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Christine Nattrass | USLHC | USA

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Adieu, for now

Tuesday, April 5th, 2011

So this is my last post, at least for a while.  Andrew Adare, a post doc on ALICE at Yale, will be the ALICE blogger for the next term.  I hope you all had as much fun as I did.  I especially love the questions.  What I hope you got out of my posts was a better understanding of why we’re interested in lead-lead collisions and a basic understanding of the Quark Gluon Plasma.  I also hope I managed to get you all excited about science in general and high energy physics in particular.

To that end, I’d like to remind you about Brookhaven National Laboratory’s Summer Sundays.  This is a series of events where the lab is open to the public and gives tours focusing on different subjects.  Every year there is one day focusing on the Relativistic Heavy Ion Collider – this year it’s August 14th.  It’s still early enough to plan a trip out to Long Island to see it!  You’ll get to see the collider and the experiments and your guides are physicists working on the experiments.  Most national laboratories have outreach programs, so if you’re near a national laboratory, it’s worth seeing what public outreach programs they have.  Many will do school tours, too.  And for undergraduates, there are paid summer internship programs at national labs.

Now I’ll turn it over to Andrew, your new guide to the exciting realm of heavy ion physics.  Welcome, Andrew!


Why run at lower energy?

Wednesday, March 23rd, 2011

Right now the LHC is about to start a short run with proton-proton collisions at a center of mass energy 2.76 TeV.  This is lower than what we ran last year and is a special request from the heavy ion physicists.  So you’ve heard a lot about why the particle physicists want to go to higher energy.  But why do we heavy ion physicists want to go to lower energy?

We want a reference for our lead-lead collisions.  If nucleus-nucleus collisions were nothing but a bunch of proton-proton collisions, what we measure in lead-lead collisions should be just some constant times what we measure in proton-proton collisions.  This is a bit simplistic, but it’s a pretty good start.  A lot of our measurements use proton-proton collisions as a reference and look for differences between proton-proton collisions and lead-lead collisions.  For instance, in the paper I discussed here we looked at the distribution of particles as a function of their momenta in lead-lead collisions and compared that to what we observed in proton-proton collisions.  For this paper we used the data from proton-proton collisions at 900 GeV and at 7 TeV to extrapolate to what we’d expect at 2.76 TeV, the same energy per nucleon as our lead-lead collisions.  As discussed here our models for proton-proton collisions are pretty good but they get some of the details wrong – and miss some features like this.   Since we depend on models to extrapolate to 2.76 TeV, we have greater uncertainty in our measurements than we would have if we had data at 2.76 TeV.  The LHC can go down to 2.76 TeV and what we need 2.76 TeV proton-proton data doesn’t require as many statistics (as many total proton-proton collisions) as what the particle physicists need to look for things like the Higgs.  So we’re having a short run with proton-proton collisions at a lower energy because it will significantly help the heavy ion physics program.  (We’ll also get some core physics measurements out of the 2.76 TeV proton-proton data, but like the paper I discussed here, these will refine our understanding but not dramatically change our understanding of proton-proton collisions.)  I hope you’re as excited as I am!


What is the Quark Gluon Plasma?

Friday, March 18th, 2011

I am overdue for a blog post because I have been way too busy lately.  I got an email from an elementary schooler, Jacob, asking about the QGP so I thought instead of replying privately I’d reply here since it may be of general interest.  The questions are from Jacob.

What is QGP going to be used for in the future when it is better controlled?

Right now we don’t think the QGP has any practical applications.  We’re studying it because we want to understand the universe in general and nuclear matter in particular.  Shortly after the Big Bang, we think that the universe went through a Quark Gluon Plasma phase.  By understanding the QGP better, we may understand how the universe expanded better.  When we do basic research, we don’t usually know what impact it will have.  What we know by looking at history is that basic research eventually leads to benefits to humanity – but we’re very bad at predicting what those benefits will be.  When Mendel studied genetics of plants, he never imagined that genetic studies would lead to all of the improvements in medical care we have now.  Einstein developed his theory of gravity not so that we could send satellites into space or so that we could all have GPS in our cars or get better TV reception – he was motivated by simple curiosity and a desire to understand our universe better.  We are still reaping new benefits from quantum mechanics, developed in the early 20th century – we now have light emitting diodes (LEDs) in traffic lights and flashlights and while LEDs existed when I was your age, they weren’t nearly as widespread, as cheap, or available in so many colors.  So it takes a long time to see the benefits of basic research.

