Particle Physics


What the Heck is a Pentaquark?

What do you get when you combine four quarks and an antiquark?

If you think this sounds like the opening of a particle physicists’ riddle, you aren’t too far off. Hypothetically, this particular quark combo makes a “pentaquark.” Despite decades of searching, physicists haven’t been able to actually find a pentaquark. Now, though, there’s a hint that two pentaquarks have unexpectedly come out of hiding.

Illustration of a possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. © CERN

If the new result holds up—a big if—the unexpected discovery would add a new species of particle to the standard model’s menagerie. But the measurements, recently announced by the team collaborating on the LHCb experiment, are truly perplexing. While the results were submitted for publication a couple of days ago, the first discussion in a large public conference occurred on July 23 at the 2015 meeting of the high energy physics division of the European Physical Society, where I had the opportunity to hear Sheldon Stone, who led the analysis, talk about the result. It’s certainly a topic of both excited and skeptical discussion here at the conference.

Pentaquarks were first predicted in 1964 by Murray Gell-man and George Zweig in the separate and competing papers in which they first hypothesized the existence of quarks. (Gell-man’s name “quark” has stood the test of time, while Zweig independently proposed the now-defunct “aces.”) Physicists have looked for pentaquarks for a long time, unsuccessfully. We don’t know why there has been no evidence for their existence for so long. Maybe they don’t exist. Or maybe they do and the LHCb experiment has finally found them.

Quarks are the building blocks of protons and neutrons and, as far as we know, they are the smallest basic units of matter. Quarks combine with other quarks according to the rules of quantum chromodynamics (QCD), which is the theory describing the behavior of the strong nuclear force, which is the strongest of the known subatomic forces. Pair a quark with an antiquark, and you’ve got a particle called a meson; three quarks make a baryon, like a proton or neutron. The new pentaquark—if it really is a pentaquark—seems to be made up of two up quarks, a down quark, and a charm quark/antiquark pair.

The announcement is the latest chapter in a somewhat dubious story of now-you-see-now-you-don’t discovery. In 2002, scientists in Japan announced the discovery of a particle with a mass about 1.5 times that of a proton. They called it the Θ+, and argued that it was a kind of pentaquark. This announcement triggered a flurry of searches by other groups of experimenters, with some groups confirming the Θ+ and finding other particles that were claimed to be different pentaquark candidates, while other researchers found no evidence for any new particles at all. The excitement continued for three years until 2005, when the community decided that the original announcement was wrong. The death knell of the Θ+ sounded when a group of scientists at the Thomas Jefferson National Accelerator Facility (TJNAF) in Newport News, Virginia, repeated the initial Japanese measurement with far more data. The TJNAF scientists saw no evidence for the existence of the Θ+, and the community consigned it to the dustbin of history as one of many particle “discoveries” that ultimately didn’t pan out.

The particles recently announced by the LHCb experiment aren’t the Θ+. Instead, the new particles have a mass of about 4.5 times that of the proton. The LHCb team wasn’t actually searching for pentaquarks when they made their measurements. Instead, they were studying how a particle called the Λb baryon decays. To their surprise, they found that a fraction of the time, some of the “daughter” particles left behind by the decay seemed to be coming from an unknown parent particle. So what the heck was it?

The LCHb team found the potential pentaquarks while investigating how a Λb baryon decays into a J/ψ meson and a Λ* baryon, which in turn decays into a K- meson and a proton (p+). In such a complicated decay mode, it is customary to look at the three daughter particles two at a time and calculate what the mass of the parent particle could have made them. In the case of the K- meson and a proton, you’d expect to see that they preferentially came from a particle with a mass of a Λ* baryon. Since the J/ψ and the proton weren’t thought to come from the decay of a single particle, you’d expect to see no particular mass looking special—but, as seen here, the researchers saw that a fraction of the time, these two particles seemed to come from a parent with a specific mass. Could pentaquarks be the culprit? Image adapted by Don Lincoln.

The LHCb team was unable to reconcile their measurements with any of the known or predicted particles of the Standard Model. They seemed to need something new. After testing out lots of hypotheses, they considered the discredited pentaquarks. (Remember that pentaquarks are a prediction of the theory of QCD, they’ve just never been seen before.) One pentaquark wasn’t enough to fit their data, but two did the trick. When they included two new pentaquark particles in their calculations, the data and theory agreed.

The two new particles have an unusual amount of quantum mechanical spin, specifically 3/2 and 5/2. (Protons, neutrons and electrons are all spin ½.) Like all particles that are bound by the strong nuclear force and decay under its rules, they live for a very short time, specifically about 10-23 seconds.

Given the checkered history of previous pentaquark searches, physicists are naturally skeptical. So it is worth dissecting the claim. The first question is whether scientists are confident that they’ve discovered some kind of new particle. Here, the claim is on firmer ground: the two detections have significance of nine and 12 standard deviations respectively. (The usual standard in particle physics to claim the discovery of a phenomenon is five standard deviations, and larger numbers mean more certainty. Nine and 12 are very strong numbers.)

