kef‹laio 6: jewr—ec bajm—dac Νικόλαος Τράκας · ac. pi s teÔoume ì ti oi...
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Θεωρητική Φυσική
Κεφάλαιο 6: Θεωρίες Βαθμίδας
Νικόλαος ΤράκαςΚαθηγητής
Σχολή Εφαρμοσμένων Μαθηματικών και Φυσικών Επιστημών
Τομέας Φυσικής
Εθνικό Μετσόβιο Πολυτεχνείο
΄Αδεια Χρήσης
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Jewr�e Bajm�da Oi summetr�e pa�zoun prwtarqikì rìlo sth Fusik . Giapar�deigma, h analloi¸thta se genikoÔ metasqhmatismoÔ suntetagmènwn od ghse ton Einstein sthn diatÔpwsh th Jewr�a th Genik Sqetikìthta . PisteÔoume ìti oiallhlepidr�sei sth fÔsh perigr�fontai apì Jewr�e Bajm�da .H diat rhsh megej¸n (ìpw fort�o, qr¸ma k.lp.) topik� (local)sundèontai �rrhkta me autè .H sqèsh summetr�a ↔nìmo diat rhsh èqei suzhthje� sthnLagkranzian je¸rhsh. Oi exis¸sei Lagrange e�nai
d
dt
(∂L
∂q̇i
)
− ∂L
∂qi= 0
ìpou qi oi genikeumène suntetagmène kai q̇i = dqi/dt kai h260
sun�rthsh Lagrange L = T − V (= Kinht. En. − Dunam. En.).Phga�nonta apì ta qi sti φ(x, t), pa�rnoume
L(q, q̇, t)→ L(φ, ∂µφ, xµ)kai oi exis¸sei Lagrange g�nontai∂µ
(∂L
∂(∂µφ)
)
− ∂L∂φ
= 0
ìpou L h Lagkranzian puknìthta kai L =∫L d3x. Ant� nagr�foume ti exis¸sei pou dièpoun ta ped�a ma , gr�foume thn
L. 'Etsi, gia par�deigma h
L =1
2(∂µφ)(∂µφ)− 1
2m2φ2
261
d�nei thn ex�swsh Klein-Gordon
∂µ∂µφ+m2φ = 0, (�2 +m2)φ = 0'Askhsh 48 De�xte ìti h Lagkrantzian
L =1
2(∂µφ)(∂µφ)− 1
2m2φ2 = 0odhge� sthn ex�swsh tou Klein-Gordon
∂µ∂µφ+m2φ = 0, (�2 +m2)φ = 0Den up�rqei t�pota to magikì ed¸! H Lagkrantzian epilèqthkeme autìn ton trìpo ¸ste na d�nei thn ex�swsh Klein-Gordon.H Lagkrantzian
L = iψ̄γµ∂µψ −mψ̄ψ262
d�nei thn ex�swsh Dirac
(i∂µγµ −m)ψ = 0'Askhsh 49 De�xte ìti h Lagkrantzian
L = iψ̄γµ∂µψ −mψ̄ψd�nei thn ex�swsh Dirac
(i∂µγµ −m)ψ = 0Ep�sh , h Lagkrantzian
L = −1
4F µνFµν − jµAµ, F µν = ∂µAν − ∂νAµ
d�nei ti exis¸sei tou Maxwell
∂µFµν = jν263
An gr�youme thn Lagkrantzian
L = −1
4F µνFµν − jµAµ +
1
2m2AµA
µ
ja p�roume thn ex�swsh tou fwton�ou ��me m�za��
(�2 +m2)Aµ = jµPoia e�nai h sqèsh an�mesa sthn Lagkrantzian prosèggish kaith diataraktik mejìdou me ta diagr�mmata Feynman? Se k�jeLagkrantzian antistoiqoÔn orismènoi kanìne Feynman. Aut h antistoiq�a g�netai w ex :1. Se k�je ìro th Lagkrantzian antistoiqoÔme diadìte kaisuntelestè koruf .2. Oi diadìte proèrqontai apì tou tetragwnikoÔ ìrou th Lagkrantzian (p.q. φ2, ψ̄ψ, (1/2)(∂µφ)2, ψ̄(iγµ∂µψ).264
