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Frank Simon | MPI for Physics | Germany

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The Spin of the Proton: It’s a Long Road

Writing and publishing a scientific paper in a high energy physics experiment and a large collaboration is usually a long road: The data analysis is complex, often many people from different institutions on different continents are involved, and the end result has to make everybody happy.

I’ve just made it through this process again in the collaboration that was my home for my PhD and post-doc times, the STAR collaboration. 10 days ago I’ve finally submitted a paper I’ve been working on with others for quite a while.

It is titled “Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV”

You can check it out here.

So, what is that about? One of the physics goal of the experiments at the Relativistic Heavy Ion Collider (STAR is one of the two remaining experiments there) is to study the spin structure of the proton. We are trying to answer the question of how the proton gets its spin of 1/2. A good question to ask, after all, the proton is not an elementary particle, but consists of many constituents, quarks, anti-quarks and gluons. And protons make up a good part of us, so understanding how a proton works is definitely a worthy goal.

Now, the proton has exactly the same spin as an electron, which is quite important for the structure of matter. But how do all the pieces of the proton come together to make the spin exactly right? An obvious solution would be that the three valence quarks, two ups and one down, each with a spin of 1/2, combine in such a way that two cancel out and 1/2 is left over for the proton. However, experiments in the late 80’s have found that the quarks only make up approximately 25% of the total spin of the proton. So other contributions have to come in. This leaves essentially the gluons, and orbital motion of the constituents within the proton.

Several experiments, among them the RHIC experiments, are now investigating the gluon contribution. At STAR, we collide polarized protons, and look for differences in particle production depending on the spin orientation of the protons. Since we know the effects from the quarks from the previous measurements, we can extract the gluon contributions to the proton spins by measuring spin asymmetries in the particle production (That is where the title of the paper comes from). There are several processes that can be used for these measurements. The one with the highest statistical power is inclusive jet production, since the cross section (i.e. production probability) is high, and a jet is relatively easy to measure in our detector. An alternative is to look at neutral pions, particles we can identify in our electromagnetic calorimeter. A neutral pion decays almost instantaneously into two photons, which we can identify in the calorimeter. From those two photons, we can reconstruct the properties of the original pion. The spin measurements with pions measure essentially the same thing as the jets, in the situation where a neutral pion is the leading particle of the jet. This extra requirement obviously reduces the available statistics, making the measurement less accurate. However, the systematic uncertainties (coming in from detector understanding, the trigger system and the like) are quite different than those for the jet measurements. Using pions as well as jets gives us thus an independent cross check for our results.

The main plot: The spin asymmetry we measured for neutral pions (the black dots), compared to calculations based on different assumed gluon contributions to the proton spin. The red curve, a scenario with very large gluon contributions, is excluded by the data. The others, which have negative, small or vanishing gluon contributions are still in. The proton spin remains a puzzle...

The main plot: The spin asymmetry we measured for neutral pions (the black dots), compared to calculations based on different assumed gluon contributions to the proton spin. The red curve, a scenario with very large gluon contributions, is excluded by the data. The others, which have negative, small or vanishing gluon contributions are still in. The proton spin remains a puzzle...

Since jets play such an important role in proton-proton collisions (trust me, you’ll hear a lot about jets once LHC results come flooding in), we’ve also looked at the relationship of the neutral pions to jets, by looking at the energy fraction the pions we have in our analysis typically carry from the jet they “live” in. To verify that the theoretical calculations we are using to interpret the results match our data, we have also measured the production cross section for neutral pions in proton-proton collisions at the RHIC energy. This measurement is also important as a reference for heavy ion collisions, the other corner stone of RHIC physics. To ensure consistency of analysis techniques across the spin physics and the heavy ion physics communities within STAR, I was working together with heavy ion guys for this paper.

So, what did we learn? Of course you can get the full story from the paper, but the short version is: The spin contribution of gluons to the overall spin of the proton is not terribly big, at least in the range that is accessible to the RHIC experiments. This was also shown by other measurements, both with jets at STAR and neutral pions from our competitors, the PHENIX experiment. We have now completed our first neutral pion analysis, but more data is available and is currently being analyzed (not by me, though), promising more precise results in the future.

I’m glad the paper is done, now I’m waiting for the referee reports, and then further down the road the publication… A happy ending for a large piece of work.

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