From ATLAS around the world: A view from Down Under

While ATLAS members at CERN were preparing for Run 2 during ATLAS week, and eagerly awaiting the beam to re-circulate the LHC, colleagues “down under” in Australia were having a meeting of their own. The ARC Centre of Excellence for Particle Physics at the Terascale (CoEPP) is the hub of all things ATLAS in Australia. Supported by a strong cohort of expert theorists, we represent almost the entirety of particle physics in the nation. It certainly felt that way at our meeting: more than 120 people participated over five days of presentations, discussions and workshops. Commencing at Monash University, our youngest researchers were exposed to a one and a half day summer school. They then joined their lecturers on planes across the Bass Strait to Tasmania where we held our annual CoEPP general meeting.

CoEPP comprises ATLAS collaborators from the University of Adelaide, University of Melbourne and University of Sydney, augmented by theory groups, and joined by theory colleagues from Monash University. CoEPP is enhanced further by international partnerships with investigators in Cambridge, Duke, Freiburg, Geneva, Milano and UPenn to help add a global feel to the strong national impact.

 

Larry Lee of the University of Adelaide talks about his ideas for ATLAS Run 2 physics analyses.

Larry Lee of the University of Adelaide talks about his ideas for ATLAS Run 2 physics analyses.

Ongoing work was presented on precision studies of the Higgs boson, with a primary focus on the process where the Higgs is produced in association with a top-antitop quark pair (ttH) in the multilepton final state and the process where the Higgs decays into two tau leptons (H->tautau). Published results were shared along with some thoughts on how these analyses may proceed looking forward to Run 2. Novel techniques to search for beyond Standard Model processes in Supersymmetry and Exotica were discussed along with analysis results from Run 1 and prospects for discovery for various new physics scenarios. CoEPP physicists are also involved in precision measurements of the top-antitop (ttbar) cross-section and studies of the production and decay of Quarkonia, “flavourless” mesons comprised of a quark and its own anti-quark (Charmonium for instance is made up of charm and anti-charm quarks). It wasn’t just ATLAS physics being discussed though, with time set aside to talk about growing involvement in the plans to upgrade ATLAS (including the trigger system and inner detector) and how we can best leverage national expertise to have a telling impact.

A dedicated talk to outline our national research computing support for ATLAS proved very helpful to many people new to the Australian ATLAS landscape.

CoEPP director, Professor Geoffrey Taylor of the University of Melbourne, in deep discussion during the poster session.

CoEPP director, Professor Geoffrey Taylor of the University of Melbourne, in deep discussion during the poster session.

I was happy to spend time with colleagues from our collaborating institutes and also to meet the new cohort of students/postdocs and researchers who have joined us over the past year. It dawns on me how the Australian particle physics effort is growing, and how we are attract some of the brightest minds to the country. It is exciting to see the expansion and to be able to play a part in growing an effort nationally. The breadth of Australia’s particle physics involvement was demonstrated with a discussion of national involvement in Belle-II and the exciting development of a potential direct dark matter experiment to be situated in Australia at the Stawell Underground Physics Laboratory. The talks rounded out a complete week of interesting physics, good food, a few drinks and a lot of laughs.

As this was the first visit to Hobart for many of us it was particularly pleasing that the meeting dinner was held at the iconic Museum of Old and New Art (MONA), just outside the centre of the city. It proved a fitting setting to frame the exciting discussion, new and innovative ideas, and mixture of reflection and progression that the week contained. Although Australia’s ATLAS members are some of the farthest from CERN there is considerable activity and excitement down under as we plan to partake in a journey of rediscovery of the Standard Model at a new energy, and to see what else nature may have in store for us.

All the CoEPP workshop attendees outside MONA, Hobart.

All the CoEPP workshop attendees outside MONA, Hobart.

 

From ATLAS around the world: Preparing for Run 2 from Colombia

I work at Universidad Antonio Nariño in Bogota, and I have been part of the ATLAS experiment since 2010. After a two-year stay at CERN, I moved back to Colombia in 2012 and since then, I have continued to do my work on ATLAS from here. Being involved in ATLAS and working from Colombia has been a great experience for me; I get to continue contributing to the physics searches I am involved in at ATLAS, and also do other things like teaching, giving seminars, and doing outreach activities.
A typical day for me starts with a videoconference meeting with one of the ATLAS groups I work with at CERN. The time difference means this is usually quite early, but things here in the city start quite early as well so it does not feel that strange anyway. After the meeting it is half way through the morning and some days I have to teach at the University at that time. Then I have all the afternoons to focus on my work on ATLAS, which is now starting to be all about preparing for Run 2 of the LHC.
For a couple of years I have been involved in a particular search for physics beyond the standard model, looking for a charged Higgs boson, different to the Higgs boson we already found at the LHC (which has zero charge). Our results with the Run 1 data are almost finished, and most of the people in the analysis group, me included, are moving to start preparing the search for the same particle with the Run 2 data. Although there is no data yet, it is important that we anticipate the issues that might arise with the new conditions of the LHC, and also that we prepare all our “machinery” (analysis software) for when the data arrives.
As one of the many improvements ATLAS has made for Run 2, the way in which we analyse and process our data has been changed, to improve its efficiency and compatibility across different analysis groups, so that we can more easily compare our results with other colleagues in a more efficient and faster way. So the first thing I am involved in for Run 2 is in understanding and adapting our Run 1 machinery to this new environment in order to be prepared when the new data arrives. This will help us get new updated results quickly, which for this search should be possible rather soon, hopefully just after the first year of data taking.
The other people in my group are also preparing themselves for Run 2. Our group also works on the system that chooses which events to keep and which ones to discard. This is called the Trigger, and here we are involved in developing a particular tool for identifying low mass particles reconstructed from electrons. Electron identification is in general something our group has been involved in for some time, and all this work will be very relevant as soon as we start taking data again, helping us identify where we need to improve and checking whether our algorithms are working as they should in the new conditions of the LHC.
Other than the research work on ATLAS, I also take remote computing shifts, where I monitor from here the behaviour of the ATLAS computing GRID for eight hours a day. This is part of the service work all of us in the collaboration do to share the load of all the different tasks needed to keep the experiment working properly all year long.
In the summer I will be in the ATLAS control room being part of the data taking

