Reidar Hahn, Fermilab Babar’S This Violation

Home , Patricia Burchat

Photo: Reidar Hahn, Fermilab BaBar’s This violation. iscalledcharge-parity asymmetry (CP) it doesnotactequally onmatterandantimatter—unlike theotherforces. rate thantheantimatter versions.That meanstheweak forceislopsided: different slightly a at decay matter of made ones the that determined force, itwouldholdtogether forever.” of MIT. Steve Sekula BaBar collaborator “Left onitsown without the weak quark to form a (or pairs: onemadeofmatter, onemadeofantimatter. abottom Each contains samplebytheendof2008. ble itsowndata antimatter partners,positrons.The BaBarteamexpects tomorethandou their and electrons together slamming by them of pairs billion a almost a called ticle The par toolfor exploring experiments’ theweakforceisaheavy primary The are seizingtheiruniqueopportunitiestodecipherthem. of theuniverse.The weakforcestillholdssecrets, andBaBarBelle force’s sometimes-odd behavior, andopengatewaystothebigmysteries stretch, theexperiments theweak areracingtolearnevenmoreabout their firsteightyearsofoperation.Nowenteringthefinaltwo-yearhome in discoveries landmark made have They Japan. in KEK at Belle and inCalifornia LinearAcceleratorCenter(SLAC) ments—BaBar atStanford detectors work, and indicate the age of ancient skeletons and tools. radioactive decays that diagnose disease and treat cancer, make your smoke heat the Earth’s core. It’s also tremendously useful. The weak force drives ourlives,drivingthenuclearreactionsthatpowersunand sustains the weakforce. The sunisshining;theEarthwarminsteadoficy. Life isgood,thanksto By HeatherRock Woods hints ofspectacularnewphysics. a meretwoyearsasithuntsfor Center looks to double its data in at StanfordLinearAccelerator TheBaBar weak force the window on Because theweakforceexists, “The Providing awindowontheweakforcearetwo“ One ofthefourknownforcesthatshapeuniverse,weakforce b ) quark, the second heaviest quark. The B B playground meson is a playground or laboratory fortheweakforce,” mesonisaplaygroundorlaboratory says B meson. The meson. B meson. B

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symmetry | volume 04 | issue 01 | jan/feb 07 Photos: David Harris

Physicists already knew CP violation existed when BaBar got underway, Top photos: Technicians work and they knew that this phenomenon must contribute to the dominance of on reconnecting elements of the BaBar detector after a matter over antimatter in the universe. The B factories confirmed that all CP major upgrade. Bottom photo: violation we see in nature comes from the weak force—but they also found Babar rests in the PEP-II that it’s not enough to explain the amount of matter in the universe today. storage ring tunnel, his name- sake detector behind the The mismatch between experimental results and the actual universe high- wall in the background. lights the incompleteness of the prevailing theory of how nature works, known as the Standard Model. Something more is at play, and BaBar and Belle have a shot at saying who the players are. “The matter-antimatter imbalance remains one of the top mysteries of the universe. The weak force appears not to be the thing that solves it for us. Whatever causes the imbalance, the effects are subtle,” says Sekula.

Precision The two pillars of the BaBar program are to deeply understand the realm where the weak force and quarks interact, and to use that precise under- standing to search for rare and new kinds of physics. BaBar Spokesperson Hassan Jawahery, professor at the University of Maryland, says, “We’re embarking to a new world, and we need to know

10 high-energy realm. high-energy the in look to where into insight keen providing are and can they cles, Butwhilethe atCERN. (LHC) that willbeachieved atnewmachines liketheLargeHadronCollider new particles.BaBarandBelleoperateatafractionoftheenergyreach see newformsofphysics,whether supersymmetry, extra dimensions,or force. theweak raredecayswouldexplainmere existence more about ofcertain and to increase the chances of seeing extremely rare events. Indeed, the inalltypesofmeasurements toreduceuncertainties both amounts ofdata to compare it with.” the old world very well so when we see something new, we have something that physicists still need to search. professor Patricia Burchat. Tightening the constraints reduces the territory future must be consistent with what we’ve observed,” says Stanford physics we reveal no inconsistencies with the Standard Model, every theory in the will still bequeath something substantial. nificantly reducetheuncertainty, cansolvethisquandary. which willincreasethenumberofallpenguin decaysandsig more data, still hasplentyofroomtorevealthecontributionsnewphysics. Only Modelpredictionsbut in thattypeofpenguindecayisclosetoStandard seen asymmetry of detector.Theamount the in observed decays meson billion one roughly the in far so times 1200 only detected have they appreciable,” saysJawahery. something newiscomingin, itmaychange inawaythat’s theasymmetry Model,aclearindicationofnew physics. dicted bytheStandard differentcase, theexperiments ratesofdecaysthanpre would observe that couldchange theamountofmatter-antimatterasymmetry. Inthat particle, oranotherkindoftheorizedparticle. tual particlescouldbethemuch-sought Higgsparticle,asupersymmetric print. unmistakable an leaves it concrete, wet in walking cat a like But, directly. observed never is particle virtual a that quickly so repaid be to has energy the However, space. of vacuum the from energy borrow temporarily to them more massivethantheoriginal almost instantly. place where “virtual” particles pop into existence and then disappear again is a decay that requires the weak force but also contains a “virtual loop”—a more commonthanothers. falls apartinoneofhundredsdifferent ways,someofwhich aremuch B betweenmeasures theamountofasymmetry about the existence of new kinds of physics. Deviations from what’s expected in the Standard Model would tell physicists Hunting penguins Rome. INFN says thepastphysicsanalysiscoordinatorforBaBar, Riccardo Faccini of before?’” come has that data the all with consistent seeing I’m what ‘Is meson lives about a trillionth of a second after being created and then then and created being after second a of trillionth a about lives meson findanewparticle,oneofthequestionswillbe, “If physicistsattheLHC What the vast collecting on depends years two next the in success BaBar’s “The Even if the chance to herald the new doesn’t materialize, the manytypesofpenguindecays,including onetype BaBar hasobserved if because nice them makes which rare, very are decays “Penguin If newparticlesdoenterpenguinloops,theywillintroduceinteractions Physicists areinhotpursuitofthesepenguindecaysbecausethevir Even thoughitsoundsimpossible,thesevirtualparticlescanbemuch One of the rare processes is called a “penguin” (see sidebar next page). It To search forsurprises,a B factories will leave a legacy ofprecisionmeasurements.Evenif factorieswillleavealegacy B factories find will be vital tofuture experiments thathopeto factoriesfindwillbevital B B factory usesitstypicalmodusoperandi:it factory B factories can’t directly observe newparti factoriescan’tdirectlyobserve mesons.Quantummechanics allows B andanti- 11 B decays.Each B factories B

