Friday, October 27, 2017

CKM Matrix Measurements Place Strict Limits On New Physics


In the Standard Model of particle physics, sometimes a quark emits a W boson, via the weak force, and changes quark flavor. Any of the three up-type quarks can transform into any of the three down-type quarks, and visa versa, conservation of matter-energy permitting. The probability of each particular type of flavor change is governed by the three by three element CKM matrix. These probabilities are determined by observing various aspects of the weak force driven decays of hadrons (i.e. meson and baryons). 

Thus, the CKM matrix is the product of many dozens of observables, many of which provide independent measurements of the same CKM matrix element. If those independent measurements aren't consistent with each other, then the theory behind the CKM matrix is wrong, or you have a serious experimental error. As it happens, decays of a meson called a kaon are particularly important in making these measurements because these measurements of one of the lightest mesons that can experience weak force decays are particular precise.

Further, "unitarity" which is just a fancy way of saying that all probabilities of an event must add up to 100% if you have a complete description of the relevant physics, provides another test of whether the CKM matrix is complete. If there are transitions of quarks from one flavor to another (perhaps to known new quark flavors) governed by new physics, then the percentages determined by adding up the individual entries should not add up to 100%.

Thus, the unitarity of the CKM matrix which is described with a summary of the data called the "Unitary Triangle", which relies on the further assumption of the Standard Model that the nine elements of the CKM matrix are fully described by four parameters, is a good global test of the completeness and consistency of the Standard Model that can probe energy scales not otherwise reachable with direct measurements.

The New Paper

A new pre-print analyzes the extent to which the available data on the CKM matrix element values rules out beyond the Standard Model Physics.

It finds that in the most rigid model dependent analysis, that new physics are excluded up to a characteristic energy scale of about 50,000 TeV. There is no practical way that a human designed experiment could reach these energy scales from an engineering perspective. These are energy scales that haven't existed anywhere in the Universe since the early moments of the Big Bang.

If the assumptions of the model are relaxed, new physics are still excluded up to a characteristic energy scale of 114 TeV. This would take an accelerator on the order of ten times as powerful as the LHC or more to test.

To allow for new physics at a characteristic energy scale of 11 TeV which is the highest that there is a realistic chance that could be discovered at the LHC, one would have to abandon assumptions that are almost certainly correct based upon the available experimental evidence.

These limits are much more strict than the energy scale limitations established by direct searches for new particles at the LHC.

Many physicists consider the Standard Model of particle physics to be a "low energy effective theory" describing a more complete theory that is true at all energy scales. But, these CKM matrix measurements suggest that "low energy" is a term that encompasses pretty much everything that happens or has happened on a regular basis in the universe for something on the order of 13 billion years.

No comments: