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?


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


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


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”


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!


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


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~


UK ATLAS Physics



Lq~Lq

Rl~

02~

Rl

1

0~ LqP

S

0

1~02 S

Σ=0

Spin-0

02~


UK ATLAS Physics



(near) Rl

θ*p

SLq~

Lq

Rl~

02~

Rl

02~


UK ATLAS Physics


**

**

1*

3*

cossin

sincos)sin()cos(ˆ

σp


(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


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


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


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~


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


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


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


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


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)


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

susy spin measurements with atlas

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