Trigger Strategy and Performance of the LHCb Detector Mitesh Patel (CERN) (on behalf of the LHCb Collaboration) Thursday 7th July 2005

Introduction LHCb experimental goals : Precision measurements of CP Violation in B decays Aim to (over-)constrain the unitarity triangle by making measurements in multiple channels b production predominately at small polar angles → LHCb optimised as single forward arm spectrometer To meet the physics goals require a trigger which can select : Multitude of signal channels in the LHCb experimental environment Channels required for calibration, alignment and systematic studies Channels that allow the efficiency of the tagging of B flavour to be evaluated Unbiased control channels System must be simple, robust and flexible 7th July 2005 HCP2005 - Mitesh Patel

The LHCb Trigger Environment LHC Bunch crossing frequency: 40 MHz Non empty bunches → 30 MHz LHCb Luminosity : 2×1032 cm-2s-1 10-50 times lower than ATLAS, CMS B decays → displaced secondary vertex, need ~1 interaction/event ‘Visible’ interactions at 10 MHz 100 kHz bb events (800 kHz cc) 15% of bb events: all decay products of at least one B in detector Branching ratio of interest : 10-3 to 10-7 Use information from a variety of LHCb’s detectors to reduce 10 MHz … 7th July 2005 HCP2005 - Mitesh Patel

Detectors in the LHCb Trigger VErtex LOcator primary vertex impact parameter displaced vertex Scintillator Pad Detector Charged multiplicity Trigger Tracker p, pT Calorimeters PID: e,, 0 Trigger on hadr. Muon System Pile-up system multiple interactions, charged multiplicity 7th July 2005 HCP2005 - Mitesh Patel

Trigger Strategy The physics goals of LHCb motivate : → Data mining Exclusive triggers : ‘hot’ physics eg. BsDsh, Bsff, B0J/y KS, B0D*p, B(s)h+h-, B0K*m+m- , B0D0 K*, Bsm+m-, BsJ/y f, Bsfg Inclusive triggers : Inclusive single-muon sample [independent of signal type ] Sample triggered independent of signal type – unbiased on the signal side Signal trigger efficiencies Inclusive di-muon sample [selected without lifetime information] Clean mass peaks for alignment, momentum (B field) calibration Proper time resolution using prompt J/ events Inclusive D* sample [selected without RICH information] Clean signal of D*D(K) Measure PID performance as a function of momentum → Data mining 7th July 2005 HCP2005 - Mitesh Patel

Trigger Overview LHCb will use three levels of trigger : 10 MHz 1 MHz Level 0 Trigger [4 ms] [hardware] ‘high’ pT particles in calorimeters and m detector Pile-up System throws away busy events Level 1 Trigger [~1 ms] [software] Partial read-out: Vertex Detector (VeLo), Trigger Tracker (TT) and L0 summary Find high IP tracks, estimate pT of tracks, link to L0 objects pT of the two highest pT tracks High Level Trigger [~10 ms] [software] Confirmation of L1 decision then full reconstruction of event Exclusive selections for most important physics channels Inclusive selections 10 MHz 1 MHz Already well developed, relatively fixed 40 kHz Now being developed 200 Hz 1800 Hz + L1 and HLT will be run on a single ~1600 CPU PC farm 7th July 2005 HCP2005 - Mitesh Patel

The Level 0 Trigger: Overview 2 highest pT muons m CHAMBERS Fast search for ‘high’ pT particles Cut on global variables L0 has 4 ms latency “L0 OBJECTS” Highest ET g, electron, p0, hadron candidates CALORIMETERS L0 decision unit L0DU SET CALORIMETERS GLOBAL VARIABLES z and # trks in 1st, 2nd vtx PILE-UP SYSTEM Charged particle multiplicity SPD, PILE-UP SYSTEM 7th July 2005 HCP2005 - Mitesh Patel

Level 0: Muon Trigger Search for high pT muons Five muon stations M1-5 Variable granularity Projective geometry 2 highest pT candidates per quadrant sent to L0 decision unit p/p ~ 20% for b-decays Typical Performance: ~88% efficiency on B→J/(µµ)X 7th July 2005 HCP2005 - Mitesh Patel

Level 0: Calorimeter Trigger ECAL: 6000 cells, 8x8 to 24x24 cm2 HCAL: 1500 cells, 26x26, 52x52 cm2 Look for high ET candidates in the calorimeters : In regions of 2x2 cells Particle identification from ECAL / HCAL energy PS and SPD information ET threshold ~ 3 GeV Sent to L0 decision unit: Highest ET candidate of each type Global variables Total calorimeter energy SPD multiplicity Typical Performance: 30-50% efficiency on hadronic channels for about 700 kHz bandwidth ECAL HCAL Pre-Shower Detector (PS) Scintillator Pad Detector (SPD) FE SPD-PreShower ECAL HCAL Validation cards Selection crates e± g p0 p0 ETtot SPD mult. hadr 7th July 2005 HCP2005 - Mitesh Patel

