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HAL Id: jpa-00221856

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Submitted on 1 Jan 1982

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LEPTONS FROM b AND c QUARKS

V. Barger

To cite this version:

V. Barger. LEPTONS FROM b AND c QUARKS. Journal de Physique Colloques, 1982, 43 (C3), pp.C3-32-C3-35. �10.1051/jphyscol:1982303�. �jpa-00221856�

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JOURNAL DE PHYSIQUE

CoZZoque ~ 3 , suppze'ment au n o 12, Tome 43, de'cembre 1982

LEPTONS FROM b AND c QUARKS

V. Barger

Department of Physics, University of Wisconsin, Madison, Wisconsin 53706, U.S.A.

1. I n t r o d u c t i o n . - The p o s s i b l e r o l e s of b and c quarks a r e reviewed i n connection with t h r e e phenomena of p a r t i c u l a r i n t e r e s t :

( i ) $I$ e v e n t s , observed i n TN c o l l i s i o n s v i a $ -t P'U- decays, ( i i ) same-sign d i l e p t o n s produced i n n e u t r i n o r e a c t i o n s ,

( i i i ) a r e p o r t e d breakdown o f ve/v u n i v e r s a l i t y i n prompt n e u t r i n o e v e n t s a s observed i n some beam dump egperiments.

The conclusions a r e a s f o l l o w s :

( i ) The $$ kinematic c h a r a c t e r i s t i c s s u g g e s t a b b production source, with sub- sequent b + s$ decays.

( i i ) The same-sign dimuon r a t e s remain unexplained by any reasonable mechanism.

( i i i ) A v / v asymmetry could a r i s e from a n o n - u n i v e r s a l i t y i n charm decays, cauzedUby t h e v i r t u a l exchange o f a charged s c a l a r boson.

2. $ Production. - The r e a c t i o n ITN + $$X h a s been observed by t h e NA3 c o l l a b o r a - t i o n [ ? ] a t e n e r g i e s = 17 and 23 GeV with a c r o s s s e c t i o n of 20-30 pb. Two p o s s i b l e sources o f $$ e v e n t s a r e t h e p r o d u c t i o n of two c c p a i r s which r e a r r a n g e t o form $$ o r t h e p r o d u c t i o n of a bb p a i r with subsequent B and B meson decays t o $.

I n p e r t u r b a t i v e QCD heavy quarks (Q) a r e produced v i a f u s i o n and f l a v o r e x c i - t a t i o n subprocesses. The heavy quarks have s o f t x d i s t r i b u t i o n s s i n c e t h e QQ p a i r o r i g i n a t e s from a gluon a t low x. F a s t hadrons with a Q o r Q c o n s t i t u e n t a r e t h e n r e a l i z e d through recombination with f a s t v a l e n c e quarks[2]. This p i c t u r e q u a l i t a - t i v e l y d e s c r i b e s t h e f e a t u r e s o f d i f f r a c t i v e charm d a t a , without any n e c e s s i t y f o r a hard i n t r i n s i c charm component[3]. I n t h i s t h e o r e t i c a l c o n t e x t t h e observed c h a r a c t e r i s t i c s o f $$ p r o d u c t i o n can be a s s e s s e d ; t h e following d i s c u s s i o n i s based on a r e c e n t a n a l y s i s by W. Y . Keung, F. Halzen and myself [UW-Madison r e p o r t PH/64

(1982)l.

In a c c mechanism, t h e $ mesons a r e formed without valence recombination a n d t h e $$ p a i r w i l l t h e r e f o r e appear p r e f e r e n t i a l l y i n t h e c e n t r a l r e g i o n . For a bb mechanism, t h e recombination o f t h e s p e c t a t o r b ( o r b) quark with a v a l e n c e quark i n t h e pion p r o j e c t i l e , l e a d s t o a B ( o r B) meson o f high momentum: s e e Fig. 1.

