ellipsix informatics

Ellipsix Informatics: the personal website and blog of David Zaslavsky.

I'm a graduate student styudying theoretical particle physics, and I also do a lot of computer programming. Find me elsewhere online:

2015
Jul
26

A Virtual Welcome to the Rencontres du Vietnam

In my last post I mentioned how the coast of Vietnam, where the Rencontres conference series is being held, looks amazing — with photo evidence. You might have guessed that that image was a promotional picture, and you'd be right: it came from the 2014 edition of the conference's website.

This one I took with my cell phone:

view of Quy Nhon including the beach and mountains

Guys. Vietnam is really pretty!

I arrived in Quy Nhon Saturday morning after an overnight trip from Wuhan, about 15 hours door-to-door. So despite having a couple of free days before the official start of conference events tonight (Sunday), all I really managed to do was catch up on some desperately needed sleep, and snap a couple more pictures of the coast.

panoramic view of the beach

Of course, I have been enjoying the food. Vietnamese cuisine, or at least what I've seen of it so far, is not as strongly flavored as what I'm used to in Wuhan, where every other dish is spicy with a thick Szechuan-style sauce. But they do some great things with a more subtle flavor palette. We have all our meals provided at the hotel restaurant, arranged by the conference organizers. Steamed vegetables with garlic sauce, fried spring rolls, beef noodle soup (pho), and many other dishes, all with a distinctly Asian style but still it's a nice variation from what I've been eating the past few months. Don't get me wrong, a lot of Chinese food is great, but even the best food gets monotonous after a little while. I really need to find some good pizza.

beef pho at the conference welcome dinner

The Rencontres conference itself kicked off tonight with a welcome cocktail for attendees, followed by a buffet-style conference dinner (where I took the photo of the pho). Talks begin tomorrow morning. It's a very small conference, only 36 participants, with everyone giving a talk during the week, and lots of unstructured time for discussions. In that respect the structure kind of reminds me of Science Online 2014, which I still count as the best conference I ever went to, so I think this is going to be a good week. I don't want to promise too much, but I'll try to get in more blog posts with the interesting physics results as the week goes on, and I'll be live-tweeting the talks under the hashtag #renviet15.

2015
Jul
23

Checking in after a busy semester

Greetings, readers!

I've been absent from the blog for a while for a few different reasons — between some issues to deal with in my personal life and a bunch of projects for work, I haven't been able to focus on a blog post for about six months. But I thought that streak has gone on long enough. Here's a quick status update:

  • My group has put out a paper, which was just accepted for publication into Physical Review D! This paper is a generalization of the same calculation I did for my PhD thesis, which I'm 2/3 of the way into a series of posts explaining. The third post is coming at some point, I promise.

  • I'll be traveling to two conferences to talk about this paper. First, the Rencontres du Vietnam workshop on heavy ion physics, held at the brand new International Center for Interdisciplinary Science and Engineering (PDF), which is taking place next week. This is the first time I've been invited to present at a real conference! The venue also looks amazing.

    ICISE aerial view

    (image from the Rencontres 2014 website, all rights reserved)

    When the official schedule includes time for "Beach and informal discussions" you know it's going to be good!

    The week after that, I head back to the US for the APS Division of Particles and Fields meeting in Ann Arbor. The DPF meeting is not a major conference in my field — in fact, I fully expect that there won't be any other specialists in saturation physics there — but that's okay, since it gives me a chance to talk about my work at a less technical level.

  • On the "life in China" front, I've spent the past semester (March to July) taking a class in Chinese through CCNU. It's very intense: four hours a day, five days a week! And even after all that time, I'm barely able to hold conversations with Chinese people. But "barely able" is a lot better than "unable", so even that minimal level of Chinese is a huge help! That's one of the things you come to miss most about living in a foreign country where you don't speak the language: just the potential to talk to random people, even if you usually don't (like me), is comforting.

  • I hope to be posting a lot more about life in China, if not while traveling, at least later on this year. It's been five months since my last trip, and there are definitely parts of the expatriate experience that don't show up until you've been away from home for months, not just weeks.

  • And of course, there's some cool science that has been happening recently! The discovery of pentaquarks by LHCb (or so they think), the production of Weyl fermions, New Horizons passing Pluto, and those are just the headlines. Maybe I can get some posts up about these discoveries, if I have time.

But work has to come first. I'll be tweeting updates on the conferences I go to, so watch that feed for more!

2015
Jan
25

About saturation

Time to kick off a new year of blog posts! For my first post of 2015, I'm continuing a series I've had on hold since nearly the same time last year, about the research I work on for my job. This is based on a paper my group published in Physical Review Letters and an answer I posted at Physics Stack Exchange.

