Quantum Diaries

Seth Zenz | USLHC | USA

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Tetrahedral Carbon Lattice

Tuesday, April 24th, 2012

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.

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More Multitasking

Friday, April 13th, 2012

I fell out of practice at multitasking at the end of grad school. For the final six months, almost all of my work went into finalizing how to present my analysis results. There were two versions of the presentation: the paper and my thesis, but the general direction of work was all the same. The previous tasks I had worked on, geared toward keeping ATLAS running, were all long since “done,” at least as far as I was concerned.

Starting a postdoc means a sudden change of gears, with more multitasking than ever before. I’ve started many new projects from scratch at the same time, and because I’m new to CMS, every one of those tasks involves tools and procedures that I don’t know. It’s easy to lose track of some of those tasks at any given time, or simply to want to focus on one thing until I understand it, but the job doesn’t work that way. Being succesful as a postdoc will mean significant contributions to the running and understanding of the detector and significant contributions to keeping my group’s analysis running and starting a new analysis (sub)channel of my own. None can be dropped, and most of the things I’m doing have deadlines in the next few months.

So I’m having to remember and improve my multitasking skills, quickly. Step one is bringing this post to a close, and asking you to wish me luck, and getting back to work!

Tags: postdoc life, student life, switching experiments
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A Grumpy Note on Statistics

Tuesday, March 13th, 2012

Last week’s press release Fermilab about the latest Higgs search results, describing the statistical significance of the excess events, said:

Physicists claim evidence of a new particle only if the probability that the data could be due to a statistical fluctuation is less than 1 in 740, or three sigmas. A discovery is claimed only if that probability is less than 1 in 3.5 million, or five sigmas.

This actually contains a rather common error — not in how we present scientific results, but in how we explain them to the public. Here’s the issue:

Wrong: “the probability that the data could be due to a statistical fluctuation”
Right: “the probability that, were there no Higgs at all, a statistical fluctuation that could explain our data would occur”

Obviously the first sentence fragment is easier to read — sorry!^[1] — but, really, what’s the difference? Well, if the only goal is to give a qualitative idea of the statistical power of the measurement, it likely doesn’t matter at all. But technically it’s not the same, and in unusual cases things could be quite different. My edited (“right”) sentence fragment is only a statement about what could happen in a particular model of reality (in this case, the Standard Model without the Higgs boson). The mistaken fragment implies that we know the likelihood of different possible models actually being true, based on our measurement. But there’s no way to make such a statement based on only one measurement; we’d need to include some of our prior knowledge of which models are likely to be right.^[2]

Why is that? Well, consider the difference between two measurements, one of which observed the top quark with 5 sigma significance and the other of which found that neutrinos go faster than light with 5 sigma significance. If “5 sigma significance” really meant “the probability that the data could be due to a statistical fluctuation,” then we would logically find both analyses equally believable if they were done equally carefully. But that’s not how those two measurements were received, because the real interpretation of “5 sigma” is as the likelihood that we would get a measurement like this if the conclusion were false. We were expecting the top quark, so it’s a lot more believable that the excess is associated with the top quark than with an incredibly unlikely fluctuation. But we have many reasons to believe neutrinos can’t go faster than light, so we would sooner believe that an incredibly unlikely fluctuation had happened than that the measurement was correct.^[3]

Isn’t it bad that we’d let our prior beliefs bias whether we think measurements are right or not? No, not as long as we don’t let them bias the results we present. It’s perfectly fair to say, as OPERA did, that they were compelled to publish their results but thought they were likely wrong. Ultimately, the scientific community does reach conclusions about which “reality” is more correct on a particular question — but one measurement usually can’t do it alone.

———————————

[1] For what it’s worth, I actually spent a while thinking and chatting about how to make the second sentence fragment simpler, while preserving the essential difference between the two. In this quest for simplicity, I’ve left off any mention of gaussian distributions, the fact that we really give the chance of a statistical fluctuation as large or larger than our excess, the phrase “null hypothesis,” and doubtless other things as well. I can only hope I’ve hit that sweet spot where experts think I’ve oversimplified to the point of incorrectness, while non-expert readers still think it’s completely unreadable.

[2] The consensus among experimental particle physicists is that it’s not wise to include prior knowledge explicitly in the statistical conclusions of our papers. Not everyone agrees; the debate is between Frequentist and Bayesian statistics, and a detailed discussion is beyond the scope of both this blog entry and my own knowledge. A wider discussion of the issues in this entry, from a Bayesian perspective, can be found in this preprint by G. D’Agostini. I certainly don’t agree with all of the preprint, but I do owe it a certain amount of thanks for help in clarifying my thinking.

[3] A systematic mistake in the result, or in the calculation of uncertainties, would be an even likelier suspect.

Tags: faster than light, Higgs boson, neutrinos, OPERA, science, Statistics
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New Information on “FTL Neutrinos”

Thursday, February 23rd, 2012

We have new information, but my position on the OPERA experiment’s FTL neutrino measurement hasn’t changed.

