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STAR Detector Relativistic Heavy Ion Collider
Science & Technology

‘Strange’ Glimpse Into Neutron Stars and Violations of Fundamental Symmetries in the Universe

Internal vertex parts of the STAR detector at the Relativistic Heavy Ion Collider (righthand view) permit scientists to hint tracks from triplets of decay particles picked up in the detector’s outer areas (left) to their origin in a uncommon “antihypertriton” particle that decays simply exterior the collision zone. Measurements of the momentum and recognized mass of the decay merchandise (a pi+ meson, antiproton, and antideuteron) can then be used to calculate the mass and binding vitality of the father or mother particle. Doing the similar for the hypertriton (which decays into completely different “daughter” particles) permits precision comparisons of these matter and antimatter varieties. Credit score: Brookhaven Nationwide Laboratory

RHIC measurements of ‘hypertriton’ and ‘antihypertriton’ binding vitality and mass discover strange-matter interactions and take a look at for ‘CPT’ violation.

New outcomes from precision particle detectors at the Relativistic Heavy Ion Collider (RHIC) provide a recent glimpse of the particle interactions that happen in the cores of neutron stars and give nuclear physicists a brand new option to seek for violations of elementary symmetries in the universe. The outcomes, simply printed in Nature Physics, may solely be obtained at a strong ion collider reminiscent of RHIC, a U.S. Division of Power (DOE) Workplace of Science person facility for nuclear physics analysis at DOE’s Brookhaven Nationwide Laboratory.

The precision measurements reveal that the binding vitality holding collectively the parts of the easiest “strange-matter” nucleus, referred to as a “hypertriton,” is bigger than obtained by earlier, less-precise experiments. The brand new worth may have essential astrophysical implications for understanding the properties of neutron stars, the place the presence of particles containing so-called “unusual” quarks is predicted to be frequent.

The second measurement was a seek for a distinction between the mass of the hypertriton and its antimatter counterpart, the antihypertriton (the first nucleus containing an antistrange quark, found at RHIC in 2010). Physicists have by no means discovered a mass distinction between matter-antimatter companions so seeing one could be an enormous discovery. It could be proof of “CPT” violation — a simultaneous violation of three elementary symmetries in nature pertaining to the reversal of cost, parity (mirror symmetry), and time.

The Heavy Taste Tracker at the heart of RHIC’s STAR detector. Credit score: Brookhaven Nationwide Laboratory

“Physicists have seen parity violation, and violation of CP collectively (every incomes a Nobel Prize for Brookhaven Lab), however by no means CPT,” mentioned Brookhaven physicist Zhangbu Xu, co-spokesperson of RHIC’s STAR experiment, the place the hypertriton analysis was executed.

However nobody has regarded for CPT violation in the hypertriton and antihypertriton, he mentioned, “as a result of nobody else may but.”

The earlier CPT take a look at of the heaviest nucleus was carried out by the ALICE collaboration at Europe’s Massive Hadron Collider (LHC), with a measurement of the mass distinction between peculiar helium-3 and antihelium-3. The consequence, exhibiting no important distinction, was printed in Nature Physics in 2015.

Spoiler alert: The STAR outcomes additionally reveal no important mass distinction between the matter-antimatter companions explored at RHIC, so there’s nonetheless no proof of CPT violation. However the proven fact that STAR physicists may even make the measurements is a testomony to the exceptional capabilities of their detector.

The only normal-matter nuclei include simply protons and neutrons, with every of these particles made of peculiar “up” and “down” quarks. In hypertritons, one neutron is changed by a particle known as a lambda, which accommodates one unusual quark together with the peculiar up and down varieties.

Such unusual matter replacements are frequent in the ultra-dense situations created in RHIC’s collisions — and are additionally doubtless in the cores of neutron stars the place a single teaspoon of matter would weigh greater than 1 billion tons. That’s as a result of the excessive density makes it less expensive energy-wise to make unusual quarks than the peculiar up and down varieties.

