Журавлева Сборник текстов для подготовки аспирантов-физиков 2011

of the connection between leptons and quarks. Superstring theory may also come up with some predictions that can be tested. But although the graviton, the quantum transmitter of the gravitational force, fits well into the superstring picture, it seems unlikely that it will be found in the near future. However, gravitational radiation is predicted from general relativity, and its existence can be inferred from the careful measurements over many years of the change in period of a binary pulsar. Several detectors for gravitational radiation are in the final stages of construction. They are large optical interferometers, with arms 0.6km to 2km in length, looking for the distortion of space caused by violent astronomical events. So the wave properties of gravity will open a new window in astronomy. There will no doubt be unexpected surprises, as there have been in the past. I predict that particle physics and its links with astrophysics and cosmology will continue to be exciting in the foreseeable future.

Research at SLAC

Elementary Particle Physics at SLAC

In a decades-long search for answers about the fundamental structure of matter and the forces between subatomic particles, SLAC scientists study the collisions of particles accelerated to nearly the speed of light.

BaBar and Antimatter

For each type of matter particle there exists a nearly identical antiparticle, but with opposite charge and other slight differences. Theories predict that the tremendous abundance of energy after the Big Bang should have created particles and anti-particles in equal amounts. Yet we live in a world of particles, not anti-particles. What happened to the antimatter? The BaBar experiment explores this asymmetry in nature by looking at the decays of short-lived subatomic particles called B mesons. If B mesons disintegrate differently than their antiparticles, B mesons, this could help explain how the universe tipped in favor of matter over antimatter. Starting in 1998, scientists used SLAC's PEP-II accelerator to speed beams of electrons and their antimatter counterparts, posi-

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trons, to a collision point within the 1200-ton BaBar detector. The abundant electron–positron collisions of this "B Factory" produced particles called Υ(4S) mesons, which decay into equal numbers of B and anti-B mesons. These further disintegrate into other exotic states. True to predictions, the Bs outlasted the anti-Bs, confirming the role of particle decay in symmetry violation—but also demonstrating that it’s not enough to explain the amount of matter in the universe today. The BaBar Collaboration - approximately 500 physicists and engineers from 10 nations - has delivered beyond expectations, providing not only a deeper understanding of asymmetric decay in B mesons, but new subatomic particles and decays. The results appear in more than 300 scientific publications so far. The BaBar detector finished collecting data and closed down in summer 2008. The collaboration continues to make new discoveries from the vast amount of data produced by this record-breaking experiment.

SLAC and the Large Hadron Collider

SLAC scientists contribute to the design, computing and scientific collaborations for the Large Hadron Collider, which recently began operations at CERN, the European particle physics center on the French/Swiss border. Since joining the collaboration in 2006, SLAC has helped to design and build one of the collider’s major detectors, called ATLAS, and continues to help plan upgraded components for the LHC accelerator systems. SLAC is one of a few dozen ATLAS Tier 2 computing centers around the world, and one of only five in the United States. At the moment, the main job of a Tier 2 institution is to simulate collisions, to examine how best to approach the deluge of future LHC data. Once the LHC turns on, the lab's role will expand to include data interpretation in search of a deeper understanding of physics at high energies. In addition, many SLAC theorists are at work modeling the behavior of physics that might—or might not—leave traces in the aftermath of LHC particle collisions. The theorists' job is to predict just what clues might be hidden in the debris, to make it easier to spot the traces of fascinating new physics.

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What will the LHC do?

The LHC will allow scientists to probe deeper into the heart of matter and further back in time than has been possible using previous colliders. Researchers think that the Universe originated in the Big Bang (an unimaginably violent explosion) and since then the Universe has been cooling down and becoming less energetic. Very early in the cooling process the matter and forces that make up our world ‘condensed’ out of this ball of energy. The LHC will produce tiny patches of very high energy by colliding together atomic particles that are travelling at very high speed. The more energy produced in the collisions the further back we can look towards the very high energies that existed early in the evolution of the Universe. Collisions in the LHC will have up to 7x the energy of those produced in previous machines; recreating energies and conditions that existed billionths of a second after the start of the Big Bang. The results from the LHC are not completely predictable as the experiments are testing ideas that are at the frontiers of our knowledge and understanding. Researchers expect to confirm predictions made on the basis of what we know from previous experiments and theories. However, part of the excitement of the LHC project is that it may uncover new facts about matter and the origins of the Universe. One of the most interesting theories the LHC will test was put forward by the UK physicist Professor Peter Higgs and others. The different types of fundamental particle that make up matter have very different masses, while the particles that make up light (photons) have no mass at all. Peter’s theory is one explanation of why this is so and the LHC will allow us to test the theory. More of the Big question are available about the universe that the LHC may help us answer.

