The University of Manchester Triggering Top Events Simon Head The University of Manchester Presenting on behalf of: Thorsten Wengler (Manchester), Attila Krasznahorkay (Debrecen / CERN), David Berge (CERN), Juergen Thomas (Birmingham), Alan Watson (Birmingham)

Motivation for Top Physics at ATLAS ATLAS will be a top factory Cross-section for top production at ATLAS is ~800 pb In 10 fb-1 (one years low luminosity running) at 14 TeV we expect 8 million tops Large mass of the top quark makes it unique and sensitive to new physics

Triggering Collisions at the LHC occur at a rate of up to 40 MHz, compared to a storage capacity of about 200 Hz. The trigger system has the “interesting” task of rejecting 99.9995% of the collision events while keeping those needed to achieve the physics goals of ATLAS. Internally the trigger identifies and makes use of Regions-of-Interest Each level of the trigger refines the decision made by the previous level and has access to more detailed information about the event ATLAS uses a 3 level trigger system First Level Trigger (LVL1) High Level Trigger (HLT) - consisting of: Second Level Trigger (LVL2) and Event Filter (EF)

The Atlas Trigger System The LHC proton bunches will cross at a frequency of 40 MHz At the design luminosity of 1 × 1034 cm−2 s−1, roughly 23 inelastic proton–proton collisions at each bunch crossing The LVL1 trigger will reduce the initial event rate to less than 75 kHz The HLT must reduce the event rate further, to O(100) Hz

Trigger and Top The ATLAS trigger is designed to fire on high pT leptons, jets and missing ET The semi-leptonic ttbar decay provides all of these Athena 11.0.5 is the first release suitable for detailed trigger studies of fully simulated data Data samples are produced for the CSC (Computing System Commissioning) The ATLAS collaboration will write a number of CSC notes (produced by the Physics and Joint Performance group)

LHC nominal running b t / tbar l v q tbar /t qbar b Concentrate on the semi-leptonic ttbar decays Distinctive signal to trigger on (high PT lepton with isolation and missing ET) Centrally produced (CSC) sample 5200 (14 TeV) – using “recotrig” AOD Make use of TopView and EventView for event selection Write custom tools to insert trigger data into the EventView

Environment for the study Work presented here is in the context of note T5: “Triggering Top Quark Events in ATLAS” Performance study Exploit the trigger setup for efficient ttbar event selection Define and test methodology to characterise trigger performance e.g. Determine trigger efficiencies from data using simultaneously fulfilled trigger signatures in ttbar events. Data must be officially produced with Athena 11.0.42 or 11.0.5. Re-run with 12.0.x using a misaligned geometry. More detailed studies will be performed in the next two to three months using TopView

TopView Extensions to EventView to do top analysis Reconstructs semi-leptonic decays for ttbar pairs by simply combining reconstructed W with a tagged b jet We run with a modified version of the ttbar analysis included with TopView https://uimon.cern.ch/twiki/bin/view/Atlas/TopView We have written tools (EventView) that so far add: LVL1 trigger information to the EventView and save to an NTUPLE Full muon slice Write out composite particles to the user data Perform some simple analysis

Available Trigger information (11.0.5) LVL1: The RoIs produced by the LVL1 system are available through the LVL1_ROI container. Information provided: eta and phi coordinates and passed threshold level. (Also: name of the threshold, and from 12.0.0 the passed threshold value in GeV). Other quantities such as the CTP Decision are also available LVL2: Among the provided information: eta and phi coordinates, and pT of the reconstructed particle EF: Not available in 11.0.5

LVL1 egamma efficiency What is the efficiency that a true electron will fire the First Level Trigger, I.e. pass a trigger threshold such as EM25i ? EM25i asks for a single isolated electron with PT > 25 GeV Common trigger object fired for many processes Electrons within |eta| < 2.5 EM25i EM cluster > 19 GeV EM isolation <= 3 GeV Hadronic core <= 2 GeV Hadronic isolation <= 2 GeV Start by plotting some simple quantities…

LVL1 egamma efficiency PT spectrum for the electron in ttbar semi-leptonic decay (from truth)

LVL1 egamma efficiency EM25i as defined above Threshold has reached about 90% efficiency at its nominal value of 25 GeV, as it should.

