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Remodeling the standard model
Fermilab findings suggest new elementary particle may soon emerge
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The Large Hadron Collider, the world’s most powerful atom smasher, may be only months away from finding a new elementary particle — a sign of a new force in nature — recent studies suggest.

The studies focus on the top quark, the heaviest of the six quarks, which are the fundamental building blocks of nature. Top quarks appear to behave badly when they are produced during proton-antiproton collisions at a lower-energy particle accelerator, the Fermilab’s Tevatron in Batavia, Ill. Compared with what the standard model of particle physics predicts, these quarks fly off too often in the direction of the proton beam and not enough in the antiproton direction.

The Tevatron finding was first reported in 2008, but the results could have been due to chance. A recent report, using additional data, boosts confidence in the result, says Dan Amidei of the University of Michigan in Ann Arbor, a member of the Tevatron’s CDF experiment. For energies above 450 billion electron volts, 45 percent of the top quarks travel along the path of the proton beam while only 9 percent are expected to do so, the CDF team reported online January 3 at Online March 10, the team reported additional evidence of the top quark’s puzzling directional preference after examining the paths of quarks generated by a different set of particle interactions.

There’s only about a 0.04 percent chance, for energies greater than 450 billion electron volts, that the top quark’s apparent directional preference is a fluke, Amidei notes. Although that percentage still doesn’t meet the threshold for what physicists consider proof, the Tevatron’s other experiment, DZero, has recently found hints of the same asymmetry, using different data and techniques.

Assuming the effect is real, the directional preference suggests the existence of a new elementary particle, not predicted by the standard model. The particle could be the messenger of a new type of force that interacts with top quarks — along with their antiparticles — in such a way as to cause the asymmetry.

Researchers are truly abuzz about the possibility that the proposed particle could be within the grasp of the Large Hadron Collider, near Geneva. If the asymmetry arises from some new type of particle just slightly too massive for the Tevatron to produce, the LHC could produce the particles directly, possibly by the end of 2011, says Markus Schulze of Johns Hopkins University.

Finding such a particle, Schulze says, “would be a beautiful and delicate signal of physics beyond the standard model.”

Theorist Moira Gresham of the University of Michigan says many explanations for the Tevatron asymmetry predict new particles that could be seen at the LHC — even though it is now running at only half its maximum energy. She and her colleagues posted a paper on the proposed properties of several such particles at on March 18, one of a flurry of articles that have appeared in response to the Tevatron finding.

Among the possible particles is a heavy cousin to the massless gluon, the particle that binds quarks together. Another idea, noted in a paper posted at on April 1, posits the existence of a particle called the Z’ boson, a heavier version of the Z boson, a messenger particle for the weak interaction. The Z’ boson would transforms one type of quark into another, leading to the asymmetry.  

“Particle physicists know that the standard model is incomplete,” Gresham says. “We've been waiting a long time to get some more concrete hints.”

It is indeed possible that the LHC might find a new particle to explain the asymmetry this year, says Aleandro Nisati of the Istituto Nazionale di Fisica Nucleare in Rome, a member of one of the main experiments at the LHC.  “If the machine performs as well as I hope,” he says.


B. Bhattacherjee, S.S. Biswal, and D. Ghosh. Top quark forward-backward asymmetry at the Tevatron and its implications at the LHC. Physical Review D, in press, 2011.

J.L. Hewett et al. $A^t_{FB}$ Meets LHC. arXiv:1103.4618v1 Posted March 24, 2011. [Go to]

CDF collaboration. CDF note 10436. Measurement of the Forward Backward Asymmetry in Top Pair Production in the Dilepton Decay Channel using 5.1 fb−1 Posted March 10, 2011. [Go to]

T. Aaltonen et al. Evidence for a mass dependent forward-backward asymmetry in top quark pair production. arXiv:1101.0034v1 Posted January 3, 2011. [Go to]

M.I. Gresham, I-W Kim and K.M. Zurek. On models of new physics for the Tevatron top AFB. arXiv:1103.3501v1. Posted March 18, 2011. [Go to]

J.A. Aguilar-Saavedra and M. Perez-Victoria. Probing the Tevatron t tbar asymmetry at LHC. arXiv:1103.2765v2 Posted March 29, 2011. [Go to]

