Lightening the dark
About 96% of the Universe is in the form of unknown matter and energy. The rest – only 4% – is the ‘ordinary matter’ that we are made of and that makes up all the planets, the stars and the galaxies we observe. The LHC experiments have the potential to discover new particles that could make up a large fraction of the Universe.
In recent years, scientists have collected various evidence of the existence of a new type of matter in the Universe. They call it ‘dark’ because it does not emit or absorb electromagnetic radiation. "One of the main proofs of its existence comes from the measurement of the rotational speed of astronomical bodies in spiral galaxies", explains Gian Giudice, a member of CERN's Theory group and the author of "A Zeptospace Odyssey
", a recent book on LHC physics aimed at the general public. According to the Newtonian laws of motion, this value varies as a function of the distance from the centre of the galaxy: more distant objects should rotate at a lower speed than those situated nearer the centre. However, back in the 1970s, astronomers found that outer stars move at a higher rotational speed than expected. “With such a velocity, the attractive gravitational force exerted by the observable mass would not be enough to keep those stars in the galaxy, and stars would simply escape”, continues Gian Giudice. Therefore, something must exist that keeps the galaxy together by exerting gravitational attraction.
"The second strong piece of evidence suggesting the existence of dark matter comes from the 'gravitational lensing' effect, in which galactic clusters bend the light coming from more distant objects. The way the light is deviated shows that the total mass contained in the clusters must be much larger than what we observe”, explains Giudice. Moreover, studies on the way in which the initial atoms and molecules formed in the Universe show that ordinary matter cannot account for more than 4% of the Universe. This fact allows scientists to exclude the possibility that invisible matter is made of massive objects such as Jupiter-sized planets. On the other hand, theory and observations do not exclude that dark matter is made of primordial black holes in which large amounts of matter could be trapped. However, this latter possibility seems very remote, and scientists tend to think that dark matter is made of a new type of particle.
How could the LHC help enlighten physicists?
"The yet undiscovered dark matter has to meet some requirements imposed by observations and theory", says Gian Giudice. "It has to be stable, it has to carry no charge, and it has to be relatively heavy”.
Through studies on the evolution of the Universe, scientists have been able to infer the mass of the dark matter constituents, situating it between 100 GeV and 1 TeV (for reference, the mass of the proton is about 1 GeV). Interestingly enough, this is exactly the same mass range in which theories beyond the Standard Model anticipate the existence of new particles.
“The LHC will explore exactly that range of energies. Therefore, if new particles exist, the LHC has a big chance of finding them”, confirms Gian Giudice. He adds: “The theoretical supersymmetric model suggests three possible candidates for dark matter: the neutralino
, the gravitino
and the sneutrino
. However, it is important to note that supersymmetry is not the only possible scenario".
Besides the whole plethora of possible alternative scenarios, even if the LHC experiments find evidence of new particles, it will not be possible to claim that they are the actual components of dark matter. For this, confirmation will be needed from other dedicated experiments (see box).
From deep inside the Earth to outer space
Other experiments are searching for the elusive dark matter particles. Some of them, such as the CDMS
experiment at the Soudan Underground Laboratory in Minnesota, and the XENON
experiments at the Gran Sasso Laboratory in Italy, are installed underground. Others, such as Pamela
, are in orbit around our planet.
by Francesco Poppi