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Archive for April, 2011

Jet spotting

Saturday, April 30th, 2011

Every week I try to take a few hours to study something different. The idea being that this will give me a broader sense of what’s going on within the ATLAS collaboration and the world of particle physics at large. Last week I was mostly watching Gavin Salam’s superb lectures on jets. They’re available as videos from here.

So what is a jet? It’s certainly nothing to do with aeroplanes. Jets are what we observe at ATLAS when a highly energetic quark or a gluon (collectively referred to here as partons) is produced in a collision.

I won’t take the time to explain the physics behind a jet and how they come into being. Those interested can see Flip’s excellent post. In essence, instead of the individual partons, what we see in the detector is a spray of collimated particles. This is what we refer to as a “jet”.

At hadron colliders, such as the LHC, jets are everywhere. In fact the vast majority of interactions at a hadron collider will result in the creation of multiple jets. They are our window on partons and on to the strong force itself.

Being so ubiquitous it’s important that we’re able to reliably identify these within our detector. Unfortunately this isn’t always such an easy task. The event display below illustrates a typical jet event. How many jets do you see?



Here is ATLAS’s answer.



In this case the jets have helpfully been colour coded. In real life, this doesn’t happen.

As you can tell the definition of a jet can be somewhat ambiguous. At ATLAS the trigger system has to quickly identify thousands of jets a second in order to pick out the interesting events to record. Identifying such a large number of jets is no easy feat.

To solve this problem we use jet algorithms. These are pieces of software which define jets based on what we see in the detector. They come in all sorts of shapes and sizes, from “simple” versions where a jet is defined as all the particles inside a cone, to more advanced versions which sequentially combine together individual particles based on their separation and energy.

Different algorithms have different strengths and weaknesses. Cone based jets are relatively simple and provide nice, round jets. Unfortunately though, the jets they identify can easily be altered by changes within the jet itself, or by small amounts of energy coming from unrelated collisions. This makes them very hard to compare to the predictions from theory. More complicated algorithms such as the “kT” algorithm remove these ambiguities, but often result in “ugly” irregularly shaped jets.

The current vogue algorithm both at CMS and ATLAS is the so called “anti-kT” algorithm. This starts from the most energetic single particles and sequentially combines them with everything nearby, stopping at some pre-defined distance. This algorithm results in the identification of nice, round jets, and does this consistently regardless of the small amounts of additional energy or the structure of the jets themselves.


New Ubuntu Linux 11.04

Friday, April 29th, 2011

So I promised myself I wouldn’t put the most common blog entry I think there is on the internet by not writing about how I am sorry that I haven’t been blogging frequently and then make (most likely false) promises about blogging more (did I just fail at that?)

Anyways, I thought I would instead talk about the new Linux release of Ubuntu that I just downloaded and finally decided to abandon the dual boot world of Windows/Linux for my laptop and instead just use Linux exclusively.


I had effectively been doing this for the last year, but always had the safety net of being able to boot in windows if necessary. Now, after a glowing experience with Ubuntu Linux I went completely without a net into this new release!

Previously I was using Ubuntu 10.04 and was very pleased. As a member of the high energy physics community I was really pleased to see that the Ubuntu community had included CERN’s program ROOT as part of their software center! For those of you outside the physics world ROOT is THE FUNDAMENTAL PROGRAM used by physicists for just about everything we do. Without ROOT, we would likely be using an abacus and graphing paper.

Needless to say I was disappointed in the new version not to find the same convenience ready to install ROOT with a “click of the mouse”. However, unlike when I was a young grad student, the installation process has become much easier and better documented. Thusly, I was surprised when ROOT complained that I didn’t have a library installed (X11 for those that care) despite me explicitly following the well documented instructions on their page (http://root.cern.ch/drupal/). What I realized is that the libraries did in fact exists just not where ROOT was looking for them. Once I found this I made a series of softlinks (computer jargon for pointed the computer to where to look) and things ran swimmingly!

