Recently in Nuclear & particle physics Category

Since its 1995 discovery in 2-TeV proton–antiproton collisions at Fermilab, the ultramassive top quark (t) has mostly been produced in top–antitop quark pairs via the strong interactions (diagram a, for example), which forbid the production of single top quarks. The standard model of particle theory also predicts single-top production via weak interactions like that in diagram b, with the weak boson W^± replacing the gluon g⁰ in the intermediate state and a bottom quark (b) emerging. But single-top production is much harder to detect than pair production amidst the overwhelming background of more pedestrian processes that can mimic either rare process. That’s because a pair gives the experimenter two chances to see the telltale signals of t decay. So why bother? Yielding a direct measure of the coupling at the tbW vertex, the cross section for single-top production provides a particularly sensitive test of aspects of the standard model such as the presumed absence of a fourth generation of quarks beyond the t and b. Furthermore, the pattern-recognition techniques developed and tested in the search for single-top production are crucial to the quest for the Higgs boson. Now the DZero and CDF detector teams at Fermilab have reported robust observations of single-top production with a cross section of about 3 picobarns (3×10^-36 cm²), consistent with the standard-model prediction. That’s almost half as big as the pair-production cross section, which is severely suppressed by the kinematic requirements for making two ultraheavy quarks. But that modest cross-section disparity also reflects the unifying tendency of the weak and strong interactions to approach each other with increasing energy. (V. M. Abazov et al., http://arxiv.org/abs/0903.0850; T. Aaltonen et al., http://arxiv.org/abs/0903.0885; both in press at Phys. Rev. Lett.)—Bertram Schwarzschild

In any Lorentz-invariant local field theory, particle and antiparticle masses must be identical. That equality has been verified to high precision for leptons and hadrons, but not for quarks. With one exception, it's impossible to measure quark masses directly because a newly created quark "dresses itself" in other quarks and gluons to form a hadron within 10⁻²² seconds. And hadron masses yield, at best, only rough estimates of the quark masses. The exception is the top quark. Almost 200 times heavier than the proton, the top is by far the most massive quark. Its lifetime of 10⁻²⁴ seconds is much too brief to form a hadron. Thus by measuring its decay products, experiments at Fermilab's Tevatron collider have determined the top mass (173 GeV) with a precision of better than 1%. Those experiments were based on the production of top–antitop pairs, and the analyses assumed that the masses were equal. Now the DZero collaboration at the Tevatron has reanalyzed its data to look for a possible mass difference between the two. One can distinguish the top from the antitop by the charge of an energetic lone decay lepton (a muon, electron, or positron) in the event. The reanalysis yields a mass difference of 3.8 ± 3.7 GeV, consistent with zero. But that wasn't a foregone conclusion. The ultimate unified theory of particle interactions might well be something other than a local field theory—perhaps a string theory. (V. M. Abazov et al., http://arxiv.org/abs/0906.1172.)—Bertram Schwarzschild

Several experiments are operating or being built to detect astrophysical neutrinos. Ranging up to about a cubic kilometer in size, those experiments are embedded in ice or in a liquid such as water, where they watch for telltale flashes of Cherenkov radiation. (See the article by Francis Halzen and Spencer Klein in Physics Today, May 2008, page 29.) But the highest-energy neutrinos, with energies of an exaelectron volt (1EeV = 10¹⁸ eV) or higher, are so scarce that installations spanning 100 km³, along with massive numbers of expensive photomultiplier tubes, would be needed to collect adequate event statistics in a reasonable time. So other detection schemes are being explored, one of which involves acoustics: When a very–high-energy neutrino interacts with water or ice, a sudden localized thermal expansion occurs and the resulting wave propagates farther than the light flashes. To explore that method, the Aachen Acoustic Laboratory was set up in late 2007 and its first experiment made a precise measurement of the speed of sound in ice that is entirely devoid of bubbles and cracks. The Aachen physicists carefully positioned an array of sensors—six detectors and one emitter—in a 3-m³ water tank (shown here) equipped with a freeze-control unit and a degassing system. The difference in arrival times of an acoustic pulse at adjacent receivers determined the speed of sound. Between 0 °C and −17 °C, where they took measurements, the speed ranged from about 3840 m/s to 3890 m/s, agreeing well with earlier laboratory experiments. The team is also part of SPATS (the South Pole Acoustic Test Setup), which is currently obtaining complementary in situ measurements. (C. Vogt, K. Laihem, C. Wiebusch, J. Acoust. Soc. Am. 124, 3613, 2008.) —Stephen G. Benka

Through its influence on evaporation rates, humidity levels, and other factors, the moisture content of soil has a significant impact on weather. Accurate measurements of that content, though important for meteorological, hydrological, and ecological forecasting, are difficult to make. Extrapolating point measurements to larger areas is inaccurate, and satellite-based remote-sensing methods are hindered by ground cover, surface roughness, and other limitations. A team from the University of Arizona and the Southwest Watershed Research Center in Tucson has shown that just above the ground surface, so-called fast neutrons with energies on the order of an MeV are quantifiably correlated with soil moisture and thus provide a noninvasive means for measuring the average moisture levels over regions several hundred meters wide and tens of centimeters deep. The neutrons are generated by cosmic rays. Upon collision with atmospheric nuclei, cosmic rays create showers of high-energy secondary particles, and those that reach Earth's surface can penetrate it, collide with nuclei there, and produce among their debris fast neutrons, some of which escape back into the atmosphere. Marek Zreda and colleagues discovered that hydrogen, mostly found in water, dominates soil's ability to moderate fast neutrons and that a strong inverse correlation, independent of soil chemistry, exists between moisture content and the intensity of the fast neutrons that escape out of the ground. The team demonstrated that with an independent measurement of the moisture content for calibration, a neutron detector a few meters above the ground can give precise measurements of soil moisture on the time scale of minutes to a few hours. As the figure shows, the hourly soil moisture determined by a cosmic-ray neutron detector (top) agrees with that determined by time-domain reflectometry probes (middle) and with the monitored daily precipitation (bottom). (M. Zreda et al., Geophys. Res. Lett. 35, L21402, 2008, doi:10.1029/2008GL035655.) — Richard J. Fitzgerald

