Quantum Diaries

Let me say this much about the Community Summer Study 2013, also known as “Snowmass on the Mississippi“: it feels like a family reunion. There are about 600 people registered for the meeting, and since in the end we are a small field, I know a lot of them. I’m surrounded by people I grew up with, people I’ve worked with before, people I work with now, and people with whom I’d really like to work someday. I find it a little overwhelming. Besides trying to learn some science, we’re all trying to catch up with each other’s lives and work.

As with any family, we have our differences on some issues. We know that there are diverse views on what the most important issues are and what are the most promising pathways to scientific discovery. But also, as with any family, there is a lot more that unites us than divides us. As Nigel Lockyer, the incoming director of Fermilab, put it, we will probably have little trouble finding consensus on what the important science questions are. Today’s speakers emphasized that we will need to approach these questions with multiple approaches, and there was mutual respect for the work being done in all of the study groups.

The challenge, of course, is how to accommodate all of these approaches within an envelope of finite resources, and how to strike a balance between near-term operations and long-term (if not very long-term) projects. As our speakers from the funding agencies pointed out, we are in a particularly challenging time for this due to national political and fiscal circumstances. Setting priorities will be a difficult job, and one that will only come after the Snowmass study has laid out all the possibilities.

The workshop continues for another eight days, and if you are interested in particle physics and the future of the field, I hope you’ll be keeping an eye on it. The agenda page linked above has a pointer to a live video stream, presentations are also being recorded for future viewing, and various people are tweeting their way along with hashtag #Snowmass or #Snowmass2013. There are a lot of exciting ideas being discussed this week, some of which can have a transformative effect on the field. Stay with us!

It’s the 4th of July 2013 – Happy Higgsdependence Day! It is exactly one year since the observation of a ‘Higgs-like particle’ was announced at CERN in the auditorium in which I now sit.

I remember stealthily watching the live stream of the announcement at my office desk last year. If you had suggested to me then that one year later I would be working at CERN I would have told you (in a broad Northern Irish accent) to ‘catch yourself on’.

Peter meet François – Professors Higgs and Englert, who collaborated during the 1960s, meet for the first time on 4 July 2012. Notice the back of the room is filled with last years summer students who camped overnight for seats for the Higgs announcement.

This of course means that the summer lecture series has started in earnest with a gentle introduction to the ‘Particle World’ from Dr Tara Shears. I understand the content of the lecture so am feeling confident, comfortable and optimistic.

Until lecture series Day 2 when Dr James Wells hits us with his lectures on the Standard Model – our best model for explaining the interaction of fundamental particles.

James is an eminent theorist and is clearly at the top of his game. He poetically weaves an intricate mathematical web to explain the subtleties of the Standard Model using an area of mathematics known as Group Theory. I nod along knowingly with the other students but am feeling a bit bamboozled by all the tensors, boosts and Lagrangians.

Next up is the über cool Prof. Cranmer and his jazzy neon shoe laces to teach us all about statistics. (See my last post “Die – electronics, Die” on the importance of statistics at CERN.)

“And that’s the difference between a quark and a fork” – me telling Dr Wells all about the Standard Model.

My favourite lectures however are on physics beyond the Standard Model. The no-nonsense Italian Dr Gian Giudice gives the clearest descriptions of the Higgs, Supersymmetry and dark energy that I have come across, while quirky Professor Gia Dvali reassuringly concludes that black holes are different from elephants. Phew.

After each morning’s three lectures there is a half hour discussion session where the students interrogate the lecturers. While some students nip off for an early lunch I find these sessions often provide the best insight into the day’s topics.

So, set yourself a mad old goal for this time next year – it just might happen…

(You can view the lecture programme and speaker’s slides for this and previous year’s CERN Summer Student Lectures at http://summer-timetable.web.cern.ch/summer-timetable/.)

My first project as a CERN summer student was to assemble an electronic die. After a few hours of soldering and burnt fingers I produce this:

My creation

How does it work?

When you tap the die on a table a piezo sensor glued to the base ‘feels’ the impact and sends a small current through the printed circuit board and into a chip.

The chip converts the analog signal into a decimal value (for example a 1.24167 volt signal is converted to the number 1.24167) and reads the least significant decimal place to generate a random number. The LEDs then light up to show a number 1 to 6.

