'Beyond the standard Model' Physics

The name beyond the standard model physics (or BSM physics for short) is pretty self-explanatory, but in case of any doubt, it really means just that: any theoretical framework or model which exhibits features which the Standard Model can't describe.  We generally lump any and all such theories into the category of BSM theories.  It's just a name.

As mentioned in the very first section, the Standard Model itself is an incredibly successful theory.  Laid out in its full glory, it's also an elegant and beautiful theory.

Unfortunately though it can't be the full story.  The Standard Model has several shortcomings – things it doesn't explain but that we've observed to be the case.  I'll give a few of the notable exceptions throughout this section.   But crucially remember that it doesn't get anything plain wrong. In fact its rather the opposite that's so striking – the precision with which it gets things right is truly incredible.  

That said, again, we know there's got to be something else out there which does an even better job.

Now in terms of proposed theoretical models which might be able to address these shortcomings, there are a great number, with some more beautiful and some more zany than others.  

Also as mentioned before, ultimately of course those models can't disagree with the parts that the Standard Model gets right.  But in addition, a successful theory need to be able to describe all that it gets right plus all that it fails to describe.  Or at least more of it.

That's why BSM models are often extensions of the Standard Model.  Or, said another way, the Standard Model could just be a low-energy approximation of one of the proposed models which describes the full (or, again, a fuller) picture.  As already mentioned before, this is analogous to Newtonian gravity being an approximation of Einstein's general theory of relativity.  They'll both give you the same answer under a certain set of circumstances, but will differ wildly under others (in which case you'd be wise to place your bet on Einstein's prediction over Newton's).


What will 'something new' look like at the LHC?

The answer to this question is that something new could manifest itself in many different ways.  There are physicists looking in various different ways for hints of something new.  But the short answer is, no one knows.  We have ideas of course, but when something new comes along, it could show up in a way that shakes our theory to its core.  That would certainly be nice.  

To illustrate one way that something new could manifest itself, let's pretend there's some even heavier version of the already quite massive Z boson.  We'll call it the Z-prime (or Z').

A hypothetical theoretically proposed Z prime boson.

The Standard Model doesn't know about it, but the new theory claims it exists just like the rest of the more familiar particles we’ve observed to date.  Let's also pretend, for argument's sake, that some theorist has laid out the ground rules for how the Z prime boson interacts with the regular Standard Model particles.  This means we add some new vertices to our list.  

Here you can see a few representative examples:

A few representative decay modes of a theoretically proposed Z prime boson.

So how do we go about testing this theory?  How do we know if something like the Z-prime boson exists?  

Based on the theory, we can run some simulations which take everything into account – the underlying theory, the production and decay of the Z primes as well as all of the less interesting stuff, and ultimately all interactions with the simulated detector – and we can then see where we might have the best chance to spot them. 

These ways of spotting them can be split into two general groups: so-called direct and indirect observations.  Again, an illustration to summarize the basic idea:

Examples highlighting the difference between a direct and an indirect observation of a new particle.

An example of a direct observation (cartoon on the left) would be looking for so-called 'resonances'.  We add up the energy of candidate objects we think might have originated from the hypothetical Z-prime boson's decay and look to see for a 'bump' in the distribution of the overall mass which, roughly speaking, should be close to that of that new particle.  In other words, we can do searches motivated by simulation and ultimately see what the data look like when the dust settles.  Is there some excess above the prediction made by the Standard Model?  There are other examples, but that's the easiest one to visualize.  Of course one crucial question you should probably be asking yourself is if we have enough energy in the proton-proton collisions from the LHC to produce a real one of these guys.  If it's a virtual particle, it can take on whatever mass it needs to, but if our collision takes place above a certain production threshold (in terms of energy), then we're in business!  And what is this not-so-secret energy threshold?  The mass of the particle times the speed of light squared!

An indirect observation (cartoon on the right) can come in many forms, so I've just drawn one example.  The basic idea though (or at least my take on it anyway), is that there are underlying diagrams as a result of the existence of the new particle, and even if we don't ever reconstruct the particle directly, its very existence alters the way the final particles in certain processes appear (where their decay particles appear in the detector on average for example, or just how many particles we have overall).  So a new particle's existence can allow for additional production diagrams for some of the heavier Standard Model particles we already know about (such as Higgs or W/Z bosons) which in turn might affect their rates of production or the directions in which they shoot out from the interaction point at the centre of the detector.  An indirect observation won't be staring us in the face as much as the direct observation, but we can nonetheless test a hypothesis against a measurement.  Sometimes an indirect measurement is a more feasible way to go.  So we might just look for ways in which this discrepancy might be easiest to spot.  If we can show that a given theory explains the exact discrepancy observed (while still describing everything else as before!), then it may be on to something!  That parenthetical remark is important: once we add some new rules to the overall theory, this new particle – whatever it is – can't make predictions that disagree with what's already been observed!  This means the constraints on new theories have some constraints (that's a good thing!).

