Looking for SUSY Dark Matter with ATLAS The Story of a Lonely Lepton Nadia Davidson Supervisor: 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 SM ParticleSpinSUSY Particle Spin Quark½Squark0 Lepton½Slepton0 Gluon1Gluino½ W1Wino½ B1Bino½ Higgs0Higgsino½ 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. is a superposition ofandstates

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).

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

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 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 q q g g q

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

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 orthen

InitialAfter SUSY cutsAfter All Cuts Signal17,1006,1751,460 SUSY background30,5003, SM background12,500,0002,271, approx no. of events in ATLAS in first half year After three months should have events

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 123 GeV 108 GeV 98 GeV 83 GeV events Lepton p T (GeV)

Mean lepton transverse momentum Signal (green) increases approx linearly. 1GeV increase in mean P T with every 2GeV decrease in mass of neutralino. Background (light blue) does not change with neutralino mass. Sensitivity of lepton P T to chargino-neutrino mass difference (GeV) chargino – neutralino mass (GeV) signal background

And after cuts: Sensitivity of lepton P T 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 P T only 1GeV increase in mean P T 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. (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 ? Invariant mass of chargino Transverse mass of chargino edge at chargino mass edge gone Result of using missing momentum of all three missing particles Can not be used due to the contribution of the 2 nd neutralino (primed) events

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: Intermediate SUSY decays Intermediate SUSY decays

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 P T 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.

Lepton P T We want to find a variable sensitive to neutralino or chargino mass. W’s P T is sensitive since in the chargino rest frame: It then aquires a boost: Similar for the lepton So how does the mean P T change with the neutralino and chargino mass?

No way of extracting the masses of the neutralino or chargino if we are using the detector data. With 2 nd neutralino After detector resolution effects are added How does the 2 nd neutralino effect the transverse mass?

Variation of m 1/2 parameter