What are atomic particles and lead nuclei?

Atomic particles

A large number of atomic particles have been discovered in the last100 years, but the most common are those which make up the atoms that are the building blocks of our familiar world; including our bodies, the air we breathe and the stars we see in the night sky. These particles

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(electrons, quarks and force-carriers) are only part of a list that includes many strange and exotic particles; some have been observed in experiments, while others are ‘known’ only from theory.

Lead nuclei

The nuclei of lead atoms contain many protons (82) and neutrons (125) and consequently are much heavier than the single proton of the hydrogen nucleus. Once lead atoms have been stripped of their surrounding cloud of electrons the remaining nucleus carries a positive charge that allows them to be accelerated in the LHC like the much smaller single proton of the hydrogen nucleus. However, their greater mass potentially allows scientists to study the results of collision sat very high energy densities, where they expect to observe quark/gluon plasma - a state of matter that existed very early in the evolution of the universe before more familiar atomic particles appeared.

How does a collider work?

Colliders have two functions, to accelerate particles to high speeds in beams about 2mm wide (small enough to pass through the 0 on a 20 pence piece) and to then direct the beams to collide head-on at the collision points at the heart of the detectors. The LHC is the world’s most powerful particle accelerator and will create collision energies 7x greater than previous machines. The particles the LHC will accelerate and collide are protons or lead nuclei, both have positive charges and this means that they can be steered by use of appropriate magnetic fields. Various types of superconducting magnets (9,300 in total) are used to steer and focus beams of particles as they race around the 27km loop of the LHC collider. The LHC carries two beams, travelling in opposite directions, in two, adjacent beam pipes. At the collision points the beams briefly share the same pipe as the magnets direct them to collide head-on. The beam pipes are enclosed in a sheath of superconducting magnets and all of this is bathed in supercold liquid helium (1.8oK). The magnets, which make up the bulk of the collider, are only one part of the story. The other task of the collider is to accelerate the particles as they travel around it. This is done at 4 locations where the particles pass

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through superconducting radio frequency (RF) cavities. Just like pushing a child’s swing, these RF cavities give the particles a push each time they pass, steadily increasing the energy of the particles prior to collision. The LHC is the last in a ‘ladder’ of accelerators that are used in sequence to accelerate low energy particles up to the LHC’s maximum energy.

How does a detector work?

Particle detectors are very simple in principle, but extremely complex in practice.

The detectors at the LHC are built around the collision points where the particle beams meet head-on and they are designed to track the motion and measure the energy and charge of the new particles thrown out in all directions from the collisions. The LHC detectors are very large, for example ATLAS is the size of a 5 storey building. Their great size is necessary firstly, to trap high energy particles travelling near the speed of light and secondly, to allow the tracks of charged particles to be detectably curved by the detector magnets.

Detectors are typically made up of layers, like an onion, with each layer designed to detect different properties of the particles as they travel through the detector. The layers nearest to the collision point are designed to very precisely track the movement of particles, especially the short-lived particles that are both the most difficult to detect and the most interesting to the researchers. Subsequent layers track the movement, and also slow down and stop, longer-lived and more energetic particles. As these particles are slowed down they release energy that is measured by these layers (the calorimeters). Detectors usually include a powerful magnet; this affects the motion of charged particles produced in collisions and from the extent of its effect researchers can measure the charge and momentum of particles.

•ALICE(A Large Ion Collider Experiment)

•ATLAS (A Toroidal LHC ApparatuS)

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•CMS (Compact Muon Solenoid experiment)

•LHCb (Large Hadron Collider beauty experiment)

Black Holes: What Are They?

Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a " singularity ". Around the singularity is a region where the force of gravity is so strong that not even light can escape. Thus, no information can reach us from this region. It is therefore called a black hole, and its surface is called the " event horizon ".

But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the Earth's orbit around the Sun would be unchanged. (Of course the Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us.) To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape. The Schwarzschild radius can be calculated using the equation for escape speed:

vesc = (2GM/R)1/2

For photons, or objects with no mass, we can substitute c (the speed

of light) for Vesc and find the Schwarzschild radius, R, to be

R = 2GM/c2

If the Sun was replaced with a black hole that had the same mass as the Sun, the Schwarzschild radius would be 3 km (compared to the Sun's radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the center of our Solar System.

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Список литературы

1.http://www.the-

scientist.com/?gclid=CIXdoonap6UCFRYv3wod5hIj5Q

2.http://www.scientificamerican.com/section.cfm?id=news

3.http://www.scientificamerican.com/physics

4.http://www.scientificamerican.com/space

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