Performance of the ALICE Experiment at the CERN LHC

ALICE Collaboration

ALICE is the heavy-ion experiment at the CERN Large Hadron Collider. The experiment continuously took data during the first physics campaign of the machine from fall 2009 until early 2013, using proton and lead-ion beams. In this paper we describe the running environment and the data handling procedures, and discuss the performance of the ALICE detectors and analysis methods for various physics observables.

Figures

Figure 1

 The ALICE experiment at the CERN LHC. The central-barrel detectors (ITS, TPC, TRD, TOF, PHOS, EMCal, and HMPID) are embedded in a solenoid with magnetic field B=0.5 T and address particle production at midrapidity. The cosmic-ray trigger detector ACORDE is positioned on top of the magnet. Forward detectors (PMD, FMD, V0, T0, and ZDC) are used for triggering, event characterization, and multiplicity studies. The MUON spectrometer covers $-4.0< \eta< -2.5$, $\eta=-\ln\tan(\theta\!/2)$

Figure 2

 Background rate observed during several fills as a function of the product of the intensity of Beam 1, N$_1$, and the sum of the measured pressures from three vacuum gauges on the left LSS2.

Figure 3

 Beam pipe pressure and background rate infill 2181. The expected background rate has been estimated using the linear parameterization shown in Fig. 2. VGPB.120.4L2, VGPB.231.4L2, and VGI.514.4L2 are the pressure gauges located in front of the Inner triplet (at 69.7 m from IP2), on the TDI beam stopper (at 80 m from IP2), and on the large recombination chamber (at 109 m from IP2), respectively.

Figure 4

 Top: minimum bias, centrality, and muon triggers as a function of time during Pb-Pb data taking (run 169721). The B mask selects the LHC bunch slots where collisions between bunches of Beam 1 and Beam 2 are expected at IP2, while the ACE mask selects slots where no beam-beam collision is expected. Bottom: ZDC-A trigger rate as a function of time in the same run.

Figure 5

 Correlation between the sum and difference of signal times in V0A and V0C. Three classes of events - collisions at (8.3 ns, 14.3 ns), background from Beam 1 at (-14.3 ns, -8.3 ns), and background from Beam 2 at (14.3 ns, 8.3 ns)- can be clearly distinguished.

Figure 6

 Correlation between reconstructed SPD clusters and tracklets. Two bands corresponding to the collisions and MIB are visible. The dashed cyan line represents the cut used in the offline selection: events lying in the region above the line are tagged as BG and rejected.

Figure 7

 Correlation between the sum and the difference of times recorded by the neutron ZDCs on either side (ZNA and ZNC) in Pb-Pb collisions. The large cluster in the middle corresponds to collisions between ions in the nominal RF bucket on both sides, while the small clusters along the diagonals (spaced by 2.5 ns in the time difference) correspond to collisions in which one of the ions is displaced by one or more RF buckets.

Figure 8

 Top: MBand trigger rate vs. beam separation in $x$ and $y$ obtained during the May 2010 van der Meer scan. Double Gaussian fits to the data are shown as lines. Bottom: Measured MBand cross section for 48 colliding bunch pairs in the March 2011 scan, as a function of the product of colliding bunch intensities N$_1$N$_2$.

Figure 9

 Instantaneous rate (top) and number of collected events (bottom) for selected triggers in the running periods from 2010 to 2013. Special running periods (Pb-Pb, p-Pb, low energy pp) are indicated by shaded areas; the rest represents pp runs at the highest available energy.

Figure 10

 Integrated luminosity in the 2013 p-Pb run, collected in the minimum bias and the rare-trigger mode (before and after January 25, respectively).

Figure 11

 Distribution of the V0 amplitude (sum of V0A and V0C). The centrality bins are defined by integrating from right to left following Eq.(5). The absolute scale is determined by fitting to a model (red line), see below for details. The inset shows a magnified version of the most peripheral region.

