The mystery of matter
The Mystery of Matter.
The Universe shouldn’t be here.
Everything scientists know about particle physics,
summed up in a theory called the Standard Model,
suggests that the big bang should have created equal quantities of matter and antimatter.
A mirror version of matter, antimatter consists of partner particles for the regular particles,
we know of, equal in every way, but with opposite charge.
When matter and antimatter particles collide, they destroy one and another,
so the mass created that the Universe was born should have been completely wiped out,
leaving an empty, featureless cosmos containing only light.
However, there was enough leftover matter, after this great annihilation to form galaxies,
stars and planets, but almost no antimatter.
This is called as the matter-antimatter imbalance.
This existential anomaly is one of the greatest outstanding mysteries of modern physics.
Scientists have formulated many hypothesis to explain this mismatch.
But we don’t know which, if any are true.
Some of them seek to offer matter in upper hand, by introducing new particles that decay,
producing more matter than antimatter in the process,
or that interact differently with matter and antimatter.
Some of these proposals include side effects that scientists can hope to detect,
thereby providing evidence for the theories.
One example, is an exotic properties of electrons called the electric dipole moment.
This is a small difference between the centre of mass of an electron,
and its centre of charge.
Such a displacement has never been detected,
and should be much smaller than current experiments could measure.
Many proposed extensions to the Standard Model,
that seek to explain in matter-antimatter imbalance,
predict much larger values for the electric dipole moment.
Recently scientists pioneered a new strategy that allowed them,
to make the most precise measurement of the electric dipole moment.
To understand what they were looking for, imagine any simple physics experiment.
Now picture repeating the experiments with all the positive charges,
replaced by negative ones, and vice versa, and the entire apparatus arranged,
in the opposite direction as if reflected in a mirror.
If we get an equivalent result with the mirror setup,
the experiment is said to conserve charge and parity symmetry, CP symmetry for short.
In 1967, scientists has shown that this symmetry is intimately connected,
to the matter-antimatter imbalance.
If our Universe as we currently find it developed from a Universe,
that was initially composed of equal parts matter and antimatter,
something must have happened to break CP symmetry.
Other scientists discovered that nature does sometime violate CP symmetry.
For instance, the weak force, responsible for radioactivity in the atomic nuclei,
slightly breaks this symmetry while it interacts with quarks.
Yet the instance of known CP violation in the Standard Model,
aren’t enough to explain the matter-antimatter imbalance.
We must find, new undiscovered physics phenomena,
that don’t conserve CP symmetry to solve the mystery.
This is where the pioneering experiment comes in.
It searches for evidence of new particles in the Universe,
by looking for subtle effects on known particles.
These effects occur because of the nature of the Standard Model,
which is a type of quantum field theory.
In quantum field theories, the basic building blocks of the Universe are fields, not particles.
There is a field for each of the particles in nature.
From common particles such as electrons and photons, to their more exotic cousins,
such as muons and gluons.
We can imagine two dimensional analogs of these fields as huge,
flexible sheets that extend through all of space,
supporting ripples like the surface of a lake does.
In quantum field, ripples can occur only in certain discrete sizes.
The smallest possible ripple in a given field is what we call a particle.
Positive ripple in the field are matter particles, and negative ripples are antimatter particles.
The amount of energy it takes to create the smallest possible ripple,
depends on the swiftness of the stretchy sheet.
This minimum amount of energy is the rest mass of the associated particle.
The different fields are linked together, or “coupled”,
so that the ripple in one field disturbs the connected fields.
For example, an oscillating ripple in the electron field creates accompanying ripples,
in the field of electro magnetic field corresponding to photons.
We make good use of this phenomena in everyday devices,
such as radio antennae and mobile phones.
Scientists’ most successful tools for discovering new fields,
and the particles associated with them have historically being particle colliders.
These machines direct two particles, protons,
for example, to fly towards each other at high speeds.
When the particles or ripples crash into each other, like two water waves meeting in a beach,
their violent interactions can cause some of their energy,
to be carried off as ripples in other fields.
If the energy of their collision is exactly equal to the energy,
needed to create a ripple in one of the other fields they are coupled to,
we get what’s called resonant enhancement.
This greatly increases the probability of a new particle being created.
Such collisions resonances were used to discover many of the fields we know about.
This includes the most recently confirmed piece of the Standard Model, the Higgs field,
with the associated particle, the Higgs boson.
The world’s most powerful accelerator, the 27 km wing of the Large Hadron Collider,
or LHC, near Geneva, is now operating at the highest collision energies it was designed for.
It has so far not discovered any other new fields.
If undiscovered fields exists, either their mass is higher than the LHC can reach,
or their coupling to their fields of the Standard Model, is too weak for the LHC to create them.
A new particle collider would be very expensive.
