HADES: Continues search for Dark Matter Beyond the Standard Model

HADES at the GSI in Darmstadt, Germany searches for dark matter candidates. 

Credit: 3-D Rendering: A. Schmah /HADES

Although Dark Energy and Dark Matter appear to constitute over 95 percent of the universe, nobody knows of which particles they are made up.

Astrophysicists now crossed one potential Dark Matter candidate, the Dark Photon or U boson, off the list in top position.

This is the result of recent HADES experiments, where researchers from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and from 17 other European institutes try to pin down the nature of Dark Matter.

These negative results, recently published in Physics Letters B, could even lead to challenges of the Standard Model of particle physics.

The interpretation of current astrophysical observations results in the striking mass-energy budget of matter in the universe: 75% Dark Energy and 20% Dark Matter. Only about 5% of the universe consists of "ordinary", baryonic matter.

Many attempts have been made to explain the nature of Dark Matter. Researchers believe that Dark Matter is comprised by hitherto unknown particles which do not fit into the Standard Model of particle physics.

The Standard Model is a theoretically sound quantum field theory with fundamental matter particles, such as quarks (bound in hadrons, e.g., baryons) and leptons (e.g., electrons and neutrinos), which interact via exchange of force-carrier quanta, called gauge bosons (e.g., photons).

Some of these species acquire their masses by the interaction with the Higgs boson.

While evidences for the Higgs boson were found recently at CERN, the Standard Model looks now complete when supplemented by some neutrino masses, and nothing else seems to be needed to understand the wealth of atomic, sub-nuclear and particle physics phenomena.

Nevertheless, Dark Matter appears not to be explained by any of the constituents of the Standard Model.

This status of the affair has initiated worldwide efforts to search for Dark Matter candidates.

Beyond the Standard Model
Searching a needle in the haystack is simpler: one knows both the wanted object (the needle) and the place (the haystack).

In the case of Dark Matter the object is unknown, and the localisation, e.g. in galactic halos, is also not constraining the loci of interest.

To specify the search goal one can envisage diverse hypothetical candidates, such as certain hypothetical particles beyond the Standard Model, which fulfill requirements qualifying them as constituents of Dark Matter.

Dark Energy drives the presently observed accelerated expansion of the universe. Dark Energy is homogeneously distributed and can be attributed to a cosmological constant or vacuum energy.

In extreme cases it may cause, in the future, such a sudden expansion that anything in the universe is disrupted, this would be the Big Rip.

Dark Matter, in contrast, is bumpy and is needed to explain the formation of the observed density distribution of visible matter in the evolving universe, evidenced by the hierarchy of structures from (super)clusters of galaxies, galaxies, stars, planets and other compact objects such as meteorites, etc.

Among the lists of candidates of Dark Matter is a hypothetical particle, often dubbed U boson or Dark Photon.

These nicknames refer to the underlying theory construction: a second unitary ("U") symmetry allows for quanta which are, in one respect, similar to photons, namely gauge bosons, but in another respect different from photons, namely attributing to these quanta a mass, making them to Dark Photons because of a very weak interaction with normal matter.

Very similar to photons the Dark Photons can decay into electron-positron pairs, if they have the proper virtuality.

Combining the chain of hypotheses one arrives at a scenario, where an "ordinary" virtual photon converts into a Dark Photon which decays afterwards into an electron-positron pair.

Background on HADES
HADES is an acronym of High-Acceptance Di-Electron Spectrometer.

It is an optimised detector system operated by a European collaboration of about hundred physicists at GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt.

HADES is aimed at investigating virtual photon signals emitted as electron-positron pairs off compressed nuclear matter to understand the origin of the phenomenon "masses of hadrons" and test it in some detail.

The highly sophisticated apparatus HADES has the capability to select electron-positron pairs, which can be attributed to primary sources, out of a huge background of other particles .

Read the full article here

More information: G. Agakishiev et al. (HADES Collaboration), Phys. Lett. B 731, 265 (2014) dx.doi.org/10.1016/j.physletb.2014.02.035

CERN Future Circular Collider (FCC): Europe's new giant particle collider

Europe's physics lab CERN said Thursday it was eyeing plans for a circular particle collider that would be seven times more powerful than the facility which discovered the famous "God particle."

