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Search for Dark Matter with ANTARES
Holger Motz
Erlangen Centre for Astroparticle Physics
Experiental Particle Physics Seminar
Kyushu University, Fukuoka 2011.10.31
Neutrino Telescope: Detection Principle
• neutrinos can penetrate Earth
• CC interaction in the vicinity of
the detector → muon with
(almost) same trajectory
• muon emits Cerenkov light
when traversing water
• position and time of Cerenkov
photons detected allow
reconstruction of muon path
The ANTARES Collaboration and Site
24 Institutes from 7 Countries
Detector located in Mediterranean near
Toulon at 2475 m depth (to shield from
atmospheric muons)
40 km submarine cable
Depth: 2475 m
Toulon
The ANTARES Collaboration and Site
Shore Station “Michel Pacha” in
La Seyne sûr Mer
40km electro-optical cable for power
and data transmission
40 km submarine cable
Depth: 2475 m
Toulon
The ANTARES Detector
• 12 Lines + IL, ~0.1km² geometric area
• each line: 25 storeys with 3 PMTs per storey
• 885 PMTs total (one sector acoustic particle detection)
Junction Box
40 km electro-optical cable to shore
~70m
~450m
Detection and Calibration Elements
Hydrophonestorey position by
acoustic triangulation(5 per line)
Optical Module(10” PMT in 17” glass sphere)
photon detection
Local Control Module (in Titanium container)
front-end ASIC, Clock, DAQ/SC,compass and tiltmeter for
measuring storey orientation
Optical Beaconwith blue LEDstiming calibration
(4 per line)
Position Calibration
lineshape formula derived from drag and buoyancy of line components fitted to data. => position and orientation of all Optical Modules at ~cm precision
triangulation of the position forfive hydrophones per line with signal from emitters at Bottom String Socket (BSS)
orientation of each storey by compass and tiltmeters
Dominated by two effects:
1. β – decay of 40 K
2. Bioluminescent organisms:
e.g.
40 K
40 Ca
e-
γ
γ
Optical Background
March 2006 – May 2008
Event Reconstruction Basics
• track hypothesis described by x, y, z, t,
zenith and azimuth varied to minimize
time residuals
• time residuals of true track not zero
due to time smearing, light scattering
etc. → PDF
• background hits should either be
included in PDF or removed by hit
selection algorithms
p
Δt
Muon Event in 12-Line Detector
Hei
ght
o f h
it O
M
Time of hit
One plot per line
Neutrino Candidate Event
Hei
ght
o f h
it O
M
Time of hit
One plot per line
Triggering and Hit Selection
Online Trigger
• “all data to shore” concept – L0 hits with threshold 0.3 pe
• 104ms timeslices distributed among datafilters
• L1 hits: one 3 pe hit or 2 hits on storey with Δt <20ns
Standard trigger algorithms
• 3D Scan: 5 L1 hits causally connected + compatible with any track from
direction scan
• 2T3: 2 clusters of 2 L1 hits on adjacent storeys with Δt <100ns or on next to
adjacent storeys with Δt <200ns
Triggered HIts
• L2 hits: Hits that are part of the triggering pattern
Cluster Hit Selection
• select all hits within a window in
time and space from a primary hit
(loop over all hits)
• calculate slopes of lines connecting
primary hit with other selected hits
in z vs t
• calculate RMS of slope distribution
• if RMS worse than user defined
upper threshold, remove worst hit
• continue until RMS good or number
of hits drops below given minimum
• applied selection combines several
cluster sizes with different hit
amplitude cuts
Hei
ght
o f h
it O
M
Time of hit
Cluster Hit Selection Efficiency and Purity
average efficiency (fraction of selected signal hits in all signal hits) for cluster: 0.78, L1: 0.30, L2: 0.60
average purity (fraction of signal hits in selected hit sample) for cluster: 0.86, L1: 0.56, L2: 0.97
ν
µνpp
quality cuts required to remove atmospheric muon background
Background from Cosmic Radiation
badly reconstructed atmospheric muon events
Aart Strategy Reconstruction
• developed for point source search – good angular resolution (0.3°)
Reconstruction steps
• linear prefit as starting point
• several prefits starting from shifted positions and directions
• final fit with full likelihood including background hits
Qualtity Cut Λ
• Λ = log(likelihood)/ndof+0.1*(number of prefits deviating <1°)
• cut depends on background rate
• Λ > -5.3 at 60 kHz → 10% additional background from
misreconstructed atmospheric muons
ANTARES Low-Energy Effective Area
60 kHz background rate from K-40 decay and
bioluminescence
Indirect Search for Dark Matter
• Dark Matter Particle: WIMP, self annihilating (e.g. SUSY Neutralino χ)
• elastic scattering → bound to massive stellar objects (Sun, Earth)
• increase of Neutralino density → Annihilation rate enhanced
• primary annihilation products (quarks, gauge bosons, leptons) decay into
neutrinos
Sensitivity to mSugra Dark Matter
•flux prediction based on a random walk scan of mSugra Parameter Space
guided by the relic density compared to WMAP prediction
•mSugra parameters: m0, m
1/2, A
0 on GUT scale as well as tan(β) and μ
•RGE code ISASUGRA calculates masses on weak scale
•cross sections, relic density, fluxes from annihilation etc. from DarkSUSY
•neutrino oscillation effects for neutrinos from the centre of the Sun
calculated using path-ordered propagators to include matter effects (MSW)
•galactic Dark Matter halo model: Navarro-Frenk-White profile (0.3GeV/cm³)
•renormalisation dependent on top quark mass: mt set to 172.5 GeV
•Integrated νμ and νμ flux
above 10 GeV threshold
