Are there BSM which do not predict proton decay?

[ohad]

Hi,

To my knowledge about BSM (which is not much), all of them require or predict proton decay.
Do you know of some of them which do not?

Thanks,
Ohad

 

[Vanadium 50]

MSSM,

 

[ohwilleke]

Proton decay is an affliction particularly common in grand unified theories (GUTs) that seek to combine all three Standard Model forces in a single unified theory, typically a single group representation.

It is typically not a problem in BSM theories that tweak one part or another of the Standard Model in a less comprehensive manner, for example, by introducing heavy right handed neutrinos to support a see saw mechanism for neutrino mass generation and as dark matter candidates, or by adding a fourth generation of Standard Model fermions. Likewise, it is not generally an issue in BSM theories that set forth a theory of quantum gravity or modify gravity, without tinkering with the Standard Model in any significant way.

As the introduction to the Wikipedia article on Proton Decay sums up the situation:

In particle physics, proton decay is a hypothetical form of radioactive decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron.[1] There is currently no experimental evidence that proton decay occurs.

According to the Standard Model, protons, a type of baryon, are stable because baryon number (quark number) is conserved (under normal circumstances; see chiral anomaly for exception). Therefore, protons will not decay into other particles on their own, because they are the lightest (and therefore least energetic) baryon.

Some beyond-the-Standard Model grand unified theories (GUTs) explicitly break the baryon number symmetry, allowing protons to decay via the Higgs particle, magnetic monopoles or new X bosons with a half-life of 10^31 to 10^36 years.

The GUTs allow proton decay (which violated baryon number conservation), neutrinoless double beta decay (which violated lepton number conservation), and flavor changing neutral currents (which can also violate conservation of one or both of these numbers) in large part by design.

These features are designed into GUTs despite the non-observation of any instance of either conservation of baryon number or conservation of lepton number, because GUT designers want to include a mechanism by which the matter-antimatter asymmetry of the universe can be reproduced following a Big Bang out of pure energy in which baryon number and lepton number are both zero and all subsequent processes respect the laws of physics set forth in the GUT.

In the Standard Model, separate conservation of lepton number and baryon number implies that those numbers have remained unchanged in the universe since the universe entered an epoch which is within the domain of applicability of the Standard Model which according to conventional cosmology estimates begins about 10 seconds after the Big Bang and perhaps even as early as 1 second after the Big Bang. https://en.wikipedia.org/wiki/Chronology_of_the_universe

But, if you are designing a GUT that is supposed to have a domain of applicability at the extremely high energies that existed in those first one to ten seconds after the Big Bang which is outside the domain of applicability of the Standard Model, and you believe that B=0 and L=0 is the only natural starting point for the Big Bang, then you need to design in processes that violate separate B and L conservation (substituting a global conservation of B-L is the usual choice) then you are going to get processes that violate separate B and L conservation like proton decay, neutrinoless double beta decay, and flavor changing neutral currents, which are not observed.

The Standard Model actually does have one or two processes that conserve B-L but not B and L separately, but those processes are not common enough, even at the extreme energy levels of the first ten seconds after the Big Bang, to generate to observed matter-antimatter asymmetries in the universe.

Another reason to design violations of B and L conservation into a GUT is that this makes it possible to have Majorana mass neutrinos which are greatly favored over Dirac mass neutrinos by GUT designers and theoretical physicists, but which violate L conservation and give rise to neutrinoless double beta decay which has not been observed either.

Of course, then you have to have some mechanism that shuts off or dramatically suppresses these processes in circumstances within the domain of applicability of the Standard Model that puts these events beyond detection with current experiments to make your GUT a viable candidate to describe reality. The first GUTs didn't even come close because their designers didn't realize that proton decay would turn out to be a problem. Later GUT designers tweaked their models to escape current experimental bounds but were over optimistic about the likelihood that B and L violation would show up just beyond current experimental bounds. Current GUT designers are well aware of the problems involved in allowing B and L violation that has been experimentally ruled out to quite extreme levels, but are overconstrained because if the B and L violation threshold is set too high, the GUT won't be able to reproduce the matter-antimatter asymmetry that is observed in our universe.

 

[ohad]

ohwilleke, thank you for the very detailed explanation.
I learned a lot from it.

 

[mfb]

In the Standard Model, separate conservation of lepton number and baryon number implies that those numbers have remained unchanged in the universe since the universe entered an epoch which is within the domain of applicability of the Standard Model which according to conventional cosmology estimates begins about 10 seconds after the Big Bang and perhaps even as early as 1 second after the Big Bang. https://en.wikipedia.org/wiki/Chronology_of_the_universe

10 seconds? The SM works nicely a microsecond after the Big Bang, probably even earlier. That is 6-7 orders of magnitude earlier than 1 to 10 seconds.

