NEW RESULTS ON WEAK INTERACTIONS - SUMMARY AND ASSESSMENT

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NEW RESULTS ON WEAK INTERACTIONS - SUMMARY AND ASSESSMENT

L. Sehgal

To cite this version:

L. Sehgal. NEW RESULTS ON WEAK INTERACTIONS - SUMMARY AND ASSESSMENT. Jour-

nal de Physique Colloques, 1982, 43 (C3), pp.C3-22-C3-31. �10.1051/jphyscol:1982302�. �jpa-00221855�

JOURNAL DE PHYSIQUE

CoZZoque C3, suppZ6ment au n012, Tome 4 3 , dbcembre 2982 page C3-22

NEW RESULTS ON WEAK INTERACTIONS

-

SUMMARY AND ASSESSMENT

L.M. Sehgal

III. PhysikaZisches I n s t i t u t , Teehnische HochschuZe, Physikzentrwn, 51 00 Aachen, F.R. G.

New r e s u l t s have been p r e s e n t e d a t t h i s Conference t h a t have been o b t a i n e d f r o m a v a r i e t y o f l e p t o n - h a d r o n processes ( n e u t r i n o i n t e r a c t i o n s , muon s c a t t e r i n g , atomic p a r i t y v i o l a t i o n and beam dump e x p e r i m e n t s ) . I s h a l l a t t e m p t i n t h i s summary t o p l a c e t h e s e r e s u l t s i n t h e c o n t e x t o f t h e general endeavour t o probe and d e f i n e t h e phenomenological p r o p e r t i e s o f t h e weak i n t e r a c t i o n s .

1. NEUTRAL CURRENTS

1.1 Weak-Electromagnetic I n t e r f e r e n c e i n pN I n t e r a c t i o n s

The NA4 e x p e r i m e n t a t CERN (BCDMS c o l l a b o r a t i o n ) ' ) has compared t h e i n c l u s i v e c r o s s s e c t i o n of p+ and p- on a Carbon t a r g e t :

I t i s f o u n d t h a t

u(w+,A) # u(v-,-A)

where A(-A) denote t h e p o l a r i z a t i o n s o f t h e p f ( p - ) beams. The beams a c t u a l l y used were n e a r l y l e f t - h a n d e d p+ and n e a r l y r i g h t - h a n d e d p-, w i t h I A

I

= 0.8 and

ED

=

200 GeV. The asymmetry ( 2 ) i s a v i o l a t i o n o f t h e one-photon exchange a p p r o x i - mation, which demands equal p + and p - c r o s s s e c t i o n s , r e g a r d l e s s o f p o l a r i z a t i o n . A charge asymmetry o f t h e t y p e ( 2 ) can a r i s e f r o m two sources:

The f i r s t i s a QED r a d i a t i v e c o r r e c t i o n t h a t can be s y m b o l i c a l l y r e p r e s e n t e d as

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982302

L.M. Sehgal C3-23

w h i l e t h e second'denotes a weak-electromagnetic i n t e r f e r e n c e e f f e c t

The r a d i a t i v e e f f e c t Brad has been c a l c u l a t e d b y B a r d i n and ~ h u m e i k o . ~ ) The weak asymmetry Bweak depends on t h e n e u t r a l c u r r e n t i n t e r a c t i o n o f muons and quarks.

y r i t i n g t h e e f f e c t i v e Lagrangian as

t h e weak asymmetry i s

The l a s t f a c t o r i n K i s a d i l u t i o n f a c t o r due t o a n t i q u a r k s , c l o s e t o u n i t y f o r t h e c o n d i t i o n s o f t h i s experiment. One t h e r e f o r e e x p e c t s Bweak t o be p r o p o r t i o n a l

2 2

t o t h e v a r i a b l e w

-Q

[ I - ( I - y ) ] / [ 1 + ( 1 - y ) ~ ] , a p r e d i c t i o n borne o u t by t h e data.

The s l o p e of t h i s v a r i a t i o n y i e l d s t h e c o m b i n a t i o n o f weak i n t e r a c t i o n parameters

w i t h

IXI

= 0.8. Note t h a t t h e measured q u a n t i t y i s a l i n e a r c o m b i n a t i o n o f a p a r i t y - c o n s e r v i n g and a p a r i t y - v i o l a t i n g term. By c o n t r a s t , t h e SLAC

$D

e x p e r i - ment3) measured a p u r e l y p a r i t y - v i o l a t i n g e f f e c t , summarized by4)

The c o n s t r a i n t s ( 6 ) and ( 7 ) a r e i l l u s t r a t e d i n F i g s . l a and I b .

