iXb:z 0Tf

CONTENTS

A PERIODICAL OF PARTICLE PHYSICS

WINTER 1991 VOL. 21, NUMBER 4

EditorsRENE DONALDSON, BILL KIRK

Contributing EditorMICHAEL RIORDAN

Editorial Advisory BoardJAMES BJORKEN, ROBERT N. CAHN, DAVID HITLIN,

STEWART C. LOKEN, JOHN REES, RONALD RUTH

MARVIN WEINSTEIN

Photographic ServicesTOM NAKASHIMA

BETTE REED

IllustrationsTERRY ANDERSON, KEVIN JOHNSTON

SYLVIA MACBRIDE, JIM WAHL

DistributionCRYSTAL TILGHMAN

FEATURES

1 THE TRIUMF KAON FACTORY

A powerful new Canadian facilityfor a broad range of particle andnuclear physics studies

Michael Craddock and Peter Kitching

8 CP VIOLATION IN B MESON DECAYS

An asymmetric electron-positron storagering called a "B Factory" can provide acritical test of the Standard Model.

David Hitlin and Sheldon Stone

14 THE RISE OF THE STANDARD MODEL

The Third International Symposiumon the History Particle Physics will beheld June 24-27, 1992, at Stanford LinearAccelerator Center.

The Beam Line is published quarterly by theStanford Linear Accelerator Center,P.O. Box 4349, Stanford, CA 94309.Telephone: (415) 926-2585BITNET: BEAMLINE@SLACVMFAX: (415) 926-4500SLAC is operated by Stanford University undercontract with the U.S. Department of Energy.The opinions of the authors do not necessarilyreflect the policy of the Stanford LinearAccelerator Center.

Cover: Measurement of the submicron beam spotsthat will be created at future linear colliders poses aformidable challenge. Physicists at SLAC are experi-menting with narrowjets of liquid metals that might beused to intercept such a beam profile. The "nozzle"shown has been formed from a glass tube heated anddrawn to pinch the opening to a submicron orifice.

DEPARTMENTS

13 DATES TO REMEMBER

16 CONTRIBUTORS

YZ" Printed on Recycled Paper

.

~~tR~r*~or

· LAONr.

by MICHAEL CRADDOCKand PETER KITCHING

: :: 0 : .. - : S; :: f:: i:gE

;;:4000::: 0~: ;g:f:C::: ::_

x ̂ \ 0 :~~~~~~~~~~~~~~~: ACS:::i

9 .~~~~~~~~~~~~L · 0 : i:fls:L

i::" :

o

rz

A powerful new Canadian facilityfor a broad range of particle and

nuclear physics studies

"There is no excellent beauty that

hath not some strangeness

in the proportion." -Francis Bacon

TRANGENESS WILL CERTAINLY BE AVAILABLE IN PROFUSION at theKAON Factory, in the form of intense beams of kaons, the lightest particlescarrying the strange quark-and through KAON we look forward to muchbeautiful physics. But this new 30 GeV, 100 ,uA facility, to be built at TRIUMFin Vancouver, British Columbia, Canada, will be a factory for more than kaons.It will also produce beams of antinucleons, hyperons, pions, muons, and:.-neutrinos, 100 times more intense, or cleaner, than are availabei3'-E.g:; '.now-hence the acronym Kaon-Antiproton-Otherhadron-. :

kN eutrino Factory. The existing 500 MeV H- cyclotron .. i.--3 .. i

at TRIUMF will provide the required 100 gA injector.;...To .accelerate this intense beam to 30 GeV will re-iy ;

ire-an interleaved sequence of two fast-cycling , .'

synchrotrons and three storage rings, with some r S j^

novel design features. One-third of the construc-tion costsf(Cdn $708 million) has been promised .

by Britis olumbia, the second third has re-cently been,-o reytm e government of Canada,and it now remainsto sere the ped e apreviousy' 0

made by other countri Un StathiesIfi W»following the model used infninanign HE A. -ii : .9-^^^^

BEAM LINE 1

AL

THE

mt =150 GeV/c 2

B-B mixing1.0

0.5

p

0

-0.5

-1.0

----��- ::---

:: : : :: :---

r;;: :: -::::::-:::: ..: .: ::::. -:::.... :

0 0.4

'1

Limits of the Kobayashi-Maskavparameters (r1, p) from presentments (solid area) and from futLK-decay experiments [B.R. (K+<2x1- 10 , B.R. (KO ->i) < 1

SCIENTIFIC PROGRAM

THE MUCH MORE INTENSE, or pure,beams of secondary particles men-tioned above will make possible abroad range of particle and nuclearphysics studies on the "precisionfrontier," complementary to the"energy frontier." Major areas ofinvestigation will be rare decaymodes of kaons, CP violation, mesonand baryon spectroscopy, meson andbaryon interactions, neutrino scat-tering and oscillations, quark struc-ture of nuclei, properties of hyper-nuclei, K+ and p scattering fromnuclei, and polarization effects.

