The hills are alive, with the sound of gravitational waves

Gravitational Wave Discovery

Presentation by Barry C. Barish on 11 Feb 2016 in the CERN Main Auditorium on LIGO and the discovery of gravitational waves caused by the merging of two black holes. IMAGE: M. Brice, © 2016 CERN.

It’s 16:00 CET at CERN and I’m sitting in the CERN Main Auditorium. The room is buzzing with excitement, not unlike the day in 2012 when the Higgs discovery was announced in this very room. But today the announcement is not from CERN, but the LIGO experiment which is spread across two continents. Many expect the announcement to be about a discovery of gravitational waves, as predicted by Einstein in 1916, but which have remained elusive until today…

LIGO uses interferometry to detect gravitational waves as they pass through the Earth. Where do gravitational waves strong enough to be detected on Earth come from? Few objects in the Universe are massive enough, but two black holes spiralling towards each other and eventually merging could give just such a strong and characteristic signal. At 16:29 CET, this is exactly what LIGO announced had been observed, followed by extended applause.

Black Holes Merging

Simulation of two massive black holes merging, based on data collected from the LIGO collaboration on 14 Sep. 2015. IMAGE: LIGO Collaboration © 2016 SXS

Scientists at CERN are excited about this discovery. Not only because it has been a much sought after treasure – with searches starting over 50 years ago with Joseph Weber – but also because it could have a direct link to some of the searches we are performing with the ATLAS detector at the LHC. Gravitational waves are described by the general theory of relativity as proposed by Einstein, and encompass massive objects (both stationary or moving very fast) at very large (cosmological) distance scales.

At CERN we are interested in a coherent and testable theory for gravity at the very small scale, so-called quantum gravity. The LHC is used to accelerate protons up to velocities very close to the speed of light, colliding them together at enormous energies within detectors placed around its 27 km circumference. Detectors such as ATLAS and CMS act as giant digital cameras and try to work out what happened during that interaction. It is in the data collected by these experiments that some theories suggest a theoretical particle called the graviton could be found. The gravitational waves mentioned in the announcement yesterday, should actually be related to a massless version of the graviton.

Z' Decay

Simulation of a Z’ boson decaying to two muons in the ATLAS Detector. IMAGE: ATLAS Collaboration © 2016 CERN

The experiments at the LHC are not sensitive to this kind of graviton or the gravity waves detected by LIGO. However, in quantum theories of gravity massive states of the graviton could also exist, being created within the ATLAS detector and subsequently decaying into pairs of particles such as electrons, muons or photons. All of these signatures of a graviton and more have been searched for using the ATLAS detector ([1], [2], [3]), and the observation of such a particle with the statistical precision that is required to claim a discovery in our field (5 sigma), would be a direct observation of quantum gravity. It is interesting to note that it is at a statistical significance of 5.1 sigma that LIGO claimed its discovery yesterday.

But gravity is a peculiar force, unlike any other we know. For one it is extremely weak – so weak that it loses in a tug of war over a metal nail, with the gravitational pull of the entire earth on one side and a small hand-held magnet (using the electromagnetic force) on the other. It is when you realise how weak gravity is that you begin to comprehend how titanic the spiraling and merging of those two black holes must have been to allow them to be detected on Earth, over a billion light years away.

It is also for this reason that most of the theories of quantum gravity involve extra spatial dimensions. It is suggested that within these extra dimensions, gravity has a similar strength to the other forces of nature, and it is just in our three known spatial dimensions that we feel its diluted strength. In the popular extra dimensional theories, the size of these other dimensions could either be small, with a warped geometry, or very large (micrometres!!!), with a flat geometry [60 second guide to extra dimensions]. It is precisely because we explore such high energy scales (and thus small distance scales!) with the ATLAS detector, that we could probe these extra dimensions (if they exist) and potentially observe a massive graviton. However, other theories suggest that gravity might not be like a normal force at all, that it is simply due to space-time geometry. This would be unlike the other forces of nature that we know of, which have particles that communicate the strength of the force during interactions (in the theory of quantum gravity, this would be the graviton).

