Archivi tag: Baryons

Baryons at low density: stellar halos around galaxies

Stellar halos are ubiquitous in luminous galaxies, but because of their faint surface brightness the detailed study of their physical properties has been difficult and, until recently, confined largely to the Milky Way (MW) and Andromeda. Since the advent of large cameras and surveys, both from ground and space, our knowledge of stellar halos is increasing. Several late and early-type galaxies had stellar halo properties traced out to hundred kiloparsecs or beyond, revealing very low luminosity extended stellar structures similar to halos of our own MW and our closest neighbor, the Andromeda galaxy. Continua a leggere Baryons at low density: stellar halos around galaxies

LHCb observes an exotic particle outside the quark model

The black points at the left image above show the ψ’π- invariant mass squared distribution of the data. The blue histogram shows the Z(4430) contribution. Credit. LHCb

I fisici che lavorano all’esperimento LHCb hanno pubblicato un articolo in merito ad una serie di misure di una particella esotica denominata Z(4430). Secondo il modello standard che descrive i quark, le particelle che sono soggette all’interazione forte, cioè gli adroni, sono formate sia da coppie quark-antiquark (mesoni) o da tre quark (barioni). Da quasi 50 anni, gli scienziati stanno cercando di identificare queste particelle, chiamate adroni esotici, che potrebbero non essere classificate secondo gli schemi tradizionali. Sono stati proposti numerosi candidati ma fino ad oggi non c’è stata alcuna evidenza sperimentale che confermasse con certezza la loro esistenza.

The first evidence for the Z(4430) particle has been presented in 2008 by the Belle Collaboration as narrow peak in the ψπ mass distribution in the B → ψ decays. In the latest Belle publication the observation of the Z(4430) particle is confirmed with a significance of 5.2σ and a 3.4σ evidence is presented that the quantum numbers JP = 1+ are favored over the other spin assignments. There are many so called charmonium cc* neutral states in this mass region. The fact that the Z(4430) is a charged particle does not allow to classify it as a charmonium state making this particle an excellent exotic candidate. The BaBar collaboration could explain the Z(4430) enhancement in their data by a possible feature of experimental analysis (so called reflections, for experts), not contradicting in the same time the Belle evidence. The LHCb Collaboration has reported today an analysis of about 25 200 B0 → ψ, ψ → μ+μ decays observed in 3 fb−1 of pp-collision data collected at √s = 7 and 8 TeV. The LHCb data sample exceeds by an order of magnitude that of Belle and BaBar together.

The significance of the Z(4430) signal is overwhelming, at least 13.9σ, confirming the existence of this state.

The Z(4430) quantum numbers are determined to be JP = 1+ by ruling out 0, 1, 2+ and 2 assignments at more than 9.7σ, confirming the evidence from Belle. The LHCb analysis establishes the, so called, resonant nature of the observed structure in the data, and in this way proving unambiguously that the Z(4430) is really a particle. The minimal quark content of the Z(4430) state is cc*du*. It is therefore a four quark state or a two-quark plus two-antiquark state.

LHCb: Unambiguous observation of an exotic particle which cannot be classified within the traditional quark model
The Conversation: Quirky quark combination creates exotic new particle

Quantum Diaries: Major harvest of four-leaf clover
arXiv:  Observation of the resonant character of the Z(4430)^- state

Large galaxy clusters contain the right proportion of visible matter

Grazie ad uno studio recente condotto da alcuni ricercatori della University of Alabama in Huntsville (UAH) guidati da Massimiliano Bonamente, i barioni, che costituiscono la materia visibile nell’Universo e che si riteneva un tempo assente dagli ammassi di galassie, sono presenti in accordo alle proporzioni attese dal modello cosmologico standard negli ammassi di galassie più grandi e luminosi.

The new research studied very large galaxy clusters and concludes that they indeed contain the proportion of visible matter that is being worked out as part of the Big Bang Theory.

The work may prompt new efforts to explain past research findings that some clusters have a deficit in baryons from what is expected.

