Dark Matter Searches
at ATLAS
Dan Tovey
University of Sheffield
Dan Tovey
1
PPC07, May 2007
ATLAS
Length: ~45 m
Radius: ~12 m
Weight: ~7000 tons
Dan Tovey
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PPC07, May 2007
Dan Tovey
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PPC07, May 2007
First Astroparticle Data…
First cosmic muons
observed by ATLAS
in the underground cavern
on June 20th 2005
(recorded by hadron
Tilecal calorimeter)
Dan Tovey
Tower energies:
~ 2.5 GeV
4
PPC07, May 2007
Event Rates
Main asset of LHC: huge event statistics thanks to high s and L
Allows precision measurements/tests of SM
Searches for new particles with unprecedented precision.
•
•
•
Process
Events/s
Events per year
Total statistics collected
at previous machines by 2008
W e
15
108
104 LEP / 107 Tevatron
Z ee
1.5
107
107 LEP
tt
1
107
104 Tevatron
bb
106
1012 – 1013
109 Belle/BaBar?
H m=130 GeV
0.02
105
?
~g
~ m= 1 TeV
g
0.001
104
---
Black holes
m > 3 TeV
(MD=3 TeV, n=4)
0.0001
103
---
L = 1033 cm-2 s-1
Dan Tovey
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PPC07, May 2007
Dark Matter @ ATLAS
• Characteristic signature
for Dark Matter
production at ATLAS:
Missing Transverse
Energy (‘MET’)
• Valid for any DM
candidate (not just SUSY)
• Observation of MET
signal necessary but not
sufficient to prove DM
signal (DM particle could
decay outside detector)
Conclusive proof of
both existence and
identity of DM by
combining LHC data
with astroparticle data
Dan Tovey
~0
c
1
MET
~
c 01
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PPC07, May 2007
• Measurements at ATLAS
complementary to direct and
indirect astroparticle searches /
measurements
scpsi (cm2)
– Uncorrelated systematics
– Measures different parameters
No. MC Experiments
Complementarity
EDELWEISS
DAMA
CDMS-II
CRESST-II
ZEPLIN-2
scpsi
GENIUS
mc
Provides strongest possible
test of Dark Matter model
DM particle mass mc (GeV)
Dan Tovey
log10 (scpsi / 1pb)
– Fit SUSY model parameters to
ATLAS measurements
– Use to calculate DM parameters
Wch2, mc, scpsi etc.
XENON
ZEPLIN-MAX
scpsi
• Aim to test compatibility of e.g.
SUSY signal with DM hypothesis
(data from astroparticle
experiments)
ZEPLIN-4
EDELWEISS 2
Wc
h2
Wch2
CDMS
ZEPLIN-I
Polesello et al JHEP 0405 (2004) 071
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PPC07, May 2007
SUSY DM Strategy
SUSY Dark Matter studies at ATLAS will proceed in
four stages:
1) SUSY Discovery phase
2) Inclusive Studies (measurement of SUSY Mass Scale,
comparison of significance in inclusive channels).
3) Exclusive studies and interpretation within specific
model framework (e.g. Constrained MSSM / mSUGRA)
– Specific model needed to calculate e.g. relic density g
general SUSY studies will be less model dependent.
4) Less model-dependent interpretation
– Relax model-dependent assumptions
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PPC07, May 2007
Stage 1:
SUSY Discovery
Dan Tovey
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PPC07, May 2007
SUSY Signatures
• Q: What do we expect SUSY events @ LHC to look like?
• A: Look at typical decay chain:
p
p
~
g
q
~
q
~
c0 2
q
~
c0 1
~
l
l
l
• Strongly interacting sparticles (squarks, gluinos) dominate
production.
• Heavier than sleptons, gauginos etc. g cascade decays to LSP.
• Long decay chains and large mass differences between SUSY states
– Many high pT objects observed (leptons, jets, b-jets).
• If R-Parity conserved LSP (lightest neutralino in mSUGRA) stable
and sparticles pair produced.
– Large ETmiss signature (c.f. Wgl).
• Closest equivalent SM signature tgWb.
