Transcript Slide 1

Discovering
(and understanding)
SUSY at the LHC…
Alan Barr
University of Oxford
… an introduction
(with apologies to the many people who’s work I have included
unreferenced and to those whom I have left out)
LHC physics is about to get
very interesting!
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ATLAS control room
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Have lots of cosmics events
(these from much earlier)
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Last chance to visit LHC
Relatively well-known German physicist takes her chance
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Motivational arguments
1/α
Visible mass
Invisible mass
+SUSY
Log10 (μ / GeV)
The value of prejudice rapidly diminishing
stop
higgs
Alan Barr, Oxford
higgs
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How to make a discovery?
cMSSM
Other
SUSY?
Extra
Dimensions?
Alan Barr, Oxford
• Which way to
search?
Who knows
what?
Explorer/experimentalists rule:
Try to COVER ALL BASES
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Signature-based hunts
Experiments see:
Jets, leptons, missing energy, b-jets
• Astro/cosmo motivation for modelindependent signatures
– We’re pretty sure there are WIMPs
out there
– LHC produces Dark Matter +
something visible
• Invisible particle could be:
– Lightest SUSY particle
– Lightest KK particle
– Lightest generic parity-odd particle
• Signature:
– Missing energy
+ Xvis + Xvis
• Benefit:
Same search finds multiple different models
• Drawback: You ain’t so sure what you’ve got when you find it
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Example SUSY search
Mass (GeV)
“Typical” SUSY spectrum
• Assume R-parity
• Look for:
– Jets from squark & gluino decays
– Leptons from gaugino & slepton decays
– Missing energy from (stable) LSPs
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SUSY event
Missing transverse momentum
Jets
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Leptons
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Heavy quarks
10
Cross-sections etc
“Rediscover”
Lower backgrounds
WW
ZZ
“Discover”
Higher backgrounds
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Precise measurement of SM
backgrounds: the problem
• SM backgrounds
are not that small
• There are
uncertainties in
– Cross sections
– Kinematical
distributions
– Detector response
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Typical search: inclusive
distributions
• Trigger on jets
+ missing
energy
• Plot “effective
mass”
Signal
i
E
 T  E T miss
i
• Look for nonSM physics at
high mass
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BG
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Standard Model backgrounds:
measure from LHC DATA
m
m
Measure in
Z -> μμ


Use in
Z -> νν
• Example: SUSY BG
– Missing energy + jets
from Z0 to neutrinos
– Measure in Z -> μμ
– Use for Z -> 
Alan Barr, Oxford
R: Z -> 
B: Estimated
R: Estimated
• Good match
– Useful technique
• Statistics limited
– Go on to use W => μ to
improve
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Estimating the backgrounds
mT2  ETl ETmiss - pTl  pTmiss
Good match to “true” background
Search region
Control Region
More from Davide Costanzo
later in this session
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Importance of detailed detector
understanding
Et(miss)
Lesson from the Tevatron
•
•
•
Simulation shows events with
large fake missing energy
– Jets falling in “crack” region
– Calorimeter punch-through
Vital to remove these in
missing energy tails
Large effort in physics
commissioning
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Reach in cMSSM?
’Focus point' region:
annihilation to gauge
bosons
mSUGRA A0=0,
tan(b) = 10, m>0
Slepton Coannihilation
region
Rule out
with 1fb-1
WMAP constraints
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'Bulk' region: tchannel slepton
exchange
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Multiple channels for discovery
Below the lines = discovered
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Different
final
states
18
What might we then know?
• Can say some things:
•
•
•
Assume we have MSSM-like
SUSY with
m(squark)~m(gluino)~600 GeV
See excesses in these
distributions
Can’t say “we have discovered
SUSY”
– Undetected particles produced
• missing energy
– Some particles have mass ~ 600
GeV, with couplings similar to QCD
• Meff & cross-section
– Some of the particles are coloured
• jets
– Some of the particles are
Majorana
• excess of like-sign lepton pairs
– Lepton flavour ~ conserved in first
two generations
• e vs mu numbers
– Possibly Yukawa-like couplings
• excess of third generation
– Some particles contain lepton
quantum numbers
– …
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• opposite sign, same family dileptons
Slide based on Polesello
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Mapping out the new world
LHC
Measurement
SUSY
Extra
Dimensions
Masses
Breaking
mechanism
Geometry &
scale
Spins
Distinguish
from ED
Distinguish
from SUSY
Mixings,
Lifetimes
Gauge unification?
