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New Particle Discovered At CERN 144

New submitter ph4cr writes with news that a new particle has been discovered at CERN that confirms theoretical predictions. A pre-print of the academic paper is available at the arXiv (PDF). From the article: "Physicists from the University of Zurich have discovered a previously unknown particle composed of three quarks in the Large Hadron Collider (LHC) particle accelerator. A new baryon could thus be detected for the first time at the LHC. The baryon known as Xi_b^* confirms fundamental assumptions of physics regarding the binding of quarks. ... In the course of proton collisions in the LHC at CERN, physicists Claude Amsler, Vincenzo Chiochia and Ernest Aguiló from the University of Zurich's Physics Institute managed to detect a baryon with one light and two heavy quarks. The particle Xi_b^* comprises one 'up,' one 'strange' and one 'bottom' quark (usb), is electrically neutral and has a spin of 3/2 (1.5). Its mass is comparable to that of a lithium atom. The new discovery means that two of the three baryons predicted in the usb composition by theory have now been observed."
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New Particle Discovered At CERN

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  • by Anonymous Coward on Saturday April 28, 2012 @08:55AM (#39831511)

    Protons and neutrons are composed of strictly up and down quarks, in (uud) and (udd) combinations for protons and neutrons respectively. Up quarks weigh about 2.5 MeV and down quarks weigh about 5.0 MeV. A strange quark weighs about 100 MeV, and a bottom Quark weighs (very) roughly 4.2 GeV. It's because of the bottom quark that Xi_b^* weighs so much.


  • chi b star (Score:5, Informative)

    by Roger W Moore ( 538166 ) on Saturday April 28, 2012 @09:03AM (#39831541) Journal
    It's pronounced "chi b star" and the discovery was by the CMS collaboration. The analysis was done by physicists from Zurich (apparently including one of my former postdocs) but they require data generated by the experiment so typically we credit the experiment. The discovery is of a new bound state of 3-quarks - not a new fundamental particle - so while interesting and definitely worthwhile it is not particularly exciting.
  • by krzysz00 ( 1842280 ) <krzysdrewniak&gmail,com> on Saturday April 28, 2012 @10:01AM (#39831803)
    Xi_b^* is actually the LaTeX code needed to generate the name of the particle. The particle's name is actually this (png image) []
  • by BlueParrot ( 965239 ) on Saturday April 28, 2012 @10:16AM (#39831865)

    It gets even more amusing when you consider that a proton has a mass of about 938MeV/c , whereas the three quarks it is made up doesn't even add up to 10MeV/c. The binding energy of protons and neutrons is immense compared to the particles they are composed of.

  • Re:chi b star (Score:4, Informative)

    by Snowtred ( 1334453 ) on Saturday April 28, 2012 @10:37AM (#39831945)

    Actually, this is the "Xi B Star" or "Cascade B Star". The "Chi b" particle has already been found, but is a completely different type of particle (meson, with quark and antiquark) than the "Xi b" (baryon, with 3 quarks)

  • Re:Question: (Score:5, Informative)

    by jmtpi ( 17834 ) on Saturday April 28, 2012 @11:41AM (#39832317) Homepage

    You're asking a couple distinct, and reasonable, questions. About "blind testing" -- I don't know the details for this particular result, but particle physicists put quite a bit of effort into making sure that they aren't fooling themselves. One of the best ways of doing it is so-called "blind analysis". The idea there is to define your entire data analysis strategy based solely on simulated data. There are pretty good simulations available of both the expected backgrounds, and of the process you are trying to actually find (the signal). So you define all of the methods you are going to use using these simulations before you look at the data. This ensures that you don't bias yourself into "finding something" in the data that isn't really there. (I don't know if a strict blinding procedure was used for this analysis, but likely something similar was done.)

    The formal peer review system will come into effect now that the result is submitted to a journal. The paper will be distributed to some anonymous referees who will try to judge the merits of the physics and decide whether it merits publication. But I should note that the peer review process in modern particle physics actually starts long before the result is made public. Although there are only 3 or 4 main analysts, the paper is signed by the entire 3000 person CMS Collaboration (of which I am a member). So we have a very stringent internal review process to ensure that the result is sound before we release it with 3000 names taking responsibility. That doesn't mean that particle physics collaborations never make mistakes, but it does mean that results are scrutinized by a number of more or less unbiased eyes before they are made public.

