# Is proton decay considered in neutron star models (and LHC)?

Although it is definitely not simple, there are many reasons to consider that baryon number can be violated, for example:

• during baryogenesis (just after Big Bang) there was created more matter than antimatter,
• hypothetical Hawking radiation (black holes) can finally turn any matter (mainly baryons) into massless radiation (photons),
• some GUT models require proton decay: https://en.wikipedia.org/wiki/Proton_decay ,
• while charge conservation is guarded by Gauss law, there is nothing like that for baryon number.

Sure, the search for proton decay in huge room temperature water tanks was unsuccessful. However, if proton can be destroyed, it would require relatively huge energy – the assumption that it can spontaneously thermally localize on a single proton in room temperature water might be just wrong (?)

In contrast, baryogenesis and Hawking radiation examples suggest that really extreme conditions would be necessary to destroy a proton (like temperature). So another candidate might be LHC, but if happening in tiny amounts, the calorimetry has no chance to catch it, and to consider it in Monte Carlo we would need the exact parameters … is proton decay considered for LHC?

More important candidate as environment with the most extreme conditions is the center of neutron star. There are real issues with understanding the huge amounts of energy released in gamma-ray bursts – from Wikipedia: “The means by which gamma-ray bursts convert energy into radiation remains poorly understood".

Or ultraluminous X-ray sources, especially the M82 X-2: pulsar radiating ~10 million times more energy than our sun, according to Wikipedia: "shining about 100 times brighter than theory suggests something of its mass should be able to".

Hypothetically, reaching extreme conditions to start statistically essential “baryon burning” (total matter -> energy conversion, >100x more energy density than from fusion) in the center of neutron star, might help explaining these extreme energy sources.

So I wanted to ask if proton/neutron decay is considered in neutron star models? Should it?

Astronomers have discovered the brightest neutron star ever found. This extremely dense object is 1,000 times brighter than researchers previously thought was possible for neutron stars (...) This is one of the questions the scientific community needs to answer in the next years

• The object in the article is an accreting neutron star in a binary system. Its luminosity is not intrinsic to the neutron star and has nothing to do with proton decay. – ProfRob Aug 15 '20 at 7:50
• So what is the reason for being "1,000 times brighter than researchers previously thought was possible for neutron stars "? How do you know the missing energy source is not some baryon decay - e.g. from vortices, shockwaves inside, or maybe directly accreting matter in extreme conditions, velocities? We didn't observe proton decay in room temperature water, but this way we could also disprove nuclear fusion - some phenomena just need extreme conditions. – Jarek Duda Aug 15 '20 at 8:02
• They are not a 1000 times more luminous than thought possible for neutron stars. They are up to 1000 times more luminous than the Eddington limit, if the flux is isotropic. – ProfRob Aug 15 '20 at 8:39

One has to clarify the meaning of "decay" as far as protons go.Here is a proton decay diagram in GUTS:

Notice that what is really disappearing is two quarks going into an antidown and an e+, to get a total baryon number of zero.

This type of proton disappearance has no need of high temperatures as it is inherent in the couplings that the theory proposes. At the moment the experimental limit is very small,~10^32 years, while supersymmetry, for example predicts 10^39 and GUTs 10^34 .

What you are talking about with your

really extreme conditions would be necessary to destroy a proton (like temperature).

is being studied in the LHC, it is called a quark gluon plasma, where all protons have disappeared and one is left with a soup of quarks, antiquarks and gluons. But baryon number is conserved in the quark gluon plasma, unless one of the disappearances of quarks ( as in figure)is included in the theory for the quark gluon plasma. The probability of this would be affected by the mass of the X (leptoquark) , but away from the resonance more energy/temperature would not make a difference .

If leptoquarks exist and their mass measured, one might try checking astrophysical data for specific energy electrons/leptons associated with collinear gamma pairs ( pi0s) for example. This would be similar to checking for antiproton proton annihilation signals for antimatter galaxies. It would still depend on the coupling measurable in accelerator experiments. Without specific information astrophysical observations are not the best laboratory for detecting new particles.

They are searching for lepto quarks at the LHC and if the new linear collider (ILC) is ever built it is one of its main projects.

• Thank you, I understand the short answer to my question in "no". However, it means there is still a real problem with e.g. M82 X-2: "shining about 100 times brighter than theory suggests something of its mass should be able to" - so what are the official hypothetical explanations of such enormous energy source? (not violating baryon number conservation) en.wikipedia.org/wiki/M82_X-2 – Jarek Duda Jan 28 '17 at 15:02
• the models of physics change as time goes on and new data from experiments arrive. I was just pointing out that it is not possible to get definitive answers from astrophysical data. It works the other way around. Obviously if existing astropysical models do not fit observations, new models must be devised. Maybe baryon number violation will be playing a role, but current particle theories and data do not offer such extrapolations. – anna v Jan 28 '17 at 15:45

Both M82-X2 and the new object referred to in the new edit are examples of ultraluminous X-ray sources, with luminosities exceeding the Eddington luminosity by 100-1000 times (if the flux is assumed to be emitted isotropically). They are neutron stars which are part of accreting binary systems and there is no need to invoke exotic particle physics to explain their properties. Accretion of material from a companion is the source of their luminosity.

The Eddington luminosity is a theoretical benchmark that limits spherically symmetric accretion onto a neutron star, because the radiation produced has a pressure sufficient to halt the accretion.

However, super-Eddington sources are common, and can be produced by channelling the accretion via a disc or through magnetic funnels. They can also be produced in accreting neutron stars via the beaming of radiation (i.e. the pulsar phenomenon) which, because the radiation is not emitted isotropically, makes detected objects appear brighter even though their total luminosity is more modest.

The rare, ultra-luminous "Super-super-Eddington" sources like M82-X2 do challenge this understanding. Current models require them to have exceptionally strong magnetic fields of $$10^{14}$$-$$10^{15}$$ G (such fields are known to exist on a type of neutron stars referred to as magnetars), which channel the accretion in an extreme way and greatly reduces the Thomson-scattering opacity of electrons. These geometric effects, combined with the reduction in the effectiveness of radiation pressure and some beaming of the emergent radiation is thought to be sufficient to explain what is seen (see Israel & Rodriguez Castillo 2019 for a short review).

There are relatively few protons inside neutron star (perhaps 1% of baryons). Their disappearance (but not decay), along with the neutrons into a quark-gluon plasma is considered in neutron star models at about 10 times the nuclear saturation density.

The disappearance of baryons is considered in papers considering the death of the universe in the far future, when much of the baryonic matter will be inside cold, degenerate objects. For example, Adams & Laughlin (1997) consider both white dwarfs and neutron stars in the far future that are powered by proton decay. They point out that the products of proton/neutron decay either themselves decay rapidly into photons, or interact with electrons to produce photons. The net result is to turn matter into radiation. Laughlin & Adams estimate a luminosity of $$\sim 10^{-24} L_{\odot}$$ and a surface temperature of $$\sim 3$$ K for such objects, which isn't relevant when considering ultraluminous accreting neutron star sources at $$\sim 10^7 L_{\odot}$$.

• It is not about protons, but general violation of baryon number, like hypothesized in baryognesis or Hawking radiation. And not claiming that it is required, only questioning if we should completely neglect such option as it is done currently. – Jarek Duda Aug 15 '20 at 9:28