One week to do it all – Days 4-7: Diffractive data taking

On Thursday morning the first fill reached “Stable Beam”. We had prepared a sequence to move the ALFA detectors so, with the push of a bottom, they were all moved to exactly the right position for loss maps. The fill had 42 bunches (as opposed to the 3 bunches used in elastic data taking), so the trigger rates were much higher than before.

The goal was no longer to take elastic events, but diffractive events which are rarer. These collisions do not need to be head-on or elastic, but can be something in-between. For example, diffractive events can occur when one proton is intact (like in elastic events), but the other proton is not (like in head-on collisions). The goal of this run was therefore to combine ALFA with the rest of ATLAS and take measurements, for example, of the intact proton in the ALFA detectors and the rest in ATLAS. Since these events are rare, more collisions are needed to statistically observe the physics. But, since there are more collisions, selecting which events should be stored for analysis has to be strict. The ALFA diffractive group had therefore set up dedicated selection criteria to decide online if an event should be stored or thrown away.

Online reconstructed tracks in the ALFA detectors. The bright line in the center of each detector is again the pattern from elastic scattered particles, but here also the area to the left have many events. These are the pattern of protons that have lost a little energy and therefore have smaller orbit and are the signature for the diffractive events with only one proton in the ALFA pots.

Online reconstructed tracks in the ALFA detectors. The bright line in the center of each detector is again the pattern from elastic scattered particles, but here also the area to the left have many events. These are the pattern of protons that have lost a little energy and therefore have smaller orbit and are the signature for the diffractive events with only one proton in the ALFA pots.

The fill was kept for just about 3 hours as it was preferred to move to a fill with even more bunches. The next fill was with 252 bunches and it was also kept for about 3 hours. These two fills were needed for safety considerations. When a beam setup is new and untested, it is safer to discover problems with fewer bunches. Normally the LHC can be filled with up to about 2800 bunches but, since ALFA has problems with bunches being very close together and with high particle flux in the detectors, this special running was limited to 685 bunches.

Late Thursday evening the first fill with 685 bunches was successfully brought to “Stable Beams” in the LHC and the real data-taking could start at full blast. I had helped the ATLAS trigger group in preparing the priorities for the run but, as always, a bit of fine tuning was needed. So I left the CERN Control Centre and hurried to ATLAS after “Stable Beams” was declared. Even though it was after midnight I found most of the trigger experts in a corner of the ATLAS control room eagerly looking at the rates and gradually changing the trigger setup to include a higher and higher fraction of interesting physics and background events. I joined them and together we continued the game of maximizing the outcome, without going over the limits. Had we gone over the limits, ATLAS would not have been able to handle it. As this special physics program was so short it was really important to get enough of both physics events and background events.

It turned out that some of the special ALFA events I had requested for background studies were a source of problems. While the events were small in size, so not a lot of bandwidth was needed, they were so numerous that we hit a new limit in the system: the processors of the PCs dealing with the storage. I therefore selected which ALFA background events we needed the least, and we stored a smaller fraction of these in order to leave resources for the diffractive physics events, our main goal.

The fill ended with a superconducting magnet problem at 03:36. Its recovery took until the next evening, giving me a chance to sleep a bit…

From Friday evening on everything went smoothly, for the most part. Monitoring the detector was mainly done by our shifters and I was called only when something went wrong. By Sunday morning we had three good fills and already more data than we had hoped for. At the 8:30 daily LHC meeting it was decided to go for one more day and a fifth very good fill was made.

The only real problem during the diffractive data-taking period was “Single-Event-Upsets” (SEU) in the ALFA electronics. When a particle passes through the electronics, there is a chance that a “bit” is flipped. If that happens the electronics can go into a bad state. ALFA was only designed for the low number of bunches seen during elastic data-taking and was therefore not very well protected from SEU. Five SEU were observed during data-taking. Each time I had to ask ATLAS to stop the run, then power cycle the ALFA detector and reconfigure it, then re-include it in the run (basically re-synchronizing the event number) and re-start the ATLAS run.

The diffractive program ended Monday morning when the final beam was dumped by the LHC operator as they needed to go back to normal physics data-taking. It had been very successful and the excellent availability of all the ATLAS sub-detectors (including ALFA) meant that ATLAS had collected about 82% of the luminosity delivered, which is extraordinary for special run, where often only 50% efficiency is observed.

One week to do it all – and we did it.

