Atoms within atoms

It’s a matter of some irony that forces that act across larger distances also give rise to lots of empty space – although the more you think about it, the more it makes sense. The force of gravity, for example, can act across millions of kilometres but this only means two massive objects can still influence each across this distance instead of having to get closer to do so. Thus, you have galaxies with a lot more space between stars than stars themselves.

The electromagnetic force, like the force of gravity, also follows an inverse-square law: its strength falls off as the square of the distance – but never fully reaches zero. So you can have an atom with a nucleus of protons and neutrons held tightly together but electrons located so far away that each atom is more than 90% empty space.

In fact, you can use the rules of subatomic physics to make atoms even more vacuous. Electrons orbit the nucleus in an atom at fixed distances, and when an electron gains some energy, it jumps into a higher orbit. Physicists have been able to excite electrons to such high energies that the atom itself becomes thousands of times larger than an atom of hydrogen.

This is the deceptively simple setting for the Rydberg polaron: the atom inside another atom, with some features added.

In January 2018, physicists from Austria, Brazil, Switzerland and the US reported creating the first Rydberg polaron in the lab, based on theoretical predictions that another group of researchers had advanced in October 2015. The concept, as usual, is far simpler than the execution, so exploring the latter should provide a good sense of the former.

The January 2018 group first created a Bose-Einstein condensate, a state of matter in which a dilute gas of particles called bosons is maintained in an ultra-cold container. Bosons are particles whose quantum spin takes integer values. (Other particles called fermions have half-integer spin). As the container is cooled to near absolute zero, the bosons begin to collectively display quantum mechanical phenomena at the macroscopic scale, essentially becoming a new form of matter and displaying certain properties that no other form of matter has been known to exhibit.

Atoms of strontium-84, -86 and -88 have zero spin, so the physicists used them to create the condensate. Next, they used lasers to bombard some strontium atoms with photons to impart energy to electrons in the outermost orbits (a.k.a. valence electrons), forcing them to jump to an even higher orbit. Effectively, the atom expands, becoming a so-called Rydberg atom[1]. In this state, if the distance between the nucleus and an excited electron is greater than the average distance between the other strontium atoms in the condensate, then some of the other atoms could technically fit into the Rydberg atom, forming the atom-within-an-atom.

[1] Rydberg atoms are called so because many of their properties depend on the value of the principal quantum number, which the Swedish physicist Johannes Robert Rydberg first (inadvertently) described in a formula in 1888.

Rydberg atoms are gigantic relative to other atoms; some are even bigger than a virus, and their interactions with their surroundings can be observed under a simple light microscope. They are relatively long-lived, in that the excited electron decays to its ground state slowly. Astronomers have found them in outer space. However, Rydberg atoms are also fragile: because the electron is already so far from the nucleus, any other particles in the vicinity, even a weak electromagnetic field or a slightly warmer temperature could easily knock the excited electron out of the Rydberg atom and end the Rydberg state.

Some clever physicists took advantage of this property and used Rydberg atoms as sensitive detectors of single photons of light. They won the Nobel Prize for physics for such work in 2011.

However, simply sticking one atom inside a Rydberg atom doth not a Rydberg polaron make. A polaron is a quasiparticle, which means it isn’t an actual particle by itself, as the –on suffix might suggest, but an entity that scientists study as if it were a particle. Quasiparticles are thus useful because they simplify the study of more complicated entities by allowing scientists to apply the rules of particle physics to arrive at equally correct solutions.

This said, a polaron is a quasiparticle that’s also a particle. Specifically, physicists describe the properties and behaviour of electrons inside a solid as polarons because as the electrons interact with the atomic lattice, they behave in a way that electrons usually don’t. So polarons combine the study of electrons and electrons-interacting-with-atoms into a single subject.

Similarly, a Rydberg polaron is formed when the electron inside the Rydberg atom interacts with the trapped strontium atom. While an atom within an atom is cool enough, the January 2018 group wanted to create a Rydberg polaron because it’s considered to be a new state of matter – and they succeeded. The physicists found that the excited electron did develop a loose interaction with the strontium atoms lying between itself and the Rydberg atom’s nucleus – so loose that even as they interacted, the electron could still remain part of the Rydberg atom without getting kicked out.

In effect, since the Rydberg atom and the strontium atoms inside it influence each other’s behaviour, they altogether made up one larger complicated assemblage of protons, neutrons and electrons – a.k.a. a Rydberg polaron.

Another window on ‘new physics’ closes

This reconstructed image of two high-energy protons colliding at the LHC shows a B_s meson (blue) produced that then decays into two muons (pink), about 50 mm from the collision point.
This reconstructed image of two high-energy protons colliding at the LHC shows a B_s meson (blue) produced that then decays into two muons (pink), about 50 mm from the collision point. Image: LHCb/CERN

The Standard Model of particle physics is a theory that has been pieced together over the last 40 years after careful experiments. It accurately predicts the behaviour of various subatomic particles across a range of situations. Even so, it’s not complete: it can explain neither gravity nor anything about the so-called dark universe.

Physicists searching for a theory that can have to pierce through the Standard Model. This can be finding some inconsistent mathematics or detecting something that can’t be explained by it, like looking for particles ‘breaking down’, i.e. decaying, into smaller ones at a rate greater than allowed by the Model.

The Large Hadron Collider, CERN, on the France-Switzerland border, produces the particles, and particle detectors straddling the collider are programmed to look for aberrations in their decay, among other things. One detector in particular, called the LHCb, looks for signs of a particle called B_s (read “B sub s”) meson decaying into two smaller particles called muons.

On July 19, physicists from the LHCb experiment confirmed at an ongoing conference in Stockholm that the B_s meson decays to two muons at a rate consistent with the Model’s predictions (full paperhere). The implication is that one more window through which physicists could have a peek of the physics beyond the Model is now shut.

The B_s meson

This meson has been studied for around 25 years, and its decay-rate to two muons has been predicted to be about thrice every billion times, 3.56 ± 0.29 per billion to be exact. The physicists’ measurements from the LHCb showed that it was happening about 2.9 times per billion. A team working with another detector, the CMS, reported it happens thrice every billion decays. These are number pretty consistent with the Model’s. In fact, scientists think the chance of an error in the LHCb readings is 1 in 3.5 million, low enough to claim a discovery.

However, this doesn’t mean the search for ‘new physics’ is over. There are many other windows, such as the search for dark matter, observations of neutrino oscillations, studies of antimatter and exotic theories like Supersymmetry, to keep scientists going.

The ultimate goal is to find one theory that can explain all phenomena observed in the universe – from subatomic particles to supermassive black holes to dark matter – because they are all part of one nature.

In fact, physicists are fond of Sypersymmetry, a theory that posits that there is one as-yet undetected particle for every one that we have detected, because it promises to retain the naturalness. In contrast, the Standard Model has many perplexing, yet accurate, explanations that is keeping physicists from piecing together the known universe in a smooth way. However, in order to find any evidence for Supersymmetry, we’ll have to wait until at least 2015, when the CERN supercollider will reopen upgraded for higher energy experiments.

And as one window has closed after an arduous 25-year journey, the focus on all the other windows will intensify, too.

(This blog post first appeared at The Copernican on July 19, 2013.)