Nuclear Physics AS27 (1991) 74%748~ North-Holland, Amsterdam

745c

1.

RECENT ARGUS RESULTS ON CHARMED MESONS AND BARYONS

J. STIEWE

Universitit Heidelberg

THE DECAY D$ -+ $a+

We have performed a new measurement reconstructing the D$ in (7’~~) - combina- tions with 17’ -t ~‘7, p” + ~+a-, and 17’ + T]~+T-, 17 + ~7.‘) Fig. 1 shows the D$ - signal with 7’ + p’y. Using the ARGUS value a(D:+D,).BR(D$ + @r+) = (7.8ztO.8f1.3)pb at fi =10.15 GeV, and taking the weighted average of both 7’ channels, we get BR(D: --+ $r+)/BR(Df + &r+) = 2.5 f 0.5 f 0.3.

OC 1.7 1.9 2.1 2.3

+b+ [GeV/c2]

Fig. 1: M($r+) with 9 + p’y.

2. THE DECAYS e+v AND D$ + qSe+v

The only source for leptons in the continuum are charm decays. We have looked for

??*’ and 4 in the presence of an electron; since only ??*‘e+, but no R*‘e- combinations can come from charm decays, one can use the latter to estimate the background contributions.

Table 1: Results for the decays D+ + ??“e+v and D$ + qie+v

K*Oe+ K*“e- &+ Combinations observed 1441 f 96 573 f 80 200 * 21

Fakes 256 f 51 259 f 52 455 9

Fragmentation 185 f 46 260 f 46 35% 11

T(45) 22Ok 52 67 f 29 16f 7

Remaining 880 f 129 (-13 f 119) 104 f 26

0375~9474/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

746~ .I. Stiewe / Charmed mesons and batyons

The resulting branching ratio is BR(D+ 4 r*‘e+~) = (5.0 f 0.7 f 1.2)s. For the

decay D$ + $e+v, we apply the analogous procedure. Here we obtain BR(Dz --+

4e+v)/BR(D$ -+ ++) = 0.57 & 0.15 k 0.15 and BR(D$ + &T+) = (3.0 f 1.2) %. The numbers of signal and background combinations are given in Table 1.

3. EVIDENCE FOR PARITY NON-CONSERVATION IN THE DECAY A; --$ AT+

In the decay of an unpolarized AZ h AT+, A b pr-, parity nonconservation gives rise to an asymmetry in the helicity angular distribution of the proton. Fitting the (A&) mass in bins of the cosine of the helicity angle using the parmetrization W(0) 0; l+or~;a~cos(d),

where oat and O(A are the decay asymmetries of At and A(o* = 0.642 = 0.642~tO.013 from PDG), we fit the slope to be (-0.62 f 0.26) and get o,,t_ = (-0.97 * 0.41) consistent with

maximum parity violation (Fig. 2). As a byproduct, we get BR(Az -+ Ax+)/BR(A$ +

pK-s+) = (18 ztz 3 f 4) %.

dN

N dcosJ.

0 ,,..! L,.. I,., 1 -1 .o -0.5 0.0 0.5 1 .o

cos 29

Fig. 2: Decay proton helicity angle distribution

4. MEASUREMENT OF THE CHARMED STRANGE BARYONS 2: and E:

We observe the charmed strange baryons Ez and Sf in the decay channels Z: + z-r+r+, zo c + Z-n+ and E:-n+r+rr-. The latter channel has been observed here for the

first time (Fig.3). The measured mass values are given in Table 2.

Table 2: Massea and 0. BR fOT S! and 52

Decay Mode Measured Mass (MeV/c)* 0. BR, z,, > 0.5 pb) u . BR, all zp (pb)

?0 -C d z-x+ 2475.7 & 6.2 f 3.2 0.48 zt 0.15 f 0.08 0.77 l 0.24 f 0.16

IC ---) ‘=o t-r+n+r- 2471.2 f 3.0 + 1.6 1.59 f 0.40 & 0.17 2.55 f 0.64 & 0.39

Ez, weighted mean 2472.1 & 2.7 f 1.6

22 -_$ x-x+K+ 2465.1 f 3.6 f 1.9 0.95 f 0.25 f. 0.10 1.50 f 0.39 f 0.23

M(q) - M(EE) -7.0 f 4.5 f 2.2

74-k

0

1.9 2.2 2.5 2.8 3.1

[CeV/F]

Adding the charmels Z$ -+ Z-&r+ and $! -+ Z-~+@rr-, we extract the sp spectrum and fit the Peterson function with q+terson = 0.24 i 0.08 . The vaiues for cross section times branching ra.tio (for zp, > 0.5 and, using the above value for cc, for the entire X~ range), are listed in Table 2. A@$> - M(Ez) = (-7.0 * 4.5 f 2.2)MeV

Our value for the isospin splitting is in excellent agreement with those obtained

by CLE02) and ACCMOR..3)

REFERENCES

1. H. Albrecht et ad., “Observation of the Decay D$ 4 q’&,” DESY preprint 90-032 (1990).

2. M. S. A&n et al., Phys. Lett. 226B (1989) 401.

3. S. Barlag et ad., Phys. Letd. 236B (1990) 495.