ALICE upgrades during the LHC Long Shutdown 2

A Large Ion Collider Experiment (ALICE) has been conceived and constructed as a heavy-ion experiment at the LHC. During LHC Runs 1 and 2, it has produced a wide range of physics results using all collision systems available at the LHC. In order to best exploit new physics opportunities opening up with the upgraded LHC and new detector technologies, the experiment has undergone a major upgrade during the LHC Long Shutdown 2 (2019-2022). This comprises the move to continuous readout, the complete overhaul of core detectors, as well as a new online event processing farm with a redesigned online-offline software framework. These improvements will allow to record Pb-Pb collisions at rates up to 50 kHz, while ensuring sensitivity for signals without a triggerable signature.


Accepted by: JINST
e-Print: arXiv:2302.01238 | PDF | inSPIRE
Figure group

Figure 1

ALICE 2 detector systems (see legend and text for details).

Figure 3

Time frame and heartbeat frame structure in continuous and triggered mode. HeartBeat (HB) triggers are issued in continuous and triggered modes to all upgraded detectors. Physics triggers can be sent to upgraded detectors in triggered mode and are sent to non-upgraded detectors in all modes. HBF and TF rates are programmable with the following nominal values; HBF: 1 every orbit, $\sim$89.4 ┬Ás/$\sim$10 kHz, TF: 1 TF every 128 HBFs/$\sim$11 ms/$\sim$100 Hz.

Figure 5

Block diagram of the common readout unit (CRU): the CRU forms the interface between the first-level processors (via PCIe), the central trigger system (via TTS), and the detectors (via TTS and FE).

Figure 6

Picture of a CRU; bottom left, FPGA cooling radiator, bottom right, power mezzanine; top row; 3 out of 8 Minipods installed; top left, fiber optics cable to MPO connector on front panel; bottom left, SFP transceivers.

Figure 11

ALPIDE sensor chip detection efficiency and fake-hit rate vs global threshold setting. Beam test results (\SI{6}{\giga eV/\it{c}} pions, orthogonal incidence). ALPIDE substrate reverse bias: \SI{-3}{\volt}.

Figure 12

ALPIDE chip hit-position resolution and average cluster size as a function of global threshold setting. Beam test results with \SI{6}{\giga eV/\it{c}} pions with perpendicular incidence. ALPIDE substrate reverse bias: ${\rm -3 V}$.

Figure 19

Schematic layout of the ITS2. The three innermost layers form the inner barrel, the middle and outer layers form the outer barrel.

Figure 21

Inner HIC seen from the sensor side. The green tabs are used for fixing and handling, and are removed before mounting the HIC.

Figure 22

Top view and bottom view of an outer barrel HIC. The yellow cables connect the HIC to the power bus.

Figure 23

Pictures of the ITS2 assembly. Top left: A view of the MAM used for chip inspection and HIC assembly. Top right: Photo of an outer barrel stave, with power bus cables opened on the two sides. Bottom: Photo of an inner barrel stave, with detailed views shown in the insert.

Figure 24

Space frame and cold plate cooling scheme (Left) Inner Barrel; (Right) Outer Barrel.

Figure 25

Azimuthal distribution of single contributions to the material budget of an inner (left) and outer (right) barrel stave layer. The relative contribution of each component to the total material budget is quoted.

Figure 26

Overview of the mechanical structure of the ITS2. The upper panel shows the Inner Barrel, the Outer Barrel staves and the MFT in the back. The lower panel shows the IB and OB conical structural shells supporting the respective services.

Figure 29

ITS2 in the clean room during on-surface commissioning. The lower left shows a zoomed-in view of the half barrels of the outer barrel (OB) and inner barrel (IB) type.

Figure 30

Fake-hit rate of an inner half-barrel as a function of the number of masked pixels.

Figure 31

Top: Outer barrel surrounding the beam pipe with the Muon Forward Tracker (MFT) in the background. Bottom: ITS2 inner barrel bottom half-barrel next to the beam pipe, outer barrel and MFT in the background.

Figure 32

Event display of a cosmic muon traversing all layers of ITS2 twice, no magnetic field.

Figure 33

Longitudinal distribution of the primary vertex positions from ITS2 tracks reconstructed online during the LHC pilot beam in October 2021.  .

Figure 34

Schematic view of the Muon Forward Tracker (left) and its integration with the central barrel (right).

Figure 36

Example of an assembled MFT ladder. Upper picture: back side of the FPC with the glue spots on which the sensor are glued. Bottom picture: front view of assembled ladder.

Figure 38

Left: Half-disk during ladder gluing. Right: Glue deposition pattern.

Figure 41

Noise occupancy, measured in hit/pixel/event as a function of the number of masked pixels over the whole MFT detector.

Figure 42

Display of the reconstructed muon tracks in the MFT from a TED shot event.

Figure 44

Schematic view of a stack with four GEM foils. The baseline settings for the voltages across the four GEMs, the transfer fields between the GEMs, and the induction field between GEM\,4 and the pad plane are indicated as well.

Figure 45

Energy resolution $\sigma(^{55}\rm{Fe})$ as a function of ion backflow (IBF) in a 4-GEM stack (S-LP-LP-S) in Ne-CO$_2$-N$_2$ (90-10-5) The gas gain is kept at 2000 in all measurements by adjusting the voltages on GEM\,3 and GEM\,4 at a fixed ratio of 0.8 or 0.95. Figure from  .