Wednesday, September 29, 2010


Charged particles and their antiparticles, such as the electron and positron, are known to have remarkably similar masses. But what about other neutral particles? According to the Particle Data Group, the neutron and antineutron have different masses. That is, they report the result

$\frac{m_n - \overline{m}_n}{m_n} = 9 \pm 6 \times 10^{-5}$

where the neutron mass is $m_n = 939.565346 \pm 0.000023$ MeV. This amounts to a difference that is many orders of magnitude greater than the neutrino mass scale! Further evidence of CPT violation.


  1. Interesting but a 1.5 sigma result is not very convincing. I wonder what it would take to measure the antineutron mass a bit better. The lifetime might be worth looking at too.

  2. Well, the $1.5$ sigma is why nobody ever mentions it, I guess. If it wasn't for MINOS, I would not have looked at the result myself!

  3. Wow! This is the kind of thing I would expect the experimentalists to be working on right now. And I was wondering about the hadrons, without some sort of difference, it's hard to justify extending Koide's formula to the neutrinos.

  4. Yeah, pretty cool, isn't it? And the more data I look at, the more anomalies keep popping up ... on a roll here!

  5. The mass difference is stated to be mn − mp = 1.2933317 ± 0.0000005 MeV. The formal accuracy is surprising. This is one roughly one fifth of u quark mass which is recently believe to be around 5 MeV. I could not find any reference to article where the result would be reported to see how the claimed difference is measured.

  6. The only url I managed to find to the mass measurement of antineutron mass was this
    Physics Letters B Volume 177, Issue 2, 11 September 1986, Pages 206-210.
    It was measured in bubble chamber.

    By putting this to the Google you get to the page which has much longer address.

  7. Sorry, I had talked nonsense above. For some reason -probably related to the fact that I am approaching my sixties- I talked about proton neutron mass difference. The claim for the fractional mass difference is .9-1.5 *10^(-4) and gives .084 MeV. In any case this mass difference looks nonsensical.


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