Experimental studies of baryon modification in nuclei using hyperons

Although the EMC effect indicates that the momentum distribution of quarks in the baryons in nuclear matter is modified from that in the free space, no clear evidence for baryon modification has been found in low energy nuclear phenomena and detailed mechanism of the modification is not understood yet. In order to challenge this problem, we plan to use $\Lambda$ hyperons embedded in a nucleus. One of the possible approaches is a measurement of $\Lambda$'s $\beta$-decay rate in hypernuclei, which is expected to be significantly reduced due to structure change of baryons in the nucleus. We will measure the lifetime and the branching ratio of the $\beta$-decay of $^5_\Lambda$He and/or $^{13}_\Lambda$C. The feasibility of this experiment is under study, in which how to suppress huge background from mesonic and nonmesonic weak decays of hypernuclei is a key point. Reliable theoretical estimates of nuclear effects and hadronic effects, including Pauli effect of nucleons, are also necessary.