The Standard Model of Particle Physics
Reminder: Have you read the page on "Particles and forces"?
The Standard Model of particle physics, combining quantum mechanics with Einstein’s theory of special relativity, has been described as “a tapestry woven by many hands”, sometimes driven forward by new experimental discoveries, sometimes by theoretical advances, a collaborative effort in the largest sense, spanning continents and decades[1]. The picture as we presently know it looks something like this:
A glance at this table reveals that it is arranged in terms of generations in categories of increasing mass for each particle across the board. The up quark, the down quark and the electron (those of Generation I on the left hand side of the table) are all that is necessary to build the matter of the universe. The four heavier quarks in columns 2 and 3 decay in fractions of a second into the lightest two. So why are they necessary at all - Who ordered them? we might ask, with apologies to the Nobel Prize-winning physicist Isador Isaac Rabi who originally asked the question in the singular when he learned of the discovery of the muon [2]?
The answer lies perhaps in the fact that the chart delineating the features of all the known elementary particles possibly betrays repeating patterns (somewhat analogous to the periodic table of chemical elements) suggesting that they may not be fundamental after all and that the differences in generations may stem from the configuration of even smaller building blocks of matter within quarks and leptons, which have been generically named preons, maybe 3 or 5 to a quark. If they do exist, this may also explain why the second and third generation particles interact more with the Higgs field than the first generation, thereby endowing them with greater mass [3].
In other words, the standard model may be considered incomplete and in no way a complete model of the way the universe operates at its tiniest scales. It does not explain why the universe is overwhelmingly composed of matter rather than antimatter when both were created equally in the aftermath of the big bang. It tells us nothing about dark matter, nor why the matter particles are arranged in three generations the way they are, albeit with a striking hierarchy of mass from the up and down quarks which “weigh” very little to the top quark which is almost as heavy as a gold nucleus. [4]
These and other related issues are the subject of ongoing scrutiny in an experiment at the Large Hadron Collider in Geneva, Switzerland, denominated the LHCb experiment, the ‘b’ standing for beauty which uses so-called beauty hadrons to look for the signatures of unseen particles that cannot be directly produced in conventional ATLAS and CMS experiments which are designed to search for new and hitherto unknown particles of considerable mass [5]. LHCb studies what happens when beauty hadrons are created in the LHC and then decay into other particles. In the process, virtual particles are created that pop into and out of existence, influencing the behaviour of conventional Standard Model particles, and thereby serving as signposts to a new physics involving behaviour that the Standard Model cannot explain.
To consider this in part, imagine a B-bar meson which decays into a W boson and then into a tau and anti-tau neutrino. The W boson is around around 16 times more massive than the original B-bar meson, thereby apparently violating the law of conservation of energy, but, under the "mysterious accounting rules of quantum accounting", this is allowed so long as it happens, and the energy “repaid”, over a sufficiently short period of quantum time. In this scenario, the W boson is said to be virtual. [6]
In a series of LHCb experiments involving the deployment of these so-called virtual particles, a number of discrepancies with the Standard Model have made their presence felt, which are capable of explanation (inter alia) by the presence of a new charged Higgs particle, a leptoquark (a hypothetical particle that can allow quarks and leptons to interact, or what is known as a Z prime particle, which could be an exotic, heavier cousin of the Z-boson, but one that decays into quarks and leptons in its own distinctive manner. For the moment, these outcomes are more intriguing hints than clear contradictions of the Standard Model. However, these LHCb experiments of an indirect and virtual perspective are continuing, and hope abounds that they may well bear fruit in the not too distant future, or the answer could well emerge more directly in the collisions at LHC’s ATLAS or CMS or at some future accelerator of even higher energy. [7]
On the following page, we will have a more detailed look at how this wonderful tapestry, as it stands to date, came into being.
[1] http://en.wikipedia.org/wiki/Standard_Model where the historical development of the theory is set out.
[2] Don Lincoln, “The Inner Life of Quarks”, Scientific American, November 2012, 25.
[3] Ibid, 25-27.
[4] Guy Wilkinson, "Measuring beauty, Scientific American, November 2017, 48 at 52.
[5] Ibid, 50, 53.
[6] Ibid, 53. And see also:
http://www.science20.com/news_articles/bbar_meson_makes_cracks_standard_model-91195
[7] Ibid, 55.