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....

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

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.

SIX!

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!

But....

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!

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!

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!