RPP 2020

Yesterday I finished the teaching in my SUSY course for this academic year. I talked (among many other things) about going beyond the MSSM (the Minimal Supersymmetric Standard Model) and modern perspectives on the future of SUSY phenomenology. To add a little from the post a couple of weeks ago, I presented three approaches that have been embraced by the community for a few years now:

1. Carry on looking for the MSSM. As I said before, the LHC has done a good job of limiting the superpartners that are coupled to the strong force, but in reality a rather poor job for electroweak-charged states. There is also a good argument that the Higgs mass alone suggests we may never see the coloured particles without a new collider anyway, but this is not watertight.
2. Look at non-minimal supersymmetric models. This is the approach I have favoured in my own work (in particular Dirac gaugino models).
3. Abandon a complete SUSY theory at low energies, and look instead at high-scale SUSY or split SUSY. In particular, the latter allows you to keep gauge coupling unification and a natural dark matter candidate. On the other hand, it seems hard to find in string theory, because of the need for an approximate R-symmetry.

In my earlier post, I stated that I would not recommend that new students exclusively study SUSY, and indeed I do not propose SUSY phenomenology as the main focus of my new students. This is at least partly a sociological statement: they would struggle to find a career in the current climate, and I strongly believe that it is important to know at least something about SUSY. But it is even more vital to learn about all of the problems of the Standard Model and the many potential solutions, and look for the most promising ways to make progress based on current and future experiments in an open-minded way.

Rencontres de Physique des Particules 2020

I'm currently at the first day of the annual French particle theory meeting. There will be some nice political discussions alongside interesting talks that represent a little of the field. Notably, in physics Beyond the Standard Model there have been talks today about indirect dark matter searches, axions and the tension in the Hubble constant, by people recruited in recent years to the CNRS; there will be more talks tomorrow and Friday by recent recruits and people hoping to be recruited (this meeting often serving as a shop window).

France has a unique way of funding research, in that the CNRS hires people to work solely on research, and supports them in "mixed labs" where there are also university professors and "maîtres de conferences" who are the equivalent of assistant or associate professors elsewhere. Unlike their university counterparts, CNRS researchers do not have to teach, and have a huge amount of liberty. For this last reason I absolutely love my job. The French system also believes in recruiting people relatively early in their careers but requires good judgement in finding the stars of tomorrow rather than people who are already established. So at this meeting you could say there is sampling of what the CNRS committee may believe (or what some people hope they believe) is the future of the field ...

Another feature of the French system is, perhaps paradoxically in the land of "liberté, egalité, fraternité," that it is ultra-elitist. The "grandes écoles" (in particular Ecole Normale and Ecole Polytechnique) are incredibly selective institutions for students, but most people outside of France have not heard of them because they barely register on the Shanghai rankings -- but only because they are small (they punch incredibly hard for their weight, and top tables of "small universities). One interesting thing we heard today, from the Vice Provost for Research at Ecole Polytechnique, was that the government aims to make Ecole Polytechnique into a French version of MIT, which would mean doubling the number of students -- but multiplying the budget by a factor of ten (this is probably a slightly unfair calculation, as it would not include the salaries of CNRS researchers, for example). Apparently the way they are trying to achieve this is to "work out how to get money out of" large multinationals, essentially using the students' skills as a "goldmine." Sadly these companies are not at all interested in basic research, and so funding for future fundamental science would have to be somehow siphoned off from that obtained to do machine learning etc.

Ah, I've just veered into cynicism, which I want to avoid on this blog, so I better go and take part in the "table ronde" discussion and save real politics for a different post ...

The KOTO anomaly

Patrick Meade pointed out some new papers about an experimental anomaly, starting with his own. The KOTO experiment at J-PARC in Japan (where they are also building a \( g-2 \) experiment) has seen 3 events when looking for the rare process \( K_L \rightarrow \pi_0 + \mathrm{invisible} \), when they expect a background of \( 0.05 \pm 0.02 \) Update: it was pointed out to me that the effective background rate is \( 0.1 \pm 0.02 \) as in Meade's paper, because the Standard Model rate is \( 0.049 \pm 0.01 \). For more details see the slides of the talk where the results are reported; there is currently no paper about the excess. This is interesting as the Standard Model process \( K_L \rightarrow \pi \overline{\nu} \nu \) has a tiny branching ratio, two orders of magnitude too small to explain the number of events.

