27.04.2012: Elementary Particles and Resonances -- what's the difference

The CMS collaboration at CERN's LHC collider announced the possible discovery of the exited Ξ0b baryon [1].

The German press realized this as the discovery of a real new particle [2,3] while emphasizing it in one line with the Higgs boson [4] and the superluminal neutrinos [5].

However, this is not true. Let's explain why: Baryons are particles consisting (unlike Mesons) of three Quarks. Baryons are the building stones of the atomic nucleus: the Proton and the Neutron. Since matter needs to be stable, the quark constitution of the particles are those of the lightest and none-decayable quarks: The Up and Down quark |u> and |d>:

Protons: |uud>

Neutron: |udd>

Quarks are -- according to our today's knowledge -- 1/3 charged and therefore are subject of the electro-magnetic interaction, the week interaction (which turns out to the same) and the strong force, mediated by Gluons.

Common with the Leptons, Quarks obey a certain scheme: Being organized in pairs (u,d) and in three generations (u,d), (c,s), and (b,t). The charge of the 'upper row' quarks is +2/3 those of the 'lower row' -1/3. The only (known) difference among the generation is their mass; the higher-mass quarks decay into lower-mass quarks by means of the weak interaction [6].

Now we start to play. We exchange a |d> quark with a |s> quark and look at the result:

Σ+: |uus>

We can continue the game using strange and bottom quarks and end up with:

Ξ0b: |usb>

Ξ-b: |dsb>

The bottom quark essentially decays in a two-step process, since first a charm |c> quark is generated, decaying in a second stage to finally into a |u> quark.

The baryon may however become excited (increase of quantum momentum). For instance the proton my become a Δ++, decaying into a proton and π+. This happens already at low energies and is well known.

We know baryons containing |s> and/or |c> quarks pretty well. For instance, exchanging one |d> quark in the neutron with a strange |s> quark yields a particle |uds> known as Λ0s, a strange baryon. Continuing the game and using now the bottom |b> quark instead of a |d> we end up in very heavy baryons known as Σ0b composed of |usb>. Again, these particles are known and most bottom baryons are already discovered.

We call the ground state any allowed the baryons compositions stable, since they only can decay by means of the weak interaction, resulting in relative long life times. However the excited states decay immediately, typically by means of a secondary meson in the final state. The physicists call the excited state a resonance.

It is this, what the CMS people did discover [1]: Ξ0*b ==> Ξ-b + π+. The '*' denotes an excited state.

The importance of this discovery is indeed not the new particle; but rather that the CMS collaboration was able to reconstruct the (complicated) decay chain precisely. This actually means, the team does understand it's detector to a high level of accuracy [7], which eventually is required to find the Higgs [4] boson; if existing.


[1] arxiv.org/abs/1204.5955v1
[2] www.spiegel.de/wissenschaft/technik/0,1518,830232,00.html
[3] www.heise.de/newsticker/meldung/Neues-Teilchen-am-CERN-entdeckt-1563144.html
[4] www.spiegel.de/wissenschaft/natur/0,1518,820135,00.html
[5] arxiv.org/abs/1109.4897k
[6] www.fehcom.net/WA21/index.html
[7] www.youtube.com/user/CERNPeople