energy plotted against
neutralino mass
Neutrino Flux from mSugra Dark Matter Annihilation in the Sun
•sensitivity calculated for five
years of taking data
•unified approach by Feldman-
Cousins used
•background from atmospheric
neutrinos plus 10% more from
misreconstructed atmospheric
muons
•3° radius search cone
Detection Rate from mSugra Dark Matter Annihilation in the Sun
mSugra Parameter Space Regions
Focus Point Region
regions where relic density close tovalue predicted by WMAP
Exclusion Capabilities of ANTARES for the mSugra Parameter Space
Excludable in 3 years at 90% CL: all some none (A
0 varied between -3m
0 and +3m
0 and tan(β) within indicated slice)
Direct Detection
•comparison to direct
detection experiments
sensitive to spin
independent WIMP-
nucleon cross section
Direct Detection
•comparison to direct
detection experiments
sensitive to spin-dependent
WIMP-nucleon cross
section and other neutrino
experiments
•Sun consists mostly of
hydrogen → spin
dependent scattering
dominant for capture →
direct relation with
annihilation rate in, and
neutrino flux from Sun
Analysing the Flux from Earth's Centre
• data from December 2010 analysed
• searching for Dark Matter annihilations in Earth
• model independent limits for various annihilation
channels and WIMP masses
• only Zenith angle needs to be reconstructed →
higher efficiency and lower energy threshold
• no off-source region → background estimation
from MC only → improvements on MC
Monte Carlo Improvements
• background and OM-
Condition from Minimum
Bias Events (MBE) – data
snapshots taken every 10
seconds
• triggering on and
misreconstruction of pure
background also simulated
using MBE
• PMT gain spread for
simulated signal hits
randomized according to
MBE amplitude distribution
t
Simulated Event
MBE4400 ns
MBE4400 ns
MBE4400 ns
TriggeredHits
Trigger Snapshot +- 2200 ns
Use this MBE'sOM Condition
OM Condition in Data and MC
black: datared: MCblue: pure bkg MC
Average Rate per OM in Data and MC
black: datared: MCblue: pure bkg MC
Hit Amplitude Distribution (good OM)
Hit Amplitude Distribution (bad OM)
Global Hit Amplitude Distribution
Global Hit Amplitude Distribution
BBFit Reconstruction
Fit Procedure
• χ² fit on time residuals
• additional term: hit amplitude * Cerenkov light travel distance
• ROOT Minuit minimizer
• single line (zenith angle) and multi line version
• track and shower version (better χ² selected)
Initial hit selection
• large hits (>2.5 pe)
• coincidences on a storey (Δt <20ns)
• hits recursively connected by the T3 cluster definition
• all hits on a storey merged (time from first hit on storey)
Zenith Angle Distribution before Cuts
BBFit X²-Cuts
• separate cuts for single line, two and three line fits
• cut defined at 100% additional background from misrecontructed atmospheric muons
1 Line
2 Line3 Line
Zenith Angle Distribution after Cuts
Data / MC Events after Cuts over zenith
Simulated Annealing Fit
Time consuming fit for low energies
• used on events for which BBFit failed and reconstructed energy < 600 GeV
Simulated annealing minimizer
• random walk with decreasing step size (analogous to cooling metal)
3 Prefits
• PDF fitted from MC track and Cluster hit selection
• wide gaussian PDF (50ns) and all Hits
• narrow gaussian PDF (5ns) and Cluster Hits merged with time of first hit on
each storey
Final Fit
• only if no prefit reconstructed events as downgoing
• 10° zenith around best prefit
Simulated Annealing Fit Quality Cut
• Δt < 15 ns:direct hit
• >4 direct hits
• ω = reconstructed energy / number of direct hits
Zenith Angle Resolution
red: Simulated Annealing Fit avg. 5.6° green: BBFit 3-Line avg. 0.9°turquoise: BBFit 2-Line avg. 1.3°blue: BBFit 1-Line avg. 5.1°
Efficiency (Zenith Angle)
red: Simulated Annealing Fitgreen: BBFit 3-Lineturquoise: BBFit 2-Lineblue: BBFit 1-Line
Efficiency (Low Energy)
red: Simulated Annealing Fitgreen: BBFit 3-Lineturquoise: BBFit 2-Lineblue: BBFit 1-Line
Expected Signal Event Calculation
• additional weights for atm. neutrino Monte Carlo Simulation
• flux calculated for 200 annihilations per ns purely into one channel
• fluxtables from WIMPSIM homepage
• absorption to Earth surface negligible
• WIMP masses [GeV]: 25, 50, 80.3, 91.2, 100, 150, 176, 200, 250,
350, 500, 750, 1000, 1500, 3000, 5000
• annihilation channels:
Best Search Cone Calculation
• model rejection factor minimized (average upper limit / exp. events)
• for each channel and WIMP mass
W-channel, 100 GeV
Expected Signal Events (best cone)
Model Rejection Factor
Limit on the Number of Signal Events
black: datared: MCblue: limit
Feldman-Cousins90% CL limit
used cone cutsoptimized from MC beforehand
Limit on the Annihilation Rate
theoretical capture rate for a 10 ³ cm² spin independent x-section⁻⁴
Limit on the Neutrino Flux
•10 GeV neutrino energy threshold•used WIMPSIM tables again
events x 60, assume to measure expected background
5-year Sensitivity
Summary
Sensitivity of ANTARES to mSugra for annihilations in the Sun
• 5 year sensitivity reaches parameter space
• complimentary to direct detection and collider approach
Analysis of neutrino flux from Earth's centre
• prototype analysis with improvements on data selection and MC
simulation
• Simulated Annealing Fit allows to reconstruct extra events
especially at low energies
• limits set for annihilation rate and neutrino flux from WIMPs
annihilating into a single channel for various WIMP masses