N. Craig, C. Kilic, and M.J. Strassler. LHC charge asymmetry as constraint on models for the Tevatron top anomaly. arXiv:1103.2127v1. Posted March 11, 2011. [Go to]

Y. Bai et al. LHC predictions from a Tevatron anomaly in the top quark forward-backward asymmetry. arXiv:1101.5203v1. Posted January 28, 2011. [Go to]

M.R. Buckley et al. Light Z' bosons at the Tevatron. arXiv:1103.6035. Posted April 1, 2011. [Go to]

Suggested Reading

F. Margaroli. Top quark pair production at the Tevatron. arXiv:1011.4679v1. Posted November 22, 2010. [Go to]

R. Cowen. Elusive Higgs particle has fewer hideouts. Science News online, March 16, 2009. [Go to]

Comments (10)

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  • Quite aside from the possibility of a new particle involved, surely the fact that the combination of directional (spatial) asymmetry involved with the preference for the top quarks to rebound in the incoming proton's direction (as opposed to the anti-proton's incoming direction) - if confirmed - says something astonishingly fundamental about TIME. It always amazes me that HEP/Standard Model physicists so reflexively wring their hands on the possibility of 'another particle' without taking the far more gigantic potential implications of the clue into consideration.
    Adolf Schaller Adolf Schaller
    Apr. 3, 2011 at 8:45am
  • Adolf, I'm not a particle physicist, so clue me in, please. What would such a particle - if confirmed - say about time?
    Robert Woodman Robert Woodman
    Apr. 4, 2011 at 3:10am
  • I'll admit, Robert, that this is what struck me also about Adolph comment. Thinking about this and the almost luminal velocities involved with the Tevatron and the LHC, particle path length in the detection chamber could give clues about the lack of directional symmetry which might turn out to be time effects and/or new particle disturbance. I think!
    PedroRoberto PedroRoberto
    Apr. 4, 2011 at 1:13pm
  • @Robert: I'll try to explain my position.

    The fact that matter and anti-matter are involved in a spatially directional preference - product jets apparently rebounding statistically more frequently in the direction the proton came from instead of in the direction of the anti-proton as viewed from the reference frame of the collision site - already hints at some fundamental relation between time and space, irrespective of the existence of some unknown particle that may be responsible for the asymmetry. Given that anti-protons may be considered protons moving 'backward' in time, the observed asymmetry, if confirmed, could be telling us something extraordinarily important about the properties of the 3 macroscopic dimensions of space we live in.

    It might imply that what we used to consider to be a symmetrically neutral 'playing field' of 3 dimensional space possesses a peculiar built-in bias for which 'direction' the arrow of time is pointed. If that's so, it would be tantamount to a cosmological issue as well as an exclusively particle physics (small-scale) issue. There are indeed many other partial clues that seem to point to an overall cosmological structure which incorporates not a unitary directionality to the arrow of time, but to an 'axis' of time common to twin universes which have an arrow of time pointing in opposite directions, yet share an origin in a common 'big bang'.

    One can think of such a cosmology as consisting of opposing relativistic 'jets' that preserve an overall symmetry - a twin-arrowed AXIS of time - yet subsequent processes in EITHER 'matter' or 'anti-matter' realms are subject to symmetry-breaking because of an apparent unitary directionality of the arrow of time. If this is the case, one 'universe' could not exist without its partner. Something even deeper may be involved, such as a 'persisting entanglement', not just between inhabitant particles in BOTH sides, as may be plausibly integrated into the Standard Model (supersymmetry [SUSY] ideas, for example, may be an avenue to just such an integration) but between the very character of space-time playing-field each 'side' supplies for the fundamental particles that inhabit them. Such a binary cosmology could at once explain the asymmetrical condition of an observed 'unidirectional' arrow of time while simultaneously preserving overall cosmological symmetry...along with explaining many other fundamental issues that have perplexed both particle physicist and cosmologists.

    As this fine article explicitly points out (with supplied quotes), the current Standard Model is obviously missing something crucially important. Indeed, everyone knows this. But reflexively attributing such a startling observation of asymmetry (if confirmed) yet again to another 'particle' (or, equivalently, what physicists also refer to as a 'resonance energy state') - as in the papers supplied in the links - while that's certainly one perfectly legitimate way of accounting for the asymmetry, may, I'm afraid, overlook the bigger picture, and suggests that physicists, perhaps because of more than a little reluctance at sticking their necks out in order to preserve their reputations, may not be thinking sufficiently far outside of the box.

