Searching for dark energy with particle colliders
The expansion of our universe is accelerating, and we don’t understand why. The observational evidence for this is so compelling that its discoverers were awarded the Nobel Prize in 2011, and yet none of our current theories explain how this can be happening. We are forced to conclude either that Einstein’s theory of General Relativity is wrong, or that there are particles in our universe totally unlike anything we have seen so far. These new particles and associated forces are called dark energy, and they make up 70% of our current universe, yet we understand essentially nothing about them.
Our failure to understand why the expansion of the universe is accelerating points to a fundamental flaw in modern physics; an inability to combine our best theories of gravity and quantum mechanics. If we lived in a universe governed entirely by classical physics, with no quantum effects, this acceleration would not be a surprise. There is a constant in Einstein’s equations of General Relativity, the cosmological constant, which can cause exactly this acceleration, all we need to do is fix it to have the correct value. However our universe also requires quantum mechanics to explain the behaviour of particles on very short scales, and this tells us that even in the vacuum of empty space there is always a non-zero probability of particles popping into existence for a short period of time and then disappearing. The consequence of these particles appearing and disappearing at all points in space, and all times in the history of the universe add up, leading to a prediction for the value for the cosmological constant appearing in Einstein’s equations. If we believe that we otherwise understand physics up to the Planck scale the value is predicted to be 120 orders of magnitude larger than what is observed. Such a universe would have expanded so rapidly, that it would not have been possible to form galaxies, let alone stars, planets and people.
We are forced to conclude that at least one, and likely both, of these theories must be fundamentally modified. Currently we have no compelling alternative theories and a dearth of observational evidence that could guide us in building them. However one common feature, that arises in almost all attempts at an explanation, is the introduction of new particles that we call dark energy. These interact with normal matter and transmit new forces. They could be directly responsible for the acceleration of the expansion of the universe, or a by-product of a solution as yet unknown. Detecting such a particle, its corresponding force, or determining how dark energy is constrained by theory and experiment, will not only tell us what 70% of our universe is made of, but also shape the form of a new and more unified theory of physics.
High precision experiments have long searched for new forces, but without success. The apparent contradiction between what cosmological theories require, and what terrestrial observations exclude has hamstrung efforts to understand dark energy until very recently. The main theoretical advance in the study of dark energy in the last decade has resolved this by allowing the properties of the dark energy particle, such as its mass and coupling constant, and the associated strength of the force that it transmits to vary with the environment. This is known as ‘screening’ and it relies on the dark energy field having complex self-interactions, meaning that the mathematical equations describing the theory become non-linear. If dark energy is sufficiently screened in all existing searches for new forces, then this explains why we haven’t detected it to date. The most popular current model is known as the chameleon, after the analogous ability of these lizards to change their appearance to avoid being seen. Understanding screening offers a unique opportunity to tailor our experimental searches to take account of this chameleonic behaviour and thereby maximise our chances of detecting dark energy.
The current approach to detecting dark energy attempts to parameterise, and then detect or constrain, possible deviations from general relativity on cosmological scales. The Dark Energy Survey (DES) is underway and the next generation of cosmological survey satellites are under construction, Euclid will launch in 2020, and the WFIRST satellite will be launched in the mid-2020s. However this is far from a comprehensive list of all possible observables that are sensitive to dark energy, which after all will have effects on all scales in the universe from the cosmological to the sub-atomic.
Frameworks exist for interpreting the LHC data in terms of constraints on many new physics models, including supersymmetry, composite Higgs and dark matter models. However no such framework exists for the study of dark energy in collider experiments. Such constraints are important; firstly because particle colliders are the only experiments that can tell us how dark energy interacts with fundamental particles. All other experiments use composite objects ranging in size from atoms to galaxies. Understanding the form of these most fundamental interactions is a necessary step in building a consistent theory of dark energy on all scales. Secondly, the LHC studies the highest energy scales that can be reached in a terrestrial experiment, meaning it can probe regimes of the dark energy parameter space that it is difficult for any other experiment to reach. This is particularly important for those models with the strongest interactions with matter, which are those for which the screening mechanism most efficiently hides the effects of the dark energy field in laboratory and astrophysical searches.
As dark energy particles have a small mass there will always be enough energy available in an LHC collision to produce such a particle, but this may only happen extremely rarely. If a dark energy particle is produced, there are two possibilities for what happens next. Either it leaves the detector without interacting with anything else, or it decays into visible particles that we can try to detect. In recent work we have shown that current searches for other types of new physics that leave missing energy signals can be recast into constraints on dark energy. This restricts the scale controlling the coupling of dark energy to matter to lie above 102 GeV. For one type of coupling this is eleven orders of magnitude more stringent than constraints obtained from cosmological and solar system tests, and excludes previously popular models that require the coupling scale to lie close to the dark energy scale, Λ ~ 10-3 eV. These existing searches are not optimised to search for dark energy, as the dark energy particle is significantly lighter than other new physics particles, and its couplings to matter can depend on the energy and momentum in the collision. Constraints could easily be improved with a targeted search.
The key advantage of laboratory searches over cosmological surveys is that if dark energy is detected in the laboratory we are able to study this new substance actively, rather than only passively through astronomical observations, leading to rapid further advances in understanding. However, this does not negate the need to ensure that cosmological observations take full account of the possible range of behaviours for dark energy. Only a combination of terrestrial and cosmological data can give us a comprehensive picture of dark energy, how it interacts with standard model particles, and how it behaves on all scales in the universe.