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THE FABRIC of the COSMOS, Brian Greene, 2004
```(annotated and with added bold highlights by Epsilon=One)
```(annotated and with added bold highlights by Epsilon=One)
Chapter 14 – Up in the Heavens and Down in the Earth
The Higgs, Supersymmetry, and String Theory
Beyond the scientific challenges of searching into the unknown, and the chance of finding evidence of extra dimensions, there are a couple of specific motivations for recent upgrades on the accelerator at Fermilab and for building the mammoth Large Hadron Collider. One is to find Higgs particles. As we discussed in Chapter 9, the elusive Higgs particles would be the smallest constituents of a Higgs field — a field, physicists hypothesize, that forms the Higgs ocean and thereby gives mass to the other fundamental particle species. Current theoretical and experimental studies suggest that the Higgs should have a mass in the range of a hundred to a thousand times the mass of the proton. If the lower end of this range turns out to be right, Fermilab stands a reasonably good chance of discovering a Higgs particle in the near future. And certainly, if Fermilab fails and if the estimated mass range is nonetheless correct, the Large Hadron Collider should produce Higgs particles galore by the end of the decade. The detection of Higgs particles would be a major milestone, as it would confirm the existence of a species of field that theoretical particle physicists and cosmologists have invoked for decades, without any supporting experimental evidence.
Another major goal of both Fermilab and the Large Hadron Collider is to detect evidence of supersymmetry. Recall from Chapter 12 that supersymmetry pairs particles whose spins differ by half a unit and is an idea that originally arose from studies of string theory in the early 1970s. If supersymmetry is relevant to the real world, then for every known particle species with spin-1/2 there should be a partner species with spin-0; for every known particle species of spjn-1, there should be a partner species with spin-1/2. For example, for the spin-1/2 electron there should be a spin-0 species called the supersymmetric electron, or selectron for short; for the spin-1/2 quarks there should be supersymmetric quarks, or squarks; for spin-1/2 neutrinos there should be spin-0 sneutrinos; for spin-1 gluons, photons, and W and Z particles there should be spin-1/2 gluinos, photinos, and winos and zinos. (Yes, physicists get carried away.)
No one has ever detected any of these purported doppelgangers, and the explanation, physicists hope with fingers crossed, is that the supersymmetric particles are substantially heavier than their known counterparts. Theoretical considerations suggest that the supersymmetric particles could be a thousand times as massive as a proton, and in that case their failure to appear in experimental data wouldn't be mysterious: existing atom smashers don't have adequate power to produce them. In the corning decade this will change. Already, the newly upgraded accelerator at Fermilab has a shot at discovering some supersymmetric particles. And, as with the Higgs, should Fermilab fail to find evidence of supersymmetry and if the expected mass range of the supersymmetric particles is fairly accurate, the Large Hadron Collider should produce them with ease.
The confirmation of supersymmetry would be the most important development in elementary particle physics in more than two decades. It would establish the next step in our understanding beyond the successful standard model of particle physics and would provide circumstantial evidence that string theory is on the right track. But note that it wouldn't prove string theory itself. Even though supersymmetry was discovered in the course of developing string theory, physicists have long since realized that supersymmetry is a more general principle that can easily be incorporated in traditional point-particle approaches. Confirmation of super-symmetry would establish a vital element of the string framework and would guide much subsequent research, but it wouldn't be string theory's smoking gun.
On the other hand, if the braneworld scenario is correct, upcoming accelerator experiments do have the potential of confirming string theory. As mentioned briefly in Chapter 13, should the extra dimensions in the braneworld scenario be as large as 10^-16 centimeters, not only would gravity be intrinsically stronger than previously thought, but strings would be significantly longer as well. Since longer strings are less stiff, they require less energy to vibrate. Whereas in the conventional string framework, string vibrational patterns would have energies that are more than a million billion times beyond our experimental reach, in the braneworld scenario the energies of string vibrational patterns could be as low as a thousand times the proton's mass. Should this be the case, high-energy collisions at the Large Hadron Collider will be akin to a well-hit golf ball ricocheting around the inside of a piano; the collisions will have enough energy to excite many "octaves" of string vibrational patterns. Experimenters would detect a panoply of new, never before seen particles — new, never before seen string vibrational patterns, that is — whose energies would correspond to the harmonic resonances of string theory.