It’s less certain whether the new particles are really pentaquarks. There are good reasons for skepticism: For one thing, the makeup of the new pentaquarks—two ups, a down, and a charm quark/antiquark pair—seems improbable. It should be easier to make a pentaquark consisting of only up and down quarks, which are lighter than charm quarks, and such a particle has never been discovered. Discovering a charm pentaquark first feels like going fishing and pulling up two sharks and no trout. A second possibility is that the new discovery is actually a sort of “molecule”: a particle called a J/ψ attached to a proton, roughly similar to how a deuteron is a proton and neutron bound together. Both have the same quark content, but only “five things in a bag” qualifies as a “real” pentaquark.

When I caught up with Sheldon Stone during the coffee break after his talk at the conference, he speculated that the higher mass of the charm quarks could make the resulting pentaquark more stable or perhaps somehow makes this sort of pentaquark more likely to form. He cautioned, however, that this was speculation on his part and more work would be required to substantiate these ideas.

Theoretical physicists are likewise skeptical. Frank Wilczek, professor of physics at MIT and winner of the Nobel Prize in physics for his contributions to the development of the theory of QCD was excited about the possibility of the existence of the pentaquark, but cautious about the measurement.

So what will it take for the community to embrace this exciting development? Well, as Carl Sagan is famous for noting, extraordinary claims require extraordinary evidence. It is also true that independent confirmation is key. Accordingly, other LHC experiments will try to repeat the analysis approach reported by the LHCb collaboration in order to see if their measurement can be replicated. In addition, theorists will try to see if they can find a mechanism within QCD that will explain why pentaquarks containing charm quarks are more likely to form than ones with lighter quarks.

Now, taking a more personal perspective, what do I think? First, Sheldon Stone made a persuasive and thorough case at his talk. I think the LHCb experiment is a world class collaboration, with some of the finest minds on the planet and ample experience in the subject matter. Further, they are well aware of the history of the pentaquark and would not lightly propose this hypothesis without adequate care. However, I am very cautious of claims of this nature, especially without confirmation from other experiments. I think the only sensible approach is to view the claim charitably, but critically. Taking a phrase from President Ronald Reagan, I “trust, but verify.” I think the next few months will be very interesting.

Go Deeper
Author’s picks for further reading

arXiv: Observation of J/ψp resonances consistent with pentaquark states in Λ0b→J/ψKp decays
Read a pre-print of the scientific paper describing the pentaquark observation.

Inside Science: Party of Five! Physicists Discover Long-Sought ‘Pentaquark’ In Stroke of Luck
Science writer Michael Greshko on the “accidental” discovery of the pentaquark.

Nature: Forsaken pentaquark particle spotted at CERN
Nature News reports on the latest chapter in the pentaquark story.

Tell us what you think on Twitter, Facebook, or email.

Don Lincoln

    Don Lincoln is a senior experimental particle physicist at Fermi National Accelerator Laboratory and an adjunct professor at the University of Notre Dame. He splits his research time between Fermilab and the CERN laboratory, just outside Geneva, Switzerland. He has coauthored more than 500 scientific papers on subjects from microscopic black holes and extra dimensions to the elusive Higgs boson. When Don isn’t doing physics research, he spends his time sharing the fantastic world of science with anyone who will listen. He has given public lectures on three continents and has authored many magazine articles, YouTube videos and columns in the online periodical Fermilab Today. His most recent book "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" tells the tale of the Large Hadron Collider, the physics and the technology required to make it all work, and the human stories behind the hunt for the Higgs boson.

    • Anonymous

      How many fundamental particles does the Standard Model predict? Or is there no limit?

      • The SM doesn’t predict any particular number of fundamental particles. It simply incorporates those that are found. (Well, so far.) However, the pentaquark isn’t a fundamental particle…it’s a composite particle. There is no limit on the number of composite particles allowed by the quark model.

    • Георги Кънев

      Quarks are
      theoretical category which it isn’t proven in reality, so physically to use
      something which we don’t sure that exists is unsoundness. Moreover according to
      USM the
      structure of the matter doesn’t need existence of such particles.

      CERN’s quark and lepton analysis of decaying channels: There suggest us the
      symmetry of leptons decay to convince us in correctness of so called “standard
      model”. About the model itself later, but now towards the suggested experiment,
      where first of all we have to take into account that the CERN detector room
      hasn’t ability to measure the momentum of so called boson particles and we
      cannot be certain whether these are particles or simply resonances……the last is
      most probable! If we don’t know this answer father analysis of decaying of such
      particles is useless and of course incorrect. But let continue to tract
      further: The searched symmetry is existing, but not because of the symmetry
      needed by the “standard model”, but because of the polarization of the space
      provoked by the essence of the field formation and the fact that the leptons
      use the resonance radius of the electron, see USM By the way the
      last suggests that such heavier particles like B+ mesons, tau and incorrectly
      equalized here Z and Higgs bosons as a identical origination, most probably are
      resonances but not particles. The same is in force about so called heaviest
      quarks with masses 170 Gev or so and so called basic quark which is 40 times or
      so lighter. Again the conception of quark comes from the supposition that each
      stable particle like a proton for instance is contains from three such quarks
      and therefore with 1/3 from the mass of proton in repose, but such particles
      nobody never have observed despite huge effort in this direction. So what is
      this “quark” actually where it isn’t measure any momentum of such particle and
      we can’t know whether it is a particle or resonance, because we don’t know what
      is the mass of repose of these particles if such mass exist at all!

