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The planet’s most substantial scientific endeavor has accomplished it once more, uncovering evidence of the most massive antimatter particle ever detected.
This signifies that the Large Hadron Collider (LHC), the most potent particle accelerator constructed, has allowed researchers to witness conditions that prevailed when the universe was less than a second old. The antimatter particle is partnered with a substantial matter particle referred to as hyperhelium-4, and its detection may assist scientists in solving the enigma of why standard matter has come to predominate in the universe, even though matter and antimatter were produced in equal proportions at the universe’s inception.
This discrepancy is termed “matter-antimatter asymmetry.” Particles of matter and antimatter obliterate upon contact, discharging their energy back into the universe. This suggests that if an imbalance between the two had not emerged early in the universe’s formation, then the cosmos might have been an exceedingly emptier and less intriguing realm.
The LHC is well-acquainted with groundbreaking findings concerning the early universe. Operating in a 17-mile (27-kilometer) circular track beneath the Alps near Geneva, Switzerland, the LHC is renowned for its identification of the Higgs Boson particle, the “messenger” of the Higgs Field responsible for bestowing mass to other particles at the universe’s dawn.
The collisions occurring at the LHC yield a state of matter designated as “quark-gluon plasma.” This dense plasma mirrors the “primordial soup” of matter that occupied the universe around one-millionth of a second post-Big Bang.
Exotic “hypernuclei” and their antimatter equivalents arise from this quark-gluon plasma, granting scientists insight into the conditions of the early universe.
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ALICE through the looking glass
Hypernuclei comprise protons and neutrons like standard atomic nuclei and also unstable particles known as “hyperons.” Similar to protons and neutrons, hyperons are constructed of fundamental particles called “quarks.” While protons and neutrons consist of two varieties of quarks known as up and down quarks, hyperons include one or more so-called “strange quarks.”
Hypernuclei were initially identified in cosmic rays, showers of charged particles that descend upon Earth from deep space approximately seventy years ago. However, they are infrequently found in nature and are challenging to produce and examine in laboratory settings. This has rendered them somewhat enigmatic.
The identification of the initial evidence of the hypernuclei, which is an antimatter counterpart of hyperhelium-4, took place at the LHC detector ALICE.
While the majority of the nine experiments at the LHC, each equipped with its unique detector, derive their findings by colliding protons at nearly the speed of light, the ALICE collaboration generates quark-gluon plasma by colliding considerably heavier particles, typically lead nuclei, or “ions.”
The collision of iron ions (try articulating that ten times quickly) is optimal for producing large quantities of hypernuclei. However, until recently, scientists engaged in heavy-ion collisions had only managed to observe the lightest hypernucleus, hypertriton, and its antimatter counterpart, antihypertriton.
This changed earlier in 2024 when researchers utilized the Relativistic Heavy Ion Collider (RHIC) in New York to identify antihyperhydrogen-4, which consists of an antiproton, two antineutrons, and a quark-containing particle termed an “antilambda.”
Now, ALICE has advanced this by detecting a heavier anti-hypernuclei particle, antihyperhelium-4, composed of two antiprotons, an antineutron, and an antilambda.
The lead-lead collision and the ALICE data that resulted in the detection of the heaviest antimatter hypernucleus thus far at the LHC actually trace back to 2018.
The signature of antihyperhelium-4 was disclosed through its decay into other particles and the identification of these particles.
ALICE scientists extracted the signature of antihyperhelium-4 from the data employing a machine-learning technique that surpasses the collaboration’s conventional search methods.
In addition to spotting indications of antihyperhelium-4 and antihyperhydrogen-4, the ALICE team could also ascertain their masses, which aligned well with contemporary particle physics theories.
The researchers also gauged the quantities of these particles produced in lead-lead collisions.
They found these figures to be consistent with the ALICE data, indicating that antimatter and matter are generated in equal proportions from quark-gluon plasma forged at the energy levels attainable by the LHC.
The cause of the universe’s matter/antimatter imbalance remains a mystery, yet antihyperhelium-4 and antihyperhydrogen-4 might yield significant insights into this enigma.
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