Ask the particle physicist

Miscellaneous and Personal Stuff I Must Share
21

Ok, I have a fairly simple 2 part question.

If all of the mass in our solar system were able to be measured, including all the possible objects in the Ort cloud, comets, dust, every bit of mass. What would the Sun’s mass represent as as percent of the total mass?

Part 2: Could the gravitational effects of ‘dark matter’ in the observable universe be explained by planets, dust, comets, etc, around every other star? If the star formation process leaves a certain amount of byproducts behind, is that enough to account for the dark matter?

Is dark matter just a large amount of detritus, or is the mass needed to explain dark matter too large to be explaned by every star having a certain percentage of it’s mass hidden as left over bodies such as planets, etc.

22

- What’s the next step if the LHC doesn’t find evidence of the Higgs Boson?
- Besides the Higgs boson, what other things of interest might the LHC find?

The LHC serves several experimental groups. The two largest groups, ATLAS and CMS, are using general purpose detectors to search for the Higgs particle, supersymmetry, extra dimensions, and a laundry list of other pet possibilities. ALICE will study quark-gluon plasma, a soup that existed shortly after the big bang and which can be produced by smashing heavy ions (e.g., lead) together. LHCb will study the B meson, which is a good test subject for probing charge*parity (CP) violation. LHCf sits in front of ATLAS to study forward-going products (stuff that blasts out of the front of ATLAS). TOTEM will probe the proton’s structure.

No particle physicist is losing sleep over the possibility that the LHC running ends with nothing new having being discovered. If it’s not the Higgs, it’ll be something else. Whatever gets seen will guide the next step, although a logical progression would be an electron-positron collider tuned to probe more precisely whatever new physics gets discovered. (The trouble with proton-proton collisions is that they are very messy and you also don’t have a constraint on the total momentum involved in the interaction. This is due to the composite nature of the proton – the “spectator” quarks that are not actually involved will carry some significant portion of the momentum of the beam. Electron-position colliders do not have these problems, but it is technologically easier to push the energy frontier with protons.)

Having said all that, if in fact nothing new is seen (again, very not expected), it is not clear what would happen to the energy frontier of particle physics. In practical terms, funding either would or wouldn’t be available for a higher energy apparatus. If it isn’t, people would have to migrate to other areas of particle physics research.

23

- What can you tell us about what you are working on?
- Where are you working, and on what experiment?

I work at Caltech, and I am currently involved in two neutrino experiments (MINOS and NOvA). Both experiments are based at the Fermi National Accelerator Laboratory (FNAL) outside Chicago.

Our goals with these experiments are manifold, but in short: the idea is to measure the properties of a neutrino beam at one point and then again at a point much farther away (~800 km). The composition of the beam will change as the neutrinos travel, and the particulars of the change tell us a lot about the underlying physics. Things we can probe with such a setup: relative masses of the neutrinos; which neutrino is the heaviest; how coupled are the different neutrinos to one another; what fundamental symmetries do neutrino respect; do antineutrinos behave the same way. In addition to this changing-while-propagating approach, we are also looking at the properties of neutrino interactions themselves. That is, when a neutrino hits a nucleus, what all can happen? How often? Do we understand what gets produced? (The answer to this last one is known to be “no” for some types of interaction.) Out of left field is that we will also be able to detector neutrinos from any supernova that happens to occur nearby. Such a data set would be immensely valuable to astrophysicists.

I’ll give a brief description of each apparatus, and I’ll expand based on follow-up questions.

The neutrino beam is produced by first smashing protons into a chunk of carbon. The products of this smashing include particles (pions, kaons) that quickly decay to produce neutrinos. A beefy pulsed electromagnet (185 kA peak current) also bends the intermediate pions and kaons in a generally forward direction so that the neutrinos they yield are heading the way you want them.

And, yeah, that was 185 kiloamps. The boom each pulse makes is impressive.

Anyway, the neutrinos head toward the detectors. (Since neutrinos interact so weakly, dirt and rock is no hinderance to the beam.) Each experiment has two detectors, one on-site at FNAL and another in northern Minnesota. This 800 km of travel is needed for the “change-while-propagating” physics.