I thought I’d post the libraries that gave me issues in case there was anyone out there in the physics world considering changing brands of Linux and wanted to give Ubuntu 11.04 a whirl (Because heaven knows we can’t have a machine and NOT have ROOT installed)


jasaadi:/usr/lib$ sudo ln -s /usr/lib/x86_64-linux-gnu/libX11.so /usr/lib/libX11.so
jasaadi:/usr/lib$ sudo ln -s /usr/lib/x86_64-linux-gnu/libX11.so.6 /usr/lib/libX11.so.6
[email protected]:/usr/lib$ locate libXft.so
jasaadi:/usr/lib$ sudo ln -s /usr/lib/x86_64-linux-gnu/libXft.so /usr/lib/libXft.so
jasaadi:/usr/lib$ sudo ln -s /usr/lib/x86_64-linux-gnu/libXft.so.2 /usr/lib/libXft.so.2
jasaadi:/usr/lib$ locate libXext.so
jasaadi:/usr/lib$ sudo ln -s /usr/lib/x86_64-linux-gnu/libXext.so /usr/lib/libXext.so
jasaadi:/usr/lib$ sudo ln -s /usr/lib/x86_64-linux-gnu/libXext.so.6 /usr/lib/libXext.so.6


Bob Peterson continues to travel with his QuarkNet  particle detector around the edge of Africa recording remnants of cosmic rays. This offers a chance to study how cosmic ray recordings differ on land and sea and at different latitudes. The data will be accessible to high school students and teachers in several countries who use similar detectors to learn about particle physics.

His first post explains why he’s taking his science to the seas and how getting a detector on a boat sounds easier than it is.

–April 20: Breakfast at 0730 and you better be on time. The galley crew frowns on late comers and they clear the tables whether you are done or not. Other scientists have boarded in preparation of our departure. There is a group of atmospheric chemists studying particles that nucleate clouds. They had a gigantic shipping container lifted aboard last night, and spent the night setting up. They look a bit bedraggled. Soon, I will be too. I want to get the detector on the air before we depart. But, still, the cosmic ray muon detector sits on the cargo deck and I can’t carry it up seven flights of stairs.

Cosmic ray muon detector aboard ship. Credit: Fermilab

–April 20, 1200: It’s noon and finally the cosmic ray muon detector (in a box) is sitting in front of the ship’s weather office. My contact, Michael Walter, from the high-energy physics laboratory DESY/Zeuthen in Germany,  does outreach with students using cosmic ray detectors and is a collaborator on IceCube. He  previously installed his own detector. My detector will live directly above. By having two detectors of the same type we can double check data and also have a backup for when one of the detectors malfunction, which has happened at times on previous trips. The afternoon will be busy: assemble, plateau, capture data, record data.

–April 20, 1950: Didn’t quite make collecting data. The cosmic ray muon detector is alive and healthy and all counters plateaued. That means that the detector has hit the sweet spot where the photo multiplier tubes are recording the optimal amount of photons. Below that spot we would be missing valuable data and above it the data would get muddy. While all this is great, I lack a computer to record data. I need drivers installed to talk to the data acquisition system, and the computer I’m using doesn’t have drivers. No driver; no data recorded. I must have the data. I will work on this problem
Right now, the pilot has arrived to shepherd the R/V Polarstern out of Cape Town and underway. I will hide in the dark corners of the bridge and stay out from under foot.

— April 20, 2001: Much is going on. The gangway has been taken in, the springs are loosed and weare headed north and west. The pilot departs with the briefest of words. It’s dark and the Southern Cross constellation  is rising in the east; Orion is settling in the west. We clear the breakwater at 2020, and, oh my, what a westerly swell. The weather officer warned me: Be ready. I am not, and my stomach rolls over. It’s been 30 years since I have felt such a thing, and I’m as green as the wreaths of Christmas. It’s going to be an unpleasant night. Quickly I retire to my bunk.

–April 21AM: The Polarstern is a big ship, but it is getting pushed around by 6-to 8-meter swells. After all we are near the roaring forties winds and the dominate westerlies. The weather guys have quickly become my friends as they point out that this will pass soon enough as we enter the southeast trade winds.
But, I can’t wait. I have to get that cosmic ray muon detector data recorded no matter my condition. If you need friends, look to your weathermen, named Klaus and Max, and your systems administrator, named Felix. Installing the driver has proven problematic for the available Ubuntu machine. Felix enters with a brand new Mac: “Can you use this?” Within minutes, I have data sliding neatly into the open file, and now I can relax. Back to my bunk where I will spend the afternoon.