  • Re:Question: (Score:4, Informative)

    by kyrsjo ( 2420192 ) on Saturday April 28, 2012 @12:09PM (#39832505)

    Additionally, if the results are real, they can be replicated. LHC collides particles not only in the heart of the CMS detector, but there is also (among others) the ATLAS detector. This detector has more or less the same goals as CMS, but is built and operated by different people using a different detector design (both on the level of individual electronic chips and sensors, and on overall design choices), as well as different and mostly independently written software.

    So I guess someone with access to ATLAS data should now write up the analysis and see if they can find it too.

    --- Physicist who did his master thesis with sensors for ATLAS tracker, now doing a PhD on accelerator cavities for the CLIC future high-energy electron-positron collider.

  • by physburn ( 1095481 ) on Saturday April 28, 2012 @12:13PM (#39832529) Homepage Journal
    It names after the Gell Mann's group of all particles that you can make with up,down and strange quarks, the Xi^0 particle is the name for the Baryon for the particle with two strange quarks and an a up quark. For heavier quarks, physicists write as a subsubscript the letter for an heavy quark replacing the strange quark.

    The Baryon multiplets are.

    Spin 1/2 (you can draw this as a hexagon)
    Xi^0 Xi^-
    Sigma^- Sigma^0 Sigma^+
    Neutron Proton

    Spin 3/2 (draw this as a triangle)
    Xi^0 Xi^-
    Sigma^- Sigma^0 Sigma^+
    Delta^- Delta^0 Delta^+ Delta^++

    There are plenty of Baryons yet to be found, including most massively of all, the Omega Triple Bottom, which is (bbb) instead of (sss)

  • Re:chi b star (Score:5, Informative)

    by FrootLoops ( 1817694 ) on Saturday April 28, 2012 @03:37PM (#39833483)

    This discussion made me wonder where the new particle falls in standard particle classifications. I've always been curious so I finally looked it up. My notes are below if anyone else is curious. I abbreviated the fundamental forces as (G)ravity, (E)lectromagnetic, (W)eak, (S)trong.

    (1) Elementary particles: indivisible (probably). Includes fundamental fermions and bosons.
            (A) Fundamental fermions: obey Pauli exclusion principle and Fermi-Dirac statistics. Includes quarks and leptons.
                    (I) Quarks: six flavors; combine in groups of two or three; interacts with GEWS. The "S" allows atomic nuclei to exist.
                    (II) Leptons: six types, three charged, three not.
                              (a) Charged leptons: mostly, the electron. Interacts with GEW. The "E" there makes chemistry work.
                              (b) Uncharged leptons: neutrinos. Interacts with GW, so not much with ordinary matter.

            (B) Fundamental bosons: obey Bose-Einstein statistics, disobey Pauli exclusion principle. Includes gauge bosons, Higgs boson, and gluons.
                    (I) Gauge bosons: force carrying particles. Photons carry E, W- and Z-bosons carry W, gluons carry S.
                    (II) Higgs boson: would explain the non-masslessness of some fundamental particles. Currently the only unobserved standard model particle.
                    (III) Graviton: would carry G. Theoretical status somewhat uncertain; not a standard model particle; currently unobserved.

    (2) Composite particles: composed of multiple elementary particles. Includes hadrons, atoms, molecules.
            (A) Hadrons: two or three quarks held together by S. Includes baryons and mesons.
                    (I) Baryons: fermions made of three quarks. Most famous examples are protons and neutrons. Huge variety--~hundreds or more depending on how you count.
                    (II) Mesons: bosons made of two quarks. All unstable. Huge variety--~hundreds or more depending on how you count..

    Note that each particle has an anti-particle, where each composite particle's anti-particle is obtained by replacing the constituent elementary particles with corresponding anti-particles.

    The \Xi_b^{*0} particle (the summary left off the 0 for some reason...) is a baryon, so it falls under (2AI) in the above list. In light of the variety of the hadrons and their composite particle nature, this story isn't terribly exciting (at least to me).

    [Please correct any mistakes; I'm not a physicist.]

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