M. Sakitt

["Measurement of the Λ→n+γ branching ratio"]

The branching ratio for the {Lambda} weak radiative decay has been measured to be {ital B}({Lambda}{r arrow}{ital n}+{gamma})/({Lambda}{r arrow}anything) =(1.78{plus minus}0.24(stat){plus minus}{sub 0.16}{sup 0.14}(syst)){times}10{sup {minus}3}. A low-energy kaon beam was used to produce the {Lambda} hyperons via the reaction {ital K}{sup {minus}}+{ital p}{r arrow}{Lambda}+{pi}{sup 0} at rest. Photons from the signal channel and {pi}{sup 0} decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.

Measurement of the. Lambda. r arrow n +. gamma. branching ratio

The branching ratio for the {Lambda} weak radiative decay has been measured to be {ital B}({Lambda}{r arrow}{ital n}+{gamma})/({Lambda}{r arrow}anything) =(1.78{plus minus}0.24(stat){plus minus}{sub 0.16}{sup 0.14}(syst)){times}10{sup {minus}3}. A low-energy kaon beam was used to produce the {Lambda} hyperons via the reaction {ital K}{sup {minus}}+{ital p}{r arrow}{Lambda}+{pi}{sup 0} at rest. Photons from the signal channel and {pi}{sup 0} decay were detected with a NaI(Tl) array. The final spectrum contains 287 events after background subtraction, an order of magnitude more events than from the only previous measurement.

π − p → π 0 π 0 n near threshold and chiral symmetry breaking

Total cross sections, angular, and mass distributions for the reaction {pi}{sup {minus}}{ital p}{r arrow}{pi}{sup 0}{pi}{sup 0}{ital n} have been measured for {ital p}{sub {pi}}{sup {minus}}(lab)=7--140 MeV/{ital c} above threshold. The threshold amplitude was used to determine a value for the chiral-symmetry-breaking parameter, {xi}, of {minus}0.98{plus minus}0.52. The {pi}{pi} scattering lengths {ital a}{sub {ital I}} for isospin {ital I}=0 and 2 are derived from this result, together with a current-algebra sum rule. The results are {ital a}{sub 0}=(0.207{plus minus}0.028){ital m}{sub {pi}}{sup {minus}1} and {ital a}{sub 2}=({minus}0.022{plus minus}0.011){ital m}{sub {pi}}{sup {minus}1}. These values are consistent with chiral symmetry broken by the Weinberg {pi}{pi} interaction and the effects of the {ital f}{sub 0}(975) scalar meson.

A measurement of the branching ratio for the∑+ →pγ decay

We have measured the weak radiative decay branching ratio (∑+ →pγ)/(∑+ →pπ0). The experiment was carried out at Brookhaven National Laboratory using a low-energy separatedK− beam stopping in liquid hydrogen. The∑+ was tagged by observing the pion fromK−K+p→∑+π− in a range telescope, and the gamma was detected in a segmented NaI detector. The result is (2.81±0.39−0.43+0.21)×10−3, which is consistent with previous measurements. This corresponds to a branching ratio for (∑+ →pγ)/(∑+ → all) of (1.45±0.20−0.22+0.11)×10−3, where the quoted errors are statistical and systematic, respectively. A total of 408 signal events were observed, doubling the previous world total.

["π-p→π0π0n near threshold and chiral symmetry breaking"]

Total cross sections, angular, and mass distributions for the reaction {pi}{sup {minus}}{ital p}{r arrow}{pi}{sup 0}{pi}{sup 0}{ital n} have been measured for {ital p}{sub {pi}}{sup {minus}}(lab)=7--140 MeV/{ital c} above threshold. The threshold amplitude was used to determine a value for the chiral-symmetry-breaking parameter, {xi}, of {minus}0.98{plus minus}0.52. The {pi}{pi} scattering lengths {ital a}{sub {ital I}} for isospin {ital I}=0 and 2 are derived from this result, together with a current-algebra sum rule. The results are {ital a}{sub 0}=(0.207{plus minus}0.028){ital m}{sub {pi}}{sup {minus}1} and {ital a}{sub 2}=({minus}0.022{plus minus}0.011){ital m}{sub {pi}}{sup {minus}1}. These values are consistent with chiral symmetry broken by the Weinberg {pi}{pi} interaction and the effects of the {ital f}{sub 0}(975) scalar meson.

