Recently the Beam Team hosted scientists from the SELDOM group, who are normally based at the University of Ferrara, Italy. Together, we wanted to test how well silicon crystals could bend beams of positively-charged particles. Usually, we have to use rather large magnets to control the direction of beams, so it’s useful to investigate new methods. To do this, we designed an experiment that used a beamline called H8 in the North Area of CERN.

The H8 beamline takes protons (p) from CERN’s Super Proton Synchrotron and fires them onto a target made from Beryllium. When the beam hits the target, protons in the beam collide with protons inside the Beryllium atoms. The resulting mixture of particles includes a beam of positive pions (π+), sometimes called pi-mesons, which is then directed toward the silicon crystals.

One equation for pion production from proton collisions is: 

p + p -> π+ + n + p

We used a thin sheet of crystals that was easily bent to form an angle that could scatter the pion beam. The sheet was 2mm thin and about 4cm wide; probably about the same width and thickness as the ruler in your pencil case. The sheet couldn’t bend as much as your ruler, though: the maximum bend angle we had was around 7 milli-radian, or 0.4 degrees.

This sheet was mounted onto a holder and placed on an adjustable stand called a goniometer. Using the holder, we bent the crystal sheet so that the front edge was aligned with the incoming pion beam. How would the beam be affected by the crystal?

To find out, we placed detectors downstream of the crystal that could precisely measure where the beam of pions ended up. We knew that, without the crystal in its path, the pion beam would go in a straight line. We expected, then, that when the beam hit the angled sheet of crystal the pions would follow the crystal structure, going in a bent path off to one side, almost in the same way that light follows a bent optical fibre.

Initial results from the experiment show that the silicon crystals do indeed bend the pion beam! It’s exciting that we can bend positive particles this much in a short distance. 0.4 degrees might not sound like a lot but when particles travel near the speed of light it gets very difficult to change their direction. Some particles, like the rare lambda-c (Λc), also don’t ‘live’ very long before they decay. If we can bend them before they decay, we can find out more about their properties.

The SELDOM group managed to test three different lengths and bend angles of crystal sheets during the one-week experiment and are still analysing their data. We’re hoping they can publish some more details about their results soon. 

The Beam Team is also analysing the data to prepare for an experiment called TWOCRYST where we will add two bent silicon crystals into the Large Hadron Collider. One of the long-term goals of the TWOCRYST experiment is to help measure the spin property of the rare lambda-c particle.