Sunday, 10 January 2021

Recasting a spell

For three successive Januaries now, since I started this blog in 2018, I posted a list of the things to look forward to, which for whatever reason didn't materialise and so were essentially repeated the next year. Given the state of the world right now some positive thinking seems to be needed more than ever, but it would be a bit of a joke to repeat the same mistake again. In particular, the measurement of the muon anomalous magnetic moment (which is apparently all I blog about) has still not been announced, and I'm led to wonder whether last year's controversies regarding the lattice QCD calculations have played a role in this, muddying the water.

Instead today I want to write a little about an effort that I have joined in the last couple of years, and really started to take seriously last year: recasting LHC searches. The LHC has gathered a huge amount of data and both main experiments (CMS and ATLAS) have published O(1000) papers. Many of these are studying Standard Model (SM) processes, but there are a large number with dedicated searches for New Physics models. Some of them contain deviations from the predictions of the Standard Model, although at present there is no clear and sufficiently significant deviation yet -- with the obvious exception of LHCb and the B-meson anomalies. Instead, we can use the data to constrain potential new theories.

The problem is that we can't expect the experiments to cover even a significant fraction of the cases of interest to us. For an important example, the simplest supersymmetric models have four 'neutalinos' (neutral fermions), two 'charginos' (charged fermions), plus scalar squarks and sleptons -- and two heavy neutral Higgs particles plus a charged Higgs; it is clearly impossible to list in a few papers limits on every possible combination of masses and couplings for these. So the experiments do their best: they take either the best-motivated or easiest to search for cases and try to give results that are as general as possible. But even then, supersymmetric models are just one example and it is possible that a given search channel (e.g. looking for pair production of heavy particles that then decay to jets plus some invisible particles as missing energy) could apply to many models, and it is impossible in a given paper to provide all possible interpretations.

This is where recasting comes in. The idea is to write a code that can simulate the response of the relevant LHC detector and the cuts used by the analysis and described in the paper. Then any theorist can simulate signal events for their model (using now well-established tools) and analyse them with this code, hopefully providing a reasonably accurate approximation of what the experiment would have seen. They can then determine whether the model (or really the particular choice of masses and coupling for that model) is allowed or ruled out by the particular search, without having to ask the experiments to do a dedicated analysis.

Ideally, recasting would be possible for every analysis -- not just searches for particular models, but also Standard Model searches (one example which I have been involved in is recasting the search for four top-quarks which was designed to observe the Standard Model process and measure its cross-section, but we could then use this to constrain new heavy particles that decay to pairs of tops and are produced in pairs). However this is a lot of work, because theorists do not have access to the simulation software used for the experiments' own projections (and it would probably be too computationally intensive to be really useful anyway) so the experiments cannot just hand over their simulation code. Instead there is then a lot of work to make approximations, which is why there is really some physics involved and it is not just a mechanical exercise. Sometimes the experiments provide pseudocode which include an implementation of the cuts made in the analysis, which helps understanding the paper (where there is sometimes some ambiguity) and often they provide supplementary material, but in general getting a good recast is a lot of work.

In recent years there has been a lot of effort by both experimentalists and theorists to make recasting easier and to meet in the middle. There are lots of workshops and common meetings, and lots of initiatives such as making raw data open access. On the theory side, while there are many people who write their own bespoke codes to study some model with some search, there are now several frameworks for grouping together recasts of many different analyses. The ones with which I am most familiar are CheckMATE, Rivet, ColliderBIT and MadAnalysis. These can all be used to check a given model against a (non-identical) list of analyses, but work in somewhat different ways -- at their core though they all involve simulating signals and detectors. There is therefore a little friendly competition and very useful cross-checking. Then there are other useful tools, in particular SModelS, which instead cleverly compares whatever model you give it to a list of simplified model results given by the LHC experiments, and uses those to come up with a much faster limit on the model (of course this loses in generality and can fall prey to the assumptions and whims of the particular simplified models results that are available, but is nonetheless very useful).

So the reason for the post today is the paper I was just involved in. It is a proceedings of a workshop where a group of people got together to recast a bunch of the latest LHC searches in the MadAnalysis framework. I didn't take part in the workshop, but did recast a search which was useful for a paper last year (if you are interested, it involves the signal in the inset picture)
so there are now 12 new reinterpretations of some of the latest LHC searches. This should be useful for anyone comparing their models to data. You can see by the number of authors involved how labour-intensive this is -- or check out the author list on last year's white paper; there are still many more searches from Run 2 of the LHC that are yet to be recast, so we still have our work cut out for some time to come before there is any new data!

If you are interested in the latest developments, there will be a forum next month.

