CERN Accelerating science

New season starts for the LHC experiments

by Panos Charitos

The last 2017 proton-proton collisions took place on 28 November and the LHC machine was shut down during the winter period to allow for planned technical interventions while the LHC experiments profit from this period to perform maintenance work on many sub-detectors.

Four months later proton beams were back at the LHC. The re-commissioning of the accelerator has proceeded very smoothly and first collisions arrived earlier than initially expected. Last April, a small number of bunches were injected to deliver test collisions inside the four LHC experiments. LHC operators declared stable beams on the 17 April and over the next days they stepwise increased the number of protons bunches per beam. On the 28 April they reached 1200 bunches. This  was a crucial step in the intensity ramp up of the LHC towards the optimal running configuration – which foresees 2556 bunches per beam.

ATLAS and CMS, the two “general-purpose” detectors, will continue to probe the properties of the Higgs boson as its peculiar properties call for further exploration including its interaction with all the other particles of the Standard Model.

An event recorded by ATLAS earlier in April, from some of the first collisions of the year with three proton bunches circulating in the LHC (Image: ATLAS/CERN)

Since the Higgs boson discovery, physicists have studied its behaviour and interactions with other particles, which have so far shown good agreement with the Standard Model. Searches will also continue for supersymmetric partners of the familiar bosons and fermions that are predicted to exist by a family of theories known as supersymmetry, which might provide us with a candidate for a dark-matter particle. ATLAS, CMS and LHCb are also searching for hints of dark matter through other means, and will add the forthcoming trove of data to their stockpiles as they advance their explorations.

Collisions with 1200 bunches of protons in each beam, recorded by the CMS detector in April 2018. The yellow lines represent reconstructed particle trajectories in the tracker, the green and blue rectangles represent energy deposits in the calorimeters, and the long red lines represent reconstructed muon trajectories (Image: Tom McCauley/CMS/CERN).

Among other searches, LHCb will continue to seek a solution to the problem of matter-antimatter asymmetry, as the Standard Model cannot adequately explain the observed abundance of matter in the universe. When matter was formed in the Big Bang, there should have been an equal amount of antimatter accompanying it; each matter-antimatter pair should then have annihilated upon contact, leaving us with a universe without stars or human beings to observe them.

ALICE, the LHC’s heavy-ion specialist, focuses on collisions of lead nuclei in order to study the strong interaction and the quark-gluon plasma. It also records proton-proton collisions to continue its investigation of the properties of collision events that contain a large number of particles produced at the same time and to serve as a baseline with which to compare lead-lead collisions.

A proton-proton collision event at a centre-of-mass energy of 13 TeV, recorded by ALICE on 30 April 2018, one of the first with proton beams containing 1200 bunches (see image above). Over the last month, ALICE carried out a few tests, starting from the timing calibration of some of the subdetectors. Data were taken with a set of triggers and special setups, as requested by some detector groups: in particular, the Electromagnetic calorimeter (EMCal) and the Di-Jet Calorimeter (DCal). A few more special setups will be used in the following weeks, before starting a long data taking period with a fixed trigger configuration, which will hopefully last for some months.

The LHC operators will keep ramping up the number of bunches, aiming to hit 2556 bunches in total. This will help them achieve their target of 60 inverse femtobarns (fb-1) of proton-proton collisions this year delivered to both ATLAS and CMS, 20% more than the 50 fb-1 achieved in 2017. In simple terms, each inverse femtobarn can correspond to up to 100 million million (1014) individual collisions between protons. The proton-proton run will be followed by the first heavy-ion run since 2016; the LHC will inject and collide lead nuclei at the end of the year.

On 5 May, during the last steps of the intensity ramp-up, when the average peak luminosity for ATLAS and CMS was close to 2.1 x 1034 cm-2s-1, while on the 11th of May, the integrated luminosity for ATLAS and CMS was already at 6.91 fb-1 (of the 60 fb-1 planned for 2018) equalling or even surpassing the record peak luminosity of 2017. From now on, the LHC is in production mode for physics. The operation of the machine will be consolidated in parallel, meaning that the machine settings will be further tweaked, beam life times optimised, performance stabilised and, if possible, increased. 

This is the last year with collisions before the LHC enters a period of hibernation until spring 2021 (Long Shutdown 2), during which the machine and the experiments will be upgraded. All four experiments will therefore hope to maximise their data-collection efficiency to keep themselves occupied with many analyses and new results over the two-year shutdown, using high-quality data collected this year.