Choice of specialization for Quantum Chromodynamics PhD?

I am entering into the 2nd year of my masters and I plan to do my phd in quantum chromodynamics. So for my specialization, would it be helpful if I choose nuclear and particle physics or quantum field theory?

Don't you need both?

PeroK said:

Don't you need both?

The thing is in nuclear physics we study some of the basics of article physics and standard model but it's more concerned with reactors and power plants.
in QFT, we mainly focus on scalar field theory and other relate stuff like kelin gordon theory, feynman diagrams etc, without covering QCD and QED.

QFT, no doubt

For QCD you need of course a good knowledge in relativistic QFT to start with. I think, it's a good idea to first study introductory QFT up to QED and then QCD. My own experience is that what one usually lacks from the introductory physics lectures is group and representation theory. I don't know, why this all-important subject is so much neglected in the standard curriculum. One has to learn it for oneself on the side :-(. So I think it's a good idea to study this first. A very good book, giving just what's needed for a physicist aiming at relativistic QFT:

R. U. Sexl and H. K. Urbantke, Relativity, Groups, Particles, Springer, Wien (2001).

Concerning the question, whether to choose particle or nuclear physics as a specialization, it depends mostly on your own interest. I'm a theorist in relativistic heavy-ion physics, where QCD is of course the fundamental theory, and it's applied to many-body QFT, i.e., thermal field theory. So that's also a possibility to get involved with QCD within a fascinating research topic, where one wants to figure out the properties of strongly interacting matter with applications not only in heavy-ion collisions but also in cosmology and astrophysics (neutron stars, neutron-star mergers, and all that).

By chance, just yesterday there was a White Paper on QCD on the arXiv (with the focus on electron-ion-collider physics):

https://arxiv.org/abs/2303.02579

VVS2000 said:

I am entering into the 2nd year of my masters and I plan to do my phd in quantum chromodynamics. So for my specialization, would it be helpful if I choose nuclear and particle physics or quantum field theory?

I hate to be mercenary, but nobody so far has mentioned a really critical point. How does the job market look in each case in the short to medium term?

(In the long term, we're all dead and it is impossible to accurately predict something that far in advance anyway.)

I don't know what the economics look like right now, and this forum is probably not a great place to get the best information on the economic side of your decision.

But, your job prospects aren't such a horrible thing to think about. If you had a really strongly held preference already, based upon the nature of the work in each of these specialties (with which you already have much more familiarity than the average person, even within physics generally), presumably, you wouldn't be asking about it now.

If you don't have a firm direction at this point, I'd ask people in each specialty what they do and don't like about their choice. Also, ask them what their take is on the economic prospects in their specialty.

The best people to ask are probably the folks in PhD programs who are a year or three ahead of you, and post-docs, as they are the most likely to have their fingers on the pulse of exactly what the job market looks like in their specialization.

Principal investigators outside of academia, senior scientists in private industry, and tenured professors, are mostly too far removed from your part of the job market to have accurate and up to date information about what the job market looks like now in their fields.

Junior scientists and assistant professors are in between when it comes to reliability and the timeliness of their knowledge, but may be more comfortable talking about it to you, since they have generally come to peace with their decisions by now and don't see you as competition.

I recognize that you probably didn't go into QCD physics because you love networking with people.

Networking with your immediate seniors in QCD may seem like a chore that you'd prefer to avoid. But doing your best to network with them, and getting out of your comfort zone to get the best quality of information possible to make this choice, in the most informed way possible, could greatly impact your life for the next several decades. So, the social anxiety/inconvenience that comes with this kind of networking is worth it.

Also, of course, you aren't networking with just anyone. These are "members of your tribe", fellow QCD physicists, with whom you have far more in common than the average person you meet at a graduate school social gathering. And, you will need to network on physics issues with them for decades to come, no matter which specialization you end up choosing, so even if the networking you do doesn't produce immediate useful results, is worth the effort involved in this networking, even if it is a big drag for you, at this point.

