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It is through the gravitational effect of dark matter on other matter in space that astronomers inferred its existence.

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The first evidence for the existence of dark matter came as early as the s [1]. Many astronomers had been observing the motion of galaxies, and found a discrepancy with respect to their expectation that only accounted for matter that was emitting light. This was corroborated in the 70s through observations of the rotational velocity of galaxies made by Vera Rubin and collaborators. From comparing the known position of the source e. Observations of gravitational lensing also pointed to additional matter with respect to what was visible.

More recently, supercomputer simulations of the structure of our universe show that only including visible matter will not reproduce the structures that are observed in the universe , while if dark matter is included then a closer agreement is obtained between observations and simulations. The presence of dark matter and its amount in the universe can also be inferred from the variations of temperature in the early universe.

However, none of the observations or simulations involving dark matter give a clear indication of what dark matter is made of. We only know that if dark matter is a particle [2] , then it must have mass, since it interacts with other matter through the force of gravity. We can hope to understand its nature by observing rare dark matter particles and their interactions from space where we have already seen its effects , and by trying to produce them in controlled laboratory conditions. Experiments at particle accelerators have revealed much about the nature of visible ordinary matter, starting from the first prototypes that aided the discovery of the proton and the antiproton to the recent discovery of the Higgs boson.

All of the particles observed so far are part of the Standard Model of Particle Physics, describing the fundamental components of matter and their non-gravitational interactions. The hope is that at the LHC we can create massive dark matter particles by colliding known particles, in the same way we create the Higgs boson in proton-proton collisions.

Particles are regularly accelerated to very high energies in the universe in "natural" particle accelerators, such as supernovae explosions, and then collide with other particles in our atmosphere.

Cosmic rays , for example, are particles that are generated in outer space and make it to Earth. We can also create a large and known number of collisions and observe them in a controlled environment. Since dark matter is dark , it will not interact significantly with instruments made of ordinary matter. For this reason, the underlying signature of dark matter production at the LHC, used by all ATLAS searches, is the presence of invisible particles in proton-proton collisions.

One might reasonably ask how invisible particles can be observed, since they are by definition undetectable! We solve this problem with a little ingenuity. Before each collision, the protons travel along the direction of the LHC beams, and not in directions perpendicular to the beams. This means that their momenta in these perpendicular directions — their "transverse momentum" — is zero. A fundamental principle of physics is that momentum is conserved and so, after the collision, the sum of the transverse momenta of the products of the collision should still be zero.

Therefore, if we add up the transverse momenta of all the visible particles produced in the collision and find it not to be zero, then this could be because we have missed the momentum carried away by invisible particles. We refer to this missed transverse momentum as "ETmiss". LHC searches for dark matter look for collisions with large values of ETmiss, where the dark matter is produced in association with other, visible particles from the Standard Model, such as photons, quarks or gluons forming "jets" of particles , or electrons, muons or tau leptons.

While ETmiss can be difficult to measure because it relies on accurate measurements of all the other particles in the collision, it is a powerful tool for observing dark matter. A further requirement for the identification of dark matter particles in collisions is that the invisible particles should not decay as they travel through the ATLAS detector.


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In order for an invisible particle to be a candidate for the "relic" dark matter produced in the Big Bang, it should have a lifetime of at least the age of the universe — of the order of 14 billion years. Particles created in LHC collisions take about 40 nanoseconds to cross the ATLAS detector, so requiring that their lifetime be longer than this is not enough, on its own, to prove they constitute the dark matter. Complementary information from astroparticle experiments searching for relic dark matter would be required.

Dark matter | CERN

However, it is a very good start! It is worth noting that other particles that are connected to dark matter might also be detected at the LHC, for example new short-lived particles that can decay both into dark matter and into known matter. Observing those would be an important complement to an observation of dark matter particles from space, as it would allow us to better understand the landscape of dark matter interactions.

Experimentally, there are very few indications of what dark matter might be. We can, however, make theoretical hypotheses on the nature of dark matter, which are useful to experimentalists. Theoretical models of dark matter can tell us more about how the interaction of dark matter with ordinary matter may take place. From that, we can predict what to expect in our detectors if that model were realised in nature. This is relevant for designing detectors sensitive to dark matter, and for deciding how to analyse the products of the collisions once they have been recorded. It is also useful to know what to look for, as we have to decide in real-time which collisions to save data from this is done using the ATLAS trigger system.