–April 21 PM: Klaus was right. Slowly the swell subsided and so did my constitution. I managed to attend dinner and felt better. Not great; just better. More good news: the cosmic ray muon detector is behaving.

— April 23: I’m managing many things now. I’ve figured out the meal schedule. I’ve learned who to go to with questions: Klaus and Max and Felix are always willing but steer clear of the cargo mate. He’d just as soon bite your head off. I’m always welcome on the bridge and Philipp is good at providing details. I know much of this, though I’m a bit rusty. It’s all coming back. And still the cosmic ray muon detector data flows. Good thing.

— April 23 PM: The crew relaxes with a movie and the bar is open. Everyone settles into the routine. Tomorrow is Easter. I will have to think about how to worship. I suspect I’ll be all alone about it.

— Bob Peterson


Related information:

You can follow the journey at:

You can e-mail Bob questions at [email protected] Please send only text, no images.


*Galley: A ship’s kitchen.

*Bridge: The elevated, enclosed platform on a ship from which the captain
and officers direct operations.

*Springs: Dock lines

*Breakwater: A barrier built out into a body of water to protect a coast or
harbor from the force of waves.

*Swell: A slow, regular movement of the sea in rolling
waves that do not break.


Intention to post…

Thursday, April 28th, 2011

Hey QDers…

So…I just spent most of my day and night coding for the ASYEOS experiment’s online monitoring. 😀 ASYEOS is a group looking at the nuclear equation of state, and how it differs for neutrons and protons, particularly in high density environments, like when we smash metal nuclei together at about 400 MeV (using SIS at the GSI facility, Darmstadt, Germany). I haven’t posted in an age, because since my new job started I’ve been busy helping with preparations for our experiment out here in Germany (and briefly publicising it in Glasgow at the IOP’s nuclear and particle physics divisional conference). Things are pretty mad at the moment and the next month will be the busiest of them all, as our experiment runs.

Our cave, which has been closed for a few weeks for another experiment, opened again this morning, and as soon as the radiation-safety door light turned green, in flooded the many members of our group, who have traveled from America, Poland, Italy, UK…our various detectors are being installed and tested over the next few days and everyone has their hands full. This is going to be an incredible month of data-taking. I promise to post about it just as soon as its over (I have had my camera out and hope to make an amateur video diary of sorts)! I promised my fiance I’d make him something of a diary to get us through the time apart. He’s been great through it – I tell you something, it’s not easy planning a wedding from abroad, especially when you’re working 13 hour days, and he’s helped out a lot!

Anyway that was all. There have been some excellent posts on here recently. I can’t wait to update you all on what’s been going on at GSI. For now though, it’s bed time in Darmstadt. Sorry!


The Hunt for the Elusive Reason

Thursday, April 28th, 2011

— By Lindsay Davies, Communications Assistant

I think it’s safe to say a good chunk of the population is not all that familiar with science – myself included, so we wonder “Why is it important to put so much money into scientific research if we don’t know that anything will come of it?”  Well that’s a main reason – we don’t know if anything will come of it, but we if we don’t try, we will never know.

Every Wednesday, TRIUMF has an open lecture for students to help us understand the concepts behind the science we do here.  Yesterday’s lecture was on the importance of fundamental research and how you never know what’s useful until you fully understand it.

The first example was from way back in the 1600s, where Antonie van Leeuwenhoek first discovered microbes.  Having a familiarity with glass, van Leeuwenhoek created small glass lenses through heating small shards of glass in a hot flame, and discovered they could magnify objects.  He created varying magnifications, which he then used to study the microorganisms in the pond water. He discovered bacteria which was then discovered to make anyone in his town of Delft in the Netherlands sick that drank it.  Over his lifetime, he ground more than 500 optical lenses along with over 250 microscopes. Through the microscopes, he made many other discoveries, including the banded pattern muscular fibers in 1682.

But if you asked someone back then to put money into researching glass to view microorganisms before anyone knew they existed – do you think they would?  They’d think you were crazy! But now we know that research can take us many places unknown.