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PPC07, May 2007
Inclusive Searches
•
•
•
•
Use 'golden' Jets + n leptons + ETmiss discovery channel.
Map statistical discovery reach in mSUGRA m0-m1/2 parameter space.
Sensitivity only weakly dependent on A0, tan(b) and sign(m).
Syst.+ stat. reach harder to assess: focus of current & future work.
5s
5s
ATLAS
Dan Tovey
ATLAS
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PPC07, May 2007
Background Systematics
• Z g  + n jets, W g l + n jets, W g
t + (n-1) jets (t fakes jet)
• Estimate from Z g l+l- + n jets (e or m)
• Tag leptonic Z and use to validate MC
/ estimate ETmiss from pT(Z) & pT(l)
• Alternatively tag W g l + n jets and
replace lepton with  (0l):
(Zll)
ATLAS
Preliminary
– higher stats
– biased by presence of SUSY
ATLAS
(Wl)
Preliminary
Dan Tovey
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PPC07, May 2007
Stage 2:
Inclusive Studies
Dan Tovey
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PPC07, May 2007
Inclusive Studies
• Following any discovery of SUSY next
task will be to test broad features of
potential Dark Matter candidate.
• Question 1: Is R-Parity Conserved?
– If YES possible DM candidate
– LHC experiments sensitive only to LSP
lifetimes < 1 ms (<< tU ~ 13.7 Gyr)
~
Non-pointing
~
~
photons from c01gGg
R-Parity
Conserved
R-Parity
Violated
ATLAS
• Question 2: Is the LSP the lightest
neutralino?
– Natural in many MSSM models
– If YES then test for consistency with
astrophysics
– If NO then what is it?
– e.g. Light Gravitino DM from GMSB
models (not considered here)
GMSB Point 1b
(Physics TDR)
ATLAS
Dan Tovey
LHC Point 5
(Physics TDR)
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PPC07, May 2007
Measuring Parameters
• First indication of mSUGRA
parameters from inclusive channels
– Compare significance in jets + ETmiss +
n leptons channels
ATLAS
• Detailed measurements from
exclusive channels when accessible.
• Consider here two specific example
points studied previously:
Point
LHC Point 5
SPS1a
m0 m1/2 A0
100 300 300
100 250 -100
Sparticle Mass (LHC Point 5)
~
qL
~690 GeV
~
c02
233 GeV
~
lR
157 GeV
~
c01
122 GeV
Dan Tovey
tan(b) sign(m)
2
+1
10
+1
Mass (SPS1a)
~530 GeV
177 GeV
143 GeV
96 GeV
15
LHC Point 5 (A0 =300
GeV, tan(b)=2, m>0)
Point SPS1a (A0 =-100
GeV, tan(b)=10, m>0)
PPC07, May 2007
Stage 3:
Exclusive studies and interpretation
within specific model framework
Dan Tovey
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PPC07, May 2007
Exclusive Studies
• With more data will attempt to measure weak scale SUSY parameters
(masses etc.) using exclusive channels.
• Different philosophy to TeV Run II (better S/B, longer decay chains) g
aim to use model-independent measures.
p
p
~
q
~g
q
~
c0 2
q
~
lR
l
~
c0 1
l
• Two neutral LSPs escape from each event
– Impossible to measure mass of each sparticle using one channel alone
• Use kinematic end-points to measure combinations of masses.
• Old technique used many times before ( mass from b decay
spectrum, W (transverse) mass in Wgl).
• Difference here is we don't know mass of neutral final state particles.
Dan Tovey
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PPC07, May 2007
Dilepton Edge Measurements
• When kinematically
accessible ~
c02 can undergo
sequential two-body decay
to ~
c01 via a right-slepton
(e.g. LHC Point 5).
• Results in sharp OS SF
dilepton invariant mass
edge sensitive to
combination of masses of
sparticles.
• Can perform SM & SUSY
background subtraction
using OF distribution
c~02
l
e+e-
c~01
l
e+e- + m+m- e+m- - m+e-
m+m-
+
Point 5
ATLAS
~
l
30 fb-1
atlfast
Physics
TDR
•m0 = 100 GeV
•m1/2 = 300 GeV
•A0 = -300 GeV
•tan(b) = 6
•sgn(m) = +1
ATLAS
5 fb-1
SU3
e+e- + m+m- - e+m- - m+e-
• Position of edge measured
with precision ~ 0.5%
(30 fb-1).