The MINOS detector on the far end is 5400 tons of alternating steel layers and readout layers. Here’s a picture. The steel provides the raw mass needed to induce neutrino interactions while the readout layers provide information on the energy and location of particles produced in each interaction. Since different neutrino types lead to different interaction products, we can (usually) determine what sort of neutrino caused a given interaction and what its energy was (both important to the questions we’re after.)

The NOvA far detector is still under construction, so I have no picture of it, but here’s a diagram. It will weigh 14000 tons, but it won’t have any steel. Instead, it is made of thousands of PVC cells filled with liquid “scintillator”. (A scintillator makes light when charged particles pass through. (Many common materials scintillate.) The design is so different from MINOS because we need to identify cleanly when a high-energy electron is produced in a neutrino interaction, and lots of dead steel makes that hard to do. (High energy electrons share their energy reqdily with other electrons in materials and also with photons that are produced during electron scattering, so you end up with a messy blob of particle activity rather than a clear, clean “electron track”. Steel amplifies these secondary interactions thanks to its density. Muons (more relevant for the goals of MINOS) are heavier and punch through the steel to make long, clean tracks. So for NOvA: no steel, and fine-grained cellular readout.)

24

- Which books would you recommend for civilians to get to know your subject(s) better? Either from a generalist viewpoint or more specific to the area of research you are focused on?
I actually don’t have a good idea, since I haven’t read many books aimed at a lay audience. Perhaps others viewing this thread have suggestions?

- What’s one thing that most folks don’t realize about working particle physicists?
We travel a lot. Two reasons: (1) experiments are sited where they need to be, in terms of infrastructure or environment, and you have to go to those places. This might be a national lab or it might be a deep underground mine shaft or it might be Antarctica. And, (2) a single experiment can have dozens or hundreds of people working on it, and you need face-to-face time, so meetings are frequent.

If I had to throw in a couple of other items: No, we don’t wear lab coats. And, we are more normal and well-rounded than many people think. Even well into a cocktail-party-type conversation, if a person learns that you do particle physics, they often give you an awkward stare that reads, “Well, good luck with all that. We’ve clearly got nothing to talk about. I’m going back over here.” I don’t mind this sort of thing too much, though, since anyone that does carry on the conversation as normal is probably someone I’d rather be talking to.

- Where do you guys think you rank in the pecking order of geek-cool, relative to billionaire software/internet developers, astronomers, and other similar folks?
Hmm… I mean, we think we’ve got some pretty solid geek-cred. Software guys don’t intimidate us too much, since we are pretty software-heavy ourselves and typically think “Yeah, I could do that, if I wanted to.” (But then the ones that do do it laugh at us physicists as they fly around in their private jets. :slight_smile: )

25

I gotta say, that’s one cool ass job.

26

- What exactly is a “magnetic bottle”?
Charged particles that move through a magnetic field follow a curved path. If you set up your magnetic fields just right, you can get a particle to loop back and forth in a small region of space – trapped in the “bottle”. A loose analogy would be a marble in a bowl. When the marble goes one way, gravity and the slope of the bowl lead to a force that pushes the marble back toward the center. Same here, although the path is more complicated. (It ends up being a loopy spirally combination pattern.)

- Is there a five day waiting period for an electron gun?
Nope, and no background check, either, at least not at Crazy Eddie’s Electron Gun and Radioisotope Emporium. (I built an electron gun once, by the way.)

- What is spin and how is it measured? I know the math behind it… but what’s physically going on during the measurement process? I don’t mean what’s happens to collapse the wave (though if you have a definitive answer, do share!), I mean how do you detect spin and how do you know when a particle has been spun once, twice, or halfway around?

Me, too! :slight_smile:

Spin isn’t the rotation of a particle. Rather, it is a property (akin to mass, charge, etc.) that behaves mathematically much like “regular” angular momentum (actual spinning), but it is not regular angular momentum. Spin does need to be counted when checking angular momentum conservation. Also, the spin of a particle determines whether a collection of such particles has a wavefunction that changes sign when two particles are interchanged. A consequence of this is that two identical particles with half-integer spin cannot occupy the same quantum state (“Pauli exclusion”).