Now let’s jump to the late 1800s.  Electrical discharges were observed within vacuum tubes (cathodes) when voltage was applied to it, later to be discovered as streams of electrons (which are now more commonly known as electron beams).  Through his experiments, Wilhelm Röntgen discovered that the rays created within the cathodes could penetrate material.  When looking into the external effects from various types of vacuum tube equipments as an electrical discharge would pass through them, he found that the cathode rays caused a fluorescent effect on a small cardboard screen with barium platinocyanide painted on it.  This led to further experiments where he covered the tubes with a light-tight cover. When testing to ensure this, he noticed a faint shimmering from the barium platinocyanide screen he was planning on using next.  He then considered the idea that a new type of ray could be possible, which he dubbed the “x-ray”.  A few weeks later, he took the very first picture of his wife’s hand with this new technology using x-rays, creating an image of her hand’s skeletal structure.

But back then, if you asked the population to fund research into the effects of running electricity through vacuum tubes, they again would have thought you were crazy.

So who’s to say what will happen in the future with research?  What if we do discover the Higgs Boson? In theory, it should resolve inconsistencies in current theoretical physics, but what else could it help discover?  For all we know it could help us determine something arbitrary, but it could help us discover something extraordinary. And what if we don’t discover the Higgs Boson, but discover something else entirely? Where will that lead us? We’ll never know unless we try.


Bob Peterson has agreed to forfeit a month of his time and his non-sea-loving stomach, for a briefer period, in the name of science.

Bob Peterson

This Fermilab employee and QuarkNet instructor will shepherd a cosmic ray muon detector through rolling ocean waves around the edge of Africa to gather data to help the world’s largest frozen particle detector, the IceCube neutrino observatory at the South Pole. The detector is the same type as those built and used by high school students in cosmic ray study projects, including QuarkNet program, which allows students throughout the world to collect and share particle physics data. QuarkNet gives students a chance to interact with physicists and get a taste of what it would be like to work on a global experiment such as IceCube.

IceCube serves as a neutrino telescope peering at neutrino particles that cascade out of collisions of high-energy cosmic rays with the Earth’s atmosphere. The trick is that IceCube only wants to look at these cosmic neutrino remnants coming from a certain direction, from the opposite hemisphere and through the Earth, not those falling directly overhead of the detector.

The data Peterson collects will help IceTop, a smaller detector set a top IceCube, to give the most precise data to the IceCube collaboration so that it knows it is focusing on the right particles. His data will be combined with data collected on a similar trip last year in Antarctica by a group of Wisconsin college physics students. This combined data will tell researchers the varying intensity levels of cosmic ray remnants as you travel from the equator to the poles. This will help the IceTop collaboration calibrate its detector to compare data to IceCube and help IceCube reject background particles from downward cosmic ray remnants that can obscure the detectors’ views of neutrinos moving upward through the Earth.

Sketch of the IceCube detector. Each cross on the surface represents two IceTop tanks. Credit: IceTop Collaboration

Studying neutrinos in one of the coldest places on Earth will help scientists get a better picture of where high-energy neutrinos originate and how they contribute to the universe’s most violent events, exploding stars called supernovaes. These explosions spit out the heavy elements necessary for the creation of life on Earth. Without these neutrino-fed explosions the universe would look very, very different.

And so Peterson, an avid sailor, who gets sea sick for the first few days of every journey, packed up his gear and headed out to learn about the universe and its most distant, violent objects by riding on a boat.

He is blogging his adventures in science and seafaring here at Quantum Diaries. A glossary of sailing terms will appear at the bottom of each post to aid readers.

23 Apr 2011, R/V Polarstern
Lat: 24-50.5S
Long: 9-42.7E
somewhere off the Namibia coast
Heading: 320degT
Speed: 12 knots

Bob’s Blog begins wherein Bob is on a voyage aboard a German research vessel to the Southern Hemisphere and discovering that using the Internet aboard a ship isn’t that simple.

–April 18: I had a really nice blog written yesterday, but the mail server timed out, and poof. No more. Then I found out from others that, oh yes, that happened to them, too. Well, I won’t do that again.

–April 19 AM: I arrived in Cape Town, South Africa, late Monday night after a 26-hour flight. The trip took me through Dulles Airport and then through Dakar across the Atlantic. I recommend South Africa Airlines; they are good with comfort and food. But still, there I was standing in CPT wondering who was picking me up. After a few nervous minutes, the driver for the ships’ agent, Randal, found me contemplating my next move. Ah, now I’m in good hands as we visit the immigration office where there is much stamping of documents.