Dark matter candidate?
• Some measurements make high demands on:
– Statistics ( time)
– Understanding of detector
– Clever experimental techniques
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SUSY mass measurements
• Extracting
parameters of
interest
– Difficult problem
– Lots of competing
channels
– Can be difficult to
disentangle
– Ambiguities in
interpretation
Try
various
decay
chains
Look for
sensitive variables
(many of them)
• Example method
shown here
• Alternatives also on
the market
Extract
masses
– Comparable precision
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Stransverse mass (MT2) method
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Measuring the shapes
• Better precision possible
than for endpoints
• Systematic uncertinties
need to be controlled
Much work here recently…
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SUSY spin measurements
• The defining
property of
supersymmetry
– Distinguish from
e.g. similar-looking
Universal Extra
Dimensions
• Difficult to measure
@ LHC
Slepton spin from
angles in Drell-Yan
production
Neutralino spin
from angles in decay
chains
~+
l
q
_
q
θ
~l
– No polarised
beams
– Missing energy
– Inderminate initial
state from pp
collision
• Nevertheless, we
have some very
good chances…
+ lots of other recent work in this area
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Other ways of measuring spin
Vector
Scalar
Gluino
Fermion
Squark
• Cross-section depends on spin
• If mass scale can be measured then spin can be inferred
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Dark matter relic density?
•
•
mSUGRA
assumed
•
Use LHC measurements to
“predict” relic density of
observed LSPs
Caveats:
– Cant tell about lifetimes beyond
detector (need direct search)
– Studies done so far in optimistic
case (light sparticles)
To remove mSUGRA assumption
need extra constraints:
1. All neutralino masses
•
Use as inputs to gaugino &
higgsino content of LSP
2. Lightest stau mass
•
Is stau-coannihilation important?
•
Is Higgs co-annihilation
important?
3. Heavy Higgs boson mass
•
Alan Barr, Oxford
More work is in progress
–
–
Probably not all achievable at LHC
ILC would help lots (if in reach)
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Covering all the bases…
• Host of other searches:
– Light stop squarks
– R-parity violating models
– Dileptons/trileptons with
missing energy
– Taus with jets & missing
energy, …
– Single photons
– Diphoton resonances
– Heavy l resonances
– Heavy flavour excesses
– Monojets
– Same sign Stops
– …
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See e.g.
CMS Physics TDR II
2006
ATLAS SUSY
discovery chapter
2008
27
10 TeV … LHC run 2008
10 TeV run need not be “just “commissioning”
Lots of physics and discovery potential
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Conclusions
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Extra rations
Gauge Mediated SUSY Breaking
•
•
Signature depends on
Next to Lightest
SUSY Particle (NLSP)
lifetime
Interesting cases:
– Non-pointing
photons
– Long lived staus
•
•
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Extraction of masses
possible from full
event reconstruction
More detailed studies
in progress by both
detectors
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R-hadrons
• Motivated by e.g. “split
SUSY”
– Heavy scalars
– Gluino decay through
heavy virtual squark very
suppressed
– R-parity conserved
– Gluinos long-lived
Alan Barr, Oxford
• Lots of interesting
nuclear physics in
interactions
– Charge flipping, mass
degeneracy, …
• Importance here is that
signal is very different
from standard SUSY
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Exotic WW scattering
• The ultimate test of electroweak symmetry breaking
– Not unitary above ~1 TeV if no new physics
BG
BG
signal
• Reconstruct hadronic +
leptonic W pair
• Require forward jets
• Veto jets in central
region
Most difficult case: continuum signal
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5- significance with 30 fb-1
in most difficult case
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Dijet masses: Contact Interactions
• Reduce systematics by
using ratio à la DZero
– New physics in the central
region
– “Calibration” sample at
higher rapidity
• Uncertainties from
proton structure not
negligible
– Improve with LHC data?
