09 January 2020

Testing a New Scintillator Detector Design at the Los Alamos National Laboratory

What's cubed, made of plastic, and detects charged sub-atomic particles? The new detector for the T2K (Tokai-to-Kamioka) neutrino oscillation experiment of course! This new detector is called the Super Fine-Grained Detector (SFGD) and has been proposed for the upgrade of the near detector of T2K. It's planned for installation in 2021. There are also plans for a similar detector called 3DST (3 Dimensional Scintillator Tracker) on the DUNE experiment, which is under construction. Two prototypes of the detector have been made, and several of us from the T2K and DUNE experiments headed over to Los Alamos, New Mexico to use their neutron beam in the Los Alamos National Laboratory (LANL).

I am a 3rd year PhD student at Imperial. Most of my work so far has been relating to the upgrade of T2K's near detector, hence why I was able to help out at this beam test. In fact, I've already been to CERN to help out with a beam test of one of the detector prototypes, where we fired lots of charged particles at it. That beam test was useful for getting to grips with the detector and measuring some of its inherent properties, such as its time resolution. At LANL, we were firing neutral particles at the detector, which cannot be directly detected in the scintillating material. Instead, we relied on the neutrons interacting with the plastic in our detector and knocking out protons or some other charged particles, which we could see in the detector. The whole design of the detector is built around trying to identify particles from neutron events as close to the interaction point as possible. This will be extremely useful for experiments looking at anti-neutrino interactions, as it is important to identify the interaction process that caused a neutron to be produced from an anti-neutrino interaction, so that the energy of the original neutrino can be calculated.

Rolling hills
Rolling hills
The Rio Grande
The Rio Grande

But enough about the physics, because I also want to talk about Los Alamos itself. It was a privilege to work in a place so steeped in history, as this was the laboratory where the original atomic bombs, Fat Man and Little Boy, were developed, with help from many famous names such as Oppenheimer and Feynman. It's also very scenic, and a totally different environment to that of London, or anywhere in the UK! The town is at an altitude of 7000 ft (that's about 3000 m for us metric users), and surrounded by a desert of rocky cliffs and mesas. I didn't get much chance to explore the area, but I went on a hike one day by a town called White Rock. We climbed down into a canyon and walked along the Rio Grande river before climbing back up the canyon (not for the faint hearted!). The views of the canyon and the skyline with mountains in the distance were spectacular. As you can see from the photos, the weather was very clear on the hike, as it was for most of my stay there. Although, when I first arrived at the end of November, there was a snow storm for the first few days and there was a good 6 inches of snow covering the area. I had rented a car for the trip, and the snow mixed with the fact that I'd never driven on the right side of the road before made for some nervous driving! I got the hang of it in the end though.

New Mexican food!
New Mexican food :)
Birthday surprise!

I may not have had many days off to explore the surroundings, but I was able to go into Los Alamos and Santa Fe to sample the local cuisine. There were a plethora of restaurants serving New Mexican food, which is similar to Mexican food but with a few twists (mainly the twist is more chile!). I'm vegetarian but there was normally plenty of choice for meat-free meals, which I was pleasantly surprised by. At most places I got the vegetarian platter so I could try as many things as possible! We went to a restaurant called Gabriel's on my birthday and I got a surprise rendition of "Happy Birthday" in Mexican (I think), with me wearing a sombrero! And to top it all off a free dessert :). Would definitely recommend a birthday in New Mexico!

Back to the beam test of the SFGD prototype, the test itself was successful. We managed to see events in the detector that coincided with the beam position, and we could see the structure of the beam extremely well with our timing electronics. There has already been some data analysis but there is still a lot more to be done before we can completely gauge how successful the beam test was. One of our aims is to measure the cross-section of the neutrons in our detector, which requires the detection of a very large number of neutrinos. We believe we've gathered enough data over the last month for an accurate measurement, but the result is yet to be calculated.

I mentioned at the start that there were two prototypes at the beam test. The SFGD prototype was in the beamline for the whole month of the beam test, yet the US-Japan prototype wasn't put in until the last day! This was due to some hardware issues, which is to be expected since this was a completely new and untested machine. Initial glances at the data we collected with it indicate it was working as expected, but again, we will have to wait until further data analysis is done before we reach any hard conclusions.

Of course, I wasn't carrying out this beam test all by myself! There were a number of other T2K/DUNE collaborators that helped out. I was one of the three run coordinators that spent the most time at LANL and became most familiar with the detector and the procedures, hence had the job of organising the tasks for the day and figuring out problems that occurred. The other two run coordinators were Guang Yang of Stony Brook University and David Last from the University of Pennsylvania. One of the benefits of working in high energy physics is the opportunity to meet people from all over the world, and I thoroughly enjoyed my time working with these two and the other volunteers that came to LANL. We had plenty of meals together and went to see a few movies too!

In summary, my visit to Los Alamos is not one I will ever forget. Friendships were formed and, perhaps secondarily, physics was performed! Now begins the long but satisfying process of data analysis! I'll leave you with some more pictures of the beautiful White Rock area and a picture of we three run coordinators next to the SFGD prototype detector. Thanks!