Figure 12

 Correlation between the total energy deposited in the zero-degree calorimeters and the ZEM amplitude. The centrality bins defined based on this distribution (lines) are compared to the centrality from V0 (colored dots).

Figure 13

 Correlation between signals in the two neutron zero-degree calorimeters. Single electromagnetic dissociation events produce a signal in only one of the calorimeters. Mutual dissociation and hadronic interactions populate the interior of the plot and can be distinguished from each other by the signal in ZEM.

Figure 14

 Resolution of various centrality estimators.

Figure 15

 Resolution of the second-order event-plane angle, $\Psi_{2}^{\rm EP}$,extracted from two- and three-detector subevent correlations for TPC, V0, FMD, and PMD.

Figure 16

 Event-plane angle, $\Psi_{n}^{\rm EP}$, resolution for $n=2,3$, and $4$, calculated with a three-detector subevent technique separately for V0A and V0C detectors.

Figure 17

 Resolution of the first-harmonic event plane estimated from spectator deflection, as measured by the two ZDCs.

Figure 18

 Event reconstruction flow.

Figure 19

 TPC track finding efficiency for primary particles in pp and Pb-Pb collisions (simulation). The efficiency does not depend on the detector occupancy.

Figure 20

 ITS-TPC matching efficiency vs. $\pt$ for data and Monte Carlo for pp (top) and Pb-Pb (bottom) collisions.

Figure 21

 Fraction of reconstructed tracks coming from the primary interaction vertex. Two sets of cuts on the track distance of closest approach ($d_{0}$) to the primary vertex are shown: "loose'' with $|{\rm d}_{0,z}|< 3$ cm, ${\rm d}_{0,xy}< 3$ cm and "strict" with $|{\rm d}_{0,z}|< 2$ cm, ${\rm d}_{0,xy}< (0.0182+0.0350 GeV/c \> \> \pt^{-1}$) cm.

Figure 22

 Resolution of the transverse distance to the primary vertex for identified particle global ITS-TPC tracks (top) and for all charged ITS-TPC tracks (bottom) The contribution from the vertex resolution is not subtracted.

Figure 23

 The $\pt$ resolution for standalone TPC and ITS-TPC matched tracks with and without constraint to the vertex. The vertex constrain significantly improves the resolution of TPC standalone tracks. For ITS-TPC tracks, it has no effect (green and blue squares overlap).

Figure 24

 Invariant mass spectra of $\mu^+\mu^-$ (top) and $e^+e^-$ (bottom) pairs in ultraperipheral Pb-Pb collisions. The solid and dotted lines represent the background (exponential) and peak (Crystal Ball) fit components, respectively. The bremsstrahlung tail in the $e^+e^-$ spectrum is reproduced in simulation. The mass resolution is better than 1%.

Figure 25

 Top: Bunch crossing (BC) ID of tracks obtained from the comparison of time of flight measured in the TOF detector and expected from the track kinematics. The ID is defined with respect to the BC in which the triggering interaction took place The peak at -15 corresponds to tracks not matched in TOF (mostly from the pileup in the TPC, outside of the TOF readout window of 500 ns). Bottom: $z$ coordinates of tracks' PCA to the beam axis in a single event with pileup; the positions of reconstructed vertices with attributed bunch crossings are shown by markers.

Figure 26

 The $x$ (top) and $z$ (bottom) projections of the luminous region obtained from reconstructed vertices in pp and Pb-Pb collisions (folded with vertex resolution).

Figure 27

 Transverse width of the final vertex distribution (solid points), decomposed into the finite size of the luminous region $\sigma_{D}$ and the vertex resolution $\alpha / \sqrt{(\dNdeta_{ch})^\beta}$ For comparison, the widths of the preliminary (SPD) interaction vertices are shown as open points.

Figure 28

 Secondary vertex reconstruction principle, with $\Kzs$ and $\Xi^{-}$ decays shown as an example. For clarity, the decay points were placed between the first two ITS layers (radii are not to scale). The solid lines represent the reconstructed charged particle tracks, extrapolated to the secondary vertex candidates. Extrapolations to the primary vertex and auxiliary vectors are shown with dashed lines.