Ultimately, some key issues both in terms of work life experience and economic prospects differ materially even between sub-specialties within the main branches of QCD physics. So will whether your employer is academic, a non-profit research lab, governmental, or a private sector employer.

But, the decision you are making now is one of the important immediate steps at which you will make a choice that determines what you will end up doing in your early career.

You made early choices in your secondary education, more important choices in choosing your undergraduate institution and major, and more choices still in make the decisions that went into pursuing graduate eduction in the field, and at the institution, that you ended up at.

You'll make another big choice choosing a specialty now.

You'll make yet another big choice when choosing a dissertation topic, and yet another pursuing your first real job in your field.

The decision you are making now, however, is probably the most free and unconstrained choice about your educational and career path that you have left to make.

Once you choose a specialty and your PhD advisor is assigned, this is going to greatly influence what topic you do your dissertation on, which will influence in turn what kind of first real job you end up chasing, and these choices combined, both of which will be quite constrained by what is available at the time, will heavily influence which sub-specialty you end up in.

Despite the fact that your final sub-specialty choice is important, the final sub-specialty choice you make is likely to be pretty happenstance. At the level of sub-specialization within these top level categories, what you end up specializing in usually turns out to be path dependent and based heavily upon who your PhD advisor is and what you do in your first post-docs and/or permanent jobs.

Another legitimate issue to consider is geography. If you want to become an experimental particle physicist, this pretty much means that your next step after your PhD, and possibly while pursuing it as well, will be one of the half dozen or so major particle colliders in the world, and within those handful of choices, you will also be heavily biased based upon considerations like which languages you speak well enough to function in.

If you pursue nuclear physics, there are a lot more places you could end up, but you're still limiting yourself to a fairly modest subset of countries and institutions.

In principle, you can do QFT almost anywhere that has a more than bare bones graduate physics program. This is a roster of places you probably already became quite familiar with when applying to graduate programs. But, in QFT or any other academic career path, the institution you work at is only half of the story.

At this point in your graduate education, you need to be starting to get to the point where you are developing a short list of senior physicists who are doing active research who you would like to work with or under. It is all about "who" you will be interacting with in your research pursuits on a day to day basis, and not where you happen to be.

If you aren't starting to develop a short list of favorite physicists in your field now, you need to start reading preprints on arXiv in QCD related fields more often. You also need to find graduate students and scientists who are talking about new research in your field and start becoming a part of those discussions, even if you are just a lurker in their discussions at first. And, you should be keeping your ear to the ground about gossip and conference scuttlebutt about these people for the time being (even if your parents beat it into your head growing up that following gossip is morally wrong, this one moment in your life is an exception to that rule and there is really no other good way to find out about the many hidden landmines that are out there waiting to make your life miserable for a few years).

How much time should you be devoting to networking and getting a sense of who in your field you would like to be working with or under?

At this critical moment in your life, you should probably be devoting as much time to these two tasks as you do to a part-time job or to a single course at a time in graduate school: maybe ten or fifteen hours a week on average for a semester or two. Based upon the experiences of my children, their peers, and other people I know who have gone down paths like these in recent years, that is how much time it takes to really do it right and to see your efforts really pay off.

If you have a significant other (SO), you should also start having some discussions with that person about all aspects of this decision even if it is beyond the depth of what your SO can really understand. Try to explain in terms that your SO can understand for each scenario, what the work will be like and how you feel about that, what your economic prospects will be, and what kind of places you are likely to end up living. This is true even if your SO really has nothing to contribute to the decision you make.

First, you do this because talking things through with someone you trust, and trying to simplify and explain the issues you are dealing with to someone less familiar with these things, can help you clarify your thoughts and decision making. Even if you don't have a SO, you should be talking your ideas through with trusted friends, family, or mentors for the same reason.

Second, you should do this because your SO won't feel "out of the loop" when a decision that affects both of you is ultimately made. If you shared your thoughts and listened to what your SO had to say, even if that input wasn't decisive, your SO will be able to take some ownership of whatever reasons came with your ultimate decision, and will also be more supportive, whether your decision ends up being a good one in hindsight or not. This is important for the quality of your long term relationship.