16 November 2017

Impressions of the HEP Group on an Imperial undergraduate

I have been working in the HEP Group on level 5 for some months now. My first encounter with particle physics was during First Year at Imperial, where a partner and I investigated a theorised supersymmetric Higgs boson with MC data from the Tevatron at Fermilab. This is how I got to know Per, who has been my link to the Group and indeed, the ideal project supervisor. During the last summer, I worked under his supervision within an Undergraduate Research Opportunity Programme (UROP), doing analysis in T2K. The primary goal of my project was to devise an analysis into the noise hit level upon the ECalorimeters of the Near Detector (ND280). Since then, I have been working in the Group on my BSc proejct about the Comet experiment.

Upon starting the work, the first thing I told myself was: "this particle physics is a tough job!".
This was not least due to the fact that on the first few days, being logged into the Group's linux machines through a small terminal with an even smaller font, I felt my eyes were gradually melting. It was hard for me to grasp that particle physicists actually spend full days in front of bulks of code..!
However, as I learned to get to know the people in the Group, I realised how good they were with computing and code. The first one to standout was Clarence, whom I found out to be a Swede only after weeks when I had thought him to be an Englishman.

Figure 1: Eyes-melting code terminal. Font would not go bigger!  
Not only he was immensely helpful in helping me fix code problems and teach me how to write Bash scripts, but also he seamed to be a genuine computer mogul. He has tried to rebuilt his laptop's hard disk with a screwdriver and some tools and even knows a bit how to programme in Assembly, virtually talking to the computer, in the latter's language. Another "fun fact" with which he managed to amaze me was that apparently GPUs are sometimes more useful than CPUs in performing the latter's job. That is, he suggested using graphics processing units to conduct data analysis - crazy!

On the first day on the job, I was told that I needed to investigate piles of this code to determine where did one data container was filled, and that I needed to modify it. When people entered our office, they were therefore not surprised to find me looking for that needle in the haystack.
Figure 2: Roy during his UROP internship. Somewhere in that haystack hid a container that was filling with neutrino and noise hit data!
The software installation process took only a week (...) but that was fine given it followed a light read of the technical report about the ND280 ECals which was witty, vibrant and thrilling, all at the same time...! Nonetheless, after the preliminaries were over, I was on-the-go, coding and running scripts in linux!

Upon starting to go about the Group's different offices, I started to realise how deeply-cemented was the programming aspect of the work, in virtually everyone. I encountered an ancient debate which dichotomically divided the Group, believed to be so antiquated a debate, to have originated between Plato and Socrates: whether to use EMacs or Vim. As I was firstly introduced to the former, people were flabbergasted when they noticed I opened a new terminal in order to code. I remember I told myself "how far does this go to?" when Phill told me one may write Latex documents using EMacs.
Thereafter, I gradually started moving "to the other side", resulting in being an ardent believer in Vim. But this was only the tip of the iceberg.

As I was getting to know the people more, I understood what a considerable part of their lives comprised of programming. The highlight of that discovery was of course Yoshi, who holds any sort of computer interface in contempt. Indeed, I was dazzled (understatement) to find he was doing EVERYTHING (literally) via the linux command line - including emailing and booking plane tickets. Before that moment I had believed such people only existed in The Matrix but I was proven wrong!

Figure 3: Yoshi about his regular business, reading emails and shopping online. 
This got me thinking on the education one has received from childhood - too many interfaces, not enough programming and command lines. I thought to myself, how good of a programmer would I have been today, had I been introduced to bleak linux command line from birth, instead of my first-ever computer, Comfy.

Figure 4: Comfy, A Computer Experience for the "little ones". 

Considering this, I thought more on whether humanity was endangered by those stupefying baby games, making whole generations unfit to face the Rise of the Machines against us humans. I thought such people as Imperial HEP Group members would partake in the army against that rise, to secure our future on Earth...!

Roughly on a weekly basis, the Group's lovely Phd students, summer students and postdocs, went to Imperial's Prince's Gardens to throw a Frisbee. This was a great way to end a week of facts, figures and code. Nonetheless, the occasional pub crawls proved to be an amazingly enjoyable experience. Most of all, I loved how people from such different backgrounds ended-up working together in the divine realm of particle physics. I thought to myself - that's brilliant, science does bring people together - another proof of its positive force in the world.

A particularly great experience was going to lunch with the rest of the Group. College had temporarily allowed for undergraduate summer students to eat at the SCR, which is regularly closed to us. It served great food for reasonable prices. This was a remarkable opportunity to enjoy good food at Imperial, and coincidentally avoid the usual poison sold at the god-forsaken rat-infested library!

Another routine I particularly enjoyed was the weekly T2K group meetings. Apart from presenting my own progress, I learned a lot from the comments and suggestions made by the other members. Moreover, listening to the work that had been done by the rest was especially captivating and enabled me to have a broader view of Imperial's invaluable contributions to this experiment.

Nonetheless, working in 536, I got to know other people also, thus to hear about the exiting experiments they were working on, like the SoLid neutrino experiment and the Comet muon experiment, which I am currently working on. I also found that a substantial part of the knowledge is shared between people working on different experiments, and that prior knowledge and experience in one experiment may well be useful to be working on others. First and foremost is ROOT, which I find rather cool, whereas I know I am in the minority, as all I have ever heard from other people about it were complaints. In any matter, lovely people from other experiments were able to help me in ROOT and advise on quicker and more efficient ways to go about things.