Dan Tovey
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PPC07, May 2007
Measurements With Squarks
•
•
•
•
Dilepton edge starting point for reconstruction of decay chain.
Make invariant mass combinations of leptons and jets.
Gives multiple constraints on combinations of four masses.
Sensitivity to individual sparticle masses.
~
qL
~
q c0 2
ATLAS
Dan Tovey
l
~
l
l
~
qL
~
c0 1
~
q c0 2
llq threshold
lq edge
1% error
(100 fb-1)
1% error
(100 fb-1)
TDR,
Point 5
ATLAS
h
b
llq edge
TDR,
Point 5
2% error
(100 fb-1)
TDR,
Point 5
ATLAS
19
~
c0 1
b
bbq edge
1% error
(100 fb-1) TDR,
Point 5
ATLAS
PPC07, May 2007
‘Model-Independent’ Masses
• Combine measurements from edges
from different jet/lepton
combinations to obtain ‘modelindependent’ mass measurements.
~
c0
ATLAS
Mass (GeV)
~c0
2
ATLAS
LHCC
Point 5
~
lR
1
Mass (GeV)
ATLAS
Mass (GeV)
~
q
L
ATLAS
Mass (GeV)
Sparticle Expected precision (100 fb-1)
~
qL
 3%
~
c02
 6%
~
lR
 9%
~
c01
 12%
Dan Tovey
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PPC07, May 2007
Measuring Model Parameters
• Alternative use for SUSY observables (invariant mass end-points,
thresholds etc.).
• Here assume mSUGRA/CMSSM model and perform global fit of model
parameters to observables
– So far mostly private codes but e.g. SFITTER, FITTINO now on the market;
– c.f. global EW fits at LEP, ZFITTER, TOPAZ0 etc.
Point
LHC Point 5
SPS1a
Parameter
m0
m1/2
tan(b)
A0
Dan Tovey
m0 m1/2 A0
100 300 300
100 250 -100
tan(b) sign(m)
2
+1
10
+1
Expected precision (300 fb-1)
 2%
 0.6%
 9%
 16%
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PPC07, May 2007
Dark Matter Parameters
• Can use parameter measurements
for many purposes, e.g. estimate
LSP Dark Matter properties (e.g.
for 300 fb-1, SPS1a)
– Wch2 = 0.1921  0.0053
– log10(scp/pb) = -8.17  0.04
Micromegas 1.1
(Belanger et al.)
+ ISASUGRA 7.69
Wc
h2
300 fb-1
ATLAS
Dan Tovey
Baer et al. hep-ph/0305191
LHC Point 5: >5s error (300 fb-1)
SPS1a: >5s
error (300 fb-1)
scp=10-11 pb
DarkSUSY 3.14.02
(Gondolo et al.)
+ ISASUGRA 7.69
scp=10-10 pb
scp
scp=10-9 pb
300 fb-1
LEP 2
No REWSB
ATLAS
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PPC07, May 2007
Target Models
• SUSY (e.g. mSUGRA) parameter space strongly constrained by
cosmology (e.g. WMAP satellite) data. mSUGRA A0=0,
tan(b) = 10, m>0
'Focus point'
region:
~
significant h
component to
LSP enhances
annihilation to
gauge bosons
'Bulk' region: tchannel slepton
exchange - LSP
mostly Bino.
'Bread and
Butter' region for
LHC Expts.
Dan Tovey
Ellis et al. hep-ph/0303043
Disfavoured by BR (b  sg) =
(3.2  0.5)  10-4 (CLEO, BELLE)
c~01
c~01
~0
c
1
l
~
lR
t~1
t
t~1
g/Z/h
Slepton Coannihilation
region: LSP ~
pure Bino. Small
slepton-LSP
mass difference
makes
measurements
difficult.
l
Also 'rapid
annihilation funnel'
at Higgs pole at
0.094  W c h2  0.129
(WMAP)
high tan(b), stop
co-annihilation
region at large A0
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PPC07, May 2007
Coannihilation Signatures
• Small slepton-neutralino mass
difference gives soft leptons
100 fb-1
– Low electron/muon/tau energy
thresholds crucial.
