麻豆淫院

Might two bent crystals pave the way to finding new physics? The Standard Model of particle physics describes our world at its smallest scales exceptionally well. However, it leaves some important questions unanswered, such as the imbalance between matter and antimatter, the existence of dark matter and other mysteries.

One method to find "new physics" beyond the Standard Model is to measure the properties of different particles as precisely as possible and then compare measurement with theory. If the two don't agree, it might hint at new physics and let us slowly piece together a fuller picture of our universe鈥攍ike pieces of a jigsaw puzzle.

An example of particles that physicists wish to study more closely are "charm baryons" such as the "Lambda-c-plus" (螞_c⁺) which is a heavier "cousin" of the proton, consisting of three quarks: one up, one down and one charm. These particles decay after less than a trillionth of a second (10^-13 s), which makes any measurement of their properties a race against time. Some of their properties have not yet been measured to high precision, leaving room for new physics to hide.

The particles' magnetic and electric dipole moments are of particular interest. In the past, precise measurements of dipole moments in other particles have provided key tests of established theories and, sometimes, uncovered surprises that pointed to new physics.

A novel experimental concept aims to measure the properties of charm baryons using a fixed target and two bent crystals. Electric and magnetic dipole moments can be measured by forcing particles on a curved trajectory. Since charm baryons decay extremely quickly, however, conventional techniques using magnetic fields are not strong enough to obtain measurable results.

An alternative approach could be to exploit the fact that the atoms inside a crystal are neatly organized as a three-dimensional lattice, forming tiny channels when viewed from certain directions. If a bent crystal is placed inside a stream of charged particles, the particles may follow these channels, experiencing deflections otherwise out of reach within such a short distance. Thus, this makes measurements on extremely short-lived particles possible.

In the full set-up, one bent silicon crystal is inserted close to the proton beam inside a stream of particles called the "secondary halo"鈥攑rotons that strayed too far from the beam center and would normally be absorbed by the LHC collimation system. This first crystal steers the particles away from the main LHC beam towards a tungsten target where the collisions produce charm baryons. A second silicon crystal then bends the path of the produced particles strongly enough that their dipole moments can be precisely measured with a specialized detector.

was conceived as a proof-of-principle experiment, designed to test whether the concept really works in practice鈥攆rom the performance of the crystals to the precision of their alignment. After only two years of preparation, TWOCRYST was installed in the LHC at the beginning of the year.

"The experimental set-up is a simplified version of a full-fledged experiment, consisting of two bent silicon crystals, a target and two 2D detectors (a pixel tracker and a fiber tracker)," explains TWOCRYST study leader Pascal Hermes. "One goal is to verify if the particles can be deflected through both crystals in sequence鈥攖he so-called 'double channeling.'"

The first TWOCRYST measurements in June at an energy of 450 GeV showed promising results. All the newly installed hardware is functional and operational and, after both silicon crystals had been carefully aligned, "double-channeled" particles were observed for the first time at the LHC and at the highest energy ever achieved. The team will now complete a set of further tests at higher energies of several TeV. Earlier related research is also on the arXiv preprint server.

All the measurements will be analyzed in detail to determine whether enough deflected charm baryons could be collected to justify a full-scale experiment. Whatever the outcome, TWOCRYST has already opened a new chapter of crystal applications at the LHC. The results from TWOCRYST may well shape the design of future fixed-target experiments and novel beam-control concepts at the LHC and beyond.