Monthly Archives: July 2010

Why do we need the LHCb?

This post is about the LHCb – the motivations and science behind the experiment, and what scientists hope to discover there.

The LHCb is one of four experiments at the Large Hadron Collider (LHC) at CERN in Geneva. It will study the decays of particles known as B mesons in the hope of discovering the answer to a problem known as matter-antimatter asymmetry.

The problem is this: at the big bang, matter and antimatter should have been, and in all likelihood were, created in equal amounts. However, according to what we currently know, if they had been created in equal amounts, then I wouldn’t be here to write this and you wouldn’t be here to read it either. We exist due to a tiny imbalance in the ratio of matter to antimatter at the beginning of time. This tiny imbalance meant that when most of the stuff created in the big bang was annihilated (when matter meets antimatter both are destroyed and lots of energy is released) a tiny amount of matter was left over, and this tiny amount makes up all the matter we see in the Universe today, including us.

Shortly after the Big Bang, there were roughly equal amounts of matter and antimatter...

... crucially, though, there was slightly more matter. This is why we live in a matter dominated Universe today.

To investigate the matter-antimatter asymmetry, physicists are looking at B mesons. These particles are so called because they each contain a b, or bottom, quark. After its discovery in 1977 there were some attempts to change the bottom quark’s name to “beauty”, but the original name stuck. Incidentally, the “b” in LHCb does stand for “beauty” as opposed to “bottom”.

Decays involving B mesons may hold the key to the matter-antimatter asymmetry problem because they exhibit a property known as CP violation. CP stands for “charge parity” and is used to describe the combination of two symmetries called charge conjugation symmetry and parity symmetry. If these symmetries were obeyed, the laws of physics would treat matter and antimatter exactly the same. CP violation occurs when matter and antimatter are treated differently, and as such might be able to explain why we live in a matter dominated Universe today.

It is the weak force that is responsible for the decay of B mesons, and it is the only one out of nature’s four fundamental forces that is known to violate CP. CP violation has been seen at experiments BaBar and Belle, which are located at the Stanford Linear Accelerator Centre (SLAC) in the US and the KEK laboratory in Japan respectively. However, the weak force alone is not enough to explain all the CP violation we see.

Scientists at the LHCb will search for rare decays of B mesons in order to try and find new physics to explain the asymmetry. They will be looking for new particles that have never been seen before as well as new physical phenomena. This new source of CP violation could be found in quarks, or it could be found in some other particle. If the Higgs boson is discovered, maybe it will point us in the right direction. We don’t yet know exactly where the answer lies, but there’s only one way to find out…

For more information see the LHCb website.

Images: US / LHC webpage

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Mapping the cosmos: Can a new technique help us learn more about dark energy?

In a paper published in Nature today, physicists detail a new method of looking at faraway galaxies that may help shed light on dark energy.

In order to learn more about dark energy – the mysterious force that is believed to be responsible for the ever increasing rate of expansion of the Universe – astronomers need to be able to peer further back in time. Luckily, due to the finite speed of light, all we need to do to see further back in time is to look at objects further away. For example, if we look at a galaxy that is ten light years away, we see it as it was ten years ago. Simple.

What is not so simple is working out how we can observe far away objects in enough detail to make the observations useful. When observing galaxies, astronomers look at their emission spectra, which are made up of spectral lines that each correspond to a certain wavelength of light that has been emitted by a particular element. From these lines astronomers can determine which elements are present, but spectral lines can have other uses too.

The emission line with a wavelength of 21cm is caused by neutral hydrogen, and is potentially very useful when it comes to probing dark energy. So far, astronomers have been able to detect the 21cm line in galaxies with a redshift up to 0.24. Cosmological redshift is a measure of how fast an object is moving away from the Earth and is a consequence of the expanding Universe. Objects with higher redshift are moving away from the Earth faster than those with lower redshifts, and are further away too. Galaxies more distant than those with a redshift of 0.24 are too faint for the 21cm line to be detected using traditional methods. It is believed that observations of the 21cm line from galaxies with redshifts between 0.5 and 2.5 would allow us to gain a better understanding of dark energy, so physicists have been trying to find a way to make these observations.

Emission spectrum for the Sun (left) compared to the spectrum from a supercluster of distant galaxies (right). Spectral lines are shifted towards the red end of the spectrum as shown by arrows, because the cluster of galaxies is moving away from us at an ever increasing speed. Image: Wikipedia

Chang et al have come up with a new technique called intensity mapping that may help to solve this problem. Rather than trying to look at the 21cm emission line from individual galaxies that are too far away, they have measured the total emission from a “cosmic web” containing thousands of galaxies. Combining this with data from the DEEP2 optical galaxy redshift survey they have produced a 3D intensity map for redshifts between z=0.53 and z=1.12, giving an insight into the 21cm emission line in the region needed to probe dark energy. In doing this they have avoided having to look at every individual galaxy and instead jumped straight to determining the large scale structure of the Universe.

