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CERN | Geneva | Switzerland

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Major harvest of four-leaf clover

The LHCb Collaboration at CERN has just confirmed the unambiguous observation of a very exotic state, something that looks strangely like a particle being made of four quarks. As exotic as it might be, this particle is sternly called Z(4430)-, which gives its mass at 4430 MeV, roughly four times heavier than a proton, and indicates it is has a negative electric charge. The letter Z shows that it belongs to a strange series of particles that are referred to as XYZ states.

So what’s so special about this state? The conventional and simple quark model states that there are six different quarks, each quark coming with its antiparticle.  All these particles form bound states by either combining two or three of them. Protons and neutrons for example are made of three quarks. All states made of three quarks are called baryons. Other particles like pions and kaons, which are often found in the decays of heavier particles, are made of one quark and one antiquark. These form the mesons category. Until 2003, the hundreds of particles observed were classified either as mesons or baryons.

And then came the big surprise: in 2003, the BELLE experiment found a state that looked like a bound state of four quarks. Many other exotic states have been observed since. These states often look like charmonium or bottomonium states, which contain a charm quark and a charm antiquark, or a bottom and antibottom quarks. Last spring, the BESIII collaboration from Beijing confirmed the observation of the Zc(3900)+ state also seen by BELLE.

On April 8, the LHCb collaboration reported having found the Z(4430)- with ten times more events than all other groups before. The data sample is so large that it enabled LHCb to measure some of its properties unambiguously. Determining the exact quantum numbers of a particle is like getting its fingerprints: it allows physicists to find out exactly what kind of particle it is. Hence, the Z(4430)- state appears to be made of a charm, an anti-charm, a down and an anti up quarks. Their measurement rules out several other possibilities.

LHCb-Z(4430)

The squared mass distribution for the 25,200 B meson decays to ψ’ π- found by LHCb in their entire data set. The black points represent the data, the red curve the result of the simulation when including the presence of the Z(4430)- state. The dashed light brown curve below shows that the simulation fails to reproduce the data if no contribution from Z(4430)- is included, establishing the clear presence of this particle with 13.9σ (that is, the signal is 13.9 times stronger than all possible combined statistical fluctuations. These are the error bars represented by the small vertical line attached to each point).

Theorists are hard at work now trying to come up with a model to describe these new states. Is this a completely new tetraquark, a bound state of four quarks, or some strange combination of two charmed mesons (mesons containing at least one charm quark)? The question is still open.

Pauline Gagnon

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For more information, see the LHCb website

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19 Responses to “Major harvest of four-leaf clover”

  1. Deepak Vaid says:

    What is so surprising about these tetraquark and pentaquark states? None of the articles mention why these states are so unexpected. A short theoretical explanation by someone in the know would be much appreciated.

  2. Mike says:

    Tetra and pentaquark states are not unexpected theoretically – i.e. they are not expressly forbidden as far as we know. They are just highly unstable (since they are more massive) – therefore being able to see them is perhaps the unexpected result.

    All the Standard Model says is that any stable hadron(particles composed of quarks) must be colourless – this is known as colour confinement. In a nutshell, all quarks have an extra “colour” property called Red, Green or Blue – and all observable hadrons must be formed of linear combinations that are colourless i.e. R+G+B works, R+ anti R works, but R+R+B does not etc. Since one can make 2 and 3 quark colourless combinations, it follows that 4 and 5 quark combinations must be possible (just by adding 2 quark combinations together for example). That’s it. The standard model does not say anything else in regards to physical particles.

    Having said all that, 2 and 3 are very nice numbers in particle physics and seemingly pop up with regularity. Further, there has never been anything other than 2 or 3 quark hadrons observed. The speed of decay of 4 quark hadrons into other things used to be thought to be instantaneous for all intents and purposes. It’ll be interesting to verify if the decay width etc is what we expect in these particles, so that we can confirm we are not seeing anything unexpected theoretically.

    Finally it gives a very nice avenue to test the colourless principle of our theory as it ties into the very complicated quantum chromodynamics (theory of the strong force) and why quarks are never observed by themselves. This last bit has historically been a very challenging and ongoing field of research, so any new result would help guide us.

  3. Vernon Nemitz says:

    With 4 quarks there is a chance that some “color” might be exposed. The articles I’ve read so far don’t say anything about that. This particular particle, however, appears to be colorless, with the charm and anti-charm cancelling out their colors, and the down and anti-up quarks also cancelling their colors (same as an anti-pion).

    If nothing else, with the preceding as a starting point, theorists should be able to predict a whole slew of 4-quark particles that are colorless –and eventually find all of them.

  4. N D Hari Dass says:

    I too don’t understand the hype..the theory does not preclude them though their production rates are likely to be very small..it is not as if colored states had been found..

  5. Gavin Flower says:

    We have 4 quarks, do we have a higher bid?

    I suspect that we’ll find many 4 quark combinations.

    My intuition (warning IANAP!) suggests that 6 & 8 quark combinations are more likely to be found than 5 & 7 quark combinations – based on naive geometrical reasoning. What do Real Physicists think?

  6. Mike says:

    The quark model with 4 quarks has already been considered by just expanding the SU(3) to SU(4) and taking the 4x4x4x4 tensor product of the representation. The symmetry is just badly broken by the mass of the c quark (which is significantly larger than 0). In any case, the theoretically possible states can and have been enumerated – probably in the 70s.

  7. Χρυσάνθη Λυκούση says:

    The results are very interesting. I wonder what new technology could come out of these observations.

    • Xezlec says:

      None. It lasts for only an unimaginably small fraction of a second and can only be produced under very extreme conditions.

    • Gavin Flower says:

      We don’t know.

      But, then again, no one had any idea what the discovery of the electron would lead to…

      Steam power was known to the ancient Greeks, but it took about 2000 years before we had steam engines that were actually useful.

      If it will lead new tech, we might not know for hundreds of years.

      However, the knowledge used to find out will almost certainly lead to other benefits in the next few years. If we had to justify science based on predicted benefits then we would still be very primitive…

  8. Fred says:

    It might be a stupid question, but how can it be a charged particle if it consists of a c and anti-c, and a d and anti-d?

    • Patrick Premont says:

      The article says down and anti-up.

    • Mike Loutris says:

      @Patrcik Premont

      You are correct regarding the down and nati-up, however as Fred correctly noted two of the suspected components are charm and anti-charm quarks. I also wonder how these two can exist, or is the particle unstable due to the charm/anti-charm being present?

  9. […] que este descubrimiento podría significar para la humanidad. Tal vez hayas escuchado que el CERN anunció el descubrimiento de una extraña partícula conocida como Z (4430), una publicaci-on que resume lo que ha sido […]

  10. […] a LHCb Quantum Diaries […]

  11. West says:

    I am curious how one gets 13.9 sigma from this measurement, particularly when one just has the plot above.
    My naive guess is:
    — a Pearson chi-squared test is used to check how extreme the result is assuming there is no Z’
    — compute the right tail p-value using the relevant chi-squared distribution
    — convert probability to #-of-stdevs using the inverse error-function.
    Do I have this right or was it a case of multiple-testing and tail-areas from Poisson distributions?

  12. […] may have heard that CERN announced the discovery (confirmation, actually. See addendum below.) of a strange particle known as Z(4430).  A paper […]

  13. David says:

    Hi, please can you help me to understand why the mass of the tetra quark is roughy four times that of a proton? Why is it so large?

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