Online the Higgs Boson Eas- That Empty Space Is fi Lled with Tions of Other Particles
Total Page：16
File Type：pdf, Size：1020Kb
NEWSFOCUS THE DISCOVERY OF THE Higgs Boson NO RECENT SCIENTIFIC ADVANCE HAS act by exchanging other particles that con- Higgs, researchers at the European particle generated more hoopla than this one. On vey three forces: the electromagnetic force; physics laboratory, CERN, near Geneva, built 4 July, researchers working with the world’s the weak nuclear force, which spawns the $5.5 billion, 27-kilometer-long LHC. To biggest atom smasher—the Large Hadron neutrinos; and the strong nuclear, which spot the Higgs, they built gargantuan particle Collider (LHC) in Switzerland—announced binds quarks. detectors—ATLAS, which is 25 meters tall that they had spotted a particle that appears But there’s a catch. At first blush, the and 45 meters long, and CMS, which weighs to be the long-sought Higgs boson, the standard model appears to be a theory of 12,500 tonnes. The ATLAS and CMS teams last missing piece in physicists’ standard massless particles. That’s because boast 3000 members each. More model of fundamental particles and forces. simply assigning masses to the than 100 nations have a hand The seminar at which the results were pre- particles makes the theory go in the LHC. sented turned into a media circus, and the news haywire mathematically. So Perhaps most impressive captured the imagination of people around the mass must somehow emerge is the fact that theorists pre- world. “[H]appy ‘god particle’ day,” tweeted from interactions of the oth- dicted the existence of the will.i.am, the singer for pop group The Black erwise massless particles new particle and laid out its Eyed Peas, to his 4 million Twitter followers. themselves. properties, right down to Yet, for all the That’s where the Higgs the rates at which it should hype, the discovery of comes in. Physicists assume decay into various combina- Online the Higgs boson eas- that empty space is fi lled with tions of other particles. (To sciencemag.org ily merits recognition a “Higgs field,” which is a bit test whether the particle really For an expanded as the breakthrough like an electric fi eld. Particles inter- is the Higgs, researchers are mea- version of this sec- of the year. Hypoth- act with the Higgs field to acquire energy suring those rates now.) Physicists have made tion, with podcast, video, links, and more, see www. esized more than and, hence, mass, thanks to Albert Einstein’s such predictions before. In 1970, when only sciencemag.org/special/ 40 years ago, the Higgs famous equivalence of the two, encapsulated three types of quarks were known, theorists btoy2012 and science- boson is the key to in the equation E = mc2. Just as an electric fi eld predicted the existence of a fourth, which was on December 24, 2012 careers.org. physicists’ explanation consists of particles called photons, the Higgs discovered 4 years later. In 1967, they pre- of how other funda- fi eld consists of Higgs bosons woven into the dicted the existence of particles that convey mental particles get their mass. Its observa- vacuum. Physicists have now blasted them out the weak force, the W and Z bosons, which tion completes the standard model, perhaps of the vacuum and into brief existence. were found in 1983. the most elaborate and precise theory in all of That feat marks an intellectual, technologi- Particle theorists offer various explana- science. In fact, the only big question hang- cal, and organizational triumph. To produce the tions of their knack for prognostication. Parti- ing over the advance is whether it marks the cle collisions are inherently reproducible and beginning of a new age of discovery in par- free of contingency, theorists say. Whereas www.sciencemag.org ticle physics or the last hurrah for a fi eld no two galaxies are exactly the same, all that has run its course. protons are identical. So when smash- The Higgs solves a basic prob- ing them, physicists need not worry lem in the standard model. The about the peculiarities of this pro- theory describes the par- ton or that proton because there ticles that make up ordi- are none. Moreover, theorists nary matter: the electrons say, in spite of its math- that whiz around in ematical complexity, the Downloaded from atoms, the up quarks standard model is con- and down quarks that ceptually simple—a make up the pro- claim that nonphysi- tons and neutrons cists might not buy. in atomic nuclei, The standard the neutrinos that model ultimately are emitted in a owes its predictive type of radioactiv- power to the fact that ity, and two sets the theory is based on of heavier cous- the notion of math- ins of these parti- ematical symmetry, cles that emerge in some theorists say. particle collisions. Each of the three forces These