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LHCb Alignment
12th April 2007S. Viret
Cosener’s Forum
« LHC Startup »
1. Introduction
2. The alignment challenge
3. Conclusions
1. Introduction
2. The alignment challenge
3. Conclusions
LHC Startup1 S. Viret
1. Introduction2. The alignment challenge3. Conclusions
New-physics signs are expected at the LHC
Why LHCb ?
1. Physics justification2. The detector3. LHCb startup program
They will be difficult to characterize
Heavy-flavor physics will provide plenty of
discriminating observables
LHC Startup2 S. Viret
Example : bs
bs
ss
b s
Involves FCNC, forbidden by
Standard ModelBsdecay
b s
W±
t
New contributions could arise and affect
observable parameters (BR, ACP, Aisospin)
b s
t
~
~
Need for loops involving heavy particles
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup3 S. Viret
If you want to study B-physics, it’s nice to have :
A precise vertex reconstruction
A very good particle ID
An efficient trigger system
A large b quark production in the acceptance
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup4 S. Viret
Beam line
The LHCb experiment :
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup5 S. Viret
b
b
b
b
Beam
b & b quark directions highly
correlated.
b quark production cross-section larger at high
100 b
Production cross-section (Pythia)
pT 230 b 105 b-hadrons per seconde at
L=2x1032cm-2s-1 (LHCb nominal lumi.)
A large b quark production in the acceptance
A forward geometry
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup6 S. Viret
Beam line
A precise vertex reconstruction An efficient trigger system
VELO (VErtex LOcator)
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup7 S. Viret
VELO
VELO :
Innermost part of LHCb. A detector very close to the beam (~8 mm). 42 detection modules in 2 boxes.
VErtex LOcator
~1m
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup8 S. Viret
VELO modules
4.2 cm 8 mm
R
Module=
2 sensors (1R/1) glued
together
x
y
z (beam)
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup9 S. Viret
VELO is a moving detector !
During LHC beam injection, each box is retracted by 3cm from its nominal position.
Then the boxes are moved back close to the beam, and data taking starts.
VELO box (empty here)
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup10 S. Viret
Beam line
A very good particle ID
RICH 1&2
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup11 S. Viret
2 complementary detectors :
1. Physics justification2. The detector3. LHCb startup program
RICH 2
RICH 1
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup12 S. Viret
RICH design :
1. Physics justification2. The detector3. LHCb startup program
Photon collected by HPD detectors (484 in total RICH 1&2)
Number of mirrors:RICH 1 : 4 sphericals / 16 planesRICH 2 : 56 sphericals / 40 planes
Some advertising…
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup13 S. Viret
Beam line
Tracking System
An efficient trigger system
A precise vertex reconstruction
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
Tracking stations
Trigger
Tracker
Outer Tracker
125.6 mm
41.4
mm
Trigger Tracker (1 station) Silicon Strips 183 m pitch 128 7-sensor ladders 4 layers: X:U(5o):V(-5o):X 128 ladders to be aligned
Inner Tracker 20% of the tracks Silicon Strips 198 m pitch 1-2 sensor ladders (336 ladders) 4 layers: XUVX
Outer Tracker (3 stations)
5.0 mm Straws Double-layer straws 4 layers: X:U(5o):V(-5o):X
Overlap regions between IT/OT to facilitate relative alignment
Inner Track
er
125.6 mm
LHC Startup14 S. Viret
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup15 S. Viret
HLT
~2 kHz
HLT (10ms): purely software. Fine event selection (streaming).
L1
40 kHz
LEVEL 1 (1ms): purely software. Look for a displaced vertex in the VELO (good detector alignment mandatory here).
Rate
(in
Hz)
107
106
105
104
103
L0
1 MHz
LEVEL 0 (4s): purely hardware. Select the events containing interesting info (, di-muons, e, & hadrons with high pT). Pile-up rejection.
Trigger strategy An efficient trigger system
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
2007: startup– Pilot run at 450 GeV per beam– Establish running procedures, align detectors in time and space – Integrated luminosity for physics ~ 0 fb–1
2008: early phase– Complete commissioning of detector and trigger at s=14 TeV– Calibrate momentum, energy and particle ID– Start first physics data taking, assume ~ 0.5 fb–1
– Establish physics analyses, understand performance
2009–20xx: stable running– Stable running, assume ~ 2 fb–1/year– Develop full physics program– Exploit statistics, work on systematics
LHC Startup16 S. Viret
1. Physics justification2. The detector3. LHCb startup program
1. Introduction2. The alignment challenge3. Conclusions
1. Introduction
2. The alignment challenge
3. Conclusions
LHC Startup17 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
The alignment problem:
A particle passes trough a misaligned
detector
What happens if track is fitted using uncorrected
geometry
With no correction, one gets a bad quality track (or even no track at all)
How could this affect LHCb results ?