So the announcement yesterday of gravitational waves being discovered is exciting, because it could help point us in the right direction when looking for a massive version of the graviton (if extra spatial dimensions exist) here at the ATLAS experiment. Do these waves exhibit a behaviour that could shed light on quantum gravity? Perhaps using wave-particle duality – a phenomenon that already describes the duplicitous nature of light as both particles (photons) and waves (electromagnetic spectrum)? Conversely, could the details of this discovery put a dent in all of our current theories of quantum gravity and require theorists to go back to the drawing board?

With the startup of the LHC again in March, collecting up to 10 times more data this year than we did last year, I might be sitting in that room again not too long from now, with a discovery announcement of our own.

Daniel Hayden Daniel Hayden is a postdoctoral researcher for Michigan State University, using the ATLAS Detector to search for Exotic new particles such as the Z’ or Graviton, decaying to two electron or muons. Born in the UK, he currently lives in Geneva, Switzerland, after obtaining his PhD in Particle Physics from Royal Holloway, University of London. In his spare time Dan loves going to the cinema, hanging out with friends, and talking… a lot.

Top 2015 – Mass, Momentum, and the Conga

The top quark conference normally follows the same basic structure. The first few days are devoted to reports on the general status of the field and inclusive measurements; non-objectionable stuff that doesn’t cause controversy. The final few days are given over to more focused analyses; the sort of results that professors really enjoy arguing about. We got a taste of this earlier than usual this year as discussion on top transverse momenta (pT) broke out at least three times before we even managed to get to the session on Thursday! As a postdoc, I do love this sort of debate at a workshop, almost as much as I enjoy watching the students arrive at 9am, desperately hungover and probably assuming they were quiet as they crept back into the Hotel at 3am (no Joffrey, we definitely didn’t hear you knock over that sun lounger).

The CMS combination of measurements of top-quark mass, currently the most sensitive in the world.

The CMS combination of measurements of top-quark mass, currently the most sensitive in the world.

DAY 3:

Top Mass is always a great topic at this conference. This year the theorists started by reminding us, for what feels like the millionth time, of the difference between various interpretations of “mass” in perturbative QCD, telling us which are well-defined and safe to use. The LHC and Tevatron experiments then showed staggeringly precise measurements using our ill-defined definition of “Monte Carlo mass” that theorists have been complaining about for decades. This year we’ve really outdone ourselves and CMS have combined their results to produce a measurement with an uncertainty of less than 0.5 GeV! Fine, we’re not sure ‘exactly’ what the Monte Carlo mass really is theoretically, but we did also provide well-interpreted pole-mass results (at the cost of having larger uncertainties), so let’s hope that’s enough to keep the theorists happy.


While it cannot yet be said that starting a conga line qualifies as a tradition at the Top conference, it does seem to occur with increasing frequency. I have my own theories about how and why this occurs (and evidence of a certain ATLAS top convenor who seems to be close to the front of the line each time it happens…) and I find that there are few things as surreal as your bosses and ex-bosses dancing around in a semi-orderly line with their hands on your hips screaming “go faster” in your ear. Though this has little to nothing to do with top physics, I enjoy mentioning it.


Predictions at leading order (LO), next-to leading order (NLO), and next-to-next-to leading order (NNLO) of the top quark transverse momentum.

DAY 4:

Once upon a time, ATLAS and CMS measured the top quark’s pT distribution in data. At first, ATLAS and CMS simulations appeared to disagree with each other, and neither agreed well with the observed data. Though most of the differences between ATLAS and CMS were eventually explained (…sort of) the data itself remained stubbornly different from the simulation. Czakon et al. and their STRIPPER program to the rescue! David Haymes presented a differential top pT distribution at full next-to-next-to leading order (NNLO), calculated using STRIPPER, that agrees nicely with all of the data, proving that next-to-leading-order doesn’t go nearly far enough when it comes to the top quark.

You’ll notice that I didn’t explain what STRIPPER actually is. In short, it is a combination of an NNLO computational algorithm, capable of providing predictions of the top quarks kinematics, and a touch of theorist humour, in the form of an extremely contrived acronym. One can only hope that STRIPPER is meant to describe the stripping away of the complexities of NNLO calculations, but I suspect that would be generous to the point of naivety. At least the speaker wasn’t wearing a horrendous anime shirt. The result itself, however, is very impressive and desperately needed in order to understand the LHC data.

