## Sunday, August 07, 2016 ... /////

### Actual lessons from the LHC null results

It seems increasingly likely by now that the LHC hasn't made any new discoveries up to this moment even though there has been a large number of high-energy collisions in 2016 and a significant perceived chance that something new could emerge out of them. Should physicists run desperate? Who should be blamed? What are the lessons we should learn and how should particle physics adjust itself given the new data?

Well, first, physicists have no reason to become desperate and no one should be blamed for anything.

This is simply how Nature works up to the given energy and luminosity of $pp$-collisions and the physicists' primary job is to learn the truth about the experimentally accessible regime. The LHC experimenters have been doing a superb job in testing – and confirming – the state-of-the-art effective field theory, the Standard Model. Maybe they missed a search strategy or two – but in that case, it's a collective fault of the whole community, including the phenomenologists. No one has been good enough in finding better strategies or at least in convincing others that they're great.

Experimenters would surely be happier – and more celebrated – if they found something new. But that outcome didn't depend on them only. It depended on Nature's decisions that were done 13.8 billion years ago, perhaps "before" any time existed. ;-) Experimenters were probably doing the best possible work given the assumption that we live in the Universe with these pre-existing laws of physics.

Even though the outcome is that "the Standard Model seems to describe even this state-of-the-art dataset", i.e. a rather boring answer (the most likely single answer people have been thinking about for some 40 years), the experimenters still need to be paid because if the sign of their wages depended on something they can't change – the laws of physics – no one would be doing the work. So the experimenters deserve no big prizes because they were not lucky – Nature wasn't cooperating with their optimistic plans – but they mostly deserve the bonuses to their salaries for their regular work or whatever. ;-)

Now, should theorists be blamed? Once again, the correct answer is obviously No, theorists can't be blamed because they can't be held responsible for the actual laws of physics, either. However, we may ask the question whether the theorists have been spending their time wisely in the light of the latest experimental results. The answer is "It depends" and the most important axis we need is the theory-phenomenology axis.

The theorists may be basically divided to

• hep-ph: high-energy physics, phenomenology
• hep-th: high-energy physics, (formal) theory
The terminology is due to Paul Ginsparg who divided the archives when he was starting xxx.lanl.gov (today: arXiv.org) in the early 1990s. Alternatively, you may think that the theorists aren't divided to two groups but instead, it's a continuum parameterized by a coordinate that runs from the pure phenomenologists to pure theorists. This classification has lots of issues and subtleties. And additional fields enter. People who are "more phenomenological than phenomenologists" are basically experimenters while the people who are "more theoretical than theorists" are mathematical physicists if not mathematicians.

What do the null results imply for the balance between hep-ph and hep-th? In my opinion, the answer is obvious:
The methods of hep-ph were shown less effective than previously thought. The assumptions of hep-ph are probably less correct than previously thought. Due to its separation from the LHC experiments, hep-th is largely unaffected.
Why is it so? It's because by definition, hep-ph is the part of the particle theorists who want to work on models that could be experimentally verified in a foreseeable future. It is in no way obvious that this is the right assumption or the viable strategy and the null results from the collider suggest that it's not such a right assumption or viable strategy.

There is simply no good reason to assume that important ideas and facts about Nature may be experimentally tested soon, that they may be proven so directly that even mathematically challenged people must admit that a proof exists.

The previous paragraph is well-appreciated and has been well-appreciated by the folks in hep-th, almost by definition of hep-th. While the hep-ph folks have always preferred to look for new physics around the corner – new physics that would be seen in a few years if it exists – hep-th theorists think that this assumption is a bias or a wishful thinking and they prefer more objective arguments to decide about the mass scales and values of other parameters.

That's why hep-th theorists often tend to assume that the bulk of the truly interesting new physics is taking place at energy scales close to the apparent most fundamental energy scale in Nature, the Planck scale (the GUT scale isn't far from it in most GUT theories, either), and this fundamental scale is almost certainly inaccessible by experiments in any imaginable future.

So if the null results mean something far-reaching for theorists, they surely mean that the assumption that new experimental facts may be discovered every $X$ years shouldn't be trusted as much as the hep-ph folks have trusted it. In other words, all theorists should better adopt the hep-th impartial thinking to some extent and probably to a larger extent than before. All theorists should also learn some extra methods known among hep-th people and surely buy at least one string theory textbook if they don't have any.

The idea that new things may be investigated by simple methods and straightforward experiments simply looks significantly less plausible now than it did just weeks ago.