Figure 29

 Invariant mass distributions of $\pi^+\pi^-$ (left panel) and p$\pi^-$ (middle panel) pairs in central Pb-Pb collisions at $\sNN$ = 2.76 TeV. The hatched areas show the regions of the $\Kzs$ and $\Lambda$ peaks and of the combinatorial background. The right-hand panel shows the reconstruction efficiencies (including the candidate selection cuts) as a function of transverse momentum for central (0-5$\%$) and peripheral (80-90$\%$) collisions.

Figure 30

 Distance of the $\Lambda$, $\bar{\Lambda}$, and $\Kzs$ decay vertex from the interaction vertex, scaled by $p/m$. The slopes of the distributions are consistent with the known lifetimes.

Figure 31

 Invariant mass distribution of $K^-\pi^+$ pairs before (symbols) and after (line)selection cuts on the relation between the secondary (D$^{0}$ decay) and primary vertices. The extracted D$^{0}$ mass and its resolution as well as the significance are shown after selection.

Figure 32

 Distribution of secondary vertices from hadronic interactions in the ALICE material. The ITS layers (r < 50 cm), the inner TPC containment vessel (60 cm < r < 70 cm), and the inner TPC field cage ($r\sim$ 80 cm) are visible.

Figure 33

 Distribution of the energy-loss signal in the ITS as a function of momentum. Both the energy loss and momentum were measured by the ITS alone.

Figure 34

 Specific energy loss ($dE/dx$) in the TPC vs. particle momentum in Pb-Pb collisions at $\sNN$ = 2.76 TeV. The lines show the parametrizations of the expected mean energy loss.

Figure 35

 Ionization energy loss ($dE/dx$) distributions in the TPC in pp (top) and Pb-Pb collisions (bottom) at $\sNN$ = 2.76 TeV. The lines represent Gaussian fits as described in the main text.

Figure 36

 Interaction time of the collision with respect to the LHC clock measured by the T0 detector (top) and the resolution of the system obtained as the time difference between T0A and T0C (bottom). The time difference is corrected for the longitudinal event-vertex position as measured by the SPD.

Figure 37

 Matching efficiency (including the geometric acceptance factor) at TOF for tracks reconstructed in the TPC in p-Pb collisions at $\sNN$ = 5.02 TeV, compared to Monte Carlo simulation.

Figure 38

 Distribution of $\beta$ as measured by theTOF detector as a function of momentum for particles reaching the TOF in Pb-Pb interactions.

Figure 39

 Distribution of $\beta$ as measured by the TOF detector as a function of momentum for particles reaching TOF in p-Pb interactions. The background of mismatched tracks is lower than in Pb-Pb.

Figure 40

 TOF $\beta$ distribution for tracks with momentum 0.95 GeV/$c$ < $p$ < 1.05 GeV/$c$. The Pb-Pb histogram is normalized to the p-Pb one at the pion peak ($\beta=0.99$). While the resolution (width of the mass peaks) is the same, the background of mismatched tracks increases in the high-multiplicity environment of Pb-Pb collisions. Both samples are minimum bias.

Figure 41

 Time resolution of pion tracks with 0.95 < $p$ < 1.05 GeV/$c$ as a function of the number of tracks used to define the start time of the collision $t_{\rm ev}$ [68]. The data are from p-Pb collisions.

Figure 42

 TOF measured in Pb-Pb collisions at $\sNN$ = 2.76 TeV. The expected time of flight for kaons is subtracted and the result is divided by the expected resolution.

Figure 43

 Matching efficiency (including the geometric acceptance factor) at HMPID for tracks reconstructed in the TPC.

Figure 44

 Mean Cherenkov angle measured by HMPID in pp collisions at 7 TeV as a function of track momentum. The lines represent parametrizations of Eq.(16) for each species.

