searching for dark matter: new results from the xenon100...

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


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


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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


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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)


Veto array: 64 PMTs

arXiv:1207.3458 [astro-ph.IM]

http://arxiv.org/abs/1207.3458

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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


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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


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


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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


→ 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)


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


(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


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!!!



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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.)




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XENON1T: The next level...


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!


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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


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Questions?


Supported in part byGRK Symmetry Breaking(DFG/GRK 1581)

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Backup Slides


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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])


Cryogenic distillation in XENON100

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Backup (II)


Observed event 1S1 Peak

S2 Peak

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Backup (III)


S1 Peak

S2 Peak

Observed event 2

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Backup (IV)


For Leff

= 0 for E < 3 keVnr

Within the relevant masses theimpact on the limit is below 5 %(mχ>6GeV /c2

)

searching for dark matter: new results from the xenon100...

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