Of Particular Significance

The New Particle at CMS, Through the Media

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 05/01/2012

CBS NEWS, today: “A never-before-seen subatomic particle has popped into existence inside the world’s largest atom smasher, bringing physicists a step closer to unraveling the mystery of how matter is put together in the universe.”

Overall, this is not a bad article — but that last bit is propaganda.  Maybe physicists made a nano-step (“nano” = one billionth).  We knew this particle — a composite object made from known particles — would be there; we just didn’t know its details.  To suggest this is a step toward a breakthrough is just silliness.  Of course all new information proves useful eventually for something, but… really!

And the first part?  Less exciting than it sounds (though a very nice bit of research!)  Here’s the link to my post about this from Friday, explaining that the CMS experiment at the Large Hadron Collider has reported very strong evidence for a new composite object — a hadron, a member of a particular subclass called a `baryon’ —  made from a single bottom (or `beauty’ — or just `b’) quark and from other known particles.

The reporter tells the usual white lie:

Baryons are particles made of three quarks (the building blocks of the protons and neutrons that populate the nuclei of atoms). Beauty baryons are baryons that contain at least one beauty quark (also known as a bottom quark). The new specimen is a particular type of excited beauty baryon called Xi(b)*, pronounced “csai-bee-star.”

A better description of baryons (including protons) is that they are made from three quarks, many gluons, and many additional pairs of quarks and anti-quarks.  The three excess quarks are the ones being referred to in the above paragraph.  See my article on what protons are and Friday’s post on this new one, which shows a sketch that gives better intuition about what protons and Xi(b)*s look like.

[Sometimes white lies get you into a lot of trouble later.  The fact that protons have gluons and anti-quarks in them is crucial to understanding how the Large Hadron Collider does its thing.]

The reporter gets this part right:

The Xi(b)* particle had been predicted by a physics theory called quantum chromodynamics, which predicts how quarks bind together to form heavy particles, but had never before been observed.

“It was expected to be more or less where it was found,” [Vincenzo] Chiochia said. “Not all of those heavy states have been discovered, so you have to look for all those particles. It may well be that the theory is not complete. In this particular case it was expected, but we have to keep looking for things that are unexpected.”

Well.  I don’t know about that, Dr. Chiochia.  As a quantum field theorist, I would agree that “quantum chromodynamics” (which does more than what the reporter writes — it is the set of equations that describes and predicts all of the interactions of quarks, gluons and anti-quarks of all types) might indeed be turn out to be incomplete.  But if you really want to look for places where it might fail, this is an especially bad place to look!  This composite particle itself may be new, but the forces and particles required to make this object have already been very well-studied in other contexts.  (Nevertheless, congratulations on nice work!)

Oh, and by the way, Wind Farms Do Not Cause Global Warming.  They may cause some local mixing of warmer air just above the ground at night down to the surface near the wind farm itself; this does not increase the overall temperature of the planet.  Read headlines (and articles) very carefully.  http://www.washingtonpost.com/blogs/ezra-klein/post/no-wind-farms-are-not-causing-global-warming/2012/04/30/gIQAMl2GsT_blog.html

Share via:


12 Responses

  1. I suppose I was wondering whether the reason it had not been done already was simply because the calculations were too difficult.

    My intuition led me to wonder whether in the stormy sea inside a hadron there would be virtual Higgs’s being created and destroyed.

    So if you had replied with: well a QCD calculation ( assuming given quark masses ) would get you 95% of the hadron mass, then taking into account the electroweak about 4%, with the virtual higgs coming in a poor third for the remaining 1%. Well I wouldn’t have gone away surprised.

    I think I saw you were about to write an article with more details about the mass of a proton – so I await with eagerness.

    1. The Higgs field interacts very weakly with all of the particles in a proton, and even with all the particles in a Xi_b. The strongest of these interactions is with the bottom quark, whose mass is a bit below 5 GeV. Every interaction between the Higgs field and a bottom quark comes with a factor

      (bottom quark mass)/(value of Higgs field) ~ (5 GeV)/(250 GeV) [rough estimates here] ~ 1/50

      and a virtual “Higgs” (i.e. a disturbance in the Higgs field) created by interacting with the bottom quark will be at most the square of this, (1/50)^2, times a geometric factor that affects processes of this type, which is of order 1/(16 pi^2), another factor of 150. Altogether that is a suppression of order 1/(2500*150) ~ 1/400,000. We don’t know the mass of the Xi_b to anywhere near this accuracy.

