This article was originally included on Conversation.

As a physicist working at the Large Hadron Collider (LHC) in Cern, one of the most common questions I am asked is “When will you find something?” Resisting the temptation to answer sarcastically: “Besides the Nobel Prize-winning Higgs boson and a bunch of new composite particles?” to the wider world.

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

Recently, however, a series of accurate measurements of already known, standard swamp particles and processes have threatened to shake up physics. And with the LHC getting ready for work at higher energy and intensity more than ever, it is time to start discussing the consequences widely.

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

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

Three key findings

Particles obey an anti-intuitive but extremely successful theory called quantum mechanics. This theory shows that particles that are too massive to be made directly in a laboratory collision can still affect what other particles do (through something called “quantum fluctuations”). However, measuring such effects is very complex and much more difficult to explain to the public.

But recent results suggesting an inexplicable new physics outside the standard model are of this second type. In detail studies from the LHCb experiment found that a particle known as a beauty quark (quarks make up protons and neutrons in the atomic nucleus) “decays” into an electron much more often than a muon, the heavier but otherwise identical electron brother. . According to the standard model, this should not happen – implying that new particles or even natural forces can affect the process.

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

Meanwhile, the Muon g-2 experiment was recently performed at Fermilab in the United States very accurate research about how muons “swing” because their “rotation” (quantum property) interacts with the surrounding magnetic fields. He found a small but significant deviation from some theoretical predictions – again suggesting that unknown forces or particles may be at work.

IN the last surprising result is a measurement of the mass of a fundamental particle called W boson, which carries the weak nuclear force that controls 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 – a deviation from an amount that would not have happened by chance in more than a million million experiments. Again, it may be that undiscovered particles add to its mass.

Interestingly, however, this is also inconsistent with some measurements with lower accuracy than the LHC (presented in this teaching and this one).

The sentence

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

Of course, almost as many new mechanisms will be proposed to explain these observations as there are theorists. Many will look for different forms of “supersymmetry“. This is the idea that the standard model has twice as many fundamental particles as we thought, with each particle having a “super partner”. These may include additional Higgs bosons (associated with the field that gives the mass of the fundamental particles).

Others will go beyond this, citing less modern ideas such as “technical color“Which would mean that there are additional natural forces (in addition to gravity, electromagnetism and weak and strong nuclear forces) and could mean that the Higgs boson is actually a composite object made of other particles. Only experiments will reveal the truth about the issue – which is good news for experimenters.

The experimental teams behind the new discoveries are well respected and have worked on the problems for a long time. However, it is not disrespectful to note that these measurements are extremely difficult to perform. Moreover, the predictions of the standard model usually require calculations where approximations need to be made. This means that different theorists can predict slightly different masses and decay rates depending on the assumptions and the level of approximation made. So it may turn out that when we make more accurate calculations, some of the new discoveries will fit into the standard model.

In the same way, it may be that researchers use subtly different interpretations and thus find inconsistent results. Comparing two experimental results requires careful verification that the same approximation level is used in both cases.

Both are examples of sources of “systematic uncertainty” and while all stakeholders are doing their best to quantify them, there may be unforeseen complications that underestimate or overestimate them.

None of this makes the current results less interesting or important. What the results illustrate is that there are many paths 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 chances that new particles will be made through rarer processes or found hidden under backgrounds that we have not yet discovered.

Roger Jones is a professor of physics and head of the department at the University of Lancaster. He received funding from STFC and is a member of the ATLAS Collaboration.


The standard model of particle physics may be broken

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