"The time has come to look even further ahead," the European Organisation for Nuclear Research (CERN) announced.

In 2012, CERN's Large Hadron Collider (LHC)—a giant lab housed in a 27-kilometre (17-mile) tunnel straddling the French-Swiss border—identified what is believed to be the Higgs boson, the long-sought maker of mass theorised in the 1960s.

The facility flushed out the so-called God particle by crashing proton beams at velocities near the speed of light. It went offline a year ago for an 18-month overhaul.

The LHC, completed in 2008, has "at least 20 more years" of life in it, the agency said.

However, the long time needed to build its successor—the LHC took a quarter of a century—means that planning should start now.

It will launch a feasibility study next week for a so-called Future Circular Collider (FCC) with a circumference of 80 to 100 kilometres (50 to 60 miles).

The FCC would probably be located in the same area and may incorporate the LHC tunnel in its infrastructure, CERN said in a statement.

The LHC after its overhaul will see collision energies reach 14 teraelectron volts (TeV) but the FCC should be capable of reaching unprecedented smashups of around 100 TeV, CERN said.

The FCC study will run in parallel with an ongoing probe into an alternative design—an 80-kilometre (50-mile) straight collider dubbed the Compact Linear Collider (CLIC).

The two studies will examine the feasibility and costs and produce conceptual designs by 2018/2019, when the European-wide strategy on particle physics research is set to be updated.

The winner will be "a worthy successor to the LHC," CERN said.

"Such an accelerator would allow particle physics to push back the boundaries of knowledge even further," it claimed.

Targets could include supersymmetry, the notion that there are novel particles which mirror each known particle.

It could also help shed light on dark matter, which comprises most of the cosmos and whose existence is inferred from their impact on ordinary matter.

Some 300 scientists will meet at the University of Geneva from February 12-15 to kick off the five-year feasibility study for the FCC.

DarkSide: Project aims to find particles of dark matter

The DarkSide-50 research team is made up of faculty, students and researchers from dozens of institutions around the world,

From left, Luca Grandi, an assistant professor at the University of Chicago, Richard Saldanha, an associate fellow in the Kavli Institute for Cosmological Physics at the University of Chicago, and Hanguo Wang, a researcher at the University of California-Los Angeles. 

While wearing protective clothing to keep the environment clean, they are working to assemble the core of the dark matter detector, an argon-filled tank with photodetectors at the top and bottom to spot the light from collisions, and copper coils to help determine where the collisions occur. 

Credit: Peter Meyers

In a laboratory under a mountain 80 miles east of Rome this fall, a Princeton-led international team switched on a new experiment aimed at finding a mysterious substance that makes up a quarter of the universe but has never been seen.

The experiment, known as DarkSide-50, is searching for particles of dark matter. For the last several decades, researchers have known that visible matter—the stuff we can see—makes up only 4 percent of the universe, while dark energy is thought to make up about 73 percent.

Dark matter is thought to make up the remaining 23 percent, and finding it, researchers say, will solidify our understanding of how the universe formed and shed light on its ultimate fate.

Peter Meyers

"This is like the search for the Higgs boson was 10 years ago," said Peter Meyers, a professor of physics at Princeton University and one of the lead scientists on the project.

"We have a good idea of what to look for, but we don't know exactly where or when we will find it."

Housed inside a cavernous chamber in Italy's Gran Sasso National Laboratory, the DarkSide-50 collaboration involves 17 American institutions as well the Italian Institute for Nuclear Physics (INFN) and other institutions in Italy, France, Poland, Ukraine, Russia and China.

The research team includes postdocs, staff researchers and several graduate and undergraduate students from Princeton.

The researchers spent last summer assembling the detector, which consists of three fluid-filled chambers nested one inside the other like Russian dolls.

Now that the experiment is up and running, the waiting begins. Unlike the massive Large Hadron Collider that discovered the Higgs, DarkSide-50 doesn't smash anything. Instead, it is designed to detect dark matter particles that drift through its chambers.

