A high-energy proton (yellow line) flung into a lower-energy proton in the hydrogen gas cloud. The green arrow represents the high-energy gamma ray that results from the proton collision. -
Artwork by Sarah BallantyneIn 2004 scientists were startled when they discovered that the center of our galaxy is emitting gamma rays.
Astrophysicists at The University of Arizona, Los Alamos National Laboratory and the University of Adelaide (Australia) have discovered a mechanism that might produce these high-energy gamma rays. The black hole at the center of our Milky Way could be working like a cosmic particle accelerator, revving up protons that smash at incredible speeds into lower energy protons and creating high-energy gamma rays, they report.
When complete, the Large Hadron Collider in Switzerland will be able to accelerate protons to seven trillion electronvolts. Our galaxy's black hole whips protons to energies as much as 100 trillion electronvolts, according to the team's new study. That's all the more impressive because "Our black hole is pretty inactive compared to massive black holes sitting in other galaxies," Ballantyne noted.
Even though the protons move close to the speed of light, their motion is so random that it takes several thousand years for the particles to travel beyond 10 light years of the black hole. After the high-energy protons escape the black hole environment, they fly off into the interstellar medium, where they collide with low-energy protons (hydrogen gas) in a smash-up so energetic that particles called 'pions' form. These particles of matter quickly decay into high-energy gamma rays that, like other radiation, travel in all directions.
"Ironically, even though our galaxy's central black hole does not itself abundantly eject hyper-relativistic plasma into the surrounding medium, this discovery may indirectly explain how the most powerful black holes in the universe, including quasars, produce their enormous jets extending over intergalactic proportions. The same particle slinging almost certainly occurs in all black-hole systems, though with much greater power earlier in the universe," Melia said.
A Particle Accelerator at Galactic Center? from Centauri Dreams
Milky Way Black Hole Is a Natural Particle Accelerator from Universe TodayMilky Way Black Hole May Be A Colossal 'Particle Accelerator' from Science Daily_______________________________________________________
Searching for Extra DimensionsPossible extra-dimensional effect observable at collides.
A graviton leaves our 3-dimensional world for a short moment of time, just to come back and decay into a pair of photons.
The search for extra dimensions is not over yet. In fact, it has only just started. We are looking for the effects of extra dimensions in collisions that produce different types of particles, such as quarks. We are also looking for events where gravitons are produced in the collisions and then leave our three-dimensional world, possibly travelling off into one of the other dimensions. This would cause an apparent non-conservation of energy from the point of view of our three dimensional world.
We very well might see them in the next generation collider.
The LHC, that has been built at CERN (near Geneva, Switzerland) will allow us to ultimately probe the theory of large extra dimensions and either find them or show that the idea is actually wrong. But we will have to wait a little longer, before we learn that.
Where are my keys - Searching for Extra Dimensionsfrom Plato @ Dialogues of Eide.
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In physics, a wormhole is a hypothetical topological feature of spacetime that is essentially a 'shortcut' through space and time. A wormhole has at least two mouths which are connected to a single throat. If the wormhole is traversable, matter can 'travel' from one mouth to the other by passing through the throat. While there is no observational evidence for wormholes, spacetimes containing wormholes are known to be valid solutions in general relativity.
I am inclined to 'believe' there is an arrow of Time.
That we cannot rewind Time (like on film, video or DVD) and, it would be peculiar to see people and cars moving backwards or rain pouring skyward.
We probably cannot 'travel' back in Time, after all even if we could travel at the speed of light and lived for a 163,000 years - it is unlikely that Supernova 1987A would have stood still waiting for us. We would still arrive there in 163,000 years Time or 326,000 years after the event took place. (unless there are shortcuts)
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Evaporating micro-black-holes from Bee @ Backreaction.
The Principle of finite imagination-ii by B @ Backreaction.
LHC detector performs first test of fundamental forces by JoAnne.
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Labels: CERN, Particle Physics, Theoretical Physics
One interesting question, connected to the scope of Lisa’s talk, is: “If there is new physics, but it lies at a higher energy scale than the one directly accessible by the machine, how do we maximize our chances to see it ?”
Historically, the reason that black holes appear so promising as compared with other possible signatures is the predicted huge cross section for their production if there is a low quantum gravity scale. Lisa ventured to compute that if quantum gravity turns on at a scale of a TeV, one gets 100 pb which for 100 fb-1 luminosity would yield ten million events.
The basic reason why this cross section is so large compared to the production of a particle with TeV mass in a typical beyond the SM theory is the lack of any small couplings, such as gauge couplings in the cross section and absence of phase space suppression factors. However, this estimate ignores several major considerations and uncertainties in the black hole production and decay cross sections.
There is no suppression from gauge couplings, so it is indeed a large signal. Also, the signature is spectacular, since these objects are predicted to decay into large multiplicity final state, with highly spherical distributions. Very distinctive, unmistakable new physics.
