The Compact Muon Solenoid (CMS) Experiment, located at the Large Hadron Collider at CERN, is a general purpose particle detector experiment with a high resolution electromagnetic calorimeter (ECAL). I report on the commissioning of the light monitoring system, which tracks changes in transparency of the lead tungstate crystals and is thus crucial for maintaining the energy resolution of the calorimeter

Orimoto, Toyoko J.

Caltech Authors - Main

2008 IEEE Nuclear Science Symposium Conference Record
The Commissioning of the CMS Electromagnetic
Calorimeter Light Monitoring System
Toyoko J. Orimoto,
On behalf of the CMS ECAL Group
N30·184
Abstract-The Compact Muon Solenoid (CMS) Experiment,
located at the Large Hadron Collider at CERN, is a general
purpose particle detector experiment with a high resolution
electromagnetic calorimeter (ECAL). I report on the commis-
sioning of the light monitoring system, which tracks changes in
transparency of the lead tungstate crystals and is thus crucial
for maintaining the energy resolution of the calorimeter.
Index Terms-LHC, CMS, electromagnetic calorimeter, crystal
calorimeter, light monitoring
I. INTRODUCTION
UNDERSTANDING the origins of electroweak symmetrybreaking, and in particular discovering the Higgs boson
predicted by the Standard Model, is the main motivation for
building the Compact Muon Solenoid (CMS) experiment at
the Large Hadron Collider (LHC) at CERN.
The CMS electromagnetic calorimeter (ECAL) was de-
signed to optimize the resolution for the discovery of the
Higgs in the distinctive, yet challenging, diphoton channel.
In the low Higgs mass range around 120 GeV, the diphoton
channel will be the most promising for discovery because
of its distinctive signature. In this mass range, the decay
width of the Higgs is expected to be very narrow, and the
discovery potential is directly related to the reconstructed mass
width, or rather, the energy and position resolution of the
electromagnetic calorimeter.
The CMS ECAL is a high-resolution, high-granularity scin-
tillating crystal calorimeter, consisting of 75,848 lead-tungstate
(PbW04) crystals [1],[2]. PbW04 crystals were chosen be-
cause of their short radiation length, small Moliere radius,
and fast speed as a scintillator. The design energy resolution
of the ECAL has a constant term of 0.5%, and to maintain this
constant term, in situ calibration and monitoring of the crystals
must be performed. Exposure at the level of the nominal LHC
luminosity causes a decrease in crystal transparency due to
radiation induced absorption. The changes in transparency are
monitored with a high precision, laser-based light monitoring
system, the performance of which is described here.
II. RADIATION INDUCED CRYSTAL TRANSPARENCY
CHANGE
At the LHC design luminosity, the CMS detector will be
exposed to a very harsh radiation environment. The PbW04
crystals are radiation hard up to a high integrated dosage, but
T. J. Orimoto is with the California Institute of Technology, Pasadena, CA
91125, USA (e-mail: toyoko@hep.caltech.edu).
nonetheless suffer from dose-rate dependent radiation damage,
resulting in a decrease in crystal transparency.
Previous studies have shown that the scintillation mech-
anism of PbW04 crystals is not affected by radiation, and
the loss of light output is due only to the absorption caused
by radiation induced color centers [3]. The time scale of the
transparency change is fast, on the order of a few hours, com-
pared to the time scale of days, weeks, or months needed for
calibration with physics events to reach sub-percent precision.
During irradiation, the annihilation and creation of color cen-
ters reach an equilibrium, depending on the applied dose rate.
Although the crystals will self-recover in absence of radiation,
this recovery takes place on the order of a week, while the time
cycle of the LHC operation is 12 hours. Therefore, changes in
crystal transparency, and therefore calorimeter response, due to
radiation damage must be corrected for to maintain the energy
resolution of the detector.
