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

As a physicist at the Large Hadron Collider (LHC) in Cern, one of the most common questions I get asked is, “When are you going to find something?” Resist the temptation to reply sarcastically, “Aside from the Nobel Prize-winning Higgs boson, and a whole host of new composite particles?” I realize the reason the question is asked so often has to do with how we’ve portrayed advances in particle physics to the wider world.

We often talk about progress in terms of discovery new particles, and it often is. Studying a new, very heavy particle helps us to view underlying physical processes, often without disturbing background noise. This makes it easy to explain the value of the find to the public and politicians.

Recently, however, a series of accurate measurements of already known, swamp-standard particles and processes threaten to shake up physics. And while the LHC gets ready to run at higher energy and intensity than ever before, it’s time to broadly discuss the implications.

in truth, particle physics has always proceeded in two ways, one of which is new particles. The other is by taking very precise measurements that test the predictions of theories and look for deviations from what is expected.

The early evidence for Einstein’s theory of general relativityfor example, by discovering small anomalies in the apparent positions of stars and by the movement of Mercury in its orbit.

Three key findings

Particles obey a counterintuitive but wildly successful theory, quantum mechanics. This theory shows that particles far too massive to be created directly in a lab collision can still influence 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 suggesting unexplained new physics beyond the Standard Model are of this second type. Detailed studies from the LHCb experiment have shown that a particle known as a beauty quark (quarks make up the protons and neutrons in the atomic nucleus) “decays” (breaks apart) in an electron much more often than in a muon — it heavier, but otherwise identical, sibling. According to the Standard Model, this shouldn’t happen – an indication that new particles or even forces of nature can influence the process.

Intriguingly, however, measurements of similar processes involving “top quarks” from the ATLAS experiment at the LHC show this decay happens at equal speeds for electrons and muons.

Meanwhile, the Muon g-2 experiment at Fermilab in the US recently very accurate studies of how muons “wobble” as their “spin” (a quantum property) interacts with surrounding magnetic fields. It found a small but significant deviation from some theoretical predictions, again suggesting unknown forces or particles at work.

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

LHCb experiment. Credit: Cern

The last surprising result is a measurement of the mass of a fundamental particle called the W bosonwho wears the weak nuclear force that controls radioactive decay. After many years of data collection and analysis, the experiment, including at Fermilab, suggests it is significantly heavier than the theory predicts — with an aberration that wouldn’t happen by chance in more than a million million experiments. Again, it may be that undiscovered particles are increasing its mass.

Interestingly, however, this is also inconsistent with some lower precision measurements of the LHC (presented in this study and this one

The verdict

While we’re not absolutely sure that these effects require a new explanation, evidence seems to be growing that new physics are needed.

Of course, almost as many new mechanisms will be proposed to explain these observations as there are theorists. Many will look at various forms of “supersymmetryThis is the idea that there are twice as many fundamental particles in the Standard Model than we thought, with each particle having a ‘super partner’. These could be additional Higgs bosons (associated with the field that gives fundamental particles their mass).

Others go further than this, invoking less recently fashionable ideas such as “technicolor“, which would imply that there are additional forces of nature (besides gravity, electromagnetism and the weak and strong nuclear forces), and could mean that the Higgs boson is in fact a composite object made of other particles. Only experiments will reveal the truth of the matter – which is good news for experimenters.

The experimental teams behind the new findings are all respected and have worked on the issues for a long time. That said, it’s no disrespect to them to note that these measurements are extremely difficult to make. In addition, Standard Model predictions usually require calculations that involve making approximations. This means that different theorists can predict slightly different masses and rates of decay depending on the assumptions and the level of approximation made. So it may be that as we make more accurate calculations, some of the new findings will fit the standard model

Likewise, the researchers may use subtly different interpretations to find inconsistent results. To compare two experimental results, it is necessary to check carefully whether the same level of approach was used in both cases.

These are both examples of sources of ‘systematic uncertainty’, and while everyone involved does their best to quantify them, there can be unforeseen complications that they underestimate or overestimate.

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

With the restart of the LHC, there are still prospects of new particles being created by rarer processes or hidden under backgrounds we have yet to excavate.


Responding to the interaction of the Higgs boson with the charm quark


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