I have described hep-ph as the part of the theorists suffering from a wishful thinking that important new ideas will be experimentally found soon. I have always disagreed with this bias. But there exist people who represent the "fanatical exaggeration" of this bias, the notorious critics of hep-th. People who have tried to claim that theorists are "forbidden" to do research and talk about things that can't be tested soon.

Lee Smolin is one of the most notorious individuals in this class. He has famously claimed that physicists should be obliged to work according to five-year plans of the Soviet type. And if they don't prove that their ideas are right within the five-year window, the commissars supervising them should ban any further research by these physicists.

Well, quite obviously, all of particle physics would have been killed by now if this Stalinist recommendation by Comrade Smolin were taken seriously, if people failed to appreciate the important fact that Smolin is just a deluded Bolshevik crackpot.

There can't be five-year plans in an effective economy and there can't be five-year plans in a healthy particle physics community. The value of "five years" is just an arbitrary quantity that some power-thirsty Bolsheviks made up (either in Moscow or in Waterloo, Canada – the difference between Stalin and Smolin isn't too big) and doesn't have any intrinsic significance. Many ideas take much longer to be established and proven. And when this actual work is ongoing, theorists simply need their own methods – independent of commissars and plans – to determine the value of various ideas, principles, research directions, or particular models.

If someone is incapable of converging to an opinion about the value of ideas without the help of direct experiments that say Yes/No, then he or she simply cannot be a theoretical physicist. It's that simple. Theorists are those who must work on these ideas before it is spectacularly clear which of them is right. The people who can only see it afterwards are self-evidently just ordinary consumers or laymen.

The people who have these direct-experiment-unrelated criteria to judge the viability of ideas in particle physics have done their job for some time and will continue to do so pretty much independently from the new LHC null results. For example, lots of the top minds will continue to think that the Standard Model is a relatively arbitrary or ugly theory that is good as an effective approximation but can hardly be considered a fundamental theory of Nature.

They're pretty sure – we're pretty sure – that new phenomena exist and can be seen at higher energies. But no one really knows what the energies are and the assumption that they must be available to this collider or the next one is nothing else than a wishful thinking, not a result of a solid derivation.

In the following part of this blog post, I want to discuss a particularly hysterical reaction to the LHC null data. Sabine Hossenfelder wrote a rant titled
The LHC “nightmare scenario” has come true
I think that I fundamentally disagree with pretty much every idea she tries to express in that rant. Let me show why.

In the first paragraph, she basically says that she decided to study college-and-PhD focusing on particle physics in 1995 when and because the top quark was discovered. This is weird. As she admits, the top quark has been believed to exist since 1973 (when I was born). Just for your counting, I was academically 3 years older than her. So I went to the college in 1992 and I already considered the current list of elementary particles – including the top quark – to be a fact. The pairs of quarks looked sensible. Maybe I was a bit brainwashed by the popularizers of physics but I found it "more obvious for the top quark to be there than not" before it was discovered and even before I fully understood the role of the electroweak symmetry. At any rate, the assumption was right.

This is also how my instructors of particle physicists framed it soon after 1992. The top quark surely exists. In the early 1990s, people began to understand that it almost certainly had to be lighter than $200\GeV$. Some extreme renormalization running would spoil the theory if the mass (and the Yukawa coupling) were higher. And the quark was lighter than that, indeed. Some string theory models have approximately predicted the top quark mass before 1995 – but the string theorists probably knew the non-stringy arguments for that mass range, too (not to mention that those models don't seem exactly right for other reasons). For a few years, they were converging towards those $175\GeV$ although many contemporary values prefer to say $172\GeV$ etc. But my main point is that the actual discovery of the quark at the Tevatron has played virtually no role in my attitudes to fundamental physics. It had to happen. It's great for Melissa Franklin et al. to have found it but it's not something that could change the world view of a good theorist. After all, it was just the sixth quark and about twelfth fundamental fermion.

In the second paragraph, she criticizes people who call the Standard Model "ugly". Again, it's still ugly. I have mostly believed in a Big Desert – no new physics scales between the Standard Model (and/or supersymmetry) scale near the LHC energies; and the GUT scale – for more than 20 years. But that fact in no way contradicted my opinion that the Standard Model is ugly. It is simply not a fundamental theory. The LHC null results change nothing about this appraisal. Quite generally, when I say that something is true, it isn't automatically including the statement that in a few years, it will be obvious to everybody that it's true. Assuming the first statement, the second statement may still be true or false.

