1. LHCb: a stroll from Trigger to Physics
Eduardo Rodrigues
NIKHEF
Seminar at CPPM, Marseille, 4th May 2007

2. LHCb: a stroll from Trigger to Physics
Contents
LHCb: the must know
Detector and simulation
Trigger system
Reconstruction and PID
Analysis: tagging and event selection
Physics sensitivity studies
Disclaimer: no complete review of all aspects; emphasis put on personal work
LHCb: a stroll from Trigger to Physics

3. LHCb: a stroll from Trigger to Physics
Jura
Geneva
9 km diameter
LHC
CERN
LHCb: a stroll from Trigger to Physics

4. LHCb: a stroll from Trigger to Physics
I. LHCb: the must know
LHCb
Goal: B-physics studies
CP violation
rare B-decays
Acceptance: 1.8 < h < 4.9
Luminosity: 21032 cm-2 s-1
Nr of B’s / year: 1012
How forward is LHCb?
B hadrons mainly produced in forward region(s)
both B’s tend to be correlated
Unique forward spectrometer
at high energy!
Excellent tracking
Excellent PID
LHCb: a stroll from Trigger to Physics

5. LHC(b) environment LHC environment Cross sections
pp collisions at ECM = 14 TeV
tbunch = 25 ns  bunch crossing rate = 40 MHz
<L> = 2x1032 cm-2 LHCb interaction region
 times lower than for ATLAS/CMS
Cross sections
Physical quantity
Value
Event rate
Yield / year
s total
~ 100 mb
s visible
~ 60 mb
~ 12 MHz
s (c-cbar)
~ 3.5 mb
~ 700 kHz
~ 7x1012 pairs
s (b-bbar)
~ 0.5 mb
~ 100 kHz
~ 1012 pairs
Expected B-signal rates
branching ratios ~ 10-9 – 10-4
 10 – 106 events / year ?
B-hadrons are heavy and long-lived !
LHCb: a stroll from Trigger to Physics

6. Ring Imaging Cherenkov LHCb: a stroll from Trigger to Physics
II. LHCb detector
Ring Imaging Cherenkov
Calorimeters
250/300 mrad
Acceptance
10 mrad
pp collision
(side view)
Muon System
« Tracking » detectors
LHCb: a stroll from Trigger to Physics

7. LHCb: a stroll from Trigger to Physics
LHCb cavern
LHCb: a stroll from Trigger to Physics

8. LHCb: a stroll from Trigger to Physics
Simulation
LHCb Monte Carlo simulation software:
Pythia, EvtGen, GEANT4 and Gaudi-based reconstruction
Detailed detector and material description (GEANT)
Pattern recognition, trigger simulation and offline event selection
Implements detector inefficiencies, noise hits, effects of events from the previous bunch crossings
LHCb: a stroll from Trigger to Physics

9. LHCb: a stroll from Trigger to Physics
III. The LHCb Trigger
Trigger overview and strategy
Level-0: first level trigger
HLT: High Level Trigger
LHCb: a stroll from Trigger to Physics

10. LHCb: a stroll from Trigger to Physics
Trigger overview
pp collisions
12 MHz
custom hardware
high ET particles
partial detector information
Level-0
1 MHz
CPU farm -> software trigger
high ET / IP particles
full detector information
HLT
~2 kHz
event size ~35 kb
Storage
LHCb: a stroll from Trigger to Physics

11. LHCb: a stroll from Trigger to Physics
Trigger Strategy
Be alert !
Two-level Trigger
L0 high ET / PT particles
hardware trigger, sub-detector specific implementation
pipelined operation, fixed latency of 4 ms
(minimum bias) rate reduction ~12 MHz -> 1 MHz
HLT: high ET/PT & high Impact Param. particles & displaced vertices & B-mass & …
algorithms run on large PC farm with ~1800 nodes
several trigger streams to exploit and refine L0 triggering information
software reconstruction on part/all of the data
 tracking / vertexing with accuracy close to offline
selection and classification of interesting physics events
 inclusive / exclusive streams
rate reduction 1 MHz -> 2 kHz
estimated event size ~ 30kb
LHCb: a stroll from Trigger to Physics

12. Level-0 Trigger strategy LHCb: a stroll from Trigger to Physics
select high ET / PT particles
 hadrons / electrons / photons / p0’s / muons
reject complex / busy events
 more difficult to reconstruct in HLT
 take longer to reconstruct in HLT
reject empty events
 uninteresting for future analysis
L0 thresholds on ET / PT of candidates
global event variables
Pile-up system
Calorimeter
Muon system
LHCb: a stroll from Trigger to Physics