3. Oi �lloi ìroi th Lagkrantzian antistoiq�zontai sti korufè allhlep�drash . O suntelest th koruf e�nai osuntelest tou ant�stoiqou ìrou th Lagkrantzian .AkoloujoÔme ton klasikì trìpo prosèggish . H klasik Lagkrantzian kbant�zetai. Ta ped�a g�nontai telestè dhmiourg�a kai katastrof . Oi allhlepidr�sei upolog�zontaimèsw th jewr�a diataraq¸n. To apotèlesma aut th diadikas�a metafr�zetai se èna sÔnolo kanìnwn Feynman.Mèsw aut¸n mporoÔme na elègqoume ti fusikè diergas�e pouperigr�fei mia Lagkrantzian . Ed¸ den ja akolouj soume ton��kanonikì formalismì��. Ja akolouj soume thn pepo�jhsh ìtita ��diagr�mmata Feynman perigr�foun poll� perissìtera apìì,ti èna aplì formalismì �� (t’Hooft kai Veltman).265
Je¸rhma Noether. Summetr�e kai nìmoi diat rhsh Gnwr�zoume ìti h analloi¸thta se qwrik met�jesh odhge� sthdiat rhsh orm , h analloi¸thta se strofè odhge� sthdiat rhsh th stroform kai h analloi¸thta se met�jesh stoqrìno odhge� sth diat rhsh th enèrgeia . Ed¸, jaendiaferjoÔme gia eswterikè summetr�e . Metasqhmatismo� poumetat�jentai me thn qwroqronik ex�rthsh th kumatosun�rthsh . Gia par�deigma, h Lagkrantzian pouperigr�fei to hlektrìnioL = iψ̄γµ∂µψ −mψ̄ψparamènei anallo�wth ston metasqhmatismìψ(x)→ eiαψ(x)266
Pr�gmati, polÔ eÔkola fa�netai ìti
ψ̄(x)→ e−iαψ̄(x), ∂µψ → eiα∂µψ(x)Oi oikogèneia twn metasqhmatism¸n f�sh U(a) = eiα apotele�thn monoparametrik abelian om�da pou sumbol�zetai me U(1).O ìro Abelian anafèretai sthn antimetajetikìthta
U(α1)U(α2) = U(α2)U(α1)En¸ autì o metasqhmatismì fant�zei aploðkì , èqei ter�stiashmas�a. H analloi¸thta k�tw apì metasqhmatismoÔ U(1),odhge� sthn diat rhsh tou reÔmato . A p�roume apeirostìmetasqhmatismì
U(α) = 1 + iα, ψ → (1 + iα)ψ267
Analloi¸thta shma�nei δL = 0. Opìte
δL =∂L∂ψ
δψ +∂L
∂(∂µψ)δ(∂µψ) + δψ̄
∂L∂ψ̄
+ δ(∂µψ̄)∂L
∂(∂µψ̄)=
=∂L∂ψ
(iαψ) +∂L
∂(∂µψ)(iα∂µψ) + (−iα)ψ̄
∂L∂ψ̄
+ (−iα∂µψ̄)∂L
∂(∂µψ̄)=
=∂L∂ψ
(iαψ)−[
∂µ
(∂L
∂(∂µψ)
)]
iαψ + ∂µ
[∂L
∂(∂µψ)ψ
]
iα−
− iαψ̄ ∂L∂ψ̄
+ iαψ̄
[
∂µ
(∂L
∂(∂µψ̄)
)]
− iα∂µ
[
ψ̄∂L
∂(∂µψ̄)
]
=
= iα
[∂L∂ψ− ∂µ
∂L∂(∂µψ)
]
ψ + ∂µ
[∂L
∂(∂µψ)ψ
]
iα−
− iαψ̄[∂L∂ψ̄− ∂µ
∂L∂(∂µψ̄)
]
− iα∂µ
[
ψ̄∂L
∂(∂µψ̄)
]
=
268
= (iα)∂µ
[∂L
∂(∂µψ)ψ − ψ̄ ∂L
∂(∂µψ̄)
]Epomènw , h apa�thsh δL = 0 odhge� sth sqèsh
∂µjµ = 0ìpou to reÔma
jµ =ie
2
(∂L
∂(∂µψ)ψ − ψ̄ ∂L
