Tarek Saab

Transcription

Tarek Saab

Dark Matter Searches
with the CDMS II Experiment
Tarek Saab
University of Florida
PPC 2013
July 9, 2013
The SuperCDMS Collaboration
Fundamentals of WIMP
Direct Detection particle nuclear
theory
Interaction Rate
[events/keV/kg/day]
o
'
2
4mr
fA
Large enhancement for heavy nuclei targets
⇡
m mN
mr =
m + mN
“Reduced mass” of WIMP-nucleus system
ER mN Ro2 /3
2
2
vmin /vo )
T (ER ) ' exp(
p
vmin = ER mN /(2m2r )
Tarek Saab - PPC 2013
2
dR
o F (ER ) ⇢o T (ER )
p
=
2
dER
m
mr
vo ⇡
2
F (ER ) ' exp
structure
local properties
of DM halo
“Form factor” (quantum mechanics of
interaction with nucleus)
Integral over local WIMP velocity distribution
Minimum WIMP velocity required to produce
a given recoil energy ER
3
Starting with very low
differential interaction rates ...
2
Differential Rate @druD, mc = 100 GeVêc2 , s = 1.2¥ 10-45 cm-45
2
dR/dE
[counts/100
kg/keV/year]
for
m
=
100
GeV/c
&
!=1x10
cm
r
!
dRêdEr @countsê10kgêkeVêyearD
dR/dEr
0.1
1.0
0.01
0.1
0.01
0.001
0.001
10-4
0
Ge
Ar
Si
analysis threshold
Xe
20
40
60
4
80
Ne
Er @keVD
100
Er [keV]
… we get threshold dependent
total interaction rates …
Total
rate
[counts/100
as a2 ,function
of -45 cm2
Total
Rateintegrated
for different
thresholds,
mc kg/year]
= 100 GeVêc
s = 1. ¥ 10
RHEthreshL
@countsê10kgêyearD
threshold
energy for m! = 100 GeV/c2 & !=1x10-45cm2
Xe
Ge
1.00
Ar 0.50
R(Er) Si
Ne 0.10
0.05
0
10
20
5
30
40
Ethresh @keVD
Ethreshhold [keV]
and strongly mass (mχ) dependent
total interaction rates …
Total
integrated
ratethresholds,
[counts/100
as 2a, function
of-45 cm2
Total
Rate
for different
mckg/year]
= 20 GeVêc
s = 1. ¥ 10
RHEthreshLthreshold
Xe
Ge
Ar
Si
10.0
1.00
5.0
0.50
R(Er) Ne
1.0
0.10
0.05
0.5
0
10
20
6
30
40
Ethresh @keVD
Ethreshhold [keV]
… making knowledge of the energy scale
and efficiency at threshold very critical.
Total
integrated
ratethresholds,
[counts/100
as 2a, function
of-45 cm2
Total
Rate
for different
mckg/year]
= 10 GeVêc
s = 1. ¥ 10
RHEthreshLthreshold
Xe
Ge 10.0
1.00
Ar 0.50
5.0
Si
R(Er) Ne
1.0
0.10
0.05
0.5
0
10
20
7
30
40
Ethresh @keVD
Ethreshhold [keV]
… making knowledge of the energy scale
and efficiency at threshold very critical.
Total
integrated
ratethresholds,
[counts/100
as 2a, function
of-45 cm2
Total
Rate
for different
mckg/year]
= 10 GeVêc
s = 1. ¥ 10
RHEthreshLthreshold
Xe
Ge 10.0
1.00
Ar 0.50
5.0
Si
R(Er) Ne
1.0
0.10
0.05
0.5
0
0
10
20
7
30
40
Ethresh @keVD
Ethreshhold [keV]
The CDMS II Detectors
Ge/Si
Anatomy of a particle interaction
✤
Electrons and holes, created by a particle interaction, are drifted
across the crystal by an electric field of a few V/cm
0V
Holes
Electrons
+3V
Ionization Sensors
9
✤
High frequency “prompt” phonons are created and begin to
propagate diffusively.
Phonon Sensors
0V
prompt phonons
+3V
10
✤
Luke phonons are created by the drifting charges, adding to the total
energy of the system: ELuke = e "V Neh
Ptot = Erecoil + ELuke
Phonon Sensors
0V
prompt phonons
Luke phonons
+3V
11
The ZIP detector
✤
Four independent phonon readout channels are instrumented on one
surface, and two independent ionization channels are instrumented
