“La Nuit des Chercheurs” (Researchers’ Night)

Evening, Friday September 23rd. I came from Saclay (near Paris) to participate in the ‘Researchers’ Night’ event taking place across CERN as part of the European Researchers’ Night initiative. Students aged 13 to 18 were on their way from all around the local area to learn about what on earth it is we do at the mysterious “Point 1” – ATLAS’s home on the LHC ring. Three different groups of 10 or so students were to stay with the ATLAS team in the experiment’s control room from 6:00 p.m. until midnight, helping shifters to take data and monitor the experiment…

Early in the evening, we gave the students an introduction to “Physics at the LHC” as well as ATLAS itself. We wanted to help them understand what our main goals here are, although it was surprising how tricky it was to find the proper words to describe our job! After their stints in the control room, each group of students was invited to fire questions and discuss informally with ATLAS physicists over cookies and soda.

Students used a 3D visualization tool to pick out and study interesting events

Talking can only do so much though. In order for the students to really get a feel for what we do as experimental physicists, we organized a workshop around one of our 3D visualization software programs. Groups of three – two students and one ATLAS physicist – split off to play with the graphical tool, which shows the tracks and energies of particles created by proton collisions. The idea was to “clean up” an event just like the ones the students had been helping to record in the control room. They had to make cuts and selections, and find ways to pull out the most interesting events from the overwhelming jumble of non-interesting events in the background.

The 3D visualization is similar to a video game; most of the students were able to master it incredibly quickly (some of them better than our ATLAS colleagues!) The hope is that, by giving these young people a glimpse of the way physicists may eventually find new particles or new phenomena, our research domain now seems more accessible to them.

Laurent Chevalier



Laurent Chevalier – a research scientist with CEA Saclay, France – helped design and build ATLAS’s muon spectrometer and is studying high mass di-muons. He is involved with measuring understanding the magnetic field in the ATLAS detector, precision mapping of the muon spectrometer, and developing 3D event visualization tools.

ATLAS never sleeps

Working in an international laboratory like CERN is incredibly exciting, and I’m not just talking about Higgs hunting. People in the outside world are endlessly curious about what happens on the sprawling two-kilometre-long site, and I get asked all kinds of questions, ranging from the funny to the profound.

I’ve been asked how many CERNs are there (?!), and whether everything happens inside the iconic Globe that’s visible from the road that passes the site, as well as from planes flying in and out of Geneva. While those questions are charming, I’m going to address another commonly asked question today. Namely – what our working hours are like, and whether we switch off our experiment before going home at night.

Yes, there are support staff who work regular hours. But for engineers directly involved in accelerator or detector operations, or for physicists making sure ATLAS is getting good uninterrupted data, there’s simply no such thing as regular working hours! The LHC’s 27 km underground tunnel contains super-cooled superconducting magnets. These must be maintained at a temperature of -271 °C, in order to provide the kind of magnetic field strengths needed to bend beams of protons around the ring. It would be totally inefficient to cool the magnets down each day, and indeed it takes weeks to cool them to that temperature in the first place.

The upshot of this is that, once everything is up and running, the proton beams are kept circulating and colliding for as long as possible. We want to record data whenever it’s being generated by collisions, so the ATLAS control room has to be staffed 24/7. Physicists take turns to do eight-hour shifts, keeping a close eye on the detector and its data recording systems. Experts for several different detector systems have to be ‘on-call’ too – at the end of a phone line, day or night – as any problem which compromises data collection or recording means we’re not making use of the collisions, defeating the whole purpose of the run.

Whilst data comes in at a furious pace at one point on the map, physicists all over the world are busy analysing data that’s already been collected. Individual physicists burning midnight oil are all too common, but we have the added benefit that the institutes making up the ATLAS Collaboration span the entire world. While our colleagues in Japan start working in the morning, others in California are calling it a night. Wherever we are, we always wake up to see emails from collaborators in a different time zone, so not only does the data-taking not stop, neither do the analyses and discussions.

The way big collaborations like ATLAS work is changing the way science is done. And no, that globe by the entrance is way too small to contain all these activities!

