Why Scientists Think Physics May Be in a Reckoning

Why Scientists Think Physics May Be in a Reckoning

As a physicist working on the Large Hadron Collider (LHC) at CERN, one of the most frequently asked questions I am asked is, “When are you going to find something?” Resisting the temptation to sarcastically reply, “Apart from the Nobel Prize-winning Higgs boson, and a slew of new composite particles?” I realize that the reason the question is asked so often is because of the way we portray the progress of particle physics to the wider world.

We often talk about progress in terms of discovering new particles, and it often is. Studying a new, very heavy particle helps us visualize 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 particles and processes already known and standardized 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 broadly discussing the implications.

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

The first evidence of 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 main findings

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

But recent results that suggest 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” (breaks apart) into an electron much more often than into a muon – the heaviest electron . , but otherwise identical, brother. According to the standard model, this should not happen, suggesting that new particles or even forces of nature can influence the process.

LHCb experiment.Cern

Interestingly, though, measurements of similar processes involving “top quarks” from the ATLAS experiment at the LHC show that this decay happens at equal rates for electrons and muons.

Meanwhile, the Muon g-2 experiment at Fermilab in the US recently made very accurate studies of how muons “oscillate” as 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.

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 theory predicts – deviating from an amount that would not happen by chance in over a million experiments. Again, it could be that as-yet-undiscovered particles are increasing in mass.

Interestingly, however, this also disagrees with some of the low-precision measurements of the LHC (featured in this study and this one).

The verdict – While we’re not absolutely sure these effects require a new explanation, evidence seems to be growing that some 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 look at various forms of “supersymmetry”. This is the idea that there are twice as many fundamental particles in the Standard Model as we thought, with each particle having a “superpartner”. These may involve additional Higgs bosons (associated with the field that gives fundamental particles their mass).

Others will go further, invoking less recent trendy ideas like “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 experimentalists.

The experimental teams behind the new findings are all respected and have been working on the problems for a long time. That said, it’s no disrespect for them to note that these measurements are extremely difficult to make. In addition, standard model predictions often require calculations where approximations need to be made. This means that different theorists can predict slightly different masses and decay rates, depending on assumptions and the level of approximation made. So it could be that when we do more accurate calculations, some of the new findings will fit the standard model.

Likewise, it could be that researchers are using subtly different interpretations and thus finding inconsistent results. Comparison of two experimental results requires careful verification that the same level of approximation was used in both cases.

Both are examples of sources of “systematic uncertainty” and while everyone involved does their best to quantify them, there can be unforeseen complications that under- or over-estimate them.

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

With the restart of the LHC, there are still prospects that new particles will be made through rarer processes or found hidden in backgrounds that we have yet to discover.

This article originally appeared on The conversation and was written by Roger Jones of Lancaster University. Read the original article here.

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