A low-mass dark matter search using
ionization signals in XENON100
Andrea Tiseni
on behalf of the XENON collaboration
IDM 2016
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The XENON Collaboration
A. Tiseni
IDM 2016
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XENON Dark Matter Program
XENON10
XENON100
Operation time = 2005-2007 Operation time = from 2008
Xenon mass = 25 kg
Xenon mass = 161 kg
Target mass = 15 kg
Target mass = 62 kg
8.8 ⇥ 10
44
cm
2
@ 100 GeV (2007)
A. Tiseni
XENON1T
XENONnT
See L.
See M. Galloway’s
Grandi’s talk
talk
See L.
Rauch + B.
Pelssers talks
IDM 2016
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Slite
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CDM
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XE 2014
LUX 015
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Su
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Low mass
WIMP
region
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WIMP−nucleon cross section ( cm )
WIMP−nucleon cross section ( zb )
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This Talk: XENON100 low mass
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A. Tiseni
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mWIMP ( GeV/c )
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D. S. Akerib et al. (LUX), Phys. Rev. Lett. 116, 161301 (2016)
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IDM 2016
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XENON100: Detection Principle
A. Tiseni
Scintillation Signal (S1)
Ionisation Signal (S2)
IDM 2016
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XENON100: Position Reconstruction
S2
time difference
S1
Light pattern in the top PMT
array.
Computed and cross-checked by
• Neural network algorithm
• χ2 minimization
• Support Vector Machine
Z position from the time difference
between S1 and S2 signals
z = vdrift x dt ; vdrift ≈ 1.74 mm/µs
Resolution σz < 0.3 mm
Resolution σX,Y < 3 mm
A. Tiseni
IDM 2016
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•
Do not require the S1 signal for an
event. Use the S2 as energy estimator
S2-only
features
Event Rate [(keV x day X kg)-1]
Ionisation (S2)-only analysis
lower energy
threshold
No Z
coordinate
Recoil Energy [keV]
expected S2 [PE]
No ER/NR
discrimination
WIMP spectrum,
mass = 7 GeV
σ=10-41 cm2
mean standard
analysis threshold
100% trigger
efficiency
Our threshold:
80 PE, 0.7 keV
A. Tiseni
E. Aprile et al. (XENON100), arXiv:1605.06262 (2016)
Recoil Energy [keV]
IDM 2016
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WIMP-nucleon cross section (cm2)
Analysis concept
• Background (electronic recoils) dominated
analysis
claim 1
• We can challenge claims or improve at low
masses.
• Assume all events that remain comes from
WIMPs
claim 2
• Set a limit.
WIMP-nucleon cross section (cm2)
Wimp mass (GeV)
A. Tiseni E. Aprile et al. (XENON100), arXiv:1605.06262 (2016)
claim 1
limit
claim 2
Wimp mass (GeV)
IDM 2016
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Data selection (I)
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WIMPs interact uniformly in the TPC: Radial selection (R < 13.4 cm).
•
If there is an S1: Z-cut (1.9 cm below liquid gas interface + 0.5 cm above the cathode)
and NR S2/S1 selection.
•
Single scatter + noise selection.
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WIMPs interact ~uniformly with time: 10 ms cut after an event.
increased rate
after an event
Photo-ionization on metal surfaces
can cause small S2 signals
appearing up to milliseconds
after an event
E Aprile et al. (XENON100), J. Phys. G: Nucl. Part. Phys. 41 (2014) 035201
A. Tiseni
E. Aprile et al. (XENON100), arXiv:1605.06262 (2016) IDM 2016
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Data selection (II)
S2 signal can be produced
by an interaction in the gas
between the anode
and the top PMT screening electrode
Top PMT array
electrode
anode
electrode
AmBe
Neutron-induced nuclear recoil calibration
(AmBe) is used to model the S2 light
collection between the top and bottom PMT
array
S2 light collection parameter
A. Tiseni
E. Aprile et al. (XENON100), arXiv:1605.06262 (2016)
IDM 2016
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Acceptance + trigger efficiency
WIMP
detection efficiency
50% at 80 pe
Threshold
Exposure = 30 kg x year
Data used = RUN II (2011-2012)
A. Tiseni
Selection
Events in Search
data
S2>80 PE + Radial
cut
537398
Z cut + NR
103914
Noise Cut
57516
Single Scatter + 10
ms cut
49041
S2 light collection
13560
E. Aprile et al. (XENON100), arXiv:1605.06262 (2016)
IDM 2016
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Energy Conversion Model
S2 = Y ⇥ Qy ⇥ E
How many photoelectrons
per electron
we see in XENON100?
Y = 20 pe/e-
How many electrons per keV
of recoil energy?
We use the Bezrukov model
with 0.7 keV cut-off
Y
Model chosen
Cut-off
E Aprile et al. (XENON100), J. Phys. G: Nucl. Part. Phys. 41 (2014) 035201
A. Tiseni
E. Aprile et al. (XENON100), arXiv:1605.06262 (2016)
IDM 2016
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Limit Calculation
•
Comparison of the WIMP S2 spectrum with the search data S2 spectrum
after cuts. Exclusion of the cross section that would cause more events
than what we see.
•
Optimum interval method used to chose in an unbiased way the interval
for the exclusion. S. Yellin, Phys.Rev. D66 (2002) 032005
mass = 6 GeV
σ=1.5 x 10-41 cm2
A. Tiseni
E. Aprile et al. (XENON100), arXiv:1605.06262 (2016)
IDM 2016
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Results
DAMA/LIBRA
CRESST-II
CDMSlite
CDMS-II
XENON10
XENON100 this work
improvement
LUX
A. Tiseni
XENON100
SuperCDMS
E. Aprile et al. (XENON100), arXiv:1605.06262 (2016)
IDM 2016
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Conclusions
•
Low mass WIMP search with XENON100.
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Improvement below 7.5 GeV compared to S1-S2 analysis.
•
Similar analysis in XENON1T?
Thank you for your attention!
A. Tiseni
IDM 2016
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A. Tiseni
IDM 2016
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How to improve in the future: XENON1T
A. Tiseni
•
No bare Cu
surfaces
compared to
XENON100
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Better
purification
system
IDM 2016
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Optimum interval
•
Optimum interval is an
extension of the maximum gap
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Consider a gap with n events
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Choose the gap that most
strongly indicates that the
proposed cross section is too
high
•
Set a limit based on this best
gap
Events
A. Tiseni
S. Yellin, Phys.Rev. D66 (2002) 032005
IDM 2016
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