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Posts Tagged ‘track jets’

My Crazy Semester: Thesis Writing

Monday, February 28th, 2011

Oops, I seem to have done things in the wrong order. The good news is that I have a job lined up for when I graduate. The… challenging… news is that I still have a few things to finish here in Berkeley: little things, like finalizing my analysis and writing my thesis! This has made for a rather busy semester, to say the least.

From the perspective of readers here, though, I’m not going anywhere fast. I will be switching universities, and (gasp!) switching experiments, but in neither case am I going far enough that I fall outside the subject of this blog. I didn’t mean “far enough” too literally, but the distances are about 2500 miles and 5.3 miles respectively.

This gives me lots to write about, so let me start with my thesis. It’s not that much fun. It has two parts: background stuff I have to look up, and describing my analysis in more detail. But it is, I have to admit, probably all wortwhile. The former part is all stuff that’s relevant to my work and I ought to be able to describe off the top of my head — and in fact, I usually can, but not with all the numbers and equations and details just right. The latter part is a good chance to really document what I did in my analysis, which is information that might not be public elsewhere. Maybe someone, someday, will want to look up what I did. Maybe I’ll look it up myself out of nostalgia. Once I get my thesis written at last, though, one thing I’m sure of: I won’t look at it again for a while. I’m already ready to move on!

While I’m working on that, here are some goodies from my thesis. The output of a helpful script I wrote:

Seth-Zenzs-MacBook-Pro:~ sethzenz$ python scripts/thesistimeleft.py

You have 75 days left to file your thesis!!!

It recalculates every day. I could make it send me automatic emails, if I really want to make myself nervous.

And here’s a bit of my (first draft) introduction, which tries to explain how my work fits into the overall context of the LHC program:

The Large Hadron Collider (LHC) was built to produce new particles and rare interactions at a high rate, but its first and foremost byproduct is sprays of low-energy hadrons. At its full design capacity, the LHC will cross proton bunches 31.6 million times per second at each interaction point, with an average of about 20 proton-proton collisions per crossing. Most of these collisions will be “soft” interactions, with relatively little energy exchanged and the outgoing hadrons having relatively little momentum perpendicular to the beam axis. These interactions are described in principle by Quantum Chromodynamics (QCD), the quantum field theory of the strong interactions. In practice, however, they are the most difficult to understand, because the theory becomes non-perturbative at low energies. Predictions can only be made via approximations and phenomenological models. This difficulty with low-energy strong interactions appears even in interactions that are initially well-described by perturbation theory. Outgoing high-energy quarks and gluons quickly “clothe” their strong color charge by evolving into jets of lower-energy hadrons, a process that again requires approximation and modelling.

The LHC’s general-purpose experiments, ATLAS and CMS, are equipped with multi-stage trigger systems that select against these common processes, for example by identifying leptons and missing energy produced in electroweak interactions. However, low-energy QCD still has a significant impact on the physics program in several areas. With so many collisions in each crossing, the most interesting collisions will have many low-energy collisions whose signals in the detector overlap with the objects of interest. In order for their effects to be subtracted, these features of these pileup collisions must be known quantitatively. The evolution of high-energy hadronic jets must also be well-understood. This is partially to account for their contribution as pileup events, but their energy must also be calibrated so they can be studied in their own right. Although even very high-energy jets are relatively common at the LHC, they can also serve as signatures of the decay of new particles.

The quantitative investigation of low-energy QCD is thus a foundational element of the LHC program, which will inform the studies and discoveries of the coming years. Initial low-energy QCD measurements have divided the problem between low-energy events and the study of higher-energy jet properties. In the former case, inclusive charged particle distributions are produced from events identified using a “minimum bias” trigger. In the latter case, higher-energy jets are triggered and studied using the calorimeter system built for the purpose.

This work focuses on the additional information to be gained in the case that the two issues are not-so easily factorized, by studying the emergence of low-energy jets from soft interactions. Particles are identified using the methods of the lowest-energy measurements, but grouped together into jets according to the algorithms used to study jets at higher energies. Low-momentum jets and their properties are measured using the ATLAS Inner Detector, the component of the ATLAS experiment that tracks charged particles, in events identified using the ATLAS Minimum Bias Trigger Scintilators.

That’s very unlikely to be final, but in any case that gives you a picture of the sort of thing I’m working on.

