Transcript Document 7159645

SUSY searches at the LHC
Arthur M. Moraes
Brookhaven National Laboratory
(on behalf of the ATLAS Collaboration)
LISHEP 2006 – Workshop on Collider Physics
Itacuruçá, April 5th 2006.
I.
Introduction: Why Supersymmetry?
II.
ATLAS and CMS;
III.
SUSY searches at the LHC:
IV.

What will SUSY events look like?

Inclusive reach in mSUGRA parameter space.

SUSY mass scale measurements.

Dilepton mass distribution.

Right-handed squark mass.

SUSY spin measurement.
Summary
Arthur Moraes
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SUSY searches at the LHC
Outline:
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Supersymmetry (SUSY): why are we interested?
SUSY has many merits:
 it is elegant;
 assuming the existence of superpartners with TeV-scale masses, the
Strong, Weak and Electromagnetic force strengths become equal at the “GUT
scale”;

from experimental limits, squarks and gluinos must be heavy (>400GeV)
 SUSY theories (RPC) provide explanation to the dark matter in the Universe:
“neutralinos” (the lightest SUSY particles – LSP).
 If SUSY is a true symmetry of Nature and it is realized at the TeV scale, it will
almost certainly be discovered by ATLAS and CMS.
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SUSY searches at the LHC
 it provides a natural explanation of why the Higgs mass can be low (< 1 TeV).
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SUSY searches at the LHC
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LHC (Large Hadron Collider):
• p-p collisions at √s = 14TeV
• bunch crossing every 25 ns (40 MHz)
 low-luminosity: L ≈ 2 x 1033cm-2s-1
(L ≈ 20 fb-1/year)
 high-luminosity: L ≈ 1034cm-2s-1
(L ≈ 100 fb-1/year)
Status of the LHC:
 Priority: complete the LHC project
(machine/detectors/LCG) by Spring 2007.
SUSY searches at the LHC
•
 April 2007: start machine
commissioning (mainly single beam)
 ~ Summer 2007: two beams in the
machine (first collisions!)
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ATLAS: A Toroidal LHC AparatuS
Muon Detectors
Tile Calorimeter
Liquid Argon Calorimeter
ATLAS and CMS are well
adapted to any new physics.
TRT Tracker
Toroid Magnets
SCT Tracker
Solenoid Magnet
Pixel Detector
SUSY searches at the LHC
• Multi-purpose detectors
coverage up to |η| = 5;
design to operate at L= 1034cm-2s-1
• The most important aspect of any detector used
for inclusive SUSY searches are the energy
resolutions and hermeticities of the calorimeters.
• Mismeasurements of jets by the ECAL and
HCAL can lead to significantly increased
rates of high-ETmiss background QCD events.
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CMS: Compact Muon Solenoid
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CMS
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SUSY searches at the LHC
ATLAS
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Physics goals and potential for the first year (a few examples…)
Expected event rates for ATLAS (or CMS) at L = 1033 cm-2 s-1
Events/s
Events for 10 fb-1
W e
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~108
Z ee
1.5
~107
-
~1
107
~106
1012 – 1013
H m=130 GeV
0.02
105
~~
gg m= 1 TeV
0.001
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Black holes
0.0001
103
tt
-
bb
Already in the first year, large statistics
expected from:
 known SM processes (understand the
detector and physics at the LHC)
 several new physics scenarios can be
tested
m > 3 TeV
(MD=3 TeV, n=4)
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SUSY searches at the LHC
Process
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Going “beyond the Standard Model” at the LHC
 ATLAS and CMS will probe physics at TeV mass scale.
ATLAS: illustration of a
black-hole event
 LHC detectors and BSM physics: we do not know what might be
found! But “any” new physics must decay to SM particles and/or
new stable ones.
- Absolutely stable particles must be neutral and weekly
interacting. Hence escape detector giving missing ET.
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LISHEP 2006 – April 5th 2006
SUSY searches at the LHC
 Goals include searching for the Higgs boson and for
other physics “Beyond the Standard Model” (BSM) such
as SUSY particles.
–But we cannot see Higgs/SUSY particles directly
as they either decay to lighter (stable) particles or
cannot be seen with any known detector.
–We have to design our detector to look for the
stable particles and signs of “invisible” particles…..
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SUSY: what will it look like?
g~
q~
02
Z
01
 SUSY could provide quite spectacular
events with many leptons as well as
jets and missing transverse energy.
 mSUGRA: minimal Supergravity
used as a standard benchmark
model.
CMS
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q
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SUSY searches at the LHC
 The SUSY cross-section at the LHC is
dominated by associated strong production
of gluinos and squarks. SUSY might be
observed with modest integrated luminosity
-However, much work will be necessary
to check our understanding of the
detectors and the physics backgrounds
before announcing any discovery!
q