Electron Trigger in Top Events Use teb to set isolation cuts More activity than Zee lower efficiency for same cuts Isolation cuts for initial running: EM isolation  3 GeV Inner Had isolation  2 GeV Outer Had isolation  2 GeV But rates for 1031 very low Tau trigger, ET > 9 GeV, no isolation  ~4 kHz 9 GeV unisolated electron would be a subset of this Use very loose trigger initially to understand performance

Top Efficiency from Electron Trigger Run 5200 MC@NLO At least 1 top  lepton decay Unisolated EM trigger  high efficiency including events with no electrons! many rejected when isolation applied Events containing > 25 GeV e Typical Offline top selection Used MC truth – no reconstruction inefficiency Trigger efficiency very high, with or without isolation

Top Efficiency from Jet Trigger All numbers very preliminary Run 5200 Expect 4 low-mid ET jets/event electrons can also fire jet trigger 1033 trigger menu not efficient 1 Jet > 200 GeV ~15% 3 Jets > 75 GeV  ~40% see box for more explanation But 1031 is a different story 1 Jet > ~100 GeV 80-90% efficiency, 200 Hz rate Efficient redundant trigger possible ttbar 6 jets Fast simulation study suggests 5 jet trigger promising, even at 1033 can we use these offline though? Plot shows actual L1 Jet ET Scale does not match offline Trigger threshold << offline ET 200 GeV jet  ~150 GeV threshold 75 GeV jet  ~ 40 GeV threshold Usually quote Jet ET for which trigger is 95% efficient, rather than (calibration-dependent) threshold

Jet Efficiency vs. ET Comparison of trigger performance against reconstructed cone 0.4 jets efficiency for individual jets

Missing ET in Top Events Inclusive ETmiss trigger? Possible at 1031? ETmiss > 40 GeV  60 Hz excluding machine backgrounds Gives ~60% efficiency for SL top events Probably more interesting to use top events to understand ETmiss trigger Maybe multijet + ETmiss could be used as a redundant control for muon triggers in top events? ETmiss is a challenging trigger Global variable, so hard for L2 to improve Combine ETmiss with other trigger or require low L1 rate Most vulnerable to machine backgrounds

Muons Work continued from last Trigger and Physics week Working on getting larger statistics Our analysis code has access to the full muon slice The plots opposite are just a small selection

Level-1 Trigger Item Overlaps in 11.0.5 RecoTrig AODs Goal of item overlap study: determine efficiencies of each item using simultaneously fulfilled trigger signatures Level-1 Trigger menu in 11.0.5 AODs: (0) Electromagnetic: (EM) 15 GeV, no isolation (1) Hadronic (HA) 20 GeV, no isolation (2)-(7): MU of 6, 8, 10, 11, 20 and 40 GeV (8): Jet (JE) 40 GeV, size 4x4 Practically all events are triggered ! Only 85 out of 24000 Ev are not. Producing ROIs for each trigger type. Histo (top) shows trigger bits sets: Each of EM, HA and JE trigger nearly every event. Only MU shows pT dependence. Legoplot (bottom) illustrates overlap of items (logical AND of bits)

Level-1 Trigger Item Overlaps: Trigger Menu 1033 Now set a trigger menu suitable for 10^33 (TDR-like items), Re-done Digi to AOD step (200 Ev). To study combined triggers. Items: Jet: (0) Single Jet 180 GeV, (1) Three Jets, 75 GeV each (2) Four Jets, 60 GeV each Hadronic (tau): (3) 20 GeV, isolation 5/10 GeV, Electromagnetic: (4) 15 GeV, isolation 3/2/2 GeV Combined: (5) Hadronic 30 GeV + Missing-ET 30 GeV, (6) Jet 75 GeV + Missing-ET 30 GeV Legoplot (bottom) illustrates overlap of items: Logical AND of item bitmap (Word0 of CTP Decision) As expected, items (3) (5) (6) overlap strongly

Conclusions Now is a good time to do trigger aware analysis The Trigger is extremely important and thus far, mostly overlooked in analysis Information is in the AOD, especially those titled “recotrig” We are developing the tools to allow access to the information for EventView analysis Information is already there, if you know how to access it These tools should make accessing it using EventView easier For a luminosity of 1031 we can use an EM trigger without isolation Some RoI’s seem to match an electron (energy) but are a long way from the electron in eta-phi – why?