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!

25 December 2011

Happy Christmas from the Imperial College High Energy Physics Group!

The other day we had our HEP group party as we do every year, with about 80 group members and guests participating. The dinner buffet is always the centrepiece of the party, with dozens of hand-made dishes and puddings brought in by group members for everyone to enjoy.

The first year PhD students provided the main entertainment for the party, with their "Pin the Higgs" game, where we all queued up in front of a Higgs Boson mass plot to pin on it our "predictions", while blindfolded—although the clustering in the 120+ GeV range did indicate that this analysis wasn't as blind as it was meant to be! Whoever is closest if the LHC experiments discover the Higgs will win something highly coveted, I hear.... This is Paula going for the win:
...and Jordan claiming the region excluded 10 years ago by the LEP accelerator:
Later on, the party was raised to a yet higher level of sophistication:
with the proceedings coming to an end with a meticulously-rehearsed performance of "Last Christmas" by the PhD students....

18 August 2011

Strangeness at LHCb

New results were published from LHCb last week which will help physicists to simulate proton-proton collisions at the Large Hadron Collider.  Members of the Imperial HEP group have measured two ratios of strange particles which give clues about how hadrons are produced.
The huge energy of LHC collisions allow physicists to look deep inside the protons to see interactions between the constituent quarks and gluons.  Good predictions can be made for these high energy interactions using the theory of Quantum Chromodynamics (QCD).
Sometimes, the interactions of partons can produce "resonances", heavy particles like Z bosons which can decay to produce a shower of quarks and gluons: 

Illustration of an LHC proton-proton collision.
These many-particle events are extremely hard to predict because of a surprising property of QCD: low energy interactions occur with more strength.  If gravity behaved like this, you could make yourself heavier by moving more slowly, or lighter by running very fast -- sort of like a fat couch potato compared with a trim Olympic athlete.

As more and more quarks and gluons are produced their share of the available energy becomes less and less and the interactions get stronger and stronger until the quarks become "trapped" in groups of two (called mesons) or three (called baryons), just like the partons originally inside the colliding protons.  This process is called hadronisation.

Hadronisation involves so many interactions that we cannot use QCD theory to predict what will happen.  Instead we use approximate models:

The predictions of the model are reasonable enough physically that we expect it may be close enough to reality to be useful in designing future experiments and to serve as a reasonable approximation to compare to data. We do not think of the model as a sound physical theory . . . ” – Richard Feynman and Rick Field, 1978

A popular model connects up all the partons with a "string" which snaps to produce mesons and baryons:
Hadronisation of a parton shower.
These models need to be tested against real experimental results.  LHCb's strange particle results are useful because the strange quarks sit in a Goldilocks zone where they are light enough to be produced by the hadronisation process and yet do not provide a net contribution to the structure of the colliding protons.

The first ratio anti-Λ/Ks compares how often strange quarks end up in groups of 3 (the anti-Λ baryon) or in groups of 2 (the Ks meson):
This ratio is much higher in data than predicted by hadronisation models, so the models must be underestimating how often strange quarks group into 3s. And this underestimate gets worse with higher particle momentum (perpendicular to the proton beams).

The second ratio anti-Λ/Λ, compares how many times anti-strange quarks group in 3s compared to strange quarks.  Protons are made of quarks, not anti-quarks (really less anti-quarks), so it should be easier to make Λ than anti-Λ.  This behaviour changes with the angle to the proton beam, or the "rapidity" -- think of large rapidity as a small angle to the proton beam and small rapidity as a large angle.
LHCb is unique amount the LHC experiments with a view of the high rapidity (small angle) region.  The anti-baryon/baryon ratio shows a significant change in behaviour across this region. At small rapidity data matches models which have already been validated at the Tevatron but at high rapidity the best match is PerugiaNOCR, a model with localised hadronisation, which uses shorter strings that don't connect all the partons together.