In the summer I will be in the ATLAS control room being part of the data taking

Another thing I enjoy doing here is outreach, trying to encourage young people in doing science and showing them a bit of what we do in ATLAS and why it is interesting and exciting. Just this month we hosted two international masterclasses, where school students come to the university one day and learn about the work that we do, and get to do a “mini” search for a particle with real ATLAS data. At the end of the day we have a video conference with other universities around the world where the same activity is being held, so that they can interact with other students in a different country and compare their results, sort of the way we do in real life in a collaboration like ATLAS.
We had a very special guest  at one of the International Masterclasses; Peter Jenni, one of the founding fathers of ATLAS came to visit us

We had a very special guest at one of the International Masterclasses; Peter Jenni, the first spokesperson of ATLAS.

These activities are important for us, because we are a rather small community here in Colombia doing experimental particle physics, so it is one of our “duties” to reach out to people and let them know about the potential of our research.
In the summer holidays I am usually able to go to CERN for a couple of months which is very nice: I can interact with many people there and hear about their ideas and get involved in new projects for when I return to my home country. This year the detector will already be taking data by the time I arrive, so it will be even better, since I will also take shifts in the control room and be part of the data taking process.
I feel very fortunate to be able to take active part in the very exciting things we do on ATLAS from my home country, and like all of us in the collaboration, looking forward to the new challenges that Run 2 will bring us.

CarlosSandoval Carlos Sandoval is a Lecturer at Universidad Antonio Nariño in Bogotá, Colombia, and is part of the ATLAS collaboration since 2010. His is currently involved in beyond standard model Higgs searches and has previously worked on jet physics and jet triggers in ATLAS.

A Week of Firsts

Me on skis!

Do I look nervous? I was nervous.

The annual conference, Moriond, is in its 50th edition this year, and I’ve had the pleasure of coming down to Aosta in Italy to participate in the QCD session for the first time. It’s actually a week of firsts for me. The conference organizers described it as being in a kind of “QCD confinement”: whereas most conferences are in big cities, where it’s easy to disappear into a museum or restaurant, stay in a hotel far from the conference, see friends, and so on, Moriond attempts to stay remote enough that the attendees are “forced” to discuss physics with each other for the week (it doesn’t take much encouragement, to be honest). Moriond has a funny schedule: talks from 8:30 to 12 in the morning, and from 5 to 7:30 in the evening, to leave a few hours in the middle of the day for skiing. Having never skied before, that’s been the most frightening first, but I am proud to say that I did manage to ski a couple of times while here, and I avoided breaking a leg this time. I even successfully got on and off a chairlift!

This was also the first time I’ve gotten to present the results of a physics analysis that I worked directly on at an international physics conference. And what a chance! That sounds a little funny, of course. ATLAS, like the other experiments, has to find a balance between getting the best, most knowledgeable speakers at each conference and ensuring that young students, people looking for jobs, and people who have been doing critical work for the experiment that isn’t a physics analysis (work we call “service work”, generally) get to go to conferences. That does sometimes mean people are asked to give talks on subjects that they’re familiar with, but the results that are ready to be presented may not be ones that they were deeply involved in producing. And that’s been the case for me – I’ve been to a couple of conferences, but up to this point it’s just not quite been at the right time or with the right topic to show something that I was working on.

Our little bump!

Our little bump. The data (black points) are about three sigma above the background (colored histogram). We have a couple of signal models (dashed histograms) on there for comparison. Is it SUSY? Or is it luck?

So what about those results? Well, one of the papers I worked on was submitted back in January, and while it was a great search covering lots of different possible signals, we didn’t really see anything. But some of you might have seen that CMS recently submitted a SUSY search that had a ~2.5 sigma excess, which caused a lot of stir. The week after, ATLAS submitted our version of the same search, which I also had the pleasure of helping out with, showing that with a signal region designed to be as close as possible to the CMS search region, we do not see a similar excess. But in the same paper, we pointed to another area with an apparent 3.0 sigma excess! There has been some discussion, both in the form of papers submitted by theorists and in the form of physics blogs.

