The particle-smashing machine has fired up again — sparking fresh hope it can find unusual results. What’s more, scientists start with a tantalizing shopping list of anomalous results — many more than at the start of the last run — that hint at where to look for particles outside the standard model.
How the revamped Large Hadron Collider will hunt for new physics
Beams of protons are once again whizzing around its 27-kilometre loop at CERN, Europe’s particle-physics laboratory near Geneva. By July, physicists will be able to switch on their experiments and watch bunches of particles collide.
In its first two stints, in 2009–13 and 2015–18, the LHC explored the known physics world. All of that work — including the triumphant 2012 discovery of the Higgs boson — reaffirmed physicists’ current best description of the particles and forces that make up the Universe: the standard model. But scientists sifting through the detritus of quadrillions of high-energy collisions have yet to find proof of any surprising new particles or anything else completely unknown.
Higher energy, brighter beams, better software for choosing events to analyze, improved instruments...There have been experiments showing hints of leptoquarks and maybe a new lepton, somewhat outside the mass range we can observe directly, plus wrong masses and wrong decay rates among B mesons and W bosons.
After renovations to its particle accelerators, the third version of the LHC will collide protons at 13.6 trillion electron volts (TeV) — slightly higher than in run 2, which reached 13 TeV. The more-energetic smashes should increase the chances that collisions will create particles in high-energy regions where some theories suggest new physics could lie, says Rende Steerenberg, who leads beam operations at CERN. The machine’s beams will also deliver more-compact bunches of particles, increasing the probability of collisions. This will allow the LHC to maintain its peak rate of collisions for longer, ultimately allowing experiments to record as many data as in the first two runs combined.
The LHC can now scan through 40 million collisions per second looking for events of interest, and has vastly improved rendering systems based on Graphics Processing Units produced by semiconductor companies for use with video games. Software has been improved with AI to pick out events outside the normal ranges predicted by the Standard Model, in the hope of picking out the true anomalies that lead to new particles and new physics.
Anomalies
- The mass of the W boson, a fundamental particle that carries the weak nuclear force involved in radioactive decay, is significantly higher than the standard model predicts.
- B-meson decays tend to produce electrons more often than they produce their heavier cousins, muons. The standard model predicts that nature should not prefer one over the other.
- One hypothetical particle has been dubbed the leptoquark, because it would, at high energies, take on properties of two otherwise distinct families of particles — leptons, such as electrons and muons, and quarks.
- A search found intriguing hints of a beyond-standard-model lepton.
- A search for hypothetical heavy, long-lived charged particles found 7 candidates at around 1.4 TeV, around 8 times the energy of the heaviest known particle.
- Collisions between lead ions (which the LHC smashes together when not working with protons) produce quark–gluon plasma. Particles in proton–proton and proton–lead ion collisions show some traits of this state of matter, such as paths that are correlated rather than random.
As far as we know now, any of these could be a chance statistical fluke, a coincidence. But any could be something really new. LHC will give us answers to all of the questions above and more.
Further Upgrades
The ATLAS and CMS experiments now have improved detectors but will not receive major hardware upgrades until the next long shutdown, in 2026. At this point, the LHC will be overhauled to create more focused ‘high-luminosity’ beams, which will start up in 2029. This will allow scientists in the following runs to collect 10 times more collision data than in runs 1 to 3 combined. For now, CMS and ATLAS have got prototype technology to help them prepare.
After 2029, the LHC is scheduled to run for ten years without interruption.