Real-time physics: novel concepts for trigger, calibration & alignment,

and data processing with LHCb

Lucia Grillo on behalf of the LHCb Collaboration

LHCP 2016Lund, 13 - 18 June

Picture: Horologium mirabile Lundense, the astronomical

clock of Lund Cathedral

1

The LHCb Experiment

• Single-arm forward spectrometer, unique pseudo-rapidity range at LHC

• LHCb is the experiment dedicated to heavy flavor physics at the LHC collider

• Goal: search for indirect evidence of New Physics in CP violation and rare decays of beauty and charm hadrons

• Requirements: excellent tracking and Particle Identification performances

LHCb Detector PerformanceInt. J. Mod. Phys. A30 (2015) 1530022

2

Tracking system

Vertex Locator (VELO)• 42 silicon micro-strip stations with R-Φ sensors• 2 retractable halves, 8 mm from beam Tracker Turicensis (TT)

• 4 planes (0 ,+5 ,-5 ,0 ) of silicon micro-strip sensors, total silicon area of 8 m

o o o o

2

Performance of the LHCb Vertex Locator JINST 9 (2014) 09007

LHCb Detector PerformanceInt. J. Mod. Phys. A30 (2015) 1530022

3

Tracking systemOuter Tracker (OT)

Inner Tracker (IT)• 3 stations with 4 planes of straw tubes • 3 stations with 4 planes of silicon micro-

strip sensors• hit resolution ~200μm

Performance of the LHCb Outer Tracker; JINST 9 (2014) P01002

• hit resolution ~50μm

LHCb Detector Performance; Int. J. Mod. Phys. A30 (2015) 15300224

Tracking systemOuter Tracker (OT)

Inner Tracker (IT)• 3 stations with 4 planes of straw tubes • 3 stations with 4 planes of silicon micro-

strip sensors• hit resolution ~200μm

• hit resolution ~50μm

Performance in Run I

Δp/p = 0.5-1.0%

Tracking efficiency > 96%

Decay time resolution ~45fs

Performance of the LHCb Outer Tracker; JINST 9 (2014) P01002LHCb Detector Performance; Int. J. Mod. Phys. A30 (2015) 1530022

5

Particle Identification: RICH detectors

RICH Detectors

• Upstream (RICH1) and downstream (RICH2) of the magnet

• Different radiators and geometry to identify particles with different kinematicsRICH1 RICH2

• C4F10 radiator

• 2<p<40 GeV/c

• CF10 radiator

• 15<p<100 GeV/c

Performance of the LHCb RICH detector ad the LHCEur.Phys.J. C73 (2013) 2431

6

Particle Identification: RICH detectors

RICH Detectors

• Upstream (RICH1) and downstream (RICH2) of the magnet

• Different radiators and geometry to identify particles with different kinematicsRICH1 RICH2

• C4F10 radiator

• 2<p<40 GeV/c

• CF10 radiator

• 15<p<100 GeV/c

Performance of the LHCb RICH detector ad the LHCEur.Phys.J. C73 (2013) 2431

PID Performance in Run I

• Kaon identification efficiency ~95%

• Pion mis-identification fraction ~10% over the full 2-100GeV/c momentum range

• Muon identification efficiency ~97%

7

Particle Identification and triggerMuon system

Calorimeter System• Electromagnetic (ECAL) and hadronic

(HCAL) calorimeters

• Scintillator planes + absorber material planes

• Used the hardware (L0) trigger selection

• 5 stations equipped with multi-wire proportional chambers• Inner part of the first station equipped with GEM detectors• Used the hardware (L0) trigger selection

Performance of the Muon Identification system JINST 8 (2013) P10020

LHCb Detector Performance; Int. J. Mod. Phys. A30 (2015) 1530022

8

Trigger - Run I face

• Preliminary alignment and calibration of the detector

Trigger (online reconstruction)

Offline reconstruction

• Faster but less performing track and PID reconstruction (only part of the PID information available)

• Full and best performing detector alignment and calibration

• Full reconstruction including full PID information

Trigger != Offline

2012: Introduction of the Deferred Trigger

• Better usage of the farm exploiting the time between physics fillsPerformance of the LHCb high level trigger in 2012,

