Science & Technology

BaBar Experiment Narrows Down the Search for Dark Photons

The BaBar detector at SLAC Nationwide Accelerator Laboratory. (Credit score: SLAC)

New evaluation of knowledge from the BaBar experiment guidelines out theorized particle’s clarification for muon thriller.

In its ultimate years of operation, a particle collider in Northern California was refocused to go looking for indicators of latest particles which may assist fill in some huge blanks in our understanding of the universe.

A recent evaluation of this knowledge, co-led by physicists at the Division of Vitality’s Lawrence Berkeley Nationwide Laboratory (Berkeley Lab), limits a few of the hiding locations for one sort of theorized particle – the darkish photon, often known as the heavy photon – that was proposed to assist clarify the thriller of darkish matter.

The most recent end result, printed in the journal Bodily Overview Letters by the roughly 240-member BaBar Collaboration, provides to outcomes from a set of earlier experiments searching for, however not but discovering, the theorized darkish photons.

“Though it doesn’t rule out the existence of darkish photons, the BaBar outcomes do restrict the place they will disguise, and definitively rule out their clarification for one other intriguing thriller related to the property of the subatomic particle generally known as the muon,” mentioned Michael Roney, BaBar spokesperson and College of Victoria professor.

Dark matter, which accounts for an estimated 85 % of the complete mass of the universe, has solely been noticed by its gravitational interactions with regular matter. For instance, the rotation fee of galaxies is way sooner than anticipated based mostly on their seen matter, suggesting there may be “lacking” mass that has up to now remained invisible to us.

So physicists have been engaged on theories and experiments to assist clarify what darkish matter is product of – whether or not it’s composed of undiscovered particles, for instance, and whether or not there could also be a hidden or “darkish” pressure that governs the interactions of such particles amongst themselves and with seen matter. The darkish photon, if it exists, has been put ahead as a doable service of this darkish pressure.

Utilizing knowledge collected from 2006 to 2008 at SLAC Nationwide Accelerator Laboratory in Menlo Park, California, the evaluation workforce scanned the recorded byproducts of particle collisions for indicators of a single particle of sunshine – a photon – devoid of related particle processes.

The BaBar experiment, which ran from 1999 to 2008 at SLAC, collected knowledge from collisions of electrons with positrons, their positively charged antiparticles. The collider driving BaBar, known as PEP-II, was constructed by a collaboration that included SLAC, Berkeley Lab, and Lawrence Livermore Nationwide Laboratory. At its peak, the BaBar Collaboration concerned over 630 physicists from 13 nations.

BaBar was initially designed to review the variations in the conduct between matter and antimatter involving a b-quark. Concurrently with a competing experiment in Japan known as Belle, BaBar confirmed the predictions of theorists and paved the method for the 2008 Nobel Prize. Berkeley Lab physicist Pier Oddone proposed the concept for BaBar and Belle in 1987 whereas he was the Lab’s Physics division director.

The most recent evaluation used about 10 % of BaBar’s knowledge – recorded in its ultimate two years of operation. Its knowledge assortment was refocused on discovering particles not accounted for in physics’ Commonplace Mannequin – a type of rulebook for what particles and forces make up the identified universe.

“BaBar carried out an in depth marketing campaign looking out for darkish sector particles, and this end result will additional constrain their existence,” mentioned Bertrand Echenard, a analysis professor at Caltech who was instrumental on this effort.

This chart exhibits the search space (inexperienced) explored in an evaluation of BaBar knowledge the place darkish photon particles haven’t been discovered, in contrast with different experiments’ search areas. The pink band exhibits the favored search space to find out whether or not darkish photons are inflicting the so-called “g-2 anomaly,” and the white areas are amongst the unexplored territories for darkish photons. (Credit score: Muon g-2 Collaboration)

Yury Kolomensky, a physicist in the Nuclear Science Division at Berkeley Lab and a school member in the Division of Physics at UC Berkeley, mentioned, “The signature (of a darkish photon) in the detector could be very simple: one high-energy photon, with out another exercise.”

