The collaboration shows how proton–proton collisions at the Large Hadron Collider can reveal the strong interaction between composite particles called hadrons.
In a paper published today in Nature, the ALICE collaboration describes a technique that opens a door to high-precision studies at the Large Hadron Collider (LHC) of the dynamics of the strong force between hadrons.
Hadrons are composite particles made of two or three quarks bound together by the strong interaction, which is mediated by gluons. This interaction also acts between hadrons, binding nucleons (protons and neutrons) together inside atomic nuclei. One of the biggest challenges in nuclear physics today is understanding the strong interaction between hadrons with different quark content from first principles, that is, starting from the strong interaction between the hadrons’ constituent quarks and gluons.
Calculations known as lattice quantum chromodynamics (QCD) can be used to determine the interaction from first principles, but these calculations provide reliable predictions only for hadrons containing heavy quarks, such as hyperons, which have one or more strange quarks. In the past, these interactions were studied by colliding hadrons together in scattering experiments, but these experiments are difﬁcult to perform with unstable (i.e. rapidly decaying) hadrons such as hyperons. This difficulty has so far prevented a meaningful comparison between measurements and theory for hadron–hadron interactions involving hyperons.
Enter the new study from the collaboration behind ALICE, one of the main experiments at the LHC. The study shows how a technique based on measuring the momentum difference between hadrons produced in proton–proton collisions at the LHC can be used to reveal the dynamics of the strong interaction between hyperons and nucleons, potentially for any pair of hadrons. The technique is called femtoscopy because it allows the investigation of spatial scales close to 1 femtometre (10−15 metres) – about the size of a hadron and the spatial range of the strong-force action.
This method has previously allowed the ALICE team to study interactions involving the Lambda (Λ) and Sigma (Σ) hyperons, which contain one strange quark plus two light quarks, as well as the Xi (Ξ) hyperon, which is composed of two strange quarks plus one light quark. In the new study, the team used the technique to uncover with high precision the interaction between a proton and the rarest of the hyperons, the Omega (Ω) hyperon, which contains three strange quarks.
“The precise determination of the strong interaction for all types of hyperons was unexpected,” says ALICE physicist Laura Fabbietti, professor at the Technical University of Munich. “This can be explained by three factors: the fact that the LHC can produce hadrons with strange quarks in abundance, the ability of the femtoscopy technique to probe the short-range nature of the strong interaction, and the excellent capabilities of the ALICE detector to identify particles and measure their momenta.”
“Our new measurement allows for a comparison with predictions from lattice QCD calculations and provides a solid testbed for further theoretical work,” says ALICE spokesperson Luciano Musa. “Data from the next LHC runs should give us access to any hadron pair.”
“ALICE has opened a new avenue for nuclear physics at the LHC – one that involves all types of quarks,” concludes Musa.
Photos of ALICE detector
Videos of ALICE detector
Outside drone footage https://videos.cern.ch/record/2027842
Views inside the detector https://videos.cern.ch/record/1987362
ALICE experiment https://home.cern/science/experiments/alice
Recreating Big Bang matter on Earth https://home.cern/news/series/lhc-physics-ten/recreating-big-bang-matter-earth
This picture shows the ALICE Miniframe being reinstalled on the detector in mid-November 2020, after a two-year-long stay at the surface for upgrades.
Weighing in at 14 tonnes and measuring 12 metres long, the Miniframe is anything but “mini”: it is a giant “plug” – a large metallic support structure, installed in front of the A-side of the ALICE Time Projection Chamber (TPC) and sitting partially in the L3 magnet. It has supported ALICE’s systems since the detector’s debut, carrying the services for the TPC and ITS (Inner Tracking System), such as power supply, cooling, gas, detector control, detector safety, trigger and data acquisition. It also houses the ALICE forward detectors, FIT-A (Fast Interaction Trigger A), the ALICE RB24 beampipe, and the compensator magnet. The Miniframe was designed to be easily removable in case the TPC needed to be extracted during long shutdown (LS) periods.
This came in handy during LS2, as the TPC was temporarily removed from the cavern for upgrades. In January 2019, the Miniframe was brought to the SX2 surface hall, where it received new services and patch panels for the new ALICE tracker, the ITS2, including kilometres of new cables for the ITS2 and TPC. The support structures and cable trays were re-engineered to accommodate the cables, including the 7000 optical fibres needed to allow continuous readout from the TPC and ITS2.
