16,173 research outputs found
Strange quark mass from e+e- revisited and present status of light quark masses
We reconsider the determinations of the strange quark mass m_s from e+e- into
hadrons data using a new combination of FESR and revisiting the existing
tau-like sum rules by including non-resonant contributions to the spectral
functions. To order alpha_s^3 and including the tachyonic gluon mass lambda^2
contribution, which phenomenologically parametrizes the UV renormalon effect
into the PT series, we obtain the invariant mass m_s=(119 +- 17)MeV leading to:
m_s(2 GeV)=(104+- 15)MeV. Combining this value with the recent and independent
phenomenological determinations from some other channels, to order alpha_s^3
and including lambda^2, we deduce the weighted average: m_s (2 GeV)=(96.1 +-
4.8)MeV . The positivity of the spectral functions in the (pseudo)scalar [resp.
vector] channels leads to the lower [resp. upper] bounds of m_s(2 GeV): (71 +-
4) MeV < m_s(2 GeV) < (151 +- 14) MeV, to order alpha_s^3. Using the ChPT mass
ratio r_3 = 2m_s/(m_u+m_d)=24.2 +- 1.5, and the average value of m_s, we
deduce:
(m_u+m_d)(2 GeV)=(7.9 +- 0.6) MeV, consistent with the pion sum rule result,
which, combined with the ChPT value for m_u/m_d, gives: m_d(2 GeV)=(5.1 +-
0.4)MeV and m_u(2 GeV)=(2.8 +- 0.2)MeV. Finally, using (m_u+m_d) from the pion
sum rule and the average value of m_s (without the pion sum rule), the method
gives: r_3= 23.5 +- 5.8 in perfect agreement with the ChPT ratio, indicating
the self-consistency of the sum rule results. Using the value: m_b(m_b)=(4.23
+- 0.06) GeV, we also obtain the model-building useful scale-independent mass
ratio: m_b/m_s=50 +- 3.Comment: Updated and improved average values. Version to appear in Phys. Rev.
Almost-Commutative Geometries Beyond the Standard Model II: New Colours
We will present an extension of the standard model of particle physics in its
almost-commutative formulation. This extension is guided by the minimal
approach to almost-commutative geometries employed in [13], although the model
presented here is not minimal itself.
The corresponding almost-commutative geometry leads to a Yang-Mills-Higgs
model which consists of the standard model and two new fermions of opposite
electro-magnetic charge which may possess a new colour like gauge group. As a
new phenomenon, grand unification is no longer required by the spectral action.Comment: Revised version for publication in J.Phys.A with corrected Higgs
masse
Delayed inhibition of an anticipatory action during motion extrapolation
Background: Continuous visual information is important for movement initiation in a variety of motor tasks.
However, even in the absence of visual information people are able to initiate their responses by using motion
extrapolation processes. Initiation of actions based on these cognitive processes, however, can demand more
attentional resources than that required in situations in which visual information is uninterrupted. In the
experiment reported we sought to determine whether the absence of visual information would affect the latency
to inhibit an anticipatory action.
Methods: The participants performed an anticipatory timing task where they were instructed to move in
synchrony with the arrival of a moving object at a determined contact point. On 50% of the trials, a stop sign
appeared on the screen and it served as a signal for the participants to halt their movements. They performed the
anticipatory task under two different viewing conditions: Full-View (uninterrupted) and Occluded-View (occlusion of
the last 500 ms prior to the arrival at the contact point).
Results: The results indicated that the absence of visual information prolonged the latency to suppress the
anticipatory movement.
Conclusion: We suggest that the absence of visual information requires additional cortical processing that creates
competing demand for neural resources. Reduced neural resources potentially causes increased reaction time to
the inhibitory input or increased time estimation variability, which in combination would account for prolonged
latency
Quantum disorder and Griffiths singularities in bond-diluted two-dimensional Heisenberg antiferromagnets
We investigate quantum phase transitions in the spin-1/2 Heisenberg
antiferromagnet on square lattices with inhomogeneous bond dilution. It is
shown that quantum fluctuations can be continuously tuned by inhomogeneous bond
dilution, eventually leading to the destruction of long-range magnetic order on
the percolating cluster. Two multicritical points are identified at which the
magnetic transition separates from the percolation transition, introducing a
novel quantum phase transition. Beyond these multicritical points a
quantum-disordered phase appears, characterized by an infinite percolating
cluster with short ranged antiferromagnetic order. In this phase, the
low-temperature uniform susceptibility diverges algebraically with
non-universal exponents. This is a signature that the novel quantum-disordered
phase is a quantum Griffiths phase, as also directly confirmed by the
statistical distribution of local gaps. This study thus presents evidence of a
genuine quantum Griffiths phenomenon in a two-dimensional Heisenberg
antiferromagnet.Comment: 14 pages, 17 figures; published versio
Manifestation of spin-charge separation in the dynamic dielectric response of one--dimensional Sr2CuO3
We have determined the dynamical dielectric response of a one-dimensional,
correlated insulator by carrying out electron energy-loss spectroscopy on
Sr2CuO3 single crystals. The observed momentum and energy dependence of the
low-energy features, which correspond to collective transitions across the gap,
are well described by an extended one-band Hubbard model with moderate nearest
neighbor Coulomb interaction strength. An exciton-like peak appears with
increasing momentum transfer. These observations provide experimental evidence
for spin-charge separation in the relevant excitations of this compound, as
theoretically expected for the one-dimensional Hubbard model.Comment: RevTex, 4 pages+2 figures, to appear in PRL (July 13
Monitoring cortical excitability during repetitive transcranial magnetic stimulation in children with ADHD: a single-blind, sham-controlled TMS-EEG study
Background: Repetitive transcranial magnetic stimulation (rTMS) allows non-invasive stimulation of the human brain. However, no suitable marker has yet been established to monitor the immediate rTMS effects on cortical areas in children.
Objective: TMS-evoked EEG potentials (TEPs) could present a well-suited marker for real-time monitoring. Monitoring is particularly important in children where only few data about rTMS effects and safety are currently available.
Methods: In a single-blind sham-controlled study, twenty-five school-aged children with ADHD received subthreshold 1 Hz-rTMS to the primary motor cortex. The TMS-evoked N100 was measured by 64-channel-EEG pre, during and post rTMS, and compared to sham stimulation as an intraindividual control condition.
Results: TMS-evoked N100 amplitude decreased during 1 Hz-rTMS and, at the group level, reached a stable plateau after approximately 500 pulses. N100 amplitude to supra-threshold single pulses post rTMS confirmed the amplitude reduction in comparison to the pre-rTMS level while sham stimulation had no influence. EEG source analysis indicated that the TMS-evoked N100 change reflected rTMS effects in the stimulated motor cortex. Amplitude changes in TMS-evoked N100 and MEPs (pre versus post 1 Hz-rTMS) correlated significantly, but this correlation was also found for pre versus post sham stimulation.
Conclusion: The TMS-evoked N100 represents a promising candidate marker to monitor rTMS effects on cortical excitability in children with ADHD. TMS-evoked N100 can be employed to monitor real-time effects of TMS for subthreshold intensities. Though TMS-evoked N100 was a more sensitive parameter for rTMS-specific changes than MEPs in our sample, further studies are necessary to demonstrate whether clinical rTMS effects can be predicted from rTMS-induced changes in TMS-evoked N100 amplitude and to clarify the relationship between rTMS-induced changes in TMS-evoked N100 and MEP amplitudes. The TMS-evoked N100 amplitude reduction after 1 Hz-rTMS could either reflect a globally decreased cortical response to the TMS pulse or a specific decrease in inhibition
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