The Search for Muon-Number Violation at LAMPF

7/25/2019 The Search for Muon-Number Violation at LAMPF

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and neutrino) can interact at a singlepoint. Ho wever, the pro cess is allowed inthe second-order approximation by amore c omplicated virtual sequence of in-teractions. The branching ratio (ratio ofdecay rates) predicted by this model forthe rare decay p - ey versus the knowndecay p evv was

where is the rate for the process. In1957, the experimental upper limit6 forthis branching ratio was < 2 x l o 5 ;therefore, there was no discrepancy be-tween theory and experiment.

However, as had long been re al i~ ed ,~the four-fermion weak interaction in-creases in strength with energy, leadingt o infinite answ ers. To av oid high-energyproblems, schwinge r8 conjectured thatan intermediate vector boson (a particle

with spin 1) mediates the weak interac-tion. In this nonlocal interaction, thebranching ratio above is calculated to be- 1 0 ' in contradiction to the ex-per imental l imit . T o reconci le th isproblem, the notion of lepton quantumnumber conservation was postulated.8'10In this scheme, assignments of leptonand muon numbers are made for themuon, the electron, and two kinds ofneutrinos. These are shown in Table IThe conservation law then states that thesum of the lepton number and the sum

of the muon number are each conservedseparately. Th is law then forbids the un-observed processes, p - e y , p' --etete , an d p Z - e + Z, butallows all observed processes includingp - e + v ~ @ .

This scheme, however, requires theexistence of distinct electron-neutrinosv e ) and muon-neutr inos v ) . Pon-tecorvo and Schw artz l1 discussedmethods to determine if there are twokinds of neu trinos. D an by et al.12 perfor-

TA B L E I A S S I G N M E N T SLepton Number Assignment

Particle Lepton Num ber

e , ~ , V ~ , v , + 1e',p',~~,V, 1All others 0

Muon Number Assignment

Particle Muon Numbe r

4-9 v, +1p+,v, -1All othe rs 0

med the two-neutrino experiment in1962 and showed that the neutrinos

produced in pion decay, by the process

interacted with matter to produce m uonsby

but did not produce electrons. Tneutrinos produced in beta decay(N -

+ e V) d o interact with matterproduce electrons.13 This observationtwo distinct types of neutrinos was sas a validation of the lepton num ber cservation law.

The experimental status of mu

number conservation in 1964 and 1is shown in Table 11. The prevailing titude after the two-neutrino experimwas expressed as follows:

The results of the ne utrino ex-periments . . . indicate thatthe normal weak interactionchannels are closed to thisdecay mode [ft. ey]. Sinceit now appears unlikely thatthis decay is lurking justbeyond present experimentalresolution, any further search

for the p y decay mode atthis time seems futile.14

Even though lepton number consertion accou nts for the failure to detectprocesses in Table 11, there is no fdamental reason for this conservatlaw to be exact. Unlike electric cha

TA B L E I1S TAT U S O F M U O N N U M B E R C O N S E RVAT IO N , 1964 A N D 198

Muon Number Process 1964 198

r ( p - eee)< 1 . 3 x l O - ' < 1 . 9 ~ 1 0 - ~

r ( p - evv)

r(g-Z - e-Z)


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conservation, which must be exact byvirtue of the gauge invariance of theelectromagnetic field wh ose qu antu m ex-citations are massless photons, leptonnumber conservation is not associatedwith a massless gauge field. A heuristicargument1 will help explain this.

Assume that black holes exist. If acharged particle falls into a black hole,the memo ry of its charge is preserved by

its electric field outs ide the black hole, sothat conservation of electric charge canbe verified. On the other hand, if amuon -neutrino falls into the black hole,it leaves no tra ce at all, so that an ex actconservation law for muon number isnot a measurable concept.

