ATLAS Experiment at CERN

Figure 1: Measured data in comparison with the forecast from the normal Model (solid red line), also from the normal Model plus various new invisible phenomena near the limit of ATLAS density (dotted/dashed lines). (Picture: ATLAS Collaboration/CERN)

As the Large Hadron Collider (LHC) contrasts collectively protons at a centre-of-mass energy of 13 TeV, it makes a rich assortment of particles that are identified via the touch of the interactions with the ATLAS detector. However, what if there are particles being produced that travel through ATLAS without interacting? All these “invisible particles” may offer the answers to a number of the greatest puzzles in physics.

1 instance is Black Matter, that seems to make up 85% of mass in the Universe but has not been identified yet. We heard of its presence via astrophysical observations, including galaxy formation and gravitational lensing. However, we know more about what it isn’t than what it is. There’s no single concept of Dark Matter; different forecasts have different implications for its properties and how it interacts.

The imperceptible particles produced in LHC collisions carry energy away, leading to an apparent imbalance at the energy/momenta of these observed visible particles. Various theories predict that, if the imperceptible particles exist, even more events with large imbalance along with other distinctive patterns of observable particles could be detected by ATLAS. Assessing the number of these occasions predicted by idea of the number of events found in the detector is a means of searching for invisible particles.

While shown for a successful approach, there are limits. Imagine if all of our theoretical versions of Black Matter are wrong? Imagine if an entirely different phenomenon is that the reason behind invisible particles? Presently, if theoretical models are shown to be incorrect, it can be tough and time-consuming to re-use the data to test new versions. To do so requires an comprehension of how these particles have been recorded from the sensors, how the events have been chosen, and how the normal Model processes that mimic these particle routines were modelled.

ATLAS physicists have developed a new measurement-led approach, which was made to be detector-independent and permits for effortless re-interpretation of their data in future.  

Tip 2: Area of Dark Matter and mediator particle masses actually excluded (around the sound purple line) and anticipated to be excluded (up to the green solid line) on this measurement. Dotted gray lines show preceding results in the similar dedicated search.   (Picture: ATLAS Collaboration/CERN)

ATLAS physicists have developed a new measurement-led approach, which was made to be detector-independent and permits for effortless re-interpretation of their data in future. Within this process, a number Rmiss is described that is sensitive to the manufacturing speed and attributes of any imperceptible particle(s). This quantity is measured versus different properties of the collision events, including the amount of momentum imbalance and the energy/momenta of these observable particles. Not simply the value of this quantity, but how it affects with these properties that are measured is found to provide sensitivity to undetectable particles. Known decays of Z bosons —  produced in LHC crashes  —  into invisible neutrinos imply this quantity is non-zero even in the lack of a new invisible phenomenon. This quantity is closely adjusted for detector inefficiencies, leaving a measurement free from experimental prejudice and independent of any fresh physics hypothesis (Figure 1). Any physicist can then compare the predictions of the model against this dimension.

To demonstrate the new approach, the measurement is used to examine three distinctly different theoretical versions of Black Matter, in which it is created either (1) via the powerful force, (2) via the decays of Higgs bosons, or (3) via the electroweak force. No signs of Dark Matter is observed and so ATLAS can place stringent constraints on such theories (Figure 2). The limitations are competitive with existing methods that aim to examine these particular theories and complementary to measurements in space-based indirect detection experiments.

So, no matter what mysteries lie in the “imperceptible” kingdom, ATLAS gets the techniques that it needs — both today and in the long run — to continue to find out more about the Universe.

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