Transcript Document 7331075

What prospects for Supersymmetry
at the Large Hadron Collider ?
Some of the techniques
with which ATLAS and CMS intend
to constrain Supersymmetry
Christopher.Lester @ cern.ch
What are we going to cover?
• Not a results talk! (WMAP etc)
• Briefly look at supersymmetry
• Look at RPV / RPC distinction from point
of view of experiment
• In no partiular order: look at a few
• inclusive/widely applicable experimental
techniques, also
• less general but perhaps more powerful
experimental techniques
July 2005
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Supersymmetry – Extra particles
• To stabilise the higgs mass NEED:
• A scalar partner for every fermion
• squark, slepton, (stop, sbottom, selectron,
smuon, sneutrino, etc)
• A fermion partner for ever boson:
• gluino,
• photino, wino, zino, higgsino
• (mix to form 4 neutralinos)
• Inexact symmetry – broken somehow
July 2005
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R-Parity: Conservation/Violation
R  (1)
3( B  L )  2 s
•
•
R=+1 for Standard Model particles
R= -1 for SUSY particles
• Two main SUSY scenarios:
• RP-Conserving
• RP-Violating
(RPV/RPC)
(L.S.P. = “lightest SUSY particle”)
How stable is the
lightest SUSY
particle (L.S.P.) ?
RPC Stable
Large
missing
energy?
Yes
Event can be Sparticle
reconstructed production
fully?
Usually not
Only in pairs
RPV Unstable
No
Yes
(decays to leptons or jets)
July 2005
Either singly,
or in pairs
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What do events look like?
RPV
(Baryon number violating)
RPC
RPV
(Lepton number violating)
RPC
So main signatures are:
• Lots of jets
• Lots of leptons
• Lots of missing energy (RPC)
• More on these a little later
• ATLAS Trigger: ETmiss > 70 GeV, 1 jet>80 GeV. (or 4
lower energy jets). Gives 20Hz @ low luminosity.
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What do we want to know?
• M.S.S.M.
Other models:
• Squark masses (12)
• The gluino mass
(1)
• Slepton masses
(9)
• Neutralino masses
(4)
•
•
•
•
Chargino masses
Spins
Mixing matrices
Phases
(2)
(?)
(?)
(?)
•
…..
(plenty)
•
RP-Violating M.S.S.M.
• RPV couplings
•
mSUGRA model
• m0, m1/2, A0, tan β, sgn μ
•
(5)
A.M.S.B. model
• m0, m3/2, tan β, sgn μ
•
(45)
(4)
G.M.S.B. model
• λ, Mmes, N5, tan β, sgn μ, Cgrav
(6)
There is no shortage of
parameters which need to be
determined!
July 2005
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What can we measure?
“Lots, but it depends…”
• The kinds of measurements which can be made,
very much depend on the SUSY model which nature
has chosen!
• Two very different approaches:
• (1) General techniques
• (2) Non general .. specific techniques
• Look at some specific RPV scenarios first
July 2005
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R-Parity Violation
• Easier than RPC?
RPV
L.S.P. = lightest SUSY particle
• The L.S.P. decays!
• No missing energy, so reconstruct full event!
• Case 1: Decays into leptons:
• Multi-lepton signature
• Case 2: Decays into jets:
• Multi-jet signature
• Case 3: Long lifetime:
• looks like RPC scenario
• Sparticles may be produced singly!
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Case 1: Lepton number violating RPV
• λ’ijk couples a slepton to two quarks
• Can have resonant sneutrino production
• Cross section can place lower bound on λ’ijk
• Expect to observe (within 3 years) either
• 900 GeV sneutrino if λ’211>0.05
• 350 GeV sneutrino if λ’211>0.01
• (present limit: ' 211  0.06 ( M d~ / 100 GeV))
Reconstructed neutralino
mass peak in mjjμ invariant
mass distribution
R
λ’ijk =0.09
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Case 2: Baryon number violating RPV
• Each L.S.P. decays to three quarks
(u,d,s) forming three jets (jjj)
• Require 2 leptons and at least 8 jets:
(j+jjj)+(j+ll+jjj)
• Look for L.S.P. / chargino peak in
mjjj / m jjjll plane msquark L = 638 ± 5 ±12 GeV
mneutralino 2
0.3
± 4 GeV
= 212 ±
mslepton R = 155 ± 3 ± 3 GeV
July 2005
mneutralino 1
= 117 ±
HCP20053: SUSY±at3 the
LHC
: [email protected]
GeV
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R-Parity Conservation
RPC
L.S.P. = lightest SUSY particle
• L.S.P. stable and weakly interacting,
and so “goes missing”
• Missing energy signature
• Usually incomplete event
reconstruction
• Need to rely on long decay chains and
kinematic variables (endpoints and
distributions)
• Sparticles are only produced in pairs
• Double the trouble 
• Missing information in BOTH halves of
event! 
