Quantum Diaries

Finally, it’s summer time! As I’ve said from the beginning, summer is a very nice time to be a professor, as we don’t have to do half of our job for these few months. But already this summer is filling up with things to do, and a lot of it involves travel. I have trips to five different destinations, two international, in the 13-plus weeks until the fall semester starts. It is a long road to be on. So you, dear reader, will be subject to my travelogues for a few months.

Today, I’m at the University of Colorado for the annual US CMS collaboration meeting. This is my first visit to Boulder, and it seems pretty nice, although it’s one of these campuses where are the buildings are of a similar style and exterior and thus it’s easy to get lost. The US CMS meeting is a chance for all of the US-based collaborators to get together and talk about what we’re doing on CMS and where we are going. Obviously, there is a lot to talk about right now. The LHC is running, there is a lot of data analysis in progress, and many public results that are having an impact about how we think about particle physics.

But what have we been devoting the most time to at this meeting? Detector upgrades! Yes, we’re talking about stuff that isn’t going to get installed until 2016, even while we might discover a Higgs boson in 2012. Why? First, it takes a long time to build detectors for particle physics. The technology tends to be pretty leading edge, often you have to build a large number of parts by hand, and you need extensive quality control. A real plan for construction, testing, and installation needs to be in place well before the detector needs to be operational. Also, we’re especially concerned that these improved detector components will be ready in time, if not early. The instantaneous luminosity of the LHC, a measure of the collision rate, is rising quickly, and within a few years we expect that it will be above the level for which the CMS detector was designed. If we want to be able to analyze future LHC collisions, we need a detector that meets the needed specifications. And finally, the finances for the construction of these detectors are still very much in the air. We might not have enough money to do everything we want on the timescale that we want to do it. So it’s important to give these projects a lot of scrutiny up front. It’s the start of a long road there, too.

Not that there isn’t any fun physics going on here. Today we had a series of talks by younger people (well, at least younger than me) on a variety of data-analysis topics. The quality of the work being done is really impressive, and there are a lot of creative and sophisticated ideas being put to use. One running theme is our ability to rely on real detector data, rather than simulations, to model the old-physics backgrounds to potential new-physics signals. And it’s worth keeping in mind that this is only possible because of the excellent detector that we’ve built. Good detector upgrades will allow us to keep doing this excellent data analysis in the future.

We are back to discussing B physics today, with the observation of the rare decay: \(B^- \rightarrow \pi^- \mu^+ \mu^-\). So what is this decay? It’s a \(B^-\) meson (made of a b and an anti-u quark) decaying into a \(\pi^-\) meson (made of a d and an anti-u quark) and two muons. And why is it so rare? Well, it’s a flavour changing neutral current decay. Which means that there’s a change in quark flavour in the decay, but not charge. This type of decay is forbidden at tree level in the Standard Model and so has to proceed via a loop, which can be seen in the centre of the Feynman diagram below.

If you look closer at the loop, you can see that for the decay to occur, a b quark needs to change flavour to a t or c quark, which then needs to change to a d quark. This is another reason why this decay is so rare. Transitions in quark flavour are governed by the CKM matrix, which I illustrate on the right, where the larger squares indicate more likely transitions. So while the transition from b to t is likely, the transition from t to d is very unlikely, and the b to c and c to d transitions are both fairly unlikely. This means, that whichever path is taken, the b to d quark transition is very very unlikely.

Okay, now to the LHCb result. Below I have a plot of the fitted invariant mass for selected \(\pi^-\mu^+ \mu^-\) candidates, showing a clear peak for \(B-\) decays (green long dashed line). Also shown are the backgrounds from partially reconstructed decays (red dotted line) and misidentified \(K^-\mu^+ \mu^-\) decays (black dashed line). Candidates for which the \(\mu^+ \mu^-\) pair is consistent with coming from a \(J/\psi\) or \(\psi(2S)\) are excluded.

We see around 25 \(B^- \rightarrow \pi^- \mu^+ \mu^-\) events and measure a branching ratio of approximately 2 per 100 million decays. This result makes this decay the rarest \(B\) decay ever observed!