Fortunately there is another way to detect new particles and fields.
This involves making precision measurements.
Because the fields of the Standard Model are coupled,
a ripple corresponding to a particle in one field always causes disturbance in other fields.
For example, an electron, a ripple in a electron field,
disturbs the electromagnetic field around it.
This disturbance in the electromagnetic field in turn disturbs the other fields,
that are coupled to it, and so on, eventually including all known fields of the Standard Model.
What we call an electron is actually a composite excitation of all these fields.
It is like a large water wave causing disturbances in the air above it.
The effect is sometimes referred to as the electron being surrounded,
by a cloud of “virtual particles”.
The accompanying disturbances of the other fields affect many of the electron’s properties.
By carefully measuring these properties, we can infer the presence of any undiscovered fields,
that are coupled to the electron.
If those fields are associated with heavier particles,
they are stiffer and therefore less disturbed by the rippling electron field.
Measuring the effects of fields with particles of higher and higher masses,
requires measurements with greater and greater precision.
One difficulty of this approach is that often the kind of change we are looking for,
is over shadowed by modification from fields of the Standard Model.
For example, an electron has an magnetic field similar to a tiny bar magnet.
The strength of this field is the electron’s magnetic dipole moment,
and it has been measured to a very high precision.
Its value is determined mostly by the magnetic moment of the bear electron field.
The largest changes arise from the electromagnetic field,
and they can be calculated with astonishing precision.
At the level of precision achieved by connect experiments, however,
the exact value of the Standard Model coupling,
between an electron and electromagnetic fields is not known.
There is some discrepancies in the measured values from different experiments.
Even if this problem is resolved, the tiny effects of interactions with quark fields,
and the strong force will be important.
These effects can be incredibly complex and difficult to calculate.
This makes our search for similar sized, or smaller effects from exotic particles challenging.
A nice way around this problem is to find a property that is zero,
or very very small in the Standard Model.
According to the theory, there should be a minuscule separation,
between an electron’s centre of mass and centre of charge, in other words,
its electric dipole moment.
The electric dipole moment of an electron, or eEDM,
the electric counterpart of the magnetic moment can essentially be caused,
only by interactions that violate CP symmetry.
The CP violations contained within the Standard Model is exceedingly small,
well below current experimental sensitivity.
In contrast, many extensions to the Standard Model,
proposed to explain matter-antimatter imbalance, predict eEDMs,
many orders of magnitude larger and within reach of near term experiments.
In contrast to the enormous infrastructure involved in particle colliders like the LHC,
experiments to measure everyday particles, such as an electron, can fit on a large table,
and be handled by a few scientists.
Surprisingly in certain cases, these tests can answer questions about fundamental physics,
that the world’s most expensive experiments cannot.
Tabletop experiments requires everyone to take a holistic view of the entire apparatus.
They must be generalist, garnering passible knowledge of many different disciplines.
To say an electron has a nonzero electric dipole moment,
is equivalent to say it has a preferred orientation in electric field.
It is just as the needle of a compass, which has a magnetic dipole moment,
as a preferred orientation in Earth’s magnetic field.
If a compass needle is briefly nudged, it will wobble backward and forward,
around magnetic north.
The frequency of this wobble is proportional to both the strength of the magnetic field,
and the size of the magnetic dipole movement of the needle.
If we measure the frequency of the wobble in a known magnetic field,
we will know the size of the needles magnetic dipole movement.
If the needle also has an EDM, perhaps we charged up one end of it somehow,
we can measure its size by simultaneously applying an electric field.
When the electric field is parallel to the magnetic field,
the needle will wobble with a slightly increased frequency.
When the electric field is pointing in the opposite direction, the wobble frequency will decrease.
The different between the two frequencies tell us the size of the needle’s EDM.
We can search for the electron’s precisely in the same way.
We first place the particle in a magnetic field, and measure the shift in its wobble frequency,
when we apply an electric field parallel and then antiparallel to the magnetic field.
We know the eEDM must be very tiny, if it exists at all.
So we know we are looking for an extremely tiny shift in the wobble frequency.
We can boost the signal by applying a larger electric field.
A powerful way to do this is to use electrons confined inside heavy atoms and molecules.
We might think that an electron in an atom or molecule won’t experience any electric field,
or else it would fly away.
This is true, however only if you ignore Einstein’s special theory of relativity.
When relativity is taken into account, it turns out that in heavy atoms,
where relativistic effects are most important because electrons,
move at close to the speed of light near the highly charged nucleus.
The effective electric fields acting on an electron, can be tremendous.
It can be around a million times larger than the strongest fields we can generate in a lab.
To take advantage of this fantastically large field for our measurement,
we need apply only enough of an electric in the lab to orient the atom or molecule.
This work turns out to be much easier with molecules.
