Surprises in Particle Physics
mark...@slip.net
>The naive reader fails to see the problem. A baryon consists of X quarks
>and Y antiquarks, where X - Y = 3. In a collision, these can be redistributed
>between final baryons and antibaryons, and perhaps correlations between
>the two will be observable.
>(This is what comes of reading those excellent Physics group FAQs,
>by the way - in which extrapolated sort-of-inherent masses are given
>for the u and d, which don't add up to anything like the nucleon
>masses. Conclusion: nucleons are mostly kinetic energy and gluons,
>how cool. But there is more than enough energy there for u/-u
>pair creation, so there is probably a significant amplitude for
>the same. Or is a little knowledge a dangerous thing :-?)
Hyperons do not consist of just u and d quarks, they also have the heavier
strange quarks - taking some of the force out of your otherwise good argument.
Mark Richardson
Tim Cox
> In article <DFtBo...@latcs1.lat.oz.au> bu...@latcs2.lat.oz.au
(Jonathan Burns) writes:
>
> >The naive reader fails to see the problem. A baryon consists of X quarks
> >and Y antiquarks, where X - Y = 3. In a collision, these can be redistributed
> >between final baryons and antibaryons, and perhaps correlations between
> >the two will be observable.
>
(clip)
>
> Hyperons do not consist of just u and d quarks, they also have the heavier
> strange quarks - taking some of the force out of your otherwise good argument.
>
> Mark Richardson
Actually, things are rather more complicated than the previous discussion
has pointed out.
The first, and most important point, is that Lambda hyperons (valence
quarks = uds) ARE produced transversely-polarized in inclusive production.
The Neutral Hyperon Group at Fermilab were the first people to discover
this, in the reaction:
p + N -> Lambda + X
where N is a nuclear target (typically a stick of beryllium, copper,
tungsten, or lead), p was 300 GeV protons, and X stands for the rest of
the mess of stuff produced in such a collision. (An 'inclusive' process is
one in which one focuses just on one particular particle-type to study,
and lumps everything else into X, 'the rest'; Feynman was one of the first
- if not the first - to emphasize the interest of such studies). Note that
the Lambda contains an s-quark; there are no valence s-quarks in a proton
(or a nucleus): high-energy production is certainly NOT just a
rearrangement of 'existing' quarks...
The fact that the Lambdas are polarized was a BIG surprise, since everyone
believed that high-energy collisions were such an incredible mess that
spin orientation was unlikely to result. Using the old Feynman analogy of
colliding Swiss watches (in the days when Swiss meant 'mechanical'),
polarization is something like having ALL of one type of cog-wheel emerge
from MANY such collisions all rotating in the same direction...)
Later this was found to be true for p + p -> Lambda + X (i.e. proton
target, rather than nuclear mix of p and n). And also for 400 GeV protons.
The sqrt(s), which describes the total amount of energy involved in a
collision, at a 400 GeV fixed-target experiment is about 20 GeV.
Polarization was also found at sqrt(s) = 62 GeV (31 GeV colliding proton
beams in the CERN ISR).
However, as mentioned by the original poster, for
p + N -> AntiLambda + X
with 400 GeV protons, the antilambdas had no measurable polarization (over
a wide kinematic range where Lambdas did show 20% polarization). Of
course, there are many fewer anti-lambdas produced than there are lambdas,
so it is a more difficult experiment.
To make a long story short - I love to use a cliche - the Neutral Hyperon
Group also found Cascade-Zero (=Xi-Zero to Europeans) to be similarly
polarized (this was part of my PhD thesis experiment) and then branched
out into charged hyperons too, and found Cascade-Minus, Sigma-Plus,
Sigma-Minus all to be polarized. The story continued with work by other
groups on Sigma-Zero, and later still Omega-Minus. And finally, where the
original poster started, on anti-hyperons (other than anti-Lambdas).
It is my understanding that these effects are still not understood - of
course it's hard to get a straight answer from some theorists :-)
Once one has one's hands on samples of polarized hyperons, one can use
them to measure the hyperon magnetic moments. I shall not get started on
that set of experiments...
Well, I am sure I have forgotten something crucial.
Tim Cox
p...@physics.rockefeller.edu