13. Scintillator Pad Detector (SPD) Pre-Shower Detector (PS)
Level-0 sub-systems
L0 Pile-up system:
2 silicon planes upstream of nominal IP,
part of the Vertex Locator (VELO)
Identifies multi-PV events:
- 2-interactions crossings identified with
efficiency ~60% and purity ~95%
Scintillator Pad Detector (SPD)
Pre-Shower Detector (PS)
ECAL
HCAL
L0 Calorimeter :
Identify high-ET hadrons / e’s / g’s / p0’s
Electromagnetic and hadronic calorimeters
- large energy deposits  ET in 2x2 cells
Scintillator Pad Detector (SPD) and Preshower (PS)
- used for charged/electromagnetic nature of clusters, respectively
L0 Muon System:
M1 – M5 muon stations (4 quadrants each)
σp/p ~ 20% for b-decays
LHCb: a stroll from Trigger to Physics

14. LHCb: a stroll from Trigger to Physics
Level-0 Decision Unit
Calorimeter
total ET in HCAL
SPD multiplicity
highest- ET candidates:
h, e, g, 2 p0‘s
Muon system
2 m candidates
per each of 4 quadrants
Pile-up system
total multiplicity
# tracks in second vertex
L0 Decision unit
cuts on global event variables
thresholds on the ET candidates
1 MHz
L0DU report
LHCb: a stroll from Trigger to Physics

15. Level-0 bandwidth division
Overall optimization given benchmark channels
Global event variables applied first …
~ 13
~ 8.3 3
Tracks in 2nd vertex
112 hits
Pile-up multiplicity
280 hits
SPD multiplicity
~ 7
5.0 GeV
S ET
Rate (MHz)
Cut
Global event cuts
Redundancy:
Sub-triggers overlap
… and then cuts on the ET / PT candidates
Decision Unit Variables
130
2.6
Photon
110
4.5
po local
150
4.0
po global
280
100
2.8
Electron
700
3.6
Hadron
1.3
Di-muon
160
1.1
Muon
Approx. rate (kHz)
Threshold (GeV)
Trigger
Di-muon trigger is special
- not subject to the global event selection
- PTmm = PTm1 + PTm2 with PTm2 = 0 possible
- “tags” clean B-signatures
LHCb: a stroll from Trigger to Physics

16. Level-0 performance studies LHCb: a stroll from Trigger to Physics
Typical Level-0 performance
Dedicated sub-triggers most relevant for each « channel type »
Bs → Ds p
B → pp
B → J/Y(mm) Ks
B → K* mm
B → K* g
B → J/Y(ee) Ks
N.B.: efficiencies with respect to events selected offline
LHCb: a stroll from Trigger to Physics

17. LHCb: a stroll from Trigger to Physics
High Level Trigger
Strategy
Independent alleys:
Follow the L0 triggered candidate
 Muon, Muon + Hadron,
Hadron, ECal streams
~2 kHz
Partial Reconstruction:
A few tracks selected per alley (cuts e.g. on PT, Impact Parameter, mass)
full reconstruction done at the end of the alleys
Summary Information:
decision, type of trigger fired, info on what triggered
LHCb: a stroll from Trigger to Physics

18. Timing and Fast Control LHCb: a stroll from Trigger to Physics
Trigger Farm FE FE FE
Event Filter Farm with ~1800 nodes (estimated from 2005 Real-Time Trigger Challenge)
Sub-divided in 50 sub-farms
Readout from Level-0 at 1 MHz
 50 Gb/s throughput
Scalable design  possible upgrade
L0 trigger
Readout network
Timing and Fast Control
CPU
CPU
CPU
CPU
CPU
permanent
storage
HLT algos CPU time tested on a real farn
 will fit in the size of the farm foreseen
LHCb: a stroll from Trigger to Physics

19. LHCb: a stroll from Trigger to Physics
Muon Alley - Strategy
Muon PreTrigger
Standalone m reconstruction: σp/p ~ 20%
VELO tracks reconstruction
Primary vertex reconstruction
Match VELO tracks and muons: σp/p ~ 5%
L0-m Entry
~200 kHz
Muon
Pre-trigger
~20 kHz
Muon Trigger
Tracking of VELO track candidates in the downstream T stations: σp/p ~ 1%
Refine m identification:
match long (VELO-T) tracks and muons
Muon
Trigger
~1.8 kHz
LHCb: a stroll from Trigger to Physics