Figure 45

 Squared particle masses calculated from the momentum and velocity determined with ITS-TPC and HMPID, respectively, in central Pb-Pb collisions at $\sNN$ = 2.76 TeV. The velocity is calculated from the Cherenkov angle measured in the HMPID. Dotted lines indicate the PDG mass values. The pion tail on the left-hand side is suppressed by an upper cut on the Cherenkov angle. The deuteron peak is clearly visible.

Figure 46

 Separation power of hadron identification in the ITS, TPC, TOF, and HMPID as a function of $\pt$ at midrapidity. The left (right) panel shows the separation of pions and kaons (kaons and protons), expressed as the distance between the peaks divided by the resolution for the pion and the kaon, respectively, averaged over $|\eta|< 0.5$. For the TPC, an additional curve is shown in a narrower $\eta$ region. The lower panels show the range over which the different ALICE detector systems have a separation power of more than $2\sigma$.

Figure 47

 Combined pion identification with TOF and with $dE/dx$ in the TPC.

Figure 48

 Invariant mass of reconstructed charged particles (pions and kaons) decaying inside the TPC volume and producing a secondary vertex (kink). The mass is calculated assuming that the track segment after the kink represents a muon and that the neutral decay daughter is a neutrino. The neutrino momentum is taken from the difference between the momenta of the track segments before and after the kink.

Figure 49

 Invariant mass distribution of $K^+K^-$ candidate pairs for reconstruction of the $\phi \rightarrow$KK decay, with and without particle identification, before (left panel) and after (right panel) background subtraction.

Figure 50

 Invariant mass distribution of K$\pi$ candidate pairs for reconstruction of the D$^0 \rightarrow$K$\pi$ decay, with and without particle identification, before (left panel) and after (right panel) background subtraction.

Figure 51

 Measured $dE/dx$ signal in the ALICE TPC versus magnetic rigidity, together with the expected curves for negatively-charged particles. The inset panel shows the TOF mass measurement which provides additional separation between$^3\overline{\rm He}$ and $^4\overline{\rm He}$ for tracks with $p/Z > 2.3$ GeV/$c$.

Figure 52

 Distribution of the residuals forthe EMCal clusters to track matching in pseudorapidity ($\eta_{\rm cluster}-\eta_{\rm track}$) vs. azimuth ($\phi_{\rm cluster}-\phi_{\rm track}$) in pp collisions at $\sNN$ = 7 TeV triggered by EMCal. Only clusters with an energy $E_{\rm cluster}>1$ GeV and tracks with a transverse momentum $p_{\rm T, track}>1$ GeV/$c$ are used.

Figure 53

 $E/p$ distributions for (a) electrons and (b) pions in pp collisions at $\sqrt{s}$ = 7 TeV, measured in the experiment (reddotted line), and compared to simulation (black full line). The samples of identified electrons and pions were obtained from reconstructed$\gamma$ conversions and $\Lambda$/$\Kzs$ decays, respectively. The simulation is a Pythia simulation with realistic detector configuration and full reconstruction.

Figure 54

 Relative resolution of $E/p$ vs. transverse momentum $\pt$ for electrons in experimental data (full dots) and from a fully reconstructed MC (open circles) in pp collisions at $\sqrt{s}$ = 7 TeV. The EMCal energy resolution deduced from the width of the ${\rm \pi^0}$ and $\eta$ invariant mass peaks (black dotted line), added in quadrature to the TPC $\pt$ resolution (green dash-dotted line), describes the measurement reasonably well (red solid line).

Figure 55

 Sum of the TRD signal (ionization energy loss plus transition radiation) as a function of momentum for protons from $\Lambda$ decays, charged pions from $\Kzs$ decays, and electrons from $\gamma$ conversions in p-Pb collisions.

Figure 56

 The most probable TRD signal as a function of $\beta\gamma$. Measurements performed in test beam runs, pp collisions at $\sqrt{s}$ = 7 TeV, and cosmic rays are compared.

Figure 57

 The ratio of the average signal of electrons to that of pions as a function of the depth in the detector (slice number; the lowest slice number is farthest away from the radiator).