In the first post of the series, I wrote about how particle physicists characterize collisions between protons. A quark or gluon from one proton (the "probe"), carrying a fraction x_p of that proton's momentum, smacks into a quark or gluon from the other proton (the "target"), carrying a fraction x_t of that proton's momentum, and they bounce off each other with transverse momentum Q. The target proton acts as if it has different compositions depending on the values of x_t and Q: in collisions with smaller values of x_t, the target appears to contain more partons.

kinematic diagram of proton composition

At the end of the last post, I pointed out that something funny happens at the top left of this diagram. Maybe you can already see it: in these collisions with small x_t and small Q, the proton acts like a collection of many partons, each of which is relatively large. Smaller x_t means more partons, and smaller Q means larger partons. What happens when there are so many, and so large, that they can't fit?

Admittedly, that may not seem like a problem at first. In the model I've been using so far, a proton is a collection of particles. And it seems totally reasonable that when you're looking at a proton from one side, some of the particles will look like they're in front of other particles. But this is one of those situations where the particle model falls short. Remember, protons are really made of quantum fields. Analyzing the proton's behavior using quantum field theory is not an easy task, but it's been done, and it turns out an analogous, but very serious, problem shows up in the field model: if you extrapolate the behavior of these quantum fields to smaller and smaller values of x_t, you reach a point where the results don't make physical sense. Essentially it corresponds to certain probabilities becoming greater than 1. So clearly, something unexpected and interesting has to happen at small x_t to keep the fields under control.

Parton branching and the BFKL equation

To explain how we know this, I have to go all the way back to 1977. Quantum chromodynamics (QCD), the model we use to describe the behavior of quarks and gluons, was only about 10 years old, and physicists at the time were playing around with it, poking and prodding, trying to figure out just how well it explained the known behavior of protons in collisions.

Most of this tinkering with QCD centered around the parton distributions f_i(x, Q^2), which I mentioned in my last post. Parton distributions themselves actually predate QCD. They first emerged out of something called the "parton model," invented in 1969, which is exactly what it sounds like: a mathematical version of the statement "protons are made of partons." So by the time QCD arrived on the scene, the parton distributions had already been measured, and the task that fell to the physicists of the 1970s was to try to reproduce the measurements of f_i(x, Q^2) using QCD.

When you're testing a model of particle behavior, like QCD, you do it by calculating something called a scattering cross section, which is like the effective cross-sectional area of the target particle. If the target were a sphere of radius r, for example, its cross section would be \pi r^2. But unlike a plain old solid sphere, the scattering cross section for a subatomic particle depends on things like how much energy is involved in the collision (which you may remember as \sqrt{s} from the last post) and what kinds of particles are colliding. The information about what kinds of particles are colliding is represented mathematically by the parton distributions f_i(x, Q^2). So naturally, in order to make a prediction using the theory, you need to know the parton distributions.

The thing is, we actually can't do that! Believe me, people are trying, but there's a fairly fundamental problem: parton distributions are nonperturbative, meaning they are inextricably linked to the behavior of the strong interaction when it is too strong for standard methods to handle. They already knew this in the 1970s. However, that didn't stop people from trying to calculate something about the PDFs which could be linked to experimental results.

perturbative and nonperturbative parton distributions

It turns out that even though the exact forms of the parton distributions can't be calculated from quantum field theory, you can calculate their behavior at small values of x, the green part on the left of the preceding diagram. In 1977, four Russian physicists — Ian Balitsky, Victor Fadin, Eduard Kuraev and Lev Lipatov — derived from QCD an equation for the rate of change of parton distributions with respect to x, in collisions with energy \sqrt{s} much larger than either the masses of the particles involved or the amount of energy transferred between them (Q, roughly). In modern notation, the equation (which I will explain later) is written

\pd{N(x, Q^2, \vec{r}_{01})}{\ln\frac{1}{x}} = \frac{\alpha_s}{2\pi}\int\uddc\vec{r}_2\frac{r_{01}^2}{r_{02}^2r_{12}^2} [N(x, Q^2, \vec{r}_{02}) + N(x, Q^2, \vec{r}_{12}) - N(x, Q^2, \vec{r}_{01})]

N is something called the color dipole cross section, which is related to f from before via an equation roughly like this:

f(x, Q^2) = \int^{Q^2}\iint N(x, k^2, \vec{r})\uddc\vec{r}\udc k^2

That's why f is often called an integrated parton distribution and N an unintegrated parton distribution. I won't go into the details of the difference between N and f, since both of them show the behavior I'm going to talk about in the rest of this post.

Anyway, the behavior that Balitsky, Fadin, Kuraev, and Lipatov analyzed comes from processes like these:

parton branching diagrams

At each vertex, one parton with a certain fraction x of the proton's momentum splits into other partons with smaller values of x. You can see this reflected in the equation: the term -N(x, Q^2, \vec{r}_{01}) represents the disappearance of the original parton, and N(x, Q^2, \vec{r}_{02}) + N(x, Q^2, \vec{r}_{12}) represents the creation of two new partons with smaller momentum fractions x. When this happens repeatedly, it leads to a cascade of lower-and-lower momentum particles as the branching process goes on. This explains why the number of partons, and thus the parton distribution functions, increase as you go to smaller and smaller values of x.

