So now that the Higgs has supposedly been discovered and an important prediction of the Standard Model confirmed, what’s next? Is it of any use or is it just going to sit on the particle physics shelf as a trophy to the success of big science? This is hard to answer now and may become easier as the properties of the Higgs are studied in more detail. (For previous posts in this series, click on the Higgs folder just below the blog post title.)
Knowing the mass of the Higgs removes one of the last big unknowns of the Standard Model of particle physics and means that more detailed studies can be made of its properties. While much of this work will undoubtedly be done at the LHC, where they will also search to see what else turns up as they ramp up the collision energy to the maximum design value of 14 TeV, plans are also currently underway to build an International Linear Collider in Japan that will have electron-positron collisions with energies starting at 250 GeV and then up to 500 GeV with the potential of reaching 1 TeV. This machine can tune its energies to focus precisely on the Higgs mass of 126.5 GeV and thus tease out detailed properties of it. The ILC is expected to also cost around $8 billion, roughly the same as the LHC, though these costs tend to rise as things get underway. There is another proposal for a different electron-positron machine called the Compact Linear Collider (CLIC) that can reach energies of 5 TeV.
In both designs, two linear colliders will accelerate the two particle beams straight towards each other. The total length of the system will be about 30 km for the ILC and 50 km for the CLIC, which would likely make Inigo Montoya say of the word ‘compact’ in the latter’s name, “I do not think it means what you think it means.”
With the mass known, calculations can be made of whether the Higgs has excited states and the rate of decay of the Higgs particle into each individual channel and those predictions can be compared with observations. If there is a discrepancy, that might signal that there is something new going on and may reveal some interesting and unexpected features. What might those be?
There is already speculation that the Higgs may shed light on the elusive dark matter. As many readers will be aware, only about 4.6% of the total energy that exists in the universe is due to the presence of matter made up of the 19 elementary particles that constitute the Standard Model and thus are known to exist. The remaining 95.4% has not as yet been directly detected. According to the best estimates, 23.3% of this is made up of what we call ‘dark matter’. The remaining 72.1% is what is known as ‘dark energy’ that is spread uniformly throughout the entire universe and is thought to be the reason why the expansion of the universe seems to be accelerating.
Neither dark matter nor dark energy has been directly detected but is postulated to exist in order to explain the large-scale properties of the universe. Dark matter provides an explanation for the otherwise anomalous behavior of stars in galaxies, since the gravitational force exerted by the known (‘visible’) matter is not sufficient to do so. Dark matter is believed to exist in a giant spherical halo that is concentric about the center of galaxies, but extends well beyond its edges. Since we are all immersed in it but don’t seem to be sense it, dark matter must interact very weakly with ordinary matter so that it passes freely through each of us and the Earth, leaving almost no trace, very much like neutrinos.
What is dark matter? We don’t really know. One suggestion is that it is a ‘weakly interacting massive particle’ or WIMP for short. ‘Weak’ in this context does not mean simply the opposite of strong but that the WIMP interacts with other particles via the weak interaction, one of the four fundamental forces. Of course, it is possible that it interacts via a new force entirely or that gravity is the only way it interacts. Currently there are searches underway in deep underground mines (to reduce background effects) where detectors consisting of liquid helium or solid germanium and silicon are looking for the recoil of particles or the tiny amount of heat generated on the rare occasions when a dark matter particle collides with one of the atoms in the detector.
This is where one of the properties of the Higgs might prove helpful. Since the strength of the Higgs particle’s interaction with other particles increases with the mass of that particle, the heavy WIMP may interact strongly with the Higgs and the resulting reaction may give rise to particles that we are familiar with and thus know how to detect. One possible mechanism is that a dark matter particle emits a Higgs particle that then gets absorbed by a quark that is inside a proton. By observing the recoil of the proton, we might be able to tell that such a collision took place. So the Higgs may serve as the gateway to detecting dark matter.
We do not know the mass of the dark matter particle we are searching for so like with the search for the Higgs, we have to do calculations for the entire range of possible masses that it could have and then use that to calculate the properties of such collisions and see if that (after subtracting out the huge background) is consistent with the data. It is not easy to do.
There are more fanciful suggestions that the Higgs particle may be the gateway to many particles that we have never seen before, kind of like a portal to a new and exotic world. But while that would be exciting, it is just speculation at this point and will have to await experimental signals. But it does suggest that the Higgs is not to be considered a mere trophy.
Next: Nobel dilemmas