Figure 58

 Pion efficiency as a function of electron efficiency (top panel,for 6 layers) and as a function of the number of layers (bottom panel, for 90% electron efficiency) for the momentum range 0.9-1.1 GeV/$c$. The results are compared for the truncated mean, LQ1D, LQ2D, and NN methods.

Figure 59

 Momentum dependence of the pion efficiency for the truncated mean, LQ1D, LQ2D, and NN methods. The results are for 90% electron efficiency and for tracks with signals in six layers.

Figure 60

 $dE/dx$ distribution of electron candidate tracks, with TOF and TRD selections (using 6 tracklets in the TRD) in pp collisions. Only tracks with six TRD tracklets are included.

Figure 61

 Invariant mass distribution for $\jpsi$ candidates from EMCal-triggered events in pp collisions at $\sqrt{s}$ = 7 TeV ($\mathcal{L} \approx 0.4$ pb$^{-1}$, 8M events). Electrons are identified by their energy loss in the TPC ($dE/dx$ > 70) and the $E/p$ ratio in the EMCal ($0.9< E/p< 1.1$) for both legs. A fit to the signal(Crystal Ball [50]) and the background (exponential) is shown in addition.

Figure 62

 $e^+e^-$ invariant-mass distribution with TPC-only as well as TPC and TRD particle identification in 0-40% centrality in Pb-Pb collisions at $\sNN$ = 2.76 TeV.

Figure 63

 Mean track matching distance (top) and RMS of the track matching distance distribution (bottom) for PHOS. The lines are fits to phenomenological parameterizations.

Figure 64

 $\lambda_0^2$ distribution of photon clusters in the EMCal with transverse energy of 6 GeV/$c$ < $E_{\rm T}$ < 8 GeV/$c$ originating from "semi-converted" ${\rm \pi^0}$s in pp collisions at 7 TeV compared to Monte Carlo simulation.

Figure 65

 Invariant mass distribution of all reconstructed secondaries (blue) and of the selected photon candidates (red) after all cuts were applied.

Figure 66

 Transverse distribution of the reconstructed photon conversion points for $|\eta|< 0.9$.

Figure 67

 Radial distribution of the reconstructed photon conversion points for $|\eta|< 0.9$ (black) compared to MC simulations performed with PHOJET (red). Distributions for true converted photons are shown in yellow. Physics contamination from true ${\rm \pi^0}$ and $\eta$ Dalitz decays, where the primary $e^+e^-$ are reconstructed as photon conversions, are shown as dashed blue histograms. Random combinatorics and true hadronic background are also shown.

Figure 68

 Invariant mass spectra of photon candidate pairs for pp collisions at 7 TeV by PCM, PHOS and EMCal.

Figure 69

 Invariant mass spectra of photon candidate pairs for 0-10% central Pb-Pb collisions at $\sNN$ = 2.76 TeV by PCM, PHOS and EMCal.

Figure 70

 Reconstructed ${\rm \pi^0}$ peak width (a) and position (b) in pp collisions at $\sqrt{s}$ = 7 TeV for PCM, PHOS, and EMCal compared to Monte Carlo simulations (Pythia for PCM and PHOS, and embedding of clusters from single ${\rm \pi^0}$ in data for EMCal).

Figure 71

 Reconstructed ${\rm \pi^0}$ peak width (a) and position (b) in 0-10% central Pb-Pb collisions at $\sNN$ = 2.76 TeV for PCM, PHOS, and EMCal compared to Monte Carlo simulations (Hijing for PCM, and embedding of clusters from single ${\rm \pi^0}$ in data for PHOS and EMCal).

Figure 72

 Total correction (efficiency and acceptance) for $|y|< 0.5$ for ${\rm \pi^0}$ reconstruction via two-photon invariant mass determination in pp collisions at $\sqrt{s}$ = 7 TeV (top panel) and in 0-10% central Pb-Pb collisions at $\sNN$ = 2.76 TeV (bottom panel) for PCM, PHOS, and EMCal.