In my last post, I described the different LHC collision setup at LHCb this year. Today, I thought I would describe the different LHCb trigger setup.

Now what is the LHCb trigger, I hear you all ask? I actually wrote a post on the topic last year, which I invite you all to read for details, here I’m just going to explain the details I need to describe the changes.

The LHCb trigger is an online electronic system that selects which collision events will be written to disk for offline analysis. On the right here, I have a schematic of the system. It consists of two levels; the first is made up of custom electronics, called L0, while the second is a computer farm, called HLT.

We call it an online system, as it runs in real time. As fast as collision events are coming in, the L0 electronics decides whether to reject an event or send it to the HLT. The HLT gets a little more time to make a decision, but it still needs to be pretty fast. However, sometimes it can’t handle all the events that the L0 is feeding it, and we lose events as the buffers fill up.

This situation is what our new trigger setup is designed to avoid. How are we going to do this? It was noticed that the HLT computing farm sits idle when there aren’t any collisions. So somebody came up with the clever idea to buffer events locally on the farm nodes and defer processing them until after the current collision period[*]. Thus the trigger now looks something like the schematic on the left[**].

This means we can record even more data!

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[*] The LHC doesn’t collide protons continuously, there’s a cycle in which protons are injected, accelerated, collided, ejected and the machine prepared for the next injection. Ideally, most of the time would be spent in collisions (in LHC speak: stable beams), but this isn’t always possible or viable.

[**] I have obviously simplified how the deferred HLT works. Like most simple ideas, it was quite complicated in practice. There were a lot of technicalities to consider, like how many events to store in the overflow, or what to do if the overflow became full, or how to avoid the scenario where we’re still processing deferred events when the next collision period starts…

Have you been paying attention to the LHC? Sure, you’ve been thinking about the scientific results being derived from last year’s data. And you are looking a few months down the road to the upcoming major conferences, now only a little more than two months away, when we might get some interesting news. But right now, the LHC has been running, and running well. Consider this: here is a plot of the integrated luminosity as a function of time for 2011:

And here is the same for 2012:

It is important to note the “preliminary” on this plot — all experiments are working to verify their luminosity calibration. But one can see that the integrated luminosity for this year at the end of April 2012, about an inverse femtobarn is about what it was for last year in the middle of June 2011. In all of 2011, we recorded “only” about five inverse femtobarns. (Dear LHC: could someone produce a plot with the integrated luminosity for both years on the same set of axes? Then I could make this point more easily.) We are recording data at a much faster pace than last year, and that can go straight into the physics bottom line. From what I have heard, the operating conditions of the LHC have been particularly good — the vacuum inside the beam pipe has been better than expected, which means that it will be easier than anticipated to increase the beam currents, and thus to increase the instantaneous luminosity more quickly.

This is important, because we’re about to hit the important big sprint of the year. The LHC has been doing machine studies and a technical stop during the past week. Regular operations for physics will restart around Monday. To be in a position to firmly observe a Higgs boson (if it exists) this year, we need to accumulate 6-7 inverse femtobarns by the “first breakpoint” in late June, under two months away. That is, we will need to accumulate as much data in the next two months as we did in all of last year’s run.

Can the LHC do it? Based on what we’ve seen so far this spring, I think we can try to be optimistic!

I gave you a golden ring to show you my love
You went to stick it in a printed circuit
To fix a voltage leak in your collector
You plug my feelings into your detector.

– Les Horribles Cernettes, “Collider”

But never mind gold. The material that’s really good for building particle detectors, for some applications, is diamond. This very strongly-bonded lattice of carbon is almost uniquely sturdy, with a high melting point and — more importantly — a very good ability to take radiation damage and keep working the same way. It can also act as a semiconductor, carrying charge deposited by high-energy particles in the same way that the LHC’s more “ordinary” silicon-based tracking detectors do.

Diamond is already used in both ATLAS and CMS, as part of the Beam Condition Monitors. These are very small detectors designed to detect when the LHC beams stray too far from their expected path; if this happens, they can automatically request that the LHC beam be dumped. This is necessary because the silicon pixel detectors at the center of ATLAS and CMS would be damaged if they were hit with a large number of protons. Of course, operating so close to the beam, the Beam Condition Monitors have to be able to take a lot of damage themselves, and that is why they are made of diamond. Particle physicists have also studied making entire tracking detector layers out of diamond, not so that they could take a direct hit from the LHC beam, but simply so they could last longer in the punishing environment of particles emerging from LHC collisions.

