Standard Model may account for Dark Matter and it may be massive

A massive cluster of yellowish galaxies, seemingly caught in a red and blue spider web of eerily distorted background galaxies, makes for a spellbinding picture from the new Advanced Camera for Surveys (ACS) aboard NASA's Hubble Space Telescope. 

To make this unprecedented image of the cosmos, Hubble peered straight through the center of one of the most massive galaxy clusters known, called Abell 1689. 

The gravity of the cluster's trillion stars, plus dark matter, acts as a 2-million-light-year-wide lens in space. 

This gravitational lens bends and magnifies the light of the galaxies located far behind it. Some of the faintest objects in the picture are probably over 13 billion light-years away (redshift value 6). 

Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter. 

Credit: NASA, N. Benitez (JHU), T. Broadhurst (Racah Institute of Physics/The Hebrew University), H. Ford (JHU), M. Clampin (STScI),G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA

The physics community has spent three decades searching for and finding no evidence that dark matter is made of tiny exotic particles.

Case Western Reserve University theoretical physicists suggest researchers consider looking for candidates more in the ordinary realm and, well, more massive.

Dark matter is unseen matter, that, combined with normal matter, could create the gravity that, among other things, prevents spinning galaxies from flying apart.

Physicists calculate that dark matter comprises 27 percent of the universe; normal matter 5 percent.

Instead of WIMPS, weakly interacting massive particles, or axions, which are weakly interacting low-mass particles, dark matter may be made of macroscopic objects, anywhere from a few ounces to the size of a good asteroid, and probably as dense as a neutron star, or the nucleus of an atom, the researchers suggest.

Physics professor Glenn Starkman and David Jacobs, who received his PhD in Physics from CWRU in May and is now a fellow at the University of Cape Town, say published observations provide guidance, limiting where to look.

They lay out the possibilities in a paper "Macro Dark Matter"

The Macros, as Starkman and Jacobs call them, would not only dwarf WIMPS and axions, but differ in an important way.

They could potentially be assembled out of particles in the Standard Model of particle physics instead of requiring new physics to explain their existence.

"We've been looking for WIMPs for a long time and haven't seen them," Starkman said. "We expected to make WIMPS in the Large Hadron Collider (LHC), and we haven't."

WIMPS and axions remain possible candidates for dark matter, but there's reason to search elsewhere, the theorists argue.

"The community had kind of turned away from the idea that dark matter could be made of normal-ish stuff in the late '80s," Starkman said.

"We ask, was that completely correct and how do we know dark matter isn't more ordinary stuff— stuff that could be made from quarks and electrons?"

After eliminating most ordinary matter, including failed Jupiters, white dwarfs, neutron stars, stellar black holes, the black holes in centers of galaxies and neutrinos with a lot of mass, as possible candidates, physicists turned their focus on the exotics.

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 ATLAS experiment: Higgs boson decays to two tau particles

The ATLAS detector, open during a recent technical stop. 

Credit: Maximilien Brice /CERN

The ATLAS experiment at CERN has released preliminary results that show evidence that the Higgs boson decays to two tau particles.

Taus belong to a group of subatomic particles called the fermions, which make up matter.

This result – measured at 4.1 sigma on the 5-point scale particle physicists use to determine the certainty of a result – is the first evidence for a Higgs decay to fermions.

On 4 July 2012, the ATLAS and CMS experiments at CERN announced the discovery of a new particle, which was later confirmed to be a Higgs boson.

For physicists, the discovery meant the beginning of a quest to find out what the new particle was, if it fit in the Standard Model, our current model of nature in particle physics, or if its properties could point to new physics beyond that model.

An important property of the Higgs boson that ATLAS physicists are trying to measure is how it decays.

The Higgs boson lives only for a short time and disintegrates into other particles. The various possibilities of the final states are called decay modes.

So far, ATLAS physicists had found evidence that the Higgs boson decays into different types of gauge bosons - the kind of elementary particles that carry forces.

The other family of fundamental particles, the fermions, make up matter. The tau is a fermion and behaves like a very massive electron.

Graphical representation of a Higgs boson decaying to two tau particles in the ATLAS detector. 

The taus decay into an electron (blue line) and a muon (red line) 

Credit: ATLAS

The Brout-Englert-Higgs mechanism was first proposed to describe how gauge bosons acquire mass but the Standard Model predicts that fermions also acquire mass in this manner, so the Higgs boson could decay directly to either bosons or fermions.

The new preliminary result from ATLAS shows clear evidence that the Higgs boson indeed does decay to fermions, consistent with the rate predicted by the Standard Model.

This important finding was made possible through careful analysis of data produced by the LHC during its first run.

Only with new data will physicists be able to determine if the compatibility remains or if other new models become viable.

Fortunately, the next LHC run, which begins in 2015, is expected to produce several times the existing data sample. In addition, the proton collisions will be at higher energies, producing Higgs bosons at higher rates.