In general, I had had a wonderful time to be working here during the summer. It was a memorable opportunity to be conducting research in particle physics and more importantly, get to know amazing talented people working in the Group, who were always very helpful and patient to me. I was glad to gain experience of how it is to be working in a physics research group generally and particularly, see what particle physics actually entails. Whilst observing the day-to-day basis of the people here, coding, discussing and presenting, I told myself "I want to be able to do this. I want to know what they know, I want to be able to do fruitful work with them". Indeed, as physicists, I felt that the people in the Group set me an example and consequently increased my motivation to pursue particle physics further. This has led me to conduct my BSc project on Comet and gain more experience whilst being an undergraduate student here. I can just hope to do more work in the field in the future, and wish it to gradually become more fruitful, more useful and more relevant. Without any physics-related wishes, I hope to keep in touch with the dear people here, wherever they or I may be.

Figure 5: Your humble servant posing on level 8's terrace, in front of the Royal Albert Hall .

02 October 2017

A Day in the Life of a Summer Student from the Far East - Part 2 -

After getting fuelled up with coffee-power that would last for another few hours, people once again absorbed themselves in exploring physics, mostly by means of coding. In that sense, our lifestyle more or less resembled that of a tech start-up, rather than a layman's imagination of physicists spending their time in a lab full of mysterious devices, although it might have been different during the developmental stage of the detectors and the electronics. (Toby, a PhD student who is actually building something called Gabor Lens, might have been the closest to a layman's physicist while I was there.)

Toby's cool Gabor Lens!

My research project was almost 100% computer work, predominantly C/C++ and ROOT. I could see how complicated the experiments and analyses nowadays have become, and accordingly I could understand why we can't help relying on computers as we probe deeper into physics. After spending two months at a HEP research group, I've come to admire computers as super-smart masochists who would do any sort of complex numerical calculation for just a little charge on electricity bill, but at the same time I've come to despise those arseholes with zero flexibility for causing all the unwanted troubles, despite the upsetting fact that most of the times I'm the one who is solely responsible for those troubles.

Computer ridiculing human stupidity.

Probably at least twice in every week, we went outside to play Frisbee around 5-6 in the afternoon when people ran out of patience to sit still in the office. Clarence the coffee-loving Swede was usually the initiator in this case, and he had developed a sort of conditioned reflex inside me so that I just couldn't resist associating a joy of running all over the grass to catch a saucer with him holding a Frisbee and making a suggestive facial expression. We usually played Frisbee at Hyde Park or at Queen's Lawn, but my favourite place to play Frisbee was Prince's Gardens where there's a university-run pub nearby so that we could enjoy Frisbee with a cup of beer in one hand. My supervisor Morgan once dropped by the pub and bought a pint for all of us playing Frisbee. That was awesome, but the best part of playing outside was always watching physicists helplessly throwing things to take down a Frisbee stuck on a tree.

Summer students taking a video of physicists being desperate with a Frisbee.

We played Frisbee for hours and then went to a pub to quench our thirst and grab a bite. Anyone ordering food like nachos and chips to share with everyone was always welcomed, although one time during the final week of my stay, people got so generous that we ended up having three big plates of pure nachos (which could easily be shared by 3-4) for just three people, as well as other miscellaneous food which were not as intimidating as nachos. Besides that experience of nachophobia, my experience in English pubs had been always very delightful, and I was always looking forward to trying out all different sorts of draft beer whenever we went to a pub. Enjoying a pint of unacquainted beer sitting on a garden table outside the union pub while feeling the English evening breeze was always a great reward after a day's work. 

I usually headed straight to dorm after work, but sometimes I went out to explore the cultural side of London for which I got several recommendations. One of my favourites was the Proms, even though I had no prior knowledge or interest in classical music. At the Proms, I could experience 3 hours of complete eargasm for just £6, which is an unimaginable price in the Far East. What was even better was that I could go see it everyday by just crossing the road, during my whole stay in London! What a perfect location and timing.

Royal Albert Hall is just around the corner!
Another favourite cultural event that I enjoyed was Championship football, which was recommended by Yoshi, who is a passionate Palace supporter himself and also the one who encouraged me to write this post. On the way from Bermondsey station to the Den, the home of Millwall F.C., I could see a whole different side of London which was in stark contrast to the Exhibition Road in South Kensington, which I believed to be what London would be like throughout the entire city. Evidently I was wrong.

When I arrived at the Den, at first I was quite intimidated by the number of white, muscular, short-haired males with beards. At the moment, I could only hope they would be nice. Some stared at me while I was passing through, as if I was an unwanted corgi in a bulldog family. That’s why I had to buy a Millwall shirt, to camouflage myself as much as I could.

My first experience of Championship football.

I'm not sure if that really worked, but it definitely helped to practise some of their chants on YouTube beforehand, because I could feel like home as I sang along those derogatory chants together with all the supporters around me. Whenever the referee blew the whistle against our team, or whenever our player made a mistake, everyone stood up from their seats and just went MAD. I could hear all sorts of creative British cursing and I thought it was actually a good way of learning natural English. The language of Millwall supporters was far from my image of English gentlemen and they behaved so differently from many people I know at Imperial or people I met at the Proms, but they were good people. When we scored, we danced and hugged each other and jumped and shouted "bbooooohhh" together. I felt like I was in a huge loving family. It was also interesting to see a little Millwall fan, seemingly terrified by all the swearing and noises by the adults, bravely stand up to shout "Go Millwall!" when the team was in danger. Yoshi was right. I could feel the unique charm of English, working-class football that I couldn't experience at a fancy stadium like Wembley.
Weekly dose of salted caramel.

