Transcript SUSY Spin Measurements - University of Cambridge

hep-ph/0405052
SUSY Spin Measurements
with ATLAS
Alan Barr
So … is it really supersymmetry?
“If it quacks like SUSY …”
“What else could it possibly be?”
“Don’t be so sure … ”
12 May 2004
Alan Barr
UK ATLAS Physics
(S)particle reminder
SM
Spin-1/2
Spin-1
Spin-0
SUSY
quarks (L&R)
leptons (L&R)
neutrinos (L&?)

Z0
W±
gluon
h0
H0
A0
H±
12 May 2004
Extended
higgs sector
Alan Barr
(2 doublets)
B
W0
squarks (L&R)
sleptons (L&R)
sneutrinos (L&?)
Bino
Wino0
Wino±
gluino
Spin-0
“Left squark” = scalar
SUSY partner of
left quark
After
Mixing
4 x neutralino
Spin-1/2
~
H0
~
H±
gluino
2 x chargino
UK ATLAS Physics
[RPC SUSY]
Gaugino mass term (GeV)
1-year low-lumi mSUGRA reach
• Sparticle decays
generate:
– Missing ET (LSP)
– Jets (squarks/gluino)
– Leptons (sleptons)
Plot by
D. Tovey
12 May 2004
Alan Barr
• “If we see these at
LHC we have
discovered SUSY”
• Not necessarily…
Scalar mass term
(GeV)
UK ATLAS
Physics
Universal Extra Dimensions
Cheng, Matchev
hep-ph/0205314
“How to get fooled at the LHC”
• TeV-scale universal extra dimension model
• Kaluza-Klein states of SM particles
– same QN’s as SM Radius of extra dimension ~ TeV-1
– mn2 ≈ m02 + n2/R2 [+ boundary terms]
– same spin as SM KK tower of masses n=0,1,…
– KK parity
• Lightest KK state stable, and weakly interacting
• 1st KK mode pair-produced
• First KK level looks a lot like SUSY
12 May 2004
Alan Barr
UK ATLAS
Physics
Dubbed “Bosonic
Supersymmetry”
Cheng, Matchev
UED KK spectrum
hep-ph/0205314
1st excited KK level
(example masses)
After adding boundary terms
-> SUSY-like spectrum
12 May 2004
Alan Barr
Example decay
spectrum
Stable lightest
KK particle
(weakly interacting)
ATLAS Physics
Need to UK
measure
spins!
SUSY decay chain
• Use mSUGRA
LHC point 5 as
example
• Well studied in
context of
sparticle masses
• Nice leptonic
signal at LHC
l {e, }
Spin-0
Spin-½
Spin-½,
mostly wino
Spin-0
Final state = jet + l+ + l- + ET
( + decay of other sparticle)
12 May 2004
Alan Barr
UK ATLAS Physics
Spin-½,
mostly bino
Spin projection factors
In rest frame of
with +ve z-axis
aligned with qL
q~L
~ 0
2
0
~
qL 
1
qL
~ 0
2
l

R
~
lR
12 May 2004 SM particles as massless
UK ATLAS Physics
Approximate
Alansince
Barr m « p
-> okay
P
S
Spin projection factors
In rest frame of
with +ve z-axis
aligned with qL
q~L
~ 0
2
2
Spin-0
l
~
lR

R
1
0
~
2 
0
12 May 2004 SM particles as massless
UK ATLAS Physics
Approximate
Alansince
Barr m « p
-> okay
S
Σ=0
qL
~ 0
0
~
qL 
1
P
S
Spin projection factors
In rest frame of
with +ve z-axis
aligned with qL
q~L
~20
l
θ*
qL
~ 0
2
l

R
~
lR
12 May 2004 SM particles as massless
UK ATLAS Physics
Approximate
Alansince
Barr m « p
-> okay
 (near)
R
p
S
Spin projection factors
In rest frame of
with +ve z-axis
aligned with qL
~ 0
2
Σ=0
l
~ 0
2
~
lR
Spin-0

