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Posts Tagged ‘ALICE’

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.


Working at heights

Thursday, December 2nd, 2010

I spent Tuesday at working at heights training.  Here you can see me in the ALICE hall on the crane track:

And here’s another picture of me hanging off the electromagnetic calorimeter insertion tool, the device used to physically install the electromagnetic calorimeter:

I need this training for when we install the rest of the electromagnetic calorimeter supermodules.  I won’t actually be working on physically installing the supermodules, but I’ll be working on installing the electronics and the cables to read out the data.  This means I may have to spend some time working where our detector is, well above the ground.  So on Tuesday I learned the proper way to use a harness, the proper way to climb a (very tall) ladder, how to secure ourselves to solid structures, and all of the rules and laws that apply at CERN.  And then we had practical exercises.  Yes this is supposed to be work, but it’s also fun!


First heavy ion papers!

Thursday, November 18th, 2010

ALICE’s first two papers on lead-lead collisions were submitted yesterday, about a week and a half after the first lead-lead collisions.

One paper is a measurement of the charged particle multiplicity.  This analogous to the multiplicity measurements in proton-proton collisions, except a more particles are produced in a lead-lead collision.  In p+p the models were off by around 10-15%.  This is a plot from the lead-lead paper:

The red point shows the ALICE measurement.  The x-axis is a measure of the number of particles produced in the collision.  (Specifically it is the number of charged particles produced in the collision per unit pseudorapidity for pseudorapidities from -0.5 to 0.5.)  The black points are different predictions.  Notice is that the predictions vary from 1000-2000 particles.  This is a rather large theoretical uncertainty, especially compared to proton-proton collisions.  So less than two weeks after our first collisions, we have already gained a much deeper understanding of lead-lead collisions.

The other new paper is a bit more abstract than the number of particles created in the collision.  Think of a heavy ion collision as like slamming two ice cubes at each other.  If you slammed two ice cubes together fast enough, they’d melt when they hit each other.  If you did this in space – where it’s about 3K (about -270 Celcius and -454 Fahrenheit) – the water would immediately freeze again.  This is roughly what happens in a lead-lead collision.  Nuclei are basically frozen quarks and gluons, and when we collide them fast enough, they melt.  But by the time we see the remnants of the collision in our detector, the quarks and gluons have frozen again.  However, they went through a phase where they were a liquid.  A liquid can flow.  We can see evidence that the liquid of quarks and gluons was flowing because we can see in our detector that the particles are all moving in a preferred direction.

This plot compares ALICE’s measurement to earlier measurements:

The x-axis is the collision energy per nucleon (proton or neutron) in the center of mass.  The y-axis is a measure of how much the liquid is flowing.  [Technical audience:  The y-axis is the coefficient of the second term of the Fourier decomposition of the distribution of particles in azimuth with respect to the reaction plane.]  Measuring how much the Quark Gluon Plasma is flowing gives us some insight into its viscosity.  There are lots of technical details and subtleties in interpreting these data that I’m skipping over.  But already – less than two weeks after the first lead-lead collisions – we have two measurements that give us deep insight into the properties of the Quark Gluon Plasma.


Live event displays

Wednesday, November 10th, 2010

You can see live event displays from ALICE here.  Perhaps you can identify the different detectors in the event display from the description of the ALICE detector.  And here’s an event display with our Time Projection Chamber in:

What you’re looking at is the charged particles we’ve seen in ALICE.  I’d tell you roughly how many we see except that I’m afraid that some theorists in our field would mistake that for a measurement.  (You know who you are!)

There’s more information about ALICE and more event displays here.

(ATLAS and CMS also have event displays.)


Very early this morning we got the first lead-lead collisions at the LHC!  I am all a twitter.  This is a very exciting time.  I just arrived at CERN today and I am very, very jet-lagged, so I’ll keep this short.

Pictures.  What you all want to see is pictures.

Here are some event displays with the first Pb+Pb collisions seen by ALICE.  This is an example:

These event displays only show information with the Inner Tracking System (ITS).  Our main tracking detector, the Time Projection Chamber (TPC), was off for these collisions.  The reason is that the beams were not perfectly stable for the first collisions and we did not want to damage our TPC.