DAY 5:

Well, it’s been a very successful conference. We’ve seen the first 13 TeV results, some of the most precise results to come out of LHC Run1, and even a few Tevatron highlights! Next year we’ll be near Prague, in keeping with the tradition of the conference being held in places famous for either alcohol or beaches. See you in the conga line!

James Howarth James Howarth is a postdoctoral research fellow at DESY, working on top quark cross-sections and properties for ATLAS. He joined the ATLAS experiment in 2009 as a PhD student with the University of Manchester, before moving to DESY, Hamburg in 2013. In his spare time he enjoys drinking, arguing, and generally being difficult.

TOP 2015 – Top quarks come to Italy!

The annual top conference! This year we’re in Ischia, Italy. The hotel is nice, the pool is tropical and heated, but you don’t want to hear about that, you want to hear about the latest news in the Standard Model’s heaviest and coolest particle, the top quark! You won’t be disappointed.

DAY 1:

Our keynote speaker is Michael Peskin. For those of you who have a PhD in particle physics, you already know Peskin. He wrote that textbook you fear. His talk is very good and accessible, even for an experimentalist like myself, and he gives us a very nice overview of the status of theory calculations in top physics, highlighting a few areas he’d like to see more work on. 

The highlights of my day though are the ATLAS and CMS physics objects talks. Normally, these can be a little dull. However this year we have performance plots for the first time at 13 TeV, and most people are closely scrutinising the performance of both experiments. All except a guy who looks suspiciously like Game of Thrones character Joffrey Baratheon, who is sitting completely upright, eyes closed and snoring lightly.


The poster session, two hours in (photo from @JoshMcfayden)

The poster session, two hours in (photo from @JoshMcfayden)

If you’ve never been to a poster session then this is how they work: a group of students and young postdocs, eager to present their own work (a rare treat in collaborations as large as ATLAS and CMS) stand around, proudly showcasing how they managed to make powerpoint do something that it really wasn’t designed to do.

My poster (approved only hours before) gets a fair bit of attention, but not as much as I expected. Suddenly I regret not slapping a huge “New 13 TeV Results!” banner on the top of it. 

After 3 hours (yes, 3 hours!) of standing by my poster I decide that everyone who wants to see it will have done by now, grab 3 (or 10) canapés and head to the laptop in the corner to cast my vote. For a brief moment I consider not voting for myself, but the moment passes and I type in my own name.

DAY 2:

I sit down next to Joffrey Baratheon and smile at him politely. It’s not his fault he’s an evil king after all. We start the morning with some theory, because we’re mostly experimentalists and everyone knows our attention spans are limited if they give us wine with lunch. As with last year, the hot topic is ever more precise calculations. 

Next we have a very professional talk from a very professional-looking CMS experimentalist. People who wear shirts and sensible shoes to give a plenary talk either means serious business or a terrified student giving their first conference talk. From the polished introduction on top cross-section, you can tell it’s the former. 

CMS have clearly put a lot of effort in to these results (and I’m secretly relieved that I already know our results are equally impressive), and despite a spine-chillingly large luminosity uncertainty of 12%, they have achieved remarkable precision. 

Finally, we’ve arrived at the talk that I’ve been waiting for; The ATLAS Run2 cross-section results. 

A summary of the latest top anti-top cross-section measurements from ATLAS.

A summary of the latest top anti-top cross-section measurements from ATLAS.

The speaker starts by flashing our already released cross-section in the eµ channel at 13 TeV. Even with an integrated luminosity uncertainty of 9%, it’s still a fantastic early result. We show an updated eµ result in which we measure the ratio with the Z-boson cross-section (effectively cancelling the luminosity uncertainty). People seem pretty impressed by that, as they should. Getting the top group to release results this early is hard enough, getting the standard model group to release an inclusive Z cross section is nothing short of a miracle. 

Now the speaker moves on to the precision 8 TeV results. Wait a minute? What’s going on? There are other 13 TeV results to show? What is he DOING?! Months of working on the ee and µµ cross section results and we’ve skipped past them? I turn to my colleague, who led the also-skipped lepton+jets cross section analysis. His face is stoic, as is his way, but inside I know he’s ready to storm the stage with me. I begin to whisper to my boss, sat one seat ahead of me, about the injustice of it all. Somehow it’s coming out as a childish tantrum, despite sounding perfectly reasonable in my head. 

