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How a Single Chemical Bond Balances Cells Between Life and Death

With SLAC's X-ray laser and synchrotron, scientists measured exactly how much energy goes into keeping a crucial chemical bond from triggering a cell's death spiral.

New Efficient, Low-Temperature Catalyst for Converting Water and CO to Hydrogen Gas and CO2

Scientists have developed a new low-temperature catalyst for producing high-purity hydrogen gas while simultaneously using up carbon monoxide (CO). The discovery could improve the performance of fuel cells that run on hydrogen fuel but can be poisoned by CO.

Study Sheds Light on How Bacterial Organelles Assemble

Scientists at Berkeley Lab and Michigan State University are providing the clearest view yet of an intact bacterial microcompartment, revealing at atomic-level resolution the structure and assembly of the organelle's protein shell. This work can help provide important information for research in bioenergy, pathogenesis, and biotechnology.

A Single Electron's Tiny Leap Sets Off 'Molecular Sunscreen' Response

In experiments at the Department of Energy's SLAC National Accelerator Laboratory, scientists were able to see the first step of a process that protects a DNA building block called thymine from sun damage: When it's hit with ultraviolet light, a single electron jumps into a slightly higher orbit around the nucleus of a single oxygen atom.

Researchers Find New Mechanism for Genome Regulation

The same mechanisms that separate mixtures of oil and water may also help the organization of an unusual part of our DNA called heterochromatin, according to a new study by Berkeley Lab researchers. They found that liquid-liquid phase separation helps heterochromatin organize large parts of the genome into specific regions of the nucleus. The work addresses a long-standing question about how DNA functions are organized in space and time, including how genes are silenced or expressed.

The Rise of Giant Viruses

Research reveals that giant viruses acquire genes piecemeal from others, with implications for bioenergy production and environmental cleanup.

Grasses: The Secrets Behind Their Success

Researchers find a grass gene affecting how plants manage water and carbon dioxide that could be useful to growing biofuel crops on marginal land.

SLAC Experiment is First to Decipher Atomic Structure of an Intact Virus with an X-ray Laser

An international team of scientists has for the first time used an X-ray free-electron laser to unravel the structure of an intact virus particle on the atomic level. The method dramatically reduces the amount of virus material required, while also allowing the investigations to be carried out several times faster than before. This opens up entirely new research opportunities.

New Perspectives Into Arctic Cloud Phases

Teamwork provides insight into complicated cloud processes that are important to potential environmental changes in the Arctic.

Illuminating a Better Way to Calculate Excitation Energy

In a new study appearing this week in The Journal of Chemical Physics, researchers demonstrate a new method to calculate excitation energies. They used a new approach based on density functional methods, which use an atom-by-atom approach to calculate electronic interactions. By analyzing a benchmark set of small molecules and oligomers, their functional produced more accurate estimates of excitation energy compared to other commonly used density functionals, while requiring less computing power.


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Chicago Quantum Exchange to Create Technologically Transformative Ecosystem

The University of Chicago is collaborating with the U.S. Department of Energy's Argonne National Laboratory and Fermi National Accelerator Laboratory to launch an intellectual hub for advancing academic, industrial and governmental efforts in the science and engineering of quantum information.

Department of Energy Awards Six Research Contracts Totaling $258 Million to Accelerate U.S. Supercomputing Technology

Today U.S. Secretary of Energy Rick Perry announced that six leading U.S. technology companies will receive funding from the Department of Energy's Exascale Computing Project (ECP) as part of its new PathForward program, accelerating the research necessary to deploy the nation's first exascale supercomputers.

Cynthia Jenks Named Director of Argonne's Chemical Sciences and Engineering Division

Argonne has named Cynthia Jenks the next director of the laboratory's Chemical Sciences and Engineering Division. Jenks currently serves as the assistant director for scientific planning and the director of the Chemical and Biological Sciences Division at Ames Laboratory.

Argonne-Developed Technology for Producing Graphene Wins TechConnect National Innovation Award

A method that significantly cuts the time and cost needed to grow graphene has won a 2017 TechConnect National Innovation Award. This is the second year in a row that a team at Argonne's Center for Nanoscale Materials has received this award.