In the third paragraph, she complains that the estimated superpartner masses were being pushed higher in the recent decades. What a surprise. This is what is happening to every parameter before the new phenomenon is discovered. The top quark mass was also being pushed higher before it was found at $175\GeV$. The estimated size of atoms was also downgraded a few times before the atoms were basically discovered. If the lower bound on the superpartner or top quark masses go up (as the experiments are becoming more potent), so must theorists' estimates of the superpartner masses because those are guaranteed to be "somewhere in the so far unexcluded region" (or nowhere) and because the unexcluded region's center-of-mass is shifting as the experiments advance. The fact that some particles with masses that are not determined haven't been found up to the year $Y$ doesn't imply that the particles don't exist.

But her fourth paragraph is where it gets vitriolically and emotionally brutal.
During my professional career, all I have seen is failure. A failure of particle physicists to uncover a more powerful mathematical framework to improve upon the theories we already have.
Well, during my years, I've seen one success after another even though the main success, the discovery of the vital new framework of string theory, already took place in the late 1960s and early 1970s. If she were seeing failures, why didn't she change her occupation? What is the value of a worker who thinks that the whole enterprise she's doing is a failure and who is even working hard to impose this delusion upon her colleagues? I am absolutely certain that her value is negative. Her assertion makes it obvious that she considers her work a failure, too. That's the correct evaluation in her case, of course, but it's too bad if physicists (or any employed people) have this poor opinion of their own work.

Wiser people than Hossenfelder have been interested mostly in things that had a not quite negligible chance of success and many of them have succeeded amazingly. Some of them didn't. But they (and I) didn't place my eggs into one basket. If everything she has been doing was a failure, surely she was doing something wrong – or was extremely unlucky. But other people haven't been. Many amazing things have happened since 1995, just like before.

In the following sentences and paragraphs, she slings mud on every idea in particle physics – e.g. grand unification and especially naturalness that have "failed again". First of all, it's complete nonsense that grand unification has failed due to the LHC results. Grand unification theories mostly predict nothing new for LHC-scale energies – they really predict new phenomena for energies $10^{14-16}\GeV$ or so, way above the LHC scales. Doesn't she know that? If she does, why does she "deduce" that GUT has been a failure because of these results?

Naturalness is one of the most popular targets of her mudslinging. The null LHC results surely mean that Nature is "less natural" according to the technical definitions that have been used by most phenomenologists. And many specific models with natural values of parameters have been excluded. But it's not true that all technically natural models have been excluded.

Even more importantly, the more broadly defined concept of naturalness hasn't been ruled out and cannot really be ever ruled out because it's basically a defining feature of natural science. Natural sciences always require us to compare explanations and say which of them is more natural than the other. Is it more natural for the dinosaur bones to be a trace of evolution; or did God's assistant design them just like we see them and no dinosaurs have ever lived? The first theory is more natural which really means that it's more likely according to the scientific algorithms to estimate the probability. The theory with God's assistant requires lots of fine-tuning in order to make the bones exactly what they are. On the other hand, the actual living-and-evolving dinosaurs produce many features of the bones (at least qualitative ones) "automatically", without additional assumptions. So the evolutionary explanation is less fine-tuned and more natural. The adjective "more natural" really means "more likely according to the methods to estimate the probability as understood by natural scientists right now".

You couldn't really decide whether you favor creationism or evolution without this notion of naturalness. And the naturalness as understood by particle physicists is just a more specific variation of the same thing. Some localized adjustments to the ways "how we estimate naturalness" may be done and people may be forced to do them. But we can't ever eliminate these things from physics. Hossenfelder must be a complete crank if she believes that theoretical physicists will avoid this adjective – the appraisal behind it – altogether in the future.

After several additional hysterical and wrong paragraphs, the last big paragraph of hers starts as follows:
That the LHC hasn’t seen evidence for new physics is to me a clear signal that we’ve been doing something wrong, that our experience from constructing the standard model is no longer a promising direction to continue.
This is really cute. The LHC has shown that the Standard Model is valid for a much greater set of situations than the average particle physicists estimated. But according to Ms Hossenfelder, it means that "our experience from constructing the Standard Model is no longer a promising direction to continue". Sorry but because the Standard Model's range of validity is broader than previously thought, our experience from constructing it is more important than before, too. Her conclusion is exactly upside down.
We’ve maneuvered ourselves into a dead end by relying on aesthetic guidance to decide which experiments are the most promising.
And what are her most promising experiments? She is a classic demagogue who criticizes but has absolutely no alternatives. It's very clear that because no new great ideas have emerged and no game-changing experimental results have been announced, physicists will have to continue with the set of ideas and methods they have had so far – including the mix of the aesthetic criteria. It's absolutely foolish to think otherwise.
I hope that this latest null result will send a clear message that you can’t trust the judgement of scientists whose future funding depends on their continued optimism.
Physicists (and, more generally, scientists) must have the freedom – and, in particle and fundamental physics, largely have the freedom – to honestly express their actual opinions resulting from their expertise-driven evaluation of all the evidence. And their funding should only depend on the excitement that their work creates among other scientists, not on the character of their research.