These results will be of great use to future developments of hadronisation models.  It is very important to have accurate predictions at the LHC in order to test the Standard Model and search for new physics.
If you want to read more, you can get a copy of the paper for free.  You may know that this is not generally the case for scientific publications. CERN has made special arrangements for all LHC results to be made freely available to the general public, in line with the spirit of its founding charter:

The Organization shall provide for collaboration among European States in nuclear research of a pure scientific and fundamental character, and in research essentially related thereto. The Organization shall have no concern with work for military requirements and the results of its experimental and theoretical work shall be published or otherwise made generally available.” – Convention for the establishment of a European organization for nuclear research, Article II, Section 1, Paris, 1 July 1953

15 June 2011

T2K’s First Ever Physics Publication!

A week after our first paper, describing the setup of the T2K Experiment, I am happy to say that we have just submitted our first physics paper, and announced the results in seminars at the host laboratories.

Physics results are what our experiments are all about, and after many years of toil (as mentioned many a time here), it is a wonderful feeling, as always, to present to the world something about our Universe that no one ever seen before.

In the T2K Experiment, we create a beam of muon neutrinos at the J-PARC laboratory at Tokai Village on the eastern coast of Japan, and send them to the Super-Kamiokande neutrino detector 295 km away in the mountains of the north-western part of the country.
The mathematics that seem to describe well the results of other experiments—including Super-K looking at neutrinos in the atmosphere, KamLAND (my previous experiment) and many earlier experiments looking at neutrinos from nuclear reactors, SNO and others looking at neutrinos from the Sun, and MINOS and K2K with neutrinos made in a similar way to T2K but with different optimisations—suggest that with the specific energy and distance that the T2K neutrino beam has, we might be able to see a small fraction of the muon neutrinos turn into electron neutrinos.

This effect would be the third type of neutrino oscillation that has been seen.
The fact that it is the third type may make it sound boring and unimportant, but actually it is quite the opposite—the aforementioned maths tells us that if we see three, we have seen them all and that means that various other phenomena can be explained when you plug the numbers, that are given by the experiments, into the maths.

One of these possible phenomena we may be able to explain is the existence of matter in the Universe today, as opposed to all the matter and anti-matter produced in the Big Bang just annihilating into almost nothing, which is one reason we think it is rather interesting to measure these things.

Neutrinos, on the rare occasions when they indicate their existence by colliding with the atoms that make up matter instead of just passing through it, tend to create the particles that they are labelled with in their names—muon neutrinos create muons, and electron neutrinos create electrons—Super-Kamiokande is very good at distinguishing muons from electrons, so we basically point the beam at Super-K and count the number of times we see electrons created by neutrinos.

Of course, it isn’t quite that simple—the beam is pretty messy to start with and hard to understand (just like almost anything that has to do with neutrinos), and lots of other things can mimic electrons created by neutrinos, and it is the job of we experimental physicists to do our best to sort these issues out, and most importantly, understand them enough that we can estimate what their effects are.

Once we do all that, we get the plot that is shown at the top of this blog entry. This is what we have worked so hard for so many years for!

We previously calculated that if this third type of neutrino oscillation doesn’t exist, we would have seen about 1.5 electron neutrinos (on average) in the data we took over the year or so since the T2K beam started. That is the shown in the plot above by the yellow, green and blue hatched areas.

One and a half.

But in fact, when we looked at the actual data collected, we saw 6, as shown by the black points in the plot above.


This is consistent with this new type of neutrino oscillation occurring!

If we put in this new neutrino oscillation at quite a large level, it looks like the red region in this plot:

which shows that the data does look a lot like neutrino oscillations!


Here I have to make it clear that what we see now amounts to what we refer to in physics as an “indication” or a “tantalising hint” to employ a common cliche.

We set up experiments to learn about the Universe, but it often doesn’t just respond with simple “yes” or “no” answers, but it gradually gives us a picture that becomes clearer with time.

In our case, it could easily be that the true average rate of electron neutrinos appearing is much smaller, but we were just lucky and a few came in in quick succession by pure chance.