I was somewhat foggy after such a long trip. Finding my ship proved not so simple because another ship on the quay was bunkering at the time of my arrival. But I finally made my way down the narrow quay to the gangway and the R/V Polarstern. My question, “Permission to board?” was met with a blank stare, and by then Randal was long gone so retreating was not an option. Just when I thought I would have to take my chances on the neighboring rust bucket (the one that was bunkering), the 2nd Mate, Philipp Gumtow, appeared welcoming me aboard; he took charge of the situation. Soon, I was surveying my cabin and eyeing a nice bunk.

–April 19: My wake-up alarm came from a departing tanker from the next quay over (not from the rust bucket). The traditional departure signal is one long blast, and they care not who is sleeping. Fine, I’m awake. Time for breakfast. Where’s the galley? I find it, and there I meet Philipp again. He tells me, “No, this table is reserved for crew; you sit over there at the science table”.  Ah, the world on the Polarstern is segregated: crew, officers, scientists. I think I fall somewhere in between. A ship is like a summer camp. It is all about procedures and everyone must abide or chaos ensues. The trick is find a friendly party that will explain those procedures before I step over the unseen boundaries. Tina was just that person. She is a biology grad student studying the effect of ocean warming on pelagic fish.

Then I try a quick tour of the ship, but within three turns of a corner I’m completely turned around. The Polarstern has seven decks connected by a central stairs and many doors with inscrutable labels all written in German. This is my handicap. Still, my exploration starts from my cabin to the galley and back and expands from there.

–April 19 AM: The Cape Town harbor sits at the bottom of the continent where 5,000-foot mountains loom. It shelters many ships bound for ports to the West, but not the East because of the threat of pirates. Cape Town traffic has increased as cargo is off loaded to rail and truck to avoid the east side of Africa, and likewise cargo is received for the journey west.

There are big ships here: tankers, container ships, general cargo, oil rigs, crew boats for the rigs and rust buckets. There are some tied to a remote pier called the graveyard. They will soon become target practice or cut up for scrap. One ship stands out; so I go ashore for pictures. With its many derricks and lots of deck plumbing, I want to get a closer look at the Peace in Africa. No kidding, that’s the name. Immediately a shore officer wants to confiscate my camera. “No way,” I say. Then I notice the owner: DeBeers. This ship mines the ocean bottom for diamonds; think Howard Hughes and magnesium nodules. Peace in Africa my foot. I beat a hasty retreat back to the safety of the Polarstern.

QuarkNet detector taken on ship. Credit: Fermilab

— April 19 AM: All the while, I’m wondering: Where is my cosmic ray muon detector? I ask Sonja who is the agent handling all the Polarstern logistics, whether she has seen it. She says that she will check and let me know.

— April 19 PM: Whoops, my first surprise. The money onboard is the euro……. I knew that, but they require cash. Now I’m in trouble; my credit card will do me no good. I must come up with cash; Sonja offers help and a driver to a downtown bank. But, this quickly goes bust as the banks refuse to exchange dollars for euros especially for someone on a ship. You must have an account and a permanent address. Hmmmm, I think they’ve been burned before? What to do?

–April 19 PM: Sonja has found my box. It’s in Cape Town. Closer, but still not onboard.

–April 19 PM: All ships use an agent for logistics and paper work. The Polarstern uses Meihizen International. They are skilled at problem solving, including my cash shortage problem, and they gladly offer help. First, I will retrieve South African rands from an ATM; then the agent will go to his bank for euros because he has permanent-resident status. My money goes from credit card dollars to rands to euros and I can feel a slice out of my funds at every step.

Still, the snafu gave me a tour of Cape Town and a genial meeting with the president of the agent company. He’s a sailor; I’m a sailor. We get along famously; I’m invited back for a cruise on his yacht. Nice yacht.

–April 19, much later PM: The day is getting late, and I learn we will depart at exactly 2000 (8 p.m.) on Wednesday with or without my cosmic ray muon detector. Sonja says the box is now at the agent’s office. It’s closer and will be delivered late afternoon. Sure enough, I see it come out of the delivery truck and get hoisted via the ship’s crane (I think: Please don’t drop it boys) to the receiving deck.

But, still I can’t have it. I am told that the cargo mate must verify the box on his manifest list. The officer will release it tomorrow morning.


*quay: pronounced “key”.  A concrete, stone, or metal platform lying
alongside or projecting into water for loading and unloading ships. Similar to a

*bunkering: fueling a ship.

