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Gavin Hesketh | CERN | Switzerland

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So you built the world’s highest energy particle accelerator

Wednesday, December 23rd, 2009

What’s the first thing you do? Find the Higgs? Well, actually, you measure charged hadron spectra (see the first paper with LHC collision data from ALICE). It’s something all the LHC experiments are doing, so In case you were wondering why (or what), thought I’d try to explain. And for some reason, only after writing all this did I see Zoe’s nice post on the ALICE analysis. D’oh. Well, there are not many LHC results to talk about… yet…

On the multi-purpose experiments like ATLAS and CMS, these measurements are made by what is usually called the “Minimum Bias” group (maybe on ALICE and LHCb too, I’m not sure). The “minimum bias” part relates to how these collisions end up being recorded: by firing a “minimum bias” trigger. Some triggers are highly biased and only record collisions where a muon was produced, for example. A minimum bias trigger only requires that something happened: that was a collision. It gets a bit more complicated, because there are also “diffractive” collisions, but this post is already going to be long enough! Normally the rate of collisions is too high to record them all, so only a (random) small fraction are actually recorded. But for the first LHC runs, the rate was low enough that as many as possible were kept. This is one of the nice thing about minimum bias (and why it was the first paper): you don’t have to run for very long to get a lot of events!

Then the measurements themselves, which are things like dN/deta and dN/dpT. So, the proton is a ball of stuff: mostly quarks and gluons. When you smash two of these together, it is really the quarks and gluons that interact. Most of the time, the interactions are fairly “soft” (low energy), and the results is a spray of particles from the remains of the two protons. A seasonal analogy: take a snowball and throw it at a wall, you’ll get a satisfying thud and bits of snowball fly everywhere. Now, if you have a friend who is also willing to take part in this experiment, try throwing two snowballs so they hit head on in the air. This is pretty much like these “soft” proton interactions: bits of snowball flying everywhere. The aim here is to measure the debris of the protons in these collisions, and quantifying it by looking at, for example, dN/deta. The N here is the number of charged particles produced in the debris; eta is an angle with eta=0 is perpendicular (transverse) to the beam direction. So, it’s a measurement of the average number of particles produced at different angles from the beam. Similarly, dN/dpT is the average number of particles produced with different momenta in the direction transverse to the beam.

Now, why is this interesting? Well, depends who you ask, but here is my bias: occasionally, a quark or gluon will be carrying a large fraction of the proton momentum, and a “hard” (high energy) interaction will take place: a Z might be produced, or (we hope) the Higgs or some new particle. This is where the snowball analogy breaks down: it’s like throwing two snowballs, and a tiger being produced when they collide (see Figure 1). Quantum mechanics is strange…

Figure 1: What happens when you stretch an analogy too far

Figure 1: What happens when you stretch an analogy too far

Anyway, even when there is a hard interaction which produces the Higgs, there will still be some debris from the remains of the protons. And as the LHC reaches high beam intensity, there will probably be a couple (or more) of soft interactions between different protons at the same time. In other words, a lot of this debris flying around. We need to understand it, because when all this stuff hits the detector it might make it harder to find the Higgs signal.

And the problem is that this kind of soft interaction is very hard to calculate. We have some excellent models, but they have been tuned to the measurements made in the past. And different models tend to diverge when extrapolated up to the energies we will see in the LHC soon. So we really don’t know exactly what the debris will look like, and repeating these minimum bias measurements is essential. And how do you know you are measuring this correctly? Well, compare to previous measurements and see if you get the same answer. So, it’s very convenient that the LHC has so far run at 450 GeV per beam (earlier CERN experiments like UA1 and UA5 measured minimum bias at this energy) and 2.3 TeV (very close to the Tevatron energy, where CDF measured minimum bias).


Still alive!

Wednesday, December 9th, 2009

A lot has happened since my last post: beam back in the LHC, then the first collisions, then the LHC sets the energy record, then the first public result on collision data (well done ALICE!). On my side, I joined the CMS experiment.

