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Rumsfeld hadrons June 20, 2007

Posted by dorigo in internet, news, physics, science.
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An inspiring title for a preprint, indeed. Frank Close just published on the ArXiV (get your own copy  here) a short paper, Hep-ph/0706.2709, with that very title. The paper is just as inspiring, and it is worth a close look if you are interested in heavy hadron spectroscopy.

The reason of citing a war criminal in the title is made clear from the abstract:

Donald Rumsfeld, in attempt to excuse the inexcusable, once (in)famously said that “there are things that we know we know; there are things that we know we don’t know; and then there are things that we don’t know that we don’t know”. Recent discoveries about hadrons with heavy flavours fall into those categories. It is of course the third category that is the most tantalising, but lessons from the first two may help resolve the third.

I found the paper very interesting to read. It is, however, a bit too technical for me to attempt a discussion of it here. Maybe I can just quote the simplest example that Frank brings to explain what we really understand well in heavy hadrons, to stay in the spirit of the last sentence of his abstract.

The example of things we do know we know comes from the B_c meson. This peculiar particle is made by a bottom and a anti-charm quark jiggling around each other. The large mass of the two quarks makes the system non-relativistic to a good approximation (heavy things move slower!), so the mass of the object must be not too different from the mass of its constituents – to the extent that such a thing is well-defined.

Now, the mass of the B_c has been measured with precision by CDF (see plot on the left, which shows the distribution of reconstructed mass of the J/psi and pion system into which the B_c decayed), and is found to be M=6276.5+-4.8 MeV , in fair agreement with theoretical calculations based on lattice QCD (F.Allison et al., PRL 94, 172001 (2005): M=6304+-12+18-0 MeV), which are basically calculations approximating space with a grid of points and computing the interactions between quarks on this grid.

What can fortify beyond doubt our confidence that we do understand the (b anti-c) system is however not so much the good agreement with lattice QCD, but a “crackpottian coincidence”: take the mass of the fundamental mesons made of (c anti-c) [the psi] and (b anti-b) [the Upsilon], add them, divide by two, and – lo and behold – the value turns out to be 6278.6 MeV, less than one part per mille away from the B_c mass.

Duh! two c-quarks worth of dough, plus two b-quark worth of dough, divided by two, is a c-quark plus a b-quark worth of dough! Or, as Groucho Marx would put it, “A child of five could understand this. Go fetch a child of five!” (for some reason I feel very grouchian lately).

What the exercise tells us is that quarks may be mysterious things when their dynamics is involved, but as heavy static sources of color field they are much less mysterious. Close makes the point that he awaits the moment when excitations of the B_c meson will be found, because it is then that more interesting effects will start posing new puzzles.

Comments

1. Alejandro Rivero - June 20, 2007

What about 0706.1726??

2. Tony Smith - June 21, 2007

Rumsfeld’s quote is far from original.
In the USA defense contractor community (places like the Lockheed Skunk Works plant
that produced cutting-edge aircraft like the US, the SR-71 Blackbird, etc,
it has long been a common saying that when you go into new territory
“you have to watch out for the UNK-UNKs”,

meaning that you know what you know, and you can take care about things that
you know are unknown, but it is the UNKnown-UNKnowns that can kill you by surprise.

In my opinion, Rumsfeld fell victim to still another category,
things he knew but chose to ignore anyway,
because
when he was told that, while 150,000 troops was sufficient to seize Iraq (as it was),
it would take at least 300,000 or really maybe at least 500,000 troops to occupy it
for more than a few months.
The general who told Rumsfeld the truth was fired, and Rumsfeld chose to igonore it
because
the politicians refused to initiate a draft necessary to raise that many USA troops,
and
the USA diplomats could not persuade any other country to provide that many troops
(effectively as mercenaries). I think that India, which has a very large standing army,
was approached, but the negotiations fell through. Maybe the India government had too
much common sense (and bad memories of the UK using Indian troops to fight for the UK empire).

Anyhow, if Rumsfeld is to be used as the example, there should be an additional
category of known-but-ignored.
I will try to restrain myself from giving examples from world of science,
but I cannot prevent myself from quoting Oppenheimer about Bohm:
“If we cannot disprove Bohm, then we must agree to ignore him.”

