Search for bad channelsSearch for bad channels

in the ATLAS EM Calorimeterin the ATLAS EM Calorimeter

N. DaciN. Daci

The LHC ExperimentsThe LHC Experiments

CMS

LHCbALICE

ATLAS

The LHC ExperimentsThe LHC Experiments

Goals :

• Search for the Higgs boson, CP violation study

• Search for new physics beyond the Standard Model : supersymetric particles, extra dimensions

CMS & ATLAS

The LHC will accelerate bunches of protons, whose collisions will produce thousands of particles. Among these events, we want to select those who are interesting.

The ATLAS detectorThe ATLAS detectorATLAS is composed of cylindrical subdetectors :

• The Muon Spectrometer : measures the momentum of muons

• The toroïdal magnet, which curves the muon’s trajectories

• The EM and Hadronic Calorimeters : identify the nature of the particles and measure their energy.

• The solenoïd magnet, which curves their trajectories

• The inner detector : measures the trajectories and momentum of charged particles

The EM CalorimeterThe EM CalorimeterAlternance of absorbant middle, and instrumented middle (Liquid Argon + electrodes) :

• in the first one e-/ loose their energy and convert into an EM shower

• the second one measures the ionization current from the secondary particles of these EM showers

• When a particle pass through the EM Calorimeter, it deposits energy in these cells. By grouping cells with significant signal into a cluster, EM objects are identified.• Aim of my work : check the reconstructed EM objects to validate some monitoring tools

Problematic behaviorsProblematic behaviors

• We can see a region containing a lot of these problematic clusters

• Even the fact that there is a lot of clusters in a region is a problem : for these cosmic datas, it is almost impossible for a muon to pass through the same region twice an hour.

• I identified 4 problematic behaviors : clusters’duplication, negative energy, cluster’s seeds containing too much energy

Duplicated clustersDuplicated clusters• The clustering algorithm may lead to several clusters, from a single real energy deposit.

• Once we understand that, we can check if these problematic clusters dissappear when we remove duplicated ones

• The surrounded region contain less clusters but is still present.

• In each event, if some clusters share cells, I kept only the higher energy cluster and removed the others.

• The number of problematic clusters decreased but not sufficiently.

Sporadic burst noiseSporadic burst noise• We can find three monitoring histograms indicating a problem in this region

• Some cells have a lot of times a significant signal, although for cosmic datas it is impossible for a given cell.

=> Although these cells are known as problematic, and supposed to be masked, they lead to the reconstruction of a lot of fake EM objects

• These cells are known as « sporadic Burst Noise » : they behave normally most of the time, and sometimes they have a peak of noise.

Sporadic burst noiseSporadic burst noise• Once we understood that problem, what happens if we remove all events containing a cluster generated by a sporadic cell ?• Much less problematic clusters, especially in the surrounded region, but in the whole detector too

ConclusionConclusion

• The correction of sporadic noise is not 100% efficient : sporadic cells are known, and masked following a quality factor, but they still induce several fake clusters

• The monitoring histograms allow to identify cells containing a significant signal too much times

• Some other histograms allow to identify regions with too much clusters• Then the cells monitoring histograms may suggest to check some cells when they appear too much