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the Milky Way such as from the active galaxy M87, which is about 60 million lightyears away. However, this kind of source seems to be unlikely since UHECRs are isotropic, and not affected by magnetic fields. The yellow line in Figure 01b is the observed cosmic ray energy spectrum. The three other lines show how suspected sources possibly contribute to the overall signature of cosmic rays. It is not unknown what causes the kinks at the "knee" and "ankle". Normally, |
Figure 01a Cosmic Rays, Origins of [view large image] |
Figure 01b Spectrum [view large image] |
cosmic rays bombard the Earth at a rate of about 1 particle/km2-sec. At ultra-high energies, above 1019 ev, the rate falls to less than 1 particle/km2-year. |
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Figure 02a Cosmic Rays Detectors [view large image] |
and could be conceivably lower than that for the high-energy cosmic rays.
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New results from the Pierre Auger Observatory in 2007 indicate that UHECRs have sky directions statistically consistent with the positions of nearby active galactic nuclei (AGN). These galactic centers are known to emit great amounts of light and are likely powered by large black holes. It also concludes that the UHECRs are protons. Figure 02b is an artist's view of a cosmic ray striking the Earth's atmosphere and creating a shower of secondary particles detectable on the surface. The small inserts are the images of the AGN Centaurus A and the detectors of the Pierre Auger Observatory. |
Figure 02b UHECR |
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A report in November 2008 reveals that there is an excess of cosmic ray electrons at energies of 300 - 800 Gev. The finding is consistent with other measurements as indicated in Figure 02c, where the new measurement is in red filled circles. It is compared with previous observations in various symbols. The solid curve is calculated with a power-law spectral index of E-3.2 (the new measurement is scaled with an index of -3.0). The dashed curve is the solar modulated electron spectrum. The source of the bump can be ascribed either from nearby astrophysical objects (such as pulsar or micro-quasar) or from the annihilation of dark matter particles (such as a KK particle with a mass of about 620 Gev). |
Figure 02c High Energy Electrons [view large image] |
Another possibility is via the annihilation of the supersymmetric neutralinos, which is its own anti-particle (Figure 02d) although it doesn't quite fit the observational data. |
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Figure 02e shows experiments over the years trying to obtain evidence for dark matter. It reveals a disparate results not fitting to any particular candidate. A theory introducing a new force between the WIMPs has been proposed to resolve the contradiction between theory and observation. It invokes a heavy force carrying |
Figure 02d Neutralino |
Figure 02e Dark Matter Experiments |
boson that weighs about the same as a proton. It claims to reconcile the many facets of the observations: |
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Particle accelerators were invented to investigate objects with size less then 10-12 cm. Accelerators are to particle physics what telescopes are to astronomy, or microscopes are to biology. These instruments all reveal and illuminate worlds that would otherwise remain hidden from our view. They are the indispensable tools of scientific progress. The earliest accelerators were simple vacuum tubes in which electrons were accelerated by the voltage difference between two oppositely charged electrodes. From these evolved the Cockcroft-Walton and van de Graaff machines (Figure 03), larger and more elaborate, but using the same principle. The modern example of this type of device is the linear accelerator, a sophisticated machine used in many scientific and medical applications. All such straight- |
Figure 03 Van de Graaff Generator [view large image] |
line accelerators suffer from the disadvantage that the finite length of flight path limits the particle energies that can be achieved. |
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The great breakthrough in accelerator technology came in 1920 with Ernest O. Lawrence’s invention of the cyclotron (Figure 04). In the cyclotron, magnets guide the particles along a spiral path, allowing a single electric field to apply many cycles of acceleration. Soon unprecedented energies were achieved, and the steady improvement of Lawrence’s simple machine has led to today’s proton synchrotrons (PS, Figure 05), whose endless circular flight paths allow protons to gain huge energies by passing millions of times through the electric fields that |
Figure 04 Cyclotron |
Figure 05 Synchrotron [view large image] |
accelerate them. |
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Until twenty-five years ago, all accelerators were so-called fixed-target machines, in which the speeding particle beam was made to hit a stationary target of some chosen substance. But early in the 1960’s physicists had gained enough experience in accelerator technology to be able to build colliders, in which two carefully controlled beams are made to collide with each other at a chosen point (Figure 06). Several colliders exist around the world today, and the technology for them is by now well established. Colliders are more demanding to build, but the effort pays off handsomely. In a fixed-target machine, most of the projectile particles continue the forward motion |