For that purpose, RHIC collisions give nuclear physicists a option to peer into the subatomic interactions inside distant stellar objects with out ever leaving Earth. And since RHIC collisions create hypertritons and antihypertritons in almost equal quantities, they provide a option to seek for CPT violation as effectively.

However discovering these uncommon particles amongst the 1000’s that stream from every RHIC particle smashup — with collisions taking place 1000’s of occasions every second — is a frightening job. Add to the problem the proven fact that these unstable particles decay virtually as quickly as they type — inside centimeters of the heart of the four-meter-wide STAR detector.

Luckily, detector parts added to STAR for monitoring completely different sorts of particles made the search a relative cinch. These parts, known as the “Heavy-Taste Tracker,” are situated very near the STAR detector’s heart. They have been developed and constructed by a workforce of STAR collaborators led by scientists and engineers at DOE’s Lawrence Berkeley Nationwide Laboratory (Berkeley Lab). These interior parts permit scientists to match up tracks created by decay merchandise of every hypertriton and antihypertriton with their level of origin simply exterior the collision zone.

“What we search for are the ‘daughter’ particles — the decay merchandise that strike detector parts at the outer edges of STAR,” mentioned Berkeley Lab physicist Xin Dong. Figuring out tracks of pairs or triplets of daughter particles that originate from a single level simply exterior the main collision zone permits the scientists to choose these indicators out from the sea of different particles streaming from every RHIC collision.

“Then we calculate the momentum of every daughter particle from one decay (based mostly on how a lot they bend in STAR’s magnetic subject), and from that we will reconstruct their plenty and the mass of the father or mother hypertriton or antihypertriton particle earlier than it decayed,” defined Declan Keane of Kent State College (KSU). Telling the hypertriton and antihypertriton aside is simple as a result of they decay into completely different daughters, he added.

“Keane’s workforce, together with Irakli Chakeberia, has specialised in monitoring these particles by means of the detectors to ‘join the dots,’” Xu mentioned. “In addition they supplied a lot wanted visualization of the occasions.”

As famous, compiling information from many collisions revealed no mass distinction between the matter and antimatter hypernuclei, so there’s no proof of CPT violation in these outcomes.

However when STAR physicists checked out their outcomes for the binding vitality of the hypertriton, it turned out to be bigger than earlier measurements from the Nineteen Seventies had discovered.

The STAR physicists derived the binding vitality by subtracting their worth for the hypertriton mass from the mixed recognized plenty of its building-block particles: a deuteron (a sure state of a proton and a neutron) and one lambda.

“The hypertriton weighs lower than the sum of its elements as a result of some of that mass is transformed into the vitality that’s binding the three nucleons collectively,” mentioned Fudan College STAR collaborator Jinhui Chen, whose PhD scholar, Peng Liu, analyzed the giant datasets to reach at these outcomes. “This binding vitality is known as a measure of the energy of these interactions, so our new measurement may have essential implications for understanding the ‘equation of state’ of neutron stars,” he added.

For instance, in mannequin calculations, the mass and construction of a neutron star is determined by the energy of these interactions. “There’s nice curiosity in understanding how these interactions — a type of the sturdy pressure — are completely different between peculiar nucleons and unusual nucleons containing up, down, and unusual quarks,” Chen mentioned. “As a result of these hypernuclei include a single lambda, that is one of the greatest methods to make comparisons with theoretical predictions. It reduces the downside to its easiest type.”

Reference: “Measurement of the mass distinction and the binding vitality of the hypertriton and antihypertriton” by The STAR Collaboration, 9 March 2020, Nature.
DOI: 10.1038/s41567-020-0799-7

This work was funded by the DOE Workplace of Science and by funders of the STAR collaboration. The workforce expressed gratitude to the Nationwide Power Analysis Scientific Computing Middle at Berkeley Lab (one other DOE Workplace of Science person facility) and the Open Science Grid consortium for offering assets and help.

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