Performance of the ATLAS tau trigger in p-p collisions at √ s = 900 GeV

ATLAS-CONF-2010-021 13July2010

ATLAS NOTE

April 10, 2010

Performance of the ATLAS tau trigger in p-p collisions at √ s = 900 GeV

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The ATLAS Collaboration

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Abstract

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This note describes the analysis of quantities used for the hadronic tau trigger, using

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data collected with the ATLAS detector at √ s = 900 GeV. A comparison of the level 1 and

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the high level trigger performance for data and Monte Carlo is done. During 2009 data

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taking, only the level 1 conditions were required online while the high level trigger was run

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in monitoring mode. A comparison of tau trigger rates between cosmic ray and collision

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data has also been performed.

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1 Introduction

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The tau lepton, being the heaviest of all known leptons (m

= 1776 . 84 ± 0 . 17 MeV), is of special impor-

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tance. Triggering on tau events will not only help in understanding Standard Model (SM) processes dur-

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ing early running but will also increase the discovery potential of the ATLAS detector through searches

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for the Higgs boson and supersymmetric particles at high luminosities. Due to its short lifetime, with

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(cτ = 87 . 11µ m), taus decay inside the beam pipe. The identification of tau is, therefore, done through

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its decay products inside the detector.

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The tau lepton decays into electrons or muons 35% of the time, while 65% of its decays include

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hadrons, mostly pions. The events where taus decay into leptons can be triggered by low transverse-

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energy-threshold electron triggers and low transverse-momentum-threshold muon triggers. A dedicated

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tau trigger has been designed and implemented at the ATLAS experiment [1] to select events where a tau

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lepton decays into one or more hadrons. A tau jet can be identified by the presence of a well collimated

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calorimeter cluster with a small number of associated tracks.

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The ATLAS trigger system [2] is divided into a hardware-based component, level 1 (L1), and

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software-based parts level 2 (L2) and event filter (EF). L2 and EF are referred to together as the high level

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trigger (HLT). The L1 trigger identifies regions-of-interest (RoI) using the information from calorimeter

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and muon systems. The decision time at L1 is ∼ 2 . 5 µ s. L2 takes these RoIs as input and refines the

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object identification using the information from all the subsystems. The latency at L2 is ∼ 40 ms. In

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the EF algorithms similar to the offline reconstruction are run to select interesting events. The allowed

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processing time at EF is approximately 4 s.

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The tau trigger is designed to select hadronic decays of the tau, which are characterized by the

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presence of one or three charged pions accompanied by a neutrino and possibly neutral pions. At L1,

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the tau trigger uses the electromagnetic (EM) and hadronic (HAD) calorimeter trigger towers of size

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∆η × ∆φ = 0 . 1 × 0 . 1 to calculate the energy in a core and an isolation region. At L2, selection criteria are

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applied using tracking and calorimeter based information. This takes advantage of narrowness and low

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track multiplicity to discriminate taus from the multi-jet background. Exploiting the same characteristics,

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the EF uses different selection criteria for 1-prong and multiprong decays in more refined algorithms

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which are similar to the reconstruction algorithms. It is challenging to keep the rates for these triggers

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low due to the high production rate of multi-jet events. Nevertheless it is advantageous to implement tau

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triggers to increase the sensitivity of searches for new physics. The details of the ATLAS tau trigger are

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described in [2, 3].

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A clean sample of real hadronic tau decays will not be available in the early data. It is therefore im-

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portant to consider whether some useful information can be extracted from fake taus which are copiously

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produced in multijet events. The goal is to collect a large enough data sample to check the performance

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of the hadronic tau trigger. The collision and cosmic ray data at ATLAS have provided valuable handles

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to optimize and commission the ATLAS detector. In this process the ATLAS tau trigger algorithms have

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been exercised and the hardware-based first level rates studied.

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The following sections describe an analysis of the variables related to the tau trigger in cosmic ray and

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900 GeV collision data.

