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Seth Zenz | Imperial College London | UK

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My Thesis Topic: Measuring Jets with the Inner Detector

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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