The Ξ (Xi) baryons, like all baryons, are particles made of three quarks. The first X baryon was discovered in cosmic rays in the 50s. More recently, Fermilab experiments discovered the Ξb particles, which contain a beauty or b quark. This week, CMS has reported the observation of a new excited state of the neutral baryon, the Ξb*0, the first of the family discovered so far.
The Ξb* baryons contain one beauty (b) quark, one strange (s) quark, and either an up (u) quark, which results in a neutral Ξb*0 baryon, or a down (d) quark, which results in a charged Ξb*. The ground states, that is the lowest-mass Ξb baryons — both charged and neutral — have been previously observed. However, none of the excited states predicted by the Standard Model had ever been seen. The Ξb*0 excited state just discovered by CMS is the first one.
The excited states of particles, including the Ξb*0, are expected to break up rapidly in a cascade of decays to lower mass particles, making the particle reconstruction particularly difficult. The CMS observation was made in a data sample of proton-proton collisions delivered in 2011 by the LHC operating at a centre-of-mass energy of 7 TeV. The sample corresponds to an integrated luminosity of 5.3 fb-1. The mass of the new excited state is measured to be 5945.0 ± 2.8 MeV, which makes it also the heaviest particle state of the family discovered so far.
The CMS result comes with a statistical significance of more than 5 standard deviations (5σ) above the expected background. This is one more piece of information contributing to building up a coherent picture of the various states that matter can form.
In December, the ATLAS collaboration had also reported the first observation of a new particle called χb(3P) made of a quark b and an antiquark b.
Detailed information about the CMS result is available here.
A clear signal revealing the presence of Ξb*0 particles (blue) above the background level (red)
EXCITED STATES OF MATTER
Matter can be formed in different energy states. The most stable one – that is, the one that survives the longest before decaying – is the so-called “ground state”, in which particles have the lowest possible energy. States with higher energy are called “excited states”. They are still allowed by Nature but they are unstable. The higher the formation energy (i.e. the mass) the more unstable they are.
Antonella del Rosso