The improved low-energy pionand muon beams will also greatlyenhance the existing programs inmuon spin resonance and cancer ther-apy. The range of physics that can bestudied at KAON is very large, andonly some of the highlights can betouched on here.

0.8 Rare Decay and CP Violation

The basic motivation for studyingrare and forbidden kaon decay pro-

mva matrixexperi- cesses is still very strong. Theexpenr-

ire emphasis will continue to be on-> rvv) attacking many of the same issues.5x10 9 ]. addressed at the energy frontier of

the high energy colliders, by search-ing for processes forbidden in theStandard Model and looking for rareprocesses that are sensitive to theeffects of virtual heavy particles. AtKAON one can envisage experimentswith sufficient sensitivity to probesuch processes at a level that willcritically test the Standard Modelpredictions. Although the mainemphasis is on looking for physicsbeyond the Standard Model, one canalso gain important information from

K decay experiments on the param-eters inside the Standard Model. Thisinformation can be used to makepredictions of increasing accuracyfor various processes, as our knowl-edge of the Standard Model param-eters and hadronic correctionsimproves. The experimental meas-urement of the rate for these proces-ses then becomes a more sensitivetest of the Standard Model.

A prime example of such a processis K+ -> + vv, one of the few second-order weak processes where reliablecalculations can be confronted byexperiment. Although this decaymode has not yet been seen, an experi-ment to search for it is now underway at the Brookhaven NationalLaboratory AGS. The predictedbranching ratio is so small (2x10-10)that even if the decay is unambigu-ously detected at Brookhaven, anaccurate measurement must awaitthe more intense beams which willbecome available at the KAONFactory. Such a measurement wouldprovide an important constraint onthe two least well-known of theKobayashi-Maskawa mixing matrixelements (p and 77) as indicated in thedrawing above.

A second example is the CP-violating decay KO -> n°VV, pre-dictedby the Standard Model to occurwith a branching ratio of a fewtimes10- 11. Since the direct Cp-violatingterm is a thousand times larger thanthe indirect term, the observation ofeven a single unambiguous event ofthis type at the predicted level wouldconfirm the existence of direct CPviolation, a process whose existenceis still uncertain in spite of manyyears of heroic experimentation. Ameasurement of the branching ratio

2 WINTER 1991

would immediately give the value ofthe Kobayashi-Maskawa mixingmatrix element A7, which is respon-sible for CP violation in the StandardModel.

In both the examples cited above,the long-range effects are small, sothat QCD corrections are much easierto handle than is usually the case inkaon decay, allowing sensitive testsof the Standard Model to be madethat are relatively free of theoreticalambiguity.

Kaon decays that are forbidden inthe Standard Model, such as KL -->ie,have been the subject of numerousexperiments because their discoverywould immediately signal new phys-ics, in particular the existence ofnew forces whose quanta mediatethe decay (such as neutral Higgs,horizontal gauge bosons, lepto-quarks or supersymmetric particles).KAON will be able to probe theirexistence even if they are a thousandtimes as heavy as the W and Z, witha sensitivity unequaled in experi-ments done so far (see the table atthe top of the page).

Neutrino PhysicsThe Standard Model assumes that

neutrinos are left handed while anti-neutrinos are right handed, bothbeing massless, and that neutrinosare point-like, with zero charge andmagnetic moment, and only partici-pate in the weak interaction. Thereis no known general principle thatrequires strict masslessness forelectrically neutral fermions, how-ever, and non-vanishing neutrinomasses arise naturally in mostextensions to the Standard Model. Ifneutrinos have finite mass, then theycan oscillate from one generation to

Process

KLO -ueKL° -- > #/KLO - eeK' -> 7rfe-

IA -> eA

HiggsBranching Scalars

Ratio (GeV/c 2)

<3.3x10-1 1

7.6x10-9

<1.2x10-10<2.1x10- 10<4.6x1 0-12

194.7141.722

another, although no such oscilla-tions have yet been seen.

The neutrino beams that will beavailable at KAON, two orders ofmagnitude more intense than at otherfacilities, will enable many of theseassumptions about the properties ofneutrinos to be tested. For example,neutrino-electron scattering can beused to search for interactions thatchange the handedness of the lep-tons and are thus forbidden in theStandard Model. The same reactioncan be used to look for evidence ofneutrino charge and magnetic mo-ment, both of which have astrophysi-cal implications. The search for neu-trino oscillations could be pushed tounprecedented levels of sensitivity(see graph at right) since the neutrinointensity from KAON would enablemuch longer baseline experiments(up to 100 km) to be performed. Therelatively high flux of neutrinosabove 3.5 GeV gives the possibilityof searching for vu -> vz oscillations(see graph on next page).

Neutrino-proton scattering willalso be a topic of great interest, sinceit has the potential to give newinformation on the weak axial-vectorform factor and on the heavy quark

PseudoscalarLeptoquarks

(TeV/c2)

143.64.60.922

VectorLeptoquarks

(TeV/c 2)

260621909.6118

10" 3

U

01)co

a71--I

0a

z

Z

2--

1011

10 9

10 7

1050 2 4

Neutrino Energy

BEAM LINE 3

Mass Bounds from Different Processes

6(GeV)

Anticipated neutrino fluxes at KAON.

