9.4 The 2014 data taking campaign
9.4.4 Coupling constant limit with the 2014 vacuum data
As stated before, in 2014 the CAST operated with vacuum in the magnet bores. The data taking was accomplished in two phases. In the first one, from 3rd of July to 25th of August, only the sunset detectors were operative; being the sunrise line in preparation for the telescope plus new Micromegas upgrade reported before. The data taking stopped on 25th August for carrying out this upgrade in one of the sunrise ports of the CAST magnet. This operation required uninstalling the sunset detectors for calibrating the optics with the laser beam from the other side of the magnet.
The physics data taking resumed on 11th of September, this time only with the sunrise Micromegas detector. The sunset ones remained uninstalled waiting for the installation in the remaining port of the sunrise side of the second telescope (ABRIXAS) and the InGrid detector. These works were finalized on mid-October, and subsequently, the sunset Micromegas were installed, resuming the data taking on 20th of October, now with all the four x-ray detectors acquiring data, until the end of the season, on 17th of November.
The sunset Micromegas performed a total of 78 evening solar trackings, for a total exposure of
9.4. The 2014 data taking campaign 151
Energy (keV)
0 1 2 3 4 5 6 7 8 9 10
Number of events
0 1 2 3 4 5 6 7 8 9 10
103
×
13% FWHM CAST-2014
Sunrise MM 55Fe source
Figure 9.22: Left: 55Fe calibration spectrum of the sunrise Micromegas detector. The main peak has been fitted to two gaussian functions (blue and magenta lines), corresponding to the Kα (5.9 keV) and Kβlines (6.4 keV). Right: gain uniformity of the sunrise Micromegas detector. The dead areas (in purple) show lower values than the unity (in green) and lie outside the axion-sensitive area.
Figure 9.23: Left: evolution of the gain and the energy resolution of the detector along the data-taking. The fluctuations, measured both at the mesh and at the strips, are less than 10%. Right:
A 2D hitmap of background events in the calibration area (outer circle) with energies between 2 and 14 keV, where the two populations of muon-induced events (muons) and the rest (non-muons) are indicated in red and blue respectively. The inner circle marks the inner circumference of the spider-web.
Figure 9.24: Left: Background energy spectra in the calibration area. The level achieved after the strips cuts (in blue) is (1.6 ± 0.2) × 10−6 keV−1cm−2s−1, and after the veto cut (in red) it is reduced to (1.0 ± 0.2) × 10−6 keV−1cm−2s−1. The total background time is 1449.2 hours. Right:
a zoom on the spectra after the analysis; the peak at 8 keV is due to fluorescence in the copper materials of the entrance pipe and the detector and the one at 3.2 keV corresponds to the escape peak of argon.
118.0 hours per detector, with a loss of 5.1 hours due to detector death time produced by electronic noise, while the sunrise Micromegas performed 51 solar trackings, for a total of 69.8 hours.
In table9.5, the results in terms of background level of the three Micromegas detectors during the 2014 CAST data taking campaign are shown. While sunset 1 level is compatible with the one measured in 2013, sunset 2 level is reduced significantly (∼30%) due to the detector replacement, as can be noticed by comparing with table9.1.
Time Level [2− 7] keV
Detector Efficiency Background Tracking Background Tracking 3 & 6 keV (hours) (10−6 keV−1cm−2s−1) Sunrise 75 & 75 1449.2 69.8 1.0† (0.8)± 0.2 (0.1) —‡ Sunset 1 75 & 75 1854.0 118.0 1.03± 0.05 0.94± 0.17 Sunset 2 75 & 75 1819.6 118.1 1.05± 0.05 0.87± 0.17 Table 9.5: Summary of background and tracking levels of Micromegas detectors in CAST 2012 data taking campaign. †background level defined in the detector area illuminated by the55Fe source. In parenthesis, the level in the projection of the whole cold bore area. ‡ zero tracking counts in the focusing spot.
The levels measured during axion-sensitive periods are also shown in table 9.5 It is found that background and tracking levels are compatible within one standard deviation in both sunset detectors. On the other hand, zero counts are found in the spot and in the energy RoI in the sunrise Micromegas detector, while 0.38 counts were expected.
It must be noted at this point that our definition of the focusing spot relies on the spot-calibrations performed during the x-ray finger runs, and the efficiency (throughput) of the optics is defined from simulations (see figure 9.25, left). Both telescope parameters will be accurately measured at the end of its operation in CAST. Ideally, the optics should have been calibrated before its installation in CAST, but the lack of free time spots in the schedule of the PANTER x-ray test facility forced its prior installation.
Figure 9.25: Left: efficiency of the x-ray telescope as a function of the energy. Right: spot-calibration hitmap in the xy-strips plane. The contours represent two possible definitions of the spot, the inner one with around 80% signal acceptance and the second with around 92% acceptance.
In this work, wee have conservatively taken the outer contour of figure9.23right, which encom-passes approximately the 92% of the x-ray events from the source (91.9 (1.3)% of the x-ray finger run number 1, 92.8 (1.7)% of run number 2, 92.2 (1.2)% of run number 3 and 91.7 (1.2)% of run number 4). The actual axion signal region is smaller than in our definition, since the x-ray finger is placed at a finite distance from the optics and the emission is not a perfectly parallel beam.
No axion signature is found in the 2014 data, so an upper limit on the axion to photon coupling strength as a function of the axion mass, gaγ(ma), is extracted (figure 9.26). The contributions of the three detectors and the combined result are plotted in the figure and the exact values