Speaker
Dr
Alessandro Feliciello
(INFN - Sezione di Torino)
Description
In the last decade several targetted, well focussed experiments provided a remarkable amount of high quality information about $s$-, $p$- and, very recently, $sd$-shell $\Lambda$-hypernuclei.
Their results contribute to significantly improve our knowledge of the field.
However, some ''surprising'' outcomes came from non-dedicated large apparatus like STAR and ALICE, originally intended for heavy ion physics.
In addition to the discovery of the ${\overline{_\Lambda^{3}{\mathrm H}}}$, they found that the lifetime ($\tau$) of the $_\Lambda^3$H (and of the ${\overline{_\Lambda^{3}{\mathrm H}}}$), though affected by large errors, is remarkably lower than the lifetime of the free $\Lambda$ hyperon, in contrast to the commonly accepted predictions.
The precise determination of the $\tau$($_\Lambda^3$H) is a crucial point and it deserves a dedicated experiment.
For this reason it is being proposed to study for the first time the ($\pi^-$,$K^0$) reaction on a liquid $^{3}$He target.
The most suitable location for this measurement is the new K1.1 pion beam line, currently in an advanced commissioning stage at the Hadron Experimental Facility of J-PARC.
For once, it will be possible to collect a large data sample in order to directly measure lifetimes with a very good precision ($\leq$ 10%), by exploiting the delayed time spectrum method.
The present apparatus design foresees the mechanical integration of some new detectors with the existing SKS complex, recently moved and installed in the K1.1 experimental area.
Actually, in order to detect and to identify the products of the asymmetric $K^0$ decay a suitably layered range detector should be installed upstream the magnetic spectrometer.
The same detector will select with good energy resolution the $\pi^-$ from the mesonic decay of the $_\Lambda^3$H.
In addition, thanks to the apparatus modular concept, it will be possible to replace the whole liquid $^{3}$He target system with liquid $^{4}$He or with a series of solid targets, machined in thin tiles and arranged along the $\pi^-$ beam axis like a Venetian blind.
This way it will be possible to perform a series of systematic measurements of the lifetime for several neutron-rich $\Lambda$-hypernuclei belonging to the $p$-shell.
A very strong variation of the $\Gamma_{\pi^-}$ value for isobars in the $p$-shell was predicted as due to shell-structure effects.
Such measurements are easier than the one with the $^{3}$He target and they could serve as general test of the reliability of the method, in particular the performance of the spectroscopy through the ($\pi^-$,$K^0$) reaction.
At the same time, such an arrangement will permit to simultaneously extract their partial weak decay widths $\Gamma_{{\pi^-}}$, $\Gamma_{p}$ and $\Gamma_{{\pi^0}}$.
Primary author
Dr
Alessandro Feliciello
(INFN - Sezione di Torino)
