Will we find new physics Wednesday

This is Day 3 of my outreach week challenge : between tantalizing hints and a plain discovery desert, where are we going?

Yesterday I discussed the importance of doing particle physics at the TeV scale, and pointed out that since the Higgs boson, we haven’t found any hint of the much sought-after “Beyond the Standard Model” (BSM) physics. Recently, renowned theoretical physicist John Ellis (this is the guy who succesfully predicted the mass of the bottom quark, did some groundbreaking work in collider phenomenology, and came up with the name “penguin diagrams” as the result of a bet and the smoking of “some illegal substance”) wrote on the topic : Where is particle physics going?.

John Ellis and the Penguin diagram

Today, I propose we read through the paper, commenting it along and trying to answer the question “what would John Ellis do?” (WWJED).

1. Introduction

The gist

The Standard Model of particle physics, which predicts twelve elementary fermions (the building blocks of matter) and four forces mediated by gauge bosons, has so far proven successful in describing results from collider experiments. Below is a summary plot from the ATLAS experiment, which shows excellent agreement between data and predictions, in QCD processes (i.e. involving quarks and gluons).

QCD looking pretty good

2. The Flavour Sector

The gist

At first glance, no big surprise in the “flavour” sector of the SM - “flavour” refers to the type of quark (up, down, charm, strange, top, and bottom), and we’re interested in how these can be transmuted via the exchange of weak gauge bosons ($W^\pm$, $Z^0$). However, the LHCb experiment at CERN, which is best suited for those searches, has noted a number of anomalies in its measurements - no certain evidence of new physics, but numbers that seem to be more or less consistently off by a bit…

Real parts of the C9 and C10 coefficients

Thankfully we have ways of quantifying those deviations, by introducing new operators in our theory, which allow for new processes to take place and modify the SM predictions. By assigning those operators a coefficient, we can see how strongly the data seem to favour this or that operator. Above is a plot of two such coefficients, $C_9^{\text{NP}}$ and $C_{10}^{\text{NP}}$ : although the phase space is not very constrained (the coloured regions are the ones still allowed), you can see that most of the overlaps (from different experiments and measurements) seem to happen not at the $(0,0)$ SM point, but are slightly offset…

WWJED?

Let’s wait for more data! Run 2 of the LHC isn’t even finished yet, and we’re used to excesses and anomalies in the data coming and going - most of the time, they’re just statistical fluctuations. However, would they persist, we’d have to make sure they are indeed signs of BSM physics, and not just a product of our poor understanding of non-perturbative QCD (the way we do computations in our theory breaks down when looking at such low-energy minute details).

3. Higgs Physics

The gist

On July 4th 2012, CERN announced the discovery of the Higgs boson, observed in its decay to four leptons at a mass of about 125 GeV :

Higgs->ZZ*->4l

Since then, no big surprise there either, but a big push for more precise measurements, especially in the $b\bar{b}$ channel.

WWJED?

Unfortunately, although $H\to b\bar{b}$ is the largest decay, it’s also the hardest to observe experimentally! So we have to wait for more data and precise measurements of the Higgs branching ratios and mass, possibly at the per mille level or lower. Since the Higgs is the second most massive particle of the SM, and couples to basically all the others, it is a very good candidate for observing BSM deviations. For that extra bit of fun : what if the Higgs violated lepton universality (e.g. preferred coupling to electrons rather than to muons)?

4. Elementary Higgs Boson, or Composite?

The gist

One of the first things we wanted to verify after discovering the 125 GeV resonance, was that it was indeed the expected Standard Model Higgs. A possible candidate needs to have a number of properties : it must be a boson, a scalar, be neutral, decay in a certain way, etc. On top of that, it is imperative that it be an elementary particle. If it were shown to be composite, that would be a major breakthrough and a sign that things are quite as they seem in the Standard Model.

As in the case of flavour anomalies, it is possible to rewrite the Lagrangian (the main piece of maths that describes the theory) of the Higgs sector to include additional contributions, expected if the Higgs is actually made up of smaller, elementary components. And as before, we set coefficients to these alternative contributions and go out to measure those. As you can see below, the couplings to gauge bosons and fermions is pretty much in agreement with the Standard Model point $(1,1)$ - the 125 GeV resonance discovered in 2012 looks indeed very much like a SM Higgs.

Higgs couplings to vector bosons and fermions

WWJED?

John Ellis spends some time early on in the article to detail this possibility of compositeness : it’s very much to show that the way forward is supersymmetry! (John is a big fan)

In fact, if the Higgs is an elementary scalar, we know from the way the theory works that its mass must receive “large contributions” from the rest of the Standard Model particles : essentially, each particle that interacts with the Higgs “pulls” its mass up a bit (and for some, quite a lot!). How then to reconcile the observed, relatively low mass of the Higgs with the potentially huge, predicted one? This conundrum goes by the name of the Hierarchy problem, and supersymmetry solves it by adding a whole new range of particles that “pull” the Higgs mass in the opposite direction, thereby averaging it out at an acceptable value. More on that on Sunday

5. Stability of the Electroweak Vacuum

The gist

This idea of the Higgs mass being “pulled” by the particles it couples too also applies to the electroweak vacuum, the value the field takes when it’s not doing anything. Again, the “top” culprit here is the top quark, because of its large mass. It is in fact so large, that for a Higgs of mass 125 GeV, we expect that at some high level of energy (equivalently, at an early time in the universe) the electroweak vacuum could be completely distorted and lead to the decay of basically everything - the end of the universe.

