Spin Measurements in Supersymmetry at the LHC

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Spin Measurements in Supersymmetry at the LHC

• Christopher Lester
• Cavendish Laboratory, Cambridge
• (on behalf of the ATLAS collaboration)

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This talk is Backwards
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not upside down !
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Conclusions

• ATLAS does not yet know the circumstances
  (models, masses, integrated luminosities) in
  which the spins of supersymmetric particles will
  be unambiguously measurable
• There are lots of good and promising ideas both
  within ATLAS and outside
• some are partly tested
• some are un-tested
• plenty of work remains to be done!

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What is Discovering SUSY ?

• What makes Supersymmetry different to Universal
  Extra Dimensional (UED) models?
• One part of the answer

SPIN
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We will see two important themes

• Mass measurements will precede() spin
  determinations
• Spin measurement() should not be confused
  with sensitivity to spin
• () or will at best be simultaneous with
• () Here spin measurement means determining
  unambiguously the correct nature (scalar,
  fermion, vector) of one or more particles in a
  decay chain or model

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The methods considered here

• Spins in QLL chain
• A.Barr hep-ph/0405052 ATL-PHYS-2004-017
• Smillie et al hep-ph/0605286 (including ATLAS
  member)
• Biglietti et al ATL-PHYS-PUB-2007-004
• (Study of second lightest neutralino spin
  measurement with ATLAS detector at LHC
• Matchev et al arXiv0808.2472
• Slepton Spin (pair production)
• A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023

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Other interesting methods

• Gluino chain spin
• Alvez, Eboli, Plehn hep-ph/0605067
• MAOS method
• Cho, Kong, Kim, Park arXiv0810.4853
• Spins in chains with charginos
• Wang and Yavin hep-ph/0605296
• Smillie hep-ph/0609296
• Spins in chains radiating photons
• Ehrenfeld et al arXiv0904.1293

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Spin Consistency Check
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Spin Consistency Check

• Consistent with
• Phase-space
• Scalar slepton (SFSF)
• Fermion KK lepton (FVFV)

Straight line
Relative Frequency
Di-Lepton Invariant Mass (GeV)
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QL Spin Determination (A.Barr)
hep-ph/0405052 ATL-PHYS-2004-017
2 problems
How can we distinguish the near lepton from the
far lepton?
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QuarkNearLeptoninvariant mass distributions for
hep-ph/0405052
ATL-PHYS-2004-017
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Experimental problem

• Cannot reliably distinguish QUARKs from ANTI
  QUARKs

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Expect QUARK and ANTI-QUARK contributions to
cancel
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But LHC is Proton-Proton machine

• More Quarks than Anti-Quarks! So get

hep-ph/0405052
ATL-PHYS-2004-017
Asymmetry!
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Far Lepton washout?(though compare with later
comments Matchev arXiv0808.2472)
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So define mjL, mjL- asymmetry
where
parton-level
Spin-½
500 fb-1
spin-0
Asymmetry A
hep-ph/0405052
ATL-PHYS-2004-017
detector-level
M(QL)
MjL / GeV ? sin ½?
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Charge asymmetry at detector level
ATL-PHYS-2004-017
Susy spins compared only with no spin (i.e. with
phase space)
Barr hep-ph/0405052
Points are for 500 fb-1
M(QL)
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Tested further in ATLAS
ATL-PHYS-PUB-2007-004
Biglietti et al
Preselection cuts
Dilepton selection cuts
Parametrised Simulation (ATLFAST)
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Di-lepton mass spectra
ATL-PHYS-PUB-2007-004
Biglietti et al
SU1 MC truth
SU3 MC truth
M(LL)
M(LL)
Here at SU1 there are two sleptons (êL 255 GeV,
êR155 GeV) below chi2 264 GeV. Chi1137 GeV
Here at SU3 there is only one accessible slepton
below chi2 Chi2219, êR155, Chi1118 GeV
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Jet-lepton mass spectra
ATL-PHYS-PUB-2007-004
Biglietti et al
SU1 MC truth
SU3 MC truth
M(QL)
M(QL)
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Truth Asymmetry
ATL-PHYS-PUB-2007-004
Biglietti et al
SU3 MC truth
A
where
30 /fb
M(QL)
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Truth Asymmetry
ATL-PHYS-PUB-2007-004
SU1 MC truth
Biglietti et al
Shows asymmetry depends on particle identities
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Backgrounds
ATL-PHYS-PUB-2007-004
Biglietti et al

