From DOE’s Brookhaven National Laboratory (US) and From CERN (CH) ATLAS : “ATLAS Confirms Universality of Key Particle Interactions”

From DOE’s Brookhaven National Laboratory (US)



July 9, 2021
Karen McNulty Walsh

Test demonstrates “lepton flavor universality” for interactions of muon and tau leptons with W bosons.

A new paper by the ATLAS collaboration at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) provides evidence that two different types of leptons interact in a universal way with particles called W bosons. This result, just published in Nature Physics, supports “lepton flavor universality,” a key prediction of the Standard Model of particle physics.

Standard Model of Particle Physics, Quantum Diaries[/caption]

The Standard Model of particle physics is the reigning theory describing all known particles and their interactions. It includes three flavors of leptons: the familiar electron—which is central to our understanding of electricity—and two heavier cousins known as muons and tau particles. According to the Standard Model, each of these leptons should “couple,” or interact, with a W boson with equal strength, commonly referred to as lepton-flavor universality.

Finding an experimental result in agreement with that longstanding prediction may not seem all that newsworthy. But decades ago, experiments at the LHC’s predecessor—the Large Electron-Positron (LEP) collider—had reported a hint of a discrepancy in the way muon and tau leptons behaved.

That result, from the 1990s, generated tension with the Standard Model.

Srini Rajagopalan, Program Manager for U.S. ATLAS and a physicist at Brookhaven National Laboratory.

“The new ATLAS measurement, which has significantly higher precision than the LEP experiments, resolves the decades-old tension,” said Srini Rajagopalan, Program Manager for U.S. ATLAS and a physicist at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory. “It is an important measurement to demonstrate that different types of leptons behave in the same way.”

Brookhaven Lab serves as the U.S. host laboratory for the ATLAS experiment. Brookhaven scientists play multiple roles in this international collaboration, from construction and project management to data storage, distribution, and analysis.

The ATLAS team’s motivation for using the powerful LHC to study leptons’ interactions with the W boson stems from the earlier discrepancy at LEP, which was also located at CERN.

LEP collided electrons and their anti-particles (positrons). These collisions provided a very clean environment for precision measurements of particle interactions and properties. The experiments measured a discrepancy in the frequency with which W bosons decayed to muon and tau leptons. The discrepancy suggested there was a difference in the strength of the W boson interactions with these two different flavor leptons—a violation of lepton flavor universality. But LEP produced a relatively low number of W bosons, which limited the measurement’s statistical precision.

The LHC, in contrast, collides high-energy protons. Compared with simple electrons and positrons, protons are more complex composite particles. Each proton is made of many quarks and gluons and each collision between two of these composite particles produces many different types of particles. But among the multitudes, more than 100 million of these collisions produce pairs of so-called top quarks, which readily decay into pairs of W bosons, and subsequently, in some cases, into leptons. Thus, the LHC provides a huge dataset for measuring W boson-to-lepton decays/interactions.

But there’s an added challenge: Some muons come directly from the decay of W bosons; and some come from a tau lepton itself decaying into a muon plus two invisible particles called neutrinos. Fortunately, these two sources of muons have different lifetimes, which lead to different signatures in the detector.

ATLAS Physics Coordinator Stephane Willocq, a physicist at the University of Massachusetts at Amherst (US). Credit: UMass Amherst.

ATLAS is sensitive enough to search for these unique signatures and cancel out additional uncertainties in the process—a key feature that enables the high precision of the measurement.

“This is a beautiful result that demonstrates that we can perform precision tests at the LHC, thanks to the huge datasets collected and the well-understood detector performance,” said ATLAS Physics Coordinator Stephane Willocq, a physicist at the University of Massachusetts at Amherst.

The new result gives the ratio of a W boson decaying to a tau or muon to be very close to 1. Such a measurement signifies that the decay to each lepton occurs with equal frequency implying that Ws couple with each lepton with equal strength—just as the Standard Model predicts. With an uncertainty half the size of the LEP measurement, this new high-precision ATLAS measurement suggests the earlier tension between experiment and theory may have been due to a fluctuation.