And check out this video of an event display (the original video is here):

And now that we have lead-lead data, we have a lot of work to do.  Expect the first lead-lead paper soon.  It will be a multiplicity paper like ALICE’s first few proton-proton papers.  We will just measure the number of charged particles in an event.  This information alone will tell us a lot about heavy ion collisions – the first estimates for how many particles we should see in an event varied by a factor of 4, from 2000-8000 tracks.


As I was writing a talk for a conference, I ran into a bunch of pictures of ALICE that I thought y’all might like to see.  In my last post I gave an overview of ALICE and described each of the subsystems in ALICE.  I don’t have a picture of every subsystem, but to give you an idea of what these pieces look like:

This is the ALICE magnet (which used to be the L3 magnet) in 2001, before any of the detectors were installed inside of it.

This is the hole inside the TPC where the ITS sits now

This is the whole ITS when it was being installed

Here you can see the PMD team in front of the PMD

This shows the PMD after it was installed in ALICE

Here you can see piece of the TRD right before it was installed

This is the HMPID right before it was installed

This is one of the trays of the TOF

And here you can see one of the EMCal supermodels right before it was installed

And this is the ALICE collaboration in front of ALICE

(Note to ALICE collaborators – if you didn’t see a picture of your favorite subsystem, email it to me.  If I get enough cool ALICE pictures for another post, I’ll do a follow up with more gratuitous cool pictures of ALICE.)


Tasting quark soup

Wednesday, October 27th, 2010

A lot of the articles in the news talk about the physics we’re trying to learn, but there’s not much discussion of how we do it.  I spend most of my day thinking about our detectors, how they work, and how to interpret the data.  Our detectors are the way we see, hear, feel, smell, and taste what goes on in proton-proton and lead-lead collisions.  Just like the Mars rover explores places that are uninhabitable to people, our detectors explore places that are too small for us.

ALICE is made up of several subsystems, each of which helps us sense collisions in a different way.  Below is a picture of ALICE with each subsystem labeled:

Some detectors are like our eyes – they help us see particles coming out of the collision.  These are our tracking detectors.  The main tracking detector is the Time Projection Chamber (TPC).  We also have an Inner Tracking System (ITS), comprising three different silicon detectors, the Silicon Strip Detector, the Silicon Drift Detector, and the Silicon Pixel Detector.  The ITS is a bit like our glasses – we can see particles with just the TPC, but with the ITS, the picture comes into sharper focus.  Tracking detectors tell us the momentum and spatial location of particles that go through our detector.

Some detectors are our taste buds – they help us determine the flavor of the particles we’re measuring.  A lot of different types of particles are produced in both a proton-proton collision and a lead-lead collision.  The Time-Of-Flight (TOF), the Transition Radiation Detector (TRD), and the High Momentum Particle Identification Detector (HMPID) are all designed to identify particles.  These detectors all work by measuring a particle’s velocity.  Momentum is velocity times mass, so if we know the velocity and the momentum of a particle (which we can get from the tracking detectors), we can determine its mass and therefore figure out what kind of particle it is.  The TOF measures the velocity of particles by measuring how long it takes for a particle to reach the TOF.  Since velocity is the change in distance over time and the distance traveled is known, this measures the velocity of the particle.  Here you can see one of our physics performance plots showing different particles identified by the TOF:

The x-axis is the momentum, as measured by the TPC and the ITS, and the y-axis is the ratio of the velocity to the speed of light in a vacuum.  Pions (π) are the lightest particle (140 MeV/c2) so at a given momentum, they have the highest velocity.  Protons (p) are the heaviest (938 MeV/c2) particle visible in the plot above so at a given momentum they have the lowest velocity.