There is no “spin meter” with which to measure spin. Rather, one must rely on the observational consequences to infer a particle’s spin. If you are looking at a new particle in a particle physics experiment, you can watch it decay. Add up the angular momentum of all the daughter partcles, and you’ve got your initial angular momentum (which is the spin of the original particle unless it had some “orbital” angular momentum, in which case you need to determine that first.) If you can arrange constraints on the orientation of the initial spin (there are ways), then the decay products may come out in some directions preferentially to others, telling you about the spin. (For example: a particle with zero spin has no sense of direction. So, its decay products must come out isotropically (i.e., random overall orientation).)
- How much stock do you put in Super Symmetry (I just finished reading Frank Wilczek’s The Lightness of Being, and he makes a rather convincing case)?
You need something to make any sense out of the Standard Model observables, and supersymmetry is just not that crazy, as these things go. So, yeah, there’s a pretty good case for it.
- Do you tinker with Super String Theory at all?
- What’s the status of the string theory today? Out of favor, still in, or what?
No, I don’t deal with string theory at all. Lots of people are still working at it, but it is still a degree or three removed from touching real life experiments. You’ll find those that think it is going to save the day, those that are indifferent, and those that think it is provably crap.

- If you could be any atomic or subatomic particle, which particle would you be?
Hmm… well, I guess I’d want to be a stable particle, so that limits me a lot. As pedestrian as it sounds, I’ll go with the photon.
- Just how deep do you think this rabbit hole goes, that is, do you think there are more layers to fundamental reality then we currently have theories on (what does your gut tell you)?
I do think there are more layers. It would feel very unsatifying if <your favorite supreme being(s)> came down and said, “So, yeah, that Standard Model thing? Just throw in a dash of oregano, and that’s basically what I wrote down when I designed it all.” One’s reaction would just be, “What? You designed this?!” History has provided us plenty of examples of complexity falling away through theoretical advancement (gravity on earth versus in the heavens, the periodic table of elements, electricity and magnetism, …), and I would bet on that happening some more as we push forward.

- Do you have to buy your own crowbar, or is one company issued?
Company issued.

27

I remember hitting spin in high school physics. I thought then what I think now: if it isn’t actually spin, but just behaves sorta the same except for these big differences, why did they call it that? Why give it a misleadingly-metaphorical name like ‘spin’?

28

I ask this in all honestly, and I promise that I won’t tell anybody if it’s so: You guys are just making it all up, aren’t you?

29

- What made you decide to go into physics, how did you get interested in your current field, and what kept you motivated through the years of graduate school, warnings from others as to the modest compensation you were likely to obtain from your degree, and the high amount of competition for the relatively few research jobs in the field?
Why physics: I have always liked tinkering and problem solving, and I like trying to learn how Nature works, so it was a pretty natural fit. I briefly considered math, but decided after one class of number theory that I just did. not. care. about trying to prove So-and-So’s Theorem. I also considered computer science, but I decided that I could learn what I wanted to learn there on my own. And, justifiably or not, physics had a much stronger reputation for deductive rigor than other scientific fields. So, physics became my undergraduate major.

Four years later, I felt strongly that I wanted to do research and teaching, which basically meant an academic life. So, grad school. My father was the biggest naysayer about the financial repercusions, not because he thought I’d be destitute, but rather that he thought I was squandering an opportunity to make millions in the computer industry or as an entrepreneur (neverminding the financial risk that an entrepreneurial lifestyle has.) But, it only came up once in a while, and my social circle was naturally mostly other grad students, so explicit motivation wasn’t really needed. (I liked solving whatever problem was next, and that was sufficient motivation day to day.)

Competition: to be honest, I just don’t think about it. I definitely hope to secure a tenured position, but if the field decides that there is no room for me, then I will just have to adapt. All I can do is do the best work I can.

Field of chice: a reductionist heart led me to particle physics. That is, particle physics felt the most fundamental. I did not appreciate at age 20 or whatever that “fundamental” is not the only way to get to “interesting”, and I think there are many fields of physics that I’d have been happy with. But particle physics is one of them, so that’s okay by me.

30

I hate to break it to you, but quarks aren’t charming, up, down, on top, or on bottom (though they are strange). Nor does quantum chromodynamics (interactions between quarks and gluons that give action to the strong force) have anything to do with actual colors, despite that it is predicated upon color charge of these particles.

