Muon anomalies shatter the laws of physics?
When German physicist Paul Kunze first observed the muon in 1933, he wasn’t sure what to
make of it.
He showed this track that was neither an electron nor a proton, which he called “a particle
of uncertain nature".
The newfound particle was a curious complication to the otherwise limited cast of subatomic
particles, but the ensuing deluge of exotic particles discovered in the decades that followed
showed that the muon was actually part of a larger ensemble, but a few years later it
will be discovered that might indeed be something strange about the muon.
“Who ordered that?”
This was the reaction, famous in particle-physics circles, of Isidor Isaac Rabi to the discovery
of the muon.
Rabi, a Nobel laureate who helped America develop the atom bomb, was reflecting physicists’
general surprise that muons, which are, to all intents and purposes, just heavy and unstable
versions of electrons, actually exist.
To an orderly physicist’s mind, they somehow seemed superfluous to Nature’s requirements.
Well, for several years now it has been this unexpected and unwanted particle to discuss
the world of atomic physics and to question the validity of the so-called "Standard Model".
Preliminary experiments had in fact shown that the muon has a magnetic moment slightly
different from that predicted by the theory.
An anomaly almost imperceptible, but that can be measured, and of which we have received
confirmation in these days.
The knowledge of how matter works is proceeding in small steps and the latest results, communicated
on April 7 by Fermilab, the main US laboratory that interrogates elementary particles, seem
to indicate only one direction: that of a new physics.
Necessary to interpret the evidence obtained from the Muon g-2 experiment, the results
of which are not predicted by the Standard Model, so far the physical theory has been
considered the best explanation of what happens in the infinitely small.
The muon is a heavier version of the electron.
It is an elementary particle that is copiously produced when cosmic rays (protons and light
nuclei) hit the upper atmosphere.
As we speak, and only slightly depending on our altitude over sea level and our geolocation,
we are traversed by a few tens of muons every second.
They whiz through the atoms of our body without leaving much behind - only some ionization
trail, mostly.
So, nothing to worry about, really.
Furthermore, even if a muon stopped inside our body, it would soon decay (on average,
we would have to wait about two microseconds for that to happen), releasing an electron
and two neutrinos.
Muons are electrically charged particles, so when they are placed in a magnetic field,
they start to spin.
Physicists can measure the frequency of that spin because of a phenomenon called precession,
in which the spin axis of the particle wobbles slightly.
The frequency at which a muon rotates when exposed to a magnetic field is determined
by its interactions with other particles and forces, represented by a number called the
“g factor“.
Using the standard model of particle physics, researchers can predict what this number ought
to be with extreme precision.
But in 2001, experimental results from Brookhaven National Laboratory in New York started to
diverge from those theoretical predictions – the muons were spinning slightly faster
than they ought to.
At the time, the finding was not robust enough because it had a statistical significance
of only 3.3 sigma: that is, which more or less means that there was a probability in
1000 that it could be an error.
Generally, in physics we tend to speak of "safe data" only in the presence of a statistical
significance of 5 sigma, or a probability of error of one in 3.5 million.
To explain these phenomena and more, researchers have been hunting for new physics—physics
beyond the Standard Model—by looking for anomalies in which experimental results diverge
from theoretical predictions.
And on April 7, in fact, the Muon g-2 experiment at Fermi National Laboratory in Batavia, Illinois,
has given as new result a value that has confirmed, refining it, that of Brookhaven National Laboratory:
the newly updated experimental value, in fact, deviates from theory by only a minuscule value
(0.00000000251) and has a statistical significance of 4.2 sigma, with a probability of 40.000
that it may not be an error.
In simplification, this happens: the Muon g-2 experiment starts with a beam of muons,
which scientists make by smashing pairs of protons together and then carefully filtering
through the subatomic debris.
This muon beam then enters a 14-ton magnetic ring that originally was used in the Brookhaven
experiment, shipped by barge and truck from Long Island to Illinois in 2013.
As the muons go round and round this storage ring, which has a uniform magnetic field,
the wobbling muons decay into particles that smack into a set of 24 detectors along the
track’s inner wall.
By tracking how often these decay particles hit the detectors, researchers can figure
out how quickly their parent muons were wobbling - a bit like figuring out a distant lighthouse’s
rotation speed by watching it dim and brighten.
