Search

We report the first observation of the Cabibbo-suppressed charm baryon decay Xi(+)(c)-->pK(-)pi(+). We observe 150+/-22+/-5 events for the signal. The data were accumulated using the SELEX spectrometer during the 1996-1997 fixed target run at Fermilab, chiefly from a 600 GeV/c Sigma(-) beam. The branching fractions of the decay relative to the Cabibbo-favored Xi(+)(c)-->Sigma+K-pi(+) and Xi(+)(c)-->Xi(-)pi(+)pi(+) are measured to be B(Xi(+)(c)-->pK(-)pi(+))/B(Xi(+)(c)-->Sigma+K-pi(+)) = 0.22+/-0. 06+/-0.03 and B(Xi(+)(c)-->pK(-)pi(+))/B(Xi(+)(c)-->Xi(-)pi(+)pi(+)) = 0.20+/-0.04+/-0.02, respectively.

The design and performance of the Fermilab E781 (SELEX) Hardware Scattering Trigger (HST) are described. This trigger functioned by distinguishing beam scattering at small angles ( >150 μ rad) from non-interacting beam. Six 50 μ m pitch silicon planes grouped in three ( x , y ) stations, two before and one after the target, were used as the particle detectors. The triggering system involved 1920 channels of readout providing data to the Fast Encoding and Readout System (FERS) with programmable trigger logic processor. The overall system was tested successfully at Fermilab during the 1996-1997 fixed target run. The encoding time of the readout part of the FERS device was 30 ns and the processor decision time was 55 ns. The HST provided an output signal 250 ns after beam traversal of the target.

We utilize the angle and momentum resolution of our apparatus to study the polarization of 375 GeV/c Sigma(+) hyperons produced by 800 GeV/c protons incident on a Cu target. By examining in detail two of our high statistics data samples, we find evidence for structure in the p(t) dependence of Sigma(+) polarization and are able to extract the x(F) dependence of the Sigma(+) polarization and compare it with x(F) behavior in the Lambda(0) and Xi(-) systems.

We have measured the branching ratio for the hyperon radiative decay ${\mathrm{\ensuremath{\Sigma}}}^{+}$\ensuremath{\rightarrow}p\ensuremath{\gamma} using the Fermilab polarized charged hyperon beam. This measurement and our previously published result on the asymmetry parameter in the same decay are part of Fermilab experiment E761. We find B(${\mathrm{\ensuremath{\Sigma}}}^{+}$\ensuremath{\rightarrow}p\ensuremath{\gamma})/B(${\mathrm{\ensuremath{\Sigma}}}^{+}$\ensuremath{\rightarrow}p${\mathrm{\ensuremath{\pi}}}^{0}$) to be [2.32\ifmmode\pm\else\textpm\fi{}0.11(stat)\ifmmode\pm\else\textpm\fi{}0.10(syst)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ with a sample of 31 901 events. The higher statistics and careful attention to systematic uncertainties make these significant improvements over previous measurements. We describe how our measurements were performed and briefly review the theoretical implications of these results.

A recent demonstration of spin precession using channeling in a bent crystal offers the interesting possibility of a technique to measure magnetic moments of short-lived charm baryons. There are significant differences among theoretical models of these moments. Accumulating experimental evidence indicates the tools used for hyperon magnetic moment measurements, polarized production and decay asymmetries, may also be available with heavier baryons. Useful measurements may be possible using the channeling technique but they will require challenging beam conditions.

We have measured the branching ratio for the hyperon radiative decay [Xi][sup [minus]][r arrow][Sigma][sup [minus]][gamma] from a sample of 211[plus minus]33 events obtained in the polarized 375 GeV/[ital c] charged hyperon beam at Fermilab. We find [ital B]([Xi][sup [minus]][r arrow][Sigma][sup [minus]][gamma]/[Xi][sup [minus]][r arrow][Lambda][sup 0][pi][sup [minus]])=(1.22[plus minus]0.23[plus minus]0.06)[times]10[sup [minus]4] where the quoted errors are statistical and systematic, respectively. We have also obtained an indication that the sign of the asymmetry parameter of this decay is positive.