After all the work and excursions, my typical day as a summer student ended with having a scoop of Jude's salted caramel ice cream before going to bed. It was indeed great British ice cream as advertised, and I bought it at Sainsbury's every week on the way home.

I miss you, T2K office!

So this is the end of my day-in-the-life story! Now I bet all of you understand how awesome my days were. I've learned a lot of things during my stay there, but the greatest lesson I've learned from my daily life at Imperial T2K was that, enjoying is the priority! 

I'm enjoying a week's holiday back in the Far East now, and some weird-looking birds are chirping in the backyard as I'm writing this blog post. I feel like everything was just a part of a summer night's dream, but I'm very sure that these dream-like days I had with this amazing group will stay longer in my memory than any other dream that I've had in my life.

20 September 2017

A Day in the Life of a Summer Student from the Far East - Part 1 -

I was this year's summer exchange student at Imperial from Seoul National University, and I studied how the sensitivities of neutrino oscillation parameters change with different far-detector locations for the future T2K experiment. I'd be very happy to explain to all of you how awesome my days were while I was at the Imperial T2K group.

My typical day as a summer student started with turning off an alarm, just like any other individual with a job. Sadly, it didn't always lead to waking up in my case, since our group was so blessed that I could work anywhere, anytime. I could get some more sleep and start working at home or at Hyde Park or anywhere else depending on the weather, or to be more precise, depending on my state of mind. Sometimes I went to the office as early as 8 in the morning (which isn't that early actually for most people), sometimes after lunch, and once, after 4 PM. I could leave the office in the morning if I wanted to, and also could stay in the office even after midnight but by risking a chance of being locked inside the building. It is actually a huge privilege that is really unheard of in the Far East, and I enjoyed it like a boss, letting go of any subconscious sense of obligation to be at work earlier than those in superior positions. 

However, there were some good reasons for me not to show up at work during nonsensical hours. One of them was Santander cycles, which I loved the most among all possible sorts of transportation in the UK (no air-conditioning on a subway, seriously?), and the problem was that the cycles at the docking station I always used were usually all gone by 10-11 in the morning because of all the commuters and tourists. Riding a Santander cycle across Hyde Park, from Queensway to the Queen's Gate and through the downhill towards Blackett Laboratory was so much fun, and I really didn't want to miss it any day. It never got old. I thought Santander was a bicycle manufacturer and wondered why there's a bicycle shop inside the student union building, until I was told to find a Santander cash machine by a street vendor at Portobello market. (I didn't have much trouble living without cash in London, and any Japanese visitor to London would be surprised!)

I love Santander cycles!

Another important event that I didn't want to miss was lunch at Imperial. The food served at SCR was exceptionally good, regardless of my prior low expectations on British food in general, and it was more affordable compared to other off-campus cafes or restaurants. Especially on Tuesdays and Fridays, lunch was something that could never be missed. There was a farmer's market every Tuesday, and fish and chips was served at SCR every Fried-ay. Thai Green Curry and Seafood Paella sold at the farmers' market were really good, and the brownie fudge sundae with two vanilla scoops was just the BEST. (I'm eating a self-made brownie fudge sundae while I'm writing this post, but it's nowhere near the BEST one. Vanilla ice cream and brownies made here taste surprisingly flavourless now!) Fish and chips with Rubicon Lychee was also one of my favourites, and it was even better than some of the fish and chips places outside the campus. 

Rubicon, I miss you!

One of the few peculiar things about the group was that, while there was no fixed working hours, the lunch time was strangely so strict that I could possibly measure the standard deviation to be smaller than 5 minutes. The holy initiator of lunch was Patrick, one of the two wonderful post-docs of the group from whom I had received much help during my stay in the UK, and even hungry Phill, the elder of the two, had to wait when Patrick said "we still have 10 more minutes 'til 12". (In the Far East, no one can dismiss a hungry elder suggesting to go for lunch. Confucius taught us not to.) There was actually some heretic movement to have lunch at some random absurd time like 12:30 while Patrick was on holiday, and now I can confess that I felt both guilty and excited for joining it.

Our Lovely Coffee Table!

After lunch, everyone drank coffee except me. Yes, I'm still talking about a British research group, and I hadn't seen anyone in our group drinking tea, except for one time when Clarence (who is Swedish and the main importer of expensive coffee beans to the office) tried to convince me that he also enjoys tea. Anyway, in our crammed office there was a separate desk for coffee machines and coffee beans (and Japanese coins for some reason) as if these were important members of the group, and people would gather around that desk every afternoon making themselves a cup of coffee. They talked about random stuffs ranging from the boring North Korean missile threats to one's PhD thesis, while ruining their own fatigue detection system and puffing out invisible smoke of heavy caffeine. As the only non-coffee-drinker in the office, I just pretended that I was holding an invisible coffee mug during the conversations. It reminded me of the old days when I was in the military where I was the only one who didn't smoke cigarettes. Although I didn't enjoy coffee or cigarettes, I loved chatting over random stuffs after lunch, and besides, I could get some good advice on my research project mostly during these conversations.

(To be continued...)