∂(∂µψ̄)
)
H stajer� eklègetai kat�llhla ¸ste to reÔma na sump�ptei methn hlektromagnhtik puknìthta reÔmato gia to hlektrìnio mefort�o −e. 'Amesa èpetai ìti to fort�oQ =
∫
d3x j0
269
e�nai mia diathr simh posìthta lìgw th analloi¸thta f�sh
U(1)
dQ
dt=
∫
d3x ∂0j0 =
∫
d3x (∂µjµ −∇j) = −
∫
d3x∇j = 0ìpou h diat rhsh tou reÔmato d�nei ∂µjµ = 0, en¸ to teleuta�oolokl rwma mhden�zetai, metatrèponta to se epifaneiakìolokl rwma tou reÔmato kai jewr¸nta ìti to reÔmamhden�zetai phga�nonta thn epif�neia sto �peiro.Gia thn Lagkrantzian enì migadikoÔ bajmwtoÔ ped�ou
L = (∂µφ∗)(∂µφ)−m2φφ∗
me mia diadikas�a tele�w an�logh me aut pou k�name gia to270
ped�o Dirac, to diathrhsimo reÔma, lìgw th analloi¸thta se
U(1), e�naijµ = (iα)
[∂L
∂(∂µφ)φ− ∂L
∂(∂µφ∗)φ∗]
pou d�neijµ = −ie (φ∗∂µφ− φ∂µφ∗)Prosèxte ìti h Lagkrantzian gia to migadikì bajmwtì ped�o
φ = (φ1 + iφ2)/√
2, gr�fetaiL(φ) = L(φ1) + L(φ2)me L(φi) h Lagkrantzian tou pragmatikoÔ bajmwtoÔ ped�ou.O metasqhmatismì ψ → eiαψ shma�nei ìti to α DEN e�naimetr simo, DEN èqei fusik shmas�a kai h tim tou mpore� nadialeqte� auja�reta, fusik� thn �dia gia ìla ta shme�a tou271
qwroqrìnou. Mil�me gia mia ��olik bajm�da�� (global gauge).Topik U(1) summetr�a bajm�da kai KHDAnag�goume thn f�sh α se sun�rthsh tou x. Opìte èqoume tonlegìmeno ��topikì metasqhmatismì bajm�da ��
ψ(x)→ eiα(x)ψ kai ψ̄(x)→ e−iα(x)ψ̄Autì o metasqhmatismì DEN af nei anallo�wth thnLagkrantzian L = iψ̄γµ∂µψ −mψ̄ψPr�gmati, o deÔtero ìro paramènei anallo�wto all� o pr¸to ìqi, mia kai
∂µψ → eiα(x)∂µψ + ieiα(x)ψ∂µα(x)An epimènoume na zht�me thn analloi¸thta th Lagkrantzian 272
k�tw apì topikoÔ metasqhmatismoÔ bajm�da , ja prèpei nabroÔme mia ��kat�llhlh�� (sunallo�wth) par�gwgo pou nametasqhmat�zetaiDµψ → eiα(x)DµψGia na to epitÔqoume autì eis�goume èna nèo ped�o Aµ kaior�zoume
Dµ = ∂µ − ieAµapait¸nta to nèo ped�o na metasqhmat�zetai w Aµ → Aµ +
1
e∂µα(x)
273
Tìte blèpoume ìtiDµψ = (∂µ − ieAµ)ψ →eiα(x)∂µψ + ieiα(x)ψ∂µα(x)
− (ie)eiα(x)Aµψ − ie1
eeiα(x)ψ∂µα(x) =
= eiα(x) (∂µ − ieAµ)ψ = eiα(x)DµψOpìte, h nèa anallo�wth Lagkrantzian gr�fetai
L = iψ̄Dµγµψ −mψ̄ψ = iψ̄∂µγ
µψ −mψ̄ψ + eψ̄γµAµψUpoqrewj kame loipìn sthn eisagwg tou ped�ou bajm�da
Aµ, pou allhlepidr� me to ψ ìpw akrib¸ to fwtìnio. An to