To make a discovery of the sort that T2K is aiming for is to contribute something new to the current understanding of how the building blocks of the Universe are, and this will affect how we build future experiments, and how we interpret the information that comes from other experiments, and how theoretical models of the Universe are built—so we don’t take it lightly.

What we do know is that if we can send more neutrinos to Super-K, we’ll be able to tell for certain what is going on.

Most importantly of all, the T2K experiment is clearly working well, and it can be seen how well it has been optimised for our measurement, which is why with just a few percent of the beam that it was designed for, we can see anything like this at all.

Here is a press release: http://www.kek.jp/intra-e/press/2011/J-PARC_T2Kneutrino.html and a copy of the paper we have submitted to the journal Physical Review Letters.

Unlike the paper from last week, the paper hasn’t been accepted yet, as it has to go through a lot of peer-review to make sure that the community will accept the results that we have shown.

The data we used for this result is from between early 2010 and the afternoon of the 11th of March 2011, and right now, the beam isn’t running because of the of the earthquake—which hit Tokai village very hard indeed.

Recovering from the earthquake and preparing for future data is what a large fraction of T2K collaborators are working on now, and this will continue for a while. It is very satisfying in the meantime, however, to be able to produce results like this that show the world what an exciting time it is for the experiment.

I’ll finish this post with a photograph of the T2K Collaboration that was taken a month ago during a hectic series of meetings when we were working on finalising this result:

Spot the Imperial group!

09 June 2011

T2K's First Ever Publication!

As avid followers of this blog will be aware, a group of us here at Imperial HEP have been working on the T2K Experiment over the past several years. Personally it has been seven years since I got started on T2K, although it certainly feels like much more!

All these years of toil, and we had nothing to show for it (especially in the "metric-based" world we live in today), so I am happy to say that we have just had our first paper accepted.

It isn't a paper on a physics result though, but what we call a "NIM paper", because it is being published in the journal Nuclear Instruments and Methods in Physics. This is where physicists describe the novel setups and techniques they are using to conduct experiments.

Often this sort of paper takes ages to get out because physicists tend to prefer to spend their time running and maintaining their detectors rather than writing about them, but somehow the T2K Collaboration has managed to get its act together and describe the experiment over 33 highly entertaining pages, before we have any physics results out!

Particle physicists invented the World Wide Web to help us exchange information freely, and in that spirit, anyone can access the paper for free on the arXiv site: http://arxiv.org/abs/1106.1238

Three of the pictures in the paper are mine, including the "Exploded ND280 Detector" picture that I made four years ago, shown above. I'd be the first one to admit that it isn't a work of art, but it seems to do the job!

So, we've described the experiment in detail in this paper, but the question is, where are the physics results?

Well, it takes time to collect enough data for an experiment to be able to start discerning new things about Nature, and it also takes time for physicists to interpret that data -- so it may be a while, but I can guarantee that we are working very hard indeed on it!

06 January 2011

Imperial High Energy Physics Group Open Day

Happy New Year!

We will be holding a Group Visit day on the 19th of January, for anyone who is considering joining our group as a PhD student. If you would like to come and talk to us, please email my colleague, Dr David Colling!

04 October 2010

My summer project at Imperial

After spending my last 3 months with this department over the summer, I wanted to share my experience with you via this blog. Either that or I was strongly encouraged by my supervisor Yoshi! I am a 3rd year undergraduate here at Imperial and I worked in HEP over last summer as part of a UROP (http://www3.imperial.ac.uk/urop) scheme.

Although not as extravagant as other blog entries on this site, with students travelling to Japan and Switzerland, my placement in the Blackett lab was just as enjoyable and rewarding. I was working with Yoshi and Ajit on the COMET experiment. In particular I was using computer simulations to optimise both a collimator and the fantastically named lagger-tagger, a name that Yoshi is still trying hard to get adopted by the particle physics community (hopefully not in vain).