What’s the point?

Over the course of a week, and much to the annoyance of my office mates, I tap the die on my desk 1500 times, compile a data set of the numbers thrown and go about analysing whether the die is fair or biased.

What’s this got to do with CERN then? 

Well statistical analysis is key to what CERN does and the discovery of the Higgs is a pertinent example.

In the Large Hadron Collider a Higgs boson is produced by approximately 1 in every 10 million particle collisions. The boson then decays in a fraction of a microsecond while the other collisions produce an array of other particles.

To make the hunt even more tricky, scientists didn’t know the exact energy level at which the Higgs would be found so they had to collide particles at a variety of energy levels.

A graph showing the energy at which the Higgs was found.

So searching for the Higgs was like looking for a tiny needle in a massive haystack full of other needles where your needle exists for a minute fraction of a second and you don’t really know what it looks like.

Scientists therefore had to statistically analyse trillions of collisions to be sure that the small bump at 125GeV in the graph above was the signature of the Higgs and not just an unlikely random fluctuation.

Before announcing the discovery of a ‘Higgs-like particle’ in July 2012 scientists were 99.9999% sure they’d found their boson i.e. there was only a one in a million chance this was not the Higgs.

What’s that got to do with the die?

In figuring out whether the die was fair I produced a relatively large data sample then used statistical techniques to conclude with 95% confidence that any bias displayed wasn’t just a random fluke. So in a way the exercise was analogous to the search for the Higgs but on a much smaller scale.

Discover anything interesting?

After some intense number crunching, detailed analysis and complex modelling I concluded (drum roll please) – bet on the number 2.

It turned out my supervisor sabotaged my die. So the key lesson learned was – don’t dice with physicists.

At this week’s EPS conference we have seen the release of new results from both CMS and LHCb on a search for a very rare decay of the B_s meson, to a muon-antimuon pair. I’ve written about this before (yikes, two years ago!); this decay turns out to be amazingly sensitive to possible new physics processes. This is in part because the decay (which violates flavor conservation, to leading order) is highly suppressed in the standard model, and thus the presence of additional particles could have a big impact on the decay rate. Just what impact depends on the particle; depending on the model, the rate for this decay could be either increased or decreased. It’s a somewhat unusual situation — you have something interesting to say either if you see this decay when you don’t expect to (because you don’t yet expect to have sensitivity due to insufficient data), or you if you don’t see the decay when you do expect to.

But in this particular case, the standard model wins again. CMS and LHCb now have essentially identical results, both claiming observation of this process at the rate predicted by the standard model. (LHCb had shown the first clear evidence of this decay last November.) I’m not going to claim any great expertise on this topic, but this result should put stronger constraints on theories such as supersymmetry, as it will restrict the possible characteristics of possible SUSY particles. In addition, this observation is the culmination of years of searching for this decay. I reproduce the CMS plot of the history of the searches below; over the course of about 25 years, our ability to detect this decay has improved by a factor of about 10,000.

But here’s what’s really on my mind: I’m thinking about this measurement in the context of the Snowmass workshop, which begins one week from today in Minneapolis. The studies of the workshop have been divided up into categories of “frontiers”, where the physics can fall into Energy, Intensity or Cosmic Frontiers. This categorization arises from the 2008 report of a US HEP program planning committee. It is certainly a useful intellectual organization of the work that we do in particle physics that is easy to explain to people outside the field. The Department of Energy budget for particle physics is now also organized according to these frontiers.

But where exactly does this B_s measurement fit? The physics of quark flavors and the search for rare decays would be considered part of the Intensity Frontier. But the measurements are being done at the LHC, which is considered an Energy Frontier facility because it has the largest collision energy of any accelerator ever built, and the process is sensitive to the effects of putative particles of very high mass. This is just one example of physics measurements that cut across frontiers. Another that comes to mind is that the LHC experiments have sufficient sensitivity to the production of potential dark-matter particles that in some cases, they can be competitive with searches done in non-accelerator experiments that are classified as being in the Cosmic Frontier.

Heading into next week’s workshop, I am hoping that we will be cognizant of the interconnected work of all the research that we do, regardless of how they might get classified for accounting purposes. We have many ways to explore each of our physics questions, and we need to figure out how to pursue as many of them as possible within the resources that are available.