It is Monday, and as can be seen, I am still aligned with my good resolution of a weekly write-up about particle physics and cosmology. Whereas fingers are still crossed for the future, my current time resources are still high enough so that I could cope with this.
I was initially unsure about what to discuss this time, and I even tried to have @agmoore helping me to choose the topic. I have indeed several interesting subjects that could be written about, like for instance the flavour anomalies (a hot topic in the high-energy community for a couple of years). Those anomalies are seen in data for several years, and have even been reinforced by recent results from the Large Hadron Collider at CERN. Although these anomalies are more and more solid year after year, it is still too early to conclude about any failure of the Standard Model of particle physics. The Standard Model then persists as being the best framework to explain (and predict) more than a century of data.
This being said, I finally opted not to write about these anomalies and keep this topic for an upcoming post. Instead, I discuss the Standard Model of particle physics itself, so that I could refer to this post in many of my future writings on STEMsocial. I follow the approach I usually take when discussing particle physics with the general audience, and start from a definition of particle physics before moving alongside historical developments.
I guess this is it for setting up the stage of this post, that I keep brief. Let’s now embark into a journey in more than 100 years of particle physics discoveries!
What is particle physics about?
In order to easily answer what is particle physics about, it is good to ask about the definition of physics itself. As a starting point, we can check in the dictionary. We will find that physics is defined as a science that studies the general properties of matter and that establishes the laws governing all material phenomena. This definition comes from a French dictionary (Le Robert), and slight variations can be found in other (in particular English) dictionaries.
When we move to particle physics, we can apply the same definition, but to the most fundamental building blocks of matter. Particle physics hence tries to determine the dynamics dictating how the most elementary particles behave in their everyday life.
And this brings us to a first very important question: what are those elementary building blocks? We can decide to answer in a very egoist and human-centred way. The human body is mainly made of six chemicals: oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus. However, humans are nothing in our universe (humans should actually keep this in mind more often, but this is another debate). At the scale of the universe, hydrogen and helium are sufficient to describe more than 99.9% of everything we see.
The above discussion is nevertheless only focused on chemical elements. Let’s stick to this for a moment.
Whereas most matter in the universe can be described by only two of these elements, we know today that we have 118 of them lying around, as illustrated in the image below showing the current version of Mendeleev’s periodic table of elements. From my own perspective, this table is just a mess. There are so many elements… This makes the story complicated enough, and for that reason (among others as a matter of fact) I decided not to become a chemist many many years ago.
[Credits: Explorersinternational (Pixabay)]
For the physicists of the first half of the 20thcentury, such a complex picture could be drastically simplified and rely only on three building blocks. It was indeed known that every single atom could be described with protons, neutrons and electrons (this is what was called the Rutherford model). In addition, early radioactivity studies demonstrated the existence of a stealthy fourth guy, the neutrino.
Therefore, in the 1930s, we could be tempted to state that physics was fully done. Absolutely all matter in the universe could be simply explained with 4 entities. Fortunately, the story is not so simple, which is why we still have many interesting things to study today.
Cosmic rays, the first accelerators and the particle zoo
We are now in the 1930s-1940s. The situation is not very different from today: physicists from that time were trying to understand the underlying dynamics of the world. However, they did not have many options for that, in particular as there were no particle accelerators like the Large Hadron Collider at CERN (as colliders were not invented yet). The best strategy then consisted to use highly-energetic cosmic rays.
When a highly-energetic cosmic ray enters the atmosphere of our planet, it usually quickly hits one of the particles comprising it. The products of such a (highly energetic) collision are secondary particles, that are still energetic enough to proceed with secondary collisions. This generates even more particles, that are then ready to undergo collisions too. And so on, and so on…
As a result, we get a cascade of particles hitting the ground (known as a cosmic ray shower), as depicted in the image below. The interesting point is that hints from the particles included in this shower can be obtained in a simple manner, through photographic emulsions.