But the problem is that the idyllic picture is not very realistic. The onset of a non-perturbative regime where black holes are produced and decay with those signatures is much above the QG scale, and this appears to be above the reach of even the LHC.
The Large Hadron Collider (LHC) at CERN has not emitted the first burp yet, and it is already criticized for being a midget. In any case, at threshold one would not see the striking signatures, but maybe something can be saved.
You have to go well above M, the energy scale of quantum gravity, to be sure to hit the striking signatures publicized in the past. The parton distribution functions of the proton drop rapidly with the fraction of parton momentum, and since we are by necessity near threshold, the value of the latter is very important in determining what the rate of the new process is going to be. To make matters worse, M is convention-dependent. Factors of - fly around easily, and although one knows these are only conventions and what one cares for is just the actual threshold, there is a big difference between 1 TeV and 2 TeV for the LHC. So the picture is fuzzy.
Bubble Chamber: Leonard Susskind & Lisa Randall
Lisa discussed some of the models and the resulting conventions and equations for the schwarzschild radius, the energy scale, and the other main characteristics.
One point which looked important is that in the models considered, the black hole lifetime is bigger than the inverse of the energy scale of quantum gravity. This drives some of the phenomenology of the black hole decay. Another point is that every degree of freedom should carry an insignificant amount of energy with respect to the total; and since we are never going to get far above threshold at the LHC, we will have to be careful to call what we produce a real classical black hole. These things have low entropy close to threshold, and the multiplicity of the decay will be affected.
A critical factor in the computation of the number of particles emitted in the black hole decay is the assumption of the dimensionality of the space: particles emitted in the bulk have more directions in which to oscillate. Furthermore, since the threshold for producing black holes is not M, but a higher energy, even if we did see a black hole, we would not be able to extract M from the total cross section, because of inelasticity effects: not all the energy of the colliding partons goes in the creation of the black hole, due to initial state radiation.
The difficult question to answer is in fact, what fraction of the energy gets trapped inside the horizon? It is of course important since the PDF fall rapidly with energy. What is clear is that the inelasticity effectively increases the threshold. The reduction in cross section due to this effect is enormous, and it is the lack of considering it, which has brought some overoptimistic predictions in the past.
So, the upshot is that BH production threshold is higher than originally thought. It means a lower production cross section, a lower reach in black hole mass, and it translates into lower entropy reach as well. The conclusion of Lisa Randall is that we will not produce classical thermal black holes at the LHC. What will we still be able to produce, then ? And what kind of multiplicities should we expect ?
Lisa discussed the calculation of the multiplicity of final state particles. She said that the calculation is totally unreliable. But one thing stays clear: low multiplicity final states will dominate even if we call it black holes. So we have to face the facts, and study 2-body final states: jets and leptons. Can they be distinguished from backgrounds by rate, kinematics, bra size? Yes. For jets, transversality is the key. QCD is dominated by t-channel exchange, i.e., forward scattering. Black hole events are isotropic. So this is really becoming like any other compositeness search: massive states produced at low rapidity.
While describing a scenario where the LHC will have to walk the walk of unclear kinematical analyses rather than being hit in the face by those firework-like signatures that experimentalists have started to dream more and more frequently as of late, Randall was careful to insist that the LHC is indeed a powerful machine, although she fell short of declaring it will make everything clear about quantum gravity.
After discussing the signature of black holes, Randall delved into the possible signatures of the same kind from alternative models of quantum gravity, such as a weakly coupled string theory. There one apparently expects a resonance behaviour, followed by a dramatic drop in transverse cross section, which can be used to distinguish the stringy behavior from the simple production of a new Z’ boson, ...
In addition to the resonance, you would also see a drop in the quantity. This could also allow to distinguish models: you could decide you are finding a stringy state, and you could even distinguish different stringy models, because the correlation between and the cross section is different for different models. In summary, black holes are not as spectacular as advertised in the past. However, they may still provide lots of information about quantum gravity, through careful studies of processes.
Lisa also said she would love to see these studies done by Atlas and CMS: energy-dependent angle studies in dijet production.
Tommaso Dorigo asked the question: "I know from previous blogging on the issue that when one reaches a quantum gravity regime, the QCD cross section of dijet production has to go down, but Lisa had not discussed this feature." She explained that before one reaches the regime when QCD 2-particle cross section gets reduced, the cross section has to go up, in any case. So the dijet cross section reduction that Sabine has first studied happens at a regime that LHC will fail to cover.
Source: Lisa Randall: black holes out of reach of LHC
by Tommaso Dorigo @ A Quantum Diaries Survivor.
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Lisa Randall: Unification in warped extra dimensions and bulk holography
Lisa Randall: Smashing open the Universe @ Prospect Magazine.
Event probability for Production of Single Top Quarks using Matrix Elements