The existence and stability of atoms rely on the fact that neutrons are more massive than protons. The measured mass difference is only 0.14% of the average of the two masses. A slightly smaller or ...larger value would have led to a dramatically different universe. Here, we show that this difference results from the competition between electromagnetic and mass isospin breaking effects. We performed lattice quantum-chromodynamics and quantum-electrodynamics computations with four nondegenerate Wilson fermion flavors and computed the neutron-proton mass-splitting with an accuracy of 300 kilo–electron volts, which is greater than 0 by 5 standard deviations. We also determine the splittings in the Σ, Ξ, D, and Ξcc isospin multiplets, exceeding in some cases the precision of experimental measurements.
We present a QCD calculation of the u, d, and s scalar quark contents of nucleons based on 47 lattice ensembles with N_{f}=2+1 dynamical sea quarks, 5 lattice spacings down to 0.054 fm, lattice sizes ...up to 6 fm, and pion masses down to 120 MeV. Using the Feynman-Hellmann theorem, we obtain f_{ud}^{N}=0.0405(40)(35) and f_{s}^{N}=0.113(45)(40), which translates into σ_{πN}=38(3)(3) MeV, σ_{sN}=105(41)(37) MeV, and y_{N}=0.20(8)(8) for the sigma terms and the related ratio, where the first errors are statistical and the second errors are systematic. Using isospin relations, we also compute the individual up and down quark contents of the proton and neutron (results in the main text).
Functional plasticity of the brain decreases during ageing causing marked deficits in contextual learning, allocentric navigation and episodic memory. Adult neurogenesis is a prime example of ...hippocampal plasticity promoting the contextualisation of information and dramatically decreases during ageing. We found that a genetically-driven expansion of neural stem cells by overexpression of the cell cycle regulators Cdk4/cyclinD1 compensated the age-related decline in neurogenesis. This triggered an overall inhibitory effect on the trisynaptic hippocampal circuit resulting in a changed profile of CA1 sharp-wave ripples known to underlie memory consolidation. Most importantly, increased neurogenesis rescued the age-related switch from hippocampal to striatal learning strategies by rescuing allocentric navigation and contextual memory. Our study demonstrates that critical aspects of hippocampal function can be reversed in old age, or compensated throughout life, by exploiting the brain's endogenous reserve of neural stem cells.
Ab Initio Determination of Light Hadron Masses Dürr, S; Fodor, Z; Frison, J ...
Science (American Association for the Advancement of Science),
11/2008, Letnik:
322, Številka:
5905
Journal Article
Recenzirano
Odprti dostop
More than 99% of the mass of the visible universe is made up of protons and neutrons. Both particles are much heavier than their quark and gluon constituents, and the Standard Model of particle ...physics should explain this difference. We present a full ab initio calculation of the masses of protons, neutrons, and other light hadrons, using lattice quantum chromodynamics. Pion masses down to 190 mega-electron volts are used to extrapolate to the physical point, with lattice sizes of approximately four times the inverse pion mass. Three lattice spacings are used for a continuum extrapolation. Our results completely agree with experimental observations and represent a quantitative confirmation of this aspect of the Standard Model with fully controlled uncertainties.
Sub-micron thick yttria-stabilized zirconia (YSZ) layers (
t
=
400–700
nm) containing 3 (3YSZ) or 8
mol.% (8YSZ) Y
2O
3 with microstructures ranging from isotropic amorphous to columnar ...polycrystalline and a variable porosity can be grown by pulsed laser deposition (PLD) varying the substrate temperature and oxygen background pressure. The dependence of the mechanical and optical properties on the film microstructure and chemical composition was investigated by nanoindentation experiments and transmission spectrophotometry. Dense amorphous YSZ layers exhibit a higher optical transmissivity, 0.2
eV lower band gap energy (5.5 vs. 5.7
eV), and up to 25% lower hardness (11.9 vs. 16.0
GPa) and reduced elastic moduli (231 vs. 278
GPa) compared with crystalline films, irrespective of the dopant level. High refractive indices,
n
600
nm, in the range 2.18–2.23, i.e. close to single crystal reference data, are obtained for the low pressure PLD regime. Within these boundaries the index decreases with increasing Y
2O
3 content and is consistently slightly smaller for amorphous layers compared with crystalline films of the same composition, due to a lower atomic packing density. The gradual decrease in the refractive index for YSZ layers grown at background pressures above 1.0
Pa marks the development of pores in the form of inter-columnar voids and can be used for sensitive quantification of the film porosity. The lattice order affects the fracture behaviour, as amorphous coatings show plastic deformation mediated by shear bands, while the crystalline layers yield hoop and surface cracks upon indentation. The crystalline 3YSZ films exhibit an enhanced fracture toughness compared with 8YSZ, which is related to the stress-induced transformation to the monoclinic phase in partially stabilized zirconia.