--

200 400 6(

PLAB (MeV/c)

content of the nucleon. This latter isof great current interest as a result ofthe SMC experiment at CERN thatseems to imply that the strange-quarkcontent of the proton is much higherthan expected.

Hadron Spectroscopy

Because the quark model remainsthe most useful tool in understand-ing hadron spectroscopy, it is rea-sonable to divide spectroscopy into.nnventional iuark model statpes (in%_11%- V 11 W11 |CC1 '1 C$1 \ LI X %- 1 JL.L s a o 1

10-1 100 which mesons are quark-antiquarkpairs and baryons consist of threequarks) and exotic gluonic and multi-

3rameters. quark (four or more) states.As far as conventional quark

model states are concerned, manymore states are predicted than havebeen observed. At least part of thereason for this state of affairs is thatthe states are often broad and over-lapping, they can and often do mix,and there is the possibility of exoticgluonic and multi-quark states inthe spectrum. Recent experimenta-

J _ __1_ _ _ 1_ _ __ _ _ _ _ j ---__ - __

non nas oeen aimea at trying to un-tangle this confused state of affairsand establish whether or not exoticgluonic and multiquark states exist.KAON has the potential to play aleading role in the future develop-ment of this subject. It will makeavailable intense beams of pions,kaons and antiprotons with unprec-edented purity and momenta up to20 GeV/c. The possible discovery of

)0 800 a glueball state, made entirely ofgluons, is but one example of thepotential of KAON to profoundly af-

7 nuclear fect our understanding of the stronginteractions.

Antiproton Physics

Present plans for the experimen-tal facilities available at KAON donot include an antiproton accumula-tor or storage ring, at least initially,though space has been left to addthem later. Nevertheless KAONcould deliver antiproton beams withintensities in the range of 106 to 108per second at momenta between500 MeV/c and several GeV/c. Theseintensities would be quite competi-tive with other antiproton facilitiessuch as LEAR, except at momentabelow 500 MeV/c. In particular, theenergy regime above the maximumLEAR energy of 2 GeV/c is relativelyunexplored. It thus appears that arich and varied program of physicscould be carried out with antipro-tons at KAON, encompassing hadronspectroscopy, tests of fundamentalsymmetries, studies of the nucleon-antinucleon force and use of the an-tiproton as a nuclear probe.

Nuclear Physics

The basic aim of the nuclear phys-ics program at KAON will be to shedlight on non-perturbative QCD andits relation to conventional nuclearphysics. The 1970s and 1980s sawthe issue of mesonic exchange cur-rents and 'non-nucleon' exotic par-ticles in nuclei emerge, with the roleof 7is and A's as nuclear constituents.Now and in the near future we seethe question of the role of quarks innuclei come to the forefront, i.e.,what are the relevant degrees of free-dom in nuclei? Can we continue tothink of nuclei as made up of protons

4 WINTER 1991

1(

10,

10-1 10-3 1-2

sin2 (2 0)

Limits on neutrino oscillation pc

7

5

3

1

Mean free paths of hadrons inmatter.

1(

and neutrons (with a few 7's and A'sthrown in), or are there nuclear phe-nomena which will force us to takethe underlying quarks and gluonsinto account explicitly? At KAON,intense beams of K+, the most weaklyinteracting of the hadrons (see figurebottom left), could be used to probethe inner regions of the nucleus. Thisis a promising technique which hasnot been made use of in the pastbecause of the feeble beams of kaonshitherto available.

A second example is the study ofhypernuclei, in which a strange quarkis deposited in the nucleus in theform of a A or Z particle. The Ahypernuclei are well established, butthe existence of Z hypernuclei, par-ticularly in long-lived states, isfraught with controversy and awaitsbetter experiments. The possibilityof forming "double" hypernuclei,containing two strange quarks, is aneven more exciting prospect openedup by KAON. The study of such states,if they exist, will require the mostintense possible beams of kaons, butcould provide information about, forinstance, the AA force, that can beobtained in no other way.

THE FACILITIES

THE TRIUMF H- CYCLOTRON, whichroutinely delivers 150 gA beams at500 MeV, provides a ready-made andreliable injector for KAON. It will befollowed by two fast-cycling synchro-trons, interleaved with three storagerings for time matching. The dia-gram above right shows the generalsite layout, with the A and B rings in

the Booster tunnel and the C, D andE rings in the main tunnel. The illus-tration on the next page indicatesthe time sequence of beam pulsesthrough the five rings.

The initial Accumulator ringgradually accumulates the regular23 MHz stream of beam bunches pro-duced by the cyclotron in forty no-tional "buckets" around the circum-ference. The process is similar tofilling a moving circular train of box-cars from regularly spaced piles on aconveyor belt. It continues for 20 ms,time enough for about 20,000 orbitsof the ring. Liouvillean prohibitionson continually injecting into thesame region of phase space are avoid-ed by stripping the H- beam from thecyclotron into a circulating beam ofprotons. Finally the forty bunchesare transferred to the Booster, whichhas the same circumference.

