In the next five months, the Large Hadron Collider (LHC), the world’s biggest particle accelerator, installed on the Swiss-French border, will function at its full capacity to produce massive data in an attempt to reveal the identity of the latest elementary particle discovered by physicists. On July 4 last, during the International Conference on High Energy Physics, one of the most prestigious annual particle physics events, researchers from the European Organization for Nuclear Research (Cern), linked to the LHC, announced that they had discovered a new elementary particle which according to all indications is the Higgs boson, the missing part needed to conclude the Standard Model, the successful physics theory. This theory explains what matter is made of and how it behaves at the subatomic level. “This is the most exciting moment in the field of particle physics since the 1970s,” physicist Joseph Incandela told Pesquisa FAPESP. He is the coordinator for one of the experiments being conducted at the LHC.
By the end of this year, the LHC is expected to provoke the collision of more than 3 quadrillion accelerated protons at close to the speed of light, inside a ring with a circumference of 27 kilometers. The ring was built 100 meters under the ground, with the objective of describing the new particle’s characteristics in detail. It might seem nonsensical, but the physicists hope that the data to be collected will show that the recently identified particle – if it really is the Higgs boson – does not behave as expected. If the particle does not behave normally, they will have found something entirely new in the field of physics for the first time in 40 years, which will enable them to get a better understanding of how the Universe developed at the beginning of its life. However, if the particle behaves as expected, they will have reached a dead end: the Standard Model will have been confirmed, but there will be no clues on how to improve it to answer unanswered questions about the Universe.
The complete Standard Model explains the existence of only 4% of what the Universe is made of. It does not explain the origin of 23% of the dark matter or 73% of the dark energy that must exist in order for the Universe to be as people envision it. In addition, the Standard Model does not provide much information on what might have occurred one second after the Big Bang, the explosion that is believed to have generated the Universe 13.7 billion years ago. The four fundamental forces of nature – gravitational, electromagnetic, weak nuclear and strong nuclear – were unleashed at this mysterious instant. These forces probably originated from a single initial force – and led to the formation of matter (see figure).
To further clarify what happened at that specific moment, physicists have developed theories that expand the Standard Model and predict the existence of more particles. As none of these particles have been detected yet, physicists do not know which of the main theories – namely, the theories of supersymmetry, compound models and extra dimensions – is the correct one (see figure). The physicists expect that, upon defining the characteristics of the Higgs boson or upon finding a new particle, they will find evidence that one of these theories is the correct one.
Physicists have been looking for the Higgs boson – a key element of the Standard Model – for at least three decades. The Standard Model, developed in the 1960s, describes what happens when subatomic particles are accelerated almost up to the speed of light, and collide with each other, which is what occurs within the LHC. According to Einstein’s famous equation, which establishes that energy is equivalent to the mass multiplied by the speed of light squared (E=mc2), the energy of these collisions can be converted into mass, causing, as if by magic, new particles to appear from the vacuum. In general, particles with a lot of mass survive for fractions of a second, disintegrate very quickly and turn into a cascade of lighter particles that leave traces in such detectors as the CMS and LHC’s Atlas, the construction of which cost approximately US$ 9 billion (see Pesquisa FAPESP no. 147).
The result of the disintegration can be calculated by using the equations of the Standard Model, whose mathematical properties describe how the particles interact. When they were first developed, these equations seemed to function very well, with the exception of one detail: they predicted that all particles should resemble photons, which are without mass, which is why these always travel at the speed of light. If this actually happened with all particles, the world as we know it would not exist, because the particles would never stop moving; the fact that particles rest enables the existence of atoms. To deal with this crucial detail, in 1964, Peter Higgs and other physicists proposed the existence of another field of force that supposedly permeated the entire outer space and interacted at a different intensity with each kind of particle, providing the particles with different masses. Proof of the existence of this field would be the discovery of a particle that could emerge from this field during high energy collisions: this particle is the Higgs boson, apparently now found at Cern (see figure).
“Everything that we have measured so far leads us to believe that we discovered the Higgs boson,” says physicist Sérgio Novaes, who leads a group of researchers at the Institute of Theoretical Physics of Paulista State University (Unesp) and at the Federal University of ABC. The group, funded by Fapesp, is participating in the analysis of the data provided by the CMS detector.
