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Emily Thompson | USLHC | Switzerland

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Calm before the storm: Preparing for LHC Run2

Wednesday, September 17th, 2014

It’s been a relatively quiet summer here at CERN, but now as the leaves begin changing color and the next data-taking period draws nearer, physicists on the LHC experiments are wrapping up their first-run analyses and turning their attention towards the next data-taking period. “Run2″, expected to start in the spring of 2015, will be the biggest achievement yet for particle physics, with the LHC reaching a higher collision energy than has ever been produced in a laboratory before.

As someone who was here before the start of Run1, the vibe around CERN feels subtly different. In 2008, while the ambitious first-year physics program of ATLAS and CMS was quite broad in scope, the Higgs prospects were certainly the focus. Debates (and even some bets) about when we would find the Higgs boson – or even if we would find it – cropped up all over CERN, and the buzz of excitement could be felt from meeting rooms to cafeteria lunch tables.

Countless hours were also spent in speculation about what it would mean for the field if we *didn’t* find the elusive particle that had evaded discovery for so long, but it was Higgs-centric discussion nonetheless. If the Higgs boson did exist, the LHC was designed to find this missing piece of the Standard Model, so we knew we were eventually going to get our answer one way or another.

Slowly but surely, the Higgs boson emerged in Run1 data

Slowly but surely, the Higgs boson emerged in Run1 data. (via CERN)

Now, more than two years after the Higgs discovery and armed with a more complete picture of the Standard Model, attention is turning to the new physics that may lie beyond it. The LHC is a discovery machine, and was built with the hope of finding much more than predicted Standard Model processes. Big questions are being asked with more tenacity in the wake of the Higgs discovery: Will we find supersymmetry? will we understand the nature of dark matter? is the lack of “naturalness” in the Standard Model a fundamental problem or just the way things are?

The feeling of preparedness is different this time around as well. In 2008, besides the data collected in preliminary cosmic muon runs used to commission the detector, we could only rely on simulation to prepare the early analyses, inducing a bit of skepticism in how much we could trust our pre-run physics and performance expectations. Compounded with the LHC quenching incident after the first week of beam on September 19, 2008 that destroyed over 30 superconducting magnets and delayed collisions until the end of 2009, no one knew what to expect.

Expect the unexpected.

Expect the unexpected…unless it’s a cat.

Fast forward to 2014, we have an increased sense of confidence stemming from our Run1 experience, having put our experiments to the test all the way from data acquisition to event reconstruction to physics analysis to publication…done at a speed which surpassed even our own expectations. We know to what extent we can rely the simulation, and know how to measure the performance of our detectors.

We also have a better idea of what our current analysis limitations are, and have been spending this LHC shutdown period working to improve them. Working meeting agendas, usually with the words “Run2 Kick-off” or “Task Force” in the title, have been filled with discussions of how we will handle data in 2015, with what precision can we measure objects in the detector, and what our early analysis priorities should be.

The Run1 dataset was also used as a dress rehearsal for future runs, where for example, many searches employed novel techniques to reconstruct highly boosted final states often predicted in new physics scenarios. The aptly-named BOOST conference recently held at UCL this past August highlighted some of the most state-of-the-art tools currently being developed by both theorists and experimentalists in order to extend the discovery reach for new fundamental particles further into the multi-TeV region.

Even prior to Run1, we knew that such new techniques would have to be validated in data in order to convince ourselves they would work, especially in the presence of extreme pileup (ie: multiple, less-interesting interactions in the proton bunches we send around the LHC ring…a side effect from increased luminosity). While the pileup conditions in 7 and 8 TeV data were only a taste of what we’ll see in Run2 and beyond, Run1 gave us the opportunity to try out these new techniques in data.

One of the first ever boosted top candidate events recorded in the ATLAS detector, where all three top decay products can be found inside a single hadronic jet.

