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The neutron halo is one of the most intriguing features in systems with a large excess of neutrons compared to protons, providing valuable information about weakly bound many-body systems near the neutron drip line. $^{17}$B has long been regarded as a two-neutron halo nucleus based on various experimental observations. However, recent studies suggest otherwise due to a dominant $d$-wave configuration of the valance neutrons and due to the presence of a thick neutron skin.
In this work, we report the first measurement of electric dipole ($E1$) excitation of $^{17}$B. A kinematically complete measurement was performed at RIBF, RIKEN, using the SAMURAI spectrometer. A $^{17}$B beam at 270MeV/u was impinged on a Pb target causing Coulomb dissociation (CD), and the breakup fragments $^{15}$B and two neutrons were detected in coincidence. From this measurement, the differential CD cross section $d\sigma_{\mathrm{CD}}/dE_x$ and the $E1$ strength distribution $dB(E1)/dE_x$ were extracted.
The observed $E1$ strength distribution indicates that $^{17}$B has a weakly developed halo. These results provide important structural information on $^{17}$B and they contribute to setting the criteria required for neutron halo formation.