… and then the speaker shows the result. My boss rolls her eyes at me and returns to her laptop, possibly rethinking my contract extension. Joffrey Baratheon scowls at the disturbance I’ve caused and I consider strangling him with his pullover.

Stay tuned for part 2! Where we learn about new single-top results, new mass measurements, and ttH!

James Howarth James Howarth is a postdoctoral research fellow at DESY, working on top quark cross-sections and properties for ATLAS. He joined the ATLAS experiment in 2009 as a PhD student with the University of Manchester, before moving to DESY, Hamburg in 2013. In his spare time he enjoys drinking, arguing, and generally being difficult.

Leptons & Photons meet Dragons, Castles and Multiverses in Ljubljana

Image by Edson Carquin Lopez.

Main tower of Ljubjana castle (image by Edson Carquin Lopez).

The XXVII edition of this classic conference (Lepton-Photon) brought together more than 200 scientists from around the world in the lovely city of Ljubljana, Slovenia. This year’s edition was a bit special, as it featured poster presentations that gave young researchers (including many ATLAS members) the opportunity to show their work. Six posters were selected for short talks and, from ATLAS, the chosen poster-talk was given by Moritz Backes on the Run 2 trigger performance and the upgrades which took place during the first LHC Long Shutdown (LS1).

Lepton-Photon mostly featured plenary talks, ranging from comprehensive summaries of Run 1 results (including New Physics searches, Higgs measurement status, Pentaquark, and much more) to encouraging early Run 2 results on performance and physics. There was a nice talk about the current status of the LHC as well as its future. Other talk subjects included heavy-ion, neutrino and dark matter physics, among others.

On Thursday, there was a public lecture given by the creator of inflation and the theory of multiverses, Alan Guth. The big rooms – normally used for both the plenary talks and the poster exposition – had to be joined and completely reorganised in order to fit more than a thousand people (mainly young Slovenians). During the 1-hour talk, Prof. Guth explained what inflation is and the evidence in favour of this scenario of the evolution of the Universe – all with almost no mathematical details and just one plot: the famous CMB angular spectrum, precisely fitted by the Lambda-CDM prediction.


Conference participants (image from Lepton Photon 2015).

There were good questions – as well as some bizarre ones – from the audience. Especially about the somewhat “crazy” idea of multiverses, which attempts to explain why the vacuum energy (cosmological constant) determined from cosmological observations is so different from what’s calculated in quantum field theory (by some 120 orders of magnitude). Prof. Guth’s talk covered the multiverse theory in detail, which describes how – shortly after the big bang, when inflation started – many other universes were created at the same time as ours. According to the theory, each of them grew from a different patch of the primordial cosmological “egg” and each of them (randomly) got a different vacuum energy. As a consequence, only a few of them (the ones with a vacuum energy compatible with a flat space) were able to create life… well, there should be a good reason why we are here! (This is what encapsulates the anthropic principle.)

So what do particle physics and the ATLAS experiment have to do with all of this? Well, it’s simple. Finding New Physics at the LHC may shed light on how the vacuum energy should be calculated from first principles by adding new contributions to it.

On Saturday, after a really amazing week, the conference was brought to a close with a really nice summary talk by John Ellis. The outcome from so many talks was synthesised in a single question: “Is there life after Higgs? Yes!”



Edson is a postdoc at the Physics institute at the Pontificia Universidad Católica de Chile (PUC), in Santiago. He started to work in the ATLAS experiment in 2010 and he’s currently interested in searches of beyond the standard model theories involving extra heavy Higgs bosons or other exotic heavy resonances, and in the studies of the properties of the Higgs particle. When he’s far from the office and computers, he likes to take long walks, read novels and essays, listen to nice music, and spend time with his family and friends.

Lepton Photon 2015 – Into the Dragon’s Lair

This was my first time in Ljubljana, the capital city of Slovenia – a nation rich with forests and lakes, and the only country that connects the Alps, the Mediterranean and the Pannonian Plain. The slight rain was not an ideal welcome, but knowing that such an important conference that was to be held there – together with a beautiful evening stroll – relaxed my mind.

The guardian.

The guardian.

At first, I thought I was somewhere in Switzerland. The beauty of the city and kindness of the local people just amazed me. Similar impressions overwhelmed me once the conference started – it was extremely well organized, with top-level speakers and delicious food. And though I met several colleagues there that I already knew, I felt as though I knew all the participants – so the atmosphere at the presentations was nothing short of enthusiastic and delightful.

