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Christine Nattrass | USLHC | USA

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Perks of the job

Friday, September 17th, 2010

Life as a high energy physicist is not without its perks.  I recently got back from my latest trip to CERN for the EMCal test beam.  I spent about a week on the midnight to 8 AM shift and then stayed a week to work with some of my collaborators in ALICE.  The hours are long and the work is hard but the company is good and there are many perks.

I’m an avid hiker so I took a day off to go hiking in the Juras.  My friend Daniel organized it and we ended up with a group of two physicists from ALICE, one from CMS, one from ATLAS, and one from a university in France.  We had one American, one Brit, one Spaniard, and two Mexicans.  A multicultural group in many ways.  Here you can see the view from the Juras:

Somewhere down there is CMS.  It was a nice hike but next time I’ll pack my good compass and get my own trail map.  We had some unintentional adventures.

After my trip to CERN I went to a conference in Sicily – which means I had to work on a talk while I was at CERN.  Of course Sicily is beautiful:

(This is the view from Taormina during the excursion.)

Then I packed up and left, first to Geneva and then back to the US.  Five flights and four countries in two days.  My luggage made it through Paris to Atlanta but then decided to take a vacation in Atlanta without me.  I’m now looking at a grueling travel schedule in the next four months.  Plans have changed and our detector, the electromagnetic calorimeter, is going in during the Christmas shutdown.  This is great news but it also defines my holiday schedule – November and part of January at CERN.  On top of that I have a few meetings and some personal travel.  I’ll be lucky if I manage to be home for two weeks in December.  However, I did not get much sympathy from my father the other day when I was complaining about how I might have to get extra pages in my passport because I’m running out of space.  Go figure.


CERN at night

Wednesday, August 25th, 2010

It’s 5:38 AM.  Do you know where we physicists are?

Right now I’m on a test beam shift for the ALICE electromagnetic calorimeter (EMCal).  The test beam delivers particles at a fixed momentum – right now a mixture of 60% electrons and 40% pions at 10 GeV/c.  We have a miniature version of our EMCal, 64 towers (8×8) complete with read out electronics.  It’s positioned in front of the beam line so that we can measure the response of the EMCal to these particles.  We move the beam around on the detector so that we can see the response of each tower to the beam.  We also try different momenta.

We have about a week to use the test beam and we want to make the most of our time, so we take shifts around the clock.  This is where I am right now:

The building to the right – the barracks – is where we sit when we take data.  Our little detector is to the left, behind the large cement blocks.  The cement blocks are there to shield people in the hall from radiation from the beam.  The beam comes from the far end of the hall.  The cables take data from our detector to the barracks.

And we are not alone – there are several other groups using data from the test beam and doing other experiments right now.  The lab that never sleeps.  Our test beam comes from the Super Proton Synchrotron – once the highest energy accelerator in the world and now both the injection source for the LHC and the beam source for multiple ongoing experiments.


Off to Geneva

Friday, August 20th, 2010

I’m off to Geneva for a couple of weeks. While I’m there I’ll work on the test beam for the ALICE electromagnetic calorimeter. I’ll tell you more about that in the next posts. But I thought I’d share with you the contents of my long trip survival kit:

A travel pillow, a bandana (which serves both as an eye mask and a lazy hair style), an outlet adapter, a netbook and mini-optical mouse, ear plugs, an mp3 player with a 30 hour battery, a hair brush and extra hair bands, two change purses (one for Euros, the other for Swiss Francs) and little mini-toothbrushes with toothpaste already on them.  I don’t deal with sleep deprivation very well so these flights are never very fun – but they’re easier to take than flights between the US and Asia.


A TeV, measured in chocolate and coffee

Saturday, August 14th, 2010

We toss around the term “TeV” – a teraelectron volt, 1012 electron volts (eV). But how much energy is it really?

An electron volt is the energy an electron gains when it is accelerated through a potential difference of one volt. An electron volt is defined as a unit of energy. (Various prefixes are defined here.)

Let’s put this in terms we can all understand. A Lindt 70% cocoa chocolate bar has 194 Calories. To convert this to electron volts:

194 Calories × 1000 calories/Calorie ×4.2J/calorie / 1.6 × 10-19 J/eV = 5 × 1024eV

(Note that the dietary unit, a Calorie, is 1000 calories, the amount of energy needed to raise the temperature of one mL of water by one degree Celsius.) So it would take a hundred billion (1011) proton-proton collisions at top energy (14 TeV in the center of mass) to get the same amount of energy as in a chocolate bar.

