HLT software trigger - Trigger and Data Acquisition

4.3 Trigger and Data Acquisition

4.3.2 HLT software trigger

Following aL1 Trigger accept, events are buffered in theROSwhile event data is partially reconstructed in a farm of ≈40 000 CPU cores before ultimately forming a final trigger decision. The final output rate is limited to an average of∼1 khz, which is allocated across a range of signatures (e.g., single-lepton, multi-lepton, jets,Emiss

T , and others).HLTreconstruction is limited to theROIs obtained from the specific L1 Triggeritem accept and often corresponds to a simplified version of the offline reconstruction.

Single lepton triggers

The largest fraction of the total trigger rate (∼266 hz of the∼1.5 khz “peak rate” recorded [84]) is dedicated to single-lepton triggers, due to their broad utility across a diverse set of analyses. Both electron and muon triggers rely on ID tracks, which are obtained in a two-step approach where fast-track finding is followed by a more precise algorithm using the previously-found space-points as seeds.

Electron triggers reconstruction operates in two stages to quickly reject poor candidates and avoid unnecessary processing. In the first, clusters are built from rectangular arrays of calorime-

ter cells within the L1 Trigger ROI and required to satisfy loose shower-shape criteria designed to reject candidates from hadronic showers. A 1 GeV matching track obtained from the fast-track finding algorithm is also required. In the second stage, an offline-like selection is used (see Sec-

tion4.4.2) with re-calibrated clusters and precision track inputs as well as a likelihood-based iden-

tification criterion. In 2016 data-taking, an isolation requirement PpT/ET < 0.1 using tracks in a cone of ∆R < 0.2 is also enforced for the lowest un-prescaled trigger. This item is denoted

e24 lhtight nod0 ivarloose, where the ET cut (24 GeV) and tight likelihood working point are labeled, as well as thepT-dependent, loose isolation and the fact that the d0 impact parameter is not included in the likelihood.

The muon trigger adds MDT measurements to the trigger chambers used in the L1 Trigger

decision. TheMS pT is obtained from a look-up table after which the MS-only track is combined with a correspondingIDtrack to produce a combined measurement. The efficiency is improved by matchingMSandIDtracks with both outside-in and inside-out strategies when necessary. Isolation requirements are also imposed in a manner similar to the electron case, withmu26 ivarmediumbeing the lowest-pTthreshold un-prescaled muon trigger in 2016.

Jet triggers

In theHLT, reconstruction of jets andEmiss

T proceeds after reading out the entire calorimeter at full granularity and running the offline topo-clustering algorithm (see Section4.4.4) to obtain inputs to each algorithm [85]. This allows jet reconstruction and calibration to proceed nearly identically to the manner in which it is carried out offline. The lowest un-prescaled jet trigger in 2016 required a 380 GeV jet, reconstructed from LC-scale clusters with the anti-k_⊥ algorithm withR = 0.4. As quarks and gluons are produced copiously at the LHC, jet trigger rates are large and so larger

ET must be required than is necessary for other reconstructed objects (e.g. a 26 GeV isolated electron) to record a manageable data-set. The primary users of the single-jet trigger are searches high-mass di-jet pairs (e.g. for a TeV-scaleZ0) and dark matter, via the “mono-jet”pp_→χχ¯+j

process. Requiring multiple jets allows lower thresholds, with items such as4j100 and 6j60 used for measurements of hadronict¯tdecays. Similar triggers are critical for theHL-LHC, specifically to target the rareSMhh→b¯bb¯bprocess.

The most significant difference between online and offline jets is the level of possible energy calibration. In the HLT, limited track reconstruction prevents three offline corrections based on reconstructed vertices and the individual tracks associated to jets. The first corrects the origin

4. Experimental Apparatus 42

of the jet four-vector based upon the location of the primary vertex (in z). The second applies a correction due to in-time PU based on the number of total reconstructed vertices in the event. The most important of the three is theGlobal Sequential Correction (GSC) [86], which primarily calibrates for non-uniformity in detector response to quark- and gluon-initiated jets. The energy calibration includes corrections based on the longitudinal structure of the jet shower and associated tracks as well as a muon segment-based “punch-through” correction irrelevant for trigger jet energies. The track inputs are the multiplicity associated to each jet as well as the angular moment

track width = P tksptkT ·∆R(jet,tk) P tksptkT . (4.1)

As tracking is computationally expensive and is not otherwise required for jet triggers, none of these corrections were not applied prior to 2016 data-taking.

