CMS Finds a New (Expected, Composite) Particle

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.]

Fig. 2: Sketch of a proton (left) and the new particle (right); one up quark from the proton has been replaced with the very heavy bottom quark, which makes up the majority of the new particle's mass and sits at its center, not moving much; one down quark is replaced with a strange quark; and the internals of the new particle are ``excited'' (moving around more than necessary, something like a vibrating spring.)

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.]

38 responses to “CMS Finds a New (Expected, Composite) Particle

  1. When you speak of this new baryon as having the same constituents as an already known baryon, but in an excited state, am I correct in assuming that you mean in the same sense as the \Delta^+, which, like the proton, is composed of two ups and a down, but with an angular momentum of 3/2 instead of 1/2?

  2. Bottom quark is related to the second ” extra-cosmic ” family of the S.M. , now as they show themselves in our cosmos in such transient and ghostly manner , why they exist , what role they perform ? it is now a kind of super-complex network , what are the relations ? what is the structure ?

  3. According to wikipedia the bottom quark mass is about 4 times the mass of the proton , so help is needed ……what is the matter?

    • What is your question? I don’t understand what you are confused about. (also the b quark mass is closer to 5 than 4 times the mass of the proton, but it is also a bit hard to define because we never observe b quarks entirely on their own.)


  5. Mario Acero O.

    I always enjoy reading your post. I like the way you write and explain everything.

    This time I got confused by your statement:
    “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”

    How do we know this? Is it an assumption considering the b-quark is so much massive than the others? This might be a naive question, but could you clarify this to me?

    Thanks a lot!


    Mario AAO.

    • Sure. It’s not really an assumption; it’s simple dynamics.

      We know how heavy bottom quarks are from many different measurements; it is hard to measure it very precisely, but no matter how you measure it you get something between 4 and 5 GeV/c-squared. [Permit me not to write “c-squared” each time; it’s implicit in what I say below.] We know this because

      a) we measure many objects that decay as though they decay a single bottom quark, and they are all in the 5 GeV mass range.

      b) we measure a class of objects that are clearly bottom quark/bottom anti-quark “atoms”, and these are in the 10 GeV range

      Now if you have any dynamical system that has one very heavy object and lots of lightweight objects — think about the sun and its planets, or the black hole at the center of a galaxy of stars, or the nucleus at the center of an atom — the heavy object will always end up in the center, barely moving. This is essentially momentum and energy conservation, nothing more. So it’s not an assumption.

  6. Good evening Professor,

    If I understood it right, the Higgs field is the cosmic medium (space) that induce standing spherical waves (massive particles) from free waves (bosons). Ex. Drop a marble in a pond, the ripples are the particle while the water is the Higgs field.

    1. Is the mass-energy of any specific particle related to the specific energy-density to coalesce to a soliton (eigenvalue).

    2. How would you characterize charge and spin in the standing sphere wave concept?

    3. If the Higgs field gives mass (standing waves) to particles, then is there a more fundamental field that induces cosmic excitation?

    4. Or is it the cooling (positive entropy), through the expansion of spacetime that creates coalescing and induces the vacuum oscillations?

    5. What are the first variables and in what order do physicist think there were created? Ex, energy – time – space – random, chaotic vortices – ordered, confined vortices – etc?

    Any good references?

    PS, Do you buy-in to the concept that the universe is a hologram composed of information that is confined to the boundary? I understand that some theoretical physicists believe that is what is happening in a black hole.

    • 0) no that’s not right. The ripples in the Higgs field are Higgs particles. The ripples in OTHER fields are the OTHER particles.

      1) No.

      2) They’re part of the field before you put a wave into it in the first place.

      3) What is cosmic excitation?

      4) ?

      5) ? The universe is what it is; there is no “First this, then that”.

      PS: It is a reasonable but not established hypothesis. And yes there are many theoretical physicists who think this; this is because of properties of general relativity, and because of the AdS/CFT correspondence of Maldacena, which shows (not quite proven but confidently established) that holography can work, in detail, for universes with a negative dark energy. Our universe has positive dark energy, but many people hope the basic idea still works, in a more subtle way.