One of the first ever boosted hadronic top candidate events recorded in the ATLAS detector, where all three decay products (denoted by red circles) can be found inside a single large jet, denoted by a green circle. (via ATLAS)

Conversations around CERN these days sound similar to those we heard before the start of Run1…what if we discover something new, or what if we don’t, and what will that mean for the field of particle physics? Except this time, the prospect of not finding anything is less exciting. The Standard Model Higgs boson was expected to be in a certain energy range accessible at the LHC, and if it was excluded it would have been a major revelation.

There are plenty of well-motivated theoretical models (such as supersymmetry) that predict new interactions to emerge around the TeV scale, but in principle there may not be anything new to discover at all until the GUT scale. This dearth of any known physics processes spanning a range of orders of magnitude in energy is often referred to as the “electroweak desert.”

Physicists taking first steps out into the electroweak desert will still need their caffeine.

Physicists taking first steps out into the electroweak desert will still need their caffeine. (via Dan Piraro)

Particle physics is entering a new era. Was the discovery of the Higgs just the beginning, and there is something unexpected to find in the new data? or will we be left disappointed? Either way, the LHC and its experiments struggled through the growing pains of Run1 to produce one of the greatest discoveries of the 21st century, and if new physics is produced in the collisions of Run2, we’ll be ready to find it.

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The Boy Scientist

Friday, December 21st, 2012

I’m back home in Southern California for the holidays, and have been enjoying the sunshine in December, filling up on cheap tacos, sushi, avocados, and doing all the things I miss when living in Switzerfrance (to be fair, the day I get back to CERN I’ll probably be filling up on wine, cheese, go for a walk by the lake and do all the things I miss after a prolonged visit to the US). A few days ago was a special treat—I went for a visit with my college buddy to the California Science Center to see the Endeavour Space Shuttle installation.

A computer scientist and a particle physicist visit the Endeavour Space Shuttle

Last October, winding its way through Los Angeles, the Endeavour was towed slowly past familiar landmarks and ultimately to its last resting place in Exposition Park just south of Downtown. I watched a lot of it from Switzerland, feeling nostalgic, and remembered watching lift-offs from Cape Canaveral on the television when I was still just a small kid. Even as a scientist now, it’s still inexplicably mind-blowing to think we live in a period of history when we can send humans into space.

Of course at the end of the day, after playing around with all the other exhibits, we headed into the gift store to check out all the cool science toys. In the books section, I happened to see this:

Turning to the forward, the last sentence read “So turn the page and begin your experimenting here with the fantastic projects and exciting, new discoveries every boy scientist should know.”

me: “huh. That’s weird. [puzzled]. Maybe they ran out of ‘The Girl Scientist'”
friend: “What would be the difference?”

So I came home and googled “The Boy Scientist” to see what this series was all about. Turns out there isn’t even book for girl scientists. They do have a book entitled “The Girl Mechanic”, with this blurb on Amazon.com:

“Classic girl power is finally here! Females of all ages will celebrate the first just-for-girls entry in the Popular Mechanics classic activity series. Like its predecessors, The Girl Mechanic presents time-tested projects that build skills, enhance creativity, and provide hours of pleasure. We’ve featured choice ideas for crafts, toys, furniture, sports, and games. The standout items include doll houses (one has an actual working elevator!), jewelry boxes, picture frames, playhouses, Christmas cards, and so much more. Some activities a child can do alone, others require a parent’s help, but all of them offer a charming glimpse at the handy world of our past—and give girls essential knowledge that will last a lifetime.”

Way to go, Popular Mechanics, finally publishing classic girl power in 2009! I didn’t have this book growing up, and admittedly, I feel lacking in essential doll-house- and jewelry-box-building knowledge. What do Boy Mechanics learn?

(from Amazon.com) “It’s vintage boyhood and a miscellany of marvelous ideas: from kites and toboggans to workbenches and birdhouses, this collection of projects from Popular Mechanics’ issues of long ago captures all the appeal of American ingenuity at the start of the last century.