Level 0 : Performance L0 Decision unit : Performance : OR of high ET candidates Applies cuts on global variables Performance : Efficiency ~50% for hadronic channels, 90% for m channels, 70% for radiative channels 1% bb → 3% after L0 8% cc → 10% after L0 Type Threshold (GeV) Rate (kHz) Hadron 3.6 705 Electron 2.8 103 Photon 2.6 126 p0 local 4.5 110 p0 global 4.0 145 Muon 1.1 Di-muon 1.3 Global Variable Cut Tracks in 2nd vtx 3 Pile-Up multiplicity 112 hits SPD multiplicity 280 hits Total ET 5 GeV 7th July 2005 HCP2005 - Mitesh Patel

The Level 1 Trigger: Overview 2D (rz) tracking in VELO Find high IP tracks (VErtex LOcator) Confirm track / Estimate pT from Trigger Tracker Link VELO tracks to L0-objects L1 algorithm run on PC Farm Average latency: 1 ms (max 50 ms) Primary Vertex search Allow up to 3 PV 2D track selection 0.15mm < IP < 3mm L0 m match Multiple routes through L1 : Generic Line : L1-Variable: log(pt1)+log(pt2) pt1,2 two highest pT tracks Muon lines : Single muon: pT>2.3 GeV, IP >0.15 mm Dimuons: J/ ± 500 MeV window OR : mµµ>500MeV and IP>0.05mm OR : mµµ>2.5GeV Photon, electron lines : L1-Variable (relaxed) + ECAL>3.1 GeV 3D (rfz) VELO tracking Confirm IP L0 m match p, pT estimation Use VELO-TT track + fringe B field OR : VELO track + L0 muon L1 decision 7th July 2005 HCP2005 - Mitesh Patel

Level 1: Event Reconstruction L1 relies on the LHCb VErtex LOcator (VELO) : Silicon tracker before the LHCb magnet Angular coverage of full angular range of downstream detector Sensors ~7mm away from beam, retractable (injection), in secondary vacuum Foils protect against RF pickup from the LHC beam 21 sensor stations : 2 R- and 2 f-measuring sensors per station Gradual increase of pitch (40 mm to 103 mm) Si Sensors RF foils Interaction region f sensor R sensor 100 cm 7th July 2005 HCP2005 - Mitesh Patel

Level 1: Event Reconstruction Fast tracking strategy : ~70 tracks/event after L0 : perform tracking in R-z view (using only R sensors) Primary vertex σZ ~ 60 mm, σX,Y ~ 20 mm Select 2D tracks with 0.15 < IP < 3 mm → 8.5 tracks/event 3D tracking for selected tracks pT measurement using Trigger Tracker TT : two layers of Si detectors with 200 mm pitch Only 0.15Tm of B field between VELO and TT → DpT / pT ~ 20-40% allows rejection of low p tracks which can fake high IP Matched L0-m : → DpT / pT ~ 5% z-vertex histogram xy-vertex Example: 2D tracks in 45o sz~60mm sx,y~20mm The Trigger Tracker 7th July 2005 HCP2005 - Mitesh Patel

Level 1 : Performance L1 decision : Performance : Take OR- of the multiple routes through L1 Tuned for retention of 4% of minimum bias L0 triggers (40 kHz L1 output rate) Performance : eg. Generic L1-Variable: log(pt1)+log(pt2) log(pt1)+log(pt2) L0 efficiency L1 efficiency L0L1 eff Channel Generic Single m Di-m J/Y Electron Photon Total Bd0→ p+p- 81.8% 1.6% 0.1% 4.3% 2.7% 82.6% Bs0→ Ds-K+ 79.8% 4.0% 0.7% 0.8% 4.4% 2.9% 80.9% Bd0→ D0(K+p-)K* 83.5% 2.0% 0.4% 5.0% 3.3% 85.4% Bs0→J/Y(m+m-)f 74.3% 42.8% 25.0% 45.5% 1.9% 1.7% 87.2% Bd0→ K*g 54.9% 2.4% 0.3% 17.3% 30.9% 67.2% Minimum Bias 0.2% 3% bb after L0 → 16% after L1 10% cc after L0 → 18% after L1 7th July 2005 HCP2005 - Mitesh Patel

The High Level Trigger: Overview High Level Trigger (HLT) : Generic Algorithm: repeat L1 then full readout of the detector Muon Highway feeds inclusive muon modes Form basic particles, composites, search for signatures of hot physics channels (exclusive), D* inclusive 10ms to run (in 2007) – algorithm run on same CPU farm as L1 Bs → ff Bd → D*p Bd → D0K* Bd → mmK* B → J/yX Bs → Dsp Bs → fg Exclusive HLT Inclusive di-m Inclusive D* Inclusive B→m Ds→KKp f →KK D0→Kp, KK Loose D0→hh Loose dimuons K*→Kp muons Photons, electrons p, K Generic HLT Muon Highway 7th July 2005 HCP2005 - Mitesh Patel

