death of the universe?!

PAllen

Even if you assume (reasonably) that the existing universe almost all matter, how on earth do you favor proton decay over differential productions rates? I'll buy that, with reasonable cosmological assumptions, SM cannot explain existence of matter in current quantity, but I find differential productions rates via unknown CP violations more plausible than proton decay.

Chalnoth

Such CP violations imply non-conservation of baryon number. And if baryon number is not conserved, then protons can decay.

twofish-quant

You can't prove that protons decay at all, but you can set limits.

http://hep.bu.edu/~kearns/pub/kearns-pdk-snowmass.pdf

You disprove it within certain limits. The limits that we have right now have already falsified a number of theories.

twofish-quant

If CP is violated then so is T, and if T is violated then you can have processes that create baryons violate baryon number conservation while requiring that the reverse processes conserve baryon number.

You can also have a situation in which you create X/anti-X bosons through pair production. The decay of X/anti-X bosons into protons would not conserve baryon number, but any processes involving protons decaying into lighter particles could.

If you have any literature that says that those scenarios are impossible, I'd be interested in seeing them.

Chalnoth

This line of reasoning presumes that CPT is a perfect symmetry, which means that you can perform the time-reversal of all processes, just with the positions of matter and anti-matter switched. So I don't think this gets you out of forcing proton decay to exist.

But then you'd still have decays with those heavier particles as intermediate steps.

PAllen

What if anti-protons can decay (slowly) but not protons? That seems to meet the requirements.

Chalnoth

Honestly, I'm not entirely sure. It just seems incredibly unlikely.

As far as I am aware, all GUT's yet proposed require proton decay in order to have baryogenesis.

PAllen

That is also true of what I know, but I don't see that as equivalent to the statement that any modification of SM to allow for baryon asymmetry must entail proton decay. It is an assumption that a GUT is true of our universe. Further, I've certainly seen proposed mechanisms for the origin of baryon asymmetry that don't entail proton decay (irrespective whether the model contains proton decay as one of its other predictions).

Haelfix

It is true in a sense, but it depends on the rates and details of the new physics to a certain degree.

In the standard model, proton decay proceeds via dimension 6 operators. This is a nonrenormalizable, baryon number violating interaction that is suppressed by 2 powers of a cutoff scale that is taken to be very high.

Postulating some sort of new physics between the electroweak scale and the Planck scale, will set this cutoff scale, and you must be very careful that these dimension 6 operators don't come dangerously close to violating the proton decay bounds (which are bounded by experiment to something like >10^33 years).

This is a typical and powerful constraint that phenomenologists use, but there are mechanisms or extra symmetries that you can introduce to tame the rates to a certain extent.

However if you violate baryon number, there will be proton decay at some level, it just might be small relative to some other new physics.

So for instance, in the standard model baryon number is an accidental symmetry and is actually violated by nonperturbative physics. Eg instanton physics will yield nonzero tunneling cross sections between degenerate weak SU(2) vacua that imply proton decay on the order of ~10^170 years or something enormous like that. So you see, it is a true statement, but it can be made essentially irrelevant for any sensible physics.

twofish-quant

CPT has to hold in order for their to be Lorenz covariance.

I'm still not understanding how CP-violation *requires* proton decay. What I'm looking for is some mathematical statement that if CP-violation exists and protons don't decay then some fundamental physical constraint (i.e. Lorenz covariance) is violated.

You can get around that. If the particles are heavy enough then any reactions that get you to the heavy particles would in the process produce at least one new proton for any one that decays. The mass scales are large enough so that's possible.

twofish-quant

And that's a limitation with GUT's rather than some fundamental constraint of the universe.

Basically the way that GUT's work is to put the SM into some larger group. If you want anything non-trivial then this involves mixing baryon and lepton groups. If you don't mix those groups, then you end up with just the SM and you have something that is non-publishable.

However this is "argument by aesthetics" or "argument by lack of imagination." I don't know of (and would be interesting to hear of) any arguments that say that CP-violation *requires* proton decay because of some fundamental physical constraint. I can think of some mechanisms that would supress proton decay. You'd end up with an ad-hoc and ugly theory, but argument by aesthetic or mathematical simplicity isn't something that i think counts for every much here.

twofish-quant

And that new physics could very well generate new protons at faster than the rate that protons decay. We are talking about energy scales much larger than the mass of the proton.

Chalnoth

Then that wouldn't be a baryon-number violating decay.

Granted, maybe it's possible to come up with a theory which allows baryon number violation in one direction only. My knowledge of high-energy physics isn't sufficient to rule something like this out. But it naively seems massively unlikely.

twofish-quant

If it's two different processes it would be.

Something like that happens in atomic nuclei. Free neutrons will decay to protons, but when you bind neutrons and protons then any n->p decay is balanced by p->n.

I think it's trivial to allow one way baryon number violation. You go into your favorite GUT, and say *God says so* and it is done. You end up with an ugly ad-hoc theory, but one that doesn't contradict any observations or settled physical principles.

Also, what I'm looking for is something "clean" like the Sahkarov arguments. You can easily show that any matter/anti-matter imbalance starting from a symmetry situation requires CP-imbalance, and that's a clean argument based on very firm physical principles.

If there is a similar argument saying CP-imbalance -> proton decay, I'd be interested. Proton decay is a generic feature of GUT's, but you can just argue "GUT's are wrong."

PAllen

You know, there is just as much an asymmetry in leptons. Why not claim electrons exist implies electrons decay? Whatever mechanism could allow for lepton asymmetry could, in some not yet described theory, allow for baryon asymmetry.

Again, there is a whole chain of assumptions here:

- Universe began with a big bang scenario (of course I don't dispute this, but it is separate from particle physics; in the past, eternal cyclic theories were considered, in which case baryon asymmetry is just a boundary condition of the infinite past).

- Any extension to SM to account for evolution of baryons asymmetry from assumed symmetric big bang state must have the character of known GUTs.

Please note: in no way do I dispute that big bang is highly likely, and GUT type phenomenology is likely in a successful BSM theory, however I prefer to be clear on chain of certainty and reasoning:

- we are confident there is baryon asymmetry (but not certain)
- the most successful cosmology models start with zero baryon number
- thus, there must be a mechanism to explain this

The above are not, IMO ironclad, and further assumptions requiring proton decay are somewhat less certain.

Chronos

I remain skeptical on the basis of lack of experimental evidence favoring proton decay.

twofish-quant

Not that I disagree with your main point, but for completeness....

One general result of GUT's is that B and L are not conserved but B-L is. The reason that people don't talk about electron decay is that there aren't any particles lighter than the electron that it can decay to without violating some conservation rule.

I think that's the big one that I find difficult to swallow. Things could change quickly if we see some strong evidence that GUT's are right, but right now GUT's have enough problems that if someone argues that we are just wrong, that I can't counterargue.

I wonder about this one. Starting with zero baryon number makes the universe nice and symmetric, but is there any physical reason way we can't start with non-zero baryon number?