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My First Day at ICHEP

Friday, July 23rd, 2010

This is my poster. There are many like it but this one is mine.
There are probably many blogs where you can read summaries of the ICHEP conference — or if not, there will be soon enough — so I’m going to limit myself to telling you about my day. Getting my poster printed and getting it to Paris in one piece was stressful, but uneventful in the end, and once I got to the conference things were easy. The poster session was the first evening, and you can see me at right standing in front of the thing, ready to explain what’s going on. (I will soon post more about the measurement shown in the poster, but here is the official ATLAS conference note, and here is an old summary of some of the concepts.) I didn’t get an overwhelming number of people asking questions — there were an awful lot of posters, which gave me a new perspective on how my work is one tiny facet of our overall effort to understand particle physics — but I did have a few good discussions with interested folks, and the psoter will be up all week.

As for the rest of the conference, I mostly went to the “early LHC experience” sessions, along with a few talks in the Standard Model session. I found the ATLAS and CMS measurments of the W and Z bosons interesting, but mostly because they show how the experiments are getting going. The theory of these particles is very well understood, and the experiments are consistent with it — in fact, if the experiments disagreed the theory at this point, we’d conclude that something had gone wrong with the experiments. When the detectors are solidly understood and working toward precision measurements, they may discover subtle differences from theoretical predictions in this area, but that will be years from now.

And now for another day of conference talks!

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Your Tax Dollars at Work at 2 AM

Friday, October 23rd, 2009

It’s two in the morning here in Geneva and I just got home.  While walking back, I had some ideas about how to understand the impact on track jets from tracking lower-energy particles, and how to better understand the efficiency of finding those track jets as a function of what part of the detector they hit.  So time to fire up the Internet and get back to work!  — Seth

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Hi, Seth here. I haven’t written much here lately, because I’ve been busy with two rather involved tasks. First, I’ve been working on the logistics of moving back to California in a few months; and second, I’ve been I’ve been getting my thesis topic in shape.  I gave a talk on it in a decent-sized ATLAS experiment meeting last week.  Now that I’ve explained my work to my collaborators, I’m ready to try my hand at explaining it to all of you.

The current title, at least for the talk I just gave, is Inclusive Jet Cross Section using the Inner Detector.  To explain what that means, I’ll have to take the title literally word by word.

What’s a cross section?

The cross section for something being created at the LHC is effectively a way of measuring how often it happens.  It may appear strange that we’re measuring the rate in what sounds like an area, but an analogy might help clear things up: if you were throwing big bunches of baseballs at each other in order to study how baseballs interact, the rate at which any two would collide would be proportional to the size of the baseballs, or more precisely to their cross-sectional area.  It’s also proportional to how many baseballs are in the bunches and how closely-packed the baseballs are, but those aren’t properties of a baseball, so you’d like to factor those things out — which is why you’d be interested in the cross section rather than just the collision rate.

Now, all baseballs can do is bounce off each other, but protons at the LHC can do lots of things: they can bounce off each other, produce pairs of high-energy quarks or gluons, or even high mass particles like (we hope) the Higgs Boson.  So we split the total cross section into the cross section for different things to happen.  You can imagine that if the baseballs had a tiny button on them somewhere that makes them explode, then the cross section for exploding would be much smaller than the cross section for just bouncing off each other any old way.  The rate at which they exploded would be proportional to the (smaller) cross section of the button.  Likewise, at the LHC the rare and interesting events have much smaller cross sections, although probability at the quantum level doesn’t really work in terms of protons having a special “make Higgs” button that you hit once in a while.  Analogies only take you so far.

So in the end, all a “jet cross section” really means is that I’m counting the number of jets in a certain number of proton-proton collisions at the LHC.  The word “inclusive” just means that I’m counting all the jets, whether they’re produced all at once or separately.  (One example of an alternative would be to measure the rate to get three or more jets, which we’d call a multijet cross section.)

But wait, what’s a jet?

