Worldwide distributed trigger system

Jet Energy Scale calibration for the top-antitop process

Initially the worldwide trigger will be used for calibration channels only. The best candidate to demonstrate the valuable contribution of a worldwide trigger system is jet energy scale calibration for the top-antitop process. The standard trigger for this process is based on the leptonic decay of one of the W-bosons. With the worldwide trigger we can also identify the fully hadronic decays, based on b-tagging and invariant mass reconstruction. On this page we outline how this can be accomplished from an analysis point of view.

Prerequisites

The production cross section for top-antitop events, σ(

_
tt

), is estimated to be 809 ± 12% ± 3.5% pb (Erik van der Kraaij).The luminosity for the ATLAS experiment in the first year of data taking is expected to be around 1 fb ^-1 (Ivo van Vulpen) for 10³² cm^-2s^-1. As a result, the number of
_
tt

events produced in 2008 will be around 800 · 10³. Almost all top quarks decay to bottom quarks with the emission of a W boson. The chance that a W boson decays to a
_
qq

pair is 2/3 according to the pdg data. Hence, the number of fully hadronically decaying top quark pairs in 2008 is about 4/9 · 800 · 10³ ≈ 350 · 10³.

Trigger for fully hadronic decays

Next, we need to determine how much of these events will pass the trigger. The following signal samples are currently retrieved with DonQuijote2 for our analysis (for a complete set of fully hadronically decaying

_
tt

events click here and ask for set 5204. Looking for a different standard model set? click here and get coffee):

Dataset/Task	Done	Completed(%)	Grid	Atlas ver
csc11.005204. TTbar_FullHad_McAtNlo_Jimmy. digit.v11004205.task	5986	99.77	SACLAY	Atlas-11.0.42
csc11.005204. TTbar_FullHad_McAtNlo_Jimmy. recon.v11004205.task	274	91.64	SACLAY	Atlas-11.0.42
csc11.005204. TTbar_FullHad_McAtNlo_Jimmy. evgen.v11004205.task	59	98.33	LCG	Atlas-11.0.42

From the wiki about release 11.0.42: The trigger works out of the box, except for TrigCaloRec. This is fixed with an upgrade to TriggerRelease-00-02-95. Furthermore, the trigger objects are not persistified, so the AOD has been reproduced from the digitized events. More information about running the trigger with an offline release can be found in the trigger tutorial and the transparencies of Monika Wielers at the PAT workshop in Japan.

Plots and numbers

A first estimate is made of the selection efficiency of the LVL1 trigger for fully hadronic events. The analysis is based on a custom made sample of 1000 fully hadronic

_
tt

events produced with Pythia and processed with Geant 4 based on a full simulation of the ATLAS detector. These events were analyzed with the LVL1 trigger simulation as present in the patched version of 11.0.41. The selection criteria are based on ATLAS note ATL-DAQ-2003-004, "Trigger Menus for the LHC Start-up Phase" (Schorner-Sadenius and Tapprogge): 1J200, 2J170, 3J90, 4J65. The selection efficiency is above 80%, which means that more than enough fully hadronic signal events will reach the LVL2 trigger to be competitive with other jet energy scale calibration channels. Note that we still need to study the LVL2 selection efficiency, but the before mentioned document does not split HLT in LVL2 and EF criteria. These studies are being redone and will be continued with the official CSC samples.

Jet Energy Scale calibration for the top-antitop process

Prerequisites

Trigger for fully hadronic decays

Plots and numbers

Contents

External

A worldwide distributed trigger system