In the next 10 ms the Boostersynchrotron accelerates the beambunches from 74% to 97% the speedof light, increasing their energy six-fold to 3 GeV. The use of a boosterstage reduces the cost of the wholesystem. Because the beam shrinks in

Proposed layout of the accelerators,showing cross sections through thetunnels.

BEAM LINE 5

Energy-time plot showing the progressof the beam through the five rings.

diameter during acceleration itenables the magnets in the muchlonger 30 GeV synchrotron to be sig-nificantly reduced in size and cost. Italso reduces the complexity and costof the radiofrequency acceleratingsystem by restricting the problem ofproviding a large frequency rise,matching that in velocity, to theBooster, a small ring where only alow radiofrequency voltage isrequired (750 kV).

The Collector is a fixed-energyring in the main tunnel, five timeslonger than the Booster, used to col-lect five successive bunch trains fromit end-to-end over aperiod of 100 ms.Special radiofrequency cavities areused to lengthen the bunches andavoid the microwave instabilitiesthat would otherwise be expected atfull intensity.

The Driver synchrotron is locatedimmediately beneath the Collectorand is used to accelerate the 200-bunch train assembled there from3 GeV to 30 GeV. The speed of theprotons increases from 97% to99.95% of the speed of light and thefrequency of the accelerating voltage(2,550 kV) must also increase by 3 %.The bunch train extracted from theDriver is about 3 gs long and can beused directly for neutrino experi-

ments, for which sharp pulses aredesirable in order to better distin-guish real signals from background.

For experiments with strongly in-teracting particles, like kaons, pionsand antiprotons, a sharp pulse wouldproduce too many simultaneous in-teractions, indistinguishable from oneanother. For these the beam is trans-ferred to the Extender. This is a delib-erately imperfect storage ring fromwhich the protons are gradually ex-tracted into the main experimentalhall over the full 100 ms cycle time.

This functional separation of ac-celeration and storage allows theBooster and Driver rings to run con-tinuous acceleration cycles withoutflat bottoms or flat tops, thus provid-ing maximum intensity and 100%duty factor. The other major differ-ence from existing machines in thisenergy range is the fast cycling rate:50 Hz for B and 10 Hz for D, com-pared to -0.5 Hz for the BrookhavenAGS or CERN PS. This increase isnecessary in order to achieve thegreater intensity desired, because thenumber of protons storable per pulseis already at the limit (a few x 1013)set by space-charge defocusing ef-fects, for reasonable magnet apertures.

The high intensity and recordbeampower (3 MW) have influenced thedesign in many ways. Highly efficientcollimation systems have been de-signed for each of the rings, and lossyprocesses have been avoided or mini-mized in the design. For instance, H-stripping is used for injection, bunchesare transferred from bucket to bucketbetween rings, the buckets are neverfilled more than 60%, transition iskept above the acceleration range toavoid both beam dynamic and rf beam-loading problems, and a highly effi-cient three-element slow extractionsystem is planned.

To provide the long straight sec-tion for the slow extraction system,a racetrack-shaped lattice is used inthe Extender. Simulations indicatethat the spills can be kept below0.2%. Similar lattices and tunes are

6 WINTER 1991

used for the rings in each tunnel-anatural choice providing structuralsimplicity, similar magnet aperturesand straightforward matching forbeam transfer. Separated-functionmagnet lattices are used, with thedispersion modulated so as to lowerits mean value and keep transitionabove top energy. The reference de-signs for the A and B rings are almostcircular, with superperiodicities of 3and 6 respectively, although racetrackdesigns are also being investigated.

Because the number of protonsstored is similar, beam stability prob-lems are expected to be similar inscope to those in existing machines.The rapid-cycling rate tends to pushthe challenges towards the hardware.Fortunately, experience with rapid-cycling synchrotrons is available fromelectron and low-energy proton ma-chines, such as the Argonne IPNS,Fermilab Booster, and especially the800 MeV 165 MA ISIS at Rutherford.Prototype construction and testinghas been an important component ofthe preparations for KAON over thelast three years.In this we have beensupported by an $11 million pre-construction study.

Under this program, prototypedipole and quadrupole magnets forthe Booster have been built, poweredat 50 MHz and had their fields mea-sured. A novel power supply allowsthe resonant frequency to be switchedbetween 33 Hz and 100 Hz eachcycle, to make the rise three timeslonger than the fall and reduce the rfaccelerating voltage requirement.

The prototype rf cavity for theBooster has recently provided thefirst full-scale demonstration of thesuperior performance achievablewith a perpendicularly biasedyttrium-garnet ferrite tuner (a LAMPFsuggestion), as opposed to conven-tional parallel-biased nickel-zincferrite. Gap voltages of 65 kV havebeen obtained with 50 Hz pulse rateand 46-61 MHz frequency swing.

