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Introduction to the Standard Model of Particle Physics

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Introduction to the Standard Model of Particle Physics Introduction to the Standard Model of Particle Physics Chakrit Pongkitivanichkul Khon Kaen University Chiang Mai University, 3 July 2019 "ig Questions ∙ What is the universe made of& ∙ What holds the% together& What is the interaction 'et(een the% ∙ What is the origin of the Universe& Ho( is it going to end& What is the universe made of& ∙ As of no(, our scienti+c knowledge spans across orders of %agnitude Cosmic Eye (Original HD Landscape Version) -rontier of kno(ledge ∙ -ro% the cosmological to ato%ic scale, each layer is 'uilt fro% a set of smaller 'uilding 'locks https://newt.phys.unsw.edu.au/einsteinlight/jw/module6_Planck.htm .ur Lego pieces 0 atoms ∙ All %atters is co%,osed of atoms ∙ A core 0 ,rotons 1 neutrons ∙ 2lectrons are or'iting around ∙ A di3erent nu%'er of ,rotons, neutrons, and electrons exhi'its di3erent chemical ,roperties ∙ Co%'ining (ith ,hysics of ho( charged ,articles interact (ith ,hoton, any ,heno%ena in %aterial science, che%istry can 'e calculated and tested5 .ur tools to study ele%entary particles Conservation laws ∙ Conservation of energy ∙ Conservation of electric charge ∙ Conservation of momentu% ∙ Conservation of s,in & angular momentu% ∙ Conservation of a quantum nu%'er 8irac’s theory of electron ∙ :oughly s,eaking, 2instein9s dispersion relation ad%its a negative energy solution 0; Co%,lete 'reaking do(n of ,hysical (orld ∙ Dirac’s e7uation 0 an attem,t to use 7uantu% %echanics (ith relativistic ,articles 0 linearisation the Ha%iltonian o,erator ∙ <he solution can still 'e negative 'ut there’s a (ay out 8irac sea - Positron ∙ <he ,article (as discovered from cosmic ray ,assing a cloud cha%'er (C5 Anderson? ∙ <he new ,article has o,,osite charge 'ut si%ilar %ass to electron @ ,ositron Missing energy fro% 'eta decay ∙ 2nergy is not balanced + s,in is not balanced Pauli’s hy,othetical particle ∙ $5 Pauli > !30) ,ro,osed that the +nal ,roducts should include a (eakly interacting (neutral) ,article (hich carries %o%entu%, energy, and spin a(ay — ‘neutrino’ ∙ Neutrino %ust 'e very light, electrically neutral, and has a C2 spin ∙ 25 -er%i > !34) constructed a theory of (eak nuclear interaction (hich is si%ilar to electrodyna%ics Catching a ghost ∙ <he existence of neutrinos (as con+r%ed 'y Co(an and :eines’ > !EF? ∙ <he nuclear reactor is the source of neutrino Gu4 ∙ <he tank is +lled (ith (ater solution of ∙ <he ,ositron annihilates (ith electrons ,roducing ,hoton ∙ <he neutron is ca,tured 'y ,roducing a secondary Gash of light Le,ton nu%'er Ho( do (e kno( that neutrino ,artici,ated in 'eta decay is antineutrino not neutrino? $e found that It is useful to introduce a new 7uantu% nu%'er to ,revent this ,rocess H Lepton nu%'er $e assign electron, neutrino (ith L 0 1 and their anti,article (ith / 0 = >,rotons and neutrons have L 0 0? Muon ∙ Cloud cham'er detecting cos%ic ray (C5 Anderson? ∙ <he ,articles are charged >1C=) (ith %ass higher than electron %ass — I%uon9 ∙ Similar properties to electron but heavier ∙ Who ordered that? ∙ <hey are unsta'le and decay in J2 %illisecond (decay to electron 1 %issing energy? 