Such applications are possible because the industrial processes that make synthetic diamonds get cheaper and more efficient all the time, as well as better at making large, flat, uniform diamonds. But It turns out that you can also cut diamond gemstones from these processes. They are entirely the same as the “real” ones made underground over millions of years, unless you study them with special equipment designed to tell the difference. Of course, the diamond mining and distribution industry would like you to appreciate that it is the rarity and naturalness of diamonds that makes them special: a synthetic one simply won’t do.

I mention this because, when my fiancée and I went ring shopping this past weekend, we decided to take this argument one step further. A few centuries ago, diamonds were a lot more difficult to come by and to process, and they rarely had the “perfect” cuts and transparency that many people expect today. Diamonds on antique rings are small and cloudy, and the rings themselves are a bit weathered, so they’re surprisingly affordable. But the point we took is this: it’s not the price or appearance of the diamond that matters, it’s how unique and special it is. Like, say, the ones on this ring:

Of course, for building particle detectors, I’ll probably stick to the synthetics.

I don’t know the original source, but there’s an image that has gone semi-viral over the past year which challenges the reader to identify several brand names based on their logos versus plant names based on their leaves. (Here’s a version at Adbusters.) The point is to contrast consumerism to the outdoors-y/science-y education that kids would get if they just played outside.

This isn’t the place to discuss consumerism, but I don’t agree with idea that the ability to identify plant names carries any actual educational value. Here’s my revision to the image:

On the right we’ve encoded all of the particles in the Standard Model in a notation based on representation theory. In fact, this is almost all of the information you need to know to write down all of the Feynman rules in the Standard Model (more on this below).

Tables that the one above are a compact way to describe the particle content of a model because the information in the table specifies all of the properties of each particle. And that’s the point: whether we name a particle the “truth quark” or the “top quark” doesn’t matter—what matters is the physics behind these names, and that’s captured succinctly in the table. Science isn’t about classification, it’s about understanding. I leave you with this quote from Feynman (which you can watch in his own words here):

You can know the name of a bird in all the languages of the world, but when you’re finished, you’ll know absolutely nothing whatever about the bird… So let’s look at the bird and see what it’s doing — that’s what counts. I learned very early the difference between knowing the name of something and knowing something.

Addendum: naming those particles

For those who want to know, the particles in the table are, from top down:

1. The left-handed quark doublet, containing the left-handed up quark and left-handed down quark
2. The anti-right-handed-up quark
3. The anti-right-handed-down quark
4. The left-handed lepton doublet, containing the left-handed electron and left-handed neutrino
5. The anti-right-handed electron (a.k.a the right-handed positron)
6. The anti-right-handed neutrino
7. The Standard Model Higgs

SU(3), SU(2), and U(1) refer to the strong force, weak force, and hypercharge. Upon electroweak symmetry breaking, the weak force and hypercharge combine into electromagnetism and the heavy W and Z bosons. Here’s how to read the funny notation:

1. Under SU(3): particles with a box come in three colors (red, green, blue). Particles with a barred box come in three anti-colors (anti-red, anti-green, anti-blue). Particles with a ’1′ are not colored.
2. Under SU(2): particles with a box have two components, an upper and a lower component. That is to say, a box means that there are actually two particles being represented. More on this below. Particles with a ’1′ do not carry weak charge and do not talk to the W boson.
3. Under U(1): this is the “hypercharge” of the particle.
4. The electric charge of a particle is given by adding to the hypercharge +1/2 if it’s the upper component of an SU(2) box, -1/2 if it’s the lower component of an SU(2) box, or 0 if it is not an SU(2) box (just ’1′).

As a consistency check, you can convince yourself that both the left- and right-handed neutrinos carry zero electric charge. Note, also, the fact that we’ve written out left-handed and right-handed particles differently. This is a reflection of the fact that the Standard Model is a chiral theory.