It’s a fun possibility for the world to think about: have we seen the first hint of something really exciting, or have we just seen a rare fluctuation, expected because of the enormous number of searches we’ve performed? One wonderful talk given during the week was all about the “Fifth Force”, an interesting and extended saga of scientists trying to understand whether the data they had collected was the sign of something absolutely critical to understanding the universe, or just an unexpected manifestation of a well-known phenomenon. One of my favorites from that talk: in order to measure very small deviations in gravity, they needed to survey a large area around the (outdoors) experiment. The researchers failed at one point to account for the fact that graduate students carrying a $50,000 piece of equipment would not want to take it near a river or up a steep hill, and that led to a bias in their results.

We’ll have to wait for the next run to find out whether the small excess persists – but we should know soon! We got the good news that beam is expected back in the machine in a few days. Run 2 is not far off!


ZachMarshall Zach Marshall is a Divisional Fellow at the Lawrence Berkeley National Laboratory in California. His research is focused on searches for supersymmetry and jet physics, with a significant amount of time spent working on software and trying to help students with physics and life in ATLAS.

Moriond Electroweak: physics, skiing and Italian food

If you’re a young physicist working in high energy physics, you realize very soon in your career that “going for Moriond” and “going to Moriond” are two different things, and that neither of the two means that you’re actually going here:

The original location of the Moriond conference series

The original location of the Moriond conference series

“Les rencontres de Moriond” is one of the main Winter conferences for our field. Starting from its original location in Moriond, it has been held around the French and Italian Alps since 1966. In the 60s and 70s, there was a clear distinction between two branches of the same conference, as “electroweak” and “QCD” physics were still done in different labs and accelerators: in those years the former had to do for example with the discovery of the W and Z bosons and their interactions, while the latter saw the developments of a model to describe the “quarks” that compose protons and neutrons, and the discovery of these constituents themselves. Nowadays, both kinds of physics are studied at the LHC and in other experiments around the world, so the results presented in the two conferences are not necessarily divided by topic anymore.

This year I was lucky enough to be contributing some results that were “going for Moriond”, which means they’d be approved by the Collaboration to be presented at this conference for the first time, but I would also be “going to Moriond” in person. This year’s “Moriond Electroweak” was held in the Italian mountain resort of La Thuile, and had a special significance. In the session that celebrated the 50 years of the conference, the founder Jean Trân Thanh Vân reminded the audience of the two pillars of this conference:

  • encourage discussions and exchanges between theoretical and experimental physicists;
  • let young scientists meet senior researchers and discuss their results.
The official Moriond EW t-shirt and the announcement of the slalom competition

The official Moriond EW t-shirt and the announcement of the slalom competition

The first point was made when theorists and experimentalists alike were asked to take part in a slalom competition. The results were not categorized by subject of study, but certainly the cheering came from and towards both parties.

The latter took place almost every evening, in the dedicated “young scientists session”. Here, students and young post-docs can apply to give a short talk and answer questions on their research topic in front of an international audience of theorists and experimentalists.

The questions and answers can be then carried on to the (abundant) dinner. As an Italian, I do appreciate the long evenings dedicated to a mixture of excellent food combined with physics discussions (and where the two can be identified with each other, as in this snapshot from a talk by Francesco Riva).

New physics matched to Italian dinner choices at the conference, according to Francesco Riva

New physics matched to Italian dinner choices at the conference, according to Francesco Riva

Back to the physics: the results I contributed to were shown in the afternoon session, before the 50th anniversary talks. They’re in the top corner of one of the slides from the summary of the ATLAS and CMS new physics searches.

Beyond the Standard Model: New results presented at the Moriond EW conference

Beyond the Standard Model: New results presented at the Moriond EW conference

That’s only the tip of the iceberg of a search that looks for new phenomena that would manifest as an excess of collimated jets of particles in the central region of the detector, and it shows that there is no new physics to be found here, nor in any of the other searches shown in the conference so far. (What we didn’t know at that time was that there would be something not consistent with expectations in the LHCb results shown just one day after, as explained in this article). Given that so far we have not found much beyond what we consider Standard (as in belonging to the predictions made by the Standard Model of Particle Physics), the conference had a special focus on searches that look for the unexpected in unexpected places. “Stealthy” is how the physics beyond the Standard Model that is particularly hard to find is characterized, and as experimentalists we want to pay particular attention to the “blind spots” where we haven’t yet looked for the upcoming LHC runs. This was highlighted in the morning talks, describing searches for Supersymmetry in blind spots and searches for particles that leave no immediate signature after the collision because of their long lifetime. There were also other ideas of how to test the Standard Model with very high precision, as highlighted in another food-related slide by Francesco Riva.

Techniques to find new physics, according to Francesco Riva

Techniques to find new physics, according to Francesco Riva

No one in the audience forgot, however, that the new LHC run will bring more energy and more data. Both will allow us to investigate new, rare processes that were not accessible in the first run. Discoveries might be just around the corner!

