Qu a r k s , QC D , a n d t h e R i s e of t he S t a nd a r d M od e l

Image is in the public domain. 8 .2 2 5 / S T S .0 4 2 , P h y s ic s in t h e 2 0 t h C e n t u r y P r o f e s s o r D a v id K a is e r , 3 0 N o v e m b e r 2 0 2 0

1. P a r t i c l e Z o o : C l a ssi f i c a t i o n

2. Q u a r ks: F a c t o r F i c t i o n ?

3. Q C D : F i e l d T h e o r y Re t u r n s

Be g i n n i n g s o o n a f t e r t h e S e c o n d W o r l d W a r , en o r m o u s p a r t i cl e a ccel er a t o r s yi el ded ev i den ce o f hu g e n u m b e r s of n u c l e a r pa r t i c l e s .

Fi r s t c h a l l e n g e : m o s t o f t h e p a r t i c l e s i n t e r a c t e d st r o n g l y wi t h e a c h o t h e r ( g 2 ~ 1 5 ra t h e r t h a n e 2 ~ 1/ 137 ), s o p e r tu r b a ti v e m e th o d s b r o k e d o w n .

Be r k e l e y Be v a tr o n , 1955

Ge o f f r e y Ch e w ’s p r o g r a m o f “ n u c l e a r dem o cr a cy” a n d t h e “ b o o t s t r a p ” : m a yb e al l o f t h e s e p ar t i c l e s w e r e bo u n d s tate s o f ea c h o t h er . A i m t o re p l a c e QF T w h i l e fo c u s i n g o n dy n am i c s : t h e s e l f - co n s i s t en t fo r c e s am o n g al l t h e n u c l e ar p ar t i c l e s .

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Se c o n d c h a l l e n g e : c o ul d a l l 1 0 0 + p a r t i c l e s r e a l l y be e q u a l l y “ e l e m e nt a r y ” or f u nd a m e nt a l ?

St e p 1 : A  pr od u ces a n at t r ac t i v e f o r c e bet w een t w o pi ons , ca u s i ng t hem t o a ppr oa c h ea c h ot her : F fo rc e ( m  , g ) .

St e p 2 : U p o n c o l l i d i n g , t h e t w o p i o n s p ro d u c e a n e w

co m p o s i t e pa r t i cl e , t he  : F re s ( m  , g ) .

We r n e r H e i s e n b e r g , “ O n t h e st r u c t u r e o f a t o m i c n u c l e i , ” 1 9 3 2

Du r i n g t h e 1 9 5 0 s a n d 1 9 6 0 s , s o me p a r t i c l e t h e o r i s t s p u r s u e d a

di s ti n c t appr o ac h to th e c h a l l e n g e o f th e p a r ti c l e z o o : cl a s s i f i ca t i o n . Th e y s e t a s i d e t h e q u e s t i o n o f d y n a m i c s t o f o c u s o n w a y s t o gr o u p va r i o u s p a r t i c l e s i n t o “ f a m i l i e s , ” f o c u s i n g o n “ i n t e r n a l sy m m e t r i e s . ”

Th i s a p p r o a c h h e a r k e n e d b a c k t o t h e i d e a o f is o s p in , f i r s t in t r o d u c e d b y We r n e r H e i s e n b e r g in 1 9 3 2 , ju s t a f e w m o n t h s a f t e r Ja m e s C h a d w i c k pr e s e nt e d e v i d e nc e of t he ne ut r o n .

Ea r l y ex p er i m en t s h a d s u g g es t ed t h a t i n n u cl ea r i n t er a ct i o n s , ne u t r ons a nd pr ot ons ha d t he sa m e i n t er a c t i o n st r en g t h , w h e t h e r o n e co n s i der ed p - p , p - n , o r n - n sc a t t e r i n g . He i s e n b e r g : m a y b e t h e y ’ r e th e sa m e p a r t i c l e in o n e o f t w o in t e r n a l s t a t e s . T h e s y m m e t r y w a s

© Springer-Verlag. All rights reserved. This content is excluded br ok e n d u e t o e l e c t r om a g ne t i c e f f e c t s — ju s t a s t h e e le c t r o n s t a t e

https://ocw.mit.edu/help/faq-fair- use/ “sp i n u p ” i s o n l y d i sti n gu i sh e d f r o m “sp i n d o w n ” i n a n e x te r n a l

ma g n e t i c f i e l d .