First, here’s what we know. Members of the OPERA experiment has been working diligently to improve their measurement, better understand their uncertainties, and look for errors. Yesterday, the discovery of some possible problems was leaked anonymously (and vaguely) in Science Insider. This compelled OPERA to release a statement clarifying the status of their work: there are two possible problems, which would have opposite effects on the results. (Nature News has a good summary here.)

The important thing to learn here, I think, is that the work is actually ongoing. The problems need further study, and their overall impact needs to be assessed. New measurements will be performed in May. What we’ve gotten is a status update whose timing was forced by the initial news article, not a definitive repudiation of the measurement.

Of course, we already knew with incredible confidence that the OPERA result is wrong. I wrote about that last October, but I also wrote that we still need a better understanding of the experiment. Good scientific work can’t be dismissed because we think it must have a mistake somewhere. I’m standing by that position: it’s worth waiting for the final analysis.

Tags: faster than light, neutrinos, OPERA
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Analyzing New Data: Never the Same Twice

Monday, February 20th, 2012

Physicists did a lot of planning for data analysis before the LHC ever ran, and we’ve put together a huge number of analyses since it started. We’ve already looked for most of the things we’ll ever look for. Of course, many of the things we’ve been looked for haven’t shown up yet; in fact, in many cases including the Higgs, we didn’t expect them to show up yet! We’ll have to repeat the analysis on more data. But that’s got to be easier than it was to collect and analyze the data the first time, right? Well, not necessarily. We always hope it will be easier the second or third time around, but the truth is that updating an analysis is a lot more complicated than just putting more numbers into a spreadsheet.

For starters, every time we add new data, it was collected under different conditions. For example, going from 2011 to 2012, the LHC beam energy will be increasing. The number of collisions per crossing will be larger too, and that means the triggers we use to collect our data are changing too. All our calculations of what the pileup on top of each interesting collision looks like will change. Some of our detectors might work better as we fix glitches, or they might work worse as they are damaged in the course of running. All these details affect the calculations for the analysis and the optimal way to put the data together.

But even if we were running on completely stable conditions, there are other reasons an analysis has to be updated as you collect more data. When you have more events to look at, you might be interested in limiting the events you look at to those you understand best. (In other words, if an analysis was previously limited by statistical uncertainties, as those shrink, you want to get rid of your largest systematic uncertainties.) To get all the power out of the new data you’ve got, you might have to study new classes of events, or get a better understanding of questions where your understanding was “good enough.”

So analyzing LHC data is really an iterative process. Collecting more data is always presenting new challenges and new opportunities that require understanding things better than before. No analysis is ever the same twice.

Tags: data, data analysis, LHC, luminosity, pileup, trigger
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Can the LHC Run Too Well?

Friday, February 3rd, 2012

For CMS data analysis, winter is a time of multitasking. On the one hand, we are rushing to finish our analyses for the winter conferences in February and March, or to finalize the papers on analyses we presented in December. On the other, we are working to prepare to take data in 2012. Although the final decisions about the LHC running conditions for 2012 haven’t been made yet, we have to be prepared both for an increase in beam energy and an increase in luminosity. For example, the energy might go to 8 TeV center-of-mass, up from last year’s 7. That will make all our events a little more exciting. But it’s the luminosity that determines how many events we get, and thus how much physics we can do in a year. For example, if the Higgs boson exists, the number of Higgs-like events we’ll see will go up, and so will the statistical power with which we can claim to have observed it. If the hints we saw at 125 GeV in December are right, our ability to be sure of its existence this year depends on collecting several times more events in 2012 than we got in 2011.

We’d many more events over 2012 if the LHC simply kept running the way it already was at the end of the year. That’s because for most of the year, the luminosity was increasing over and over as the LHC folks added more proton bunches and focused them better. But we expect that the LHC will do better, starting close to last year’s peak, and then pushing to ever-higher luminosities. The worst-case we are preparing for is perhaps twice as much luminosity as we had at the end of last year.

But wait, why did I say “worst-case”?

Well, actually, it will give us the most interesting events we can get and the best shot at officially finding the Higgs this year. But increased luminosity also gives more events in every bunch crossing, most of which are boring, and most of which get in the way. This makes it a real challenge to prepare for 2012 if you’re working on the trigger, because have to sift quickly through events with more and more extra stuff (called “pileup”). As it happens, that’s exactly what I’m working on.

Let me explain a bit more of the challenge. One of the triggers I’m becoming responsible for is trying to find collisions containing a Higgs decaying to a bottom quark and anti-bottom quark and a W boson decaying to an electron and neutrino. If we just look for an electron — the easiest thing to trigger on — then we get too many events. The easy choice is to ask only for higher-energy electrons, but beyond a certain points we start missing the events we’re looking for! So instead, we ask for the other things in the event: the two jets from the Higgs, and the missing energy from the invisible neutrino. But now, with more and more extra collisions, we have random jets added in, and random fluctuations that contribute to the missing energy. We are more and more likely to get the extra jets and missing energy we ask for even though there isn’t much missing energy or a “Higgs-like” pair of jets in the core event! As a result, the event rate for the trigger we want can become too high.

