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Christine Nattrass | USLHC | USA

View Blog | Read Bio

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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  • Congratulations, it’s fun. Although the statement that it’s the “first quenching in a single event” depends on how you count a “single event”.

    From a high-energy perspective, any such QGP-related event is a sequence of hundreds of events involving quarks and gluons 🙂 so you have just simplified the arrangement of these hundreds of quarks and gluons.

    Best wishes

  • Hi…

    That…2’nd Pic Missing Peaks Is Interesting, Nice Post…^_^ I Wonder Why Is That Happen ??? Hmmm…

  • Jonathan Clift

    Not being a physicist, one of the things that interested me when this was announced was the palpable sense of excitement by the physicists reporting it in blogs. It’s also interesting because, although no doubt the underlying theory is difficult, the method and result are quite easy to understand in a general hand-wavy kind of way.

    Could I ask a few questions from down here in the valley of ignorance?

    a) Is it certain that the QGP exits? Your post reads as if it’s a given and that the result we see here is exploring the nature and/or the extent of it (rather than being evidence of its existence). Isn’t there the small danger you are seeing what you want to see? Aren’t there other possible mechanisms for what you see – for instance, isn’t it possible that one of the jets could have collided with one of the other lead ions in the bunch and been dispersed as a result?

    b) One of the earlier reports I read said that we were looking at a quark-quark collision here (no reason was given). Above you leave it open as to any combination of quark or gluon. Does theory say anything about the probability of each of the permutations? And, would the outcomes be measurably different or would you get the same mess of particles in your detectors in each case?

    c) I don’t quite understand how the pair of quarks collide? Well I do – you’ve just collided two lead ions together – but why aren’t all the quarks/gluons in the plasma colliding if the plasma is as dense as you say?
    The way I see it, the quarks and gluons have enough energy to separate out, they’re very small, there’s a lot of space between them, so why don’t they fly through each other with the probability of any collision being very small? Why isn’t the sequence of events roughly: head on collision producing the mess of particles going all over the place, leading then to a high enough concentration of energy to give the quark-gluon plasma, followed by a late collision between a couple of quarks/gluons within the plasma?

    d) Do you see double collisions sometimes, ie two full jets and two quenched jets?

  • Great post, Christine! I was reading Seth’s post the other day and thought to myself, “I don’t think I understand what jet quenching is…” Now I’m much happier. 🙂

  • Christine Nattrass

    Hi Jonathan

    Thanks for the great questions.
    a) Obviously these results alone isn’t enough to conclude that there’s a QGP. In 2004 the four collaborations at the Relativistic Heavy Ion Collider (RHIC) each put out papers detailing the evidence and concluding that the QGP was present. The top RHIC energy was 200 GeV per nucleon in the center of mass, or 1/14th of the collisions so far at the LHC, so if it’s produced at RHIC it’s definitely produced at the LHC. There is always dissent in the scientific community – it’s healthy – so you could definitely find outliers, but there is a strong consensus in the scientific community that the QGP is produced in nucleus-nucleus collisions at RHIC and LHC energies.

    Some of the main evidence for the QGP is:
    1. Jet quenching – This was observed in two ways at RHIC. One method was similar to the ALICE result described in this post, but the suppression of single hadrons wasn’t as high as observed by ALICE. The other method looks at correlations between two high momentum particles, likely to be produced in jets. (This is similar to the method in the CMS paper described here: http://blogs.uslhc.us/exciting-new-results-from-cms) The concept is similar to the ATLAS measurement described above, but single particles serve as proxies for fully reconstructed jets. A similar result was observed using this method, but the results were more ambiguous because jets were not fully reconstructed.
    2. Flow – We see evidence that the QGP is a fluid, moving collectively. It flows, like water flows through pipes. (The first ALICE measurement of this effect is the second paper described here: http://blogs.uslhc.us/first-heavy-ion-papers) The data are consistent with ideal hydrodynamics – meaning a fluid with no viscosity. And there is evidence that what is flowing is quarks, not hadrons (composite particles made of quarks such as protons).
    3. Thermal & chemical equilibrium – the yields of particles are consistent with chemical and thermal equilibrium and the temperature we extract from the data is high enough to produce a QGP, as determined from theory. Even strange quarks are observed to be in equilibrium. Proton-proton collisions do not produce particles at thermal equilibrium so this must come from something which is not present in proton-proton collisions.
    4. Energy density – measurements of the transverse energy give us a measure of the energy density in the collision, and the energy densities we observe are well above the energy densities required to form a QGP.