on the other.
y
0V
1 cm
±3V
PA
PD
x
PC
4 Phonon channels
PB
z
Qo Qi
2 Ionization channels
7.6 cm
12
Phonon Readout
Al
Al
Al
Al Fins
Ge/Si Crystal
W TES
Broken Cooper Pairs
Egap Al
Egap W
R [W]
Phonons
3
2
1
0
40 60 80 100 120
T [mK]
13
Electron Recoil Discrimination
Calibration γs
Normalized Yield
30
Surface βs
20
10
Signal Region
0
-10
-20
Charge threshold
Calibration neutrons
14
-10
0
10
Normalized Timing Parameter (μs)
20
The CDMS-II Experiment
The ZIP Detectors
✤
Z-sensitive Ionization and Phonon
mediated
✤
Ge (~230 g) or Si (~106 g) crystals:
1 cm thick, 7.5 cm diameter
✤
Photolithographically patterned to
collect athermal phonons and
ionization signals
✤
Direct xy-position imaging
✤
Surface (z) event rejection
from pulse shapes and timing
✤
30 detectors stacked into
5 towers of 6 detectors
7.6 cm (3”)
1 cm
15
✤
✤
Surface events: 0.82
✤
Cosmogenic neutrons: 0.04
✤
Radiogenic neutrons: 0.057
Probability of 2 or more
leakage events : ~23%
ts
re
su
l
“o
ld
”
Estimated backgrounds:
Recoil Energy (keV)
ar
e
✤
Equivalent to 194.1 kg-d for a
60 GeV/c2 WIMP (10 -100
keV analysis energy range)
Candidate 2
Tower 3, det 4 (T3Z4)
Aug 5, 2007
se
✤
Normalized Ionization Yield (σ)
612 raw kg-days.
Candidate 1
Tower 1, det 5 (T1Z5)
Oct 27, 2007
Th
e
✤
True Ionization Yield
Results from the
Ge detectors
16
Normalized Timing Parameter (µs)
Recent results from the
Si detectors
60
Data set:
✤
2
n
Estimated background from neutrons
✤
✤
140.23 kg-days in 8 Si detectors
accumulated between
July 2007 - Sep. 2008
< 0.13 expected events
Neutrons
40
−
✤
2
✤
20
0
Surface Events
−20
20
40
60
Recoil Energy (keV)
80
Estimated background from surface events
✤
0.47 events based on
✤
Phonon timing discrimination, optimized in 3 energy bins:
7-20, 20-30, 30-100 keV
17
100
Data selection efficiency
WIMP Efficiency
1
0.8
0.6
0.4
0.2
0
0
20
100% Efficiency
+Good Nuclear Recoil
+Fiducial Volume
+Phonon Timing
40
60
80
100
Recoil Energy (keV)
18
The Si Data: Before the timing cut
Shades of blue indicate the three separate timing cut energy ranges.
7-20 keV
19
20-30 keV
30-100 keV
The Si Data: After the timing cut
20
The Si Data: Yield vs Timing
25
Normalized Yield
20
15
10
Surface Event
Distribution
5
0
Neutron
Distribution
−5
−10
−4
−2
0
2
4
Normalized Timing
21
6
8
The Si Data: Yield vs Timing
25
Candidate 1
Candidate 2
Candidate 3
Normalized Yield
20
15
10
Surface Event
Distribution
5
0
Neutron
Distribution
−5
−10
−4
−2
0
2
4
Normalized Timing
21
6
8
The Si Data: Three events
Detector T4Z3
25
Normalized Yield
20
Candidate 1
Candidate 2
Candidate 3
15
10
5
Surface Event
Distribution
0
Neutron
Distribution
−5
−10
−2
0
2
Normalized Timing
4
Detector T5Z3
25
Normalized Yield
20
15
10
5
Surface Event
Distribution
0
Neutron
Distribution
−5
−10
−2
22
0
2
Normalized Timing
4
ST
10-41
10-5
DAMA
10-42
10-6
N1
00
CD
EDE
LWE
I
MS
SS L
T
CRE
SST
II G
e
Optimal gap analysis sets a
10-43
5 6 7 8 9 10
15
20
30
limit for the SI cross-section of
2.4x10-41cm2 for a WIMP mass of 10 GeV/c2.
WIMP Mass @GeVêc2 D
Probability of background fluctuation resulting in
three or more events anywhere in the signal region is 5.4%.
10-7
40 50