J. Phys. Conf. Ser. 513 (2014) 0120019

Trigger - Run I face

• Preliminary alignment and calibration of the detector

Trigger (online reconstruction)

Offline reconstruction

• Faster but less performing track and PID reconstruction (only part of the PID information available)

• Full and best performing detector alignment and calibration

• Full reconstruction including full PID information

Trigger != Offline

2012: Introduction of the Deferred Trigger

• Better usage of the farm exploiting the time between physics fillsPerformance of the LHCb high level trigger in 2012,

J. Phys. Conf. Ser. 513 (2014) 012001

Novel strategy: physics on the HLT output

• No need for offline data processing (the best possible selection of signal candidates is already there)

• Smaller event size allows to reduce pre-scales

more physics with the given resources

10

Trigger - Run II face

• All events are buffered to disk before running second software level of trigger

• Perform calibration of PID detectors and alignment of the full tracking system in real-time

• Last trigger level runs the offline-quality reconstruction

Trigger == Offline

➜ Same alignment and calibration constants in the trigger and offline

➜ The same full reconstruction in the trigger and offline: the best performance

11

Trigger - Run II face

• All events are buffered to disk before running second software level of trigger

• Perform calibration of PID detectors and alignment of the full tracking system in real-time

• Last trigger level runs the offline-quality reconstruction

Trigger == Offline

➜ Same alignment and calibration constants in the trigger and offline

➜ The same full reconstruction in the trigger and offline: the best performanceNovel strategy: physics on the HLT output

Challenges:

• Offline quality reconstruction in few ms

• Full detector alignment and calibration in real time

12

Alignment and calibration: how important!

Physics performance relies on spatial alignment and calibration of the detector

• Examples: VELO alignment is essential for PV discrimination, IP and proper time resolution,

First σIP (high pT ) = 14.0 µm

Latest σIP (high pT ) = 11.6 µm

13

Alignment and calibration: how important!

Physics performance relies on spatial alignment and calibration of the detector

LHCb Preliminary

LHCb Preliminary

• Examples: VELO alignment is essential for PV discrimination, IP and proper time resolution, better alignment improves mass resolution,

First alignment σΥ = 92 MeV/c2

Latest alignment σΥ = 49 MeV/c2

First σIP (high pT ) = 14.0 µm

Latest σIP (high pT ) = 11.6 µm

14

Alignment and calibration: how important!

Physics performance relies on spatial alignment and calibration of the detector

LHCb Preliminary

LHCb Preliminary

• Examples: VELO alignment is essential for PV discrimination, IP and proper time resolution, better alignment improves mass resolution, use of PID allows for more exclusive selections

Trigger+PID+tighter PID

Trigger+PID

CS: D+→π+π-π+

DCS: D+→K+-K-K+

First σIP (high pT ) = 14.0 µm

Latest σIP (high pT ) = 11.6 µm

First alignment σΥ = 92 MeV/c2

Latest alignment σΥ = 49 MeV/c2

15

Real-time alignment & calibration

• Alignments: VELO, Trackers, RICH mirrors, Muon

• Calibrations: RICH refractive index and HPDs, OT time, Calorimeters

16

Alignment & calibration framework• Strategy:

• Automatic evaluation at regular intervals (per run or per fill depending on the task)

• Dedicated sample to perform alignment or calibration collected with a specific trigger selection

• Compute new alignment or calibration constants (few minutes for the alignment tasks, run at the beginning of each fill when the VELO is closed)

• Update the constants if necessary

• New constants will be used in the trigger and coherently in offline reconstruction

• Alignment

• Iterative process

• Analyzer (multiple nodes) perform reconstruction

• Combining output, fits/χ2 minimization done on a single node→ new constants

Tracks reconstruction using current align. constants

Compute a new set of constants by minimizing a

global χ2

Iterate until χ2 is below threshold

17

Alignment number [a.u.]10 20 30 40

m]

µV

aria

tion

[

20!

15!

10!