Why some of us remember events more clearly than others and why memory loses precision over time is a major focus in memory research. Here, we show that the recruitment of specific neuroanatomical ...pathways within the medial temporal lobe (MTL) of the brain defines the precision of the memory recalled over the lifespan. Using optogenetics, neuronal activity mapping, and studying recent to very remote memories, we report that the hippocampal subfield CA1 is necessary for retrieving the gist of events and receives maximal support from MTL cortical areas (MEC, LEC, PER, and POR) for recalling the most remote memories. In contrast, reduction of CA3’s activity alone coincides with the loss of memory precision over time. We propose that a shift between specific MTL subnetworks over time might be a fundamental mechanism of memory consolidation.
Display omitted
•Recalling the gist of an event depends on CA1 independent of the age of the memory in mice•CA1 receives support from MEC, LEC, PER, and POR for recalling very remote gist memories•CA3’s role in memory recall is time limited and restricted to the precision of the memory•The precision of a memory might depend on which medial temporal lobe subnetwork is recruited
Atucha et al. show that, in mice, recalling the gist of an event requires CA1 that receives support from medial temporal lobe (MTL) areas when memories are remote, whereas CA3 is necessary for memory precision. Thus, a shift within MTL subnetworks over time might be a fundamental mechanism of memory consolidation.
Perception greatly benefits from integrating multiple sensory cues into a unified percept. To study the neural mechanisms of sensory integration, model systems are required that allow the ...simultaneous assessment of activity and the use of techniques to affect individual neural processes in behaving animals. While rodents qualify for these requirements, little is known about multisensory integration and areas involved for this purpose in the rodent. Using optical imaging combined with laminar electrophysiological recordings, the rat parietal cortex was identified as an area where visual and somatosensory inputs converge and interact. Our results reveal similar response patterns to visual and somatosensory stimuli at the level of current source density (CSD) responses and multi-unit responses within a strip in parietal cortex. Surprisingly, a selective asymmetry was observed in multisensory interactions: when the somatosensory response preceded the visual response, supra-linear summation of CSD was observed, but the reverse stimulus order resulted in sub-linear effects in the CSD. This asymmetry was not present in multi-unit activity however, which showed consistently sub-linear interactions. These interactions were restricted to a specific temporal window, and pharmacological tests revealed significant local intra-cortical contributions to this phenomenon. Our results highlight the rodent parietal cortex as a system to model the neural underpinnings of multisensory processing in behaving animals and at the cellular level.
We give details of our precise determination of the light quark masses
m
ud
= (
m
u
+
m
d
)/2 and
m
s
in 2 + 1 flavor QCD, with simulated pion masses down to 120 MeV, at five lattice spacings, and ...in large volumes. The details concern the action and algorithm employed, the HMC force with HEX smeared clover fermions, the choice of the scale setting procedure and of the input masses. After an overview of the simulation parameters, extensive checks of algorithmic stability, autocorrelation and (practical) ergodicity are reported. To corroborate the good scaling properties of our action, explicit tests of the scaling of hadron masses in
N
f
= 3 QCD are carried out. Details of how we control finite volume effects through dedicated finite volume scaling runs are reported. To check consistency with SU(2) Chiral Perturbation Theory the behavior of
M
π
2
/m
ud
and
F
π
as a function of
m
ud
is investigated. Details of how we use the RI/MOM procedure with a separate continuum limit of the running of the scalar density
R
S
(
μ
,
μ
′) are given. This procedure is shown to reproduce the known value of
r
0
m
s
in quenched QCD. Input from dispersion theory is used to split our value of
m
ud
into separate values of
m
u
and
m
d
. Finally, our procedure to quantify both systematic and statistical uncertainties is discussed.
Indirect CP violation in K→ππ decays plays a central role in constraining the flavor structure of the Standard Model (SM) and in the search for new physics. For many years the leading uncertainty in ...the SM prediction of this phenomenon was the one associated with the nonperturbative strong interaction dynamics in this process. Here we present a fully controlled lattice QCD calculation of these effects, which are described by the neutral kaon mixing parameter BK. We use a two step HEX smeared clover-improved Wilson action, with four lattice spacings from a≈0.054 fm to a≈0.093 fm and pion masses at and even below the physical value. Nonperturbative renormalization is performed in the RI-MOM scheme, where we find that operator mixing induced by chiral symmetry breaking is very small. Using fully nonperturbative continuum running, we obtain our main result BKRI(3.5 GeV)=0.531(6)stat(2)sys. A perturbative 2-loop conversion yields BKMS¯-NDR(2 GeV)=0.564(6)stat(3)sys(6)PT and BˆK=0.773(8)stat(3)sys(8)PT, which is in good agreement with current results from fits to experimental data.
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