Both the CMS and the Atlas have received signs of the existence of a new boson, whose mass ranges from 125 to 126 giga-electron volts (GeV) – one GeV corresponds to one billion electron volts, the unit of energy used to measure the particle’s mass. The probability that the measured signal is the result of chance is one in 3.5 million. The new particle also seems to disintegrate, as predicted by the Standard Model, but the researchers must still analyze many more collisions to establish, with the same degree of certainty, the boson’s disintegration pattern as well as its other properties. Incandela estimates that the LHC will be able to establish the characteristics of this new particle more accuracately by the end of the year. Uncertainties in this respect are expected to be clarified more consistently from 2015 onward, when the LHC should go back into operation, after being shut down for two years for adjustments. The adjustments will increase the energy of its collisions from 8 to 13 tera-electron volts (TeV) and increase the number of collisions accumulated until 2018 ten times.
If the Higgs boson is confirmed, it will be the first elementary particle of a special class that is an enigma to theoreticians. “This class of particles is quite mysterious, because it is very difficult for their mass to stabilize,” says Incandela, referring to the Standard Model’s Achilles’ heel, referred to as the hierarchy problem.
The hierarchy problem appears when the Standard Model is viewed as a theory that explains how particles and forces have interacted since the Universe’s first moments – at time zero – when energy levels were trillions of times more powerful than the ones achieved nowadays in the LHC. In physics, this is referred to as the Planck scale, which depicts the most powerful energy that can exist in the Universe. Under these conditions, gravitational force, which generally does not affect particles because it is less powerful than the other three forces, starts to affect them. The theory hypothesizes that certain interactions of the Higgs boson – with itself and with the other particles – might trigger an enormous growth of its mass, the size of which would be much greater than expected. The Standard Model only provides the Higgs boson’s correct mass when it is assumed that some unknown effect counterbalances the enormous increase of the mass.
In the opinion of many physicists, the nature of this effect could be revealed in collisions of particles conducted in the energy range that the LHC explores nowadays. This was the second most important reason for building the LHC – the first reason was to search for the Higgs boson, often referred to as the “God particle.” Physicists dislike this nickname, which was suggested by the publisher of the book The Goddamn Particle- later renamed The God Particle – written in 1993 by physicist Leon Lederman and science writer Dick Teresi. “The hierarchy issue leads us to reflect on why and how to extend the Standard Model,” says Gustavo Burdman, a professor at the Physics Institute of the University of São Paulo (USP).
The search for a solution to the hierarchy problem has guided the work of theoretical physicists for many years. They have attempted to explain this phenomenon in different ways under the theories of supersymmetry, compound models, and extra dimensions. Each theory has a strategy to stabilize the mass of the Higgs boson.
Supersymmetry is by far the most extensively studied theory. It hypothesizes that there is an additional particle for each particle of the Standard Model. This additional particle is referred to as the superpartner particle. The supersymmetric particles subtract part of the Higgs boson’s mass, in the same proportion in which the Standard Model particles add mass to the boson, thus eliminating the hierarchy problem.
Supersymmetry became popular among physicists because of its mathematical elegance, which facilitates calculations and provides solutions to problems other than the hierarchy problem. The lighter superpartners might constitute the dark matter. In addition, supersymmetry makes it possible to unify the Standard Model’s electromagnetic, weak nuclear, and strong nuclear forces on an energy scale that is closer to the energy in place during the first seconds after the Big Bang (see figure).
The existence of supersymmetry is necessary for the consistency of the superstring theory, that attempts to unify all the forces, including the gravitational force. “It is a beautiful theory, with very good properties; it would be very interesting if it existed,” says physicist Oscar Éboli, from USP, who is looking for evidence of supersymmetry in the data from the LHC.
There are countless versions of supersymmetry. The simpler models predicted that as soon as the LHC was activated in 2008, the superpartners would show up in profusion. However, no sign of these superpartners has been seen yet. To complicate matters, the mass that seemed to be that of the Higgs boson is larger than the one predicted in those models. “The more obvious models are in a very difficult situation,” says Éboli. This means that the theory might be much more complicated and the mass of the superpartners larger than expected. “The larger the mass, the less interest certain fields of physics have in it, because this would no longer explain the hierarchy problem,” says Ricardo Matheus, a professor at Unesp’s Institute of Theoretical Physics.