2 types of neutrinos? <he %uon=electron track is /=shaped → %issing energy neutrino is not enough due to Lepton nu%'er conservation @ neutrino 1 antineutrino Are they the sa%e ty,e& Later, the ,rove ca%e fro% the ,roduction of neutrino (ith %uon $hat is inside a nucleus? Kuka(a’s theory ∙ A new force is needed to overcome the electrostatic repulsion 'et(een ,rotons and neutrons — strong nuclear force ∙ Protons and neutrons are attracted to one another 'y a +eld (si%ilar to electric/gravitational +eld? ∙ It is effective (ithin fe%to%eter ∙ <he +eld %ust 'e 7uantised and its 7uantu% is LPionM (ith %ass J 40 MeN Pion ∙ Pion (as ,redicted 'y )5 Kuka(a to 'e the ,article that holds nuclei together ∙ 8iscovered in cos%ic ray (C5 Po(ell? ∙ It is unsta'le and decay in 0.01 %illisecond ∙ Muon and Pion are dicovered in cos%ic rays and have %asses 'et(een electron and ,roton H (later 'eca%e kno(n as %eson 0 %ediu%=(eight) A jungle of Legos ∙ <he invent of high energy accelerators in 960s 'rings a nu%'er of strongly interacting ,articles ∙ <hey can 'e classi+ed into 2 classes 'y charge, %ass and strangeness ∙ <he 'aryons ∙ <he %esons ∙ It see%s that they should not 'e fundamental ,articles — (e found a hidden structure The Eightfold way Hidden structure #uarks model ∙ <he 2ightfold (ay J ,eriodic ta'le ∙ #uantu% 'ehaviour of electrons is the ex,lanation for the ,eriodic structure ∙ Si%ilarly, 3 ty,es of 7uarks can 'e used to ex,lain the 2ightfold structure ∙ u,, do(n, strange 7uarks are ,ostulated ∙ $hy don9t (e see them& 8o quarks really exist& ∙-iring high energetic electrons into nucleus 0 deep inelastic scattering ∙<he scattering angle sho(s that ,roton has su'structure $hy don’t we see quarks? ∙#uarks are strictly 'ounded inside nucleus ∙Con+nement 0 Asy%,totic freedo% #uarks are colourful ∙<he electron=,ositron collision ex,eri%ent suggested that ,articles (ith charged = /3 and 2/3 are inside nuclei 1 each ty,e of 7uark has 3 co,ies → 3 colours ∙Colours are la'eled LredM, LgreenM, and L'lue” 0 equivalent to electric charge in strong nuclear force The second family of particles ∙ Muon is a copy of electron 'ut heavier ∙ Strange 7uark is a co,y of down 7uark 'ut heavier ∙ Char% 7uark >JCPsi? (as discovered in !PD A a copy of u, 7uark st family 2nd family There are 3 families (so far? ∙ Nature ,resents us another set of ele%entary ,articlesQ tau lepton > !PE?, 'ottom 7uark > !PP?, top 7uark > !!E?, tau neutrino >2000? ∙ $hy so %any& st family 2nd family 3rd family How do particles interact& <he fra%e(ork is quantum + relativity • In ,article ,hysics, (e (ould like to ex,lain a small stu3 >7uantu% %echanics) (hich is traveling very fast >special relativity? • <he theory is called “Quantum Field Theory” — (here (e instead use the concept of +elds instead of ,article • <he +eld is every(here in space • .nly its 7uantu% Guctuation 'eco%es ,article <he force mediator ∙ <hro(ing 'all 0 transfer %o%entu% → force acting on each ,layer ∙ Si%ilarly ,articles exert force 'y e4changing a force %ediator ∙ <hey are virtual ,articles >'orro(ing energy from Heisen'erg9s uncertainty ,rinci,le) Electromagnetic force ∙ 2lectro%agnetic force is responsi'le for al%ost everything in our daily life ∙ Photon +eld is the force %ediator of