Assuming the anomaly is just statistics, the probability of observing three or more events would be of the order of one chance in \( 10,000 \) if we take the more generous estimate of the background. On the other hand, it is apparently only roughly two-sigma evidence for an anomalous \( K_L \rightarrow \pi_0 + \mathrm{invisible} \) signal. Moreover, the central value of the required signal is just above (but well within errors of) the Grossman-Nir bound, which says that if something generates \( K_L \rightarrow \pi \overline{\nu} \nu \), it should also generate \( K^+ \rightarrow \pi^+ \overline{\nu} \nu\) in the ratio $$ \frac{\mathrm{Br} (K_L \rightarrow \pi_0 \overline{\nu} \nu)}{\mathrm{Br}(K^+ \rightarrow \pi^+ \overline{\nu} \nu)} = \sin^2 \theta_c$$ where \( \theta_c \) is the Cabbibo angle, provided that the interactions respect isospin. Since the charged process is not observed, the observed anomaly might be in slight tension with this bound.

So far I can find three papers seeking to explain this anomaly, through light scalar extensions of the Standard Model (with masses less than 180 MeV) and the inevitable two-Higgs doublet model. Since such scalars must couple to quarks/mesons they look a bit like axion-like particles and there are many astrophysical and beam-dump experiments that exclude large swathes of the potential parameter space, but this is quite exciting as, if the anomaly is confirmed, it should also be possible to easily look for it in (many) other experiments.

How to make progress in High Energy Physics

Before I start, just following from my previous post about B-mesons, today I saw a CERN press release about lepton universality in B-baryons (i.e. particles made of three quarks, at least one of which is a bottom, rather than B-mesons, which have two quarks, at least one of which is a bottom). It seems there is a \( 1 \sigma \) deviation in $$ R_{pK}^{-1} \equiv \frac{\mathrm{BR} (\Lambda_b^0 \rightarrow p K^- e^+ e^-)}{\mathrm{BR} (\Lambda_b^0 \rightarrow p K^- J/\psi(\rightarrow e^+ e^-))} \times \frac{\mathrm{BR} (\Lambda_b^0 \rightarrow p K^- J/\psi(\rightarrow \mu^+ \mu^-))}{\mathrm{BR} (\Lambda_b^0 \rightarrow p K^- \mu^+ \mu^-)} $$ While, by itself, it is amazing that this gets a press release heralding a "crack in the Standard Model", it does add some small evidence to the picture of deviations from Standard Model predictions; no doubt the interpretation in terms of a global fit with other observables will appear soon on the arXiv. So it's a positive way to start this entry.

Polemics on foundations of HEP

Recently an article appeared by S. Hossenfelder that again makes the claim that "fundamental physics" is stuck, has failed etc, that theorists are pursuing dead-end theories and "do not think about which hypotheses are promising" and "theoretical physicists have developed a habit of putting forward entirely baseless speculations." It is fairly common and depressing to see this message echoed in a public space. However what riled me enough to write was the article on Not Even Wrong discussing it, in which surprise is expressed that people at elite institutions would still teach courses in Beyond the Standard Model Physics and Supersymmetry. This feels to me like an inadvertent personal attack, since I happen to teach courses on BSM physics and SUSY at an elite French institution (Ecole Polytechnique) ... hence this post.

Physicists are not sheep

Firstly though I'd like to address the idea that physicists are not aware of the state of their field. While "maverick outsiders" might like to believe that HEP theorists live in a bubble, just following what they are told to work on, it upsets me that this message has cut through enough that a lot of students now feel that it is true, that if they do not work on what they perceive to be "hot topics" then they will be censured. The truth is that now, more than perhaps at any time since I entered the field, there is a lack of really "hot topics." In previous decades, there were often papers that would appear with a new idea that would be immediately jumped on by tens or hundreds of people, leading to a large number of more-or-less fast follow-up papers (someone once categorised string theorists as monkeys running from tree to tree eating only the low-hanging fruit). This seems to me to be much less prevalent now. People are really stepping back and thinking about what they do. They are aware that there is a lack of clear evidence for what form new physics should take, and that the previously very popular idea of naturalness is probably not a reliable guide. Some people are embracing new directions in dark matter searches, others are trying to re-interpret experimental data in terms of an effective field theory extension of the Standard Model, others are seeing what we can learn from gravitational waves, still others are looking at developing new searches for axions, some people are looking instead at fundamental Quantum Field Theory problems, cosmology has made huge progress, etc etc (see also this thread by Dan Green). There is a huge diversity of ideas in the field and that is actually very healthy. There is also a very healthy amount of skepticism.

