Johannes Haller Thomas Schörner-Sadenius Hamburg University Summer Term 2009

The Large Hadron ColliderMachine, Experiments, PhysicsTrigger + Data Acquisition (+ muon chambers) Johannes HallerThomas Schörner-Sadenius Hamburg UniversitySummer Term 2009

NECESSITY OF TRIGGER (1) Luckily … … we are mainly interested in “high-pT” signatures. Example for muons: Interesting (non-minimum-bias) physics starts only at relatively high transverse momenta pT. New particles are expected to be heavy (Higgs > 100 GeV)  high-pT decay products! Same is true for jets, photons, electrons, … Cross-section overview: – Small cross-sections and branching ratios for new processes! – Large rates for SM processes and up to 30 overlay events at high lumi! – SM processes as backgrounds to new physics (Wbb, ttbb, W/Z pairs) understand!!!!! – Use SM calibration processes Zl+l-, W jj, …

NECESSITY OF TRIGGER (2) bunch crossing rate: 40 MHz total interaction rate: ~1 GHz event size: ~ 1.5 MB affordable: ~ 300 MB/s storage rate: ~ 200 Hz  online rejection: 99.9995% total interaction rate storage rate discoveries

… MORE COMPLICATIONS? ppHZZ(*)e+e-μ+μ-

… MORE COMPLICATIONS! ppHZZ(*)e+e-μ+μ- At 1034cm-2s-1 up to 23overlay pp collisions.~1700 charged particles!

NECESSITY OF TRIGGER (2) Definition “trigger”:The trigger is the central component of the data acquisition that decides about selection and rejection of events “online”. The trigger can be realised as fast hardware (DSPs, transputer, FPGAs, …) or as software algorithms running on computer farms. Main problem for a trigger:How to acquire, in the short time available, sufficient information to come to a solid decision (remember LHC bunch crossing 25 ns, length of experiment 40 m ~ 120 ns, 108 channels …)?Typical solution:Build a multi-layer trigger system with increasing latency requirements for each successive layer. Provide only very rough information to early levels and train them to reject as many events as possible. Finer and finer filtering with more and more information in higher trigger levels. Very often early levels implemented as hard, later levels as software. Will discuss ATLAS in some detail, some aspects of ZEUS and/or H1 also mentioned. In addition:… many interesting processes have distinct features (“signatures”) like MANY jets, MANY b quarks, MANY leptons: Consider for example SUSY cascade decays: So events might be identified by counting number of high-pT objects:

OVERVIEW: WHAT TO TRIGGER ON? Trigger menue … more details later (rates in kHz) Consider various signatures: – inclusive and di-leptons (electrons, muons): - Gauge boson pairs (W,Z)  calibration etc.- single and pair top production- direct Higgs production with HZZ*/WW*- associated Higgs production with WH, ZH, ttH- MSSM Higgs decays- new gauge bosons with decays to leptons. - SUSY and leptoquark searches – Photons: - Hγγ – High-pT hadronic jets - SUSY- leptoquarks- resonances- compositeness models- top, W – missing transverse energy (MET)- Supersymmetry- top, W- …  Trigger on high-pT leptons, photons, jets, and MET.

THE ATLAS TRIGGER: OVERVIEW Multi-layered, pipe-lined system! } - Hardware-based (FPGAs and ASICs)- Coarse granularity from calo/muon- 2s latency (pipelines) L1 } - ‘Regions-of-Interest’- ‘Fast rejection’- Spec. algorithms- Latency ~10ms L2 } EF - Full event- Best calibration- Offline algorithms- Latency ~seconds

THE LEVEL1-TRIGGER Candidates forelectrons/photons,taus/hadrons,jetsabove pT thres-holds. Muoncandidatesabove pTthresholds Energy sumsabove thresholds Multiplicities Regions-of-Interest Interface to highertrigger levels/DAQ:objects with pT,, Event decisionfor L1 Interface tofront-end

THE CALORIMETER TRIGGER 1 The calorimeter trigger provides four different types of objects: – electron/photon candidates from clusters in the electromagnetic section (and vetoes on the hadronic section) of the calorimeter. – τ leptons or single hadrons based on cluster in EM and HA calorimeters. – Jets of hadrons, defined from EM+HA energies in the calorimeters. – Missing transverse momentum, based on all calorimeter energy in a defined pseudo-rapidity range. Example electron/photon trigger (picture right): – Use “sliding-window” technique to find 2×2 towers in EM calorimeter with maximum ET = E∙sinθ.This “cluster” defines the “region of interest”. – Find sub-cluster of 2 towers in RoI cluster for definition of transverse energy ET of cluster. – Define isolation criteria for ring of 12 EM towers around the cluster. – Reject candidate clusters with too much hadronic energy behind in 4×4 tower region. Main parameters: – size of cluster: containment of EM showers, sharp trigger threshold, rejection of jets. – trigger thresholds  trigger rates. – up to 8 different ET thresholds for EM candidates implemented in ATLAS trigger chain (later).