Overall, the “Rencontres de Moriond” conferences have the effect of leaving everyone enthusiastic for the discussion and eager for more results: in particular, next year’s edition may see some of the first results of the upcoming LHC run. And of course, the results will be best discussed on skis and over dinner.


Caterina Doglioni Caterina Doglioni is a post-doctoral researcher in the ATLAS group of the University of Geneva. She got her taste for calorimeters with the Rome Sapienza group in the commissioning of the ECAL at the CMS experiment during her Master’s thesis. She continued her PhD work with the University of Oxford and moved to hadronic calorimeters: she worked on calibrating and measuring hadronic jets with the first ATLAS data. She is still using jets to search for new physics phenomena, while thinking about calorimeters at a new future hadron collider.

The Ties That Bind

A few weeks ago, I found myself in one of the most beautiful places on earth: wedged between a metallic cable tray and a row of dusty cooling pipes at the bottom of Sector 13 of the ATLAS Detector at CERN. My wrists were scratched from hard plastic cable ties, I had an industrial vacuum strapped to my back, and my only light came from a battery powered LED fastened to the front of my helmet. It was beautiful.

Cleaning the ATLAS detector

Beneath the ATLAS detector – note the well-placed cable ties. IMAGE: Claudia Marcelloni, ATLAS Experiment © 2014 CERN.

The ATLAS Detector is one of the largest, most complex scientific instruments ever constructed. It is 46 meters long, 26 meters high, and sits 80 metres underground, completely surrounding one of four points on the Large Hadron Collider (LHC), where proton beams are brought together to collide at high energies.  It is designed to capture remnants of the collisions, which appear in the form of particle tracks and energy deposits in its active components. Information from these remnants allows us to reconstruct properties of the collisions and, in doing so, to improve our understanding of the basic building blocks and forces of nature.

On that particular day, a few dozen of my colleagues and I were weaving our way through the detector, removing dirt and stray objects that had accumulated during the previous two years. The LHC had been shut down during that time, in order to upgrade the accelerator and prepare its detectors for proton collisions at higher energy. ATLAS is constructed around a set of very large, powerful magnets, designed to curve charged particles coming from the collisions, allowing us to precisely measure their momenta. Any metallic objects left in the detector risk turning into fast-moving projectiles when the magnets are powered up, so it was important for us to do a good job.

ATLAS Big Wheel

ATLAS is divided into 16 phi sectors with #13 at the bottom. IMAGE: Steven Goldfarb, ATLAS Experiment © 2014 CERN

The significance of the task, however, did not prevent my eyes from taking in the wonder of the beauty around me. ATLAS is shaped somewhat like a large barrel. For reference in construction, software, and physics analysis, we divide the angle around the beam axis, phi, into 16 sectors. Sector 13 is the lucky sector at the very bottom of the detector, which is where I found myself that morning. And I was right at ground zero, directly under the point of collision.

To get to that spot, I had to pass through a myriad of detector hardware, electronics, cables, and cooling pipes. One of the most striking aspects of the scenery is the ironic juxtaposition of construction-grade machinery, including built-in ladders and scaffolding, with delicate, highly sensitive detector components, some of which make positional measurements to micron (thousandth of a millimetre) precision. All of this is held in place by kilometres of cable trays, fixings, and what appear to be millions of plastic (sometimes sharp) cable ties.

Inside the ATLAS detector

Scaffolding and ladder mounted inside the precision muon spectrometer. IMAGE: Steven Goldfarb, ATLAS Experiment © 2014 CERN.

The real beauty lies not in the parts themselves, but rather in the magnificent stories of international cooperation and collaboration that they tell. The cable tie that scratched my wrist secures a cable that was installed by an Iranian student from a Canadian university. Its purpose is to carry data from electronics designed in Germany, attached to a detector built in the USA and installed by a Russian technician.  On the other end, a Japanese readout system brings the data to a trigger designed in Australia, following the plans of a Moroccan scientist. The filtered data is processed by software written in Sweden following the plans of a French physicist at a Dutch laboratory, and then distributed by grid middleware designed by a Brazilian student at CERN. This allows the data to be analyzed by a Chinese physicist in Argentina working in a group chaired by an Israeli researcher and overseen by a British coordinator.  And what about the cable tie?  No idea, but that doesn’t take away from its beauty.

There are 178 institutions from 38 different countries participating in the ATLAS Experiment, which is only the beginning.  When one considers the international make-up of each of the institutions, it would be safe to claim that well over 100 countries from all corners of the globe are represented in the collaboration.  While this rich diversity is a wonderful story, the real beauty lies in the commonality.

All of the scientists, with their diverse social, cultural and linguistic backgrounds, share a common goal: a commitment to the success of the experiment. The plastic cable tie might scratch, but it is tight and well placed; its cable is held correctly and the data are delivered, as expected. This enormous, complex enterprise works because the researchers who built it are driven by the essential nature of the mission: to improve our understanding of the world we live in. We share a common dedication to the future, we know it depends on research like this, and we are thrilled to be a part of it.