    One may make a fair argument that the sort of boldness exemplified by individual theorists in writing papers a century ago (and which happened to lead to a series of revolutionary and progressive steps over a timescale measured in years or decades instead of generations) is not very often seen today with papers that are authored by HUNDREDS of physicists. It seems that over the last half-century, as the necessity for high-energy accelerator experiments has grown, physics has become mired in the mud of community consensus rather than the flash of individual theoretical genius and audacity.

    One thing seems assured, however: if the Standard Model is to advance at all significantly out of its current doldrums (and, no, I respectfully do not think the long-lasting impasse is entirely due to having reached some arbitrary limit of theoretical wherewithal, regardless of necessary limitations imposed on experiment), its practitioners will have to get MUCH better at examining the bigger picture and openly entertain 'wild' theoretical ideas that CAN be revolutionary without any fear of academic community backlash. We may pause to remember that it USED to be that theoretical physics was driven by individuals who had nothing to lose, not by cliques, the 'individuals' of which have an established standing to protect.
    Adolf Schaller Adolf Schaller
    Apr. 4, 2011 at 5:55pm

  • Google in the input: = or ==you can find many brand names, even more surprising is that he will sell you the unexpected o(∩_∩)o

    shuhua xu shuhua xu
    Apr. 6, 2011 at 3:01pm
  • Excellent post Adolf.
    Marc Ricciardi Marc Ricciardi
    Apr. 6, 2011 at 9:07pm
  • It is little-considered (Oakley, Rethinking Physics, 2009) that both the proton and the anti-proton are quantum black holes (QBH) constituted by relativistic-speed circulations of energy curving space-time around the event horizon of the QBH.

    The shape of the QBH is of a torus with the circulation racing in an involute path (helical 'slinky', twice around per wavelength) just inside (proton) or outside (antiproton) of the QBH event horizon.

    The energy localization changes the local index to make the energy circulation relativistic (less than c), and makes the QBH appear convex to an observer, with the curvature causing a tangential tension (giving rise to the space-time curvature of ‘gravity’ and perceived 'mass'=E/c2) balanced by radial stress (giving rise to the electric field).

    The (anti)up – (anti)up – (anti)down ‘quarks’ manifest as the (anti)proton toroid’s 3 defining field radii related as a-2/3. a-2/3. a-13, where a is the fine structure constant, but of course become meaningless when the (anti)torii are annihilated.

    Yet in observer space there appear the 2 x 938 MEv products of annihilation. As the circulations unleash each other from their respective self-created QBHs, it is interesting that there would appear some preferred directionality to the reaction products.

    This is only surprising if you assume that a (anti)proton has no real internal structure.

    This might be the evidence that Oakley's QBH theory is looking for, since everything else it predicts is just the physics world we have come to know (but with Unification and without the Large Number Problem, of course).
    Lionel O'Gauge Lionel O'Gauge
    Apr. 7, 2011 at 3:15am

  • Google in the input: = or ==you can find many brand names, even more surprising is that he will sell you the unexpected o(∩_∩)o
    shuhua xu shuhua xu
    Apr. 7, 2011 at 1:58pm
  • I'm no physicist, but science continues to find more and more subatomic particles, what if the nucleus divided infinitly? I find it ironic that physicists strongly believe that the universe started with an infinitly small and dense singularity, many times smaller than an atom and yet conceding that the atomic nucleus is infinitly dense is scientific heresy! I believe that as people create bigger and bigger colliders, the particles will just get smaller and smaller, maybe the standard model should be more of novelty and not an absolute.
    Michael Langston Michael Langston
    Apr. 8, 2011 at 3:41pm
  • The new elementary particle of Fermilab at 144 GeV is the hidden lepton outside of the three lepton families in the standard model. Unable to exist alone outside of the standard model, the hidden lepton exists in the pair mixture of the hidden and the anti-hidden leptons, resulting in producing the dijet of leptons along with W boson as observed. (facebook of d-y chung)
    d-y chung d-y chung
    Apr. 15, 2011 at 12:16am
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