LHCb, UCD physics and the last symmetry

This weekend I was back at my Alma Mater, the UCD school of physics, for a conference for physics teachers organised by the Institute of Physics.

I arrived late and sneaked into the back row of Theatre E of the UCD science block, just as I used to all those years ago. Such a strange feeling to be back in that same seat in that same lecture hall (one difference is that the grounds of the college are beautiful now). The feeling increased when I was joined by two of my former professors, the best teachers I ever had; Alex Montwill (who taught courses in formal quantum theory and high energy physics) and Ann Breslin (special relativity and experimental high-energy physics). Alex was Ireland’s foremost experimental particle physicist for many years, but is probably best remembered for a well-known series of public lectures on modern physics on national radio.

There were some very nice talks, including one on the discovery of The Antikythera Mechanism by Mike Edmunds of Cardiff University and one on the work of the UCD high-energy astrophysics group by John Quinn. However, I was mainly there to hear Ronan McNulty, the leader of the new experimental particle physics group at UCD (UCD has always been strong in fundamental areas of physics such as astrophysics and particle physics). Ronan’s group has been involved with the DØ experiment at the Tevatron, the L3 experiment at LEP and now with the LHCb detector at the LHC (they got some nice attention recently when they were one of the few groups able to report some preliminary measurements from the September switch-on). The work of the group is very important not just because of its fundamental nature, but because it is the only group in the Republic of Ireland that has an official involvement with the LHC (thanks to our non-membership of CERN, see post below). I’m sure Alex and Ann are very proud to see this large and very successful experimental particle physics group at UCD as they themselves had a successful particle physics group at UCD many years ago, measuring particle tracks in emulsions sent over from CERN (I did my final-year project with their research group, estimating the mass of the muon from pion decay tracks).

Ronan gave a superb talk, ranging from a basic introducton to particle physics up to the search for asymmetry in matter/antimatter decay and their contribution to the VELO detector of the LHCb experiment – all within the paltry 20 minutes he was allotted on the program. You can find the slides from the talk here. At question time, I asked him his view of the likelihood of seeing supersymmetry at the LHC: like many experimentalists, he seemed pretty sceptical, pointing out that there has been absolutely no hint of supersymmetric particles up until now.

At lunchtime, we all had a great chat, ranging from supersymmetry to ‘progressive’ ideas in university administration, to Ireland’s continued non-membership of CERN. It’s always great to catch up with the people who taught you and to hear their perspective on things as an adult. I particularly enjoy talking to Alex and Ann as they are among the very few people who understand Lochlainn’s work in gauge symmetry and the impact it had at the time. Re CERN, it seems negotiations on the issue are continuing…

In other news, although they are now officially retired, Alex and Ann have just written a book (real academics don’t do retirement): Let there be lightis due to be published by Imperial next month. I got a sneak preview and it looks superb, as you might expect of the culmination of a lifetime’s reflection on physics by two highly respected physicists. The book is pitched at a level somewhere between undergraduate and the layman and is an introduction to pretty much all of modern physics from the perspective of the study of the nature of light – from optics to wave theory, from wave/particle duality to light quanta, from electromagnetism and light to special relativity, etc. The book will be officially launched at UCD next month, so I’ll discuss it in detail then.



Supersymmetry has been in my mind all day today, sparked by a comment Ronan made yesterday. He mentioned that as he understands it, one of the reasons mathematicians are keen on SUSY is that it’s the last remaining symmetry under the Poincare symmetry group. I think that’s right and in fact I once heard Julius Wess comment that he sometimes wished he had used the term ‘ultimate gauge symmetry’ in the original paper (he made the comment at a Memorial Syposium two years ago). Sadly, Julius, one of the last of the supersymmetry pioneers, passed away himself last year.

Julis Wess of the Wess-Zumino model of supersymmetry.

All day I’ve been thinking thatThe Last Symmetry would be a great title for a popular book on particle physics, if supersymmetric particles do turn up – possibly a better title than The Story of Atoms . Either way, I didn’t do too much work on my imaginary book over the summer, must get back to it. On the subject of language, I also wonder about the term super-matter…I’ve never heard the term but it’s a nice word and immediately hints at an analogy with antimatter (if SUSY does exist, it must involve a broken symmetry, just like matter/antimatter decay).

As to whether SUSY really exists, a philosophical point has also been on my mind – as far as I know, there is no path to a unified field theory of the interactions without some sort of symmetry betweenfermions (leptons and quarks, the constituents of matter) and bosons (the force carriers). In the 1960s, the unification program ran into a formidable mathematical wall with the emergence of a series of no-go theorems (McGlynn, O’Raifeartaigh, Coleman and Mandula) that showed that the strong interaction could not be incorporated into a single scheme with the other interactions using the methods that had been so successful in electro-weak theory. Mathematicians were hugely relieved when SUSY, a radical new symmetry between the two most fundamental classes of particles, suggested a possible way around the problem and even hinted at the inclusion of gravity. Without some form of SUSY, it’s not clear whether unification can happen, and without unification, the picture of a single ‘superforce’ in the early universe condensing out into the fundamental interactions we see today can’t happen – a major blow for cosmology as well as for particle physics. So I like to think that either we will see SUSY sometime, or if we don’t, we haven’t got our predictions right because we simply haven’t developed the right model of supersymmetry breaking yet…

Also, there is one famous mathematical clue. When theoreticians plot the magnitude of the coupling constants of the three strongest fundamental interactions as a function of energy, they converge – but not to a single point. That is, unless SUSY is included in the calculation – in which case they converge very nicely. Hmm…

PPS: . Last week was the hundreth anniversary of Minkowski space-time. I completely forgot about this, but Stefan and Bee have a great post on it on Backreaction


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2 responses to “LHCb, UCD physics and the last symmetry

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