The B + QX decay of t h e f a s t B g i v e s a l e a d i n g 9. This decay mode was

*

advocated by F r i t z s c h a s a s i g n a t u r e

f o r b-quarks; v a r i o u s t h e o r e t i c a l D

1.-

e s t i m a t e s a g r e e on a B + $X branching

b (spectator)

-

f r a c t i o n of o r d e r 2% [ 4 ] .

b (interacting) I n t h e c c mechanism one e x p e c t s a s u b s t a n t i a l f r a c t i o n o f t h e events t o have low v a l u e s o f t h e l o n g i t u d i n a l quark in target s c a l i n g v a r i a b l e x = pL/pBEAM of t h e

$$ system, whereas a l l observed e v e n t s have x($$) 2 0.4. The i n v a r i a n t mass Fig. 1: Typical f l a v o r e x c i t a t i o n o f $$ would b e expected t o peak n e a r diagram f o r b-quark production. 2m$, but t h e d a t a a r e d i s t r i b u t e d over

a broad range from 6.5 t o 8.5 GeV. I f t h e charm p a i r s a r e produced v i a f u s i o n graphs [as suggested by R. E . E c c l e s t o n e and D . M. S c o t t , Cambridge p r e p r i n t (1982)], t h e n t h e $-mesons would occur approxi- mately back-to-back, b a l a n c i n g pT; t o t h e c o n t r a r y , t h e d a t a i n d i c a t e a s u b s t a n t i a l p ~ of t h e $$. Hence t h e c h a r a c t e r i s t i c s of t h e d a t a suggest t h a t some o t h e r source a l s o e x i s t s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982303

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V. Barger C3-33

The b-quark mechanism gives one fast B, associated with valence recombination, and one slow B. From the b + $s decays a fast $ and a secondary $ result, with the

$$ system at high-longitudinal x. The $$ mass is smeared over a broad range and the pT of the $$ is substantial. A quantitative calculation of the b-excitation mechanism gives the salient characteristics of the $$ data as illustrated in Fig. 2.

Fig. 2: Comparison of b-flavor excitation model with nN + $$X data.

If this interpretation is correct, the study lo3 of b-production will be feasible in experi-

ments which measure only muons. Figure 3 shows the sizeable rates expected for b- production at higher energies[S].

dpb) b-quark

Fig. 3: Perturbative QCD predictions 16' l0lRfor sections b- and in t-quark nN or NN production collisions. cross The /<-quark $$ data have been converted to a b-

quark cross-section using a branching 101 lo2 lo3 fraction B(B + $X) = 2%.

3. Same-Sign Dileptons. - Opposite-sign dilepton production by neutrinos conforms to the parton model, both in rates and distributions. The production subprocesses are shown in Fig. 4.

Opposite-sign dileptons result from the semileptonic decays c + 11+ and

c

+ 2.-

where R = e or 1-1. The rates relative to single muons are about_l~-~.

If same-sign (SS) dileptons are due to heavy quarksl then c or must also be produced in v,, interactions and decay via c + p- or b + c + p-. The problem is that the observed rates[6], of order relative to single muons, are an order of magnitude higher than that expected. The observed lepton distributions in same-sign events are similar to corresponding distributions in opposite-sign d i - lepton events.

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C3-34 JOURNAL DE PHYSIQUE

The s t a n d a r d model mechanisms f o r SS d i l e p t o n s a r e i l l u s t r a t e d i n Fig. 5 .

Fig. 5 : Gluon brems- s t r a h l u n g , gluon f u s i o n , and i n t r i n s i c charm - production o f c and b i n v c o l l i s i o n s .

I n a l l c a s e s t h e secondary l e p t o n s from t h e heavy quark decays a r e s o f t and muon acceptance c u t s s i g n i f i c a n t l y reduce t h e p r e d i c t e d r a t e s . Figure 6a compares p r e - d i c t i o n s [ 7 ] , which i n c l u d e Ep > 4 GeV acceptance c o r r e c t i o n s , with t h e p-p- d a t a .

A l l p r e d i c t i o n s a r e f a r below t h e observed r a t e s . A maximal ( ~ , b ) ~ coupling s t r e n g t h of

lucb

1

= 0.5 has been assumed.

The i n t r i n s i c charm c a l c u l a t i o n assumes a 1% hard charm d i s t r i - b u t i o n , whereas p e r t u r b a t i v e QCD g i v e s s o f t charm. I f t h e charm i s i n f a c t s o f t , slow r e s c a l i n g and acceptance c u t s would reduce t h e r a t e from t h i s source.

A phenomenological mecha- nism f o r d i f f r a c t i v e cc produc- t i o n based on t h e diagram of Fig. 7 h a s been suggested [8]

.

E,(GeV) E,(GeV) The u-quark was assumed t o ffag-

ment a t t h e 1% l e v e l i n t o c , c which c a r r y t h e u-quark energy.