particles inter- in the standard model is related to and, in some Pieced together. In this par- sense, necessitated by a dif- ticle collision, it appears that ferent symmetry. The Higgs a Higgs boson decays into two mechanism itself was invented electrons and two positrons (red). to preserve such symmetry while MCCAULEY CERN/L. TAYLOR/T. CREDIT: 1524 21 DECEMBER 2012 VOL 338 SCIENCE www.sciencemag.org Published by AAAS BREAKTHROUGH OF THE YEAR 2012 | NEWSFOCUS giving mass to force-carrying particles like theory is obviously incomplete, as it doesn’t In fact, scientists have no guarantee that the W and the Z. Simply put, symmetry argu- incorporate the force of gravity. And the the- any new physics lies within the reach of the ments are powerful predictive tools. ory itself suggests that interactions between LHC or any conceivable collider. The stan- No matter the reason for particle physi- the Higgs and other particles ought to make dard model could be all of the inner workings cists’ predictive prowess, with the Higgs the Higgs hugely heavy. So physicists suspect of the universe that nature is willing to reveal. boson apparently in the bag, they have no sim- that new particles lurking in the vacuum may The discovery of the Higgs is a breakthrough. ilar prediction to test next. They have plenty counteract that effect. But those arguments Will particle physicists ever score a similar of reason to think the standard model is not aren’t nearly as precise as the one necessitat- breakthrough again? the fi nal word on fundamental physics. The ing the Higgs boson. –ADRIAN CHO A HOMe RUn for ancient DNA Two years ago, paleogeneticists made our short list for Breakthrough of the Year for publishing the complete sequence of the nuclear genome of the Neandertals. In 2011, the same lab shared our spotlight for piecing together the genome of the Denisovans, an archaic human that lived in Siberia at least 50,000 years ago. But those ancient DNA sequences and others were blurry snapshots next to the high-resolution genomes that researchers can now sequence from living people. Much of the fragile DNA from fos- sils is degraded into single strands that auto- on December 24, 2012 matic sequencers can’t copy. Researchers were resigned to deciphering only parts of the code of ancient genomes, whether from archaic humans, animals, or pathogens. This year, however, a persistent post- doc developed a remarkable new method that enabled his team to revisit the Deniso- van DNA and sequence it 31 times over. www.sciencemag.org The resulting genome, of a girl who lived in Siberia’s Denisova Cave, reveals her genetic material in the same sharp, rich detail that researchers typically get from the DNA of living people. This techno- logical feat promises to give a major boost to the fi eld of ancient DNA, as researchers Single-minded. Postdoc Matthias Meyer (above) developed a new method to prepare single strands of ancient begin to apply the method to other samples DNA; the technique gave researchers an unprecedented view of an ancient girl’s genome. Downloaded from and species. Ancient DNA researchers typically have of the genome 20 times—the benchmark for evolved, providing a “near-complete” cata- adapted the tools used to sequence DNA reliably identifying nucleotide positions. log of the handful of genetic changes that from living humans, which start with sam- The results confi rmed that Denisovans separate us from Denisovans, who were ples of double-stranded DNA. But ancient interbred with the ancestors of some liv- close kin to Neandertals. DNA usually breaks into single strands. ing humans; people living in parts of island These details are all the more remark- So postdoc Matthias Meyer at the Max Southeast Asia have inherited about 3% of able because the Denisovans are so poorly Planck Institute for Evolutionary Anthro- their nuclear DNA from Denisovans. The known from fossils: Only a tiny scrap of pology in Leipzig, Germany, set out to genome literally offers a glimpse of the finger bone and two molars have been sequence single-stranded ancient girl, suggesting that she had brown reliably assigned to them so far. In DNA from scratch. He failed at eyes, brown hair, and brown skin. contrast, the Neandertals are known from first, but then managed to bind It also allowed the team to use hundreds of fossils but from a much less special molecules to the ends DNA to estimate that the girl died complete genome. of a single DNA strand, hold- between 74,000 and 82,000 years Neandertal experts may catch up soon. ing it in place for sequencing. As ago—the fi rst time researchers had Meyer and colleagues have been trying a result, using only 6 milligrams of used genomic information to date “Matthias’s method” on fossil samples that bone from the Siberian girl’s pinky fi n- an archaic human.