ATLAS
• Study point chosen within region:
– m0=70 GeV; m1/2=350 GeV; A0=0;
tanß=10 ; μ>0;
Preliminary
~
~
• Decays of c~02 to both lL and lR
kinematically allowed.
– Double dilepton invariant mass
edge structure;
– Edges expected at 57 / 101 GeV
100 fb-1
• Stau channels enhanced (tanb)
– Soft tau signatures;
– Edge expected at 79 GeV;
– Less clear due to poor tau visible
energy resolution.
Dan Tovey
• ETmiss>300 GeV
• 2 OSSF leptons
PT>10 GeV
• >1 jet with PT>150
GeV
• OSSF-OSOF
subtraction applied
Preliminary
24
ATLAS
• ETmiss>300 GeV
• 1 tau PT>40
GeV;1 tau PT<25
GeV
• >1 jet with
PT>100 GeV
• SS tau
subtraction
PPC07, May 2007
Focus Point Signatures
• Large m0  sfermions are heavy
• Most useful signatures from heavy neutralino decay
• Study point chosen within focus point region :
– m0=3550 GeV; m1/2=300 GeV; A0=0; tanß=10 ; μ>0
~0 → c
~0 ll
• Direct three-body decays c
n
1
~0 )-m(c
~ 0 ) : flavour subtraction applied
• Edges give m(c
n
1
~0 → ~
~
c01 ll
c 02 → ~
c01 ll c
3
Z0 → ll
M = mA+mB m = mA-mB
ATLAS
300 fb-1
Preliminary
Dan Tovey
25
Parameter
Without cuts
Exp. value
M1
68±92
103.35
M2-M1
57.7±1.0
57.03
M3-M1
77.6±1.0
76.41
PPC07, May 2007
Stage 4:
Less model-dependent interpretation
Dan Tovey
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PPC07, May 2007
Dark Matter in the MSSM
• Can relax mSUGRA constraints to obtain
more ‘model-independent’ relic density
estimate.
• Much harder – needs more measurements
• Not sufficient to measure relevant (co-)
annihilation channels – must exclude all
irrelevant ones also …
• Stau, higgs, stop masses/mixings important
as well as gaugino/higgsino parameters
Nojiri, Polesello & Tovey, JHEP 0603 (2006) 063
Wc
σ(mtt)=
5 GeV
Wch2
300 fb-1
σ(Wch2) vs
σ(mtt)
h2
σ(mtt)=
0.5 GeV
300 fb-1
SPA point
Dan Tovey
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PPC07, May 2007
Heavy Gaugino Measurements
• Potentially possible to identify dilepton
edges from decays of heavy gauginos.
• Requires high stats.
• Crucial input to reconstruction of MSSM
neutralino mass matrix (independent of
SUSY breaking scenario).
ATLAS
SPS1a
ATLAS
100 fb-1
Dan Tovey
ATLAS
100 fb-1
28
ATLAS
100 fb-1
SPS1a
PPC07, May 2007
Summary
• Following a (SUSY) discovery ATLAS will aim to test the (SUSY)
Dark Matter hypothesis.
• Conclusive result only possible in conjunction with astroparticle
experiments (constraints on LSP life-time).
• Estimation of relic density and direct / indirect DM detection
cross-sections in model-dependent scenario will be first goal.
• Less model-dependent measurements will follow.
• Ultimate goal: observation of neutralinos at LHC confirmed by
observation of e.g. signal in (in)direct detection Dark Matter
experiment at predicted mass and cross-section.
This would be major triumph for both
Particle Physics and Cosmology!
