Life is good at 13 TeV

The second run of the LHC will start in 2015, when the beams of protons will reach 6.5 TeV of energy and the collisions will happen at 13 TeV. What will physicists look for after the discovery of the Higgs boson?

Life beyond the discovery of the Higgs boson is full of exciting possibilities. “When we were looking for the Higgs boson, we knew it would be difficult to discover but we also knew where to search and how,” says Ignatios Antoniadis, Head of the Theory Unit of CERN’s Physics Department. “The electroweak precision tests of LEP and other experiments had already suggested that the Higgs boson should exist and should be light, not very far from the LEP boundary. Now that we have completed the picture of the Standard Model, we are entering a totally new territory.”

The new territory is what scientists call “beyond the Standard Model” physics. It is populated by open-ended questions, including: What is Dark Matter made of? Why is the universe made of just matter and no anti-matter? Are there any new scalar bosons similar to the Higgs? How can the difference in masses and mixings between quarks and leptons of different generations be explained? Does Supersymmetry exist in nature? Does the graviton exist and why is gravity so weak compared to the other known fundamental interactions? Are we living in a world with 10 or 11 dimensions?

These questions are extremely difficult to address, both from an experimental and a theoretical point of view. However, physicists do have an ace in the hole. “Although each of these phenomena could be interpreted with a variety of theoretical models, many of them have one thing in common: if they exist in nature, their signature in our detectors will be linked to what we call 'missing energy',” says Antoniadis.

“Missing energy” is what happens when there is a discrepancy between the energy at the beginning and at the end of a given process. “In the Standard Model, this discrepancy is associated with neutrinos, those particles that escape from the detector without leaving any tracks,” explains Antoniadis. “However, in the physics beyond the Standard Model, the missing energy could also be the signature of many undiscovered particles, including the Weakly Interacting Massive Particles (WIMP) that are very strong candidates for Dark Matter. If WIMPs exist, the collisions at 13 TeV may reveal them and this will be another huge breakthrough.”

Since no Supersymmetric theoretical model has yet to be ruled out, the high-energy collisions might also eventually unveil the Supersymmetric partners, at least the lighter ones. The “missing energy” could also account for the graviton escaping in extra-dimensions or for other exotic particles. “We have a variety of possibilities and this implies that we will have to abandon the old paradigm of dealing with experimental data, while having in mind a defined theoretical model that shows us where to look and how,” says Antoniadis. “We’ll have to look at data in a completely unbiased and model-independent way. Thanks to recent studies we now know the Standard Model so well that, if we observe an unknown source of missing energy, we can be confident that this is due to new physics.”

Additional scalar bosons – relatives of the Higgs boson – complete the plethora of opportunities that will open up with the second run of the LHC. “Theory does not constrain the total number of scalar fields that might exist in Nature,” concludes Antoniadis. “Finding the first one required 40 years of technological progress and fundamental research. Who knows? Maybe the next ones do exist and will be easier to find!”

by Antonella Del Rosso