pi-N charge exchange and pi(+)-pi(0) scattering at low energies

pi-N and pi-pi interactions near threshold are uniquely sensitive to the chiral symmetry breaking part of the strong interaction. The pi-N sigma-term value with its implications for nucleon quark structure and the recent controversy concerning the size of the scalar quark condensate have renewed the experimental interest in these two fundamental systems. We report new differential cross sections for the reaction $pi^-p \to \pi^0n$ at 27.5 MeV pion incident kinetic energy, measured between $\theta_{CM} = 0^\circ$ and $55^\circ$. Our results are in excellent agreement with the existing comprehensive pi-N phase shift analysis. We also report on a Chew-Low analysis of exclusive $\pi^+ p \to \pi^+\pi^0p$ data at 260 MeV pion incident energy.Comment: Talk given by D. Pocanic at QULEN97, Osaka, 20-23 May 1997; 4 pages, 2 PostScript figures, writen in LaTeX 2e, uses packages "epsfig" and "espcrc1

Measurement of stopping beam distributions in the PIBETA detector

Precise calculation of the geometrical acceptance of a large solid angle detector with an integrated stopping target relies on precise knowledge of the beam geometry. We describe four alternative methods that we used to measure the beam stopping distributions in the PIBETA detector active target: (i) light response of segmented target elements to incident beam particles, (ii) back-tracking of charged particles from pi+ and mu+ decays using multi-wire proportional chambers, (iii) volume distribution of the Dalitz decay (pi0->gamma e+e-) event vertices, and (iv) the opening angle distribution of two pi0 photons originating from the beta decay of pi+ at rest. We demonstrate consistent results obtained by these four independent approaches and show how particular beam stopping distributions affect the detector's geometrical acceptance.Comment: 38 pages, 16 postscript figures, 2 tables, LaTeX, submitted to Nucl. Instrum. Meth.

Impact of Neutron Decay Experiments on non-Standard Model Physics

This paper gives a brief overview of the present and expected future limits on physics beyond the Standard Model (SM) from neutron beta decay, which is described by two parameters only within the SM. Since more than two observables are accessible, the problem is over-determined. Thus, precise measurements of correlations in neutron decay can be used to study the SM as well to search for evidence of possible extensions to it. Of particular interest in this context are the search for right-handed currents or for scalar and tensor interactions. Precision measurements of neutron decay observables address important open questions of particle physics and cosmology, and are generally complementary to direct searches for new physics beyond the SM in high-energy physics. Free neutron decay is therefore a very active field, with a number of new measurements underway worldwide. We present the impact of recent developments.Comment: 13 pages, 6 figures; Proceedings of the 5th International BEYOND 2010 Conference, Cape Town, South Africa (2010), World Scientific, accepted for publication; Corrected typo

The automatic gain-matching in the PIBETA CsI calorimeter

Segmented electromagnetic calorimeters are used to determine both the total energy and direction (momentum components) of charged particles and photons. A trade off is involved in selecting the degree of segmentation of the calorimeter as the spatial and energy resolutions are affected differently. Increased number of individual detectors reduces accidental particle pile-up per detector but introduces complications related to ADC pedestals and pedestal variations, exacerbates the effects of electronic noise and ground loops, and requires summing and discrimination of multiple analog signals. Moreover, electromagnetic showers initiated by individual ionizing particles spread over several detectors. This complicates the precise gain-matching of the detector elements which requires an iterative procedure. The PIBETA calorimeter is a 240-module pure CsI non-magnetic detector optimized for detection of photons and electrons in the energy range 5-100 MeV. We present the computer-controlled, automatic, in situ gain-matching procedure that we developed and used routinely in several rare pion and muon decay experiments with the PIBETA detector.Comment: 28 pages, 13 postscript figures, LaTeX, submitted to Nucl. Instrum. Meth.

