Dark Matter Searches with ATLAS
David Berge – CERN – TeV Particle Astrophysics 2009 (SLAC)
On behalf of the ATLAS collaboration
Setting the stage: Dark Matter and Particle Physics
•
Dark Matter evidence:
– Based on astronomical observations
– Purely gravitational interactions
•
‘Particle-izing’ dark matter, assumptions:
– 1 (and only 1) neutral heavy particle, stable on
cosmological timescales (‘WIMP’)
– Can be pair produced and annihilate in pairs to
establish a thermal equilibrium
•
Then: WIMP density <sv> ≈ 1 pb
– Typical cross section for expected new physics at the
LHC!
•
NASA/CXC/M.Markevitch et al. | NASA/STScI; Magellan/U.Arizona/D.Clowe et al. | NASA/STScI; ESO
WFI; Magellan/U.Arizona/D.Clowe et al.
Assuming coupling is of the order of the weak /
elm. coupling constant, expected particle mass is
O( 100 GeV )
– Electroweak Symmetry breaking!
•
Most popular models: SUSY, Universal Extra
Dimensions, Little Higgs
– Will focus on SUSY, most studied in ATLAS
– Experimental signatures are generic for WIMPs
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NASA/ESA/HST 2
Supersymmetry (SUSY)
The light scalar Higgs boson is unprotected at GUT/ Planck scales
Fermion and boson loops contribute with different signs to the Higgs radiative corrections:
existed a symmetry relating these two, this could protect the masses of the scalar !
if there
Supersymmetry realisesFermion
this byloop
transforming bosons  fermions
SUSY transforms for example a scalar boson into a spin-½ fermion, whose mass is protected
Hence, the scalar mass is also protected (precisely through SUSY)
This solves the naturalness and the hierarchy problems of the SM (at least technically)
To avoid proton decay, a new symmetry is introduced: R-parity
Boson loop
SUSY particles must be produced in pairs, and decay into other SUSY particles
The lightest SUSY particle is stable  dark matter candidate !
SUSY also naturally achieves Grand Unification of the forces at ~ 1016 GeV
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Hadron Collider Observables
(sx1x2)
x1 p
proton
[ “Hard scattering partons” ]
x2 p
proton
proton beams
10
q
X
p
p
g
Typical SUSY cascade at
a proton-proton collider
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LSP escapes
 missing ET
qL
 20
q
q
10
10
R
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The Large Hadron Collider
LHC has 4 large experiments: ATLAS, CMS, LHCb, ALICE
LHC
LHCb
PS
ATLAS
SPS
ALICE
CMS
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The ATLAS Experiment
• Except for muons and
neutrinos all hightransversemomentum particles
are eventually
absorbed in the
calorimeters
• Calorimeters are
crucial for dark matter
22 m
searches via missing
ET!