Before the beginning of the conference, the ATLAS detector just started getting the first data from proton collisions at 13 TeV center-of-mass energy, with a proton bunch spacing of 25 ns. The conference’s opening ceremony was followed by two excellent talks: Dr. Mike Lamont presented the LHC performance in Run 2 and Prof. Beate Heinemann discussed the ATLAS results from Run 2.

Furthermore, at the start of the Lepton Photon 2015 conference, the ALICE experiment announced results confirming the fundamental symmetry of nature (CPT), agreeing with the recent BASE experiment results from lower energy scale measurement.

The main building of the University of Ljubljana

The main building of the University of Ljubljana

The public lecture by Prof. Alan Guth on cosmic inflation and multiverse was just as outstanding as expected. He entered the conference room with a student bag on his shoulder and a big, warm smile on his face – the ultimate invitation to both scientists and Ljubljana’s citizens. His presentation did an excellent job at explaining, to both experienced and young scientists, the hard work of getting to know the unexplored. While listening to Prof. Guth’s presentation, it seemed like the hour passed in only a few minutes – so superb his talk was.

I was also impressed by some of the participants. Many showed great interest in the lectures, and asked tough, interesting questions.

To briefly report on the latest results, as well as the potential of future searches for physics beyond the Standard Model, the following achievements were covered during the conference: the recent discovery by the LHCb experiment of a new class of particles known as pentaquark;, the observed flavor anomalies in semi-leptonic B meson decay rates seen by the BaBar, the Belle and the LHCb experiments; the muon g-2 anomaly; recent results on charged lepton flavor violation; hints of violation of lepton universality in RK and R(D(*)); and the first observation and evidence of the very rare decays of Bs and B0 mesons, respectively.

The conference centre.

The conference center.

The second part of the conference featured poster sessions, where younger scientists were able to present their latest working achievements. Six of them were selected and offered the opportunity to give a plenary presentation, where they gave useful and well prepared talks.

The ending conference lecture was given by Prof. Jonathan Ellis, who provided an excellent closing summary and overview of the conference talks and presented results, with an emphasis on future potential discoveries and underlying theories.

To conclude, I have to stress that our very competent and kind colleagues from the Josef Stefan Institute in Ljubljana (as well as other international collaborative institutes) did a great job organizing this tremendous symposium. They’ve set a high standard for the future conferences to come.

pic_tatjana_jovin1 Tatjana Agatonovic Jovin is research assistant at the Institute of Physics in Belgrade, Serbia. She joined ATLAS in 2009, doing her PhD at the University of Belgrade. Her research included searches for new physics that can show up in decays of strange B mesons by measuring CP-violating weak mixing phase and decay rate difference using time-dependent angular analysis. In addition to her fascination with physics she loves hiking, skiing, music and fine arts!

Getting ready the next discovery

I’m just on my way back home after a great week spent in Ljubljana where I joined (and enjoyed!) the XXVII edition of the Lepton-Photon conference.


Ljubljana city center (courtesy of Revital Kopeliansky).

During the Lepton-Photon conference many topics were discussed, including particle physics at colliders, neutrino physics, astroparticle physics as well as cosmology.

In spite of the wide spectrum of scientific activities shown in Lepton-Photon, the latest measurements by the experiments at the Large Hadron Collider (LHC) based on 13 TeV proton-proton collision data were notable highlights of the conference and stimulated lively discussions.

The investigation of the proton-proton interactions in this new, yet unexplored, energy regime is underway using new data samples provided by LHC. One of the first analyses performed by ATLAS is the measurement of proton-proton inelastic cross section; this analysis has a remarkable relevance for the understanding of cosmic-ray interactions in the terrestrial atmosphere, thus offering a natural bridge between experiments in high-energy colliders and astroparticle physics.

Dragon sculpture in the Dragon Bridge in Ljubljana.

Dragon sculpture in the Dragon Bridge in Ljubljana.

While we are already greatly excited about the new results based on the 13 TeV collisions provided by LHC, it is also clear that the best is yet to come! As discussed during the conference, the Higgs boson discovered in 2012 by the ATLAS and CMS collaborations still has many unknown properties; its couplings with quarks and leptons need to be directly measured. Remarkably, by the end of next year, the data provided by LHC will have enough Higgs boson events to perform the measurements of many Higgs-boson couplings with good experimental accuracy.