The upgrades were completed just in time for the reinstallation in November 2020, when the fully refurbished Miniframe was lowered back into the cavern and inserted in front of the detector. Since then, work has been continuing to connect the services, with a view to getting the TPC operating by the end of the year.
MADMAX is preparing for a stopover at CERN from 2022. Mel Gibson, his artillery and quest for revenge will not be there, but instead a handful of physicists armed with an aged magnet will be searching for dark matter in CERN’s North Area (not to be confused with a post-apocalyptic wasteland).
Indeed, the MADMAX collaboration (MAgnetized Disks and Mirror Axion eXperiment, external to CERN), humbly proposes to identify the nature of dark matter and to solve the enigma of the absence of CP symmetry violation in the strong sector, while detecting a particle that has eluded physicists for decades: axions.
To do so, the collaboration has developed a brand-new concept using a booster composed of dielectric disks and mirrors. The booster acts as a resonator to amplify the photon flux that hypothetical axions would produce under a magnetic field, if these axions exist. In order to validate the concept, a prototype needs to be tested under a magnetic field before the launch of the experiment, planned to be located at DESY in Germany.
Although such a magnetic field is difficult to obtain, the collaboration can rely on CERN's assistance. On 16 September, CERN's Research Board agreed that the MADMAX prototype could use an old magnet previously used by the ATLAS experiment. The “Morpugo” magnet is located in the North Area, generates a field of up to 1.6 Tesla, and although it arrived at CERN in 1979, is still used to test ATLAS subdetectors. MADMAX physicists will jump in to mount and test their prototype during the inter-beam period, when ATLAS is not using the magnet. A solution that suits everyone: for MADMAX, a magnet that meets the prototype's criteria is provided free of charge, and for ATLAS, the space around the magnet is reorganised and optimised, which is necessary for the installation of the prototype.
The recycling and repurposing of equipment is common at CERN, in the spirit of pragmatism and sustainability. With successive generations of equipment, state-of-the-art accelerators go on to become injectors for their successors, and old magnets are reused for new experiments. This is the case, for example, with the CAST experiment, which uses an old LHC dipole prototype in its search for, once again, axions.
However, allowing external researchers to use CERN equipment, as in the case of MADMAX, is far from trivial. According to Pascal Pralavorio, the MADMAX contact person at CERN, this helps to develop new ideas: "Today, particle physicists are searching for new physics in many different directions, which naturally leads to experiments based on novel concepts. To validate them, we must make the most of the equipment that’s already available, and that is what MADMAX and CERN are doing with the Morpugo magnet.”
CERN's endeavours to benefit science around the world have long been visible whether through collaborations, prototyping, donating equipment and more, and this is set to continue. Although we don’t need another hero, we wish the MADMAX researchers well in their quest for axions.
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On 23 September, the ALICE collaboration celebrated its best PhD theses, which are selected based on the excellence of the results obtained, the quality of the thesis manuscript, and the importance of the contribution to the collaboration.
Out of the 11 outstanding PhD theses received by the selection committee, two theses stood out: Fabrizio Grosa’s, entitled “Strange and non-strange D-meson production in pp, p-Pb, and Pb-Pb collisions with ALICE at the LHC”, and Arild Velure’s - “Design, Verification and Testing of a Digital Signal Processor for Particle Detectors”.
The winners were congratulated by ALICE Spokesperson, Luciano Musa, the Collaboration Board Chair, Silvia Masciocchi, and the Chairs of the Thesis Committee, Giuseppe Bruno and Philippe Crochet. Luciano awarded the certificates and prizes to Fabrizio and Arild, who then presented their work.
Fabrizio Grosa (Turin Polytechnic) analysed vast data on the production of several particle species in different colliding systems as part of his doctoral research. His results contributed to five published ALICE papers, with two more on the way. Fabrizio has also made significant contributions to the upgrade project of the ALICE Inner Tracking System for the forthcoming LHC runs, working on alignment procedures and physics performance studies.
Hailing from Bergen University, Arild Velure worked on the design of the so-called SAMPA ASIC, a complex mixed-signal chip that has become the state-of-the-art readout for gaseous detectors like the ALICE Time Projection Chamber and the muon tracking chambers. He made significant contributions to the ASIC specifications as well as to the design and implementation of the detector front-end cards. Arild’s research is of paramount importance to the success of the forthcoming ALICE high-rate data-taking campaign.