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
July 2009 Archives
The late 19th century saw a radical innovation in horse racing: Jockeys abandoned a comfortable, upright posture for the hunched-over, short-stirrup style seen today at racetracks (and in the figure). By 1910, when the style was universally adopted, race times had dropped by more than 5%; the improvement in the first decade of the 20th century was greater than in the hundred years since. One might think that the reduced aerodynamic drag of the new style led to the faster times, but Thilo Pfau and colleagues at the University of London’s Royal Veterinary College suggest that the way the modern jockey moves in response to the horse’s galloping makes the more significant contribution. The London group measured the motion of jockeys and horses and observed that jockeys do not suffer much vertical displacement as a horse races—a consequence of the way they absorb the horse’s motion by strenuously pumping their legs while riding. So, though the horse supports the jockey’s weight, it does not expend unnecessary energy lifting and lowering its cargo. Moreover, the horse’s forward speed varies over the course of a gallop cycle. When the horse is moving faster than on average, the jockey moves slower. That out-of-phase response could help the horse to execute a smoother, more energy-efficient gallop. (T. Pfau et al., Science 325, 289, 2009; photo by Pharaoh Hound) —Steven K. Blau
In principle, setting a droplet in motion inside a microfluidic channel is straightforward: Apply pressure and the liquid flows. In practice, however, precise control of droplet flow simultaneously along multiple channels is technically challenging; conventional pressure pumps are not feasible for microfluidic systems. Inspired by the potential of finely tunable acoustic-pressure generators, a group of engineers at the University of Michigan set out to control droplet motion with music. First, they composed a computer-synthesized sequence of single notes and chords. That signal was then sent to four resonance cavities that were tuned according to their lengths to extract and amplify narrow, non-overlapping frequency bands from the input tones. As shown in the figure and the movie, unidirectional droplet flow was generated from the difference between positive air pressure in the oscillating cavity and relative negative pressure at vent ports near the cavity's outlet. Although the relatively high frequencies of the selected tones produced steady flow, the researchers adjusted the relative amplitudes of the input tones as needed to compensate for variations in average flow velocity. Maybe someday, conducting complex lab-on-chip microfluidic operations will be as simple as stringing together a few musical notes. (S. M. Langelier et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0900043106.) — Jermey N. A. Matthews
Today's best fundamental theories—whether for gravity, electrodynamics, or elementary particles—say that the laws of physics are identical for all inertial observers, independent of their speed or direction of motion. That so-called local Lorentz invariance has been well tested for quantum field theories (see Physics Today, July 2004, page 40). To date, however, the LLI of gravitational interactions has received little attention, mostly because the weakness of gravity requires exquisitely sensitive experiments. In general, LLI tests are examined within the "standard model extension," which incorporates a series of coefficients, nine of which reflect gravitational effects. Any nonzero coefficients demonstrate violations of LLI and could reveal clues about quantum gravity, variants on general relativity, or other physics beyond the standard model. Some previously undetermined coefficients have now been pinned down by Holger Müller of the University of California, Berkeley, and his colleagues. Using an atom interferometer with an atomic fountain, they looked for anomalous variations in the gravitational acceleration g as Earth revolves through space. The physicists combined new results with those from previous experimental runs and with lunar-ranging data (see Physics Today, May 1996, page 26). The bottom line? Of the nine independent gravitational coefficients, five are now known to be zero to within parts per billion, and three to parts per million. One remains undetermined. The team also established that further improvements can come from using horizontal devices—perhaps guided atoms. (K.-Y. Chung et al., Phys. Rev. D 80, 016002, 2009.) —Stephen G. Benka
The types of galaxies that populate our universe have changed over time. The population evolution can be at least partly unraveled by studying the cores of rich clusters of galaxies, then comparing them across cosmic time—at different cosmological redshifts z. But few such clusters are known with z > 1. At those higher redshifts, the most prominent spectral feature of the elliptical galaxies at rich clusters' cores—the 4000-Å break in the continuum that occurs when there is a dearth of hot, blue stars—is shifted into the IR. A proven detection method is to scan the sky in two wavelength bands, one on either side of the 4000-Å break, but only recently have near-IR detectors become available that allow ground-based telescopes to see the "blueward" side of the shifted spectral break in distant clusters. A new study, named the Spitzer Adaptation of the Red-sequence Cluster Survey, found hundreds of new galaxy cluster candidates using relatively modest observational resources. The astronomers surveyed high-z galaxies by combining wide-field observations from telescopes in Hawaii and Chile with archival IR data from the Spitzer Space Telescope. Follow-up observations of the first three candidates confirmed them to be massive clusters at z ~ 1.2–1.3, firmly in the realm where evolutionary effects can begin to be studied. The authors expect to confirm many additional distant clusters from the survey. (A. Muzzin et al., Astrophys. J. 698, 1934, 2009; G. Wilson et al., Astrophys. J. 698, 1943, 2009.) —Stephen G. Benka