Optics: general-purpose scintillator light response simulation code

We present the program optics that simulates the light response of an arbitrarily shaped scintillation particle detector. Predicted light responses of pure CsI polygonal detectors, plastic scintillator staves, cylindrical plastic target scintillators and a Plexiglas light-distribution plate are illustrated. We demonstrate how different bulk and surface optical properties of a scintillator lead to specific volume and temporal light collection probability distributions. High-statistics optics simulations are calibrated against the detector responses measured in a custom-made cosmic muon tomography apparatus. The presented code can also be used to track particles intersecting complex geometrical objects.Comment: RevTeX LaTeX, 37 pages in e-print format, 12 Postscript Figures and 1 Table, also available at http://pibeta.phys.virginia.edu/public_html/preprints/optics.p

Otpornost na zračenje dijelova detektora PIBETA

We have examined long-term changes in signal amplitude gain, energy resolution and detection efficiency of the active components of the PIBETA detector system. Beam-defining plastic scintillation counters were operated in a ∼ 1 MHz stopped π + beam for a period of 297 days, accumulating radiation doses of up to 2 · 106 rad. Detectors in the charged particle tracking system — a pair of cylindrical multi-wire proportional chambers and a thin plastic scintillation barrel-shaped hodoscope array — were irradiated during the same running period with an average dose of ∼ 4 · 104 rad. Individual CsI (undoped crystal) calorimeter detectors received an average dose of ∼ 120 rad, mainly from photons, positrons and protons originating from π + hadronic interactions as well as from π + and µ + weak decays at rest in the active target.Ispitivali smo trajne promjene amplitude, energijskog razlučivanja i učinkovitosti detekcije aktivnih dijelova detektorskog sustava PIBETA. Plastični scintilacijski detektori koji definiraju snop ozračeni su do 2 Mrad. Par višežičanih proporcionalnih komora i tanak valjak plastičnih scintilacijskih detektora ozračeni su do 40 krad. CsI kalorimetrijski detektori ozračeni su do 120 rad

Otpornost na zračenje dijelova detektora PIBETA

We have examined long-term changes in signal amplitude gain, energy resolution and detection efficiency of the active components of the PIBETA detector system. Beam-defining plastic scintillation counters were operated in a ∼ 1 MHz stopped π + beam for a period of 297 days, accumulating radiation doses of up to 2 · 106 rad. Detectors in the charged particle tracking system — a pair of cylindrical multi-wire proportional chambers and a thin plastic scintillation barrel-shaped hodoscope array — were irradiated during the same running period with an average dose of ∼ 4 · 104 rad. Individual CsI (undoped crystal) calorimeter detectors received an average dose of ∼ 120 rad, mainly from photons, positrons and protons originating from π + hadronic interactions as well as from π + and µ + weak decays at rest in the active target.Ispitivali smo trajne promjene amplitude, energijskog razlučivanja i učinkovitosti detekcije aktivnih dijelova detektorskog sustava PIBETA. Plastični scintilacijski detektori koji definiraju snop ozračeni su do 2 Mrad. Par višežičanih proporcionalnih komora i tanak valjak plastičnih scintilacijskih detektora ozračeni su do 40 krad. CsI kalorimetrijski detektori ozračeni su do 120 rad

Light response of pure CsI calorimeter crystals painted with wavelength-shifting lacquer

We have measured scintillation properties of pure CsI crystals used in the shower calorimeter built for a precise determination of the pi+ -> pi0 e+ nu decay rate at the Paul Scherrer Institute (PSI). All 240 individual crystals painted with a special wavelength-shifting solution were examined in a custom-build detection apparatus (RASTA=radioactive source tomography apparatus) that uses a 137Cs radioactive gamma source, cosmic muons and a light emitting diode as complementary probes of the scintillator light response. We have extracted the total light output, axial light collection nonuniformities and timing responses of the individual CsI crystals. These results predict improved performance of the 3 pi sr PIBETA calorimeter due to the painted lateral surfaces of 240 CsI crystals. The wavelength-shifting paint treatment did not affect appreciably the total light output and timing resolution of our crystal sample. The predicted energy resolution for positrons and photons in the energy range of 10-100 MeV was nevertheless improved due to the more favorable axial light collection probability variation. We have compared simulated calorimeter ADC spectra due to 70 MeV positrons and photons with a Monte Carlo calculation of an ideal detector light response.Comment: Elsevier LaTeX, 35 pages in e-print format, 15 Postscript Figures and 4 Tables, also available at http://pibeta.phys.virginia.edu/~pibeta/subprojects/csipro/tomo/rasta.p

Theoretical Predictions for Pionium Searches

Characteristic properties of pionium $A_{2\pi}$ and associated low energy s--wave cross sections $\sigma(\pi^0\pi^0\to \pi^0\pi^0)$, $\sigma(\pi^+\pi^-\to\pi^0\pi^0)$ and $\sigma(\pi^0\pi^0\to \pi^+\pi^-)$ are investigated with a coupled channels potential model. Some experimental results and conclusions are to be reconsidered.Comment: 10 Pages REVTeX using epsfig.sty. Submitted to PR