5!05

101520 x-translation

y-translationLHCb VELOPreliminary

23/04/2016 - 06/06/2016

Alignment of the tracking system

• Automatic alignment procedure (sequence of VELO, Tracker and Muon alignment tasks) runs at the beginning of each fill

• ~700 elements to be aligned• Each task takes ~7 minutes

• Constants automatically updated when needed• VELO once every 2-3 fills• Tracker (TT, IT, OT) once every several fills and

at magnet polarity change• Muon runs as monitoring, constants updated only

after hardware intervention

new 2016 plots, improved control of the

automatic updates

Alignment number [a.u.]5 10

X V

aria

tion

[mm

]!

1.5"

1"

0.5"

0

0.5

1

1.5M1M2M3M4M5

LHCb Muon A-sidePreliminary

14/05/2016 - 06/06/2016

Alignment number [a.u.]10 20 30

X V

aria

tion

[mm

]!

0.2"

0.15"

0.1"

0.05"

0

0.05

0.1

0.15

0.2IT1 ASideIT1 CSideIT1 TopIT1 Bottom

LHCb TrackerPreliminary

06/05/2016 - 06/06/2016

18

Rich mirrors alignment

• Cherenkov photons focused on photon-detector plane by spherical and flat mirrors

Misaligned Aligned

• Center of Cherenkov ring corresponds to the intersection point of the track.

• Alignment is determined by fitting the Cherenkov opening angle as a function of the azimuthal angle of the ring

• 110 mirror pairs to align: 1090 constants

• Runs only as monitoring every fill

new for 2016: optimization and speed up

19

Calibration of the RICH detectors

• Refractive Index calibration

σΔθ=0.67 mrad

Stability of Cherenkov angle resolution

RICH2

Calib.runs

EM 25 nsup

• Hybrid photon detector (HPD) calibration

• evaluated each run by fitting the anode images cleaned using the Sobel filter to detect the edges

• affected by magnetic and electric fields• 1940 parameters

• depends on the gas mixture, temperature and pressure

• evaluated each run by fitting the difference between reconstructed and estimated Cherenkov angle

• 2 parameters

20

Run number [a.u.]0 200 400 600

[ns]

0t!

0.4"

0.2"

0

0.2

0.4 Updated0t Not updated0t

LHCb OT Preliminary 23/4/2016 - 4/6/2016

Calibration of the Outer Tracker time

• Measured drift time is different from time estimated from the distance of the track to the wire due to the readout electronics

• The dominant effect is a global offset due to the difference between the collision time and the LHCb clock, which is time dependent

• A global shift of 0.5 ns leads to tracking inefficiency of ~2.5‰

• The time offsets per module are stable in time, besides hardware interventions

drift time residuals [ns]-20 -10 0 10 20

Goo

d tra

cks/

(0.4

ns)

02468

1012141618

610×

LHCb OT

2016 plots

21

Calibration of the calorimeters

• Relative calibrations• Raw occupancy method: comparing the

performance of each cell with a reference• LED monitoring system allows to detect

aging of the PMTs, every fill

!"#!

!"#$

%"!!

%"!$

%&'!$'%( ()'!$'%( !&'!*'%((%'!$'%( !#'!#'%$ (+'!#'%$ +!'!#'%$%*'!#'%(!"#!

!"#$

%"!!

%"!$

,-./012-34/15

,-./012-34/15

%"%! %"%!

67893:;-.1<15/;= 67893:;-.1<15/;=

2012: no correction 2015: applied new π0 calibration applied new LED corrections

• Absolute calibrations

• π calibration for ECAL: compute di-photon invariant mass.

• Cs source scan for HCAL, evaluated during the technical stop

0

22

Optimization of reconstruction

• Code optimization (vectorization, memory access)• New reconstruction chain in HLT1 and simplified geometry in Kalman filter

• Re-implementation and/or re-tuning of the algorithms (HLT1 and HLT2)• Run I offline reconstruction 2 times faster

Offline Run I reconstruction can run in the Trigger in Run II, with same or better performance

• New resources: farm nearly doubled wrt Run I, 10PB disk space to buffer the events between HLT1 and HLT2

• Big effort in speeding up the reconstruction:

Callgrind graphs(Area ∝ CPU time)