Figure 73

 SSh trigger efficiency in pp collisions at $\sqrt{s}$ = 2.76 TeV. Efficiency for single EM clusters (left panel) and reconstructed jets (anti-$\kT$, $R=0.4$, right panel) for data (red points) is well reproduced in simulation (black dashed line). See text for details.

Figure 74

 PYTHIA particle-level simulation of jet-by-jet energy shift due to unobserved contributions from neutrons and $\rm{K^0_{L}}$.

Figure 75

 Probability distribution of $R_{\rm corr}$ (Eq.(18)) for various intervals of $\sum_p$, measured in MB and EMCal-triggered pp collisions, compared to detector-level simulations based on PYTHIA.

Figure 76

 Mean transverse momentum, $\left< \pt\right>$, of constituents measured in reconstructed jets in 2.76 TeV pp collisions (anti-$\kT$, $R=0.4$) vs. jet $\pt$. Left: charged tracks; Right: neutral clusters. Data are shown for MB and SSh triggers, and are compared to detector-level simulations.

Figure 77

 Mean total number of constituents (left) and mean neutral energy fraction (right) measured in reconstructed jets in 2.76 TeV pp collisions (anti-$\kT$, $R=0.4$), vs. jet $\pt$. Data are shown for MB and SSh triggers, and are compared to detector-level simulations.

Figure 78

 Instrumental effects on jet energy measurement (Eq.(19)). Upper panel: jet-by-jet distribution for various intervals in jet $\pt$. Lower panels: Mean and median (left) and standard deviation (right) of these distributions.

Figure 79

 Event display of a central Pb-Pb collision containing a high $\pt$ jet in the EMCal acceptance. The event was triggered using the EMCal SSh trigger.

Figure 80

 Probability distribution of $R_{\rm corr}$ (Eq. (18)) in two different intervals of $\sum_p$, measured in central (0-10%) and peripheral (70%-80%, left panel only) Pb-Pb collisions. Also shown are detector-level simulations for MB pp collisions based on PYTHIA (same distributions as Fig. 75), and for central PbPb collisions based on HIJING (left panel only).

Figure 81

 Invariant mass distribution of $\mu^+ \mu^-$ pairs measured by ALICE for pp collisions at $\sqrt{s}$ = 7 TeV ($\mathcal{L}$ = 1.35 pb$^{-1}$, corresponding to the full 2011 dimuon-triggered data sample).

Figure 82

 Measured muon track reconstruction efficiency in Pb-Pb collisions as a function of the collision centrality.

Figure 83

 Muon spectrometer acceptance times efficiency for $\jpsi$ within $-4.0 < y < -2.5$ during the Pb-Pb 2011 campaign, as a function of the $\jpsi$ transverse momentum (left) or rapidity (right).

Figure 84

 Unlike-sign dimuon trigger efficiency for $\jpsi$, calculated using a realistic (filled squares) and ideal (open squares) chamber efficiency. The ratio of the two curves is shown in the bottom panel.

Figure 85

 Muon spectrometer resolution measured as a function of the centrality of the collision. The main contributions come from the cluster resolution and the residual misalignment of the tracking chambers.

Figure 86

 Invariant-mass distribution of $\mu^+ \mu^-$ pairs in 0-10% most central Pb-Pb collisions at $\sNN$ = 2.76 TeV with the $\jpsi$ peak fitted by an extended Crystal Ball function. The combinatorial background was determined by the event mixing method and subtracted.

Figure 87

 Invariant-mass distribution of $\mu^+ \mu^-$ pairs in Pb-Pb collisions at $\sNN$ = 2.76 TeV with the $\Upsilon(1S)$, $\Upsilon(2S)$, and $\Upsilon(3S)$ peaks fitted by the sum of three extended Crystal Ball functions with identical relative widths and identical relative displacements from the PDG mass values. The tail shape is fixed by the embedding-MC simulation and the combinatorial background is parametrized by an exponential.

Figure 88

 Centrality dependence of $\jpsi$ invariant mass fit parameters.