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Inclusive reach in mSUGRA parameter space
 mSUGRA framework: five free parameters: m0,
m1/2, A0, tan(β), sgn(µ)
 Map of discovery potential corresponding to a 5σ
excess above background in mSUGRA m0 – m1/2
parameter space for the ATLAS experiment.
only statistical errors included;
detector effects simulated
using fast simulation;
no pile-up included;
 jets + ETmiss channel (no lepton
ATLAS
requirement) gives greatest discovery
potential!
 next greatest discovery potential: lepton veto
channel (‘0l’).
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LISHEP 2006 – April 5th 2006
SUSY searches at the LHC
SM background taken into account:
- W+jet, Z+jet and QCD
tt,
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Inclusive reach in mSUGRA parameter space
 Map of discovery potential corresponding to a 5σ
L = 1033 cm-2 s-1
excess above background in mSUGRA m0 – m1/2
parameter space for the ATLAS experiment.
~1 year → ~2200 GeV
~1 month → ~1800 GeV
ATLAS
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few days (< one week) → ~1300 GeV
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SUSY searches at the LHC
jets + ETmiss channel
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Inclusive reach in mSUGRA parameter space
 Map of discovery potential corresponding to
a 5σ excess above background in the mSUGRA
parameter space for the CMS experiment.
 Signal selection: performance similar to
that found in ATLAS studies!
Best channel: jets + ETmiss channel
(no lepton requirement)
 A factor two increase or decrease in total
background cross-section results in small
effect on overall discovery potential (few tens
of GeV).
SUSY searches at the LHC
 Similar reach in gluino and squarks mass
should apply to any model in which they decay
into an invisible and relatively light LSP.
 If R parity is violated the presence of
additional leptons make the discovery easier!
For example:
~10  l l 
~10  qq l
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SUSY mass scale measurements: MSUSY
m0=70GeV, m1/2=350GeV, A0=0, tan β=10
 Define effective mass variable (measurement):
20.6fb−1
ATLAS
M eff  ETmiss   | p Tjeti |
SUSY signal
(full simulation)
i
(pTjeti: transverse momentum of jet i)
 The effective Mass gives a handle on the SUSY
mass scale (Hinchliffe et al., Phys. Rev. D55 (1997) 5520):
-
M SUSY  min( m~g , m~q )
SM background
(PS event generator
& fast simulation)
 The peak of the distribution of Meff values for SUSY
events should lie at around twice MSUSY due to the
kinematics of the SUSY particle decay processes.
Arthur Moraes
tt
W+jet
Z+jet
QCD
 Cuts to reject SM background
4 jets with pT > 50GeV
2 jets with pT > 100GeV
ETmiss > max(0.2Meff,100GeV)
no lepton
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SUSY searches at the LHC
low mass
scales (~TeV)
M eff  M SUSY
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SUSY mass scale measurements
 The presence of a high mass Lightest Supersymmetric Particle (LSP) reduces
the number and pT of observed jets.
 In practice, preferable to consider measurements of an effective mass scale
(MeffSUSY) which takes the LSP mass into account.
 ( M SUSY 
M 2
M SUSY
)
SUSY searches at the LHC
M
eff
SUSY
systematic errors
not included;
Scatter plots:
correlation between
Meff and MeffSUSY
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SUSY mass scale measurements
 Signal / background separation: looks possible but need to
verify new multi-parton (ME-PS matching) generators.
 PS event generators: good in the collinear region but
problem in high-pT region!
“1 lepton mode”
10fb−1
10fb−1
Meff (GeV)
Meff (GeV)
 Background increases by a factor of
 Signal reduced to 20 – 40% of no lepton mode.
2 – 4 compared to PS prediction.
Backgrounds are suppressed by a factor of 20 – 30!
 Depending on Meff signal/background
 One lepton mode gives clean discovery.
separation can be severely affected!
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SUSY searches at the LHC
Number of events / 400GeV
Number of events / 400GeV
“no lepton mode”
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Dilepton invariant mass
 After initial discovery of SUSY the measurement of
the sparticle masses will be the next step.
 In mSUGRA and most SUSY models, all SUSY
~ 0
particles decay to invisible
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no mass peaks!
 Specific decays can often be
identified.
~ 
0
~
 2  l l  ~10l  l 
M (l l  )  M ( ~20 )  M ( ~10 )
 Note: ATLAS and CMS have comparable acceptance for e and μ.
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SUSY searches at the LHC
 Use kinematic endpoints to measure
mass combinations.
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Dilepton invariant mass
 Remove SUSY/SM BG using Opposite Flavor/Opposite Sign (OF/OS) pairs.
Plot:
 
 
 
m(e e )  m(μ μ )  m(e μ )
Event selection: Electrons and muons with PT ≥ 20 GeV
Separate leptons from jets by ΔR > 0.4
m0=100GeV, m1/2=300GeV, A0=-300, tan β=6
4.2fb−1



ATLAS
Only SUSY signal events.
No SM background cuts.
Fit a triangular shape distribution
convoluted with a Gaussian.
mllmax = 100.31 GeV (input)
Medge = 100.25 ± 1.14 GeV (edge
position)