THE CALORIMETER TRIGGER 2 Calorimeter trigger implementation: – The τ/hadron trigger: – Use “sliding-window” technique to find 2×2 towers in EM+HA calorimeter with maximum ET = E∙sinθ. This “cluster” defines the “region of interest”. Find subcluster and check isolation: The jet/energy trigger: – 2×2 cluster in 2×2 or 3×3 or 4×4 region of elements of 0.2×0.2 in η×φ space. analog sums of cells  7200 triggertowers of 0.1•0.1) digitisation,presumming to jetelements with0.2•0.2 granularity cluster processor:Find e/, /hadroncandidates in 6400TT (||<2.5) • - Find jet candidates in 30•32 jet elements for ||<3.2 • Build total ET sum up to ||<4.9.

THE MUON TRIGGER Idea: pT from hit coincidences in successive detector layers: – Trigger chambers: • 3 RPC stations for ||<1.05• 3 TGC stations for 1.05<||<2.4. • 2 , layers per station (TGC 2/3) – Procedure: • Put predefined ‘roads’ through all stations (width in  ~ pT). • If hit coincidences in 2(3) stations  muon candidate for pT threshold corresponding to ‘road’. • ‘Roads’ can be defined for 6 different pT thresholds (for which multiplicity counts are delivered to the CTP) Very complex logic due to high number of predefined “roads”. Data from all sectors of the detector are digested in MUCTPI – muon-to-CTP interface … precision chambers trigger chambers ATLAS quadrant in rz view

THE CENTRAL TRIGGER PROCESSOR Implementation: – Basically one big FPGA (“field programmable gate array”) that contains the conditions and items. – In addition services, communication, timing, … The LVL1 decision in ATLAS: – CTP receives multiplicities of electron/photon, tau/hadron, jet, missing-ET, and muon candidates for different thresholds. – These multiplicities are discriminated against “trigger conditions” like 2EM10 or 1JT90 … These trigger conditions are grouped into larger conditions that are physics motivated. If any of these “trigger items” is fulfilled, “LVL1 Accept” is set and decision and objects with fourvecs is passed to HLT (pipeline readout).

L1 SIMULATION AND PLANNING The planned trigger menue for the LVL1 agian: The Simulation of trigger in software necessary for … – generation of MC events for analysis purposes – rate and efficiency estimates – Inputs for HLT tests and configuration and hardware tests.

THE HIGH-LEVEL TRIGGER (HLT) Good example for solid software process.

HLT: DESIGN OVERVIEW HIGH-LEVEL TRIGGER (HLT) Event- Filter EventFilter (EF) Level1 (L1) ~102 kHz LEVEL 2 (LVL2) ~1 kHz Selection ~102 Hz Classification Offline High-Level Trigger: Design Hardware Implementation Read-Out Subsystem Modules Simplified subsystem view

HLT: SELECTION SOFTWARE Running in Level2 Processing Units (L2PU)+EF. Set-up by HLT configuration EventFilter PESA Core Software Level2 PESA Algorithms Offline Reconstruction Offline Architecture & Core Software

HLT PRINCIPLES AND DECISION Example for actual decision: HLT selection principles: – Regions of interest: Selection and rejections starts with localized LVL1 objects – RoIs  limited amount of data to be processed! – Step-wise procedure: Stepwise more and more correlated data from muon or calo system or other detectors (tracking!) are retrieved to guide and aide the decision. – Fast rejection: After each step of refining the information check whether trigger conditions are still fulfilled  optimal use of HLT processor farm! – Flexible boundary between L2 and EF  distribution of load and optimal use of computing resources! – Use of offline reconstruction algorithms  use of common software facilitates understanding of trigger behaviour (rates, efficiencies). Use of common “event data model” (EDM) makes life easier for everybody.

HLT SELECTION: TRIGGER MENU Optimization of efficiency/rejection and CPU load / data volume. Rate·Event size (1.6MB)  needed band widths / storage volumeRate·CPU time  number of processors (500?)