ATLAS Collaboration

ATLAS Collaboration members in discussion. What discoveries are in store this year?  IMAGE: Claudia Marcelloni, ATLAS Experiment © 2008 CERN.

This spring, the LHC will restart at an energy level higher than any accelerator has ever achieved before. This will allow the researchers from ATLAS, as well as the thousands of other physicists from partner experiments sharing the accelerator, to explore the fundamental components of our universe in more detail than ever before. These scientists share a common dream of discovery that will manifest itself in the excitement of the coming months. Whether or not that discovery comes this year or some time in the future, Sector 13 of the ATLAS detector reflects all the beauty of that dream.


Steven Goldfarb Steven Goldfarb is a physicist from the University of Michigan working on the ATLAS Experiment at CERN. He currently serves as the Outreach & Education Coordinator, a member of the ATLAS Muon Project, and an active host for ATLAS Virtual Visits. Send a note to info@atlas-live.ch and he will happily host a visit from your school.

The Art of Rediscovery

When I tell people I’m a particle physicist, one of the most frequent questions I get asked is: “So, have you discovered anything?” Funnily, I’ve spent much of the past two years trying to rediscover something that’s already been seen before. In today’s world, which fetishizes the New, this may seem slightly lame, but just because we’ve discovered something, doesn’t mean we’ve fully understood it.

I worked on a search by ATLAS for the production of a pair of bosons (the so-called W and Z bosons), which is an important test of the Standard Model. I focused on the specific process where a W decays to a lepton and neutrino, and a W or Z decays to two jets, which is a particularly difficult process to measure, and which had not been measured by ATLAS before. The real nastiness of this measurement is the presence of two jets, which are sprays of particles originating from quarks. Despite jets’ inherent coolness, “jets” remains a four-letter word for many physicists.

The problem is that jets are produced abundantly at the LHC, and the vast majority of the jets do not originate from the bosons we’re looking for. So, when we scan through all the collision data recorded by ATLAS, filtering out the recorded collisions (or “events” as we call them) that resemble a boson-pair (i.e. showing the presence of a lepton, neutrino, and two jets), we run into the problem that most of these events do not actually contain the boson-pairs we’re looking for. These imposter events are called “background,” as opposed to the “signal” boson-pairs we’re seeking.

The key to the analysis is finding a property of the signal events that allows us to distinguish them from the background. The invariant mass is a quantity that can be calculated for any jet pair, which has the useful property of being close to the W/Z boson mass if the jets actually came from a W/Z boson decay (the W and Z bosons have similar masses). So great, we can look at the invariant masses of the jet pairs of the events, and use this to tell the real signal events from the background. After sifting through all our data, we ended up with the following boson-pair candidate events:

fig_01b

The boson-pair candidates that pass our initial filter. The red sliver on the top of the “hill” is what we’re trying to measure.

This is the kind of plot that makes physicists flee for the nearest exit. On the x-axis is plotted the invariant mass of the two jets in the event. The black dots show the data. The blue and green histograms give the predicted amount of background, and the red histogram on top shows the prediction for the signal we’re looking for. Yes, you read that correctly. The red histogram—I’ll pause while you fetch your glasses—that little red sliver is what we’re looking for. As you can see, the signal is dominated by the background. There is a glimmer of hope, though: the background peaks at an invariant mass of about 70 GeV, while the signal peaks at 80-90 GeV.

To measure the signal, the idea is basically to predict how many of the events are from background, then subtract this background from the data, and whatever is left is the signal we’re trying to observe. Sounds easy, but it’s ridiculously hard. Physicists have been working for decades on predicting these backgrounds, developing ever-more-complex computer simulations, but there remains substantial uncertainty in these predictions.

In the end, here’s what we see after subtracting our best estimate of the background from the data:

fig_03b

After subtracting off the background from the data, the boson-pairs are clearly visible in the data!

Voila! The points show the background-subtracted data, and for comparison the red line shows the shape expected for our boson-pair signal: a peak near the W/Z mass. Excellent agreement, showing clear evidence that we are in fact seeing boson-pair production. In the end, the result isn’t surprising: this process has been seen before, and our measurement is consistent with predictions. Nevertheless, it’s a nice confirmation of the Standard Model, and the fact that we’re able to pick out a signal from seemingly overwhelming backgrounds bodes well for future searches at ATLAS.


Brian Lindquist Brian Lindquist is a postdoctoral researcher for Stony Brook University in New York. His research has included measurements of multi-boson production and quality assurance for the IBL, an upgrade of ATLAS’s pixel detector. Prior to joining ATLAS, he got his PhD from Stanford University, working on the BaBar experiment.

Defending Your Life (Part 3)

This is the last part of my attempt to explain our simulation software. You can read Part 1, about event generators, and Part 2, about detector simulation, if you want to catch up. Just as a reminder, we’re trying to help our theorist friend by searching for his proposed “meons” in our data. The detector simulation gives us a long list of energy deposits, times, and locations in our detector. The job isn’t done though. Now we have to take those energy deposits and turn them into something that looks like our data – which is pretty tricky! The code that does that is called “digitization”, and it has to be written specially for our detector (CMS has their own too).