The difference is how much space we pack that energy into. A proton has a volume of roughly 1 fm3, or about 10-39 cm3. A Lindt chocolate bar is about 10 cm x 1/2 cm x 20 cm = 100 cm3. A chocolate bar then has an energy density of about 194 Cal/100 cm3, or around 2 Cal/cm3. A proton-proton collision at 14 TeV has an energy density around 14 x 1012 eV/10-39 cm3 x 1.6 × 10-19 J/eV *1000 Calorie/4.2J = 5 x 1035 Cal/cm3. So our proton-proton collisions have an energy density about 1035 times a chocolate bar.

We also use an electron volt as a unit of temperature. An atom in a monatomic (helium, argon, etc.) ideal gas has a kinetic energy of 3/2kBT where kB is the Boltzmann constant. The factor in front (3/2) is different for different systems. For instance, it’s 5/2 for a diatomic gas, such as hydrogen (H2), oxygen (O2), or nitrogen (N2). But the energy is usually kBT times some factor between 1-10. So to convert an electron volt into a unit of temperature, we use eV=kBT and T=eV/kB=11604 Kelvin.

So how hot is a cup of coffee in electron volts? When I worked at a coffee shop in high school, we made our cappuccinos and lattes at 160°F (71°C). This works out to be 344K, or 0.03 eV. So a proton moving at 7 TeV is about 100,000,000,000,000 (1014) times more energetic than the average molecule in a cup of coffee.

The Quark Gluon Plasma created at the Relativistic Heavy Ion Collider is at a temperature of about 170 MeV. (Note this is the temperature of the medium produced, not the energy of the incoming beam.) The fluid we’ll create at the LHC will be hotter – over ten billion (1010) times hotter than a cup of coffee.

These collisions are hot stuff!

[Note these are all what we call “back-of-the-envelope” calculations. The goal is to figure out the right order of magnitude for various quantities, not to do a detailed, precise calculation.]


Tour a particle collider

Tuesday, August 3rd, 2010

This weekend I’ll be headed up to Long Island, where I’ll be one of the volunteers for the Brookhaven National Laboratory Summer Sundays public tours of the Relativistic Heavy Ion Collider.  It’s free and no reservations are required.  Details are available here.  I’d recommend it to anyone interested in particle accelerators.

The Relativistic Heavy Ion Collider (RHIC) is a little over a kilometer in diameter.  By comparison, the LHC is about 8.5 kilometers in diameter.  The top center of mass energy at RHIC is 500 GeV for proton-proton collisions and 200 GeV for heavy ion collisions, about 1/28th of the top LHC energies.  While the LHC can collide protons at the top energy in the world, RHIC is the only machine that can collide polarized protons.  Currently RHIC can collide heavy ions at the highest energy in the world – until this fall, when we expect our first heavy ion collisions at the LHC.  RHIC can produce collisions at center of mass energies as low as 7 GeV.  Additionally, RHIC can collide deuterons with gold.  With RHIC and the LHC combined, we can study different regions of the phase diagram of nuclear matter.

There are two main experiments still taking data at RHIC, STAR and PHENIX.  (I was on STAR as a PhD student; I am now a member of PHENIX.)  During the tours, you’ll be able to see part of the collider tunnel and both the STAR and PHENIX experiments.  You’ll be guided by physicists working on the collider and on STAR and PHENIX.  (I will be giving tours of the PHENIX experiment.)

If you’ve never seen an accelerator or a particle physics experiment and you’re in the area, I’d strongly recommend you make the trip out to Long Island.  Hope to see you on Sunday!


The size of the proton

Monday, July 12th, 2010

There is a new measurement of the size of the proton and it turns out that protons are smaller than we thought they were.

At some point in your education you probably got introduced to the Bohr model of the atom.  The nucleus is made up of protons and neutrons, and electrons orbit around the nucleus.  In the Bohr model, electrons orbit the nucleus in circular orbits like the Earth orbits the Sun, but these orbits are only allowed to have some radii (which correspond to an integer number of de Broglie wave lengths).  Electrons can transition between these levels and when they do, they either absorb a photon (in the case of an electron being excited from, say, the ground state to an excited state) or emit a photon (in the case of an electron going from an excited state to a lower state.)  This is shown below:

The Bohr model isn’t exactly right – but it’s close enough to get some feel for what’s going on.  In a more precise quantum mechanical picture, the electron isn’t actually orbiting the nucleus – it’s smeared out in what we call a wave function.  The square of the wave function tells us how likely we are to find the electron in a given place.  The ground state orbital (the shape of the wave function of the electron in the atom) is spherical.  The lowest excited state has four different possible orbitals, one spherical (S) and three which are shaped like a dumbbell (P), a sort of 3D figure-8.