Finally, I said above that the table of particles almost specifies the structure of the Standard Model completely, the additional pieces of information required are:

1. Which of the above particles are fermions and which are scalars (the gauge bosons are implied)
2. Write down the most general ‘renormalizable’ theory (we write only the simplest interaction vertices)
3. Specify the pattern of electroweak symmetry breaking (the Higgs)
4. Specify the flavor symmetries (three of each type of matter  particle)

From this one can write the complete mathematical expressions for the Standard Model. One then just has to fill in the observed numerical values to be able to calculate concrete predictions for actual processes.

In the past I’ve made it known that I’m a politically-engaged person — and not without some commentator controversy. While I generally prefer to keep my science and politics separate, they inevitably intersect in the matter of governmental funding of scientific research and conflicts between groups driving the national dialogue on science policy. Unfortunately, scientists are often left behind in this conversation, resulting in a serious disconnect with the public.

It’s not hard to find embarrassing stories about how Americans are ignorant of basic scientific knowledge: roughly half believe dinosaurs and humans coexisted, 1 in 5 adults believes the Sun revolves around the Earth, and when it comes to acceptance of evolution, we’re out of step with much of the world. On many topical issues — global climate change, nuclear energy, genetically-modified foods, vaccination, cell phones — an abundance of misinformation drowns out the science, or at least muddies the waters. And even worse, many Americans don’t understand how scientists draw their conclusions, i.e. the scientific method, nor do they apply it in their daily lives. A much-quoted survey from 2007 found that 70% of Americans are “scientifically illiterate” (though that distinction, as well as the statistic, is misleading: scientific literacy is not on a binary scale).

I realize that I’m probably preaching to the choir here: You all have made the effort to read a physics blog written by physicists about highly technical topics, which suggests to me that you are either totally awesome science enthusiasts or… scientists. Thanks for reading! But from whom does the rest of the country not following Quantum Diaries get its science information?

Well, for starters, there’s Hollywood and the entertainment industry, where scientists are commonly portrayed as mad/evil or awkward geniuses — people to fear or mock, perhaps, but not befriend or idolize — and scientific accuracy is typically thrown out the window in favor of more explosions. There’s also the Internet, where people can and do say pretty much whatever they want without the need for peer review or, you know, facts. Do you remember when unfounded fears that CERN was going to create an Earth-devouring black hole ricocheted around the web? Although the Internet is an incredible resource for information and personal research, it’s treacherously inconsistent. The public also learns about science from the news/media, where sensationalism is routine and “fair and balanced” reporting means giving equal time to scientific fact and wild speculation. Recently, a chemistry publication entitled “Evidence for the Likely Origin of Homochirality in Amino Acids, Sugars, and Nucleosides on Prebiotic Earth” made headlines when, in its final two sentences, the author suggests that advanced, potentially dangerous, dinosaurs could exist elsewhere in the universe. Take a guess on how the media covered it. Unfortunately, most Americans don’t learn about science from scientists, and given the abject mess of these other sources, it’s a wonder that a quarter of the population is “scientifically savvy and alert.”

Well, an explainable wonder: America is the only country in the world that requires undergraduates to take a year of general education — and it makes a difference! Education works, who would’ve guessed? However, there is serious cause for concern, particularly with regards to K-12 education. One of the great legacies of the Bush Administration, the “No Child Left Behind Act” of 2001, tied federal funding of public schools to student performance on annual, standardized tests in math and reading (laughably, the law stipulates that all children are to perform above average). Perhaps not surprisingly, educators under pressure are more likely to “teach to the test” to improve scores at the expense of other subjects and skills, such as science and critical thinking. Should we worry about what will happen when the NCLB generation makes it to Physics 101…?

It’s worth pointing out that a post-secondary education in physics, for instance, is also subject to distorted priorities: Our training is extremely focused on skills needed to continue in Academia and fundamental research, while statistics show that a significant fraction of us go on to careers in industry immediately after grad school, and that the most-used skills are not properly developed in the curriculum. Furthermore, in the long term, most of us end up working outside of Academia. Are we better off learning electrodynamics from a glorified textbook on special topics in mathematical methods? I think not.

More to come!

— Burton