experimental An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs when a ...

particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

, missing energy refers to

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...

that is not detected in a

particle detector In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing particles, such as those produced by nu ...

, but is expected due to the laws of conservation of energy and

conservation of momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...

. Missing energy is carried by particles that do not interact with the electromagnetic or strong forces and thus are not easily detectable, most notably

neutrinos A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is ...

. In general, missing energy is used to infer the presence of non-detectable particles and is expected to be a signature of many theories of physics beyond the Standard Model. The concept of missing energy is commonly applied in

hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the e ...

collider A collider is a type of particle accelerator which brings two opposing particle beams together such that the particles collide. Colliders may either be ring accelerators or linear accelerators. Colliders are used as a research tool in particl ...

s. The initial momentum of the colliding partons along the beam axis is not known — the energy of each hadron is split, and constantly exchanged, between its constituents — so the amount of total missing energy cannot be determined. However, the initial energy in particles traveling transverse to the beam axis is zero, so any net momentum in the transverse direction indicates missing transverse energy, also called missing ''E''_T or MET. Accurate measurements of missing energy are difficult, as they require full, accurate, energy reconstruction of all particles produced in an interaction. Mismeasurement of particle energies can make it appear as if there is missing energy carried away by other particles when, in fact, no such particles were created.

References

Experimental particle physics {{Particle-stub