Read Sune’s previous ALFA blog posts: “Day 1: Setting Up“, “Day 2: Elastic data-taking” and “Day 3: Preparing for Stable Beam


Bio_photo Sune Jakobsen is a fellow at CERN shared between the ATLAS experiment and Beam Instrumentation. Sune joined ATLAS in 2008 while doing his Master’s at the University of Copenhagen, but was immediately and permanently stationed at CERN to contribute to making and commissioning the hardware for the ATLAS sub-detector ALFA. Currently he is ALFA trigger coordinator, ALFA run coordinator and ALFA technical coordinator besides being responsible for the operation and development of the LHC luminosity detector called BRAN. In addition to his fascination for physics and detectors, Sune loves traveling – especially to Africa and Asia – and hiking in the mountains.

One week to do it all – Day 3: Preparing for Stable Beam

Tuesday at 23:55 I called the ATLAS shift leader and told them to stop the elastic physics run and ramp down the inner detector as the elastic program was over. But that’s when the problems started. For some reason, the inner detector could not ramp down and ATLAS requested – for the safety of the inner detector – that the LHC team touch nothing until the problem was solved. While this actually gave us more time taking data for elastic physics, LHC operators and representatives from the other experiments in the CERN Control Centre were really not too happy about the situation.

At 00:40 the call finally came in: ATLAS was ready and LHC could move on with the program.

The next step was to run the “loss maps”. This is one of the steps the LHC needs before using a large number of bunches and declaring “Stable Beams”. The idea is to use a low intensity beam – exactly as we had for the elastic physic program – and then simulate different scenarios under which the beam could get lost.

Figure 4: Distribution of beam losses over the 27km of LHC as measured for one “Beam Loss” exercise. Note the logarithmic scale. Main losses are observed at the primary collimators in octant 7, exactly where they should be. Almost no losses are observed at the ALFA detectors in octant 1.

Distribution of beam losses over the 27km of LHC as measured for one “Beam Loss” exercise. Note the logarithmic scale. Main losses are observed at the primary collimators in octant 7, exactly where they should be. Almost no losses are observed at the ALFA detectors in octant 1.

The idea of the “loss maps” is to see where those particles are lost. Some positions are safe, like the primary collimators that are made to handle losses, but some may become unsafe places like the ALFA detector which could get damaged and/or damage nearby equipment in cases of large beam losses. Therefore experts purposefully create losses with a low intensity beam and observe where the losses go. To measure these losses, they use beam loss monitors, like the ones used for the ALFA alignment. The goal is to measure the fraction of losses that go to safe positions relative to unsafe positions, and scale the result to the full beam intensity to see if it is safe or not to run with the ALFA detectors very close to the beam.

Before they started we had to move the ALFA detectors to the position foreseen for “Stable Beam”. Originally the LHC safety board (Machine Protection Panel, MPP) and the collimation group had requested the ALFA detectors at about 7.8 mm from the beam. But the ALFA detectors were originally designed to be closer and, at 7.8 mm, the alignment system would be at the edge of its sensitivity and might not be able to get a good alignment. Also, every small step inwards means many more physics events and more sensitivity to small-angle scattered protons. In the end, a compromise was reached: for the loss maps, ALFA could move to about 6.5 mm from the beam. If losses were observed for the loss maps, the ALFA detectors would need to be retracted further and the loss maps re-done.

I was therefore anxious to see the results when the ALFA detectors were put into position and the first loss maps made. Would the beam hit us hard? Would we need to re-do the loss maps and maybe even refill for it… which would take at least 3 hours of beam time?

Luckily my fear was in vain: all loss maps were performed without any significant losses in the ALFA detectors and ALFA was approved to have the detectors at 6.5 mm from the beam for Stable Beam operation. This was a big victory and extremely good for the alignment and physics program to follow.

All of the loss map runs that needed the ALFA detectors were performed during the end of the elastic physics fill, which was new since normally 2-3 fills were needed to do everything. The fill ended at about 03:00 Wednesday morning. Finally – time for some more sleep!

As most of Wednesday’s daytime was used for an emergency access to the ATLAS cavern and Wednesday evening was used for additional loss maps without the ALFA detectors, I actually managed to sleep and recover for the next part.

To be continued…

Read Sune’s previous ALFA blog posts: “Day 1: Setting Up” and “Day 2: Elastic data-taking


Bio_photo Sune Jakobsen is a fellow at CERN shared between the ATLAS experiment and Beam Instrumentation. Sune joined ATLAS in 2008 while doing his Master’s at the University of Copenhagen, but was immediately and permanently stationed at CERN to contribute to making and commissioning the hardware for the ATLAS sub-detector ALFA. Currently he is ALFA trigger coordinator, ALFA run coordinator and ALFA technical coordinator besides being responsible for the operation and development of the LHC luminosity detector called BRAN. In addition to his fascination for physics and detectors, Sune loves traveling – especially to Africa and Asia – and hiking in the mountains.

