253 research outputs found
Search for exchange-antisymmetric two-photon states
Atomic two-photon J=0 J'=1 transitions are forbidden for
photons of the same energy. This selection rule is related to the fact that
photons obey Bose-Einstein statistics. We have searched for small violations of
this selection rule by studying transitions in atomic Ba. We set a limit on the
probability that photons are in exchange-antisymmetric states:
.Comment: 5 pages, 4 figures, ReVTeX and .eps. Submitted to Phys. Rev. Lett.
Revised version 9/25/9
Precise study of the resonance at Q0=(1,0,0) in URu2Si2
New inelastic neutron scattering experiments have been performed on URu2Si2
with special focus on the response at Q0=(1,0,0), which is a clear signature of
the hidden order (HO) phase of the compound. With polarized inelastic neutron
experiments, it is clearly shown that below the HO temperature (T0 = 17.8 K) a
collective excitation (the magnetic resonance at E0 \approx 1.7 meV) as well as
a magnetic continuum co-exist. Careful measurements of the temperature
dependence of the resonance lead to the observation that its position shifts
abruptly in temperature with an activation law governed by the partial gap
opening and that its integrated intensity has a BCS-type temperature
dependence. Discussion with respect to recent theoretical development is made
Career Success in Different Countries : Reflections on the 5C Project
fi=vertaisarvioitu|en=peerReviewed
Pressure Collapse of the Magnetic Ordering in MnSi via Thermal Expansion
The itinerant quasi-ferromagnetic metal MnSi has been studied by detailed
thermal expansion measurements under pressures and magnetic fields. A sudden
decrease of the volume at the critical pressure Pc ~1.6 GPa has been observed
and is in good agreement with the pressure variation of the volume fraction of
the spiral magnetic ordering. This confirms that the magnetic order disappears
by a first order phase transition. The energy change estimated by the volume
discontinuity on crossing Pc is of similar order as the Zeeman energy of the
transition from the spiral ground state to a polarized paramagnetic one under
magnetic field. In contrast to the strong pressure dependence of the transition
temperature, the characteristic fields are weakly pressure dependent,
indicating that the strength of the ferromagnetic and the Dzyaloshinskii-Moriya
interactions do not change drastically around Pc. The evaluated results of the
thermal expansion coefficient and the magnetostriction are analyzed
thermodynamically. The Sommerfeld coefficient of the linear temperature term of
the specific heat is enhanced just below Pc. The magnetic field-temperature
phase diagrams in the ordered and paramagnetic phases are also compared.
Comparison is made with other heavy fermion compounds with first order phase
transition at 0 K.Comment: 9 pages, 13 figures, accepted to be published in JPS
Assembly and dynamics of the bacteriophage T4 homologous recombination machinery
Homologous recombination (HR), a process involving the physical exchange of strands between homologous or nearly homologous DNA molecules, is critical for maintaining the genetic diversity and genome stability of species. Bacteriophage T4 is one of the classic systems for studies of homologous recombination. T4 uses HR for high-frequency genetic exchanges, for homology-directed DNA repair (HDR) processes including DNA double-strand break repair, and for the initiation of DNA replication (RDR). T4 recombination proteins are expressed at high levels during T4 infection in E. coli, and share strong sequence, structural, and/or functional conservation with their counterparts in cellular organisms. Biochemical studies of T4 recombination have provided key insights on DNA strand exchange mechanisms, on the structure and function of recombination proteins, and on the coordination of recombination and DNA synthesis activities during RDR and HDR. Recent years have seen the development of detailed biochemical models for the assembly and dynamics of presynaptic filaments in the T4 recombination system, for the atomic structure of T4 UvsX recombinase, and for the roles of DNA helicases in T4 recombination. The goal of this chapter is to review these recent advances and their implications for HR and HDR mechanisms in all organisms
Phylogenetic Distribution of Intron Positions in Alpha-Amylase Genes of Bilateria Suggests Numerous Gains and Losses
