DIRAC observes dimeson atoms and measures their lifetime

CERN

Published 2012 | Version v1

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DIRAC observes dimeson atoms and measures their lifetime

CERN

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A general view of the downstream part of the DIRAC experiment (upper image) beyond the magnet (orange), with the yellow boxes of the Cherenkov counters in the foreground. The lower image shows the section between the beam shielding and the magnet, with the vacuum chamber (blue), the beginning of the flat chamber (yellow) and the magnetic screen (orange) in front of the magnet. The beam pipe for the primary protons (silver) lies beneath the blue vacuum chamber. md5:c0cd0fe247a6d945ae5bd9d2ebc2e2ea	1.1 MB	Preview Download
Fig. 1. Pionium (A2π), produced in nickel, is detected through its break-up (ionization). Alternatively, the A2π annihilates (decays). NA denotes the total number of produced atoms, whereas nA is the number of “atomic pairs” from atom break-up. md5:abeb5406028ef980a1a772971a1f6629	49.1 kB	Preview Download
Fig. 2. The DIRAC double-arm spectrometer. The microstrip gas chambers (MSGC), the scintillating-fibre detector (SFD) and the scintillation-ionization hodoscope (IH) provide initial track data. Downstream of the spectrometer magnet, drift chambers (DC) measure final tracks to determine momenta. The vertical and the horizontal-scintillation hodoscopes (VH and HH) are used for trigger purposes and time measurement. Cherenkov detectors (Ch) containing nitrogen, preshower detectors (PSh) and scintillation muon detectors (Mu) behind the iron absorber help to distinguish pions from electrons and muons. md5:3afcfb230b6236187f5dd74487349777	71.1 kB	Preview Download
Fig. 3. (left) QL distributions of π+π– pairs from data (black dots) and simulation (solid line). Top: the experimental spectrum in comparison with the simulated background components of “Coulomb” (NC), “non-Coulomb” (NnC) and “accidental” (Nacc) pairs. Bottom: the residuals for the “atomic pairs” (nA) – the pionium signal – after background subtraction and the corresponding simulation. md5:d7e0cd814cdeea26f260fcda1920029f	92.2 kB	Preview Download
Fig. 4. (above) Pionium energy-level scheme and 2s–2p energy splitting. The decay widths (Γ1s, Γ 2s) and the energy shifts (ε1s, ε2s, ε2p) are presented for the states with principal quantum numbers n=1 and 2. md5:79a61d2cf12de96a97c41ff03451a89c	67.9 kB	Preview Download

Additional details

CDS: 1734766
CDS Report Number: DIRAC-PUB-2012-01

Issued: 2012-03

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DIRAC observes dimeson atoms and measures their lifetime

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