    These links have more details and are accessible to the general public:

    In principle it would be possible to have jets quenched by a lead ion not involved in the collision, but the probability of this happening is incredibly low – too low to explain the data. In a particle collider, particles go around in bunches. The beams of particles are not like streams of water coming out of a fire hose. Particles travel around the collider in bunches, like beads on a string. Each beam is like a beaded necklace with each bead containing several nuclei. Collisions happen when the bunches in each beam cross. The luminosity – the rate of collisions – was so low that we didn’t get more than one collision in each bunch crossing. (At least not at a statistically significant rate.) If density of nuclei in a bunch is so low that we don’t get more than one collision in each bunch crossing, the odds that something could get hit out of one of the nuclei and hit a third nucleus is even smaller. Imagine you and a friend are hitting baseballs at each other trying to get two baseballs to collide – and then you’re trying to get one baseball to ricochet off the other and hit a third baseball. Not very likely.

    We have pondered whether other explanations could explain jet quenching – but we’ve now seen it in three different ways (methods like the ALICE result, methods like the CMS result, and methods like the ATLAS result) and it’s been seen by 7 experiments at two different colliders – ALICE, ATLAS, CMS, PHENIX, STAR, PHOBOS, and Brahms. So the experimental evidence is pretty overwhelming.

    b) All combinations are possible and it’s possible to calculate which are most probable. The detailed calculations are complicated but some details are here: http://en.wikipedia.org/wiki/Parton_%28particle_physics%29
    There are a lot more gluons in the incoming nuclei than quarks – there are a few quarks sitting in a gluon pudding. So actually the most probably is a gluon-gluon collision. And yes there are differences between quark and gluon jets – but they’re not easy to measure. So it will take a while for us to do these measurements.

    d) (Answering out of order) Yes!! We often get more than two sets of di-jets in a collision. This is why the ATLAS measurement required a 100 GeV jet and then looked for a jet with at least 25 GeV at an angle of 180 degrees away from the 100 GeV jet. The higher energy a jet is, the more rare it is. For example, there might be one jet with at least 10 GeV in every event on average, but only one 100 GeV jet in every 100,000 events. (These numbers are made up – don’t draw any physics conclusions from them.) By looking for a very rare jet and then looking for another jet where we’d expect it, roughly 180 degrees away from the first jet, ATLAS was able to significantly reduce the background from other jets.

    c) Yes!! We do get multiple interactions between quarks and gluons. But jets, especially high energy jets, select a very small percentage of those interactions. Hard scatterings are those where the momentum transfer is large.
    But actually the QGP is very, very dense. The quark gluon plasma is liquid of quarks and gluons.

    And thanks to everyone else for the great comments!

  • Hi…

    Nice Explanation Christine Anyway I Wonder If (b) Explaination Have Something To Do With The Universe Because The Characteristic For Gluon + Gluon = Star (Universe) Because From What I Can Imagine It Show 4 Fundamental Force…Is That Jet Show Electroweak Theory For The Higg’s ???

  • Jonathan Clift

    Hello Christine,

    Thanks for such a comprehensive reply.

    One problem I had was getting a feel for what happens as the collision occurs and the QGP comes into existence. For anyone else in the same situation, this web page has some useful graphics:


    One thing I’d missed was the flattening of the ion because of Lorentz contraction. I can now visualise how a hard collision between, say, a quark in each ion can occur within a QGP that’s already forming around it.

    The other thing that helped was re-reading Flip’s last post (the one immediately preceding this one) and the others on QCD. [The last section of the last one is an interesting summary of the different view theorists and experimenters have of these collisions and is worth reading in its own right.]

  • Great post Christine, thanks for sharing!

  • student

    Hello, can I use the proton-proton collision picture for a t-shirt design or is it under copyright protection? Thank!

  • Christine Nattrass

    Hi student – parts of the figure might be problematic. Both the nucleus and the QGP are borrowed from elsewhere. I don’t remember where. The QGP is actually a cropped image of the sun.

  • E.Chaniotakis

    very nice post! I would like to ask about jet quenching measured in the jet+gamma channel.
    We see that the jet is actually quenched while the gamma comes ‘untouched’ *as expected* by the QGP.
    The question is rather conceptual:
    We want the jet+gamma to be produced by the same parton collision (and therefore their pT to be balanced).
    The mechanism of their production is the deep virtual gluon compton scattering?
    Meaning: q+g->q+gamma ?
    Is there any other process that I am not aware of that would produce the same results?
    Thank you!

  • Christine Nattrass

    Hi E. Chaniotakis

    There are a number of other processes which, experimentally, can look a lot like a gamma jet. Fragmentation photons are one such process. Also, an extremely high momentum pi0, which decays into two photons very close together, can look like a photon.