Spin Measurements in Supersymmetry at ATLAS

LHC SUSY SPIN MEASUREMENTS SPIN PRAHA 20091

Spin Measurements in Supersymmetry at ATLAS

Christopher LesterCavendish Laboratory, Cambridge

(on behalf of the ATLAS collaboration)


This talk is Backwards


not upside

down !


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!


What is “Discovering SUSY” ?

•What makes Supersymmetry different to Universal Extra Dimensional (UED) models?

•One part of the answer:

SPIN


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


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 arXiv:0808.2472

•Slepton Spin (pair production)

• A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023


Other interesting methods

•Gluino chain spin

• Alvez, Eboli, Plehn hep-ph/0605067

• MAOS method

• Cho, Kong, Kim, Park arXiv:0810.4853

•Spins in chains with charginos

• Wang and Yavin hep-ph/0605296

• Smillie hep-ph/0609296

• Spins in chains radiating photons

• Ehrenfeld et al arXiv:0904.1293


Spin Consistency Check


Spin Consistency Check

Di-Lepton Invariant Mass (GeV)

Rel

ativ

e F

req

uen

cy Straight lineConsistent with:

• Phase-space

• Scalar slepton (SFSF)

•Fermion KK lepton (FVFV)


QL Spin Determination (A.Barr)

“NEAR”

“FAR”

How can we tell from ?

2 problems:

How can we distinguish the ‘near’ lepton from the ‘far’ lepton?

hep-ph/0405052 ATL-PHYS-2004-017


Quark+NearLeptoninvariant mass distributions for:

sin ½θ*

Back to backin 2

0 frame

QL+

QL-

Phase space(spin-0)

Pro

babili

ty d

en

sity

QUARKS

L+ L- and

sin ½θ*

Back to backin 2

0 frame

QL-

QL+

Phase space(spin-0)

Pro

babili

ty d

en

sity

ANTI-QUARKSL+ L- and

__

_

hep-ph/0405052ATL-PHYS-2004-017


Experimental problem

Cannot reliably distinguish QUARKs from ANTI QUARKs

In experiment, can only distinguishRED(QL+,_ L+) from BLUE(QL-,_L-)Can only distinguish lepton chargeRED(QL+,QL+) from BLUE(QL-,QL-)

Expect QUARK and ANTI-QUARK contributions to cancel:

QL+QL+_

QL-QL-

_

SUM jL+

SUM jL-

But LHC is Proton-Proton machine

•More Quarks than Anti-Quarks! So get:

QL+QL+_

QL-QL-

_

SUM

SUM

jL+

jL- Asy

mm

etry

!h

ep

-ph

/04

05

05

2

ATL-

PH

YS-2

004-0

17


“Far” Lepton washout?(though compare with later comments Matchev arXiv:0808.2472)

“NEAR”

“FAR”


jL+

So define mjL+, mjL- asymmetry

parton-level

detector-levelAsy

mm

etr

y “A

”

spin-0

Spin-

½

MjL / GeV sin ½θ*

where

jL-h

ep

-ph

/04

05

05

2

ATL-

PH

YS-2

004-0

17 500 fb-1

M(QL)

Charge asymmetry at detector level

Susy spins compared only with no spin

(i.e. with phase space)

Points are for 500 fb-1

detector level

no spin

parton level x 0.6

Barr

hep-p

h/0

40

50

52

ATL-

PH

YS-2

004-

017

(similar to SU3)

M(QL)


Tested further in ATLASATL-PHYS-PUB-2007-004

Biglietti et al

Preselection cuts

Dilepton selection cuts

Parametrised Simulation (ATLFAST)


Di-lepton mass spectraATL-PHYS-PUB-2007-004

Biglietti et al

SU1 – MC truth SU3 – MC truth

Here at SU1 there are two sleptons (êL =255 GeV, êR=155 GeV) below chi2 =264 GeV. Chi1=137 GeV

Here at SU3 there is only one accessible slepton below chi2

Chi2=219, êR=155, Chi1=118 GeV

M(LL) M(LL)


Jet-lepton mass spectraATL-PHYS-PUB-2007-004

Biglietti et al

SU1 – MC truth SU3 – MC truth

M(QL) M(QL)

Truth AsymmetryATL-PHYS-PUB-2007-004

Biglietti et alSU1 – MC truth

Near lepton

Left slepton

Near lepton

Right slepton

Far lepton

Right slepton

Far lepton

Left slepton

A A

AA

220 /fb

220 /fb

220 /fb

220 /fb

M(QL) M(QL)

M(QL) M(QL)

Shows asy

mmetry

depends on parti

cle

identities

BackgroundsATL-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


Fast simulation resultsATL-PHYS-PUB-2007-004

Biglietti et al

SU1 – ATLFAST SU3 – ATLFAST

Note ... Flavour subtraction


Fast simulation resultsATL-PHYS-PUB-2007-004

Biglietti et al

A A


Note ... Flavour subtraction

30 /fb100 /fb

Sufficient power to eliminate null hypothesis

M(JL) M(JL)


Safety checkATL-PHYS-PUB-2007-004

Biglietti et al


Expect to see NO ASYMMETRY in these OFOS lepton plots. Confirmed!

30 /fb100 /fb

M(JL) M(JL)


QUACK !

QUACK !

SU

SY

UED

Not all things that quack are ducks!


UEDUED

SUSY SUSY

SPS1a masses

UED type masses

UED and SUSY not distinguished by dilepton mass spectrun

he

p-p

h/0

60

52

86

Compare ratios of dilepton invariant mass distributions for

different spin configurations (SUSY is horizontal) :

M(LL)2 M(LL)2


UED masses SPS 1a masses

Similar form, different magnitudeNot detectable for UED masses

Jet + lepton asymmetry not good at distinguishing UED and

SUSY either.


“ANY-Spin” Determination (Smillie et.al.)

Later fuller follow-up (Matchev, Kong, et al)

F

F FF

S FS

he

p-p

h/0

60

52

86

arX

iv:0

80

8.2

47

2

M(JL)2

(SUSY)(UED)

rela

tive

pro

bab

ilit

y

Cannot distinguish:


01~

01~Rl

~

Rl~

l

l

Direct di-slepton productionA.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023

Event selection: •two single leptons•transverse missing energy > 100 GeV•no jets with pT> 100 GeV•no b-jets•mT2(l+, l-) < 100 GeV•Mll< 150 GeV•|missing pT+ pT(l+), pT(l-)| <100 GeV•Lepton pT(l1) > 40 GeV, pT(l2) > 30 GeV


Look at slepton production angleA.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023


Have some access to desired angle

Distribution of is correlated with decay angle

A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023


Fast simulation plot

Outer error bars: after SUSY & SM background subtraction

Significance strongly dependent on mass spectrum

A.Barr hep-ph/0511115 ATL-PHYS-PUB-2005-023


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

Documents

Spin Measurements in Supersymmetry at ATLAS