Preliminary Results from Highly Energetic Cosmic Rays in the ATLAS Calorimeter

[SumET]

Distribution of the scalar sum of the transverse energy in all cells measured in the calorimeter (Sum ET) from the cosmic L1Calo stream (run 90272 in Sep. 2008) and cosmic Monte Carlo simulation. The MET_Base algorithm (i.e. all cells with |E|>2 sigma are considered for the calculation) is used for the Sum ET calculation. Energy is at the electromagnetic scale. Monte Carlo distribution is normalized to data in the 100-300 GeV range. Only events which have a jet with ET>20 GeV are included. Trigger effects are not considered in the simulation. The measured Sum ET distribution is well described by the simulation. At high ET, more events are found in the data than in the MC. This might be explained by the limited MC statistics and by air shower events not included in the simulation.

[Jet Transverse Energy]

Distribution of jet transverse energy from the cosmic L1Calo stream (run 90272 in Sep. 2008) and cosmic Monte Carlo. The same normalization factor as for the figure above is applied. The ATLAS Cone Jet algorithm with a cone size 0.4 is used. Calorimeter clusters reconstructed with the topological clustering algorithm are the inputs for the jet reconstruction. Only jets with ET>20 GeV are shown. The jet energy is at the electromagnetic scale. The shape of the distribution is well described by the simulation. At high ET, more events are found in the data than in the MC. This might be explained by the limited MC statistics and by air shower events not included in the simulation.

[Jet EM Fraction]

The jet EM fraction is the ratio of the energy deposited in the EM calorimeter (Presampler, EM Liquid Argon, and the 1st layer of Forward Calorimeter) and the whole calorimeter. The jet EM fraction from the cosmic L1Calo data stream (run 90272 in Sep. 2008), a cosmic Monte Carlo simulation, and QCD di-jet Monte Carlo samples simulating proton-proton collisions are shown. Only jets with ET>20 GeV are included. The distributions were normalized by the total number of jets (areas are 1). The most likely value for the EM fraction is 0 or 1 for fake jets from cosmics, since the high energy deposit from photons originated from high energetic muons will localize either in the EM or the hadronic calorimeter. The QCD jets have a broad distribution of EM fraction peaking around 0.8. EM fractions less than 0 or larger 1, are due to small negative energy contribution coming from noise. Good separation between real QCD jets and fake jets from cosmics is observed. Selection cuts around 0 and 1 can remove most of the fake jets while keeping most of the jets produced in proton-proton collisions.

[Number of Calorimeter Clusters]

The number of calorimeter clusters in jets from cosmic L1Calo data stream (run 90272 in Sep. 2008), a cosmic Monte Carlo simulation, and QCD di-jet Monte Carlo samples simulating proton-proton collisions. Only jets with ET>20 GeV and |eta| < 3.2 are shown, since the number of clusters do not have much fake rejection power in the forward region due to large granularity. Fake jets tend to have small numbers of clusters. By requiring low cluster multiplicity, most of the fake jets can be removed while keeping most of the jets produced in proton-proton collisions.

[Jet Transverse Energy Distrubution after Cleaning Cuts]

The distribution of the jet transverse energy from cosmic L1Calo data stream (run 90272 in Sep. 2008) and cosmic Monte Carlo. Only jets with ET>20 GeV are included in the figure. The blue points are from data, and the red solid lines are from the Monte Carlo. The blue square marks and the red solid distributions are before jet cleaning cuts. The blue open circles and the red dotted lines show distributions after applying a cut on the EM fraction (jets remaining after 0.2 < EM fraction <0.97 is required). The blue triangular marker and the red dash lines show distributions after applying the cut on the EM fraction (0.2 < EM fraction <0.97) and the number of clusters (jets remain when number of clusters >=7). The cuts are used as an illustration of the possible performance of the cleaning cuts.

[Event EM Fraction]

The Event EM fraction from cosmic L1Calo stream (run 90272 in Sep. 2008), a cosmic Monte Carlo simulation, and QCD di-jet Monte Carlo samples simulating proton-proton collisions. The Event EM fraction is defined for each event as a jet ET weighted-average of the jet EM fraction. Jets with ET>20 GeV are considered. Good separation between real QCD jets and fake jets from cosmics were observed. Event EM fraction is a promising observable to remove events with fake missing ET.

[Event Display from a Single Muon Event]

Event display of an event with a single cosmic muon passing through the detector and depositing more than 1 TeV in Tile Calorimeter cells. The green lines show the muon segments. The muon segments in X-Y view are pointing to the interaction point, due to the lack of RPC hits. The yellow boxes indicate energy deposits in the Tile Calorimeter. The direction of the reconstructed jet is shown by the gray marker. The light blue dotted line is the direction of Missing ET. The highlights in the MDT indicate hits (yellow for any hits, and orange when tracks are reconstructed). No track was reconstructed in the inner detector for this event. The X-Y projection and the R-Z projection are shown.

[Event Display from an Air Shower Event]

Event display of an air shower event. The green lines show the muon segments. The yellow boxes indicate energy deposits in the Tile Calorimeter. The direction of the reconstructed jet is shown by the gray marker. The light blue dotted line is the direction of Missing ET. The yellow highlights in the MDT indicate hits. The X-Y projection and the R-Z projection are shown.