Two designs are being evaluatedfor the ceramic beam pipe needed

within the fast-cycling magnets. One,built in the UK, follows theRutherford ISIS scheme with a sepa-rate internal wire shield to provide alow impedance path for the imagecurrents. The other, being developedby Science Applications Interna-tional Corporation (San Diego), hassilver conducting stripes laid downin grooves on the internal surface ofthe pipe.

The kicker prototype follows theCERN PS transmission-line designand, in a collaborative effort, hasbeen shown to be capable of operat-ing at 50 Hz. Another pulsed deflec-tion device that is quite novel is the1 MHz chopper needed to remove10% of the pulses from the cyclo-tron, to leave beam gaps for kickeroperation in the new rings. The pro-totype uses a 0.5 ps coaxial delay linecable for energy storage and hasachieved 6 kV pulses with rise and falltimes below the 40 ns specification.

Because H- ions are normally ex-tracted from the cyclotron by strip-ping, a new resonant extraction sys-tem has had to be developed to ex-tract the ions whole. The rf deflec-tor, electrostatic septum, and firstmagnetic channel have already beeninstalled and tested, demonstratingapproximately 90% extraction effi-ciency; the 10% lost is interceptedby a special stripper, avoiding anyirradiation of the extraction elements.

The slow extracted proton beamwill be shared between two lines,each with two production targets.Each target will feed at least twoforward K and p channels, and insome cases backward u channels.The six charged kaon channels willhave maximum momenta of 0.55,0.8, 1.5, 2.5, 6 and 21 GeV/c. Withsolid angle and momentum accep-tances ranging from 8 msr x 6% forthe lowest momentum channel to0.1 msr x 1% for the highest, themaximum fluxes range from 0.6 to3.7 x 108 K+/s and from 0.7 to 11 x107 p/s. A dedicated line and area isprovided for polarized proton beams.

The neutrino production target, fedby the fast extracted beam, is locatedin the main experimental hall forgood crane access, but the neutrinoexperimental area is in a separatebuilding. Target development hasincluded both modification of anexisting rotating graphite target(driven and cooled by water) fromgraphite to tungsten, and the con-struction of a prototype target ro-tated by a flexible cooling line.

Environmental, industrial andeconomic impact studies have alsobeen completed and the cost esti-mates and schedule updated. Theplan is to share the total cost of Cdn$708 million equally among the gov-ernments of Canada, British Colum-bia, and other participating coun-tries, along similar lines to the fund-ing of HERA. The $236 million fed-eral and provincial shares have eachnow been approved, subject to suc-cessful negotiations on the operat-ing costs and successful negotiationsabroad for the remaining one-third.In view of the pledges made in previ-ous international discussions, theprospects for this look good on thetime scale envisaged.

In the U.S. there is a potential usercommunity of several hundred, andthe Nuclear Science Advisory Com-mittee (NSAC) recommended strong-ly in favor of a U.S. $75 million con-tribution in its recent Long RangePlan; this has been included in DOEbudget planning. Germany, France,and Italy have promised financialsupport proportional to the numberof their potential users. Participa-tion is also expected from Japan,where there is strong scientific sup-port. A number of other countries-Israel, PR China, South Korea, UK,and USSR-will contribute man-power towards design and construc-tion and equipment for experiments.The negotiations to confirm thesepledges are expected to take severalmonths, but we look forward withconfidence now to starting construc-tion in 1992. O

BEAM LINE 7

CP VIOLATION IN B MESONi

by David Hitlin and Sheldon Stone

An asymmetric electron-positronstorage ring called a "B Factory"can provide a critical testof the Standard Model. A

accelerator physicists as well as to experimenters. We will brieflydescribe the physics motivation for the widespread interest in BFactories, as well as some of the unique characteristics of theexperiments to be done at these new accelerators.

8 WINTER 1991

JAYS

CP Violation

BY CP VIOLATION WE MEAN a lackof symmetry between the weakdecays of particles and antiparticles.Until 1957, it had been assumed thatcertain symmetries of nature wereself-evident. The discovery of thenon-conservation of parity in theweak interactions upset this notion.The violation of parity meant that it ispossible to distinguish between anevent viewed directly and its mirrorimage, for most of us a non-intuitiveconcept. It was then proposed that asemblance of intuition could be re-covered by asserting that symmetrycould be restored by comparing thedirect view of a particle with themirror image of its antiparticle, aconcept known as CP symmetry.This notion held until 1964 whenChristensen, Cronin, Fitch, andTurlay found CP-violating decays ofthe KL meson. This lack of symme-try in the weak interactions is quitesmall (two parts per thousand), asopposed to parity violation, which ismaximal. Nonetheless, the lack ofCP symmetry in nature has far-reaching consequences.

