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Section 8: The Origin of Mass

The mystery of CP violation and the origin of our matter-dominated universe represent two of the basic issues in 21st century physics. But thousands of physicists are working night and day to solve an even more fundamental problem: How do particles acquire mass? Although many of us would like to have less mass, particle theorists find it extremely difficult to explain how we have any at all.

Scottish theorist Peter Higgs postulated that particles acquire mass by scattering off of a particle that fills all space, now called the Higgs boson. The heavier the individual particle, the more often it will interact with the Higgs. Think of a politician moving through a crowd. The more popular she is, the more people will try to shake her hand. In analogy, the heavy top quark interacts constantly by scattering off of Higgs particles, while the light electron moves through the crowd with only an occasional handshake.

Physicists have sought the Higgs boson for decades, hoping to find it each time. A new, more powerful accelerator opened up another window on the production of heavier particles. CERN's $9 billion Large Hadron Collider (LHC) is the latest and greatest vehicle, replacing Fermilab's Tevatron as the most powerful accelerator on Earth. Many hopes ride on the LHC. However, the collider's promise suffered an early blow. In July 2009, less than ten months after the machine generated its first proton beams, physicists identified problems in its electrical connections that threatened its ability to run at full power. Those problems delayed the LHC's experimental timetable. In doing so, it increased the—admittedly small—chance that the Tevatron might find the first evidence for the Higgs boson.

Early in 2009, scientists working at the Tevatron reported precise studies of the mass of the W boson, which carries the weak nuclear force. Those measurements put strict bounds on the mass of the Higgs boson, suggesting that it is probably quite light, and implying that the LHC will have some difficulty detecting it. Plainly, the race for the Holy Grail of particle physics will continue unabated.

Presenting limits on the Higgs boson's mass.

Figure 24: Presenting limits on the Higgs boson's mass.

Source: © Fermilab. More info

Of course, it is quite possible that neither the Tevatron nor the LHC will observe the Higgs boson. There may even be several Higgs particles, in addition to new partners for all of the known fundamental particles. And, if neutrinos are confirmed to be their own antiparticle in double beta-decay experiments, the Higgs mechanism cannot explain neutrino masses, replacing one mystery with another. This may provide the most exciting scenario of all for particle physicists: the opportunity to discover new particles and the laws that govern them.