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  8. Hi Professor,

    If an electron is a fundamental particle and neutrons and protons are composed of different quarks (two ups and 1 down, or 2 downs and 1 up) how can they change from one to the other through beta decay or electron capture? I mean, how does the electron change the quark from up to down or vice versa?

    • The ability to change a particle from one type to another is a crucial property of the weak nuclear force. The basic transformation does not involve a quark changing to an electron; it is that a down quark changes to an up quark and in the process a disturbance in the W field is created (called a “virtual W particle”, even thought it’s not a particle) and this disturbance can convert into a neutrino and a positron (i.e. an anti-electron).

      Now you can ask how a W virtual particle can convert into a neutrino and a positron. And the answer is that one property of the universe is that the W field, the electron field and the neutrino field can interact with one another, and this permits a disturbance in the W field to create disturbances in the electron and neutrino fields; it permits a real W particle to decay to a real positron and a neutrino, and it permits a more general disturbance tp convert into a neutrino and a positron.

  9. One of the fundamental Q.s is : how the extra- cosmic families 2 and 3 by disturbing the fields of the first family — virtual particles — direct / control / structure our cosmos ?
    We appreciate very much your elaboration on this dilemma , for ex. how this new particle share in this huge task?

    • I made a comment above to an earlier question, which I think answers this question too.

      • With this respect allow me to mention some points , there are 2 levels of understanding ;
        1- The mechanistic physical one , and here as you said we do not know why 3 generations or why fundamental fields…..
        2- This a why Q. which is beyond science , but we understand that the properties of our cosmos required the existence of fields and 3 generations.
        3- So we actually do not know and really we do know , it is a kind of to be or not to be , that is the Q.

  10. But you said that the 2nd and 3rd generations are the causing factors for our world to be as we observe , you said that without it our world would be totally different , so when you state now that we do not know why 3 generations exist , according to what you said WE DO KNOW , 2nd and 3rd ones are the causal mechanism to implement properties of 1st generation world .
    Is is a catch 22 ?

  11. P.S. : Maybe you mean that we do not know what mechanism is responsible for generating 2nd and 3rd generations ?????

    • We don’t know why there are any generations at all, much less 1, 2, 3, 4, 5, or 2384. No insights as of yet. Specific speculative models have given various answers, but we don’t know which one might be right (or if some other idea is needed.)

      But it is true that without the second and third generation’s effects on the gluon field, the world for the physics of the first generation would be completely different. This is through non-obvious indirect quantum effects. Fields affect each other in complicated ways.

  12. Marcel van Velzen

    Prof Strassler,

    Could you give any reason to call an excited state of a known particle a new particle?

    • a bad physics student

      @Marcel van Velzen
      I think maybe even i can answer that question(if i got it right).
      Please do correct me if i am wrong.(i am just learning and trying to understand). Please everyone corrent any mistake i made

      I do not know if this is the right way to explain it. If one looks for unitary representations of the poincare group one can characterize those representations by the values casimir operators(operators that commute with every object in the lie algebra of a lie algebra associated to a given lie group(the lemma of schur guarantees that such operators are a complex number times the identity)) are associated with. For the Poincare group spin is something that characterizes a representation(rest-mass also does). A representation of the Poincare group is associated with a hilbert space. Vectors from this hilbertspace describe particles. Hilbert spaces with different valued casimir operators describe different particles thus. An excited state ussually has a higher rest mass than a non-exicted state thus has to be treated as a different particles also lives in a different hilbert space.

      • Marcel van Velzen

        Thanks for your long answer. Not bad for a bad physics student!

        I think what you say is true if you have to describe a composite particle as an elementary particle, when it is to difficult to describe it as a composite particle. The elementary particles are characterized in this way. But, for example, the hydrogen atom can be described as a composite particle (non-pertubatively) and no one calls an hydrogen atom with the electron in a higher orbit a different particle. However, maybe these complex composite particles are more or less described as elementary particles and that is why some refer to it as a new particle. I never considered this, so many thanks for your answer!