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup18 S. Viret
Example 1 : proper-time estimation
= d · mB c · |pB|
Proper-time
Detector
Primary vertexB-decay vertexTracks
ddnew
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup19 S. Viret
Example 2 : trigger efficiency
BsKK events
HLT trigger efficiency
Y axis
With 0.5 mrad tilt of one VELO box, 30% less events selected
These events are definitely lost!!!
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup20 S. Viret
A 3 steps procedure :
Complete survey of every sub-detector and of all the structure when installed in the pit
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
Hardware alignment (position monitoring):
Stepping motors information during VELO boxes closing
OT larges structures positions constantly monitored (RASNIKs system)
Laser alignment for RICH mirror positioning
Software alignment
Alig
nm
en
t p
reci
sion
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup21 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
Software alignment strategy :
Align all sub-detectors (VELO, IT, OT, RICHs) internally
Align the sub-detectors w.r.t. the VELO (Global alignment). Start IT & OT, then TT (not alignable internally), RICH and finally Ecal, Hcal and Muon.
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup22 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
VELO alignment : how to proceed ?
Alignment should be designed to be FAST (few minutes)and PRECISE (<5
m precision)
Data taking
Residuals monitoring
End of run : VELO is open
Start of run : VELO is closed
If necessary…
Software alignment procedure
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup23 S. Viret
Step 2
Aligned VELOAlign the boxes using global fit again on primary vertices,
overlapping tracks,...
Step 1
Internally-aligned VELO
Global fit applied on tracks (classic & beam
gas/halo) in the two boxes
Step 0
Misaligned VELO
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
VELO: the strategy
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup24 S. Viret
Residuals are function of the detector resolution, but also of the misalignments
From this…
The geometry we are looking for is the one which minimizes the tracks residuals (in fact there are many of them but there are ways to solve this problem).
… to that
Global fit ?
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
1. Introduction2. The alignment challenge3. Conclusions
xclus = ∑i∙i + ∑aj∙j
LINEAR sum on misalignment constants
i
LINEAR sum on track parameters i
(different for each track)
LHC Startup25 S. Viret
GLOBAL FIT IDEA : Express the residuals as a linear function of the
misalignments, and fit both track and residuals in the meantime:
Taking into account the alignment constants into the fit implies a simultaneous fit of all tracks (they are now all ‘correlated’):
We get the solution in only one step.
The final matrix is huge (Ntracks∙Nlocal+Nglobal)
xclus = xtrack + x
LINEAR sum on track parameters i
(different for each track)
xclus = ∑i∙i + x
LOCAL PART GLOBAL PART
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
But inversion by partitioning (implemented in V.Blobel’s MILLEPEDE algorithm), reduces the problem to a Nglobal x Nglobal matrix inversion !!!
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup26 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
VELO : MC results (STEP 1 : modules alignment)
x
y
Before After Resolution on alignment constants (with ~20000
tracks/box) are 1.2 m (x and
y) and 0.1 mrad ()
Algorithm is fast (few minutes on a single CPU)
Code integrated into LHCb software. MC tests made with different misaligned geometries.
1. Introduction2. The alignment challenge3. Conclusions
STEP 2 results also within LHCb requirements
LHC Startup27 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
VELO : testbeam results (Nov.06)
The testbeam setup
10 modules installed
4 configurations (6 modules cabled) tested
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup28 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
VELO : testbeam results
Track residuals
Before alignment
After alignment
• +26% vertices in target 1• +10% vertices in target 2
1 2
Vertexing
1. Introduction2. The alignment challenge3. Conclusions
LHC Startup29 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
Tracking system alignment
Use the same method as the VELO (global fit via Millepede) via a common alignment software framework (currently under development).
Interface with Millepede is more complex than in the VELO, due to different track shapes (parabolas inst. of straight tracks). On the other hand detectors are not moving, alignment might be less frequently processed.
1. Introduction2. The alignment challenge3. Conclusions
IT and OT are internally aligned separately, then w.r.t. each other using the overlap areas.
Work is ongoing. Results expected soon.