09 November 2015

A Neutrino Buzz in the Air

Today is a special day for those who have been working in the area of neutrino oscillations.

We were still celebrating the recent awarding of the Nobel Prize to Drs Art McDonald and Takaaki Kajita, who are leaders of the SNO and Super-K experiments respectively—it is a fantastic feeling to know that colleagues in the area of physics we study have been recognised in one of the most visible ways possible.

But a different award was announced today—or rather on the evening of Sunday 8th November in California where a flashy presentation ceremony was held, with Seth Macfarlane as the host—the 2016 Breakthrough Prize in Fundamental Physics.

From the official announcement page:
The 2016 Breakthrough Prize in Fundamental Physics to be Awarded to Seven Leaders and 1370 Members of Five Experiments Investigating Neutrino Oscillation: Daya Bay (China); KamLAND (Japan); K2K / T2K (Japan); Sudbury Neutrino Observatory (Canada); and Super-Kamiokande (Japan)"
The Nobel Prize is famously awarded to up to only three individuals per prize, and there is always much discussion before and after as to who ought to receive the prize, and, inevitably, who missed out unfairly. There is usually no controversy about whether the actual recipients deserved their prizes, but there are cases where many of us feel that it would have been fairer to relax the three-winner requirement a little, a constraint that was only officially introduced in the late 1960s.

One of the many differences between the Nobel Prize and the Breakthrough Prize is that the latter not only allows more than three people to win the prize, but that it acknowledges the important role that collaborative work plays in modern science. Therefore, the $3M prize goes not just to the top few leaders of an experiment (although such leaders are also recognised explicitly; with seven physicists in this year's case being honoured his way), but is shared by all those who worked together to produce the seminal journal papers in which these experiments reported their findings.

At Imperial, we are delighted that many past and present HEP group members are laureates for the T2K experiment, and as it happens I also receive the prize for my work with KamLAND when at Stanford University.

In our field, collaborations can be all-consuming parts of our lives; in the early days of T2K, I vividly remember my colleagues working day and night, week after week, to help design the detectors we would be building, and long hours spent in the lab, testing and assembling detector components; we would discuss and argue over and over again about how best to do things, and toiled to make sure that the fruits of our work in 2005 would still be worthwhile in 2015 (and now, we are hoping they will continue to be useful in 2025).

Collaborations can continue working together for many years, with individuals receiving their PhDs, becoming postdocs and obtaining academic positions, and generally growing old together, all while pursuing the same common goal—to make their experiment successful. Of course many people will move on to different things, be they jobs in industry or work at other experiments (and in new collaborations), but I think the bond between people who have worked on these experiments together during the most intense times is quite unique and long-lasting.

Today I received an email that was sent out to the roughly 100 prize recipients of the KamLAND Collaboration who worked on the papers from early 2000s where we demonstrated that neutrinos actually oscillate, rather than disappearing in other ways. The list of names on its own brings back memories to me of stressful, but also exhilarating, days and nights spent deep in a mine—in fact all of the experiments which received the prize today involve some kind of underground part to their set-ups—trying to get the experiment to perform as well as it needed to, and arguing over how to analyse the data. Yes, we do spend a lot of time arguing with each other!

Super-K and SNO, whose leaders received the Nobel Prize, showed definitively how neutrinos change identity as the travel; but one needs to put together the discoveries made by all five experiments which won the Breakthrough Prize to form the current picture that we have of neutrino oscillations, and it is an interesting distinction that has been made by the respective prize committees.

All the buzz that surrounds our field is made even more exciting by the fact that the discoveries we have made point to more possible progress in the next several years, and here at Imperial we are working on the future Hyper-K and LBNF/DUNE experiments as well as other neutrino projects, all as part of international collaborations. As proof of this, this month we are hiring three postdoctoral researchers (we are currently going through the selection process) to join the T2K and Hyper-K effort, and we hope that some of the new cohort of PhD students that have just arrived at Imperial will also join us (but that is up to them!).

So while these prizes do help us look back to savour the amazing physics discoveries that we have made in this field over the last couple of decades, it is the future that really excites us—not only in neutrinos, but in all the other areas in which we are building experiments that have the ability to make breakthrough discoveries that tell us more about the universe we live in.

18 June 2014

My World Tour of Particle Physics

My World Tour of Particle Physics

The great irony of particle physics is that in order to see the microscopic world ( and by microscopic, I really mean femtoscopic ), we must build machinery that is larger and more complicated than has ever been seen before. This technology is so big, that it's unusual for it to be constructed by a single country, let alone a single institution. And so, as a result of this need for collaboration, particle physics comes with a lot of travel.
That said, the 5 months I spent last winter, were probably a little extreme in that sense, especially for a student.


Before I go any further, for those who don't know me, let me introduce myself. I am a second year PhD student here with the Imperial College High Energy Physics (HEP) group. I am working on an experiment called COMET, an experiment involving some 120 collaborators from 12 different countries ( and that's considered small in this field! ). COMET is searching for a process known as COherent Muon to Electron Transitions ( or muon to electron conversion, but that wouldn't make such a good acronym ). Although the signal ( the thing we're looking for ) in COMET is quite simple -- an electron with an energy of 104.9 MeV -- there is some uncertainty of the various background processes that could fake this signal due to the fact that this measurement has never been done in this way before. Because muon-electron conversion is expected to be so rare, if it does exist, it is vital that we have extremely good control over any such background processes. Since late last year, I have taken part in several activities to refine our understanding of some of these backgrounds for COMET. A more detailed description of the experiment can be found on the Imperial web-page.

