241 research outputs found
Double radiative pion capture on hydrogen and deuterium and the nucleon's pion cloud
We report measurements of double radiative capture in pionic hydrogen and
pionic deuterium. The measurements were performed with the RMC spectrometer at
the TRIUMF cyclotron by recording photon pairs from pion stops in liquid
hydrogen and deuterium targets. We obtained absolute branching ratios of for hydrogen and for deuterium, and
relative branching ratios of double radiative capture to single radiative
capture of for hydrogen
and for
deuterium. For hydrogen, the measured branching ratio and photon energy-angle
distributions are in fair agreement with a reaction mechanism involving the
annihilation of the incident on the cloud of the target proton.
For deuterium, the measured branching ratio and energy-angle distributions are
qualitatively consistent with simple arguments for the expected role of the
spectator neutron. A comparison between our hydrogen and deuterium data and
earlier beryllium and carbon data reveals substantial changes in the relative
branching ratios and the energy-angle distributions and is in agreement with
the expected evolution of the reaction dynamics from an annihilation process in
S-state capture to a bremsstrahlung process in P-state capture. Lastly, we
comment on the relevance of the double radiative process to the investigation
of the charged pion polarizability and the in-medium pion field.Comment: 44 pages, 7 tables, 13 figures, submitted to Phys. Rev.
Observation of double radiative capture on pionic hydrogen
We report the first observation of double radiative capture on pionic
hydrogen. The experiment was conducted at the TRIUMF cyclotron using the RMC
spectrometer, and detected --ray coincidences following stops
in liquid hydrogen. We found the branching ratio for double radiative capture
to be . The measured
branching ratio and angle-energy distributions support the theoretical
prediction of a dominant contribution from the
annihilation mechanism.Comment: 4 Pages, 4 Figures. accepted for publication in Phys. Rev. Let
The CAST Time Projection Chamber
One of the three X-ray detectors of the CAST experiment searching for solar
axions is a Time Projection Chamber (TPC) with a multi-wire proportional
counter (MWPC) as a readout structure. Its design has been optimized to provide
high sensitivity to the detection of the low intensity X-ray signal expected in
the CAST experiment. A low hardware threshold of 0.8 keV is safely set during
normal data taking periods, and the overall efficiency for the detection of
photons coming from conversion of solar axions is 62 %. Shielding has been
installed around the detector, lowering the background level to 4.10 x 10^-5
counts/cm^2/s/keV between 1 and 10 keV. During phase I of the CAST experiment
the TPC has provided robust and stable operation, thus contributing with a
competitive result to the overall CAST limit on axion-photon coupling and mass.Comment: 19 pages, 11 figures and images, submitted to New Journal of Physic
A new limit of T-violating transverse muon polarization in the decay
A search for T-violating transverse muon polarization () in the
decay was performed using kaon decays at rest. A
new improved value, , was
obtained giving an upper limit, . The T-violation parameter
was determined to be Im giving
an upper limit, Im.Comment: 5 pages, 4 figure
New solar axion search in CAST with He filling
The CERN Axion Solar Telescope (CAST) searches for conversion in
the 9 T magnetic field of a refurbished LHC test magnet that can be directed
toward the Sun. Two parallel magnet bores can be filled with helium of
adjustable pressure to match the X-ray refractive mass to the axion
search mass . After the vacuum phase (2003--2004), which is optimal for
eV, we used He in 2005--2007 to cover the mass range of
0.02--0.39 eV and He in 2009--2011 to scan from 0.39--1.17 eV. After
improving the detectors and shielding, we returned to He in 2012 to
investigate a narrow range around 0.2 eV ("candidate setting" of our
earlier search) and 0.39--0.42 eV, the upper axion mass range reachable with
He, to "cross the axion line" for the KSVZ model. We have improved the
limit on the axion-photon coupling to (95% C.L.), depending on the pressure settings. Since 2013, we
have returned to vacuum and aim for a significant increase in sensitivity.Comment: CAST Collaboration 6 pages 3 figure
Search for chameleons with CAST
In this work we present a search for (solar) chameleons with the CERN Axion
Solar Telescope (CAST). This novel experimental technique, in the field of dark
energy research, exploits both the chameleon coupling to matter () and to photons () via the Primakoff effect. By reducing
the X-ray detection energy threshold used for axions from 1keV to 400eV
CAST became sensitive to the converted solar chameleon spectrum which peaks
around 600eV. Even though we have not observed any excess above background,
we can provide a 95% C.L. limit for the coupling strength of chameleons to
photons of for .Comment: 8 pages, 12 figure
A search for two body muon decay signals
Lepton family number violation is tested by searching for
decays among the 5.8 positive muon decay events analyzed by the
TWIST collaboration. Limits are set on the production of both massless and
massive bosons. The large angular acceptance of this experiment allows
limits to be placed on anisotropic decays, which can arise
from interactions violating both lepton flavor and parity conservation.
Branching ratio limits of order are obtained for bosons with masses
of 13 - 80 MeV/c and with different decay asymmetries. For bosons with
masses less than 13 MeV/c the asymmetry dependence is much stronger and
the 90% limit on the branching ratio varies up to . This is
the first study that explicitly evaluates the limits for anisotropic two body
muon decays.Comment: 7 pages, 5 figures, 2 tables, accepted by PR
CAST solar axion search with 3^He buffer gas: Closing the hot dark matter gap
The CERN Axion Solar Telescope (CAST) has finished its search for solar
axions with 3^He buffer gas, covering the search range 0.64 eV < m_a <1.17 eV.
This closes the gap to the cosmological hot dark matter limit and actually
overlaps with it. From the absence of excess X-rays when the magnet was
pointing to the Sun we set a typical upper limit on the axion-photon coupling
of g_ag < 3.3 x 10^{-10} GeV^{-1} at 95% CL, with the exact value depending on
the pressure setting. Future direct solar axion searches will focus on
increasing the sensitivity to smaller values of g_a, for example by the
currently discussed next generation helioscope IAXO.Comment: 5 pages, 2 figures. Last version uploade
Solar axion search with the CAST experiment
The CAST (CERN Axion Solar Telescope) experiment is searching for solar
axions by their conversion into photons inside the magnet pipe of an LHC
dipole. The analysis of the data recorded during the first phase of the
experiment with vacuum in the magnet pipes has resulted in the most restrictive
experimental limit on the coupling constant of axions to photons. In the second
phase, CAST is operating with a buffer gas inside the magnet pipes in order to
extent the sensitivity of the experiment to higher axion masses. We will
present the first results on the data taking as well as the
system upgrades that have been operated in the last year in order to adapt the
experiment for the data taking. Expected sensitivities on the
coupling constant of axions to photons will be given for the recent run just started in March 2008.Comment: Proceedings of the ICHEP 2008 conferenc
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