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Discrete Symmetries and CPViolation This page intentionally left blank Discrete Symmetries and CP Violation From Experiment to Theory M. S. Sozzi University of Pisa 1 3 GreatClarendonStreet,Oxfordox26dp OxfordUniversityPressisadepartmentoftheUniversityofOxford. ItfurtherstheUniversity’sobjectiveofexcellenceinresearch,scholarship, andeducationbypublishingworldwidein Oxford NewYork Auckland CapeTown DaresSalaam HongKong Karachi KualaLumpur Madrid Melbourne MexicoCity Nairobi NewDelhi Shanghai Taipei Toronto Withofficesin Argentina Austria Brazil Chile CzechRepublic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore SouthKorea Switzerland Thailand Turkey Ukraine Vietnam OxfordisaregisteredtrademarkofOxfordUniversityPress intheUKandincertainothercountries PublishedintheUnitedStates byOxfordUniversityPressInc.,NewYork ©MarcoSozzi2008 Themoralrightsoftheauthorhavebeenasserted DatabaserightOxfordUniversityPress(maker) Firstpublished2008 Allrightsreserved.Nopartofthispublicationmaybereproduced, storedinaretrievalsystem,ortransmitted,inanyformorbyanymeans, withoutthepriorpermissioninwritingofOxfordUniversityPress, orasexpresslypermittedbylaw,orundertermsagreedwiththeappropriate reprographicsrightsorganization.Enquiriesconcerningreproduction outsidethescopeoftheaboveshouldbesenttotheRightsDepartment, OxfordUniversityPress,attheaddressabove Youmustnotcirculatethisbookinanyotherbindingorcover andyoumustimposethissameconditiononanyacquirer BritishLibraryCataloguinginPublicationData Dataavailable LibraryofCongressCataloginginPublicationData Sozzi,M.(Marco) DiscretesymmetriesandCPviolation:fromexperimenttotheory/M.S.Sozzi. p.cm. ISBN978–0–19–929666–8 1.CPviolation(Nuclearphysics) 2.Symmetry(Physics) 3.Mirrorsymmetry. I.Title. QC793.3.V5S592007 (cid:1) 539.725—dc22 2007034660 TypesetbyNewgenImagingSystems(P)Ltd.,Chennai,India PrintedinGreatBritain onacidfreepaperby BiddlesLtd.,King’sLynn,Norfolk ISBN 978–0–19–929666–8 AEdoardo:luce. This page intentionally left blank PREFACE AsfarasIsee, allaprioristatementsinphysics havetheirorigininsymmetry. H.Weyl Thisworkoriginatesfromtheconstructiveinterferenceofseveralconsiderations. As usual, lecture notes for courses given over several years to undergraduate andgraduatestudentswerethestartingpoint.InteachingthebasicsofCPviolation I tried to put some emphasis on ‘how’our current knowledge was obtained, and ‘why’thingsweredoneastheywere;intheprocessIfoundthatnoneoftheseveral very good books on the subject provided to the student all the aspects I felt to be important. First, as the subject is not an easy one, sometimes students struggling to find theirwayamongthemyriadofphasesandamplitudesmightbeleftwiththefeeling that eventually everything is so nicely ‘explained’within the Standard Model of Particle Physics (SM in the following) that no other possibility could actually be conceived: the ‘poor researchers’ of times past just didn’t have a chance to understand what is now clear and pristine. While this is clearly a naive and non- historical view, my objection is mostly to the fact that it tends to dim the critical attitude of the student – a danger to which all researchers are exposed from the great successes of the Standard Model – and which must be resisted for progress tooccur.Ifeelthatwhilelearningasubjectitisimportanttokeepabroaderview initially, to pick connections and analogies with known phenomena; I have often encountered young graduate students, very deeply into the details of their Ph.D. workonsomeCP-violatingeffect,lackingasolidgraspofthebasicconnectionwith their neighbour’s work. Older textbooks often presented (quite understandably) a broader treatment of the basics; such books are however long out of print and of coursehopelesslyoutdatedinmostoftheircontents. Second,manytextbookslackanyemphasisontheexperimentsfromwhichour understanding emerged (which can be partially excused when trying to keep the size of a comprehensive book below the 1,000- page mark). Still, to understand how ‘the numbers’are obtained, why some of them are easy to get and some are not,whythingsaremeasuredinonewayratherthananother,orwhatdistinguishes a successful experiment from another one, is a fundamental part of the formation of any physicist, and several of my experimental colleagues would agree that too oftenthisisthemostdifficulttoconvey.Again,youngworkersinthefield(notonly viii PREFACE thosewithhightheoreticalinclinations)sometimesgetthefeelingthatinorderto make progress what is needed is just a brilliant theory, and then somebody has to gooutandmeasureallitsparameterstocheckit.Ithinkathoroughappreciationof the experimental principles is really crucial for any new generation of physicists, evenmoresoatthepresenttimeinwhichexperiments(atleastinwhatwenowcall high-energyphysics)areevolvingtowardslife-longenterprises:itisnotuncommon nowadaystohaveayoungexperimentalphysiciststartingworkonaspecificissue (be it an instrumentation, a simulation or a physics analysis topic) and be stuck withthatformanyyears,withoutevergettingachancetolearntheprinciplesfor