I spent most of my time working with Ajit, who was very helpful, giving me lots of his time and expertise. His crash course in particle physics allowed me, who had not studied the subject yet, to understand enough of what was going on to do my project. I started by getting familiar with some of the tools of the high-energy physicist. First on the list was the hilariously steep learning curve required to use ROOT, and its 20 years worth of quirky workarounds, providing endless fun for the data analyst (I also wonder if the Windows version of ROOT is called Administrator). Second was G4beamline, a brilliant piece of software, with documentation so in depth and confusing that presumably only the person who wrote it can use all of its myriad features with any degree of confidence. Joking aside these are impressive programs, testament to group collaboration over years, and they allowed me to complete my project without much hassle at all.

The next part of my work was at the Daresbury Laboratory, working as part of the team building the detectors for T2K. This was useful as I could see another stage of an experiment, its actual construction, rather than its design. When I received the email telling me that the detector I had worked on had been shipped out to Japan it gave me the feeling that I had provided something real to an huge multinational experiment.

Another enjoyable part of my work over the summer was the opportunity to be a part of a research group, thankfully the High Energy physics group was welcoming and I got on well with all that I met, meeting for lunches and the occasional night out. The experience has convinced me to do a similar project next summer and to apply for a PhD place after my degree.

All in all I had a great time doing my UROP placement here, it was hard work, but very rewarding. Looking around this blog, I have only one regret, that I didn’t take more photos of myself smiling, standing in front of physics equipment.

Thanks to Yoshi and Ajit for their time.


19 April 2010

CERN and first collisions at LHCb

The last few months have been very exciting here at CERN. Ravi and me are currently on a long term attachment (LTA) at CERN in Geneva, working on the LHCb experiment. Both of us have been out here for nearly a year now and a lot has been happening during this period. We experienced the entire process of the LHC being repaired, new start up dates getting announced, etc... and of course working with monte carlo simulated data only so far!
But since last year things have changed as I am sure most of the readers of this blog will know. In November we saw the first beam circulating in the LHC after its repair. The LHCb detector was in good shape: Before Christmas LHCb was recording its very first data (it could not detect cosmics before due to its horizontal alignment)! Meanwhile the LHC people were testing their machine extensively in order to be ready for the official date of first 3.5 on 3.5 TeV collisions a few weeks ago (30.3.2010)... And it was a huge success! Since then LHCb has been taking several million events, stored and ready to be analysed (for example by PhD students like Ravi and me). So far we have done a pretty good job :) I want to point out Ravi being the first one to see hints of a D0 peak in the collaboration (and me hunting J/Psi peaks)!
The following picture was taken on 30th March, at 12.59, in the LHCb control room, showing Andrei Golutvin (in the red jumper, LHCb spokesperson and my supervisor) watching the LHCb event display showing first collisions (notice the two green muon tracks, coming from a J/Psi?):

It is a very exciting time to be here at the moment. Being part of this unique science community is truly special. Everyone who has been at CERN knows what I mean - at this place history in particle physics has been written. This becomes obvious every day - for example when walking past the Gargamelle bubble chamber, Tim Berners-Lee's office (where the "web" was born) or Jack Steinberg in person (actually, he is in the office opposite to mine and sometimes asks for help with his computer) - just to mention a few occasions. CERN also organises events and lectures: To celebrate the LHC an incredible number of Nobel Prize winners of the field came together last year and gave highly interesting lectures stretched over two days. Gerard 't Hooft was even so kind to be in a picture with a few IC students (and ones that used to be):

Of course CERN's location between the Swiss/French alps and the Jura offers many opportunities to throw yourself down a mountain on some sort of ski or snowboard. So during the long wait for the turn on date a common activity of the UK PhD students here at CERN was to organise numerous trips to Chamonix and other resorts. It should be mentioned that Ravi won the prestigious CERN ski club downhill race in a new record time (~2.13 minutes)- well done! However, the season is over now, and the summer is about to start here in Geneva. The LHC is running, LHCb is recording data... Everything seems to be working very well, and - touch wood - hopefully it will stay like that! An interesting time lies ahead, and Ravi and me hope to witness signs of new physics here at LHCb!