This BFKL model has been tested in experiment after experiment for many years, and it works quite well. For example, in the plot below, from this paper by Anatoly Kotikov, you can see that the predictions from the BFKL equation (solid lines) generally match the experimental data (dots with error bars) quite closely.

comparison of F2 experimental data and BFKL predictions

The plot shows the structure function F_2, which is a quantity related to the integrated parton distribution.

Parton recombination

However, there is one big problem with the BFKL prediction: it never stops growing! After all, if the partons keep splitting over and over again, you keep getting more and more of them as you go to lower momentum fractions x. Mathematically, this corresponds to exponential growth in the parton distributions:

\mathcal{F}(x, Q^2) = \ud{f}{Q^2} \sim \frac{x^{-4\bar{\alpha}_s\ln 2}}{\sqrt{\ln\frac{1}{x}}}

which is roughly the solution to the BFKL equation.

If the parton distributions get too large, when you try to calculate the scattering cross section, the result "breaks unitarity," which effectively means the probability of two partons interacting becomes greater than 1. Obviously, that doesn't make sense! So this exponential growth that we see as we look at collisions with smaller and smaller x_t can't continue unchecked. Some new kind of physics has to kick in and slow it down. That new kind of physics is called saturation.

The physical motivation for saturation was proposed by two physicists, Balitsky (the same one from BFKL) and Yuri Kovchegov, in a series of papers starting in 1995. Their idea is that, when there are many partons, they actually interact with each other — in addition to the branching described above, you also have the reverse process, recombination, where two partons with smaller momentum fractions combine to create one parton with a larger momentum fraction.

parton recombination

At large values of x, when the number of partons is small, it makes sense that not many of them will merge, so this recombination doesn't make much of a difference in the proton's structure. But as you move to smaller and smaller x and the number of partons grows, more and more of them recombine, making the parton distribution deviate more and more from the exponential growth predicted by the BFKL equation. Mathematically, this recombination adds a negative term, proportional to the square of the parton distribution, to the equation.

\pd{N(x, Q^2, \vec{r}_{01})}{\ln\frac{1}{x}} = \frac{\alpha_s}{2\pi}\int\uddc\vec{r}_2\frac{r_{01}^2}{r_{02}^2r_{12}^2} [N(x, Q^2, \vec{r}_{02}) + N(x, Q^2, \vec{r}_{12}) - N(x, Q^2, \vec{r}_{01}) - N(x, Q^2, \vec{r}_{02})N(x, Q^2, \vec{r}_{12})]

When the parton density is low, N is small and this nonlinear term is pretty small. But at high parton densities, the nonlinear term has a value close to 1, which cancels out the other terms in the equation. That makes the rate of change \pd{N}{\ln\frac{1}{x}} approach zero as you go to smaller and smaller values of x, which keeps N from blowing up and ruining physics.

By way of example, the following plot, from this paper (my PhD advisor is an author), shows how the integrated gluon distribution grows more slowly when you include the nonlinear term (solid lines) than when you don't (dashed lines):

plot of BFKL and BK solutions

So where does that leave us? Well, we have a great model that works when the parton density is low, but we don't know if it works when the density is high. That's right: saturation has never really been experimentally confirmed, although it's getting very close. In the third and final post in this series (not counting any unplanned sequels), I'll explain how physicists are now trying to do just that, and how my group's research fits into the effort.

2015
Jan
01

A look back at 2014 on the blog

Every New Year's Eve I do a review of my favorite blog posts from the past year. And normally I have too many good physics posts to make a top 10 list like so many other sites seem to do. But not this year. It's been a pretty quiet year for blogging, especially for physics blogging (unless you count that one really big blog post they call a dissertation).

Therefore, New Year's resolution #1: write more blog posts about interesting physics. This is one I actually think I can keep.

For now, here is a short list of my favorites out of the 32 blog posts I wrote this year.

2014
Dec
16

Adventures in China: The Christmas

Guess where this is?

pile of presents in restaurant lobby

This is the restaurant where I went to dinner last night. A fancy, yet very definitely Chinese restaurant. In China.

News flash: Americans aren't the only ones obsessed with Christmas.

Okay, to be fair, nobody turns Christmas into an obsession quite like the United States. I think the frantic rush to start making preparations in September is a uniquely American tradition. But the celebration is catching on among the Chinese, especially young people, in a big way. From what I hear, a lot of Chinese are taking Christmas as an occasion to spend more time with their families. And businesses are capitalizing on the spirit by putting up holiday-themed decorations — lights, presents, and even decorated trees are everywhere.

ornamented stairs at Best Western

tree at Best Western

As I write this, I've been sitting in the Beijing airport for five hours listening to a loop of "Santa Baby," "There's No Place Like Home For The Holidays," "Silver Bells," "Jingle Bells," and a rather Hawaiian-sounding rendition of "Let It Snow" (notable for the contrast with the complete lack of snow outside).

I guess the lesson is, if you're tired of the Christmas frenzy, you might be able to hide, but you can't run. It's everywhere.