N ELSEVIER Nuclear Physics A675 (2000) 345c-348c

M U Ig L E A I ~ = H Y S I E ] S A

www.elsevier.nl/locate/npe

Fi r s t m e a s u r e m e n t of b r a n c h i n g r a t i o and gl/fl of the n e u t r a l cascade

h y p e r o n us ing final s t a t e p o l a r i z a t i o n

Roland Winston ~ (for the KTeV Collaboration)

Department of Physics and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637

The KTeV collaboration has made a first observation of neutral cascade beta decay ( E0 _~ Z+ e-V). We present measurements of the branching ratio and form factors for this semi leptonic process. The results agree well with the Cabibbo SU(3)/theory and do not favor attempts to include symmetry breaking effects.

1. I N T R O D U C T I O N

Since the bare quarks are not accessible for carrying out direct measurements, the Hyperon Octet affords the best available laboratory for studying the weak interactions between the light quarks, u,d,s and the leptons (Fig. 1). Up to now, neutral cascade beta decay was the last missing entry of the obviously accessible processes. This decay is of particular interest because it is the SU(3)/analog of the well-studied neutron decay. This is therefore an excellent case for probing SU(3)] symmetry breaking effects. The KTeV collaboration has now observed this decay and reports results on both branching and form factors.

2. E X P E R I M E N T A L M E T H O D

The KTeV apparatus is designed to study CP violation in neutral kaon decay as well as rare kaon decays. In addition, there is a sufficient flux of neutral cascade Hyperon to also study semi-leptonic and radiative decays. To study the hyperons, we instrumented the beam region in order to trigger on the high momentum forward proton characteristic of all decay channels of interest.

Our events of interest are neutral cascades that decay to ~+, electron, antineutrino followed by the subsequent decay of E + --+ p ~r ° . The decay E + -* p ~r ° has a decay asym- metry of 98% making this the equivalent of an essentially fully polarized beta decay mea- surement. Only the semileptonic decay produces a E+which thereby serves as a unique signature.

3. B R A N C H I N G R A T I O

Fig. 2 shows our two-body and beta decay event samples. The back ground is about 10% and well simulated as resulting from neutral kaon decays. Our result is: B.R. =

0375-9474/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0375-9474(00)00280-3

346c R. W~nston/Nuclear Physics A675 (2000) 345c-348c

2.54 5= O.11(stat) 5= O.16(sys) x 10 -4 which agrees well with the SU(3)/value of 2.61 + 0.11 × 10 -4.

4. F O R M F A C T O R S

The observables in our beta decays in addition to branching ratios and energy spectra are several angular correlations: electron neutrino, electron proton and neutrino proton. To interpret these in terms of the form factors of the hadron current, fl , gl requires expressing the angular distribution in terms of the polarization of the final state baryon. Accordingly we have derived highly transparent expressions which are accurate to second order in q/M of the decay, that is to a few percent. (S. Bright et al, to be published in Phys Rev D) To demonstrate our ability to measure angular correlations we show the proton lambda spin correlation for the 2-body mode ( figure 3 ). Our result a~oaa = -.2884-O.O07(stat)+O.O16(sys) agrees well with the accepted PDG value of -0.2645=0.013. Our beta decay events are remarkably low in background (only 2 %) and the background is well simulated from KLdecays. We study our beta decay correlations in the E+rest frame. Our electron correlation is as large as possible. Combining all observed correlations with the branching ratio provides a 2-dimensional fit to f l and gl ( figures 4,5 ).

5. D I S C U S S I O N

Our results are in better agreement with the Cabibbo SU(3)] value (same as neutron decay) than with any of the various models that attempt to include SU(3)/breaking effects. While we await more data from our ongoing experiment to sharpen the precision one may already observe that the beta decays of the octet of hyperons agree remarkably well with the original Cabibbo SU(3)/predictions.

/2 n • ~ >'P

g /

Figure 1. The KTeV detector ( left ) and a diagram of the octet baryons, with the F and D coefficients for the observed hyperon beta decays ( right ).

R. Winston~Nuclear Physics .4675 (2000) 345c-348c 347c

-2=,z,,,/~ s']~b~

K -

Figure 2. Mass peaks for the signal E°--* E + e - Y ( left ) and normalization mode ~o _+ ATr 0 with A -+ pTr- ( right ) .

~ot

o ~ s

o • ~ : . - ; ~ . . ~ . . g ~ . . • - . . : : . . ~ , . • . ~ . . . ~ . .

~(p~A) ( A]~ )

e,~.s

uos

o

9B/

~ ( p,e ) (Z+l~me)

Figure 3. Acceptance corrected proton-electron correlation for ~o _+ E+ e - ~ ( left ), and Acceptance corrected longitudinal A polarization forT~ ° --+ ATr ° with A -+ per-( right ).

R. Winston/Nuclear Physics A675 (2000) 345c-348c 348c

t

i : o ° ° ° °

"o - %

)

" ' % . °

i i ~1

i ., .. ~,

'1 ° % ,

. u

" " ; ~ ' " ' ; " ~ " " ; ' " ' h ' "

Figure 4. The polarization of the E+in the electron and neutrino directions ( left ) and the electron-neutrino correlation ( right ) for :~0 _+ E+ e- ~ .

g i m l e t V a r l a N ~ for ~ ~ ~+ e" v Do~s = DATA.Ht~o = M C )

....... I ~ I ~-4-~---r--~._s._~A ~ * 1 ~,~

°~,,,, ............................... ~-:d p*e ( ~ frame ) 1~

'° F ~ l " - - - t - ~ - - - t - - . I - ~ ]-,-~ L_2 I '= ,o L- -+'[-- i i

o k-,-~ ............................... ~-,-~-:-I -1 -~s .o.~ ~s.~ .~.2 o o.2 o.4 o~ o.s 1 1

I ~ V ~ ( ~-e F r ~ u e ) .o r ---t - -~-+--~ 4 ~ ,,s ,0 [_ r__l__~ 1 - "-1___~__]

o ? . ' : ' ~ . , , ~ , , . ~ . , , i . , . i , . . , , , . , . . , i . . . ~ ' : - . v . i .o.s -o.~ .x~ .05 o o.z ~4 o.~ o.s 1

]~nm Factor l~lt for ~p d e ~ v ( S m m e r )

Us ing B.E. o f 2 ..~ ~- i D.11 ~ :~ 0 . 1 6 q t x 1 0 ~

t'.~ e ~ p r e ! i m i n ~ r ~

• E~met SU($) ( i ~ p e" v ) KT®V

," & ~ su(3) D ~ l d ~ g W~s ~ l o ~ M ~ d i c ~ a et aL ,. A-I I su(3) Bre~kl .$ fits ~ t e U ~

e5 0A 0~ u 1 15 1~ 1.6 1~ g l ~ . f l t~(q= =0 )

Figure 5. Kinematic distributions used in obtaining gl / f l ( left ) and fit for f l and g~ using measured gl/fland rate ( l e f t ) .