Tuesday 12 Sept 2023 – 12:30
modemily
Hello, welcome to the CERN live Chat!
Patrick
I’m Patrick, and I do nuclear physics at ISOLDE (Isotope Separation Online DEvice)
Mary
Hey everyone, I’m Mary! I’m a 3rd year PhD student at the University of Edinburgh, currently based at CERN to work on the LHCb experiment 🙂
Vichayanun
I’m Vichayanun, a PhD student from Bangkok working with @Joel from CMS Collaboration.
Vichayanun
I’m also in the process of graduating very soon, so wish me luck!
Jenny
Hello! I’m Jenny and I study particles with charm quarks at the LHCb experiment. I also work on detector development.
Joel
I am an academic at the University of Bristol, and I mainly work on tracking particles at the CMS experiment.
Dan
I also work on LHCb, now in my 4th year of my PhD, i am looking for rare processes that shouldn’t exist in our model of the universe but if they do, that’ll be exciting!
Gustavo
Hi! I am Gustavo and I’m working on the design of future accelerators.
Dora
I’m Dora and I am a PhD student in accelerator physics
NicholasS
@Jenny Are there any unique properties of charm quarks?
Jenny
@NicholasS: Yes! Charm quarks are one of the heavier partners of up quarks (along with the top quark). Charm is unique as it is the only one of these up-type quarks to form mesons we can study oscillations in (mesons = particles made of two quarks).
Jenny
@NicholasS: It is also interesting as it is one of the hardest quarks to make theoretical predictions about, so the data that we measure in our experiments is very useful for developing our understanding of the Standard Model.
LeilaA
What does CERN do to be more green?
Dan
@LeilaA: Currently at LHCb we reclaim some of the waste heat from the experiment and use it to heat water for the nearby town… and i believe CERN is planning to do more reclaiming like this in future
Joel
@LeilaA: The main thing it does is to make the accelerators themselves more efficient, as these are the biggest component of the power budget
Patrick
@LeilaA: They write a biennial report about their environmental efforts: https://hse.cern/environment-report
LeilaA
@Patrick: thank you I will look at that.
Edward P
A couple of years ago, I was at a talk and the speaker was Harry Cliff. He said that everything (or most things) we thought we knew about physics is wrong. If you agree, could you elaborate on this? How does this affect your work?
Mary
@Edward P: This is an interesting question because we don’t actually know how good our current best theories are unless we test them extensively. The Standard Model of particle physics is what we are probing at the LHC to extremely good precision
Mary
@Edward P: The standard model holds extremely well for a lot of things but there are things it doesn’t explain, for example the nature of neutrinos (partners of electrons) is not well understood. We also don’t have a description of dark matter
Edward P
@Mary: Are there other models of physics as well as the standard model? If so, how are they different?
Mary
@Edward P: one of the favourites that hasnt been observed yet is supersymmetry – this means every particle we have in the standard model has a ‘supersymmetric partner’ particle. We’ve not seen this, though!
Edward P
@Mary: Although you haven’t seen the “supersymmetric partner” particle, how would you describe it? Is it the exact same, or is each supersymmetric particle different to its partner, and if it is, how so?
Mary
@Edward P: So they would be similar, but particles have a property known as ‘spin’. We have fermions that have half-integer values of spin, and bosons have integer values of spin. In supersymmetry the values swap
Mary
@Edward P: This is a bit of a complex idea, but the supersymmetric fermions would go to integer spin, and supersymmetric bosons to half integer spin. They would also be a lot heavier than the standard model particles
MaxB
@Mary: What do you mean by spin and half spin in this context
Mary
@MaxB: so spin is a quantum mechanical property of particles – it is not easy to visualise I’m afraid.
Mary
@MaxB: one of my favourite ‘physics memes’ explaining spin – “imagine a ball is spinning except its not a ball and its not spinning”
Mary
@MaxB: It is something that all quantum mechanical objects have, it’s called a ‘quantum number’
MaxB
@Mary: So spin is the angle at which these orbit types are at?
