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Christine Nattrass | USLHC | USA

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Jet quenching

There have been a lot of exciting results lately and I haven’t gotten a chance to write about them because I’ve been too busy.  Today I’ll tackle jet quenching, which Seth touched on in one of his posts.

You may have done absorption spectroscopy in a chemistry lab.  In absorption spectroscopy, light from a calibrated source passes through a sample and changes in the light after passing through the sample are used to determine the properties of the sample.  For example, you may have a liquid that absorbs blue light but lets orange light through.  This tells you something about the properties of the liquid.  We want something like that for studying the Quark Gluon Plasma (QGP).  Perhaps we could try shining light on the QGP to see what it does to the light, how much is absorbed?  The problem with that is that the QGP formed in a nucleus-nucleus collision doesn’t live very long – about 10-24 seconds.  Trying to aim light at the QGP would be like trying to hit a fighter plane at top speed with a Nerf gun – by the time you aimed, the plane would be long gone.

Fortunately, photons (light) are created in the lead-lead collisions.  Since they are produced in the collision, we know they went through the QGP so we can use them and study how they’re affected by the QGP to determine its properties.  This is analogous to determining what a store sells by looking at what people have in their shopping bags when they leave the store rather than by going in the store yourself.  This is one of the measurements we’ll see at some point.  But photons only interact through the electromagnetic force and many of the features of the QGP we’re trying to study come from the interaction of quarks and gluons through the strong force.  To study these properties, we need something like a photon, but that interacts through the strong force.  We can use quarks and gluons.

There are quarks and gluons in the incoming lead nuclei, and a quark or gluon in one nucleus can scatter off of a quark or gluon in the other nucleus.  We’re particularly interested in hard scatterings, where they hit each other and bounce off like billiard balls.  This process happens early in the collision, and then the partons travel through the medium, as shown below:


But there’s a complication.  We can’t see individual quarks and gluons – they’re always bound in hadrons, states made of two quarks (mesons) or three quarks (baryons), a property called confinement.  After the parton gets knocked out of the nucleon, it hadronizes – it breaks up into several mesons and baryons.  These are actually what we observe in our detector.  For each parton, we have a cone of hadrons called a jet.  This is an event display from the STAR experiment showing two jets in a proton-proton collision:

In a proton-proton collision, it’s easy to see jets, but in a heavy ion collision they’re in events like these:

So it’s not as easy to find jets in heavy ion collisions.  One thing we can do is look at very fast moving hadrons.  These are more likely to have come from jets.  This is the subject of the most recent ALICE paper.  This is the main result from that figure:

The x-axis is the momentum of the hadron perpendicular to the beam, called the transverse momentum.  The y-axis is something called RAA, which is the ratio of the number of hadrons we measure in lead-lead collisions to the number we would expect if a lead-lead collision were just a bunch of nucleon-nucleon collisions.  We take what we measure in proton-proton collisions and scale it by the number of proton-proton, proton-neutron, and neutron-neutron collisions we would expect.  (Yes, I’m skipping lots of technical details about how that scaling is done.)  Another way of putting it is that it’s what we get divided by what we expect.  If RAA were exactly 1.0, it’d mean there’s no physics in lead-lead collisions that isn’t in proton-proton collisions.  An RAA less than one means we see way fewer particles than we expect.  In the figure, the open points are what we measure for peripheral collisions, where the nuclei just barely graze each other.  The solid points show what we measure for central – head-on – collisions.  The big, obvious feature is the bump which peaks for particles with a transverse momentum of about 2 GeV/c.  There’s a lot of physics in there and it’s really interesting but it’s not what I’m talking about today.  Look at what it does at higher momenta – above about 5 GeV/c.  This is where we trust our theoretical calculations the most.  (At lower momenta, there’s much more theoretical uncertainty in what to expect.)  We see only about 15% of the number of particles we expect to see.  This was already observed at the Relativistic Heavy Ion Collider, but the effect is larger at the LHC.

This happens because the QGP is really, really dense.  It’s harder for a parton to go through the QGP than it’ll be to walk through a Target store on the day after Christmas.  The parton loses its energy in the QGP.  Imagine shooting a bullet into a block of lead – it’d just get stuck.

ATLAS’s recent paper exhibits this more directly.  Here’s a lead-lead event where the lead nuclei barely hit each other.  Here you can see two jets, like what you’d expect if neither parton got stuck in the QGP:

The φ axis is the angle around the beam pipe in radians, the η axis is a measure the angle between the particle and the beam pipe, and the z axis is the amount of energy observed in the calorimeter.  Imagine rolling this plot up into a tube, connecting φ=π to φ=-π and that would show you roughly where the energy is deposited.  The peaks are from jets, like in the event display from STAR above.  The amount of energy in each peak is about the same – if you added up each block in the peak for both peaks, they’d be about equal.  And here’s a lead-lead event where one of the partons got stuck in the medium:

In this plot one of the peaks is missing.  One of the jets is quenched – it got absorbed by the QGP.  This is the first direct observation of jet quenching in a single event.  It’s causing quite a buzz in the field.

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