707 research outputs found
Nonmesonic decay of the Lambda-hyperon in hypernuclei produced by p+Au collisions
The lifetime of the Lambda-hyperon for the nonmesonic decay Lambda N ---> N N
has been determined by a measurement at COSY Juelich of the delayed fission of
heavy hypernuclei produced in proton - Au collisions at T_p=1.9 GeV. It is
found that heavy hypernuclei with mass numbers A= 180 +- 5 and atomic numbers
Z= 74 +-2 fission with a lifetime
130ps +- 13ps (stat.) +- 15ps (syst.) . This value together with the results
obtained for other heavy hypernuclei in previous investigations indicates (on
the confidence level of 0.9) a violation of the phenomenological Delta I = 1/2
rule for Lambda N ---> NN transitions as known from the weak mesonic decays of
kaons and hyperons. PACS:
{13.30.-a}{Decays of baryons}
{13.75.Ev}{Hyperon-nucleon interaction}
{21.80}{Hypernuclei}
{25.80.Pw}{Hyperon-induced reactions}Comment: 3 pages, 2 Postscript figures, uses svepj.clo and svjour.cls.
submitted to European Physical Journal
The lifetime of the Lambda hyperon bound in hypernuclei produced by p+U collisions
The nonmesonic decay of the Lambda hyperon has been investigated by
observation of delayed fission from heavy hypernuclei produced in proton-U
collisions at Tp = 1.9 GeV. The lifetime of heavy hypernuclei with masses A
approximately 220 obtained in the present work, i.e.
tau = 138 +- 6 (stat.) +-m 17 (syst.) ps,
is the most accurate result for heavy hypernuclei produced in proton and
antiproton induced collisions on a U target so far.
PACS: {13.30.-a}{Decays of baryons} {13.75.Ev}{Hyperon-nucleon interaction}
{21.80}{Hypernuclei} {25.80.Pw}{Hyperon-induced reactions}Comment: 16 pages, 4 Postscript figures, uses file appolb.cls (included),
submitted to Acta Physica Polonica B, http://th-www.if.uj.edu.pl/act
Nonmesonic decay of the Lambda hyperon in nuclear matter - implications on the weak Lambda-N interaction
The lifetime of the Lambda hyperon in heavy hypernuclei as measured by the
COSY-13 Collaboration in proton - Au, Bi and U collisions has been analysed to
yield tau(Lambda) = (145 +- 11) ps. This value for tau(Lambda) is compatible
with the lifetime extracted from antiproton annihilation on Bi and U targets,
however, much more accurate. We find that the dependence of the lifetime
tau(Lambda) on the mass of hypernuclei indicates a violation of the
phenomenological Delta I = 1/2 rule known from the weak mesonic decays of
strange particles. PACS: {13.30.-a}{Decays of baryons}
{13.75.Ev}{Hyperon-nucleon interaction} {21.80}{Hypernuclei}
{25.80.Pw}{Hyperon-induced reactions}Comment: 21 pages, 11 PostScript figures, EPJA in prin
Bulk meltwater flow and liquid water content of snowpacks mapped using the electrical self-potential (SP) method
Our ability to measure, quantify and assimilate hydrological properties and processes of snow in operational models is disproportionally poor compared to the significance of seasonal snowmelt as a global water resource and major risk factor in flood and avalanche forecasting. We show here that strong electrical self-potential fields are generated in melting in situ snowpacks at Rhone Glacier and Jungfraujoch Glacier, Switzerland. In agreement with theory, the diurnal evolution of self-potential magnitudes ( ∼ 60–250mV) relates to those of bulk meltwater fluxes (0–1.2 × 10−6m3s−1) principally through the permeability and the content, electrical conductivity and pH of liquid water. Previous work revealed that when fresh snow melts, ions are eluted in sequence and electrical conductivity, pH and self-potential data change diagnostically. Our snowpacks had experienced earlier stages of melt, and complementary snow pit measurements revealed that electrical conductivity ( ∼ 1–5 × 10−6Sm−1) and pH ( ∼ 6.5–6.7) as well as permeabilities (respectively ∼ 9.7 × 10−5 and ∼ 4.3 × 10−5m2 at Rhone Glacier and Jungfraujoch Glacier) were invariant. This implies, first, that preferential elution of ions was complete and, second, that our self-potential measurements reflect daily changes in liquid water contents. These were calculated to increase within the pendular regime from ∼ 1 to 5 and ∼ 3 to 5.5% respectively at Rhone Glacier and Jungfraujoch Glacier, as confirmed by ground truth measurements. We conclude that the electrical self-potential method is a promising snow and firn hydrology sensor owing to its suitability for (1) sensing lateral and vertical liquid water flows directly and minimally invasively, (2) complementing established observational programs through multidimensional spatial mapping of meltwater fluxes or liquid water content and (3) monitoring autonomously at a low cost. Future work should focus on the development of self-potential sensor arrays compatible with existing weather and snow monitoring technology and observational programs, and the integration of self-potential data into analytical frameworks.ISSN:1994-0416ISSN:1994-042
Measurement of the isospin-filtering dd -> 4He K+ K- reaction at Q=39 MeV
The total cross section for the dd -> 4He K+ K- reaction has been measured at
a beam momentum of 3.7 GeV/c, corresponding to an excess energy of 39 MeV,
which is the maximum possible at the Cooler Synchrotron COSY-J\"{u}lich. A