I n t h e s t a n d a r d SU(2) x U ( 1 ) t h e o r y , t h e parameters a p p e a r i n g i n ( 6 ) have t h e v a l u e s

JOURNAL DE PHYSIQUE

S L A C i ' D

,$.,<"I.; €"Aidl

F i g . l a : C o n s t r a i n t s on weak muon-quark c o u p l i n g s imposed F i g . I b : C o n s t r a i n t s on weak e l e c t r o n - q u a r k c o u p l i n g s b y charge asymmetry i n PI-Carbon s c a t t e r i n g imposed b y SLAC p o l a r i z a t i o n asymmetry (Ref. 3) (BCDMS c o l l a b o r a t i o n , Ref. 1 ).

F i g . 2: C o n s t r a i n t on e l e c t r o n - q u a r k c o u p l i n g s imposed by p a r i t y v i o l a t i o n measurement i n Cesium

(Ref. 5 ) and SLAC $D experiment (Ref. 3).

Ouark-Mix~ng Parameters

lucll I

F i g . 3: M i x i n g parameters of t h e charmed quark d e r i v e d from dimuon data (CDHS c o l l a b o r a t i o n , Ref. 14).

L.M. Sehgal

The measured asymmetry determines

I n a s l i g h t l y more general c o n t e x t i n which t h e

~6

i s allowed t o have a non-zero i s o s p i n 1 3 R ( ~ ) , the expressions i n ( 7 ) a r e replaced by

-

1/2 + 1 3 R ( ~ ) + 2sin 2 @ and

-

1/2

-

13R(p) r e s p e c t i v e l y . Taking s i n 2 O = 0.23, the data g i v e

c o n s i s t e n t w i t h t h e standard assumption o f right-handed s i n g l e t s .

It i s amusing t o note t h a t the weak parameters given i n ( 8 ) c o i n c i d e w i t h t h e c o u p l i n g constants CV and CA which occur i n a completely d i f f e r e n t n e u t r a l c u r r e n t sector, namely v e s c a t t e r i n g . This i s a r e f l e c t i o n o f the f a c t o r i z a t i o n p r o p e r t y

U

i n h e r e n t i n a l l s i n g l e - Z models, combined w i t h the s p e c i f i c quantum number

1 1 1

assignment IQL = (?,

-

?, 7 , - -2) f o r (vy, e, ^U, d ) i n the standard theory.

1.2 P a r i t y V i o l a t i o n i n Cs-

An experiment by Bouchiat e t a1.5) has observed a p a r i t y - v i o l a t i n g e f f e c t i n t h e s c a t t e r i n g o f c i r c u l a r l y p o l a r i z e d l i g h t from Cesium vapour. An i n t e r f e r e n c e e f f e c t i s seen between a p a r i t y - v i o l a t i n g ampl i t u d e

E P ~

and a known p a r i t y - c o n s e r v i n g Stark-induced ampl i t u d e BEo, w i t h the r e s u l t

This c o n s t i t u t e s a s t r i k i n g 6 0 e f f e c t w i t h o n l y 8% systematic u n c e r t a i n t y . The t h e o r e t i c a l e x p e c t a t i o n f o r s i n 2 O = 0.219

+

.014 i s s t a t e d t o be -1.73k.07 c

lo%,

t h e l a s t u n c e r t a i n t y being due t o atomic physics. I n terms o f t h e parameters o f t h e n e u t r a l c u r r e n t Lagrangian ( 4 ) the q u a n t i t y measured i s

My assessment o f the above r e s u l t i s t h a t Q = - 55.2 +- 11.5, y i e l d i n g t h e

c o n s t r a i n t

i wICs

E ~ ~ ( u ) + 1.12 ~ ~ , , ( d ) = 0.147 i .031

.

⁽¹³⁾

It i s i n t e r e s t i n g t o note t h a t t h i s combination i s almost orthogonal t o t h e one determined by t h e " i n t e r c e p t " measured i n the SLAC

ZD

experiment 3).

.

The c o n s t r a i n t s (13) and (14) are i l l u s t r a t e d i n F i g . 2.

C3-26 JOURNAL DE PHYSIQUE

1.3 Results from Neutrino Physics

There has been refinement a s well a s d i v e r s i f i c a t i o n in t h e study of neutral c u r r e n t s i n n e u t r i n o processes. Some of t h e new r e s u l t s a r e a s follows.