What you probably learned in school was that these S and P orbitals have exactly the same energy – and they almost do.  In a simple model, the nucleus is just a point particle – meaning it exists just at a single point, with no size in any dimension.  But protons aren’t point particles – they’re just very small.  In the S orbitals, the electron spends most of its time near the nucleus, but in the P orbitals, the electron spends less of its time near the nucleus.  This difference in how much time the electron spends near the nucleus leads to a very small shift in the energy of the orbitals, called the Lamb shift.  The Lamb shift is measured by measuring the photon emitted when an electron goes from the P to the S orbital in the second shell.  It depends on the mass of the electron and the size of the proton.  (Here’s the explanation of the Lamb shift on the experiment’s web site.)

In this new measurement, they looked at hydrogen with a muon (the heavier cousin of the electron) instead of an electron.  Because the muon is about two hundred times heavier than the electron, it spends more time near the nucleus than the electron, meaning it’s more sensitive to the Lamb shift than the electron.  Previously, the best measurement of the diameter of the proton was 0.877±0.007 femtometers (m) and this measurement measured it to be 0.8418±0.0007 fm.  A femtometer is 10-15 meters.  If you were a proton (you’re somewhere between 1-2m tall), this would mean traveling one millimeter would be like traveling from the Earth to the Sun (1011 m).  This measurement would be like finding out that you’re 5’5″ instead of 5’8″ by looking at how long it takes for you to walk between Milwaukee, WI and Chicago, IL (150 km) and Milwaukee, WI and Madison, WI (141 km)*.

The fact that this measurement is so far off from our expectations indicates one of the following:

  • The precise calculation we’re comparing to is flawed.The proton is actually a really complicated object – perhaps we forgot an important component.
  • The measurement has some flaw we haven’t figured out yet.  Maybe there was some systematic shift that wasn’t taken into account.
  • Our theory is flawed.  This could indicate some physics beyond the Standard Model – exactly what we’re looking for at the LHC.

We have to seriously consider the first two options, but the third would obviously be very exciting.

So why I am writing about this here?  First, it illustrates that there are other ways of studying fundamental particle physics than by slamming things together.  Second, it’s an interesting result that may hint at exciting new physics we’re hoping to see at the LHC.  Third, it’s a great segue into my next post…

*Yes, this analogy breaks down at some point.  Don’t take it too far.


Anti-proton to proton ratio – ALICE’s 4th paper submitted!

Tuesday, June 29th, 2010

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).


Conference season

Wednesday, June 23rd, 2010

It’s that time of year again – conference season.  There are dozens of conferences and meetings at the beginning of the summer, when professors and grad students have a break from teaching responsibilities so can handle extensive travel to multiple meetings.  Just scanning the list of talks I see posted on the ALICE web page I see at least 15 conferences in June, July, and August.  Almost every in the field does at least a little travel over the summer.  I am still on the road from a summer school on jet physics at Lawrence Berkeley National Lab, giving a seminar at UC Davis, and teaching some undergrads at Cal Poly about ALICE software.  (This is why I haven’t posted in a while.)  I’ve tried to minimize my work travel this summer but I’ve already spent two weeks in California and may spend another 2-3 weeks at CERN at the end of the summer.  This conference season will be particularly exciting because of all of the new results from the LHC.  ALICE has several papers in the pipeline and several new results.  I’ll try to highlight these when I get back.


The cost of a PhD

Thursday, June 3rd, 2010

It costs a lot of money to produce a PhD scientist.  A rough estimate, based on my education:

  • Primary and Secondary education:  For simplification, let’s say I spent all of my k-12 years in Colorado.  Colorado ranks roughly 42nd in per-pupil spending, but it still costs $8,600/pupil/year for k-12 education.  Therefore, my high school diploma cost roughly $112,000.
  • Bachelor’s degree:  I went to Colorado State University for my undergraduate degree – a large state university.  Colorado State was a great bargain and when I started there, in-state tuition was roughly $2000/year.   Most of this was covered by scholarships, so was actually paid by some branch of government.  However, CSU spent roughly $20,000/student on undergraduates, with the difference made up from the general fund.  I spent five years in undergrad, so just the tuition for my degree was worth roughly $100,000.  Fort Collins, CO was pretty cheap to live and I was an overwhelming cheapskate.  My cost of living averaged about $10,000/year, adding another roughly $50,000.  Additionally, I participated in four summer undergraduate research programs.  One program was at CSU and my participation (salary and other expenses, excluding the salaries of my supervisors) cost roughly $4000.  One program was at UNC Chapel Hill and I got paid $3,000 plus room and board and transportation to Chapel Hill, so this cost roughly $5,000.  One program was in the Netherlands for five months and this probably cost roughly $10,000.  One program was in Switzerland for two and a half months and this probably cost roughly $10,000.  So the cost of my supplementary training as an undergraduate was roughly $29,000.  Therefore the total cost of my undergraduate degree was roughly $179,000.
  • Doctorate:  The average time in graduate school in physics in the United States is six years.  I spent six years and two months in grad school.  Grad students in physics don’t pay for their tuition, but tuition is paid to the university by the grant.  At Yale, my tuition was about $20,000/year.  In addition, my stipend, my supplementary salary from teaching, the cost of my health insurance, and overhead added up to at least $40,000/year.  This adds up to at least $360,000.  On top of that, I took trips to conferences and to take shifts.  My travel for my research definitely pushed the cost of my graduate degree to at least $400,000.

