Supersymmetry

Introduction

SUSY is the most popular extension of the Standard Model right now. It is a theory that predicts a boson for every known fermion and vice-versa. The main motivation for this theory:
  • It removes fine tuning and offers ultra‐violet completeness
    • Large radiative corrections of superpartners cancel each-other
  • It offers the possibility of force unification
    • Not exactly possible with SM
  • It provides a cold dark matter candidate
    • If the lightest supersymmetric particle (LSP) is stable
  • It connects the theory to superstrings
    • Incorporation of gravity
  • It offers the possibility of radiative electroweak symmetry breaking
    • As an alternative to spontaneous breaking

Chargino-Neutralino Production











The chargino and neutralino are the result of the mixing of gauginos and higgsinos, the SUSY partners of the gauge bosons and the Higgs. If R-Parity is conserved, the SUSY particles have to be produced in pairs and the lightest one (LSP) will not decay and escape detection. The lightest neutralino is the LSP assuming mSUGRA. The chargino and neutralino will decay either through gauge bosons or sleptons, the leptonic decay of which will result to three leptons and missing ET (MET). This is considered to be the golden signature for discovery of Supersymmetry at the Tevatron!

5.8 fb-1 trilepton analysis

I am currently responsible for the most sensitive SUSY search at the Tevatron, utilizing the full extend of the CDF detector, all possible reconstructible final objects and the most complete kinematic range. We expect low-pT (<20 GeV/c) leptons and low mass (<40 GeV) resonances from SUSY. The reasons are
  • the preferential production of staus (lightest sleptons) and their subsequent decays to taus and soft leptons
  • the cascade decays of SUSY particles that result to light resonances
In this analysis we use:
  • Lepton momentum as low as 5 GeV and dilepton mass as low as 10 GeV (without excluding high pT/mass)
  • Forward (|η|>1) as well as central (|η|<1) electrons and muons
  • Stubless muons
  • Hadronic taus
  • Isolated tracks
  • Single-lepton high-pT triggers
  • High integrated luminosity
  • New dilepton triggers
Very low pT, very low mass, forward electrons, and hadronic taus are used for the first time and offer increased sensitivity to new physics.

The results of this extremely important analysis were blessed in August 2011 and were presented at the SUSY-11 international conference. We achieved the highest yield of trileptons ever detected in the signal region, and we observed a surplus of events at low dilepton mass. At the same time, we set the most stringent limit on chargino mass at the time of release. To learn more about this analysis, please visit the public analysis web page and read our public CDF note

1 fb-1 trilepton analysis

I have completed the 1 fb-1 (and earlier the 300 fb-1) analysis of inclusive-low-pT trilepton analysis. This analysis was the first one to include leptons with pT as low as 5 GeV. In this analysis I used:
  • Lepton momentum as low as 5 GeV and dilepton mass as low as 10 GeV (without excluding high pT/mass)
  • Central electrons and muons (|η|<1)
  • dilepton low-pT triggers
The challenge of this analysis is the heavy-flavor quark and fake-lepton (i.e., light-flavor punch-through or decay-in-flight) backgrounds. These QCD backgrounds are especially critical in the low-pT region. I used an innovative technique, using CDF data, for the determination of this difficult background.
The final results are statistically consistent with the Standard Model. At the same time, I discovered the first and only trimuon+MET event of CDF.



This event is extremely interesting, due to the kinematics of the muons (all pointing to close &phi directions). In addition, all three dimuon masses have similar values. It is also interesting to notice the presence of two electromagnetic clusters that could be photons. In the GMSB scenario, the LSP is the gravitino, which means that the lightest neutralino will decay to a gravitino and a photon. In other words, the signature would be three leptons, two photons and missing ET. Isn't this interesting?...

To learn more about this analysis, please visit the public analysis web page.

The paper was published in Phys. Rev. D (Phys. Rev. D 79, 052004 (2009)) and attracted recent theoretical interest (Fermilab wine and cheese, November 6, 2009).

Finally, this analysis was combined with other chargino-neutralino subanalyses, to result to a strong limit on chargino mass and production cross section. For a particular region of parameter space, the mass of the chargino is more than 129 GeV and the production cross section (times branching ratio to leptons) is less than 0.25 pb at 95% Cl. To learn more about our combined analysis, please visit the public analysis web page. The result was published in PRL (Phys. Rev. Lett. 99, 191806 (2007)). This work was also featured in the "Result of the Week" column of Fermilab Today.