Ordering electron and nuclear spins in quantum wires

Nuclear and electron spins in a quantum wire may spontaneously form an ordered state at very low temperatures, according to work recently carried out by an international team of physicists. The team was studying the conductance of gallium-arsenide quantum wires and discovered that, at temperatures of 0.1 K and lower, the conductance of the wires dropped below the universal quantized value. This reduced quantization is explained using a theoretical model that proposes that the nuclear and electron spins order themselves in a helical formation at these temperatures.

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Shedding light on the masses of exotic nuclides

The masses of 33 rare, exotic neutron-heavy nuclides have been measured with high precision by scientists at the Argonne National Laboratory’s CAlifornium Rare Isotope Breeder Upgrade (CARIBU) facility in the US. The findings are crucial to understanding how elements that are heavier than iron might have formed. Following the mass measurements, the researchers also compared simulations of astrophysical nuclear reactions using both the measured masses and theoretical models.

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Supernova origin of galactic cosmic rays confirmed

The first direct evidence that galactic cosmic rays are accelerated within supernova remnants has been provided by observations by the Fermi Large Area Telescope collaboration. The results make use of four years of data collected by the telescope observing two supernova remnants – IC 443 and W44 – within our galaxy. The observations fit very neatly with predictions of neutral pion decay.

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Muon-capture measurement backs QCD prediction

The rate at which protons capture muons has been accurately measured for the first time by the MuCap collaboration at the Paul Scherrer Institute (PSI) in Switzerland. This process, which can be thought of as beta decay in reverse, results in the formation of a neutron and a neutrino. The team has also determined a dimensionless factor that influences the rate of muon capture, which was found to be in excellent agreement with theoretical predictions that are based on very complex calculations.

Muons are cousins of the electron that are around 200 times heavier. Beta decays demonstrate the weak nuclear force in which a neutron gets converted into a proton by emitting an electron and a neutrino. Now, replace the electron with the heavier muon and run the process backwards: a proton captures a muon and transforms into a neutron while emitting a neutrino. This process – known as ordinary muon capture (OMC) – is crucial to understanding the weak interaction involving protons.

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BaBar makes first direct measurement of time-reversal violation

The BaBar collaboration has made the first direct observation of time-reversal (T) violation. The results are in agreement with the basic tenets of quantum field theory and reveal differences in the rates at which the quantum states of the B0 meson transform into one another. The researchers say that this measured lack of symmetry is statistically significant and consistent with indirect observations.

The BaBar detector at the PEP-II facility at SLAC in California was designed to study the collisions of electrons and positrons and to determine the differences between matter and antimatter. In particular, physicists working on the experiment are interested in the violation of the charge–parity symmetry (or CP violation). Although the detector was decommissioned in the spring of 2008, data collected during the period of operation continue to be analysed.

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