semileptonic \Bz-meson decays to charged spin-1
charmed mesons. We measure five angular-asymmetry observables as functions of
the decay recoil that are sensitive to lepton-universality-violating
contributions. We use events where one neutral \B is fully reconstructed in
\PUpsilonFourS{} \to\B\overline{B} decays in data corresponding to \lumion
integrated luminosity from electron-positron collisions collected with the
\belletwo detector. We find no significant deviation from the standard model
expectations
Measurement of asymmetries and branching-fraction ratios for and with using Belle and Belle II data
We measure asymmetries and branching-fraction ratios for and decays with , where
is a superposition of and . We use the full data set of the
Belle experiment, containing pairs, and data from the
Belle~II experiment, containing pairs, both collected
in electron-positron collisions at the resonance. Our results
provide model-independent information on the unitarity triangle angle .Comment: 26 pages, 8 figure
Planck 2015 results. XIV. Dark energy and modified gravity
We study the implications of Planck data for models of dark energy (DE) and modified gravity (MG), beyond the cosmological constant scenario. We start with cases where the DE only directly affects the background evolution, considering Taylor expansions of the equation of state, principal component analysis and parameterizations related to the potential of a minimally coupled DE scalar field. When estimating the density of DE at early times, we significantly improve present constraints. We then move to general parameterizations of the DE or MG perturbations that encompass both effective field theories and the phenomenology of gravitational potentials in MG models. Lastly, we test a range of specific models, such as k-essence, f(R) theories and coupled DE. In addition to the latest Planck data, for our main analyses we use baryonic acoustic oscillations, type-Ia supernovae and local measurements of the Hubble constant. We further show the impact of measurements of the cosmological perturbations, such as redshift-space distortions and weak gravitational lensing. These additional probes are important tools for testing MG models and for breaking degeneracies that are still present in the combination of Planck and background data sets. All results that include only background parameterizations are in agreement with LCDM. When testing models that also change perturbations (even when the background is fixed to LCDM), some tensions appear in a few scenarios: the maximum one found is \sim 2 sigma for Planck TT+lowP when parameterizing observables related to the gravitational potentials with a chosen time dependence; the tension increases to at most 3 sigma when external data sets are included. It however disappears when including CMB lensing
Precise measurement of the lifetime at Belle II
We measure the lifetime of the meson using a data sample of 207
fb collected by the Belle II experiment running at the SuperKEKB
asymmetric-energy collider. The lifetime is determined by fitting the
decay-time distribution of a sample of
decays. Our result is \tau^{}_{D^+_s} = (498.7\pm
1.7\,^{+1.1}_{-0.8}) fs, where the first uncertainty is statistical and the
second is systematic. This result is significantly more precise than previous
measurements.Comment: 7 pages, 4 figures, to be submitted to Physical Review Letter
Search for a resonance in events with the Belle II experiment
We report the first search for a non-standard-model resonance decaying into
pairs in events in
the 3.6-10 GeV/ mass range. We use a 62.8 fb sample of
collisions collected at a center-of-mass energy of 10.58 GeV by the Belle II
experiment at the SuperKEKB collider. The analysis probes three different
models predicting a spin-1 particle coupling only to the heavier lepton
families, a Higgs-like spin-0 particle that couples preferentially to charged
leptons (leptophilic scalar), and an axion-like particle, respectively. We
observe no evidence for a signal and set exclusion limits at 90% confidence
level on the product of cross section and branching fraction into pairs,
ranging from 0.7 fb to 24 fb, and on the couplings of these processes. We
obtain world-leading constraints on the couplings for the leptophilic scalar
model for masses above 6.5 GeV/ and for the axion-like particle model over
the entire mass range
First measurement of as an inclusive test of the anomaly
We measure the tau-to-light-lepton ratio of inclusive -meson branching
fractions , where indicates an electron or muon, and thereby test
the universality of charged-current weak interactions. We select events that
have one fully reconstructed meson and a charged lepton candidate from
of electron-positron collision data collected with the
Belle II detector. We find , in agreement with standard-model expectations. This
is the first direct measurement of
Measurement of branching fractions and direct asymmetries for and decays at Belle II
We report measurements of the branching fractions and direct
asymmetries of the decays , , , and , and use these for testing the standard
model through an isospin-based sum rule. In addition, we measure the branching
fraction and direct asymmetry of the decay and
the branching fraction of the decay . The data are
collected with the Belle II detector from collisions at the
resonance produced by the SuperKEKB asymmetric-energy collider
and contain bottom-antibottom meson pairs. Signal yields are
determined in two-dimensional fits to background-discriminating variables, and
range from 500 to 3900 decays, depending on the channel. We obtain for the sum rule, in agreement with the standard model
expectation of zero and with a precision comparable to the best existing
determinations
Measurement of asymmetries in decays with Belle II
We present a measurement of time-dependent rate asymmetries in decays to search for non-standard-model physics in transitions. The data sample is collected with the Belle II
detector at the SuperKEKB asymmetric-energy collider in 2019-2022
and contains bottom-antibottom mesons from
resonance decays. We reconstruct signal events and
extract the charge-parity () violating parameters from a fit to the
distribution of the proper-decay-time difference of the two mesons. The
measured direct and mixing-induced asymmetries are
and , respectively, where the first
uncertainties are statistical and the second are systematic. The results are
compatible with the asymmetries observed in
transitions
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