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And so about cascade baryons… June 15, 2007

Posted by dorigo in news, physics, science.

So it is D0, not CDF, that is credited for discovering cascade b-baryons first in the end. A Fermilab press release  explains that the result has been submitted to Physical Review Letters on June 12th.

Congratulations to D0 for pulling this off! After the discovery of Sigma_b baryons by CDF, it seems only fair to have D0 get their own share of credit for the fantastic amount of information on the Standard Model that the Tevatron is producing.

The Xi_b particle (not Chi_b, with which I sometimes confuse myself – see comment by Lubos Motl below) is a baryon composed of a (bds) quark triplet. Three down-type quarks, each of electrical charge -1/3, but of very different properties: the d-quark weighs only a few MeV, the s-quark weighs thirty times more, and the b-quark weighs still a factor of thirty more than the s-quark. An odd, massive object (almost 6 times heavier than the lightest baryons, proton and neutron), whose decay is driven by the characteristics of the b-quark, which has a long lifetime. It is through that peculiarity (among others)  that Xi_b baryons, although quite rare, can be identified.

My hands are itching, but I will abstain from saying what CDF has to say about these particles… I decided to play by the rules of our experiment, one of which asks bloggers to refrain from posting new results until the authors have had a chance to announce it first.

UPDATE: today at 1PM CST, D0 and CDF will give two back-to-back seminars on the discovery of the Xi_b baryon. You can follow the presentation via live stream video from this site (click on Services Offered: Streaming Video)

UPDATE 2: seems like the correct link is this one .

UPDATE 3: now that the CDF result has been presented by its authors, I break no rules if I release information about it here – I had a post out last Sunday, but decided to take it offline once I realized that the B group conveners in CDF had asked the result to be kept confidential until a formal release. In the meantime, D0 sent their own result out directly to Physical Review Letters, beating CDF in the race in a rather odd way (important results should be coordinated between the two experiments). Anyway, it does not matter much who made the announcement first – the important thing is that these particles are seen. I will discuss the details of the two analyses soon in another post.


1. Not Even Wrong » Blog Archive » The Triple-Scoop - June 15, 2007

[…] baryon, since it contains one quark from each of the three generations. Tommaso Dorigo has a new posting about […]

2. Lubos Motl - June 15, 2007

It’s Xi, not Chi!

3. dorigo - June 15, 2007

Hi Lubos, you are right. I think I will correct it above, it makes little sense to leave it there.


4. Kea - June 15, 2007

Thanks for the news! Though I do wish you people would slow down a bit…..

5. gordonwatts - June 16, 2007

Slow down!? Ha! Never.

You know, I don’t know what the proceedure is — we had to tell the director about 1 week before the paper submission that this was going to happen. But I don’t know what the director is supposed to do when both CDF and D0 tell him that they are discovering the same thing?

So — I’m on owl shift (joy joy). Why wasn’t the CDF result submitted? Also, have you guys submitted the lamba-b paper yet?

6. dorigo - June 16, 2007

Hi Kea,

you are welcome as always.

Hi Gordon,

Kea means to say we are spoiling the predictive power of the theory she is putting together if we determine every particle’s mass with too good accuracy…

And you are right – I think it was a piece of sloppiness on our part this time, if I understand it correctly.

Cheers all,

7. Gordon Watts - June 17, 2007

T — yeah, I knew she was working with the results. But I hope we’ll never slow down. 😉

8. Louise - June 17, 2007

This is very exciting. Tevatron is becoming the little engine that could.

9. nc - June 21, 2007

Here’s a copy of a relevant comment submitted to Not Even Wrong (in moderation), predicting this new hadron mass from a theory of integer numbers of massive Higgs field type mass-giving quanta published some time ago:

Baryon and meson masses are related closely by the empirical formula mass = 35n(N+1) MeV, where n is the number of confined quarks and N is an integer. This fits particle masses very accurately, far better than random numbers. (The multiplier of 35 comes is the electron mass i.e. 0.511 MeV divided by twice alpha.)

