The tell-tale missing neutrino

10 December 1997

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Last week's Picture of the Week showed an electron turned backwards by its collision deep within a proton. This week we show a similar collision, but one in which the electron has disappeared! The image, from the H1 detector at the DESY laboratory in Hamburg, shows the aftermath of a head-on collision between a high energy electron (from the left) and a proton (from the right) travelling unseen through a beam pipe, across the centre of the image. The electron has struck one of the proton's constituents - a quark - which has set off upwards and transformed to create the spray of particles in the upper half of the image. But what happened to the electron? In its interaction with the quark it changed into a neutrino - in fact, a closely related "electron neutrino". The neutrino has no charge and so remains invisible in the detector - its presence is inferred from the imbalance in momentum, which is also clearly seen by looking end on at the detector. "Charge changing" interactions like these are related to nuclear beta-decay, and occur through the weak force.

Credit: DESY / H1

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