With the rawest of materials, a minimum of technology, and a maximum of ingenuity, men and boys in the early 1900s dedicated themselves to crafting wonderful items, both practical and fanciful. It was a highly valued skill that revealed the measure of a man, and Popular Mechanics honored it and led the way in instructing these handy creators. Take a look back at those simpler, good old days—and at what we may have lost in our high-tech era—through these engaging projects, all published in the magazine during the first two decades of the 20th century. The range is simply amazing, and bound to appeal to woodworkers who love classic ideas. They include tools, like T-squares and sawhorses; an animal-proof gate latch and a birdhouse made from an old straw hat; household gadgets and handcrafted furniture; camping gear (including a screen door for a tent); and toys and games. And many of these appealing trellises, decoys, puzzles, and tents are quite doable today. Inveterate do-it-yourselfers will be astonished at the resourcefulness required to build a stove for a canoe and even a houseboat.”

(also here's a fun sociological experiment: try to google-image search "the boy mechanic" and then "the girl mechanic")

Well gee, that sounds like waay more fun…to me anyway. Digging further in to see what Popular Mechanics was all about, I had a look at the editors page:

Well, OK, everyone knows this is a magazine by men, for men. This doesn’t bother me…there are plenty of magazines targeted just to the women demographic. On their site, they write: “Our typical reader is male, about 37 years old, married with a couple of kids, owns his own home and several cars, makes a good salary and probably works in a technically oriented profession.”

But this one book in the California Science Center really irked me…what makes something a “boy” project or a “girl” project? Blue vs pink?

One of the first toys I remember having as a small child was a paper model of the solar system that I could lay out on the floor and learn the order of the planets. Later I had legos, a chemistry set and build-it-yourself robot kits. My dad let me use all the tools in his garage workbench, and when I was old enough, he taught me how to use power tools. My mom took me to summer classes at the Youth Science Center, a local K-8 extracurricular program, where I got to hold snakes and tarantulas, make a working electromagnet and a flashlight, built a model rocket and launched it…and the list goes on. Never once was I labeled as a “girl scientist”. I was always just a scientist.

Emily the JPL rocket scientist, Halloween, age ~10.

A popular explanation for why there aren’t enough women in science cites the lack of role models, but I don’t think this is the fundamental problem. There have been many successful women in science (not saying there shouldn’t be more!): Marie Currie, Jane Goodall, Rosalind Franklin, Sally Ride, just to name a few off the top of my head. And also let’s not forget our ATLAS Spokesperson, Fabiola Gianotti, runner up of Time Magazine’s Person of the Year!

I really think the solution to the gender gap in science and technology disciplines lies with early childhood development. We need more parents inspiring their children like mine did, and as a society, need to admit that there is no place for gender labels which are destructive and backwards-thinking. The Boy Scientist. While standing in the gift shop, I tried to imagine myself as a child seeing that book sitting next to The Girl Mechanic with the doll bed, and I wondered if some small kernel of doubt would have risen up, with my robotics kits and model rockets, that I was not being the pink-loving girl I was supposed to be.

So I still can’t imagine why the California Science Center Explorastore would carry such a book, by editors who while selling “vintage boyhood” are reinforcing vintage gender stereotypes. Isn’t inspiring the next generation of scientists of all genders, races, or backgrounds what a science museum is supposed to be all about? Why take a chance that a young girl on her first trip to see the Endeavour space shuttle could see “The Boy Scientist” and wonder if science is just for the boys, even if only subconsciously planting these kinds of labels in her mind? What would Sally Ride say?

(from www.smbc-comics.com)

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Happy Birthday ATLAS!

Monday, October 1st, 2012

Today marks the 20th anniversary of the submission of the experiment’s Letter of Intent – the first published document officially using the name ATLAS.

Here’s to many exciting years to come!

More on the history: ATLAS News

Higgsdepenence Day cake, celebrating the 5 sigma discovery of a higgs-like boson

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BOOST! Part 2: Jet Mass

Thursday, September 6th, 2012

In this post I’ll describe how we can use jet mass to identify a large jet containing the decay products of a boosted particle versus your every-day jet coming from a light quark (ie: any quark but the top) or a gluon.