The reason the term “spin” is applied to this characteristic is that it is, like mechanical rotation, a degree of freedom about an orientable axis. Although the particle is not physically rotating (being itself a blob of probability with a locus with well-defined areas of uncertainty and doubt) the general concept is similar. There is actually an interesting backstory on how it became known as spin, a description that Wolfgang Pauli initially rejected but ended up formalizing.

Sorry to butt in; the o.p. can probably provide a more rigorous answer to the question. But it is a good question, not only in the sense of what is really physically going on, but the etymology of physics and technical jargon.

Stranger

31

Would it be worse if he confirmed your suspicions, or if he acknowledged that despite years of enormous mental effort and all-absorbing training, in fact he barely knows more about how the world actually works at a fundamental level than you do?

Stranger

32

- What does your work consist of, and was it what you had in mind when you were in college?
Particle physics experiments have many subsystems, both physical and, er, analytical. On the physical side, there are various detector components, monitoring systems, electronics and readout, data aquisition computers and infrastructure, calibration systems, etc. Each of these needs to be designed, built, commissioned, and maintained. This can lead you all sorts of places. As an example, I once spent many months studying the light scattering properties of a particular blend of mineral oil, because it turned out that the details of how light gets scattered affected how well we could distinguish certain interactions in our detector. (This was a different experiment from the two mentioned a few posts back.)

On the analytical side, there is a lot of algorithm development needed to go from bits streaming out of the detector to a statement like: “There was an interaction involving a muon neutrino with 5.3 GeV of energy. A W-boson was exchanged, and the recoil system included a neutral pion. Probably.” These routines take many person-years to put together and optimize.

A critical tool on most particle physics experiments is a very detailed Monte Carlo ray tracing simulation of the experimental components and particle interactions within them. Such simulations attempt to include every physical process that could be relevant. A design goal of an experiment is to make it such that the simulation is not critical but rather serves as a handy guiding instrument in your analysis building. But, sometimes you simply require a calculation of how, say, an electron will look traversing component X, and that calculation is done by simulating the process of interest. (In the end, you never trust the simulation out right. If you’ve set things up cleverly enough, shortcomings in the simulation drop out of the picture before you reach your real answer, and where they don’t, much effort must be put into assessing a systematic error on the answer due to the residual effects of any shortcomings. When you have to do this, it is often the hardest part of the game.)

So at any given point, I might be working on any of these things. It depends where in the arc of the experiment you look. Then there’s other stuff like mentoring students or working on grant reviews.

- How do you get a job in physics? Is it just a matter of going to school, connecting with the existing physics community, and then being part of the social network, working with existing physicists while a student, or even being groomed for a position as you progress through school? Can someone from outside who has the necessary technical skills (electronics or machining, say) but knows nothing of high-energy physics get a job at a Famous Lab?
To be a Physicist-with-a-capital-P, there is one basic route. College, then grad school, then (almost always) postdoctoral work, and then either get hired into a research scientist track or a professorial track (with later switchover possible, but less common.) Without a Ph.D. in physics, the career ceiling is pretty low as far as doing the physics itself.

On the other hand, we hire in particle physics lots of people with technical skills, and these folks can be well paid and have excellent careers. They don’t have really any input on the physics side, and they are not typically attached to particular experiments. At a big accelerator lab, a large staff is devoted to accelerator operations and maintenance, and those people could be physicists by training but are often not.

- Are there many women in physics? There was at least one female Doper who was in physics and who was hot. It’s that combination of brains and beauty… I’m thinking of Angua? (Astrophysics, female, and South Asian. Man, what a combination.)
There are female physicists, but I wouldn’t say there are many. The imbalance begins way before you would call anyone a physicist, though, e.g. advanced physics and math enrollment in high school.

- Did they laugh at you when you presented your theory? Are you going to show them all?
Just a few more tweaks, and you’ll see… YOU’LL SEE!!!

- If you manage to open up that black hole, would you be a pal and see if I left my car keys and umpteen multiple lost socks in there?
Since a key has about 10[sup]20[/sup] times more energy than the LHC collisions will have, I don’t think the micro-blackholes it might produce will have your keys, but I’ll take a look anyway.