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2 Analysis

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2.1 Data and MC samples

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The collision data used in the analysis described in this and the following two sections consist of 12 runs

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recorded in December 2009 at a center of mass energy of 900 GeV. Two different datasets using data

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streams based on the L1 calorimeter and the Minimum Bias Trigger Scintillators (MBTS) decisions were

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produced. To reject beam backgrounds, coincident signals from the MBTS were selected by requiring

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the difference in time measured by the two MBTS systems to be less than 10 ns in both data and MC. To

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ensure the quality of data, a “good run” selection was applied. This requires stable beam conditions, run

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declared good by the calorimeter

and the inner detector to be on.

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The minimum bias (MB) Monte Carlo (MC) sample used in this analysis was produced using

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PYTHIA [4] and processed through a GEANT4 [5] based simulation of the ATLAS detector. The trigger

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menu consistent with the one that ran online to collect data was used to simulate the trigger in MC.

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2.2 Event selection

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In order to reduce the presence of non-collision events in data, the events were required to have fired the

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MBTS trigger [6]. Furthermore, to optimize the analysis procedure, data were filtered by requiring the

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following selection criteria:

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• at least one offline tau candidate reconstructed using the tauRec algorithm [2], with

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– |η| < 2 . 5

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– at least one track associated to it. The track should have

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∗ P

> 1 GeV

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∗ a minimum of one hit in the pixel detector and six hits in the Semi-Conductor Tracker

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(SCT).

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In a data sample of 2247 events, 1249 events had at least one offline tau candidate satisfying the

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above mentioned requirements. A total of 1407 offline tau candidates were found in the selected events

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in data.

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3 Tau trigger plots

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The tau trigger quantities analyzed in this section are documented in [2, 3].

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In Figs. 1-9 we compare 900 GeV data with MB MC for relevant variables of the tau RoIs at the

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different trigger levels, where the MC distributions have been normalized to the number of entries in

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the data distributions. In all of these plots the L1 trigger object with transverse energy above 5 GeV

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(L1 TAU5) is required to match to an offline tau candidate. The requirements for offline tau candidates

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were described in the previous section. The matching between the L1 and offline object is done by

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requiring ∆R < 0 . 3 where ∆R =

q (∆η )

+ (∆φ )

. A total of 50, out of 1407, offline tau candidates

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satisfied the matching requirement. The number of matched tau candidates is MC sample was found to

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be 3762.

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• Fig. 1 shows the L1 transverse energy (E

) distribution, obtained around the L1 position

adding

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the contributions from the following two regions in the EM and HAD calorimeters respectively:

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(i) a two-tower region (with granularity 0 . 1 × 0 . 1 in η × φ ) and (ii) a 0 . 2 × 0 . 2 region in η × φ . In

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this and following figures, when no data points are present in some of the bins, this indicates the

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absence of data entries in that interval.

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1A green flag assigned by the Liquid Argon calorimeter means that there are no errors seen in the front end boards, no unmasked noisy or dead channels, and the shape of the signal is as expected. For the Tile calorimeter, a green flag means that the data, calibration and detector description are in agreement with current understanding of the detector.

2PTis the momentum measured in the plane transverse to the beam direction.

3The L1 position is the center of the 4×4 towers inη×φ(with granularity 0.1×0.1) of the L1 tau RoI determined by the L1 readout.

• Fig. 2 shows the L1 EM isolation quantity, obtained for the EM towers between 0 . 2 × 0 . 2 and

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0 . 4 × 0 . 4 in η × φ around the previous L1 position.

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• Fig. 3 shows the L2 E

distribution, calculated in a region of ∆R < 0 . 1 around the L2 position.

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• Fig. 4 shows the L2 EM fraction, which is the fraction of EM energy compared to the total L2

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E

in a region of ∆R < 0 . 1 around the L2 position. The values below zero and above one appear

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because the EM and HAD energies can be negative due to noise fluctuations. Due to the relative

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volume of the EM and HAD calorimeters, cosmic ray events are more likely to deposit energy

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in the HAD calorimeter than in the EM calorimeter. This explains the peak seen at 0. The peak

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observed at 1 is due to the collision events which have large energy deposits in the EM calorimeter.

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• Fig. 5 shows the number of tracks obtained with the L2 tracking algorithm IdScan [2] with a P

>

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1.5 GeV in a cone of ∆R < 0 . 1 about the L2 direction.