Last time I introduced the top quark decay mode into jets and described why it’s necessary to consider these boosted particles in future LHC analyses. To underline the point, one of our recent ATLAS searches for a new heavy resonance decaying into a top/anti-top pair has included boosted tops, where the three hadronic decay products of one of the tops were unresolvable as individual jets.

We haven’t found new physics….yet….but we were able to show that the sensitivity to new physics increases by a lot once boosted tops are added in:

“Resolvable” hadronicly-decaying tops only:

Limit of Z'->ttbar mass in the semi-leptonic channel with "resolvable" jets only: M(Z') > 880 GeV

Including highly-boosted tops as well:

 

Limit of Z'->ttbar mass in the same channel when including boosted hadronic tops: M(Z') > 1.15 TeV

 

The lower-mass limit of a resonance like a new Z’ boson improved by a few hundred GeV! So what’s the trick?

Jet Mass

Until now, mass has been a relatively uninteresting property of a jet. When all the jets are separately reconstructed in the detector, we care a lot more about their energy and momentum….you just add them up and get back the invariant mass of the interaction that produced them, or in our case, the top quark that decayed into them. We don’t have such a luxury if the only thing we have to work with is one large jet containing indistinguishable decay products.

Ultimately, however, what we really want is the mass of the boosted top; the three individual jets don’t matter as much by themselves. So just like before when we could combine the energies of the three jets to infer the mass of the interaction, we can get the boosted top mass by adding up all the bits which compose the large jet, where in this case “bits” = calorimeter energy clusters.

Here, where E_i and p_i are the energy and three-momentum of the “ith” jet constituent. And it totally works! Using the jet mass is great for discriminating these from other normal jets in the detector; a light-quark or gluon jet will have a steeply-falling mass spectrum, but a jet from a boosted top will have a mass peaking around 173 GeV.

Comparison of the data and the Standard Model prediction for the large-R jet mass distribution after a background subtraction.

You may notice in this plot that the mass peaks somewhat higher than expected. This is because you never get a jet with just the energy deposits from the top decay products….it is an experiment after all and nothing is ever that clean. The detector is filled with a whole mess of other junk coming from the rest of the proton-proton interaction (called the “underlying event”), not to mention the high luminosity conditions at the LHC causing extra jets from other softer proton interactions in the same event (called “pile-up”).

How to deal with that is a whole topic in and of itself, so I’ll save that discussion for next time.

New jet substructure techniques on the horizon

Besides the mass, large jets which contain the decay products of a boosted object like the top quark are expected to have a much different internal structure than your typical jet from a light-quark or gluon. Jet substructure calculations represent the core of new developments in the field of boosted physics, and their discussion occupied a large chunk of the agenda at the BOOST conference in Valencia.

As an experimentalist, part of the fun attending the conference was getting to meet the theorists who are working on this. In high-energy particle physics, when performing a test on some theoretical model, the usual feedback time to go from theory paper to experimentalists announcing “I found it!” (or more likely, “I didn’t find it…”) is decades.

This is not the case when it comes to boosted physics. Some of the papers outlining new jet substructure models came out less than 5 years ago, and we already have experimental results! I get the impression that a lot of this has to do with the spirit of the BOOST conference series, which is a forum for open communication and productive discussion between theorists and experimentalists.

In fact, this HEP subfield has really exploded since the first BOOST conference was held in 2009. Theorists are coming up with all kinds of new ideas that go way beyond jet mass as a way of determining whether or not a jet is a good boosted candidate. There was a great Venn diagram by Matthew Schwartz of Harvard to describe the “happy medium” theorists are trying to achieve between models which were calculable by them, measurable by us, and/or useful to everyone.