Andrei Sakharov showed in 1967that CP violation, operating at timesof the order of 10-35 seconds afterthe big bang, as the universeentered thermal equilibrium, wascapable of explaining why theuniverse we inhabit is dominated bymatter, rather than being a mixure ofmatter and antimatter, as it wasinitially. This connection with the CP-violating decays seen in KL decaysgave the phenomenon a role ascentral to cosmology as it hadacquired in elementary particlephysics. Ironically, this observation,one of the primary motivations forour desire to understand whether themechanism of CP violation can beexplained by the so-called "StandardModel," ipso facto requires physicsbeyond the Standard Model, in thatas yet unobserved baryon-number-violating interactions are alsorequired to describe the parametersof the known universe.

A /

Vud

(

Bs -pKs

VtdVtb BdBd

I VdVc'b I

A=( p,n)

C.

Two representations of the unitatriangle. The upper figure showsunitarity relation is representedclosed triangle. The three angletriangle, a, /, and y can be direcmeasured by measuring CP-violasymmetries in B° meson decallower figure shows the unitary trdescribed by the two Wolfenste,paramaters p and rl. If the basetriangle is placed along the p axscaled to length one, then the althe triangle (A) lies at the pointy

The program of CP-violationsurements seeks to overdetermiunitarity triangle. That is, since cquantities suffice to determine athe measurement of a fourth or 1quantity allows one to make a vwtriangles from the experimentalties, any three of which must fortriangle consistent with any of thtriangles made from different cctions of three quantities. If they (the Standard Model will be over

Wolfenstein's observation that thesize of the elements decreased with

-. VKs distance from the diagonal of thematrix. The "Wolfenstein parametri-

Ž B zation" has become a standard nota-tion for discussions of the CKM ma-trix. In this approach, there are, ofcourse, still four parameters. The." , · . -· . 11 .1 * * 1

first, A, is essentially the originalCabibbo angle; the second, A, is re-lated to the lifetime of B mesons.

, B=(1,0) The other two, 77 and p, are less well1 . * ,

understood; a series of measurementsof weak decays is required for their

lry determination.how the The CKM matrix is a unitary

as a matrix,which imposes certain rela-s of thectly tions between its elements. In par-rating ticular/. The/angle VudVVub + VcdV cb + VtdV tb = 0,inof the which is just the definition of the

7is and unitarity triangle. The six CKM ma-pex of trix elements related by the unitary

mena- triangle relation are not preciselyine the known, for several reasons. The first)nly three is experimental uncertainty. For ex-triangle, ample, Vcb is related to the measured

fifth rate of B meson semileptonic decay,riety of which has an associated experimental

quanti- error. The second is theoretical un-'mame a certainty in relating measurements7mbina- to CKM matrix elements. An exampleJo not, is Vub, which suffers from model-thrown. dependence in relating the matrix

element to the measured highmomentum end of the B-to-leptondecay spectrum. A third uncertaintyis our lack of knowledge of the topquark mass, which could be remediedwithin the next few years by experi-ments at the Tevatron. The net resultof these uncertainties is that the exactshape of the unitarity triangle is notprecisely known. Taking into

account the values of A and A, andassuming the top quark mass to be120 GeV, we can say that the point r7,p must lie in the shaded region of thefigure at the right. Improved measure-ments in the next few years canreduce the area of the shaded regionto some extent. Our knowledge ofthe three sides of the unitarity tri-angle, while it does not completelyspecify the geometric figure, doeslimit the possible range of values ofthe angles of the triangle. For ex-ample, with a top quark mass of120 GeV, the angle /f can be nosmaller than 15° and no larger than150°. That is where CP-violatingasymmetry measurements enter;these asymmetries are directly re-lated to the angles of the unitaritytriangle. Thus measurements of theseasymmetries have the potential totest the self-consistency of the Stan-dard Model: if the angles fall withinthe permitted range, we will haveshown that the imaginary parameterof the six-quark CKM matrix candescribe CP violation. If they falloutside the permitted range, thiswould represent the first failure ofthe Standard Model and potentiallythe first hint of the new physics thatlies beyond it.

T HE DISCOVERY of unexpectedlylarge B0 -B° mixing by ARGUS

in 1987 made it possible to considerthe measurement of CP violation inthe neutral B meson system as aplausible goal. Indeed, the expectedCP-violating asymmetries could beas large as many tens of percent. Thedifficulty is that the modes in whichthese large asymmetries could occurhave very small decay rates. Since

10 WINTER 1991

--,u luI

mt = 120 GeV

2

the branching fraction for thesemodes is expected to be of the orderof 10- 4 to 10-5, measuring these largeasymmetries requires large samplesof B mesons, as many as 108. In orderto acquire a sufficient number of Bmesons in a reasonable time, it isnecessary to build a storage ring thatcan operate at the Y(4S) resonancewith a luminosity 15-20 times thatachieved thus far. The Y(4S) decaysinto either B°B° or B+B- pairs. Eventhis does not suffice, however; it isalso necessary that in the B Factoryaccelerator the energies of the elec-tron and positron beams be unequal,hence the name "asymmetric BFactory." The motivation for thiscomes from a combination of physicsand technology. The CP-violatingasymmetries we wish to measure atthe Y(4S) resonance are proportionalto sin(ti - t2), where t1 and t2 are thetimes at which the B0 and B0 mesonsdecay. Since the two B mesons candecay in either time order, a measure-ment that includes all possible timesand time orders is in effect anintegration from -oo < tl - t2 < 0,which is an even function of t1 - t2Since sin( tl -t2) is an odd function oftl - t2, such a measurement of neces-sity yields an answer of zero.

