Transcript Vadym Zhuravlov ATLAS MDT seminar 30 Jan 2007

Vadym Zhuravlov
ATLAS MDT seminar 30 Jan 2007
Contents:
1. Introduction: what is SUSY and why SUSY
2. Models, points, spectra
3. Production and decay of SUSY particles in
ATLAS
4. Inclusive searches – missing Et sigmature
5. SUSY spectroscopy
6. Spin measurement
7. Stransverse mass
8. Conclusion
•A symmetry which relates bosons and fernions and represented
by operator
Q |BOSON> = |FERMION> and Q |FERMION> = |BOSON>
• Q does not change the particle quantum numbers
• Even if there is no “WHY” still there is a question:
why there are two classes of
particles in nature –
bosons and fermions
• Invented more then 30 years ago and still not discovered
( Higgs also)
• Provides unification of gauge coulings:
(requires SUSY masses below few TeV)
• Provides a good candidate for
Dark Matter – lightest
neutralino (R-parity is
conserved)
• Solves “hierarchy” problem
ΔMHiggs ~ Λ
ΔMHiggs ~ Λ2
SM is effective theory at
E<<Λ (~1019 GeV)
MHiggs(tree level) ~ 1038 GeV
ΔMHiggs ~ log Λ
SUSY:
Fine tuning needed!
“Not natural…”
=0
Barnett Newman “Broken Obelisk”
SUSY fields and particles
•SM: 28 bosonic and 96 fermionic DOF – highly nonsupersymmetric!
•Fields -> superfields
• 2 complex Higgs fields: h, H, A, H+, Htanb = V1/V2
MSSM – 124 parameters.
SUSY is a broken symmetry.
• Non of MSSM fields can develop non-zero VEV to break SUSY.
Hidden sector where SUSY is broken.
• Messenger: transmit broken SUSY to visible sector.
1. Gravity mediated SUSY breaking: gravitino mass ~ EW mass
mSUGRA parameters: m0, m1/2, A0, tanb, sign(m)
1. Gauge mediated SUSY breaking: messinger sector consists of
particles with SU(3)xSU(2)xU(1) quantum numbers
2. Anomaly mediation: SUSY is broken in another brane.
BULK
superfields
GeV
SUGRA: DM favored regions
1. Bulk region: light susy particles.
cc → ffbar t-channel slepton
exchange
2. Stau coannihilation region:
masses of stau and neutralino
are almost degenerate.
Neutralono-stau co-annihilation
ct → t* → tg
3. Focus point: heavy squarks and
sleptons and light neutralino
(almost higgsino) cc → WW, ZZ,
qqbar
4. Funnel region: mA=2mc.
Resonant annihilation cc → A(H)
→ ffbar
R = (-1)3(B-L)+2s
“+” for ordinary particles
“-” for supersymmetrical partners
If R-parity is conserved, SUSY-particles are
created in pairs, LSP is stable
Under R-parity the lepton and barion numbers are
conserved
Signature:
•High missing energy (LSP is undetected)
•High-Pt jets and leptons
0-lepton mode
1-lepton mode
2-lepton mode
MET > 100 GeV
1 jet with Pt>100 GeV
4 jets with pt>50 GeV
Transv. Spher. > 0.2
MET > 100 GeV
2 jet with Pt>100 GeV
4 jets with pt>50 GeV
Transv. Spher. > 0.2
m or e pt>10 GeV
MET > 100 GeV
2 jet with Pt>100 GeV
4 jets with pt>50 GeV
2 m or e pt>20 GeV
Meff = Sjjets|Pt| + Et miss correlated to MSUSY
MSUSY = SMisi / Ssi
No lepton mode
ATLAS TDR
S/B = 10
S/B = 2
Matrix Element calculation VS Parton Showering
General idea: no-lepton estimates from tagged lepton samples
Z  :
- Z  l+l- : easy, but suffers from statistics
- W  l with l+ into pseudo-missing-ET
- MC based estimates
W  l:
- W  l with lepton tagged
- W + 1 jet (pure), extrapolation to >= 4 jets with
MC reweighting and Z+jets samples
- MC based estimates
ttbar:
bbqql with lepton tagged
why lepton missed:
60% tau
35% e, acceptance
5% isolation
Background: ttbar -> lnln (one l is missing) and ttbar -> qqbar ln
W -> ln
General ideas:
other variable
SUSY signal
plus bg
bg
A
D
B
C
bg
bg
Missing ET
Bg in D = A x C/B
normalize to data
decay resimulation
Select pure ttbar sample, reconstruct kinematics
u
quark
d
antiquark
Resimulate
W decay and
replace in
original event
Now have bbll event
decay resimulation:
•
•
•
Goal is to model complex background events using samples of
tagged SM events.
Initially we will know:
– a lot about decays of SM particles (e.g. W, Z)
– a reasonable amount about the (gaussian) performance of
the detector.
– rather little about PDFs, the hard process and Underlying
Events.
Philosophy
1. Tag ‘seed’ events containing Z/W/top
2. Reconstruct 4-momentum of Z/W/top (x2 if e.g. ttbar)
3. Decay/hadronise with e.g. Pythia
4. Simulate decay products with atlfast or fullsim
5. Remove original decay products from seed event
6. Merge new decay products with seed event (inc. ETmiss)
7. Perform standard SUSY analysis on merged event
Background: ttbar->bblnln
Bbqqln with second
lepton from b/c decay
Inclusive reach in mSUGRA parameter space
Reach sensitivity only weakly depends on tanb, A0 and m
SUSY spectroscopy
• Inclusive searches: Meff =
Sjets|Pt| + Etmiss correlated to
MSUSY=SMisi / Ssi
MeffSUSY = MSUSY-Mc2/MSUSY
SUGRA: excelent correlation,
MSSM: acceptable
10% measurement @ 100fb-1
SUSY spectroscopy
• Due to R-parity conservation all SUSY events contain 2 neutralino
which escape the detector.
• Since neutralino are not detected, one can measure the kinematic
end-points rather then mass-peaks.
p
p
~
g
q
~
q
q
~
c0 2
~
c0 1
~
l
l
l
SUSY spectroscopy (2)
p
p
~
g
q
~
q
~
c0 2
q
Di-lepton invariant mass:
e+e- + µ+µ- - e±µ± ATLAS
Preliminary
~
c0 1
~
l
l
Minv
l
~
c~20  l  l   c~10l  l 
M ll
max

