Z bb measurement in ATLAS. Iacopo Vivarelli, Alberto Annovi. Scuola Normale Superiore,University and INFN Pisa, University of Athens. Introduction. We studied the Z decay into bb Signal and background generation LVL1 requirements (with full simulation) Kinematical selection
Iacopo Vivarelli, Alberto Annovi
Scuola Normale Superiore,University and INFN Pisa, University of Athens
The final signal to background ratio is expected to be about 1%. To avoid the background shape prediction with the MonteCarlo, it is necessary to subctract the background directly from the data, i.e., to have a lower and a higher sideband to allow the background evaluation.
Too low thresholds have to be considered on the two b-jets if one consider the di-jet final state. Too large trigger rates foreseen for threshold low enough to have safe sidebands.
A different strategy has to be followed: look at boosted Z
We require a leading non-b jet. This decreases the LVL1 rate, and moves at low masses the trigger turn-on in the background invariant mass distribution
The reason is the following: requiring the leading jet to be non-b, one strongly reduces the contribution from direct bb production and selects mainly gluon splitting events (mainly ggggbbg). They are characterized by low invariant mass of the bb couple, because of the small angle between the b-jets
To reduce further the LVL1 rate & the trigger turn-on mass, a hard selection on the leading jet has been tried. A leading jet of PT > 80 GeV is required for the plots below. This reduces the value of the peak in the background invariant mass.
My εb is 50%
Full simulated QCD di-jet events with QT > 17 GeV with lumi02 pileup included have been used.
Efficiencies have been computed on QCD jets. To be repeated for the signal.
Athena 8.5.0 has been used. LVL1 jets have been reconstructed on a 8x8 window. The truth is defined as jets reconstructed using a cone algorithm (R=0.4) on MC particles. It is the standard truth reconstruction for jets. (The rate of the LVL1 multijet triggers of the HLT TDR can be reproduced with a 10% accuracy)
Use only LVL1 jets within 2.4 (it can be done), since at further trigger levels they have to be identified as b-jets.
Check the rate for a di-jet trigger. Consider as acceptable a LVL1 rate of 5 KHz.
Match the LVL1 jet with the MC jet if R between them is less than 0.4. Fill histograms with the ET at LVL1 for a given MC transverse energy. Then, find the ET of the LVL1 jet to be 95% efficient for the corresponding MC energy.
The discussed threshold is 95% efficient on 190 GeV MC jets.
With the same definition of matching between the LVL1 and the MC jet, a threshold at 10 GeV (at LVL1) is 90% efficient on a 40 GeV jet
A rate of 4.5-5 KHz can be obtained with a single threshold of 190 GeV at LVL1.
This corresponds (with high efficiency) to MC jets of 190 GeV.
The soft jet (40 GeV) can be identified as a RoI if a 10 GeV threshold is applied at LVL1.
LVL2: ask 2 b-tag with reduced rejection. With the present performances, the LVL2 rate is about 100 Hz. If one considers offline performances the rate is 10 Hz.
The question now is: can the LVL2 b-tag at that rate if the tracking is made by the LVL2 itself? Which is the maximum rate that can be analyzed at LVL2 if the tracking is done by FTK? It needs more understanding
The parametrized distribution for the background and the signal have been normalized with the corrected statistics and then