Sunday, 4 October 2020

Zoom on the Universe

Like everyone else in the field I've been enjoying the new benefits and disadvantages of Zoom meetings since the lockdown in March. They've given access to talks from all around the world, and made it even easier to carry on working on my own stuff while ignoring someone else talking in the background (it's amazing how motivating being in a talk can be).

However, as part of the Fête de la Science, this week some eminent members of the particles and strings groups at my lab are getting together to do an Ask Me Anything for anyone - adults or chidren - interested to ask questions about the deep questions in the universe. They'll be on Monday/Wednesday/Friday at 17h30 CET. The first two will be in French, and the last one in English. I'll be taking part in the one on Friday, mainly putting on my string theory cap. We had a rehearsal last week which was a lot of fun, I'm hoping the real thing will be even better.

Please circulate the link here and visit us to ask anything you ever wanted to know about string theory, cosmology, dark matter, physics Beyond the Standard Model, ... from researchers working at the cutting edge!

Tuesday, 10 March 2020

Muon g-2: lattice salad

Muon g-2: lattice salad

A couple of weeks ago, a new lattice QCD calculation by a group known as BMW tried to tip the BSM community into depression by reporting that they could resolve the tension between theory and experiment. Their new result had a tiny uncertainty (of 0.6%), much smaller than any previous lattice computation.

As I've mentioned here several times, the anomalous magnetic moment of the muon is one of the most precisely measured quantities in the world, and its prediction from theory has for several years been believed to be slightly different from the measured one. Since the theory was thought to be well understood and rather "clean", with uncertainty similar to the experimental one (yet the two values being substantially different) it has long been a hope that the Standard Model's cracks would be revealed there. Two new experiments should tell us about this, including an experiment at Fermilab that should report data this year with potentially four times smaller experimental uncertainty than the previous result; An elementary decription of the physics and experiment is given on their website.

However, there were always two slightly murky parts of the theory calculation, where low-energy QCD rears its head appearing in loops. A nice summary of this is found in e.g. this talk from slide 33 onwards, and I will shamelessly steal some figures from there. These QCD loops appear as

Hadronic light-by-light, and
hadronic vector polarisation (HVP) diagrams.

The calculation of both of these is tricky, and the light-by-light contribution is believed to be under control and small. The disagreement is in the HVP part. This corresponds to mesons appearing in the loop, but there is a clever trick called the R-ratio approach, where experimental cross-section data can be used together with the optical theorem to give a very precise prediction. Many groups have calculated this with results that agree very well.

On the other hand, it should be possible to calculate this HVP part by simulating QCD on the lattice. Previous lattice calculations disagreed somewhat, but also estimated their uncertainties to be large, comparable to the difference between their calculations and the experimental value or the value from the R-ratio. The new calculation claims that, with their new lattice QCD technique, they find that the HVP contribution should be large enough to remove the disagreement with experiment, with a tiny uncertainty. The paper is organised into a short letter of four pages, and then 79 pages of supplementary material. However, they conclude the letter with "Obviously, our findings should be confirmed –or refuted– by other collaborations using other discretizations of QCD."

Clearly I am not qualified to comment on their uncertainty estimate, but if the new result is true then, unless there has been an amazing statistical fluke across all groups performing the R-ratio calculation, someone has been underestimating their uncertainties (i.e. they have missed something big). So it is something of a relief to see an even newer paper attempting to reconcile the lattice and R-ratio HVP calculations, from the point of view of lattice QCD experts. The key phrase in the abstract is "Our results may indicate a difficulty related to estimating uncertainties of the continuum extrapolation that deserves further attention." They perform a calculation similar to BMW but with a different error estimate; they give a handy comparison of the different calculations in this plot:

The new result is LM 2020 (with BMW 2020 the result from two weeks ago). Of course this cannot be the final word, and my personal bias makes me hope without justification that it is the latter paper that is correct; it is certainly interesting times for these lattice computations!

Update 12/03/20: A new paper yesterday tries to shed some new light on the situation: apparently it has been known since 2008 that an HVP explanation of the muon anomalous magnetic moment discrepancy was unlikely, because it leads to other quantities being messed up. In particular, the same diagrams that appear above also appear in the determination of the electroweak gauge coupling, which is precisely measured at low energies from Thomson scattering, and then run up to the Z mass: $$ \alpha^{-1} (M_Z) = \alpha^{-1} (0) \bigg[ 1 - ... - \Delta \alpha^{(5)}_{\mathrm{HVP}} (M_Z) + ... \bigg] $$ where the ellipsis denotes other contributions. Adding the BMW lattice contribution there at low energies and extrapolating up, the new paper finds that the fit is spoiled for the W-boson mass and also an observable constructed from the ratio of axial and vector couplings to the Z-boson: $$ A_{\ell} = \frac{2 \mathrm{Re}[ g_V^{\ell}/g_A^{\ell}]}{1 + (\mathrm{Re} [g_V^{\ell}/g_A^{\ell}])^2}$$ The key plot for this observable is:

