9 Nuclear Emulsions
405
Such an accuracy allows using CS tracks made of only 3 out of the possible 4
track segments, thus increasing the track finding efficiency. Thanks to the CS,
the bricks wrongly identified by the scintillator trackers are not disassembled but
put back in the target with a fresh CS attached to them. This avoids useless film
handling, processing and scanning of the misidentified bricks, and minimizes the
corresponding waste of target mass. Moreover, whenever the electronic detector
reconstruction is compatible with two or more “candidate” bricks, these are ordered
by probability and their CS are scrutinized accordingly. This significantly increases
the event finding efficiency.
If one or more “event related” tracks are found, the selected brick is exposed
to cosmic-rays for about 12 h, thus providing a set of tracks to be used for
precise correction of local deformations as required for precision topological
and kinematical measurements. The brick is then disassembled and its films are
developed. The tracks measured in the CS analysis provide predictions for the
so-called scan-back procedure. The latter consists of following a predicted track
upstream in the ECC brick until it “disappear”. This procedure is initiated in the
most downstream film of the brick.
The disappearance of a scan-back track indicates a possible neutrino interaction
vertex. A wide area scan is performed over a volume of about 1 cm 3 around the track
stopping point, looking for partner tracks and/or secondary decays with a dedicated
decay search procedure [103]. This procedure, developed for the tau neutrino
search, was successfully applied to the search for charmed hadron production
induced by neutrinos. The latter process was indeed used as a control sample to
check the efficiency for the detection of the tau lepton, given the similar lifetime
of charmed hadrons (about 10 −12 s). The application of this procedure to muon
neutrino interactions led to the observation of 50 decay candidates [103], in good
agreement with the expected charmed hadron yield (54±4), derived from the value
measured by the CHORUS experiment [104]. Good agreement was found also in
the shape of the relevant kinematical and topological variables, like the angle in
the transverse plane between the charmed hadron and the muon and the impact
parameter of the decay daughter particles with respect to the primary neutrino
interaction vertex [103].
Unlike the experiments using “bulk” emulsions like CHORUS where the visual
inspection of the primary and decay vertices allows rejecting most of the residual
background, the ECC structure prevents the direct check of the vertices for
the majority of the events. However, one can still exploit precise kinematical
measurements for background suppression. For interesting event topologies, in
fact, a detailed kinematical analysis is performed in OPERA by means of the
electromagnetic shower energy measurement in the downstream part of the brick,
the determination of the momentum by Multiple Coulomb Scattering measurement
in the lead/emulsion structure [105], and the connection of tracks in consecutive
target walls.
During the five CNGS production runs from 2008 to 2012, OPERA collected
about 1.8×10 20 protons on target and more than 19,000 neutrino interactions. The
first tau neutrino candidate was reported in 2010 [106] and the display of its event
405
Such an accuracy allows using CS tracks made of only 3 out of the possible 4
track segments, thus increasing the track finding efficiency. Thanks to the CS,
the bricks wrongly identified by the scintillator trackers are not disassembled but
put back in the target with a fresh CS attached to them. This avoids useless film
handling, processing and scanning of the misidentified bricks, and minimizes the
corresponding waste of target mass. Moreover, whenever the electronic detector
reconstruction is compatible with two or more “candidate” bricks, these are ordered
by probability and their CS are scrutinized accordingly. This significantly increases
the event finding efficiency.
If one or more “event related” tracks are found, the selected brick is exposed
to cosmic-rays for about 12 h, thus providing a set of tracks to be used for
precise correction of local deformations as required for precision topological
and kinematical measurements. The brick is then disassembled and its films are
developed. The tracks measured in the CS analysis provide predictions for the
so-called scan-back procedure. The latter consists of following a predicted track
upstream in the ECC brick until it “disappear”. This procedure is initiated in the
most downstream film of the brick.
The disappearance of a scan-back track indicates a possible neutrino interaction
vertex. A wide area scan is performed over a volume of about 1 cm 3 around the track
stopping point, looking for partner tracks and/or secondary decays with a dedicated
decay search procedure [103]. This procedure, developed for the tau neutrino
search, was successfully applied to the search for charmed hadron production
induced by neutrinos. The latter process was indeed used as a control sample to
check the efficiency for the detection of the tau lepton, given the similar lifetime
of charmed hadrons (about 10 −12 s). The application of this procedure to muon
neutrino interactions led to the observation of 50 decay candidates [103], in good
agreement with the expected charmed hadron yield (54±4), derived from the value
measured by the CHORUS experiment [104]. Good agreement was found also in
the shape of the relevant kinematical and topological variables, like the angle in
the transverse plane between the charmed hadron and the muon and the impact
parameter of the decay daughter particles with respect to the primary neutrino
interaction vertex [103].
Unlike the experiments using “bulk” emulsions like CHORUS where the visual
inspection of the primary and decay vertices allows rejecting most of the residual
background, the ECC structure prevents the direct check of the vertices for
the majority of the events. However, one can still exploit precise kinematical
measurements for background suppression. For interesting event topologies, in
fact, a detailed kinematical analysis is performed in OPERA by means of the
electromagnetic shower energy measurement in the downstream part of the brick,
the determination of the momentum by Multiple Coulomb Scattering measurement
in the lead/emulsion structure [105], and the connection of tracks in consecutive
target walls.
During the five CNGS production runs from 2008 to 2012, OPERA collected
about 1.8×10 20 protons on target and more than 19,000 neutrino interactions. The
first tau neutrino candidate was reported in 2010 [106] and the display of its event