The simple idea is to change the energies into whatever it is that the detector reads out – usually times, voltages, and currents, for example, but it can be different for each type of detector. We have to build in all the detector effects that we care about. Some are well known, but not well understood (Birks’ law, for example). Some are a little complicated, like the change in light collected from a scintillator tile in the calorimeter depending on whether the energy is deposited right in the middle or on the edge. We can use the digitization to model some of the very low-energy physics that we don’t want to have to simulate in detail with Geant4 but want to get right on average. Those are effects like the spread and collection of charge in a silicon module or the drift of ionized gas towards a wire at low voltage.

Z to mu mu at high pileup

One of our events with lots of “pile-up” – many standard proton-proton collisions, one dot for each, on top of one that we’re interested in (the one with the yellow tracks)

Digitization is where some other effects are put in, like “pile-up“, which is what we call the extra proton-proton collisions in a single bunch crossing. Those we usually pre-simulate and add on top of our important signal (meon) events, like using a big library. We can add other background effects if we want to, like cosmic rays crossing the detector, or proton collisions with remnant gas particles floating around in the beampipe, or muons running down parallel to the beamline from protons that hit collimators upstream. Those sorts of things don’t happen every time protons collide, but we sometimes want to study how they look in the detector too.

Now we should have something that looks a lot like our data – except we know exactly what it is, without any ambiguity! With that, we can try to figure out if our friend’s meons are a part of nature. We can build up some simulated data that includes all the different processes that we already know exist in nature, like the production of top quarks, W bosons, Z bosons, and our new Higgs bosons. And we can build another set that has all of those things, but also includes our friend’s meons. The last part, which is really what our data analysis is all about, is trying to figure out what makes events with meons special – different from the other ones we expect to see – and trying to isolate them from the others. We can look at the reconstructed energy in the event, the number of particles we find, any oddities like heavy particles decaying away from the collision point – anything that helps. And we have to know a little bit about the simulation, so that we don’t end up using properties of the events that are very hard to get right in the simulation to separate meons from other particles. That really is the first part of almost all our data analyses. And the last part of most of our analyses (we hope), is “unblinding”, where we finally check the data that has all the features we want – passes all our requirements – and see whether it looks more like nature with or without meons. Sometimes we try to use “data-driven methods” to estimate the backgrounds (or tweak the estimates from our simulation), but almost every time we start from the simulation itself.

Some of our data with a few different guesses as to what new physics might look like (here different dark matter models). The data look more like our expectation without them, though – so no new physics today!

The usual thing that we find is that our friend told us about his theory, and we looked for it and didn’t find anything exciting. But by the time we get back, our theorist friends often say “well, I’ve been thinking more, and actually there is this thing that we could change in our model.” So they give you a new version of the meon theory, but this time instead of being just one model, it’s a whole set of models that could exist in nature, and you have to figure out whether any of them are right. We’re just going through this process for Supersymmetry, trying to think of thousands of different versions of Supersymmetry that we could look for and either find or exclude. Often, for that, you want something called a “fast simulation.”

To make a fast simulation, we either go top-down or bottom-up. The top-down approach means that we look at what the slowest part of our simulation is (always the calorimeters) and find ways to make it much, much faster, usually by parameterizing the response instead of using Geant4. The bottom-up approach means that we try to skip detector simulation and digitization all together and go straight to the final things that we would have reconstructed (electrons, muons, jets, missing transverse momentum). There are even public fast simulations like DELPHES and the Pretty Good Simulation that theorists often use to try to find out what we’ll see when we simulate their models. Of course, the faster the simulation, normally, the fewer details and oddities can be included, and so the less well it models our data (“less well” doesn’t have to be “not good enough”, though!). We have a whole bunch of simulation types that we try to use for different purposes. The really fast simulations are great for quickly checking out how analyses might work, or for checking out what they might look like in future versions of our detector in five or ten years.

So that’s just about it – why we really, really badly need the simulation, and how each part of it works. I hope you found it a helpful and interesting read! Or at least, I hope you’re convinced that the simulation is important to us here at the LHC.


ZachMarshall Zach Marshall is a Divisional Fellow at the Lawrence Berkeley National Laboratory in California. His research is focused on searches for supersymmetry and jet physics, with a significant amount of time spent working on software and trying to help students with physics and life in ATLAS.

Defending Your Life (Part 2)

I’ve been working on our simulation software for a long time, and I’m often asked “what on earth is that?” This is my attempt to help you love simulation as much as I do. This is a follow up to Part 1, which told you all about the first step of good simulation software, called “event generation”. In that step, we had software that gave us a list of stable particles that our detector might be able to see. And we’re trying to find some “meons” that our friend the theorist dreamed up.