[Credits: A. Chantelauze, S. Staffi and L. Bret (Pierre Auger Experiment)]
The principle is very simple: we patiently wait for a shower to happen above us in the sky, with a photographic plate. The plate is expected to record the tracks of any charged particle passing through it, which happens when the shower gets close to the ground. While such an apparatus is super cheap to set up, the problem is elsewhere. There is no way to control the moment at which a cosmic ray would arrive at the right spot (well aligned with the location at which the plate is gently waiting to record anything). However, with patience, everything is possible (and this worked back in the days)!
A bit later, let’s say roughly in the 1940s-1950s, the first accelerators appeared. A good example of those machines is the Cosmotron at Brookhaven National Laboratory in the US. The great advantage of particle accelerators relative to using cosmic rays is that highly-energetic collisions are controlled thanks to electric fields (acceleration of the particle beams) and magnetic fields (control of the trajectories of the beams). There is moreover no need to wait for ages to have a cosmic ray hitting the atmosphere at the right place. We can instead simply turn on the accelerator and have collisions exactly where and when we want.
Cosmic ray and accelerator studies were more or less the same thing: high-energy physics collisions yielding the production of new subatomic particles. These collisions solely rely on Einstein’s special relativity. Energy and mass are equivalent. Therefore, with a lot of energy, we can produce more massive objects and potentially new particles not discovered so far.
At the end of the 1950s, this yielded the discovery of a plethora of new subatomic particles: dozens of pions, kaons, 𝛺’s, 𝜮’s, 𝜩’s, 𝜔’s, 𝚫’s, etc., and of course the muon, a heavy cousin of the electron, and the muon neutrino (the big fat brother of the electron neutrino).
In other words, we were back to step one, a picture of the microscopic world that is as busy as Mendeleev’s table…
Cleaning the mess: from the quarks to the Standard Model
The situation started to change in the mid-sixties, when Gell-Mann and Zweig proposed that all known particles, with the exception of the electron, the muon and the two neutrinos, were composite systems made of three fundamental entities called quarks. Glashow and Bjorken refined the picture. All existing subatomic particles (still with the exception of the electron, the muon and the two neutrinos that are elementary) can be described with four quarks named the up, down, strange and charm quarks. In contrast, the other four guys and girls (the electron, the muon, the electron neutrino and the muon neutrino) are called leptons.
The dozens of known particles (the pions, kaons, 𝛺’s, 𝜮’s, 𝜩’s, 𝜔’s, 𝚫’s, etc.) could be classified on the basis of their quark content, and new states emerging from this classification were predicted (and discovered later). The symmetry properties underlying the classification were indeed pointing to missing particles. Consequently, this offered to physicists clear directions to where to search for new particles.
This idea stopped being just an idea in 1968, when the proton structure was discovered at the Standard Linear Accelerator Center (SLAC). This demonstrated that the quark model could be a correct description of nature. This feeling becomes even stronger after the so-called November Revolution in 1974, which has nothing to do with any political movement (the word ‘revolution’ is sometimes different from what it seems). This revolution is the way the discovery of the charm quark is coined in the field of high-energy physics.
Despite of these successes of the quark model, observations were still teasing physicists in the mid 1970s. During the 1930s-1940s, subatomic particles called kaons were discovered (see above), all these kaons being unstable and decaying after some time. In the 1970s, it turned out that the properties of kaon decays were not described correctly by the quark model.
To correct this issue, Harari postulated in the mid 1970s that six quarks were lying around. Such an assumption may indeed be more elegant than sending to the graveyard a theory that was working so well for many other things. This six-quark option was of course the correct hypothesis. The fifth quark, the bottom quark was discovered at Fermilab in 1977. We however had to wait until 1995 for the last of the quarks (the top quark) to be found, also at Fermilab. Its very large mass (it is an elementary particle that is as heavy as a gold atom) hid it from accelerators during more than 20 years (a lot of energy was needed to produce it, as dictated by special relativity, so that powerful accelerators were in order).