( M c~2 0  M ~l2 )( M ~l2  M c~2 0 )
2
1
M
2
~
l
 100.31 GeV
•Event selection: Electrons and muons
with PT ≥ 20 GeV
•Separate leptons from jets by ΔR > 0.4
• Fitted endpoint: 100.25 ± 1.14 GeV
which is consistent with the expected
value within the error
SUSY spectroscopy (3) – more endpoints
p
p
~
g
q
~
q
q
~
c0 2
~
c0 1
~
l
l
l
llq edge
lq edge
1% error
(100 fb-1)
1%
error
(100 fb1)
SUSY spectroscopy (3) – mass peaks
p
p
~
g
q
~
q
~
c0 2
q
~
c0 1
~
l
l
l
The 4-momentum of the c02 can be
reconstructed from the approximate
relation
p(c02) = ( 1-m(c01)/m(ll) ) pll
valid when m(ll) near the edge.
The c02 can be combined with b-jets
to reconstruct the gluino and
sbottom mass peaks from
g→bb→bbc02
Sparticle Expected precision (100 fb-1)
~L
q
 3%
~
c02
 6%
l~R
 9%
~
c01
 12%
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
•charge asymmetry of lq pairs
measures spin of c02
•shape of dilepton invariant mass
spectrum measures slepton spin
Measure
Angle
Spin-0
Spin-½
Polarise
Spin-½,
Spin-0
mostly wino
A

l  l
  
l l
Spin-0 flat
Spin-½,
mostly bino
stransverse mass
Transverse mass Mt– endpoint is a mass of decaying particle (W)
Stransverse mass Mt2– endpoint is a mass of c
stransverse mass – direct slepton production
Signature: two opposite sign
same flavor leptons and missing
Et
Endpoint of stransverse mass is
a function of mass difference
of slepton and LSP
MT2
Right-Handed Squark Mass
Determine the mass of righthanded squarks from: q~R  c~10q
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
Other topics:
1.
2.
3.
4.
5.
R-hadrons
Tau-signatures
Gaugino direct production
Study of gauge-mediated SUSY
R-parity violating processes
Conclusion:
• LHC is last chance to discover SUSY
• SM uncertainties in the BG estimation is a limiting factor
• Many models, parameters, preferable points: lot of work