Speaker
Dr
Alessandra Filippi
(TO)
Description
Data on two-nucleon K^- absorption in mesonless final states, $K^- + N + N\rightarrow Y N$, are very scarce and dated. Most of the existing
capture rates have been inferred indirectly from mononucleonic absorptions,
and only results for $^4$He, $^{12}$C, Ne and CF$_3$Br exist, obtained by
bubble chamber experiments [1]. The general understanding is that pionless
two-nucleon $K^-$ absorption could account for a fraction of all captures as
large as 20\%.
The knowledge of the capture rates of such reactions is important as they
represent the major background source in hypernuclear studies, as the phase
space for the decay particles from produced hyperons overlaps with the
spectra of hypernuclear formation pions and of the hypernuclear non-mesonic
decay products [2,3]. Especially in case of rare decays, these reactions could
give an overwhelming contribution, difficult to be separated from the
searched signals. So, the knowledge of their occurrence rate is fundamental
for correct background estimations.
At the DA$\Phi$NE $\phi$-factory kaons from the $\phi$ decays were stopped in
the FINUDA experiment in a series of thin targets, to perform hypernuclear
spectroscopy studies as well as investigations on kaon absorption mechanisms
on single and many nucleons. The pionless absorption reactions
$K^- A\rightarrow \Sigma^- p A'$ and
$K^- A\rightarrow \Lambda (\Sigma^-0) n A'$,
occurring on a $(np)$ pair embedded in the $A$ nucleus, were studied for
some $p$-shell nuclei, namely $^6$Li, $^7$Li, $^9$Be, $^{13}$C and $^{16}$O.
The $\Sigma^-$ hyperons in the final states were reconstructed via their
$n\pi^-$ decay mode, while the $\Lambda$'s by means of their charged decay,
the charged pions and protons being detected and identified by the FINUDA
magnetic spectrometer. Neutrons, on the other hand, were detected by a large
scintillator array completing externally the apparatus, featuring an
efficiency of $\sim$ 10\%.
The emission rates for the mentioned reactions, as a function of the mass
number $A$, were measured for the first time. In this presentation, they
will be discussed and compared to the existing measurements and theoretical
expectations, also in connection with the recent measurements of FINUDA of
the analogous rates for single nucleon kaon absorptions,
$K^-_{stop}A \rightarrow \Sigma^\pm \pi^\mp A'$ [4].
References
[1] C. Vander Velde-Wilquet et al., Nuovo Cim. 39 (1977), 538, and
references therein
[2] FINUDA Collaboration, M. Agnello et al., Phys. Lett. B698 (2011), 219
[3] FINUDA Collaboration, M. Agnello et al., Phys. Lett. B685 (2010) 247
[4] FINUDA Collaboration, M. Agnello et al., paper submitted to
Phys. Lett. B (2011)
Primary author
Dr
Alessandra Filippi
(TO)
