SUSY Spin Measurements with ATLAS
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Transcript of SUSY Spin Measurements with ATLAS
12 May 2004Alan Barr
UK ATLAS Physics
SUSY Spin Measurementswith ATLASAlan Barr
“What else could it possibly be?”
“Don’t be so sure … ”
hep-ph/0405052
“If it quacks like SUSY …”
So … is it really supersymmetry?
12 May 2004Alan Barr
UK ATLAS PhysicsExtended higgs sector(2 doublets)
(S)particle reminderSM SUSY
quarks (L&R)leptons (L&R) neutrinos (L&?)
squarks (L&R)sleptons (L&R)sneutrinos (L&?)
Z0
W±
gluon
BW0
h0
H0
A0
H±
H0
H±
4 x neutralino
2 x chargino
AfterMixing
gluino
Spin-1/2
Spin-1
Spin-0
Spin-1/2
Spin-0
BinoWino0
Wino±
gluino
~
~
“Left squark” = scalar SUSY partner of
left quark
12 May 2004Alan Barr
UK ATLAS Physics
• Sparticle decays generate:– Missing ET (LSP)
– Jets (squarks/gluino)– Leptons (sleptons)
• “If we see these at LHC we have discovered SUSY”
• Not necessarily…
Scalar mass term (GeV)
Gaugin
o m
ass
term
(G
eV
)
Plot byD. Tovey
[RPC SUSY]1-year low-lumi mSUGRA reach
12 May 2004Alan Barr
UK ATLAS Physics
Universal Extra Dimensions
• TeV-scale universal extra dimension model• Kaluza-Klein states of SM particles
– same QN’s as SM– mn
2 ≈ 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
hep-ph/0205314 Cheng, Matchev
“How to get fooled at the LHC”
Radius of extra dimension ~ TeV-1
KK tower of masses n=0,1,…
Dubbed “Bosonic Supersymmetry”
12 May 2004Alan Barr
UK ATLAS Physics
UED KK spectrum
1st excited KK level (example masses)
Example decay spectrum
Stable lightestKK particle(weakly interacting)
hep-ph/0205314 Cheng, Matchev
After adding boundary terms-> SUSY-like spectrum
Need to measure spins!
12 May 2004Alan Barr
UK ATLAS Physics
SUSY decay chain• Use mSUGRA
LHC point 5 as example
• Well studied in context of sparticle masses
• Nice leptonic signal at LHC
Spin-½, mostly wino Spin-0
Spin-½
Spin-0
Spin-½, mostly bino
Final state = jet + l+ + l- + ET
( + decay of other sparticle)
},{ el
12 May 2004Alan Barr
UK ATLAS Physics
Spin projection factorsIn rest frame of with +ve z-axis aligned with qL
Approximate SM particles as massless-> okay since m « p
Lq~Lq
Rl~
02~
Rl
1
0~ LqP
S
02~
12 May 2004Alan Barr
UK ATLAS Physics
Spin projection factorsIn rest frame of with +ve z-axis aligned with qL
Approximate SM particles as massless-> okay since m « p
Lq~Lq
Rl~
02~
Rl
1
0~ LqP
S
0
1~02 S
Σ=0
Spin-0
02~
12 May 2004Alan Barr
UK ATLAS Physics
Spin projection factorsIn rest frame of with +ve z-axis aligned with qL
Approximate SM particles as massless-> okay since m « p
(near) Rl
θ*p
SLq~
Lq
Rl~
02~
Rl
02~
12 May 2004Alan Barr
UK ATLAS Physics
Spin projection factorsIn rest frame of with +ve z-axis aligned with qL
**
**
1*
3*
cossin
sincos)sin()cos(ˆ
σp
Approximate SM particles as massless-> okay since m « p
(near) Rl
θ*p
SLq~
Lq
Rl~
02~
Rl
1
0~ LqP
S
0
1~02 S
Spin projection factorSpin-0
mql - measure
*210
2 sin~ Rl
02~
Σ=0
12 May 2004Alan Barr
UK ATLAS Physics
lnearq invariant mass (1)
m/mmax = sin ½θ*
Back to backin 2
0 frame
θ*
quark nearlepton
χ20
Phase space -> factor of sin ½θ*Spin projection factor in |M|2: l+q -> sin2 ½θ* l-q -> cos2 ½θ*
l+
l-
Phase space
Pro
bab
ility
)(sincos122 *212max2*2 lqqlqllq mppm pp
12 May 2004Alan Barr
UK ATLAS Physics
lnearq invariant mass (2)
squarkdecay
anti-squarkdecay
l+
l+
l-
l-
(Lepton from 2 decay only – not directly measurable)
Obvious asymmetry! Opposite asymmetry!
-> if same # squarks as anti-squarks effect cancels
0 1*
21sin
0 1*
21sin
12 May 2004Alan Barr
UK ATLAS Physics
How to measure?• lnearq shows nice charge asymmetry
– Excellent probe of spin of
• 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
02~
12 May 2004Alan Barr
UK ATLAS Physics
Production asymmetry
Peak near 0.1, 0.1Valence quarks
Parton distribution functionhas more q than q-bar for Bjorken x ≈ 0.1
-> pp collider produces more squarks than anti-squarks
x1 and x2 sampled by MC
(Twice as much for our point)
qg -> squark + gluino
12 May 2004Alan Barr
UK ATLAS Physics
Parton Level
ll
llA
difference/sum
l+l-
Experimentally measurable-> q and q-bar-> near and far leptons
Chargeasymmetry
Shape indicates that2
0 is spin-½
spin-0=flat
12 May 2004Alan Barr
UK ATLAS Physics
After detector simulation (ATLFAST)
l+
l-Change in shapedue to charge-blind cuts parton-level * 0.6
-> Charge asymmetry survives detector sim-> Same shape as parton level (but with BG and smearing)
detector-levelInvariant mass
Ch
arg
e a
sym
metr
y,
For cuts see C. Lester thesis
spin-0spin-½
Even
ts
12 May 2004Alan Barr
UK ATLAS Physics
Cross-checkNo asymmetry if spincorrelations turned off
• 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)
spin-½ -> 1)sin(2)(sin *21
~~*21 qqAA
spin-0 -> 0)(sin *21 A production asymmetry
spin-0=flat
12 May 2004Alan Barr
UK ATLAS Physics
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
Shape after different-familysubtraction
Back-to-backin slepton frame
(phase-space)
12 May 2004Alan Barr
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 • 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
*21sin