On the other hand, there is no reason to have six quarks and only four leptons. Tsai therefore postulated the existence of the tau lepton in the 1970s. It was discovered slightly later at SLAC and Berkeley.
As for the top quark, the last neutrino (the tau neutrino) hid itself for a long period, but not because of its mass (as it is almost massless). Being weakly interacting and always related to the production of tau leptons, it was very hard to see it directly in an experiment. In 2000, the physicists of the DONUT experiment (at Fermilab again) revealed the existence of the last neutrino of the Standard Model with a direct proof. They recorded 12 events in which a tau neutrino produced a tau lepton, out of a trillion of tau neutrinos that did nothing; this is what we can definitely call looking for a needle in a needle stack.
The matter sector of the Standard Model, i.e. the elementary building blocks sufficient to explain all phenomena at the fundamental level, is hence made of 12 entities: 6 quarks (up, down, charm, strange, bottom and top), 3 charged leptons (electron, muon and tau) and 3 neutrinos (electron, muon and tau neutrino). As shown above, getting to those conclusions took us already a few years, the last missing bit having been unravelled only 21 years ago, in July 2000.
The fundamental interactions
Before closing this post, there is still an important point that we need to address. We have so far discussed the elementary particles and how they have been postulated and discovered. However, we didn’t discuss how they drink, sing and dance (aka their interactions).
In the Standard Model, this is implemented through what we call gauge symmetries. Without entering into details (as this goes way beyond the purpose of this post), this tells us that each of the fundamental interactions, namely electromagnetism (connected to electricity, magnetism, biology, chemistry, etc.), weak interactions (related to radioactivity and the life of stars) and the strong force (binding protons and neutrons in atomic nuclei) are all modelled through the exchange of a mediator between elementary particles.
In other words, particles exchange force carriers to be said to interact via a specific fundamental interaction. We hence say that electrically-charged particles interact electromagnetically when they exchange photons. In addition, particles interact weakly when they exchange W-bosons or Z-bosons, and quarks are strongly interacting through gluon exchanges.
Is this simple vision of the interactions the right one (let’s forget about the complexity behind it, that I shamelessly hide under the carpet)? Well, a picture is better than 1000 words (even if this post has already a couple of thousands of words, I know). In the plot below, we have a comparison of theoretical predictions of the Standard Model (with its gauge interactions) and experimental data from the Large Hadron Collider at CERN.
[Credits: The ATLAS collaboration (CERN)]
The rates of many processes that could arise in proton-proton collisions at the Large Hadron Collider are displayed (each circle/square/triangle represents one process in which two protons produce the final state given on the x-axis). The colour bands correspond to experimental data, and the grey bands stand for the associated theory predictions. The higher we are vertically, the more common is the process considered. And conversely, the rightmost part of the figure concerns rarer processes (that are thus harder to measure as less common, and hence associated with larger error bars).
This plot is a clear proof that we have a theory (the Standard Model) making predictions over 14 orders of magnitude that are in excellent agreement with data! And this is of course not the only example (but this is the only one I show in this post).
Is this it?
In this post, I shared how I usually introduce the Standard Model of particle physics when I discuss it with the general audience. I have explained how physicists came, in more than 100 years, with a theory containing 12 entities (6 quarks, 3 charged leptons and 3 neutrinos) that interact electromagnetically, weakly and strongly (through so-called gauge interactions).
This theory, the Standard Model, is the current paradigm to explain how our universe works at its most fundamental level.
In addition, I have also tried to convey why the Standard Model is so great. In two words, because it works. It is as simple as that. I could share many other pictures such as the last one of this post. In any of these pictures, we would have a clear proof that theoretical predictions and experimental data match.