But, if all of these considerations seem like too much, and it is making you so anxious that it is giving you decision making paralysis (this is a big deal for some people and a non-issue for others) because you want to be certain you get it right, don't sweat it too much either.

Anybody who can hack it well enough to make it into the second year of a master's program in QCD physics isn't too far off the path that they belong upon. This part of STEM is one of the most ruthless disciplines of all at culling out people who don't belong there and you are not one of them.

There isn't necessarily one right choice, although there might be one or more wrong choices for you. Avoiding a choice that is clearly wrong for you, if there even is one at this point, is far more important than making the the "one right choice" among several basically O.K. possibilities that you can learn to like over time.

The only person you are really answering to for your choice is you (and maybe your SO), so it is O.K. to make your pick from your finalist options on a basis that may seem whimsical or arbitrary.

If you've always wanted to learn to sail, and your choice is between a PhD program at CU-Boulder in Colorado in hadron physics and a nuclear physics PhD program in the Bay Area of California, there is nothing wrong with choosing the program where you can spend your free time sailing instead of skiing.

If the professor who inspired you to love physics went to Carnegie Mellon where your favorite nuclear physics researcher teaches, it is perfect fine to decide to pursue that path, rather than a QFT specialty in Waterloo, Canada at the Perimeter Institute, for this nostalgic reason alone.

Research has shown that making your final choice based on gut instinct instead of logic, after doing some basic due diligence first, tends to leave people feeling better about their decision in the long run, than making a final choice based upon cold hard logical analysis and analytically weighing the pros and cons of your choice.

So, go talk to people in each specialty. Talk it over with people you trust. Muse over all sorts of considerations that are partially whimsical and may seem irrelevant that are personal, geographic, money, and relationship driven, in addition to thinking about the nature of the work involved. If necessary, take a few days off or a vacation away from anything that is part of your usual work and study routine, where you can have room to let your mind drift and make subconscious connections between all the data you've soaked up to provide you with guidance in making this decision.

Then, when you reach a point where you have to make a decision, or you reach your own self-imposed deadline, go with your gut, make a choice, and don't look back.

As I've told my children (who are now finished with their STEM field educations from top schools with good grades, and are just starting out in their careers at first or second real jobs out of college that are better than it should be possible to have for people in their young twenties), you don't want to be a "flat squirrel" who ends up getting run over by a car because they can't decide which way to jump. Almost any choice will be better than making no choice at all.

Or, if you are someone who almost inevitably seems to change your mind as soon as you make a decision, make that first gut decision sooner rather than later, so that you have some slack to change your mind once or twice before your decision is absolutely final if your gut starts churning.

ohwilleke said:

I hate to be mercenary, but nobody so far has mentioned a really critical point. How does the job market look in each case in the short to medium term?

I'm working in theoretical relativistic heavy-ion physics, and we have a lot of BSc, MSc, and PhD students, working in various subtopics, all related to QCD. Most of the research is pretty much concerned with numerical simulations, but there's also considerable analytic work using relativistic QFT (thermal and off-equilibrium many-body theory), quantum transport equations, Fokker-Planck and Langevin equations, relativistic (magneto-)hydrodynamics, as well as general relativistic applications in the physics of neutron stars and neutron-star-neutron-star, black-hole-neutron-star mergers.

All this sounds pretty far from any direct "practical applications", however you don't need to worry about your chances at the job market. I don't know a single absolvent (either on the MSc nor the PhD level), who didn't find immediately a job after graduation. Also the variety of jobs is remarkable: They went into technical development (optics industry, autonomous driving) or into various kinds of consulting (mostly IT/software development).

I think it doesn't matter too much in which branch of physics you work. What's appreciated by the job market are the skills in applied mathematics and the ability to quickly work into the various problems related with these skills, being critical in evaluating outcomes of simulations and being "frustration tolerant". Thus, if you want to pursue a PhD in physics, don't look too much at the job market but for a topic, which really interests you to know about.

ohwilleke said:

(In the long term, we're all dead and it is impossible to accurately predict something that far in advance anyway.)

