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Triggering on green frogs April 25, 2007

Posted by dorigo in humor, personal, physics, science.

You think physicists like to talk with formulas and complicated lingo ? Think better. You should have followed today’s meeting on high-level triggers for CMS, at the underground conference room of building 40.

The speaker was discussing triggering strategies for collecting multijet events. Basically that has to do with the thing we call the “Trigger” (ok, yes, some lingo here): a complicated set of filters that select the 100 or so proton-proton collisions worthy of being recorded to tape among the 40 millions happening in the core of the detector every second. The decision is based on the characteristics of the event as they can be inferred through a extremely fast reconstruction. Our data acquisition system cannot record more than about 100 events per second, and sadly, the rest has to go.

Because of its potential of throwing away whole categories of interesting events for the benefit of others, the trigger is a very sensitive issue, determining what physics one will be able to do and what will not be done because of the lack of a specific filter selecting the relevant data. In 15 years of particle physics, I have never seen people fighting, calling each other names, discussing philosophy, or just acting funny as in trigger meetings. An example ? Melissa Franklin’s famous “F*** everybody” wish before walking out the door, at a CDF meeting in 1992 when her favorite dataset was being penalized by raised calorimeter thresholds.

Ok, back to CERN. During today’s talk, a question from the audience sparked a lively discussion on the design details of the triggers collecting jet events. You would think you stand no chance of following the discussion, right ? Well, you could have, since at one point people stopped using technical language and started talking funny. Here is a rough account of the discussion:

“… So, to make it clear: if you filter the event at level 1 because you see two jets and a green frog, you better be sure that at level two you require the presence of the same green frog, at the very least.”

“Yes, but I am still free to require two green frogs at level two, can’t I ?”

“Sure, two green frogs are fine. But beware, at level two you will not be allowed to require two yellow frogs.”

“Ok, but what about more than two jets and a yellow frog then?…”

The discussion went on for quite a while until people from the audience (including yours truly) started suggesting scenarios with lizards and other creatures. These kinds of exchanges are not uncommon during the long meetings of the “Physics Days” of CMS… I should come here more often!


1. jeff - April 25, 2007

excellent post!

2. Fred - April 25, 2007

I agree with jeff. The rough account has potential for becoming a poor-physicist’s version of a Dr. Seuss book. T.S. Geisel would be proud. You’d also better copyright this work along with your plot from 2 posts past. (island was doing you a favor by instantly increasing the value of your work with just a few words) By the way, newspaper’s over here are offering the same amount to writers as they were 25 years ago, so you’re not alone. You can thank the Rupert Murdochs of the world, although, they’ll just reply that it’s the going rate, “Take it or leave it.” You’re doing a hell of a job no matter what we say about you.

3. Kea - April 25, 2007

LOL! Thanks for this insight into the workings of the venerable CERN. Do we need someone to kiss the frog when it emerges?

4. dorigo - April 26, 2007

Hi folks,

thanks for the nice words Jeff, Fred. I always try to look for the involuntary humor always present in my job (as in any one else)…

Fred, your comment on the offers to writers comes surprisingly timely, as I have been dealing with the people offering to blog for them just the other day. I will post about it when the matter is over.

Kea, in principle yes, promoting a candidate prince to a well-defined one is one of the aims of the trigger system. Only, I wouldn’t know how to kiss a high-Pt lepton or something of the kind…


5. The Dude - April 26, 2007


So I am curious. Why does the tevatron collide protons-antiprotons, but the LHC goes for proton-proton? This has been puzzling me for some time, and even google has no anwsers 😦

6. dorigo - April 26, 2007

Hello dude,

running protons against antiprotons has the advantage of allowing you to use the same magnets to bend both beams, circulating in opposite directions, in the same way. The Lorentz force experienced by a moving charge in a magnetic field changes sign for the opposite direction and also for the opposite charge, and the net effect is that both protons and antiproton can travel in the same beam pipe .- thus saving magnets, vacuum structures, and a lot of infrastructure.

What is the drawback ? It is that producing antiprotons is a maddeningly hard task. You produce antiprotons by smashing protons against a target, and sifting through the emerging bodies downstream, with magnet optics that select them and patiently direct and store them in an accumulation ring.

The art of producing antiproton beams has been perfected at the Tevatron in the last twenty years, but it is just impossible to reach the intensity required to achieve the rate of collisions that CMS and Atlas at LHC need to investigate very rare processes. Producing a proton beam is easy: you take hydrogen, strip electrons off, and there you go….

Finally, one must mention that colliding protons versus protons is not exactly the same thing as colliding protons versus antiprotons, as far as the physics output is concerned. Strong interactions do not care whether the projectiles are particle or antiparticle: what they care about is the color charge of quarks and gluons, which is the same in hadrons and anti-hadrons. But electroweak interactions do, because they are sensitive to the flavor of the quarks (electroweak processes do not “see” the gluons, by the way). So the relative rate, and kinematics, of electroweak processes is different at the two accelerators.

To summarize: deciding on protons versus antiprotons at the Tevatron was -as far as I understand it – a matter of cost versus effectiveness when the accelerator was designed. The Main Ring, a fixed-target (only protons circulating only in one verse) 400 GeV synchrotron which had helped discover the Upsilon mesons and the bottom quark at the end of the seventies, could be used to run hadron-hadron _collisions_, provided one injected antiprotons in one way and protons in the other. This yielded the possibility to try and discover the top quark without a complete redesign of the machine (although, well, major upgrades were needed anyways). The LHC, being a totally new machine, was instead built with two separate beampipes, and separate magnets in the bending tracts, thereby allowing proton-proton operation, with intensities that should exceed by two orders of magnitude those of the Tevatron – and allow to study 100-times rarer processes.

Hope that helps…

7. The Dude - April 26, 2007


Thanks for the explanation.

8. Tripitaka - April 26, 2007

“Ok, but what about more than two jets and a yellow frog then?…”

9. island - April 26, 2007

I’ll delete the, “MYplot” file once the post falls off the main page.

10. David Heffernan - April 27, 2007

You can’t just leave the story about Melissa dangling. We want to know what happened. By the way, what data set was she working on?

11. dorigo - April 27, 2007

Hi David,

I’m going to tell the story in a post.


12. At a trigger meeting 15 years ago... « A Quantum Diaries Survivor - April 27, 2007

[…] in physics, politics, science, personal. trackback David asks me to recall the story behind the famous sentence uttered by Melissa Franklin at a CDF trigger meeting 15 years ago. I comply below. By the way, I […]

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