Honeywell UOP and Argonne Seek Research Collaborations in Catalysis Under Technologist in Residence Program

Researchers at Argonne are collaborating with Honeywell UOP scientists to explore innovative energy and chemicals production.

Follow the Fantastic Voyage of the ICARUS Neutrino Detector

The ICARUS neutrino detector, born at Gran Sasso National Lab in Italy and refurbished at CERN, will make its way across the sea to Fermilab this summer. Follow along using an interactive map online.

JSA Awards Graduate Fellowships for Research at Jefferson Lab

Jefferson Sciences Associates announced today the award of eight JSA/Jefferson Lab graduate fellowships. The doctoral students will use the fellowships to support their advanced studies at their universities and conduct research at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) - a U.S. Department of Energy nuclear physics laboratory managed and operated by JSA, a joint venture between SURA and PAE Applied Technologies.

Muon Magnet's Moment Has Arrived

On May 31, the 50-foot-wide superconducting electromagnet at the center of the Muon g-2 experiment saw its first beam of muon particles from Fermilab's accelerators, kicking off a three-year effort to measure just what happens to those particles when placed in a stunningly precise magnetic field. The answer could rewrite scientists' picture of the universe and how it works.

Seven Small Businesses to Collaborate with Argonne to Solve Technical Challenges

Seven small businesses have been selected to collaborate with researchers at Argonne to address technical challenges as part of DOE's Small Business Vouchers Program.

JSA Names Charles Perdrisat and Charles Sinclair as Co-Recipients of its 2017 Outstanding Nuclear Physicist Prize

Jefferson Science Associates, LLC, announced today that Charles Perdrisat and Charles Sinclair are the recipients of the 2017 Outstanding Nuclear Physicist Prize. The 2017 JSA Outstanding Nuclear Physicist Award is jointly awarded to Charles Perdrisat for his pioneering implementation of the polarization transfer technique to determine proton elastic form factors, and to Charles Sinclair for his crucial development of polarized electron beam technology, which made such measurements, and many others, possible.


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Oxygen: The Jekyll and Hyde of Biofuels

Scientists are devising ways to protect plants, biofuels and, ultimately, the atmosphere itself from damage caused by an element that sustains life on earth.

The Rise of Giant Viruses

Research reveals that giant viruses acquire genes piecemeal from others, with implications for bioenergy production and environmental cleanup.

Grasses: The Secrets Behind Their Success

Researchers find a grass gene affecting how plants manage water and carbon dioxide that could be useful to growing biofuel crops on marginal land.

New Perspectives Into Arctic Cloud Phases

Teamwork provides insight into complicated cloud processes that are important to potential environmental changes in the Arctic.

Mountaintop Plants and Soils to Become Out of Sync

Plants and soil microbes may be altered by climate warming at different rates and in different ways, meaning vital nutrient patterns could be misaligned.

If a Tree Falls in the Amazon

For the first time, scientists pinpointed how often storms topple trees, helping to predict how changes in Amazonia affect the world.

Turning Waste into Fuels, Microbial Style

A newly discovered metabolic process linking different bacteria in a community could enhance bioenergy production.

Department of Energy Awards Six Research Contracts Totaling $258 Million to Accelerate U.S. Supercomputing Technology

Today U.S. Secretary of Energy Rick Perry announced that six leading U.S. technology companies will receive funding from the Department of Energy's Exascale Computing Project (ECP) as part of its new PathForward program, accelerating the research necessary to deploy the nation's first exascale supercomputers.

Electrifying Magnetism

Researchers create materials with controllable electrical and magnetic properties, even at room temperature.

One Step Closer to Practical Fast Charging Batteries

Novel electrode materials have designed pathways for electrons and ions during the charge/discharge cycle.


Q&A: SLAC's Vera Luth Discusses the Search for New Physics

Article ID: 676063

Released: 2017-06-07 14:05:47

Source Newsroom: SLAC National Accelerator Laboratory

  • Credit: Dawn Harmer/SLAC National Accelerator Laboratory

    Vera Lüth, professor emerita of experimental particle physics at SLAC.