Brookhaven Lab’s role in this research was funded by the DOE Office of Science.

See the full article here .


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Quantum Diaries

One of ten national laboratories overseen and primarily funded by the DOE(US) Office of Science, DOE’s Brookhaven National Laboratory (US) conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University(US), the largest academic user of Laboratory facilities, and Battelle(US), a nonprofit, applied science and technology organization.

Research at BNL specializes in nuclear and high energy physics, energy science and technology, environmental and bioscience, nanoscience and national security. The 5,300 acre campus contains several large research facilities, including the Relativistic Heavy Ion Collider [below] and National Synchrotron Light Source II [below]. Seven Nobel prizes have been awarded for work conducted at Brookhaven lab.

BNL is staffed by approximately 2,750 scientists, engineers, technicians, and support personnel, and hosts 4,000 guest investigators every year. The laboratory has its own police station, fire department, and ZIP code (11973). In total, the lab spans a 5,265-acre (21 km^2) area that is mostly coterminous with the hamlet of Upton, New York. BNL is served by a rail spur operated as-needed by the New York and Atlantic Railway. Co-located with the laboratory is the Upton, New York, forecast office of the National Weather Service.

Major programs

Although originally conceived as a nuclear research facility, Brookhaven Lab’s mission has greatly expanded. Its foci are now:

Nuclear and high-energy physics
Physics and chemistry of materials
Environmental and climate research
Energy research
Structural biology
Accelerator physics


Brookhaven National Lab was originally owned by the Atomic Energy Commission(US) and is now owned by that agency’s successor, the United States Department of Energy (DOE). DOE subcontracts the research and operation to universities and research organizations. It is currently operated by Brookhaven Science Associates LLC, which is an equal partnership of Stony Brook University(US) and Battelle Memorial Institute(US). From 1947 to 1998, it was operated by Associated Universities, Inc. (AUI) (US), but AUI lost its contract in the wake of two incidents: a 1994 fire at the facility’s high-beam flux reactor that exposed several workers to radiation and reports in 1997 of a tritium leak into the groundwater of the Long Island Central Pine Barrens on which the facility sits.


Following World War II, the US Atomic Energy Commission was created to support government-sponsored peacetime research on atomic energy. The effort to build a nuclear reactor in the American northeast was fostered largely by physicists Isidor Isaac Rabi and Norman Foster Ramsey Jr., who during the war witnessed many of their colleagues at Columbia University leave for new remote research sites following the departure of the Manhattan Project from its campus. Their effort to house this reactor near New York City was rivalled by a similar effort at the Massachusetts Institute of Technology (US) to have a facility near Boston, Massachusettes(US). Involvement was quickly solicited from representatives of northeastern universities to the south and west of New York City such that this city would be at their geographic center. In March 1946 a nonprofit corporation was established that consisted of representatives from nine major research universities — Columbia University(US), Cornell University(US), Harvard University(US), Johns Hopkins University(US), Massachusetts Institute of Technology(US), Princeton University(US), University of Pennsylvania(US), University of Rochester(US), and Yale University(US).

Out of 17 considered sites in the Boston-Washington corridor, Camp Upton on Long Island was eventually chosen as the most suitable in consideration of space, transportation, and availability. The camp had been a training center from the US Army during both World War I and World War II. After the latter war, Camp Upton was deemed no longer necessary and became available for reuse. A plan was conceived to convert the military camp into a research facility.

On March 21, 1947, the Camp Upton site was officially transferred from the U.S. War Department to the new U.S. Atomic Energy Commission (AEC), predecessor to the U.S. Department of Energy (DOE).

Research and facilities

Reactor history

In 1947 construction began on the first nuclear reactor at Brookhaven, the Brookhaven Graphite Research Reactor. This reactor, which opened in 1950, was the first reactor to be constructed in the United States after World War II. The High Flux Beam Reactor operated from 1965 to 1999. In 1959 Brookhaven built the first US reactor specifically tailored to medical research, the Brookhaven Medical Research Reactor, which operated until 2000.

Accelerator history

In 1952 Brookhaven began using its first particle accelerator, the Cosmotron. At the time the Cosmotron was the world’s highest energy accelerator, being the first to impart more than 1 GeV of energy to a particle.