With a statistical significance of 4.2 sigma, researchers cannot yet say they have made
a discovery.
But the evidence for new physics in muons - in conjunction with anomalies recently observed
at the Large Hadron Collider Beauty experiment at CERN near Geneva - is tantalizing.
The results weren’t statistically significant enough to prove that the standard model was
wrong, but they were a cause for concern.
Despite its remarkable success in explaining the fundamental particles and forces that
make up the universe, the Standard Model’s description remains in fact woefully incomplete.
It does not account for gravity, for one thing, and it is similarly silent about the nature
of dark matter, dark energy and neutrino masses.
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What are the explanations for this discrepancy from the Standard Model?
Maybe the interaction with unknown subatomic particles or energy forms inconceivable today.
It could be very massive particles, with mass beyond the energy limits of the produced collisions,
or particles so small that they have almost no interaction with matter.
There is talk of dark photon or even a second Higgs boson.
No one can tell yet.
In fact, what an anomaly implies is ambiguous.
There might be something not accounted for by the Standard Model, and it could be a difference
between electrons and muons.
Or there could be a similar effect in electrons that is too small to currently see.
The mass of a particle is related to how much it can interact with heavier unknown particles,
so muons, which have about 207 times the mass of electrons, are much more sensitive.
When a muon travels through space, that space is not really empty.
Instead it is a sizzling and swarming soup of an infinite number of virtual particles
that can pop in and out of existence.
The muon has some small chance of interacting with these particles, which tug on it, influencing
how it behaves.
Calculating the virtual particles’ effect on the muon’s spin - the rate at which its
hour hand turns - requires a series of equally arduous and incredibly precise theoretical
determinations.
If we can venture a comparison, it seems to be back to the time when based on the orbital
perturbations of Uranus observed and quantified by the astronomers of the time, in 1846 Urbain
Le Verrier was able to calculate the existence and position of a new planet that would later
be called Neptune.
OK, but...What happens now?
We will have to repeat the experiments until we reach sigma 5.
And start thinking about new explanations.
The hypotheses that arise are those that foresee unknown particles or forms of energy.
The exciting thing for physicists is that none of these are foreseen by the Standard
Model.
We are talking about a new physics, and who knows which is not the one that, one day,
will be able to give shape to the holy grail, the Theory of Everything: There are basically
two possibilities that of very massive particles, which the LHC of Cern has not yet discovered
why they have very small effects and therefore difficult to observe.
Or a particle with a very small mass and which, like neutrinos, interacts little with matter
and could be the particle of dark matter: the dark photon, which, however, has been
searched for in other experiments and has not been found.
track.
The hypothesis that a second Higgs boson exists is also a possibility.
More Higgs bosons are continually being sought, which could also explain dark matter.
It is a possibility on par with many others.
But…
Even if all of these efforts confirm there is new physics at work in muons, however,
they will not be able to reveal what, exactly, that new physics is.
The needed tool to reveal its nature may be a new collider - something many physicists
are clamoring for via proposals such as the International Linear Collider and the High-Luminosity
LHC.
In the past few months, interest has surged around a muon collider, which multiple papers
predict would guarantee physicists the ability to determine the properties of the unknown
particle or force affecting the muon.
However, even as many particle physicists are likely to be celebrating — and racing
to propose new ideas that could explain the discrepancy - a paper published in the journal
Nature casts the new muon measurement in a dramatically duller light.
The paper, which (with a certain “perfidy") appeared just as the Fermilab team unveiled
its new measurement, suggests that the muon’s measured wobbliness is exactly what the Standard
Model predicts.
In the paper, a team of theorists known as BMW presents a state-of-the-art supercomputer
calculation of the most uncertain term that goes into the Standard Model prediction of
the muon’s magnetic moment.
BMW calculates this term to be considerably larger than the value adopted last year by
the consortium, a group known as the Theory Initiative.
BMW’s larger term leads to a larger overall predicted value of the muon’s magnetic moment,
bringing the prediction in line with the measurements.
If the new calculation is correct, then physicists may have spent 20 years chasing a ghost.
But the Theory Initiative’s prediction relied on a different calculational approach that
has been honed over decades, and it could well be right.
In that case, Fermilab’s new measurement constitutes the most exciting result in particle
physics in years.