Project HiLumi LHC (2): What will the more powerful CERN accelerator reveal?

“It’s as if you were replacing a 50-watt light bulb in your living room with a 500-watt one,” says a CERN technician in one of the many promotional videos made for the high-luminosity LHC accelerator. “With much more light, you can see much more.’”

With the ongoing upgrade of the LHC to a high-luminosity LHC, physicists suddenly have an LHC that’s ten times brighter. That means ten times more experimental data to test existing theories and search for as-yet-undiscovered details and possible new particles.

Both of CERN’s large detectors—ATLAS and CMS—have outlined their physics expectations for the HiLumi LHC in the recent European Strategy Update. The main message is that the HiLumi LHC promises a wide range of new physics.

Higgs boson physics remains the primary goal of the HiLumi program. The particle was discovered at CERN in 2012, providing evidence for the existence of a field that gives particles their mass. However, countless fundamental questions remain about the particle itself.

Over its entire lifetime, the HiLumi LHC is estimated to produce 380 million Higgs particles for study in ATLAS and CMS. This wealth of data is sufficient to obtain a relatively sharp picture of unusual but extremely rare Higgs decays into muons or Z particles.

Significantly better statistics are also expected in the measurements of the Higgs particle’s interactions with other elementary particles from the Standard Model, with uncertainties of only about one percent for W and Z bosons, photons, gluons, tau particles, and the top quark. ATLAS and CMS expect that experimental measurement errors will no longer be the limiting factor; rather, it is theoretical uncertainties that will constrain the analyses.

It is also expected that the abundance of Higgs particles from the HiLumi LHC will provide a better picture of the Higgs self-coupling—the extent to which Higgs particles interact with each other. This can be studied through proton-proton collisions in which two Higgs particles are produced simultaneously, known as di-Higgs events.

Cautious indications of this have already been observed in the LHC, but the HiLumi LHC is expected to provide hard evidence (a solid 7-sigma confidence level) for di-Higgs events. The high event rate makes it possible to study this phenomenon with greater precision.

It is expected that even the simultaneous production of three Higgs particles in collisions will become visible with reasonably high precision in the abundance of measurement data from the HiLumi LHC. Such extremely rare events provide important clues about the Higgs potential—the mathematical form of the Higgs field that gives particles mass.

In the Standard Model, the simplest version of this is assumed, often depicted as a Mexican hat. Deviations from that simple shape could have major implications for particle theory and even for cosmology. In addition, with more Higgs particles, the width of the Higgs potential can be determined much more precisely, providing a more exact measure of the particle’s extremely short lifetime (on the order of 10^-22 seconds).

Proton collisions yield much more than just Higgs particles. For example, measurements of the interactions between produced W and Z bosons will, for the first time, provide a direct test of the Higgs mechanism, which describes how a universal field gives the elementary particles in the Standard Model their specific masses. The Higgs particle is a manifestation of this, but the mass mechanism itself remains unproven.

Using data from the HiLumi LHC, physicists can also search for extremely rare particle processes involving top quarks, the heaviest quarks in the Standard Model. In particular, processes involving four top quarks and those involving a top quark, a Z particle, and an antitop quark have attracted the interest of theorists, as these are highly sensitive to any unknown new particles. Such particles are too heavy to be detected directly, but they can sometimes influence measurable processes.

In a number of programs, ATLAS and CMS are constantly searching for clues to as-yet-unknown particles and forces. In some cases, this will require a combination of data from ATLAS and CMS—in principle, two independent experiments at the CERN accelerator.

According to the report on the European Strategy, measurements using the intense beams of the HiLumi LHC could also provide a test for theories about the early universe and the formation of matter during that phase. The formation of matter may have begun with a phase transition, but this is inconsistent with the relatively large mass of the Higgs boson. New data are expected to test theories regarding such phase transitions.

The HiLumi LHC, it is expected, will open a new chapter in particle physics with its statistical precision. However, what a more powerful CERN accelerator will not provide is higher collision energy. Just as in the LHC, this will remain at a maximum of 14 TeV—a record, but also a limitation in the search for heavier particles.

Another persistent obstacle is that the entire physics program of the HiLumi LHC, just like that of the LHC, relies on colliding protons—composite particles that inevitably produce messier collisions than point particles such as electrons. Only when CERN actually builds the planned FCC electron accelerator will it be possible to study the particle world directly at higher energies.