4,718 research outputs found

    High-power radiating plasma

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    The physical principles underlying the use of radiating plasmas for the optical pumping of lasers are described. Particular consideration is given to the properties of radiating plasmas; radiation selectivity; the dynamics, equilibrium, and stability of radiating plasmas; the radiative Reynolds number; and experimental results on radiating discharges

    The place of the Sun among the Sun-like stars

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    Context. Monitoring of the photometric and chromospheric HK emission data series of stars similar to the Sun in age and average activity level showed that there is an empirical correlation between the average stellar chromospheric activity level and the photometric variability. In general, more active stars show larger photometric variability. Interestingly, the measurements and reconstructions of the solar irradiance show that the Sun is significantly less variable than indicated by the empirical relationship. Aims. We aim to identify possible reasons for the Sun to be currently outside of this relationship. Methods. We employed different scenarios of solar HK emission and irradiance variability and compared them with available time series of Sun-like stars. Results. We show that the position of the Sun on the diagram of photometric variability versus chromospheric activity changes with time. The present solar position is different from its temporal mean position as the satellite era of continuous solar irradiance measurements has accidentally coincided with a period of unusually high and stable solar activity. Our analysis suggests that although present solar variability is significantly smaller than indicated by the stellar data, the temporal mean solar variability might be in agreement with the stellar data. We propose that the continuation of the photometric program and its expansion to a larger stellar sample will ultimately allow us to constrain the historical solar variability.Comment: 10 pages, 5 figures, accepted for publication in Astronomy&Astrophysic

    Theory of Z boson decays

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    The precision data on Z boson decays from LEP-I and SLC colliders are compared with the predictions based on the Minimal Standard Theory. The Born approximation of the theory is based on three most accurately known observables: G_mu -- the four fermion coupling constant of muon decay, m_Z -- the mass of the Z boson, and alpha(m_Z) -- the value of the running fine structure constant at the scale of m_Z. The electroweak loop corrections are expressed, in addition, in terms of the masses of higgs, m_H, of the top and bottom quarks, m_t and m_b, and of the strong interaction constant alpha_s(m_Z). The main emphasis of the review is focused on the one-electroweak-loop approximation. Two electroweak loops have been calculated in the literature only partly. Possible manifestations of new physics are briefly discussed.Comment: 80 pages, 16 figures, accepted in Reports on Progress in Physic

    Electroweak radiative corrections in Z boson decays

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    Contribution to A.D.Sakharov memorial volume. A detailed review of the electroweak radiative corrections to the Z-boson decays in the framework of the Minimal Standard Modelm (MSM) is presented. After a short historical introduction we describe the optimal parametrization of the MSM, especially of the Born approximation, and derive expressions for the one-loop electroweak corrections. Finally a global fit of all relevant experimental data is performed, resulting in fitted values of the top-quark mass, m_t, and strong coupling constant /alpha_s. Allowed range for the value of the Higgs mass, m_H, is discussed. Various details of calculations are described in 16 appendices.Comment: 98 pages,21 EPS and PS figures,uses epsf.sty, separate file with the tared, compressed and uuencoded figures is awailable at http://wwwtheor.itep.ru/~vysotsky/figures.u

    Charmed penguin versus BAU

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    Since the Standard Model most probably cannot explain the large value of CP asymmetries recently observed in D-meson decays we propose the fourth quark-lepton generation explanation of it. As a byproduct weakly mixed leptons of the fourth generation make it possible to save the baryon number of the Universe from erasure by sphalerons. An impact of the 4th generation on BBN is briefly discussed.Comment: 13 pages, 3 figures, version to be published in JETP Letter

    Extending the Higgs sector: an extra singlet

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    An extension of the Standard Model with an additional Higgs singlet is analyzed. Bounds on singlet admixture in 125 GeV h boson from electroweak radiative corrections and data on h production and decays are obtained. Possibility of double h production enhancement at 14 TeV LHC due to heavy higgs contribution is considered.Comment: 18 pages, 7 figures. v2: one equation added; references received after the publication of v1 are adde

    New Physics at 1 TeV?

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    If decays of a heavy particle S are responsible for the diphoton excess with invariant mass 750 GeV observed at the 13 TeV LHC run, it can be easily accomodated in the Standard Model. Two scenarios are considered: production in gluon fusion through a loop of heavy isosinglet quark(s) and production in photon fusion through a loop of heavy isosinglet leptons. In the second case many heavy leptons are needed or/and they should have large electric charges in order to reproduce experimental data on σ(pp→SX)⋅Br(S→γγ)\sigma(pp \to SX) \cdot \mathrm{Br}(S \to \gamma \gamma).Comment: 7 pages, 4 figures, 1 tabl

    On the search for 50 GeV neutrinos

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    Using the computer code CompHEP we estimate the number of events and the background, at LEP II and TESLA, for the reaction e+e−→NNˉγe^+ e^- \to N\bar{N}\gamma, where NN is a hypothetical Dirac neutrino with mass of the order of 50 GeV.Comment: Standard LaTeX, 9 pages, 2 tables, 3 figure

    Precision measurements, extra generations and heavy neutrino

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    The existence of extra chiral generations with all fermions heavier than MZM_Z is strongly disfavored by the precision electroweak data. The exclusion of one additional generation of heavy fermions in SUSY extension of Standard Model is less forbidden if chargino and neutralino have low degenerate masses with Δm≃1\Delta m \simeq 1 GeV. However the data are fitted nicely even by a few extra generations, if one allows neutral leptons to have masses close to 50 GeV. Such heavy neutrino can be searched in the reaction e+e−→NNˉγe^+ e^- \to N\bar{N}\gamma at LEP-200 with total final luminosity of 2600pb−12600 pb^{-1}.Comment: 5 pages, 6 figures, Proceedings ICHEP2000 Osaka conferenc