To summarize, we assume there's some heavier, as-of-yet unobserved particle, and we set about looking to see if it's there.  Will it be quite clear when we've see it?  It basically comes down to statistics – have we suppressed the backgrounds sufficiently such that we'd be able to observe any substantial deviation from the Standard Model expectation?  If yes, we've got a good shot (assuming it's really there).  We can look for it directly (try to reconstruct an object associated with the particle itself) or indirectly (by, for example, looking for discrepancies in some other distributions compared to the Standard-Model-only scenario).

Sometimes we can do both types of searches – direct and indirect – at the same time.  Come at it from several angles, so to speak.

Dark Matter and Dark Energy

If you're heard of dark matter or dark energy, you might know that they're two things we've assigned names to, but which we know very little about!  We do know (based on several measurements) that the universe we're able to observe directly behaves very differently from the way it should if normal matter were the full story.  Part of what the Planck Experiment observed was that normal, everyday matter, if you were to add it all up, accounts for just under 5% of the total matter-energy of the universe.  That's a tiny fraction!

The remaining ~95% we categorize as dark matter and dark energy and assign different fractions to each of the two (based on the results of the same experiment).  Both are dark in the sense that they don't interact directly or indirectly with photons (particles of light), or at least, that if they do it's on such a small level that we can't tell anyway for most purposes.  In other words, dark matter particles definitely represent something that's non-Standard-Model.  Dark matter is matter (as the name suggests) – it has mass – and its mass is spread out or distributed in such a way that we can get a clear picture of where it's concentrated, or should be concentrated.  We just don't know what the particles are, or what their properties are!  Dark energy is more difficult to visualize, but it affects the overall rate at which the universe is seen to be expanding.

Breakdown of the universe into normal matter, dark matter, and dark energy (as measured by the Planck experiment).

If we focus on dark matter for a second, it could be that dark matter particles, whatever they are, do interact with Standard Model particles, even if extremely weakly.  Maybe they interact more at higher energies, which would make the LHC a suitable tool to look for them.  If their interactions with normal matter only happens very rarely, that's ok, we can run our machine for a long time and just wait to see what comes out.  After all, even though processes might be rare, we generate incredibly huge numbers of proton-proton interactions when measured on our timescales of seconds, hours, or years.  Eventually by analysing the LHC data we might see something that serves as conclusive evidence for dark matter.

It should also be pointed out that there are many ongoing dark matter searches that don't involve colliders such as the LHC.  We might see it first, or they might see it first.  Or, if the universe is cruel, maybe no one will be able to prove or disprove its existence.

Of course if we finally do manage to produce dark matter particles at the LHC, they probably won't interact with our detector.  So again, the topic of missing transverse energy that we talked about in Part VII is very relevant here!


Supersymmetry (SUSY)

One possible candidate for dark matter is the lightest so-called stable supersymmetric particle.  Supersymmetry (or SUSY for short) is an extension of the Standard Model which, at a very basic level, adds a whole set of new particles that obey a given set of rules, much the way the original particles obey the rules of the Standard Model.

The dark matter candidate is a spin-off – it's not the underlying reason for the theory, but it just turns out we might kill two birds with one stone.

To get a basic picture of SUSY, remember the idea of matter and anti-matter and how that led to the inclusion of anti-matter counterparts to several of the Standard Model particles:

Examples of matter (top quark) and anti-matter (anti-top quark) particles.

Along the same lines, SUSY presents us with yet another set of particles for our fundamental particle menu.  We call them supersymmetric particles or SUSY particles.  And you can see below that we're getting less creative in our naming schemes.  

Here are just a few to give the basic idea:

A depiction of a few supersymmetry (SUSY) particles together with the Standard Model counterparts.

At this point of course I'm saying nothing about the actual theory, and simply flashing cartoons of some of the additional particles introduced by the theory.  But the basic idea is that we've got even more particles, and (if the theory holds), the new guys should be producible at the LHC.  There are variations on the basic SUSY model and many ways in which it can be tweaked or adjusted, but all of these variations are said to fall under the same general SUSY umbrella.

Tying back to the earlier idea, we might be able to produce SUSY particles directly:

A Feynman diagram showing a possible gluino pair production mode at the LHC. Gluinos (supersymmetric counterparts to the massless gluons of the Standard Model) have not been observed experimentally.