Mary
@MaxB: not quite, sorry I’m finding it quite hard to explain XD it is related to the orbit and particles angular momentum, but exactly how requires a course in undergraduate quantum mechanics 🙂
MaxB
@Mary: I do not yet have that! hope to do it sometime in the future.
Mary
@MaxB: yes dont worry!! This is complicated stuff that requires a lot of study to understand fully
MaxB
@Mary: Thank you!
Joel
@MaxB: Spin is a property (quantum number) of an individual particle, such as an electron. Every electron in the universe has spin-1/2.
MaxB
@Joel: As in 1 or 2 electrons?
Joel
@MaxB: I meant that every single electron in the universe has a spin value of one half. This is just as fundamental as it having an electric charge of -e.
MaxB
@Joel: Wheer did this spin value get found out about? What does this one half mean?
Joel
@MaxB: The concept of spin was really figured out in the early 20th century by theoretical physicists such as Paul Dirac, and some great experiments (check out Stern-Gerlach).
MaxB
@Mary: When does this phenomenon occur?
MaxB
@Mary: That is very odd
MaxB
@Mary: Ah, so I know this from Chemistry – objects are electrons and other subatomic particles, and their orbit / location is l, for example s or p and their quantity?
Mary
@MaxB: yes this is the same thing! 🙂
Vichayanun
@MaxB: Basically these s, p, d, and f orbitals can be explained using quantum mechanics.
Joel
@Vichayanun: And if electrons were bosons they would all live in the same orbital and atomic physics would be very boring
Vichayanun
@MaxB: And with quantum mechanics we also know that there can be more types of orbitals than s, p, d, and f, but we haven’t had an atom with that much electrons yet.
Mary
@Edward P: (but I’m not a supersymmetry expert so that is the limit of my understanding I’m afraid!)
Edward P
@Mary: Thank you it is all very interesting! What is a fermion and boson? How would you describe their features?
Joel
@Edward P: Particles are allowed to have a spin quantum number that is any multiple of 1/2: 0, 1/2, 1, 3/2 etc. Bosons are particles that have integer spin values, the others are fermions. They have very different behaviours
Dan
@Joel: I guess a few examples help, the higgs boson is, unsuprisingly a boson but all quarks and leptons are fermions
Vichayanun
@Joel: For example, fermions are not allowed to be in the same energy level, while bosons can.
Mary
@Edward P: But this is really seen as an extension of our current Standard Model (since it holds up to experiment so well in a lot of areas)
Joel
@Edward P: Most of the “models” we look for are extensions or supersets of the standard model. There are a few that aren’t (e.g. string theory)
Patrick
@Edward P: I think it would be better to say that our knowledge is incomplete rather than “wrong”. As scientists, we devise models to explain certain phenomena, and then stress-test them using experiments. We know that general relativity and quantum mechanics don’t fit together nicely, so we know that they’re “wrong” in that sense, but I’d argue that our knowledge is incomplete instead – a bit like how Newton’s theory of gravity is not the whole story, but fails when trying to describe really big heavy things
Edward P
@Patrick: Could you explain a bit about quantum mechanics and why they don’t fit together nicely?
Patrick
@Edward P: it’s outside my area of expertise, but my understanding is that general relativity (GR) is very good at describing big and heavy objects, and quantum mechanics (QM) is very good at describing small and light things. When you try and bring the two together, you can’t use the theories as they stand at the moment, and you need some kind of quantum gravity or string theory to fill in the gaps
Patrick
@Edward P: One major marker of the breakdown between these two is the “cosmological constant problem”
Edward P
@Patrick: Could you explain a bit more about the “cosmological constant problem”
Patrick
@Edward P: I’ll give it a go…there’s something called a vacuum energy density, which is how much underlying energy exists in a given volume of empty space. This is described by Einstein’s cosmological constant in GR, but can also be predicted by quantum field theory (QFT). Comparing these two theories leads to a discrepancy of between 10^50 and 10^120 (i.e. 1 with 50-120 zeroes afterwards). This shows that the theories, as they stand, are incompatible
Edward P
@Patrick: What is “empty space” and “underlying energy?”