Aµ e�nai fusikì ped�o, qrei�zetai kai kinhtikì ìro. O ìro autì e�nai (1/4)F µνFµν , pou e�nai anallo�wto k�tw apì tonmetasqhmatismì tou Aµ. 'Etsi, katal goume sthn Lagkrantzian 274
th Kbantik Hlektrodunamik (KHD)
L = iψ̄∂µγµψ −mψ̄ψ + eψ̄γµAµψ −
1
4F µνFµνParous�a m�za gia to Aµ antistoiqe� ston ìro M 2AµAµ pouìmw den paramènei anallo�wto ston metasqhmatismì tou Aµ.To fwtìnio den mpore� na èqei m�za.Perimèname thn emf�nish enì nèou ped�ou. An all�xoume thf�sh (α(x)) topik�, ja dhmiourgoÔse mia metr simh diafor�f�sh , an den mporoÔse na antistajmiste�. To endiafèron e�naiìti autì to antist�jmisma g�netai apì to Aµ. Perimènoume ep�sh to ped�o autì na èqei �peirh akt�na dr�sh (to fwtìnio �mazo),mia kai to antist�jmisma prèpei na g�nei se ìla ta shme�a touqwroqrìnou. 275
Sumperasmatik�, apait¸nta analloi¸thta se topik allag f�sh th eleÔjerh Lagkrantzian , odhgoÔmaste se miajewr�a me allhlepidr�sei , thn KHD. Autì pou f�ntaze san miaparaxeni� th jewr�a tou Maxwell, g�netai t¸ra shmantikìstoiqe�o.Mh abelianè jewr�e bajm�da kai Kbantik Qrwmodunamik H Kbantik Qrwmodunamik bas�zetai sthn om�da SU(3). HeleÔjerh Lagkrantzian e�naiL = q̄j (iγµ∂µ −m) qj = 0, ìpou j = 1, 2, 3, ta tr�a qr¸mataJa sugkentrwjoÔme pro stgm se mia ��geÔsh�� kouark.
276
O metasqhmatismì tou ped�ou q(x) e�nai t¸ra
q(x)→ Uq(x) = eiαj(x)Tjq(x)ìpou U e�nai èna monadiakì (unitary) 3× 3 p�naka . Oi p�nake
Tj e�nai oi 8 gen tore th SU(3) (j = 1, 2, ..., 8) kai αj oipar�metroi tou metasqhmatismoÔ.'Askhsh 50 Gia monadiakì metasqhmatismì (U †U = 1), isqÔeiìti detU = eiφ me φ ∈ R. An perioristoÔme stou monadiakoÔ p�nake me detU = +1 (eidikoÔ monadiakoÔ , special unitary),tìte Tr(Tj) = 0, kai αiTj = α∗jT
†j .H om�da SU(3) den e�nai abelian
[Ti, Tj] = ifijkTk277
ìpou fijk e�nai oi stajerè dom (structure constants) th om�da . Oi stajerè autè e�nai pragmatikè kai pl rw antisummetrikè stou de�kte i, j, k.'Askhsh 51 De�xte ìti oi stajerè autè e�nai pl rw antisummetrikè stou de�kte i, j, k.Jewr¸nta apeirostoÔ metasqhmatismoÔ , pa�rnoume
q(x)→ (1 + iαj(x)Tj) q(x)
∂µq(x)→ (1 + iαj(x)Tj) ∂µq(x) + iTjq(x)∂µαjAkolouj¸nta thn �dia diadikas�a ìpw sthn KHD, jaeis�goume 8 nèa ped�a Gµj pou metasqhmat�zontaiGµ
j → Gµj −
1
g∂µαj
278
kai ja eis�goume thn sunallo�wth par�gwgo
Dµ = ∂µ + igTjGµjOpìte, h Lagkrantzian g�netai
L = q̄(iγµ∂µ −m)q − g(q̄γµTjq)GµjA doÔme p¸ metasqhmat�zetai k�je ìro