The HMPID and the TRD both work on the same principle.  The speed of light in a vacuum is constant, but the speed of light in a medium can be lower.  For example, the speed of light is lower in water than in air – this is why images get distorted when you look through water.  If a fast particle moves through a medium faster than the speed of light in that medium, it will emit photons – called Cherenkov radiation – until it slows down to the speed of light in the medium.  At a given momentum, lighter particles go faster, so lighter particles will emit photons at a larger angle relative to their path.  The medium in the TRD is optimized so that only electrons (0.5 MeV/c2) will radiate photons, so the TRD can be used to distinguish electrons from everything else.  The HMPID is a ring imaging Cherenkov detector.  The photons emitted by a particle are emitted in a cone and the radius of that cone depends on the velocity of the particle.  The HMPID is optimized for distinguishing pions, kaons, and protons.  Here you can see the signal from the HMPID:

The x-axis is the momentum and the y-axis is the angle of the cone of light emitted by the particle.  At a given momentum, a pion is going faster than a kaon or a proton.  The radius of the cone of light emitted by the particle is larger the further the particle’s speed is from the speed of light in the medium, so at a given momentum the pion band is above the kaon band, which is above the proton band.

The tracking detectors, the TPC and the ITS, can also identify particles.  They work by measuring how much energy a particle loses as it travels through the detector.  A heavier particle will loss more energy than a light particle.  Think of one of those ball pits for kids.  If you threw a tennis ball in, it would knock some of the balls out of the way.  If you threw in a bowling ball, a lot of balls would get knocked around.  We know the bowling ball lost more energy than the tennis ball because the lighter balls got knocked around more.  We can distinguish between heavier particles and lighter particles like this.  If the TOF, the HMPID, and the TRD are the way we taste the particles created in the collision, the ITS and the TPC help us smell them.  Below you can see the signal from the TPC:

The x-axis is rigidity, which is the momentum over the charge.  Charge is in units of the electron charge.  All of the particles here have a charge of ±1.  Positively charged particles are on the right and negatively charged particles are on the left.  The y-axis is proportional to the energy lost by the particle in the TPC.  We see the same three particles we saw before – pions, kaons, and protons – but now we also see deuterons and tritons.  At a given momentum, heavier particles lose more energy, so as you go up the y-axis the mass of the particles increases.

My last post was on the Electromagnetic Calorimeter (EMCal).  A calorimeter is used to measure particles’ energy.  This is a way of feeling the collision – it’s like laying in the sun.  When you lay in the sun, you don’t feel photons hitting you but when photons hit you, they warm you up.  Particles hitting the calorimeter do the same thing – they hit the calorimeter and deposit their energy.  (Everything loses energy except muons – muons travel right through the calorimeters.)   We look at the energy deposited in the calorimeter to determine how much energy the particle had.  (See my post on the electromagnetic calorimeter for more details.)  We have two more calorimeters in ALICE,  the Photon Spectrometer (PHOS) and the Zero Degree Calorimeter (ZDC).  The PHOS is optimized to measure photons.  The ZDC is a calorimeter very close to the beam pipe far away from the interaction point, at an angle close to zero degrees from the beam pipe.  The ZDC is useful in lead-lead collisions for both measuring nucleons which did not participate in the collision.  These particles are called “spectators” and are not deflected by the magnetics that keep the beam in the beam pipe because the spectators do not have the same charge to mass ratio as lead nuclei.  We can figure out of the collision was head-on or just glancing using this information.

We hear the collision in the VZERO, a scintillator detector.  When a particle hits it, the scintillator emits photons and we know there was a collision when we see these photons.  Think of it as like a fire alarm – it’s what tells us there was a collision.

There’s a few detectors that don’t really fit into this metaphor but I want to mention them anyways.  The Photon Multiplicity Detector (PMD) measures the multiplicity of photons at angles close to the beam pipe.  The muon arm measures muons, the heavy cousin of the electron.  The ALICE Cosmic Ray Detector (ACORDE) is designed to trigger on cosmic rays so that the rest of ALICE can be used to study cosmic rays.  Cosmic rays were used to calibrate ALICE before the first collisions at the LHC.

Each of these detectors helps us understand proton-proton and lead-lead collisions in a different way.  When we put them all together, we have a sort of Quantum Chromodynamics rover that helps us explore exotic places – the insides of protons and nuclei – that are near us all the time.


