Sunday, February 9, 2014

Diminishing Returns Part I

Diminishing returns (Part I)

When I started my PhD I worked on the BaBar experiment at SLAC.  By LHC standards this was a very low energy experiment, optimised to produce huge numbers of B mesons.  This allowed physicists to study CP violation and the matter-antimatter asymmetry in the universe, which is one of the deepest questions we face today.  (For those interested, it turns out that a precise measurement was made of the only known source of CP violation that we know of, and it was found to be too small by a factor of about a million to explain the current asymmetry.)  Alongside the B mesons and the "golden" channel of CP violation ,the dataset gave us access to a huge range of other phenomena, including production of charm mesons and tau leptons.  We knew the centre of mass energy as well, so we could look for missing particles recoiling against the rest.  To make things even more interesting, many measurements only made sense in the context of others, so we could combine many measurements searching for any discrepancy.  With all this in mind it would be possible to spend one's whole life working with the BaBar dataset and keeping finding new things and making new measurements.  When you spoke to other BaBar physicists about your work you'd find connections between what you were doing, what it implied about higher energy phenomena.  This lead to all kinds of stimulating discussions, some of them about questions that went back decades.  As a PhD student, SLAC was one of most fascinating places I'd ever been and each week I'd learn something new.  The collection of low energy mass phenomena were just complicated and rich enough to keep me interested and present a challenge, while also being small enough and unified enough for me to develop a good understanding of them.  I would have happily stayed on BaBar, but funding was scarce, all the latent was moving to the LHC, and that's where I should have moved to as well.

When I finished my PhD I moved on to my first postdoc position on the ATLAS experiment at CERN.  Everything about the experiment was new, the hardware, the energy, the luminosity and the discoveries.  There were high hopes for the discovery of the Higgs boson, and with good reason, since everything we thought we know about the fundamental forces would collapse without it.  Some people had been waiting about 20 years to see this dataset, and the media machine at CERN had gone into overdrive to share excitement with the world.  Since I enjoyed talking about the physics I wanted to be a part of the excitement, and started making blogs posts and videos, following the progress of the Higgs search.  Eventually it was discovered, after much hard work, and it was a great time to be at CERN.  Unfortunately this time I felt the discovery was a little disappointing, because after discovering the boson and measuring its properties there's not much else to be done.  The Higgs bosons doesn't come with a whole new sector, or bound states to explore, none of the many spectra seems to show any serious interference effects.  It appears to be just a single boson that fits our expectations, and now that it's been discovered we're left wondering what else there is to find out there.

In the meantime all other searches for physics beyond the Standard Model have ended in failure.  It's quite straightforward to say something about the phenomenology of Run II and venturing into the unknown, and channels with high discovery potential, and answering unanswered questions, but honestly, I'm finding it hard to muster the enthusiasm for that.  Let's suppose that Run II of the LHC gives us nothing new, what would happen then?  There are still a handful of Standard Model measurements that have yet to be performed, but the problem is that they're very hard to measure and not particularly interesting.  So that would leave us with a situation where we're analysing data for no real reward.  Given how well the Standard Model has done for decades I'm not at all confident that we'll find anything new, which means this is probably a good time to leave the field and move on to something else.

I've also found that, for whatever reason, people at CERN are less likely to talk about the more abstract and exotic ideas of particle physics over lunch.  The few times that this has happened have been either rare and immensely rewarding, or slightly less rare and demoralising.  I get the impression that not only do most students not know how their measurements fit into the "big picture", but they're also missing some vital knowledge because it's not seen as being as important as the Higgs physics that currently surrounds us.  One of the reasons for this is probably the sizes of the collaborations we have to deal with today, but that's a topic for a different post.

So when I said "Physics is no more or less fascinating than it was ten years ago" I was actually telling a white lie.  The concepts themselves are just as fascinating as ever, but (with the exception of the Higgs boson discovery) the latest developments have been getting steadily less and less interesting.  We're probably about to face a long period of "bread and butter" physics where nothing interesting happens until we get a linear collider.  If there's ever to be a best time to leave the field it must be now.

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