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by: Maureen Roob IV


Marketplace > University of Florida > Physics 2 > PHZ 6355 > ELEM PARTICLE PHYS 1
Maureen Roob IV
GPA 3.88

Andrey Korytov

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Andrey Korytov
Class Notes
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This 6 page Class Notes was uploaded by Maureen Roob IV on Friday September 18, 2015. The Class Notes belongs to PHZ 6355 at University of Florida taught by Andrey Korytov in Fall. Since its upload, it has received 8 views. For similar materials see /class/206872/phz-6355-university-of-florida in Physics 2 at University of Florida.




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Date Created: 09/18/15
Introduction to W m quot1 my Particle Physics Note 18 Page 1 of6 The remaining story of leptons Neutrino vs antineutrino Lepton numbers Discovery of Muon Neutrino Discovery of Tlepton Discovery of Tneutrino Lepton Universality Introduction to W m quot1 try Particle Physics Note 18 Page 2 of6 Neutrino vs antineutrino Neutrino is a neutral particleia legitimate question are neutrino and antineutrino the same particle Compare p oton is its own antiparticle no is its own antiparticle neutron and antineutron are different particles Dirac neutrino 1f the answer is di erent neutrino is to be calledDirac neutrino Majorana neutrino 1f the answer is the same neutrino is to be calledMajorana neutrino 1959 Davis and Harmer conducted an experiment to see whether there is a reaction 17 n p e could occur The reaction and technique they used v 37Cl 9 e 37Ar was proposed by B Pontecorvo in 1946 The result was negative Lepton number However this was not unexpected 1953 Konopinski and Mahmoud introduced a notion of lepton number L that must be conserved in reactions 0 electron muon neutrino have L o antielectron antimuon antineutrino have L 71 This new ad hoc law would explain many facts 0 decay of neutron with antineutrino is OK n p e 17 pion decays with single neutrino or antineutrino is OK Tl gt u 17 but no pion decays into a muon and photon if gt u 7 which would require no decays of muon with one neutrino u gt e V which would require L 1 gt L 1 i 1 0 or 2 no processes searched for by Davis and Harmer which would require L T O L b H L O H But why there are no decays u gt e 7 22 Splitting lepton numbers 1959 Bruno Pontecorvo suggests that the lepton numbers for electrons and muons should be treated separately 0 electron and electronneutrino have L5H their antiparticles have Lfl39 o muon and muonneutrino have L 139 their antiparticles have Lfl o THUS there should be two types of neutrinos electronkind and muonkind Rewrite all decays 7 f hink f d h n Hp e V l onet s in terms 0 Feynman s 1agrams 1t1s a rat er If H M v natural proposition when muons or electrons participate in 7 a weak interaction by exchanging with W and change into neutrinos these neutrinos might be of two different kinds too 1 However the question on neutrino vs antineutrino turned out to be much more complicated than one would think The if culty came from the act that parity quantity related to mirror re ection symmetry was discovered not to conserve in the weak interactions We will discuss this nonconservation later It was discovered in 1956 and caught everybody by surprise One of the affecting neutrinos besides gravity Therefore the observation that the two particles interact differently might not result from their intrinsic differences but simply because of them being born with different handedness or helicity However if neutrinos do have a mass as the recent observations of neutrino avor oscillations tend to indicate then their handedness is not uniquely de ned or Lorentz invariant and one can set up special very delicate experiments to tackle this question We will discuss these issues later For now we will assume that neutrinos are of Dirac kind ie described by the Dirac equation very much the same way as electrons 39 39 that quot A haveany quot th39 ero err mass 15 very close to z the only 2 Feynman s rule of thumb whatever is not explicitly forbidden is MANDATORY Introduction to Elementary Particle Physics Nate 1 8 Page 3 of 6 Discovery of Muon Neutrino 1962 Lederman Schwarts Steinberger Brookhaven m e an experiment 0 check Whether muon and electron neutrino indeed different One has to start from muon neutrinos and see Whether all they can make are muons 4quot ap V14 P It 1 n a Pions is an excellent source of muon neutrinos The idea of neutrino beams actually belongs to Pontecorvo 0 15 GeV protons gt Be target gt lots ofpions 15 1539 1 0 20 m decay distance not too short good fraction ofpions decay not too long muons do not decay 7539 