( i ) D i f f r a c t i v e p0 Production. An experiment a t F N A L ~ ) r e p o r t s t h e observation of d i f f r a c t i v e p0 production by neutral c u r r e n t s :

7

+ N ⁺

7

+ N + pO. Comparing with t h e charged c u r r e n t process

7

+ N ⁺ pt

+

N + p - , and using vector meson dominance, i t i s p o s s i b l e t o determine t h e parameter ' a ' of t h e neutral hadron c u r r e n t 4 ) :

The r e s u l t i s a = 0.44

+

0.18. (Recall t h a t in t h e standard model a = 1

-

2sin 2 0.) ( i i ) Coherent TO Production : The Aachen-Padova experiment has observed a signal f o r t h e coherent process v + ~ 1+ 'vV + ~ A I + ~TO. ~7, This r e a c t i o n i s l a r g e l y

1-I

determined by t h e divergence o f t h e a x i a l i s o v e c t o r c u r r e n t , and so s e l e c t i v e l y determines t h e parameter 6. A preliminary e s t i m a t e y i e l d s

where t h e e r r o r includes a l a r g e u n c e r t a i n t y from theory. The r e s u l t i s compatible with t h e standard expectation

6

= 1.

( i i i ) Single n Production: A new a n a l y s i s of old measurements of s i n g l e pion production has been made by Fogli8), with a view t o e x t r a c t i n g t h e i s o s c a l a r couplings y and 6. The r e s u l t s a r e

t o be compared w i t h t h e standard p r e d i c t i o n y = -2sin 0/3, 6 2 = 0.

( i v ) Inclusive S c a t t e r i n g o f f Protons and Neutrons: Recent experiments in deuterium bubble chambers have measured t h e c r o s s s e c t i o n r a t i o oNC/oCC on protons and neutrons s e p a r a t e l y . The r e s u l t s , converted i n t o the c h i r a l couplings of u and d quarks, a r e

L L

-

-

0.19

+

.06 0.13

+

^.04 ( 1 5 ' , Ref. 9) 0.14

+

.04 0.18

+

^.05 (BEBC, Ref. 1 0 )

.

L.M. Sehgal C3-27

The s t a n d a r d t h e o r y p r e d i c t i o n IdL

1

> /uL

I

i s n o t y e t d e c i s i v e l y demonstrated by t h e above measurements. The r i g h t - h a n d e d c o u p l i n g s uR and dR remain, as b e f o r e , poor1 y r e s o l ved.

( v ) C o u p l i n g s o f s t r a n g e quarks: I n a comparative s t u d y o f t h e y - d i s t r i b u t i o n o f CC and NC i n t e r a c t i o n s , t h e CHARM experiment1') has d e t e c t e d a c o n t r i b u t i o n f r o m t h e NC coupl i n g o f s t r a n g e quarks p r e s e n t i n t h e s s sea. The coupl i n g s t r e n g t h , when r e f e r r e d t o t h a t o f t h e d-quark i s

c o n s i s t e n t w i t h G I M symmetry, which p r e d i c t s u n i t y .

( v i ) C o u p l i n g s o f charmed quarks: The CDHS experiment has observed t h e d i f f r a c t i v e process v + N + v

+

N

+

@.I2) Comparing t h e c r o s s s e c t i o n w i t h t h a t o f

y + N -t y + N + ^I), and assuming v e c t o r and a x i a l v e c t o r c u r r e n t s t o c o n t r i b u t e w i t h equal w e i g h t (as i n t h e Z-gluon f u s i o n model) one a r r i v e s a t an e s t i m a t e o f t h e NC c o u p l i n g s t r e n g t h o f charmed quarks

c o m p a t i b l e w i t h u n i t y . It remains t o be understood why a f u s i o n mechanism,

i n v o l v i n g t h e e m i s s i o n o f a c o l o u r e d quantum f r o m t h e nucleus, s h o u l d b e r e l e v a n t t o a d i f f r a c t i v e channel i n which t h e t a r g e t s u f f e r s l i t t l e o r no e x c i t a t i o n . ( v i i ) N e u t r i n o - E l e c t r o n S c a t t e r i n g : A new measurement o f t h e r a t i o o f v e and e

lJ lJ

c r o s s s e c t i o n s has been r e p o r t e d by t h e CHARM experiment.13) T h i s r a t i o d e l i v e r s a v a l u e o f s i n 2 O r e l a t i v e l y f r e e o f n o r m a l i z a t i o n e r r o r s :

s i n 2 O = 0.215

+

.040 ( s t a t . ) + 0.015 ( s y s t . )

.

