More on the SMOG system

Between 2015 and 2018, the SMOG system collected data samples with circulating proton and lead ions and with injected helium, argon and neon at the highest energy ever reached for a fixed-target experiment (Fig. 1) . The analysis of such samples is giving unprecedented experimental inputs covering different fields of interest, such as heavy ion, cosmic-ray physics, nucleus characterization and more.

Figure 1: Physics sample collected between 2015 and 2018 exploiting the circulating LHC proton and lead beams and the injection of helium, neon and argon. The colour indicates the beam energy, the y axis the number of particles impinging on the targets and the x axis the data-taking year and the nature of the sample (the colliding particle on the left and the target element on the right)

The gas was free to spread in 40 metres around the LHCb nominal interaction point (Fig. 2), where pumps serving the VELO detector module extracted it. Because of such a wide covered region, the pressure and the gas species had to be limited for safety reasons. Also, the overlap between the beam-beam and beam-gas interaction regions forced LHCb beam-gas data to be collected in dedicated short periods or simultaneously with the beam-beam ones but only with a small fraction of the LHC bunches.   

Figure 2: Gas injection in proximity of the LHCb beam-beam interaction point (IP) with the SMOG, used in 2011-2018, and the SMOG2 systems, which will operate from 2022. The corresponding gas target pressures are indicated.

Within the new SMOG2 cell, whose installation upstream the nominal LHCb interaction point (Fig. 2) was accomplished in August 2020, the gas target area density could be increased by up to two orders of magnitude. The cell is composed of two retractable halves (Fig. 3), following the opening and closing procedures of the VELO detector during beam development periods and the nominal data-taking, respectively. The cell material is appropriately treated to avoid instabilities, while the new gas injection system will be able to fully replace the gas in some minutes and to inject all noble gases, also the heavy species such as Kr and Xe, and H2, D2, O2  and more. The separation between the beam-beam and beam-gas interaction regions firstly opens the possibility to simultaneously acquire data in two collision systems at different energies. The related physics prospects offer a further extension of the LHC complex physics reach, pioneering what is indicated as a key objective for the future in the 2020 European Strategy for Particle Physics Update. A new window on QCD studies will be opened, precisely addressing the quark and gluon content of the nucleons and providing unique experimental inputs to the heavy-ion and cosmic-ray phenomenology. With the gas injection, a unique laboratory for QCD studies at the LHC will operate from 2022. Stay tuned!

Figure 3: Opening procedure of the SMOG2 cell, ten times faster than in reality.