33

Not at all. Feel free to contribute!

34

How much did the LHC cost, and who paid for it?
Most of the $10 billion price tag came from participating European countries, but I don’t know offhand the breakdown. The US contribution is about $0.5 billion.

If all of the mass in our solar system were able to be measured, including all the possible objects in the Ort cloud, comets, dust, every bit of mass. What would the Sun’s mass represent as as percent of the total mass?
The sun is nearly all of it. 99.9%.

Part 2: Could the gravitational effects of ‘dark matter’ in the observable universe be explained by planets, dust, comets, etc, around every other star?
Not in the way you are thinking. Part of the evidence for dark matter comes from the rotational speed of objects around their galactic core. The pattern of speeds suggests that the dark matter is not distributed the same way as the visible matter, which goes against the idea that it is star “shavings”. Your idea that dark matter is just a bunch of rocks and Jupiters does have a name, though: massive compact halo objects (MACHOs). However, there are indirect constraints on what fraction of the universe’s matter can by baryonic (i.e., protons and neutrons), and these constraints pretty strongly rule out MACHOs as the sole dark matter source. (They can be there, but you still need something else.) These constraints come independently from (a) the ratios of light isotopes in the universe, (b) the cosmic microwave background fluctuations (which reflect matter distributions in the early universe), and (c) the pattern of galaxy clusters across the sky.

I remember hitting spin in high school physics. I thought then what I think now: if it isn’t actually spin, but just behaves sorta the same except for these big differences, why did they call it that? Why give it a misleadingly-metaphorical name like ‘spin’?
Addressed by Stranger above. Indeed, the name isn’t too crazy, in that it reminds one that spin is a form of angular momentum (if not due to actual spinning). It sounds like your instructor could have been a little more explicit in his/her explanations.

I ask this in all honestly, and I promise that I won’t tell anybody if it’s so: You guys are just making it all up, aren’t you?
My answer: xkcd: Science . (Here’s the figure being referenced, showing outstanding agreement between observation and prediction for the spectrum of CMB photons.)

It really is amazing that we can say the things we can say about things we can’t see, but if you step lightly and carefully and try not to let any bad assumptions or prejudices creep in, then after a half-century or two you can really get somewhere.

Here’s another excellent plot: Number of light neutrinos. I won’t talk about the details unless someone asks, but basically you smash electrons and positrons together at increasing energy. The rate of interaction goes up and then back down. The exact shape of that increase and decrease depends on the number of neutrinos (which, incidentally, are invisible in these experiments.) You know a priori that “number of neutrinos” should be an integer, but just for the heck of it, you let it vary freely and see what value leads to the best agreement between the predicted shape and the measured rates. That value? 2.984 +/- 0.008. In other words, three, an integer.

35

I think I’m caught up. Let me know if I missed your question, and of course ask more if you’ve got 'em!

Also, just because I’ve given a certain level of explanation somewhere, don’t be shy in asking for a different level (either more or less advanced). It’s impossible to write for all audiences at once, so multiple passes on some items could make sense.

36

Where does particle physics end, and quantum physics begin? Or is their a wide swathe of crossover?

37

Where does particle physics end, and quantum physics begin? Or is there a wide swathe of crossover?
The framework we use for describing particles and their interactions is basically quantum mechanics plus special relativity. In most cases, the energies involved are high enough that simple quantum mechanics is insufficient. However, I would probably treat the term “quantum physics” loosely enough to say that all of particle physics is quantum physics – it’s just a particular application of it. (You might have been after something less semantic?)

38

Which schools offer the best physics programs? Do you have a top 10? Do you have a hottie neighbor who works in the Cheesecake Factory?

39

Which schools offer the best physics programs?
The U.S. News and World Report rankings are usually pretty reasonable, I find, if you keep in mind that the top few rearrange themselves year to year.
Here’s the list for particle physics.
Here’s the list for physics in general.

Do you have a hottie neighbor who works in the Cheesecake Factory?
Nope. But, I did meet the guy who does most of the physics consulting on the show. (He’s a professor at UCLA.) Completely unrelatedly, I attending a live taping of it once, too.

40

Who are your top 10 favorite physicists?

Are you an atheist?

Is Ed Whitten the smartest physicist alive?