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Synthesis, X-ray structure and strong in vitro cytotoxicity of novel organoruthenium complexes
Nuclear Instruments and Methods in Physics Research A 876 (2017) 13–16
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A
journal homepage: www.elsevier.com/locate/nima
Real-time calibration and alignment of the LHCb RICH detectors
MARK
Jibo HE
University of Chinese Academy of Sciences (UCAS), Beijing, China
A R T I C L E I N F O
A BS T RAC T
On behalf of the LHCb RICH collaboration
In 2015, the LHCb experiment established a new and unique software trigger strategy with the purpose of
increasing the purity of the signal events by applying the same algorithms online and offline. To achieve this,
real-time calibration and alignment of all LHCb sub-systems is needed to provide vertexing, tracking, and
particle identification of the best possible quality. The calibration of the refractive index of the RICH radiators,
the calibration of the Hybrid Photon Detector image, and the alignment of the RICH mirror system, are reported
in this contribution. The stability of the RICH performance and the particle identification performance are also
discussed.
Keywords:
RICH
Cherenkov detectors
Calibration
Alignment
LHCb
1. Introduction
The LHCb experiment [1] is one of the four large particle physics
experiments at the Large Hadron Collider (LHC), and is designed to
search for physics beyond the Standard Model by precision study of the
beauty and charm hadrons. The Ring Imaging CHerenkov (RICH)
detectors of the LHCb experiment employ the C4F10 and CF4 radiators to
provide particle identification (PID) of charged particles in the
momentum range of 2–100 GeV. This is essential for the LHCb core
physics program. The centre-of-mass energy of the LHC has been
increased from 8 TeV to 13 TeV in 2015, and the LHCb experiment will
be upgraded [2,3], starting in 2019, and run at a 4-times higher
luminosity than in LHC Runs I and II. A new and unique software
trigger strategy has been established at the LHCb experiment with the
purpose of increasing the purity of the signal events by applying the
same algorithms online and offline. This requires ultimate quality of
vertexing, tracking, and PID. Therefore, real-time calibration and
alignment of all LHCb sub-systems is required online.
The calibration of the refractive index of the RICH radiators, the
calibration of the Hybrid Photon Detector (HPD) image, and the
alignment of the RICH mirror system are reported here. The LHCb
trigger strategy in Run-II (2015–2018) has been changed with respect
to that in Run-I (2010–2013), as shown in Fig. 1. The online event
reconstruction in Run-I was simpler and faster than that used offline,
and did not have the latest detector calibration and alignment
constants applied. In the Run-II data-taking, the events selected by
the first stage of the software trigger are buffered on local disks, then an
automatic detector calibration and alignment is performed, and the
resulting calibration and alignment constants are used in the final stage
of the software trigger. As the full offline event reconstruction is run in
the final stage of the software trigger, its output can be used for physics
analysis directly without further offline processing.
Fig. 1. Schematic diagram of the LHCb trigger data-flow in Run-I data taking (left)
compared to the data-flow in Run-II (right).
2. RICH optical layout and reconstruction
The LHCb RICH system has two detectors, one upstream and one
downstream of the magnet, covering the low momentum range ∼2–
60 GeV/c and the high momentum range ∼15–100 GeV/c respectively
[1]. The side view of the RICH detector upstream of the magnet is
shown in Fig. 2. The Cherenkov photons emitted by charged particle
tracks in the radiator are reflected and focused by the combination of
spherical and secondary mirrors out of the LHCb geometrical acceptance and then are detected by the Hybrid Photon Detectors (HPD).
To reconstruct the photon candidate and the Cherenkov angle, one
http://dx.doi.org/10.1016/j.nima.2016.12.041
Received 1 November 2016; Received in revised form 20 December 2016; Accepted 21 December 2016
Available online 24 December 2016
0168-9002/ © 2016 Elsevier B.V. All rights reserved.
Nuclear Instruments and Methods in Physics Research A 876 (2017) 13–16
J. HE
Fig. 3. Difference between the reconstructed and expected Cherenkov angle before the
calibration. The mean value, determined from the fit shown, is then converted to a
correction factor that is applied to the expected refractive index.
Fig. 2. Side view of the LHCb RICH detector upstream of the magnet.
takes the spatial position of the HPD pixel hits as the detection point,
and the middle point of the associated track in the radiator as the
emission point, then solves a quartic equation that fully describes the
reflections of the photon, given the RICH geometry [4]. The “global
likelihood algorithm” [5] is used to determine the PID, where
information from all tracks, all radiators and all pixel hits within a
given event are considered simultaneously, and the likelihood is
maximized by comparing the observed Cherenkov angles with those
expected under different PID hypotheses.
To achieve the best PID performance, one needs to align the RICH
mirrors, detector planes and the tracking system, and calibrate the
refractive index of radiators and the HPD image with good precision.
These factors are all time-dependent, necessitating real-time calibration and alignment of the LHCb RICH detectors, and the tracking
system.
Fig. 4. Schematic drawing of the Hybrid Photon Detector (HPD).
the end of each run. Then a dedicated task is used to fit the histograms
merged run-by-run1 and produce calibration constants to be used by
the RICH reconstruction in the final stage of the software trigger.
3.2. Calibration of the HPD images
The Hybrid Photon Detector is used to detect Cherenkov photons.