I admit it, the shortest bit is the trickiest.  A jet is a bunch of particles that form around a high-energy quark or gluon; the LHC will make the quarks and gluons, but what we can see in the detector are the jets.   Why is that?  Well, the very short answer is that “naked” quarks and gluons aren’t allowed to exist for very long; they have to be confined in particles called hadrons.  (Hadron just means “particle made of quarks”; it also happens to be the “H” in the LHC.)  But the reason that this isn’t allowed is a little more complicated, and it was actually a big mystery in particle physics a few decades ago: people realized that many properties of the hadrons they had seen could be explained if they were really just combinations of a few quarks, in much the same way that protons and neutrons explain many properties of the Periodic Table of Elements.  But then they had to ask: where are all the quarks?  Why haven’t we ever seen one by itself?  The answer to that question also tells you what a jet is.

Quarks turn out to have a property called color charge, which is sort of like electrical charge.  Just as two electrically-charged objects have a force between them, quarks also have a force.  It turns out to be a very strong force called, well, the strong force.  The strong force, unlike the electromagnetic force, doesn’t get weaker as two quarks get further and further apart; more and more energy builds up between the quarks, and eventually that energy is enough to produce a new quark-antiquark pair.  And then those will usually start flying apart too.  So if a collision at the LHC produces a high-energy quark-antiquark pair, they will quickly fly apart and produce many other quarks and antiquarks.  The LHC may also produce pairs of gluons, or a quark and a gluon.  Gluons are little packets of the strong force itself  — just like photons (light particles) turn out to be packets of the electromagnetic force.  But unlike photons, which are electrically neutral, gluons have color charge just like the quarks do.  As two gluons or a quark and a gluon fly apart, they make more quarks and gluons just the way the quark-antiquark pair does.

So when you start with quarks and gluons, you end up with lots of quarks and gluons.  The splitting process stops after a while, when the energy is low enough that the quarks get bound together into hadrons.  Around the original course of each high-energy quark or gluon you started with, you have a whole bunch of hadrons.  It’s that mess we call a jet.

What’s the Inner Detector?

So far we’ve talked about what I want to measure; now we’re moving on with how, experimentally, I want to measure it.  The Inner Detector the system for tracking charged particles in the ATLAS detector, which is the experiment I work on.  I’ve explained in some detail how tracking works in a previous post, but the short version is that it’s a system for measuring the momentum (direction and “quantity of motion”) of electrically-charged particles as they curve in a magnetic field.  The Inner Detector is a very precise instrument, but it’s actually a very unusual choice for measuring jets for one critical reason: it only tracks the electrically-charged particles.  The part of ATLAS that’s actually designed to measure jets is called the calorimeter; it stops almost any particle that hits it — including any hadron — and measures how much energy was left behind.  So almost all the energy in a jet is caught and measured in the calorimeter, whether the individual particles are electrically charged or neutral.  The Inner Detector, by contrast, will measure only the part of the jet composed of charged particles.

This difference is very important, because the electrical charge of the particles in jets is, in some sense, random.  A jet consisting of three neutral pions could just as well have contained a neutral pion, a negative pion, and a positive pion.  (A pion is the lightest kind of hadron, made of a quark and an antiquark.  The two alternatives I just listed can be built out of the same three quarks and three antiquarks.)  The calorimeter will see either possibility in roughly the same way, but the tracker won’t see the jet made of only neutral pions at all, while in the other case it will see two of the three particles.

Trying to measure jets with only the charged particles means that for any given jet I have no idea how much energy is missing — the jet’s energy might all be in charged particles, or it might mostly be in neutrals, and there will be all-neutral jets that can’t be seen with tracks at all.  The only way to make an accurate measurement is to correct for the missing neutral energy on average.   That turns out to be very tricky indeed — both in terms of mathematics and understanding the experimental errors introduced — and it’s what a lot of my work is focused on.

So why am I doing things the hard way?

There are some reasons why it’s good for the ATLAS collaboration to have a member working on such a measurement, and there are some reasons why it’s interesting for me in particular.  ATLAS benefits because a jet measurement has uncertainties that are very different from the uncertainties associated with the calorimeter measurement and provide a possible cross-check; furthermore, the track-based measurement allows lower energy jets to be studied.  I like my approach personally because it lets me apply the work I’ve done on tracking over the years; it’s also a little ways “off the beaten path,” so I get to figure more things out for myself; finally, it can be done with relatively little data, which is important to me because I’m already in my sixth year of graduate school and would like to finish as quickly as I can do a good measurement.

Will it work?  I honestly don’t know yet.  But it will be fun to try!

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