Deepak Kar is a postdoctoral research fellow with TU Dresden. His physics interest is soft-quantum chromodynamics, and he is currently involved in underlying event analysis activities in ATLAS.

Re-hashing reconstruction

Now that the big summer conferences are under our belts, we’re busy reprocessing the data ATLAS has taken so far in 2011. The raw data we collect at ATLAS – basically millions of electrical signal values from the different bits of the detector (see Nick’s post for details of how the detector collects raw data) – has to be treated (‘reconstructed’) to turn it into meaningful physics data that can be analyzed for signs of new physics. The reconstruction transforms the raw electrical signals left by particles traversing the detector following a proton collision, piecing together the clues to build up a picture of their trajectories, energy and momentum. The bottom line is: without the reconstruction, we can’t produce any physics results.

Once or twice a year we re-run the reconstruction on the original raw data. Those ‘re’s can get confusing, but I’m about to add another, to simplify (!) things… We call the re-reconstruction ‘reprocessing’. You might wonder why we re-hash things like this (there’s another…). The answer is that the software we use to turn those electrical signals back into a picture of the real world is constantly improving. We’re learning and tweaking as we go along; six months after we first reconstruct a dataset, the algorithms and calibrations are more sophisticated than before, and can significantly improve the ‘physics potential’ of the data.

For example, this time around we’ll be able to measure the momentum of charged particles more accurately, because we’re using a more accurate alignment of the detector – we know in more detail which bits of the detector are shifted a millimeter to the left or right of perfect, and can correct for it.

This rather technical looking plot shows the reconstructed mass of the Z0 boson decaying to 2 muons. You can see the improvement in detector alignment that we get using the reprocessed data. The red points are the reprocessed data and the black points the data with the old reconstruction. The grey shaded histogram shows what we would expect with perfect tracker alignment.


I’m one of the people organizing the reprocessing which is a quite a challenge. It involves people from all the different parts of the experiment, and from all over the world, so a lot of coordination – and a LOT of meetings – are needed! So far, apart from the odd minor hiccup, things have gone well – which is a big relief.

Many different parts of the experiment contribute to the reprocessing. New software and calibrations need to be carefully validated by each of the different physics and detector groups, and the computing experts are responsible for actually running the reprocessing at computing centers all over the world, using Grid technologies. Finally, the data-quality group carefully checks the reprocessed data to make sure it looks roughly as expected. Together, these groups add up to hundreds of people working hard to make sure the reprocessing is successful. In the latest ‘campaign’, we ran over about a billion events and produced more than a petabyte (that’s a million gigabytes!) of output data; it really is a huge undertaking.

Although reprocessing brings clear and important improvements to the physics data, it’s always difficult planning when the best time to carry it out is. It takes some time for the physicists doing analysis to study and understand the data that the updated software spits out, so each time we switch to a newer version, there’s a delay before physics results start appearing. This time, we chose to do it immediately after the Lepton-Photon conference, because the gap in the conference schedule gives us a bit of time to get used to the changes before presenting new results with the reprocessed data.

Reprocessing is a huge job, both computationally and organizationally, but every incremental improvement to the software and calibrations brings us closer to exciting new physics, like the Higgs boson or evidence of dark matter, and that makes it all worthwhile!

Jamie Boyd


Jamie Boyd is a CERN Staff Scientist based in Geneva. He is ATLAS’s Deputy Data Preparation Coordinator, ensuring that billions of events worth of data are processed quickly and correctly so that hundreds of physicists can get to work analyzing them, and is searching for signs of the elusive dark matter.


Ars Atlastronica

So I’m back from the Ars Electronica 2011 festival in Linz, Austria. This year the guest of honor was CERN, to kickstart a cultural partnership which will endure over the next three years. The event was amazing, and the organization spotless. As Claudia mentioned in a previous post, CERN was well represented visually at the festival, mainly via a strong display of ATLAS multimedia throughout the many exhibit halls and events.