On the other hand, as I mentioned in my previous post, there are some tentative pieces of evidence pointing to new physics at accessible scales; and whatever explains dark matter, it should be possible to probe it with some form of experiment or observation. This is the reason for continued optimism in the field of real breakthroughs. We could be on the verge of overturning the status quo, via the (apparently outdated) method of doing experiments, and then we will race to understand the results and interpret them in terms of our favourite theories -- or maybe genuinely new ones. Of course, maybe these are mirages; as physicists we will continue to look for new and creative ways to search for new phenomena, even if we do not have a new high energy collider -- but if we don't build a new collider we will never know what we might find.

What courses should you take

Coming now to the idea of what students entering the field should learn, in the current negative climate it needs repeating that the Standard Model is incomplete. I'm not just talking about a lack of quantum gravity, but there is a laundry list of problems that I repeat to my students at the beginning of the course:

1. Quantum gravity.
2. Dark matter, or something that explains rotation curves, the CMB, etc.
3. Dark energy -- no, it hasn't been ruled out by one paper on supernovae. It was awarded the Nobel Prize because people already expected it from other observations.
4. Inflation, or something else that solves the same problems.
5. The strong CP problem. We have phases in the quark Yukawas, so we should have a neutron electric dipole moment \(10^{10} \) times greater than we observe. Most people believe this should be solved by an axion -- which might also be dark matter -- hence a lot of effort to find it, and ADMX (among other experiments) might be getting close.
6. Baryogenesis. The Standard Model Higgs is too heavy to have electroweak baryogenesis. There is apparently not enough CP violation in the Standard Model either.
7. Neutrino masses. We can't write them into the Standard Model because we don't even know if neutrinos are Majorana or Dirac! Maybe a heavy right-handed neutrino can give us Baryogenesis through leptogenesis. There is a huge amount going on in neutrino physics at the moment, too ...

Nearly all of these topics are not generally covered in a standard set of graduate courses (at least here). I try to present the evidence and some possible solutions. So the first time a lot of students encounter these issues is through popular press articles, and oblique references in "standard" courses. And if we are going to make progress on solving some of these fundamental issues, should students not have some idea on what attempts have been made to solve them?

Turning now to supersymmetry, I would not recommend that a beginning student in particle phenomenology make it the sole focus of their work (unless they really have a good motivation to do so). But there are many reasons to study it still:

1. It is hugely important in formal applications -- to give us a handle on strongly coupled theories, allowing us to compute things we could never do in non-SUSY theories, as toy models, \( N=4 \) SYM being the "simplest field theory" (as Arkani-Hamed likes to reiterate) etc etc.
2. It seems to be necessary for the consistency of string theory. I personally prefer string theory as a candidate framework for quantum gravity; if you want to study it, you need to study SUSY.
3. A lot of the difficulty with the formalism for beginning students is just understanding two-component spinors -- these are actually very useful tools if you want to study amplitudes in general.
4. It allows us to actually address the Hierarchy problem, and related to this, the idea of the vacuum energy of the theory being related to a cosmological constant. This is a subtle (and maybe heated) discussion for another time.
5. The gauge couplings apparently unify in the simplest SUSY extensions of the Standard Model. If this is just a coincidence then I feel that nature is playing a cruel joke on us.
6. The Standard Model appears to be at best metastable (there is some dispute about this). It has been further suggested (e.g. here) that black holes might seed the vacuum decays, so that if it is not absolutely stable then it should decay much quicker than we would otherwise think; and in any case the standard calculation has to assume that there are no quantum gravity contributions (giving higher-order operators). New physics at an intermediate scale (below \( \sim 10^{11} \) GeV) such as supersymmetry would then be necessary to stabilise the vacuum.
7. It genuinely could still be found at accessible energies; the LHC is actually very poor at finding particles that don't couple to the strong force, and new electroweak states could easily be lurking in plain sight ...
8. ... related to this, it's just about the only "phenomenological framework" for new physics that addresses lots of different problems with the Standard Model.

Of course, nowadays as a community we are trying to hedge our bets: there is much more ambivalence about what theories might be found just around the corner, hence my own work on generic pheneomenology, and a lot of interest in the Standard Model EFT.