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symmetry | volume 04 | issue 01 | jan/feb 07 The origin of penguins at Harvard. One evening, she, I, and Serge went Told by John Ellis: to a pub, and she and I started a game of darts. We made a bet that if I lost I had to put the word “Mary K. [Gaillard], Dimitri [Nanopoulos], and I first penguin into my next paper. She actually left the got interested in what are now called penguin darts game before the end, and was replaced by diagrams while we were studying CP violation in Serge, who beat me. Nevertheless, I felt obligated the Standard Model in 1976… The penguin name to carry out the conditions of the bet. came in 1977, as follows. For some time, it was not clear to me how to In the spring of 1977, Mike Chanowitz, Mary K. get the word into this b quark paper that we and I wrote a paper on GUTs [Grand Unified were writing at the time…. Later…I had a sud- Theories] predicting the b quark mass before it den flash that the famous diagrams look like was found. When it was found a few weeks later, penguins. So we put the name into our paper, Mary K., Dimitri, Serge Rudaz and I immediately and the rest, as they say, is history.” started working on its phenomenology. That summer, there was a student at CERN, John Ellis in Mikhail Shifman’s “ITEP Lectures in Particle Melissa Franklin, who is now an experimentalist Physics and Field Theory”, hep-ph/9510397

Antibottom Antistrange Quark Quark W-Boson

Antitop Antitop Quark Quark

Gluon

Strange Antistrange Quark Quark

More than a B factory BaBar physicists have multiple sources of urgency for the final two years, because this may be the last chance to make certain kinds of measurements. Although the LHC will have a b-quark experiment (LHCb), the exist- ing B factories can do certain measurements that will be difficult or impos- sible in future experiments. BaBar and Belle also serve as excellent factories for other particles, espe- cially charm mesons (containing a charm, or c, quark) and tau particles (the heaviest relative of the electron). In fact, physicists are using the plethora of tau particles to look for new physics in a completely different way. They are currently seeking tau decays that end without producing a tau neutrino—find- ing such decays would be iron-clad proof that something new is going on. It might also help unravel the mystery of how neutrinos change from one kind to another. “The detector is so multipurpose that we can make a wide range of measurements that touch on a wide array of topics,” says Princeton professor Jim Olsen, physics analysis coordinator for BaBar. “We’re learning things that will change textbooks.”

12 Photos: David Harris

Much more to come Top photos: The BaBar control room is staffed 24 hours per day Both B factories are continually pushing their limits in the quest for greater with collaboration members from luminosities, or number of events they produce. around the world visiting to take At SLAC, the PEP-II accelerator, which provides the electron and posi- shifts overseeing operation and data collection. Bottom photo: tron beams for the BaBar experiment, has already exceeded its original Babar visited the BaBar experi- goal for luminosity by a factor of four. The BaBar collaboration and the mental hall during upgrades to PEP-II team recently made a number of major upgrades to the detector the detector. and the accelerator to further increase luminosity by a dramatic 70 per- cent, and to make the detector even more sensitive. The final experimental stretch will run from January 2007 through September 2008. symmetry | volume 04 | issue 01 | jan/feb 07 symmetry | volume 04 issue 01 jan/feb “We have at least 100 kinds of measurements to do in the next two years,” says Olsen. He expects around 150 analyses to be published in this time. “The amount of physics coming out of BaBar is huge.” Long after the last event flings its offspring through the BaBar detector, physicists in the 600-person international collaboration will be quarrying their mountains of data in a quest to solve the mysteries of the weak force and more.