In other words, particle physics accumulated data for more than 100 years, and we have a single theory capable to explain everything that has been recorded so far. But the beauty of the Standard Model does not end there. We also have a theory capable to make predictions relative to what we should observe in current and future experiments. This is the exact definition of a theory, somehow: making predictions.
There are two things that I didn’t discuss at all in this article.
- First of all, the long-awaited Higgs boson has been discovered almost 10 years ago. This guy being a little bit special, I decided to keep its story for another post (maybe next week, or maybe not; we will see).
- Second, I mostly do research on beyond the Standard Model physics. This means that despite of having an excellent theoretical framework, there are reasons to go beyond it. Again, this is a topic that deserves a full post and I decided not to address it here. After all, this is the cornerstone of my research…
I guess it is now time to stop writing for today (even if I am still far from the block size limit). Feel free to ask clarifications about anything that could be unclear, and also suggest topics you would like me to address. See you next Monday, for a new episode. The winter break is approaching, but I will still be here for more or less 2 weeks before decoupling (in Belgium) for 2-3 days!
Wow! I know a master teacher when I come across one. The first principle of teaching you observe in this article: Do not assume your audience knows anything about the subject. Start with the fundamentals and build up. Then, define your terms...jargon will kill a lesson faster than you can say, "Oops!".
I have missed these lessons. I retain enough (remember) of your earlier blogs to place much of the terminology and explanation in context. However, here you sum up the basic elements of the discussion as you progress. And then you come to a succinct review of what has been described:
I was ready for that summation when I read it. It made sense to me. However, my favorite line in this blog:
We would save ourselves so much grief if we recognized this.
I don't know how long it took to write the blog. I think it could be used by any high school physics teacher to explain the purpose of a course about which many kids ask, "Why do I need this?" I think if I had read this blog I might not have avoided 12th grade physics when the guidance counselor recommended it. Never too late to catch up, is it?
Thank you, @lemouth for filling in the gaps in my education.
Thanks a lot for this very nice comment (especially the last few words ;) ).
This text is really the result of how I introduce particle physics to the general audience today. The content has evolved over time, before getting to its current version. Even in this STEMsocial version, I changed a few things relative the previous version. I had so far had the chance to give it as a lecture here in Paris (several times), but also in Johannesbourg and Montreal. A much earlier version of it (that has now very little common grounds with this one) was presented at the SteemSTEM meetup at CERN (almost) 4 years ago.
I will try something a bit different soon at local middle schools. There, I will focus a bit less on research but more on the job of being a physicist that includes research, but that is not restricted to research. Actually, this could also be a nice topic for a blog (I note it for the future).
In terms of time, I must say it took me like two days (but I didn't work on the text on one go; it was scattered all over the week-end and a bit on Monday).
Thanks a lot for this very interesting post 👍
Is a photographic plate used for the same type of experiment as a Wilson cloud chamber or are they totally different experiments? If different (what I think because if I remember, a Wilson cloud chamber does not allow to see any collisions but only the trail of the particles), the 2 are complementary or for totally different research fields?
If mass is not an intrinsic value but an acquired value, what other property or knowledge would we need to be able to know that these 12 entities are in fact an aggregation of other elements?
Thanks a lot for these questions, which I will try to answer below.
Photographic plates, cloud chambers, bubble chambers. They are different devices that do the same thing: recording the tracks of charged particles passing through them. The collision that has given rise to the tracks does not necessarily need to happen on the plate (but could).
The disadvantage of the plate... is that you needed to develop them. The advantage is that they were cheap, compact and easy to carry on. Their modern versions are used today in various detector (for instance those of the neutrino experiments).
There are analyses trying to unravel the substructure of what we consider as elementary particles today. However, they have not found anything so far. Also, quantum mechanical properties (the uncertainty relations) make it a bit hard to conceive. Let me explain this last point which I find particularly striking.
