Experiment Proposal - opera - Infn

search for the scattering vertices inside the brick and checking whether or not they represent genuine
kinks.
In order to explore rates of large angle scattering down to ≤10 −5 N µ for a 2 mm lead scatterer, the
equivalent number of muons on the 28 mm stack has to be of the order of 10 5 or more. However, for a a
reliable track prediction with our trackers, the track density should not exceed 10 cm −2 or, taking into
account the beam profile, ∼ 500 tracks over the useful area of 8 × 8 cm 2 . To overcome this limitation,
we have used the technique of a movable emulsion sheet, in which a separate sheet of emulsion pressed
against the upstream surface of the target was periodically shifted during the exposure by 100 µm across
the beam, by means of a PC-controlled stepper mechanism. This technique permitted to identify the
incoming tracks stored in the upstream emulsion film at every movable sheet position. With this method,
the total exposure was limited only by the number of the shifter steps (200 in our case), and the required
level of 10 5 muons could be reached.
To increase further increase the statistics the same brick target was exposed three times, at three
different angles (−6 ◦ , 0 ◦ ,and+6 ◦ ) to the beam axis. The trigger rate during these exposures was
about 50 events per spill and the movable sheet was shifted every 10 spills. The dead time due to data
acquisition was 2%.
Besides this measurements with the lead/emulsion brick target, we took data with passive 2 mm
and 7 mm thick lead targets, as well as without target (empty target), to estimate the large-angle muon
scattering rate from electronic measurements alone. The empty-target rate was subtracted from the rates
with real targets, to remove effects of the tracker resolution, δ-rays and scattering outside the target. For
alignment and calibration purposes, empty-target data were also taken at different beam momenta, with
and without dump.
Fig. 168 shows the angular and positional resolutions at the target, for tracks measured in emptytarget
runs. The scattering angle is measured with the average accuracy of better than 1.6 mrad, while
the impact parameter with the accuracy of about 0.5 mm. The measurements with passive targets were
done at a highest possible recording rate (210 events per spill, on average).
The event statistics accumulated in this test experiment is summarised in Table 37. Fiducial area
and incoming track quality cuts retain about 72% of the raw data.
Table 37: Event statistics of the muon scattering measurement.
Target, thickness Recorded events (/1000) after cuts (/1000)
Lead-emulsion, 28 mm 307 221
Empty, 0 mm 2453 1828
Lead, 2 mm 4058 2948
Lead, 7 mm 502 366
Fig. 169 shows an example of a kink candidate event with the measured p t of 267 MeV/c.Theraw
rate of such events observed in electronic trackers is consistent with the limit of 4.1 × 10 −5 N µ derived
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