mq̄q → mq̄ (1− iαj(x)Tj) (1 + iαi(x)Ti) q = mq̄q +O(α2)
iq̄γµ∂µq → iq̄ (1− iαi(x)Ti) γµ∂µ (1 + iαj(x)Tj) q =
= −iq̄γµ∂µq + q̄γµTjq∂µαj(x) +O(α2)
279
−gq̄γµTjqGµj → −gq̄ (1− iαi(x)Ti) γµTj (1 + iαm(x)Tm) q·
· (Gµj −
1
g∂µαj) =
= −gq̄γµTjqGµj − igαmq̄γµ[Tj, Tm]qGµ
j−
− g 1
gq̄γµTjq∂
µαj(x) +O(α2) =
= −gq̄γµTjqGµj + gαmq̄γµfjmrTrqG
µj−
− q̄γµTjq∂µαj(x) +O(α2)Blèpoume ìti ma mènei o ìro gαmq̄γµfjmrTrqGµj pou qal�ei thnanalloi¸thta. All�, an anabajm�soume to metasqhmatismì touped�ou bajm�da
Gµj → Gµ
j −1
g∂µαj − fjmsαmG
µs280
ston metasqhmatismì tou ìrou −gq̄γµTjqGµj ja emfaniste� oepiplèon ìro
gαmq̄γµfjmsTjqGµsK�nonta thn allag stou ��tufloÔ �� de�kte j → r kai s→ j,pa�rnoume
gαmq̄γµfrmjTrqGµj = −gαmq̄γµfjmrTrqG
µjìpou qrhsimopoi same ìti frmj = −fjmr. Opìte, blèpoume ìti oìro pou qaloÔse thn analloi¸thta exale�fetai. Bèbaia, sthnLagkrantzian
L = q̄(iγµ∂µ −m)q − g(q̄γµTjq)Gµjja prèpei na prosjèsoume ton kinhtikì ìro tou ped�ou bajm�da ,281
ton ant�stoiqo tou −(1/4)F µνFµν . 'Etsi èqoume
L = q̄(iγµ∂µ −m)q − g(q̄γµTjq)Gµj −
1
4Gµν
j Gµνjìpou ìmw , o anabajmismèno metasqhmatismì tou ped�oubajm�da ma upoqre¸nei na or�soume
Gµνj = ∂µGν
j − ∂νGµj − gfjrsG
µrG
νsH parap�nw Lagkrantzian perigr�gei ��ègqrwma�� kou�rk kaigklouìnia kai e�nai anallo�wth se metasqhmatismoÔ th om�da
SU(3). Ta gklouìnia e�nai kai p�li �maza. Qrhsimopoi¸nta thnsunallo�wth par�gwgo mporoÔme na gr�youmeL = q̄(iγµDµ −m)q − 1
4Gµν
j Gµνj282
Poio e�nai to kainoÔrgio stoiqe�o me ta gklouìnia? O kinhtikì ìro perièqei ìro auto-allhlep�drash metaxÔ twn gklouon�wn:
gGGG kai g2GGGG.283
Mh abelianè jewr�e - Yang-Mills jewr�e H mikr akt�na dr�sh twn asjen¸n allhlepidr�sewn ma odhge�sto sumpèrasma ìti ta swmat�dia upeÔjuna gia aut n thnallhlep�drash (ta ant�stoiqa swmat�dia bajm�da ) prèpei naèqoun m�za, pr�gma pou den epitrèpetai sti jewr�e bajm�da (J.B.). Opìte, h eisagwg twn J.B. tan apì omorfi� mìno?MporoÔme na agno soume ti J.B. kai na b�loume ìro m�za , giap.q. M 2W µWµ, sthn Lagkrantzian ? An to efarmìsoume, jasunant soume apeir�e stou upologismoÔ ma pou denmporoÔme oÔte na ��di¸xoume�� oÔte na ��krÔyoume��.Sto parak�tw di�gramma ��enì brìqou��, an ta swmat�dia me thnkumatist gramm e�nai fwtìnia, k�je diadìth tou fwton�ousuneisfèrei ìro ∼ 1/q2 en¸ k�je diadìth tou hlektron�ousuneisfèrei ìro ∼ 1/q, ìpou q e�nai h orm sto brìqo pou e�nai284