Therefore my PhD cost roughly $691,000.  This is not a precise calculation and one could certainly quibble with details.  I’m sure that people with more knowledge about grants would say I’m actually underestimating a lot of costs.  A PhD at Yale is probably more expensive than at other schools, but it still easily costs well over half a million dollars to produce a PhD.  That’s a huge investment for society to make in a person – and I’m very grateful.

I benefited significantly from scholarships and grants.  Other than paying taxes like everyone else, my family and I probably paid less than 5% of that cost.  Some costs were picked up by private organizations through grants, awards, and scholarships, but most of it was paid for by some branch of the government.  My teaching, tutoring, and research does have economic value – I don’t see myself as a leech on society – but I do owe my education and the opportunities I’ve had to the kindness of taxpayers.  If we did not live in a society that at least strives to create equal opportunities for all, I would not be where I am.  Because of the debt I owe society, I feel it is my responsibility to give back – to use my education to explain what I do to the public and to help inspire and train the next generation.

At the same time, society benefits from having highly educated people.  I am doing basic research that will most likely not lead to a marketable product in my lifetime.  But basic research is crucial to future economic developments.  Research in high energy particle and nuclear physics has led to cheaper and better particle detectors which can be used for medical technologies.  CERN played a crucial role in the development of the internet – certainly more than Al Gore – and it still does.  All of the experiments at the LHC use a computing infrastructure called the grid and developing the grid took substantial improvements in networking and distributed computing.  Studying the Quark Gluon Plasma will not directly feed the hungry or cure cancer, but we move the boundary of what is possible and this benefits humanity.


Introducing Irakli, doctoral candidate

Thursday, May 27th, 2010

Earlier today one of our University of Tennessee graduate students, Irakli, successfully defended his thesis proposal, officially becoming a doctoral candidate.  To do this, Irakli had to write a proposal for his thesis and give a presentation to his committee.  His written proposal was about 35 pages long and discussed relevant past results, proposed a measurement, argued for the relevance and value of the measurement, presented results from simulations demonstrating that the measurement he’s proposing will be feasible, presented results from test beam data demonstrating the performance of the detector, and laid out a time line with detailed steps in the analysis.  His presentation was about an hour long and he was grilled by the committee on various aspects of his proposal.

Irakli will measure heavy flavor production (meaning charm and beauty quarks) through the measurement of non-photonic electrons in proton-proton collisions in ALICE, focusing on measurements at high momenta using the electromagnetic calorimeter.  While Irakli is a heavy ion physicist, he’s doing a proton-proton measurement for a few different reasons.  We have proton-proton data already, so we are certain these data will be available.  Irakli will spend the next couple years working on data analysis, checking and cross-checking his results.  If he were trying to measure data that wouldn’t be available for a couple years, he’d have to do finish his analysis quickly or spend a long time in grad school.  The proton-proton result will be a way of checking theoretical calculations.  It’s also a good way to test our measurement technique, since proton-proton collisions are much simpler than heavy ion collisions.  Also, the proton-proton result will serve as a baseline measurement for heavy ion collisions.

Most universities in the US have some variation on this procedure.  There are two main goals – to ensure that the student is making sufficient progress and understands the measurement he’s trying to do and how it fits in to larger physics goals and to ensure that the advisor is directing the student towards a meaningful and feasible project which can be completed on an acceptable time line.  It is not only the student in the hot seat.  What these procedures tend to do is focus the student (and the advisor) on a clearly articulated goal.  Before the thesis proposal, students may try out various subjects, familiarize themselves with the experiment, and learn about possible thesis topics.  Irakli worked on the test beam calibration of our electromagnetic calorimeter, took shifts as a member of the team running the detector, tested front end electronics boards for the electromagnetic calorimeter, worked on the physics performance report for the electromagnetic calorimeter, and took classes.  These all provide valuable experience, but now that his goals are clearly articulated, his work will likely focus more on work directly related to his thesis.

So, congratulations Irakli!