We have measured the magnetic moments of the SIGMA+ and SIGMABAR- hyperons produced by 800-GeV protons incident on a Cu target. We determine the SIGMA+ magnetic moment to be (2.4613 +/- 0.0034 +/- 0.0040)mu(N) where the uncertainties are statistical and systematic, respectively. In this first measurement we determine the magnetic moment of the SIGMABAR- to be -(2.428 +/- 0.036 +/- 0.007)mu(N). The magnetic moments of the SIGMA+ and SIGMABAR- are consistent with each other in magnitude but opposite in sign as required by CPT invariance.

We have measured the polarization of 375-GeV/c SIGMA+ and SIGMABAR- hyperons produced by 800-GeV/c protons incident on a Cu target. We find that the SIGMA+ polarization rises with increasing p(t) to a maximum of 16% at p(t) = 1.0 GeV/c and then decreases to 10% at p(t) = 1.8 GeV/c. We compare this SIGMA+ polarization with data at lower energies. The SIGMABAR- polarization has been measured for the first time. It has the same sign as the SIGMA+ but smaller magnitude in a similar kinematical region.

We have measured the asymmetry parameter (${\mathrm{\ensuremath{\alpha}}}_{\ensuremath{\gamma}}$) in the hyperon radiative decay ${\mathrm{\ensuremath{\Sigma}}}^{+}$\ensuremath{\rightarrow}p\ensuremath{\gamma} with a sample of 34 754\ifmmode\pm\else\textpm\fi{}212 events obtained in a polarized charged hyperon beam experiment at Fermilab. We find ${\mathrm{\ensuremath{\alpha}}}_{\ensuremath{\gamma}}$=-0.720\ifmmode\pm\else\textpm\fi{}0.086\ifmmode\pm\else\textpm\fi{}0.045, where the quoted errors are statistical and systematic, respectively.

We have searched for the rare hyperon radiative decay Omega- to Xi- gamma, using the Fermilab Proton Center 375 GeV/c charged hyperon beam. Our measurement of the Omega- beam fraction at 13.6 m from production is (4.8 +_ 0.4)10**(-5). No signal was found and we determine a new upper limit of (4.6 x 10**-4) at 90% CL for the Omega- to Xi- gamma branching ratio.

Pion virtual compton scattering (VCS) via the reaction pi(-)e-->pi(-)egamma was observed in the Fermilab E781 SELEX experiment. SELEX used a 600 GeV/c pi(-) beam incident on target atomic electrons, detecting the incident pi(-) and the final state pi(-), electron and gamma. Theoretical predictions based on chiral perturbation theory are incorporated into a Monte Carlo simulation of the experiment and are compared to the data. The number of reconstructed events (=9) and their distribution with respect to the kinematic variables (for the kinematic region studied) are in reasonable accord with the predictions. The corresponding pi(-) VCS experimental cross section is sigma=38.8+/-13 nb, in agreement with the theoretical expectation of sigma=34.7 nb.

We have searched for the production and decay of light super-symmetric baryons produced in 800 GeV/c proton copper interactions in a charged hyperon beam experiment. We observe no evidence for the decays R+(uud \g^~) -> S(uds \g^~) pi+ and X-(ssd \g^~) -> S(uds \g^~) pi- in the predicted parent mass and lifetime ranges of 1700-2500 Mev/c2 and 50-500 ps. Production upper limits for R+ at xF=0.47, Pt=1.4 GeV/c2 and X- at xF=0.48, Pt=0.65 GeV/c2 of less than 10^-3 of all charged secondary particles produced are obtained for all but the highest masses and shortest lifetimes predicted., 9 pages, uuencoded postscript 4 figures uuencoded, tar-compressed file (submitted to PRL)

Received 14 June 1982 (Revised 19 November 1982) Absolute differential cross sections for pp elastic scattering have been measured at kinetic energies of 648, 746, 795, 843, 892, 942 and 992 MeV and for momentum transfer 0.006 < Itl < 0.040 (GeV/c) 2. Both scattered and recoil protons were detected in coincidence. The slope parameters of the diffraction cone and the contribution of the spin-spin amplitudes to forward elastic pp scattering were determined.