The way around this conundrumwas proposed in 1987 by Pier Oddoneof Lawrence Berkeley Laboratory.Clearly, the way to come up with anon-zero result is to make a mea-surement that is not symmetric intl - t 2. How can we do this? Such ameasurement requires the ability tomeasure distances less than the aver-age flight path of a B0 meson before itdecays. In a conventional storage ring,with the Y(4S) stationary in thelaboratory, B0 mesons produced in

1

r7

0

-1 0 1

p

The shaded area shows the region of the (p, r7) plane in which the apex of theunitarity triangle must fall (based on the measurement of CP-violating asymmetries)if the Standard Model description of CP violation is correct. The dashed-line trianglerepresents the largest value of f allowed (i.e., the largest CP asymmetry in thedecay Bd -> J/lKs°). The white triangle represents the smallest value of p allowed.

BEAM LINE 11

2

Y(4S) decay travel a mean distanceof 19 um before decay. In spite of therapid progress in precision vertexdetection over the last several years,this accuracy is well beyond anythingwe can hope to achieve in the for-seeable future. Oddone proposed toresolve this problem by takingadvantage of the Lorentz boost in thelifetime of a particle that is movingrapidly with respect to the laboratorysystem. That is, if the Y(4S) statewere to be made in motion in thelaboratory, the B0 and B° mesonsproduced when it decays would alsobe moving rapidly in the lab. If thevelocity of the Y(4S) were to be aroundhalf the speed of light, the meandistance traveled in the laboratorybefore a B meson decayed could beboosted to perhaps 200 um, a distancethat is readily measurable. Now itwould be possible to effectively makethe limits of integration unsymmetri-cal, so that the measured CP asym-metry is non-zero. This can be accom-plished by making the energies ofthe electron and positron beams un-equal, typically with a ratio of ener-gies of about three to one.

A N EXPERIMENT TO MEASURECP violation in the B meson

system requires a detector that isconceptually similar to existing e+e-

detectors, such as CLEO II nowoperating at the CESR storage ring atCornell. A new detector must copewith higher crossing frequencies ofthe beams (there are, for example,1658 bunches in the proposed PEP-based B Factory at SLAC, whichmeans that there would be a beamcrossing every four nanoseconds),much higher event rates, and higher

12 WINTER 1991

backgrounds and radiation levels. Inaddition to the conventional driftchamber tracking system in a 1 to1.5 Tesla solenoidal magnetic fieldand a high quality electromagneticcalorimeter, such as the CLEO IIcesium iodide device, a B factorydetector must have a high precisionvertex detector capable of measuringpositions of charged tracks to a spatialaccuracy of 15 uim and, in order toreduce backgrounds in B decay finalstates, a system for positively identi-fying charged particle species. A dataacquisition system capable of copingwith very high event rates is, ofcourse, also needed. A possible detec-tor is shown schematically on theprevious page. It consists of the vertexdetector followed by a charged par-ticle tracking chamber, the chargedparticle identification system, andthe electromagnetic calorimeter.These devices are contained in asolenoidal coil, which is in turn insidea magnetic flux return that incor-porates a system for identifyingmuons. Development work on thesedetector subsystems is underway inlaboratories throughout the world.

The measurment of CP violationin B° meson decay presents one ofthe most exciting experimentalchallenges in high energy physics forthe coming decade. Groups in theUnited States (at SLAC and Cornell),at KEK in Japan, in Novosibirsk inRussia, and in Europe are currentlyworking on B factory initiatives. Weare confident that one or more ofthese efforts will come to fruition. AB Factory presents our best hope forconfronting the Standard Model witha novel test in the '90s: ascertainingwhether it can indeed explain thecrucial phenomenon of CP violation.

BEAM LINE 13

Fermilab and SLACCo-Sponsor History Symposium

THE RISE OF THESTANDARD MODEL

The ThirdInternationalSymposium on theHistory of ParticlePhysics will be heldJune 24-27, 1992, atStanford LinearAccelerator Center

IN LATE JUNE 1992, SLAC WILL HOSTthe Third International Symposium on the History ofParticle Physics, which it will sponsor jointly with Fermilab.At this gathering, a group of eminent physicists will meetwith historians, philosophers, and sociologists of science toexplore the major scientific, technological, and socialdevelopments of the 1960s and 1970s that led to thecurrently dominant Standard Model of elementary particlesand forces.

14 WINTER 1991

This symposium will be the thirdin a series that began at Fermilab in1980. The first, titled "The Birth ofParticle Physics," explored the earlyroots of the field-from about 1930through the discovery of the pion in1947. In 1985, the second conference,"Pions to Quarks," picked up wherethe first left off and examined develop-ments that occurred during the 1950s,when particle physics passed throughits "adolescence" and became an in-dependent subfield; this period endedin 1963 with the formulation of thequark hypothesis.