The large scale structure, which describes the distribution of galaxies in the Universe on scales of millions of light years, can give us an insight into the effects of dark energy on the Universe. In developing a new method that allows us to look at the 21cm emission line for even further away galaxies, and therefore find the large scale structure at times closer to the big bang, Chang et al have given us a first taste of what we may be able to do with further study of the large scale structure of our Universe.

Reference:
Chang, T., Pen, U., Bandura, K., & Peterson, J. (2010). An intensity map of hydrogen 21-cm emission at redshift z ≈ 0.8 Nature, 466 (7305), 463-465 DOI: 10.1038/nature09187

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Does one size fit all when it comes to star formation?

It is widely known (among astrophysicists at least!) that disks of accumulated matter are an essential component in the formation of low mass stars. These disks form when a rotating cloud of dust and gas collapses and, after formation, they direct material from the cloud onto a protostar at the centre. This protostar keeps accreting more and more material until it reaches a temperature high enough for it to fuse hydrogen. At this point it starts its life on the main sequence – the longest stage in the evolution of a star, and the one that our own Sun is currently at.

This all works fine for stars with less than about ten times the mass of the Sun, but above that limit our knowledge of the formation of stars gets a little hazy. In the above process, the protostar is constantly emitting light and therefore exerting pressure outwards. This radiation pressure works against the gravity that is causing more material to be added to the protostar. It is believed that no stars with a mass higher than about ten solar masses could form by the above method, as the radiation pressure from higher mass star would become more powerful than gravity and halt the formation. For this reason, it has been suggested that high mass stars form when two smaller stars merge, and not through the formation of disks like their less massive counterparts.

However, researchers working on the Very Large Telescope Interferometer at the European Southern Observatory have found evidence of a disk of material around a young star about twenty times as massive as the Sun. The star IRAS 13481-6124 is in the constellation Centaurus and it about ten thousand light years away from us.

Below you can see a video of IRAS 13481-6124 and its disk:

Kraus et al detected two bow shocks, caused by the interaction between an outflow of gas and dust from the star and the interstellar medium. The existence of this outflow is evidence for a disk around the star, as jets of material are a common feature of accretion disks around protostars.

This disk surrounding IRAS 13481-6124 is very hot and compact, and is very similar to disks observed around low mass stars, suggesting that the formation of high mass stars may not be so different after all.

Reference:
Kraus, S., Hofmann, K., Menten, K., Schertl, D., Weigelt, G., Wyrowski, F., Meilland, A., Perraut, K., Petrov, R., Robbe-Dubois, S., Schilke, P., & Testi, L. (2010). A hot compact dust disk around a massive young stellar object Nature, 466 (7304), 339-342 DOI: 10.1038/nature09174

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Thousand light year long bubble surrounds black hole in nearby galaxy

The Eddington luminosity is the exact brightness a black hole has when the outwards and inwards forces on it balance. It may seem strange to talk about the brightness of a black hole, as usually we think of them as not letting anything – including light – escape their gravitational pull, but in reality this is not the case. Black holes do draw in all the material that surrounds them, but as they do this they become surrounded by disks of gas in a process known as accretion. This gas can become so hot that it emits vast amounts of X-rays.

So, black holes above the Eddington luminosity emit light radiation that is more powerful that their gravitational pull. Such black holes are rare and not much is known about them. Very bright X-ray sources are the most likely candidates to harbour these types of black hole, so researchers from the University of Strasbourg and University College London went looking for X-ray sources around unusually large remnants of supernovae. They found what they were looking for in a galaxy not too far away…

NGC 7793, a spiral galaxy in the constellation Sculptor. Image: NASA

There is a spiral galaxy known as NGC 7793 12.7 million light years away from us in the Sculptor constellation. In one of its spiral arms is a very bright nebula with a power source that is almost certainly a black hole with a luminosity above the Eddington limit. Unusually for a black hole such as this, it appears to dispel most of its accretion-generated energy mechanically rather than through radiation. This means that coming out of the black hole’s disk in opposing directions are two huge jets of very energetic particles – something that has not been seen very often before in these types of nebulae. These jets are the most powerful of their kind ever found, and are ten thousand times more energetic than the X-rays emitted from the core of the black hole. Around the jets has formed a bubble of plasma that is a thousand light years long.

X-ray/optical image of the plasma bubble surrounding the jets. 190pc (parsecs) is equal to approximately 600 light years. Image: Pakull, M., Soria, R., & Motch, C.

Above is an image of the bubble of plasma that surrounds the jets. In the image you can see the core and the northern and southern hotspots, where the jets interact with the ambient medium. Jets such as these are needed to explain many things in astrophysics, but their origin is not yet well understood.

Reference:
Pakull, M., Soria, R., & Motch, C. (2010). A 300-parsec-long jet-inflated bubble around a powerful microquasar in the galaxy NGC 7793 Nature, 466 (7303), 209-212 DOI: 10.1038/nature09168

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