The standard model of particle physics may be broken

As a physicist working on the Large Hadron Collider (LHC) at CERN, one of the most frequent questions I get asked is “When are you going to find something?” Resisting the urge to sarcastically reply “Apart from the Higgs boson, which won the Nobel Prize, and a host of new composite particles?”, I realize that the reason the question is asked so frequently is due to to how we have portrayed progress in particle physics to the rest of the world.

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

Recently, however, a series of precise measurements of known standard bog particles and processes have threatened to shake up physics. And with the LHC gearing up to run with greater energy and intensity than ever before, it’s time to start discussing the implications at length.

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

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

Three key findings

The particles obey a counterintuitive but hugely successful theory called quantum mechanics. This theory shows that particles too massive to be created directly in a laboratory 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 that hint at unexplained new physics beyond the Standard Model are of this second kind. Detailed studies of the LHCb experiment found that a particle known as a beauty quark (quarks make up the protons and neutrons in the atomic nucleus) “decays” (falls apart) into an electron much more frequently than into a muon: the electron is heavier, but otherwise identical, bro. According to the standard model, this should not happen, suggesting that new particles or even forces of nature may influence the process.

Image of the LHCb experiment.
LHCb experiment. Image Credit: Cern

Interestingly, however, measurements of similar processes involving the “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 has recently done very precise 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 that unknown forces or particles may be at work.

The latest surprising result is a measure 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 theory predicts, deviating by an amount that would not happen by chance in more than a million experiments. Once again, it may be that as yet undiscovered particles add to its mass.

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

The verdict

While we’re not absolutely sure these effects require a novel explanation, evidence seems to be mounting that some new physics is needed.

Of course, there will be almost as many new mechanisms proposed to explain these observations as there are theoretical ones. Many will look for various forms of “supersymmetry”. This is the idea that there are twice as many fundamental particles in the Standard Model as we thought, and each particle has 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 recent buzzwords like “technicolor,” which would imply that there are additional forces of nature (besides gravity, electromagnetism, and the strong and weak nuclear forces), and could mean that the Higgs 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.

All the experimental teams behind the new findings are highly respected and have been working on the problems for a long time. That said, it is not disrespectful to them to point out that these measurements are extremely difficult to perform. In addition, the predictions of the standard model generally require calculations in which approximations must be made. This means that different theorists may predict slightly different masses and decay rates depending on the assumptions and the level of approximation made. So it may be that when we do more precise calculations, some of the new findings fit the standard model.

Similarly, it may be that the researchers are using subtly different interpretations and therefore finding inconsistent results. Comparison of two experimental results requires careful verification that the same level of approximation has been used in both cases.

Both are examples of sources of “systematic uncertainty” and while everyone involved does their best to quantify them, unforeseen complications can arise 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 all need to be explored.

With the LHC restarting, there are still prospects of new particles being produced through rarer processes or found hidden under backgrounds that we have yet to unearth.The conversation

This article is republished from The conversation under a Creative Commons license. Read the original article.

Image credit: Reidar Hahn/wikipedia, CC BY-SA

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