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Search for the associated production of 

Chargino-and Neutralino in the final state with one muon and two additional leptons (muon or electron)

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Supersymmetry (SUSY) is a proposed symmetry of Nature which introduces a fermion (boson) for each SM boson (fermion) with the same quantum numbers but the spin. A discrete multiplicative symmetry, called R-parity, is defined as RP=(-1)(2S+3B+L) such that a SM particle carries RP=+1 and a SUSY particle RP=-1. Supersymmetric particles have not been observed yet implying that SUSY is a broken symmetry. The minimal supergravity model (mSUGRA) with R-parity conservation, is a favored breaking model for SUSY. In the mSUGRA scenario the superparticles are produced in pairs and the lighter charginos and neutralinos,  mixed state of electroweak gauginos and higgsinos, and the sleptons,  are less massive than gluinos and squarks. If SUSY is a broken symmetry, it predicts a low mass Higgs boson in accord with the electroweak fits, accommodates gravity, unifies the gauge interactions and provides an excellent candidate for Dark Matter. In particular for the mSUGRA model, the lightest neutralino is identified as the candidate for Dark Matter,being neutral and the lightest stable sparticle (LSP). In this paper we report on the search for the associated production of chargino and neutralino when these particles decay leptonically into three charged leptons and two neutralinos which escape the detection causing a significant missing transverse energy in the event. This channel is reckoned as the Golden Mode for SUSY at a hadron collider.

The associated production of chargino and neutralino  is expected to occur via two modes which interfere destructively: a dominant s-mode, through virtual W exchange and a suppressed t-mode, through virtual squark exchange. The charginos and neutralinos can decay into charged leptons via virtual sleptons or virtual W/Z. The mSUGRA benchmark point selected for performing the analysis corresponds to a chargino mass at the boundary of the LEPII exclusion limit. The mass of the chargino is m= 113 GeV/c2. The corresponding mSUGRA parameters are the following: m1/2 = 180 GeV/c2 ; m0= 100 GeV/c2; tanbeta = 5; mu > 0; A0 = 0. The next-to-leading production cross section is sigma = 0.642 pb. The fully leptonic branching ratio obtained with is 0.25.

Contacts: Anadi Canepa, Else Lytken

Analysis summary

The analysis proceeds as a counting experiment by comparing the SM prediction to the observed data in kinematic regions where the SUSY signal is expected to be negligible (``control regions''). It is performed as a "statistically unbiased" analysis.   "Statistically unbiased" analysis means that the region of the data where the SUSY signal is enhanced with respect to the SM background (``signal region'') is investigated only if the agreement between the expectation and the observation is yielded in the control regions.

We explore the inclusive High pT muon dataset. We first investigate dilepton events (either dimuon or muon+electron events). We then require a third lepton (electron or muon). The first lepton in the event must be the high pT trigger muon. The second lepton can be an either a high pT muon or an intermediate pT stub muon or a CMIO; the analysis is now extended to events where the next to leading lepton is an electron belonging to any of the following categories: central tight electron, plug electron or phoenix electron. The third lepton can satisfy any of the ID criteria listed above. It can also be either a loose central electron, or a plug electron with lower ET threshold (ET> 5 GeV)  or a intermediate pT stub muon with loose isolation cut (total lepton isolation < 2 GeV).

We use the following set of mSUGRA parameters as benchmark point:

mSUGRA parameter
180 GeV/c2
100 GeV/c2
> 0
Chargino1 Mass 
113 GeV/c2
Neutralino2 Mass 
118 GeV/c2
Neutralino1 Mass 
64 GeV/c2

Plots and Tables:

The definition of the control regions is based on the analysis cuts:

Dimuon event selection

Control Region
Events with opposite sign muons
Azimutal distance between the leading muons  .eps .gif
Missing Transverse Energy  .eps .gif
Transverse Momentum of the dimuon system  .eps .gif
Jet Multiplicity jet ET > 20 GeV/c2 .eps.gif
Invariant Mass of opposite sign muons  .eps.gif
Events in control Region "G"
Muon Transverse Momentum  .eps .gif
Events in control Region "A"
Missing Transverse Energy  .eps .gif
Events with muon invariant mass: 15 < M < 76 or M > 106 GeV/c2
Missing Transverse Energy  .eps .gif
Events with two muons 
Invariant Mass of opposite sign muons  .eps.gif

Dilepton events: one muon + one electron

Missing Transverse Energy  .eps.gif
Muon pT  .eps   .gif
Electron Et  .eps  .gif

Prediction from the muon + Central Electron : 181 +/- 2 +/-  20; observed data : 182
Prediction from the muon + Plug Electron : 110 +/- 2 +/-  13; observed data : 118

Trilepton events: two muons + lepton (electron/muon)

Event Selection
Events with three leptons
Invariant mass of opposite sign muons  .eps.gif
Events with three leptons and invariant mass of opposite sign muons in [15;76] and > 106 GeV/c2
Jet Multiplicity jet ET > 20 GeV/c2 .eps.gif
Events three leptons and invariant mass of opposite sign muons in [15;76] and > 106 GeV/c2  number of jets ET > 20 GeV/c2 is N < 2
Missing Transverse Energy .eps.gif

Trilepton events: one muon + one electron + lepton (electron/muon)

Missing Transverse Energy  .eps  .gif



We observe one event in the dimuon+lepton channel. This observation is consistent with the expectation from the SM background.


 COT view .gif    COT view .eps                                                RZ view .gif   RZ view .eps


Anadi Canepa, Else Lytken


Last modified: Mon Jun 27 16:04:42 CDT 2005
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