The properties of these particles and the relationships between them would show unmistakably that they're all part of the same cosmic score, that they're all different but related notes, that they're all distinct vibrational patterns of a single kind of object — a string. For the foreseeable future, this is the most likely scenario for a direct confirmation of string theory.
Another major goal of both Fermilab and the Large Hadron Collider is to detect evidence of supersymmetry. Recall from Chapter 12 that supersymmetry pairs particles whose spins differ by half a unit and is an idea that originally arose from studies of string theory in the early 1970s. If supersymmetry is relevant to the real world, then for every known particle species with spin-1/2 there should be a partner species with spin-0; for every known particle species of spjn-1, there should be a partner species with spin-1/2. For example, for the spin-1/2 electron there should be a spin-0 species called the supersymmetric electron, or selectron for short; for the spin-1/2 quarks there should be supersymmetric quarks, or squarks; for spin-1/2 neutrinos there should be spin-0 sneutrinos; for spin-1 gluons, photons, and W and Z particles there should be spin-1/2 gluinos, photinos, and winos and zinos. (Yes, physicists get carried away.)
No one has ever detected any of these purported doppelgangers, and the explanation, physicists hope with fingers crossed, is that the supersymmetric particles are substantially heavier than their known counterparts. Theoretical considerations suggest that the supersymmetric particles could be a thousand times as massive as a proton, and in that case their failure to appear in experimental data wouldn't be mysterious: existing atom smashers don't have adequate power to produce them. In the corning decade this will change. Already, the newly upgraded accelerator at Fermilab has a shot at discovering some supersymmetric particles. And, as with the Higgs, should Fermilab fail to find evidence of supersymmetry and if the expected mass range of the supersymmetric particles is fairly accurate, the Large Hadron Collider should produce them with ease.
The confirmation of supersymmetry would be the most important development in elementary particle physics in more than two decades. It would establish the next step in our understanding beyond the successful standard model of particle physics and would provide circumstantial evidence that string theory is on the right track. But note that it wouldn't prove string theory itself. Even though supersymmetry was discovered in the course of developing string theory, physicists have long since realized that supersymmetry is a more general principle that can easily be incorporated in traditional point-particle approaches. Confirmation of super-symmetry would establish a vital element of the string framework and would guide much subsequent research, but it wouldn't be string theory's smoking gun.
On the other hand, if the braneworld scenario is correct, upcoming accelerator experiments do have the potential of confirming string theory. As mentioned briefly in Chapter 13, should the extra dimensions in the braneworld scenario be as large as 10^-16 centimeters, not only would gravity be intrinsically stronger than previously thought, but strings would be significantly longer as well. Since longer strings are less stiff, they require less energy to vibrate. Whereas in the conventional string framework, string vibrational patterns would have energies that are more than a million billion times beyond our experimental reach, in the braneworld scenario the energies of string vibrational patterns could be as low as a thousand times the proton's mass. Should this be the case, high-energy collisions at the Large Hadron Collider will be akin to a well-hit golf ball ricocheting around the inside of a piano; the collisions will have enough energy to excite many "octaves" of string vibrational patterns. Experimenters would detect a panoply of new, never before seen particles — new, never before seen string vibrational patterns, that is — whose energies would correspond to the harmonic resonances of string theory.
The properties of these particles and the relationships between them would show unmistakably that they're all part of the same cosmic score, that they're all different but related notes, that they're all distinct vibrational patterns of a single kind of object — a string. For the foreseeable future, this is the most likely scenario for a direct confirmation of string theory.