Life as a Physicist

Not everyone is satisfied with ROOT as the “tool” to analyze HEP data. Back in D0’s Run I all the data was loaded into a commercial database.

So, before you roll your eyes - you are right. HEP is littered with database train wrecks (can anyone say Objectivity?). However, most of those had to do with trying to store every single last bit of data that came off the data acquisition system in the database. And then also store reconstructed data. And then, in some cases, even the analysis level objects. In fact, ROOT grew out disagreement with this vision (and you can tell who won…).

This project, however, was different. The goal was to store only the high level physics information. For a reconstructed jet, for example, they had the four vector and some other quantities (like electromagnetic fraction of calorimeter energies - 28 values in all). They had separate markers for tight very high quality electrons and loose, lower quality, electrons. Same for muons, jets, etc. To understand the limitations of this — and what you might or might not do with this tool: if you changed your jet energy scale you would have to completely re-load the database. This is not something you do frequently, but you get the idea: this is to do your final selection - the last mile of your analysis. Indeed, the test case was to repeat the Run 1 top discovery analysis. However, if you can do selection quickly imagine the power for scanning over a large SUSY parameter space!

How much data? About 62 million events. As a raw ntuple it was 62.4 GB of ntuples (small by today’s standards, of course!). It took almost 1000 hours to generate these ntuples - applying jet energy scale, etc. After being inserted into the database it was 80 GB of raw data, and another 30 GB of database index data.

They used Microsoft’s SQL Server for this. On a qual 450 MHz Pentium II with 256 MB of memory. Does that tell you how long ago this experiment was done!?

Actually, their DB design was pretty clever. All electrons in one table, all jets in another. Then another table which just listed all tight electrons, and another one that listed all loose electrons, etc.

So, how fast did this thing run? So, looking for a Z boson goes to two electrons took about 7 seconds. It found about 6000 events - the right number. Looking for a W boson decaying to an electron and neutrino took about 18 seconds to find 86,000 events. That is pretty darn good!

Are there plans to do this in ATLAS? Well, perhaps. We have a physics summary database - but it isn’t complete (e.g. doesn’t have all the jets in an event). It its design goal is different: you use it to select a sample of events you actually want to run over.

The project was lead by Rich Partridge at Brown University (with a lot of help from an undergraduate Matt Bowen). For more raw information you can see a talk by Rich at a SLAC meeting the other day (CERN ATLAS agendas, look for meetings on Feb 27, the SLAC ATLAS forum).

At any rate, this was something I’ve been meaning to write about for a while. Unfortunately for an approach like this, about 95% of an analyzer’s time is spent trying to understand what exactly is a tight electron - and its fake rate. However, anything that makes for fast turn around is a boon in my book!

I great way to get rid of marketing calls: “Sorry, I only barely speak French!” And then the next words out of their mouth are “Oh, I only speak French - can someone else contact you next time?”. The conversation is over in 30 seconds. I’m going to have to try it in French when I’m back in the USA!

About one or two years ago I had to fix a bug in the D0 DAQ Supervisor. The Supervisor is responsible for coordinating the configuration of 400 or 500 farm nodes and about 80 front end crates that generate the data. It is massively multi-threaded. When it is at its busiest it has over 200 threads running. Most are simply me being too lazy to do anything but block while trying to send data to the Internet. Back in the day it ran on a slow dual-core machine under Linux and I did my best to avoid all locks that I could in my multi-threaded code - because locking is expensive, and the Supervisor needed every bit of speed help it could get back then (on a modern machine it is plenty fast enough).

My code was basically some initialization like the following:

global_a = 1.0;

global_b = 5.0;

global_inited = true;

Once global_inited was set to true, then I knew it was safe for the rest of my other threads to look at a and b:

if (global_inited) {

  use-global a…

}

Unfortunately this didn’t always work - sometimes the program behaved as if random values had been entered for a and b. I was never able to reproduce this either. It would happen only once in a while, and restarting the supervisor usually fixed it. Eventually, to fix this bug, I re-structured my code so that all the initialization happened before any other thread was started. After that I never saw the bug again. But I never understood why I was seeing the bug!

A guy who works deep in the stack at Microsoft recently started a blog. One of this first posts explains, possibly, what bit me: the compiler and the CPU (both!!) are allowed to reorder the order that global_a, global_b, and global_inited are set!! Since this bug was not reproducible it was probably done by the CPU, though at the time I never tested that (or ever really figured out what caused this).

Today (Sunday) is 2nd round election day in Marseille. Actually, all over France. Many people are waiting to see if the left will win - Sarkozy is from the right and his popularity has hit lows more quickly than almost any other French President.

However, all these are local elections. And it really is about getting out the vote. Posters (like this one shown) are everywhere. In fact, the city has put up special wooden stands that political’s can stick their posters to. We don’t have a TV, so we miss that aspect, but everything else is pleasingly personal. Since the only other city I ever lived in was Chicago, I don’t think I’ve ever seen a tight local race that is was seen as important (in Chicago, the machine choose, not the voter).

But Marseille is funny - at least when it comes to electing its Mayor. Marseille is divided up into sectors. In each sector 40 or so people are elected. The top 10 from each sector come together and elect the Mayor. There is no direct election of the mayor. This means that everyone runs on a list - “Gaudin’s list” or “Guerini’s list”, or someone else’s.