• Main background is SUSY
• Mostly defeated by flavour subtraction
• Main Standard Model backgrounds are
• Di-top jets
• W / Z jets

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Fast simulation results
ATL-PHYS-PUB-2007-004
Biglietti et al
Note ... Flavour subtraction
SU1 ATLFAST
SU3 ATLFAST
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Fast simulation results
ATL-PHYS-PUB-2007-004
Biglietti et al
Note ... Flavour subtraction
SU1 ATLFAST
SU3 ATLFAST
A
A
30 /fb
100 /fb
M(JL)
M(JL)
Sufficient power to eliminate null hypothesis
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Safety check
ATL-PHYS-PUB-2007-004
Biglietti et al
Expect to see NO ASYMMETRY in these OFOS lepton
plots. Confirmed!
SU1 ATLFAST
SU3 ATLFAST
30 /fb
100 /fb
M(JL)
M(JL)
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Not all things that quack are ducks!
QUACK !
UED
SUSY
QUACK !
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UED and SUSY not distinguished by dilepton mass
spectrun
Compare ratios of dilepton invariant mass
distributions for different spin configurations
(SUSY is horizontal)
hep-ph/0605286
M(LL)2
M(LL)2
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Jet lepton asymmetry not good at distingusihing
UED and SUSY either
UED masses
SPS 1a masses
Not resolvable for UED masses, maybe for SUSY
Charge asymmetry due to quark vs antiquark excess
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Jet lepton asymmetry not good at distinguishing
UED and SUSY either.
UED masses
SPS 1a masses
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ANY-Spin Determination (Smillie et.al.)Later
fuller follow-up (Matchev, Kong, et al)
(SUSY)
(UED)
hep-ph/0605286
relative probability
arXiv0808.2472
M(JL)2
Cannot distinguish
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Direct di-slepton production
A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023

• Event selection
• two single leptons
• transverse missing energy gt 100 GeV
• no jets with pTgt 100 GeV
• no b-jets
• mT2(l, l-) lt 100 GeV
• Mlllt 150 GeV
• missing pT pT(l), pT(l-) lt100 GeV
• Lepton pT(l1) gt 40 GeV, pT(l2) gt 30 GeV

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Look at slepton production angle
A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023
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Have some access to desired angle
A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023
Distribution of
is correlated with decay angle
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Fast simulation plot
A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023
Outer error bars after SUSY SM background
subtraction
Significance strongly dependent on mass spectrum
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End Notes

• QLL chain
• Some spin sensitivity but no strong UED/SUSY
  separation
• Reduced discriminatory power when considering
  general couplings (Matchev/Kong).
• Di-slepton production
• Better chance of separating UED/SUSY
• Still model dependent
• Both require large cross sections

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Backup slides
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MT2-assisted spin determination
assign 4-momenta
SUSY
UED
SUSY
UED
Cho, Choi,Kim,Park, 0810.4853
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Cross sections imply spins
Higher spins mean higher cross sections (for
given masses)
Datta, Kane, Toharia hep-ph/0510204
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SU1 (stau co-annihilation) point
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SU3 bulk point
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SU3 bulk point
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Helicity dependence
Process 1 (SUSY)
Process 1 (UED, transverse Z P /P 2x)
T
L
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Example mass spectra

• SPS1a mass spectrum (GeV)
• UED-type mass spectrum (GeV)
• (R-1 800 GeV)