The Large Hadron Collider (LHC) is the largest and most complex particle accelerator ever built and it’s the flagship experiment of CERN. The LHC has produced many new breakthroughs, but its crowning achievement so far is the discovery of the Higgs boson, the particle that gives everything else mass.
But CERN is a forward-looking institution, so it doesn't just rest on the laurels of its great discoveries. Currently, the LHC is concluding its impressive second run of observations, and researchers are busy analyzing the data coming out of the experiments. And the institution is already thinking about the next phase.
A major overhaul of the facility is planned to begin in just a few years so that the laboratory can continue to make new physics discoveries. How will these changes impact the different experiments? What can we hope to find using the LHC in the near future?
The international laboratory is planning to invest in Hi-Lumi, the nickname for the next major upgrade of the LHC that, by 2025, will become the High-Luminosity LHC. Luminosity in particle accelerators is a value proportional to the number of collisions, so higher luminosity means more collisions and more collisions mean a lot more data. Hi-Lumi will increase the luminosity of the LHC by a factor of 10. This means that it could produce 15 million Higgs bosons every year, compared to the 1.2 million from 2011-2012 when it was discovered.
The increased production of Higgs bosons will be key to probing new areas of physics, as well as improving our understanding of what we know so far. The Higgs boson is a very unstable particle and can decay in multiple ways, and each mode is worth studying in detail.
“We needed more energy and we needed more data. And that’s one of the main topics in the Higgs sector for round two," Dr Richard Polifka, a spokesperson for the ATLAS experiment, told IFLScience. "And of course re-testing all the measurements of the standard model we had so far. “Whenever you reach a new energy frontier, you look for new physics and you re-measure everything we have learned so far.”
ATLAS is one of the two general-purpose experiments in the LHC. The other one is called the CMS (Compact Muon Solenoid). Their job is to detect everything produced in the collisions. The other two experiments are ALICE, which focuses on studying an extreme state of matter known as the Quark-Gluon Plasma, and LHCb, which is being used to discover why the universe is made of matter instead of antimatter.
“The aim in general for all the experiment of the LHC is to look for new physics, the so-called physics beyond the standard model,” Dr Silvia Gambetta, a spokesperson for LHCb, told IFLScience. “LHCb has been working amazingly. The project started with fixed goals but while it was operating we discovered that we could probe so many more areas. That’s because the detector has been performing very well.”
LHCb will be upgraded between 2019 and 2021, during the next long technical shutdown. This will allow the detector to perform even better and deliver more groundbreaking science. Currently, there’s a limit to what it can test. The current level of precision of the natural aging of the experiment doesn't allow us to test certain predictions of the decay of some particles well enough.
“We are very happy with the results but if we want to see new physics we need to upgrade it and make it even better,” Dr Gambetta continued. “The aim of the upgrade is to break this barrier and to get closer to the theoretical predictions. There is no other experiment that could do what LHCb is doing.”
LHCb is not alone in getting a revamp. ALICE will also have some incredible new tech installed to push it beyond its current limits. "The ALICE experiment will have a significant upgrade in 2019-2020, which will increase the data-taking rate by a factor 100 compared to the current situation," Dr Jan Fiete Grosse-Oetringhaus, a spokesperson for ALICE, told IFLScience. "For this purpose, we replace the innermost detector, called the Inner Tracking System, completely. This allows [production of] very thin sensors (50 microns thick, thinner than a human hair) with the advantage that the particles traversing the detector are only minimally perturbed."
After Run 3, which will start in the early months of 2021, the focus will shift to Hi-Lumi. This will be a huge deal and mark a significant overhaul of the LHC. The LHCb collaboration has submitted a proposal for a further upgrade of the detector, which would allow it to withstand the more intense run expected in High-Lumi. ATLAS will also be tech’d up for Hi-Lumi.
“Hi-Lumi will be a major upgrade of the machine, but also of the detector. The central region of the detector will be redesigned and the way data is read out will change as well," Dr Polifka explained. "We will have a lot more signals and we will need a fast way to select the right events to focus on.”
The new chip they are planning to install will have a time response in the order of picoseconds (one-trillionth of a second), which will allow them to select the correct events to focus on since the collisions actually happen on a much slower timescale. For ATLAS and CMS, Hi-Lumi will make it possible to have more statistics on rare Higgs decay and hopefully to discover new physics as well.
“One of the interesting things is the process we call ‘Higgs decaying to invisible’, one of the potential ways to detect dark matter,” Dr Polifka added.
Dark matter particles have mass, so it is reasonable to expect them to interact with the Higgs boson. This is just an intuitive idea rather than soemthing that will definitely be discovered. But the teams are obviously intrigued by the possibility of witnessing these more unusual processes.
"The upgraded ALICE detector together with the high-luminosity upgrade allows for the first time the study of heavy probes (particles known as D and B mesons) traversing the Quark-Gluon Plasma (QGP)," Dr Jan Fiete Grosse-Oetringhaus explained. "The QGP is a state of matter as it existed in the early universe, microseconds after the Big Bang." Studying the movement of particles like the D and B mesons through the QGP makes it possible to characterize the process that occurs during this extreme state of matter.
The next decade is full of promise for the LHC, and discussions have already started on what to do after Hi-Lumi. There are proposals to tweak the LHC to enhance its energy. The High-Energy LHC will require new magnets to increase its energy to twice the current record, which would allow scientists to create even more exotic particles. The future of the most complex experiment on Earth is clearly bright.