The Cosmotron was retired in 1966, after it was superseded in 1960 by the new Alternating Gradient Synchrotron (AGS).

The AGS was used in research that resulted in 3 Nobel prizes, including the discovery of the muon neutrino, the charm quark, and CP violation.

In 1970 in BNL started the ISABELLE project to develop and build two proton intersecting storage rings.

The groundbreaking for the project was in October 1978. In 1981, with the tunnel for the accelerator already excavated, problems with the superconducting magnets needed for the ISABELLE accelerator brought the project to a halt, and the project was eventually cancelled in 1983.

The National Synchrotron Light Source (US) operated from 1982 to 2014 and was involved with two Nobel Prize-winning discoveries. It has since been replaced by the National Synchrotron Light Source II (US) [below].

After ISABELLE’S cancellation, physicist at BNL proposed that the excavated tunnel and parts of the magnet assembly be used in another accelerator. In 1984 the first proposal for the accelerator now known as the Relativistic Heavy Ion Collider (RHIC)[below] was put forward. The construction got funded in 1991 and RHIC has been operational since 2000. One of the world’s only two operating heavy-ion colliders, RHIC is as of 2010 the second-highest-energy collider after the Large Hadron Collider(CH). RHIC is housed in a tunnel 2.4 miles (3.9 km) long and is visible from space.

On January 9, 2020, It was announced by Paul Dabbar, undersecretary of the US Department of Energy Office of Science, that the BNL eRHIC design has been selected over the conceptual design put forward by DOE’s Thomas Jefferson National Accelerator Facility [Jlab] (US) as the future Electron–ion collider (EIC) in the United States.

In addition to the site selection, it was announced that the BNL EIC had acquired CD-0 (mission need) from the Department of Energy. BNL’s eRHIC design proposes upgrading the existing Relativistic Heavy Ion Collider, which collides beams light to heavy ions including polarized protons, with a polarized electron facility, to be housed in the same tunnel.

Other discoveries

In 1958, Brookhaven scientists created one of the world’s first video games, Tennis for Two. In 1968 Brookhaven scientists patented Maglev, a transportation technology that utilizes magnetic levitation.

Major facilities

Relativistic Heavy Ion Collider (RHIC), which was designed to research quark–gluon plasma and the sources of proton spin. Until 2009 it was the world’s most powerful heavy ion collider. It is the only collider of spin-polarized protons.
Center for Functional Nanomaterials (CFN), used for the study of nanoscale materials.
BNL National Synchrotron Light Source II(US), Brookhaven’s newest user facility, opened in 2015 to replace the National Synchrotron Light Source (NSLS), which had operated for 30 years.[19] NSLS was involved in the work that won the 2003 and 2009 Nobel Prize in Chemistry.
Alternating Gradient Synchrotron, a particle accelerator that was used in three of the lab’s Nobel prizes.
Accelerator Test Facility, generates, accelerates and monitors particle beams.
Tandem Van de Graaff, once the world’s largest electrostatic accelerator.
Computational Science resources, including access to a massively parallel Blue Gene series supercomputer that is among the fastest in the world for scientific research, run jointly by Brookhaven National Laboratory and Stony Brook University.
Interdisciplinary Science Building, with unique laboratories for studying high-temperature superconductors and other materials important for addressing energy challenges.
NASA Space Radiation Laboratory, where scientists use beams of ions to simulate cosmic rays and assess the risks of space radiation to human space travelers and equipment.

Off-site contributions

It is a contributing partner to ATLAS experiment, one of the four detectors located at the Large Hadron Collider (LHC).

It is currently operating at CERN near Geneva, Switzerland.

Brookhaven was also responsible for the design of the SNS accumulator ring in partnership with Spallation Neutron Source at DOE’s Oak Ridge National Laboratory (US), Tennessee.

Brookhaven plays a role in a range of neutrino research projects around the world, including the Daya Bay Neutrino Experiment (CN) nuclear power plant, approximately 52 kilometers northeast of Hong Kong and 45 kilometers east of Shenzhen, China.