A crucial observation was made that the sameL1 Triggerseed trigger used for single-jet events also seeds b jet items in the HLT, and that one might re-use these tracks for further jet energy corrections. After requiring a jet of moderate energy (ET∼200 GeV),b jet triggers further reduce theQCDrate by requiring a tightb-tag score, as computed from tracks associated to the jet. These tracks are obtained in a two-stage procedure, illustrated in Figure4.9. First a fast tracking algorithm processes hits in a narrow φ window, but significant coverage in z. This allows the jet vertex to be identified so that a second, more precise algorithm can be run to identify all tracks in the jet

ROIin a region narrow in z. Using these tracks, the HLT jet energy calibration can be improved through the application of theGSC. For jet energies near the single-jet threshold (_≈400 GeV), the correction from calorimeter-only corrections is significant, as shown in Figure4.10, while for those near the multi-jet (_≈100 GeV) the track-based correction is much more significant.

Emiss

T triggers Emiss

T reconstruction is particularly challenging in theHLT, as true offline-like reconstruction would require complete reconstruction, calibration, and ambiguity resolution of many species of reconstructed objects. Calorimeter-based quantities are used in the trigger to approximate the offline calculation with a variety of different strategies:

• Cell Emiss

T (xe) is constructed directly from the vector sum of all calorimeter cells with 2σ noise cuts applied, and treated as massless.

• Topo-cluster Emiss

T (xe tc lcw) is built using a similar method, directly from topo-clusters after the local hadronic calibration scheme is applied (see Section4.4.5).

4. Experimental Apparatus 43 Not reviewed, for internal cir culation only [GeV] T Offline track p 5 678 10 20 30 40 50 ₁₀2 ₂_×₁₀2 Ef ficie n cy 0.96 0.97 0.98 0.99 1 1.01 1.02 Precision Tracking Fast Track Finder Precision Tracking Fast Track Finder

ATLASfor approval

Data 2015 √s = 13 TeV 6 GeV Muon Trigger

@ (a) [GeV] T Offline p 5 678 10 20 30 40 50 ₁₀2 ₂_×₁₀2 resolution [mm]₀ d 0.014 0.015 0.016 0.017 0.018 0.019 0.02 0.021 Precision Tracking Fast Track Finder Precision Tracking Fast Track Finder

ATLASfor approval

Data 2015 √s = 13 TeV 6 GeV Muon Trigger

(b)

Figure 11: The ID trigger muon tracking performance is shown with respect to loose muon candidate tracks from the 6 GeV muon trigger withpT>6 GeV; (a) the efficiency versus the offline reconstructed muonpT, (b) the resolution on the transverse impact parameter,d0versus offline reconstructed muonpT. The offline reconstructed muon tracks are required to have at least two pixel clusters, and at least six SCT clusters are required to lie in the region|⌘_|<2.5 andpT >6 GeV. The closest matching trigger track within a cone of R<0.05 of the offline reconstructed track is selected as the matching trigger track. Bayesian uncertainties are shown.

the fast tracking again followed by the precision tracking, but this time in a wider RoI in both⌘and , but

397

centred on thezposition of the leading track identified by the first stage and limited only to| z| <10 mm

398

with respect to this leading track. For evaluation purposes, the tau lepton signatures can also be executed

399

in a single-stage mode, running the fast track finder only once in a wide RoI in⌘and and extended along

400

the beam line, followed by the precision tracking, again in this wider RoI. The RoIs from these different

401

single-stage and two-stage strategies are illustrated in Fig.12.

402

M Sutton - IDTrigger performance in 13 TeV collisions

TGM 16th September 2015

Understanding the tau timing

One-stage tracking RoI

Two-stage tracking: 1st_{stage RoI} Two-stage tracking: 2nd_{stage RoI} Plan view Beam line • Main saving in time in one-stage tracking comes from narrower φ range in first stage, and narrower z range in second

stage

• Narrower η extent in first stage has a smaller impact because of large z extent ! (Beware Δη in the trigger is almost never results in pyramid shaped RoIs - they are nearly always the wedge shapes illustrated here)

• Two stage tracking RoIs not entirely different in volume - FTF timing may not be dissimilar

Figure 12: A schematic illustrating the RoIs from the single-stage and two-stage tau lepton trigger tracking, shown in plan along the transverse direction with the beamline horizontally through the centre of the figure and in perspective view.

Figure13shows the performance of the tau two-stage tracking with respect to the offline tau tracking for

403 27th July 2016 – 16:37 17 beam line Not reviewed, for internal cir culation only DRAFT [GeV] T Offline track p 5 6 7 8 10 20 30 40 50 ₁₀2 ₁₀2 × 2 Ef ficie n cy 0.96 0.97 0.98 0.99 1 1.01 1.02 Precision Tracking Fast Track Finder Precision Tracking Fast Track Finder ATLASfor approval Data 2015 √s = 13 TeV 6 GeV Muon Trigger

@ (a) [GeV] T Offline p 5 6 7 8 10 20 30 40 50 ₁₀2 ₁₀2 × 2 resolution [mm]₀ d 0.014 0.015 0.016 0.017 0.018 0.019 0.02 0.021 Precision Tracking Fast Track Finder Precision Tracking Fast Track Finder