After SM background cuts & dilepton
selection.
Reduced statistics but triangular
shape still visible.
Fitted value after cuts:
mllmax = (99.8±1.2) GeV
Events

SUSY searches at the LHC
4.37fb−1
ATLAS
mll (GeV)
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CMS
 This technique (kinematic endpoints) can be combined with the analysis of
different end-points to constrain sparticle masses!
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SUSY searches at the LHC
 Dilepton endpoints observable over wide range of mSUGRA parameter space
scanned with fast simulation.
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Right-handed squark mass
jet2
~
qR
 mSUGRA (R-parity conserved): sparticles are pair produced
and cascade decay to the LSP.
 mSUGRA – right handed squark: usualy large
BR( ~
qR  q~
χ10 )
~
χ10
 The signal is two hard jets plus large ETmiss
 Event selection: ETmiss > 200 GeV
Two jets with ET >150 GeV
No reconstructed electrons or muons
 Calculate the stransverse mass of the two hard
jets. The endpoint gives the mass of right-handed
squarks
~
qR
jet1
ATLAS
min {max[ mT ( pT, j1, p T,1 ; M (~
χ10 )), mT ( pT, j2 , p T,2 ; M (~
χ10 )]}
p T,1  p T,2  p T
~0
 Take M ( χ1 ) from dilepton and dilepton+jet measurements.
qR ) is obtained from the endpoint
χ10 ) is known, M (~
 If M (~
of the M T2 distribution.
 Fitted endpoint:
M q~R  619  8.2 GeV for M ~ 0  100 GeV (input )
1
M q~R  614  6.7 GeV and 601.8  8.8 GeV for  10% error on M ~ 0
1
 Good agreement with actual value:
Arthur Moraes
M u~R  611.8 GeV , M d~  610.7 GeV
SUSY searches at the LHC
M T22 
~
χ10
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SUSY Spin Measurement
 If SUSY signals are observed at the LHC, it will be vital to
measure the spins of the new particles to demonstrate that
they are indeed the predicted super-partners.
 Angular distributions in sparticle decays lead to charge
asymmetry in lepton-jet invariant mass distributions. The
size of the asymmetry is proportional to the primary
production asymmetry between squarks and anti-squarks
LHC will generate more squarks
than anti-squarks (pp collider)!

mSUGRA
 l(near)q invariant mass distribution measure angular
χ 20 decays, and hence ~
χ 20 spin.
distribution of products of ~
ATLAS
A

l  l
 
l  l
 Cuts to reject SM background
4 jets with pT > 50GeV
1 jets with pT > 100GeV
ETmiss > max(0.2Meff,100GeV)
Meff > 400GeV
2 leptons: same family & opposite
signs, pT > 10GeV
Arthur Moraes
Parton-level
result (rescaled!)
Detector simulation
(500fb-1)
fast simulation
Spin correlations
suppressed!
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SUSY searches at the LHC
Lepton charge asymmetry
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Summary
 Standard Model is very successful but fails to address several crucial issues.
Models of physics beyond the Standard Model at TeV mass scale have been
ongoing for at least 25 years. SUSY is a good candidate!
 The final word can only come from experiments capable of probing TeV mass scale.
LHC can potentially start answering some of our questions on physics BSM at some
point next year.
 SUSY discovery is possible in other models which I have not covered here:
Gauge Mediated Supersymmetry Breaking (GMSB)
Anomaly Mediated Supersymmetry Breaking (AMSB)
R-Parity Violation
Arthur Moraes
LISHEP 2006 – April 5th 2006
SUSY searches at the LHC
ATLAS and CMS:
Observe squarks and gluinos below ~2.5 TeV
Accurately measure squark, slepton and neutralino masses using cascade
decays (provided chains are sufficiently long and rates are favourable)
Determine spin of neutralinos
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SUSY searches at the LHC
Backup
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Searching for SUSY
SUSY searches at the LHC
 A key signature for SUSY is large
missing transverse energy
associated with the non-interacting
Lightest SUSY Particle (LSP) that is
stable under the assumption of Rparity conservation
– Ingredients for good missing-ET
resolution are good hadronic
calorimeters and with
“hermetic” coverage
– Note that LSP is candidate for
dark matter
 Need calorimeter coverage up to |η| ~ 5,
otherwise high-pT jets outside acceptance
give large fake missing ET signature
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LISHEP 2006 – April 5th 2006
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mSUGRA parameters:
SUSY searches at the LHC
• m0 – common scalar mass
• m1/2 – common gaugino mass
• tan β – ratio of the vacuum expectation values of the two
Higgs doublets in the model
• A0 – common trilinear coupling
• sgn μ – higgsino mass parameter
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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:
(RPV/RPC)
RPC
RPV
How stable is the
lightest SUSY
particle?
Stable
Large
missing
energy?
Yes
Unstable
No
(decays to leptons or
jets)
Arthur Moraes
Event can be
reconstructed
fully?
Usually not
Sparticle
production
Yes
Either singly,
or in pairs
Only in pairs
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SUSY searches at the LHC
– RP-Conserving
– RP-Violating
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