The Higgs boson — a particle thought to explain how other particles get their mass — is tiny, but it may not be the tiniest particle yet. Theories have long predicted the existence of even smaller particles that might make up the Higgs, and recent research suggests these pip-squeaks, dubbed techni-quarks, are likely lurking in the universe.
However, it will take the upgrade of the Large Hadron Collider (LHC) — the world's most powerful particle accelerator — or the next generation of colliders to spot these Higgs components, said Thomas Ryttov, a particle physicist at the University of Southern Denmark.
"We have nailed it down to only a few theories that have the right properties and characteristics to explain the Higgs particle and the Higgs mechanism," Ryttov said.
The Higgs boson was discovered in 2012 at the Large Hadron Collider at CERN, in Switzerland. Scientists Peter Higgs, from the United Kingdom, and Francois Englert from Belgium — who both worked on the theory of the Higgs — received the 2013 Nobel Prize for physics for their research.
But there is a problem with just a single fundamental Higgs.
This particle is supposed to explain why the most basic building blocks of matter have mass. However,the vacuum — as physicists understand it through the framework of quantum field theory, the mathematical theory on which all results in particle physics are based— is not empty, but consists of a multitude of invisible "virtual" particles that constantly pop in and out of existence. Virtual pairs of particles are created and then quickly annihilated.
When Higgs particles pass through the vacuum, they are supposed to interact with all of these virtual particles while, in the process, increasing their own mass to huge values — some 100 million billion times greater than the one measured at the LHC. Therefore, their mass should then be comparable to what is known as the Planck mass, which is the fundamental unit of mass in the system of Planck units, equal to 2.18 × 108 kilograms.
"The question is, then, why the Higgs' measured mass is so much lighter than the Planck mass," Ryttov said. "This is exactly the problem."
For this mass increase not to happen, the reigning theory of particle physics — called the Standard Model — requires a high degree of fine-tuning, to correct for the differences in the measured Higgs mass and its so-called "bare mass," or the heavier mass.
This need to fine-tune is known as the naturalness problem — "a thorn in the eye of theoretical particle physicists," Ryttov said. "The theory is not as beautiful and elegant as we would expect from a theory that, in principle, should describe all matter at the most fundamental level. The Standard Model needs a tremendous amount of fine-tuning," he added.
To remove the need for fine-tuning and still answer the Higgs-mass question, physicists have suggested extensions of the Standard Model, the most popular of which is supersymmetry. This theory proposes a heavier superparticle, or "sparticle," for every particle in the Standard Model. Sparticles would then cancel out the effect of the virtual particles in the vacuum, bringing down the Higgs mass and removing the need for any fine-tuning.
None of these hypothetical supersymmetric particles have been observed so far, though.
But there are many theoretical indications that the Higgs particle could be a composite one — made of some other, smaller, particles, called techni-quarks, Ryttov said. "The problem evaporates if the Higgs particle is composed of smaller bricks of nature that bind together via a new force — the technicolor force— to form the Higgs, similar to quarks binding together to form protons and neutrons," he said.
Here's how techni-quarks would solve the mass issue: Huge corrections to the mass of the Higgs in the Standard Model are needed because it is supposed to be a fundamental particle — in other words, not made of something else — with vanishing, or zero, spin.
Techni-quarks are particles with a spin of one half, Ryttov said, so by combining two techni-quarks, it is possible to make a composite particle with vanishing spin, such as the Higgs. "It turns out that theories with only techni-quarks have no naturalness problem," Ryttov said.
The idea of techni-quarks has been around since the late 1970s, but recently, there have been several important developments and refinements of the original models.
In their latest paper, detailed on the prepublish site Arxiv, Ryttov and his colleagues have argued once again that the Higgs must have an inner structure, nailing down a handful of theories that "have the right properties to fix the problem of fine-tuning in the Standard Model and bring the subatomic world into harmony again," the researchers said.
To do so, Ryttov has examined a number of theories dealing with a composite Higgs, to see whether there could be any weaknesses in them that have been overlooked. However, "They all came out strong, indicating that there could be something real about a Higgs made out of yet more building blocks," he said.
- Katia Moskvitch, LiveScience
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