HLT Generic Redo “L1” with improved: momentum resolution muon matching HLT generic reconstructs ~1/3 of tracks in the event and redoes L1 in ~4 ms Reduces rate from 40 kHz input from L1 to 10 kHz: 16% bb after L1 → 38% after HLT Generic 18% cc after L1 → 27% after HLT Generic Then have ~24 ms for further HLT selections 7th July 2005 HCP2005 - Mitesh Patel

HLT Exclusive HLT Exclusive being tuned for ~10 core physics channels In these channels cuts tuned to take ~15Hz MB / channel B mass resolutions ~30 MeV Mass windows >±500 MeV Bs→ DsK Bs→Dsp Bd→D*p Bd→pp Bs→KK Ds→KKp Bs→mm Bd→Kp Bs→pK B → hh reconstructed as B→pp 7th July 2005 HCP2005 - Mitesh Patel

Efficiencies w.r.t. Offline and L0xL1 selected signal HLT : Performance HLT still under study, efficiencies on L0L1 and offline selected events 60-90% Philosophy to try and trigger channels in many ways Further inclusive triggers will make us more robust to the unknown Limited time available for online tracking → different to offline : inefficiency in high multiplicity channels – strategies being explored to resolve this … Channel Efficiencies w.r.t. Offline and L0xL1 selected signal Generic Tracking Total Efficiencies Excl. B mm m D* Total Bs  m+m- 99% 93% 91% 90% 94% Order of 1% 98% Bd  K*m+m- 82% 73% 62% 58% Bd,s  h+h- 95% 88% Order of 0.5% Order of 2% Bs  fg 71% 61% Bs  Dsh 60% Bd  D*p 48% 43% 55% 7th July 2005 HCP2005 - Mitesh Patel

→ HLT exclusive selection efficiency e = 0.71 Bs→ff selection on L0L1 and offline selected events : Online tracking etracking = 0.73 HLT selection cuts eselection = 0.97 → HLT exclusive selection efficiency e = 0.71 Select only 3 of the 4 tracks (fK), use online RICH to control the background : Online tracking etracking = 0.93 HLT selection cuts eselection = 0.94 → HLT exclusive selection efficiency e = 0.87 1200MeV 1000MeV Before RICH cut 3 track 4 track After RICH cut fK mass ff mass 7th July 2005 HCP2005 - Mitesh Patel

Overall Trigger Performance Bs  J/Y f ~70% Bd  p+p- ~37% Bs  DsK ~23% Level-0 Level-1 HLT Total 2 KHz bb 900 Hz cc 600 Hz 7th July 2005 HCP2005 - Mitesh Patel

Trigger Robustness Several scenarios considered : Event multiplicity Noise, misalignment, resolution Increased material LHC beam position LHC background Size of the CPU farm The performance of L0 is stable within 10%, L1 is stable within 20% The execution time and L1 event size is stable to within 30% 7th July 2005 HCP2005 - Mitesh Patel

Real Time Trigger Challenge But will it work … ?! The Real Time Trigger Challenge : Operate one (few) subfarms of the DAQ under realistic conditions 44 double CPU boxes Full-speed Data Input Long-term operation (hours) Exercise realistic Level-1/HLT code Exercise/evaluate realistic overheads Establish performance of ‘modern’ CPUs compared to (today’s) standard CERN Happening now ! 7th July 2005 HCP2005 - Mitesh Patel

Conclusions LHCb will use three level of trigger to deliver inclusive and exclusive samples of B decays suitable for it’s physics goals : Exclusive selection of core physics channels Samples to allow calibration/alignment studies Data that allows the trigger efficiencies to be determined The L0 and L1 triggers are well developed and performance is good The High Level Trigger continues to evolve as our understanding of the LHCb physics potential evolves The trigger system is robust and flexible 7th July 2005 HCP2005 - Mitesh Patel

Level 0: Pileup System B A Silicon r-sensors RA ZPV - ZA RB ZPV - ZB Two planes of Silicon sensors upstream of the interaction point Used to identify and reject multi-Primary-Vertex events : Measure R coordinate (-4.2<<-2.9) From hits on two planes  produce a histogram of z on beam axis Remove hits contributing to largest peak, look for 2nd peak above threshold L0 Decision Unit cuts on # of tracks in the second peak + hit multiplicity Performance : Vetoes 60% of double interactions keeping 95% of single interactions RA ZPV’ k’ ZB ZA RB ZPV B A k Silicon r-sensors RA ZPV - ZA RB ZPV - ZB = k  7th July 2005 HCP2005 - Mitesh Patel