Last Sunday was the first election. This week is the run-off. It is down to Gaudin and Guerini for mayor. Gaudin has been running this place for 10 years. Friends of mine at work have said things like “Don Corleone” when describing him. He is very much a creature of the right and apparently basically ignores needs of students or the universities or labs (even though Marseille is a major research hub in France). On the other hand, it is very hard to deny that Marseille is doing well. I’ve been coming here on and off for the last 5 years and I can see how much it has improved. Sure, it is still dirty - but lots better than before. New construction everywhere. New stores. New tram. Metro open to a reasonable time of the night. That last one happened just before the election… makes you say “Hmmmm…”

Gaudin’s past victories have never been in doubt. This one, however, is neck-in-neck - so close no one is willing to call it. Heck they even had a debate - a first! I suspect there will be a party somewhere on the Vieux Port tomorrow. On one side if Gaudin (the current mayor) wins, and one on the other side if Guerini wins.

There are lots of old superstitions - some of them we still live our lives by. Running a large experiment like D0 is no different. For example, there are a set of ducks along the console - the rumor is if they aren’t there then the whole system will cease to operate. I don’t think anyone has been brave enough to remove them…

I pulled the following quote from a recent shift report:

Beam was nice for a while.  Then while talking to Bill Lee about losing the beam, we lost the beam, thereby illustrating Bill’s spooky powers in the control room.

Bill has long been making our control room run smoothly, and should know the lesson: don’t talk about loosing the beam! You’ll jinx it!! [Technical reason: apparently an important power supply went out of allowed operating range].

Thirsting for some stunning pictures of the LHC? Check out this article in the National Geographic magazine. Make sure to look at the photo-gallery that comes along with the article. Some of the pictures — like the ALICE and CMS detector pictures are really stunning (ATLAS too, of course, but we’ve seen that one already).

BTW — the ATLAS detector is no longer all that photogenic on a grand scale - the cavern is now so full of bits of the detector it is quite difficult to get an idea of how big it is - all your site lines are blocked!

The picture is one of mine of ATLAS. The ones from ATLAS are much better!

My apartment was broken into last year. Today my insurance company dropped by to inspect the place and interview me. They asked me all sorts of detailed questions about the items that were taken - cost, how old, etc. At the end, they told me how much money they would give me… and then wrote a check on the spot! Amazing!

Yesterday, at the Moriond QCD conference, Uli Heintz gave the Tevatron Top Mass talk (pdf). New mass results have made the Higgs a little less mysterious. Summer results had a measured top mass of 171.3+-1.7 GeV and now it is 172.8 +- 1.4 GeV. This is good news for the Standard Model.

One of the beauties of the Standard Model is that it holds together so well as a theory. It predicts many different experimental measurements. And all those measurements must be in line with each other — the model cannot accommodate a measurement that is out of whack. And the better we make these measurements the better we can tell when one is out of whack.

The Higgs mass is no different. Even though we’ve not seen it yet, the Standard Model predicts its mass. With the new top mass result, the predicted mass is 87 +36-27 GeV. Using last summer’s top mass the predicted Higgs mass was 76 +33-24 GeV. Note the very large errors on those numbers - there is a lot of slop in that measurement!

This is good for the Standard Model because of work done at the LEP collider. They searched for the Higgs and didn’t see it - they know that the Higgs mass is more than 114 GeV. The summer’s prediction put the Standard Model more out of whack than the current one - the new predicted value of the top mass is more in line with the LEP Higgs search.

Still - I’d love to know where that thing is hiding (along with everyone else)! The latest Higgs results should have been released - but I’ve not seen them publicly posted yet.

N.B. That plot is the summer plot — I couldn’t find the plot with the most recent results - I found it only in Uli’s talk.

A friend  of a friend was investigating the Tunguksa explosion. Mirror matter has been offered up as an explanation. I’ve never heard of mirror matter before (wikipedia has a very quick introduction). The idea, basically, is to restore CP as a fundamental symmetry of our world. And, because the mirror matter would only weakly interact with our world, it might be a good dark matter candidate. There are other predictable effects as well — all having to do with astro observations. And there are even papers in real journals about it (but not many; it never seemed to really catch on the way some theories do).

All good so far. So how come I’ve never heard of it? Do I just live in a hole? Or is it because it is really only testable in an astrophysics experiment and I’ve not paid them enough attention?

As anyone who as worked at a US national lab knows, safety is job #1. The constant reminders and cross checks and forms that have to be signed before work can be done can sometimes be a little overbearing. And a bit frustrating - but there is a reason they are there; they seem to work.

Still, I can’t help but cringe when I see one of those posters that shows those cartoon characters “Hey, safe work is happy work!” or some such blather.

Fermilab seems to have come up with a better approach. I think the way this happened was they asked the graduate student association for some safety quotes. These were then turned into posters and have been distributed around the site. They work because they are out of the ordinary, and, for the most part, clever. Heck, I want to read them. I took photos of the ones I saw on my last visit:

• Take accidents to absolute zero.
• Beamline and spine, keep them aligned.
• Superconduct yourself in a safe manner.
• Accidents can happen in a blur! Stay focused.