From the Theory Group:
Measuring the Masses of Invisible Particles
Simulation of a supersymmetric event as viewed by the Large Hadron Collider's ATLAS detector.
Dark matter particles could be revealed as missing elements
in the aftermath of such collisions. (Image courtesy CERN.)
Astrophysical observations indicate that 80 percent of the mass in the universe is made of a neutral, very weakly interacting type of matter called "dark matter." Dark matter is composed of elementary particles, but these must be of a new type, not found in the Standard Model of particle physics. Many models of dark matter have been proposed. In most of these, dark matter is composed of a heavy, neutral, stable particle that can be produced at colliders of sufficiently high energy. Now the Large Hadron Collider is running at CERN, and this accelerator could well be capable of producing dark matter particles. If researchers can identify the events with dark matter production, we will have a unique opportunity to study the elementary quanta of dark matter in the laboratory.
However, the essential property of dark matter—its weak interaction with ordinary matter—makes its particles difficult to study. Dark matter particles are expected to leave no signals or energy deposits in the detectors at the LHC. If theory holds true, they will be visible only as a missing element in the aftermath of LHC collisions, for example, as events in which the observed particle tracks have unbalanced momentum. Still, we will want to know what the LHC observations can tell us about the missing particles. Is the invisible particle produced in the laboratory the same as that in the dark matter in space? To answer this question, scientists need to measure the properties of the dark matter particle, such as its mass, spin and interaction strengths.
Reminder for Exempt Employees
Are you an exempt employee? Do you complete a yellow effort/time sheet? If so, this reminder is for you!
For regular exempt employees, including faculty, who are paid through the payroll system, pay is considered to be remuneration for
all work that benefits SLAC. For full-time employees, this is 100
percent full-time equivalence, or FTE. For exempt employees, 100 percent FTE does not equate to any set number of hours, e.g., 40 or 50 hours
a week; it equates to the totality of SLAC-compensated effort. Although many SLAC systems use standard hours of 40 to indicate 100
percent FTE, this does not mean that exempt employees who are 100 percent FTE work only 40 hours per week.
When completing your yellow effort/time sheet, please make sure your effort distribution is based on the total effort for the period, including any time in excess of 40 hours per week.
In other words, each project that you worked on should be assigned a percentage of effort based on the total actual hours worked for the period. Also, remember to only distribute effort to charge numbers that benefited from the effort. Please contact Payroll at
firstname.lastname@example.org if you have any questions. Thank you!
Seen Around SLAC: Peninsula Bridge Summer Program Student Tour
Joanne Stubbs explains radioactivity and demonstrates a Geiger counter. Photo by Lori White.)
Thirty-one bright, energetic fifth-grade girls from Peninsula Bridge Summer Program at Castilleja School in Palo Alto invaded SLAC on Tuesday, July 13, for a tour of the campus and a glimpse of the exciting possibilities science could offer in their own lives.
The girls began their adventure at the Visitors' Center, where SLACers Zoe Van Hoover and Kelen Tuttle introduced them to the lab's history and science. Apurva Mehta and Deborah Bard then explained X-rays and astrophysics. The afternoon offered opportunities for hands-on activities at the Stanford Synchrotron Radiation Lightsource Building.
"A major aim of our summer program is giving academically-motivated fifth-grade girls from low-income backgrounds access to and knowledge about exciting scientific exploration," said Mary Hurlbut, the girls' teacher. She said she was thrilled by their enthusiasm.
"Exciting" was only one of the words the girls used to describe their visit. They also used "Fun", "cool", "interesting" and "amazing."
Hurlbut hopes the benefits of the trip are more subtle and long-lasting than a fun day away from the classroom. In addition to exposing her students to cutting-edge science, Hurlbut wants them to understand their gender is no barrier to joining in the exploration. The lesson seemed to be sinking in. As one student, Selenne, put it, "Yesterday I learned that nail polish is a part of science."