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This article is from the In-Depth Report The Higgs Boson at Last?

Could the Large Hadron Collider Discover the Particle Underlying Both Mass and Cosmic Inflation?

If the LHC discovers the Higgs boson or other theoretical particles, their existence could help explain inflation, one of the universe's great mysteries
LHC, large hadron collider, higgs boson



CERN

Within a sliver of a second after it was born, our universe expanded staggeringly in size, by a factor of at least 10^26. That's what most cosmologists maintain, although it remains a mystery as to what might have begun and ended this wild expansion. Now scientists are increasingly wondering if the most powerful particle collider in history, the Large Hadron Collider (LHC) in Europe, could shed light on this mysterious growth, called inflation, by catching a glimpse of the particle behind it. It could be that the main target of the collider's current experiments, the Higgs boson, which is thought to endow all matter with mass, could also be this inflationary agent.

During inflation, spacetime is thought to have swelled in volume at an accelerating rate, from about a quadrillionth the size of an atom to the size of a dime. This rapid expansion would help explain why the cosmos today is as extraordinarily uniform as it is, with only very tiny variations in the distribution of matter and energy. The expansion would also help explain why the universe on a large scale appears geometrically flat, meaning that the fabric of space is not curved in a way that bends the paths of light beams and objects traveling within it.

The particle or field behind inflation, referred to as the "inflaton," is thought to possess a very unusual property: it generates a repulsive gravitational field. To cause space to inflate as profoundly and temporarily as it did, the field's energy throughout space must have varied in strength over time, from very high to very low, with inflation ending once the energy sunk low enough, according to theoretical physicists.

Much remains unknown about inflation, and some prominent critics of the idea wonder if it happened at all. Scientists have looked at the cosmic microwave background radiation—the afterglow of the big bang—to rule out some inflationary scenarios. "But it cannot tell us much about the nature of the inflaton itself," says particle cosmologist Anupam Mazumdar at Lancaster University in England, such as its mass or the specific ways it might interact with other particles.

A number of research teams have suggested competing ideas about how the LHC might discover the inflaton. Skeptics think it highly unlikely that any earthly particle collider could shed light on inflation, because the uppermost energy densities one could imagine with inflation would be about 10^50 times above the LHC's capabilities. However, because inflation varied with strength over time, scientists have argued the LHC may have at least enough energy to re-create inflation's final stages.

It could be that the principal particle ongoing collider runs aim to detect, the Higgs boson, could also underlie inflation.

"The idea of the Higgs driving inflation can only take place if the Higgs's mass lies within a particular interval, the kind which the LHC can see," says theoretical physicist Mikhail Shaposhnikov at the École Polytechnique Fédérale de Lausanne in Switzerland. Indeed, evidence of the Higgs boson was reported at the LHC in December at a mass of about 125 billion electron volts, roughly the mass of 125 hydrogen atoms.

Also intriguing: the Higgs as well as the inflaton are thought to have varied with strength over time. In fact, the inventor of inflation theory, cosmologist Alan Guth at the Massachusetts Institute of Technology, originally assumed inflation was driven by the Higgs field of a conjectured grand unified theory.

The virtue of so-called Higgs inflation models is that they might explain inflation within the current Standard Model of particle physics, which successfully describes how most known particles and forces behave. Interest in the Higgs is running hot this summer because CERN, the lab in Geneva, Switzerland, that runs the LHC, has said it will announce highly anticipated findings regarding the particle in early July.

The problem with many of these models is that when they are run, the Higgs decreases in energy too quickly and therefore would not generate fluctuations seen in the cosmic microwave background radiation. As such, they require the existence of additional fields to accomplish all the effects of inflation, ruining the simplicity one would desire from such models in the first place.

A Higgs inflation model that Shaposhnikov and colleague Fedor Bezrukov at the University of Connecticut proposed in 2007 (pdf) eliminated the need for such extra fields by suggesting the Higgs interacts with gravity in a different way than other particles. This would allow the Higgs to keep its energy long enough to result in the kind of universe we see today. Mazumdar says the difficulty this idea faces is why the Higgs would have this special relationship with gravity to begin with when no other particle does, or why it would interact very weakly as proposed.

Instead, Mazumdar and others have suggested that other particles the LHC could detect might shed light on inflation. The models they suggest are rooted in the theory of supersymmetry, which connects the two known basic types of particles, the ones that make up matter (fermions) with those that carry the fundamental forces (bosons), predicting that each fermion has a heavier bosonic counterpart and vice versa. The discovery of supersymmetric particles or "sparticles" at the LHC could help solve key mysteries within the Standard Model. For instance, the invisible dark matter thought to make up most of the mass in the universe could be a kind of sparticle known as a neutralino. If the inflaton is also a sparticle, Mazumdar says it must have ended up with a relatively low energy density, one potentially detectable by the LHC; otherwise, the inflaton would have helped generate a lower ratio of normal to dark matter than we see in the universe.

If the LHC does prove supersymmetry correct by finding sparticles, Mazumdar's analysis suggests that the inflaton would be a sparticle with a mass of about 1,000 billion electron volts. In comparison, the LHC is capable of energies of up to 7,000 billion electron volts. In May, Mazumdar's team submitted research on how the LHC might discover the inflaton to the Journal of Cosmology and Astroparticle Physics.

Alternatively, Mazumdar says there are supersymmetry scenarios where the Higgs is the inflaton, either by interacting with sparticles such as the superpartner of the neutrino or by itself, work detailed in 2007 in Physical Review Letters and in 2011 in the Journal of Cosmology and Astroparticle Physics, respectively.

Recent data from the LHC, however, might suggest that many current models of supersymmetry may be wrong, because experiments have not yet uncovered any of the sparticles the models predict.

"Ultimately, if the LHC discovers the Higgs boson and nothing else, for me that would favor a Higgs inflation model; if the LHC discovers supersymmetric particles or any other type of new physics, the model would not be so attractive," Shaposhnikov says. "Hopefully we should learn a little more either way when CERN gives an update on the Higgs search on July 4, and on searches for new physics at the International Conference on High Energy Physics later in July in Australia."

Guth feels it is quite likely that inflation's energy scales are well beyond those available at the LHC. "But we do not know that," he says, adding that researchers should use the European particle collider to explore these ideas. "It is exciting that there is a possibility that the LHC could actually discover the field that drives inflation," Guth concludes.

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