*galley: kitchen

*manifest: A document giving comprehensive details of a ship, its cargo
and other contents, passengers,and crew for the use of customs officers.

— Bob Peterson


I know, it has been a while since my last post. I also realize that I’m writing openings like this all too frequently lately: Too much work to do, in particular many things that involve writing of some document or other, but of course also a few trips here and there. Right now, I’m on my way to Hamburg for two days, and last week, I spent two days at CERN. As usual, two days packed with various meetings, but I also had the time to see how the preparations for this years CALICE Tungsten HCAL test beams are going.

After a first, very successful beam period last year, this year we are going for high energies at the CERN SPS. First beam is less than two weeks away, so we are busy with preparations. These tests are absolutely crucial: The Tungsten HCAL (for a very brief answer to the obvious question: Why Tungsten? See here… ) we are testing is, after all, a prototype for a calorimeter at CLIC, where we expect very high-energetic particles and jets. These high energies are actually the motivation for using expensive Tungsten, so we better also test our detectors with highly energetic particles.

Puting things back together: Installing Tungsten plates for the CALICE Tungsten HCAL test beam due to start in less than two weeks.

Last year, our first commissioning test was performed at the CERN PS, where we got various particles at energies up to 10 GeV. There are several good reasons to start there: Testing at low energies is interesting, in particular from the point of view of understanding the shower physics and comparing to simulation models. Also, testing at the PS is more relaxed: While the test beam time at the SPS is notoriously oversubscribed, causing very tight schedules, things are a bit easier at the PS, allowing for more time to understand complex systems that get operated for the first time.

Now, we are getting ready for the SPS, with energies all the way up to several hundred GeV, which will show us how big the advantage of using Tungsten instead of Steel really is. A first step of of that I could see last week: The Tungsten absorber structure is getting reassembled, with a few more layers compared to last year. Each of the absorber layers weighs more than 100 kg, so they are lifted in place by crane. What I was looking at in particular was the last layer, clearly visible in the picture: This is just an empty aluminum frame, which will hold a special little experiment run by my group, which already took data last year. From that, we have first results, and I promise to write a post about that, too… This time, I hope you will not have to wait too long.


De pico à femto

Wednesday, April 27th, 2011

Par ce beau dimanche de Pâques ensoleillé et fleuri, nous sommes seize dans la salle de contrôle à garder un œil sur des dizaines d’écrans, pendant que l’équipe du  Grand Collisionneur de Hadrons, le LHC, pulvérise un record après l’autre.

Ça a été un weekend incroyable, chaque nouveau cycle d’opération générant un son nouveau record. Tout a commencé jeudi soir quand le LHC a brisé le record de luminosité avec 4.67 x 1032cm-2s-1, dépassant du coup celui de 4.024 x 1032cm-2s-1 établi par le Tevatron en 2010.

Notre équipe est en place depuis trois jours, un groupe étonnant déjà par sa simple composition puisque nous venons de Malaisie, Ukraine, Russie, Japon, Canada, USA, Pakistan, Royaume-Uni, Portugal, Italie, Chine et la République Tchèque. Comme je suis cheffe d’équipe, j’ai l’occasion de parler avec tout le monde, ce qui est encore plus intéressant.

Chaque après-midi, en venant prendre notre quart de travail à 15:00, l’équipe précédente se vante d’avoir été en place alors que le LHC établissait un nouveau record que ce soit la plus haute intensité des faisceaux (i.e. le plus grand nombre de protons mis en circulation) ou la plus haute luminosité (le nombre de collisions obtenues par seconde) ou encore le plus gros échantillon de données récoltées dans un cycle d’opération. Heureusement, nous pouvions nous aussi faire de même à 23:00 quand la relève arrivait!