So we don’t know what applications this research will have in the future.  That said, there are a lot of spin off benefits to this research.  In high energy physics, we are always building the fastest and most precise detectors possible.  To do this we often have to develop and test new detector technologies.  Once we’ve developed the technology, these detectors can be used elsewhere too.  Particle detectors are used in hospitals in x-ray and MRI machines.  They are used in chemical and biomedical research to study the images of proteins and the structures of solids.  They are used in national security for detecting radioactive materials.

Basic research moves the boundary of what is possible.  Once we have done that, there are a lot of benefits.  But since we’re working on doing things that have never been done and studying things never studied before, we can’t predict exactly how it will be useful.  Put another way, if we knew what would happen, we wouldn’t call it an experiment.

What attributes does it have that other matter does not have?

This is a difficult question to answer as worded – it depends on what you mean by “attributes”.  When I think of the properties of a particular form of matter, I think about its density, its opacity to different probes (like if you shine light through it does the light come out the other side?)…  All forms of matter have a density.  So I’m going to answer a slightly different question – what makes a QGP unique?  What makes the QGP unique (among the forms of matter we’ve studied in the laboratory) is that the quarks and gluons interact through the strong force.  There are four fundamental forces in nature

1. Gravitation
2. Electromagnetism
3. Weak interaction
4. Strong interaction

The first two are the most familiar.  Gravity is the reason why you stay on the ground instead of floating through the air.  It’s also the reason the Earth orbits the Sun.  The electromagnetic force is ultimately responsible for basically every other force you feel or see.  When you sit in a chair, the reason you don’t fall through the chair is ultimately due to interactions between your atoms and the atoms of the chair.  It’s also behind light and electricity.  It’s how your microwave and your TV work.  The most familiar thing we can attribute to the electroweak decay is beta decay – a particular kind of decay of a nucleus.   The strong force is what holds nuclei together.  If we only had the electromagnetic force, the protons in the nucleus would not be bound.

So a QGP is a liquid of quarks and gluons bound together by the strong force.  Water molecules, for instance, primarily interact through the electromagnetic force.  The properties of water are determined by the way water molecules interact through the electromagnetic force.  To understand the QGP, we have to understand how quarks and gluons interact through the strong force.  This turns out to be a very difficult computational problem.  But by studying the QGP, we can try to calculate what we would expect and then compare what we expect from our theories to what we see in the laboratory.

In addition to that, it is the hottest, densest form of matter ever created in the laboratory.  And it appears to have the lowest viscosity of any form of matter ever created in the laboratory.  Viscosity is a way of measuring how much a fluid resists flowing.  Honey, for instance, is much more viscous than water.

How will QGP affect modern or future physics?

I don’t know exactly.  It depends on what we learn.  Already we’ve learned a lot about relativistic fluids – where the individual particles in the fluid are traveling close to the speed of light.  As I said in the first answer, we don’t know exactly what we’ll learn – because if we did, we wouldn’t call it an experiment. One thing I hope – and maybe you can help me out here – is that we’ll inspire the next generation to go into science, math and engineering.

Also, what state of matter is it?  I know that it is called plasma but I’ve also read that it is very similar to both liquid and gas.

A QGP is a new state of matter.  We believe it is a liquid – indeed, a liquid that probably has the lowest viscosity of anything we’ve ever measured.  We thought it’d be a gas, but it turned out to be a liquid.  Here I have a post describing what we know about the QGP and its phase diagram.

I also could not verify what temperature it occurs at because there is so much different information on the internet.