Fig. 6 : Same-sign dimuon r a t e s from (a) This model reproduces t h e SS standard model mechanisms and from (b) a dimuon r a t e , b u t it a l s o p r e d i c t s (c, b) weak c u r r e n t . hadronic charm p r o d u c t i o n r a t e s

t h a t a r e an o r d e r of magnitude h i g h e r than o b s e r v e d [ 3 ] . The r a t e f o r n e u t r i n o production o f trimuons i s a l s o l i k e l y t o b e too high, s i n c e b o t h t h e c and c a r e e n e r g e t i c i n t h i s model.

Beyond t h e s t a n d a r d model, two p o t e n t i a l ex- p l a n a t i o n s o f S S d i l e p t o n s have been considered.

The f i r s t i s Do-Do mixing which i s n e g l i g i b l e i n -

+i

t h e s t a n d a r d model. The b e s t upper l i m i t s on Do-Do mixing a r e d e r i v e d from an upper l i m i t on t h e t r i - muon event r a t i o u ( ~ + N + p'p p + x ) / o ( p + ~ + p+p-p+x) p l a c e d by t h e European Muon C o l l a b o r a t i o n . This i n t u r n i m p l i e s t h a t o(vp + p-p-)/o(v + p-p+) < 3x10- 2 a t t h e 95% conf i d e n c e l e v e l [ 7 ] . ~ { o u ~ h n o t com- p l e t e l y excluded, Do-Do mixing i s t h e r e b y an u n l i k e - l y e x p l a n a t i o n of t h e SS d i l e p t o n s .

Another p o s s i b i l i t y i s a right-handed ( c , b ) Fig. 7: A proposed d i f f r a c - weak c u r r e n t , f i r s t considered by M. E. Peskin and t i v e mechanism f o r same-sign S.-H. H . Tye i n connection with b-decays. Figure d i l e p t o n s . 6b shows t h e corresponding p r e d i c t i o n s [ 7 ] f o r SS

d i l e p t o n r a t e s with a 1% i n t r i n s i c charm source[9]

o f 6 and with c 6 from t h e gluon f u s i o n source. The SS d i l e p t o n r a t e s w i t h a ( c , b l R r a t e s , due t o t h e more f a v o r - c u r r e n t a r e about t e n times t h e corresponding (C b)

a b l e y-dependence of t h e c r o s s s e c t i o n and t o

I U , ~ ~

= 1. Only t h e ( ~ , b ) ~ model remains i n c o n t e n t i o n a s a p o s s i b l e o r i g i n f o r t h e SS d i l e p t o n r a t e s .

Any p o t e n t i a l e x p l a n a t i o n of SS d i l e p t o n s can be s u b j e c t e d t o f u r t h e r t e s t s . For example, t h e p r e d i c t e d c r o s s s e c t i o n f o r n e u t r i n o production of t r i l e p t o n s must not exceed t h e experimental r a t e .

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V . Barger C3-35

Figure 8 shows t h e p r e d i c t e d 3p r a t e [ 7 ] f o r t h e ( ~ , b ) ~ model with 1% i n t r i n s i c charm. Another t e s t concerns t h e number of kaons observed i n p-e- e v e n t s . The very l i m i t e d s t a t i s t i c s on u-e- e v e n t s i n d i c a t e s t h a t t h e r e a r e fewer V0 ( i . e . , n e u t r a l kaons) t h a n i n y-e+ e v e n t s [ l o ] . That would b e impossible t o e x p l a i n with

b any c c o r cb model.

Unlike same-sign d i l e p t o n s , n e u t r i n o - produced trimuons a r e p l a u s i b l y understood 0 100 200 300 [ l l ]

.

About 259 of t h e r a t e a ( v -t y - p + ' ~ - ) /

a ( v + y-) % lo-' i s of r a d i a t i v e o r i g i n . E,(GeV) Most o f t h e remaining s i g n a l i s compatible

with a dimuon p a i r of hadronic o r i g i n . Q u a n t i t a t i v e c a l c u l a t i o n s based on

Fig. 8: Neutrino trimuon production dU(vN + ' J ( ~ ~ ) x ) / u ( v N + yX) = Ado(~iN + ' J ~ x ) / r a t e due t o a ( ~ , b ) ~ c u r r e n t and atot(nN) r e q u i r e a fudge f a c t o r o f h = 2.5.

i n t r i n s i c charm. The h a d r o n i c s o u r c e s o f t h e yp a r e p, w, 4

and a low mass continuum.

