Ny bestemmelse af neutronens levetid.

Fora ASTRO-FORUM NYT FRA VIDENSKABEN Ny bestemmelse af neutronens levetid.

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  • #317745
    Bjarne
    Bjarne
    Moderator
    • Super Nova

    Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection

    Abstract

    The precise value of the mean neutron lifetime, τn, plays an important role in nuclear and particle physics and cosmology. It is used to predict the ratio of protons to helium atoms in the primordial universe and to search for physics beyond the Standard Model of particle physics. We eliminated loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls. As a result of this approach and the use of an in situ neutron detector, the lifetime reported here [877.7 ± 0.7 (stat) +0.4/–0.2 (sys) seconds] does not require corrections larger than the quoted uncertainties.

    τn is defined by n(t) = n(0)e-t/τn.

    Measurement of free neutron decay to a proton, electron, and antineutrino provides information about the fundamental parameters of the charged weak current of the nucleon and constrains many extensions to the Standard Model at and above the tera–electron volt scale. Knowledge of the mean neutron lifetime, τn, to an accuracy of better than 1 s is necessary to improve big-bang nucleosynthesis predictions of elemental abundances and to search for physics beyond the Standard Model of nuclear and particle physics.

    The neutron lifetime has recently been measured with two different techniques: counting the surviving ultracold neutrons after storage in material-walled traps, with a most precise result of 878.5 ± 0.8 s, and counting the number of decay products emerging from a passing beam of cold neutrons, with a result of 887.7 ± 2.2 s. The results of these techniques disagree by 9.2 s, or 3.9 standard deviations.

    Our experiment was designed to reduce systematic uncertainties by using ultracold neutrons (UCNs) trapped in a storage volume closed by magnetic fields on the bottom and sides and by gravity on top, as previously demonstrated in. In this work, we have used an asymmetric trap to reduce the population of long-lived closed neutron orbits with kinetic energies over the storable energy threshold in the trap. We have also introduced in situ detection of the surviving neutrons to eliminate uncertainties associated with transporting the neutrons to an ex situ detector. Recent storage experiments used storage traps with variable volumes to extrapolate to infinite volume in an attempt to reduce uncertainties associated with losses of neutrons caused by interactions with the material walls. Our experiment had no detectable losses of neutrons caused by interactions with the magnetic and gravitational “walls” of the trap and thus required no extrapolation. In addition, we used a number of techniques to diagnose and eliminate effects of quasi-trapped neutrons. These neutrons have kinetic energies above the trapping potential but nevertheless can reside in the trap in quasi-stable orbits for hundreds of seconds, skewing the long storage time measurements.
    The result presented here does not require corrections to the measured lifetime that are larger than the quoted uncertainties. This result agrees with the previous best measurement of the lifetime for neutron decay by using UCNs stored in a material trap and disagrees with the lifetime for neutron β-decay determined by using the beam technique.

    • Dette emne blev ændret 2 uger, 2 dage siden af Bjarne.
    • Dette emne blev ændret 2 uger, 2 dage siden af Bjarne.
    #317750

    Bjørn Sandåker
    Participant
    • Neutron star

    Og jeg som troede at en døgnflue levde kort … Til gengeld merker jeg at dansk er blevet så meget lettere at læse! 🙂

    Mvh,
    Bjørn

    #317753
    Bjarne
    Bjarne
    Moderator
    • Super Nova

    Jeg husker en tid, hvor neutronens levetid var ganske usikker. Usikkerheden er nu ned under 1 s. Så er der jo spørgsmålet: Er der tale om halveringstiden eller tiden for et fald på en faktor e. Jeg har desværre ikke fået læst artiklen, så jeg nøjes med det engelske Abstract.

    #317758
    Bjarne
    Bjarne
    Moderator
    • Super Nova

    Artiklen er mere interessant end ventet. Levetiden har selvfølgelig stor betydning for dannelsen af de lettere grundstoffer; men den har også interesse i forbindelse med mulige afvigelser fra standardmodellen. Neutronens levetid er nemlig blevet målt på to forskellige måder: (a) ved, som ovenfor beskrevet, at måle levetiden for neutroner fanget i en fælde eller (b) ved at måle alle kendte henfaldsprodukter fra en neutronstråle. Problemet er, at (b) giver en henfaldstid, som er 9 sekunder længere end henfaldstiden fra (a). Der var i dag en artikel på arXiv.org, som foreslår, at tidsforskellen skyldes henfaldsprodukter til en hidtil ukendt mørk partikel. Problemet med det mørke stof er, at man grundlæggende ikke ved, hvilken partikel man skal lede efter.

    #317760
    Bjarne
    Bjarne
    Moderator
    • Super Nova

    Her er så den omtalte artikel, som sammenkæder neutronens henfald med den mulige eksistens af selv-vekselvirkende mørkt stof, som kan forklare problemerne med strukturdannelsen for små afstande.

    Small-scale structure from neutron dark decay

    It was recently proposed that the disagreement in the experimental measurements of the lifetime of the neutron might be eradicated if the neutron decays to particles responsible for the dark matter in the Universe. In this paper we construct a proto- type self-interacting dark matter model which, apart from reproducing the correct relic abundance, resolves all small-scale problems of the ΛCDM paradigm. The theory is compatible with all present cosmological observations and astrophysical bounds.
     

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