# Are neutrinos much more abundant than atoms?

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1. May 14, 2015

### Stephanus

Dear PF Forum,

I once read in several links just a couple of days ago, that the number of neutrinos exceeds the number of baryon in the universe by several orders of magnitude.
1. Is that true, that neutrinos are much more abundant than atoms?
2. Do neutrinos have mass? Not that they are catholic
3. If they have mass, I know I should have calculated it myself, which is more massive? Neutrinos or atoms in the universe?
4. Baryon asymetriy.
Does that includes electron and neutrino?

Thanks for any explanation.

2. May 14, 2015

### Orodruin

Staff Emeritus
Yes, at least according to the standard picture we have of the Universe. It should be said that most neutrinos are very low energy background neutrinos and this background has yet to be experimentally verified. If true, and again this is a must based on standard cosmology, the only particle more abundant would be photons due to the CMB.

Yes. The observation of neutrino oscillations verifies this.

Baryons are far heavier than neutrinos. Based on your question, you are looking for which has the higher mass density, the baryons still win - a proton is at least 1000000000 times more massive than a neutrino.

No. Neutrinos and electrons are not baryons.

3. May 14, 2015

### Stephanus

Thanks Orodruin for your answer

About baryon asymmetry. Okay... electrons and neutrinos are not baryon, they're lepton.
But according to CERN,
As you may already know, this happens in baryogenesis, less than 1 second after big bang.
But if the number of electrons doesn't not match or closely match to the number of protons, should the universe contains highly ionized atoms?

Thanks for your response.

4. May 14, 2015

### Orodruin

Staff Emeritus
That there is a baryon asymmetry does not imply that there is no lepton asymmetry. In fact, in order for the Universe to be overall neutral there needs to be roughly as many electrons as protons. This does not mean that the lepton and baryon numbers are equal, a lot of the asymmetry may "hide" in neutrinos. The cosmic neutrino background contains both neutrinos and anti-neutrinos (assuming that they are different particles).

5. May 14, 2015

### Stephanus

Thanks again Orodruin for your answer

May I ask further?
It turns out that there are leptogenesis as well
http://en.wikipedia.org/wiki/Leptogenesis_(physics)
But WHY does leptogenesis asymmetry closely matches baryon asymmetry?
1. Is leptogenesis and baryogenesis asymmetry match - still -one of the science mystery?
2. Is leptogenesis science mystery?
3. Is baryogenesis science mystery?

Thanks for any response.

6. May 14, 2015

### Orodruin

Staff Emeritus
The Standard Model of particle physics allows for a non-perturbative effect called "sphalerons". During sphaleron processes, lepton and baryon numbers are not preserved but are exchanged (B-L is preserved while B+L changes). This leads to baryon number being converted into lepton number and vice versa as long as the sphaleron processes are active (as long as the Universe is hot enough) and the ratio will be fixed as long as they are fast enough to ensure that the process is in equilibrium.

We do know of several mechanisms through which baryogenesis via leptogenesis (producing the lepton asymmetry first and then having it "spill over" to baryons through sphalerons) could work. The big problem here is that leptogenesis requires the existence of some heavy leptons which have not been observed yet, meaning that it still remains a possible solution only and not a tested hypothesis. Currently, we are not sure how baryogenesis happened, there are some different alternatives, but nothing is confirmed.

7. May 14, 2015

### Stephanus

Thanks Orodruin for you explanations.

Perhaps my English is poor (and also my math), but..
If lepton and baryon numbers are exchanged, shouldn't it B + L = C, constant?
On the other hand, if B - C = C, constant, should we say that , the difference between Baryon and lepton numbers are constant, although B/C might changes. Or the equation is wrong?

Thanks for your answer.

8. May 14, 2015

### Staff: Mentor

You exchange a lepton for an antibaryon or vice versa. The difference is constant, right.

There are models where B-L is not exactly conserved, but that is a different thing.

9. May 14, 2015

### ChrisVer

If you have no baryon so $B=0$ and a lepton with $L=1$ and you say there is a $B-L$ symmetry...

Then $B-L=-1$ , so if conserved you can have as a result an antibaryon where $B=-1$ and no lepton $L=0$ (a lepton has changed into a antibaryon)...
The $B+L = 1$ at first, then $B+L =-1$

10. May 14, 2015

### Stephanus

Perhaps my English is poor (and also my math), but..
If lepton and baryon numbers are exchanged, shouldn't it B + L = C, constant?
On the other hand, if B - L = C (not B - C = C, edited), constant, should we say that , the difference between Baryon and lepton numbers are constant, although B/L (not B/C, edited) might changes. Or the equation is wrong?

Bold red is EDITED!

11. May 14, 2015

### Stephanus

Thanks ChrisVer for your answer,
But it's Orodruin phrase that I like confirmation:
Perhaps my understanding in English (or Math) is incorrect.
IF ...lepton and baryon numbers are not preserved but are exchanged, shouldn't it B + L = constant?
It's not the physic that I like to know, but the phrase. So I can get the correct answer.

12. May 14, 2015

### Staff: Mentor

Did you read my answer in post 8?
The "exchanged" in Orodruin's post didn't specify the sign.

13. May 14, 2015

### Orodruin

Staff Emeritus
This.

I considered the sign evident from the conservation of B-L.

14. May 14, 2015

### Stephanus

Oh, it's the "subatomic number", I think it's the "amount" of baryon.
Okay, I think this is really particle physics. Sorry, I really have no background in this area.
But, thanks for the answer.

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