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COMPASS at CERN
CERN - Geneva, Switzerland
Matthias Grosse Perdekamp, Caroline Riedl, Jen-Chieh Peng, Naomi Makins

COMPASSCOMPASS (COmmon Muon Proton Apparatus for Structure and Spectroscopy) is a fixed-target experiment at CERN. Since the year 2002, it has been using the muon or hadron beam of the Super Proton Synchrotron (SPS) to scatter off a target of unpolarized or spin-polarized protons or deuterons. The SPS is also used to inject beams into the Large Hadron Collider (LHC).

One of the important missions of COMPASS is the unraveling of the nucleon spin structure - how does the spin of one half (in units of the reduced Planck constant) come along? What is the dynamic inner structure of the proton in terms of longitudinal and transverse momenta, orbital angular momentum, and spin of its constituents, the quarks and gluons? Our group is in particular interested in the quark's transverse spin and transverse momentum, and their correlations. The underlying theory is based on a rather recent extension of Quantum Chromo Dynamics (QCD), the theory of the strong interaction, and introduces new so-called structure functions of the nucleon descriptive of the non-perturbative part of the particle interactions: Transverse-Momentum Dependent Parton Distribution Functions, in short TMDs.

At CERN, we are currently a team of two graduate students, one postdoc researcher and one faculty. In 2014, our group has built a large-area drift chamber - DC5 - to replace an ageing detector in the COMPASS spectrometer. We are currently analyzing the data collected in 2015 - the world wide first ever polarization-dependent Drell-Yan measurement using a negatively charged pion beam and a transversely polarized proton target. We are responsible for certain detectors and software tasks that the entire collaboration profits from. We are working in a highly international environment - COMPASS has collaborators from 13 countries all over the world - and are immersed in the vibrating atmosphere at CERN.

At the end of April 2016, we are ready for the next beam time - in 2016 and 2017, COMPASS will collect data using the SPS polarized muon beam, an unpolarized proton target, and a detector to measure the recoiling target proton. From these data, the goal is to extract observables related to Generalized Parton Distributions (GPDs), which provide a picture of the nucleon that is complementary to that from TMDs.

COMPASS Interview



COMPASS DC5 Pictures

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Milling begins on a G10 strip to proper thickness at the Nuclear Physics Laboratory on the University of Illinois' campus.

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Detail of a ready milled "lap joint" or frame corner.

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Milling details "holes and grooves" into a G10 strip.

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Ready frame corner after glueing and polishing.

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Aligning and glueing printed circuit boards on a G10 anode frame.

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Mixing a specialized epoxy glue.

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DC5 wiring in the NPL clean room.

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DC5 wiring in the NPL clean room.

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DC5 wiring in the NPL clean room.

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Completed wired frame.

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Old Dominion University clean room showing proper space and equipment.

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Setting up anode frames for wiring in ODU clean room.

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DC5 wiring in the ODU clean room.

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DC5 wiring in the ODU clean room.

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Wiring continues after arrival at CERN.

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Stretching and glueing of a DC5 cathode mylar.

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DC5 stiffening frame is lifted after stretching of the gas window.

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Testing DC5 in the COMPASS clean area at CERN.

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Testing the front end electronics of DC5.

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DC5 cabling.

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More testing of DC5.

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Preparations to DC5 for installation.

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Craning DC5 into position.

Lowering DC5 (middle) into the COMPASS spectrometer between DC4 (right) and Straw3 (left).


Visit the COMPASS DC5 Story webpage for a narrative.