Looking for SUSY Dark Matter with ATLAS

1. Looking for SUSY Dark Matter with ATLAS The Story of a Lonely Lepton Nadia Davidson Supervisor: Elisabetta Barberio

2. 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

3. not allowed (couplings blow up) not allowed (vacuum unstable) scale up to which SM valid 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

4. is a superposition of and states Supersymmetry • A symmetry between fermions and bosons • Each Standard Model particle is given a superpartner with spin differing by a half • For 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.

5. 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.

6. 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).

7. Contours of total energy density of the Universe. Source: Baer. M., Phys Rev. D, 53:597. 1996 One out of several points chosen by LHCC is:

8. g q q g q Production of supersymmetric particles • 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… • Produced in pairs: • Replace lepton and baryon number conservation with R-parity conservation • Standard Model particles have odd R-parity • Supersymmetric particles have even R-parity • Conservation implies a stable LSP and production of supersymmetric particles in pairs

9. Intermediate SUSY decays  My Decay Channel POW Intermediate SUSY decays

10. 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.

11. or then 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

12. events approx no. of events in ATLAS in first half year After three months should have

13. 123 GeV 108 GeV events 98 GeV 83 GeV Lepton pT (GeV) Does lepton transverse momentum change with? • W is given more energy in the rest frame of the chargino when decreases • One average this increase is passed onto the lepton. Distribution of lepton transverse momentum

14. <pT> (GeV) signal background chargino – neutralino mass (GeV) Mean lepton transverse momentum Sensitivity of lepton PT to chargino-neutrino mass difference • 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.

15. <pT> (GeV) Signal+background (full cuts) Signal+background (some cuts) chargino – neutralino mass (GeV) And 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. Sensitivity of lepton PT to chargino-neutrino mass difference after cuts

16. events events Can a model-independent variable be found? Result of using missing momentum of all three missing particles Transverse mass of chargino • Why not use the transverse mass since ? edge gone edge at chargino mass Can not be used due to the contribution of the 2nd neutralino (primed) Invariant mass of chargino

17. Intermediate SUSY decays Intermediate SUSY decays 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:

18. 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.