Cosmologic philosophy: physics current dead end (Introduction)

by David Turell , Tuesday, June 19, 2018, 20:18 (2137 days ago) @ David Turell

More dead-end commentary:

https://aeon.co/essays/has-the-quest-for-top-down-unification-of-physics-stalled?utm_so...

"[At] this time, though, none of the more exotic particles and interactions that theorists hoped to see has been forthcoming. No ‘stop squarks’, no ‘gluinos’, no ‘neutralinos’. The null results are now encrusting the hull of the Standard Model, like barnacles on a beautiful old frigate, and dragging her down to the ocean floor. It looks like the centuries-long quest for top-down unification has stalled, and particle physics might have a full-blown crisis on its hands.

"Behind the question of mass, an even bigger and uglier problem was lurking in the background of the Standard Model: why is the Higgs boson so light? In experiments it weighed in at 125 times the mass of a proton. But calculations using the theory implied that it should be much bigger – roughly ten million billion times bigger, in fact.

***

"My colleagues and I watched the LHC closely for such tell-tale signs of superpartners. None have been found. We started to ask whether we might have missed them somehow. Perhaps some of the particles being produced were too low in energy for the collisions to be observed. Or perhaps we were wrong about dark matter particles – maybe there was some other, unstable type of particle.

"In the end, these ideas weren’t really a ‘get-out-of-jail-free’ card. Using various experimental analysis techniques, they were also hunted out and falsified. Another possibility was that the superpartners were a bit heavier than expected; so perhaps the mass of the Higgs boson did have some cancellation in it (one part in a few hundred, say). But as the data rolled in and the beam energy of the LHC was ramped up, supersymmetry became more and more squeezed as a solution to the Higgs boson naturalness problem.

***

"Perhaps the bleakest sign of a flaw in present approaches to particle physics is that the naturalness problem isn’t confined to the Higgs boson. Calculations tell us that the energy of empty space (inferred from cosmological measurements to be tiny) should be huge. This would make the outer reaches of the universe decelerate away from us, when in fact observations of certain distant supernovae suggest that the outer reaches of our universe are accelerating. Supersymmetry doesn’t fix this conflict. Many of us began to suspect that whatever solved this more difficult issue with the universe’s vacuum energy would solve the other, milder one concerning the mass of the Higgs.

"All these challenges arise because of physics’ adherence to reductive unification. Admittedly, the method has a distinguished pedigree. During my PhD and early career in the 1990s, it was all the rage among theorists, and the fiendishly complex mathematics of string theory was its apogee. But none of our top-down efforts seem to be yielding fruit. One of the difficulties of trying to get at underlying principles is that it requires us to make a lot of theoretical presuppositions, any one of which could end up being wrong. We were hoping by this stage to have measured the mass of some superpartners, which would have given us some data on which to pin our assumptions. But we haven’t found anything to measure.

***

"Some of us are busy speculating on what these findings might mean. Excitations of two different types of new, unobserved, exotic particles – known as Z-primes and leptoquarks, each buried deep within the bottom mesons – could be responsible for the bottom mesons misbehaving. However, the trouble is that one doesn’t know which (if either) type of particle is responsible. In order to check, ideally we’d produce them in LHC collisions and detect their decay products (these decay products should include muons with a certain energy). The LHC has a chance of producing Z-primes or leptoquarks, but it’s possible they’re just too heavy. In that case, one would need to build a higher energy collider: an ambitious plan for a beam of energy of seven times the intensity of the LHC would be a good option.

***

"We began with an experimental signature (the particular bottom meson decays that disagree with Standard Model predictions), then we tried to ‘bung in’ a new hypothesised particle to explain it. Its predictions must be compared with current data to check that the explanation is still viable. Then we started building an additional theoretical structure that predicted the existence of the particle, as well as its interactions. This theory will allow us to make predictions for future measurements of decays, as well as search for the direct production of the new particle at the LHC. Only after any hints from these measurements and searches have been taken into account, and the models tweaked, might we want to embed the model in a larger, more unified theoretical structure. This may drive us progressively on the unification road, rather than attempting to jump to it in one almighty leap."

Comment: The theorists are stuck and need a more powerful LHC or new theories. String theory is not the answer.