….though some people argued a little about which models were actually “useful” or not. ;)

As you can see, there are quite a few ideas. Jet mass still turns out to be the most powerful tool for finding boosted particles, but there is still a lot of unused information inside a jet. Any extra help in telling apart jets with substructure versus those without is extremely valuable in extending our discovery reach for new physics.

Since this post is already getting pretty long I’ll come back to discuss some of these ideas in more detail in a couple of weeks.

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Biking to CERN

Tuesday, September 4th, 2012

A few days ago, I left the CERN Meyrin site on my bicycle at the same time as the tram towards Geneva and managed to be only a few minutes behind when it arrived at my stop. The next night, the tram was still sitting at CERN when I left it in my dust, easily beating it home. The total distance door to door is approximately 7 km with only a few hills, and the route is marked for bikes on almost the entire path (except for a terrifying part over the freeway).

Gare Cornavin to CERN Meyrin site, with mile markers 1 and 4, courtesy of veloroutes.org

Elevation of route per mile (not as bad as it looks...note the scale)

If I’m not racing a tram, it still only takes me about 30 minutes one-way. Factoring in the time it takes to walk from my apartment to the tram stop, the wait for the tram, and the walk from the tram stop to my office, cycling only takes at most an extra 20 minutes a day. Or as I see it, I give up 20 minutes for an hour of exercise.

By the way, I know it’s said a lot but if you get on a bicycle, WEAR A HELMET. Even if the ride is <5 minutes. Even if you think you’ve done that same route umpteen million times. I know of so many people who have gotten into accidents around CERN, most notably at the giant round-a-bout of death coming out of the small French town of Saint-Genis-Pouilly. I’m no exception…I got into one a few years ago while still a grad student. In my case I was already at CERN, turning right at such a high speed that my back tire slid out when I tried to avoid a car (and to this day, I still slow down more when turning right than left). It was so fast, my hands didn’t even have time to leave the handle bars when I face-planted into the road….aaaand I wasn’t wearing a helmet. If you DON’T want to see what an x-ray of a nose broken in two places looks like, then DON’T click here. At least I got a good week or two of “you should’ve seen the other guy” jokes out of it.

Now I find that if I just take it easy, and ride under the assumption that cars can’t see me at all and thus always have to look out for them, I feel pretty safe. Biking has tons of health benefits, especially for people like us who sit in front of a computer all day, and I’m always in a great mood by the time I get to work. Plus I can stop and see cool things like this along the way:

Plane landing at GVA. Don't deny it, everyone loves watching planes land.

There’s a huge biking culture at CERN too….at least four people on my office floor alone regularly bike to work. CERN sponsors a “Bike to Work Challenge”, where groups or individuals compete for honor to see who can rack up the most kilometers between March and the end of the year (I just started not too long ago, so I’m still at the “Sneaky Muon” level).

Biking is really the best thing ever. Everyone should do it! And if you’re driving and see a cyclist on the road, give them some room!

More information:
Bike to Work Switzerland
CERN cycling safety information

Pro-tip:
– Showers at CERN (pdf)

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BOOST!

Sunday, August 5th, 2012

A couple weeks ago, about 80 theorists and experimentalists descended on Valencia, Spain in order to attend the fourth annual BOOST conference (tag-line: “Giving physics a boost!”). On top of the fact that the organizers did a spectacular job of setting up the venue and program (and it didn’t hurt that there was much paella and sangria to be had) overall I’d have to say this was one of the best conferences I’ve attended.

so....much.....sangria......

Differing from larger events such as ICHEP where the physics program is so broad that speakers only have time to give a cursory overview of their topics, the BOOST conferences have more of a workshop feel and are centered specifically around the emerging sub-field of HEP called “boosted physics”. I’ll try to explain what that means and why it’s important below (and in a few subsequent posts).

Intro to top quark decay

In order to discuss boosted physics, something already nicely introduced in Flip’s post here, I’m going to use the decay of the top quark as an example.