Precision measurements of the Higgs boson properties offer a way to look for new physics at LHC, complementary to direct searches for new particles in the data. Direct searches for new particles, or new physics, at LHC will play a major role in the coming months and years.

A few “hints” of possible new-physics signals were already observed in the data collected by ATLAS at lower energy in 2011 and 2012. Unfortunately such hints are still far from any confirmation and the analysis of the 13 TeV proton-proton collision data will clarify the current intriguing scenarios.

Although LHC is in its main running phase, with many years of foreseen operation ahead of us, the future of particle physics is already being actively discussed, starting from the future world-wide accelerator facilities.

During Lepton-Photon, many projects were presented including proposals for new infrastructure at CERN, in Japan and in China. All these proposals show a strong potential for major scientific discoveries and will be further investigated, posing the basis for particle physics for the next fifty years to come.

Social dinner during the Lepton-Photon conference.

Social dinner during the Lepton-Photon conference.

Without a doubt one of the most inspiring moments of this conference was the public lecture about cosmological inflation given by Alan Guth. It attracted more than one thousand people from Ljubljana and stimulated an interesting debate. In his lecture, Alan Guth stressed the relevant steps forward taken by the scientific community in the understanding of the formation and the evolution of the Universe.

At the same time, Alan Guth remarked on our lack of knowledge of many basic aspects of our Universe, including the dark matter and dark energy puzzles. Dark energy is typically associated to very high energy scales, about one quadrillion times higher than the energy of protons accelerated by LHC; therefore, it is expected that dark energy can’t be studied with accelerated particle beams. On the other hand, dark matter particles are associated with much lower energy scales, and thus they are within the reach of many experiments, including ATLAS and CMS!

miapittura_new Nicola joined the ATLAS experiment in 2009 as a Master’s student at INFN Lecce and Università del Salento in Italy, where he also contributed to the ATLAS physics program as PhD student. He is currently a postdoctoral researcher at Aristotle University of Thessaloniki. His main research activity concerns the ATLAS Standard Model physics, including hard strong-interactions and electroweak measurements. Beyond particle physics, he loves traveling, hiking, kayaking, martial arts, contemporary art, and rock-music festivals.

BOOST Outreach and Particle Fever

Conferences like BOOST, which my colleagues Cristoph and Tatjana have written about already, are designed to bring physicists to think about the latest results in the field. When you put 100 experts from around the world together into a room for a week, you get a fantastic picture of the state of the art in searches for new physics and measurements of the Standard Model. But it turns out there’s another great use for conferences: they’re an opportunity to talk to the general public about the work we do. The BOOST committee organized a discussion and screening of the movie “Particle Fever” on Monday, and I think it was an enormously successful event!


For those who haven’t seen it, Particle Fever is excellent. It is the story of the discovery of the Higgs Boson, and its consequences on the myriad of theories that we are searching for at the LHC. It presents the whole experience of construction, commissioning, turning on, and operating the experiments, from the perspective of experimentalists and theorists, young and old. People love it – it has a 95% rating on Rotten Tomatoes – and nearly all my colleagues loved it as well. It’s rare to find a documentary that both experts and the public enjoy, so this is indeed a real achievement!

Getting back to BOOST, not only did we have a screening of the movie, but also a panel discussion where people could ask questions about the movie and about physics in general. One question that an audience member asked was really quite excellent. He asked why physicists think that movies like Particle Fever, and events like this public showing, were important. Why did we go to the trouble of booking a room, organizing people, and spending hours of our day on a movie we’ve already seen many times before? And let’s not forget that David Kaplan, a physicist and a producer of the movie, spent several years of his life on this project full time. He essentially gave up research for a few years in order to make a movie about doing research – not an easy task for a professor!

So why do we do it? Why is Particle Fever important?

The answer, to me, is that we have a responsibility to share what we know about the Universe. We study the fundamental nature of the Universe not so that we as individuals can understand more, but so that humanity as a whole understands more. On an experiment as big as ATLAS, you quickly become extremely aware of how much you depend on the work and experience of others, and on the decades and centuries of scientists who came before us. Doing science means contributing to this shared knowledge. And while some details may only be important to a few individuals (not many people are going to care about the derivation of the jet energy scale via numerical inversion), the big picture is something that everyone can appreciate.