20. Muon Alley – Performance LHCb: a stroll from Trigger to Physics
Muon PreTrigger
bμ ~11%
Signal efficiency: ~88%
~20 kHz
Muon Trigger
Single muon
PT> 3GeV and IPS> 3
Bm content 60%
Dimuon
mass >0.5GeV and IP>100mm
J/y: mass>2.5GeV (no IP cut!)
Signal efficiency: ~87%
< 1s of LHCb
J/Ψ
~1.8 kHz
dimuon mass (MeV)
LHCb: a stroll from Trigger to Physics

21. LHCb: a stroll from Trigger to Physics
Exclusive Selections
Exclusive selections
Use common available reconstructed and selected particles (Ds, D0, K*, Φ,..)
Wide B-mass windows (typically ~ 500 MeV)
Efficiency: e.g. ~90% for Bππ
Bs→Dsp
Bs→ DsK
4 tracks
~200 Hz
Bs → fg
f →KK
p, K
Bs → ff
Ds→KKp
Bs → Dsp
Off-line B → pp
On-line B → pp
LHCb: a stroll from Trigger to Physics

22. IV. Reconstruction and PID LHCb: a stroll from Trigger to Physics
Tracking: pattern recognition and fitting
Vertexing
RICH, Calo. and Muon reconstruction
Particle Identification (PID)
LHCb: a stroll from Trigger to Physics

23. LHCb: a stroll from Trigger to Physics
Tracking strategy
|B| (T)
A set of PR algorithms
Multi-pass strategy
A typical event:
26 long tracks  best quality for physics: good P & IP resolution
11 upstream tracks  lower p, worse p resol., but useful in the RICH1 pattern recognition
4 downtream tracks  improves Ks finding efficiency (good p resolution)
5 seed/T tracks  used in the RICH2 pattern recognition
26 VELO tracks  used in primary vertex reconstruction: good IP resolution
B-field measured to ~ 0.03% precision!
Z (cm)
LHCb: a stroll from Trigger to Physics

24. Pattern recognition - strategy LHCb: a stroll from Trigger to Physics
VELO tracking
(VELO seeds)
Seeding
(T seeds)
Matching
(long tracks)
VELO-TT
(upstream tracks)
Forward tracking
(long tracks)
Ks tracking
(downstream tracks)
Clone
killing
Set of “best” tracks
LHCb: a stroll from Trigger to Physics

25. Pattern recognition - performance
Performance on B-signal samples for “long” tracks:
Efficiency ~ 95 % for p > 10 GeV
Ghost rate ~16.7 %
Dip due to beam-pipe material
Efficiency
Efficiency
p (GeV) h
LHCb: a stroll from Trigger to Physics

26. LHCb: a stroll from Trigger to Physics
Track fitting
Tracks
- pattern recognition attributes detector measurements to a track
- modelled with straight-line segments, the (track) states
Track fitting
- determine the best track parameters and corresponding covariance matrix
given the measurements on the track
- take into account multiple scattering and energy losses in the detector material
Description of a realistic detector with mis-alignments
- introduction of trajectories to model “measurements”
(concept adapted from BaBar)
- trajectory = 3D curve in space (of arbitrary shape)
- detector shapes locally described with trajectories
- a track state (straight-line segment) can also be
modeled as a trajectory
- any trajectory can provide a parabolic approximation
of its shape at any point along itself
 closest approach track state – measurement
becomes a general point-of-closest-approach problem!
“banana-shaped” Outer Tracker modules
LHCb: a stroll from Trigger to Physics

27. Track fitting « à la Kalman » LHCb: a stroll from Trigger to Physics
LHCb uses the Kalman filter technique:
Mathematically equivalent to least c2 method
Adds measurements recursively starting from a initial track estimate
Reconstructs tracks including multiple scattering and energy losses
We use a “bi-directional” fit …
Bi-directional fit
direction of the filter
direction of the reverse filter
track prediction
filtered track
track prediction
filtered track
Filter in both directions … smoothed result = weighted mean of both filters
LHCb: a stroll from Trigger to Physics

28. Fitting from a seed (track) state
Measurements
(hit & error)
Seed State
(position, direction & momentum)
Prediction
(adaptive 5th order Runge-Kutta method solves motion in the inhomogeneous B)
Kalman Filtered
(update prediction with Measurement info)
LHCb: a stroll from Trigger to Physics

29. Fitter: prediction and Kalman filter
Velo TT
IT&OT
LHCb: a stroll from Trigger to Physics

30. Fitter: Kalman smoother LHCb: a stroll from Trigger to Physics
Velo TT
IT&OT
LHCb: a stroll from Trigger to Physics

31. Bi-directional fitting LHCb: a stroll from Trigger to Physics
Velo TT
filtered state
weighted mean
IT&OT
LHCb: a stroll from Trigger to Physics