Two groups of cosmic-ray observers have reported unexpectedly large fluxes of high-energy electrons and positrons. Those excesses suggest either that there are undiscovered astrophysical sources such as radio-quiet pulsars surprisingly nearby or that the positrons and electrons are annihilation products of WIMPs—weakly interacting dark-matter particles hundreds of times more massive than the proton. Standard cosmology predicts that dark nonbaryonic matter dominates the material content of the cosmos. But its constituent particles have yet to be identified. The ATIC balloon collaboration, led by John Wefel of Louisiana State University, reports a significant enhancement in the spectrum of cosmic-ray electrons, peaking near 600 GeV. The peak suggests that 600-GeV WIMPs of the kind predicted by extra-dimensional extensions of standard particle theory might be annihilating with each other to create e⁺e^– pairs in very dense concentrations of dark matter not far from our solar system. The ATIC detector cannot distinguish positrons from the much more abundant cosmic-ray electrons. But the magnetic spectrometer aboard the orbiting PAMELA satellite can. Positrons are routinely produced in collisions between cosmic rays and ordinary interstellar matter. The ratio of such positrons to cosmic-ray electrons was expected to fall steeply with increasing energy. Instead, the PAMELA collaboration, led by Piergiorgio Picozza of the University of Rome “Tor Vergata,” reports that the positron fraction grows steadily with energy from 10 GeV to 100 GeV. So it appears that there must be some additional source of high-energy positrons. The collaboration will continue taking data for at least another year, hoping to find spectral structure suggestive of WIMPs or anisotropy pointing to a nearby astrophysical source. Both WIMP annihilations and pulsars are expected to produce high-energy gamma rays. So for the moment, all eyes are on the recently launched Fermi Gamma-ray Space Telescope (originally called GLAST), which is designed to pinpoint gamma-ray sources and spectral features but can also confirm the ATIC electron result with higher statistics. (J. Chang et al., ATIC collaboration, Nature 456, 362, 2008; O. Adriani et al., PAMELA collaboration, http://arxiv.org/abs/0810.4995.) — Bertram Schwarzschild

The Ω^– baryon played an important role in the evolution of particle theory. Its much heralded discovery in 1964 at precisely the mass (1.67 GeV) predicted by symmetry arguments about charge and strangeness led promptly to the quark model of the strongly interacting particles. The quark model described the Ω^– as a bound state of three strange (s) quarks. The relatively straightforward “naïve” quark model has long since been incorporated into quantum chromodynamics, a much more complete theory from which, however, precise predictions are notoriously hard to extract. But QCD does predict that the Ω^– should have a heavy-quark analogue, called Ω_b^– , with a mass of about 6.0 GeV—more than six times that of the proton. In the Ω_b^– , one of the three s quarks is replaced by the much heavier bottom (b) quark. Now, having combed through 10¹⁴ proton–antiproton collisions accumulated at Fermilab’s Tevatron collider over the past four years, the collaboration that runs the collider’s D0 detector complex has reported finding 18 events in which the expected decay of an Ω_b^– to an Ω^– plus a charmonium meson is clearly discerned. The discovery was difficult not only because so very few collisions produce an Ω_b^– , but also because the newly discovered baryon is so short-lived that it moves only about a millimeter before decaying. The 6.1-GeV mass extracted from the observed events is reassuringly close to that predicted by the number-crunching lattice-gauge calculations to which QCD theorists have to resort. (V. Abasov et al., D0 collaboration, http://arxiv.org/abs/0808.4142.) — Bertram Schwarzschild

In the final run of its fruitful decade-long operation, the PEPII electron–positron collider at SLAC has revealed the lowest-energy state of bottomonium, the heaviest family of mesons. All the bottomonium mesons are bound quark–antiquark pairs of the bottom quark b. And they all have masses near 10 GeV, roughly the mass of a boron atom. Until now, the only known bottomonium mesons have had the spins of the two spin-½ quarks aligned to form triplet spin-1 configurations. Their lower-energy singlet spin-0 hyperfine partners had to exist, but 30 years of looking for them had been in vain. So there was almost no empirical information about the spin dependence of the strong force between b quarks. Quantum chromodynamics, the relevant theory, should in principle predict the hyperfine mass splitting. But QCD predictions are notoriously hard to calculate. Computational tricks based on QCD are presumed to do best with the heaviest quarks. Because top quarks are too short-lived to form mesons or any other particles, b-quark states afford the best opportunity for comparing theory with measurement. Having now discovered the spin-singlet ground state, labeled η_b, the BaBar detector collaboration at PEPII has measured its mass to lie just 71±4 MeV below that of Y(1S), the lowest-mass spin-triplet state, whose discovery in 1977 first revealed the existence of the b quark. BaBar’s measurement of the hyperfine mass splitting provides an important validation of the lattice-QCD computational technique that predicted 61±14 MeV. (A. Aubert et al., http://arxiv.org/abs/0807.1086.) — Bertram M. Schwarzschild