deuterium cluster-jet target and the ANKE forward magnetic spectrometer, placed
inside the storage ring, have been employed in this investigation. We find a
total cross section of sigma(tot) < 14 pb, which brings into question the
viability of investigating the dd -> 4He a0(980) reaction as a means of
studying isospin violation.Comment: Five pages with three eps figure
The Near-Threshold Production of Phi Mesons in pp Collisions
The pp->pp phi reaction has been studied at the Cooler Synchrotron
COSY-Juelich, using the internal beam and ANKE facility. Total cross sections
have been determined at three excess energies epsilon near the production
threshold. The differential cross section closest to threshold at epsilon=18.5
MeV exhibits a clear S-wave dominance as well as a noticeable effect due to the
proton-proton final state interaction. Taken together with data for pp
omega-production, a significant enhancement of the phi/omega ratio of a factor
8 is found compared to predictions based on the Okubo-Zweig-Iizuka rule.Comment: 4 Pages, 3 Figures, 1 Table, submitted to Phys. Rev. Let
Kaon Pair Production in Proton--Proton Collisions
The differential and total cross sections for kaon pair production in the
pp->ppK+K- reaction have been measured at three beam energies of 2.65, 2.70,
and 2.83 GeV using the ANKE magnetic spectrometer at the COSY-Juelich
accelerator. These near-threshold data are separated into pairs arising from
the decay of the phi-meson and the remainder. For the non-phi selection, the
ratio of the differential cross sections in terms of the K-p and K+p invariant
masses is strongly peaked towards low masses. This effect can be described
quantitatively by using a simple ansatz for the K-p final state interaction,
where it is seen that the data are sensitive to the magnitude of an effective
K-p scattering length. When allowance is made for a small number of phi events
where the K- rescatters from the proton, the phi region is equally well
described at all three energies. A very similar phenomenon is discovered in the
ratio of the cross sections as functions of the K-pp and K+pp invariant masses
and the identical final state interaction model is also very successful here.
The world data on the energy dependence of the non-phi total cross section is
also reproduced, except possibly for the results closest to threshold.Comment: 12 two-column pages, 12 figures, 1 tabl
Non-equilibrium emission of complex fragments from p+Au collisions at 2.5 GeV proton beam energy
Energy and angular dependence of double differential cross sections
d/ddE was measured for reactions induced by 2.5 GeV protons
on Au target with isotopic identification of light products (H, He, Li, Be, and
B) and with elemental identification of heavier intermediate mass fragments (C,
N, O, F, Ne, Na, Mg, and Al). It was found that two different reaction
mechanisms give comparable contributions to the cross sections. The
intranuclear cascade of nucleon-nucleon collisions followed by evaporation from
an equilibrated residuum describes low energy part of the energy distributions
whereas another reaction mechanism is responsible for high energy part of the
spectra of composite particles. Phenomenological model description of the
differential cross sections by isotropic emission from two moving sources led
to a very good description of all measured data. Values of the extracted
parameters of the emitting sources are compatible with the hypothesis claiming
that the high energy particles emerge from pre-equilibrium processes consisting
in a breakup of the target into three groups of nucleons; small, fast and hot
fireball of 8 nucleons, and two larger, excited prefragments, which
emits the light charged particles and intermediate mass fragments. The smaller
of them contains 20 nucleons and moves with velocity larger than the CM
velocity of the proton projectile and the target. The heavier prefragment
behaves similarly as the heavy residuum of the intranuclear cascade of
nucleon-nucleon collisions. %The mass and charge dependence of the total
production cross %sections was extracted from the above analysis for all
observed %reaction products. This dependence follows the power low behavior
%(A or Z)
The energy dependence of the pp->K+ n Sigma+ reaction close to threshold
The production of the Sigma+ hyperon through the pp->K+nSigma+ reaction has
been investigated at four energies close to threshold, 1.826, 1.920, 1.958, and
2.020 GeV. At low energies, correlated K+pi+ pairs can only originate from
Sigma+ production so that their measurement allows the total cross section for
the reaction to be determined. The results obtained are completely consistent
with the values extracted from the study of the K+-proton correlation spectra
obtained in the same experiment. These spectra, as well as the inclusive K+
momentum distributions, also provide conservative upper limits on the Sigma+
production rates. The measurements show a Sigma+ production cross section that
varies roughly like phase space and, in particular, none of the three
experimental approaches used supports the anomalously high near-threshold
pp->K+ nSigma+ total cross section previously reported [T. Rozek et al., Phys.
Lett. B 643, 251 (2006)].Comment: Submitted to PR
- …