REFINING EFFECT OF HLT: EXAMPLE 1 Pion decay to two γ Real photon: in first sampling: Excellent photon efficiency with this kind of trigger: – Single-photon efficiency > 90% (function of ET!) – Rate few 100 Hz at L2. – Jet rejection of about 3000! Other example: Tracking-assisted muon ID: Use of tracking for improved muon ID (possible only in HLT!) improves rejection and resolution: L1: about 20 kHz muon triggers L2: about 200 Hz muon triggers! Example: backgrounds to photons: – from π0γγ and narrow hadronic jets. – Identification of photons mainly based on ET, hadronic leakage, shower shape, and cluster structure (track veto possible?). – Variables: EM-ET in 3∙7 cells, HA-ET, lateral shape in second sampling, lateral shape in first sampling for low energies, … Back sampling(0.05•0.025): 2-12X0. Second sampling(0.025•0.025): 24X0. First sampling with finer cell granularity for 0 rejection (0.003•0.1): 6X0.

REFINING HLT EFFECT (2)

TEST BEAM RESULTS: MUON TRIGGER end-caps (TGCs): barrel (RPCs): threshold efficiency after chamber shifting position in precision muon chambers vs. position in RPCs efficiency and BCID • Triggered Bunch • Next Bunch • Previous Bunch • total efficiency • pT threshold 6  pT threshold 5  pT threshold 4 • nice correlation between RPC and MDT position measurement • trigger efficiency at test-beam (3/4, phi): 99.4% • efficiency for correct identification of bunch crossing: 99.5% • chamber was shifted to emulate the effect of deflection in magnetic field • coincidence algorithm works • big timing margin where e(correct bunch) high and e(bunches before and after) tiny

TEST BEAM RESULTS: CALORIMETER TRIGGER L1Calo setup PreProcessor ROD CPMs/JEMs Receivers Correlation of energy in LAr calo. and CPM Detector slice with parts of all components in test beam: – Constituted about 1% of final capacity. . – Checks of data consistency successfull! – Picture from the counting room: – Good correlation of energy values measured in calorimeter and received in trigger chain. – No events below EM trigger threshold of 20 GeV selected “offline”  Calorimeter trigger did work!

DATA ACQUISITION Higher trigger layers running on large PC farms, latency O(1s). ~30 PCs  Storage

THE ZEUS TRIGGER – Inputs from calorimeter CAL, jet chamber CTD, and other components. Note tracking input at first level. – Pipelined read-out: about 60 BC can be stored to leave enough time for component FLTs (first-level triggers) and global FLT decision (GLFT). – GFLT accept can be rechecked using “fast clear”. – After GFLT accept data transferred to GSLT – this is low-level software running on transputer networks. – After GSLT accept the full event is assembled in the event builder, and the then (PC farm) TLT is started and does the final decision. Note difference in triggering strategy between H1 and ZEUS (detail): – ZEUS has only HIGH and LOW luminosity trigger configurations to adapt the data taking to the beam situation. “Prescales” are fixed for each configuration. – H1 employs an auto-prescale scheme – the mixing of trigger slots is (the composition of the trigger menu) is adjusted on a 30-second basis. Also here three-layer system:

THE CMS TRIGGER SYSTEM (1)

THE CMS TRIGGER SYSTEM (2)

energy energy energy muon electron missing energy Minimum bias event rejected by Trigger Z→ e+e accepted by Trigger H→ e+eµ+µ accepted by Trigger SUSY event accepted by Trigger Micro Black Hole ? accepted by Trigger Simulation von charakteristischen Ereignissen im ATLAS Detektor electron electron electron lots of ’s and tracks

2.2.3: Detektoren: Triggersysteme Typisches Beispiel für erste Triggerstufe: • Beachte: Trigger-Entscheidung (~ms) dauert länger als Zeit zwischen zwei Wechselwirkungen (25ns). • Trotzdem: alle 25ns wird eine Trigger-Entscheidung gefällt. • Jeder Auslesekanal hat Pipeline- Memory • Bis zur Ankunft der Triggerentscheidung am Detektor: Speicherung der Ereignisse in Memories • Tiefe der Pipeline muss so lang wie Latenzzeit sein, sonst Mischung von Ereignissen • „Langsamer“ Trigger lange Pipelines  teuer! • L1 sollte schnell sein  Elektronik • Ereignisse die von LVL1 akzeptiert werden, werden von der Computer Farm (L2 und L3) weiterprozessiert Calorimeter trigger Muon trigger Anzahl von e, Taus, Jets Anzahl von Muonen Latenzzeit: 2.5 ms = 100 BC Central Trigger Processor (CTP) L1A signal TTC TTC TTC TTC TTC …

MUON CHAMBERS: MOTIVATION Muons are easily recognised and may therefore also serve valuable calibration and luminosity purposes: Resolution of dimuon mass in Zμμ or Hμμ events might be important! Also new heavy gauge bosons etc. There are many muons at the LHC! Many interesting (new-physics) processes involve muons in the final state. Examples:

MUON CHAMBERS: REQUIREMENTS – At least 16 hadronic interaction lengths everywhere for safe muon measurement! – Need to trigger on muons from few to about 100 GeV transverse momentum up to pseudorapidities of 2.1! – About 10% (30%) stand-alone momentum resolution at 10 GeV (1 TeV) muon momentum. – After track matching 1% (10%) momentum resolution at 10 GeV (1 TeV). – Spatial position matching muon/tracking of the order of 1 mm. – 99% correct charge assignment up to 7 TeV energy!– Radiation hardness!