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• Fig. 6 shows the EF E

distribution, calculated from cells associated with the tau trigger candidate

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in a region of ∆R < 0 . 4 around the EF direction.

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• Fig. 7 shows the EF EM radius, a measure of the shower size in η -φ obtained from an energy

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weighted ∆R of the cells associated with the tau trigger candidate around the EF position.

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• Fig. 8 shows the EF HAD radius, calculated in a similar way to the EF EM radius but for the HAD

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cells associated to the tau trigger candidate.

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• Fig. 9 shows the number of tracks at the EF using the offline tau reconstruction algorithm [7], in

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a region of ∆R < 0 . 3 around the tauJet seed direction as calculated from topological clusters.

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• Fig. 10 shows the fraction of the offline tau candidates that satisfy the selection criteria described

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in Section 2.2 which match to a L1 trigger object with E

> 5 GeV. The fraction is shown as a

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function of the E

of the offline tau candidate. The dashed (solid) line represents fit to the data

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(MC) points.

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• Fig. 11 shows the relative energy difference between EF and offline using two different methods

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for calculating E

during the offline reconstruction. The first method uses the energy obtained in

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the detector using digital signal processors (DSPs). This approach is more similar to the one used

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by the HLT algorithms. The second method recalculates the E

using the analog signal coming

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from samples of cells which is less similar to the one used by the HLT, however more precise than

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the former method. More details about calculating the offline E

are given in [8].

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The selection criteria for Figure 11 require at least one offline tau candidate with |η | < 2 . 5 and

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no track requirements imposed. A matching between the offline and EF tau candidate is required

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(∆R < 0 . 3). There were 75 EF tau candidates in data that passed the event selection and the

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matching criteria. The raw uncalibrated energy was used for both EF and offline tau candidates

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to make the plot. One can see that a better comparison between the offline and EF energies is

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observed when offline energy is calculated from DSPs.

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4The L2 position is obtained after weighting the position of each cell by its energy. The cells in a region of∆R<0.15, around the L1 position are considered and energy is calculated using all of the calorimeter layers.

5Topological clusters are obtained with a topological algorithm which clusters together neighboring cells, as long as the signal in the cells is significant compared to noise. More information on this algorithm can be found in [2].

[GeV]

L1 E

0 2 4 6 8 10 12 14 16 18 20

Number of tau RoIs/GeV

0 2 4 6 8 10 12 14 16 18 20

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Minimum Bias MC

ATLAS Preliminary

Figure 1: Comparison of the L1 tau candidate E

distribution for 900 GeV data and MB MC. The cut off at 6 GeV corresponds to the L1 TAU5 threshold.

L1 EM isolation [GeV]

0 1 2 3 4 5 6 7 8 9 10

Number of tau RoIs/GeV

0 5 10 15 20 25 30 35 40

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Minimum Bias MC

ATLAS Preliminary

Figure 2: Comparison of the L1 tau candidate EM isolation distribution for 900 GeV data and MB MC.

[GeV]

L2 E

0 2 4 6 8 10 12 14 16 18 20

Number of tau RoIs/GeV

0 2 4 6 8 10 12

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Minimum Bias MC

ATLAS Preliminary

Figure 3: Comparison of the tau candidate E

distribution at L2 for 900 GeV data and MB MC.

L2 EM fraction

-0.2 0 0.2 0.4 0.6 0.8 1 1.2

Number of tau RoIs/0.05 counts

0 2 4 6 8 10 12 14 16 18

= 900 GeV) s

Data 2009 ( Minimum Bias MC

ATLAS Preliminary

Figure 4: Comparison of the tau candidate EM fraction distribution at L2 for 900 GeV data and MB MC.

The values below zero and above one appear because the EM and HAD energies can be negative due to

noise fluctuations.

Number of tracks at L2

0 1 2 3 4 5 6

Number of tau RoIs

0 5 10 15 20 25 30 35 40 45

= 900 GeV) s

Data 2009 ( Minimum Bias MC

ATLAS Preliminary

Figure 5: Comparison of the tau candidate number of tracks distribution at L2 for 900 GeV data and MB MC.