Radiative hyperon processes

SummaryWeak as well as pure electromagnetic radiative decays of hyperons are discussed. The radiative decays of excited hyperon states can provide information on the quark wave functions inside the hyperons. The weak (strangeness-changing) decays represent the last low-q2 frontier of weak-interaction physics. The theoretical motivation of these studies is reviewed, as well as the existing experiments. Special emphasis is placed on recent experiments at the Brookhaven AGS and at Fermilab. Radiative K− at rest on the deuteron is also discussed, and new results from Brookhaven are presented.RiassuntoSi discutono i decadimenti radiativi elettromagnetici puri nonché quelli deboli. I decadimenti radiativi degli stati eccitati degli iperoni possono fornire informazioni sulle funzioni di onda dei quark all’interno degli iperoni. I decadimenti deboli (che cambiano la stranezza) rappresentano l’ultima frontiera a bassoq2 della fisica delle interazioni deboli. Si passano in rassegna la motivazione teorica di questi studi e gli esperimenti esistenti. Si pone particolare enfasi sui recenti esperimenti all’AGS di Brookhaven e a Fermilab. Si discute anche la cattura di K− radiativa a riposo sul deuterone e si presentano nuovi risultati di Brookhaven.РеэюмеОбсуждаются слабые и чисто злектромагнитные радиационные распады гиперонов. Радиационные распады воэбужденных гиперонных состояний могут дать информацию о волновых функциях кварков внутри гиперонов. Слабые (с иэменением странности) распады представляют последнюю нижнююq2 границу для фиэики слабых вэаимодействий. Предлагается теоретическая мотивация зтих исследований и сушествуюших зкспериментов. Особое внимание уделяется последним зкспериментам в Брукхейвене AGS и в лаборатории Ферми. Также обсуждается радиационный K−-эахват в покое на дейтроне и приводятся новые реэультаты, полученные в Брукхейвене.

π−p→π0π0nnear threshold and chiral symmetry breaking

Total cross sections, angular, and mass distributions for the reaction {pi}{sup {minus}}{ital p}{r arrow}{pi}{sup 0}{pi}{sup 0}{ital n} have been measured for {ital p}{sub {pi}}{sup {minus}}(lab)=7--140 MeV/{ital c} above threshold. The threshold amplitude was used to determine a value for the chiral-symmetry-breaking parameter, {xi}, of {minus}0.98{plus minus}0.52. The {pi}{pi} scattering lengths {ital a}{sub {ital I}} for isospin {ital I}=0 and 2 are derived from this result, together with a current-algebra sum rule. The results are {ital a}{sub 0}=(0.207{plus minus}0.028){ital m}{sub {pi}}{sup {minus}1} and {ital a}{sub 2}=({minus}0.022{plus minus}0.011){ital m}{sub {pi}}{sup {minus}1}. These values are consistent with chiral symmetry broken by the Weinberg {pi}{pi} interaction and the effects of the {ital f}{sub 0}(975) scalar meson.

pi. sup minus p r arrow. pi. sup 0. pi. sup 0 n near threshold and chiral symmetry breaking

Total cross sections, angular, and mass distributions for the reaction {pi}{sup {minus}}{ital p}{r arrow}{pi}{sup 0}{pi}{sup 0}{ital n} have been measured for {ital p}{sub {pi}}{sup {minus}}(lab)=7--140 MeV/{ital c} above threshold. The threshold amplitude was used to determine a value for the chiral-symmetry-breaking parameter, {xi}, of {minus}0.98{plus minus}0.52. The {pi}{pi} scattering lengths {ital a}{sub {ital I}} for isospin {ital I}=0 and 2 are derived from this result, together with a current-algebra sum rule. The results are {ital a}{sub 0}=(0.207{plus minus}0.028){ital m}{sub {pi}}{sup {minus}1} and {ital a}{sub 2}=({minus}0.022{plus minus}0.011){ital m}{sub {pi}}{sup {minus}1}. These values are consistent with chiral symmetry broken by the Weinberg {pi}{pi} interaction and the effects of the {ital f}{sub 0}(975) scalar meson.

pi- p ---> pi0 pi0 n near threshold and chiral symmetry breaking

Total cross sections, angular, and mass distributions for the reaction {pi}{sup {minus}}{ital p}{r arrow}{pi}{sup 0}{pi}{sup 0}{ital n} have been measured for {ital p}{sub {pi}}{sup {minus}}(lab)=7--140 MeV/{ital c} above threshold. The threshold amplitude was used to determine a value for the chiral-symmetry-breaking parameter, {xi}, of {minus}0.98{plus minus}0.52. The {pi}{pi} scattering lengths {ital a}{sub {ital I}} for isospin {ital I}=0 and 2 are derived from this result, together with a current-algebra sum rule. The results are {ital a}{sub 0}=(0.207{plus minus}0.028){ital m}{sub {pi}}{sup {minus}1} and {ital a}{sub 2}=({minus}0.022{plus minus}0.011){ital m}{sub {pi}}{sup {minus}1}. These values are consistent with chiral symmetry broken by the Weinberg {pi}{pi} interaction and the effects of the {ital f}{sub 0}(975) scalar meson.

Weak radiative hyperon decays

New measurements of the Σ+ and Λ weak radiative decays are discussed. The hyperons were produced at rest by the reaction K−p → Yπ where Y = Σ+ or Λ. The monoenergetic pion was used to tag the hyperon production, and the branching ratios were determined from the relative amplitudes of Σ+ → pγ to Σ+ → pπ0 and Λ → nγ to Λ → nπ0. The photons from weak radiative decays and from π0 decays were detected with modular NaI arrays.