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Search Results: 1 - 10 of 461875 matches for " A. Nyffeler "
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Effective Field Theory of the Linear $O(N)$ Sigma Model
A. Nyffeler,A. Schenk
Physics , 1994, DOI: 10.1006/aphy.1995.1065
Abstract: The low energy structure of a theory containing light and heavy particle species which are separated by a mass gap can adequately be described by an effective theory which contains only the light particles. In this work we present a thorough analysis of the effective field theory of the linear $O(N)$ sigma model in the spontaneously broken phase. In particular, we present a detailed discussion of two techniques, a short-distance expansion and a method based on loop-integrals, which can be used to explicitly evaluate the functional relationships between the low energy constants of the effective theory and the parameters of the underlying theory. We furthermore provide a detailed analysis of the matching relation between the linear sigma model and its effective theory, in order to clarify some discrepancies which can be found in the literature.
Effective Field Theory for a Heavy Higgs Boson: Matching and Gauge Invariance
A. Nyffeler,A. Schenk
Physics , 1996,
Abstract: For large values of the Higgs mass the low energy structure of the gauged linear sigma model in the spontaneously broken phase can adequately be described by an effective field theory. We present a manifestly gauge-invariant functional technique to explicitly evaluate the corresponding effective Lagrangian from the underlying theory.
Effective Field Theory for a Heavy Higgs: a Manifestly Gauge Invariant Approach
A. Nyffeler,A. Schenk
Physics , 1995, DOI: 10.1103/PhysRevD.53.1494
Abstract: For large values of the Higgs boson mass the low energy structure of the gauged linear sigma model in the spontaneously broken phase can adequately be described by an effective field theory. In this work we present a manifestly gauge invariant technique to explicitly evaluate the corresponding effective Lagrangian from the underlying theory. In order to demonstrate the application of this functional method, the effective field theory of the abelian Higgs model is thoroughly analyzed. We stress that this technique does not rely on any particular property of the abelian case. The application to the non-abelian theory is outlined.
QCD at Large $θ$ Angle and Axion Cosmology
N. Evans,S. D. H. Hsu,A. Nyffeler,M. Schwetz
Physics , 1996, DOI: 10.1016/S0550-3213(97)00174-0
Abstract: We use the chiral Lagrangian to investigate the global properties of the $N$-flavor QCD vacuum as a function of the $\theta$ parameter. In the case of exact quark degeneracy we find evidence for first order phase transitions at $\theta = \pi \cdot ({\rm odd~ integer})$. The first order transitions are smoothed by quark mass splittings, although interesting effects remain at realistic quark masses. We emphasize the role of the $\eta'$ condensate in our analysis. Finally, we discuss the implications of our results on the internal hadronic structure of axion domain walls and axion cosmology.
Hadronic light-by-light scattering contribution to the muon g-2: an effective field theory approach
M. Knecht,A. Nyffeler,M. Perrottet,E. de Rafael
Physics , 2001, DOI: 10.1103/PhysRevLett.88.071802
Abstract: The hadronic light-by-light contribution to a_{mu}, the anomalous magnetic moment of the muon, is discussed from the point of view of an effective low-energy theory. As an application, the coefficient of the leading logarithm arising from the two-loop graphs involving two anomalous vertices is computed, and found to be positive. This corresponds to a positive sign for the pion-pole contribution to the hadronic light-by-light correction to a_{mu}, and to a sizeable reduction of the discrepancy between the present experimental value of a_{mu} and its theoretical counterpart in the standard model.
Hadronic light-by-light scattering in the muon g 2: impact of proposed measurements of the π0 → γγ decay width and the γ*γ → π0 transition form factor with the KLOE-2 experiment
Nyffeler Andreas
EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20123705002
Abstract: We discuss, how planned measurements at KLOE-2 of the π0 → γ γ decay width and the γ*γ → π0 transition form factor can improve estimates for the numerically dominant pion-exchange contribution to hadronic light-by-light scattering in the muon g 2 and what are the limitations related to the modelling of the off-shellness of the pion.