III. THE LASER MONITORING SYSTEM
The CMS ECAL utilizes a laser monitoring system to
monitor the light output of the crystals. With this system,
we can measure the change in transparency of each crystal
continuously during LHC operation, with very high precision.
In order to maintain the constant term in the ECAL resolution
to 0.5%, the measurement of the transparency change must
be made with an accuracy of greater than 0.2%. The ECAL
readout electronics require that the laser pulse width be shorter
than 40ns and stable at a level better than 10%. Sychronization
with the LHC clock requires that the laser pulse timing be
stable to a few ns. In addition, the pulse energy must be on
the order of 1mJ/pulse, which is equivalent to 1.3 TeV in the
dynamic range of the system, to provide sufficient light to
monitor the entire ECAL in approximately 20 minutes.
To meet these requirements, a dual-stage, laser system has
been designed, constructed and commissioned by the Caltech
and Saclay groups [4]. The monitoring light source consists of
two sets of Nd:YLF laser pumped Ti:Sapphire lasers, which
provide two wavelengths, available using a 3 x 1 optical switch.
The selected wavelength is then sent to each ECAL monitoring
element using a 1 x 88 optical switch.
In 2006, a software feedback system was implemented so
that the long term stability of the laser pulse intensity and
FWHM width are maintained at the level of a few percent,
with a laser pulse timing jitter of less than 2ns. The long term
stability was observed in more than 2,000 hours of test beam
data, as shown in Fig. 1. With an integrating sphere based light
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1400=-------·-·------..,
1200r D..=ata=-----l
E..-
'Mun
1000 'lWS
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Fig. 3: Energy resolution of an ECAL barrel crystal, before
(left) and after (right) applying the transparency correction
determined in Fig. 2.
operation. As shown in Figure 3, the energy resolution before
and after the monitoring corrections, for 120 OeV electrons,
reconstructed by summing the energy in 3 x 3 crystals, is
0.93% and 0.53% as measured with test beam data.
,- Nov. 2006
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1°Koj
4.2 5.8
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o
1----+--+-~I___+_-_t____t-_+____4 23 32
17r~j
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Time (hours)
::i
.!.
e; 5.INIRll......
~
w
Fig. 1: Stability of the laser monitoring system, as measured
in test beam.
distribution system, the stability of the normalized monitoring
response is about 0.1%. With such performance, even small
changes in transparency can be monitored with precision.
Typical variations in crystal transparency measured by using
blue laser pulses are at the level of a few percent. The
corresponding variations in the crystal light output, measured
by using an electron beam, can be as large as 5% in the ECAL
barrel and even greater in the endcaps for radiation levels at
LHC design luminosity.
V. LASER MONITORING DATAFLOW
During LHC running, laser data are collected during "abort
gap" events, periods in absence of collisions lasting for 3J-Ls
every 90ps. Fig. 4 shows a diagram of the laser data flow,
during in situ operation of the monitoring system. The abort
gap events, containing laser data amongst other data, are sent
to a laser PC farm, at which point the events are sorted
and analyzed to extract data relevant for the transparency
correction. The data are inserted into the "online" database,
located at CMS Point 5, and then transferred to the "offline"
database, during which any necessary corrections are applied
to the laser data. During the offline reconstruction of CMS
data, the values in the database are used to compute and apply
the transparency correction to physics events.
IV. PERFORMANCE RESULTS FROM TEST BEAMS
The performance of the monitoring system and the trans-
parency correction were tested during dedicated irradiation
runs at test beams in 2006 and 2007 [5]. These dedicated
studies were performed by emulating the dose rates expected
at the LHC by irradiating the crystals with an electron beam
at high rates. During the irradiation periods, the crystals were
regularly monitored with laser runs. Fig. 2a shows the changes
in crystal transparency during the damage and recovery cycles
in test beam data.
