Looking for SUSY Dark Matter with ATLAS The Story of a Lonely Lepton Nadia Davidson Supervisor: Elisabetta Barberio
Jan 04, 2016
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