        • The reason no one calls a hydrogen atom in an excited state a different particle from the ground state of hydrogen is historical. And the reason that we call excited states of the proton (such as the Delta) different particles is also historical. There is no inherent reason. So let us not argue about words.

    • Yes.

      Experimentally, they both appear as a peak on a plot, so you can’t be sure, at first, whether you have a new elementary particle or a new composite particle, such as an excited state.

      In fact, there have been theories (mostly discredited, but they made sense at the time before there was more data) that the muon and the tau were excited states of the electron. In other words, what you think is elementary today may turn out to be composite tomorrow.

      It can be even worse than this; the distinction between elementary and composite can actually be ill-defined.

      In the end, a “particle” is a ripple in fields that retains its identity as it travels, and has mass, energy and momentum. And an internally excited state of a particle is a particle too; whether you call it “new” or not is a matter of convention.

      • Marcel van Velzen

        Hello Prof. Strassler,

        I get the impression this discussion got you in an excited state also 🙂

        The reason I asked the question is that the particle physics community has to be very careful these days how to communicate discoveries. We should not forget that high energy physics got a very heavy blow from the faster than light neutrino business. This is especially true for the Italian physicists (although Dutch I lived in Italy for many years so I can imagine how bad they collectively feel about this).

        A few weeks ago the discovery of the Majorana wavefunction at zero K was communicated as a new Majorana elementary particle and mixed with theories of the Majorana neutrino by the university that made the discovery! I had to convince a lot of people that the zero K wave function was not the same as the Majorana neutrino and that it was not an elementary particle. The other day on Dutch television an ex-minister of economics (a smart guy) was in a talk show with the scientist (Leo Kouwenhoven) who discovered the Majorana wave function and the scientist was asked if this discovery was expected to remain (a question nobody would have asked in the past). The ex-minister interrupted to say: “there aren’t any Italians working with you, are there?”. Of course, it was a joke but this is how things go. I really feel sorry because Italians actually standout for their contributions to high energy physics!

        I got the impression you were putting things about this discovery in the right perspective also. On the CERN web site it reads: CMS discovers new particle
        while you wrote: CMS Finds a New (Expected, Composite) Particle.

        I now feel many people will say “yeah sure” when the Higgs gets (more convincingly) discovered especially because they hear so often now about the discovery of an elementary or new particle. Experimental and theoretical physicists are doing really great things these days (far above my capabilities at least) and there is no need whatsoever to mystify the excellent work they are doing!

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  14. Prof. Strassler,
    By a “heavy” particle, I assume you mean a particle with relatively large inertial (rest) mass. Since we are studiously avoiding any gravitational interactions in QCD, using the physics slang term “heavy” can be particularly misleading. (pun intended 🙂

    Also, just using the term “particle” is one of physics’ historical accidents, due primarily to the order of discovery 100 years ago. We’ve known for decades that we can devise multi-path experiments where a “particle” does not take path A, nor path B, nor both paths, nor any other path, and yet moves predictably through the apparatus. Not something a “particle” (or a wave, for that matter) should be able to do. Since Halvorson and Malament showed in the 1990s that it is logically impossible to have what we think of as localized “particles” in any relativistic field theory, what we observe is really an illusion of a “particle”. A peak in a plot, a transfer of momentum, the detection of a flash on our retinas when we “see” a single photon in a very darkened room — calling that “lumpiness” of quantum properties over space and time a “particle” can be very misleading. But we’re stuck with it.

    • Yes, you are right that the word “heavy” is dangerously ambiguous.

      I agree about “particle”; elsewhere I have emphasized on this site that “quantum” is a much better term, and that “particles” are best thought of as the ripples in a field of smallest possible intensity. Actually I still have to write a proper article about it.

      What’s the Halvorson and Malament reference? I don’t think I’ve ever seen their argument (though the result sounds rather obvious to me.)

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