It was 2016, physicists worked tirelessly. Four years ago, the LHC confirmed the existence of the Higgs boson predicted by the Standard Model. Everything went to the fact that the LHC must find other new particles - nature itself, it seemed, required them. But all the data collected by scientists only smashed their dreams to smithereens. The Standard Model and General Relativity work great, but physicists sense there is a catch somewhere. They think that these theories are incomplete, do not correlate with one another, and sometimes lead to paradoxes for which no cure has yet been found. There must be something else. But where to look?
Caches with new phenomena are becoming less and less. But physicists have not yet exhausted all possibilities. Here are the most promising areas in which searches are currently underway.
Weak interaction
Particle collisions at high energies, such as those achieved with the LHC, can produce all existing particles up to the energies that the colliding particles had. But the number of new particles depends on the strength of their interaction. A particle that interacts very weakly can be born so rarely that it has not yet been seen.
Physicists have proposed many new particles that fall into this category because weakly interacting material as a whole looks very similar to dark matter. In particular, this includes weakly interacting massive particles (WIMPs), sterile neutrinos and axions (also a strong candidate for dark matter).
Such particles are sought both through direct measurements - observing large reservoirs in underground mines in anticipation of rare interactions - and looking out for unexplained astrophysical processes that could act as an indirect signal.
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High energies
If these particles were not of the weakly interacting type, we would have noticed them already, unless their mass is outside the energy that we have been able to achieve with particle accelerators at the moment. In this category, we have all supersymmetric partner particles, which are much heavier than the standard model particles because supersymmetry is broken. In addition, at high energies one could see the particle excitations that are present in models with compactified extra dimensions. These excitations are shown at certain discrete energy levels that depend on the size of the extra dimension.
Strictly speaking, an important role in the question of the possible detection of such a particle is played not by mass, but by the energy required to produce such particles. Strong nuclear forces, for example, exhibit "confinement," meaning that it takes a lot of energy to break apart quarks, even if their mass is not very large. Consequently, quarks must have constituents - they are often called "preons" - that have an interaction - "technicolor" - similar to the strong nuclear one. The most obvious technicolor models came into conflict with the data decades ago. But the idea continues to live on, and while surviving models are not particularly popular, it should not be discounted.
These phenomena are sought at the LHC and in high-energy cosmic rays.
High accuracy
High-precision process testing of the Standard Model complements high energy measurements. They can be sensitive to minute effects from virtual particles that are too energetic to be produced in accelerators, but very important at low energies due to quantum effects. Examples of this are proton decay, neutron-antineutron oscillations, muon g-2, kaon oscillations. For all of these examples, there are experiments looking for deviations in the Standard Model, and the accuracy of these measurements is continually increasing.
Another high-precision test is the search for neutrinoless double beta decay, which would demonstrate that neutrinos are Majorana particles, an entirely new type of particle.
A long time ago…
During the young universe, matter was much denser and hotter than we could ever hope to achieve in our particle accelerators. Therefore, the signatures left over from these times could give us new food for thought. Temperature fluctuations in the cosmic microwave background could test scenarios of inflation or its alternatives, could our Universe experience a “big bounce” instead of a “big bang”, and whether gravity was quantized at that time.
…far away from here…
Some of the signatures of new physics appear at large distances, not at small ones. An unresolved question remains, for example, the shape of the universe. Is it infinitely large, or is it closing in on itself? And if so, how exactly? One of the studies devoted to this issue is to look for repeating patterns in temperature fluctuations of the cosmic microwave background (CMB). If we live in a multiverse, the two universes could accidentally collide, which would leave a mark on the CMB. Another phenomenon that can manifest itself at large distances is the fifth force, which can lead to slight violations of general relativity.
… and right here
Not all experiments are large and expensive. While knee-high discoveries are becoming less and less likely simply because much has already been tried and done, there are areas where small laboratory experiments could lead us on a new trail. This is especially true of quantum mechanics, in which tiny mechanisms and detectors allow previously impossible experiments. Maybe one day we can resolve the dispute over the "correct" interpretation of quantum mechanics by simply measuring which one is correct.
Physics is far from complete. It is becoming increasingly difficult to test new fundamental theories, but we are gradually expanding the boundaries of many existing experiments. There may be new physics out there somewhere; we just need to increase energies, accuracy and look for ever more subtle effects. If nature is kind to us, in this decade we can destroy the Standard Model and travel to a new universe beyond.
ILYA KHEL