eleÔjerh (met� thn diat rhsh th orm se k�je koruf ), opìtekai ja prèpei na oloklhr¸soume thn orm aut : ∫ d4q∫
d4q1
q2
1
q2
1
q/+m
1
q/+m
Gia meg�la q to olokl rwma den parousi�zei kanèna prìblhma.An ìmw ant� fwtìnia èqoume swmat�dia me m�za, o diadìth e�nai
−gµν + qµqν
M2
q2 −M 2
q→∞−→ qµqνM 2q2285
opìte, to di�gramma ja d�nei
∫
d4q1
q/+m
1
q/+m
qµqνM 2q2
qρqσM 2q2To olokl rwma ìmw t¸ra apokl�nei gia meg�la q. Bèbaia, jamporoÔsame na b�loume èna p�nw ìrio sto olokl rwma, w miapar�metro pou ja ma d¸sei to pe�rama. All�, phga�nonta sediagr�mmata me perissìterou brìqou , oi apeir�e qeirotereÔounkai qreiazìmaste ìlo kai kainoÔrgie paramètrou . Mia tètoiajewr�a onom�zetai mh ��anakanonikopoi simh�� kai den mpore� naèqei probleyimìthta.
286
Krummènh summetr�a - Aujìrmhth parab�ash th summetr�a A jewr soume thn Lagkrantzian
L = T − V =1
2(∂µφ)(∂µφ)−
(1
2µ2φ2 +
1
4λφ4
)
h opo�a èqei mia summetr�a φ→ −φ. To λ ja prèpei na e�naijetikì gia na mporoÔme ne èqoume el�qisto. Gia µ2 > 0, hLagkrantzian perigr�fei èna bajmwtì swmat�dio me m�za µ kaiautoallhlepidr�sei . H kat�stash qamhl¸terh enèrgeia (ground state) e�nai φ = 0 kai h kat�stash aut upakoÔei thsummetr�a φ→ −φ. Gia µ2 < 0, ìmw , blèpoume ìti φ = 0 dene�nai olikì el�qisto.287
Pr�gmati∂V
∂φ= φ(µ2 + λφ2) = 0→ φ = ±
√
−µ2
λ≡ ±v
Efarmìzoume jewr�a diataraq¸n gÔrw apì èna apì ta el�qista,epilègonta to φ = +v
φ(x) = v + η(x)ìpou η(x) perigr�fei ti kbantikè diataraqè gÔrw apì to288
el�qisto. H Lagkrantzian g�netai
L′ =1
2(∂µη)(∂
µη)− λv2η2 − λvη3 − 1
4λη4 + stajer�O deÔtero ìro ma de�qnei ìti to η èqei m�za
mη =√
2λv2 =√
−2µ2
All�, h L kai h L′ e�nai isodÔname . 'Ena metasqhmatismì denmpore� na all�xei th fusik pou perigr�fei h Lagkrantzian . AnmporoÔsame na ��lÔsoume�� pl rw thn L kai h L′ ja br�skame ta�dia apotelèsmata. All�, sth jewr�a diataraq¸n upolog�zoumediataraqè gÔrw apì èna el�qisto. Me thn L, h seir� th jewr�a diataraq¸n den ja sunèkline giat� ja anaptÔsame gÔrwapì èna astajè el�qisto, φ = 0.289
Aujìrmhth parab�ash olik jewr�a bajm�da H Lagkrantzian gia migadikì ped�o φ = (φ1 + iφ2)/√
2 e�nai
L =1
2(∂µφ
∗)(∂µφ)−(µ2φ∗φ+ λ(φ∗φ)2
)=
= (∂φ1)2 + (∂φ2)
2 − 1
2µ2(φ2
1 + φ22)−
1
4λ(φ2
1 + φ22)
2
Gia λ > 0 kai µ2 < 0, sto q¸ro twn (φ1, φ2) up�rqei miaperifèreia φ21 + φ2
