NA62 Spectrometer: A Low Mass Straw Tracker

The NA62 experiment at CERN, aiming at a precision measurement of the ultra-rare decay K+ → π+νν, relies on kinematical rejection up to 105 (≈ 1012 is needed in total). One of the limiting factors to achieve this goal is the multiple scattering in the magnetic spectrometer for kaon decay products; therefore an almost massless (≈1.5% X0) straw tracker has been designed to operate in vacuum, to be able to install it inside the decay volume. A vacuum tight prototype was built and tested in 2010: efficiency (≈99%), rate capability and single straw resolution (≈200 μm) were verified. The construction of the first chamber started in 2011.