The reason what you find on the internet is somewhat unclear is that the answer is somewhat unclear.  First, it doesn’t exist at just one temperature.  Think about water.  Water can be cold, warm, hot, etc.  It depends.  There’s a temperature where ice melts and becomes water and below that you can’t have water.  That temperature is called the melting point.  But then once you have water, you can heat it up and you have to heat it up a lot before it boils and becomes a gas.  That also occurs at a special temperature – the boiling point.  The problem is, these temperatures depend on pressure and volume.  Water boils at a lower temperature at high altitude.  Analogously, we have a melting point and a boiling point for the QGP.  We think the melting point at the baryochemical potential at RHIC is about 170 MeV – but there’s a fairly large uncertainty in that number.  We think we’re well above that at RHIC and we’ll be even further above it at the LHC (but we haven’t yet had enough time to analyze the data at the LHC to say how hot it is). This gets to a crucial issue – we don’t have a thermometer to measure a QGP.  If you put a thermometer like the one you have in your house into a vat of QGP (if we could ever create that much of it) it’d melt.  So we have to come up with other ways of measuring the temperature.  We can look at the energies of particles created in the collision, for instance.  But it takes more work than just using a thermometer.

Many thanks to Jacob for the great questions!


Home again

Sunday, February 20th, 2011

I’m now back home after spending about a month at CERN. I was trying to think of how to describe a trip to CERN. They’re usually right at the board line between manageable and overwhelming. There’s always someone you run into whom you should speak to about something. Sometimes it’s a someone from another institute who’d benefit from your expertise on a given topic. Sometimes it’s someone who has expertise on a topic you’d benefit from. Sometimes it’s someone you’re working with on an analysis with a group of people and you need to figure out the best way to proceed. There’s always friends – and there is no sharp line between friends and coworkers. Many of my collaborators are also friends. People who aren’t collaborators now may be at some point in the future, and anyone can provide useful insight into physics. Sometimes you hang out with friends with completely different physics interests and you end up talking about some physics topic which is really useful to both of you. If you’re not based at CERN, you always need to get as much done as possible since you won’t be back for a while. There is something you can’t get done before you leave. There is always work to do and you have to prioritize.

There is also the physical environment at CERN. CERN sits in the valley between the Juras and the Alps. It’s beautiful. There is a a vinyard right next to the lab. Wine from the region is exceptional. And the lab has old buildings named by numbers alone, with numbers that have no relation whatsoever to either their location or their function. At best the buildings are boring. When I stay at the dorms, I walk about 100 feet to work and about 50 feet to get to the cafeteria. It is really convenient to just eat, sleep, and work.  Which is basically what I did the last month.

Trips to CERN are always highly productive and incredibly exhausting. During this trip I:

  • Completed the training required to work inside the magnet
  • Attended a three day meeting on the status of the electromagnetic calorimeter
  • Tested and repaired dozens of front end electronic cards
  • Worked on getting the newly installed electromagnetic supermodules installed and ready for data
  • Met with some visiting journalism students to discuss what we do in ALICE
  • Worked with collaborators on our data analysis
  • Attended ALICE meetings
  • Attended phone meetings for a pending paper
  • Attended group meetings over the phone

The last two sets of meetings were meetings in the US, so I would call in to them at 9 PM at night. Most Mondays, Tuesdays, and Wednesdays I had phone meetings at 8 or 9 PM, after a full day of work.

So I worked on many things on this trip and worked long hours every day. This is normal for trips to CERN – they are exhausting. And productive. Every day is different. No days are easy. There is nothing about this job that is routine. I spent time climbing around inside the ALICE magnet fixing electronics, worked on outreach to the public, fixed electronics, discussed our analysis method, worked on writing a paper and an analysis note… I learned, taught, listened to others give talks, gave talks… All trips to CERN are really busy, but this was a busier trip than usual. And really productive.

I am developing a love-hate relationship with the CERN cafeteria. It is a much better cafeteria than most cafeterias in the US – definitely better than either the Oak Ridge National Lab or the Brookhaven National Lab cafeterias, both of which I know far too well. But it is still cafeteria food. It’s really convenient so when I need to get a lot of work done it is really easy to just eat at the cafeteria rather than going out or trying to use the kitchens in the dorms to cook. Right now they’re remodeling the main kitchen in the dorm so it’s even tougher than usual to cook at CERN. The first meal I had at home was fajitas with lots of guacamole and hot salsa that is actually spicy.  Next up:  BBQ.