^{( 2 0 )}

Combined w i t h t h e e a r l i e r measurement o f

o(T

e ) , t h e new r e s u l t a l s o g i v e s a lJ

measure o f t h e o v e r a l l s t r e n g t h parameter p o f t h e NC Lagrangian, p = 1 - 1 2

+

0.12 ( s t a t . )

+

0.11 ( s y s t . )

i n agreement w i t h t h e v a l u e p = 1 i n t h e s t a n d a r d model.

2. CHARGED CURRENTS

2.1 Quark M i x i n g Parameters f r o m h+y- d a t a

The CDHS e x p e r i m e n t has made a d e t a i l e d s t u d y o f v- and <-induced p + ~ - e v e n t s o r i g i n a t i n g f r o m t h e p r o d u c t i o n and decay o f charmed p a r t i c l e s . ' 4 ) The o b j e c t i v e i s t o determine t h e s t r e n g t h o f t h e (AC ( = 1 c r o s s s e c t i o n and t h e r e b y deduce t h e quark m i x i n g parameters Ucd and UcS. I n a w o r l d c o n t a i n i n g u,c,d,s quarks o n l y ,

C3-28 JOURNAL DE PHYSIQUE

these parameters would simply be Ucd = sinOc, Ucs = cosOc, Oc being t h e Cabibbo angle; b u t given t h e existence o f a t l e a s t t h r e e quark doublets, these parameters must be considered unknown, s u b j e c t o n l y t o t h e u n i t a r i t y c o n s t r a i n t

The i n c l u s i v e cross s e c t i o n s f o r

%'

^{+ N -t} p- + H+ + X a t asymptotic energies, assuming complete acceptance, are

do(2p) - G ~ M E

- - - 2 2

dxdy n

[

^IUcd

¹

^{q + IUcs}

1

qs I B p

--- 2

1

dii(211)

G'E

[ ^{1ucd !$}

^{+ /Ucs}

¹

^{qs !Bp}

dxdy 71

--

where q = x (u+d), = x (G+d), qS = x ( z s ) , i n f a m i l i a r n o t a t i o n , and B i s the I.1 average branching r a t i o o f t h e charmed p a r t i c l e m i x t u r e produced i n these r e a c t i o n s . Comparing w i t h t h e s i n g l e muon cross sections

2

d d l i r ) - G ME

[

^{( lUud}

¹

^{2 t}

^IUCd\

² (

lUusl

2 ⁺ u s \ 2 )qs ⁺ < ( ' - Y ) ~

dxdy n

1

₍₂₃₎

2 2

dz(l~) =

[ q ( ~ - y ~ 2 + (

1uudl2

+ I U ~ ~ ~ ) ? ~ ⁺ (

1uUs~

⁺ I U ~ ~

]

I ~ ) ~ ~

dxdy n

one can, i n p r i n c i p l e , determine Ucd and Ucs. I n p r a c t i c e , t h i s i s achieved o n l y a f t e r a c a r e f u l c o r r e c t i o n o f t h e dimuon cross sections f o r e f f e c t s o f t h r e s h o l d and acceptance.

F o l l o w i n g t h i s procedure, t h e CDHS experiment1 4, f i n d s

( i i ) lUCd / l U C s

1

^{= (0.33 i 0.03)}

[

2 ~ / ( 6 + ]Ii2

.

The l a t t e r r e s u l t i n v o l v e s t h e r a t i o o f strange t o non-strange sea c o n t r i b u t i o n s t o t h e nucleon momentum. A safe upper l i m i t i s 2 ~ / ( c + D) < 1, w h i l e a rough lower l i m i t zs/(U+D) > 0.16 i s obtained by combining the CDHS measurement o f -

U

+ D

w i t h the NC y - d i s t r i b u t i o n measured by CHARM.") The r e s u l t i n g c o n s t r a i n t i s

The allowed domain o f Ucd and Ucs i s i l l u s t r a t e d i n F i g . 3.

L.M. Sehgal

2.2 L i m i t s on right-handed c u r r e n t s

The y - d i s t r i b u t i o n o f t h e r e a c t i o n

5

+ N + D+ + X has been analysed i n t h e framework o f a Lagrangian c o n t a i n i n g b o t h V-A and V+A c u r r e n t s :

The CDHS experiment o b t a i n s t h e l i m i t 15)

Likewise, t h e c h i r a l s t r u c t u r e o f the charmed c u r r e n t has been analysed by studying the y - d i s t r i b u t i o n o f y'p- events. Assuming the i n t e r a c t i o n t o have the form

two experiments have obtained the r e s u l t s

2 2 2

gR/(gL + gR) = 0.15

+

0.10 (CHARM, Ref. 16)

< 0.07 (95% C.L.) (CDHS, Ref. 14)

.