AlCap at PSI

The Alcap collaboration
It all began in November last year, when I caught a plane to Zurich. Not far from there, in a valley on the Aare river, is the Paul Scherrer Institute (PSI) which hosts one of the most intense muon beams in the world ( until we build COMET :p ). Using this beam and an aluminium target, the Alcap collaboration ( a joint effort between COMET and our Fermilab cousins, Mu2E ) reproduced the situation of COMET, albeit with much lower statistics, by stopping the beam in an Aluminium target. Several different detectors then observed the types of particles that were subsequently produced and from this we build up various different spectra. You can see the setup in the images below.
The chamber, beamline and detectors at PSI
And so for 5 weeks a team of about 15 of us, mostly fellow students, worked to set this experiment up and get the detectors working. This was new ground for me. Real hardware work, getting my hands dirty making ( and breaking ) cables, using different radioactive sources to calibrate the detectors and so on. And that was just the setup. Once we started running with the beam we worked around the clock in shifts. On a couple of occasions we would arrive at 9am one day, only to leave at 9am the next. Occasional trips for dinner across the border in Germany were about the only respite.
But it was worth it. From running and developing a data-acquisition (DAQ) system, to making tight vacuum seals; from wrestling with electrical grounding issues, to building a vacuum safety interlock; from gamma ray emission spectra, to just how good Swiss roestis really are, you couldn't help but learn. And more importantly, despite several set backs we managed to take enough data that a decent analysis will be possible. 
The Alcap setup as seen from above.

CM13 and Technical Review at KEK

That all ended just in time for the holidays, which mostly involved catching up on sleep and work on the simulation of COMET. And, after a brief trip home, I was back on a plane jetting over to Fukuoka, Japan, for the 12th COMET collaboration meeting. These meetings are essentially a conference for everyone working on the experiment from all round the world to come together, share their updates in person and discuss the next steps. As my work within COMET itself had mostly involved development of the simulation, I gave a short presentation of the situation there.
Much of this presentation was then shown again two weeks later at KEK in Tsukuba, just north of Tokyo, to an independent review panel that was making sure COMET was being properly developed. Talks were shown covering the whole experiment and it was fantastic to have the opportunity to present the COMET software as a part of this.

ECAL Pile-up Studies

Blending in...
No sooner had this review finished than was I back on a plane heading for Novosibirsk, the capital of Siberia, Russia. The Budker Institute of Nuclear Physics (BINP) is helping to build the Electromagnetic Calorimeter for COMET ( commonly referred to as the ECAL ). This is the part of the detector which measures the energy of a particle once it reaches the end of the system, and is therefore a crucial part of the experiment.

The Problem

Remember that for COMET we are looking for electrons with an energy of 104.9 MeV, ( I tried to put this into real terms, but however you look at it, it's a small number: about the kinetic energy of an apple moving 5 cm per hour or the energy consumed by a 40 watt bulb in about 0.4 picoseconds ). The difficulty arises because a similar process, where the muon decays to an electron and 2 neutrinos ( the Standard Model process, which happens all the time ) is also able to produce such electrons. This is only true because this other process occurs from the orbit of a nucleus, which is why we call it Decay In Orbit (DIO). If the nucleus recoils against the electron, extra momentum can be given to it, until it reaches the 104.9 MeV of the mu-e conversion process. As we get closer and closer to the signal energy the probability of this happening gets much much smaller so we see fewer and fewer electrons coming from DIO.
An example pile-up pulse as might be seen by the ECAL.
Now imagine that an electron from DIO arrives at the detector with about 100 MeV. All the time in the experiment we see lots of low energy particles coming from various processes. If one of these other particles, with 5 MeV were to arrive at the detector at the same time as the 100 MeV electron, then suddenly our system would think it's seen mu-e conversion! We set about writing our discovery, publishing everything and putting out the press releases whilst in reality we had only seen 2 well understood processes.
This problem, known as pile-up, is what I was studying in Russia. How can we identify such occurrences and what can we then do to obtain the individual particle energies? The detector itself outputs waveforms, a bit like on a heart monitor in a hospital. The challenge is to find ways that analyse these waveforms to give the right information regardless of the overlap of two incoming particles. 


We started by looking at the literature, looking at how other experiments have handled similar issues. Two techniques were found and carried through for further studies.
The first, known as the g-2 fit, from the experiment that first developed it, produces the shape of a single pulse by merging the response of many waveforms to give a 'template' pulse. Then each waveform is fitted with this template and the agreement between the recorded waveform and the fitted template is then checked. If the two don't agree well we add a second template pulse and see if things now agree better. If they do, we say the pulse suffered from 'pile-up' and take the values from fitting two pulses to work out the energy of each pulse.
The second technique, known as a Matched Finite Impulse Response (FIR) filter, scans across the waveform and produces an output based on some combination of adjacent samples. The combination is a weighted-sum, where each sample is multiplied by some value ( which changes depending on the time of the current sample ), called the weight, and adds each of the results together. The key part is how these weights are chosen. The aim is to choose the weights such that we undo the effect of the detector on the waveform and obtain a truer estimate for the energy of each particle as a result.
Variation of the quality of fit vs. pile-up separation

Reconstruction Studies

From these two techniques, we began to look at the g-2 fit method first by creating some fake data. This was done by using a pulse generator to create many events with shapes similar to the real thing but with a constant height. We then averaged all of these pulses to produce a template pulse. Two of these template pulses were then stacked on top of each other, although each one was scaled to a different height and separated by a small amount of time. We then added noise ( more-or-less random fluctuations ) on top of this and finally fitted the template pulse against the resulting waveform. An example of one such pulse is shown in the plot above.
What's interesting from a pile-up point of view, was how well this process could distinguish a pile-up event from a clean one. The plot in the image below shows how the agreement varies for different separations between the first and second pulse in a pile-up event. Each line in the plot is a different possible electronics configuration.