2 = −µ2/λ ≡ v2 pou h dunamik enèrgeia e�naiel�qisth. S' aut n thn perifèreia, epilègoume èna shme�o
φ1 = v, kai φ2 = 0kai gr�foume to φ(x)
φ(x) =
√
1
2[v + η(x) + iξ(x)]290
-1.0
-0.5
0.0
0.5
1.0-1.0
-0.5
0.0
0.5
1.0
-0.2
0.0
0.2
ìpou η e�nai h ��aktinik �' diataraq kai ξ h diataraq sthnkateÔjunsh th perifèreia tou el�qistou, gÔrw apì toel�qisto pou dialèxame. H Lagkrantzian gr�fetai
L′ =1
2(∂µξ)
2+1
2(∂µη)
2+µ2η2+stajerè +(kubiko� ìroi se η kai ξ)
blèpoume p�li to ped�o η na èqei m�za mη =√
−2µ2, all� to291
ped�o ξ e�nai �mazo. E�nai to legìmeno ��mpozìnio�� Goldstone.Epomènw , prospaj¸nta na parabi�soume th summetr�a,epilègonta to sugkekrimèno el�qisto, d¸same m�za se ènaswmat�dio all� ma mènei kai èna �mazo. To teleuta�o toperimèname. To ξ e�nai sthn dieÔjunsh th perifèreia touel�qistou. S' aut th dieÔjunsh den up�rqei el�qisto.To par�deigm� ma e�nai mia efarmog tou Jewr mato
Goldstone, pou anafèrei ìti ìpote parabi�zoume aujìrmhta miaolik summetr�a, emfan�zontai �maza bajmwt� ped�a.Epomènw , ti mpore� na g�nei me thn asjen allhlep�drash, pouta swmat�dia bajm�da W kai Z èqoun m�za qwr� thn emf�nish�mazwn swmatid�wn?Mhqanismì HiggsPhga�noume t¸ra se mia Lagkrantzian pou upakoÔei mia topik 292
summetr�a bajm�da , φ(x)→ exp[iα(x)]φ(x), kai fusik�qreiazìmaste thn sunallo�wto par�gwgo Dµ = ∂µ − ieAµ me
Aµ → Aµ + (1/e)∂µα(x). H Lagkrantzian g�netai
L = (∂µ + ieAµ)φ∗(∂µ − ieAµ)φ− µ2φ∗φ− λ(φ∗φ)2 − 1
4FµνF
µν
Gia µ2 > 0, an exairèsoume to ìro (φ∗φ)2, èqoume thn legìmenh��bajmwt �� Kbantik Hlektrodunamik . Gia µ2 < 0 ja èqoumeaujìrmhth parab�ash th summetr�a . K�nonta ta �dia ìpw kaisthn per�ptwsh th olik summetr�a , ja p�roume
L′ =1
2(∂µη)
2 +1
2(∂µξ)
2 − λv2η2 +1
2e2v2AµA
µ − evAµ∂µξ−
− 1
4FµνF
µν + (ìroi allhlepidr�sewn)293
Blèpoume, loipìn, ìti
mξ = 0, mη =√
2λv2, mA = evall� up�rqei kai èna ��par�xeno �� ìro , mh diag¸nio :
−evAµ∂µξ. O lìgo e�nai ìti to Aµ, afoÔ p re m�za, apèkthse(apì 2) 3 bajmoÔ eleujer�a . H L′ den e�nai ��kal��� grammènhkai mperdeÔei ti anex�rthte katast�sei twn ped�wn. Kat'arq , autì den e�nai kakì, arke� na mporoÔme na diakr�noume poie e�nai oi fusikè katast�sei .MporoÔme na broÔme k�poio sugkekrimènh bajm�da, pou na ma de�qnei ekpefrasmèna autè ti fusikè katast�sei ?Parathr ste ìti
φ =
√
1
2(v + η + iξ) ≃
√
1
2(v + η)eiξ/v
294