How do we deal with this? Well, there are a few choices:

1. Increase the amount of momentum required for the electron (again!)
2. Increase the amount of missing energy required
3. Increase the minimum energy of the jets being required
4. Get smarter about how you count jets, by trying to be sure that they come from the main collision rather than one of the extras
5. Check specifically if the jets come from bottom quarks
6. Find some way to allocate more bandwidth to the trigger

There’s a cost for every option. Increasing energies means we lose some events we might have wanted to collect — which means that even though the LHC has produced more Higgs bosons, it’s counterbalanced by us seeing fewer of the ones that were there. Being “smarter” about the jets means more time spent by our trigger processing software on this trigger, when it has lots of other things to look at. Asking for bottom quarks not only takes more processing, it also means the trigger can’t be shared with as many other analyses. And allocating more bandwidth means we’d have to delay processing or cut elsewhere.

And for all the options, there’s simply more work. But we have to deal with the potential for extra collisions as well as we can. In the end, the LHC collecting much more data is really the best-case scenerio.

Tags: CMS, Higgs, LHC, luminosity, pileup, trigger
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Location, Location, Location

Thursday, January 19th, 2012

If I had to pick one thing that’s definitely better on my old experiment, ATLAS, than on my new experiment, CMS — and especially if I had to pick something I could write publicly without getting into trouble — it would be this: the ATLAS detector is across the street from the rest of CERN. I’m not sure how that was decided, but once you know that, you know where CMS has to be: on the other side of the ring, 5 or 6 miles away. That’s because the detectors have the same goals and need the same beam conditions; two opposite points on the LHC are where a duplicate performance is easiest. The pre-existing caverns from the LEP collider, whose tunnel the LHC now uses, probably also helped determine where the detectors are.

In any case, it used to be that when I wanted to work on my detector, I had only to go across the street. Now I have to drive out of Switzerland and several miles into France. Except, I don’t like driving. So I’ve been working on alternate means of transportation. A few months ago I walked. Last night I had to go to downtown Geneva, so I took the bus. It’s actually pretty good, although the bus stop is a mile away from CMS. There’s also the shift shuttle, which runs from the main CERN site to CMS every 8 hours via a rather roundabout route. And I can bike, once the weather gets better and I get myself a little more road-worthy. To be honest, every option for getting here is much slower than driving, but I enjoy figuring out ways to get places enough that I’m going to keep trying for a while.

I have plenty of chances to try, because I’ll be here in the CMS control room a lot of the time over the next few weeks. Right now, I’m learning and helping with the pixel detector calibration effort. (We’re changing the operating temperature, so all the settings have to be checked.) Soon I’ll be learning to take on-call shifts. So the more I stay here, the more I learn. I got here this morning, and I won’t leave tonight until about 11 pm. I could take the shift shuttle back — or maybe I’ll just get a ride.

Tags: ATLAS, CMS, control room, LHC, pixel, shift, switching experiments, travel
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Numerical Family Connections

Wednesday, December 21st, 2011

Just a brief random thought at the start of the first winter break in my life where I’m not visiting or living with my parents… Whenever I need the number π — that is, the ratio between a circle’s circumference and its diameter — in computer analysis code I’m writing, I always write it out like this:

3.141592654

That’s not exactly π, but it’s quite close. What I really should do is look up where it’s already defined in the math library I’m using, but this is more than accurate enough for any reasonable purpose. It’s too many digits, in fact, although I know a few more. So why do I always write out exactly that many places? Well, after thinking about it for a minute a little while ago, I remembered the answer: it’s the number of digits of π my dad taught me when I was a kid.

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Walking Across the LHC

Monday, November 28th, 2011

About a month ago, I walked back to Saint-Genis-Pouilly, France from the CMS experiment site after my last meeting of the day, which basically amounts to walking the width of the LHC ring: about 6 miles. Here are a few pictures from the walk:

More pictures, and commentary, on Google+…

Tags: CMS, control room, LHC, travel
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Have we Found the Higgs Yet?

Monday, November 21st, 2011

Along with a bunch of important people who actually know how to give interviews, I answer that question in this video:

The video goes along with this Nature News article. You may also be interested in the recent combined ATLAS and CMS Higgs result, which uses only the first half of this year’s data.

By the way, when I talk about a “minimal Higgs, that only does the part we know that something like the Higgs has to do,” I’m referring to the so-called fermiphobic Higgs. It plays the usual role of the Standard Model Higgs boson in breaking electroweak symmetry, but doesn’t couple to quarks and leptons (i.e. fermions). We already know from the way the weak and electromagnetic forces work that the relationship between them has its origins in something like the Higgs — but we have less reason to be certain that the same particle takes care of quark and lepton masses too. This version of the Higgs boson is more difficult to find, but perfectly sensible, and we’ll probably hear a lot more about it in coming years if we don’t have a big discovery this year or next.

Tags: Higgs, publicity
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