Thursday, May 14, 2015
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Press Release

Two Large Hadron Collider experiments first to observe rare subatomic process

This event display from the CMS experiment at the Large Hadron Collider shows a collision that produced a candidate for the rare decay of the Bs particle, predicted and observed to occur only about four times out of a billion. Image: CMS collaboration

Two experiments at the Large Hadron Collider at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland, have combined their results and observed a previously unseen subatomic process.

As published in the journal Nature this week, a joint analysis by the CMS and LHCb collaborations has established a new and extremely rare decay of the Bs particle (a heavy composite particle consisting of a bottom antiquark and a strange quark) into two muons. Theorists had predicted that this decay would only occur about four times out of a billion, and that is roughly what the two experiments observed.

"It's amazing that this theoretical prediction is so accurate and even more amazing that we can actually observe it at all," said Syracuse University Professor Sheldon Stone, a member of the LHCb collaboration. "This is a great triumph for the LHC and both experiments."

LHCb and CMS both study the properties of particles to search for cracks in the Standard Model, our best description so far of the behavior of all directly observable matter in the universe. The Standard Model is known to be incomplete since it does not address issues such as the presence of dark matter or the abundance of matter over antimatter in our universe. Any deviations from this model could be evidence of new physics at play, such as new particles or forces that could provide answers to these mysteries.

"Many theories that propose to extend the Standard Model also predict an increase in this Bs decay rate," said Fermilab's Joel Butler of the CMS experiment. "This new result allows us to discount or severely limit the parameters of most of these theories. Any viable theory must predict a change small enough to be accommodated by the remaining uncertainty."

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In Brief

Main Ring health and fitness

Walking around the ring is fun with friends. Photo: Cindy Arnold
How many wheels are in this picture? Photo: Cindy Arnold
There is almost no situation that cannot be improved by the company of a beshaded dog. Photo: Cindy Arnold

Fermilab employees turned out for the laboratory's annual Health and Fitness Day on Tuesday, walking and wheeling their way around the Main Ring.

In the News

Mapping dark matter

From Sky and Telescope, May 7, 2015

Observations show the universe to be a cosmic spider web: galaxies and clusters of galaxies are strung along its nodes and filaments like so many caught flies. Yet the thread — dark matter, which makes up 85 percent of the universe's mass — is largely invisible, fully visualized only in simulations.

Scientists are finding ways to map this unseen backbone of the universe, plotting its effect on light coming from distant galaxies and even from the remnant glow of the Big Bang, the cosmic microwave background.

Two projects making the invisible visible are the Dark Energy Survey, led by Josh Frieman (Fermilab) and conducted at the Cerro Tololo Inter-American Observatory in the Chilean Andes, and the Atacama Cosmology Telescope polarization survey, also in Chile and high in the Atacama Desert. These complementary surveys are taking on the universe on scales big and small.

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Physics in a Nutshell

Accelerators without batteries

Accelerators use cavities (the structures running through the center of the tunnel) to kick particle beams to higher and higher energies. Photo: Reidar Hahn

In my last column I discussed scattering experiments as a major tool for gathering information about our world. I suggested that we could get more detailed information than our light beams, in combination with our eyes, can provide if we devised more powerful beams and detectors. We concluded that providing beam energy by using flashlight batteries is not only impractical, but is innovative only in a literary sense. Powerful beams require technical innovation.

The upgrade of our beams from visible light to higher-energy particles began about 100 years ago when two scientists named Cockcroft and Walton built an electrostatic generator capable of producing 700,000 volts. By this time we were already using electron beams and beams from natural radioactive sources to investigate nature. For example, in 1909 Ernest Rutherford discovered the nucleus of the atom using a beam of alpha particles from a radioactive source.

Cockcroft and Walton used their impressive voltage generator to investigate the details of the atomic nucleus, discovering the neutron in the process. One disadvantage of this brute-force technique is that it works only for voltages less than a million volts, corresponding to 1 million electronvolts of energy to the beam, limiting the detail that could be studied.

Early particle physicists overcame the voltage constraints by using two techniques still used in modern accelerators. The first technique resembles the battery solution that proved so impractical in my last column. This time I will be more clever: The second technique makes use of a series of accelerating stations, made up of resonant cavities arranged end to end and tuned to oscillate at a particular frequency.

An oscillating voltage rapidly cycles from positive to negative. Coupled to each cavity, it provides an acceleration kick to a bunch of beam particles timed to arrive at the right point in the cycle. The resonant cavities maximize the acceleration available from the power supplied from the wall plug. In this manner we can achieve peak accelerating voltages of more than a million volts depending on the size and shape of the cavity and the frequency of the oscillations. Particles arriving nearer to the voltage peak receive a larger acceleration. By arranging the phase of the voltage oscillations with respect to the beam particles, we can give a larger kick to the late arriving particles and a smaller kick to the early arrivals. (Sometimes Einstein and his relativity theory demand that we do just the opposite. I will explain this in a subsequent column.)

We now have the primary innovation that we need to construct a linear accelerator, in which beam runs in a straight line from one end of the cavity string to the other exactly once. However, linear accelerators have their limitations as well as advantages. Next time we will move from straight-line to circular acceleration. By now I hope it is obvious that running the beam through the same accelerating cavities over and over might be a good idea. If not, stay tuned!

Roger Dixon

Photo of the Day

Bright orange line

A leafless tree stands stark against the sunset by Lake Law. Photo: Sudeshna Ganguly, University of Illinois at Urbana-Champaign
In the News

Everything you need to know about dark matter

From The Week, May 2, 2015

Scientists recently released a new map of the universe. But don't count on relying on it for space travel. The subject of this map, drawn by the Dark Energy Survey (DES), is dark matter, stuff that is invisible to us (thanks to the fact it has not been found to absorb or emit any kind of electromagnetic radiation) but that still plays a huge role in shaping our world. It is thought that dark matter is the essential ingredient that keeps galaxies from being torn apart by their own rotation. Just how else it might affect reality is still under investigation.

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