Calibration of the CMS Electromagnetic

1
Calibration of the CMS Electromagnetic
Calorimeter with LHC collision data Maria
Margherita Obertino on behalf of the CMS
Collaboration
Introduction The electromagnetic calorimeter
(ECAL) of the CMS experiment is an homogeneous,
hermetic detector with high granularity. It is
made of 75,848 lead-tungstate (PbWO4) crystals.
The central barrel calorimeter (EB) is organized
into 36 supermodules (SM) and it is closed at
each end by an endcap calorimeter (EE) consisting
of two dees. For the light collection it is
equipped with avalanche photodiodes (APD) in the
barrel part and vacuum phototriodes (VPT) in the
endcaps. A silicon/lead pre-shower detector (ES)
is installed in front of the crystal calorimeter
in the endcaps in order to improve the g/p0
discrimination and the vertex reconstruction for
photons. The CMS ECAL is one of
the highest resolution electromagnetic
calorimeters ever constructed, but relies upon
precision calibration in order to achieve and
maintain its design performance. Variations in
light collected from the lead tungstate crystals,
due to intrinsic differences in
crystals/photodetectors, as well as variations
with time due to radiation damage for example,
need to be taken into account. Sophisticated and
effective methods of inter-crystal and absolute
calibration have been devised, using collision
data and a dedicated light injection system. For
inter-calibration, low mass particle decays (p0
and eta) to two photons are exploited, as well as
the azimuthal symmetry of the average energy
deposition at a given pseudorapidity. Absolute
calibration has been performed using Z decays
into electron-positron pairs. The light injection
system monitors the transparency of the crystals
in real-time and enables the re-calibration of
the measured energies over time. This is
cross-checked by the comparison of E/p
measurements of electrons from W decays (where
the momentum is measured in the CMS tracker)
with/without these re-calibrations applied.
amplitude change in order to cope with laser
maintenance interventions. The response is
normalized to the signal of a PN installed in the
same optical ?ber fan-out, whose stability and
linearity have been studied with dedicated laser
amplitude scans. The plot shows the relative
response to laser light (440 nm) measured by the
ECAL laser monitoring system, averaged over all
crystals in bins of pseudo rapidity ? for the
2011 data taking. The observed transparency loss
has an exponential behaviour and reaches a
saturation level which depends on the dose rate.
The average response change is about 23 in the
Barrel and reaches 40 for ? 2.7 in endcap,
in agreement with the expectations for the
achieved instantaneous luminosity. The
spontaneous recovery of the crystals in periods
without irradiation is clearly visible.
SM

Single crystal intercalibration
The strategy to intercalibrate The ?-symmetry
method is based on the assumption that for a
large number of minimum bias events the total
transverse energy (ET) deposited should be the
same for all crystals in a ring at fixed
pseudorapidity (?). Inter-calibration in ? can be
performed by comparing the total transverse
energy (SET) deposited in one crystal with the
mean of the total SET collected by crystals at
the same absolute value of ?. In the barrel,
crystals are arranged in 85 pairs of rings
(positive and negative sides) with 360 crystals
in each ring. In the determination of the
transverse energy sum, only deposits lying
between a low and high thresholds are considered.
The former is applied to remove the noise
contribution, the latter is meant to avoid that a
few very high ET hits bias significantly the
value of the summed transverse energy. The ?
inhomogeneities of the detector are taken into
account introducing a data-driven correction. In
the p0/h method, the invariant mass of photon
pairs from p0/h ? ?? is used to obtain the
inter-calibration constants. The photon
candidates are reconstructed using a simple 3?3
window clustering algorithm. The cluster energy
is computed as the sum of crystal energies S9
S 3?3 ci ? Ei where ci denotes the calibration
constant and Ei the energy deposited in each i-th
crystal. The shape of the cluster energy
deposition should be consistent with that of an
electromagnetic shower produced by a photon. The
selected sample contains about 1.4?107 signal p0
decays.
The crystal-by-crystal calibration constants
obtained with ?-symmetry and p0 methods
independently are combined together with the
beam dump constants. The precision of the
resulting inter-calibration constants is shown in
the lower plot as a function of crystal
pseudorapidity. The 0.5 test beam precision is
not negligible with respect to the combination
precision and therefore subtracted in quadrature.
For the central barrel (crystal ? index 45)
the combined inter-calibration precision is found
to be 0.6.
References CMS Collaboration, The CMS experiment
at the CERN LHC, J. Inst. 3 S08004 (2008) CMS
Collaboration, The Electromagnetic Calorimeter
Technical Design Report, CERN/LHCC 97-33
(1997) M. Bonesini et al., Intercalibration of
the electromagnetic calorimeter with cosmic
rays, CMS NOTE 2005/023 L. Agostino et al.,
Inter-calibration of the CMS electromagnetic
calorimeter with isolated electrons, J. Phys.
G 33(2007) N67-N84 P. Adzic et al., Energy
resolution of the barrel of the CMS
electromagnetic calorimeter, J. Inst. 2 P04004
(2007) P. Adzic et al., Intercalibration of the
barrel electromagnetic calorimeter of the CMS
experiment at start-up, J. Inst. 3 P10007
(2008) CMS Collaboration, Performance and
operation of the CMS electromagnetic
calorimeter, J. Inst. 5 T03010 (2010) CMS
Collaboration, Electromagnetic calorimeter
calibration with 7 TeV data, CMS PAS EGM-10-003
(2010)
Summary At start-up the calibration precision of
the electromagnetic calorimeter in the CMS
experiment was between 1.5 and 2.5 in the
barrel and of about 5 in the endcaps. The level
of precision reached is such that Z width
measurement is not affected in EB, and is already
almost insensitive to residual mis-calibration
even in EE Using the first 250 nb-1 collected
with the CMS detector in 2010 at a center of mass
energy of 7 TeV a channel-by-channel calibration
is achieved with a precision of about 0.6 in the
central barrel. The global energy response scale
of the ECAL is also studied, and found to agree
with the expectation to within about 1 in the
barrel and 3 in the endcaps. The calibration of
the CMS ECAL will continue with the upcoming LHC
data, with collection of large samples of neutral
pion decays as well as W and Z bosons decaying
into energetic and isolated electrons.