The End of the Standard Model in Particle Physics?

The End of the Standard Model in Particle Physics?
LHCb Experiment at CERN. Credits: CERN

As a physicist working at the Large Hadron Collider (LHC) at CERN, one of the questions I get asked most often is “When will you find something? With the exception of the Nobel Prize-winning Higgs boson and a bunch of new compound particles, “resist the temptation to answer sarcastically?”. Yes, these types of questions are frequently asked to our author, as well as to physicists working at CERN or other collider units at various times.

Our author speculates that the reason the question is asked so often is due to how we describe the progress in particle physics in the social environment.

There is often talk of progress in terms of the discovery of new particles. Examining a new, very heavy particle helps us see the underlying physical processes.

However, recent measurements on standard particles have had a profound impact on physics.

And as the LHC prepares to operate at higher energy and intensity than ever before, it seems it's time to begin a broad discussion of its implications.

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

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

Particles obey a counter-intuitive but highly successful theory called quantum mechanics.

This theory suggests that particles too large to be created directly in a laboratory collision can still influence what other particles do (through a process known as "quantum fluctuations").

However, measurements of such effects are very complex and much more difficult to make public.

However, recent results pointing to new physics that cannot be explained beyond the standard model are of this latter type.

Detailed studies from the LHCb experiment discovered that a particle known as the beauty quark (quarks make up protons and neutrons in the atomic nucleus) decays into an electron much more often than a muon.

According to the standard model, this shouldn't happen – implying that new particles or even forces of nature can influence the process.

Interestingly, 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 USA has recently done very conclusive studies of how muons "swing" when their "spin" (quantum property) interacts with surrounding magnetic fields.

He found a small but significant deviation from some theoretical predictions. He again suggested that unknown forces or particles might be at work.

The latest surprising result is the measurement of the mass of an elementary particle called the W boson, which carries the weak nuclear force that governs radioactive decay.

After years of data acquisition and analysis, Experiment also done at Fermilabshows that the theory is significantly more controversial than it predicts. This corresponds to an amount of deviation that would not have happened by chance in more than a million experiments.

Again, the overlooked possibility of the mass of yet-to-be-discovered particles may lead to this result.

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

While we are not absolutely sure that these effects require a new explanation, they may indicate a need for new knowledge in physics.

Of course, to explain these observations, theorists can concentrate on "supersymmetry".

This is the idea that there are twice as many elementary particles as we thought in the standard model, and that every particle has a "super partner."

These may contain additional Higgs bosons (associated with the field that gives elementary particles their mass).

Others will go beyond that, invoking less trendy ideas like "technicolor", implying that there are additional forces of nature (in addition to gravity, electromagnetism, and the weak and strong nuclear forces), meaning that the Higgs boson is actually a composite object made of other particles.

.Only experiments will reveal the truth of matter. This is good news for those who are into experimental physics.

However, it should be noted that these measurements are extremely difficult to make.

Moreover, the estimates of the standard model often require calculations to be made of approximations.

This means that different theorists may predict slightly different masses and decay rates, depending on the assumptions made and the level of approach.

Therefore, when we make more accurate calculations, it may be that some new findings fit the standard model.

Likewise, researchers may be using vastly different interpretations and thus finding inconsistent results.

Comparing two experimental results requires careful checking that the same zoom level is used in both cases.

Both of these are examples of sources of "systematic uncertainty". While all researchers do their best to quantify these, there can be unforeseen complications that underestimate or overestimate them.

None of this makes the present results any less interesting or important.

What the results show is that there are multiple avenues for a deeper understanding of the new physics, and they all need to be explored.

With the Lhc restart, there is still the possibility that new particles will be revealed via rarer processes, or be found lurking under backgrounds that we haven't uncovered yet.








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