Tony Smith

3. jim - June 21, 2007

The United States, as you probably have heard, is a free country, and one of our most cherished freedoms is freedom of speech. We also have a system of justice whereby everyone is presumed innocent unless, and only if, proven guilty in a court of law. The so-called court of public opinion, however, does not qualify as a legally constituted venue.

To the best of my knowledge, Donald Rumsfeld has not been convicted of any crimes; certainly not any crime as serious as that which constitutes a war crime. In referring to Mr. Rumsfeld as a war criminal you may be exercising American freedom of speech, though I am not sure if your own country is as liberal in that regard. But you would also be branding yourself a slanderer in our country, according to our understanding of that term. Perhaps your own country is more liberal in that regard, since your country has a fascist past not unfamiliar with war crimes of its own.

4. jim - June 21, 2007

The United States, as you probably have heard, is a free country, and
one of our most cherished freedoms is freedom of speech. We also have
a system of justice whereby everyone is presumed innocent unless, and
only if, proven guilty in a court of law. The so-called court of
public opinion, however, does not qualify as a legally constituted
venue.

To the best of my knowledge, Donald Rumsfeld has not been convicted of
any crimes; certainly not any crime as serious as that which
constitutes a war crime. In referring to Mr. Rumsfeld as a war
criminal you may be exercising American freedom of speech, though I am
not sure if your own country is as liberal in that regard. But you
would also be branding yourself a slanderer in our country, according
to our understanding of that term. Perhaps your own country is more
liberal in that regard, since your country has a fascist past not
unfamiliar with war crimes of its own.

5. dorigo - June 21, 2007

Hi Alejandro, I printed the four-page paper this morning. It is a quite precise determination of several QCD parameters, and I appreciate the fact that these lattice calculations are becoming really reliable. But I am too ignorant of the details to comment!

Tony, thanks for the input – I imagined Rummy’s sentence was not his own concoction, but he is (in)famous enough to have tied it to his name. I agree with your analysis of the importance, and the damage from underestimating, the fourth category…

Cheers,
T.

6. dorigo - June 21, 2007

Hi Jim,

I agree with everything you said. Whether Rummy is a war criminal or not is an opinion, not a fact.

Cheers,
T.

7. jeff - June 21, 2007

How foolish Close was, and in this case you too Tommaso, for having started off with a political statement. He and you shifted the interest away from the physics. Physics is too profoundly interesting to be corrupted by politics. Politics might be interesting but it is certainly not quite as profound.

jeff

8. dorigo - June 21, 2007

Hi Jeff,

I agree… But the temptation to be multi-disciplinary is always there when one discusses about physics.

Cheers,
T.

9. Fred - June 21, 2007

Jeff,
Your gesture is solid but physicist and chessplayer are the launching points in the subtitle of this blog. I took his word for it.

10. Fred - June 21, 2007

Jeff,
Your gesture is very solid but physicist and chessplayer are the launching points stated in this blog’s subtitle. I took Tommaso’s word for it. We’re aware of the endless combinations that chess promotes as well as the rediculous immensity of physics. Comparitively so, the importance of politics is certainly reduced and becomes a sideshow. It’s also nice to read Tony’s understanding of military history and matters. Maybe he’ll relate a civil war story for us one day.

11. jim - June 21, 2007

Undoubtedly, multidisciplinary research is where exciting discoveries are made. But it is cynical to insinuate that politically motivated opinion is a discipline. Anyone with half a brain has politically motivated opinions, and the web is littered with them. Of course, everyone is entitled to their own bad taste. But to broadcast one’s predisposition for pandering is to follow the lead of such luminaries as Cindy Sheehan, Barbra Streisand, the Dixie Chicks, and many other such multi-disciplinarians.

12. Alejandro Rivero - June 22, 2007

“there should be an additional category of known-but-ignored.”

Hmm, in the tone of Close paper (ie hadrons and widths) we have my last year example, which Dorigo was kind to upload here: https://dorigo.wordpress.com/2006/09/14/a-mistery-behind-the-z-width/


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