For the past decade or so all the leading experiments of this type,
have used electrons in heavy molecules made of two atoms.
In one experiment scientists used hafnium mono-fluoride molecules.
This is because hafnium, with 72 protons in its nucleus,
is one of the heaviest metals in the periodic table, that isn’t radioactive.
Even with enormous electric field, the change in the wobble frequency of the electron,
that we might expect from a realistically sized EDM is still very tiny,
corresponding to about one extra wobble every 7 hours or so.
To detect such a minuscule change, we need to measure two frequencies,
with the electric field parallel and then opposite to the direction of the magnetic field,
extremely precisely.
The longer we monitor a frequency, the more wobbles we can measure,
and therefore the more precise we can make our measurements.
Our timing is limited to how long our molecules last.
For these kind of experiments we must use molecules that have free, unpaired electrons.
This makes them highly reactive -
the electrons are eager to bond with any other atoms they encounter.
We have to keep our molecules in vacuum chambers,
where they come into contact with other particles or the walls of the chamber.
Previous experiments have used beams of molecules,
travelling at hundreds of meters per second down a long vacuum chamber,
with scientists observing the molecules in free flight.
In this setup, the measurement time is limited by how long the beam of molecules,
can travel before it starts spreading out too much and the signal is lost.
Typically this happens within about a meter, or around one millisecond.
Scientists wanted to be able to observe the electrons for longer.
They decided to use trapped molecular ions - charged molecules -
which were held in position by electrical fields.
These measurements require that we expose our molecules to electric fields,
if the molecules are charged ions, the electric fields should cause them to accelerate away.
Some scientists suggested that they could rotate the electric field fast enough,
that instead of flying away the ions just trace out small circles within the trap.
This method let scientists measure the molecules for three seconds,
which was a great improvement.
The measurement time was limited mainly by the time it took for the molecules,
to decay into lower energy states.
The ion trap technique had a drawback.
Because they could trap only so many ions at once,
the experiment measured fewer electrons in each run, then typical beam experiments.
Scientists were able to observe a few hundred electrons per shot.
Over two months they measured more than hundred million electrons in total.
Gathering the data was the quick part.
The real challenge of a precision experiment spent looking for systematic errors.
These errors might convince them that they had measured an eEDM when in fact they had not.
Scientists spent about two years hunting for and measuring such flaws.
An important source of errors in EDM experiments is the level of control over the magnetic field.
They were looking for a difference in the wobble frequency of an electron in a magnetic field,
when electric field is applied parallel to and then opposing the direction of that magnetic field.
The problem is that the wobble frequency depends on the strength of that magnetic field.
If that field drifts slightly between the two measurements, the results will look like an EDM.
To address this possibility scientists found a way to do,
both electric field measurements simultaneously.
They took a cloud of molecules and prepared half,
with their internal electric field aligned with the external magnetic field,
and half with their electric field anti-aligned.
They then measured the wobbles of electrons in both groups simultaneously.
Because both are in the same trap at the same time,
they experienced the same magnetic field to a very high precision.
Another source of systematic error is an experimenter bias.
All scientist are human beings, despite their best efforts,
can be biased in their thoughts and decisions.
This fallibility can potentially effect the results of experiments.
A well studied example is in measurements of the speed of light.
In the late 19th century, attempts to determine this constant over estimated it significantly.
Later measurements tended to under estimate the value.
This led some scientists to suggest that the speed of light was changing.
It wasn’t until the scientists had a better grasp on the true size of their errors,
that the various measurements converged on what we now think is the correct value.
Scientists used special techniques to ensure that there is no error in the EDM experiment.
It seems that they have no evidence that the electron has an EDM.
Though perhaps not as exciting as a non zero value,
a new upper limit on the positive size of the eEDM has substantial consequences.
If we assume that any CP violating field couples to electrons,
with a strength similar to that of the electromagnetic field,
the measurements means that the mass of its associate particles,
must be roughly 40 tera electron volts.
This result is surprising to many who expected new fields to exists below this energy scale.
One possible explanation is that the fields coupled to the Standard Model,
in such a way that their contribution to the eEDM is only indirect,
and therefore smaller for a given mass than the estimate assumes.
Scientists may be able to confirm this possibility by making complimentary measurements,
of EDM’s in other particles built from quarks where the coupling is likely to be different.
Such experiments are underway for neutrons and for mercury nuclei,
and many more are planned.
Another possibility is that the new fields are at just higher energies or smaller couplings,
out of reach of the current experiments,
but accessible to the next generation of eEDM measurements.
Ultimately scientists hope to either detect an electric dipole moment in an electron,
or limit its possible size enough to effectively rule out the types of fields and particles,
scientists has envisaged to explain our antimatter mystery.
We know there must be some reason for the Universe of matter we live in to be the way it is.
The question is how long we will take to discover it.