Searching for Dark Matter:New results from the XENON100 exper iment
Boris BauermeisterJohannes Gutenberg Universität Mainz
24.09.122nd Annual Retreat/Bad Kreuznach
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Outline
● Short introduction into Dark Matter● The XENON Dark Matter Project
● XENON100 – The experiment● XENON100 – Data analysis● XENON100 – Results
● The next level: XENON1T● Summary
Boris Bauermeister: New results from XENON100
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Bullet Cluster
Short introduction to Dark Matter (I)
Rotation curve
Boris Bauermeister: New results from XENON100
CMB (WMAP)
Theory: WIMPs are a promising candidate for Dark Matter – Weakly interacting massive particlesfrom small to large scales
Evi
de
nts
fo
r D
ark
Ma
t te
r: ● Measuring Rotation curves of Galaxies● A famous example: Bullet Cluster
● Determine the mass indirectly by gravitational lensing effect● Determine the mass from the luminosity (stars)● Determine the mass of the gas (X-ray)
→ A further component can be found: Dark Matter● Cosmic Microwave Background (WMAP, Planck)
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Short introduction to Dark Matter (II)
Three (independent) ways of Dark Matter detection:
Annihilation:Indirect detection by lookingfor annihilation products.e.g. ICECUBE
Production:e.g. in accelerators (LHC)→ Missing energy
Scatter:Direct detection by measuringthe nuclear recoil of an atom.e.g. XENON
Boris Bauermeister: New results from XENON100
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The XENON Dark Matter Project
15 institutes worldwide participatein the XENON collaboration
The XENON experiment is located at theGran Sasso National Laboratory (LNGS)in Italy.
Collaboration meeting April 2012
Boris Bauermeister: New results from XENON100
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XENON100 (I)
Direct Dark Matter detection in noble liquids:
→ Measuring energy spectrum of elasticDM-nucleus scattering at xenon atoms:
● ρ = 3 g/cm3
● Self-shielding properties● High mass number for high WIMP rate● 161 kg used in the XENON100 experiment● Low intrinsic radioactivity
→ Measuring light and charge with photomultiplier-tubes (PMTs)
● 242 PMTs are used for XENON100
→ PMT arrays design for charge and light detection!
Top array: 98 PMTs
Bottom array: 80 PMTs
XENON100 timeprojection chamber
PTFE Shield(Mirror for UV light)
Boris Bauermeister: New results from XENON100
Veto array: 64 PMTs
arXiv:1207.3458 [astro-ph.IM]
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XENON100 (II)
Two phase time projection chamber (TPC):
● WIMP/neutron hits a xenon atom● Excited and ionized xenon atom
● Scintillation light (178 nm) is produced by dis-excitation
● S1 signal● Observed at top and bottom array
● Recombination of the electrons is suppressed by an electric field between the cathode and the gate mesh ( ~530 V/cm)
● Electrons drift upwards● Electrons are extracted from the liquid to the gaseous phase
● Higher electric field accelerates electrons● Proportional scintillation● S2 signal at the top array
gate mesh
Boris Bauermeister: New results from XENON100
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XENON100 (III)
S1 Hitpattern
S2 Hitpattern
Hitpattern in the XENON100 TPC● X/Y resolution ~3 mm
Signal discrimination:● Electron drift velocity (v
d = 1.73 mm/µs)
separates S1 and S2 signal● Z resolution ~3 mm
● S2/S1 ratio determine electronic recoil (ER) and nuclear recoil (NR)● Background rejection: Double scatter events with Δz < 3 mm
Waveforms of S1 and S2 hitpattern
Boris Bauermeister: New results from XENON100
Full 3D reconstruction!