  • Credit: Peter Ginter/SLAC National Accelerator Laboratory

    View of the BABAR detector (about six meters in diameter) with staff scientist Michael Kelsey analyzing problems during a shutdown of SLAC’s PEP-II electron-positron storage rings.

  • Credit: SLAC National Accelerator Laboratory

    An electron-positron annihilation producing a pair of B mesons as recorded by the BABAR detector at the PEP-II storage rings. Among the reconstructed curved particle tracks is a muon (bottom left). The direction of the associated anti-neutrino (dashed arrow) is identified as missing momentum. Both particles originate from the same B-meson decay.

The Standard Model of particle physics describes the properties and interactions of the constituents of matter. The development of this theory began in the early 1960s, and in 2012 the last piece of the puzzle was solved by the discovery of the Higgs boson at the Large Hadron Collider (LHC) at CERN in Switzerland. Experiments have confirmed time and again the Standard Model’s very accurate predictions.

Yet, researchers have reasons to believe that physics beyond the Standard Model exists and should be found. For instance, the Standard Model does not explain why matter dominates over antimatter in the universe. It also does not provide clues about the nature of dark matter – the invisible substance that is five times more prevalent than the regular matter we observe.

In this Q&A, particle physicist Vera Lüth discusses scientific results that potentially hint at physics beyond the Standard Model. The professor emerita of experimental particle physics at the Department of Energy’s SLAC National Accelerator Laboratory is co-author of a review article published today in Nature that summarizes the findings of three experiments: BABAR at SLAC, Belle in Japan and LHCb at CERN. 

What are the hints of new physics that you describe in your article?

The hints originate from studies of an elementary particle, known as the B meson – an unstable particle produced in the collision of powerful particle beams. More precisely, these studies looked at decays of the B meson that involve leptons – electrically charged elementary particles and their associated neutrinos. There are three charged leptons: the electron, a critical component of atoms discovered in 1897; the muon, first observed in cosmic rays in 1937; and the much heavier tau, discovered at the SPEAR electron-positron (e+e-) storage ring at SLAC in 1975 by Martin Perl.

Due to their very different masses, the three leptons also have very different lifetimes. The electron is stable, whereas the muon and tau decay in a matter of microseconds and a fraction of a picosecond, respectively. A fundamental assumption of the Standard Model is that the interactions of the three charged leptons are the same if their different masses and lifetimes are taken into account.

Over many years, different experiments have tested this assumption – referred to as “lepton universality” – and to date no definite violation of this rule has been observed. We now have indications that the rates for B meson decays involving tau leptons are larger than expected compared to the measured rates of decays involving electrons or muons, taking into account the differences in mass. This observation would violate lepton universality, a fundamental assumption of the Standard Model.

What does a violation of the Standard Model actually mean?

It means that there is evidence for phenomena that we cannot explain in the context of the Standard Model. If such a phenomenon is firmly established, the Standard Model needs to be extended – by introducing new fundamental particles and also new interactions related to these particles.

In recent years, searches for fundamentally new phenomena have relied on high-precision measurements to detect deviations from Standard Model predictions or on searches for new particles or interactions with properties that differ from known ones.

What exactly are the BABAR, Belle and LHCb experiments?

They are three experiments that have challenged lepton universality.

Belle and BABAR were two experiments specifically designed to study B mesons with unprecedented precision – particles that are five times heavier than the proton and contain a bottom or b quark. These studies were performed at e+e- storage rings that are commonly referred to as B factories and operate at colliding-beam energies just high enough to produce a pair of B mesons, and no other particle. BABAR operated at SLAC’s PEP-II from 1999 to 2008, Belle at KEKB in Japan from 1999 to 2010. The great advantage of these experiments is that the B mesons are produced pairwise, each decaying into lighter particles – on average five charged particles and a similar number of photons.

