The standard model of particle physics can be broken, says the expert

As a physicist working on the Large Hadron Collider (LHC) in Cern, one of the most common questions I am asked is “When will you find something?” Resisting the temptation to sarcastically answer “Apart from the Higgs boson, which won the Nobel Prize, and read through new composite particles?” I understand that the reason why the question is so often asked is how we have shown advances in particle physics to the wider world.

We often talk about progress in terms of discovering new particles, and often it is. Studying new, very heavy particles helps us see the underlying physical processes – often without annoying background noise. This makes it easier to explain the value of the discovery to the public and politicians.

Recently, however, a series of precise measurements of already known particles and processes standard for wetlands have threatened to shake up physics. And as the LHC prepares to work with more energy and intensity than ever before, it’s time to start discussing the implications widely.

True, particle physics has always taken place in two ways, one of which is new particles. The second is to perform very precise measurements that test the predictions of theories and look for deviations from expectations.

Early evidence for Einstein’s general theory of relativity, for example, came from the discovery of small deviations in the apparent positions of stars and from the motion of Mercury in its orbit.

Three key findings

Particles obey a counterintuitive but extremely successful theory called quantum mechanics. This theory shows that particles that are too massive to be made directly in a laboratory collision can still affect what other particles do (through something called “quantum fluctuations”). However, measurements of such effects are very complex and much more difficult to explain to the public.

But recent results pointing to inexplicable new physics beyond the standard model are these other types. Detailed studies from the LHCb experiment found that a particle known as a beauty quark (quarks make protons and neutrons in the atomic nucleus) “decays” (decays) into an electron much more often than a muon – the electron is heavier but otherwise identical, brother and sister. According to the standard model, this should not happen – suggesting that new particles or even forces of nature may affect the process.

Intriguingly, however, measurements of similar processes involving “top quarks” from the ATLAS experiment at the LHC show that this decay occurs at the same rate for electrons and muons.

Meanwhile, the Muon g-2 experiment at Fermilab in the US recently did very precise studies on how muons “oscillate” while their “spin” (quantum property) interacts with surrounding magnetic fields. A small but significant deviation from some theoretical predictions has been found – again suggesting that unknown forces or particles may be at work.

The standard model of particle physics can be broken, says the expert

LHCb experiment. Credit: Cern

The last surprising result is the measurement of the mass of a fundamental particle called the W boson, which carries a weak nuclear force that controls radioactive decay. After many years of data collection and analysis, the experiment, also at Fermilab, suggests it is significantly heavier than the theory predicts – deviating by an amount that would not have happened by chance in more than a million million experiments. Again, it may be that undiscovered particles add to its mass.

Interestingly, however, this is also inconsistent with some lower-precision measurements from the LHC (presented in this and this study).


Although we are not entirely sure that these effects require a new explanation, there seems to be growing evidence that some new physics is needed.

Of course, almost as many new mechanisms will be proposed to explain these observations as there are theorists. Many will look for various forms of “supersymmetry”. It is the idea that in the standard model there are twice as many fundamental particles than we thought, with each particle having a “super partner”. These may include additional Higgs bosons (associated with the field that gives the particles their mass).

Others will go beyond this, referring to less modern ideas such as “technicolor”, which would imply that there are additional forces of nature (besides gravity, electromagnetism and weak and strong nuclear forces), and could mean that the Higgs boson is actually a composite object made of other particles. Only experiments will reveal the truth – which is good news for experimenters.

The experimental teams behind the new discoveries were appreciated and worked on the problems for a long time. However, it is not disrespectful to note that these measurements are extremely difficult to perform. Moreover, standard model predictions usually require calculations where approximations must be made. This means that different theorists can predict slightly different masses and decay rates depending on the assumptions and the level of approximation made. So, it may happen that when we do more accurate calculations, some of the new discoveries will fit into the standard model.

Equally, it may be that researchers use subtly different interpretations and thus find inconsistent results. A comparison of the two experimental results requires careful verification that the same level of approximation was used in both cases.

These are both examples of sources of “systemic uncertainty,” and while everyone involved is doing their best to quantify them, unforeseen complications can arise that underestimate or overestimate them.

None of this makes the current results less interesting or important. What the results illustrate is that there are multiple paths to a deeper understanding of new physics and all need to be explored.

With the restart of the LHC, there is still the prospect of creating new particles through rarer processes or hidden beneath a background yet to be excavated.

Focusing on the interaction of the Higgs boson with charm quark

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