Yes, it’s true what you’ve read; the CMS experiment at the Large Hadron Collider has found a new particle. However, this isn’t one to get excited about. Or rather, it’s the particle that’s excited, not the rest of us. It’s a nice result; a neat result; but this particle is a slightly more massive version of a hadron that we already knew about, a composite object similar to a proton, built out of more fundamental particles we discovered over 30 years ago. So in the grand scheme of things, this is minor news; no big mysteries to resolve here. Nevertheless, congratulations to CMS! Finding such particles always involves reconstructing them from their decay products, and since this one decays in a very complicated way, the result represents a technical tour-de-force!
This is a similar story to one from last December, when ATLAS announced that it had found, with confidence, a new particle. I explained to you then that there are particles and there are particles;
- there are (apparently)-fundamental particles such as top quarks, electrons, Z particles and neutrinos [“apparently” because someday we might learn they have a structure, but right now we know they’re smaller than a billionth of a billionth of a meter.]
- there are composite particles that are built from smaller and apparently-fundamental particles; atoms are one class of examples; hadrons, particles built from quarks, antiquarks and gluons, are another. [All hadrons have a size of about a millionth of a billionth of a meter.]
As I explained carefully in December, what ATLAS found was a hadron that is an excited state of a sort of “atom” made from a bottom quark and a bottom anti-quark. Since we found the bottom quark in the 1970s, this is very different indeed from finding, say, the Higgs particle, which would be really new. [That might happen soon, and certainly we have hints that suggest discovery of the Higgs may be imminent, but the evidence is still, in my view, pretty weak.]
Now CMS has found, with high confidence (over 5 standard deviations above background), a hadron that is an excited state of something just a bit more complicated than what ATLAS found. It contains a single bottom quark, a strange quark, and an up quark, along with a whole mess of gluons and quark/anti-quark pairs (as is the case for most hadrons). [Sometimes people still refer to the top and bottom quarks as “truth” and “beauty” quarks, which is why you’ll read people calling this a “beautiful hadron” or a “beauty particle”. It makes for good puns.]
This is to be compared with a proton, which has two up quarks, a down quark, and a whole mess of gluons and quark/anti-quark pairs. The class of hadrons that have three quarks plus gluons plus quark/anti-quark pairs are called baryons; the proton is the lightest one, with a mass of 0.938 GeV/c2, but there are hundreds and hundreds of them. This one is called (tentatively, needing verification) the Ξ*b0; it is electrically neutral (like a neutron) and has a mass of about 5.945 GeV/c2 [with an uncertainty of about 0.003 GeV/c2]. The main differences between the newbie and the proton is that
- the bottom quark is so heavy that it makes up, by itself, most of the mass of this hadron; and it sits dead center and barely moves while all the other quarks and gluons and anti-quarks whiz around it
- the new particle has its quarks and gluons moving around with more energy than necessary (this is what it means to be “excited” — it is the same notion as for an excited state of an atom; compare a spring sitting still to a spring that is vibrating) so there is a hadron of smaller mass that contains exactly the same constituents, just moving around with less energy.
- the new particle decays within a trillionth of a trillionth of a second or so, cascading down in several steps to particles that live long enough to be measured in CMS’s electronic tracking devices.
[Reminder: I’m giving a public lecture about the Large Hadron Collider on Saturday, April 28th, 1 p.m. New York time/10 a.m. Pacific, through the MICA Popular Talks series, held online at the Large Auditorium on StellaNova, Second Life; should you miss it, both audio and slides will be posted for you to look at later.]