[GeV]

EF E

0 5 10 15 20 25 30 35 40

Number of tau RoIs/2 GeV

0 2 4 6 8

10

Minimum Bias MC

ATLAS Preliminary

Figure 6: Comparison of the tau candidate E

distribution at EF for 900 GeV data and MB MC.

EF electromagnetic radius

-0.1 0 0.1 0.2 0.3 0.4 0.5

Number of tau RoIs/0.02 counts

0 2 4 6 8 10

12

Minimum Bias MC

ATLAS Preliminary

Figure 7: Comparison of the tau candidate EM radius distribution at EF for 900 GeV data and MB MC.

EF hadronic radius

-0.1 0 0.1 0.2 0.3 0.4 0.5

Number of tau RoIs/0.02 counts

0 2 4 6 8 10

12

Minimum Bias MC

ATLAS Preliminary

Figure 8: Distribution for the tau candidate HAD radius at EF for 900 GeV data and MB MC.

Number of tracks at EF

0 1 2 3 4 5 6 7 8 9 10

Number of tau RoIs

0 5 10 15 20

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Minimum Bias MC

ATLAS Preliminary

Figure 9: Distribution for number of tracks associated to the tau candidate at EF for 900 GeV data and MB MC. The difference between the number of tracks distribution at L2 (Fig. 5) and that at EF (Fig. 9) is due to the difference in the requirement for the minimum p

of the track. At L2 the minimum p

requirement is 1.5 GeV, while at the EF it is 1 GeV.

of offline tau [GeV]

E

0 5 10 15 20 25 30 35 40 45 50

Fraction of offline taus passing L1

0 0.2 0.4 0.6 0.8 1

=900 GeV) s

Data 2009 ( Minimum Bias MC

ATLAS Preliminary

Figure 10: Fraction of the offline tau candidates matched to a L1 trigger object with E

> 5 GeV as a

function of the E

of the offline tau candidate. The dashed (solid) line represents fit to the data (MC)

points.

T [%]

)/Offline E - Offline E T

(EF E T

-20 -10 0 10 20

Events/1%

0 5 10 15 20 25 30 35 40 45 50

= 900 GeV) s

Data 2009 ( offline from DSP offline from samples

ATLAS Preliminary

Figure 11: Relative difference between EF and reconstructed transverse energy, when offline energy is

calculated using DSPs (dashed line) and digital signals from samples of cells (solid line).

4 Cosmic ray and collision data comparison

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In this section a comparison between cosmic ray and collision data taken during 2009 is shown.

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4.1 Data samples

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For the cosmic ray reference samples, runs 135356 and 140056 from October and November 2009 have

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been selected with 362,886 and 43,213 recorded events, respectively. The definition of the tau trigger

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item with the lowest threshold was slightly different in both runs, but is accounted for in the normal-

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ization. In the case of collision data, a list of good runs and luminosity blocks has been assembled in a

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similar way to that described in Section 2.1. All data events are selected from the L1 calorimeter stream

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by requiring at least one L1 tau RoI with E

> 5 GeV.

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4.2 Cumulative L1 tau rate vs. L1 tau E

threshold

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In Figure 12, the cumulative L1 tau object rate is plotted as a function of different E

thresholds for

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cosmic ray and collision data. For a given threshold value, only tau objects with E

larger than the

135

threshold are taken into account.

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The histograms for the runs containing cosmic ray data are normalized to the number of bunches that

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were used in the trigger item configuration for cosmic ray data taking. An additional correction for the

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trigger dead-time has been applied which is shown as the average efficiency in Table 1.

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The normalization of the collision data histograms is derived from the overall lifetime of the L1 TAU5

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trigger item which is about 93577 seconds. This is very close to the integrated length of all luminosity

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blocks considered (about 94160 seconds). Each entry in those histograms has been weighted by the

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number of paired bunches in the corresponding luminosity block.