The existing (still slightly sketchy) theoretical derivation is here and a table comparing it to meson and baryon masses is here.

For a baryon (n=3) of mass 5.774 GeV, we get: 5774 = [105(N+1)], so N = 53.99. Close to an integer! So I’d like to thank the experimentalists for adding this new ‘crackpot coincidence’…

This prediction doesn’t strictly demand perfect integers to be observable, because it’s possible that effects like isotopes to exist, where the different individuals of the same type of meson or baryon can be surrounded by different integer numbers of Higgs field quanta, giving non-integer average masses. (The number would be likely to actually change during a high-energy interaction, where particles are broken up.)

The early attempts of Dalton and others to work out an atomic theory were regularly criticised and even ridiculed by the fact that the measured mass of chlorine is 35.5 times the mass of hydrogen, i.e., nowhere near an integer!

10. dorigo - June 21, 2007

Hi Nc,

your formula is interesting, but I failed to find the time to read your whole documentation. I reckoned that I was not knowledgeable enough to either dismiss the theory or buy it… What is the predicted value for the Omega_b, anyways ? I think it is going to be discovered at the Tevatron in the near future… Could be a good test.


11. gordonwatts - June 21, 2007

I mispoke above. Not only did I mispell Lambda, but I meant Sigma_b — the one that you guys talked about around 8 months ago… why hasn’t that been submitted yet?

12. dorigo - June 21, 2007

Hi Gordon,

I think the paper is in review. CDF’s internal review process is about as tedious as a colonoscopy, but lasts longer.


13. nc - June 22, 2007

Hi Tomasso,

If the Omega_b is a baryon, it’s mass should be close to an integer when expressed in units of 105 MeV (3/2 multiplied by the electron mass divided by alpha: 1.5*0.511*137 = 105 MeV).

If it is a meson, it’s mass should be close to an integer when expressed in units of 70 MeV (2/2 multiplied by the electron mass divided by alpha: 1*0.511*137 = 70 MeV).

If it is a lepton apart from the electron (the electron is the most complex particle), it’s mass should be close to an integer when expressed in units of 35 MeV (1/2 multiplied by the electron mass divided by alpha: 0.5*0.511*137 = 35 MeV).

This scheme has a simple causal mechanism in the quantization of the ‘Higgs field’ which supplies mass to fermions. By itself the mechanism just predicts that mass comes in discrete units, depending on how strong the polarized vacuum is in shielding the fermion core from the Higgs field quanta.

To predict specific masses (apart from the fact they are likely to be near integers if isotopes don’t occur), regular QCD ideas can be used. This prediction doesn’t replace lattice QCD predictions, it just suggests how masses are quantized by the ‘Higgs field’ rather than being a continuous variable.

Every mass apart form the electron is predictable by the simple expression: mass = 35n(N+1) MeV, where n is the number of real particles in the particle core (hence n = 1 for leptons, n = 2 for mesons, n = 3 for baryons), and N is is the integer number of ‘Higgs field’ quanta giving mass to that fermion core.

From analogy to the shell structure of nuclear physics where there are highly stable or ‘magic number’ configurations like 2, 8 and 50, we can use n = 1, 2, and 3, and N = 1, 2, 8 and 50 to predict the most stable masses of fermions besides the electron.

For leptons, n = 1 and N = 2 gives the muon: 35n(N+1) = 105 MeV.

For mesons, n = 2 and N = 1 gives the pion: 35n(N+1) = 140 MeV.

For baryons, n = 3 and N = 8 gives nucleons: 35n(N+1) = 945 MeV.

For leptons, n = 1 and N = 50 gives tauons: 35n(N+1) = 1785 MeV.


14. nc - June 22, 2007

Whoops, I wrote fermion to cover all the particles, but I include mesons which are of course bosons. Sorry.

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