CMS MUON SYSTEM – OVERVIEW (1) Importance of magnetic field:- σx is single-point precision.- higher B (stronger curvature) improves resolution!- Important: lever arm L: muon chambers at outside of experiment!- Resolution is important for many measurements!

CMS MUON SYSTEM – OVERVIEW (2) • Segmentation: • 4 stations of muon chambers in both barrel and forward regions.- barrel: drift tubes plus resistive plate chambers- forward: cathode strip chambers + RPCs. - in barrel: three “superlayers” with each four staggered layers of drift chambers per station (60/70 tubes/station).

CMS BARREL DRIFT CHAMBERS Wire at +3.6kV Aluminum at ground potential Strip electrodes, +1.8kV I-beams (aluminum) at -1.8kV Insulating plastic – At larger radii + in the barrel, low rates, low particle multiplicities  slow drift chambers are okay (≤20mm drift lenth, corresponding to up to 300 ns). – Tubes have different advantages, for example protection of chamber against wire breaking (in contrast to MWPC). – Tubes decouple neighbour channels electronically. – High number of tubes (about 200k) together provide excellent time and spatial resolution. – Layout of one station – note the mechanical, electrical, gas-supply and other “trivial” problems! – spatial resolution: 200 μm!

CATHODE STRIP CHAMBERS IN ENDCAPS muon wires cathodes strips wire Ind. charge avalanche Principle and motivation: – Monitored drift tubes have rather large dimensions (diameter and length)  problems for readout in high-occupancy environment. – Especially in forward region higher particle fluxes  need different design. – CMS: All endcap muon precision chambers are CSCs, in ATLAS only the very low-angle region (rest MDT) – Trapecoidal shape, arranged in rings around beam pipe; typically six layers of chambers. Rather large strip widths (O(1cm)  lower number of channels. – 2-dimensional readout (wires and strips) – Small wire distance ~3 mm  schnell! – high spatial precision (interpolation over various strips). – Simple phi measurement through strips (wedge shape).

CMS RESISTIVE PLATE CHAMBERS (RPCs) Resistive Plate Chambers are fast:– Use for generating fast trigger signals!– Principle: Ionisation at upper edge of proportionality region! – In addition the chambers provide easy, cheap and fast readout  optimal for trigger! – Strip structure allows for high segmentation  even in trigger good muon pT resolution! Realisation:– two chambers back-to-back sharing one set of strips form on RPC module! All in all 6 such chambers in barrel and forward region for each pseudorapidity  good efficiency and coverage for trigger purposes. ATLAS … … use Thin Gap Chambers (TGCs) for triggering in the endcaps. – Similar to MWPCs, but – … wire pitch larger than wire-cathode distance (wires on positive potential) – Operated with fast gas in saturation mode (very high field)  stable signal.

CMS MUON SYSTEM: PERFORMANCE (1) Important for drift chambers:Linearity of drift  proportionality of drift distance to drift time. Results for CMS: Very good! Efficiency of muon system as function of pseudo-rapdity η ( detector structure): … and that is what you will see:

CMS MUON SYSTEM: PERFORMANCE (2) Z: µ+tracker Z: only µ Z’: 150 GeV Z’: 300 GeV Invariant mass resolution of muon pairs from decays of heavy particles:– Usage of tracker information improves resolution drastically! – Resolution few GeV! Excellent muon system for discoveries, calibration, … pT resolution :– again combination of tracking muon system delivers optimum result (<1%)! Efficiency: Shown is the trigger efficiency as function of pT for different pT trigger thresholds.

THE ATLAS MUON SYSTEM Cathode Strip Chambers - 67000 wires- only for ||>2 in first layer- space=60m, t=7ns Resistive Plate Chambers - 354000 channels- space=1cm- trigger signals in 1ns Thin Gap Chambers - 440000 channels- ~MWPCs Monitored Drift Tubes - 3 cylinders at R=7, 7.5, 10m- 3 layers at z=7, 10, 14 m- 372000 tubes, 70-630 cm- space=80m, t=300ps

Johannes Haller Thomas Schörner-Sadenius Hamburg University Summer Term 2009