G. Gidal

Evidence for the decay sigma+ --> pmu+ mu-.

We report the first evidence for the decay Sigma(+)-->pmu(+)mu(-) from data taken by the HyperCP (E871) experiment at Fermilab. Based on three observed events, the branching ratio is B(Sigma(+)-->pmu(+)mu(-))=[8.6(+6.6)(-5.4)(stat)+/-5.5(syst)]x10(-8). The narrow range of dimuon masses may indicate that the decay proceeds via a neutral intermediate state, Sigma(+)-->pP(0),P0-->mu(+)mu(-) with a P0 mass of 214.3+/-0.5 MeV/c(2) and branching ratio B(Sigma(+)-->pP(0),P0-->mu(+)mu(-))=[3.1(+2.4)(-1.9)(stat)+/-1.5(syst)]x10(-8).

Observation of the decay K- ---> pi- mu+ mu- and measurements of the branching ratios for K+- ---> pi+- mu+ mu-

Using data collected with the HyperCP (E871) spectrometer during the 1997 fixed-target run at Fermilab, we report the first observation of the decay K--->pi(-)mu(+)mu(-) and new measurements of the branching ratios for K+/--->pi(+/-)mu(+)mu(-). By combining the branching ratios for the decays K+-->pi(+)mu(+)mu(-) and K--->pi(-)mu(+)mu(-), we measure Gamma(K+/--->pi(+/-)mu(+)mu(-))/Gamma(K+/--->all) = (9.8+/-1.0+/-0.5)x10(-8). The CP asymmetry between the rates of the two decay modes is [Gamma(K+-->pi(+)mu(+)mu(-))-Gamma(K--->pi(-)mu(+)mu(-))]/[Gamma(K+-->pi(+)mu(+)mu(-))+Gamma(K--->pi(-)mu(+)mu(-))] = -0.02+/-0.11+/-0.04.

Search for CP violation in charged-Xi and Lambda hyperon decays.

We have compared the p and p angular distributions in 117 x 10(6) Xi- -->Lambdapi- -->ppi-pi- and 41 x 10(6) Xi+ -->Lambda pi+ -->p pi+pi+ decays using a subset of the data from the HyperCP experiment (E871) at Fermilab. We find no evidence of CP violation, with the direct-CP-violating parameter AXiLambda identical with (alphaXialphaLambda-alpha Xialpha Lambda)/(alphaXialphaLambda+alphaXialphaLambda)=[0.0+/-5.1(stat)+/-4.4(syst)] x 10(-4).

Search forCPViolation in Charged-ΞandΛHyperon Decays

We have compared the p and p angular distributions in 117 x 10(6) Xi- -->Lambdapi- -->ppi-pi- and 41 x 10(6) Xi+ -->Lambda pi+ -->p pi+pi+ decays using a subset of the data from the HyperCP experiment (E871) at Fermilab. We find no evidence of CP violation, with the direct-CP-violating parameter AXiLambda identical with (alphaXialphaLambda-alpha Xialpha Lambda)/(alphaXialphaLambda+alphaXialphaLambda)=[0.0+/-5.1(stat)+/-4.4(syst)] x 10(-4).

Observation of Parity Violation in the Omega-minus -> Lambda + K-minus Decay

The alpha decay parameter in the process Omega-minus -> Lambda + K-minus has been measured from a sample of 4.50 million unpolarized Omega-minus decays recorded by the HyperCP (E871) experiment at Fermilab and found to be [1.78 +/- 0.19(stat) +/- 0.16(syst)]{\times}10^{-2}. This is the first unambiguous evidence for a nonzero alpha decay parameter, and hence parity violation, in the Omega-minus -> Lambda + K-minus decay.

Observation of parity violation in the Ω−→ΛK− decay

The alpha decay parameter in the process Omega-minus -> Lambda + K-minus has been measured from a sample of 4.50 million unpolarized Omega-minus decays recorded by the HyperCP (E871) experiment at Fermilab and found to be [1.78 +/- 0.19(stat) +/- 0.16(syst)]{\times}10^{-2}. This is the first unambiguous evidence for a nonzero alpha decay parameter, and hence parity violation, in the Omega-minus -> Lambda + K-minus decay.

Search for the lepton-number-violating decay Xi- ---> p mu- mu-

A sensitive search for the lepton-number-violating decay Xi(-)-->pmu(-)mu(-) has been performed using a sample of approximately 10(9) Xi(-) hyperons produced in 800 GeV/c p-Cu collisions. We obtain B(Xi(-)-->pmu(-)mu(-))<4.0x10(-8) at 90% confidence, improving on the best previous limit by 4 orders of magnitude.

CP violation in strange baryon decays: A report from Fermilab experiment 871

Fermilab experiment 871, HyperCP, is a search for direct CP violation in Ⅺ and Λ hyperon decays. A non-zero value in the asymmetry parameter A, defined in terms of the decay parameter products αⅪαΛ and αⅪαΛ, would be unambiguous evidence for direct CP violation. The first data-taking run finished at the end of 1997 and accumulated over one billion Ⅺ− and Ⅺ+ decays. A sensitivity in A of ≈10−4 is expected. A review of CP violation in hyperon decays is given, the HyperCP detector is described, and the status of the data analysis is discussed.

Study of the rare hyperon decay Ω∓→Ξ∓π+π−

We report a new measurement of the decay \Omega^- \to \Xi^- \pi^+ \pi^- with 76 events and a first observation of the decay \Omega^+ \to \Xi^+ \pi^+ \pi^- with 24 events, yielding a combined branching ratio (3.74 ^{+0.67}_{-0.56}) \times 10^{-4}. This represents a factor 25 increase in statistics over the best previous measurement. No evidence is seen for CP violation, with B(\Omega^- \to \Xi^- \pi^+ \pi^-)=4.04^{+0.83}_{-0.71} \times 10^{-4} and B(\Omega^+ \to \Xi^+ \pi^+ \pi^-)=3.15^{+1.12}_{-0.89} \times 10^{-4}. Contrary to theoretical expectation, we see little evidence for the decays \Omega^- \to \Xi_{1530}^{*0} \pi^- and \Omega^+ \to \Xi_{1530}^{*0} \pi^+ and place a 90% C.L. upper limit on the combined branching ratio B(\Omega^-(\Omega^+) \to \Xi^{*0}_{1530}(\Xi^{*0}_{1530}) \pi^\mp)<7.0 \times 10^{-5}.

Study of the rare hyperon decay

We report a new measurement of the decay \Omega^- \to \Xi^- \pi^+ \pi^- with 76 events and a first observation of the decay \Omega^+ \to \Xi^+ \pi^+ \pi^- with 24 events, yielding a combined branching ratio (3.74 ^{+0.67}_{-0.56}) \times 10^{-4}. This represents a factor 25 increase in statistics over the best previous measurement. No evidence is seen for CP violation, with B(\Omega^- \to \Xi^- \pi^+ \pi^-)=4.04^{+0.83}_{-0.71} \times 10^{-4} and B(\Omega^+ \to \Xi^+ \pi^+ \pi^-)=3.15^{+1.12}_{-0.89} \times 10^{-4}. Contrary to theoretical expectation, we see little evidence for the decays \Omega^- \to \Xi_{1530}^{*0} \pi^- and \Omega^+ \to \Xi_{1530}^{*0} \pi^+ and place a 90% C.L. upper limit on the combined branching ratio B(\Omega^-(\Omega^+) \to \Xi^{*0}_{1530}(\Xi^{*0}_{1530}) \pi^\mp)<7.0 \times 10^{-5}.