Because of this lack of evidence, theories other than supersymmetry have been more extensively explored in the last few years. One class of such theories is referred to as compound models, which hypothesize that the Higgs boson and possibly other particles of the Standard Model are constituted of even more elementary particles. The fact that the Higgs boson is formed from other particles would change its properties, thus eliminating the accumulating effect of the mass that causes the hierarchy problem.
However, if this theory proves to be correct, the history of physics would be repeated once again. Until the 1960s, the general belief was that protons, neutrons, and other particles such as the pion, discovered by Brazil’s César Lattes in 1947, were elementary particles. The Standard Model made it clear that they were constituted of quarks, which are even more basic particles. Like supersymmetry, the compound models predict the existence of new particles that have not been observed yet – some versions of these models have already been discarded. “For a very long time, the compound models have been at odds with the data,” says Incandela, “but we cannot discard them entirely.”
Another solution for the hierarchy problem might be the existence of extra spatial dimensions, a hypothesis that so far lacks experimental proof. These dimensions, which are hard for even physicists to grasp, might, in principle, be detected in the LHC, provided that the particles responsible for gravitational force – the gravitons – exist at the tera-electron volt level of energy. In fact, the energy of the Big Bang at time zero should have been trillions of times lower than the energy currently calculated. In other words, the Planck scale would be wrong and the Higgs boson would not accumulate mass because it would have already achieved the biggest possible mass. One of the consequences of this theory is that the Universe would be one second younger.
The team led by Novaes is looking for signs of extra dimensions in the data obtained by the DZero detector, from the recently de-activated North American accelerator Tevatron, and by the LHC. In March of this year, the DZero collaboration published an analysis conducted by Angelo Santos, who is doing a doctoral program under Novaes. The analysis, published in Physical Review Letters, establishesthe first experimental limits for the existence of a certain extra dimension model.
However, both the Tevatron and the LHC have already discarded extra large dimensions, big enough to be perceived as having energies of up to 2 TeV. Perhaps, when increasing the energy of the collisions, the LHC might find evidence in the next few years of the existence of extra smaller dimensions. Many of the extra dimension theories, however, make predictions that are nearly identical to those of the compound models, which would not allow one to be distinguished one from the other. “This is an issue that will come up at some point,” says Burdman.
“So far we haven’t seen anything that clashes significantly with the expectations of the Standard Model’s Higgs boson,” says Incandela of the results presented on July 4. However, he acknowledges that there are a few signs indicating that the Higgs boson might not be behaving as expected. “These signs could become significant by the end of this year, but they could also disappear easily,” he adds.
The transformation of the new boson into pairs of photons, an event that seems to occur at a higher proportion than initially expected, has attracted the bulk of scientists’ attention so far. The Standard Model predicts into which particles the Higgs boson can be transformed and how frequently each one appears (see figure).
In an analysis published one day after the announcement of the discovery of the Higgs boson, Éboli and his colleagues from the United States and Spain also showed that the production of the probable Higgs boson in the LHC is about half of the production predicted by the Standard Model.
Many papers published since July 4 speculate that the excess of photons as well as the production of the Higgs boson have been provoked by the influence of superpartner particles. Éboli compares the confirmation of these signs to a test to verify whether a coin is counterfeit or not; –when a coin is tossed upwards, it has an equal chance of showing heads or tails when it falls down. “If a coin is tossed ten times upwards and the result is seven heads and three tails, it could be presumed that there is a slight indication that the coin might be a counterfeit coin. For the same reason, it will only be possible to confirm if the disintegration of the boson into photons is happening at a normal rate if many more collisions are analyzed. “The experimental error in the disintegrations is still significant and we need to obtain more data to check that it is really the Higgs boson of the Standard Model,” says Éboli.
“This year, the entertainment of the theoretical physicists is to pay attention to the data and respond swiftly,” says Matheus, from Unesp. He compares the physicists´ current situation to that of Christopher Columbus about to discover America. “Burdman agrees: “Physics can change from one day to the next.”
São Paulo Regional Analysis Center (nº 2008/02799-8); Modality Regular Funding for Research Project; Coordinator Sergio F. Novaes – Unesp; Investment R$ 2,023,838.68 (FAPESP).
ABAZOV, V. M. et al. Search for Universal Extra Dimensions in pp- Collisions. Physical Review Letters. 30 Mar. 2012.
CORBETT, T. et al. Constraining anomalous Higgs interactions. http://arxiv.org/pdf/1207.1344.pdf.