Half an event
• More general techniques available!
July 2005
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Inclusive reach in mSUGRA
RPC
CMS 100 fb-1 (~3 years)
(Slide stolen from G.Polesello – SUSY2004)
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Squark/gluon mass scale
M eff  E
missing
T
 | p
jet i
T
|
i
events
What you
measure:
RPC
Signal
S.M.
Background
Peak of Meff distribution correlates well with SUSY scale “as defined
above” for mSUGRA and GMSB models. (Tovey)
M eff (GeV)
Kinematic edges: l+l- edge
RPC
• EXAMPLE:
• l+l- edge
• The l+l- invariant mass from the
decay chain (right) has a
kinematic endpoint.
• For 100 fb-1, edge measured at
109.10±0.13(stat) GeV
• Dominant systematic error on
lepton energy scale also ~0.1%
• Maximum dilepton invariant mass
is related to sparticle masses
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Plenty of other kinematic endpoints!
RPC
Sequential
Branched
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Edge positions
Fitted distributions
ll
llq
ll
llq
lq high
lq low
lq high
lq low
llq
Xq
llq
Xq
Endpoint structure …
What different invariant mass
distributions look like for a selection of
plausible supersymmetric models.
( hep-ph/0410303 )
Note that some edges are not simple!
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Coverage of edges / Problems
However ...
RPC
… different processes can produce the same
final state.
• Can the process be identified?
• Detailed study of the shape of the distributions
can provide clues
• Likely coverage?
• Lepton edge observable over significant region
of m0, m1/2 parameter space (CMS plot left)
• See also hep-ph/0410303 and hepph/0501033 for more detailed analysis
• Likely outcome?
July 2005
• Precise sparticle mass differences – 1%
if lucky with which chains are open
• When chains are long enough, resolution on
absolute mass scale improves and can
measure mass of L.S.P.
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The sort of measurement you get
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Measuring spins in SUSY
•
•
•
•
hep-ph/0405052
pp-collider!
Protons have more quarks
than antiquarks
So LHC will make more
squarks than antisquarks!
Spin-1/2 neutralino can
tell the difference
between:
1. q+l or qbar+lbar, and
2. q+lbar or qbar+l
•
•
Look for asymmetry
between 1. and 2.
5 years’ data
1.5 years data (HL)
500 fb-1
150 fb-1
Asymmetry not washed out
(completely) by lepton ambiguity!
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Non-edge RPC methods
• For LONG enough decay
chains (4 or more 2-body
decays) kinematics of decaying
system are over-constrained
by observed momenta
• So can determine masses from
small sample of events
• O(N) events needed to
determine N unknown masses
• “Mass Relation Method” -proof of principle using 1000
events (hep-ph/0402295)
July 2005
For LONG decay chains.
At least 4 decays.
Mass relation method
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Mass Relation Method Results
mother (0 to 1000 GeV)
Gluino
Squark
Neutralino2
Slepton
Very good measurements of
mass differences < 1%
Reconstructed sparticle
masses as function of
reconstructed LSP mass
mneutralino (0 to 600 GeV)
July 2005
Correlations still make overall
mass scale hard to
determine, without input
from LC or say some other
independent LHC technique
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Putting it all together!
• Want to make fewer model assumptions
• Huge parameter spaces / model spaces need to
be explored
• Have large number of different measurements
we can make
• Need Markov Chain techniques to explore
likelihood surfaces efficiently
July 2005
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Exploring non-linear
experimental
constraints upon
susy model spaces
July 2005
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LHC & Supersymmetry
• What can the LHC provide if SUSY exists?
• DISCOVERY ? ………………………………. YES!
• Excellent prospects
• Might even be “easy” !
• Largely model-independent
• PRECISE MEASUREMENTS ? …….... Plenty!
• but more likely to be model-dependent
July 2005
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The End
•
We can expect ATLAS and CMS to
• Observe squarks and gluons
below 2.5 TeV and
observe sleptons below 300 GeV
in inclusive measurements.
• Accurately measure squark, slepton
and neutralino masses using
cascade decays (provided chains are
sufficiently long and rates are
favourable)
Other areas of completed and ongoing research
which there was not time to discuss:
•
•
•
•
•
•
•
•
•
N.L.S.P. lifetime in G.M.S.B. models
(Non-pointing photons / slow heavy leptons)
A.M.S.B. models
Lepton flavour violation (via slepton mixing)
Measuring the gaugino mixing matrix
Direct slepton production
Non-minimal models
SUSY Higgs sector
Everything else which I have forgotten to mention ...
• Determine spin of neutralinos
•
Success is expected in both RPV
and RPC scenarios
•
Precise measurements:
many can
be made in principle, but which of them can
measured in practice will depend strongly
on the model which nature has chosen
July 2005
CMS
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Missing energy – early reach
ATLAS
(Slide stolen from G.Polesello – SUSY2004)
RPC
Cross sections and rates
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