Recently in Nuclear and particle physics Category

When relativistic heavy nuclei collide, they fleetingly interact to produce hot, dense matter—often interpreted as the quark–gluon plasma—containing roughly equal numbers of quarks and antiquarks. As the matter cools, it changes phase to a hadronic gas that includes nucleons and their antiparticles. And those antinucleons, when close enough in position and momentum, can form a stable bound state. The STAR collaboration, a team of hundreds of scientists from 54 institutions worldwide, has now found evidence for antihelium-4 in the debris created in high-energy collisions at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider. Consisting of two antiprotons and two antineutrons, ⁴He is the heaviest antinucleus yet detected. The experiment’s central detector, situated in a solenoidal magnetic field, is used to image the ionization trail left by charged particles and antiparticles as they traverse a gas-filled chamber. From measurements of the energy loss and the time of flight for the antiparticles to reach a secondary detector composed of 23 000 sensors, the collaboration unambiguously identified 18 ⁴He nuclei in a sample of 10¹² tracks from a billion gold-on-gold collisions; the figure shows such tracks, including one (red) from a ⁴He nucleus, for a typical event. The yield is consistent with expectations from thermodynamic and nucleosynthesis models and provides a benchmark for any future observations of ⁴He, or even heavier antimatter nuclei, from cosmic radiation. (H. Agakishiev et al., STAR collaboration, Nature, in press, doi:10.1038/nature10079.)—R. Mark Wilson

Several lines of evidence suggest that dark matter, the mysterious substance that makes up 83% of the mass of the universe, is made up of subatomic particles known generically as WIMPs (weakly interacting massive particles). On cosmic scales, the presumed influence of WIMPs is easy to spot; the particles' collective gravity controls the distribution and motion of stars in galaxies. Detecting single WIMPs is far harder. After running continuously for 100 days, the XENON100 experiment has now yielded the most stringent limits on WIMP properties. Situated in a cavern in Italy's Apennine mountains, XENON100 consists of a bank of photomultiplier tubes that monitor 100 kg of liquid xenon. If a WIMP—or any other energetic particle—were to collide with one of the xenon atoms, the resulting scintillation would be detected. The overlying rock prevents cosmic muons from reaching the xenon, but the rock also contains radioactive elements, whose gamma-ray emission can beget scintillations. Although XENON100 incorporates ways to discriminate between the two sources of scintillations, some residual, unremovable background remains. In a paper submitted to Physical Review Letters, the XENON100 team report detecting three events that survived all the discrimination criteria. Given that the expected background was 1.8 events, the three events don't constitute a firm detection, but they do lower the limits on the WIMP mass and the WIMP–nucleon cross section. Supersymmetry, an extension to the standard model of particle physics, predicts the existence of WIMPs of order 100 GeV/c ², which is both consistent with the XENON100 results and within reach of the Large Hadron Collider. (E. Aprile et al., http://arxiv.org/abs/1104.2549.)—Charles Day

The Crab nebula, shown here in a Hubble Space Telescope image, is the remnant of a supernova explosion so nearby that Chinese astrologers took note of it in AD 1054. Powered by the rapidly spinning neutron star at its heart, the nebula shines with extraordinary luminosity at all photon energies from radio to TeV gammas. The apparent stability of the Crab’s brightness over that entire range has made it a convenient reference for calibrating astronomical instruments; the brightness of other sources is often quoted in “millicrabs.” But now two teams that operate orbiting gamma-ray telescopes—the Italian Space Agency’s AGILE and NASA’s Fermi—have reported seeing powerful short-duration flaring of the Crab nebula at gamma energies ranging from 100 MeV to 1 GeV. Lasting a few days or weeks, and apparently occurring once or twice a year, the outbursts manifest synchrotron radiation by electrons and positrons in the nebula that have somehow been accelerated to extremely high energies on an astonishingly rapid time scale. The flares rudely challenge the prevailing theory of how charged particles are accelerated, very gradually, in shock fronts within supernova remnants. The theory goes back to Enrico Fermi’s 1949 attempt to explain the origins of cosmic rays. Now the Crab has been put under close surveillance by orbiting x-ray and optical telescopes as well as AGILE and Fermi, in hopes of pinpointing longer-wavelength clues within the nebula during the next gamma outburst. (M. Tavani et al., AGILE collaboration, Science, in press, doi:10.1126/science.1200083; A. A. Abdo et al., Fermi collaboration, Science, in press, doi:10.1126/science.1199705.)—Bertram Schwarzschild