The Universe is composed of about 75 percent dark energy and 25 percent matter. Of the portion that is matter, about 16 percent is the familiar visible matter that is all around us and the remaining 84 percent is dark matter. “We call it dark matter because we don’t know what it is made of, but it is made of some type of particles and it doesn’t seem to emit visible energy”, said Bonamente. Together dark energy, dark matter and ordinary baryonic matter form a pie chart of the mass of the Universe, where everything has to add up to 100 percent. “We don’t know what dark matter is”, he said, “but we have the means to put the pie together”. While dark energy has a repulsive energy, dark matter and baryonic matter have an attractive force where “everything likes to clump together” to form stars and planets and galaxies. Using x-rays, astrophysicists discovered that there is a diffuse hot plasma gas that fills the space between galaxies. “Basically, the space between galaxies is filled with this hot plasma that is 100 million degrees in temperature”, said Bonamente. Because the gas is so diffuse, it has very low heat capacity. “It is like if I posed this question to you: Which would you rather put your finger in, a boiling cup of water or a room that had been heated to 212 degrees Fahrenheit? You choose the room because the temperature inside it is more diffused than it would be in the concentrated cup of water, and so you can tolerate it”. So why doesn’t the hot gas simply escape? “It is bound to the cluster by gravity”, said Bonamente. “With hot gas, you can do two things. You can measure the regular matter, which is the baryon content. And two, since the hot gas is bound, you can measure how much matter it would take to hold the gas and therefore you can tell how much dark matter there is. “All of a sudden, there is something really wonderful about the hot gas”, he said. “You can have your cake and eat it, too”. Theoretically, the Universe should contain the same proportions of visible and dark matter regardless of where it is sampled. Using cosmic microwave radiation readings, astrophysicists have been able to do a type of forensics of the Universe’s past, and those findings have shown the proportions that were present at the Big Bang or shortly thereafter (post). “Because it started in the Big Bang, that ratio should persist”, Bonamente said. “It is like if I go to the ocean with a scoop. The scoop of water I get should have the same concentration of salt as the rest of the ocean, no matter where I get it”.

But past research had indicated that some clusters were short on the expected percentage of baryons, posing the question of where they were.

Since recently, people believed that clusters had less than 16 percent of baryons, so there were missing baryons”, Bonamente said. “We said no, they are there. So, how did we find clusters with this correct ratio? We studied the most luminous ones, because they have more mass and retain more baryons”.

The findings could open new areas of investigation into why the deficits in baryons were recorded in past research.

Bonamente suggests one theory. “We know that some smaller clusters do have lower concentrations of baryons than the larger ones”, he said. Perhaps because of weaker gravitational forces, the hot gases escaped in similar fashion as planets that have no atmosphere. “Maybe the gas can be bound but maybe a little bit can fly off if there is just not quite enough gravity”. For further studies on smaller clusters, Bonamente looks forward to the arrival of new faculty member Ming Sun, formerly at the University of Virginia, who is an expert on groups having less than 16 percent baryons. “I am excited that Ming has decided to join our research group”, says Bonamente. “With him on board, UAH is poised to continue making discoveries on the makeup of the universe, and that is the most exciting question to answer that I can think of”.

UAH: UAH findings on makeup of universe may spawn new research
arXiv: Chandra Measurements of a Complete Sample of X-ray Luminous Galaxy Clusters: the Gas Mass Fraction

Sesto 2013: Tracing Cosmic Evolution with Clusters of Galaxies

Following the Sesto-2001 and Sesto-2007 Conferences, we are organizing a new conference in Sesto Pusteria, in the heart of the Italian Dolomites, dedicated to studies of the evolution and formation of cosmic structures with galaxy clusters, with the title “Sesto 2013 – Tracing Cosmic Evolution with Clusters of Galaxies“.

In recent years, X-ray and radio observations have revealed in ever increasing detail the complexity of the Intra-Cluster Medium, while still leaving unexplained the riddle of cool cores.  A growing number of optical/IR studies of clusters above z~1 and up to z~2 have shed new light on the formation history of cluster galaxies, revamping at the same time the role of galaxy clusters in constraining the nature of dark matter and dark energy.  Searches for and studies of clusters based on the SZ effect now flourish. The systematic use of gravitational lensing with spectacular HST data has enabled the dark matter mass distribution in the inner cores of cluster halos to be explored in detail. Recent numerical simulations, with an increasing level of realism and more sophisticated implementation of relevant physical processes, have led to better understanding of both the achievements and shortcomings of the current modeling of galaxy clusters in the cosmological framework. At a time when a new generation of large volume, multi-wavelength cosmological surveys is unfolding, and CMB observations from Planck are establishing the cosmological scenario of structure formation with unprecedented accuracy, this conference aims to bring together both theoretical and observational astronomers working at different wavelengths to discuss recent results and future prospects in the study of cosmic evolution through galaxy clusters.

The meeting will focus on three main topics:
  1. The matter distribution in clusters from different methods
  2. The cycle of baryons in clusters  and their galaxy populations
  3. Cosmology and large-scale structure of the Universe