MaxB
@Patrick: Did you learn this from Uni or your own research?
Patrick
@MaxB: University a good 7 or 8 years ago now!
Patrick
@MaxB: My research is not on this at all – I work on nuclear physics now, and less of these fundamental physics problems
MaxB
@Patrick: Wow, so finding out about things to do with atoms and their makeup, or fusion and fission?
Patrick
@MaxB: I stick neutrons to atoms to work out how the protons and neutrons in the nucleus are organised. It’s only in recent years that we’ve been able to do this on neutron-rich = unstable nuclei. Probing at the limits of nuclear stability enables us to improve our understanding of how nuclei fit together!
MaxB
@Patrick: As in you disrupt the nucleus arrangement to see how it changes?
Patrick
@MaxB: We fire a heavy nucleus at a 2H target (proton + neutron). The neutron sticks to the nucleus in a single step, and we measure the energy of the proton. Using energy and momentum conservation, we can work out the properties of the neutron in the final nucleus from the energy and momentum of the proton. This gives us a direct probe of the nucleus. If you do this along a chain of isotopes, you can see how the structure evolves as you add more and more neutrons
MaxB
@Patrick: That sounds very impressive
Patrick
@MaxB: The big picture of science experiments is often very impressive – the day-to-day nitty gritty looks a lot less glamorous!
MaxB
@Patrick: How do you isolate these nuclei to carry out these experiments?
Patrick
@MaxB: We take 50% of CERN’s protons (the rest go to the LHC) and fire them at a target enriched with a heavy nucleus e.g. uranium. This produces loads of nuclei. We then use electric and magnetic fields, as well as some lasers, to select the nucleus we want which we can then use as a beam to fire at our target.
Patrick
@MaxB: I should also mention that the 2H is just in a very thin plastic target, replacing the standard 1H in a polymer with 2H
MaxB
@Patrick: I am not too sure what difference this maces, but it sounds interesting
Patrick
@MaxB: In the reaction, it is the source of the proton and the neutron
MaxB
@Patrick: Wow, thanks
Patrick
@Edward P: That’s a good question, and stretching my knowledge 😅. If you imagine taking a region of space, and trying to remove all of the particles, cooling it down and making it as non-energetic as possible, that region will have a total energy equal to the vacuum energy. It’s impossible to have a region of space with “no energy” in it
Edward P
@Patrick: I just want to say thank you as this has been very informative (although it might take me a while to get my head around it😅 )
Patrick
@Edward P: To be honest, I’m not sure I ever fully got my head around it 😂
MaxB
Please confirm my knowledge: The LHC is the large hadron collider that hits protons together at almost light speed to break them apart to see what they are made of?
Vichayanun
@MaxB: Basically yes. For a more detailed explanation, protons already contain a bunch of particles like quarks and gluons which can interact when collided together and give rise to new particles via particle interactions.
Vichayanun
@Vichayanun: Also the speed at which we accelerate those protons are so high that when we add more energy to those particles the speed do not really increase anymore, according to Einstein’s Special Relativity.
MaxB
@Vichayanun: So is this for research to build a clearer model of the makeup of atoms, and are you planning on using this knowledge in some way?
Vichayanun
@MaxB: I’m pretty sure that we already have a clear picture of what protons are made of, which are quarks and gluons tying those gluons together. However, we still do collide them to see some rare processes that gives particles normally not in atoms.
Vichayanun
@MaxB: Like interactions giving top quarks and charm quarks for example. We do not usually see them in everyday matter.
MaxB
@Vichayanun: Is there an aim to this, as in to use this knowledge for a wider benefit?