art 1 103 M4 1 D 10 s 7 K 39 ll 1 395 s gt lots of photons pions muons and high energy muon neutrinos and antineutrinos 0 13 m of steel to absorb all particles but neutrinos spark chamber With Al sheets 0 cover detectors With scintillator to veto cosmic rays gt 10M muon neutrinos and antineutrinos muons should produce nice tmcks running throughout the entire detector 29 events electrons should produce showers as they cross Al sheets none Tmquot t I I aim target pm an axe era in i rr r l v w 7 w 7 7 i 39 r if detector plmesnn melanin spark lumber beam The aarderatol the neutrino beam and he delecml pm of Ihe tircullr samurai Bmoliham in wliidl the Drum um amalgam I iiimesons hid were re uxed In in mm collisions wi n me use inf concrete iiquot gt5 l m I 39 P c exclp e Dene ml naiui MW imam rental in the 62mm and viva rise to mums which am um observed in the and My am m n vnwnq In 5mm mum um um Mass limit from direct mass measurements lt200 keV 1988 Lederman Schwarts Steinberger receive the Nobel Prize quotfor the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino Inquot J 39 to Fquot yParticle Physics Note 18 Page 4 of 6 Discovery of 39clepton 1975 MarkI experiment at SPEAR Stanford Linear Accelerator Center claims discovering new lepton Most of the work on this analysis was credited to Martin Perl The tau leptons can be created whim temperature and density are high annug su as was the casein Bl Bang The gure shows the decay of a pnir of tnuans Into a munn electron u and quotinvisihlequot neuIrinas v DieribuLion 0139513 4 5 GeV 21mm 1891 crilen a pt gt05 eVc umpl gt20 Tam Charge 2 Number photons X o X i 55 o 0 0 up 16 15 6 0 0 0 sh 18 23 52 2 fl 3 1111 15 16 1 4 U 5 111 13 ll 30 10 4 6 Sum 126 184 162 lb 8 17 Table 1 From Perl 1975a llm caption read quotllisidcnlllmuon probabilities or 43 Gov sample Momentum range GeVc PM2 1 th rh h 05119 130t00 161t006 7091012 09 12 160 009 2151 13271020 12 16 205016 216017 5781029 16 24 269031 211027 5201043 39 Weighted average using h hh wand 91 evenu 18230117 198007 519012 In the SPEAK collider electrons and posihons 2 crash headon in a little quotBig Bang whereupon among many possible reactions tau are created in pairs T lepton has a mass of 1777 GeV and lifetime of 29103913 s It has lots of modes of decays 18 Tquot a e39 V vI 47 139 a u 3 v 64 Iquot a hadrons vI Now WHO the hell ordered THAT 1995 Per receives the Nobel Prize quotfor the discovery of the T lepton Lu J quot to F vParticle thsics Note 18 Page 5 of6 Discovery of I neutrino 2000 DONUT experiment at Fermilab claims discovering T neutrino spokesmen Lundberg and Paolone 6000000 candidates on the tape 4 clean T decay events truly one in a million DONUT Detector mennrcsum or mm canny m u my Calmmeter uecarmrnes energy or decay Dm ws Dmr crumhers record decay paw Hacks Magnetsueaas inadq or mad Parades Emmsron was wrlh planes or sanlrllatran bers block was nine in Sleei shield in r r Ammo DONUT Detector for direct observation of tau neutrinos 14 r W m m EXP DONUT M m 3039rcr191o I 3 if 1 E400 MO D Ecc1 h quot Hadron Unknown F L 5290 u m ammomorau P414 Mevrc parsgg cewc re Detecting a Tau Neutrino Tau mummy Pamciu Msrmnmmrm immlau Nammo pmduuas rm mmquot lepton Emuismn rm beam mmquot m layers may rams recorded V quotmunquot quot r wuP39 v J 9 X 2quot 4 quotw MWquot v 4 39u n4 we wuW a nilquotMM Man quota a Du quotrgtaha him 33 quotmt oooou Nquot s 0 4 IlI ll 42 6 quot lVIass limit from direct mass measurements in T lepton decays lt24 MeV different experiments Introduction to W m quot1 my Particle Physics Note 18 Page 6 of6 Lepton Universality Muon and tau lepton lifetimes r gt e Tev rHeW 27411412 p GeV 1 Gev GeVS p must have dimentions of GeVS While the only energy variable available is tau mass m me and m M can be neglected 1 er zwzm FHM 01784 Fm Laife ma i 0172845 Tram 27DG m if gt 176111 FHW 2721Gij 7 lifetime 1 PM 2726ij 7 m5 Prediction 4 017844 133 x10 7 7 m 713 Experiment 3 132 X 10 7 7 22x10 it Branching ratios of tau lepton decaying into a muon or electron 1quot gt u39T ig Pram litiMlzp 272Gsz r eew Puma4Msz 2nczm Fra vv Br7 gt I llV Pram Frae vv 377 gt 27quot Fm Br739 gt eTv F Prediction 7 M 1 Br e WV KW B 01884 Experiment M 7 2 L03 Br139 gt uTv 01737 Note this does not work for pion decays into muon or electron ay v 9999 7 a e v 0 01 There is a very special reason for that to be discussed later It is related to parity violation in weak interactions When there is a twobody decay the parity violation effect overrides What one might expect from simple phase space considerations the phase space considerations remain valid they are just not the only factor in the game


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