Most eukaryotes have at least some genes interrupted by introns. While it is well
accepted that introns were already present at moderate density in the last
eukaryote common ancestor, the conspicuous diversity of intron density among
genomes suggests a complex evolutionary history, with marked differences between
phyla. The question of the rates of intron gains and loss in the course of
evolution and factors influencing them remains controversial. We have
investigated a single gene family, alpha-amylase, in 55 species covering a
variety of animal phyla. Comparison of intron positions across phyla suggests a
complex history, with a likely ancestral intronless gene undergoing frequent
intron loss and gain, leading to extant intron/exon structures that are highly
variable, even among species from the same phylum. Because introns are known to
play no regulatory role in this gene and there is no alternative splicing, the
structural differences may be interpreted more easily: intron positions, sizes,
losses or gains may be more likely related to factors linked to splicing
mechanisms and requirements, and to recognition of introns and exons, or to more
extrinsic factors, such as life cycle and population size. We have shown that
intron losses outnumbered gains in recent periods, but that “resets”
of intron positions occurred at the origin of several phyla, including
vertebrates. Rates of gain and loss appear to be positively correlated. No phase
preference was found. We also found evidence for parallel gains and for intron
sliding. Presence of introns at given positions was correlated to a strong
protosplice consensus sequence AG/G, which was much weaker in the absence of
intron. In contrast, recent intron insertions were not associated with a
specific sequence. In animal Amy genes, population size and
generation time seem to have played only minor roles in shaping gene
structures
A new measurement of the K ± → π ±γγ decay at the NA48/2 experiment
The NA48/2 experiment at CERN collected two data samples with minimum bias trigger conditions in 2003 and 2004. A measurement of the rate and dynamic properties of the rare decay from these data sets based on 149 decay candidates with an estimated background of events is reported. The model-independent branching ratio in the kinematic range is measured to be , and the branching ratio in the full kinematic range assuming a particular Chiral Perturbation Theory description to be
Precise tests of low energy QCD from Ke4 decay properties
We report results from the analysis of the K\ub1 \u2192 pi+ pi 12 e\ub1 \u3bd (Ke4 ) decay by the NA48/2 collaboration at the CERN SPS, based on the total statistics of 1.13 million decays collected in 2003\u20132004. The hadronic form factors in the S- and P-wave and their variation with energy are obtained. The phase difference between the S- and P-wave states of the pion pion system is accurately measured and allows a precise determination of a00 and a02 , the I = 0 and I = 2 S-wave pion pion scattering lengths: a00 = 0.2220 \ub1 0.0128stat \ub1 0.0050syst \ub1 0.0037th , a02 = 120.0432 \ub1 0.0086stat \ub1 0.0034syst \ub1 0.0028th . Combination of this result with the other NA48/2 measurement obtained in the study of K\ub1 \u2192 pi0 pi0 pi\ub1 decays brings an improved determination of a00 and the first precise experimental measurement of a02, providing a stringent test ofChiral Perturbation Theory predictions and lattice QCD calculations. Using constraints based on analyticity and chiral symmetry, even more precise values are obtained: a00 = 0.2196 \ub1 0.0028stat \ub1 0.0020syst and a02 = 120.0444 \ub10.0007stat \ub1 0.0005syst \ub1 0.0008ChPT
Search for the dark photon in pi0 decays
Abstract A sample of 1.69
7 10 7 fully reconstructed \u3c0 0 \u2192 \u3b3 e + e 12 decay candidates collected by the NA48/2 experiment at \{CERN\} in 2003\u20132004 is analyzed to search for the dark photon ( A \u2032 ) production in the \u3c0 0 \u2192 \u3b3 A \u2032 decay followed by the prompt A \u2032 \u2192 e + e 12 decay. No signal is observed, and an exclusion region in the plane of the dark photon mass m A \u2032 and mixing parameter \u3b5 2 is established. The obtained upper limits on \u3b5 2 are more stringent than the previous limits in the mass range 9 \ua0 MeV / c 2 < m A \u2032 < 70 \ua0 MeV / c 2 . The NA48/2 sensitivity to the dark photon production in the K \ub1 \u2192 \u3c0 \ub1 A \u2032 decay is also evaluated
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