ALICE is made up of several detectors, each of them designed for a different purpose.  Here you can see a drawing of ALICE with the different detectors labeled:

In this post I introduced you to the Time Projection Chamber (TPC).  The US is mostly involved in the electromagnetic calorimeter (EMCal), which is positioned outside of the TPC (labeled “EMCAL”).  I work on the EMCal.

At some point in a chemistry or physics lab you probably did a lab with a calorimeter.  The standard calorimeter experiment involves heating something up, submerging it in cool water, and measuring the change in the temperature of the water in order to determine the specific heat of the object.  A calorimeter measures the energy deposited by an object.

In high energy physics calorimeters are used to measure the energy of particles.   When a high energy particle hits our detector, it initiates an electromagnetic shower, emitting electron/positron pairs and photons.  As the shower propagates through the calorimeter, it deposits more and more of its energy, until it either stops or comes out the other side of the calorimeter.   An electronic calorimeter is most sensitive to photons and electrons.

Of course sometimes a high energy particle will hit one of the nuclei of one of the atoms in the calorimeter and deposit some of its energy this way.  Hadrons – such as protons and pions – are more likely to do this than electrons because they interact strongly (through the strong force), unlike electrons and photons.  This is how a hadronic calorimeter works.  This would not happen in an ideal electromagnetic calorimeter, but no detector is ideal so some hadrons will leave a shower in our detector.

The TPC is used for measuring the tracks left by charged particles and can measure their momentum.  The EMCal can measure particles’ energy.  We can use the EMCal to measure photons, which the TPC cannot see.  We can also use the EMCal to distinguish electrons from hadrons because electrons will leave most of their energy in the calorimeter while hadrons will not.  The EMCal can be used to measure many neutral particles, whereas the TPC can only see charged particles.  By combining the information from both of these detectors, we can characterize events better.

A sampling calorimeter is made up of layers of something which produces the shower and something which can collect the energy from the shower.  In the ALICE EMCal we have 77 alternating layers of lead and scintillators.  A high energy particle produces an electromagnetic shower in the lead.  The photons released in the shower are collected by the layers of scintillators.  The light from the scintillators is collected by fiber optic cables, converted into an electrical signal, and read out as digital signals.

The ALICE EMCal is made up of 12,288 towers, each of which is about 6 cm2.  It covers about 107° around the beam pipe.  This is what it looks like:

Each green square is one tower.  For scale, in this post our piece of the EMCAL is 4×4 towers.  We have to read out data from the EMCal towers.  This takes a lot of cables and what we call Front End Electronics – electronic boards which manage the data until we record it.  All of these pieces have to be carefully tested before they’re installed.  Here you can see me testing one of the Front End Electronics boards at CERN:

Right now only about 36% of the EMCal is installed.  We got great news recently – we’ll be able to install the rest of it in January!  This is wonderful – but it means we have a lot of work to do.  I’ll be spending November at CERN testing parts and helping prepare the EMCal for installation.  Luckily there are plenty of places to get a Thanksgiving turkey in Geneva!


ALICE has just submitted its fourth paper, on the anti-proton to proton ratio in p+p collisions, to Physical Review Letters.  This is a really cool measurement because it is one way of quantifying how many of the particles we create in our collisions – as opposed to how many of the particles we see are remnants of the beam.

A proton has three valence quarks, two up quarks and one down quark.  The proton’s electric charge is +1.  An anti-proton has three valence anti-quarks, two anti-up quarks and one anti-down quark.  The anti-proton’s electric charge is -1.  The anti-proton is the proton’s anti-particle.  When a proton and an anti-proton come together, they annihilate.

A baryon has three valence quarks –  examples are protons (two up quarks and a down quark) and neutrons (two down quarks and an up quark).  There are many more exotic baryons – my favorites are the Λ (an up quark, a down quark, and a strange quark) and the Ω (three strange quarks) . A proton is a baryon, while an anti-proton is an anti-baryon.  Baryon number is the net number of baryons in a system and it is conserved in all processes we have observed in the laboratory.  In our p+p collisions, the baryon number is 2 because there are two incoming baryons.  Because the anti-proton is an anti-baryon, it had to be created in the collision.  Moreover, because there were no (net) anti-quarks in our incoming protons, all three anti-quarks in any anti-proton we see had to be created in the collision.  If we just look at protons, we can’t tell if they were created in the collision or if they are remnants of the beam.