The LHCb experiment is continuing to operate at the proton-proton collider LHC with energies that exceed the ones of B factories by more than a factor of 1,000. At this higher energy, B mesons are produced at a much larger rate than at B factories. However, at each crossing of the beams, hundreds of other particles are produced in addition to B mesons. This feature tremendously complicates the identification of B meson decays.  

To study lepton universality, all three experiments focus on B decays involving a charged lepton and an associated neutrino. A neutrino doesn’t leave a trace in the detector, but its presence is detected as missing energy and momentum in an individual B decay.

What evidence do you have so far for a potential violation of lepton universality?

All three experiments have identified specific B meson decays and have compared the rates of decays involving an electron or muon to those involving the higher mass tau lepton. All three experiments observe higher-than-expected decay rates for the decays with a tau. The average value of the reported results, taking into account the statistical and systematic uncertainties, exceeds the Standard Model expectation by four standard deviations.

This enhancement is intriguing, but not considered sufficient to unambiguously establish a violation of lepton universality. To claim a discovery, particle physicists generally demand a significance of at least five standard deviations. However, the fact that this enhancement was detected by three experiments, operating in very different environments, deserves attention. Nevertheless, more data will be needed, and are expected in the not too distant future.

What was your role in this research?

As the technical coordinator of the BABAR collaboration during the construction of the detector, I was the liaison between the physicists and the engineering teams, supported by the BABAR project management team at SLAC. With more than 500 BABAR members from 11 countries, this was a challenging task, but with the combined expertise and dedication of the collaboration the detector was completed and ready to take data in four years.

Once data became available, I rejoined SLAC’s Research Group C and took over its leadership from Jonathan Dorfan. As convener of the physics working group on B decays involving leptons, I coordinated various analyses by scientists from different external groups, among them SLAC postdocs and graduate students, and helped to develop the analysis tools needed for precision measurements.

Almost 10 years ago, we started updating an earlier analysis performed under the leadership of Jeff Richman of the University of California, Santa Barbara on B decays involving tau leptons and extended it to the complete BABAR data set. This resulted in the surprisingly large decay rate. The analysis was the topic of the PhD thesis of my last graduate student, Manuel Franco Sevilla, who over the course of four years made a number of absolutely critical contributions that significantly improved the precision of this measurement, and thereby enhanced its significance.

What keeps you excited about particle physics?

Over the past 50 years that I have been working in particle physics, I have witnessed enormous progress in theory and experiments leading to our current understanding of matter’s constituents and their interactions at the most fundamental level. But there are still many unanswered questions, from very basic ones like “Why do particles have certain masses and not others?” to questions about the grand scale of things, such as “What is the origin of the universe, and is there more than one?”

Lepton universality is one of the Standard Model’s fundamental assumptions. If it were violated, unexpected new physics processes must exist. This would be a major breakthrough – even more surprising than the discovery of the Higgs boson, which was predicted to exist many decades ago.  

What results do you expect in the near future?

There is actually a lot going on in the field. LHCb researchers are collecting more data and will try to find out if the lepton universality is indeed violated. My guess is that we should know the answer by the end of this year. A confirmation will be a great event and will undoubtedly trigger intense experimental and theoretical research.

At present we do not understand the origin of the observed enhancement. We first assumed that it could be related to a charged partner of the Higgs boson. Although the observed features did not match the expectations, an extension of the Higgs model could do so. Another possible explanation that can neither be confirmed nor excluded is the presence of so-called lepto-quarks. These open questions will remain a very exciting topic that need to be addressed by experiments and theoretical work.

Recently, LHCb scientists have reported an interesting result indicating that certain B meson decays more often include an electron pair than a muon pair. However, the significance of this new finding is only about 2.6 standard deviations, so it’s too early to draw any conclusions. BABAR and Belle have not confirmed this observation.

At the next-generation B factory, Super-KEKB in Japan, the new Belle II experiment is scheduled to begin its planned 10-year research program in 2018. The expected very large new data sets will open up many opportunities for searches for these and other indications of physics beyond the Standard Model.


SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, Calif., SLAC is operated by Stanford University for the U.S. Department of Energy's Office of Science. For more information, please visit slac.stanford.edu.

SLAC National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.