Ars Electronica 2011 "Origin" logo

Ars Electronica 2011 "Origin" logo

I participated in two panels on Saturday. The first one was on ‘Open Research’. I talked about ‘Open Educational Resources’, and introduced the audience to a new set of applications we have been developing for outreach purposes and use in Masterclasses, called AVION and CAMELIA. The latter is a kind of ATLAS simulator, where users can navigate interactively through a realistic looking model of the experiment, view it in a beautiful artistic transparent rendition (we call it “jelly”), and load and display events. These events can then be analyzed with tools built into the application. And you can run it in a Mac, Windows PC, Android, iOS or even in a browser, following the installation of a small plugin.

One feature of CAMELIA which makes it a very original open educational resource is its ability to record, edit, upload and share actions trough a system called ‘DIVE’. It’s like an automated guided tour of an event analysis, complemented with subtitles, and will in the future include multimedia explanations of physics concepts. Your very own electronic tutor! In a few months it’ll be possible to plan and share whole lectures or Masterclasses trough this system. We already plan to release a public beta of this application by October.

CMS Physicist Michael Hoch using an ATLAS video in "Deep Space"

The other panel was called ‘Inside CERN’. There I was in amazing company, sided by CERN colleagues Michael Doser, Dietrich Liko and Werner Riegler. Together we presented an overview of what we do at CERN, the history of particle physics, what is it like to work in an experiment involving such large collaborations spanning across many countries and institutions, what are the fundamental questions we aim to answer by doing fundamental research in High Energy Physics, what do we really see inside a detector, and how do we filter that data… It was a two-hour long presentation and session of questions and answers, which kept a very interested audience amazingly engaged.

I made my talk entirely using the multimedia slide system embedded in CAMELIA, and the inbuilt interactive event analysis tools. This was preceded by a virtual fly-by of the ATLAS site and cavern using  AVION. The talk took place in a special room at the Ars Electronica Center called ‘Deep Space’. It consists of a gigantic 16-meters-wide by 9-meters-high display, ideal for colorful multimedia presentations. Physicists from other experiments took advantage of that fact too, as well as ATLAS’s extensive multimedia collection, to spice-up their presentations (as you can see in the picture above).

Then on Sunday night, something very special and beautiful happened. In a concert session during the Große Konzertnacht at the Brucknerhaus (the city’s biggest concert hall), the Bruckner Orchestra of Linz interpreted ‘Mysterious Mountain op. 132‘ by composer Alan Hovhaness. And what did they project on the Cinema screen behind..? Just check out the video below:

Those tracks are LiveEvents (TM) from ATLAS! They’re visualizations made by digital artists Daito Manabe, Motoi Ishibashi and Satoru Higa with help from myself. They also captured the maestro and front row musicians’ 3D outline with Kinect devices, and superimposed it over our live data. Absolutely fantastic. I had never seen anything like this before. I’ll keep in touch with the artists, and make sure we’ll have more interactions like this.

I came back full of fresh ideas to enhance our outreach arsenal, and contacts of renowned artists interested in collaborating in our future multimedia projects. This was also the first public trial of CAMELIA and AVION outside CERN. The comments from the public and artists on the new set of tools were encouraging and inspiring. Watch this space!

Joao Pequenao


Joao Pequenao is a visualization expert and digital artist with background in physics. He works for the Lawrence Berkeley National Laboratory and the ATLAS Experiment. His main focus is producing multimedia portraying scientific concepts in an artistically appealing and accurate way.

Science and art collide at Ars Electronica

Located in Linz, Austria, Ars Electronica is an exhibition centre and creative lab which “has been investigating the consequences of the Digital Revolution” since the late 1970’s. Ars Electronica holds a yearly festival that attracts thousands of people from Austria, Germany and the rest of the world. This year, the theme of the festival, which is happening in collaboration with CERN, is ‘Origin – how it all begins’.

As we approached the downtown area of Linz on Wednesday, we started to see ATLAS event displays all over: on posters, flags in the main street, and up high on tall buildings. Soon the cubic and modernist Ars Electronica centre was in sight. As we walked in, we could feel an excitement in the air. A line of people was already forming at the door. We met a couple of the people from the Ars’ team who had visited CERN previously, and they gave us a quick tour of the place.