The most exciting development of thepast decade in theoretical physics hasbeen the successful unification of theweak and electromagnetic interactionswithin the framework of non-Abeliangauge theories.16 (Electromagnetic in-teractions alone are described by theAbelian gauge theory quantum elec-trodynamics.) At present, there is con-siderable latitude in the exact composi-tion and structure of the correct gaugemodel: this freedom can be reduced onlyby accumulating more experimentalfac ts . The gauge models not onlysuggest that lepton number conservationis not exact, they also predict that muon -number violating processes may occurat rates somewhat below experimental

limits. If we can measure the rates orreduce the experimental limits, we willgo a long way toward pinning down thecorrect unified model of weak and elec-tromagnetic interactions and place ad-ditional constraints on the many grandunified models recently proposed to uni-fy the w eak, electromagnetic,and stronginteractions.

In 1977, a group of physicists fromthe Los Alamos Scientific Laboratory,the University of Chicago, and Stanford

G R P H I T E

P R O D U C T I O N

T R G E T L M P F8 M e V

P R O T O N B E M

L M P F

STOPPED

M U O N C H N N E L

C RY S T L B O X D E T E C T O R

Schematic view of the experiment at LAM PF. The 800- M eV pro ton beam strikgraphite production target producing pions. Som e TT * 's low down and stop near surface of the target and decay into C s . Th e muons are then transported by the sped muon c hannel to the detec tion apparatus: Th e channel consists of dipole bendmagnets and quadrupole ocussing magnets. The muons com e to rest in the stoptarget in the center of the cyrstal box detector and then decay. A l e^ y evenshown. Positrons and electrons are detected in the drift chamb er and hodoscope coters and their energy is measured in the N d :photons do not register in either the dchamber or the hodoscope counters but do deposit energy in the N dAll informat

from the detectors is processed by an on-line computer and stored on magnetic tfor further off-line analysis.

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The crystal box that will be used to detect rare decays of the muon

university1 mounted an experiment at1.9 x 10-lo ,LAMPF to search for the decay p - l (p - v v

e+y. The h igh-intensity beams of mu onsavailable a t LA M PF are particularly about an order of magnitude more sen-well suited to a search for this rare sitive than any previous search .decay. This experiment did not detect Now a new collaboration from thethe deca y but did push the experimental same three institutions has embarked onupper limit1 down to an experiment to search for the muon-

number violating processes p - e+p - e e e, and pt - e^yy withlarge new experimental facility knownthe crystal box

As shown in the conceptual drawof the app aratus , the basic design of detection system calls for a large soangle modular sodium iodide detec

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weighing -2000 kg, surroundin g a thintarg et in which the rriuons sto p anddecay, a cylindrical drift chamber, andtrigger hodoscope (plastic scintillation)c o u n t e r s . T h e a p p r o x i m a t e ly 4 0 0sodium iodide modules will detect 53-MeV positrons and photons with essen-tially 100 % efficiency, an ener gy resolu-tion of -2 MeV (FW HM ) and a t imingresolution of 0.5 ns (FW H M ) (1 ns =

l o 9 s ). The dr if t chamber will recordthe passage of charged particles with ap o s i t io n r e s o lu t i o n o f - 2 0 0 p M(F W H M ) in each o f e igh t laye r s.Photons produced in the events will beidentified by detecting energy depositedi n t h e s o d i u m i o d i d e w i th n ocorresponding response from the driftchamber or hodoscope counters; elec-trons an d positrons are detected by all ofthese systems.

The three processes, ,LL - 'y, ,L -e 'e '^e , and l - 'yy, will be stud iedsimultaneously with a sensitivity tob ra nc hin g r a ti os of a bo ut 1 0 . T hisrepresents an improvement of- 0, 100,and 5000, respectively, over present ex-perimental limits.) Events will be selectedby a hard-wired processor designed touse both the analog and digital informa-tion from the detector and to make adecision within 250 ns.

This speed will enable the apparatusto operate at a flux of 5 x 10' 1'1s an dwill provide an immediate suppression of

accidental coincidences from the or-dinary d eca ys of several muons. W e willbegin setting up the experiment in late1980 and will begin taking d ata by mid-1981.

