1. Plans for checking hadronic energy
depositions in the ATLAS calorimeters
N. Davidson (The University of Melbourne)
On behalf of the ATLAS Collaboration
Abstract
The first data from the ATLAS detector at the Large Hadron Collider (LHC) is due to be collected later this year. This first phase will play a vital role in understanding the detector and its response, in-situ. Jet reconstruction is important for identifying new physics as well as making precision measurements of standard model physics. The fine granularity of the ATLAS calorimeters can be used to gain information about a jet's shape and the classification of energy deposits, which allows a better estimate of the jet energy to be made and in particular correction for the non-compensating nature of the calorimeter using so-called calibration weights. The classification algorithm and weights are presently calculated using simulation.
In this poster we describe an important step in the validation of ATLAS's jet calibration using charged tracks reconstructed in the inner detector and their inter-calibration with the clusters reconstructed in the calorimeters.
Selecting isolated pions in minimum bias events
Minimum bias events are inelastic proton collisions which generally produce only soft or low pT particles. As most events at the LHC will be of this nature, the collection of statistics is only limited by the rate of the trigger and data acquisition system. Minimum bias event have been allocated 10Hz3.
The feasibility of using minimum bias events as a source of tracks for E/p measurements was studied4. Monte-Carlo equivalent to approx. 1 week of data at a luminosity of L=1032cm-2s-1 were used to check the sensitivity of the method, since data was not available.
Isolated hadrons were identified by the following criteria:
Track were isolated from other tracks
Track were required to have larger momentum than other tracks in the event
The pion shower was isolated in the hadronic calorimeter
Transverse momentum reach of tracks in minimum bias (solid curve) scaled to 1μb-1.
Good quality tracks were selected (dashed curve) with a fit of χ2/ndof < 1.5 ,no more than 1 missing hit and a hit in the first layer of the Inner detector. This removed fake tracks and pions with strong interactions in the Inner Detectors.
E/p method
The ATLAS track momentum scale and resolution should be known to within much less than 1%1,2 at energies below approximately 100 GeV allowing an inter-calibration between the inner tracker and calorimeters in-situ. E/p distributions of isolated charged hadrons will be compared to the Monte-Carlo used to derive the hadronic calibration in ATLAS. This will allow validation of, for example, the hadronic shower simulation.
Background
The energy deposited by selected hadrons was contaminated
by energy from neutral particles accompanying the selected
charged hadron.
A data-driven background estimate procedure was employed:
Hadrons were classified as either early showering or late showering based on the energy deposition in the hadronic calorimeter compared to the electromagnetic calorimeter (in the green core region).
The contaminating energy was measured in the electromagnetic calorimeter (blue) region for late shower pions.
The contaminating energy distribution was unfolded from the distribution for all pions (late and early showering)
Calorimeters
Energy (E) is taken as the sum of cells or clusters within a cone volume centred around the track position when extrapolated to the calorimeter.
Inner Detector
Momentum (p) is taken from track of charged hadron
Range: |η|< 2.5
pT= GeV
pT= GeV
pT=4-6 GeV
E/p distributions for selected pions in minimum bias Monte-Carlo (black solid curve), all tracks in minimum bias Monte-Carlo (black dashed curve) and isolated pions without background (red dashed curve). All samples were weighted to have the same distribution in η.
Results
All values of the mean E/p were found to be statistically consistent with the isolated pion sample without background. Systematic effects were studied such as hadron species (pion, kaon or proton) and the limitations of the background estimation technique. The estimated systematic effects were statistically dominated, but always below 10%
Early Showering hadrons
There was overlap between hadron
showers and showers of other particles
Late showering hadrons
There was little overlap between hadron
Showers and showers of other particles
pT= GeV
pT= GeV
pT=4-6 GeV
Hadron shower
Had. Cal. EM
Cal.
Hadron shower
Had. Cal.
core cone
The final E/p distribution obtained for pions from minimum bias Monte-Carlo (black solid curve) and isolated pions without background (red dashed curve) . The energy was calculated as the sum of topologically clustered cells, calibrated to the hadronic energy scale. The E/p here is below 1 because energy is lost in cells outside the cluster, and these looses are significant at these low energies.
Showers from
other particles
Region where
background
was measured
Showers from
other particles EM
Cal.
1. ATLAS Collaboration, ATLAS Performance and Physics Technical Design Report, ATLAS TDR 15, CERN/LHCC/99-15 (25 May 1999) ATLAS Collaboration, “Tracking” in Expected Performance of the ATLAS Experiment: Detector, Trigger and Physics, CERN (2008)
2. ATLAS Collaboration, “Trigger” in Expected Performance of the ATLAS Experiment: Detector, Trigger and Physics, CERN (2008) ATLAS Collaboration, “Jets and Missing Transverse Energy ” in Expected Performance of the ATLAS Experiment: Detector, Trigger and Physics, CERN (2008)