Could new physics beyond the Standard Model be discovered thanks to crystals? This is the idea behind the TWOCRYST project at CERN’s LHC accelerator. TWOCRYST was conceived as a proof of principle: it was designed to test a new experimental approach based on the use of crystals to study extremely short-lived particles. After only two years of preparation, TWOCRYST was installed in the LHC at the beginning of this year and has now successfully carried out its measurements.
The Standard Model of particle physics describes our world at its smallest scales with exceptional accuracy. However, it leaves some important questions unanswered, such as the asymmetry between matter and antimatter, the nature of dark matter, and other mysteries. One way to discover this “new physics” beyond the Standard Model is to measure the properties of various particles as precisely as possible and then compare the measurements with theoretical predictions. If there is a discrepancy between the two, it could point to new physics that helps build a more complete picture of our universe, like adding new pieces to a puzzle.
An example of particles of interest for this purpose is charmed baryons (containing the charm quark), because some of their properties have not yet been measured with high precision, leaving room for potential new discoveries in physics. These particles, however, decay in less than one picosecond (10⁻¹² seconds), making any measurement of their properties a race against time. Of particular interest are their magnetic and electric dipole moments. In the past, precise measurements of dipole moments in other particles have provided key tests of well-established theories and, at times, revealed surprises that opened the way to new discoveries in physics.
Now there is a new experimental approach that aims to measure the properties of charmed baryons using a fixed target and two bent crystals. The magnetic and electric dipole moments can be measured by forcing the particles to follow a curved trajectory. However, since charmed baryons decay extremely rapidly, conventional techniques using magnetic fields are not strong enough to produce measurable effects. An alternative approach could exploit the fact that atoms inside a crystal are arranged in an orderly three-dimensional lattice, forming tiny channels when viewed along certain directions. If a bent crystal is inserted into a stream of charged particles, the particles can follow these channels, experiencing deflections otherwise impossible to achieve over such short distances. This would make it possible to carry out measurements on particles with extremely short lifetimes.
In the full experimental setup, a bent silicon crystal is placed close to the proton beam inside a stream of particles known as the “secondary halo”, composed of protons that have strayed too far from the centre of the beam and would normally be absorbed by the accelerator’s collimation system. This first crystal deflects the particles from the main LHC beam toward a tungsten target, where collisions produce charmed baryons. A second bent silicon crystal then curves the trajectories of the produced particles with enough force to allow precise measurement of their dipole moments using a specialised detector.
The first TWOCRYST measurements, conducted in June at an energy of 450 GeV, have shown promising results. All the newly installed hardware is functioning and operational, and after careful alignment of both silicon crystals, “double-channelled” particles were observed for the first time at the LHC and at the highest energy ever achieved. The TWOCRYST collaboration has just completed a further series of tests at higher energies, reaching several TeV.
“All the preliminary measurements and tests carried out so far are very promising, enough to justify a large-scale experiment. However, it will be useful to wait for the final results to gain a complete picture. TWOCRYST has already begun a new chapter in crystal applications at the LHC, and its results could pave the way for the design of future fixed-target experiments and new beam-control approaches at the LHC and in future accelerators”, said Nicola Neri, national coordinator of the TWOCRYST collaboration.
The TWOCRYST scientific collaboration includes CERN, INFN, IFIC – University of Valencia, IJCLab – Orsay, the University of Malta, UCAS, Warsaw University of Technology, IFJ PAN.
CERN news: Towards new physics with bent crystals.