4. Beam Dum v / v Nonuniversality. - S e v e r a l CERN beam dump experiments[l2] have r e p o r t e d a pzom$t Heutrino f l u x r a t i o o f o r d e r ve/vp = 0.6 r 0.2 which h a s no conventional e x p l a n a t i o n . A s p e c u l a t i v e e x p l a n a t i o n i s n o n u n i v e r s a l i t y i n t h e charm decays which a r e t h e source of t h e neutrino-flux.-The v i r t u a l exchange of a charged s c a l a r boson t h a t couples o n l y t o t h e c s and v,,y could g i v e an e f f e c t of t h e r e p o r t e d s i z e [ l 3 ] . Such an i n t e r p r e t a t i o n has s e v e r a l t e s t a b l e consequences:

( i ) The muon spectrum from charm decay w i l l b e h a r d e r than t h e e l e c t r o n spectrum.

( i i ) The F+ + y+vP branching f r a c t i o n w i l l be of o r d e r 25%. ( i i i ) Charm p r o d u c t i o n by v w i l l have a small component due t o charged s c a l a r exchange which i s propor- t i o n a l t o y2. 'J

Acknowledgments. - I wish t o thank R. J . N . P h i l l i p s and K . Hagiwara f o r comments during t h e p r e p a r a t i o n of t h i s r e p o r t . The r e s u l t s summarized h e r e a r e l a r g e l y based on c o l l a b o r a t i o n s with F. Halzen, W. Y . Keung, S. Pakvasa, and R . J . N . P h i l l i p s .

This r e s e a r c h was supported i n p a r t by t h e U n i v e r s i t y of Wisconsin Research Committee with funds g r a n t e d by t h e Wisconsin Alumni Research Foundation, and i n p a r t by t h e Department of Energy under c o n t r a c t DE-AC02-76ER00881.

References

rl] BADIER, J. e t a l . , Phys. L e t t . % (1982) 457.

[2] ODORICO, R . , Phys. L e t t . (1981) 112.

[3] HALZEN, F., r a p p o r t e u r r e p o r t i n t h e s e proceedings.

[4] FRITZSCH, H . , Phys. L e t t .

86B

(1979), 164, 343; WISE, M. B . , Phys. L e t t .

89B

(1980) 229; DE GRAND, T. and TOUSSAINT, T., Phys. L e t t .

89B

(1980) 256;

KUHN, J . , NUSSINOV, S . , and RUCKL, R . , Z e i t f u r Phys. C 5 (1980) 117.

[5] BARGER, V . , KEUNG, W. Y . , and HALZEN, F., Phys. Rev. D "J (1982) 112.

[6] TRINKO, T. etg., Phys. Rev. D 23 (1981) 1889; ALLABY, J . i n Proc. of I n t e r n . Conf. on Neutrino Phys., ed. by J . Cence, E . Ma, and A . Roberts, Hawaii (1981), p . 429; NISHIKAWA, K.

s.,

Phys. Rev. L e t t . 46 (1981) 1555;

DE GROOT, J . G . H . e t a l . , Phys. L e t t .

86B

(1979) 103.

[7] BARGER, V , , KEUNG, W. Y . , and PHILLIPS, R . J . N . , Phys. Rev. D 25 (1982) 1803;

[8] GODBOLE, R. M. and ROY, D. P., Phys. Rev. L e t t . 48 (1982) 1711-

[9] BRODSKY, S. J . , PETERSON, C . , and SAKAI, N . , Phys. Rev. D 2 (1981) 2745.

[ l o ] MURTAGH, I.I. J . , i n Proc. of 1979 I n t e r n . Symposium on Lepton and Photon I n t e r - a c t i o n s a t High Energies, ed. by T. Kirk and H . Abarbanel, Fermilab, p . 277.

[ l l ] BARGER, V . , GOTTSCHALK, T., and PHILLIPS, R . J . N . , Phys. Rev. D

18

(1978) 2308.

[12] Sources of t h e beam dump d a t a a r e given i n Refs. 3 and 13.

[13] BARGER, V., HALZEN, F . , PAKVASA, S . , and PHILLIPS, R.J.N., Phys. L e t t . ( i n p r e s s )

.

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