109 research outputs found
Systematic Study of Two-Pion Production in NN Collisions -- from Single-Baryon to Di-Baryon Excitations
The two-pion production in nucleon-nucleon collisions has been studied by
exclusive and kinematically complete experiments from threshold up to =
1.36 GeV at CELSIUS-WASA. At near-threshold energies the total and differential
distributions for the and channels are dominated by
Roper excitation and its decay into and channels. At beam
energies 1.1 GeV the excitation governs the two-pion
production process. In the channel evidence is found for the
excitation of a higher-lying I=3/2 resonance, favorably the . The
isovector fusion processes leading to the deuteron and to quasi-stable He,
respectively, %with the production of an isovector pion-pair exhibit no or only
a modest ABC-effect, {\it i.e.} low-mass enhancement in the -invariant
mass spectrum, and can be described by conventional -channel
excitation. On the other hand, the isoscalar fusion process to the deuteron
%with the production of an isoscalar pion-pair exhibits a dramatic ABC-effect
correlated with a narrow resonance-like energy dependence in the total cross
section with a width of only 50 MeV and situated at a mass 90 MeV below the
mass.Comment: Proceedings HADRON0
Encounters with di-baryons - from the ABC effect to a new resonance?
The ABC effect, an intriguing low-mass enhancement in the invariant mass spectrum, is known from inclusive measurements of two-pion production in nuclear fusion reactions to the few-body systems d, He and He. It was first observed 1960 by Abashian, Booth and Crowe in the inclusive pd He X reaction. Its explanation has been a puzzle since then. In an effort to solve this long-standing problem by exclusive and kinematically complete high-statistics experiments, we have measured the fusion reactions to d, He and He with WASA-at-COSY. These measurements cover the full energy region, where the ABC effect has been observed previously in inclusive reactions. In a recent kinematically complete measurement of the reaction we have shown that the ABC effect in this basic double-pionic fusion reaction is correlated with a narrow structure in the total cross section with quantum numbers I(J) = 0(3), a mass of 2.37 GeV and a width of about 70 MeV. The mass is about 90 MeV below 2 times the mass of , the mass of a system, and the width is three times narrower than expected from a conventional t-channel process. In the double-pionic fusion reaction to the helium isotope He again the ABC effect is observed to be correlated with the appearance of a resonance-like structure in the total cross section at the same excess energy. From a previous exclusive experiment at CELSIUS-WASA it is known that the double-pionic fusion to He also exhibits a pronunced ABC effect. New data from COSY on the He reaction scanning the full ABC region are presented as well as the status of measurements in other reaction channels, where the new resonance might contribute
Double-Pionic Fusion of Nuclear Systems and the ABCEffect -- Aproaching a Puzzle by Exclusive and Kinematically Complete Measurements
The ABC effect - a puzzling low-mass enhancement in the invariant
mass spectrum - is well-known from inclusive measurements of two-pion
production in nuclear fusion reactions. Here we report on first exclusive and
kinematically complete measurements of the most basic double pionic fusion
reaction at 1.03 and 1.35 GeV. The measurements, which
have been carried out at CELSIUS-WASA, reveal the ABC effect to be a
channel phenomenon associated with both a resonance-like
energy dependence in the integral cross section and the formation of a
system in the intermediate state. A corresponding simple
s-channel resonance ansatz provides a surprisingly good description of the
data
Optical nanofibers and spectroscopy
We review our recent progress in the production and characterization of
tapered optical fibers with a sub-wavelength diameter waist. Such fibers
exhibit a pronounced evanescent field and are therefore a useful tool for
highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist
or of the medium surrounding. We use a carefully designed flame pulling process
that allows us to realize preset fiber diameter profiles. In order to determine
the waist diameter and to verify the fiber profile, we employ scanning electron
microscope measurements and a novel accurate in situ optical method based on
harmonic generation. We use our fibers for linear and non-linear absorption and
fluorescence spectroscopy of surface-adsorbed organic molecules and investigate
their agglomeration dynamics. Furthermore, we apply our spectroscopic method to
quantum dots on the surface of the fiber waist and to caesium vapor surrounding
the fiber. Finally, towards dispersive measurements, we present our first
results on building and testing a single-fiber bi-modal interferometer.Comment: 13 pages, 18 figures. Accepted for publication in Applied Physics B.
Changes according to referee suggestions: changed title, clarification of
some points in the text, added references, replacement of Figure 13
Power dependence of supercontinuum noise in uniform and tapered PCFs
An error was made in the calculation of the relative intensity noise (RIN) because of an incorrectly specified value of the photodetector DC transimpedance gain
Cross section ratio and angular distributions of the reaction p + d -> 3He + eta at 48.8 MeV and 59.8 MeV excess energy
We present new data for angular distributions and on the cross section ratio
of the p + d -> 3He + eta reaction at excess energies of Q = 48.8 MeV and Q =
59.8 MeV. The data have been obtained at the WASA-at-COSY experiment
(Forschungszentrum J\"ulich) using a proton beam and a deuterium pellet target.
While the shape of obtained angular distributions show only a slow variation
with the energy, the new results indicate a distinct and unexpected total cross
section fluctuation between Q = 20 MeV and Q = 60 MeV, which might indicate the
variation of the production mechanism within this energy interval.Comment: 9 pages, 9 figure
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