DIRAC experiment at CERN: results and plans

Perex

The main task of the DIRAC experiment is to check precise predictions of low-energy QCD.

At present, theory predicts the ππ s-wave scattering lengths with a precision of 1.5-2.5% for a0, a2, a0-a2. At present, the precision of ππ scattering lengths measurements is significantly worse then the theoretical precision. The DIRAC experiment takes data to obtain a precision of about 3% for |a0-a2|, measuring the life-time of pi-pi atoms.

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Now, theory predicts πK scattering lengths with a precision of about 10%. Direct measurements of πK scattering lengths do not exist. The DIRAC experiment will observe πK atoms, to measure the lifetime of this atom and to obtain the first evaluation of the s-wave scattering length combination |a1/2- a3/2|.

In 2010 DIRAC collaboration is planning to present an Addendum with the aim to observe the long-lived states of ππ atoms in 2011. Further data taking will allow us to obtain experimentally the Lamb shift ΔE2s-2p in this atom. The measurement of ΔE2s-2p allows determining in a model-independent way the combination of ππ scattering lengths 2a0+a2.

From the data collected in 2008, 2009 and 2010 it will be possible to observe the Coulomb enhancement in the production of K+K- pairs, and πμ pairs and thus to determine, in a model independent way, the number of K+K- atoms and number of πμ-atoms to be produced at the same time. This analysis will allow us to assess the feasibility to observe these atoms.

The new possibilities to check the predictions of the low-energy QCD will be available after installation of the DIRAC setup on a 450 GeV/c SPS proton beam. Simulations were based on FRITIOF6, which gives correct π and K meson spectra in the dynamic range of the DIRAC spectrometer. At the same intensity of the secondary particles across the forward detectors the number of detected ππ atoms will be 15 times higher than the one at 24 GeV, the number of K+π- atoms 25 times and the number of K-π+ atoms 32 times higher. This enhancement in atom yields will allow us to give the crucial check of low-energy QCD predictions.