DarkSide Document 1926-v1

Detailed Characterization of Nuclear Recoil Pulse Shape Discrimination in the DarkSide-50 Direct Dark Matter Experiment

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Erin E Edkins
Updated by:
Erin E Edkins
Document Created:
27 Apr 2017, 14:28
Contents Revised:
27 Apr 2017, 14:28
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10 Feb 2018, 13:47
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While evidence of non-baryonic dark matter has been accumulating for decades, its exact nature
continues to remain a mystery. Weakly Interacting Massive Particles (WIMPs) are a well motivated
candidate which appear in certain extensions of the Standard Model, independently of dark matter
theory. If such particles exist, they should occasionally interact with particles of normal matter,
producing a signal which may be detected. The DarkSide-50 direct dark matter experiment aims
to detect the energy of recoiling argon atoms due to the elastic scattering of postulated WIMPs.
In order to make such a discovery, a clear understanding of both the background and signal region
is essential. This understanding requires a careful study of the detector’s response to radioactive
sources, which in turn requires such sources may be safely introduced into or near the detector
volume and reliably removed.
The CALibration Insertaion System (CALIS) was designed and built for this purpose in a joint
effort between Fermi National Laboratory and the University of Hawaii. This work describes the
design and testing of CALIS, its installation and commissioning at the Laboratori Nazionali del
Gran Sasso (LNGS) and the multiple calibration campaigns which have successfully employed it.
As nuclear recoils produced by WIMPs are indistinguishable from those produced by neutrons,
radiogenic neutrons are both the most dangerous class of background and a vital calibration source
for the study of the potential WIMP signal. Prior to the calibration of DarkSide-50 with radioac-
tive neutron sources, the acceptance region was determined by the extrapolation of nuclear recoil
data from a separate, dedicated experiment, ScENE, which measured the distribution of the pulse
shape discrimination parameter, f 90 , for nuclear recoils of known energies. This work demonstrates
the validity of the extrapolation of ScENE values to DarkSide-50, by direct comparison of the f 90
distribution of nuclear recoils from ScENE and an AmBe calibration source. The combined accep-
tance as defined by ScENE and the in-situ AmBe calibration were used to establish the best WIMP
exclusion limit on an argon target. Unfortunately, radioactive sources used for the calibration of
DarkSide-50 are universally accompanied by gamma decays, which obscure the low energy region
where most WIMP interactions are expected to occur and seem to make continuing dependence on
an external measurement such as ScENE inevitable. However, this work presents a novel method
of nuclear recoil calibration employing event selection, unique to the design of DarkSide-50, which
produces a nearly pure sample of nuclear recoils. Further, it describes the execution of a neutron
calibration campaign, from planning to analysis, which yielded a valuable data set for defining the
acceptance region. Together with the event selection techniques, this allows for the definition of the
acceptance region independent of ScENE values. Two analytical models of the f 90 distribution are
described and their results for nuclear recoils are compared. Finally, a detailed study of integrated
noise in nuclear and electron recoil events is presented, which demonstrates a difference between
these classes of events for the first time.
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