3. SEARCHES FOR ANOMALOUS BEHAVIOUR 3.1 Flavour-Changing Neutral Currents

An experiment a t FNAL (CCFRS c ~ l l a b o r a t i o n ) ' ~ ) has s t u d i e d prompt p p r o d u c t i o n i n a beam dump, and has searched f o r y i u + and

u-u-

events among a sample o f dimuons.

A t 90% confidence, an upper 1 i m i t

P - V - + P+U+ < 0.8% (30)

u-v+

0 -0

i s obtained, which, i f i n t e r p r e t e d i n terms o f p o s s i b l e

D

- D m i x i n g gives

thus c o n s t r a i n i n g a p o s s i b l e AC = 1 n e u t r a l c u r r e n t . 3.2 Scalar Coup1 ings

The p o s s i b i l i t y t h a t s c a l a r quanta p l a y a r o l e i n weak i n t e r a c t i o n s prompts a continued search f o r s c a l a r couplings i n lepton-hadron processes. One s i g n a t u r e

i s a y term i n t h e y - d i s t r i b u t i o n o f n e u t r i n o r e a c t i o n s . The CHARM experiment

2

11

JOURNAL DE PHYSIQUE

r e p o r t s 1 i m i t s

2 2

gS,p/gV,A < 0.06 f o r charged c u r r e n t s

( 9 5 % c.L.) ( 3 2 )

< 0.03 f o r n e u t r a l c u r r e n t s

Another l i m i t may be o b t a i n e d by n o t i n g t h a t s c a l a r NC i n t e r a c t i o n s , w h i c h f l i p h e l i c i t y , c a n n o t c o n t r i b u t e t o weak-electromagnetic i n t e r f e r e n c e i n e l e c t r o n - q u a r k s c a t t e r i n g b u t can a f f e c t n e u t r i n o - q u a r k processes. The f a c t t h a t

suggests t h a t such c o u p l i n g s a r e s m a l l . Note t h a t i f < s i n 2 O> i s d e r i v e d f r o m a comparison o f NC and CC r e a c t i o n s t h r o u g h a Paschos-Wolfenstein t y p e r e l a t i o n 1 8 ) vq

such a d e r i v a t i o n i s i n s e n s i t i v e t o t h e e x i s t e n c e o f s c a l a r CC o r NC c o u p l i n g s , s i n c e any s c a l a r c o n t r i b u t i o n s cancel o u t i n t h e numerator as w e l l as denominator o f (34).

3.3 e-p U n i v e r s a l i t y

The v a l i d i t y o f e-p u n i v e r s a l i t y i n charm-decay has come under q u e s t i o n 1 9 ) i n c o n n e c t i o n w i t h t h e CERN beam dump "puzzle", namely e/p

2

^{0.6 t} 0.2. A new e x p e r i - ment a t FNAL") y i e l d s a r a t i o 1.0

+

0.3, c o m p a t i b l e w i t h u n i v e r s a l i t y . Any s u g g e s t i o n o f a d i f f e r e n c e i n t h e e- and u-branching r a t i o s o f charmed p a r t i c l e s must contend w i t h t h e n e a r l y equal r a t e ( w i t h i n 10%) o f

u-u+

and p-e' e v e n t s i n n e u t r i n o i n t e r a c t i o n s 1 4 ) . Small d e p a r t u r e s o f e/p f r o m u n i t y c o u l d a r i s e f r o m r a d i a t i v e c o r r e c t i o n s t h a t produce d i f f e r e n c e s i n t h e v e and v s p e c t r a o f charm

L' decay p r o d u c t s .

3.4 Composite Q u a r k s ?

One s i g n a t u r e f o r composite quarks i s a non-zero r a d i u s . Experiments on e'e- ⁺ qq l i m i t t h e e l e c t r o m a g n e t i c r a d i u s t o R < (100 G ~ v ) - ' . A n o t h e r m a n i f e s t a t i o n c o u l d b e t h a t t h e weak charged c u r r e n t v e r t e x i s n o t y,(1 + y 5 ) b u t y,(1

+

gAy5). I t i s n a t u r a l t o e x p e c t t h a t t h i s renormal i s a t i o n o f t h e a x i a l c o u p l i n g would be v e r y s m a l l , i . . I

-

g A I ² m R x However, a d i r e c t a n a l y s i s o f t h e charged c u r r e n t d a t a y i e l d s o n l y t h e weak c o n s t r a i n t 9

L . M . S e h g a l

References

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