How we should make progress

Finally we get to the topic of the post. In the original article that I linked to above, Hossenfelder does make (as she has made elsewhere) the positive suggestion that physicists should talk to philosophers. [ In France, this is amusing, because there is a fantastic tradition of famous philosophers, and every schoolchild has to study philosophy up to the age of 18 ] It is good to make suggestions. In the article though is the idea that people cannot recognise promising new ideas amidst a sea of "bad" ones, so people are either following old dead ends or endlessly making ridiculous suggestions. I admit that, superficially, this is the impression people could have got once upon a time, but I would argue is not the state of the field now. I disagree that the problem is a fundamental one about how people think, or that there is a system censuring of "good" radical ideas. I don't think there is only one way to make progress: if I had a suggestion how the scientific creative process should work I would be applying it like crazy before advertising the benefits publically! And of censureship of "good" ideas, there are a lot of people willing to take a risk on new concepts. Research is hard, and creativity is not something that is easily taught. But I am constantly amazed by the creativity and ingenuity of my peers, the diversity of their ideas, and it is heartbreaking to see their effort denigrated in popular articles.

Indeed, repeatedly making the claim in public that one group of scientists are dishonest (or kidding themselves) about progress, that the field has failed etc, helps no-one. It deeply worries me to read that Dominic Cummings has Not Even Wrong on his blog roll; and I have already seen that people in other fields often hold very wrong opinions about the state of fundamental physics due to this filtering through (most people only see wildly speculative and hagiographic articles on one side and hugely negative pessimism on the other). It has been an issue when deciding about grant funding for at least a decade already. And it also filters through to students when they are deciding what to do, who, as I pointed out above, usually haven't really seen enough about the fundamentals of the field before they have to make a decision on what they want to study.

Finally, coming back to the suggestion that physicists do not think about what they are doing or why, there are two very important times when we emphatically do do this: when we are teaching, and when we are writing grant proposals. The preparation of both of these things can be hard work, but they are rewarding, and more reasons that I have faith in my fellow physicists ability to genuinely try to challenge the big problems in our field.

Let's try that again

It's been suggested that if you go to send someone a "Happy New Year" message and notice that the last one you sent was the same thing last year, that maybe you don't need to send the message. Well, I'm going to try to prove that wrong and once again kick start this blog.

My writing in 2018 was hamstrung partly by illness. I had much better excuses last year, which was actually much worse. But this blog is called "Real Self Energy" and I am always trying to look on the bright side and see the positives, so if we look at what I was looking forward to last year in physics, in fact this year we could say almost exactly the same things:

1. We're still waiting for the results on muon \( g-2 \). At the French intensity frontier GDR meeting in November we had a great talk on this by Marc Knecht, and the theory challenges in the future, which really convinced me that theorists do have a good handle on the calculation and we are just waiting for the experiments to have their final say.
2. The B-meson anomalies lost a bit of their lustre with an update that just preserved the status quo: the measurement of \( R(K) \equiv \frac{\mathrm{BR}(B \rightarrow K \mu \mu)}{\mathrm{BR}(B \rightarrow K e e)}\) by LHCb moved closer to the Standard Model value while the uncertainty shrank, keeping the deviation about the same, while a preliminary measurement by Belle of \( R(K^*) \equiv \frac{\mathrm{BR}(B \rightarrow K^* \mu \mu)}{\mathrm{BR}(B \rightarrow K^* e e)} \) was consistent with the SM value, but with much poorer uncertainty than the (anomalous) values from LHCb. Again we discussed this extensively at the GDR in November and there is still a lot of excitement and anticipation that looks set to continue for some time, with many experiments set to report data over the coming years. A good reference of the current status is this paper.
3. The CMS and ATLAS collaborations seem to be taking their time with analyses of the full dataset of Run 2, so we are still waiting for lots of new results to come out. From the theory perspective, I have recently been involved in putting collider limits on new theory models ("recasting") and every time new experimental results come out, there is a lag before they are implemented in the various theory tools. One of the interesting questions for me will be which theory tool emerges as the winner in the long run from this effort, or if the experiments will first make their analyses completely unreproducible (e.g. by moving from cut-based analyses to neural networks)!
4. Regarding the Higgs, the mass has already been experimentally determined to an accuracy much better than we "need" (compared e.g. to the top quark), and the accuracy of the coupling measurements will only be incrementally improved with more data. There has been a lot of interest in the production cross-section, for both single and double Higgs events, where I learnt recently that the prediction in the Standard Model is now more accurately known than it can ever be determined at the LHC. This is an interesting effort that one of my LPTHE colleagues got into last year: here and here.

None of this mentions dark matter, neutrinos or axions, where there are lots of interesting things going on. And I was going to mention last year's politics, but I have run out of time for today, and, since that's a rather personal and bleak post, I will leave it for later!