“ nu c l e o n ” : o n e t y p e o f p ar t i c l e t h at c o u l d o c c u r i n o n e o f t w o i n te r n al s tate s o f “ i s o s p i n ”

Du r i n g t h e 1 9 5 0 s a n d 1 9 6 0 s , s o me p a r t i c l e t h e o r i s t s p u r s u e d a

di s ti n c t appr o ac h to th e c h a l l e n g e o f th e p a r ti c l e z o o : cl a s s i f i ca t i o n . Th e y s e t a s i d e t h e q u e s t i o n o f d y n a m i c s t o f o c u s o n w a y s t o gr o u p va r i o u s p a r t i c l e s i n t o “ f a m i l i e s , ” f o c u s i n g o n “ i n t e r n a l sy m m e t r i e s . ”

Th i s a p p r o a c h h e a r k e n e d b a c k t o t h e i d e a o f is o s p in , f i r s t in t r o d u c e d b y We r n e r H e i s e n b e r g in 1 9 3 2 , ju s t a f e w m o n t h s a f t e r Ja m e s C h a d w i c k pr e s e nt e d e v i d e nc e of t he ne ut r o n .

Ea r l y ex p er i m en t s h a d s u g g es t ed t h a t i n n u cl ea r i n t er a ct i o n s , ne u t r ons a nd pr ot ons ha d t he sa m e i n t er a c t i o n st r en g t h , w h e t h e r o n e co n s i der ed p - p , p - n , o r n - n sc a t t e r i n g . He i s e n b e r g : m a y b e t h e y ’ r e th e sa m e p a r t i c l e in o n e o f t w o in t e r n a l s t a t e s . T h e s y m m e t r y w a s br ok e n d u e t o e l e c t r om a g ne t i c e f f e c t s — ju s t a s t h e e le c t r o n s t a t e “sp i n u p ” i s o n l y d i sti n gu i sh e d f r o m “sp i n d o w n ” i n a n e x te r n a l ma g n e t i c f i e l d .

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Af t e r t h e w a r , p h y s i c i s t s n o t i c e d t h a t s o m e o f t h e n e w pa r t i c l e s — du b b ed “ s t r a n g e , ” b eca u s e t h ey w er e un e x p e c t e d a n d un f a m i l i a r — al w a y s s e e m e d t o b e pr od u c e d to g e th e r du r i n g p a r t i cl e co l l i s i o n s , s u c h a s t h e K an d  pa r t i c l e s or K an d  pa r t i c l e s :

Mu r r a y G e l l - Ma n n an d Ab r a h a m P a i s su g g e st e d t h a t a n e w qua n t um n um b e r , “ st r a n g en ess ” c h a r g e , e x i ste d a n d m u st b e co n s er v ed: .

In 1 9 6 0 , Ge l l - Ma n n an d ( s e p ar at e l y ) Yu v a l N e ’ e m a n fo u n d t h at a c e r t ai n c o m b i n at i o n o f t h e s e n e w “c h a r g e s” c o u l d a c c o u n t f o r e v e n l a r g e r p a tte r n s .

“ Hy p e r c h a r g e ” : Y = B + S E le c t r ic c h a r g e : Q = I + Y /2

ba r y on st r a n g e n e ss is o s p in hy p e r c h a r g e

pr ot on : I = + ½ , Y = ( 1 + 0 ) , s o Q = ( ½ + ½ ) = +1

ne ut r o n : I = - ½ , Y = ( 1 + 0 ) , s o Q = ( - ½ + ½ ) = 0

“ E i g h tfo ld W ay ” * : G e l l - M an n f o u n d t h at h e c o u l d ar r an g e g r o u ps o f p ar t i c l e s by hy p e r c h a r g e an d is osp in

* G e l l - Ma nn bor r o w e d t he t e r m f or t he B u d d hi s t ’ s 8 - st e p p a t h t o a c h i e v i n g n i r v a n a .

