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CERN | Geneva | Switzerland

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How is new physics discovered?

Finding an experimental anomaly is a great way to open the door to a new theory. It is such a good trick that many of us physicists are bending over backward trying to uncover the smallest deviation from what the current theory, the Standard Model of particle physics, predicts.

This is the approach the LHCb collaboration at CERN is pursuing when looking at very rare decays. A minute deviation can be more easily spotted for rare processes. One good place to look is in the rate of K meson decays, a particle made of one strange quark s and one anti-down quark d.

There are in fact two sorts of K mesons: short-lived ones, K0S (called K-short) and long-lived ones, K0L (“K-long”). In the early 1970’s, scientists discovered that the K0L were decaying into a pair of muons 10 000 times less often than the theory predicted. At the time, the theory knew of only three quarks: u, d and s. This hinted three theorists, Sheldon Glashow, John Iliopoulos and Luciano Maiani to propose a mechanism that required the existence of a new, unknown quark, the charm quark c, to explain how this rate could be so suppressed. This explanation is now called the GIM mechanism, an acronym based on their last names.

This major breakthrough on a theoretical level was soon confirmed by the discovery of the charm quark in 1974.

Recently, the LHCb collaboration has turned its attention to measuring the decay rate of the short-lived kaons K0S, the only K mesons decaying fast enough to be seen with precision in their detector.

To make this measurement, they had to select billions of muon pairs and see if any was coming from the decay of a K0S. One can reconstruct the mass of a decaying particle by adding together the mass and momentun of all its fragments. If these muons were coming from the decays of K0S, the reconstructed mass would be the K0S mass. An accumulation of events would appear near this value in the distribution of all the recombined masses.

But as can be seen in the figure below, no such accumulation appears in the region around 500 MeV, the K0S mass value. This allowed the LHCb collaboration to estimate how often a K0S can decay into two muons, a quantity called the branching ratio. They placed a limit at less than 9 times in a billion, or in scientific notation, BR(K0S →μμ ) < 9 x 10-9 with 90% confidence level using all of the 2011 data. Since no peak appears anywhere on this curve, it means the muon pairs were produced in a variety of decays where other particles were also produced.

They have a long way to go since it is still about 2000 times larger than what the Standard Model predicts, namely a branching ratio of 5×10-12. Nevertheless, LHCb is getting closer to the theoretical prediction and eventually, given enough data, they might be able to test it.

Not easy to get to the next layer of the theory when the current one makes predictions requiring thousands of billions of events to be tested.

Pauline Gagnon

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  • “You shall not find any new physics, because all physical events are interpreted well-known particles (leptons, quarks, gauge bosons) and forces which have long known (electroweak, gravity, strong interactions)”. http://arxiv.org/pdf/0708.2322v2

  • Uncle Al

    The greatest obstacle to understanding reality is not ignorance but the illusion of knowledge. New physics requires heresy. Newton suffered Maxwell, then SR, GR, and QM. Dirac’s equation met Otto Stern measuring the nuclear magneton. Dirac failed. Yang and Lee were pariahs Christmas 1956 but prodigies New Year’s Day 1957.

    No contrast of measurable observables violates the Equivalence Principle. Strong field GR and QM horrors in pulsar-solar star binaries give orbit, periastron precession, and gravitation radiation orbital decay validating GR. Postulating boson photons’ vacuum mirror-symmetry for fermionic matter is a disaster of parity violations and symmetry breaking patches. SUSY is not empirical.

    The vacuum is observed trace chiral toward fermionic matter. Opposite shoes fit into chiral vacuum with different energies. Opposite shoes vacuum free fall along non-identical minimum action trajectories, violating the Equivalence Principle. Physical chirality cannot be measured. Optical rotation ignores atomic mass distribution: norboran-2-one 29.8° and norbornen-2-one 1146° are identically shaped.

    Section 2: Eleven pairs of enantiomorphic space groups are crystallography’s opposite shoes
    Two geometric Eötvös experiments.

    Particle theory and quantum gravitation are riddled with holes. The worst a massed geometric test of spacetime geometry can do is succeed, observing new physics in 90 days. Do it.

  • Hello,

    well, this is precisely what we want to test. Is there anything else or not? The fact that we see dark matter and that the Standard Model has no explanation for that is a good indication there must be something else. But we shall see in the coming years what we find. So stay tuned.