Pretty Cold Weather

That's the physics at least, but as for the experience of being in Siberia, it was incredible. Never have I been in such a cold place. The tone was set on landing by the announcement, "ladies and gentlemen, welcome to Novosibirsk, where the weather today is 22 degrees ... [dramatic pause] ... below zero." And by the end of my first week the temperature had reached -35C, which I probably wouldn't have noticed if it wasn't for the 20 minute walk to the institute ( you'll never feel as rugged as arriving at work with frost in your beard ). On top of that, seeing my supervisor try to cross country ski in a business suit with a camera around his neck ( Japanese stereotype anyone? ) and freezing my toes off whilst watching the sunset over the Ob sea ( that's not a stock photograph below ) are experiences I will never forget.
Just a short 20 minute walk from where I was staying (which was to the back of me and not the igloo you can see).

ECAL Beam Test

Blending in once more...
But abruptly it came to an end and a short 24 hours travelling and I found
myself back in Japan. COMET's Electromagnetic CALorimeter (ECAL) sub-group was running a beam test and had allowed me to join in to help with the set-up and running of the experiment. This was a very different experience to Alcap, and not just because it was in Japan. With only 2 detectors to operate things were simplified a fair bit. Of the two detectors, one was to define when and where a particle came from ( the Beam Definition Counter, BDC), and another which was the ECAL itself. That said, the ECAL is divided into 49 individual crystals ( arranged in 7 rows and 7 columns ) and with 64 fibres making up the BDC there were considerably more individual data channels than Alcap.
The primary purpose for this beam test was to select a material for the ECAL crystal. There are currently two candidates for COMET: GSO ( Gadolinium Silicate ) and LYSO ( Lutetium Yttrium Silicate ). LYSO has a much better light yield and a faster response time which is to say, if a particle enters the crystal, you get more photons produced in a shorter time. As it's these photons we convert to electrical signals, if they're more numerous and appear more quickly the final electrical signal is easier to distinguish from just a random fluctuation. The downside is that LYSO is considerably more expensive.
Wrapping the LYSO crystals in Teflon then aluminised Mylar
So we ran for one week with a week or so beforehand for preparation. I was lucky in that I got to help with the wrapping of each crystal ( at KEK, not Tohoku U. ) and then help with their mounting into the actual setup. During the run we scanned through 5 or 6 different momentum points ( from 65 to 145 MeV/c ) to check each crystal's performance along the whole momentum range that we might need to measure. We also moved and rotated the setup to be able to check the performance as a function of the incoming particle's position and direction. You can see some of the setup in the pictures below.

The setup of the ECAL beam test.  The electron beam entered from the left, passed the BDC standing upright an then reached the ECAL crystals in the very centre.
Connecting the crystals to the electronics readout

 Alcap Collaboration Meeting

The 23rd of March arrived, the date I expected to head back home. Except instead I found myself in Chicago at the Fermi National Laboratory (Fermilab), with Alcap, for a collaboration meeting which had been scheduled after I left in January.
Somehow it snowed every place I visited...
The main aims of the meeting were to summarize the work we had done in Switzerland before Christmas, come up with an analysis strategy for processing the data and work out our next steps.
It was a very useful week, starting with a summary of the work that we did and the data we had taken. We'd run with 4 different configurations as well as taking several calibration data sets for the whole detector. From this, some preliminary analysis was discussed; things were looking good. We see a very clean proton signal as well as deuteron and triton spectra. What's more, the timing for these processes looks exactly right to be coming from an Aluminium target and not the shielding or other parts of the setup, so we can be confident that we are seeing the right processes.
Next steps are to finish off the analysis which requires writing the code to perform it in a rigorous and systematic manner. We also need to run simulations to check how much uncertainties in the setup will impact our results. For instance, we know the alignment of things to within a millimetre or so. It's therefore important to quantify how much our results change if we shift the positions of the detectors and target around by that much. And with all of these steps completed and the analysis done we're hoping to publish our results properly, so watch this space!!