Dan Tovey
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PPC07, May 2007
BACK-UP SLIDES
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PPC07, May 2007
Supersymmetry
• Supersymmetry (SUSY) fundamental
continuous symmetry connecting
fermions and bosons
Qa|F> = |B>,
Qa|B> = |F>
• {Qa,Qb}=-2gmabpm: generators obey anticommutation relations with 4-mom
– Connection to space-time symmetry
• SUSY stabilises Higgs mass against loop
corrections (gauge hierarchy/fine-tuning
problem)
LEPEWWG
Winter 2006
mH<207 GeV
(95%CL)
– Leads to Higgs mass  135 GeV
– Good agreement with LEP constraints from
EW global fits
• SUSY modifies running of SM gauge
couplings ‘just enough’ to give Grand
Unification at single scale.
Dan Tovey
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PPC07, May 2007
SUSY Spectrum
• SUSY gives rise to partners of SM states with opposite spin-statistics
but otherwise same Quantum Numbers.
• Expect SUSY
partners to
have same
masses as SM
states
– Not
observed
(despite best
efforts!)
– SUSY must
be a broken
symmetry at
low energy
• Higgs sector
also expanded
Dan Tovey
h
A
H0
H±
u
d
e
e
c
s
m
t
b
t
g
Z
W±
~
~0 0 ~~0 + ~0~ ~
~
0
0
c
H 1u cH2 d cH3 u c H4 d
m
~
u
~
c
~
d
~
s
~
e
~
m
~
e
~

t
~
t
~
b
~
t
~
t
g
~
~~
±~ ±
~
c
cW
g ± 1Z
2
~
g
G
~
G
32
m
PPC07, May 2007
Model Framework
• Minimal Supersymmetric Extension of the Standard Model (MSSM)
contains > 105 free parameters, NMSSM etc. has more g difficult to
map complete parameter space!
• Assume specific well-motivated model framework in which generic
signatures can be studied.
• Often assume SUSY broken by gravitational interactions g
mSUGRA/CMSSM framework : unified masses and couplings at the
GUT scale g 5 free parameters
LHCC
(m0, m1/2, A0, tan(b), sgn(m)).
mSUGRA
• R-Parity assumed to be conserved.
Points
• Exclusive studies use benchmark
3
points in mSUGRA parameter space:
•
•
•
•
LHCC Points 1-6;
Post-LEP benchmarks (Battaglia et al.);
Snowmass Points and Slopes (SPS);
etc…
Dan Tovey
33
5
1
2
4
PPC07, May 2007
SUSY & Dark Matter
• R-Parity Rp = (-1)3B+2S+L
• Conservation of Rp
(motivated e.g. by string
models) attractive
mSUGRA A0=0,
tan(b) = 10, m>0
Ellis et al. hep-ph/0303043
Disfavoured by BR Universe
(b  sg) =Over-Closed
-4
(3.2  0.5)  10 (CLEO, BELLE)
– e.g. protects proton from
rapid decay via SUSY states
• Causes Lightest SUSY
Particle (LSP) to be
absolutely stable
• LSP neutral/weakly
interacting to escape
astroparticle bounds on
anomalous heavy elements.
• Naturally provides solution to
dark matter problem
• R-Parity violating models still
possible  not covered here.
Dan Tovey
c~01
c~01
~0
c
1
l
~
lR
t~1
t
t~1
g/Z/h
l
0.094  W c h2  0.129
(WMAP-1year)
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PPC07, May 2007
Stop Mass
• Look at edge in tb mass distribution.
• Contains contributions from
–
–
–
~ ~ tbc
~+
gtt
1
1
~
~+
~
gbb
1btc 1
SUSY backgrounds
mtbmax = (443.2 ± 7.4stat) GeV
Expected = 459 GeV
120 fb-1
ATLAS
• Measures weighted mean of end-points
• Require m(jj) ~ m(W), m(jjb) ~ m(t)
LHCC Pt 5
(tan(b)=10)
mtbmax = (510.6 ± 5.4stat) GeV
Expected = 543 GeV
120 fb-1
ATLAS
LHCC Pt 5
(tan(b)=10)
• Subtract sidebands from m(jj)
distribution
• Can use similar approach with
~
~
gtt1ttc~0i
– Di-top selection with sideband
subtraction
• Also use ‘standard’ bbll analyses
(previous slide)
Dan Tovey
35
PPC07, May 2007
Preparations for 1st Physics
• Preparations needed to ensure efficient/reliable searches
for/measurements of SUSY particles in timely manner:
–
–
–
–
–
Initial calibrations (energy scales, resolutions, efficiencies etc.);
Minimisation of poorly estimated SM backgrounds;
Estimation of remaining SM backgrounds;
Development of useful tools.
