LHC Is Hinting at a Tantalising New Particle. We May Even Need New Physics to Explain It

There was a huge amount of excitement when the Higgs boson was first spotted back in 2012 – a discovery that bagged the Nobel Prize for Physics in 2013.

The particle completed the so-called standard model, our current best theory of understanding nature at the level of particles.

Now scientists at the Large Hadron Collider (LHC) at Cern think they may have seen another particle, detected as a peak at a certain energy in the data, although the finding is yet to be confirmed.

Again there’s a lot of excitement among particle physicists, but this time it is mixed with a sense of anxiety. Unlike the Higgs particle, which confirmed our understanding of physical reality, this new particle seems to threaten it.

The new result – consisting of a mysterious bump in the data at 28 GeV (a unit of energy) – has been published as a preprint on ArXiv. It is not yet in a peer-reviewed journal – but that’s not a big issue.

The LHC collaborations have very tight internal review procedures, and we can be confident that the authors have done the sums correctly when they report a “4.2 standard deviation significance”.

That means that the probability of getting a peak this big by chance – created by random noise in the data rather than a real particle – is only 0.0013 percent.

That’s tiny – 13 in a million. So it seems like it must a real event rather than random noise – but nobody’s opening the champagne yet.

What the data says
Many LHC experiments, which smash beams of protons (particles in the atomic nucleus) together, find evidence for new and exotic particles by looking for an unusual build up of known particles, such as photons (particles of light) or electrons.

That’s because heavy and “invisible” particles such as the Higgs are often unstable and tend to fall apart (decay) into lighter particles that are easier to detect.

We can therefore look for these particles in experimental data to work out whether they are the result of a heavier particle decay. The LHC has found many new particles by such techniques, and they have all fitted into the standard model.

The new finding comes from an experiment involving the CMS detector, which recorded a number of pairs of muons – well known and easily identified particles that are similar to electrons, but heavier.

It analysed their energies and directions and asked: if this pair came from the decay of a single parent particle, what would the mass of that parent be?

In most cases, pairs of muons come from different sources – originating from two different events rather than the decay of one particle.

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