32. Track fitting performance
Momentum resolution for “long” tracks
dp/p = 0.35 – 0.55 %
dp/p = 0.5%
LHCb: a stroll from Trigger to Physics

33. Vertexing performancebt Bs K K
,K  Ds
Primary vertex p
Bs vertex resolution
sz,core = 168 mm
Primary vertex resolutions
sx = 8 mm
sz = 47 mm
Typical resolutions
LHCb: a stroll from Trigger to Physics

34. LHCb: a stroll from Trigger to Physics
RICH reconstruction
RICH1
RICH2
Algorithm:
Input: all types of tracks available
Search for hits assuming different hypotheses
Global ring fit
Cherenkov angle resolutions in the range 0.6 – 1.8 mrad
LHCb: a stroll from Trigger to Physics

35. LHCb: a stroll from Trigger to Physics
RICH performace
Particle identification needed over momentum range GeV/c
Ex.: need to distinguish Bd pp from other similar topology 2-body decays
K  p
p  p
Particle Momentum (GeV/c)
K  K or p
p  K or p 20 40 60 80
100
p  e, m or p
K  e, m, or p
p identification
K identification
LHCb: a stroll from Trigger to Physics

36. Calorimeter reconstruction (1/2)
electrons:
Identification with e~95% based on
- c2 of match calo. cluster energy – position of extrapolated track
- Energy deposited in the preshower detector
- Energy deposited in the hadronic calorimeter
- Bremsstrahlung correction
EPS(MeV)
True Electrons
backgrounds
ECAL
Magnet
example
LHCb: a stroll from Trigger to Physics

37. Calorimeter reconstruction (2/2)
Neutral pions:
Resolved p0 :
- reconstructed from isolated calo. photon clusters
- mass resolution ~10 MeV, e~30%
Merged p0 :
- Several clustes merged into single (high energy) cluster
- mass resolution ~15 MeV , e~20%
all π0s
Merged π0
π0 from B
π0 mass (Mev/c²)
Resolved π0
Bd→ π+π-π0 events
Merged cluster
Resolved cluster
LHCb: a stroll from Trigger to Physics

38. LHCb: a stroll from Trigger to Physics
Muon reconstruction
Extrapolate well-reconstructed tracks to the Muon system
Look for compatible hits in a “field of interest” around the extrapolated tracks
eff ~ 94 %
misid ~ 1.0 %
Typical performances
for b  J/Y(mm) X
 ~ 10 MeV/c²
em m  ~ 94 % and ep  m  ~ 1.0 % for tracks in Muon detector acceptance
LHCb: a stroll from Trigger to Physics

39. Particle Identification (PID) LHCb: a stroll from Trigger to Physics
No PID
Hadrons:
RICH1 and RICH2
Electrons & neutrals:
Calorimeter system
Muons:
Muon system
 invariant mass
 invariant mass
K invariant mass
With PID
LHCb: a stroll from Trigger to Physics

40. LHCb: a stroll from Trigger to Physics
V. Physics analysis
Flavour tagging
Event selection & background estimation
- example of the decay Bs Ds h
LHCb: a stroll from Trigger to Physics

41. LHCb: a stroll from Trigger to Physics
Flavour tagging (1/2)
Purpose / need:
Determination of flavour of B-mesons at production (at primary vertex)
tagging gives (best estimate of) the flavour by examining the rest of the event
time-dependent analyses require flavour tagging
Quantifying the tagging performance :
tagging efficiency
wrong tag fraction
CP asymmetries diluted
with dilution factor
LHCb: a stroll from Trigger to Physics

42. LHCb: a stroll from Trigger to Physics
Flavour tagging (2/2)
Dz/bgc = Dt
Bs0
rec
t =0 b s u K+
Dt picoseconds after leaving the primary vertex, the reconstructed B decays. PV
b-hadron
l + (e+, m+)
l - (e-, m-)
K -
Uses flavour conservation in the hadronization around the Brec
eD2  1% (B0) , 3% (Bs)
Same-side tag
Assume:
Opposite side tag
eD2  5%
LHCb: a stroll from Trigger to Physics

43. bt Event selection: example of Bs  Ds h (1/3) Bs → Ds- p +
Bs -> Ds∓ K± bt Bs K K
,K  Ds
Primary vertex
Requirements
Trigger on the B-decay of interest : high-PT tracks and displaced vertices
efficient trigger
Select the B-decay of interest while rejecting the background
good mass resolution
Tag the flavour of the B-decay
efficient tagging and good tagging power (small mistag rate)
Time-dependent measurements
good decay-time resolution
LHCb: a stroll from Trigger to Physics