The theme and title of this thirdsymposium is "The Rise of the Stan-dard Model." Most of the theoreticalfoundations for this major advance inour thinking about elementary par-ticles were laid down in the 1960s,followed immediately by a decade ofexperimental discovery and consoli-dation. Although there are plenty ofloose ends remaining, the StandardModel has endured in essentially itspresent form for more than a decade.The time has come to begin reviewingthe events and processes that led to itsestablishment as the dominant theoryof particle physics today.

The focus of the Symposium willbe on the major advances that occurredduring the core period from 1964 to19 79, but related events that happenedbefore or after this time will be in-cluded in the program. Several theo-retical developments that began inprior years-including gauge fieldtheories and spontaneous symmetrybreaking-reached their full floweringduring this period. Colliding-beamtechniques came to the fore, replacingfixed-target experiments as theprincipal means of probing the high-energy frontier, while the 4 pi

electronic detector replaced the bubblechamber as the instrument of choice.To build this increasingly massiveand costly equipment, the experimen-tal collaborations grew to almost ahundred physicists hailing from anumber of different institutions.

The Symposium will begin onWednesday, June 24, and continuethrough Saturday, June 27. In addi-tion to opening and closing sessions,there will be five plenary sessionsorganized around the following top-ics: Heavy Quarks and Leptons; Ac-celerators, Detectors, and Laborato-ries; Toward Gauge Theories; TheSearch for Electroweak Unification;and From Quarks to QCD. There willalso be two panel discussions: one onSymmetry Breaking and the other onScience Policy and the Sociology ofBig Laboratories.

Steven Weinberg will deliver theopening talk, and Sheldon Glashowwill summarize the Symposium; thefeatured banquet speaker is MurrayGell-Mann. Other physicists speak-ing in the sessions include: JamesBjorken, Jerome Friedman, Gerhard't Hooft, Makoto Kobayashi, LeonLederman, Burton Richter, SamuelTing, and Robert Wilson. Among thescholars of science who will sharetheir insights on this period are PeterGalison, John Heilbron, John Krige,Michael Redhead, Sylvan Schweber,and Sharon Traweek.

The Symposium is being organizedby LillianHoddeson, Michael Riordan,Laurie Brown, Max Dresden, and NinaAdelman Stolar. For further informa-tion, contact Ms. Stolar at SLAC,MS 70, Box 4349, Stanford, CA 94309,telephone (415) 926-2282; telefax(415) 926-4999; and the BITNET ad-dress is NINA@SLACVM. 0

BEAM LINE 15

CONTRIB UTORS

Michael Craddock Peter Kitching David Hitlin Sheldon Stone

MICHAEL CRADDOCK is head of the KAON Factory Accelerator Division at TRIUMF and deputy project leader. He

joined the faculty at the University of British Columbia in 1964, played a major role in preparing the propopsal for

the TRIUMF cyclotron, and became group leader for beam dynamics. Since 1977 he has worked on designing andpromoting the KAON Factory and still believes that he will see it working before he retires.

PETER KITCHING is a professor of physics and Director of the Centre for Subatomic Research at the University ofAlberta. He was Associate Director and Head of the Science Division of TRIUMF from 1983 to 1988 and acted asHead of the Science Division of the KAON Factory Project Definition Study. As a part of this study, jointly funded

by the Canadian federal government and the British Columbia provincial government and completed in 1990, heorganized a series of workshops on the science that could be pursued at the KAON Factory. These workshops wereheld in North America, Europe, and Japan. His main scientific interest is the experimental search for rare kaondecays, and he is currently a member of the collaboration carrying out an experiment to look for the decay of a kaoninto a pion and a neutrino-antineutrino pair (E787) at Brookhaven National Laboratory.

DAVID HITLIN is Professor of Physics at Caltech. He has been active in experiments at SLAC since 1969, emphasizingthe study of the weak decays of KL mesons and of charmed particles and the J/y with the Mark II and Mark IIIdetectors at SPEAR. He also managed the design and construction of the SLAC Large Detector Liquid Argon

Calorimeter system. He has for the past few years been heavily involved in the effort to upgrade the PEP storage ring

into a high-luminosity asymmetric-energy B Factory for the study of CP violation. He is the current chairman of

the SLAC users organization.

SHELDON STONE did his undergraduate work at Brooklyn College and his graduate work at the University ofRochester. He has been a member of the CLEO collaboration since its beginning. He has made a contribution to bothhardware development and physics analysis. He participated in the construction of the dE/dx wire proportionalchambers for the original detector and the construction of the CsI calorimeter for CLEO II. His analysis work hasbeen mostly in the areas of B meson and charm meson decays. This work includes the discovery of the B and Ds

mesons and measurement of many properties of their decays. He is now a professor at Syracuse University.

BEAM LINE 16

-

0

.2

cf

c.(6