TopBox: Method

Monday, August 24, 2009

12:31

 



 



M2 and M3 using MC Matching Selction in 100/pb of ttbar single muon events.

 

Notice that the center of the M2 and M3 distributions are actually at 88 GeV and 180 GeV repectively. There could be a selection bias since not all Jets are used to reconstruct these invariant masses. The distribution above is only a subset of all semileptonic ttbar events. If we reduce the Jet Pt cut from 30 GeV to only 5 GeV, we will see a shift back to the actual W-boson and Top-quark masses as show in the plot below.

 



M2 and M3 using MC Matching Selection in 100/pb of ttbar single muon with no Jet Pt Cut.

 

Notice W-boson is still slight above the PDG define value of 80.6 GeV. There be other bias or Jet reconstruction effect causing a slightly higher "Effective W-boson Mass." However, we are experimentally constriant to use a sensible Jet Pt cut, otherwise one would be swarm by detector fluxuations since every HCAL cell passing a low read-out threshold will effectively become a Jet. From this point on we will use an effective W-mass of 90 GeV and effective Top-mass of 180 GeV.

 

From the first M2 and M3 distributions we can get an idea of the M2 and M3 cuts used to identify semileptonic  ttbar events. Using these distribution we can choose a first order cut of 70 < M2 < 110 and 150 < M3 < 210 in GeV. Using these cuts we run through all events passing the 1 muon 0 electron, 4 Jets with the leading Jet's Pt > 80 GeV.  All event which pass the M2 and M3 are consider ttbar-like and place in the TopBox as a ttbar-like sample.

TopBox: Self Validation

Thursday, August 27, 2009

13:54

TopBox: Validation PsuedoExp

Using 2/fb of data we can conduct 20 Psuedo Experiment of 100/pb.

 

First we like to check that the MET Predictions are consistent through out the different "runs." 1

 

 

TopBox: W+Jets Contribution

All previous plots were shown with only ttbar events, however in data the selection of 1 Muon, and 4 Jets will have constributions. Some were due to mismeasurements while others are due to real processes such as W+Jets, where the W decay leptonically. In this section we will examine how much W+Jets occur in the TopBox.

Using a sample of 100/pb of W+Jets, where the W is force to decay into a muon in order to reduce the sample size. The selection cut actually reduce theW+Jets constribution to 194 events.

 

 

TopBox: MET Bias From Leptonic M3

Wednesday, September 02, 2009

15:18

The TopBox method can be easily expanded or contracted into more selector variable. One example is the leptonic side of ttbar decay. If we assume the MET is entirely due to the neutrino from ttbar decay, we use the W-boson's mass and the muon's  four-momentum to calculate 2 possible solution of neutrino's Pz. Then we can loop over all remaining jets in the event to find the one that reconstruct a M3 nearest to the top mass.

 

 



MET Distribution for Hadronic and Had+Lep selected events for 1/fb at 10 TeV using only ttbar sample

 

Since this sample is pure ttbar one should expect the prediction to match unless there's a selection bias in the method. If we integral the events with MET above 200 GeV, we see an unestimate of number of events with MET greater than 200 GeV.

Actual MET > 200 = 395

HadTBox MET > 200 = 343

Lep+Had TBox MET > 200 = 222

 

The Leptonic M3 cut seems to reject high MET events. A possible reason for this is because higher MET events are easier to mismeasure due to larger absolute Pt error with higher Pt Jets. High MET events also tends to have more Jets, since any mismeasurement in each Jet's energy contribution to MET.  The selection purity of the method is highly dependent of the Jet Multiplicity of the event due to combinatoric backgrounds.

 

 

 



MHT Distribution for Hadronic and Had+Lep selected events for 1/fb at 10 TeV.

 

 



Muon Pt Distribution for Hadronic and Had+Lep selected events for 1/fb at 10 TeV.

 

 

 



SumET Distribution for Hadronic and Had+Lep selected events for 1/fb at 10 TeV.