46 m
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10 September 2008: LHC Startup
ATLAS main control room
Setting up the L1 trigger…
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‘Counting room’ close to ATLAS
David Berge - CERN
• 10:30: full turn beam 1
• 15:00: full turn beam 2
• 22:00: hundreds of turns
beam 2
Beam energy 450 GeV, 2 x
109 protons per bunch
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First beam-related events: splashes
±140m before
the detectors
are collimators
in both LHC
beam lines,
which – when
closed – stop the
beam
Accelerator
team chose stepwise procedure:
first brought
onto
• 2 xprotons
109 450
GeV protons
collimator,
dumped
into collimators
adjusted
• Fixed target-like setup,
positioning, then
spray of secondary
into next LHC
particles
sector…smashing into
ATLAS from one side
• Up to 1 PeV energy
depositions measured
• Very useful events for
initial timing
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The incident 19 September 2008
QV
QV
Q
SV
D
D
D
Cold-mass
Vacuum vessel
Line E
Cold support post
Warm Jack
Compensator/Bellows
Vacuum barrier
Q
•
•
Busbar •
QV
D
D
D
PT
Q
SV
D
D
D
Q
QV
D
D
D
QV
Q
During 5 TeV commissioning in one of the LHC sectors (ramp
to 9.3 kA – 5.5 TeV), a resistive zone appeared in a
superconducting busbar between two magnets
An electrical arc developed and punctured the helium
enclosure
Large amounts of Helium gas (6 tons in total) were released into
the insulating vacuum of the cryostat:
– Self actuating relief valves opened, releasing large amount of He in
the tunnel, but could not handle huge pressure
– Large pressure waves traveled along the accelerator both ways
– Large forces exerted on the vacuum barriers located every 2
machine cells
– These forces displaced several quadrupoles by up to ~50 cm
– Beam pipes broke as well, vacuum contaminated
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Hardware repair
Collateral damage
•
•
53 magnets repaired / replaced since 30 April
Further preventive measures:
New support jacks
– enhanced quench protection system (all sectors)
– Exchange pressure relief valves (4 warm sectors,
partly 4 cold sectors, remaining work next
shutdown)
– Strengthen anchoring system
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LHC Schedule 2009 / 2010
• New schedule:
High electrical cost
• Plan ‘Base’: additional 20 weeks of physics in the Winter months, at the expense of ~8
MCHF additional electricity costs
• Center-of-mass energy in 2009 and 2010 limited to 10 TeV
• Latest news: maybe down to 8 TeV depending on resistance measurements this summer
• Target restart October 2009
• The presently installed collimation system limits the total intensity to 10-20% of the
nominal intensity
• Integrated luminosity (collected data) target of 200-300 pb-1
• At design luminosity: expect 100 fb-1 per year!
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Measuring missing ET is tough
•
•
Energy sum of all objects in event
– Jets, electrons, muons, taus, …
Detectors not perfectly hermetic
– Beamline, cables, services
Many sources of fake missing ET
– Mis-measured jets, fluctuations, resolution, etc.
– Electronics noise
– Dead / un-instrumented regions of detector
– Punch-through
– Physics (minimum bias, jets) overlap with other
backgrounds
•
•
•
If jet 2 rec.
energy
undershoots
true, or
direction
deviates,
missing ET is
faked
Φ-plane
Jet 1
ETmiss
Jet 2
Cosmic ray muons undergoing hard bremsstrahlung
Beam-halo, other machine background
Events with real missing ET (neutrinos!)
Radiation length
Material before EM calorimeter
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Interaction lengths
•
EM calorimeter ‘thickness’
η
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Simulated jet punching through the ATLAS calorimeter 12
Missing ET and cosmic rays
• Extended cosmic ray commissioning
campaigns on-going
• Noise contributions to missing ET well
modeled by simple Gaussian
• On the other hand, cosmics generate
fake missing ET
Random events,
cosmics commissioning
m
≈ TeV ‘jet’
≈ TeV ‘missing ET’
if no cuts
applied…
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Missing ET cleaning cuts
• Exploit 1ns time resolution of
hadronic calorimeter
• Cosmic-ray transit time from
top to bottom about 18ns
• And use precision muon
chambers of ATLAS to identify
cosmic rays passing through
from above
QCD dijets
t=0
Cosmic-ray MC
t = -18 ns
• Jet electromagnetic signal fraction (0 or 1 for muons undergoing hard Bremsstrahlung)
• Number of calorimeter clusters (1 cluster = smallest jet ‘unit’)
• Can also use tracking (not shown)
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Missing ET cleaning cuts
Left: effect of hardware failures on missing ET in QCD dijet simulations. Region 1 has 3 front-end readout crates turned
off, region 2 has 2 crates off, region 3 has the whole detector functional. Right: Jet EM fraction cut cleans sample.
• Missing ET is susceptible to various detector deficiencies (high-voltage, lowvoltage, electronics readout problems, etc.)