Obligatory Particle Zoo plushie portraying the top quark in a happy state

The most massive of all known fundamental particles by far, weighing in at around 173 GeV/c2, the top quark has an extremely short lifetime….much shorter than the time scale of the strong interaction. Thus the top quark doesn’t have time to “hadronize” and form a jet…instead, it will almost always decay into a W boson and a b quark (more than 99% of the time), making it a particularly interesting particle to study. The W boson then decays into either a lepton and a neutrino or two lighter quarks, and the full top decay chain is colloquially called either “leptonic” or “hadronic”, respectively.

From the experimental point of view, top quarks will look like three jets (one from the b and two from the light quarks) about 70% of the time, due to the branching fraction of the W boson to decay hadronically. Only 20% of tops will decay in the leptonic channel with a jet, a muon or electron, and missing energy. (I’m ignoring the tau lepton for the moment which has it’s own peculiar decay modes)

In colliders, top quarks are mostly produced in top/anti-top (or “t-tbar”) pairs….in fact, the top-pair production cross section at the LHC is about 177 pb (running at sqrt(s)=7 TeV), roughly 25 times more than at the Tevatron!! Certainly plenty of tops to study here. Doing some combinatorics and still ignoring decay modes with a tau lepton, the whole system will look:

  1. “Fully hadronic”: two hadronically-decaying tops (about 44% of the time)
  2. “Semi-leptonic”: one leptonically-decaying and one hadronically-decaying top (about 30% of the time)
  3. “Fully leptonic”: two leptonically-decaying tops (only about 4% of the time)

Branching fractions of different decay modes in t-tbar events (from Nature)

 

The point: if a t-tbar event is produced in the detector, it’s fairly likely that at least one (if not both) of the tops will decay into jets! Unfortunately compared to the leptonic mode, it turns out this is a pretty tough channel to deal with experimentally, where at the LHC we’re dominated by a huge multi-jets background.

What does “boost” mean?

If a t-tbar pair was produced with just enough energy needed to create the two top masses, there wouldn’t be energy left over and the tops would be produced almost at rest. This was fairly typical at the Tevatron. With the energies at the LHC, however, the tops are given a “boost” in momentum when produced. This means that in the lab frame (ie: our point of view) we see the decay products with momentum in the same direction as the momentum of the top.

This would be especially conspicuous if, for example, we were able to produce some kind of new physics interaction with a really heavy mediator, such as a Z’ (a beyond-the-Standard-Model heavy equivalent of the Z boson), the mass of which would have to be converted into energy somewhere.

Generally we reconstruct the energy and mass of a hadronically-decaying top by combining the three jets it decays into. But what if the top was so boosted that the three jets merged to a point where you couldn’t distinguish them, and it just looked like one big jet? This makes detecting it even more difficult, and a fully-hadronic t-tbar event is almost impossible to see.

At what point does this happen?

It turns out that this happens quite often already, where at ATLAS we’ve been producing events with jets having a transverse momentum (pT) of almost 2 TeV!

A typical jet used in analyses in ATLAS has a cone-radius of roughly R=0.4. (ok ok, the experts will say that technically it’s not a “cone,” let alone something defined by a “radius,” as R is a “distance parameter used by the jet reconstruction algorithm,” but it gives a general idea.) With enough boost on the top quark, we won’t be able to discern the edge of one of the three jets from the next in the detector. Looking at the decay products’ separation as a function of the top momentum, you can see that above 500 GeV or so, the W boson and the b quark are almost always within R < 0.8. At that momentum, individual R=0.4 jets are hard to tell apart already.

The opening angle between the W and b in top decays as a function of the top pT in simulated PYTHIA Z'->ttbar (m_Z' =1.6 TeV) events.

 

We’ll definitely want to develop tools to identify tops over the whole momentum range, not just stopping at 500 GeV. The same goes for other boosted decay channels, such as the imminently important Higgs boson decay to b-quark pairs channel, or boosted hadronically-decaying W and Z bosons. So how can we detect these merged jets over a giant background? That’s what the study of boosted physics is all about.