The problem is that we’re apparently in a region of meta-stability, as depicted below. Although the Higgs is happily sitting in the minimum of his potential, a sudden, violent perturbation (or simply quantum tunneling, red arrow) could throw it over the top, rolling to a more exotic vacuum or straight down an infinite hill as the universe dissipates around us…

WWJED?

One can think about the problem in these terms : if such a disaster could happen at high energies, and the universe was once at such high energies, why hasn’t it happened? A possible solution is to say that it did in fact happen at most places in the universe, but a few lucky ones were spared. These patches then underwent so-called cosmological inflation, which blew them up to epic proportions, essentially making the visible universe we observe today. But that seems like appealing to either naturalness or the anthropic principle : those spared parts would have to be suspiciously lucky, or we only know this sort of physics because we’re here today to do it.

No, John Ellis would rather take this as evidence that some new phenomena must take over at these energies, to literally “save the day”. The theory even gives us an estimate of the scale at which they should appear : about $10^9$ GeV! That’s perfectly in line with what people have tried so far - including, you guessed it, supersymmetry…

6. The SM Effective Theory

The gist

Effective theory essentially means “approximation of some deeper underlying theory” : that’s a fair assumption to make about the Standard Model. We know perfectly well that it’s not the end of the story, but at the same time it works too well at low energies to be just a lucky guess. So why not start from what we know, and simply add more operators, i.e. more types of interactions? Again, this is what we’ve seen before, with that whole coefficient and operators business. Crucially, the idea of the “SMEFT” is not to add new fields (particles), but rather to allow us to do more things with what we already have.

SMEFT

As you can see, data pretty much still agrees with the Standard Model point at $(0,0)$.

WWJED?

Although the SMEFT is not in itself a “deeper underlying theory”, if any of its coefficients is ever found to not be zero, we will know immediately what that new physics look like, and start building a coherent model. That’s reason enough to keep trying!

7. The Standard Model is not Enough

Bad James Bond pun, drum roll…

8. Supersymmetry

There we go.

I will come back to this on Sunday, so don’t want to say too much now. The essential information is the following : supersymmetry is the best-looking candidate BSM theory we have, but we still haven’t found even a hint of the predicted “sparticles”. John Ellis would argue that we still haven’t looked everywhere yet, and that the fact that SUSY is not one model but a whole family of them makes things harder for experimental physicists. Some recent models are also slightly outside of the range of the LHC, which perhaps is motivation enough for building larger, more powerful accelerators.

9. Direct Dark Matter Searches

The gist

There are three different ways of (possibly) observing Dark Matter, that elusive substance that makes up about 25% of the energy content of the universe, outlined by the diagram below :

Dark Matter interaction diagram

The two most promising directions are production at colliders (smashing two SM protons at the LHC with fingers crossed) and direct detection (looking for signs of Dark Matter colliding with, and exciting or otherwise perturbing SM particles). The latter is achieved, usually, by setting up a big tank of your favourite liquid underground, so as to be shielded from cosmic particles and those pesky neutrinos, and patiently waiting for a potential Dark Matter particle to pass by, scatter off your tank, leaving behind some electric or luminous signature, for instance. Many such experiments exist, and more are planned, which brings us to John’s next point.

WWJED? 10. A Plea for Patience

The estimated cross-sections (that is, the likelihood of observing an interaction between particles) of Dark Matter processes are ridiculously low. Essentially, that means all you can do is wait, or build bigger tanks and colliders, then wait some more. That’s not say there’s nothing to be found - after all, the LHC is still young, and the Higgs boson was discovered 48 years after being first theorized (for gravitational waves, that number goes up to a hundred years!). Wait and see…

11 & 12. Future Colliders

As I’ve mentioned now several times, to see something as rare and energetic as Dark Matter or supersymmetry, you need experiments that can either record more data or perform at higher energies - or a careful mix of both. For John Ellis, the answer is clear cut : let’s build a very large LHC (possibly 100 km in its circumference!) that will operate at 100 TeV. More than an incredible engineering feat, that would be the particle physicist’s heaven : precision measurements of all SM processes, possibilities to directly observe new physics up to about 50 TeV, and indirectly to much higher up! The point of these two sections of the article is essentially that, not having any hints of BSM physics at the moment, there’s no point in building a precision linear collider to study those (non-existent) hints further; as John concludes, “the answer to the question in the title may well be round in circles”.

I’d like to end this post with a final remark : there are obvious limits to building ever bigger colliders - money, space, technology, environmental costs, etc. To me, this should be seen not as the end of high energy particle physics, but rather as a motivation for young experimentalists to come up with fresh ideas, new ways of building experiments. Hopefully, by the time the LHC reaches the end of its service life we’ll have come up with some new, bright ideas and been able to finally say something about what lies beyond the Standard Model.

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