Whereas quarks and leptons are elementary particles, we in fact only have an upper bound on their size. For the electron, it is less than 10-18 m (the proton is 1000 times this). The momentum uncertainty (in the quantum mechanical sense, that is close to the statistical sense) of a constituent of the electron would then be about 400,000 times larger than the electron mass (50,000 times for a quark of the same size). Here is the problem: how could light quarks or leptons be made of smaller entities that would have an enormous energy arising from their momenta, this energy being much larger than the mass of the composite system?
Thanks for your comment!
Hahaha you killed me, next time I will wait for the French post (not sure it will change something LOL!). The paracetamol company thanks you, I just ordered a few boxes from them.
Thank you for these clarifications on the different devices and this very interesting last paragraph of your answer. I believe that science still has wonderful and exciting discoveries to make!
As for quantum mechanics, I think that as human, it's really difficult to conceive because on the one hand our senses (sight, hearing, smell, touch) have conditioned our brain to have a distorted representation and on the other the knowledge taught is sometimes not in conformity with the reality because of the lack of technology. For me it took years of deconditioning before I could finally visualize our solar system not as a sun stationary with the planets revolving around it as learned with a 2D plan on a book but as the sun evolving in the universe with the planets following it in a spiral around it.
Noooooooo!!!! I didn't want to kill you, really. ^^
As a funny coincidence, a fresh bachelor student asked me and colleagues earlier today what quantum mechanics was. It is very hard to explain but the impact are easier to grasp. It explains why matter is coherent and not falling apart. It also explains while although any stuff (object or being) is mainly made of vacuum, it has a solid envelope somehow. And it is damned counter-intuitive in its first approach.
If you are interested, I wrote a bachelor's level primer book on quantum mechanics (in French). Otherwise, I am afraid that I do not know what to answer without having the question more specific.
Don't worry, my job has made me used to big headaches.
One thing that helps me put things in perspective is to remember that when I sit down, my ass never touches the atoms of the chair Hahaha
Yes, I am interested :)
Where have you heard that sentence? We used it yesterday too :)
Would there be a quantum entanglement between France and Thailand? 🤔 Hahaha... I probably heard it on Youtube, video of scientific popularization or replay of a conference (Alain Aspect, Julien Bobroff, Roland Lehoucq, Marc Lachièze-Rey, Étienne Klein...)
Not directly related to this post, but do you use distributed computing in analysing the data? If so, do you use general purpose frameworks or you have dedicated one?
Thanks for the question. This is a very good one. I assume you refer to what we do today. Here, distributed computing is definitely in order, at least for specific experiments.
For what concerns the LHC, most analyses rely on the Worldwide LHC Computing Grid that allows us to store, distribute and analyse the dozens of petabytes of data available (this consists in what has been recorded, most collisions being ignored because of the electronic speed and the much higher collision rates; see here or there for more information). In practice, we are dealing with hundreds of thousands of computers, spread in more than 100 computing centres all over the world.
On my side, as a theorist, I do not need such a huge computing power and I can make my life easy enough with O(100) CPU cores for most of my research work.
Thank you @lemouth for the post. I have had a sumptuous meal of introduction to particle physics. I will be glad if you could spare some of your precious time to throw more light on two things that seems unclear to me in the post viz:
My question about your statement above is this: If Standard Model physics is accurate and infallible its prediction and explanation of what goes on in the universe, what is the motivation for probing beyond it?
My second question has to do with a statement you made in passing about humans. I know that is not the focus of the article and that you acknowledged that the statement is debatable, nevertheless, I would appreciate if you could explain what you mean by
This is important to me because I think and believe that humans are the most important thing in the universe. Everything else in the universe exist because of us!
The Standard Model works well, but it includes conceptual issues and has limitations. For instance, there is no dark matter in the Standard Model, or neutrinos are predicted to be massless (which is contradicted by experimental data). For that reason, we believe that the Standard Model is only the tip of an iceberg. The challenge is to get information of the hidden part of this iceberg.
Note that I purposely didn't enter into details, as this will be the topic of one of my next blogs.
