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.
COMET
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
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| 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.
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| 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.
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| 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
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| 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.
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| 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.
Solutions
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.
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| 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.
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| 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
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| 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.
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| 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.
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| 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. |
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| 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.
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| 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!
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| I wonder whether I picked up more radiation whilst flying or in the test beam facilities... |