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Searches for dark matter at the LHC are commonly guided by theoretical models that would allow us to explain the relic density of dark matter with one or a few kinds of particles. A class of models that satisfies these requirements includes a dark matter particle that only interacts weakly with ordinary particles and has a mass within the energy range that can be probed at the LHC — a Weakly Interacting Massive Particle WIMP.

This is especially important when you consider that the content of dark matter in the universe is five times the content of ordinary matter, and ordinary matter is described by a variety of different particles and interactions.

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At the LHC, we have begun our tour into possible theoretical models of dark matter [4] hoping that the few most prominent components and interactions of dark matter will be detected first, just as the electron, proton and electromagnetic interaction were discovered before all other particles of the Standard Model.

The simplest models one can build in terms of particle content are those where the dark matter particle is added to the Standard Model. In these models, the interaction between visible and dark matter must proceed through existing particles, such as the Z or Higgs boson. This means that the Z or Higgs boson could decay into two dark matter particles [5] , in addition to their ordinary decay modes involving Standard Model particles. While models with a Z boson portal are fairly constrained by precision measurements, including those done at the LEP collider at CERN during the s, now is the first time in the history of particles that we can study the properties of the Higgs boson in detail.

In addition to dark matter, one can also conceive of another particle not included in the Standard Model that acts as a portal particle. In the simplest versions of these models, the mediator is an unstable heavy particle that is produced directly from the interaction of Standard Model particles, such as quarks at the LHC.

THE EVIDENCE

Therefore, it must also be able to decay into those same particles, or into a pair of dark matter particles. If a model of this kind occurs in nature, we have a chance to directly discover this mediator particle at the LHC, as we would be able to detect its Standard Model decay products. While these models are commonly used to interpret the results of many LHC searches in terms of dark matter, they are too simple to represent the full complexity of a dark matter theory.

The WIMP is Dead. Long Live the WIMP!

However, they are still useful as building blocks for more complete theories with more ingredients. An appealing feature of supersymmetry is that it also solves a stability problem of the relatively low mass of the Higgs boson and other electroweak particles of the Standard Model around GeV compared to the Planck scale 10 19 GeV , at which gravity is expected to become strong and the Standard Model must break down. Quantum field theories like the Standard Model naturally prevent such large differences in energy scale from developing, so a physical mechanism is required to generate them.

SUSY models provide such a mechanism and, in many cases, predict the existence of a new stable, invisible particle - the lightest supersymmetric particle LSP - which has exactly the right properties to be a WIMP dark matter particle. If produced in LHC collisions, these particles could decay to produce a variety of Standard Model particles that can be observed in the ATLAS detector, together with two escaping LSP dark matter particles that generate the characteristic ETmiss signature discussed above.

Many other theories, of various degrees of completeness and complexity, contain dark matter particle candidates. Some of them predict new particles similar to the Higgs boson that can decay into dark matter, while others go beyond the WIMP paradigm and include mediators with extremely feeble interactions with known particles that only decay after traveling significant distances inside or outside! It is important for LHC searches to cover all this ground, while also preparing for unexpected, not-yet-theorised discoveries.

No stone must be left unturned! As shown in the diagram in Fig. A very similar technique can be used for detecting the presence of dark matter particles. If we take the process in Fig. The detector signature of the processes shown in Fig. Since we cannot distinguish the processes on a collision-by-collision basis, we have to take a different approach. We start by collecting a large number of events that have a large amount of missing transverse momentum and a highly energetic object.

So far, we have not found any excess with respect to backgrounds in this kind of search,as shown in Fig. Adding data and improving the experimental precision of future searches will enable us to search for even weaker dark matter interactions yielding processes that are still rarer than those to which we are already sensitive. The advantage of this kind of search is that it makes no specific assumption about the nature of the invisible particles, other than that they are produced in association with a Standard Model particle. The mediator particle can also decay to visible particles, leading to a peak or "resonance" in the total mass of those particles.

Searches for new particles using resonances in the total mass of visible particles have led to numerous discoveries at colliders, including, most recently, the Higgs boson at the LHC. Given that the LHC is the highest-energy laboratory particle collider, the most obvious goal is to search for extremely massive particles that could not have been produced before. Virtually every aspect of modern dark-matter research is covered, with the wide authorship providing detailed but consistently readable contributions. I can report that my colleagues, on seeing the book, have more often than not attempted to steal it away to lose themselves in its depths.

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