Prior studies have shown that the decrease in scintillation
signal response due to a particle traversing a crystal is di-
rectly proportional to the decrease in the injected laser light
monitoring signal. The loss in the response of the crystals to
electron beam runs can be correlated with the loss in response
to laser runs, by the equation, E(t)/E(to) = [L(t)/ L(to)]Q,
where E(t)/E(to) corresponds to the normalized electron
beam response, L (t) / L(to) is the normalized response to laser
runs, and Q is the correlation constant which relates these two
responses. Fig. 2b shows the fit for Qt for one of the ECAL
barrel crystals, from test beam data. The spread of Q values,
as shown in Fig. 2c, has been measured to be around 5% in a
sample of 28 crystals. Such a small dispersion will allow us to
use the same a parameter for all ECAL crystals during LHC
VI. COMMISSIONING OF THE LASER MONITORING
SYSTEM AND FIRST LHC DATA
Two laser systems (blue/green and IR/red) have been in-
stalled and commissioned in the CMS underground cavern
at Point 5, and first laser data has been collected. During
the commissioning phase of CMS and first LHC beam, we
have not reached radiation levels which would induce trans-
parency changes in the calorimeter crystals. However, the laser
monitoring system has been used for the commissioning of
ECAL. For instance, the laser system provides a precise way
eMS PointS
Offtln.
'I"'
+--j:~
Fig. 4: Schematic diagram of the laser monitoring data flow
from the eMS detector to offline reconstruction.
2259
Teat Beam Deta
8kI.1_(~~....._)
tlectl<>n de.. t 1't"GI'W .... «IfNd!'-
(a)
I R for crystal 168 I
I
8. 1
I
~95
w
0.985 0.99 0.995 1
.....r R.sponse (Interp)
(b)
10
Dispersion of a for 28 srcp crystals
61 2002
• 2003
• 2004 (fill)
• 2004(....
(c)
Fig. 2: (a) The changes in responses to electron beam and laser runs during the damage and recovery cycles of the crystals, as
seen in test beam data. (b) The correlation between the responses to electron and laser test beam data, for one of the ECAL
barrel crystals. (c) The distribution of ex values for 28 different ECAL crystals.
to evaluate the timing of the ECAL signal. Utilizing laser data
and first LHC beam data, we have adjusted for differences in
timing between different ECAL readout units. In addition, the
laser data has confirmed that the timing of all ECAL readout
units is within 2ns of the mean value.
VII. CONCLUSION
The CMS ECAL laser monitoring system has been tested
at test beams at CERN, and commissioned at CMS Point
5 underground. The laser monitoring system will provide
corrections for light output fluctuations caused by variations in
crystal transparency during the radiation damage and recovery
process. The stability of the system has been exhibited to be on
the order of 0.1%. With such performance, even small changes
in transparency can be monitored with precision.
ACKNOWLEDGMENT
I would like to acknowledge the CMS ECAL Collaboration
for their effort and dedication. I would also like to thank my
colleagues from the laser monitoring group, particularly A.
Bornheim and R. Y: Zhu, for their input and support.
REFERENCES
[1] "The Electromagnetic Calorimeter Technical Design Report", CMS Col-
laboration, CERNILHCC 97-33, 1997.
[2] R Adolphi et al. [CMS Collaboration], "The CMS experiment at the
CERN LHC," JlNST 3. S08004 (2008).
[3] "Radiation Induced Color Centers and Light Monitoring for Lead
Tungstate Crystals", X. Qu, L. Zhang, R Y. Zhu, IEEE Trans. Nucl. Sci.
NS-47 (2000).
[4] "Performance of the Monitoring Light Source for the CMS Lead
Tungstate Crystal Calorimeter", IEEE Trans. Nucl. Sci. Vol. 52 No.4
(2005) 1123-1130.
[5] "ECAL Monitoring Light Source at H4", D. Bailleux, A. Bomheim, L.Y.
Zhang, KJ. Zhu, RY. Zhu, and D. Liu, CMS IN 2003/045.
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Toyoko J. Orimoto

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