If there really were some spontaneously broken symmetry that linked the strong and electroweak forces, then there would have to exist new heavy particles in order to fill out the complete family of force-carrying particles along with the W, Z, photon, and gluons. In this case, the energy of a few million billion billion volts could be identified as the energy contained in the mass of these new heavy particles. As we shall see, in today’s superstring theories there is no need to assume the existence of a separate new symmetry linking the strong and electroweak forces, but it is still the case
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