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SpinQuest Experiment

The SpinQuest experiment E 1039 at Fermilab is a collaborative effort to solve the missing spin puzzle of the proton. The collaboration includes several universities across the United states and accross the globe.

Experiment

View of the CMS endcap through the barrel sections. The ladder to the lower right gives an impression of scale.

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Overview

Every proton and neutron is made up of tinier particles called quarks and gluons, and we continue to explore how the quarks and gluons behave therein. Both quarks and their anti-matter cousins the anti-quarks play a role in creating protons and neutrons. The SpinQuest experiment is designed to explore a very particular aspect of proton and neutron structure: Are the sea quarks orbiting around the spin axis of the nucleon?

Physics motivation

At the heart of every atom is a nucleus, and every nucleus is constructed from just two particles; the proton and the neutron. Collectively, we call the proton and neutron “nucleons” since they are the residents of every nucleus. Nucleons are themselves made up from another set of particles called quarks and gluons.

If you wanted to build a proton from scratch, for example, you would need two “up” quarks (uu) and one “down” quark (d). As soon as you put those three quarks together, a very strong attractive field of gluons would bind them together. The field of the gluons is very strong indeed; most of the mass of the proton comes from the field of gluons, and not from the quarks themselves.

The quarks have electric charges, by the way; up-quarks have +2/3 charge (measured in units of the electron charge) while down-quarks have a -1/3 charge. However, the electric force is NOT responsible for the main attraction felt between the quarks; the gluon field creates a force thousands of times stronger that binds the quarks together.

Another thing that happens, after you put the three quarks together and the gluon field is created, is the gluons create pairs of quarks and anti-quarks! For example, a pair of up (u) and anti-up (u) quarks might be created. This is possible because the field of the gluons contains a lot of energy, more than enough energy to create these particle pairs, and the up- and down-quarks have very little mass. These quark and anti-quark pairs do not exist for very long, and they eventually recombine to create a gluon, but a lot of them are created and on average they have a presence in the nucleon that cannot be ignored. We call the quarks and anti-quarks created in this way the “quark sea” — an ocean of additional quarks created by the gluon field.

To complete our picture of the nucleon, we have to mention that the constituent particles, the quarks and gluons, have an interesting property called “spin”. We call this property “spin” because particles with “spin” appear to be rotating, but it is not like a top spinning; there is no actual rotation involved. Yet this “spin” has real consequences; if a particle with “spin” has electric charges in it, then the “spin” will create a magnetic field!

Target

The polarized target system to be used in SpinQuest is a high-cooling-power evaporation refrigerator connected to a large pump stack (14,000 m3/hour) and a microwave generator used to dynamically polarize the nucleons in the target. The magnet has a 5-T field with a homogeneous region of 8 cm and will be used to polarize protons and neutrons in the sample. Here the target system is shown the polarized target lab at the University of Virginia where the system was setup for testing and optimization. This system has the longest (along the beam-line) target cell to date for an evaporation refrigerator; this requires a unique microwave distributing horn and three NMR coils per target-cell to further reduce systematics in the polarization measurement. This system was configured with the intention of having the greatest instantaneous luminosity of any previous evaporation refrigerator system by having a beam intensity of 3×1012 protons/sec for 5 seconds while using a large pump system (14,000 m3/hour). We are also running an experiment with both polarized protons and neutrons which share different sensitivities to the overall figure of merit of the Drell-Yan asymmetry.

Etymology

The term Compact Muon Solenoid comes from the relatively compact size of the detector, the fact that it detects muons, and the use of solenoids in the detector. "CMS" is also a reference to the center-of-mass system, an important concept in particle physics.

See also

• List of Large Hadron Collider experiments

References

• CMS Collaboration (Bayatian, G.L. et al.) (2006). "CMS Physics Technical Design Report Volume I: Software and Detector Performance" (PDF). CERN. {{cite journal}}: Cite journal requires |journal= (help) (mirrors: inspire, CDS)

External links

• CMS home page
• CMS Outreach
• CMS Times
• CMS section from US/LHC Website
• The assembly of the CMS detector, step by step, through a 3D animation
• The CMS Collaboration, S Chatrchyan; et al. (2008-08-14). "The CMS experiment at the CERN LHC". Journal of Instrumentation. 3 (8): S08004. Bibcode:2008JInst...3S8004T. doi:10.1088/1748-0221/3/08/S08004. (Full design documentation)
• Copeland, Ed. "Inside the CMS Experiment". Sixty Symbols. Brady Haran for the University of Nottingham.

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