Volume 93B, number 4 PHYSICS LETTERS 30June1980

W-EXCHANGEDOMINANCEINNEUTRALB-MESONDECAY

S.P. ROSEN PLtrdue University, West Lafayette, Indiana 47907, USA

Received 3 March 1980

Revised manuscript received 11 April 1980

Tests of W-exchange dominance in neutral B-meson decay are developed from the isospin and SU(3) properties of the,

final state. For B” (d), it is shown that the decay rates into (D+ plus anything) and into (F+ plus anything) are always equal

to one another and never more than 3/2 times the rate for (Do plus anything). A similar analysis is applied to Be(s), and it is

suggested that any of the four possibilities {$, qc) + (a, q’ ) would make a good signal for this meson.

To explain the differing lifetimes of charged and neutral D-mesons [ 11, several authors [2] have sug-

gested that nonleptonic decay is dominated by diagrams in which the quark and anti-quark constituents of the charmed meson exchange a charged weak bosonbe- tween themselves. As long as the quark charge spec- trum is of the form (0, 0 - l), such diagrams occur only in the decays of neutral particles [3], and hence the Do will be shorter lived than the D+. Here we take up the suggestion [4] that a similar phenomenon migh occur in the decay of B-mesons, and we develop a number of tests for it. These tests are derived from the isospin and SU(3) properties of the final state, and they consist of relations between the partial rates for different exclusive decay modes, and bounds on the ratio of D+ to Do production in all decay modes. We also discuss briefly the effects of go-B0 mixing and penguin diagrams.

We begin by considering the neutral meson Be(d) constructed from a b-quark and a down anti-quark. If the b-quark decays principally into a c-quark and a negative weak boson,

b+c+W- (1)

then Be(d) will, through W-exchange, undergo a transi-

tion into a (c + ii) system in the Cabibbo-allowed limit at the light quark vertex (see fig. la). Quark-anti-quark pairs will be extracted from the gluon field so that this system can materialise in the form of physical mesons

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b C

‘\,w-

s c

(a)

Fig. 1. W-exchange diagrams for Cabibbo-allowed decays: (a)

B” (d)decay ; (b) B” ($-decay.

like (cq) and (qu), where q = u, d, s, and c. The likeli- hood that q is a charmed quark rather than a light one is small, namely (5):

[a,(4m,2)/~,(4m,2)] 2 = 5% (n = u, d, s) (2)

and so the rates for decay into final states $D” and ncDu will be much smaller than the rates for decay into a single charmed meson plus ordinary hadrons, for ex- ample Dn, FK, and Dnn. In fact, modes like $D” and ncDo will be much closer in rate to modes which are Cabibbo-suppressed at the light quark vertex than to ones which are allowed.

This result has immediate consequences for B- meson searches based upon the Fritzsch mode [6] $&r. For charged B-mesons, this mode occurs when the W- of eq. (1) materialises as a (Cs) pair (see fig. 2) instead of being re-absorbed by the other constituenl of the meson, and its branching ratio has been estimat-

Volume 93B, number 4 PHYSICS LETTERS 30 June 1980

W-EXCHANGE DOMINANCE IN NEUTRAL B-MESON DECAY

S.P. ROSEN Purdue University, West Lafayette, Indiana 47907, USA

Received 3 March 1980 Revised manuscript received 11 April 1980

Tests of W-exchange dominance in neutral B-meson decay are developed from the isospin and SU(3) properties of the. final state. For B ° (d), it is shown that the decay rates into (D + plus anything) and into (F + plus anything) are always equal to one another and never more than 3/2 times the rate for (D O plus anything). A similar analysis is applied to B°(s), and it is suggested that any of the four possibilities {qJ, nc} + (~, r~' ) would make a good signal for this meson.

To explain the differing lifetimes of charged and neutral D-mesons [1], several authors [2] have sug- gested that nonleptonic decay is dominated by diagrams in which the quark and anti-quark constituents of the charmed meson exchange a charged weak boson.be- tween themselves. As long as the quark charge spec- trum is of the form (O, Q - 1), such diagrams occur only in the decays of neutral particles [3], and hence the D O will be shorter lived than the D +. Here we take up the suggestion [4] that a similar phenomenon migh occur in the decay of B-mesons, and we develop a number of tests for it. These tests are derived from the isospin and SU(3) properties of the final state, and they consist of relations between the partial rates for different exclusive decay modes, and bounds on the ratio of D + to D O production in all decay modes. We also discuss briefly the effects of B 0 - B 0 mixing and penguin diagrams.

We begin by considering the neutral meson B0(d) constructed from a b-quark and a down anti-quark. If the b-quark decays principally into a c-quark and a negative weak boson,

b ~ c + W - (1)

then B0(d) will, through W-exchange, undergo a transi- tion into a (c + fi) system in the Cabibbo-allowed limit at the light quark vertex (see fig. la). Quark-anti-quark pairs will be extracted from the gluon field so that this system can materialise in the form of physical mesons

b , \ c

~W - (a)

b ,\ C

\xW- Co)

Fig. 1. W-exchanee diagrams for Cabibbo-allowed decays: (a) B ° (d)-decay; CO) B ° (s)-decay.

like ( c ~ and (qfi), where q -= u, d, s, and c. The likeli- hood that q is a charmed quark rather than a light one is small, namely (5):

[OLs(4m2)/as(4m2)] 2 ~ 5% (~ = u, d, s) (2)

and so the rates for decay into final states 6D 0 and r/cD0 will be much smaller than the rates for decay into a single charmed meson plus ordinary hadrons, for ex- ample Dlr, FI~, and DTr~r. In fact, modes like ~D 0 and rtc D0 will be much closer in rate to modes which are Cabibbo-suppressed at the light quark vertex than to ones which are allowed.

This result has immediate consequences for B- meson searches based upon the Fritzsch mode [6] ~g.Tr. For charged B-mesons, this mode occurs when the W - ofeq . (1) materialises as a (~s) pair (see fig. 2) instead of being re-absorbed by the other constituenl of the meson, and its branching ratio has been estimat-

492