Perfect timing, right? I have to admit it hasn’t quite been what I expected. Having been on D0 at the Tevatron for several years so knew a lot about the detector, software and physics. Now I’ve joined CMS, spent a week without a desk (office space is a premium here!), spent a while thinking about what to get involved in, now am slowly learning the software. So while there is a lot of excitement here about the first collisions, I’ve also had the frustration of not even being able to do the most basic things, like get access to the data and make some plots (the basic currency of particle physics). As a physicist, the main thing I want to do is get stuck in and work with these exciting first collisions! Instead, I’ve been grappling with python scripts, French classes, the congested Geneva apartment market, and how it’s impossible to find tomatillos (it’s the little things that make all the difference).

Well, the difficulty of changing continents, laboratories and experiments is mostly over now, and I’m getting down to some real work…


meetings, biscuits, and other important things

Wednesday, October 21st, 2009

I’m still at CERN running around trying to talk to as many people as possible before deciding which experiment to join. And one thing about large collaboration like atlas or cms (over 2000 people on each) is the number of meetings – of working groups, sub-groups, sub-sub groups, task-forces… you could spend all your time in meetings and never do any work. And as “the bureaucracy expands to meet the needs of the expanding bureaucracy”, the regular meetings of the Meeting Optimisation Committee have achieved an almost legendary status here.

Yesterday, my source for the news that the LHC is now colder than space (the BBC news website), reported on the optimal meeting time: apparently, it’s 3pm on a Tuesday (while this is not exactly cutting edge research, at least it is a step up from the formula for the optimal length of time to dunk a biscuit in a cup of tea). Anyway, being this side of the Atlantic, most of the meetings are in the afternoon, so our US colleagues can video/phone in. In the US, all meetings have to be at 8, 9, 10 am to avoid being too late in Europe. I know which one I’m happier with!

And soon I have to pick an experiment, so I will start writing about physics…


Where is my bargepole?

Wednesday, October 14th, 2009

Today two of my non-scientist friends independently pointed out this story in the New York Times:

I don’t want to devote many words further raising the profile of these ideas when they have already been deconstructed elsewhere – for example, the NYT article provided this link:

The thing that got me was that this was in the NYT, and written by a physics graduate journalist. I don’t understand the motivation behind the article – I honestly can’t tell if Overbye is taking the ideas seriously, or it’s just a long-winded attempt to make a bad joke about being a Red Sox fan (that punchline at least makes the whole thing sound more like astrology than science). Anyway, a bad week for science journalism – though from the way this one has spread around the web, a good one for tabloid journalism. Is it still true that any publicity is good publicity?


Tour de CERN

Tuesday, October 13th, 2009

I have the first 3 weeks of my Fellowship here at CERN to choose which experiment to join. This is really a nice situation to be in, but it is going to come down to a difficult decision.

Over the last week I’ve been visiting the various experiments, apart
from ATLAS – many ATLAS people were in Barcelona for a collaboration week (life is hard as a physicist!). It turns out the CERN group on CMS is made up of quite a few people I know from D0, so it is good to see familiar faces as well as learn about the detector. And I was definitely geeking out a bit this week about the CMS detector, and the is really quite impressive.

Also got to visit a couple of the experimental halls: for CMS and LHCb. Turn out I was very lucky with LHCb, the hall is now closed for beam injection studies. And I learned a few new facts to add to my
collection: there is apparently more iron in CMS than in the Eiffel
Tower (this can’t be correct, surely??); and there is more energy stored in the huge CMS magnet than in the entire rest of the LHC; and: the CMS silicon tracking system has more silicon than all previous particle physics experiments combined. But, no photos – the cable for my camera must still be sitting in a box in Chicago waiting to be shipped over.

One more thing – I can confirm also that LHCb is not doing “nuclear physics”, despite the recent press. I’m not really sure what to make of this coverage: obviously we do not yet know the full story behind Dr Hicheur’s arrest, but is the media coverage because the LHC is in the public eye these days, or is it because of some “angles & demons” type conspiracy theory? As Paul says, the liberal use of the word “nuclear” suggests the latter.


CERN and statistics.