Tous ces records n’auraient pu être atteints si on n’avait pas fait une pause de dix jours pour procéder à un grand récurage. Et depuis, un succès n’attend pas l’autre. Bien sûr, il y a eu quelques couacs, mais rien de sérieux. Il faut tout de même compter trois ou quatre heures pour chaque cycle, c’est-à-dire remplir l’accélérateur de protons, les amener à pleine énergie puis mettre les deux faisceaux en collision. Pendant ce temps, dans les salles de contrôle des quatre grandes expériences utilisant les données du LHC, soit ALICE, ATLAS, CMS et LHCb, nous gardons l’œil pour ne pas perdre une goutte de données. Hier, on a eu le plus fructueux cycle pour l’accumulation de données et récolté 24 picobarn inverse en 16 heures. Bien sûr, pico peut sembler petit puisque cela correspond à un millionième de millionième de barn (grange en anglais), cette drôle d’unité de mesure. Mais il faut comparer avec ce que l’on a collecté en une année entière d’opération, soit 45 picobarn inverse. Et oui, une année avec 15 à 20 personnes en tout temps dans la salle de contrôle et ce pour ATLAS seulement… Pas une mince affaire. Ça fait donc immensément plaisir de voir les données rentrer beaucoup plus vite maintenant.

Au tout début du LHC en 2010, on comptait les données en nanobarn inverse, une unité mille fois plus petite. Et sous peu on devrait commencer à parler en femtobarn inverse, une unité mille fois plus grande! Déjà pour 2011, on a plus du double des données de l’an dernier, y’a de quoi être satisfaite!

Eh bien, pour un weekend de Pâques, c’est réussi! Et c’est sans compter tous les œufs qu’on a trouvé dans la salle de contrôle!

Pauline Gagnon


From pico to femto

Wednesday, April 27th, 2011

By the most radiant sunny Easter Sunday, sixteen of us are in the ATLAS control room, keeping an eye on several dozens computer screens while the Large Hadron Collider (LHC) is breaking one record after another.

It’s been an amazing weekend, with new records being established at each new fill. First of all, around midnight on Thursday, April 22, the LHC set a new world record for beam intensity at a hadron collider when it collided beams with a luminosity of 4.67 x 1032cm-2s-1, exceeding the previous world record of 4.024 x 1032cm-2s-1 set by the Tevatron in 2010. What was most amazing is that at each new fill, it got even better.

Our team has been on shift for the last three days. It’s a great crew, with people coming from Malaysia, Ukraine, Russia, Japan, Canada, USA, Pakistan, UK, Portugal, Italy, China and the Czech Republic. Being shift leader, I get to talk with everybody, so it’s a lot of fun.

Every time we came on shift at 15:00, the leaving crew bragged about having witnessed a new record: highest intensity for the beams (that’s how many protons were circulating in the accelerator), or highest luminosity (how many collisions created per second) and now, highest data sample collected during one fill. But we got our chance to do just the same at 23:00 when our replacements came in!

Following ten days of down time dedicated to an intensive scrubbing period to allow pushing the machine higher than ever, it’s been one successful fill after another. Well… there have been a couple aborted attempts but that’s part of the game. It takes about three to four hours to fill up the accelerator with fresh beams, accelerate them at full energy then bring them into collisions. Meanwhile, in the four LHC detector control rooms for ALICE, ATLAS, CMS and LHCb, the four large experiments collecting collision data from the accelerator, we are following each step, waiting for the safe moment where we can turn on the innermost layers of our detectors and start recording the proton collisions. The last run started around noon on Saturday and ended 16 hours later at 6:00 am this morning. During this time, ATLAS and CMS collected about 24 inverse picobarn of data. Well, pico might not seem much to you since it is one millionth of a millionth of that weird unit of a barn, but to put it in perspective, compare that to the 45 inverse picobarns total we got during the whole of last year! Yes, that means, the best of a year with 15-20 people on shift, 24 hours a day, waiting to collect this data. A huge effort, so we are thrilled to see the data coming in at a much-increased rate now.

At the very start of the LHC in March 2010, we were keeping records of the data taken in units of inverse nanobarns, that is measuring it in units one thousand times smaller. So in the course of only one year of operation, we went from counting each inverse nanobarns we received to using inverse picobarns, and very soon, we will be estimating our data sample in femtobarns, a unit a million times bigger. Already this year, we have more than doubled what we had last year and we should reach one inverse femtobarn in the next couple of months.

So this has been a great way to spend the Easter weekend, with lots of data collected, not to mention a very sizeable crop of Easter eggs too!