      In some models with a more complicated set of Higgs fields, this effect can be enhanced by a factor of 1000. It’s still pretty small even then.

      Any effect from the other quarks and gluons engaging with the Higgs field will be thousands of times smaller than for the bottom quark.

      Finally, and just as bad, the effect is the same on all hadrons that contain a bottom quark; it will shift the mass of all of them by about the same amount. Since the way we measure the bottom quark mass is by studying its hadrons, there’s no way to separate an effect due to the Higgs from a small mismeasurement of the bottom quark mass. Basically it would be almost impossible to detect this way.

      So you can forget all hope of seeing an effect of virtual Higgs processes in the masses of bottom hadrons.

      Rare bottom hadron decays, however, have offered opportunities, and that’s because (since they’re rare) even tiny effects that enhance them can make a big difference. See for example http://profmattstrassler.com/articles-and-posts/tevatron-news/a-rare-and-interesting-decay-of-a-b-meson/ .

  2. How accurately is QCD able to calculate the mass of the Xi(b)*?

    Would such a calculation have to use details of the underlying Higgs mechanism?

    In other words, would this experimental result along with better theoretical tools be another way to better understanding of the higgs field(s)? I guess the same question would apply to all other Baryon masses.

    1. You can’t learn anything about the Higgs mechanism by studying hadrons (either baryons or mesons). If it were possible, it would already have been done many years ago.

      What can be calculated using QCD about the Xi(b)* is the difference of its mass from that of other b hadrons, such as the Xi(b) or Sigma(b) or Lambda(b).

      If you knew the bottom quark mass in advance, you could calculate the Xi(b)* mass using QCD. But we only obtain that mass from experiment, not from QCD itself.

      The Higgs mechanism only determines that the bottom quark is allowed to have a mass; the Higgs mechanism by itself gives no insight into why the bottom quark has the specific mass that it does.

  3. Is it reasonable to always blame or question the journalists integrity? Could it be that journalists are parroting an overhyped narrative put forward by a scientist seeking publicity or self promotion? Both parties are mere humans after all!

    1. I recommended an article about it last week, see http://profmattstrassler.com/2012/04/26/some-good-reads/ .

      Quoting from the current article:

      “The fantastic new thing is that this decision to entangle two photons can be done at a much later time,” said research co-author Anton Zeilinger, also of the University of Vienna. “They may no longer exist.”

      Zeilinger is one of the best in his field, quite famous. His co-authors I don’t know.

      Such an experiment had first been predicted by physicist Asher Peres in 2000, but had not been realized until now.

      In other words, it’s something you can predict from quantum theory, and it makes sense to me given what I know about quantum mechanics, but it is something I haven’t studied so I don’t have any special insights into how to explain it to you. Whether it’s groundbreaking…? Not really. It just uses the laws of quantum mechanics and follows them to their logical conclusion. It won’t change what we know about physics, that’s for sure. But it might help us understand better what we know.

  4. Does “csai-bee-star” really aid in pronunciation? I thought I knew how to pronounce “Xi”, but I have no clue as to how to pronounce “csai”.

    Based on some of the reporting and typography I’ve seen, I’d almost have thought it would be pronounced “Xi-star-bee-zero”.

    1. Yeah, I wondered about that too! I would have written “ksai”, that might have helped.

      Let me assure you, this particle is not so important that knowing in which order the star, bee and zero should go is going to affect anyone’s life, except the discoverers.

  5. The press, by its nature, injects as much excitement into a report as possible. Perhaps journalists think they do Science a favor by making it more interesting. Perhaps they do. But I think it’s common for witnesses to be disappointed with the reporting.

Leave a Reply


Buy The Book

A decay of a Higgs boson, as reconstructed by the CMS experiment at the LHC


Although I’ve been slowly revising the Higgs FAQ 2.0, this seemed an appropriate time to bring the Higgs FAQ on this website fully into the

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/15/2024

The particle physics community is mourning the passing of Peter Higgs, the influential theoretical physicist and 2013 Nobel Prize laureate. Higgs actually wrote very few

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/12/2024