Since anti-protons don’t exist prior to collision, one way of quantifying how many particles were created in the collisions, as opposed to how many are beam remnants, is their ratio.  If this is near zero, most of the particles we observe are remnants of the beam.  If this is near one, most of the particles we see were created in the collision.  At low energies, the anti-proton to proton ratio is closer to zero, but we expect it to be almost one at LHC energies.  Here you can see the collision energy dependence of the anti-proton to proton ratio (Figure 4 of the new paper):

The y-axis is the anti-proton to proton ratio.  The upper x-axis is the center-of-mass energy of the collision.  The different data points are measurements from different experiments.  The line shows a fit to the data.  The lower y-axis is a little more complicated – I’ve put an explanation below, but you can skip it and just look at the top x-axis.  You can see that the anti-proton to proton ratio is very close to one at LHC energies.  But of course, we have to quantify how close the anti-proton to proton ratio is to one.  Specifically, we measured it to be 0.957 ± 0.006(statistical) ± 0.014(systematic) at 0.9 TeV and 0.991 ± 0.005(statistical) ± 0.014(systematic) at 7 TeV.  Most of the work went into determining the uncertainty.  We could reduce the statistical uncertainty by just taking more data, but the systematic uncertainty is limited by the method and the experiment.

What do we learn from this measurement?  It helps us test and refine our understanding of baryon production in proton-proton collisions.  We can compare to models for proton and anti-proton production and this lets us constrain some models and exclude others.

To give a feel for how complicated it can be to do the measurement, I’ll explain one of the details that has to be considered to do this measurement right.  If we see an anti-proton, we’re pretty sure it was really created in the collision.  But we have billions and billions of protons in our detector.  A very fast particle created in the collision could knock a proton out of our detector.  If we measure a proton, how can we be sure that it didn’t come from our detector?  We have accurate enough charged particle tracking to see where the proton came from.  This figure (Figure 2 from the paper)

shows the distribution of the distance of closest approach (dca) of protons and anti-protons to the collision vertex.  Real protons and anti-protons created in the collision will mostly be close to the collision point (near a dca of 0), so this shows up as a peak around a dca of 0.  Our largest background is from protons knocked out of the beam pipe by a fast particle created in the collision.  These protons don’t get close to the collision vertex – their dca is larger.  This is why the proton peak on the left sits on top of a plateau.  But we can’t knock anti-protons out of the beam pipe – so we don’t see the same plateau under the anti-proton peak.  Protons knocked out of the beam pipe will also be slower on average than protons created in the collision.  This is why we see the plateau from protons knocked out of the beam pipe on the left (for protons with momentum p≈0.5 GeV/c) but we don’t see it on the right (for protons with roughly twice the momentum, p≈1.0 GeV/c).  To get an accurate anti-proton to proton ratio, we have to subtract off the protons knocked out of the beam pipe.  We can tell where particles travelling practically at the speed of light went to within a few mm – and we need to in order to do our measurements.

Isn’t that cool?  ALICE is a wonderful detector!

Explanation of the lower x-axis of the anti-proton to proton ratio plot:

This is the difference between the beam rapidity, y, and the rapidity where the measurement is done (|y|<0.5).  You can calculate the beam rapidity using

y = 1/2 ln((E+pz)/(E-pz))

where pz is the momentum along the beam axis and E=√(E2+m2) is the total energy.  If you plug in the numbers, you’ll see that the beam rapidity is about 7.6 for 900 GeV and about 9.6 for 7 TeV.  I have fudged over a detail, which is that it matters where we do the measurement.  If we look closer to the beam axis, we’ll see a lower anti-proton to proton ratio and we’ll get the highest anti-proton to proton ratio at rapidities close to zero (roughly perpendicular to the beam axis).