One week to do it all – Day 2: Elastic data-taking

No time to waste after the alignment. We had moved the detectors to about 2.8 mm from the beam, but the rates of particles passing the detectors indicated a very high background (mainly particles from the beam halo) so we decided to move the detectors out to about 3.5 mm. Now it was time for data taking. Since the detectors were so close to the beam, the LHC could not declare “Stable Beam”. Therefore ATLAS was prepared to manually override the normal safety feature, which only allows the tracking detectors to be powered fully once the LHC declares “Stable Beam”. Tracking detector experts were standing by in the ATLAS control room and my call to the ATLAS shift leader triggered them to override the normal safety procedure and power up the detectors fully. After only a few minutes of ramping the voltage of the ATLAS inner detectors, ALFA was taking data.

The trigger setup – which basically selects which events to store – worked great and all the important events could be stored (read this blog post to learn more about the ATLAS Trigger). ATLAS even had bandwidth left over so that all the requests were satisfied; the rates were lower than expected and all the events could be taken. This, however, had to be fine-tuned, so I helped the trigger expert to figure out which type of event needed which pre-scale before being stored.

Online reconstructed tracks in the ALFA detectors. The bright line in the center of each detector is the pattern from elastic scattered particles.

Online reconstructed tracks in the ALFA detectors. The bright line in the center of each detector is the pattern from elastic scattered particles.

The main purpose of data taking was “Elastic Physics”. Usually we study physics from protons colliding head-on (inelastic) as this is what can potentially make Z bosons, Higgs etc. that can be observed in the central ATLAS detector. However a large number of the protons do not collide head-on. Instead some of them just barely collide and bounce away intact; what we call an elastic collision. They get a small push perpendicular to the beam and can leave the beam envelope where they are detected by ALFA; the closer the detector to the beam, the better the physics. This is why the ALFA detectors need to be aligned and moved so close to the beam. A lot of information is obtained by measuring the rates versus the production angle of these elastic events, such as how likely it is to get a collision and how many collisions have happened per unit of time and surface. ALFA’s design goal is to measure these quantities normally referred to as “total cross section” and “absolute luminosity”.

The next hours were spent looking at the online monitor (see image). Are all the channels giving data, are the triggers working correctly, and are the rates stable or do we need to re-optimize the trigger setting, etc…?

After about two hours of running, a synthetic voice said the words we feared: “Dump beam1, dump beam2”. The beam was lost and, after a few minutes of digging, the LHC operators found the reason: an earth fault on a magnet.

Access was needed to fix it. It is always hard to predict how long that can take, but at least several hours. It was now Tuesday morning at 7:30 and I still had not slept since the start of the program. I called the ATLAS control room and ATLAS Run Coordination to give the update: “good data taking so far, but we need more time to get enough elastic data”.

After a few hours of sleep, I received a call saying the LHC was repaired and getting ready to fill again. I rushed to the CERN Control Centre where I found there was a rather relaxed atmosphere and it was clear that the next beam injection was still some time away. That gave me a chance to hear what happened during the day and to learn of one particular thing that was really not good: during the day, when experts had looked into the LHC settings of the night, they realized that the beams had been crossing at an angle in ATLAS. This is something needed when LHC uses many bunches, but for the elastic program it is really not nice as it complicates how to measure the angles of the elastic scattered protons. Good thing it was discovered! Action was taken to have the next fill without the crossing angle; in the end, we were actually happy that the beams were dumped in the morning.

Around 16:00 the LHC finally injected beam again. It went relatively smoothly and at 19:20 we inserted the ALFA detectors to the same positions as the fill before. The trigger rates were similar to the fill before. A call to the ATLAS control room triggered the ramp-up of the inner detectors and we were back in data-taking mode.

The next few hours were once again spent checking that all the online monitor looked good. No problems. I got a call from the LHC physics coordinator and we agreed that the ALFA elastic data taking would continue until midnight.

We continued monitoring and everything went smoothly for the entire evening. The elastic data-taking had been a huge success and essentially ended that second day.

To be continued…

Read Sune’s previous ALFA blog post: “Day 1: Setting Up


Bio_photo Sune Jakobsen is a fellow at CERN shared between the ATLAS experiment and Beam Instrumentation. Sune joined ATLAS in 2008 while doing his Master’s at the University of Copenhagen, but was immediately and permanently stationed at CERN to contribute to making and commissioning the hardware for the ATLAS sub-detector ALFA. Currently he is ALFA trigger coordinator, ALFA run coordinator and ALFA technical coordinator besides being responsible for the operation and development of the LHC luminosity detector called BRAN. In addition to his fascination for physics and detectors, Sune loves traveling – especially to Africa and Asia – and hiking in the mountains.