I was actually referring to the fact that many humans behave as they would be kings of the universe, and do crazy stuff for that reason. In fact, humans are just nothing compared to everything that lies in the universe (and we actually do not even know precisely what most of this everything is). For that reason, I believe humans should be a little bit humbler once in a while.
Does it clarify? Do you agree?
Thank you for the clarification. I got your first point but I have reservation about the answer you gave to the second question. Yes, I agree that human should be humble and stop doing crazy things but where I disagree with you is the part where you say:
I am convince humans are truly the kings of the universe! Yes, we should be humble and not do crazy thing but we are still the king and will remain so as long as the universe exist.
You probably we have to write your proposed post sooner than planned to educate me and others like me on your stand.
I strongly disagree with you.
How can you say that humans are the kings of the universe, once you account for the fact that we do not know nothing about the universe? Moreover, I recall that in terms of exploration, we never went very far.
In other words, for which reason(s) do you consider that humans have all rights on the universe?
It is for the simple reason that the universe exist to serve humans as a matter of fact. The sun and the planets all exist to serve human.
Humans are the only personality capable of exploring and manipulating the universe. Humans are capable of harnessing the marvels of the universe to serve its own end. Therefore human is the king of the universe.
I await your blog on the subject. I am willing to see from your perspective.
There is no proof of that. That's a belief, and not a scientific statement. By the way, the sun and the planets were there well before humans. That's a fact :)
This statement cannot be proved as well. The universe is so vast and we only checked a little (negligible in fact) part of it.
PS: I hope you don't mind, but I am tagging @agmoore as we debated on the same sentence of this post earlier.
Checking a little or negligible part of it is called exploring and manipulating!
Perhaps you need to answer these questions: (1) Why does the universe exist? (2) What is the purpose of your research as a physicist?
Since the Manhattan project, particle physic has attracted the brightest minds because this is where the money went, building an atomic bomb. But the important objectives have been completely missed. We still don't know how to bring fresh water to thirsty children in Madagascar, we don't know how to provide a cheap energy that could prevent climate change.
By not working on the important topics, particle scientists will be responsible for million of deaths in the coming years.
That's not the role of particle physicists. Not all scientists should work on the same things, and not every one is good at the same things. Moreover, who are you to decide what is important and what is not? I am not saying helping Madagascar children is not important. I only say that there is no relation with particle physics research, and that the fact other problems are urgent is not a good reason to forget about all the rest. Why cannot both be addressed, especially as both require different skills? That's a point you seem to forget: science is vast and so specialised that it is impossible to be expert everywhere.
PS: particle physics it not about building a bomb... You didn't get it right at all.
@chrisaiki particle physic is neither good nor bad. It's like many things, what we do with it that is good or bad. Maxwell in the 19th-century didn't work for the Manhattan Project as far as I know ;)
Do you know the VOSS system (concrete flywheel to store solar energy)? Do you know the ITER project (electricity generated by nuclear fusion). Science itself is dependent on its own progress.
If today we are able to make more and more advanced simulations (rising waters, climate change, fluid mechanics and so on) it's thanks to a ridiculously small component called transistor that comes straight from particle physics (quantum mechanics).
Science is part of knowledge, to transmit knowledge to those who don't have access to it is one of the key.
Let's stop being hypocrites, for those who die of thirst and hunger every day, we are all responsible for the simple fact of our inaction, no need to look for another culprit than ourselves. If 1 million people went to Madagascar with a shovel, how long would it take to bring them water?
I fully agree with most of your remarks, except that science itsetf is dependent on its own progress.
None of the involved scientists in the Mahattan project decided on their own to work on the atomic bomb.
The oil industry and the tobacco industry have falsified science for decades.
I am not speaking of big pharma who prevent public researchers to study phytotherapy.
I am a European citizen and I would like to have my word about how public money is spent.
Having a word or an opinion is different from deciding as being king of the world.
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