  We present the first detection of hydrogen chlorine in a protostellar shock,
by observing the fundamental transition at 626 GHz with the Herschel HIFI
spectrometer. We detected two of the three hyperfine lines, from which we
derived a line opacity < 1. Using a non-LTE LVG code, we constrained the HCl
column density, temperature and density of the emitting gas. The hypothesis
that the emission originates in the molecular cloud is ruled out, as it would
imply a too dense gas. Conversely, assuming that the emission originates in the
10"-15" size shocked gas previously observed at the IRAM PdB interferometer, we
obtain: N(HCl)=0.7-2 x 10(13) cm-2, temperature > 15 K and density > 3 x 10(5)
cm-3}. Combining with the Herschel HIFI CO(5-4) observations allows to further
constrain the gas density and temperature, 10(5)-10(6) cm-3 and 120-250 K, as
well as the HCl column density, 2 x 10(13) cm-2, and, finally, abundance: 3-6 x
10(-9). The estimated HCl abundance is consistent with that previously observed
in low- and high- mass protostars. This puzzling result in the L1157-B1 shock,
where species from volatile and refractory grains components are enhanced,
suggests either that HCl is not the main reservoir of chlorine in the gas
phase, against previous chemical models predictions, or that the elemental
chlorine abundance is low in L1157-B1. Astrochemical modelling suggests that
HCl is in fact formed in the gas phase, at low temperatures, prior to the
occurance of the shock, and that the latter does not enhance its abundance.