Next: Finding boosted objects using jet “mass” and looking for jet substructure

Next next: Pileup at the LHC….a jet measurement nightmare.

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from PhD to postdoc

Tuesday, July 31st, 2012

Hello!  I’ll probably write more technical posts later, but since I’m a new US LHC blogger, I thought I would spend this first post talking a little about my background and how I decided what kind of postdoc position to look for.  Lately, quite a few of my friends in their last year of graduate school have been asking about the latter. ;)

I’ve been with Columbia University for almost a year now, having defended my PhD thesis at the University of Massachusetts, Amherst on June 10, 2011.  I knew I wanted to stay in the field after graduating, if only for the fact that it would have been a shame to look at only 42 pb-1 of data, see no new physics, and miss out on being around for (what eventually became!) the biggest particle physics discovery in decades. Just to give you an idea of how much data I had for my thesis compared to what we have now, see this histogram showing the integrated luminosity recorded by ATLAS in 2010, 2011 and so far in 2012:

I was that green line.

Before applying for postdoc positions, I needed to decide what kind of research to do in the next stage of my career, and where I would want to do it. Almost all the work I had done as a graduate student was related to the muon spectrometer on ATLAS; from helping in the installation and commissioning of the muon precision chambers, to muon reconstruction performance studies, to measuring the first Z→μμ cross section at sqrt(s)=7 TeV and finally performing a search for new physics in the high-mass tail of the mu-mu invariant mass spectrum. Muons were my thing.

The advice I got from most of my colleagues at the time, including my adviser, was to switch experiments. The reasoning made sense. If you stay with the same experiment for your postdoc, you miss out on a free pass to do research on something completely new. It’s a rare opportunity to start from scratch while still having some allowance for time to catch up.

But that was the thing….most of the people I was seeking advice from had come from other experiments to the LHC, not the other way around. In fact, I was one of the first US students able to write a thesis on LHC data (the delay partly due to the incident in 2008….let’s not talk about that). So where could I have gone from here? If I wanted to stay in collider physics, I needed to stay at the LHC.

Knowing I wanted to come back to CERN, it also took some time to figure out exactly what kind of analysis I wanted to work on after the PhD. I talked to a lot of people that semester, asking who would be working on what and getting lots of advice. I certainly had many interesting options for research, but it wasn’t until I was sitting in a talk about the evidence for forward-backward asymmetry of the top quark when I thought now hey, top physics…

In the end, I decided to make as big of a switch as possible while still staying on ATLAS, moving from the muon spectrometer and dimuon analyses to work on top quark physics and jets at an institute responsible for the liquid argon calorimeter electronics. The move seemed to cover the best of all possible scenarios…I didn’t need to worry about the year-long wait to qualify for authorship or to figure out ATLAS software, but I did get the opportunity to learn something ultimately different when it came to hardware work and physics analysis. However, because of the size of the collaboration, where each subdetector community has roughly the same number of people as one Tevatron experiment, it took some time to get enough exposure to be recognized for the new work I was doing. That will be the case whenever you start a new job, no matter what.

Even more difficult was going from feeling like an expert in my thesis topic to suddenly being thrown in the deep end of a new topic amongst other experts. I found I wasn’t the only one who experienced that.  Before I began, a few senior postdoc friends of mine who wrote their PhDs at the Tevatron said that their first year at the LHC felt just like being a brand new graduate student all over again and that it was hard to feel like anything really substantial had gotten done during that time, just because there was the additional learning curve thrown in. When I looked a little sad, one of them said “well for you, since you’re staying on ATLAS…maybe only 6 months.”

My advice to anyone wrapping up their graduate studies and thinking about getting a postdoc would be to talk to as many people as possible and get as many opinions as possible. My experience is just one of many! I can say though that the more I knew going in, the easier the transition was, and now one year later everything is going really smoothly.

Anyway, have a look at my upcoming posts, where I’ll talk about jet substructure, new physics searches involving the top quark, and whatever other cool beyond-the-Higgs stuff is happening at CERN.

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