We report the results of a polarized-${\ensuremath{\Sigma}}^{\mathrm{\ensuremath{-}}}$ beta-decay experiment carried out in the Fermilab Proton Center charged-hyperon beam. These results are based on 49 671 observed ${\ensuremath{\Sigma}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${\mathrm{ne}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{} decays. The ${\ensuremath{\Sigma}}^{\mathrm{\ensuremath{-}}}$ beam had a nominal momentum of 250 GeV/c and was produced by 400-GeV/c protons im- pinging on a Cu target. At a production angle of 2.5 mrad, the polarization was (23.6\ifmmode\pm\else\textpm\fi{}4.3)%. The decay asymmetries of the electron (${\ensuremath{\alpha}}_{e}$=-0.519\ifmmode\pm\else\textpm\fi{}0.104), neutron (${\ensuremath{\alpha}}_{n}$=+0.509\ifmmode\pm\else\textpm\fi{}0.102), and antineutrino (${\ensuremath{\alpha}}_{\ensuremath{\nu}}$=-0.230\ifmmode\pm\else\textpm\fi{}0.061) were measured and used to establish sign and approximate magnitude of the axial-vector-to-vector form-factor ratio ${g}_{1}$/${f}_{1}$. The form-factor ratios \ensuremath{\Vert}${g}_{1}$/${f}_{1}$\ensuremath{\Vert} and ${f}_{2}$/${f}_{1}$ were determined most sensitively from the neutron and electron center-of-mass spectra, respectively. We obtain \ensuremath{\Vert}${\mathit{g}}_{1}$/${\mathit{f}}_{1}$-0.237${\mathit{g}}_{2}$/${\mathit{f}}_{1}$\ensuremath{\Vert}=0.327\ifmmode\pm\else\textpm\fi{}0.007\ifmmode\pm\else\textpm\fi{}0.019 and ${\mathit{f}}_{2}$(0)/${\mathit{f}}_{1}$(0)=-0.96\ifmmode\pm\else\textpm\fi{}0.97\ifmmode\pm\else\textpm\fi{}0.13, where the stated errors are statistical and systematic, respectively. A general fit that includes the asymmetries and makes the conventional assumption ${g}_{2}$=0 gives the final value ${g}_{1}$(0)/${f}_{1}$(0)=-0.328\ifmmode\pm\else\textpm\fi{}0.019. The data are also compatible with positive values for ${g}_{2}$/${f}_{1}$ combined with corresponding reduced values for \ensuremath{\Vert}${g}_{1}$/${f}_{1}$\ensuremath{\Vert}. .AE

We have used the spin-precession technique to measure the ..sigma../sup -/ magnetic moment (..mu../sub ..sigma../). A ..sigma../sup -/ beam with a polarization of 22% was produced by a 400-GeV proton beam striking a Cu target at nominal production angles of +- 3 mrad. We simultaneously recorded 21X0 ..sigma../sup -/..-->..ne/sup -/nu-bar decays and 650X0 ..sigma../sup -/..-->..n..pi../sup -/ decays at ..sigma../sup -/ beam momenta of 253 and 308 GeV/c. We find that ..mu../sub ..sigma../ = -1.166 +- 0.014 +- 0.010 nuclear magnetons, where the quoted errors are statistical and systematic, respectively.

We have measured the production polarization and magnetic moment of a sample of 89{times}10{sup 3} {Xi}{sup {minus}} hyperons produced in the inclusive reaction {ital p}(400 GeV/{ital c})+Cu{r arrow}{Xi}{sup {minus}}+X. The weighted average of the polarization is {minus}0.070{plus minus}0.008{plus minus}0.010 at a {ital p}{sub {ital t}} of 0.63 GeV/{ital c}. The {Xi}{sup {minus}}'s magnetic moment yields the value {mu}{sub {Xi}}={minus}0.661{plus minus}0.036{plus minus}0.036 nuclear magnetons. The first error is statistical, the second systematic.