One little problem with those wonderful event generators is that they don’t know anything about our experiment, ATLAS. We need a different piece of software to take those particles and move them through the detector one by one, helping model the detector’s response to each one of the particles as it goes. There are a few pieces of software that can do that, but the one that we use most is called Geant4. Geant4 is publicly available, and is described as a “toolkit” on their webpage. What that means is that it knows about basic concepts, but it doesn’t do specifics. Like building a giant lego house out of a bag of bricks, you have to figure out what fits where, and often throw out things that don’t fit.

One of the detector layouts that we simulate

The first part of a good detector simulation is the detector description. Every piece of the detector has to be put together, with the right material assigned to each. We have a detector description with over five million (!) volumes and about 400 different materials (from Xenon to Argon to Air to Aerogel and Kapton Cable). There are a few heroes of ATLAS who spend a lot of time taking technical drawings (and photographs, because the technical drawings aren’t always right!) of the detector and translating them into something Geant4 can use. You can’t put every wire and pipe in – the simulation would take an eternity! – so you have to find shortcuts sometimes. It’s a painstaking process that’s still ongoing today. We continuously refine and improve our description, adding pieces that weren’t important at the beginning several years ago but are starting to be important now (like polyboron neutron shielding in our forward region; few people thought early on that we would be able to model low-energy neutron flux in our detector with Geant4, because it’s really complex nuclear physics, but we’re getting so close to being able to do so that we’ve gone back to re-check that our materials’ neutron capture properties are correct). And sometimes we go back and revise things that were done approximately in the beginning because we think we can do better. This part also involves making a detailed magnetic field map. We can’t measure the field everywhere in the detector (like deep in the middle of the calorimeter), and it takes too much time to constantly simulate the currents flowing through the magnets and their effect on the particles moving through the detector, so we do that simulation once and save the magnetic field that results.

A simulated black hole event. But what do meons look like?

Next is a good set of physics models. Geant4 has a whole lot of them that you can use and (fortunately!) they have a default that works pretty well for us. Those physics models describe each process (the photoelectric effect, Compton scattering, bremsstrahlung, ionization, multiple scattering, decays, nuclear interactions, etc) for each particle. Some are very, very complicated, as you can probably imagine. You have to choose, at this point, what physics you’re interested in. Geant4 can be used for simulation of space, simulation of cells and DNA, and simulations of radioactive environments. If we used the most precise models for everything, our simulation would never finish running! Instead, we take the fastest model whose results we can’t really distinguish from the most detailed models. That is, we turn off everything that we don’t really notice in our detector anyway. Sometimes we don’t get that right and have to go back and adjust things further – but usually we’ve erred on the side of a slower, more accurate simulation.

The last part is to “teach” Geant4 what you want to save. All Geant4 cares about is particles and materials – it doesn’t inherently know the difference between some silicon that is a part of a computer chip somewhere in the detector and the silicon that makes up the sensors in much of our inner detector. So we have to say “these are the parts of the detector that we care about most” (called “sensitive” detectors). There are a lot of technical tricks to optimizing the storage, but in the end we want to write files with all the little energy deposits that Geant4 has made, their time and location – and sometimes information (that we call “truth”) about what really happened in the simulation, so later we can find out how good our reconstruction software was at correctly identifying photons and their conversions into electron-positron pairs, for example.

The fun part of working on the simulation software is that you have to learn everything about the experiment. You have to know how much time after the interaction every piece of the detector is sensitive, so that you can avoid wasting time simulating particles long after that time. You get to learn when things were installed incorrectly or are misaligned, because you need those effects in the simulation. When people want to upgrade a part of the detector, you have to learn what they have in mind, and then (often) help them think of things they haven’t dealt with yet that might affect other parts of the detector (like cabling behind their detector, which we often have to think hard about). You also have to know about the physics that each detector is sensitive to, what approximations are reasonable, and what approximations you’re already making that they might need to check on.

That also brings us back to our friend’s meons. If they decay very quickly into Standard Model particles, then the event generator will do all the hard work. But if they stick around long enough to interact with the detector, then we have to ask our friend for a lot more information, like how they interact with different materials. For some funny theoretical particles like magnetic monopoles, R-hadrons, and stable charginos, we have to write our own Geant4 physics modules, with a lot of help from theorists.

The detector simulation is a great piece of software to work on – but that’s not the end of it! After the simulation comes the final step, “digitization”, which I’ll talk about next time – and we’ll find out the fate of our buddy’s meon theory.


ZachMarshall Zach Marshall is a Divisional Fellow at the Lawrence Berkeley National Laboratory in California. His research is focused on searches for supersymmetry and jet physics, with a significant amount of time spent working on software and trying to help students with physics and life in ATLAS.

Doing Physics in Vietnam

Beach next to the conference center

One of the perks of working in our field is the opportunities we get to go to exotic places for conferences. I always felt the HEP-MAD conference in Madagascar would top this list, but the one some of us went to in Vietnam can’t be too far behind.

The Rencontres du Vietnam conference series has been organised in the coastal town of Quy Nhon since 2011, covering different physics topics. This year, one of them was titled Physics at the LHC and Beyond, where I had the privilege of presenting ATLAS soft QCD results.