The electroweak chiral Lagrangian revisited
Andreas Nyffeler
Physics , 1999,
Abstract: Using a manifestly gauge-invariant approach we show that the set of low-energy constants in the electroweak chiral Lagrangian currently used in the literature is redundant. In particular, by employing the equations of motion for the gauge fields, one can choose to remove two low-energy constants which contribute to the self-energies of the gauge bosons. The relation of this result to the experimentally determined values for the oblique parameters S,T and U is discussed. We then evaluate the matching relation between gauge-invariant Green's functions in the full and the effective theory for the case of the Standard Model with a heavy Higgs boson and compare the results for the independent low-energy constants with those for a simple Technicolor model. Since the pattern of the low-energy constants is very different in these two models it may be misleading to mimic any strongly interacting symmetry breaking sector by a heavy Higgs boson. From our investigation we conclude that current electroweak precision data do not really rule out such strongly interacting models.
Rho meson properties in nuclear matter from QCD sum rules
Andreas Nyffeler
Physics , 2000,
Abstract: We study the properties of rho mesons in nuclear matter by means of QCD sum rules at finite density. For increased sensitivity, we subtract out the vacuum contributions. With the spectral function as estimated in the literature, these subtracted sum rules are found to be not well satisfied. We suppose that Landau singularities from higher resonance states in the nearby region in this channel are the cause for this failure.
Chiral Dynamics in the electroweak sector
Andreas Nyffeler
Physics , 2000,
Abstract: Using a chiral Lagrangian we show that strongly interacting models of electroweak symmetry breaking are not in conflict with precision data. Such models, like Technicolor, need not lead to a heavy Higgs-like signal. Furthermore, the allowed values for the low-energy constants in the effective Lagrangian, derived from bounds on the oblique correction parameters S,T,U, are not unnatural. Finally, we point out that there are some problems with gauge invariance, if one tries to relate the oblique parameters to the low-energy constants in the ordinary chiral Lagrangian for QCD of Gasser and Leutwyler. In particular, S cannot be identified with l_5^{GL}.
Eye Movements Discriminate Fatigue Due to Chronotypical Factors and Time Spent on Task – A Double Dissociation
Dario Cazzoli, Chrystalina A. Antoniades, Christopher Kennard, Thomas Nyffeler, Claudio L. Bassetti, René M. Müri
PLOS ONE , 2014, DOI: 10.1371/journal.pone.0087146
Abstract: Systematic differences in circadian rhythmicity are thought to be a substantial factor determining inter-individual differences in fatigue and cognitive performance. The synchronicity effect (when time of testing coincides with the respective circadian peak period) seems to play an important role. Eye movements have been shown to be a reliable indicator of fatigue due to sleep deprivation or time spent on cognitive tasks. However, eye movements have not been used so far to investigate the circadian synchronicity effect and the resulting differences in fatigue. The aim of the present study was to assess how different oculomotor parameters in a free visual exploration task are influenced by: a) fatigue due to chronotypical factors (being a ‘morning type’ or an ‘evening type’); b) fatigue due to the time spent on task. Eighteen healthy participants performed a free visual exploration task of naturalistic pictures while their eye movements were recorded. The task was performed twice, once at their optimal and once at their non-optimal time of the day. Moreover, participants rated their subjective fatigue. The non-optimal time of the day triggered a significant and stable increase in the mean visual fixation duration during the free visual exploration task for both chronotypes. The increase in the mean visual fixation duration correlated with the difference in subjectively perceived fatigue at optimal and non-optimal times of the day. Conversely, the mean saccadic speed significantly and progressively decreased throughout the duration of the task, but was not influenced by the optimal or non-optimal time of the day for both chronotypes. The results suggest that different oculomotor parameters are discriminative for fatigue due to different sources. A decrease in saccadic speed seems to reflect fatigue due to time spent on task, whereas an increase in mean fixation duration a lack of synchronicity between chronotype and time of the day.
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