Direct dark matter searches and
low mass WIMPs
C. Nones
CEA/DSM/IRFU/SPP
1
Outline
 Dark matter candidates → (low-mass) WIMP
 Direct detection of WIMPs in general
 Direct detection of WIMPs in the low-mass case
 Signatures and background
 Detector classification and experiment overview
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What do we know about DM?
• The mass and cross section range span many orders of magnitude
• Experimentalists are strongly guided by theorists
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What do we know about DM?
• The mass and cross section range span many orders of magnitude
• Experimentalists are strongly guided by theorists
I will focus on WIMPs (mainly low mass)
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How to detect WIMPs
FERMI-GLAST
LHC
Man-made COLLIDER
production
Indirect detection
Relic annihilation in the
cosmo
Relic WIMP-nucleon
scattering
DIRECT
DETECTION
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Direct WIMP detection: the principle
Detection of the energy
deposited due to elastic
scattering off target nuclei
• Elastic scattering of a WIMP deposits small
amount of energy into recoiling nucleus
(~ few 10s of keV)
• Expected rate:
< 1 interaction per kg per year
• Radioactive background of most materials
gives higher rate
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Scattering rate
Spin-dependent scattering scales with the spin of the
nucleus. Spins of Individual nucleons can cancel
NO COHERENT EFFECT!
Heavy nucleai have the advantage!
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Scattering rate
Spin-dependent scattering scales with the spin of the
nucleus. Spins of Individual nucleons can cancel
NO COHERENT EFFECT!
Heavy nucleai have the advantage!
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Scattering rate for a heavy WIMP
(100 GeV)
Implications for
experimentalists:
1. Need of O(100) kg detector
running O(1) year to see one
event !
2. Rates are higher the lower
your threshold
3. Huge natural background
from natural radioactivity at
these rates and energies.
Scattering rate for a light WIMP
(10 GeV)
Lighter targets and lower thresholds are better
when you look for WIMPs with mass < 10 GeV
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Dark matter signature
① Rate and shape of recoil spectrum depend on target material
② Motion of the Earth causes:
(a) Temporal variation in
the rate: June-December
rate asymmetry: 2-10 %
(b) Direction modulation
asymmetry: 20-100% in
forward-backward event rate
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Typical backgrounds
13 12
Managing backgrounds
Active muon veto: reject event from cosmic rays
Use passive shieldings:
Pb (shielding from radiocativity gammas) /
Polyethyene: moderate neutrons produced
from fission decays and from (α,n) interactions
resulting from U/Th decays
Use clean materials
Since you have O(106) more electron recoils than expected WIMP scatters in your detector,
make sure to have some ability
to distinguish ER from NR
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Managing backgrounds
Active muon veto: reject event from cosmic rays
Use passive shieldings:
Pb (shielding from radiocativity gammas) /
Polyethyene: moderate neutrons produced
from fission decays and from (α,n) interactions
resulting from U/Th decays
Use clean materials
Since you have O(106) more electron recoils than expected WIMP scatters in your detector,
make sure to have some ability
to distinguish ER from NR
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Current and future experiments
Need at least 1000 m rock overbuden
Site experiment underground
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Many technologies…
NEWS
DAMIC
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Many technologies…
NEWS
DAMIC
My selection
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Why do we need so many
experiments?
Different targets and different technologies have different advantages in
background rejection, WIMP scattering sensitivities and ease of construction or
operation
The nature of dark matter and its interarctions with standard model particle is
unknown (maximazation of exploration of possible candidates over a broad range of
theoratical frameworks)
Direct detection is hard
• Background control
• Detailed understanding of instrumentation
• Overlapping sensitivities allow experiments to cross-check each other
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The WIMP SI-landscape
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Low mass region
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The DAMA/LIBRA low-mass region
250 kg target made of ultra-radio-pure NaI crystal scintillators – located in LNGS
The DAMA/LIBRA signal remains robust and generally consistent with a dark
matter interpretation (period = 1 year, phase = June 2 ± 7 days)
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“Standard” allowed regions for the DAMA/LIBRA signal
Scattering off Na
Scattering off I
Analysis in Phys. Rev. D 84, 055014 (2011)
A0 halo model
v0 = 170 km/s
ρo = 0.18 GeV/cm3
With channeling
With energy-dependent quenching factor
Analysis in Phys. Rev. D 84, 055014 (2011)
A0 halo model
v0 = 270 km/s
ρo = 0.45 GeV/cm3
With channeling
With energy-dependent quenching factor
Dual phase Xe detector
Leading the field in SI sensitivity and SD coupling to neutrons
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Dual phase Xe detector
Not ideal for low masses
The exclusion curves are very steep in the low-mass side of the excluded region
Extending sensitivity below 5 GeV is problematic
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Cryogenic bolometers at T∼ 10 mK
Collaboration between
SuperCDMS and EURECA
(EDELWEISS+CRESST) at
SNOLAB
~ 100 kg target level
EDELWEISS
CRESST
CDMS
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The EDELWEISS-III experiment
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EDELWEISS-III – low mass WIMPs
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EDELWEISS-III projections
R&D on HV (Neganov-Luke amplification)
x100 reduction on Heat-Only events
100 eV (RMS) ion & heat
350 kg-days
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CDMSlite – low ionization threshold
experiment
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CDMSlite – Results
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SuperCDMS SNOLAB – projected
sensitivity
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Exploring low-mass WIMPs with
CRESST
They have seen an excess in CRESST-II
Background reduction mandatory
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TUM40 results & CRESST future
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DAMIC – Dark Matter in CCDs
a novel experiment optimized to explore the low-mass WIMP region with low threshold
Phys. Lett. B 711 (2012) 264
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DAMIC-100 sensitivity
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Radial TPC with spherical proportional
counter read-out
Saclay-Thessaloniki-Saragoza
NEWS
5.9 keV 55Fe
signal
E=A/R2
15 mm
• Simple and cheap
C≈ Rin= 7.5 mm < 1pF
• Large volume
• single read-out
• Robustness
• Good energy resolution
• Low energy threshold
• Efficient fiducial cut
A Novel large-volume Spherical Detector with Proportional Amplification read-out,
I. Giomataris et al., JINST 3:P09007,2008
• Low background capability
NEWS
Search for light dark matter
Detector installed at LSM end 2012: 60 cm, Pressure = up to 10 bar
Gas targets: Ar, Ne, He, CH4
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NEWS: from LSM to SNOLAB
HV
Calibration rod/stringGas/vacuum/recovery
DAQ
Supporting rods
80 mm Cu pipe
8m
Pure copper vessel
Cu Rod with 0.1 mm HV wire
Sensor, 4-10 mm Ø Cu/Si ball
Gas CH4/He/Ne
2m sphere
8m
NEWS-SNOLAB project
Kingston, Saclay, Grenoble, LSM, Thessaloniki…..
2 m detector at 10 bar
Pure water shield
Funded by Canadian grant of excellence
LOI recently approved by SNOLAB committee
NEWS-SNO projections
Background free limits obtained for around 100 kg.d with Ne/He/CH4 , taking into account
anticipated background from materials, with threshold set at 1 electron (ie 30 to 40 eVee),
& quenching factors extrapolated down to 100-200 evNR
Conclusions
 Dark matter → viable candidate to explain
astrophysics observation
 Hunt for (low-mass) WIMPs
 Leading technology not perfectly adapted to the
low-mass case
 Arsenal of methods / experiments to reach the
neutrino floor in the low-mass region
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DAMIC challenges
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