And Particle Fever helps with that. The movie is funny, smart, and understandable – all things that we strive to be as science communicators, but which we sometimes fail at. Every particle physicist owes David Kaplan and the director Mark Levinson a tremendous debt, because they have done such a tremendous job of communicating the excitement of fundamental knowledge and discovery. CERN has always sought to unite Europe, and the world, through a quest for understanding, and Particle Fever helps the rest of the world join us on that quest.

Conferences like BOOST are a great time to focus on the details of our work, but they’re also an opportunity to consider how our physics relates to the rest of the world, and how best we can communicate our understanding. Particle Fever has made me realize just how much work it takes to do a really wonderful job, and I’m extremely happy that such a great guide is available to the public. With any luck, we’ll have more movies coming out soon about the discovery of supersymmetry and extra dimensions!

“Max Max is a postdoctoral fellow at the Enrico Fermi Institute at the University of Chicago, and has worked on ATLAS since 2009. His work focuses on searches for supersymmetry, an exciting potential extension of the Standard Model which predicts many new particles the Large Hadron Collider might be able to produce. When not colliding particles, he can be found cycling, hiking, going to concerts, or playing board games.

A boost for the next discovery

I arrived in Chicago for my first conference after the first long LHC shutdown, where new results from the two big experiments ATLAS and CMS were to be shown. Before the beginning of the conference on Monday, I had one day to fight against jetlag and see the city – certainly not enough time to see everything!


Chicago at night.

The focus of the BOOST conference is to discuss the physics of objects produced at very high momenta. New, very heavy particles can be produced at LHC. Such particles decay very quickly and we only observe their daughter particles, or their granddaughter particles in the detector. These decay products are special – they get the boost from the mother particle and have very high momenta.

The reconstruction of such highly boosted particles is a big challenge. Around 100 experts from different institutes around the world meet for one week to discuss the discovery techniques and strategies for these new heavy particles at LHC. Compared to some other much larger conferences, it is a very familiar atmosphere!


Boost 2015 in Chicago: Many fruitful discussions among theorists and experimentalists from ATLAS and CMS.

We had many interesting discussions during the presentations and coffee breaks. For me, it was a great opportunity to meet colleagues from around the world that I’d known for a while but had never met in person. Theorists and experimentalists from ATLAS and CMS exchanged new ideas on the reconstruction and identification of high momenta particles, and discussed not only the discovery potential of both experiments for the next run period but also the prospective designs of new very high energy machines that could be built in 20-30 years.

The University of Chicago organised a public event during the conference: a screening of the Particle Fever movie and a discussion panel. For me, it was like going back in time before the Higgs boson discovery – feeling once again the excitement about the first beam at LHC. It was a great time and I am sure we will have more great moments aheard. It’s time to boost for a new discovery!

F00062 Tatjana Lenz is research assistent at the University of Bonn, Germany. She joined ATLAS in 2005, doing her Master’s at the University of Wuppertal. Her research includes searches for new physics which can show up in decays of top quark pairs and studies of Higgs boson properties like spin and parity. Currently she focuses on the measurement of the Higgs boson couplings to bottom quarks and new physics searches involving this topology. In addition to her fascination for physics she loves diving, climbing, hiking, skiiing and working in her garden!

Boost and never look back

Seen in Chicago the day before the start of the BOOST conference.

Seen in Chicago the day before the start of the BOOST conference. (Photo: Christoph Anders)

When I arrived in Chicago this last Sunday for the BOOST conference I had a pretty good idea what new results we were going to show from ATLAS. I also had some rough ideas of what our friends from the other experiments and theory groups would be up to. What I didn’t expect was to see an ad that would fit the conference so nicely!

Now, why do we care about “boost” and what is it?

Let’s say we want to find a pair of top quarks in our detector. You need enough energy in the collision to produce the top quark pair, and if it is just enough they will be “at rest”, i.e. just sit there. Now let’s picture the decay of the two top quarks as two water balloons exploding. The splashes you see are what we would see in the ATLAS detector. But imagine you want to understand which splashes came from which of the two water balloons… This is hard, as there are many different combinations, the water mixes, etc.

That is where “boost” will help!