Ge l l - Ma n n gr o u p e d o t h e r p a r t i c l e s b y h y p e r c h a r g e a n d is o s p in , a n d f o u n d a 1 0 - pa r t i c l e pa t t e r n wi t h a g a p . N o t o n l y d i d it s e e m lik e ly t h a t a s in g le p a r t ic le w it h Y = - 2 , I = 0 sh o u l d ex i s t , h e a l s o n o t ed a p a t t er n a m o n g t h e p a r t i cl es ’ ma s s e s .

m as s ( M e V / c 2 ) 1232

1385

1530

[1 6 8 0 ]

At a c o n f e r e n c e i n 1 9 6 2 , Ge l l - Ma n n pr e d i c t e d t ha t s u c h a pa r t i c l e w ou l d be f ou nd w i t h th e s e s p e c i f i c p r o p e r ti e s . I n 1 9 6 4 , e x p e r i m e n ta l i s ts a n n o u n c e d i ts d i s c o v e r y : th e  - pa r t i c l e , wi t h m a s s 1 6 7 2 M e V / c 2 .

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Qu e s t i o n s ?

Y

d

1/ 3

u

- 1/ 2

1/ 2

I

s

- 2/ 3

Wi t h i n 1 3 d a y s o f e a c h o t h e r i n J a n u a r y 1 9 6 4 , Ge l l - Ma n n an d (s e p a r a te l y ) Ge o r g e Z w e i g pr opos e d t ha t t he s y m m e t r i e s a m ong n u c l e a r pa r t i c l e s c ou l d be a s s oc i a t e d w i t h th r e e c o n s ti tu e n t par ti c l e s (“ qua r k s ” o r “ ac e s ”).

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d

1/ 3

u

- 1/ 2

1/ 2

I

s

- 2/ 3

“ Hy p e r c h a r g e ” : Y = B + S

E le c t r ic c h a r g e : Q = I + Y /2

Wi t h t h e s e a s s i g n m e n t s , Ge l l - Ma n n an d Zw e i g co u l d p + = u u d ac c o u n t fo r al l th e o c te t a n d d e c u p l e t p a tte r n s a m o n g n 0 = u d d nu c l e a r p a r t i c l e s : e a c h b a r y o n c o u l d b e a c c o u n t e d f o r a s  - = s s s

a s e t o f 3 c o n s t i t u e n t q u ar k s , w h i l e m e s o n s w e r e q u ar k - π + = u d

an t i q u ar k p ai r s .

π - = u d

d

1/ 3

u

- 1/ 2

1/ 2

I

s

- 2/ 3

“ Hy p e r c h a r g e ” : Y = B + S

E le c t r ic c h a r g e : Q = I + Y /2

Bu t th e s e a s s i gn m e n ts r a i s e d n e w q u e s ti o n s :

p + = u u d

1. fr a c t i o n a l el ect r i c c h a r g es ? n 0 = u d d

2. P a u l i e x c l u s i o n p r i n c i p l e f o r b i d s b o u n d s t a t e s o f 3  - = s s s

id e n t ic a l s p in - 1/ 2 p a r t i c l e s (  - = sss ?) +

3. dy n am i c s : h o w d o t h e s e o b j e c t s i n t e r a c t w i t h e a c h

ot he r ? π - = u d

d

1/ 3

u

- 1/ 2

1/ 2

s

- 2/ 3

Bu t th e s e a s s i gn m e n ts r a i s e d n e w

ti

p + = u u d

1. fr a c t i o n a l el ect r i c c h a r g es ? n 0 = u d d

2. P a u l i e x c l u s i o n p r i n c i p l e f o r b i d s b o u n d s t a t e s o f 3  - = s s s

id e n t ic a l s p in - 1/ 2 p a r t i c l e s (  - = sss ?) +

3. dy n am i c s : h o w d o t h e s e o b j e c t s i n t e r a c t w i t h e a c h

ot he r ? π - = u d

SL A C - MI T E x p e r i me n t s

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Th e St a n f o r d L i n e a r A c c e l e r a t o r L a b o r a t o r y (S L A C ) b e g a n ope r a t i on i n 1 9 6 6 . O ne of t he f i r s t s e t s of e x pe r i m e nt s w a s di r ect ed b y M I T ’ s Je r r y F r i e d m a n an d He n r y K e n d a l l : s c a t t e r hi g h - en er g y el ect r o n s o f f o f p r o t o n s .