One week to do it all – Day 1: Setting up

I have the pleasure to work for a very special sub-detector of ATLAS, called “Absolute luminosity For ATLAS” or ALFA in short. ALFA aims to measure protons at very small angles relative to the beam. To measure these small angles, ALFA is installed on the beam pipe about 240 m away from the interaction point (IP) of ATLAS. The ALFA detectors can move inside the beam pipe in order to get very close to the beam. The detector is only used a few days out of the year when LHC is running with a very special beam setup. In my blog posts, I will try to portray what happened during these special days in 2015 when we took data for elastic and diffractive physics (read my post about Day 2 to find out more).

Figure 1: Sketch of normal optics compared to high betaoptics.

Figure 1: Sketch of normal optics compared to high betaoptics.

After about 3 years of modifications, improvements and optimizations since the last real ALFA run, it was finally the day! The LHC was once again setup to run in a very special way for the so-called “high β* program”. I arrived Monday 12 October 2015 at the LHC morning meeting, eager to hear if everything was ready for the startup or if there would be delays. To my relief everything was on track and the LHC would change to the special program on time. Next followed the ATLAS Run Coordination meeting, where the final details of the run were discussed and everything seemed to be set. All of ATLAS had changed from its normal setting to a setup especially developed only for the sake of “my run”.

As ALFA is so far away from the rest of ATLAS, that other data needs to be kept on hold while waiting for ALFA signals to arrive before deciding if the data should be kept or not. On that day, all the other sub-detectors needed to adapt the standard use of their storage buffers to instead wait for ALFA signals (i.e. not running at very high rates).

After the meetings I was finally free to go to the CERN Control Centre (CCC) to follow the last steps of the commissioning. Most people in ATLAS have never been in CCC and only the ATLAS shift leader communicates with the CCC over the phone when there are problems – but since ALFA was operating so close to the beam, much more direct communication was needed and I therefore preferred to be at  the CCC during most of ALFA’s operation. When I arrived the operators were just about to inject the beam. I greeted everyone and started the ALFA monitor to keep an eye on the status of the detector. Although this first fill would only be used for beam commissioning, it was still a good opportunity to see the full system running smoothly. A few channels were not giving signal and were soon recovered.

The fill was the final validation of the special setting of LHC. Instead of making the beam size as small as possible at the IP (to make as many collisions as possible), the beam size was made larger to make it almost unchanged between the IP and the ALFA station. The particles with small angles therefore leave the beam envelope and can be measured in the ALFA detectors. This is called “high β* beam optics”. This is illustrated in Figure 1. The fill was successful and ended in the late afternoon. There were a few delays, so I had time to sneak in a quick dinner before the next fill.

Figure 2. Top: Beam Loss Monitor signal showing losses at the end and the threshold to stop the movement. Bottom: Detector position. The stop of the movement is visible. The figure is an online display actually used for the alignment.

Figure 2: Top: Beam Loss Monitor signal showing losses at the end and the threshold to stop the movement. Bottom: Detector position. The stop of the movement is visible. The figure is an online display actually used for the alignment.

The next fill was the real deal: a fill for ALFA alignment. A few collimators needed to be aligned first, so it was only our turn around 01:00 in the morning. Time to shine. To align the ALFA detectors to the beam we moved each of the detectors in very small steps. When a detector reached the halo edge of the beam, a lot of particle showers were made and observed in a beam loss monitor. Then the movement was stopped and we knew where the beam was and could move on to the next detector.

We did this eight times and it was one of the most stressful tasks you can imagine: if the detector is not stopped or takes a too large step into the beam halo, the beam is dumped and many hours of beam time is wasted. Also, the time wasted would come out of the already very short data-taking time. Thanks to past experience, we had early warnings that the detectors were coming close to the beam. We had learned that the trigger rates are a very strong indicator of this and, by observing these rates, we were able to predict the edge a few steps earlier, allowing us to be completely ready to push “Stop” manually in case of problems. This was not needed though, as new stopping tools installed since the previous ALFA run worked nicely, and the detectors stopped automatically without any problems. In two hours all was done and we were ready to take data.

To be continued…


Bio_photo Sune Jakobsen is a fellow at CERN shared between the ATLAS experiment and Beam Instrumentation. Sune joined ATLAS in 2008 while doing his Master’s at the University of Copenhagen, but was immediately and permanently stationed at CERN to contribute to making and commissioning the hardware for the ATLAS sub-detector ALFA. Currently he is ALFA trigger coordinator, ALFA run coordinator and ALFA technical coordinator besides being responsible for the operation and development of the LHC luminosity detector called BRAN. In addition to his fascination for physics and detectors, Sune loves traveling – especially to Africa and Asia – and hiking in the mountains.

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.

CONFERENCE DINNER:

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.

NNLO_top_pt

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.

POSTER SESSION:

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.

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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!”


ecarquin

 

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.

revi_pic_2

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.