Standard model of particle physics may be broken

(ORDO NEWS) — Professor Roger Jones of the Large Hadron Collider at Kern looks at the future of discoveries in physics.

As a physicist working at the Large Hadron Collider (LHC) at Kern, one of the most common questions I get asked is “When are you going to find something?”.

Resisting the temptation to quip, “Apart from the Nobel Prize-winning Higgs boson and a whole host of new compound particles?” I realize that the reason for the question being raised so often lies in how we present progress in particle physics to the wider world.

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

Recently, however, a series of precise measurements of already known, standard particles and processes threatened to shake physics. And as the LHC prepares to operate at a higher energy and intensity than ever before, it’s time to start discussing the implications broadly.

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

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

Three key takeaways

Particles obey a non-intuitive but hugely successful theory called quantum mechanics. This theory shows that particles too massive to collide directly in the lab can influence what other particles do (through what is called “quantum fluctuations”). Measurements of such effects, however, are very complex and much more difficult to explain to the general public.

But the latest results, hinting at inexplicable new physics beyond the Standard Model, are precisely of this second type.

Detailed studies of the LHCb experiment showed that the 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 into a muon – the electron’s heavier but otherwise identical brother.

According to the Standard Model, this should not happen, which hints that new particles or even forces of nature can influence this process.

However, oddly enough, measurements of similar processes involving “top quarks” carried out by 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 (USA) has recently made very precise studies of how muons “oscillate” as their “spin” (a quantum property) interacts with surrounding magnetic fields.

A slight but significant deviation from some of the theoretical predictions was found, which again suggests that unknown forces or particles may be at work here.

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 years of collecting and analyzing data, an experiment also performed at Fermilab showed it to be significantly heavier than theory predicts – a deviation by an amount that could not happen by chance in more than a million million experiments. Again, it is possible that yet undiscovered particles increase its mass.

Interestingly, however, this is also inconsistent with some of the less accurate LHC measurements.

Although we are not absolutely sure that these effects require a new explanation, there is growing evidence that new physics is needed.

Of course, almost as many new mechanisms will be 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 “superpartner”. These could be additional Higgs bosons (associated with the field that gives the fundamental particles mass).

Others will go further, putting forward ideas that are less fashionable in recent times, such as “technocolor”, which implies the existence of additional forces of nature (besides gravity, electromagnetism, weak and strong nuclear forces), and may mean that the Higgs boson is actually a composite object.

Made up of other particles. Only experiments will show the truth in this matter – which is good news for experimenters.

The experimental groups behind the new results are well-deserved and have been working on these problems for a long time.

However, it is not disrespectful to note that these measurements are extremely difficult to make. Moreover, the predictions of the Standard Model usually require calculations in which approximations must be made.

This means that different theorists may predict slightly different decay masses and rates depending on the assumptions made and the level of approximation. Therefore, it is possible that when we make more accurate calculations, some of the new finds will fit the standard model.

In addition, it may turn out that researchers use completely different interpretations and therefore get conflicting results. Comparing two experimental results requires careful verification that the same level of approximation was used in both cases.

These are both examples of sources of “systematic uncertainty” and while all stakeholders do their best to quantify them, unforeseen complications can arise that underestimate or overestimate them.

All this does not make the current results less interesting or important. The results show that there are many paths to a deeper understanding of new physics, and all of them should be explored.

After the LHC restart, there are still prospects that new particles will be produced by rarer processes or found hidden under a background that we have not yet discovered.


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