Background Model Status and
Improvements for the LUX
Experiment Dark Matter Search
Brian Tennyson
Yale University
On behalf of the LUX Collaboration
LU
APS April Meeting 2015 — Baltimore, MD —12 April 2015
The Large Underground
Xenon Experiment
Low-radioactivity
titanium cryostat
Copper shield
PMT array
Active xenon
volume
59 cm
49 cm
370 kg total xenon mass
250 kg active liquid xenon
118 kg fiducial mass
PTFE reflector
panels and
field cage
PMT array
Copper shield
2
How the LUX Detector
Works
3
Signal and Background in
LUX
•
Dark matter signal we’re looking for: Single-scatter nuclear
recoils (NR) with energy less than 25 keVnr (~5.3 keVee)
•
Our backgrounds: Events that will mimic a dark matter response
in the detector, such as:
•
•
Neutrons that scatter once in the detector
•
Electron recoil (ER) events, statistically leaking (0.4%) into
NR area of parameter space (The focus of this talk)
We construct models for candidate WIMP signals and these
expected backgrounds and use a Profile Likelihood Ratio (PLR)
analysis to achieve our science result
4
ER Background Model
•
•
Needs to include:
•
Sources of gamma radiation near the detector
(i.e. construction materials)
•
Sources of beta radiation and gamma radiation
within the xenon itself (e.g. activated xenon lines)
Values and limits for these activities are reached
by counting activity in the construction material
before construction
5
Sources of activity in model
(mBq/unit)
Units Amount
226R
232Th
40K
60Co
46S
85Kr
127Xe 220Rn 222Rn
PMTs
PMT
122
a
9.5
2.7
66
2.6
c
-
-
-
-
-
PMT Bases
Base
122
1.4
0.13
1.2
0.03
-
-
-
-
-
Reflector Panels
kg
9.3
5.0
1.3
-
-
-
-
-
-
-
Field Shaping Rings
kg
28
0.5
0.8
-
0.3
-
-
-
-
-
Field Ring Supports
kg
33.5
0.5
0.35
-
-
-
-
-
-
-
Field Grids
kg
4.5
1.4
0.23
0.4
1.4
-
-
-
-
-
Grid Supports
kg
15.5
3.0
1.0
-
-
-
-
-
-
-
Cryostats (Ti)
kg
231
0.37
0.8
1.6
-
4.4
-
-
-
-
PMT Mounts (Cu)
kg
169
2.2
2.9
-
1.7
-
-
-
-
-
Copper Shields
kg
414
2.2
2.9
-
1.7
-
-
-
-
-
Weir (Cu)
kg
3.2
0.4
0.2
-
0.17
-
-
-
-
-
Superinsulation
kg
2.2
73
14
640
-
-
-
-
-
-
Thermal Insulation
kg
6.0
130
55
100
-
-
-
-
-
-
Xenon
kg
370
-
-
-
-
-
6
0.0013 0.49 0.007 0.049
How the background model
is generated:
This flowchart represents the
steps taken in constructing
the ER background model.
Energy Deposition Only
Simulations
Each step has can be (and
has been) checked for
consistency.
Optical Simulations
(only events with energy below
20 keVee)
Data Processing Chain
PDFs for PLR
7
Updates to the background
model
•
An earlier version of the background model was constructed for
our original science result in 2013
•
Our work represents an improvement over this model in the
following ways:
•
•
Increased simulated statistics to 10x live time
•
Added step of processing simulated background data
through same data processing chain as real data
These additions to the process increase our confidence in the
model’s ability to accurately predict the backgrounds observed
in the LUX detector
8
Background Model: Xenon
External Backgrounds
118 kg fiducial volume, with an S1 signal < 50 phe
S2
Counts/phe
Counts/phe
S1
10
16
14
8
12
10
6
8
4
6
4
2
2
0
5
10
15
20
25
30
35
40
0
45
50
S1 Area (phe)
0
500
1000
1500
2000
2
Counts/10cm
Counts/cm
50
40
20
10
10
5
20
25
4000
4500
5000
S2 Area (phe)
20
15
15
3500
25
30
10
3000
R2
Z
0
2500
30
35
40
45
Z Position (cm)
9
0
50
100
150
200
250
300
350
400
Radius Squared (cm2 )
Background Model: Xenon
Internal Backgrounds
118 kg fiducial volume, with an S1 signal < 50 phe
S2
Counts/phe
Counts/phe
S1
8
7
10
6
8
5
6
4
3
4
2
2
1
0
0
5
10
15
20
25
30
35
40
0
45
50
S1 Area (phe)
0
500
1000
1500
2000
2500
3500
4000
4500
5000
S2 Area (phe)
R2
Z
2
8
Counts/10cm
Counts/cm
3000
7
6
12
10
5
8
4
3
6
2
4
1
0
10
15
20
25
30
35
40
45
Z Position (cm)
10
0
50
100
150
200
250
300
350
400
Radius Squared (cm2 )
Future Work
•
•
For our re-analysis of the first science run:
•
Comparing to data and updating the model as need.
•
Continue to scrutinize and perform more checks on the model
•
(If time allows) Increase the simulated background statistics
•
Expand energy range to allow for analysis to include of nonWIMP dark matter signals
In preparation for analysis of our ongoing second science run:
•
Understand how activities in the detector will change (decay
away of activated Xenon, for example)
11
Summary
•
Signals and backgrounds in the LUX Detector
•
ER background model generation
•
Improvements to the model
•
Extension of background model for further analysis
and second science run
12