12 May 2004 Alan Barr

1
SUSY Spin Measurementswith ATLAS
hep-ph/0405052

• Alan Barr

So is it really supersymmetry?
If it quacks like SUSY
What else could it possibly be?
Dont be so sure
2
(S)particle reminder
SM
SUSY
Left squark scalar SUSY partner of left
quark
quarks (LR)leptons (LR) neutrinos (L?)
squarks (LR)sleptons (LR)sneutrinos (L?)
Spin-1/2
Spin-0
AfterMixing
?Z0 W gluon
BinoWino0Wino gluino
BW0
Spin-1
4 x neutralino
Spin-1/2
gluino

h0 H0 A0 H
H0H

2 x chargino
Spin-0
Extended higgs sector (2 doublets)
3
RPC SUSY
1-year low-lumi mSUGRA reach

• Sparticle decays generate
• Missing ET (LSP)
• Jets (squarks/gluino)
• Leptons (sleptons)
• If we see these at LHC we have discovered SUSY
• Not necessarily

Gaugino mass term (GeV)
Plot by D. Tovey
Scalar mass term (GeV)
4
Universal Extra Dimensions
Cheng, Matchev
How to get fooled at the LHC
hep-ph/0205314

• TeV-scale universal extra dimension model
• Kaluza-Klein states of SM particles
• same QNs as SM
• mn2 m02 n2/R2 boundary terms
• same spin as SM
• KK parity
• Lightest KK state stable, and weakly interacting
• 1st KK mode pair-produced
• First KK level looks a lot like SUSY

Radius of extra dimension TeV-1
KK tower of masses n0,1,
Dubbed Bosonic Supersymmetry
5
UED KK spectrum
Cheng, Matchev
hep-ph/0205314
Example decay spectrum
1st excited KK level (example masses)
Stable lightestKK particle(weakly interacting)
After adding boundary terms -gt SUSY-like spectrum
Need to measure spins!
6
SUSY decay chain
Spin-0

• Use mSUGRA LHC point 5 as example
• Well studied in context of sparticle masses
• Nice leptonic signal at LHC

Spin-½
Spin-½, mostly wino
Spin-0
Spin-½, mostly bino
Final state jet l l- ET ( decay of
other sparticle)
7
Spin projection factors
P
In rest frame of with ve z-axis aligned with qL
S
Approximate SM particles as massless -gt okay
since m p
8
Spin projection factors
P
In rest frame of with ve z-axis aligned with qL
S
S0
S
Spin-0
Approximate SM particles as massless -gt okay
since m p
9
Spin projection factors
In rest frame of with ve z-axis aligned with qL
?
p
S
Approximate SM particles as massless -gt okay
since m p
10
Spin projection factors
P
In rest frame of with ve z-axis aligned with qL
mql - measure
S
?
S0
p
S
S
Spin projection factor
Spin-0
Approximate SM particles as massless -gt okay
since m p
11
lnearq invariant mass (1)
Back to backin ?20 frame
nearlepton
quark
Probability
l
?
Phase space
?20
l-

• Phase space -gt factor of sin ½?
• Spin projection factor in M2
• lq -gt sin2 ½?
• l-q -gt cos2 ½?

m/mmax sin ½?
12
lnearq invariant mass (2)
(Lepton from ?2 decay only not directly
measurable)
0
1
0
1
squarkdecay
anti-squarkdecay
l
l-
l
l-
Obvious asymmetry!
Opposite asymmetry!
-gt if same squarks as anti-squarks effect
cancels
13
How to measure?

• lnearq shows nice charge asymmetry
• Excellent probe of spin of
• Experimental problems in measurement
• Cant tell near lepton from far lepton
• Plot lq and lq instead -gt contributions from
  both near and far
• Cant tell quark jet from anti-quark
• Sum of q and q-bar is measurable
• pp collider -gt get squark asymmetry from PDFs

14
Production asymmetry
qg -gt squark gluino
x1 and x2 sampled by MC
Valence quarks
Peak near 0.1, 0.1
Parton distribution functionhas more q than
q-bar for Bjorken x 0.1
-gt pp collider produces more squarks than
anti-squarks
(Twice as much for our point)
15
lfarq invariant mass
(Lepton from slepton decay only not directly
measurable)
squarkdecay
anti-squarkdecay
l-
l
l
l-
Some asymmetry inherited from boost of slepton
in ?20 rest frame
16
Parton Level
l-
l
spin-0flat
Chargeasymmetry
Experimentally measurable-gt q and q-bar -gt near
and far leptons
difference/sum
Shape indicates that?20 is spin-½
17
After detector simulation (ATLFAST)
Change in shape due to charge-blind cuts
l-
parton-level 0.6
Events
spin-½
Charge asymmetry,
spin-0
l
detector-level
Invariant mass
-gt Charge asymmetry survives detector sim-gt Same
shape as parton level (but with BG and smearing)
For cuts see C. Lester thesis
18
Cross-check

• Monte Carlo is HERWIG (6.5)
• Can turn off spin correlations
• distribution for scalar ?20
• consistent with flat
• not consistent with spin-½ ?20 of previous page
  (linearly increasing)

No asymmetry if spincorrelations turned off
spin-0flat
spin-½ -gt
spin-0 -gt
production asymmetry
19
Further evidence - slepton spin

• Dilepton invariant mass.
• Right-handed slepton
• l and l- are right handed
• might expect pronounced spin effects (as for lq
  distributions)
• none because slepton is scalar
• Scalar particle carrying lepton number
• sounds like a slepton to me!

Straightline distn
(phase-space)
Back-to-backin slepton frame
Shape after different-family subtraction
20
Conclusions

• Method for measuring spins of SUSY particles at
  the LHC
• Measure lq invariant mass distributions
• equivalent to angular distributions in
• Look for lepton charge asymmetry
• Requires initial asymmetry in squark vs
  anti-squark production
• valence quarks in PDFs at x 0.1
• Measure spin-½ nature of ?20
• Also spin-0 slepton from mll
• Should be enough to kill straw man e.g.
  universal extra dimensions