✤
A profile likelihood analysis favors a WIMP+background hypothesis over the known
background estimate at the 99.81% confidence level (~3!, p-value: 0.19%).
✤
The maximum likelihood is at a WIMP mass of 8.6 GeV/c2 and cross-section of 1.9x10-41cm2.
We do not believe this result rises to the level of a discovery,
but does call for further investigation.
23
✤
WIMP-nucleon cross section @pbD
ES
NO
WIMP-nucleon cross section @cm2 D
CR
XE
✤
10-4
S2
✤
10-40
0
N1
Three events observed in a
140.23 kg-day exposure with an
expected background of
< 0.7 events.
DAMA
NO
✤
10-3
XE
so, … what
are they?
10-39
A Closer Look
DAMA
10-40
10-3
10-4
CR
N
XE
ES
1
ON
ST
0
S2
10-41
10-5
DAMA
NO
10-42
10-6
XE
EDE
LWE
I
0
N1
CRE
SST
SS L
T
0
10-39
CD
MS
II G
e
10-43
5
6 7 8 9 10
15
20
24
30
10-7
40 50
What do we
do next?
✤
SuperCDMS @Soudan
✤
✤
iZIP’s solve the problem of surface
events observed in CDMS II
iZIP
Installation complete Nov. 8, 2011
✤
9 kg of Ge iZIP operating since
March 2012
✤
Expected to improve sensitivity to
SI WIMP-nucleon interactions by
a factor of ~10 over CDMS II
results after 3 years of operation.
25
Crystal Bulk
h+
-2V
e0V
Vacuum
Surface Rejection with the iZIPs
✤
Since March 2012, 87,723 events (71,525 #s & 16,258 Pb recoils) were collected during
the current Soudan run. Using information only from the charge channel we have a
demonstrated rejection of < 1.7×10-5 with a 50% cut efficiency.
Surface electron recoils
(from Pb decay #s)
206Pb
26
recoils
<0.6 surface events over a 0.3 ton-year
exposure. Good enough for a 200 kg
experiment run for 4 years at SNOLAB!
Using Ge detectors to investigate
low mass WIMPS
✤
CDMSlite
✤
Low mass searches require a lower energy threshold.
This requires amplifying the signal without increasing the noise.
✤
e/h have “terminal velocity”corresponding to ~30 meV KE
✤
Under bias, O(10 eV) ≫ 30 meV.
This “excess energy” is radiated as
Luke phonons: ELuke = (e "V) Neh
0V
prompt phonons
Luke phonons
✤
Noise is independent of "V
+3V
27
CDMSlite
in action
✤
Operating with a voltage bias of 69 V:
✤
Detector calibration with 252Cf
produces activation lines in Ge of
1.3 keV and 10.4 keV
✤
Lines observed with an energy resolution of 3.3% and 1.3%,
indicating successful amplification of phonon signal.
✤
We pay the price by giving up electron vs. nuclear recoil discrimination
✤
Expect to achieve a 170 eVee threshold (& hope to make it to 85 eVee)
28
Reach of SuperCDMS @ Soudan
CDMSLite
&
II Ge
DM
-lit
DAMA
70
T
SL
e1
T
eL
II G
erC
MS
10-4
lite
ee
eV
S85
ee
eV
10-3
CDMS
Sup
CD
M
CD
10-40
MS
CD
10-39
SuperCDMS LT
Co
CR
E
Ge
NT
SS
T
10-41
10-5
SuperCDMS Soudan
Projections:
CR
E
10-6
threshold, ~170 eVee CDMSlite
Anticipated Fall: iZIP low threshold,
~2 keVnr analysis
3
4
5
6
SS
T
10-42 Anticipated Summer: first ultra-low
10-43
2
✤
7 8 9 10
29
15
20
10-7
30
So, when are we going
to see WIMPs for sure?
✤
To borrow a phrase from a very patient man:
“The strongest of all warriors are these two — Time and Patience.”
% Leo Tolstoy, War and Peace
In others words, maybe soon,
…… or maybe not.

Tarek Saab

Transcription

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