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The black points represent all L1 tau trigger objects taken from the L1 calorimeter stream without

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imposing any requirement on the MBTS trigger. From this distribution two subsets have been selected

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by requiring the same MBTS trigger and timing constraints as described in Section 2.1 (blue boxes) and

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vetoing on the MBTS trigger decision (red triangles). The former distribution should therefore consist

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mostly of the tau trigger objects from collision events whereas the latter should be dominated by cosmic

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ray events. A good agreement is observed between cosmic ray and non-MBTS collision data.

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Run # bunches Avg. efficiency Overall scale factor

135356 3492 0.90 3 . 18 · 10

140056 3543 0.89 3 . 17 · 10

Table 1: Normalization for histograms from dedicated cosmic ray runs in Fig. 12. The last column contains the final normalization factor for the histograms.

5 Tau trigger data quality

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The task of the ATLAS Tier-0 system [9] is to perform the prompt reconstruction of the raw data com-

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ing from the online data acquisition system. The monitoring setup at Tier-0 provides an efficient way

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of monitoring trigger related variables, to assure not only the expected performance of the trigger but

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also the quality of data. The plots made during Tier-0 processing are a quick way to look at different

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distributions and decide whether there are problems with the data before starting the bulk reprocessing

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of the data.

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threshold [GeV]

L1 tau E

5 10 15 20 25 30 35 40 45 50

Cumulative L1 tau rate [Hz/1 GeV]

10

10

10

10

10

ATLAS Preliminary Data 2009

Cosmic ray data (run 135356) Cosmic ray data (run 140056)

=900 GeV) s

Collision data (

Collision data (MBTS veto) Collision data (MBTS req)

Figure 12: Cumulative L1 tau trigger rate as a function of the L1 tau object E

threshold, normalized to one colliding bunch pair. For a given threshold value, the objects considered have a transverse energy greater than this value.

Monitoring of variables related to the tau trigger at Tier-0 not only assures the quality of data but also

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provides a way to see the difference in the properties of cosmic ray and collision events. Two example

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plots are shown in Figs. 13 and 14. These plots are made using the data that are promptly reconstructed

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at Tier-0 without applying any data quality or offline selection criteria. Figure 13 demonstrates that the

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energy variable alone does not discriminate between cosmic ray and collision events at 900 GeV. From

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the data quality point of view, an exponentially falling distribution will show that the software and the

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hardware related to the trigger are performing as expected. Fig. 14 shows that very few (or no) tracks

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are expected in cosmic ray events selected by the tau trigger, while collision events occasionally contain

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some tracks. This is important to monitor both online and offline as the absence of tracks in case of

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collision events will be an indication of a hardware or software based problem.

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6 Conclusions

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We have shown an analysis of the cosmic ray and 900 GeV collision data collected with the ATLAS de-

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tector in 2009. The distributions observed in data are compared with the expectations from MC samples.

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Figures in Section 3 show that there is a reasonable agreement between data and MC expectations for

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the variables used in the tau trigger selection.

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The L1 turn-on curve is also shown and its behavior matches the MC expectation. The L1 rate plot

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described in Section 4 demonstrates a good agreement in the rate for L1 TAU5 between cosmic ray and

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non-MBTS collision data. Finally, Section 5 is an example of how to efficiently identify some problems

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relevant to the tau trigger.

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of calorimeter cluster at L2 [GeV]

E

0 20 40 60 80 100

Number of clusters at L2 / 1.62 GeV

0 10 20 30 40 50 60

Cosmic ray data

Collision+cosmic ray data

ATLAS Preliminary Data 2009

Figure 13: E

of the calorimeter cluster found at L2. The solid histogram represents a run with cosmic ray and collisions events while the dashed filled histogram represents a cosmic ray run only. The data used in this plot have no data quality requirements or offline selection criteria applied.

Number of tracks associated with a cluster at L2

0 1 2 3 4 5

Number of clusters

0 50 100 150 200 250

Cosmic ray data

Collision+cosmic ray data

ATLAS Preliminary Data 2009

Figure 14: Number of tracks associated with a cluster at L2. The solid histogram represents a run with

cosmic ray and collisions events while the dashed filled histogram represents a cosmic ray run only. The

data used in this plot have no data quality requirements or offline selection criteria applied.

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