The Standard Model of Particle Physics Could Be Broken, Expert Says

As a physicist working at CERN’s Large Hadron Collider (LHC), one of the most common questions I get asked is “When are you going to find something?” Resist the temptation to answer sarcastically “Apart from the Higgs boson, which won the Nobel Prize, and a whole host of new composite particles?” I realize that the reason the question is asked so often has to do with how we have presented advances in particle physics to the rest of the world.

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

Recently, however, a series of precise measurements of particles and processes already known and conforming to bog standards have threatened to shake up physics. And with the LHC poised to run at higher energy and intensity than ever before, it’s time to start discussing the implications broadly.

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

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

Three key findings

Particles obey a counter-intuitive but hugely successful theory called quantum mechanics. This theory shows that particles far too massive to be made directly in a collision in the laboratory can still influence what other particles do (through what are called “quantum fluctuations”). The measures of these effects are, however, very complex and much more difficult to explain to the public.

But recent results hinting at new unexplained physics beyond the Standard Model are of this second type. Detailed studies of the LHCb experiment have revealed that a particle known as the beauty quark (quarks make up the protons and neutrons in the atomic nucleus) ‘decays’ (decays) into an electron much more often than in a muon – the electron is heavier, but otherwise identical, brother. According to the Standard Model, this should not happen, hinting that new particles or even forces of nature could influence the process.

Curiously, however, measurements of similar processes involving ‘top quarks’ from the ATLAS experiment at the LHC show that this decay occurs at equal rates for electrons and muons.

Meanwhile, the Muon g-2 experiment at Fermilab in the US recently carried out very precise studies of how muons “oscillate” when their “spin” (a quantum property) interacts with surrounding magnetic fields. He found a small but significant deviation from some theoretical predictions, again suggesting that unknown forces or particles might be at work.

LHCb experiment. Credit: CERN

The latest surprising result is a measurement of the mass of a fundamental particle called the W boson, which carries the weak nuclear force that governs radioactive decay. After many years of data collection and analysis, the experiment, also at Fermilab, suggests that it is significantly heavier than the theory predicts, deviating by an amount that would not occur by chance in more of a million million experiences. Again, there may be as yet unknown particles adding to its mass.

Interestingly, however, this also disagrees with some lower precision LHC measurements (presented in this study and this one).

The verdict

While we’re not absolutely certain that these effects require a new explanation, it seems increasingly clear that new physics is needed.

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

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

The experimental teams behind the new findings are all highly respected and have been working on the problems for a long time. That said, it is not disrespectful to them that these measures are extremely difficult to do. In addition, 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 level of approximation made. So it may be that when we do more precise calculations, some of the new findings fit the Standard Model.

Likewise, researchers may use subtly different interpretations and thus find inconsistent results. Comparing two experimental results requires careful verification that the same level of approximation has been used in both cases.

These are two examples of sources of “systematic uncertainty”, and although all parties involved do their best to quantify them, there can be unforeseen complications that underestimate or overestimate them.

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

With the restart of the LHC, there are still chances that new particles will be made by rarer processes or found hidden under backgrounds that we have not yet discovered.


Focus on the interaction of the Higgs boson with the charm quark


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