Or we might first try to look for indirect evidence.

Some extensions of the Standard Model also predict the existence of additional Higgs bosons – the one we discovered in 2012 would then just be the first of many siblings (five in the case of a standard SUSY model).  Some of these new Higgs bosons, according to the proposed theory, would be electrically charged, whereas others (including the one we've already discovered) would be electrically neutral.  This, as well as their actual masses (which we don't know) would have consequences in terms of their potential decay products.  

Additional Higgs bosons as predicted by Supersymmetry.

Just as an example, if we were to suppose that the additional four Higgs-like bosons above exist, we might be able to look for one of them according to the following signal process:

The decay of two Standard Model Higgs bosons which were produced from the decay of a 2nd type of heavier Higgs boson as predicted, for example, by some supersymmetric (SUSY) extensions of the Standard Model.

Would we be able to see that with ATLAS?  If you've read the rest of these sections, you might be able to appreciate that the answer is a resounding... maybe.  It depends!  Among other things, it depends on the backgrounds, it depends on the mass of the heavier Higgs cousin, and it depends on how that heavier Higgs 'couples' or interacts to the Standard Model Higgs boson (the vertex 'strength').  

Supposing we were to go looking for the above signal process.  Well, one obvious background process we'd be faced with in such cases is simply the production of two nothing-special Standard Model Higgses.  So nothing new, just the plain old vanilla Standard Model.  We haven't yet observed even this process at the LHC since it's statistics limited (we were only able to produce Higgses one at a time in 2012).  We should see double Higgs production at the LHC, but we just can't yet, though we know it should certainly happen.  And we'd better understand that well if we're to hope to see something non Standard Model-like on top of it.

At any rate, here's at least one way this can come about with just the Standard Model alone (first one that came to mind):

One possible production and decay mode of two Standard Model Higgs bosons at the LHC.

A bit of a mess, but just to emphasize that it's certainly possible.  

There are many additional diagrams I could have chosen to draw (an infinite number in fact) instead of this one.

They key point is that those extra Higgs bosons (as predicted by the new theory) could exist, but if we want to see them, we have to show that there's a significant excess above everything the Standard Model itself would predict!


The four fundamental forces we know about are certainly pretty different from one another.  Or are they?  Certainly we know a lot about them all – even gravity, despite its not being included in the framework of the Standard Model.  If you remember, way back we mentioned that the forces work via force carriers or mediators.  In the case of the electromagnetic force, the force mediator is the photon.  When we treat the photon as a wave rather than a particle, we give it names like X-raygamma ray, visible light, infrared radiation, and so on.  But they're all just different names for electromagnetic radiation in some form, and the corresponding particle is always the photon.

Even though they're less commonly known compared with the photon, there are mediators for the other forces as well.  The gluons mediate the strong nuclear force.  The W/Z bosons mediate the weak nuclear force.  We even have a name for the mediator for the gravitational force – the graviton – although we haven't seen any evidence of its existence!  (Gravitational waves fit in to the 'classical'  theory of gravity as formulated by Einstein, so they're a different concept from the graviton).

Some forces allow particles to interact with or 'speak to' one another regardless of their physical separation!  Gravity and electromagnetism are good examples.  We say that they have an infinite range. In contrast, two quarks separated by a distance larger than roughly that of a proton simply cannot interact with any considerable amount via the strong nuclear force.  And the range of the weak nuclear force is roughly on the order of ten to a hundred times smaller that that.

The four known fundamental forces of nature together with some of their properties.

Part of the reason for the differences in scales is the difference in masses of the force mediators – you can see that the W/Z bosons are in fact quite massive, and it's their large mass which limits the range of the weak nuclear force.  The electromagnetic and the strong nuclear forces differ (in part) for the reason that photons don't directly 'speak' to one another in the Standard Model, whereas gluons do.

Two examples of so-called triple-gauge vertices. The first, involving photons, is not allowed according to the Standard Model, whereas the triple-gluon vertex is. In other words, gluons can interact with one another based on a single vertex. Photons can still interact with one another, but it requires a more complicated (so-called higher-order) diagram.

So what?

Well, actually it turns out the forces might not be as different as we think they are.  In fact they can behave differently at different energy scales.  The electromagnetic and weak nuclear force are said to be unified once you get to a high enough energy level – the so-called electroweak unification scale.  We then refer to them collectively as one force: the electroweak interaction.

The natural question following that is if, at even higher energies, the other forces unify in a similar way?

In other words, something along these lines:

The (possible) unification of the four known forces of nature at higher energies.

