First observation of the doubly charmed baryon Xi(+)(cc)

We observe a signal for the doubly charmed baryon Xi(+)(cc) in the charged decay mode Xi(+)(cc)-->Lambda(+)(c)K-pi(+) in data from SELEX, the charm hadroproduction experiment at Fermilab. We observe an excess of 15.9 events over an expected background of 6.1+/-0.5 events, a statistical significance of 6.3sigma. The observed mass of this state is 3519+/-1 MeV/c(2). The Gaussian mass width of this state is 3 MeV/c(2), consistent with resolution; its lifetime is less than 33 fs at 90% confidence.     

Formation of a narrow baryon resonance with positive strangeness in K + collisions with Xe nuclei

The data on the charge-exchange reaction K ⁺Xe → K ⁰ pXe′, obtained with the bubble chamber DIANA, are reanalyzed using increased statistics and updated selections. Our previous evidence for formation of a narrow pK ⁰ resonance with mass near 1538 MeV is confirmed. The statistical significance of the signal reaches some 8 (6) standard deviations when estimated as $ {S \mathord{\left/ {\vphantom {S {\sqrt B \left( {{S \mathord{\left/ {\vphantom {S {\sqrt {B + S} }}} \right. \kern-0em} {\sqrt {B + S} }}} \right)}}} \right. \kern-0em} {\sqrt B \left( {{S \mathord{\left/ {\vphantom {S {\sqrt {B + S} }}} \right. \kern-0em} {\sqrt {B + S} }}} \right)}} $ . The mass and intrinsic width of the Θ⁺ baryon are measured as m = 1538 ± 2 MeV and Γ = 0.39 ± 0.10 MeV.     

Data Acquisition and Diagnostic System for Prototype of PANDA Pellet Target

Physics of Atomic Nuclei - In the PANDA experiment of the FAIR project, it is proposed to use internal targets based on the hydrogen isotopes that provide a monodisperse regime of flow generation...     

Observation of two-neutrino double-beta decay in 136Xe with the EXO-200 detector

We report the observation of two-neutrino double-beta decay in (136)Xe with T(1/2) = 2.11 ± 0.04(stat) ± 0.21(syst) × 10(21) yr. This second-order process, predicted by the standard model, has been observed for several nuclei but not for (136)Xe. The observed decay rate provides new input to matrix element calculations and to the search for the more interesting neutrinoless double-beta decay, the most sensitive probe for the existence of Majorana particles and the measurement of the neutrino mass scale.     

Observation of a baryon resonance with positive strangeness in K + collisions with Xe nuclei

The status of our investigation of low-energy K ⁺Xe collisions in the xenon bubble chamber DIANA is reported. In the charge-exchange reaction K ⁺Xe → K ⁰ pXe′, the spectrum of K ⁰ p effective mass shows a resonant enhancement with M=1539±2 MeV/c ² and Γ≤9 MeV/c ². The statistical significance of the enhancement is near 4.4σ. The mass and width of the observed resonance are consistent with expectations for the lightest member of the antidecuplet of exotic pentaquark baryons, as predicted in the framework of the chiral soliton model.     

Search for Neutrinoless Double-Beta Decay in ^{136}Xe with EXO-200

We report on a search for neutrinoless double-beta decay of ^{136}Xe with EXO-200. No signal is observed for an exposure of 32.5?kg?yr, with a background of ～1.5×10^{-3} kg^{-1}?yr^{-1}?keV^{-1} in the ±1σ region of interest. This sets a lower limit on the half-life of the neutrinoless double-beta decay T_{1/2}^{0νββ}(^{136}Xe)>1.6×10^{25} yr (90% C.L.), corresponding to effective Majorana masses of less than 140-380?meV, depending on the matrix element calculation.     

Aleksei Borisovich Kaidalov (July 20, 1940–July 25, 2010)

Physics of Atomic Nuclei -     

New Measurement of the π0 radiative decay width

High precision measurements of the differential cross sections for π0 photoproduction at forward angles for two nuclei, 12C and 208Pb, have been performed for incident photon energies of 4.9-5.5 GeV to extract the π0→γγ decay width. The experiment was done at Jefferson Lab using the Hall B photon tagger and a high-resolution multichannel calorimeter. The π0→γγ decay width was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value for the decay width is Γ(π0→γγ)=7.82±0.14(stat)±0.17(syst) eV. With the 2.8% total uncertainty, this result is a factor of 2.5 more precise than the current Particle Data Group average of this fundamental quantity, and it is consistent with current theoretical predictions.