If we have more energy, we can give both top quarks/balloons a kick – we call them “boosted”. They will fly away from each other and when they decay/explode, the splashes will go into two different directions! Now we can tell them apart! Of course it is more complicated in real life. We have other processes that look similar to the “splashes” the two boosted top quarks make, but we can distinguish them by analyzing the inner structure of the splashes.

This is what the BOOST conference is about, in essence: understanding how we can use the boost to our advantage when searching for new phenomena.

It actually does work and it is a lot of fun!
Boost and never look back!

Christoph Anders Christoph is a postdoc at the University of Heidelberg’s physics institute in Germany. He has been working in ATLAS since 2007. His research is currently focussed on the identification of very energetic heavy particles, e.g. top quarks. When he is not looking for physics beyond the Standard model, he likes reading, listening to music, photography, cooking, traveling with his wife, playing board games with friends and sports.

A summer evening in the ATLAS control room

The sun has already set over Geneva when I finally walk out from the ATLAS control room. We have been waiting for beams to be injected into the machine since the early hours of the afternoon, but without much success so far. First a issue with high voltage in one of the accelerator sectors, then some problems in controlling the cooling temperature, and finally a UFO dumping the beams (not what you are thinking of – rather a microscopic dust particle in the beam pipe, creating instabilities in the beam orbits). Just a regular day for the most ambitious particle accelerator mankind has ever built, but a pretty boring afternoon for our entire shift crew.

Don't feed physicists peanuts!

Don’t feed physicists peanuts!

With the 50 ns data-taking now half-way over, the ATLAS control room is no longer crowded as it was back in early June, when rivers of champagne were flowing out to celebrate the first Run 2 milestones and Discovery Channel cameras were filming physicists in their natural habitat. Now that more routine data-taking has been established, only the six shifters are there permanently, plus a few experts to assist them… and the Run Manager, which is me this week.

The Run Manager acts as a layer between the Run Coordinators and the rest of the team, making sure the plan of the day is respected and steering ATLAS operations towards a happy ending. But what if there are no operations to be steered? The excitement for us starts with the first hint of proton collisions – before that, we need to get ready and then wait patiently.

Luckily the messages from the LHC team keep us awake: they regularly announce beams “very soon”… and we always believe it. Under the vigilant eye of the Shift Leader, the trigger shifter keeps on checking constant particle rates, the shifters from the sub-detectors (the inner detector, the calorimeters and the muon systems) stare at green flags, the shifter at the Run Control desk enjoys the flux of harmless warning messages, and the Data Quality shifter gets ready to monitor the goodness of the data we might be collecting tonight.

Two more hours, and they will all be released, while the next shift crew comes in. Only the Run Manager – now sitting at the CERN restaurant nervously checking her phone – had better stay, to make sure all parameters are set correctly before a long fruitful run starts. In such a complex detector as ATLAS, every configuration has to be checked by multiple pairs of eyes; we can’t afford the luxury of making mistakes.

Long hours spent staring at the screen of LHC Page 1.

Long hours spent staring at the LHC Page 1 screen.

I am always surprised by how little competition reigns here in the control room. We are all part of this huge wonderful mechanism, that wouldn’t work without the tiniest piece – and we feel it. We are always happy to help new people getting started (so that they may eventually cover some night shifts, a privilege often reserved for students…) and ensure that they are never ashamed to ask trivial questions. I would say it’s like being part of a funny, weird family… if only it would not certainly condemn me to the eternal mockery of my colleagues! Friendships have been built over coffee, eating the Sunday croissants together (a small reward for being here over the weekend), and during the daily rushes to get our slides ready for the next presentations. Long days and long nights that we hope will bring new exciting discoveries.

The sunset has now made room for a warm summer night, but still no beams are circulating in the machine. I will need to call the CERN Control Center to ask what their plans are. It’s late but I don’t feel that tired. With a sigh and a smile I walk back to the control room, while the sky over the Alps turns into pink and then lilac and then blue.

Manuela Venturi Manuela Venturi is a research assistant at the University of Victoria (British Columbia). She is originally from Florence (Italy). She joined ATLAS in 2009, while doing her Master’s at the University of Roma Tor Vergata, and has been happy of that choice ever since. Together with the rest of the team, she searched for and discovered the Higgs boson. She likes to walk, run, dance, cook, write, read and the colour pink.