Looking for WIMPs 
The evidence for dark matter dates to the 1930s, when astronomers realized that the amount of matter we can see—as planets, stars and galaxies—falls far short of what must be out there to give galaxies their characteristic spiral shapes and clustering patterns.

Without this missing matter, the galaxies should have flown apart long ago. Matter provides the gravity that keeps the stars in rotation around the galaxy's center.

Unless our theories of gravity are wrong—and a minority of physicists think that is a possibility—dark matter must exist.

Cristiano Galbiati

"Finding dark matter particles would help confirm our understanding of the universe," said Cristiano Galbiati, an associate professor of physics at Princeton.

"And, whether or not we find it, we will have learned a great deal about how to go about looking for it. This is as exciting a moment in the search for dark matter as there has ever been."

Although no one knows for sure what dark matter is made of, the DarkSide-50 team and many other scientists think the most likely candidate is a particle so weak that it is called a WIMP, which is short for "weakly interacting massive particle."

As the name suggests, WIMPs barely interact with their surroundings. They simply drift through walls like ghosts.

If you cup your hands together, you will surround—but never trap—a few of these ethereal beings. Scientists suggest that a WIMP can be detected when it smacks into the nucleus of an atom such as argon, which is found in air.

When this happens in a chamber of densely packed argon atoms, the stricken atom recoils and creates a track of excited argon atoms in its wake.

This track appears as a fleeting trail of light, which can be detected by devices called photodetectors. But these collisions are rare—just a few WIMPs are detected each year.

Because other particles also give off light when they collide with argon, DarkSide-50 is located nearly a mile beneath Gran Sasso mountain ("gran sasso" is Italian for "great stone").

The rock shields out cosmic-ray particles that routinely bombard the Earth.

To read the full article go here

CERN LHC: New results indicate that Higgs boson particle is discovered

Event display of a H -> 4mu candidate event with m(4l) = 124.1 (125.1) GeV without (with) Z mass constraint. 

The masses of the lepton pairs are 86.3 GeV and 31.6 GeV. 

The event was recorded by ATLAS on 10-Jun-2012, 13:24:31 CEST in run number 204769 as event number 71902630.

Zoom into the tracking detector. Muon tracks are coloured red. 

Credit: ATLAS Experiment © 2012 CERN

At the Moriond Conference today, the ATLAS and CMS collaborations at CERN's Large Hadron Collider (LHC) presented preliminary new results that further elucidate the particle discovered last year.

Having analysed two and a half times more data than was available for the discovery announcement in July, they find that the new particle is looking more and more like a Higgs boson, the particle linked to the mechanism that gives mass to elementary particles.

It remains an open question, however, whether this is the Higgs boson of the Standard Model of particle physics, or possibly the lightest of several bosons predicted in some theories that go beyond the Standard Model. Finding the answer to this question will take time.

Whether or not it is a Higgs boson is demonstrated by how it interacts with other particles, and its quantum properties. For example, a Higgs boson is postulated to have no spin, and in the Standard Model its parity -- a measure of how its mirror image behaves -- should be positive.

CMS and ATLAS have compared a number of options for the spin-parity of this particle, and these all prefer no spin and positive parity. This, coupled with the measured interactions of the new particle with other particles, strongly indicates that it is a Higgs boson.

"The preliminary results with the full 2012 data set are magnificent and to me it is clear that we are dealing with a Higgs boson though we still have a long way to go to know what kind of Higgs boson it is." said CMS spokesperson Joe Incandela.

"The beautiful new results represent a huge effort by many dedicated people. They point to the new particle having the spin-parity of a Higgs boson as in the Standard Model. We are now well started on the measurement programme in the Higgs sector," said ATLAS spokesperson Dave Charlton.

To determine if this is the Standard Model Higgs boson, the collaborations have, for example, to measure precisely the rate at which the boson decays into other particles and compare the results to the predictions.

The detection of the boson is a very rare event -- it takes around 1 trillion (1012) proton-proton collisions for each observed event. To characterize all of the decay modes will require much more data from the LHC.

The above story is reprinted from materials provided by CERN, the European Organization for Nuclear Research.