R
1
0
~
2 
0
*

cos

pˆ  σ  cos( * ) 3  sin( * ) 1  
sin  *

 ~20 l R  sin 12  *


12 May 2004 SM particles as massless
UK ATLAS Physics
Approximate
Alansince
Barr m « p
-> okay
S
mql - measure
l
qL
q~L
0
~
qL 
1
P
θ*
S
 (near)
R
p
S
Spin projection factor
sin  * 

*
 cos 
invariant mass (1)
near
lepton
quark
Back to back
in 20 frame
Probability
near
l q
l+
θ*
Phase space
χ20
l-
Phase space -> factor of sin ½θ*
Spin projection factor in |M|2:
l+q -> sin2 ½θ*
l-q -> cos2 ½θ*

  
mlq2  2 pl  pq  2 pl p q 1  cos *  mlq2
12 May 2004
Alan Barr
max
m/mmax = sin ½θ*
sin 2 ( 12  * )
UK ATLAS Physics
lnearq invariant mass (2)
(Lepton from 2 decay only – not directly measurable)
0
sin 12  *
squark
decay
sin 12  *
0
1
l+
1
anti-squark
decay
l+
l-
Obvious asymmetry!
Opposite asymmetry!
-> if same # squarks as anti-squarks effect cancels
12 May 2004
Alan Barr
l-
UK ATLAS Physics
How to measure?
• lnearq shows nice charge asymmetry
0
~
– Excellent probe of spin of 
2
• Experimental problems in measurement:
– Can’t tell near lepton from far lepton
• Plot l+q and l–q instead -> contributions from
both near and far
– Can’t tell quark jet from anti-quark
• Sum of q and q-bar is measurable
• pp collider -> get squark asymmetry from PDFs
12 May 2004
Alan Barr
UK ATLAS Physics
Production asymmetry
qg -> squark + gluino
x1 and x2 sampled by MC
Valence quarks
Peak near 0.1, 0.1
Parton distribution function
has more q than q-bar
for Bjorken x ≈ 0.1
12 May 2004
Alan Barr
-> pp collider produces more
squarks than anti-squarks
UK ATLAS Physics
(Twice as much for our point)
Parton Level
l-
A 
l+
l l
  
l l
spin-0=flat
Charge
asymmetry
Experimentally measurable
-> q and q-bar
-> near and far leptons
12 May 2004
Alan Barr
difference/sum
Shape indicates that
0
UK 
ATLAS
Physics
2 is spin-½
l-
Change in shape
due to chargeblind cuts
l+
parton-level * 0.6
Charge asymmetry,
Events
After detector simulation (ATLFAST)
Invariant mass
spin-0
detector-level
-> Charge asymmetry survives detector sim
-> Same shape as parton level (but with BG and smearing)
12 May 2004
Alan Barr
UK ATLAS Physics
For cuts see C. Lester thesis
Cross-check
• Monte Carlo is
HERWIG (6.5)
• Can turn off spin
correlations
No asymmetry if spin
correlations turned off
– distribution for
“scalar 20”
– consistent with flat
– not consistent with
spin-½ 20 of previous
page (linearly
increasing)
spin-0=flat
~q
~
q
spin-½ ->
A (sin  )  A
spin-0 ->
A (sin 12  * )  0
12 May 2004
Alan Barr

1
2
*
2 sin(  ) 1
1
2
*
production asymmetry
UK ATLAS Physics
Further evidence - slepton spin
• Dilepton invariant mass.
Straight
line distn
(phase-space)
Back-to-back
in slepton frame
Shape after different-family
subtraction
12 May 2004
Alan Barr
– “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!
UK ATLAS Physics
Conclusions
• Method for measuring spins of SUSY
particles at the LHC
• Measure l±q invariant mass distributions
 
*
– equivalent to angular distributions in sin 12 
• Look for lepton charge asymmetry
• Requires initial asymmetry in squark vs antisquark 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
12 May 2004
Alan Barr
UK ATLAS Physics