electro%agnetic force ∙ Classically the ,hoton +eld is equivalent to the electro%agnetic ,otential Gauge symmetry ∙ >Classical? gauge transfor%ation 0 transfor%ing ,otential 'ut leaving ,hysics invariant ∙ Gravitational ,otential, electric ,otential, %agnetic ,otential all have this gauge transfor%ation ,ro,erty ∙ There is a si%ilar conce,t in 7uantu% +eld theory Gauge symmetry ∙ 2M ,otential can 'e transfor%ed di3erently at each ,oint in s,ace and ti%e Gauge symmetry ∙ In 7uantu% +eld theory, you cannot Oust only change electro%agnetic ,otential >,hoton +eld) (ithout changing ,hysics ∙ <o leave ,hysics invariant, (e need to rotate the internal clock >,hase) of the electron +eld accordingly ∙ <he 2M theory has a gauge sy%%etry A U( ! symmetry ∙ <he theory ex,laining electro%agnetic force at the 7uantu% >+eld) level is called “Quantum "lectrodynamics” Quantum Electrodynamics ∙ -or%ulated 'y <o%onaga, Sch(inger, -eyn%an and Dyson ∙ <he theory is extremely successful since it gives extremely accurate ,redictions J 0 digits agree%ent (ith ex,eri%ent (spin %agnetic %o%ent — g factor? ∙ Photon is called the gauge 'oson of the U(1?EM gauge sy%%etry -eynman diagrams ∙ <o visualise interactions 'et(een +elds >,articles), (e use -eyn%an diagrams techni7ue ∙ /ines re,resent ,article ,ropagations ∙ Nertices re,resent interactions -eynman diagrams ∙ <his techni7ue is also hel,ful in the calculation for ,hysical 7uantities in 7uantu% +eld theory ∙ <his is a very ,o(erful tool for the calculation $eak Interaction ∙ Can (e repeat the sa%e success of #ED in (eak interaction& H SU(2) gauge grou, ∙ No$$$ the %ediators of (eak interaction are %assive >short ranged force) — no com,ensation 'et(een 7uark +elds and gauge 'osons ∙ Gauge sy%%etry for (eak nuclear interaction cannot 'e there Symmetry breaking ∙ Should (e give u, on this idea of sy%%etry& ∙ $hat if the gauge sy%%etry is there 'ut so%eho( hidden fro% us? symmetric non=symmetric )iggs mechanis% ∙ $e ,ro,ose a new scalar +eld (Higgs +eld) (ith a ,otential in a sha,ed of a Mexican hat ∙ <he averaged +eld sits at the %ini%u% (hich 'reaks the sy%%etry ∙ S,ontaneous sy%%etry 'reaking 0 the sy%%etry still e4ists 'ut inaccessi'le to us )iggs mechanis% ∙ <he non=zero value of the Higgs +eld can 'e used to generate the %ass for gauge 'osons ∙ <he Higgs +eld is also responsi'le for 7uark and lepton %asses ∙ <he gauge sy%%etry for (eak nuclear interaction is ,ossi'le!T ∙ <he %echanis% (as ,roposed in !FD 'y 2nglert, "rout, Higgs, Guralnik, Hagen, Ki''le The Electro(eak theory ∙ In !F7, Sala%, $ein'erg, Glasho( successfully incor,orated the %echanism in unifying the (eak nuclear interaction and electro%agnetic interaction A this %arks the 'irth of the Standard %odel of particle physics ∙ It turns out that 2M force is the left over fro% sy%%etry 'reaking of the 'igger theory A the electro(eak theory ∙ <he gauge sy%%etry for 2$ theory is SU(2?/ 4 U(1?K ∙ Massive gauge 'osons are $eak bosons ∙ involve in 'eta decay ,rocess ∙ are %assive >'roken ,arts of the electro(eak theory? ∙ neutrinos, 7uarks, le,tons interacts via exchanging (eak 'osons $eak bosons ∙ $eak 'osons (ere discovered at SPS (Su,er Proton Synchrotron at C2:N) 'y UA1 and UA2 detectors in
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