Homeward Bound

And then I came home. Sort of. I did have to fly the wrong way round to get there, because I'd had to keep my original flight to Japan. Given that Japan and the USA are roughly equidistant to the international date-line I'd hoped the jet-lag from each place would cancel out. Unfortunately if they did, they only put me somewhere in the middle of the pacific, or 12 hours out of sync with time in London. Fortunately, I was now well trained from the beam tests in getting little sleep...
It really was an incredible experience, and not one I ever expected to have when I started this PhD. I got to see a huge range of physics, in so many different places, surrounded by so many different languages and working in so many different cultures.
But perhaps most importantly, I got to work with a lot of different people. Without them I would never have been able to do such a trip. So thank you to Yoshi, Imperial and the STFC for funding much of this. Thank you to the Alcap collaboration for letting me join in, despite only knowing a few of you. Thank you to Dima Grigoriev and the rest of the BINP students and professors who looked after me in Novosibirsk ( Dima even lent me his own son's thick coat when he saw I'd turned up with just a flimsy leather jacket, so thank you to Dima's son as well! ). Thank you to Junji and the ECAL sub-group for letting me get involved with their work and taking part in the beam test, again without having worked with them before. And a huge thank you to Yoshi Kuno at Osaka University who funded my travel to Chicago and the rest of the month in Japan. He even dropped me at the airport in person! 
I wonder whether I picked up more radiation whilst flying or in the test beam facilities...

19 July 2013

New Discovery from the T2K Experiment (7.5 sigma for Electron Neutrino Appearance)!

It has been pointed out to me that this has become more of a historical record than a blog....

If I were to blame it partly on a former student who has been promising to write an extensive article about his finishing his PhD on T2K and then going on to work on the LUX dark matter experiment in the States, that would be very poor form indeed, so I would not do such a thing!

Anyway, today is a big day for us on the T2K experiment (the photo above is the T2K Imperial group from a couple of years ago), and in particle physics as a whole. The early T2K observation from a couple of years back that muon neutrinos actually oscillate into electron neutrinos has become a fully-significant discovery (and is being presented to the European Physical Society meeting in Stockholm today by our friend and colleague Mike Wilking):

While the most obvious change is that the number of electron-neutrino candidate events has gone from 6 to 28, which is because of the longer period of time T2K has been running, and the higher intensity of the beam (plus of course the size of the underlying physics which causes the process), it is just as important that a lot of work has gone into understanding of the beam and the detectors and the ways neutrinos interact, to increase our certainty on the inputs that go into the physics statement that we can make.

That is to say that the likelihood that random fluctuations (which is to say just bad luck) could make us see what we see is less than one in a trillion. We are certain at that level that what we are seeing is the appearance of electron neutrinos in a muon neutrino beam, once they have travelled the 295 km from J-PARC to the Super-K detector! In the plot above, the green shows the electron neutrino candidates we'd expect to see if this new physics didn't happen. So much work goes into working out the size of the green histogram, so that we know that the actual electron neutrino candidates we see (in the black dots with uncertainties shown in the vertical bars) must mostly be caused by new physics, shown in pink.

We will still be able to make this measurement better, providing a high-precision value of the parameters that describe this in our current model of neutrino oscillations. This, combined with measurements made by other experiments of related but different processes, and other measurements by T2K itself, will continue to help us learn if our models do indeed describe the universe well, and what their implications are. That could be the topic of a future blog post—are there any finishing PhD students' who'd like to contribute an extensive article on this?

PS. This is the work of many many people, and here I quote the bit in our press release today that describes this:

The T2K experiment was constructed and is operated by an international collaboration. The current T2K collaboration consists of over 400 physicists from 59 institutions in 11 countries [Canada, France, Germany, Italy, Japan, Poland, Russia, Switzerland, Spain, UK and US]. The experiment is primarily supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). Additional support is provided by the following funding agencies from participating countries: NSERC, NRC and CFI, Canada; CEA and CNRS/IN2P3, France; DFG, Germany; INFN, Italy; Ministry of Science and Higher Education, Poland; RAS, RFBR and the Ministry of Education and Science of the Russian Federation; MICINN and CPAN, Spain; SNSF and SER, Switzerland; STFC, U.K.; DOE, U.S.A.

10 July 2012

NEUTRINO2012 in Kyoto

For the last week or so the Higgs has been hitting the headlines, but it's also been an amazing year in the world of neutrinos, and last month, a group of us from Imperial attended the Neutrino 2012 conference in Kyoto, which is where the whole community comes together to report and discuss our work, and think about the future.

This was the 25th in the Neutrino series of conferences, which are held every other year and are the biggest and most prestigious in the field of neutrinos. In 2014 it will be held jointly by Boston University, Harvard, MIT and Tufts, and in 2016 it will be those of us here at Imperial College London who will be hosting*. We have already started making plans for 2016, so this year at Kyoto, my colleagues and I were thinking not just about the physics, but also the logistics of the conference, the good things we encountered, and any issues that we might be able to improve when it is our turn:
Anyway, this year, over 600 physicists participated, making it the best attended Neutrino conference ever, and the physics results from the past two years that were reported are really reshaping our view of the Universe and also how we should perform experiments in the future to learn even more. Here are some pictures of Kyoto as found in slides shown by some of the speakers during the conference:
And finally, one of the most beautiful slides of all:
Well, one last photo that is not quite as pretty:
Overall, it was a fabulous conference, with all sorts of ideas sprouting forward from the community on the sorts of things we can do next to take the next steps forward—many of these will result in new experiments, and many will result in new interpretations for previous and current experiments, including these ones we are working on here at Imperial.
The big question with the Higgs and the LHC is “are we seeing something beyond the Standard Model”, but in neutrinos we've been looking well beyond the Standard Model, and now that we actually know all three mixing angles, it may not be long before we uncover a few more fundamental mysteries of the Universe....
*which is to say the Neutrino conferences from now till then are following me round the world!