From among 10¹³ proton–proton collisions at 7 GeV in its first year of operation, the Large Hadron Collider (LHC) at CERN has as yet yielded no evidence of black hole production. The detectable creation of microscopic black holes at the LHC follows from speculative but attractive theories that seek to explain the puzzling weakness of gravity by positing curled-up extra spatial dimensions accessible only to gravitons. In such theories, the intrinsic strength of gravity would be comparable to those of the electromagnetic and weak interactions at energies near 1 TeV, where electroweak unification occurs. But now the collaboration that runs the LHC’s Compact Muon Solenoid (CMS) detector, having found no evidence of black holes, has published the first experimental lower limits on their masses. A black hole produced in a 7-TeV collision would decay by Hawking radiation within 10^-27 s into perhaps half a dozen extraordinarily energetic particles—mostly quarks and gluons manifesting themselves as jets of hadrons. Such a spectacular decay would be conspicuous not only by the number of emerging ultrahigh-energy jets, but also by their unusually isotropic distribution. Even in the absence of true black hole events, however, 10¹³ collisions will create many imposters. So determining limits on black hole production requires painstaking estimation of the resulting backgrounds. The figure shows the minimum black hole mass deduced from the CMS null result as a function of two parameters of the extra-dimension theories: the number n of extra spatial dimensions, and the characteristic mass scale M_D of the putative unification of the gravitational and electroweak interactions. (CMS collaboration, http://arxiv.org/abs/1012.3375.)—Bertram Schwarzschild

With diameters of less than 30 km, neutron stars harbor the densest matter in the cosmos. Much of it is surely just neutrons packed together at nuclear densities. But there has been lively speculation that more exotic phases—nuclear matter with hyperons, Bose–Einstein condensates of mesons, or plasmas of free quarks—reside at neutron-star cores. Much of that speculation has now been laid to rest by just one precision mass determination at the National Radio Astronomy Observatory in Green Bank, West Virginia. The NRAO measurements of the millisecond pulse train from a rapidly spinning neutron star orbited by a white-dwarf companion revealed that the neutron star’s mass was a record 1.97 ± 0.04 times that of the Sun. This unexpected record mass exceeds the neutron-star mass limits prescribed by almost all of the proposed equations of state that yield the exotic phases. The precision measurement exploits an unusually strong manifestation of a general relativistic effect first pointed out by Irwin Shapiro in 1964: Light is not only bent but also delayed as it passes near a massive object. In this case, as shown in the figure, radio pulses along the line of sight from the neutron star are manifestly delayed once every nine days when they pass very close to the orbiting companion. The happy accident that the binary system’s orbits are seen almost perfectly edge-on makes possible the unusually clear Shapiro-delay signal that yields the precise measurement of both stellar masses. The white dwarf is about ¼ as heavy as the neutron star. (P. B. Demorest et al., Nature 467, 1081, 2010.)—Bertram Schwarzschild

An accelerator experiment at Los Alamos National Laboratory in the late 1990s reported the observation of neutrino flavor oscillation on a laboratory length scale—tens of meters. All prior oscillation observations had been over much larger distances. The disquieting implication of the LANL claim was that so small an oscillation length required the existence of a “sterile” neutrino flavor impervious to the weak interactions, which would clutter the attractively neat prevailing theory. So experimenters designed the MiniBoone facility at Fermilab to confirm the LANL result or lay it to rest. In 2007, the MiniBoone collaboration reported that its results were incompatible with the LANL claim. But now a new MiniBoone result suggests that the sterile neutrino’s obituary was premature. The new result looks a lot like the original LANL data. Like the LANL experiment, it was based on a beam of muonic antineutrinos. The 2007 refutation, however, was based on a muonic neutrino beam. The standard theory assumes that neutrinos and antineutrinos oscillate identically. But the new result, though not yet statistically robust, appears to show that antineutrinos, unlike neutrinos, do indeed oscillate on a distance scale that implies one or more sterile neutrino states. In fact, theorists are already considering how interference between two sterile states of different mass might explain such a neutrino–antineutrino asymmetry. The photo shows the photomultiplier tubes in the oil-filled MiniBoone detector that discern the flavors of neutrinos by recording the Cherenkov light of the charged leptons they create in collisions with nucleons. (A. Aguilar-Arevalo et al., http://arxiv.org/abs/1007.1150.)—Bertram Schwarzschild.

Willis Lamb’s 1947 measurement of the tiny splitting between the 2s and 2p states of atomic hydrogen gave a crucial impetus to the development of quantum electrodynamics (QED). That “Lamb shift” from the Dirac hydrogen spectrum is a 4-μeV increase in the 2s energy level due primarily to vacuum fluctuations of the electromagnetic field. Now Randolf Pohl (Max Plank Institute for Quantum Optics, Garching, Germany) and coworkers at the Paul Scherrer Institute (PSI) in Switzerland have finally measured the analogue of the Lamb shift in the muonic H atom—a proton orbited by a μ⁻ instead of an e⁻. Muons live only microseconds, but they are 200 times heavier than electrons, and their atomic orbits are correspondingly tighter. The muonic Lamb shift is about 200 meV, and its precise value is particularly sensitive to the proton’s finite size. The PSI experiment was accomplished with precision laser excitation of μ⁻ p atoms created by an intense μ⁻ beam stopping in a small volume of H₂ gas at very low pressure. The team measured the muonic Lamb shift to a part in 10⁵ and compared it with elaborate QED calculations that parameterize the proton’s finite size with an effective charge radius R_p. They find an R_p about 4% smaller than that measured, with less precision, by conventional H spectroscopy and e⁻–p scattering experiments. The discrepancy is 5 standard deviations. Either the proton really is smaller than previously thought, argue Pohl and company, or there’s something wrong with the QED calculations or their input constants. But the proton is a quark composite whose size and shape are quantum-chromodynamic manifestations beyond the purview of QED. Several QCD theorists suggest that at the extraordinary precision achieved by the PSI experiment, it may not be possible to describe proton-size effects adequately with a single length parameter. (R. Pohl et al., Nature 466 , 213, 2010.)—Bertram Schwarzschild