Vichayanun
@MaxB: Some groups of physicists are also looking for hypothetical particles like dark matter or supersymmetric particles and see if our current Standard Model is wrong.
Vichayanun
@MaxB: As for the plan to use these knowledge, I don’t really know about that yet, but just look at electricity. During the time when electricity is discovered no one had any idea of what they could use it.
Vichayanun
@MaxB: These days we can chat over a long distance using the knowledge of electricity!
MaxB
@Vichayanun: I understand – so you are trying to improve our knowledge in the hopes of it to be put to use, not essentially immediately?
Vichayanun
@MaxB: So I would speculate that in the future someone would be able to use these knowledge to apply in a new technology in like, I don’t know, hundreds of years?
Vichayanun
@MaxB: But the side benefits from our experiments are already there, since we also have to overcome a lot of technological issues during our experiments, like how do we share the information or how do we design a detector that can withstand radiation.
MaxB
@Vichayanun: Thank you, that explains some things.
Vichayanun
@MaxB: So let’s put it this way. We are trying to understand what our universe is made of, but we do not have hopes that we can “use” the knowledge immediately or even in like a decade.
MaxB
@Vichayanun: Very nice
Joel
We hope that most of what we “know” now is wrong, as that is how science moves forward. (Of course, Harry might have been referring to school-level physics that is often only approximately right)
LeilaA
Do you think that the 100 km collider which could potentially be built is worth it for the amount of money it would cost?
Mary
@LeilaA: Personally, yes. There are many technological advancements that come from these ambitious worldwide projects – the first web server came from CERN! But not only that, the jobs it creates and the opportunites you can have access to
LeilaA
@Mary: Thanks
Joel
@LeilaA: I will let the others answer this – I will be retired by the time it is built (_if_ it is built)
Jenny
@LeilaA: This is a widely debated question. To build the 100km collider we need lots of technology developments, some of which may benefit other parts of society, for example medical physics.
Dora
@LeilaA: There are many different options under consideration for future colliders, the 100km FCC is only one of them. They all have their benefits and drawbacks, but the interest in building one or more of them is high because there are certain things we just cannot do with current machines. So if we want to try to push for new physics we need to build new machines
Mary
@LeilaA: For instance at CERN, there are non-physics related opportunities for jobs that allow people from all over the world to network and share knowledge – there is great value in this!
Gustavo
@LeilaA: I think it is! It is in the right direction to allow experiments of the future to keep exploring new phisics. A large sum of this money is spent developing technologies that will end up improving mecical, and material sciences amongst others
NicholasS
@Jenny When you say that the charm quark is partners with other quarks, what does that mean exactly.
Jenny
@NicholasS: Our everyday particles (protons and neutrons) are formed from up and down quarks. These have a small mass. We also know there are four more quarks, two of which are like the up quark but heavier (charm and top) and two are like the down quark (strange and bottom).
Jenny
@NicholasS: They are partners in that they share some properties such as their electrical charge (+2/3 e for up-type quarks and -1/3 e for down-type quarks).
Jenny
@NicholasS: The different pairs of quarks (up-down, charm-strange, top-bottom) are sometimes called the three ‘generations’.
NicholasS
Do you imagine there will be successful fission energy within the near future?
Patrick
@NicholasS: They’re making progress…hopefully it will arrive within the next century, if that counts as the near future 😂
NicholasS
@Jenny Thank you for your explanation
MaxB
Great, thanks both
Edward P
@all What is the most challenging problem you have faced this year in your work? If you have solved it (or are in the process of solving it), what did you do?
Vichayanun
@Edward P: It might not be related to Physics, but in my case it’s how to write my PhD thesis and submit to the committee before the deadline.
Edward P
@Vichayanun: 😅 What is your thesis about?
Vichayanun
@Edward P: My thesis is about finding a very rare particle interaction involving four top quarks within the CMS detector and using that interaction and neural networks to measure a constant in the Standard Model called top quark Yukawa coupling.