ATLASfor approval Data 2015 √s = 13 TeV 6 GeV Muon Trigger

(b)

Figure 11: The ID trigger muon tracking performance is shown with respect to loose muon candidate tracks from the

6 GeV muon trigger withpT>6 GeV; (a) the efficiency versus the offline reconstructed muonpT, (b) the resolution

on the transverse impact parameter,d0versus offline reconstructed muonpT. The offline reconstructed muon tracks

are required to have at least two pixel clusters, and at least six SCT clusters are required to lie in the region|⌘_|<2.5

andpT>6 GeV. The closest matching trigger track within a cone of R<0.05 of the offline reconstructed track

is selected as the matching trigger track. Bayesian uncertainties are shown.

the fast tracking again followed by the precision tracking, but this time in a wider RoI in both⌘and , but

397

centred on thezposition of the leading track identified by the first stage and limited only to| z|<10 mm

398

with respect to this leading track. For evaluation purposes, the tau lepton signatures can also be executed

399

in a single-stage mode, running the fast track finder only once in a wide RoI in⌘and and extended along

400

the beam line, followed by the precision tracking, again in this wider RoI. The RoIs from these different

401

single-stage and two-stage strategies are illustrated in Fig.12.

402

M Sutton - IDTrigger performance in 13 TeV collisions

TGM 16th_{September 2015}

Understanding the tau timing

2 One-stage tracking RoI

Two-stage tracking: 1st_{stage RoI} Two-stage tracking: 2nd_{stage RoI} Plan view beam line

• Main saving in time in one-stage tracking comes from narrower φ rangein first stage, andnarrower z rangein second stage

• Narrower ηextent in first stage has asmaller impact because of large z extent ! (BewareΔηin the trigger is almost never

results in pyramid shaped RoIs - they arenearly alwaysthe wedge shapes illustrated here)

• Two stage tracking RoIs not entirely different in volume - FTF timing may not be dissimilar

Figure13shows the performance of the tau two-stage tracking with respect to the offline tau tracking for

403

27th July 2016 – 16:37 17

Figure 4.9: Regions considered in the two-step tracking procedure used for jet triggers is shown. First, a fast algorithm finds tracks in a region long inzand narrow inφ(pictured in blue) with the sole purpose of reconstructing the primary vertex associated to the jet. Next a more precise tracking algorithm is run over hits in a window narrow inz about the selected vertex, but comparable to the jetROIin η_×φ(pictured in green).

380 400 420 440 460 480 500 520 540

[GeV] T p

Leading offline jet 0 0.2 0.4 0.6 0.8 1 1.2

Per-event trigger efficiency > 450 GeV

T p HLT, 2016 calibration

2017 calib., calorimeter only 2017 calib., with tracks

ATLAS Preliminary −1 = 13 TeV, 21.9 fb s Data 2017, |<2.8 η 1 jet with | ≥ Offline selection:

Figure 4.10: Efficiencies are compared for three configurations of single-jet HLTtrigger accepting 400 GeV jets. In the green squares no GSC is applied, while in the red closed circles only the calorimeter-based corrections are applied and in blue open circles theIDtrack corrections are also applied. A more complete correction leads to tighter correspondence between the online and offline jet energies and thus a lower rate of events collected with offline jetpTbelow the trigger threshold. Figure reproduced from Ref. [87].

4. Experimental Apparatus 44

• Topo-clusterEmiss

T withη-dependentPUcorrection(xe tc pueta) is constructed as above, but with average pileup densities computed from<2σtopo-clusters in ten regions ofη, which are subsequently subtracted from the topo-clusters used to build theETmiss.

• Pile-up fit Emiss

T (xe tc pufit) extends the η-dependent correction scheme by tiling the calorimeter into 0.8_×0.8 regions which are divided in hard-scatter and PU dominated regions according to a total energy threshold criterion. The pileup densities in each hard-scatter region are obtained in a fit (assuming no Emiss

T from PU) with the remaining energy used to compute the Emiss

T . • Jet-based Emiss

T (xe tc mht) is computed using all R = 0.4 anti-k⊥ jets reconstructed as de- scribed above withpT>7 GeV. Retaining low-pTjets gives an estimate of the soft-term. The proliferation of methods is due to the significant dependence ofEmiss

T resolution onPU, when in- formation from theIDis not used in the computation. Alternate methods have differing sensitivities to the level ofPU, and also make requirements on energies calibrated at different scales (EMscale for cellEmiss

T , Local Hadronic Calibration (LC) scale for topoclusters, and particle-scale for fully

calibrated jets). Figure4.11shows the efficiency for triggers of all types as well as the dependence of trigger rates on the average number of collisions per event. The increase in luminosity from 2015 to 2016 (_hµ_i= 13 to 25) required a more complicated trigger strategy, where combined requirements on differentHLTEmiss

T quantities were necessary.

In document Targeting Natural Supersymmetry With Top Quarks (Page 69-73)