Thursday, 6 February 2020

Brexit and me

When I set up this blog I imagined that I would post things about my Brexit experiences in the build up to it actually happening. In the end it seemed like everything that could be said about it was written elsewhere, and in terms of consequences for me personally there was so much uncertainty about what would actually happen that it was not worth it. In the aftermath of Brexit day I received an email from the French government stating that, having already applied for a titre de séjour, I do not have to do anything for now and should not go to my local préfecture: you get the impression they are just annoyed by the whole Brexit thing and are trying to avoid hoards of confused anglais turning up demanding documents. I spent the weekend feeling a sense of loss, which seems to have replaced the rollercoasters of anger and hope of the last years. So I now feel that it's time to share my thoughts.

In case you can't guess, since I'm a Brit living and working in France who will be directly impacted in myriad ways by Brexit, I am vigorously opposed to it, but I would have been so even if I had never emigrated.  In the UK, there has always been a nasty nationalistic undercurrent among a sizeable minority, with foreigners (especially French) being contemptible/lazy/inferior, and it seemed you could never mention Germany without someone referring to Nazis, "Two World Wars and one World Cup," etc, but then my teenage experiences visiting France and Germany made me realise how blinkered this was and that as Europeans we have so much more in common than we have differences. These formative experiences came not long after the founding of the EU in '92, when there was also a strong sense that all of Europe was coming together to work in a common interest and be stronger together, and Europe was being referred to positively in at least part of the public conversation in the UK.

In my field, there are not many British people actually working in Europe. (I often wonder if this is true, and it may just be my perception -- I'd be interested to see some statistics). I think this starts with the fact that very few British students seem to go abroad for their PhDs. Then there are a large number of PhD students trained at UK universities, but they start their doctoral training at a (much) younger age than their European counterparts, and a PhD in the UK is 3, or if they are lucky, 4 years, rather than 5+ in the US. This means that only a small proportion of UK PhD students are actually competitive internationally when they are applying for postdocs: they often just do not have the same level of experience or publications. Partly it will also be because the UK has a large (and very international) academic jobs market, so more British people tend to be absorbed back there than elsewhere for permanent jobs.

... anyway, I have got used to being different, and I enjoy it, even as I feel and try to become more and more native as time goes on. The French system is quite open to non-French permanent researchers, so my lab is rather international. But I am currently the only Englishman, and I don't form part of any sort of British enclave or cabal. So as regards Brexit, I have been somewhat insulated from the apparently toxic atmosphere in the UK for the past three and a half years. Instead I have had a certain stifling sense of worry about the future, because I am one of the people who will be directly affected by the elimination of freedom of movement, and who the British government seems indifferent to (not least because my right to vote in the UK will expire). At the moment this is coupled with anxiety about the French government's pension reforms (which will almost certainly affect me rather severely -- it's perhaps damning that it is still not clear what the reform will actually do in detail) and reforms of the funding of research (the LPPR which aims to install "academic darwinism") ...

So what do French people, and more specifically French academics, think of Brexit? Well, for the large part it is both viewed as tragic and hilarious. It seems to have dispelled talk about "Frexit" (even among the hard-right Rassemblement National). And while at first I was debating Brexit most days with colleagues, now it is regarded as old news and barely merits headlines. People were far more interested in the goings-on in Parliament before Christmas, with my Italian colleagues crowing that they had been overtaken as the country with the most dysfunctional politics. On Friday 31st January when it was mentioned on the radio, they quickly segued into the old anecdote about how "God save the King/Queen" was actually written to celebrate the successful recovery of Louis XIV from surgery for an anal fistula, then stolen by (German-born) Handel and translated for an English audience. The other mention in the news was the storm in a teacup when Guernsey forbade French boats from fishing in their waters; they climbed down immediately when the French ports refused Guernsey boats access.

On a practical level, as I mentioned at the beginning, for the time being I will need a titre de séjour to prove that I can stay, or more specifically exchange the EU one that I was granted last year for some new Brexit card. The French government has set up a brexit website to help inform people of their rights, but since nothing has been firmly decided beyond the transition period the uncertainty will carry on -- although there is probably less uncertainty here about what will actually happen than in the UK. Indeed, I get the impression that there is a relief that perfidious Albion will be out of the European decision-making process, and there is an opportunity to attract businesses and people here. In particular, there were 10 special positions opened in the CNRS across all disciplines this year, which were unofficially intended to attract people fleeing the UK. I have heard of cases of academics doing exactly that already.

Wednesday, 29 January 2020

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 ...

Friday, 24 January 2020

The KOTO anomaly

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.

Thursday, 16 January 2020

How to make progress in High Energy Physics

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.