Pauline Gagnon

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Depuis son redémarrage le 13 mars dernier, et jusqu’à la semaine dernière, le LHC, le Grand Collisionneur de Hadrons, avait produit très peu de collisions pour les milliers de physiciens et physiciennes comme moi, impatients de mettre la main sur de nouvelles données afin de préparer de nouveaux résultats pour les conferences d’été. Pour la simple raison qu’il fallait tout d’abord que l’équipe du LHC puisse reproduire les performances de la fin de l’année dernière, et tout récemment, réussir à nettoyer à fond le tuyau à vide de l’accélérateur.

Mais puisque le LHC est tout neuf, pourquoi un tel nettoyage? Le fait d’être tout neuf est justement le problème. C’est comme lorsqu’on achète une nouvelle voiture: ça sent le neuf! Et le LHC devait perdre cette odeur de neuf.

Quiconque a déjà travaillé avec un système à vide ultra pur sait que lorsqu’on commence à pomper pour en évacuer l’air, ses matériaux relâchent peu à peu les molécules qui s’étaient logées à leur surface. Comme les vinyles et autres composants des voitures neuves relâchent les matières volatiles, les parois relâchent peu à peu les molecules qui s’y sont logées. Même en utilisant des matériaux non poreux comme le verre ou l’acier inox, il y a toujours des molécules qui réussissent à s’incruster à la surface. Ce phénomène est crucial pour le LHC puisqu’on y fait un vide équivalent à moins de un millionième de millionième de la pression atmosphérique.

Au cours de la première année d’opération en 2010, alors qu’on gagnait en expertise, on a réussi à augmenter l’intensité des faisceaux pratiquement à chaque semaine, surmontant les difficultés qui se présentaient à chaque étape. Mais vers la fin de l’année, les molécules relâchées dans l’accélérateur ont commencé à poser problème. Prenons l’exemple d’un boyau d’arrosage. Sous l’effet de la pression de l’eau dans le tuyau, si de la poussière ou des débris s’y trouvent, ils seront libérés et contamineront l’eau, surtout si on double, décuple, voire multiplie par cent mille la pression! Et c’est justement ce qui s’est passé l’an dernier: les molécules libérées, nous empêchaient d’augmenter encore plus la “luminosité”, l’équivalent de la pression dans notre cas.

Parce qu’on a affaire non pas à de l’eau dans notre tuyau, mais bien à des protons qui ont une charge électrique, ça se complique encore un peu plus car ils ionisent ces molécules, c’est-à-dire elles libèrent des électrons qui se retrouvent dans les pattes des faisceaux. Des nuages d’électrons se sont donc formé, empêchant le LHC d’atteindre sa vitesse de croisière. Moins de luminosité se traduit par moins de chance de faire une découverte intéressante.

D’où ce grand ménage printanier. Pendant dix jours, on a injecté des faisceaux de hautes intensités mais basse énergie pour aller récurer la paroi du tuyau de l’accélérateur. On a même mis à contribution ces mêmes électrons qui nous embêtaient. Profitant de la répulsion électrique induite au passage des protons, on projette les électrons avec force contre les parois, délogeant du coup toute molécule de contaminants. En répétant cette opération à plusieurs reprises, et en augmentant à chaque fois l’intensité des faisceaux, on a pu nettoyer à fond toutes les parois et constater qu’elles n’émettaient plus rien. Pour reprendre l’analogie du boyau d’arrosage, l’eau n’était plus contaminée.

Et les bénéfices ne se sont pas fait attendre! Dès le départ, l’équipe du LHC a réussi à injecter des faisceaux beacoup plus intenses. En une semaine à peine, on a pu accumuler autant de données que durant tout 2010, et même briser tous les records!

Maintenant, l’équipe du LHC va continuer à ajouter de plus en plus de protons regroupés en grappes, séparées les unes des autres de 50 nanosecondes (eh oui, seulement 50 milliardièmes de secondes). En ce moment, ils réussissent à injecter jusqu’à 480 grappes mais l’ojectif est de se rapprocher autant que possible de 1318 grappes, le nombre maximum qu’on peut introduire dans la machine. Mais plus facile à dire qu’à faire. C’est un peu comme jongler avec des centaines de balles à la fois! Et il y a deux faisceaux circulant en sens contraire. Tout ça dans le seul but de maximiser le nombre de collisions produites. Pour nous les physiciens et physiciennes, plus de collisions signifie plus de chances d’observer quelque chose de rare et intéressant. Nous sommes donc sur la bonne voie!

Pauline Gagnon

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