 

[ohwilleke]

10 seconds? The SM works nicely a microsecond after the Big Bang, probably even earlier. That is 6-7 orders of magnitude earlier than 1 to 10 seconds.

Baryogenesis, which is beyond the domain of applicability of the Standard Model, continues up to 1 second according to the source I cite. Parts of the SM may work, but it isn't enough by itself to explain that. It is unclear to me whether or not leptogenesis is taking place through 10 seconds, which is why I hedge on that point.

Honestly, the big takeaway is that the SM works perfectly for 13.6 billion years or so minus an exceedingly short period of time following the Big Bang. Given that pretty much everything is moving at relativistic speeds in that early moment, I'm not even sure how meaningful an estimate like "one second" or ten seconds" or a "microsecond" is at that point. The conclusion is model dependent, can't be directly observed and is stated in such a manner that I don't even clearly know which frame of reference it is being measured in, so the fine details don't really matter.

The trick, from the perspective of a GUT designer, is how to get almost perfect separate conservation of B and L through the SM era of the universe which makes up almost all of the history of the universe, while achieving the desired baryogenesis and leptogenesis outcomes with BSM physics in just a few moments of the early universe, presumably with a trigger that is a function of energy/momentum scale or perhaps space-time curvature. And, obviously to solve the matter-antimatter issues you need not just violation of B conservation and violation of L conservation. You also need CP violation that grows much more extreme in the UV.

 

[mfb]

Baryogenesis, which is beyond the domain of applicability of the Standard Model, continues up to 1 second according to the source I cite. Parts of the SM may work, but it isn't enough by itself to explain that. It is unclear to me whether or not leptogenesis is taking place through 10 seconds, which is why I hedge on that point.

Wikipedia is not a proper source for a good reason, I changed the misleading description in the table. You shouldn't use "stuff I read at Wikipedia" in an A-level thread (and ideally not in other threads either). The net baryon number didn't change any more after the first microsecond, what happened afterwards is just the cooling of the quark gluon plasma, hadronization and annihilation of all the antibaryons. All those processes are studied at colliders, and well described by the SM. Same for the lepton asymmetry.

I'm not even sure how meaningful an estimate like "one second" or ten seconds" or a "microsecond" is at that point.

It is very meaningful, the time is measured for comoving observers. I would not call 7 orders of magnitude "fine details". The model-dependence is a tiny effect at that epoch.

 

[kodama]

Wikipedia is not a proper source for a good reason, I changed the misleading description in the table. You shouldn't use "stuff I read at Wikipedia" in an A-level thread (and ideally not in other threads either). The net baryon number didn't change any more after the first microsecond, what happened afterwards is just the cooling of the quark gluon plasma, hadronization and annihilation of all the antibaryons. All those processes are studied at colliders, and well described by the SM. Same for the lepton asymmetry.
It is very meaningful, the time is measured for comoving observers. I would not call 7 orders of magnitude "fine details". The model-dependence is a tiny effect at that epoch.

has colliders like LHC found any differences in matter/antimatter production from collisions or are they produced in exactly the same amount?

 

[mfb]

No experiment has ever seen a violation of baryon number conservation.

The LHC is a proton-proton collider, so the initial state is not symmetric - every collision has two baryons more than antibaryons, which leads to a slight asymmetry in the baryon/antibaryon distributions observed. The Tevatron collided protons with antiprotons, there no asymmetry existed (if you summed over all directions).

 

[kodama]

No experiment has ever seen a violation of baryon number conservation.

The LHC is a proton-proton collider, so the initial state is not symmetric - every collision has two baryons more than antibaryons, which leads to a slight asymmetry in the baryon/antibaryon distributions observed. The Tevatron collided protons with antiprotons, there no asymmetry existed (if you summed over all directions).

should Tevatron have detected an asymmetry, on the order that followed the big bang?

 

[mfb]

No, the energy was orders of magnitude too low, but even with the right energy the asymmetry is so incredibly tiny (~ parts per billion or trillion) that there would have been no hope of detecting it. The detector is made out of matter, so detection asymmetries are unavoidable, and lead to systematic uncertainties on the permille level.

 

[kodama]

No, the energy was orders of magnitude too low, but even with the right energy the asymmetry is so incredibly tiny (~ parts per billion or trillion) that there would have been no hope of detecting it. The detector is made out of matter, so detection asymmetries are unavoidable, and lead to systematic uncertainties on the permille level.

are there any theories you know of that perhaps tie baryogenesis with desitter spacetime and that in a universe with anti-desitter spacetime, the asymmetry would favor antimatter?

 

[mfb]

I have never heard of such a theory.

 

[kodama]

I have never heard of such a theory.

suppose electron and neutron EDM values, in the future, are reported to be at 10^-32 or lower, SM values. the level of CP violation as the result of neutron EDM is only SM-level, not any BSM level like SUSY. would this mean
Sakharov conditions conditions are violated?