Th i s w a s e s s e n t i a l l y a r e - pl a y of Ru t h e r fo r d s c a t t e r i n g . T h e re s u l t s f o r s c a t t e ri n g

ra t e s v e rs u s a n g l e w e re co n s i s t en t w i t h

in t e r n a l s t r u c t u r e wi t h i n ea c h p r o t o n : t i n y sc a t t e r i n g si t e s .

© source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use/

© source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, s ee https://ocw.mit.edu/help/faq-fair-use/

A p r o t o n a t l o w e n e r g y w o u l d be a bi g m e s s : s t ro n g i n t e r n a l f o rc e s , a j u m b l e o f m o v i n g p ar t s .

As s e e n b y t h e s p e e d i n g e l e c t r o n , t h e p ro t o n wo u l d u n d e r g o le n g t h c o n t r a c t i o n an d i t s in t e r n a l d y n a m ic s w o u ld b e s lo w e d b y tim e

di l at i o n . P a r t o n s w o u l d e f f e c t i v e l y b e h a v e l i k e

A v i c t o r y f o r q u a r k s ? N o t r i g h t a w a y fr e e (n o n - in t e r a c t in g ) s t a t io n a r y t a r g e t s .

© source unknown. All rights reserved. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/help/faq-fair-use/

A p r o t o n a t l o w e n e r g y w o u l d be a bi g m e s s : s t ro n g i n t e r n a l f o rc e s , a j u m b l e o f m o v i n g p ar t s .

As s e e n b y t h e s p e e d i n g e l e c t r o n , t h e p ro t o n wo u l d u n d e r g o le n g t h c o n t r a c t i o n an d i t s in t e r n a l d y n a m ic s w o u ld b e s lo w e d b y tim e

di l at i o n . P a r t o n s w o u l d e f f e c t i v e l y b e h a v e l i k e

A v i c t o r y f o r q u a r k s ? N o t r i g h t a w a y

fr e e (n o n - in t e r a c t in g ) s t a t io n a r y t a r g e t s .

Qu e s t i o n s ?

Be t w e e n 1 9 7 2 - 74, Mu r r a y G e l l - Ma n n , Ha r a l d F r i t z s c h , a n d co l l ea g u es dev el o p ed a n ew dy n am i c al th e o r y o f th e s tr o n g n u c l e a r fo r c e . C as t i n an al o g y t o q u an t u m e l e c t r o d y n am i c s ( Q E D ) , t h e ne w m od e l f oc u s e d on qua r ks (a n a l o g o u s to e l e c tr o n s ) i n te r a c ti n g by e x c h a n g i n g f o r c e - ca r r yi n g gl u o n s (a n a l o g o u s to p h o to n s ).

Ne w i d e a : “ c o l o r ” c h a r g e . E a c h q u a r k c a r r i e s ( y e t a n o t h e r ) i n t e r n a l q u a n t u m n u mb e r ( re d , gr e e n , o r bl ue ), s o th e y c a l l e d th e i r n e w m o d e l “ qua nt um ch r o m o dy na m i c s ” (Q C D ). A fe w as s u m p t i o n s : c o l o r c h ar g e i s co n s e r v e d ; f r e e p a r t i c l e s m u s t h a v e a n e x a c t bal an c e am o n g th e c o l o r c h a r g e s ; a n d th e i n te r a c ti o n s a m o n g q u a r k s a r e sy m m et r i c wi t h r e s p e c t t o pe r m u t a t i ons of t he c ol or c ha r g e .

Th i s h e l p e d r e s o l v e p u z z l e s l i k e t h e  - . S u c h a p a r t i c l e co u l d be a bou nd s t a t e of 3 s qua r k s wi t h o u t v i o l a t i n g t h e P a u l i e x c l u s i o n p r i n c i p l e :  - = s s s .