• Cleaning procedure relies on identifying dead detector regions or e.g. cut on
jet energy fraction in electromagnetic signal
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Missing ET validation planned with data
Logical flow
1. Event cleaning cuts (noise, dead regions, cosmics, beam background)
2. Basic reconstruction (photons, electrons, jets, muons, etc.)
3. Validate missing ET in data without real missing ET (‘minimum bias’, QCD dijets)
4. Validate missing ET in data with real missing ET (Z->tt, W->mn) – towards 30-100 pb-1
Take minimum bias events without hard,
just soft interactions. Measure resolution
and bias of x and y component of missing
ET, compare to simulations.
Missing ET resolution in events without
intrinsic missing ET. Will be verified with
early data and serve as validation.
Soft QCD (minimum bias)
QCD dijet events
QCD dijet events
QCD dijet events
QCD dijet events
These 2 plots can
be obtained in situ
purely from data!
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SUSY benchmark points
•
Choose ‘representative’ SUSY scenario
(mSUGRA)
–
–
•
Nice because predictive
On the other hand as good a choice as many
other scenarios
m1/2
500
SU1
300
Respect cosmic dark matter density
200
•
Run full detector simulation
100
•
Assume 14 TeV c.m. and 1fb-1 (!)
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“coannihilation point”
400
Pick a few “characteristic” points
•
[GeV]
SU3
SU6
“Higgs funnel”
“bulk region”
SU4
“low mass point”
100 200 300 300 500
David Berge - CERN
“focus point”
SU2
3500
m0 [GeV]
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Searches for Supersymmetry
•
•
•
R-parity conserving SUSY has clear signature – with the right mass and model
parameters it could be discovered early on!
Analyses shown here require basically
– 0/1 lepton + 4 high-energy jets + large missing ET
Important: need to estimate backgrounds from data due to Monte Carlo uncertainties!
Plots for mSUGRA with m0=100 GeV, m1/2=300 GeV, tanb=6, m>0, A0=-300GeV, 1 fb–1
0 lepton mode
1 lepton mode
Excellent understanding of jet and missing ET scale and resolution necessary ! – Think of it as ‘1fb-1 of well understood data’!
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ATLAS discovery reach
•
•
•
•
mSUGRA parameter scan
Fast detector simulation to
interpolate between the fully
simulated ‘SU’ benchmark points
Best sensitivity in 0 lepton + jets +
missing ET channel (despite more
background than the others)
Initial discovery channel: 1 lepton +
jets + missing ET
–
L = 1 fb–1
less QCD background, which will take
time to understand
Tevatron reach (CDF)
Reach for squark & gluino
0.1 fb–1 → M ~ 750 GeV
masses using 4-jets + 0-lepton 1 fb–1
→ M ~ 1350 GeV
–1
mode:
10 fb
→ M ~ 1800 GeV
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mSUGRA @ 95% C.L.
T. Aaltonen et al. [arXiv:0811.2512]
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Interlude: SUSY at 10 vs. 14 TeV LHC energy
g
q
g
q
Quark-flavour
production
High-pT QCD jets
g
g
W, Z production
q
W ,Z
q
g
gluon-to-Higgs fusion
t
H0
g
g
q
g
q
squarks, gluinos
(m ~ 1 TeV)
Production cross section steeply falling with decreasing c.m. energy!
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SUSY – Dark Matter?
Once we have a missing ET excess beyond the Standard Model, how do we get to
the cosmic WIMP? This is not at all trivial, at the LHC we can never prove the
existence of a particle of cosmic lifetimes!
•
•
•
ATLAS TDR
Need combined approach with direct and indirect Dark Matter
detection experiments!
Need to confirm a concrete model of beyond the Standard
Model theory and measure particle properties (effective mass,
mass, spin, couplings) – hard at the LHC!
Exclusive searches might give us a clue about particle masses –
‘dilepton edges’
SUSY mass scale vs. effective
mass (Meff = Σ|pTi| + Etmiss)
q
~
qL
q

l+
l-
Di-lepton kinematic endpoint:

mmax 
1
m
m
2
20
 10
~
l
0
2
 m2
R
 m
2
R
 m20
1

R
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