Human skin, the largest organ in our body, is a sensitive detector of both pressure and temperature. Efforts to develop similar sensors for electronics are widespread, and many of the tools are already well known: Piezoelectric materials generate electrical signals in response to changes in applied pressure, and pyroelectrics are sensitive to changes in temperature. Unfortunately, most materials that fall into one of those categories also fall into the other, which makes it difficult to discriminate between pressure and temperature changes. But an international team is reporting a nanocomposite that separates the two sensitivities. The bifunctional material features nanoparticles of the piezoelectric ceramic lead titanate embedded in a ferroelectric polymer that can be pressed into a film 30 µm thick. The polarizations of the two constituents can be configured independently. In particular, an alternating voltage can be used to orient the polymer's polarization with respect to the ceramic's. When the polarizations are parallel, the piezoelectric coefficients of the polymer and composite cancel, whereas the pyroelectric response is enhanced; when antiparallel, the material displays only a piezoelectric response. By controlling the phase of the last cycle of the AC voltage applied to different parts of their composite film, the researchers defined regions that were sensitive to either pressure or temperature. The film thus prepared could be mounted to a flexible foil containing silicon or organic transistors; the figure shows a prototype with two sensor regions. Initial results showed linear responses by the pressure- and temperature-sensitive regions with only limited cross-sensitivities. (I. Graz et al., J. Appl. Phys., in press.) —Richard J. Fitzgerald
Iceland, located on top of the Mid-Atlantic Ridge, is one of the most geologically active places on Earth. Its Katla volcano—buried deep under a huge glacier—is particularly large and active: It tends to explosively erupt every 50 or 60 years, although its most recent eruption was back in 1918. One type of nearby seismic activity is so-called long-period (lp) earthquakes, which have shallow origins and magnitudes of less than 3.3. Swarms of those quakes are sometimes thought to indicate an imminent eruption; after more than 900 lp events took place in October 2002, local authorities developed evacuation plans. But Kristín Jónsdóttir (shown here making field measurements) and her colleagues at Uppsala University in Sweden conclude that the glacier, not the volcano, is the culprit. As the glacier gradually flows down a long valley, part of it reaches a cliff where gargantuan 80-meter-thick ice sheets break off and drop 100 meters, carrying more than enough energy to account for the seismicity. The researchers analyzed data from more than 14 000 lp events in 1991–2007. The events' locations, waveforms, seasonal variations, and other variables all pointed to the calving outlet glacier as the seismic source. The authors note that continued global warming could increase such seismicity. For more on glaciers and earthquakes, see Physics Today, September 2008, page 17. (K. Jónsdóttir et al., Geophys. Res. Lett. 36, L11402, 2009, doi:10.1029/2009GL038234.) —Stephen G. Benka
Familiar software-based “random number generators” rely on deterministic algorithms, so their outputs are not actually random. For some applications, such as Monte Carlo simulations or randomized music playlists, that’s not a problem. But for others, such as secure communications or online gaming, it’s important to use numbers that are truly unguessable, such as can be generated from measurements of stochastic physical processes. One candidate is the chaotic output of a semiconductor laser: When a certain fraction of laser light is fed back into the laser, small intensity fluctuations (thought to be quantum in origin) are amplified into large, irregular, subnanosecond oscillations, as shown in the figure. Last year, Atsushi Uchida (Takushoku University, Tokyo) and colleagues used digitized measurements of two semiconductor lasers to generate random binary sequences at a rate of 1.7 billion bits per second—much faster than any previous method based on a physical system (A. Uchida et al., Nat. Photonics 2, 728, 2008). Small drifts in the lasers’ average intensities can throw off the balance of ones and zeros, but with the lasers suitably tuned, the sequences meet statistical criteria for randomness. Now, Michael Rosenbluh, Ido Kanter, and their students at Bar-Ilan University in Ramat-Gan, Israel, have developed a new method, using just one laser, that avoids the problem of intensity drift. From the differences between successive intensity measurements, they can generate 12.5 billion random bits per second. (I. Reidler et al., Phys. Rev. Lett., in press.) —Johanna Miller
A gamma-ray burst detected in April by NASA’s Swift orbiter has a higher redshift (z = 8.26 ± 0.08) than any other celestial entity for which a redshift has been measured—except for the cosmic microwave background (CMB) at z ≈ 1100. That means the massive star whose collapse to a black hole the GRB is presumed to manifest was significantly more distant than any star or galaxy yet observed. Its demise provides a glimpse of the cosmos just 625 million years after the Big Bang. Beyond revealing that such stars already existed back then and providing a first approximation to their formation rate, the discovery adds a potentially powerful new probe to the search for the first generation of stars and the investigation of how UV radiation from early stars reionized the intergalactic medium. After the first moment of cosmic transparency, signaled by the CMB, and before there were stars, almost all the primordial hydrogen and helium was unionized. To reconstruct the history of cosmic reionization, one seeks to measure the absorption by neutral atomic hydrogen of light arriving from sources at various very high redshifts. Such observations with quasars have revealed that cosmic reionization was essentially complete by z = 6 (950 Myr after the Big Bang). But high-redshift GRBs seem to be essential for tracing its earlier stages. GRBs are briefly luminous enough to be seen at much greater distances than quasars. (N. R. Tanvir et al., http://arxiv.org/abs/0906.1577; R. Salvaterra et al., http://arxiv.org/abs/0906.1578.)—Bertram Schwarzschild