      the delusion of the standard model approach is the supposition that the
      Universe has begun with huge infinite energy (big bang) which it isn’t correct,
      because our space (our universe) in fact has begun with energy equal to zero
      and the energy of our space appears together with the very first mass in our
      space. Moreover the standard model doesn’t give us any idea what is the essence
      of the field and quantity and quality connection between the known stable
      fields, which created the known stable particles and the essence of the
      polarization of the space. These answers give us the part I and part II USM So the standard model simply it
      isn’t correct. This doesn’t obstruct existence of the particle which already is
      called Higgs particle but this has nothing to do with the standard model.
      Similar particles must to exist until to 200 Gev according to USM. To observe
      these particles there is necessary new far more powerful accelerator for
      example joromachine
      which has possibility to reach kinetic energy up to 10 raising to a
      power 30 (ev). When we build this machine which has another more useful
      proposes, only then we can discovery all particles in the area until 200 Gev
      and beyond we can found out the resonances of the layer structure of the proton
      or neutron which exist but not quarks or something like that. Now I remember
      again that the experiment of CERN doesn’t prove any Higgs field because the
      experiment itself is colliding of two high energy protons so there it isn’t
      some purely beginning energy how it is in big bang supposition but two real
      mass particles which must to appear again on the end of the process because of
      the barion rule of preservation which means that the spin of this new particle
      must to be zero, but not because it is some father particle. Finally according
      to USM all particles which we call bosons in fact are resonances, see nuclear
      particles USM

    • Георги Кънев

      About CERN
      new beginning: Let examine first assertion that reached energy of impact is
      already 13 Tev. I immediately ask which energy this one in laboratory
      coordinate or in the center masses and haw they measure this energy and whether
      it is reliable and in what extend? First is it possible to measure directly the
      energy of two beams protons impact in coordinate center masses? That is
      possible only in one case if the two beams are from particles and respectively
      anti particle, which it isn’t possible in reality. So haw they know what
      exactly is the energy of impact in the center masses? The answer is only
      “probably” it is equal to the energy in laboratory coordinate. Let give one
      possible method of measuring this energy: Because these accelerated particles interact
      with the accelerating magnets there appears weak resonance radiation and
      measuring its frequency we can estimate the velocity of the beam by the known
      formula about wave spreading. And another way is to estimate the centrifugal
      force of the beam because there is the limit of bending of the trajectory and
      interaction with the withstanding magnets. So this measurement is in laboratory
      coordinate and obviously it isn’t so accurately. So how in CERN know the
      exactly energy of impact in the coordinate center masses? They didn’t, because
      they measuring the energy of emit in the process particles through calorimeter
      and eventually the bending of the trajectory of this particles in strong
      magnetic field if these particles are charged, but if there are not? Then what?
      So let be precise and take into account the next:

      What is
      doing in CERN and whether it is lies or simple delusion? Let analysis what is
      the experiment itself? Simply it is proton – proton dispersing in motion
      (accelerating one proton and another in opposed direction and colliding each
      other). We have existing now theory
      about dispersing, see Complement 5 page 402 USM , which is fully
      applicable in examined case. On the page 414 is shown the applicable theory of
      dispersing of (п)
      mesons by nucleons so called phase’s analysis, which is the same about low
      energy p-p dispersing. About high energy dispersing (as it is in CERN case), we
      have to include the high energy wave spates and the task practically can to be
      determinate about any “so called new boson discovery”. So what actually in CERN
      shown to us: the resonances in high energy p-p dispersing or asserted newly
      found out particles – bosons? To answer this question the detector room in CERN
      accelerator must to be full developing as to show the momentums of beginning
      particles the two protons, all momentums of appearing in process particles
      including these ones of bosons (if they asserted that these are particles but
      not resonances) and finally again the momentums of exiting two protons because
      of the rule of barion preservation. Unfortunate we cannot see anything like
      this, contrary they said us that such measurements are impossible because of
      the high energy process, which it isn’t correct – simply the construction of
      detector room must to be different and fare larger than used now. Only then
      when the detector room represents full skill experimental data these physicists
      can assert and more importantly proof their cause whether it is right or wrong!

      About Higgs
      particle: If this is particle giving mass of other particles as the physicists
      on CERN assert then why this particle is so rare, we must to wait trillions of
      collisions to see eventually one such particle rather than these event have to
      be dominant, or these gentleman simply lies or they are in delusion about
      existence of such particle at all? G.Kanev

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