There were talks covering all aspects of LHC physics, a dedicated session on detector performance with ATLAS and CMS speakers going one after the other, and intense discussion on future colliders. Nobel Laureate Francois Englert was the guest of honour at the conference, and he talked about the history of the Brout-Englert-Higgs mechanism.

Charming hilltop temple

The conference was held at the relatively new International Center of Interdisciplinary Science Education (ICISE), a beautiful facility right by the sea, with its own beach. The food was amazing too – with extensive buffets for breakfast (with fried rice and noodles no less!), lunch and dinner. At the conference dinner, we even got green coconuts filled with water. We were also taken to hill-top Cham temples, and saw local dance/martial arts performances.

Jean Trân Thanh Vân, who is the founder of the renowned Rencontres de Moriond conference series, deserves a big thanks for organising this conference in Vietnam – which surely helps in making particle physics popular in south-east Asia.


Deepak Kar is a research associate in Glasgow. He is involved in soft-QCD measurements, Monte-Carlo tuning, and jet substructure studies.

Defending Your Life (Part 1)

Eur. Phys. J C Cover

Our ATLAS Simulation Paper

Having spent many hours working on the simulation software in ATLAS, I thought this would be a good place to explain what on earth that is (H/T to Al Brooks for the title). Our experiment wouldn’t run without the simulation, and yet there are few people who really understand it. So that I don’t have to grossly over-simplify, I’ll try to make this a three-part post. Our “simulation” runs in three steps, so it seemed only appropriate. If you want to read a lot more, albeit a bit of a technical description, you can try the ATLAS simulation paper that we wrote a few years ago (ATLAS’s first cover article!).

Say your friend comes up with a new theory about how the world works. “This theory we have now (the Standard Model) is pretty good,” your friend says, “but it would work way better if we added some meons. See, if we just add meons, we can explain so much more! And if I’m right, then the LHC is making meons every minute!!” You, a member of ATLAS, think your friend is nuts – and you don’t particularly like the name “meons” – so you decide you are going to prove that he is wrong (or, if he’s lucky, help him win the Nobel Prize!).

Unfortunately, you can’t just demand that ATLAS find meons – we wouldn’t know what we were looking for!! So we need to know what reality would look like if there were meons, and if there weren’t meons. Then we can check which one of those our data looks like, and we can say (with some confidence) “Yes, there probably are meons,” or “No, there probably aren’t meons.” In steps simulation, your new hero!

The first step to any good simulation is called “event generation.”

The LHC collides protons. The theory that describes what happens when those protons collide is called Quantum Chromodynamics. It describes how the quarks and gluons inside the protons scatter off one another, how they might create new particles, and how those new particles behave after they’ve been created. Needless to say, it’s really complicated. In fact, for various reasons, there are some things that you just can’t calculate in the theory. There are many event generators, and many are set up to do only one particular thing very, very well. Some of them are wonderfully generic, but often you have to string them together to get a full description of a single collision between two protons at the LHC. FeynRules + MadGraph5 + Pythia8 + EvtGen is one of my favorite combinations lately. FeynRules is particularly cool, because it lets a theorist write down the Lagrangian of their theory in Mathematica (which is a piece of software that almost all theorists use), and then it translates it into rules for Feynman Diagrams, and from there into code that an experimentalist can use.

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The Sherpa authors make nice simplified pictures like these of their events. The real things look like that, but a few hundred times more complicated!

These event generators give you a list of all the particles that come out of a collision between two protons. If you read these blogs, you’ve probably seen notes about how particles interact with the detector and decay on their own sometimes, so what you really need is a list of particles that stick around for at least one hundredth of a billionth of a second or so. Those are “stable” enough that we should worry about their interacting with the detector. The event generators usually do rather complicated things to get the number of particles of each type right, but they are kind enough to leave us with a record of what they’ve done, like the one in the little picture on the left there. Inside of that record you can often find the original top quarks, or meons in our case, and see whether they decayed, what they decayed into, and what observable particles were produced from them. In fact, it’s a numerical model of a quantum mechanical process, so even we physicists have to remind ourselves not to cheat and look at the internal record from the event generator too often – we should be able to tell a Higgs boson from a top quark from an meon only by looking at the final particles that we can observe in our detector (pions, electrons, muons and so on).

So, to massively oversimplify things, all you do is tell your friend “write down how your meons interact in a program you like”, and after a little bit of work (shortest turn-around I’ve managed is several months), you can have a complete description of all the particles that come out of a collision between two protons when an meon is produced. Now you’re ready for the next step, detector simulation, which will be our next post.

If you are excited about it, you can try running event generation yourself. All the software is publicly available. I would recommend trying to download MadGraph5. Those guys are good at interfaces, and it has cute modules to let you try out various things. It’ll be a little bit of a struggle to understand everything that’s going on if you aren’t a particle physicist, but you can make a lot of pretty cool pictures if you are willing to spend a little time (and it really is doing event generation just like we do!).


ZachMarshall Zach Marshall is a Divisional Fellow at the Lawrence Berkeley National Laboratory in California. His research is focused on searches for supersymmetry and jet physics, with a significant amount of time spent working on software and trying to help students with physics and life in ATLAS.