If any of these processes is observed,it will be obvious evidence of the failureof the conservation of muon number.The strength of the failure will provide agreat deal of information as to what isthe correct model of the basic interac-tions. Should none of these processes be

y Hoffman left) and Minh Duong-Van right) examine a prototype drift chambecomponent of the crystal box detector that will be used to search or rare decays ofmuon.

observed. the experiment will force tightconstraints on many potential modelsand eliminate many others. If theprocess l e'y is not obser ved in thecrystal box. we plan to reconfigure thesodium iodide modules inside a largemagnet and continue the search formuon-number violation with at least an

order of magnitude greater sensitivity.The search for muon-number viola-tion is being pursued at L A M P F withseveral orders of magnitude greater sen-sitivity than anywhere else in the world.

t must be stressed that theory neitherpredicts nor forbids this violation. Theoutcom e of these experiments will have agreat bearing on the way we view theworld. Perhaps on e of the few conserva-tion laws that we believed to be exactwill turn out to be violated after all.

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References

1. E. Noether, Nachrichten Akad, W iss. Goettingen, Math P hysik, Kl. IIa, Math. Phys. Chem . Abt. 191 8, 235 1918).

2. C. D. Anderson and S. H. Neddermeyer, Phys. Rev. 51, 884 1937) and J. C. Street and E Stevenson, Phys. Rev. 51, 1005 1937).

3. H. Yukawa, Proc. Phys. Math. Soc. Japan, 17, 48 1935).

4. R. P. Feynm an, The Theory of Fundamental Processes W. A. Benjamin 1966), p 66.

5. See, for example. G. Feinberg, Phys. Rev. 110, 1482 1958) and the references therein.

6. S. Lokanathan and J. Steinberger, Phys. Rev. 240 1955).

7. W. Heisenberg. Zeit. Phys. 101, 533 1936).

8. J. Schwinger, Ann. Phys. 2 407 1957).

9. M. Gell-M ann and R. P. Feynman, A m. Phys. Soc. Meeting, 1957 unpublished).

10. K . Nishijma. Phys. Rev. 108, 907 1957) andS. Bludman, Nuovo C imento 9,43 3 1958). The first lepton number scheme was proposed by K onopinskMahmoud. Phys. Rev. 92. 1045 1953).

1. B. Pontecorvo. JETP 10, 1236 1960) and M. Schwartz, Phys. Rev. Letters 4, 306 1960).

12. G. Dan by, J-M . Gaillard. K. Goulianos. L. M. Lederman, N. Mistry, M. Schwartz, and J. Steinberger, Phys. Rev. Letters9, 36 1962).

13. F. Reines and C L. Cowan. Phys. Rev. 90, 492 1953).

14. S. Frankel. W . Frati. J . Halpern. L. Holloway, W. Wales, and 0 Chamberlain, Nuovo Cimento 27, 894 1963).

15. A. De Rujula. H. Georgi. and S. L. Glashow, Phys. Rev.D12, 147 1975).

16. See, for example. A. D e Rujula. H . Georgi. S.L. Glashow, and H. R. Quinn, Rev. Mod. Phys. 46,39 1 1974) andE S. Abers and B. W. Lee. Phys. Rep9C. 3 1973).

17. J . D. Bowman. M. D. Cooper. M. Hamm, C. M. Hoffman, R. E. Mischke, D. E. Nagle,J S. Sarracino, P. A. Thomps on, H. L. Anderson.W . W . KmnisH. S. Matis. S. C Wright. R. L. Carrington. R. A. Eichler, R. Hofstadter, E. B. Hughes, and T. McPharlin, Phys. Rev. L etters 42, 556 1979

The collabo rators for this experiment are H. L. Anderson, J. D. Bowman,R. Carlini, M. D. Coope r, M. Duong -Van, J. Fra nk, C.M. Hoffman, G Hogan, WW. Kinnison, R. Macek, H. Matis, R McKee, R E Mishcke, D. E. Nagle, V. D. Sandberg, G H. Sanders, R Talaga, and R A. Williams of LASL; Grosnick and S. C Wright of the University of Chicago; and R. Hofstadter,E. B. Hughes, and S. Wilson of Stanford University. Technical support is bprovided by R. Bolton, C Dalton, G Hart, E. Iverson, G. Krausse, M. A. Johnson, J. Sandoval,J Sena, and H. P Von Gun ten of LASL, and by J. RolfeStanford University.

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The Search for Muon-Number Violation at LAMPF

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