Definition of prescale (calibration) trigger strategy
• Different situation to Run II (no previous s measurements at same s)
• Will need convincing bckgrnd. estimate with little data as possible.
• Background estimation techniques will change depending on
integrated lumi.
• Ditto optimum search channels & cuts.
• Aim to use combination of
– Fast-sim;
– Full-sim;
– Estimations from data.
• Use comparison between different techniques to validate estimates
and build confidence in (blind) analysis.
Dan Tovey
36
PPC07, May 2007
Black Hole Signatures
• In large ED (ADD) scenario, when impact
parameter smaller than Schwartzschild
radius Black Hole produced with
potentially large x-sec (~100 pb).
• Decays democratically through Black
Body radiation of SM states – Boltzmann
energy distribution.
w/o pile-up
Mp=1TeV, n=2, MBH = 6.1TeV
ATLAS
ATLAS
w/o pile-up
• Discovery potential
(preliminary)
– Mp < ~4 TeV  < ~ 1 day
– Mp < ~6 TeV  < ~ 1 year
• Studies continue …
Dan Tovey
37
PPC07, May 2007
Dan Tovey
38
PPC07, May 2007
Dan Tovey
39
PPC07, May 2007
Sbottom/Gluino Mass
• Following measurement of squark, slepton
and neutralino masses move up decay
chain and study alternative chains.
• One possibility: require b-tagged jet in
addition to dileptons.
• Give sensitivity to sbottom mass (actually
two peaks) and gluino mass.
~0 mass
• Problem with large error on input c
1
remains g reconstruct difference of gluino
and sbottom masses.
~
~
• Allows separation of b1 and b2 with 300 fb-1.
p
p
~g ~
b
b
Dan Tovey
~0
c2
b
~
lR
l
~
~0 )
m(g)-0.99m(c
1
= (500.0 ± 6.4) GeV
300 fb-1
ATLAS
SPS1a
~)
~
m(g)-m(b
1
= (103.3 ± 1.8) GeV
~
~
m(g)-m(b
2)
= (70.6 ± 2.6) GeV
ATLAS
300 fb-1
~0
c1
SPS1a
l
40
PPC07, May 2007
RH Squark Mass
• Right handed squarks difficult as rarely decay via ‘standard’ ~
c02 chain
~ gc
~0 q) > 99%.
– Typically BR (q
R
1
• Instead search for events with 2 hard jets and lots of ETmiss.
• Reconstruct mass using ‘stransverse mass’ (Allanach et al.):
mT22 =
min
c(1)
c(2)
miss
[max{mT2(pTj(1),qTc(1);mc),mT2(pTj(2),qTc(2);mc)}]
q
+q
=E
~
0
• Needs c 1 mass measurement as input.
• Also works for sleptons.
T
T
T
~
c0 1
q
ATLAS
ATLAS
30 fb-1
30 fb-1
Right
squark
SPS1a
~
qR
ATLAS
100 fb-1
SPS1a
SPS1a
Right
squark
Left slepton
Precision ~ 3%
Dan Tovey
41
PPC07, May 2007
Physics Strategy
• December 2007(?): 900 GeV calibration run
– commence tuning trigger menus / in situ calibration
• Summer 2008: first 14 TeV physics run (L < 1032 cm-2 s-1, Lint ~ 1 fb-1)
– commence tuning trigger menus / in situ calibration
– First SM measurements: min bias, PDF constraints, Z / W / top / QCD
• 2008/9: physics run (L ~ 2x1033 cm-2 s-1, Lint ~ 10 fb-1)
– First B-physics measurements & rare decay searches (e.g. Bs→J/yf)
– First searches: high mass dilepton / Z’, inclusive SUSY, Black Hole
production, Higgs in ‘easier’ channels e.g. H→4l
• 2009/10: physics run (L ~ 2x1033 cm-2 s-1, Lint = 10 fb-1/year)
– First precision SM & B-physics measurements (systematics under control)
– Improved searches sensitivity
– Light Higgs searches (ttH, H→gg, VBF qqH(H→tt) etc.)
