Abstract
After observing and investigating the double-exotic (a double-exotic atom is a bound system, in which both oppositely charged components are unstable particles like μ,π,K,…) π+π− atom with the ground state lifetime τ of about 3×10−15 s, the upgraded DIRAC experiment at the CERN PS accelerator observes for the first time long-lived states of the same atom with lifetimes of about 10−11 s and more. The number of characteristic pion pairs resulting from the breakup (ionisation) of long-lived π+π− atoms amounts to 436±61, corresponding to a signal-to-error ratio of better than 7 standard deviations. This observation opens a new possibility to measure energy differences between p and s atomic states and so to determine ππ scattering lengths.
Highlights
The experimental program of the DIRAC Collaboration comprises the observation and detailed analysis of dimesonic atoms, which are produced by protons interacting with target nuclei [1]
The data analysis has yielded a first measurement of the π K atom lifetime and π K scattering lengths [4]
The observation of long-lived π +π − bound states gives the possibility to study the Lamb shift and a new π π scattering length combination
Summary
The experimental program of the DIRAC Collaboration comprises the observation and detailed analysis of dimesonic atoms, which are produced by protons interacting with target nuclei [1]. By measuring the value of Ens−np, one can determine the numerical value of Enst0r, as all other terms in Enl have been calculated with high precision: the strong interaction effects contribute up to 80% of the full energy shift. In order to measure the shift of the vertical component Q Y , which is enlarged by the horizontal magnetic field, e+e− Dalitz pairs generated in the Be target as well as e+e− pairs produced in the Pt foil by photons have been investigated. Q Y of 12.9 MeV/c, whereas a pair produced in the Pt foil gets a smaller Q Y , caused by the fringing field, of only 2.3 MeV/c This difference in the Q Y shift for pairs from Be and Pt allows to suppress significantly the background level for observing atomic pairs from long-lived A2Lπ. With a spill duration of 450 ms the beam intensity has been (28–30) · 1010 protons/spill
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