I want to get back to my morning runs, which aren’t so easy when I’m at CERN. The weather in Tennessee should be really nice for hiking soon and I missed the Smokies. I won’t miss 9 PM phone meetings. (I might have to call into some 8 AM phone meetings instead…) So it’s good to be home.



Thursday, February 3rd, 2011

The electromagnetic calorimeter is now fully installed but there’s still work to do before we start running.  We now have to make sure we’re able to read all of the data.  I’ve spent most of the last week in, on, and next to ALICE troubleshooting (along with several of my colleagues working on the calorimeter.)  Here I am sitting inside the magnet on top of  the support structure next to the front end electronics (the boards that read out the data) for the calorimeter.  I’m on the phone with someone upstairs who’s trying to take a pedestal run to see if we’ve fixed a problem reading out data from one of the new supermodules.  (A pedestal run is a run you take without proton-proton or lead-lead collisions to see what the background in your detector is.  It’s useful for troubleshooting because the detector has to send data.)

Now that we’re getting close to the start of the run, they’re putting the concrete shielding in.  In total 30 or 40 tons of concrete blocks sit above ALICE.  Here you can see one of the last blocks going in:

And just to go along with the preposition theme, here’s a picture under ALICE (in the magnet but under the TPC, TRD, and TOF):


Inside ALICE

Sunday, January 30th, 2011

I am currently at CERN to work on getting the electronics for the electomagnetic calorimeter working now that the rest of it is installed.  I got to see the ALICE detector in person for the first time on Thursday, which was very exciting.

This is a picture of me in front of the detector:

But that was part of a tour and to work in the detector I needed a lot of training.  I needed to take

  • Radiation safety training – there can always be residual radiation from things that have been activated by the beam and there may be radioactive sources in the area.  I have to recognize the appropriate placards and understand any dangers that may be present.
  • Working at heights training – the electromagnetic calorimeter is not at ground level and working on the electronics requires me to work well above ground.  I have to know how to use a harness properly.
  • Confined space training – the doors of the magnet are closed now so that they can start replacing the shielding around ALICE and I need to work inside the magnet.  This is a confined space.  There is a risk of oxygen deficiency – the amount of oxygen can drop rapidly and I have to to be aware of potential dangers and ready to respond.
  • Biocell training – The biocell is a small container of oxygen which I have to carry with me at all times in case the oxygen levels rapidly drop.  I have to be trained to use this properly because I may need to use it to save my life.

I also have to wear a dosimeter (which measures how much radiation I’ve been exposed to), a hardhat with a headlamp (in case the power goes out), and safety (steel-toed) shoes.  No shorts are allowed.  Inside the magnet there are high voltage sources, risks of falling, risks of falling objects, and detectors using flammable and/or toxic gases which could leak.  We are required to have at least two people working inside the magnet at a time – so that if someone gets hurt, the other person knows and can get help – and to have a 3rd person outside the magnet as a watcher keeping track of who is inside and where they are so that if anyone gets hurt or there is an emergency there is someone who can call the fire brigade and tell them how many people are inside and where they are.

I haven’t had the opportunity to take any pictures inside ALICE yet – and safety always has to come first so I may not be able to – but this is the hole we use to enter the magnet:

It is about 60 cm in diameter.  To get down to ALICE, you first have to go through this door:

(This is Soren Sorensen, my boss, coming down to see ALICE.) To go through this door, I have to scan my dosimeter on a card reader.  This says who I am and whether or not I have access to “the cavern” – the space underground where the detector is.  Then the outer doors open, I walk in, and I’m closed inside.  They scan one of my eyes and weigh me to make sure that I really am the person who owns the dosimeter.  Only then am I allowed in.  Inside there’s an elevator that takes us the 70m down to ALICE.  (It is easier to go down to see the cavern as a visitor than to work on the detector – one does not need training but must be supervised.)

This is why we tested as many components of the electromagnetic calorimeter  as possible before the EMCal was installed.  However, there will always be something which doesn’t work quite right and we want to fix it if we can.  It’s really exciting work, but we have to stay alert and stay safe.


Some tough problems to solve

Monday, January 17th, 2011

Given that today is Martin Luther King Day it seems like a good time to reflect on the under-representation of African Americans in physics.  This is a chart showing the number of African Americans who earned PhDs in physics in the US:

African Americans earn approximately 2% of PhDs in physics earned by US citizens in the US.  You can find more statistics on African Americans in physics here.  African Americans are about 12% of the US population.  Clearly something is wrong here.