Dan
@Edward P: fab question, I’ve been trying to figure out and completely understand how we correct our “simulations” of our experiment to look and act as much like real data as possible… many times I’ve thought that I’d understood it and then tried to explain what I understand it it all unravels 🙂
Edward P
@Dan: So can simulations, intended to provide realistic data, sometimes provide data very far from reality? Could you give an example of one of your simulations?
Jenny
@Edward P: Hopefully the results of the simulation are not too far from what we observe in the detector! I also work on simulating photons travelling via total internal reflection through a thin glass plate. We also model how the photons are detected.
Dan
@Edward P: Yes! So the simulation is there to emulate what your detector would do, but this depends on how well you understand your detector! For example, we have a complex particle identification system that, in simulation, isn’t that well described, so we correct our simulation using “standard candles”
Jenny
@Edward P: The simulations can always be improved by accounting for more physics effects and using better inputs, but we also want them to be quick so sometimes a compromise between performance and speed is made.
Dan
@Edward P: where standard candles for us mean very high decay rate particle decays that we can look at in data and compare to in simulation and figure out where things are going wrong
Dora
@Edward P: For me the biggest challenges are usually related to creating good computer simulations. The things I try to simulate take a very long time, so I often need to find a good balance between creating simulations that run reasonably quickly and also still give enough and accurate results.
Dan
@Edward P: But I think I’m there with it now!
Joel
@Edward P: At the moment, I am trying to figure out how to fit an extremely complicated detector into a very small space. The most challening bit is how to connect it up to electricity and cooling….
Gustavo
@Edward P: So this 100km accelerator is actually two different accelerators. One after the other first colliding ellectrons and positrons and then protons. But sharing the same tunnel (saving money). My task was to make the proton proton ring overlap with the electron positrion one in some places down to the micrometer. That was challenging. Took a lot of trial and error as wel as lots of discussions with colleages.
Joel
In many ways, spin behaves like angular momentum. The value of one half means that an electron has half of the angular momentum of a spin-1 particle like a photon
MaxB
@Joel: Thank you – I will do some research and maybe join on in a couple of months once I am more accustomed on the knowledge that you are telling me.
Joel
@MaxB: Feel free to post questions in the discussion forum too – we have more space and time to answer them there
MaxB
@Joel: Thank you – I need to still learn how to use the site!
Dora
From a different perspective: we do a lot of simulations in accelerator physics too. We try to simulate how particles move around in the machine, whether the hit parts of the machine or not, etc. We have some pretty good models of the machines, but in reality the machines are not perfect. Every element in them can and most often does have imperfections (e.g. a magnet with a field that is not quite what you expect). Quite often we don’t know what these imperfections are, so you can find that your simulations don’t mach the measurements exactly. That is when you try to find out where the differences come from to adjust your models.
Edward P
I’ve heard a story where a man accidentally walked into a particle accelerator when it was running. What would actually happen to you?
Mary
@Edward P: yes, the guy you refer to had some health issues related to this but has lived a long life. Not sure how well you’d come away from the LHC, though…
Dan
What kind of subjects is everyone thinking of doing at uni? if that’s your post year 13 plan?
MaxB
@Dan: I am interested in Maths and Physics, as well as entrepreneurship – I think an Engineer may fit the bill
MaxB
@Dan: What courses I should study at uni I still need to research
Dan
@MaxB: I am quite bias towards physics clearly but I recommend doing the thing you enjoy learning about the most at a level !
Edward P
@all Thank you for all the responses. It’s been really interesting
Mary
Thanks everyone for your super good questions – really tested my knowledge today!
Joel
Great questions @all
MaxB
Thank you very much everyone! This hac been very insightful
Patrick
Thanks for all the questions 🙂
MaxB
Bye
Dan
thanks everyone!
Vichayanun
Thanks everyone!
