Look who's back (back again).
The last post saw me attempting to give a somewhat coherent and extremely condensed introduction to the Standard Model of particle physics. As a quick recap (this is me practicing for when-or if- I ever become a professor), we saw that the SM consists of two classes of particles- fermions and bosons. The fermions are matter particles and the bosons carry forces that mediate the interactions between these fermions. All stable matter is made up of first generation fermions; and three of the four fundamental forces of nature have corresponding bosons. The gravitational force hasn't made it through the selection committee to the team yet.
Clearly, the SM is not the ultimate theory to rule them all, and we cannot put a ring on it. The long-standing feud between Standard Model and General Relativity is just one of the many problems that makes it inherently an incomplete theory. Let's take a look at some of the rest one at a time:
The Curious Case Of Dark Matter
Previously, I wrote that the SM explains how all observable matter in the universe interacts via the strong, weak and electromagnetic forces. The key-word there was "observable". Cosmological observations suggest that stars orbit around in the Milky Way at speeds that seem almost independent from their distance from the center of the galaxy- which is at odds with what Kepler's laws tell us.
A possible explanation for this could be an additional source of gravity that we cannot "see", hence the name dark matter. Dark matter doesn't emit any light (by which I mean the entire spectrum of radiation), but it makes itself manifest by its gravitational force on bodies around it. It seems to interact very weakly with all the other Standard Model fields, and none of the particles within the SM has these properties. A quick Google search will tell you that Dark Matter is believed to constitute as much as 27% of matter density in the universe, while the SM particles constitute a meager 4%. This means that together with Dark Energy, there remains an unknown 96% of matter/energy density in the universe.
What, Neutrinos Are Not Massless? AKA 'My Whole Life Is A Lie'
Alright kids, it's time to forget the textbook definition of "Neutrinos are chargeless, massless particles..." that earned you those 2 points in your B.Sc. finals. Even though the Standard Model says that neutrinos are massless particles (like the photon), Neutrino Oscillation experiments beg to differ. In a nutshell, neutrino oscillation implies that a neutrino of one flavour can transform into that of another. This means that an electron neutrino can turn into a tau neutrino can turn into a muon neutrino.
This was first observed in the Davis Experiment in the 1960s, which attempted to measure the number neutrinos being emitted by the sun. The nuclear reaction in the sun results in the emission of electron neutrinos, and this makes by far the largest contribution to the neutrinos passing through earth. The experiment saw a deficit of almost two-thirds between the expected number of solar electron neutrinos and the observed number of neutrinos. However, when they accounted for all flavours of neutrinos, not just the electron neutrinos, the observed and expected number of neutrinos agreed with each other. This meant that the neutrinos that started out as electron neutrinos from the sun had oscillated into the other two flavours by the time they reached earth- the three flavours of neutrinos were clearly more-than-just-friends. In theory, this implies that these particles are not massless after all, and it's already Strike-3 for the standard model.
PSA- Some Jargon Ahead: This oscillation takes places when mass and flavour eigenstates mix with each other. The CKM mixing matrix makes an appearance yet again, and the Lagrangian does all kinds of really cool stuff. But to cut to the chase, I hope you will take my word when I say that neutrino oscillation definitely points to a non-zero neutrino mass.
Fun Fact: There are roughly 100 billion (that's 100000000000) solar neutrinos passing through every square centimeter of your body EVERY SECOND. Look at the numbers again, and let it sink in.
Mother Nature Has A Favourite Kid: The Matter/Anti-Matter Asymmetry
The Standard Model likes to play fair. It predicts that in the beginning matter and anti-matter were created in equal amounts; at the very least there is no inherent reason why there should be more of one kind and less of another. But it is evident that present day universe consists mostly of matter, and very little anti-matter. But wait a second: how do we know that there doesn't exist an unobserved corner of the universe that is made up entirely of anti-matter? That's a good question, and it has a surprisingly elegant answer. Matter and anti-matter famously don't get along (hence the names) and annihilate when they come in contact with each other. Dramatic, I know. So if there indeed does exist a part of the universe made entirely out of anti-matter, there would also be a definitive boundary that would separate these regions of matter-universe and anti-matter-universe, and this boundary would see large amounts of radiation being emitted as a result of the annihilation. We can be pretty sure that such magnitudes of radiation would be hard to miss. So while it is plausible- it is still improbable that there is an anti-universe out there we are yet to observe.
This matter/anti-matter asymmetry arises out of what is called Charge-Parity (CP) Violation. CP-Violation essentially means that the laws of physics are not mirrored for matter and anti-matter. The SM does account for a tiny amount of CP-Violation, but nowhere close to the amount of asymmetry we see in the universe. (Side note: CP-Violation was pretty much the topic of my Masters' Thesis so if you're interested in hearing me talk over-animatedly about this for hours feel free to shoot me an email).
The Bulimic Higgs
The last of the short-comings of the Standard Model that I am going to discuss in this blog post is the Hierarchy Problem of the Higgs mass. 4th July 2012 saw the Higgs (or "Higgs-like" particle to be politically correct) at 126 GeV being unveiled to the world by scientists at CERN in comic-sans, for reasons best known to whoever made the presentation. However, this is anything but the end of the Higgs-chapter for the SM. As it turns out, the theoretical and observed mass of the Higgs Boson only agree within the standard model when we allow for extreme fine-tuning of the cancellation between the bare-mass of the Higgs and the quadratic radiative corrections that push its mass to the Planck scale. In jargon-free language this roughly translates as follows: The Higgs boson has a "bare" or basic mass, to which one needs to add corrections that account for its interactions with the other Standard Model particles. The heavier the SM particle, the larger is this correction. The biggest of these corrections comes from the interaction of the Higgs with the Top quark, which weighs in the neighbourhood of 170 GeV.
In principle one can add these quantum corrections to the Higgs mass all the way up to the Planck scale, which is the energy scale at which we know that the SM comprehensively breaks down. This means that the radiative corrections would result in a Higgs with a mass of 1000000000000000000 GeV (that's 1 followed by 18 0s, which looks more dramatic than 10 to the power of 18), unless the "bare" mass and the corrections cancel out to an incredible degree of precision, giving us the observed 126 GeV Higgs. There is no physical reason why such a fine-tuning is impossible, but there is no physical reason why the terms in the Higgs mass should cancel out so wonderfully either. It is an ad hoc assumption and looks unnatural, to say the least. This is the reason to believe that there are perhaps additional, undiscovered symmetries at play that normalize the Higgs mass, without having to resort to fine-tuning the free-parameters in the theory to arbitrary levels of precision.
So there you have it, we have compelling evidence that points us towards physics beyond the Standard Model. The journey is far from over. Our adventures at the terascale have just begun- and the universe is turning out to be stranger, more complex and much more beautiful than we could have previously dared to imagine.