I am not the right person to talk about what it is like to be a black physicist.  I can’t tell you what it feels like because I’m white.  Even though I’m not the ideal person to talk about this, someone has to say something because the problem is huge.  It is too big to ignore.  There are only 56 African American women with PhDs in physics.  I only know two black physicists in heavy ion physics, neither of them are originally from the US, and one is still a graduate student.  We have to do something because it threatens the viability of scientific research.  We are not recruiting the best and the brightest to the field.

This is a really complicated problem and I don’t have a solution.  Changing this – inspiring young black girls and boys to pursue careers in science and technology – will not be easy.  But we have to try.


The incredible flying electromagnetic calorimeter

Wednesday, January 5th, 2011

I introduced you to the ALICE electromagnetic calorimeter (EMCal) a while ago, and told you about some additional training I had so that I could work on the detector after the EMCal is physically installed.  Over the winter shut down – right now – the EMCal is being installed inside ALICE.

There are several steps in this process.  First the EMCal was assembled, partially calibrated, and tested – this was done in November.  There were several stages of testing.   We tested that each individual cell works.  We tested that each individual electronic card for reading out the data works.  We assembled everything exactly how it would be installed inside ALICE and tested it again, making sure that all of the parts (including the wires) worked together.  We partially calibrated the detector by taking data on cosmic rays.  We’ve had all of the six supermodules we’re adding waiting at CERN until we could get access to ALICE to install them.

Now they’re physically installing the supermodules and our amateur EMCal documentarian, Federico, has taken some videos of the process.  (It might help to go back to this post, where I introduce each of the detectors and explain what they do, and this post, where I show you some pictures of each of the detectors.  Then maybe you can identify the different parts of ALICE in the video.)  Note the action in the videos is very slow because it’s very important not to damage anything while installing the detector.

The first step is to put the supermodule in the EMCal insertion tool.  This is a specialized device for installing EMCal supermodules.  Here you can see a video of that step:

[youtube 75Olhr4YoUw&NR]

And then once the supermodule is in the insertion tool, it gets installed in ALICE:

[youtube 0es9Qcdj-H8]

And now some gratuitous cool pictures of the process:

ALICE ready for the installation of EMCal supermodules

Looking up from the cavern.  We sit at the top when we take data and the detector is far below us.  The EMCal supermodules have to be lowered down.

Checking everything twice to make sure there are no mistakes.

Getting the EMCal insertion tool ready for a supermodule

Getting ready to strap the support onto the supermodule

Sliding the support onto the supermodule

Now it’s in and they’re strapping it onto the crane

Up goes the supermodule…

…and into the insertion tool.

And it’s rotated to the correct angle…

Waiting on the support structure in ALICE to make sure it goes in properly…

Get it in the right position…

Now loosen it from the EMCal tool and in it goes!

And now we do the next one.

Many thanks to Federico for the great pics!


Merry Quark Mass and Happy Glue Year!

Thursday, December 30th, 2010

It’s been a busy year at the LHC and in heavy ion physics.

In ALICE we’ve gotten out nine papers on data, including:

The links above are to explanations of the papers.  I haven’t quite gotten to writing a post on Bose-Einstein Correlations, but they are another way of measuring the size of the proton.  The spectra of charged hadrons is basically a measurement of how many particles are created in a collision and how fast are they going.  It’s been a good year for ALICE!  And a good year to be a heavy ion physicist.  ATLAS observed jet quenching and CMS observed the ridge, a feature previously only observed in heavy ion collisions, in proton-proton collisions.

And all of our hard work hasn’t gone unnoticed – The Onion, while declaring Snooki to be one of the most important people of 2010, said that those of us working on the LHC would be more deserving.   And my favorite radio show Wait Wait Don’t Tell Me even mentioned the first lead-lead collisions in one of their Listener Limerick challenges.  (It’s the second time my field came up on the show – the first was when I was a contestant.)

So Merry Quark Mass and Happy Glue Year!


Jet quenching

Monday, December 13th, 2010

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.