Complete list of massless particles
Doug B Sweetser
According to my Particle properties data book circa 1992, the mass of
the photon is less than 3x10^-33 MeV, and the gluon is zero
theoretically, but could be a few MeV. Now that more data is showing
up that neutrinos mix - which can only happen if they have mass - the
three flavors of neutrinos have too much mass to make the massless
cut.
There was a thread recently why massless particles had to be neutral,
and I will not repeat the logic of that here.
There is not yet a viable quantum theory of gravity. There are good
reasons to believe that a particle called the graviton for such a
theory will have spin 2 and be massless. The reason it must be
massless is that the field extents to infinity.
So my complete list of massless particles looks like this:
The photon - massless. Evidence for this is rock solid.
The gluon - good reason and good data, but will get more with bigger
colliders.
The graviton - will have to wait for either a direct observation or a
more solid theoretical framework.
The neutrinos have mass, and more data about this mass will await
further experimentation.
Thanks for any comments/additions/refinements,
Jeffery
[unnecessary text deleted]
> So my complete list of massless particles looks like this:
>
> The photon - massless. Evidence for this is rock solid.
>
> The gluon - good reason and good data, but will get more with bigger
> colliders.
>
> The graviton - will have to wait for either a direct observation or a
> more solid theoretical framework.
>
> The neutrinos have mass, and more data about this mass will await
> further experimentation.
According to the current view, the known massless particles are the
photon, gluon, and graviton. You could come up with other theories
with other massless particles if you wanted to. Most people assume the
graviton is massless. I have heard some people claiming the gluon has
mass. I think they are including the gluon binding energy as the
"mass" of the gluon.
[Moderator's note: since gluons do not exist as free particles, it
is more tricky to define their mass - the same applies to quarks.
However, since gluons are gauge bosons for an unbroken gauge symmetry,
nobody I know feels troubled by assigning them mass zero. - jb]
Chong Yidong
> The neutrinos have mass, and more data about this mass will await
> further experimentation.
IIRC, the neutrino oscillation experiments only measure mass
differences, so it is possible that not all the neutrinos have mass.
Is this correct?
Doug B Sweetser
The graviton is not in my 1992 Particle Properties Data Book (the
"Gauge and Higgs Bosons" section where I would expect it), although
the idea of the graviton is quite old. I guess there is no active
search for it, which is probably an interesting issue in itself.
The photon is listed like so:
I (J^PC) = 0,1(1^--)
Would someone kindly provide a key to this information? I don't see
it in the book or the web site of the Particle Data Group.
Thanks,
doug
quaternions.com
[Moderators note: there is an active search for gravitational
waves, which are to light as gravitons are to photons. You're
not going to see the graviton in your Particle Properties Data
Book anytime soon. - jb]
Doug B Sweetser
>[Moderators note: there is an active search for gravitational
>waves, which are to light as gravitons are to photons. You're
>not going to see the graviton in your Particle Properties Data
>Book anytime soon. - jb]
The Particle Properties Data Book discusses the search for the Higgs
in the section on gauge and higgs bosons. The gravitons did not make
the cut. It was also not mentioned in the current "searches" page.
The gravitational wave detection is a classical experiment, not
quantum in nature (and it is a tough classical experiment at that!).
What is more interesting than this observation on an editorial
decision is that both of these so-far-unseen particles, the Higgs
boson and the graviton, are directly related to inertia (I know I
should say mass, but sometimes it is fun to switch words around :-)
The Higgs does two things: it gives all formly massless particles in
the standard model mass, and it politely breaks the symmetry of the
standard model (the issue of spontaneous symmetry breaking has been
discussed in the newsgroup before).
The Higgs is about mass and symmetry, but not about geometry. This
sounds to my ear like it is in conflict with general relativity, where
inertia is only about geometry because mass density only curves
spacetime. If physicists succeed at pinning an MeV on the Higgs, then
mass is about the link to the Higgs field, not geometry.
One thing mass density does is create a gravitational field which is
infinite in its extent. Changes in the field propogate at the speed
of light. That task may be accomplished by the graviton, a massless
particle with spin 2 (the reason to use "may" as a verb is that we
have yet to detect it). The gravitational waves that the LIGO
experiment is trying to detect must make their way accross the galaxy
as particles. These particles are created by a major disturbance in a
mass density, or the Higgs field. Nothing is quite as tricky as
trying to figure out the link between these two proposed particles.
That calculation will not be Particle Properties Data Book anytime
soon, if at all.
Particle physics is a mature area of study. It is extraordinarily odd
that the two so-far-unseen particles are intimitely connected to mass.
doug <swee...@theworld.com>
quaternions.com
dumbo
> Hello:
> Now that more data is showing up that neutrinos mix -
> which can only happen if they have mass - (...)
not exactly: see
L.Wolfenstein: Phys. Rev: D, year 1978, vol 17, p. 2369
showing that massless neutrinos can oscillate in the presence
of matter provided that the weak leptonic current has an
off-diagonal part connecting different neutrino types;
and
V. De Sabbata & M Gasperini : Il Nuovo Cimento, year 1981,
vol 65 A, pp. 479 - 499
showing that massless neutrinos can oscillate due to the torsion
of Einstein-Cartan's spacetime.
bye
Corrado