LHC Startup30 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
RICH alignment : principle
1. Introduction2. The alignment challenge3. Conclusions
: Expected Cherenkov angle
: Measured angle
: Distortion due to mirror tilts
Mirror tilt
LHC Startup31 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
RICH alignment : results
1. Introduction2. The alignment challenge3. Conclusions
Fit those distributions for all the mirrors combinations in order to get
their individual orientation (tilt around X and Y axis).
Alignment code implemented
Minimization using MINUIT
Measured - expected
x
0.1 mrad resolution obtained, well within
requirements
RICH 2
LHC Startup32 S. Viret
1. What’s the problem with alignment ?2. LHCb alignment strategy3. Sub-detectors overview
Global alignment
1. Introduction2. The alignment challenge3. Conclusions
Most critical step is tracking system alignment:
VELO to T-stations (IT & OT) TT to VELO/IT/OT
Strategy for step has been defined (match tracks fitted independently in both tracking systems) and successfully tested on MC. Has to be extended to step
Work is ongoing.
1. Introduction
2. The alignment challenge
3. Conclusions
LHC Startup33 S. Viret
1. Introduction2. The alignment challenge3. Conclusions
LHCb has been designed to hunt New Physics sign in the heavy flavours sector.
But in order to reach our objectives, a perfect understanding of the detector will be necessary.
In particular, a very good alignment is required (for trigger, particle ID, tracking,…)
LHC Startup34 S. Viret
1. Introduction2. The alignment challenge3. Conclusions
LHCb alignment strategy has been presented (available in CERN-LHCb-2006-035 ). It has to take into account LHCb unique specificities (RICH, moving VELO) and requirements (online vertex trigger)
Work is ongoing on many fronts, and some nice results have already been obtained (VELO testbeam alignment, RICH alignment,…). A common framework is taking shape.
We are now waiting the first beams (as we can’t play with cosmics )
Backup Slides
LHC StartupA0 S. Viret
Align with what ?
Alignment algorithm feeding has to be taken seriously !
First alignment will be determined using magnet OFF data (very important for tracking systems).
Then this first alignment will be updated with magnet ON data.
Specific trigger scheme for those events necessary after 2008 (work ongoing).
Extensive use of specific types of tracks* (beam halo/ beam gas), mass resonances (e.g. J/),…
* No cosmics in LHCb.
LHC StartupA1 S. Viret
Methodology for the tests:
200 runs of 25000 events (5000 min.bias + 20000 pseudo-halo) were passed trough LHCb software with the following misalignments scales (all 6 degrees of freedom are taken into account at each level):
Misaligned events are produced via LHCb geometry framework. No momentum cut applied for track selection (try to rely on VELO information only)
Translations (in m)
(x, y, z)
Rotations (in mrad)
(,,)
Module 30 2
Box 100 1Misalignment scales chosen using misalignment studies and hardware information
LHC StartupA2 S. Viret
The huge matrix we had to invert is very sparse:
Inversion by partitioning (implemented in V.Blobel’s MILLEPEDE algorithm), reduces the problem to a Nglobal x Nglobal matrix inversion !!!
As Nglobal 100 for the VELO, the problem could be solved in few sec. !!!
kCkglobal Hk
HkT
k
=0
0
Cklocal 00
0
0 …
…
……
……
kwkxk
kwkk
… ………
Nglobal Nlocal x Ntracks
LHC StartupA3 S. Viret
How to linearize the system ?
Millepede is interesting, but linearity is a key point. Obviously VELO sensors R/ geometry is not the most linear thing in the world…
Could consider module as a rigid object and thus transform (R/) into (X,Y) point. Module is then the basic
detector element to align.
R
But R and sensors are precisely bonded together within a module (~10m precision), and also precisely surveyed (~ few m precision).
LHC StartupA4 S. Viret
VELO : MC results (STEP 2 : boxes alignment)
PV
Overlap
Results obtained with limited statistic ( ~1500 PV’s and ~300 overlap tracks ) :
offset (PV) = 17m tilt (PV) = 99rad offset (Overlap) = 13mtilt (Overlap) = 40rad
Results could still be improved but are already well within trigger requirements.
Track fit: bi-directional Kalman fit Tracking efficiency (p>5GeV) ~94% (ghost rate
~16%) Proper time resolution ~ 40 fs B Mass resolution ~ 15 MeV
125.6 mm
LHC StartupA5 S. Viret
p/pMomentum Resolution
p [GeV]
= 14.8m+30.4
m/p t (ip)
Impact Parameter Resolution
pT
1. Physics justification2. The detector3. LHCb startup program
Tracking performance:
1. Introduction2. The alignment challenge3. Conclusions