Numerous the darkish photon theories predict that the related darkish matter particles could be invisible to the detector. The one photon, radiated from a beam particle, alerts that an electron-positron collision has occurred and that the invisible darkish photon decayed to the darkish matter particles, revealing itself in the absence of another accompanying vitality.

When physicists had proposed darkish photons in 2009, it excited new curiosity in the physics group, and prompted a recent take a look at BaBar’s knowledge. Kolomensky supervised the knowledge evaluation, carried out by UC Berkeley undergraduates Mark Derdzinski and Alexander Giuffrida.

“Dark photons may bridge this hidden divide between darkish matter and our world, so it might be thrilling if we had seen it,” Kolomensky mentioned.

The darkish photon has additionally been postulated to elucidate a discrepancy between the commentary of a property of the muon spin and the worth predicted for it in the Commonplace Mannequin. Measuring this property with unprecedented precision is the purpose of the Muon g-2 (pronounced gee-minus-two) Experiment at Fermi Nationwide Accelerator Laboratory.

Earlier measurements at Brookhaven Nationwide Laboratory had discovered that this property of muons – like a spinning high with a wobble that’s ever-slightly off the norm – is off by about 0.0002 % from what is anticipated. Dark photons had been advised as one doable particle candidate to elucidate this thriller, and a brand new spherical of experiments begun earlier this yr ought to assist to find out whether or not the anomaly is definitely a discovery.

The most recent BaBar end result, Kolomensky mentioned, largely “guidelines out these darkish photon theories as an evidence for the g-2 anomaly, successfully closing this explicit window, however it additionally means there’s something else driving the g-2 anomaly if it’s an actual impact.”

It’s a typical and fixed interaction between idea and experiments, with idea adjusting to new constraints set by experiments, and experiments searching for inspiration from new and adjusted theories to seek out the subsequent proving grounds for testing out these theories.

Scientists have been actively mining BaBar’s knowledge, Roney mentioned, to make the most of the well-understood experimental situations and detector to check new theoretical concepts.

“Discovering an evidence for darkish matter is one among the most vital challenges in physics at present, and searching for darkish photons was a pure method for BaBar to contribute,” Roney mentioned, including that many experiments in operation or deliberate round the world are searching for to handle this drawback.

An improve of an experiment in Japan that’s much like BaBar, known as Belle II, activates subsequent yr. “Finally, Belle II will produce 100 occasions extra statistics in comparison with BaBar,” Kolomensky mentioned. “Experiments like this could probe new theories and extra states, successfully opening new potentialities for extra exams and measurements.”

“Till Belle II has amassed important quantities of knowledge, BaBar will proceed for the subsequent a number of years to yield new impactful outcomes like this one,” Roney mentioned.

The examine featured participation by the worldwide BaBar collaboration, which incorporates researchers from about 66 establishments in the U.S., Canada, France, Spain, Italy, Norway, Germany, Russia, India, Saudi Arabia, U.Ok., the Netherlands, and Israel. The work was supported by the U.S. Division of Vitality’s Workplace of Science and the Nationwide Science Basis; the Pure Sciences and Engineering Analysis Council in Canada; CEA and CNRS-IN2P3 in France; BMBF and DFG in Germany; INFN in Italy; FOM in the Netherlands; NFR in Norway; MES in Russia; MINECO in Spain; STFC in the U.Ok.; and BSF in Israel and the U.S. People concerned with this examine have obtained help from the Marie Curie EIF in the European Union, and the Alfred P. Sloan Basis in the U.S.

Publication: J. P. Lees et al. (BaBar Collaboration), “Search for Invisible Decays of a Dark Photon Produced in e+e Collisions at BaBar, ” Phys. Rev. Lett. 119, 131804, 2017; doi:10.1103/PhysRevLett.119.131804

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