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Victor Stenger

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The Disappointing Higgs and Sterile Neutrinos

Posted: 09/14/2012 12:25 pm

So far, the Higgs boson has been a disappointment. Of course, it was a major discovery that generated worldwide attention. The two independent experiments at the Large Hadron Collider that reported the discovery on July 4, involving thousands of physicists, engineers, and technicians, were conducted with praiseworthy skill and effort.

However, all that has been learned so far is that its mass is 125-126 GeV (billion electron-volts). Otherwise, the data look exactly as predicted by the standard model of elementary particles and forces that was formulated in the 1970s and has agreed with all observations since.

Scientists had anticipated that the LHC would point the way to new physics beyond the standard model. And, this will hopefully still happen, especially after its energy is doubled in 2013. One expectation that has not yet been fulfilled is the production of new particles predicted by a theoretical hypothesis called supersymmetry (SUSY). This principle postulates that the laws of physics are the same for integer spin particles, called "bosons," and half-integer spin particles called "fermions." SUSY predicts that every boson will have a fermion partner and every fermion will have a boson partner of the same mass. So, the spin ½ electron will have a spin zero "spartner," the selectron. And, the spin 1 photon will have a spin ½ spartner, the photino. My favorite is the spartner of the quark, which is called the "squark."

Since squarks, selectrons, and photinos have never been observed, physicists are forced to conclude these sparticles, if they exist at all, are so much more massive than their normal partners that they could not be produced by previous particle accelerators. It was anticipated that the energy of the LHC would be sufficient to reach the regime where supersymmetry comes into play and sparticles would appear. Theorists had, in fact, expected sparticles to appear by now, but it hasn't happened. By contrast, the Higgs showing up so soon was a pleasant surprise.

Supersymmetry has been a favorite idea among theoretical physicists for decades. It appears to be essential for any future quantum theory of gravity. A generation of young theorists has spent their careers developing the SUSY-based String theory, which they hope to be the ultimate theory-of-everything (TOE). If SUSY is falsified, it is unlikely that String theory will survive.

Actually, that would not be all that bad. Any TOE would mean the end of physics.

One of the big questions in science today is the nature of the dark matter. For years, the favorite candidate has been WIMPs, weakly interacting massive particles. And the most popular WIMPs are supersymmetric particles referred to generically as neutralinos.

While reports that SUSY is dead are exaggerated, other options for the dark matter are currently receiving renewed attention. A recent cover story in New Scientist (September 10, 2012) talked about the role that neutrinos play in the search for physics beyond the standard model. Of particular note are sterile neutrinos, which could constitute both the ingredient of dark matter and a pointer to new physics. (See Nature News Vol 464, March 18, 2010).

In 1998, an underground experiment in Japan called Super Kamiokande reported the first evidence that neutrinos have mass. I played a small role as a collaborator on this experiment, my final research endeavor before retiring from the University of Hawaii in 2000.

Massless particles with spin have the feature of always spinning either in the same direction as their motion, like a right-handed screw, or opposite. Neutrinos are observed to have left-handed "helicity," that is, to spin opposite to their direction. Antineutrinos are right-handed, like a normal screw.

However, when a particle has mass it can have either helicity. It follows that neutrinos must have a small right-handed component while antineutrinos have a small left-handed part. Since these components are not observed, they may possibly be "sterile," meaning, they interact only gravitationally. In that case, they are good candidates for dark matter.

To constitute the dark matter, a sterile neutrino would have to have a mass of at least 1 keV (thousand electron-volts) and a lifetime of billions of years. These features are not ruled out by any known physics.

Interest in sterile neutrinos has also been piqued by several hints in neutrino experiments and astronomical observations. None are sufficiently significant, however, to claim a discovery.

In any case, over 20 experiments are now planning to search for evidence of sterile neutrinos. For all you could want to know about sterile neutrinos, including details on each of these proposals, see the draft of a white paper under preparation by over 200 physicists. As someone who worked on neutrino physics and astrophysics for 30 years, I'm delighted to see neutrinos continue to be a crucial factor in our understanding of nature.

 
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