44. Event selection: example of Bs  Ds h (2/3)
Bs→DsK : Bs reconstructed mass signal and main background
Bs→Dsπ : Bs reconstructed mass signal and main background
Bs mass resolution ~14 MeV
LHCb note
LHCb: a stroll from Trigger to Physics

45. Event selection: example of Bs  Ds h (3/3)
Event yields for 2fb-1 (defined as 1 year)
Bs→Ds- π+
140k ± 0.67k (stat.) ± 40k (syst.)
Bs→DsŦ K±
6.2k ± 0.03k (stat.) ± 2.4k (syst.)
Results for B/S ratios, no trigger applied
Channel
B/S at 90% CL
(bb combinatorial)
(bb specific)
Bs→Ds-π+
[0.014,0.05]
C.V 0.027±0.008
[0.08,0.4]
C.V 0.21±0.06
Bs→DsŦ K±
[0,0.18]
C.V 0.0
[0.08,3]
C.V 0.7±0.3
(central values used for sensitivity studies)
LHCb note
LHCb: a stroll from Trigger to Physics

46. VI. Physics sensitivity studies The example of Bs  Ds h
Motivations
Sensitivity studies with RooFit
LHCb: a stroll from Trigger to Physics

47. LHCb: a stroll from Trigger to Physics
Motivations (1/2)
B-decay channels Bs→Ds-π+ and Bs→DsŦK±
Topology is very similar
Physics is different:
Bs→Ds-π+ can be used to measure Δms , ΔGs
CDF measurement Δms = ± 0.1(stat.) ± 0.07(syst.) ps-1
Bs→DsŦK± can be used to extract the CP angle g +fs
Standard Model prediction g ≈ 60°
LHCb: a stroll from Trigger to Physics

48. LHCb: a stroll from Trigger to Physics
Motivations (2/2)
Bs  Ds p mode:
Flavour-specific decay
(2 decay amplitudes only)
Bs  Ds K mode:
4 decay amplitudes of interest:
Bs, Bs -> Ds+K-, Ds-K+
-> 2 time-dependent asymmetries for the 2 possible final states
Ratio of amplitudes of order 1
-> large interference and asymmetry expected
Bs → Ds- p +
Bs -> Ds∓ K±
LHCb: a stroll from Trigger to Physics

49. Physics sensitivity studies LHCb: a stroll from Trigger to Physics
LHCb Sensitivity studies
Measurement of g+fs with combined Bs  Ds p and Bs  Ds K samples
Complete fit allows to obtain Dms, mistag rate, etc.
RooFit implementation for Toy MC:
Simultaneous Bs  Ds p and Bs  Ds K fit: correlations taken into account
All decay rate equations included (see next page)
Using latest 2004 data challenge selection results (LHCb-note )
Toy in propertime and mass includes:
propertime acceptance function (after trigger & selection)
per-event propertime error
smearing due to mis-tagging
background (rough estimate/description)
Fit with tagged events, and also with untagged Bs  Ds K events
LHCb: a stroll from Trigger to Physics

50. Decay rates with RooFit LHCb: a stroll from Trigger to Physics
Final state f : Ds-π+ or Ds-K+
For charge conjugate final states:
B → B , f → f , λf → λf , Af → Af
p/q → q/p
LHCb: a stroll from Trigger to Physics

51. Description in propertime LHCb: a stroll from Trigger to Physics
Proper time per-event error distribution
Acceptance function
(after trigger & sel.)
mean value 33fs
most probable value 30fs
Combined sample, tagged events, 5-year statistics
LHCb: a stroll from Trigger to Physics

52. LHCb: a stroll from Trigger to Physics
Description in mass
Signal & background, 5-year statistics
LHCb: a stroll from Trigger to Physics

53. Results with untagged events LHCb: a stroll from Trigger to Physics
2 Dsπ equations, 4 DsK equations, simultaneous fit.
DC04 input, collected data from 400 “experiments” of 5y tagged data,
scaled results to 1y
Fit a Gaussian to the fitted values distribution, pull distribution
Parameter
Δms (ps)-1 ω
Arg(λf) rad
Arg(λ f ) rad
|λf|
g+fs °
ΔT1/T2 °
input value
17.5
0.328
1.047
-1.047
0.37 60
fitted value
1.06
-1.04
60.14
0.28
resolution 5y
0.003
0.001
0.17
0.14
0.03
5.47
5.37
resolution 1y
0.007
0.26
0.32
0.06
12.2
12.0
Sensitivity to g
Error on g + fs (scaled to 1 year) ~ 12°
LHCb: a stroll from Trigger to Physics

54. LHCb: a stroll from Trigger to Physics
Thank you
LHCb: a stroll from Trigger to Physics