 

[ohwilleke]

Wikipedia is not a proper source for a good reason, I changed the misleading description in the table. You shouldn't use "stuff I read at Wikipedia" in an A-level thread (and ideally not in other threads either). The net baryon number didn't change any more after the first microsecond, what happened afterwards is just the cooling of the quark gluon plasma, hadronization and annihilation of all the antibaryons. All those processes are studied at colliders, and well described by the SM. Same for the lepton asymmetry. . . . It is very meaningful, the time is measured for comoving observers. I would not call 7 orders of magnitude "fine details". The model-dependence is a tiny effect at that epoch.

For my purposes, it doesn't really matter if it is a femtosecond or a week, which is why I wasn't very concerned about perfect accuracy in sourcing. I only specified a specific time, rather than rigorously discussing it, for example, in terms of some constant Tt (for time of transition), because it is more readable to discuss the idea that way. If it materially mattered exactly how long after the Big Bang conventional cosmology window placed baryogenesis and leptogenesis for the point I was making, for example, to do back of napkin calculations with it, I agree that Wikipedia wouldn't be a very good source. But, in this case, that detail is immaterial, and it is likewise unimportant to define what I mean by a statement that X happens at time Y, because the exact value (even up to many orders of magnitude) doesn't matter for this purpose.

The point that I care about is that there exists some brief period of time in the very early universe immediately after the Big Bang (much less than one trillionth of the age of the universe) when the SM would be beyond its domain of applicability, but a GUT would apply; while for the overwhelming share of the remaining 13.6 billion years +/-, the SM applies. Therefore, you have to fit your baryongenesis and leptogensis that gives rise to matter-antimatter asymmetry into the small window of pre-SM domain of applicability if you insist that you are starting at B=0 and L=0, which makes it important for a GUT to undergo what amounts to a phase change in B and L symmetry are broken in just the right circumstances.

Obviously, someone coming up with a GUT does not to have a very good handle on when the symmetry breaks. But, I just need an extremely crude sense of when that happens and the fact that it does to explain what I want to communicate.

 

[ohwilleke]

has colliders like LHC found any differences in matter/antimatter production from collisions or are they produced in exactly the same amount?

While there is no evidence of B/L violation, there are CP asymmetries which are measured, none of which differ in a statistically manner from the SM predictions.

 

[mfb]

suppose electron and neutron EDM values, in the future, are reported to be at 10^-32 or lower, SM values. the level of CP violation as the result of neutron EDM is only SM-level, not any BSM level like SUSY. would this mean
Sakharov conditions conditions are violated?

It could also mean CP violation hides at a different place. The sources of CP violation we know are too small.

 

[ohad]

In the following lecture of Leonard Susskind:

at minute 30 +- 1 he says that every unified theory predicts baryon number violation.

 

[ohwilleke]

I think there is a cause and effect issue. Cosmology doesn't assume baryon number violation because unified theories predict it. Unified theories are designed to have it because cosmology suggests that it is necessary.

 

[lpetrich]

There is one proposed GUT that does not have baryon-number violation at tree level. Trinification: gauge group SU(3)*SU(3)*SU(3). One of the SU(3)'s becomes the QCD SU(3), while the other two become the weak-isospin SU(2) and the weak-hypercharge U(1). I make the tree-level qualification because the Standard Model conserves baryon number at tree level. "Tree level" means no loops and no nonperturbative effects.

In all of the other proposed GUT's that I know about, one finds both baryon-number violation and lepton-number violation.

Here are the Standard Model's left-handed elementary-fermion multiplets. The quantum numbers are (QCD multiplicity, WIS multiplicity, WHC value = Y, Baryon # = B, Lepton # = L)
Q = (3,2,1/6,1/3,0) -- left-handed quark
U* = (3*,1,-2/3,-1/3,0) -- right-handed up quark
D* = (3*,1,1/3,-1/3,0) -- right-handed down quark
L = (1,2,-1/2,0,1) -- left-handed lepton
N* = (1,0,0,0,-1) -- right-handed neutrino
E* = (1,1,1,0,-1) -- right-handed electron

Georgi-GlashowSU(5) EF multiplets, with -(4/5)*Y+B-L value
5* = L + D* = -3/5
10 = Q + U* + E* = 1/5
1 = N* = -1

Pati-Salam SU(4)*SU(2)*SU(2) = SO(6)*SO(4) EF multiplets, with 3*B+L value
(4,2,1) = Q + L = 1
(4*,1,2) = U* + D* + N* + E* = -1

So in GG, -(4/5)*Y+B-L is conserved, while in PS, 3*B+L is conserved. In SO(10), however, neither is conserved, and B and L can vary separately.