Looking for SUSY Dark Matter with ATLASThe Story of a Lonely Lepton

Nadia DavidsonSupervisor: Elisabetta Barberio

The Standard Model In high agreement with experimental results Two classes of particles

Fermions with half integer spin – Regular matter Bosons with integer spin – Force carriers

Includes three of the four fundamental forces: W and Z – weak g – strong γ – electromagnetism

Higgs boson is required to give masses to the particles

Problems with the Standard Model

Fine tuning problem

Divergence problem

No explanation for Dark Matter Makes up approx. 23% of the density of the Universe

scale up to which SM valid

not allowed

(couplings blow

up)

not allowed (vacuum unstable)

Supersymmetry A symmetry between fermions and bosons

Each Standard Model particle is given a superpartner with spin differing by a halfFor each fermion a boson and for each boson a fermion.e.g. Lepton (spin ½) have superpartners “sleptons” (spin 0). Introduces many new particles, however, none of these have been observed.

The symmetry is broken if it exists because supersymmetric masses are very large compared to those in the standard model.

What is the mechanism for symmetry breaking?

In Supergravity (SUGRA) Gravity involved in the symmetry breaking so

all four forces incorporated into the model.

How does SUGRA explain Cold Dark Matter? Dark Matter could be explained with a

WIMP (weakly interacting massive particle) SUGRA contains such a WIMP

The neutralino:

Made up of a superposition of the superpartners to the W, B and Higgs Bosons

All supersymmetric particles decay into the neutralino. This is the lightest supersymmetric particle (LSP).

01

~

One out of several points chosen by LHCC is:

Contours of total energy density of the Universe.

Source: Baer. M., Phys Rev. D, 53:597. 1996

0,1.2tan,300,300,100 02/10 GeVAGeVmGeVm

Produced in pairs

Each of these then undergoes a cascade of decays into the lightest supersymmetric particle (the neutralino).

My Decay Channel:

This was chosen because it has a high branching ratio But, it is a difficult to study…

Production of supersymmetric particles

q

q

q~

q~g

q~

q~

g~

g

q

W011~~

ll

01

~

My Decay Channel

IntermediateSUSY decays

IntermediateSUSY decays

g~g~POW

01

~W

l

1~

IntermediateSUSY decays

p p

W011~~

ll

How will this decay be seen?

ALTAS will begin collecting data in 2007 In the mean-time use Monte-Carlo simulations of

the supersymmetric events and simulations of the ATLAS detector. Then: Separate the signal from Standard Model

background. So we know if we’ve seen SUSY Find a variable sensitive to the supersymmetric

particle masses. So we know what kind of SUSY we’ve found.

Removal of Background

Standard Model Background Competing Processes

Selection cut on transverse missing energy > 600GeV

Selection cut on transverse mass of lepton and missing energy > 350GeV

llW Z Z

lblt ll or then

Initial After SUSY cuts After All Cuts

Signal 17,100 6,175 1,460

SUSY background 30,500 3,135 550

SM background 12,500,000 2,271,000 180

approx no. of events in ATLAS in first half year

After three months should have

even

ts

Does lepton transverse momentum change with ?01

~m

• W is given more energy in the rest frame of the chargino when decreases• One average this increase is passed onto the lepton.

01

~m

Distribution of lepton transverse momentum

123 GeV

108 GeV

98 GeV

83 GeV

even

ts

Lepton pT (GeV)

Mean lepton transverse momentum

• Signal (green) increases approx linearly. 1GeV increase in mean PT with every 2GeV decrease in mass of neutralino.• Background (light blue) does not change with neutralino mass.

Sensitivity of lepton PT to chargino-neutrino mass difference

<p T

> (

GeV

)

chargino – neutralino mass (GeV)

signal

background

And after cuts:

Sensitivity of lepton PT to chargino-neutrino mass difference after cuts

• With full cuts (red), no trend can be seen• With all but final two SUSY cuts (blue), trend noticeable, however:

• overall translation to higher mean PT

• only 1GeV increase in mean PT with approx. 4GeV in neutralino mass decrease.

• Large statistical error. Approx 1,000 events or half a year of data collection• We would really prefer a variable which is not as sensitive to cuts and initial chargino boost.

<p T

> (

GeV

)

Signal+background (full cuts)

Signal+background (some cuts)

chargino – neutralino mass (GeV)

Can a model-independent variable be found? Why not use the transverse mass since ?2

~2

~11

mm

T

Transverse mass of chargino

edge at chargino mass edge gone

Result of using missing momentum of all three missing particles

011

011

011

~~~~2~

2~

2 22 TTTTTTTlmiss ppEEmmm

Can not be used due to the contribution of the 2nd neutralino (primed)

even

ts

)(2 GeVmTlmiss

even

ts

)(2 GeVmTlmiss

Future Work Perhaps a model-independent variable can be found:

Allow both supersymmetric branches to decay in the same way Give each particle which escapes detection a dummy momentum:

1Tq

n

missT

nT pq

)(

IntermediateSUSY decays

p

IntermediateSUSY decays

p

1

~

Wl

l

01

~

1

~

W

l

l

01

~

2Tq

3Tq

4Tq

Conclusion SUSY could be quickly seen with ATLAS if it

exists but we would not know the symmetry breaking mechanism and model parameters.

By studying the decay we would like to find the masses of the particles involved. Lepton PT was found to depend on neutralino mass.

With further work we could find a better variable that is only sensitive to the chargino and neutralino masses.

W011~~

ll