Epomènw , sthn arqik Lagkrantzian epilègoume na k�noumeton metasqhmatismì bajm�da
φ(x)→√
1
2(v + h(x))eiθ(x)/v
Aµ → Aµ +1
ev∂µθ(x)Epilègonta aut n thn bajm�da, perimènoume thn anexarths�a apìto θ, mia kai e�nai h f�sh. Pr�gmati, h nèa Lagkrantzian gr�fetai
L′′ =1
2(∂µh)
2 − λv2h2 +1
2e2v2AµA
µ − λvh3 − 1
4λh4+
+1
2e2h2AµA
µ + ve2hAµAµ − 1
4FµνF
µν
To mpozìnio Goldstone den emfan�zetai pia. O bajmì eleujer�a 295
tou e�nai ��plastì �� giat� antistoiqe� sthn eleujer�a toumetasqhmatistoÔ bajm�da . Autì o bajmì eleujer�a dìjhkesto Aµ pou t¸ra èqei m�za kai èqei 3 bajmoÔ eleujer�a . Autì e�nai ��o mhqanismì Higgs��.Aujìrmhth parab�ash th summetr�a SU(2)A p�roume thn Lagkrantzian L = (∂µφ)†(∂µφ)− µ2φ†φ− λ(φ†φ)2
me
φ =
φα
φβ
=1√2
φ1 + iφ2
φ3 + iφ4
Apait¸nta analloi¸thta se metasqhmatismoÔ bajm�da th 296
SU(2)
φ→ φ′ = eiαj(x)τj/2φ, ìpou τj oi p�nake tou Paulija eis�goume thn sunallo�wth par�gwgo
Dµ = ∂µ + igτj2W µ
jGia apeirostoÔ metasqhmatismoÔ φ→ (1 + iα(x) · τ/2)φ
W µ →W µ −1
g∂µα−α×W µ(jumhje�te ìti gia thn SU(2) oi stajerè dom th om�da
297
fijk = ǫijk). Opìte, h Lagkrantzian g�netai
L =(
∂µφ+ igτ
2W µφ
)† (∂µφ+ ig
τ
2W µφ
)
−V (φ)− 1
4W µνW µνme
W µν = ∂µW ν − ∂νW µ − gW µ ×W ν
V = µ2φ†φ+ λ(φ†φ)2
Gia λ > 0 kai µ2 < 0, to el�qisto tou dunamikoÔ epitugq�netaiìtan
φ†φ = (φ21 + φ2
2 + φ23 + φ2
4) = −µ2
λEpilègoume
φ21 = φ2
2 = φ24 = 0, φ2
3 = −µ2
λ≡ v2
298
AnaptÔssonta gÔrw apì to el�qisto
φ0 =1√2
0
v
kai epilègonta th bajm�daφ(x) = eiτ·θ(x)/2
0
v+h(x)√2
299
gia apeirostoÔ metasqhmatismoÔ pèrnoume
φ(x) =1√2
1 + iθ3/v i(θ1 − iθ2)/v
i(θ1 + iθ2)/v 1− iθ3/v
0
v + h
=
=1√2
θ1 + iθ2
v − h− iθ3
ant�stoiqa me autì pou k�name sthn prhgoÔmenh per�ptwsh.
Sthn L′, ta θ1, θ2 kai θ3 ��exafan�zontai�� kai mènei mìno to h. Tatr�a θ èdwsan to bajmì eleujer�a tou sta W1, W2 kai W3, pouapèkthsan m�za.Poia e�nai h m�za twn W ? Autì ja to broÔme apì to300
tetragwnikì ìro w pro ta W∣∣∣∣ig
1
2τ ·W µφ
∣∣∣∣
2
=g2
4
∣∣∣∣∣∣
W µ
3 W µ1 − iW µ
2
W µ1 + iW µ
2 −W µ3
φ0
∣∣∣∣∣∣
2
=
=g2v2
8
[(W µ
1 )2 + (W µ2 )2 + (W µ
3 )2]
kai h m�za e�nai
M =1
2gvkatal goume, loipìn, me 3 W me m�za M kai èna bajmwtì h mem�za √−2µ2 =
√2λv2.
301
Χρηματοδότηση
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συγχρηματοδοτείται από την Ευρωπαϊκή ΄Ενωση (Ευρωπαϊκό Κοινωνικό
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