Thursday, July 16, 2015
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From symmetry

Miraculous WIMPs

What are WIMPs, and what makes them such popular dark matter candidates? Image: Sandbox Studio with Ana Kova

Invisible dark matter accounts for 85 percent of all matter in the universe, affecting the motion of galaxies, bending the path of light and influencing the structure of the entire cosmos. Yet we don't know much for certain about its nature.

Most dark matter experiments are searching for a type of particles called WIMPs, or weakly interacting massive particles.

"Weakly interacting" means that WIMPs barely ever "talk" to regular matter. They don't often bump into other matter and also don't emit light — properties that could explain why researchers haven't been able to detect them yet.

Created in the early universe, they would be heavy ("massive") and slow-moving enough to gravitationally clump together and form structures observed in today's universe.

Scientists predict that dark matter is made of particles. But that assumption is based on what they know about the nature of regular matter, which makes up only about 4 percent of the universe.

WIMPs advanced in popularity in the late 1970s and early 1980s when scientists realized that particles that naturally pop out in models of supersymmetry could potentially explain the seemingly unrelated cosmic mystery of dark matter.

Supersymmetry, developed to fill gaps in our understanding of known particles and forces, postulates that each fundamental particle has a yet-to-be-discovered superpartner. It turns out that the lightest one of the bunch has properties that make it a top contender for dark matter.

"The lightest supersymmetric WIMP is stable and is not allowed to decay into other particles," says theoretical physicist Tim Tait of the University of California, Irvine. "Once created in the big bang, many of these WIMPs would therefore still be around today and could have gone unnoticed because they rarely produce a detectable signal."

When researchers use the properties of the lightest supersymmetric particle to calculate how many of them would still be around today, they end up with a number that matches closely the amount of dark matter experimentally observed — a link referred to as the "WIMP miracle." Many researchers believe it could be more than coincidence.

Read more

Manuel Gnida

In Brief

Interior window washing at Wilson Hall - July 20-27

Next week, Clorica Management workers will wash Wilson Hall's interior windows. The cleaning schedule is as follows:

Monday, July 20: floors 12, 13, 14, 15
Tuesday, July 21: floors 9, 10, 11
Wednesday, July 22: floors 7, 8
Thursday, July 23: floors 3, 4, 5, 6
Friday, July 24: floors 1, 2, mezzanine

The insulated interior windows on floors 7, 8 and 9 will be removed in order to be cleaned.

Clorica will wash the atrium and ground-floor windows the evening of June 23.

Please clear all items from in front of windows on days work is scheduled for your floor. Contact Enixe Castro at x2798 with questions.

Photos of the Day

Coyote glimpses

A coyote sits quietly in the mown grass north of Wilson Hall. Photo: Thais Gomes
Nary a critter in sight, but this coyote appears to be chasing something near Wilson Hall. Photo: Chris Olsen, AD

Tom Jordan memorial service

A memorial service for Tom Jordan, project coordinator for QuarkNet from its inception, will take place Sunday, July 19, at 3 p.m. at Kuhn Barn.

Read Jordan's obituary.

In the News

Large Hadron Collider discovers new pentaquark particle

From BBC News, July 14, 2015

Scientists at the Large Hadron Collider have announced the discovery of a new particle called the pentaquark.

It was first predicted to exist in the 1960s but, much like the Higgs boson particle before it, the pentaquark eluded science for decades until its detection at the LHC.

The discovery, which amounts to a new form of matter, was made by the Hadron Collider's LHCb experiment.

Read more

Frontier Science Result: DZero

Hadron assembly instructions

Determining the precise details of how the strong nuclear force assembles quarks into hadrons is not always easy.

Disponible en español

Method A:
3 quarks
chromodynamic glue

Step 1. Glue the quarks together.
Step 2. Voila!

Method A will produce a baryon; most of your more massive hadrons are baryons. The type of baryon will be determined in part by the flavor of the quarks used and how the quarks are aligned. For example, if you have glued together one up quark, one down quark and one strange quark, you have created a Lambda baryon or maybe a Sigma baryon. If you glued together two up quarks and a down quark, you have created a proton. (Of course, the work going on now out at the Fermilab Proton Assembly Building has nothing to do with any of this.)

Method B:
1 quark
1 antiquark
chromodynamic glue

Step 1. Glue the quark and antiquark together.
Step 2. Voila!

Method B will produce a meson, typically a lighter kind of hadron. Again, the type of meson will be determined by the flavors of the specific quark and antiquark. If for example you glued together an up quark to a strange antiquark, you have created a K+ meson. (Of course, the work going on now out at the Fermilab Meson Assembly Building has nothing to do with any of this.)

Method C:
There is no Method C
Or is there?

Chromodynamics is the theory of the forces — the glue — that binds quarks and antiquarks to make hadrons. The theory has a great deal of experimental support, but there are times when it is hard to make exact computations using it. In particular, theory doesn't clearly say that baryons and mesons are the only kinds of hadrons that can be constructed. Starting a little over a decade ago, increasingly strong experimental evidence emerged in favor of the existence of other hadrons — neither baryons nor mesons. But, as is so often the case in science, different experiments do not always get the same result, and those results need to be confirmed.

An example is the X(4140). This particle, sometimes called the Y(4140), was first reported by the CDF experiment in 2009. It does not seem to be the result of following either Method A or Method B. It seems to be some other sort of thing — perhaps it is two quarks and two antiquarks, or perhaps some other method was followed in assembling this hadron.

Shortly afterwards, the Belle experiment in Japan, using a different method, reported that they did not see the X(4140). LHCb at CERN also did not see it. But the CMS experiment reported that they did see it. Last year, DZero published the results of our first search for the X(4140): We saw it.

The three experiments with positive results all looked for X(4140) created as a result of the decay of a B+ meson. If the X(4140) really exists, it should have been possible to create it at the Tevatron directly, without making first a B+ meson.

DZero has recently searched for the X(4140) without requiring it to be the result of a B+ meson decay, and we discovered that there were 1,964 ± 248 such events in the Tevatron data. The number of X(4140) from meson decays is only 809 ± 175, so this is evidence that X(4140) can indeed be created directly.

Further study is needed before we can be completely certain of all of the properties of the X(4140). But we now have already strong evidence that there is a third way to assemble a hadron.

Leo Bellantoni

Avdhesh Chandra (Rice University) and Daria Zieminska (Indiana University) are the primary analysts for this measurement.
The DZero collaboration relies upon many of its collaborators to carefully review analyses for scientific quality before they are released. This analysis was guided by Editorial Board Chairs Brendan Casey and Gene Fisk (both of Fermilab).
In the News

PIP-II to advance intense particle beam physics

From Cold Facts, July 8, 2015

With the power of PIP-II, Fermilab (CSA CSM) is planning to construct and operate the foremost facility in the world for particle physics research utilizing intense beams.

In 2014, the Particle Physics Prioritization Panel published a report that examined ways to keep the US at the forefront of particle physics and recommended investing a larger portion of the DOE budget in the construction of new experimental facilities, and especially muon and neutrino programs. The panel advocated that building a world-leading neutrino program hosted in the US should be one of the nation's top priorities.

Read more