23

Run II performance: Tracking and PID

Same or improved performance as offline in Run I, but directly in the trigger

IP resolution

Track reconstruction efficiency

Pion misidentification efficiency vs Kaon identification efficiency

24

Run-II performance: Trigger

2015

2012

LHCb Preliminary

Efficiency for B+→D0π+ is ~75% Efficiency for B+→D0π+ is >90%

2011

Efficiency of the HLT2 inclusive beauty trigger as a function of B pT

• Improvement of the trigger efficiency thanks to e.g.• Best reconstruction already in the trigger• Detector fully aligned and calibrated• Use of PID information

2015

LHCb Detector Performance; Int. J. Mod. Phys. A30 (2015) 1530022

25

Real-time physics: “Turbo” stream

• Part of the physics programme needs billions of recorded candidates (e.g. charm measurements): but with no need for the rest of the event.

➜ “Turbo” stream

• Reduced event size (5kB vs 70kB)• No offline processing• ~2.5Hz of the output stream

(10kH for full)

Analyses can be done directly on the trigger output

26

Turbo and Turbo++

Even more analyses can be done on the trigger output

new for 2016

K

PV

D

!+

-

0

!+D*+

• Allows other reconstructed objects from the event to be saved, in addition to those selected by the trigger

New features:

• Saves only objects selected by the trigger

• Output limited to a standard set of variables

• Allows to create and save new variables (i.e. hits in a cone region around the track)

• Aim: according to the physics channel and desired measurement, choose how much (and which variables) of the event need to be saved

Out of the 420 HLT2 lines in 2016 physics programme, 150 choose Turbo, ~60 new lines wrt 2015

Turbo candidate

K

D

!+

-

0

!+PV *+D

Tracks from others PVs Other tracks

from trigger PV

+γ, π0Turbo++ candidate

27

Real-time physics: very first Turbo stream results

Few weeks after taking data

]2c [MeV/ -µ+µm2950 3000 3050 3100 3150 3200

2 cC

andi

date

s per

5 M

eV/

2

4

6

8

10

12

310!

-1 =3.05 pbintL = 13 TeV, sLHCb

c < 3 GeV/T

p2 < < 3.5y3 <

m(K−π+π+) [MeV/c2]1850 1900

Can

did

ate

s/(1

MeV/c

2)

0

50

100

×103

D+

Fit

Sig. + Sec.

Comb. bkg.

LHCb√s = 13TeV

m(K−π+) [MeV/c2]1800 1850 1900

Can

did

ate

s/(1

MeV/c

2)

0

50

100

150

×103

D0

Fit

Sig. + Sec.

Comb. bkg.

LHCb√s = 13TeV

28

Conclusions

• New data taking strategy implemented at LHCb experiment for Run II

• Detector calibration and alignment are provided in few minutes

• Best performance of the detector and full reconstruction, including PID, are exploited already in the trigger selection

• Thanks to the Turbo stream we are able to increase our physics program using the same resources, and to analyze data ~24h after having taken them

• We keep improving and speeding up the reconstruction, alignment, calibration and data processing

• Next challenge: upgrade!

• New development for 2016: Turbo++, to choose how much, and which variables of the events to be computed and saved according to the physics measurement

Alignment number [a.u.]10 20 30 40

m]

µV

aria

tion

[

20!

15!

10!

5!05

101520 x-translation

y-translationLHCb VELOPreliminary

23/04/2016 - 06/06/2016

]2c [MeV/ -µ

+µm

29503000

30503100

31503200

2c

Cand

idat

es p

er 5

MeV

/

2

4

6

8

10

12310!

-1 =3.05 pb

intL

= 13 TeV, sLHCb

c < 3 GeV/

Tp2 <

< 3.5y3 <

K

D

! +-

0

!+PV

*+D

Turbo++ candidate

29

Thanks for your attention

Alignment number [a.u.]10 20 30 40

m]

µV

aria

tion

[

20!

15!

10!

5!05

101520 x-translation

y-translationLHCb VELOPreliminary

23/04/2016 - 06/06/2016

]2c [MeV/ -µ

+µm

29503000

30503100

31503200

2c

Cand

idat

es p

er 5

MeV

/

2

4

6

8

10

12310!

-1 =3.05 pb

intL

= 13 TeV, sLHCb

c < 3 GeV/

Tp2 <

< 3.5y3 <

K

D

! +-

0

!+PV

*+D

30