As shown in Fig. 4, the photoelectron produced at the photocathode is
accelerated by a high voltage of up to 20 kV onto a reverse-biased
pixellated silicon detector, with a de-magnification factor of about 5
[6]. The HPD anode images are affected by the magnetic and electric
fields, and have been observed to move and change their size, possibly
due to changes in these residual fields when the high voltage is cycled
each LHC fill. Such changes could degrade the reconstruction of the
Cherenkov angle and affect the PID performance. Therefore the centre
and radius of all the HPD images are calibrated run-by-run. Fig. 5
shows the calibration process. First, the centre of the image is cleaned
to eliminate ion feedback. Then a Sobel filter is used to detect the edges
of the image that are fitted to determine the centre and the radius of the
image, which are used by the RICH reconstruction in the final stage of
the software trigger. As only the raw HPD data needs to be decoded,
more than 500 Hz of events are processed run-by-run.
3. Calibration and alignment
3.1. Calibration of the refractive index of the RICH radiators
The refractive index of the gas radiators depends on the ambient
temperature and pressure, and the exact composition of the gas
mixture; so it can change in time. These quantities are monitored by
hardware to compute an expected refractive index, but this does not
have a precision that is high enough for the physics analysis, therefore
it needs to be further corrected. As shown in Fig. 3, the distribution of
the difference between the reconstructed and expected Cherenkov
angle is fitted to obtain the shift, which is then converted to a scale
factor of the expected refractive index according to studies based on
simulation.
About 50 Hz of events are sent to multiple online reconstruction
tasks, which run in parallel, and the resulting histograms are merged at
1
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The maximum run length is one hour.
Nuclear Instruments and Methods in Physics Research A 876 (2017) 13–16
J. HE
Fig. 5. Calibration process of the HPD image: (a) HPD image for a typical run. (b) The
centre of the HPD image is cleaned to eliminate ion feedback. (c) Edge of the HPD image
detected by the Sobel filter. (d) Radius and centre of the HPD image returned by the fit.
Fig. 7. Time dependence of the mirror alignment parameters for the RICH detector
downstream the magnet for the 2016 data sample, (upper) spherical mirror, (bottom)
secondary mirror. The shadow regions show the range of alignment parameters for all the
mirror pairs, the points show the average value for all the mirror pairs.
3.3. Alignment of the RICH mirror system
The Cherenkov photons emitted by the charged particles passing
through the RICH detectors are focused onto the photon-detector plane
by the spherical and secondary mirrors. In case of misalignment the
centre of Cherenkov ring would not correspond to the intersection
point of the charged track, and this would introduce a dependence of
the difference between the measured and expected Cherenkov angle on
the azimuthal angle of the ring, as shown in Fig. 6. The alignment
constants for each mirror are determined by the fit of the Cherenkov
angle difference as a function of the azimuthal angle on the ring. The
correlation between the different mirror pairs is also taken into
account. The procedure is evaluated by an iterative procedure implemented in a dedicated framework, which makes it possible to run
the alignment in parallel using about 1800 nodes of the software trigger
farm. The alignment of the RICH mirror system has been found to be
stable enough to not affect the PID performance, as shown in Fig. 7,
and it runs routinely as a monitoring task.
3.4. Time alignment
In order to maximise the photon collection efficiency of the LHCb
RICH detectors, the HPD readout must be synchronised with the LHC
bunch crossing to within a few nanoseconds. The initial time alignment
was performed in the absence of beam using a pulsed laser, and has
been improved further with dedicated timing scan data taken during
physics collisions. As shown in Fig. 8, all the HPDs have been time
aligned to about 1 ns.
Fig. 6. Difference between the measured and expected Cherenkov angle, ΔθC plotted as a
function of the azimuthal angle ϕ and fitted with θx cos(ϕ) + θysin(ϕ) , for one side of the
RICH 2 detector [6]. The upper plot is prior to alignment, and shows a dependency of the
angle θC on the angle ϕ. The bottom plot is after the alignment correction, and ΔθC is
uniform in ϕ.
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Nuclear Instruments and Methods in Physics Research A 876 (2017) 13–16
J. HE
alignment and with the dedicated run-by-run calibrations. It has been
found to be stable for the full 2015 data-taking, as shown in Fig. 9.
4.2. Particle identification performance
Fig. 10 shows the kaon efficiency (kaons identified as kaons) and
pion misidentification (pions misidentified as kaons), as a function of
particle momentum, obtained from imposing two different requirements on this distribution in the Run-II data. One can see that the
RICH detectors provide excellent particle identification. Further details
on the particle identification performance in the Run-II data-taking of
the LHCb experiment can be found in Ref. [7].
Fig. 10. Kaon identification efficiency and pion misidentification rate measured on data
as a function of track momentum. Two different Δlog3 (K − π ) requirements have been
imposed on the samples, resulting in the open and filled marker distributions,
respectively.
5. Summary
Fig. 8. Distribution of the midpoints of timing scans in the upstream RICH (upper) and
downstream RICH (bottom) after time alignment with pp collisions.
Novel real-time calibration and alignment of the LHCb RICH
detectors have been implemented in the Run-II data-taking of the
LHCb experiment. This includes the run-by-run calibration of the
refractive index of the RICH radiators, and that of the Hybrid Photon
Detector image, and the alignment of the RICH mirror system. The
real-time calibration and alignment works well, and provides excellent
particle identification for the online trigger and offline analysis.
4. Performance
4.1. Stability of the RICH performance
The stability of the RICH performance is monitored by the
Cherenkov angle resolution obtained with the same RICH mirror
References
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[3] LHCb collaboration, LHCb PID Upgrade Technical Design Report, CERN-LHCC2013-022.
[4] T. Ypsilantis, J. Seguinot, Theory of ring imaging Cherenkov counters, Nucl.
Instrum. Methods Phys. Res. A 343 (1994) 30.
[5] R. Forty, RICH pattern recognition for LHCb, Nucl. Instrum. Methods Phys. Res. A
433 (1999) 257.
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these proceedings.
Fig. 9. Time dependence of the Cherenkov angle resolution for the RICH detector
downstream of the magnet for the 2015 data sample.
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