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XENON100 (IV)
Overview about datasets:
Dark Matter data
2011/2012
225 live days
Blind analysis
NR calibration dataAmBe
At the begin and endof the run (5.6 days)
ER calibration dataTh232 and Co60
twice a week Electron lifetimeCs137
once a week
PMT Calibration(LED)
once a week
Data analysis
Boris Bauermeister: New results from XENON100
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XENON100 (V)
Background in XENON100:
NR background:→ Muon introduced neutrons from above/rock→ Neutron and α background from detector
components & shieldER background:→ Radioactivity (β/γ) from detector components.→ Krypton (85Kr) contamination in xenon (β/γ)→ Radon contamination
Boris Bauermeister: New results from XENON100
→ Screening of all detectorand shield components(arXiv:1103.5831 [physics.ins-det])
→ Monte Carlo simulation ofXENON100 background(E. April et al. (XENON100)Phys. Rev. D83: 082001, 2011)
→ natKr in xenon is lowered to19 ppt by cryogenic distillationin compare to the last run
Background energy spectrum:
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XENON100 (VI)
Boris Bauermeister: New results from XENON100
60Co and 232Th (gammas)
AmBe (neutrons)
Detector calibration: AmBe, 60Co and 232Th
Background prediction:
Neutron source:→ Calibration with 241AmBe source (begin/end of
the run)→ Geant4 simulation with XENON100 geometry
and detector and shield screening→ Expected:
Gamma source:→ Calibration with 60Co and 232Th weekly→ 35 times more statistics than in Dark Matter data→ Expected:
0.17−0.07+0.12 events
0.79±0.16 events
Total Background: (1.0±0.2)events
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XENON100 (VII)
Signal and background:● XENON100 did a blind analysis● Event discrimination by
● ER/NR separation:
● Defined benchmark region:● S1 within 3 – 30 pe ● S2 threshold cut (S2 > 150 pe)● 99.75 % ER rejection line
● Event rejection by defining a 34 kg ellipsoid● Events from self-shielding excluded
Electronic recoilNuclear recoil
S2 thresholdS1 threshold
Boris Bauermeister: New results from XENON100
(S2S1
)α ,n
≪(S2S1
)e−,
γ
arXiv:1207.5988 [astro-ph.CO]
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XENON100 (VIII)
Profile Likelihood analysis [2]:● S1 range from 3 – 30 pe● Signal region above 97% NR quantile
Boris Bauermeister: New results from XENON100
Open the box:After unblinding 2 events are found in the benchmark WIMP search region!
● Both events are located between 3 pe to 4 pe● Waveforms are of high quality● Both events at the lower edge of the NR band● Probability that 2 events fluctuate over the backgroundexpectation is 26.4%
NO WIMP SIGNAL!!!
arXiv:1207.5988 [astro-ph.CO]
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XENON100 (IX)
New results:
Spin-independent WIMP-nucleon cross-section: σ = 2 x 10-45 cm2 @55 GeV/c2 (90% C.L.)
Boris Bauermeister: New results from XENON100
arXiv:1207.5988 [astro-ph.CO]
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XENON1T: The next level...
Boris Bauermeister: New results from XENON100
Need to increasing the sensitivity!
Construct a new detector!
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XENON1T: The next level...
More is better:• TPC size increases to ~1 m3
• Three tons of liquid xenon (1t fiducial mass)
• Water Cherenkov Muon Veto:• Water tank will have a diameter of ~9.6 m and a
height of 10 m• 84 PMTs will be installed (active veto)
• Background 100 times lower than in XENON100• Less than 2 events per year
• Improve sensitivity: σ = ~2 x 10-47 cm2
• Need more space: Hall B at the LNGS
• First results in 2017 expected!
Boris Bauermeister: New results from XENON100
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Summary
Analysis with 2011/2012 data:
→ In 225 days are 2 events observed in thesignal region where 1 event is expected!
→ No Dark Matter found!
→ The lowest exclusion limit for Dark Matter!
→ XENON100 postulate a new exclusion limit of σ = 2 x 10-45 cm2
(@55 GeV/c2 (90% C.L.) for spin-independent WIMP-nucleusscattering.
→ XENON1T construction will start at the end of 2012 (Hall B@LNGS)Its goal is a sensitivity of σ = 2 x 10-47 cm2
Boris Bauermeister: New results from XENON100
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Questions?
Boris Bauermeister: New results from XENON100
Supported in part byGRK Symmetry Breaking(DFG/GRK 1581)
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Backup Slides
Boris Bauermeister: New results from XENON100
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Backup (I)
Krypton contamination:→ natKr concentration in xenon is lowered to
19+-4 ppt by cryogenic distillation in run10.(Measureded by rare gas mass spectrometry)
→ From the decay chain: 85Kr → 85mRb → 85Rb (β-γ coincidences)18+-8 ppt
β
γ
Example for a β-γ coincidences(arXiv:1207.3458v2 [astro-ph.IM])
Boris Bauermeister: New results from XENON100
Cryogenic distillation in XENON100
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Backup (II)
Boris Bauermeister: New results from XENON100
Observed event 1S1 Peak
S2 Peak
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Backup (III)
Boris Bauermeister: New results from XENON100
S1 Peak
S2 Peak
Observed event 2
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Backup (IV)
Boris Bauermeister: New results from XENON100
For Leff
= 0 for E < 3 keVnr
Within the relevant masses theimpact on the limit is below 5 %(mχ>6GeV /c2
)
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