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Hot Paper #3 reports the measurement of (g – 2)/2 , the so-called anomalous magnetic moment, for the positive muon, carried out at Brookhaven National Laboratory by a group of 70 researchers from 11 institutions. Because the muon is 207 times more massive than the electron, it is 40,000 times more sensitive as a probe of new physics beyond the Standard Model. The muon g-factor differs from the Dirac value by one part in 800. This arises because its magnetic moment interacts with virtual particles and photons. Electromagnetic, weak, and strong interactions all conspire to create the muon anomaly, the value of which is predicted from the Standard Model with a precision of 0.6 ppm. In the Brookhaven experiment, polarized muons are injected into the world’s largest superconducting magnet, the muon storage ring, 15m in diameter. The rate of precession of the spin is directly proportional to the magnetic anomaly. To follow the precession rate a measurement is required, and this is provided through the beta-decay of the muon which emits a detectable positron. Paper #3 describes the analysis of 4 billion positron decays captured in the year 2000. The resulting value has an experimental error of 0.7 ppm, a tenfold improvement for Brookhaven, and in good agreement with other experiments. Although theory and observation have similar uncertainties, the formal probability that the error envelopes are packaging identical values is 2.7 standard deviations, or about 1%. What’s important about #3 is that theory and experiment are diverging as both traditions improve their technique. As the gap has opened up the excitement has increased as physicists get a scent of a new quarry. They have not been slow in conjuring up extensions to the 30-year-old Standard Model. Noting that the value of g – 2 for the proton is 3.6 on account of its complex structure (three quarks and gluons), they are asking if the far smaller deviation of the muon might also indicate a substructure, which would mean the muon is not a truly elementary particle, and neither is the electron. A heady speculation! Supersymmetry, which pairs all the particles we think we understand with shadowy superpartners, is another speculative arena beyond the Standard Model. At Brookhaven, Dr. Bill Morse, the resident spokesperson for the g – 2 collaboration, tells Science Watch of a more recent development. "Besides the paper at #3 on the positive muon, we recently had a paper on the negative muon (see G.W. Bennett, et al., Phys. Rev. Lett., 92(16): 161802, 23 April 2004). Both values agree with each other, as required by the Standard Model, but we still disagree with that model’s calculated value." This result concludes experiment E821 at Brookhaven. The collaboration now plan a new measurement, and they will continue to evaluate the theoretical calculation. Hot Paper #9 reports a significant advance in our understanding of gamma-rays bursts (GRB), transient outbursts lasting from seconds to minutes. Most GRBs are located at cosmological distances, which implies that the energy they release in a few seconds is larger than that of the Sun during its entire lifetime. Since 1998, astrophysicists have suspected a connection between supernova explosions and GRBs, but they lacked proof. That changed dramatically on 29 March 2003. NASA’s High Energy Transient Explorer registered a very bright
GRB. Within 90 minutes of this detection, rapid follow-up operations on
the ground, with a 1-m optical telescope in Australia, detected an optical
afterglow at the position of the GRB. Next the Very Large Telescope in
Chile joined the campaign by securing spectra which documented the
unfolding outburst. In #9, Jens Hjorth (University of Copenhagen) and his
colleagues attribute the event to a rare hypernova explosion, in which the
core of a massive evolved star has collapsed catastrophically to form a
black hole. The explosive backlash then shatters the outer layers of the
star, releasing an intense flood of gamma rays. Dr. Simon Mitton is the
Senior Fellow of
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n
particle physics, the study of spins (angular momentum) and magnetic
moments has played an important role in illuminating the structure of
subatomic matter. In 1928, Paul Dirac found a way to describe the electron
through an equation that satisfied both quantum mechanics and relativity,
in a breakthrough still celebrated as one of the great landmarks of the
history of science. The concept of spin lay at the heart of the new
equation. From the charge, mass, and spin, Dirac derived a value for the
magnetic moment in an expression that included a dimensionless constant,
the g-factor, for gyromagnetic ratio. For a point-like electron in
complete isolation Dirac calculated a g-factor of exactly 2. For
half a century experimentalists have chased ever-higher precision
measurements of g – 2 because the extent of its deviation from
the Dirac value probes the structure of matter.
.