Up and down stairs, we found ourselves inside a room with educational material about CERN, the LHC and its experiments. However all the attention was stolen by the main projection of the ATLAS experiment in the end of the room. The brilliant idea was: collisions triggered by sound! Visitors would come in and play at the pool table in front of the projection and as the balls collided, event displays were produced in the background. Wouldn’t it be great if science could copy art? One of the guides of the exhibit approached us offering an explanation, but as soon as he heard we were from CERN, it all turned into questions.

As the night approached, hundreds of people started to gather outside for the opening events. One of the main attractions was called ‘Game Boy Music Club’; artists making music from and playing with game boys, while the Ars Electronica building lit itself rhythmically in the background. The other main attraction was yet to come: Tesla Orchestra.

It was only the first day and there are a great number of interesting pieces in the program with some trying to answer questions about origins and others giving origins to new ideas. The program has conferences, events, concerts, performances, exhibits, open labs and workshops. In the list of speakers from CERN are Rolf Rouer and Sergio Bertolutti among others, and from ATLAS: Fabiola Gianotti and Joao Pequenao. Lisa Randall also will make an appearance.

Stay tuned!

Claudia Marcelloni


Claudia Marcelloni is a creative communications expert working at the interface between ATLAS and the public, and the photographer behind the book ‘Exploring the Mystery of Matter’.


Photographs courtesy of Claudia Marcelloni, for more see here.


Philosophising physics

Last Monday (August 22), within a tight 35-minute allocation, ATLAS’s Henri Bachacou presented the entirety of the results from ‘Beyond the Standard Model’ searches for BOTH the ATLAS and CMS experiments, to the Lepton Photon conference in Mumbai, India. This included results of studies on Supersymmetry, strong gravity, heavy resonances and long-lived particles, and was a staggering amount of information to convey in an extremely limited amount of time. Henri did a great job, firing through slides, and guiding the audience through the most up-to-date results from the wide range of exotic topics. He did have one thing on his side, however… from each search, from each physics topic and from each experiment, the results came back the same: Has the LHC seen anything beyond the standard model yet?  Nope.

As of the consequences of the lack of evidence for new physics, combined with the exotic locale, what we received instead was a monsoon of limits. The table below summarizes these quite colorfully. As impressive as this table looks, these are just the limits set by ATLAS, so the full salvo delivered is roughly twice this amount, when CMS limits are included.

Infographic showing the mass reach of ATLAS searches, detailing the current status of the limits ATLAS are able to set on a large range of physics models and final states, for a representative selection of ATLAS results.


As we’ve all heard for the last few years, these are exciting times in particle physics, it’s just that nature is not making it easy for us. Perhaps there was some deeper wisdom and appropriateness in holding this year’s Lepton Photon conference in India. In the ten yamas, or the controls and restraints every ideal Hindu should follow, the fifth restraint stresses kshama, or patience. It teaches the restraining of intolerance toward circumstances. At the present time we are at the edges of our seats in the LHC’s theater. However, I think we shouldn’t be impatient towards the physics and results coming out of the LHC. We are not owed anything from the machine and detectors; we have to earn it.

The staggering amount of results presented by Henri and all of the speakers at this conference took an unimaginable amount of dedicated work from not only from physicists conducting these analyses, but also from the collective efforts of everyone involved in making the LHC reality over the last 15-20 years. Furthermore, a large part of the limits being produced, and shown above, are the current ‘world best’, edging beyond the horizon set by our predecessors after only one year of data taking. So as the fifth restraint in the tenth yama suggests, let’s have ksharma, enjoy the abundance of results appearing now, and allow nature to reveal itself at it’s own time, as I’m sure we will not be disappointed.

Eric Williams


Eric Williams is a final year PhD student with Columbia University and has been based at CERN since January 2009. He worked on estimating missing energy in proton collisions and is now searching for new particles decaying to W bosons as part of the Diboson Resonance group.