So if physicists work on something that creates no excitement, they see it, their hiring and funding prospects deteriorate, and they are more likely to try different avenues. The low excitement by others may be considered to be a mistake of the community by some researchers and those may try to continue despite the hostility or indifference of the community. But they are under increasing pressure – and that's the right thing, too. Some of them succeed, anyway, and then deserve to be called heroes. Most people considered misguided don't succeed.

It is absolutely incorrect for a Ms Hossenfelder to try to "engineer" the right level of optimism among particle physicists. And it's evil to invent conspiracy theories about particle physicists who pretend their optimism. I think it is unlikely that new 5-sigma discoveries resulting from the first 10 inverse femtobarns of the $\sqrt{s}=13\TeV$ data will be announced in coming months. You may call it a "pessimism" but it's an obvious "rather justified" conclusion of the papers and talks we saw in recent days. Basically every particle physicist says the same thing. It is just a malicious lie for Ms Hossenfelder to try to accuse particle physicists of some dishonesty in the optimism they display.

At a $\sqrt{s}=100\TeV$ collider, the Standard Model may very well be the victor again. And because it has happened now, we know that it may happen again and we may in no way discard it. We know that the Standard Model ultimately breaks down at some scale but the scale where it "has to take place" is far higher than $100\TeV$. But on the other hand, there is no solid evidence that it should happen. I believe that the energy frontier – the efforts to simply increase the energy of the collisions – remains the most promising, neutral, and justified direction in the experimental search for new physics.

I am convinced about many other things that haven't changed. For example, supersymmetry remains the single most likely "kind of new physics" that may be experimentally discovered in the future. We mustn't forget that the LHC null results are, well, null results. As the word makes obvious, it means that "nothing has really changed" so far (at least not qualitatively). If something has changed qualitatively, it could have been more exciting but it simply hasn't.

That's why most physicists – and an overwhelming majority of hep-th physicists – will change nothing qualitative about the big-picture thinking about particle physics due to the LHC null results. No qualitatively new evidence has arisen – so there's no rational reason for a qualitative change of the opinions and expectations. It's just particular theories of new physics (and their classes and parts of parameter spaces) that have been ruled out. It's not particle physics as a discipline or its basic methods and values that have been ruled out – even though science haters of Ms Hossenfelder's kind would love to claim exactly that.

The Standard Model is a potent effective theory just like it was before. Its range of validity has increased by a few steps relatively to where it was located just weeks ago. But none of these steps are game-changing. A discovery would be game-changing, the lack of a discovery is not. The absence of a discovery is the most boring business-as-usual.

The comment section beneath her article is full of people suggesting that particle physicists should be desperate and switch to condensed matter physics and all this stuff. Nothing against condensed matter physics. I like it and be sure that I've gone through graduate courses on them, got an A, and was considered among the best students. From many viewpoints, it's a sibling of particle physics that is using similar methods etc. Some of the obsession with the "classification of phenomena" is rather similar, too.

But I just can't get really excited about it. There are too many things to study there and none of them is "truly fundamental". They're too applied or too close to "engineering" for me. All of the things are some effective theories describing situations whose ensemble is basically unlimited. John Baez has partly hired himself as a promoter of the environmental doomsday scenarios etc. He may think it's OK but I have been taught by very wise people to call these things "intellectual prostitution" and whether he kindly allows me or not, I will keep on considering him an intellectual slut for that reason. The fact that he is also a fanatical leftist isn't an excuse. Aside from being a leftist, he is also a person with some knowledge of mathematics and related thinking that is enough for him to know that the doomsday stuff has absolutely no scientific value.

Again, my recommendation to hep-ph folks who feel disappointed is to grow up, abandon the wishful thinking, and realize that Nature isn't obliged to cooperate and make things easy to be experimentally discovered. And buy a string theory textbook if you don't have one and try to study it as if you were a student – even if you are not one. It has become more likely after the new wave of the LHC results that the new physics may occur in the realm where truly new things from string theory do matter.

Obviously, if you have psychological problems with any of these things and you find it "essential" for experiments (or an "obligation" for Nature and the properly defined science) to confirm theoretical ideas according to five-year plans of the Smolin type, then indeed, it may be a better idea for you to leave the field.