Calibration of the photon spectrometer PHOS of the ALICE experiment

The procedure for the energy calibration of the high granularity electromagnetic calorimeter PHOS of the ALICE experiment is presented. The methods used to perform the relative gain calibration, to evaluate the geometrical alignment and the corresponding correction of the absolute energy scale, to obtain the nonlinearity correction coefficients and finally, to calculate the time-dependent calibration corrections, are discussed and illustrated by the PHOS performance in proton-proton (pp) collisions at $\sqrt{s}$ = 13 TeV. After applying all corrections, the achieved mass resolutions for $\pi^0$ and $\eta$ mesons for $p_{\rm{T}} > 1.7$ GeV/$c$ are $\sigma_m^{\pi^0} = 4.56 \pm 0.03$ MeV/$c^2$ and $\sigma_m^{\eta} = 15.3 \pm 1.0$ MeV/$c^2$, respectively.


JINST 14 (2019) 05, P05025
e-Print: arXiv:1902.06145 | PDF | inSPIRE

Figure 1

[Color online] ALICE cross-sectional view in Run 2, PHOS modules are located at the bottom of the setup.

Figure 2

[Color online] Left: Part of a cell matrix of one module; Right: A detector element comprising a {\rmPbWO}$_4$ crystal, APD photodetector and preamplifier.

Figure 4

[Color online] The amplitude of the LED peak for different APD bias voltages, for oneexample channel.

Figure 5

[Color online] Left: The dependence of the APD gain on applied bias voltage, for three different channels. Typical and two extreme cases are presented. Right: The distribution of the APD bias voltages, for all PHOS cells,for an APD gain of 29.

Figure 6

[Color online] Invariant mass distribution of cluster pairs after APD gain equalization in pp collisions at $\sqrt{s}=13$ TeV for $\pT>1.7 \GeVc$. The red curve is a fit of the spectrum with the sum of a Gaussian and a second-order polynomial function. The green dashed line is the background contribution only.

Figure 7

[Color online]Study using a toy Monte Carlo simulation of the convergence of the iterative calibration procedure based on equalization of the $\pi^0$ peak position. The residual de-calibration $\sigma_{\rm c}$ is shown as a function of the iteration number. Two values of calorimeter energy resolution are considered, standard ($\sigma_{E}$) and twice as poor ($2\sigma_{E}$).

Figure 8

[Color online] Left: Residual de-calibration in the toy model simulation with default energy resolution versus iteration number for several values of power $n$. Right: Residual de-calibration versus power $n$ for several iterations.

Figure 9

[Color online] Dependence of the $\pi^0$ peak width on the iteration number for photon pairs with $\pT>1.7 \GeVc$ in four PHOS modules.

Figure 10

[Color online] Distribution of the cluster energy to track momentum, $E/p$ ratio, for two ranges of cluster energies $E_{\rm clu}$ in one PHOS module. A peak around unity due to the electron contribution is visible.

Figure 12

[Color online] An illustration of the dependence of $\langle dz \rangle$, from equation (\ref{eq:dz-def}), with $z$, in aradially shifted detector. The magnetic field of 0.5 T is along the $z$ direction.

Figure 13

[Color online] Dependence of the mean distance between track extrapolation to the PHOS surface and cluster position in the cluster coordinate on the PHOS plane along (left) and perpendicular(right), to the beam and magnetic field direction. In the left plot contributions of electrons and positrons are combined. The dependencies are fitted with linear functions and the resulting slopes are shown in both legends.

Figure 14

[Color online] Estimation of PHOS nonlinearity using symmetric $\pi^0$ decays defined by $|E_{\gamma,1}-E_{\gamma,2}|< 0.05(E_{\gamma,1}+E_{\gamma,2})$. Data fit with function (\ref{fitNL}). The final tuned nonlinearity is shown with a dashed curve.

Figure 15

[Color online] Left: the $\pi^0$ peak position as a function of the transverse momentum for several values of nonlinearity parameters ($d$, $e$), with default values for $a$, $b$ and $c$. Right: the deviation from a constant value of the $\pi^0$ peak position expressed in $\chi^2/NDF$ as a function of the nonlinearity parameters ($d$, $e$).

Figure 16

[Color online] Example of the dependence of the $\pi^0$ peak position on the run number for 400 sequential runs recorded during 3 months of the 2017 data taking campaign.

Figure 17

[Color online] Invariant mass distributions of cluster pairs for $\pT>1.7 \GeVc$ in the $\pi^0$ (left) and $\eta$ (right) mass region after calibration with per-channel $\pi^0$ peak equalization. For the $\pi^0$ data, the solid curve shows the fitting function using thesum of the Crystal Ball and a polynomial function. For the $\eta$ data, the solid curve shows the fit function composed of a Gaussian and a polynomial function. The dashed lines represent the background contributions in both plots.

Figure 18

[Color online]Peak position and width for $\pi^0$ (left) and $\eta$ mesons (right) as a function of transverse momentum. Vertical error bars represent fit uncertainties.