designing a new experiment, which might be asked from her or him sometime in thefuture. Also,theappreciationofhowscientificprogressandtechnologicaladvancesare deeplyintertwinedinbothdirectionsissomethingwhoseimportancetranscendsthe formationofaphysicist.AsV.L.Fitch,oneoftheprotagonistsofthediscoveries discussed in this work, puts it: ‘What always paces the rate of discovery is the developmentofnewinstruments–newdevicesforprobingphenomena,eitherwith anewlevelofprecisionorbyextendingtherangeofobservationintorealmsnever beforeexplored’.AndalreadytwocenturiesagoSirH.Davystated:‘Nothingtends somuchtotheadvancementofknowledgeastheapplicationofanewinstrument. Thenativeintellectualpowersofmenindifferenttimesarenotsomuchthecauses of the different success of their labours, as the peculiar nature of the means and artificialresourcesintheirpossession’(Davy,1839). Alltheabovewouldnothavebeenasufficientreasonformetowriteanewbook; whatreallygotthisstartedwasthefeelingthattoomanystudentsdonotgetexposed to the challenging aspects of experimental physics: if a smart student chooses to pursueacareerintheoreticalphysicsthismustnotbebecauseshe/hewasnotgiven achancetoappreciatehowdeep,beautifulandhighlystimulatingtheexperimental endeavourcanbe.TheabovepointwasbestexpressedbyV.Telegdiinaniceshort essay(Telegdi,1990),inwhichheremarkedhowthereisoftenasmuchintellectual contentinanexperimentasinitstheoreticalinterpretation;heconcludedthat‘we mustteachcoursesinwhichbrilliantexperimentsofgreatsignificanceareanalyzed insomedetail’. Now, the experimental investigation of tiny symmetry violations in physics is reallyafieldwhichisfullofchallengingandcarefulexperiments,oftenwithalong history of improvements and dedication; it encompasses many different branches of physics and even more vastly different approaches; finally, it is a very active field, with several new fundamental results being obtained in the past few years. It appears to the author as an ideal subject to convey the depth and excitement of experimentalphysics. The patient reader can now appreciate the ideas behind the conception of this book:whileitwouldbehighlypretentiousoftheauthortothinkofaddressingall PREFACE ix theaboveissueswithhiswork,hehopesthatthismodestattemptcanbeseenasa stepinthedirectionsdiscussedabove,andthattherelevanceoftheimportantgoals willthrowsomelightontheinevitableshortcomingsoftheactualresult. ThebookdealsmostlywiththeC,P,T discretesymmetries:otherdiscretesymme- triessuchastranslationalsymmetryincrystalsareoutsideitsscope.Whiletrying tobeself-contained,itattemptstotakeasomewhatnovelapproachtothesubject: moreemphasisthanusualisputonexperimentalaspects,tryingtoprovideasome- what wider picture at the expense of some depth, and to convey the intellectual challenge of the connection between phenomenology and experiment. Excellent research-level books exist for the interested reader to dig more details on specific topics, and the rather extensive bibliography provided might be helpful for this purpose. Inmostchapterssomerelevantexperimentsarediscussedinsomedetail,trying toprovidetheaveragereadermoreinsightthanisobtainedjustbyreadingtheorigi- nalpaper,oftenwrittenforexpertswellversedintheapproachesandresearchissues ofthetime.Thefocusisnotsomuchonthedetectiontechniques(whichgrowold) as on the experimental approaches (which do not). The choice of experiments is tosomeextentarbitrary:itdidn’talwaysfallonthefirstexperimenttoobservean effect,noronthelastorthemostprecise,butoftenononewhichwasfelttowell exemplify some experimental issue. Many more beautiful and successful experi- mentswouldhavedeservedtobediscussedindetail,butthiswouldhaverequired several volumes (and several authors too).Also, while the most recent values for experimental measurements are listed, many of them will grow old quickly, and the most up-to-date figures should clearly be looked for in the appropriate place (Yaoetal.,2006). Asanexperimentalist,Icannotdofulljusticetothebeautyoftheoryand,need- less to say, the balance of topics is due to a personal judgement of their relative importanceandaffectedbymyownresearchactivityinhigh-energyphysics:read- erswhoareexpertsinsomefieldmightwanttoskipthebasicexplanationofsome techniqueswhicharewellknowntothem.Also,thebalanceissomewhatinfavour of more ‘established’ physics issues, with more recent studies still under active investigation(forwhichtheoverallpictureisstillinthemaking,andtheappraisal of the relevance of individual measurements still lacks the benefit of time) being discussedatamoreshallowlevel. The level of the book was originally intended to be as accessible as possible, andifthisgoalwasnotfullyachievedtheauthorisinexcellentcompany,asOkun’s masterful(butprettyhardforanovice)work(Okun,1982)wasstartedwiththepur- poseofbeing‘readableeveninatownbus’.Thetargetaudienceareundergraduate and graduate students in physics; young researchers working in particle physics and interested in getting a somewhat wider picture on topics related to their own might also find this work of some interest.Ageneric undergraduate background