I don't know what the economics look like right now, and this forum is probably not a great place to get the best information on the economic side of your decision.

At least in Germany it's a golden age for employees. There's a lack of people with STEM expertise, and all kinds of companies are glad to find well-educated people in this area.

... and ... you can always become a teacher afterwards ;)

malawi_glenn said:

... and ... you can always become a teacher afterwards ;)

That's not a given. As usual, that will depend on market conditions when the OP is looking for employment: number of openings vs number of candidates.

In Germany you can become a teacher almost without any prerequisites. In the STEM subjects the lack of teachers is that large that they ask us to advertise to 1st-semester students to become a substitute teacher. It's of course not a permanent job, and one better doesn't think about the quality of the teaching...

ohwilleke said:

I hate to be mercenary, but nobody so far has mentioned a really critical point. How does the job market look in each case in the short to medium term?

(In the long term, we're all dead and it is impossible to accurately predict something that far in advance anyway.)

The OP is in the second year of a master's program and is planning to complete a PhD. So the OP is several years away from entering the job market. I've pointed out several times previously that job markets can invert within a year or two. During my career, I've personally experienced three major market inversions (I'm in the US): the semiconductor meltdown of the early 1990's; the InterNet Bubble Burst of the early 2000's; the financial crisis of ~2007/2008. And look at the wild market swings caused by the COVID pandemic, from ~March 2020 to now.

I'll cite one concrete example (which, unfortunately, is not an isolated one). In the late 1990's, towards the maximum inflation of the InterNet Bubble, there was a shortage of scientists and engineers in the field of optoelectronics. At the time I was volunteering as an industry mentor for STEM students. As of late 1999, I had colleagues calling me up to see whether I could send promising grads their way. By ~mid 2000, new hiring had ground to a halt. And by ~mid 2001, there were massive layoffs across the industry.

So my advice to students seeking a PhD in Physics has been this: the PhD program needs to have value in and of itself; it is not necessarily a stepping stone to a long-term (whatever that means) career as a research physicist. A PhD in physics will lay a strong, broad-based foundation for jobs in many fields ... if you are willing to be flexible, if you are willing to adapt, if you are willing to pivot.

vanhees71 said:

In Germany you can become a teacher almost without any prerequisites. In the STEM subjects the lack of teachers is that large that they ask us to advertise to 1st-semester students to become a substitute teacher. It's of course not a permanent job, and one better doesn't think about the quality of the teaching...

I'm in the US. Just to clarify, I'm talking about teachers in elementary and secondary schools, not university professors. In the US, there is also a shortage of teachers (also particularly in STEM), but also a shortage of funds. Here, local public schools are heavily funded by local (municipal) property taxes, along with state and federal aid. A while back there was such a critical shortage of STEM teachers in my state, that the state offered a special training program for people who had a STEM background, but not the usual teaching background (such as a degree in education). The program took about two years to complete. Three of my colleagues completed the program ... just as a severe financial crisis caused many school districts state-wide to layoff teachers.

CrysPhys said:

That's not a given. As usual, that will depend on market conditions when the OP is looking for employment: number of openings vs number of candidates.

One should not take "always" too litterally. After all, the sun will swallow the Earth at some point in time :D

malawi_glenn said:

One should not take "always" too litterally. After all, the sun will swallow the Earth at some point in time :D

Sure. But "always" does imply at least a high degree of certainty (however you wish to define it) that, upon completion of a PhD in physics, you can land a job as a teacher. That may be true of specific locales at specific times (Reply #10 indicates that this is true in Germany at this time). It was true of my state in the US at specific times in the past (but not at other specific times in the past and not at the present). We don't know where the OP is (at least he has not indicated so in this thread or in his profile; I'm not rooting through other posts). Regardless, we don't know what his options for job locations will be several years from now when he completes his PhD, and what the market for teachers (or any other job) at those locations at that time will be (which is what really matters).