Moving from left to right in the above diagram, we spoke of going to higher and higher energies, but this is equivalent to speaking of higher and higher temperatures (temperature after all is a measure of the average kinetic energy of particles).  And the further back we go in time, the closer we approach the conditions of the big bang which featured unimaginably enormous amounts of energy in an incredibly small volume of space.  You might hear people speak of the LHC as a time machine, just in the sense that it is creating conditions more similar to those in the short moments after the big bang.  Just 'how short' exactly you're talking about depends on the actual energy.

One of the (many) reasons why SUSY, for instance, is so appealing is that although the basic Standard Model predicts the forces to have similar strengths at higher energies, one form of SUSY leads to a unification at a fixed energy.  It might turn out that's simply not how the universe behaves (beautiful theory or not), but it's certainly tempting for us as humans to want to believe it, given that we're naturally hard-wired to enjoy simplicity and symmetry.

One then speaks of Grand Unification Theories (GUT) in the case of the unification of three of the forces (all save the force of gravity) and a so-called Theory of Everything if all four were to unite.

Unification of the four fundamental forces of nature in the case of the Standard Model (left) and together with the incorporation of one form of SuperSymmetry (right).

Since the four forces are entirely different from one another, naively it's not obvious how to get to an apples-to-apples type comparison.  It comes down to those little diagrams we've drawn and the 'strength' of each vertex, depending on which force it's associated with.  The upshot is that gravity is by far the weakest, and the (appropriately named) strong force is the strongest.

"Wait, how can gravity be so weak?", you might be thinking.  After all, it's the force that keeps you and I on the surface of this blue planet of ours, it keeps the structure of the solar system more or less in place, and it's even responsible for creating supermassive black holes!  All of these things are true, and make no mistake: gravity can do some pretty crazy stuff, but only on enormous cosmic scales where the other forces are irrelevant or can't contribute.  It simply is, by comparison, the weakest of the forces.

The 'effective strength' of each of the fundamental forces changes with energy: as the energy of whatever interaction you're interested in increases, so too does the effective strength of the electromagnetic force, whereas the weak and strong nuclear forces decrease in effective strength.

Some theorists argue that gravity is a short-range force because of mini extra dimensions wrapped up on scales so small we don't have a way to perceive them in our normal 4-dimensional lives.

This brings us to another thing we're looking for at the LHC: the possible existence of extra dimensions – dimensions in addition to our 3 space-like dimensions and 1 time-like dimension which we group together and call space-time.

Are there additional space- or time-like dimensions in our universe?

New Fundamental Forces?

Even more bizarre (and exciting) would be the discovery of some entirely new fundamental force.  We can only speculate on what kind of features such a force would have (and I won't even do that), but nothing's ruled out – if there's some new force (or forces) out there, we might just catch glimpses of it at the LHC, though recognizably it might take us some years to figure out what exactly it is we're seeing!

Are there additional forces of nature in the universe that we've just not observed yet?

How fundamental is fundamental?

As one last side note here, you may at some point in all this have been wondering: wait, how do we know that electrons and quarks are, in actual fact, fundamental?  Couldn't they too be made of even smaller pieces and we just don't know it yet?  After all, the word atom comes from ancient Greek for not able to be cut up or divided any further.  Not only has that concept been refuted, but the pieces within the atom themselves can be subdivided (crucially the atom is split into electrons and a nucleus, the nucleus into protons and neutrons, and those are finally split into quarks and gluons).

Couldn't it be taken one more step?  Or (perhaps more frighteningly) an infinite number of steps?

Well, the answer is that at this point we believe that the particles we've listed in our tables (including electrons and quarks) truly are fundamental: they are mathematical points in space with no volume but a finite mass. 

We've not seen evidence to the contrary (though many people have looked for the telltale signatures analogous to those Rutherford and his graduate students observed by scattering alpha particles off of gold foil to learn about the structure of the atom!).

However, it could be that our 'fundamental particles' really aren't fundamental after all but we just don't know it yet.  If they are made up of other, more fundamental pieces, we should eventually be able to see hints of it, but not at the energy of our accelerators today.  Even stranger: there's a lower observable limit on their dimensions such that if we look at smaller and smaller scales, eventually we reach a limit called the Planck length which is somewhat (ok, very crudely) analogous to the upper limit on the speed of light.  It's a number so small that it is simply unmeasurable.  Now, I'm not saying they are that small, just saying they could be.  And if it turns out to be the case that they do have internal substructure but that it's below that value, than does it even matter if we can never see it?  Tree in a forest sort of thing. 

In other words, we leave it for the philosophers.