Saturday, October 3rd, 2009

I arrived in Switzerland on Thursday, and right now am sitting outside at the CERN cafe, feeling pretty jetlagged. With no internet in my apartment, I realise how essential it is to everyday life now: skype has become the main way I stay in touch with friends and family. Going straight to CERN after a week of packing and a transatlantic flight meant I also ended up with my worst ID photo yet…

Still there are good things: the sun us shining, there are mountains in the distance, and I have a 20 minute walk to the lab instead of a 45 minute drive. I found podcasts were the best way to deal with that drive, and I usually enjoyed “More or Less” from Radio 4. It’s about statistics. But in a good way.

Based on this photo of Chicago, how many CCTV cameras are there in the USA? And how many Higgs bosons?

Based on this photo of Chicago, how many CCTV cameras are there in the USA? And how many Higgs bosons?

On the show, they investigate the sources of various claims that come up in the media, like: Britain has 1 CCTV camera for every 14 people. It turns out this is based on a a survey of two streets in London. The number of CCTV cameras in those two streets was extrapolated to the whole of London, then the whole of the UK. So even if 1:14 is the right number, this study arrived at it completely accidentally – and nobody really knows the right answer. Yet this number crops up every now and then as though it is true.

At the Tevatron, I worked on a Higgs search, where we looked for a handful of possible Higgs particles amongst millions of collisions that look very similar in the detector. A careful statistical analysis of those data is needed to be able to determine if the Higgs is really there or not. This is probably the reason I’ve heard particle physics referred to as glorified statistics (and it wasn’t meant as a complement!), and I’ll admit it’s not the kind of thing I imagined physics was about when studying at university. But I do find it very interesting: working out exactly what we can say about nature, based on the data we have. And yes, we do check our analysis more thoroughly than the CCTV claim.

Now I have to work out how to make time for all those podcasts…


Last shift

Friday, September 25th, 2009

Wednesday evening I took my last D0 shift. I’m sure shifts have been covered here before, but we basically need people here 24 hours a day to make sure the detector is recording data, and fix any problems that come up. At D0, we have 4 people in the control room: 2 detector experts, 1 person making sure the collisions are being recorded properly, and 1 “captain” (the shifts I take) to make sure the good ship D0 stays on course. Or at least doesn’t stray too far off course.

My shift was 4pm to midnight, and frankly it was a little dull. But this is good – it means there were no problems, and we took lots of data. So at least I didn’t go out with a bang… Once a collision has been recorded, it is saved on tape at the Feynman Computing Centre here at Fermilab. Then, it gets moved to a computing farm to be “reconstructed”: turning all the 1s and 0s from the readout into more meaningful quantities, like the signal of a high energy particle. After reconstruction, it is ready to be analysed: I think the record for shortest time between a collision being recorded to appearing in an physics analysis that went public is just under 1 month. Occasionally we come across something unexpected when analysing a data sample, and have to trace it back to what was happening with the detector a few months or even years ago.

So we have to stay alert on shifts – not always easy at 4am on an overnight! If things really go wrong, we get the experts in: this is a picture from a much more exciting shift I had a few months ago.D0 Control Room


First post!

Wednesday, September 23rd, 2009

well, it took me a while to get started – I timed my first post to coincide with the Tevatron restarting of course…

Really, I’ve been trying to organise everything here for my move to CERN – exactly two weeks today. High energy physics is so international that people are coming and going all the time, and I’ve grown used to saying goodbye over the last few years. And because it is a fairly small community you always meet people again, which makes it easier. But it’s different when you are the one moving! I’ve lived in the same Chicago apartment for the last 4 years, which means a lot of memories and a lot of junk to sort through.

So, I got thinking about ways to summarise all that time. And this got me thinking about some of of those facts/stats that come up every now and then in physics to try to summarise a concept, or relate it to real life, like: write a year’s worth of LHC data on dvd’s you’d make a pile higher than the Eiffel Tower; or: the energy in a Tevatron collision is equivalent to two mosquitoes in a head on collision. I especially like ones where the analogy is stranger than the physics (I mean, we all know what a head-on mosquito collision is like, right?) and I’d kind of like to start a collection of these, so if you know a good one, I’d love to hear it – physics related is preferred, but not essential!