• Detector crosscalibration uncertainties
to be determined from
data
– Estimates here
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RS Gravitons & heavy bosons
• Randall -Sudrum
graviton spin
e
graviton
p
p
θ
e
Angular distributions
1.5 TeV Randall-Sundrum
graviton -» e+e-
• Discovery
– Find mass peak
Graviton
is spin-2
• Characterisation
– Measure spin
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Spectacular states : micro Black Holes
• Large EDs
• Micro black hole decaying
via Hawking radiation
– Photons + Jets + …
• We will certainly know
something funny is
happening
–
–
–
–
sphericity
Alan Barr, Oxford
Large multiplicities
Large ET
Large missing ET
Highly spherical
compared to BGs
• Theory uncertainty limits
interpretation
– Geometrical information
difficult to disentangle
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Black hole interpretation?
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Slide from Lester
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Some of the sources
•
CMS Physics TDR, Volume II
(recent)
–
CERN-LHCC-2006-021
•
SUSY Spin:
•
Exotic SUSY
•
Dark Matter
•
R-hadrons
–
Barr
–
Parker
–
Nojiri et al
–
–
Kraan et al
Hellman et al
–
Stefanidis
–
Zalewski, Prieur
–
Charybdis, Tanaka, Brett, Lester
–
Stephanidis
•
ATLAS Physics TDR (older)
•
Physics at the LHC 2006
•
SLAC School 06
•
SUSY06
•
WW scattering
•
Missing ET tails:
•
GMSB
•
SM background
•
•
WMAP constraints
•
SUSY mass extraction
–
–
CERN-LHCC-99-015
Programme
–
Polesello, Hinchliffe
–
Polesello, Spiropulu
–
Paige
–
Okawa et al,
–
Ellis et al
•
RS Graviton:
Allanach et al,
Traczyk
Black Holes
–
Gjelsten et al
•
WW scattering
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Constraining masses with crosssection information
• Edges best for mass
inclusive
cross-section
ptmiss > 500
differences
– Formulae contain
differences in m2
– Overall mass- scale
hard at LHC
• Cross-section
changes rapidly with
mass scale
– Use inclusive variables
to constrain mass
scale
– E.g. >500 GeV ptmiss
Alan Barr, Oxford
edges
combined
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Combine
with
Markov
Chain MC
Lester, Parker, White
hep-ph/0508143
40
SUSY Dark Matter
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.
Alan Barr, Oxford
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   c h2  0.129
(WMAP)
high tan(b), stop
co-annihilation
region at large A0
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More on GMSB
• Negligible contribution from the SM backgrounds (consistent with
TDR)
 Trigger efficiencies of the signal is crucial for the discovery
potential
(background rejection, rate estimates would be the next step)
G1a (L=90TeV)
G1a (L=90TeV)
BG Total
g1
Leading Photon Pt (GeV)
Alan Barr, Oxford
BG Total
g2
<After Requiring>
Meff > 400GeV
EtMiss>0.1Meff
two leptons
2nd Leading Photon Pt (GeV)
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Baryonic R-Parity Violation
• Use extra information from
leptons to decrease background.
0
• Sequential decay of q~L to c˜ 1
~
0
through c˜ 2 and lR producing
Opposite Sign, Same Family
(OSSF) leptons
c˜ 20
q~L
q
~
lR
l
Alan Barr, Oxford
c˜ 10
l
Decay via ~lR allowed where
0
~
m( c˜ 2 ) > m( lR )
Test point
q
q
q
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Leptonic R-Parity Violation
Stau LSP
Alan Barr, Oxford
RPV has less missing Et
Neutralino -> stau tau
stau -> tau mu qq
Large rate of taus - smoking gun
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Phillips
44
Light stops
•
•
•
Stop pair production: 412 pb (PROSPINO, NLO)
Dominant (~100%) stop decay: t → c+ b → c01 W* b
Final state is very similar to top pair production events.
– 4 jets, 2 of which b-jets, one isolated lepton, missing energy
– All of them softer (on average) than in top pair production
– Invariant mass combinations will not check out with top, W masses
M(bjj)
1.8 fb-1
M(bl)
1.8 fb-1
GeV
Alan Barr, Oxford
Points: simulated data
Histograms:
signal events08
(MC truth)
NORDITA
GeV
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New vector boson: W’
• Transverse mass
plot for
W’ => μ
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