• 2010/11: High luminosity running (L ~1034, Lint = 100 fb-1/year)
– High precision measurements of New Physics (e.g. Higgs/SUSY/ED
properties)
Dan Tovey
42
PPC07, May 2007
Inclusive SUSY
• First SUSY parameter to be measured may
be mass scale:
– Defined as weighted mean of masses of
initial sparticles.
• Calculate distribution of 'effective mass'
variable defined as scalar sum of masses
of all jets (or four hardest) and ETmiss:
Meff=S|pTi| + ETmiss.
• Distribution peaked at ~ twice SUSY mass
scale for signal events.
• Pseudo 'model-independent'
measurement.
• With PS typical measurement error
(syst+stat) ~10% for mSUGRA models for
10 fb-1 , errors much greater with ME
calculation  an important lesson …
Dan Tovey
43
Jets + ETmiss + 0 leptons
ATLAS
10 fb-1
PPC07, May 2007
SUSY Spin Measurement
• Q: How do we know that a SUSY signal is really due to SUSY?
– Other models (e.g. UED) can mimic SUSY mass spectrum
• A: Measure spin of new particles.
• One possibility – use ‘standard’ two-body slepton decay chain
– charge asymmetry of lq pairs measures spin of ~c02
– relies on valence quark contribution to pdf of proton (C asymmetry)
– shape of dilepton invariant mass spectrum measures slepton spin
Spin-0
Measure
Angle
A
Spin-½

l  l
=  
l l
Point 5
Point 5
ATLAS
150 fb -1
mlq
spin-0=flat
Polarise
Spin-½,
Spin-0
mostly wino
Dan Tovey
150 fb -1
ATLAS
Straight
line distn
(phase-space)
Spin-½,
mostly bino
44
PPC07, May 2007
Processing the Data
40 MHz
Detectors
COLLISION RATE
Charge
Time
16 Million channels
3 Gigacell buffers
Pattern
100 kHz
LEVEL-1 TRIGGER
Energy
Tracks
1 MegaByte EVENT DATA
1 Terabit/s
200 GigaByte BUFFERS
(50000 DATA CHANNELS)
500 Readout memories
EVENT BUILDER
500 Gigabit/s
Networks
20 TeraIPS
EVENT FILTER
Gigabit/s SERVICE LAN
Grid Computing
Service
PetaByte ARCHIVE
300 TeraIPS
• Many events
– ~109 events/experiment/year
– ~3 MB/event raw data
– several passes required
 Worldwide LHC computing
requirement (2007):
– 100 Million SPECint2000
(=20,000 of today’s fastest
processors)
– 12-14 PetaBytes of data per
year (=100,000 of today’s
highest capacity HDD).
• Understand/interpret data via numerically intensive simulations
– e.g. 1 event (ATLAS Monte Carlo Simulation) ~20 mins/3 MB
• Use worldwide computing Grid to solve problem
Dan Tovey
45
PPC07, May 2007
Event rate in ATLAS :
N = L x s (pp)  109 interactions/s
Mostly soft ( low pT ) events
Interesting hard (high-pT ) events are rare
Each accompanied by 20 soft events
Also additional soft interactions (UE)
ATLAS
Dan Tovey
46
PPC07, May 2007
Background Estimation
• Inclusive signature: jets + n leptons + ETmiss
• Main backgrounds:
–
–
–
–
Z + n jets
W + n jets
QCD
ttbar
Jets + ETmiss + 0 leptons
ATLAS
10 fb-1
• Greatest discrimination power from ETmiss
(R-Parity conserving models)
• Generic approach to background
estimation:
– Select low ETmiss background calibration
samples;
– Extrapolate into high ETmiss signal region.
• Used by CDF / D0
• Extrapolation is non-trivial.
– Must find variables uncorrelated with ETmiss
• Several approaches being developed.
Dan Tovey
47
PPC07, May 2007