Researchers in Russia and the US have collaborated to synthesize a new element with atomic number Z = 117. They’ve done it using the same technique they used over the past dozen years to make elements 113–116 and 118: bombarding an actinide target—berkelium in this case—with a beam of calcium-48 ions energetic enough to fuse with the actinide nuclei. Six atoms of element 117, five with 176 neutrons and one with 177, became lodged in position-sensitive detectors, where each underwent a series of alpha decays followed by a spontaneous fission. The timing and energy pattern of the observed decays allowed the researchers to identify new atoms. Along the alpha-decay chains were new isotopes of elements 115 and 113, roentgenium (Z = 111), meitnerium (109), bohrium (107), and dubnium (105), all with more neutrons than the previously known isotopes of those elements. The greater neutron numbers N brought with them greater stability and longer half-lives, in agreement with the theory that predicts an island of stability somewhere around Z = 114–126 and N = 184. In particular, the new isotopes of element 113 each lived for several seconds, long enough that the researchers hope to be able to probe the element’s electronic—that is, chemical—properties. With electrons moving at relativistic speeds, element 113 might not behave at all like thallium, the element just above it in the periodic table. (Y. T. Oganessian et al., Phys. Rev. Lett. 104, 142502, 2010.)—Johanna Miller

The binding energy of a nucleus—almost 1% of its mass—provides important information about its configuration of protons and neutrons. Theorists particularly want to know the binding energies of transuranic nuclear species approaching the so‑called island of stability predicted to lie not far beyond the most massive element yet discovered—with atomic number Z = 118. Despite the name, the island’s denizens would not be truly stable, just significantly longer‑lived than their offshore neighbors. But until now the masses, and therefore the binding energies, of transuranics have been determined only indirectly, by measuring the energies of α particles released in long α‑decay chains down to nuclei of well‑measured mass. Such indirect determinations can suffer from significant uncertainties and limitations. Now however, Michael Block and coworkers at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, have reported the first direct measurement of transuranic masses. The team measured the masses of three short‑lived isotopes of nobelium (Z = 102) created by the fusion of calcium ions from GSI’s heavy‑ion accelerator with target lead nuclei. The least stable of the three has a half‑life of two seconds. Their masses were measured to within a few parts in 10⁸ in a precision Penning trap, an electromagnetic device that confines charged particles in a small cavity and determines their masses by measuring their cyclotron‑orbit frequencies in the trap’s strong magnetic field. What makes the technique particularly challenging for those transuranics whose creation requires fusion is primarily their painfully slow production rates. (M. Block et al., Nature 463, 785, 2010.) —Bertram Schwarzschild

The cosmologists’ widely accepted “concordance” model asserts that only about 15% of the mass of matter in the cosmos is baryonic—made of protons and neutrons. The “dark matter” that predominates is thought to consist of particles yet unknown. Particle theory provides an attractive candidate: weakly interacting massive particles (WIMPs) predicted by supersymmetric extensions of the theory’s standard model. Presumably created in the Big Bang, those stable neutral particles, about 100 times heavier than the proton, would be abundant enough to account for gravitational effects observed in the rotation and clustering of galaxies. The Cryogenic Dark Matter Search (CDMS) collaboration has now released its analysis of all the data taken by its CDMSII detector in three years of looking for WIMPs deep inside an old Minnesota iron mine. CDMSII is a 5-kg cryogenic array of germanium and silicon crystals micro-instrumented to detect the recoil of a nucleus in a rare collision with a WIMP as Earth sweeps through the halo of dark matter presumed to envelop the Milky Way. An instrument of CDMSII’s limited mass was predicted to find, at most, a statistically marginal handful of WIMP collisions in a three-year run. The community is working to decide which of several competing detector technologies can best be scaled up to provide a robust result. In its final year, the detector recorded two collision events that showed no evidence of coming from the enormous background of electron recoils or from a neutron collision. But the group calculates a 23% chance that both events were background imposters that squeezed past the analysis cuts that reduced backgrounds a millionfold. So the paper makes no claim of significant evidence for WIMP interactions. But it does present the most stringent upper limits to date on the WIMP-nucleon scattering cross section. The figure shows those limits as a function of the putative WIMP mass, together with a range of predictions from supersymmetric theories. (Z. Ahmed et al., CDMS collaboration, http://arxiv.org/abs/0912.3592. —Bertram Schwarzschild

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