Figure 06 Collider |
with the debris after impact on the target. In a collider, on the other hand, two particles of equal energy coming together have no net motion, and collision makes all their energy available for new reactions and the creation of new particles. |
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It is realized that the mass-energy relation (E = mc2) provides a new way to get information about particles. If particles could be made very energetic and then used to collide with other particles, some of their energy could be converted into the creation of previously unknown particles. When particles are produced in a collision, they are not particles that were somehow inside the colliding ones. They are really produced by converting the collision energy into mass, the mass of other particles (Figure 07). Which particles will be produced is partly determined by their mass - the lighter they are, the easier it is to |
Figure 07 High Energy Collision [view large image] |
produced them, other things being equal - and also by the probabilities calculated from the Feynman diagrams. |
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Particle with energy about 1 Gev (109 ev) is required to probe the structure inside proton. Higher energy is required for smaller system - about 1000 Gev is needed to probe into the quarks. The same amount of energy is required to create many of the hypothetical particles. Currently, the Fermilab's Tevatron has enough energy to produce the top quark (~170 Gev). Figure 08 shows a schematic diagram of a top quark event and the actual observation. Since there is no free quark, the result is a jet of hadrons (particles affected by |
Figure 08 Top Quark Events [view large image] |
strong interaction) emerging in the direction of the original quarks. Up to 14 Tev (1012 ev) will be available by the LHC at CERN in 2008. Table 01 below summarizes some features of the major accelerators in the world (all of them are colliders): |
| Accelerator | Colliding Particles | Total Energy | Major Accomplishment |
|---|---|---|---|
| Stanford Linear Collider (SLC) in Palo Alto | electron, positron | 100 Gev | provided first look at Z0 |
| Large Electron Positron Collider (CERN-LEP) in Geneva |
electron, positron | 200 Gev | discovered Z0, W in 1983;shut down in 2002 |
| Relativistic Heavy Ion Collider (BNL-RHIC) in Brookhaven |
heavy ions | 200 Gev | created quark-gluon plasma (Mini Big Bang) |
| Tevatron (FNAL) in Chicago | proton, antiproton | 2 Tev | confirmed Z0, W ,discovered top quark |
| Large Hadron Collider (CERN-LHC) in Geneva |
proton-proton; ion-ion | 14 Tev | planned for 2008 - LEP replacement |
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The detection of high-energy particle or photon started with its darkening effect on photographic plates. Other methods for detecting radiation were soon developed, including scintillation techniques, electroscopes and the Geiger counters. Then a great breakthrough came in 1912 when the cloud chamber was invented. This device induces visible water droplets to form around the atoms, which have been ionized by the passage of the radiation through air. This provides a plan view of the path of the radiation and so gives a clear picture of what is happening. Figure 09 shows the three components of the cloud chamber - the radioactive source, the chamber, and the magnetic field, which is perpendicular to, and out of the plane of the picture. The magnetic field B, the velocity v, the radius of the |
Figure 09 Cloud Chamber [view large image] |
circular orbit R, the mass m, and the charge q are related by the formula: R = mv / Bq. If the direction of the initial velocity is not perpendicular to the field, the charged particle will move in a helix. |
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The bubble chamber used in high-energy accelerator experiments is the sophisticated variants of the cloud chamber. It is normally made by filling a large cylinder with a liquid just below its boiling point; at the top of the chamber a camera looks in. The whole chamber is subject to a constant magnetic field. As the particles enter the chamber, a piston suddenly decreases the pressure in the chamber. This brings the liquid to a superheated state, in which a tiny effect, such as the passing of a charged particle near an atom, is sufficient to nucleate a bubble of vaporized liquid. At this moment, the camera records the picture. Figure 10 shows the tracks of the charged particles in the |
Figure 10 Bubble Chamber [view large image] |
electroweak process ![]() + p - + p + + and the subsequent decay of + to +, and to e+. Since ![]() is a neutral particle. Although it is shown as a dotted line in the map, it leaves no track in the bubble chamber photograph. |
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Bubble chambers have largely been replaced by wire chambers, which allow particle energies to be measured at the same time. It consists of very large number of parallel wires (Figure 11a), where each wire acts as an individual detector similar to the proportional counter (the low voltage version of Geiger counter, such that there is no avalanche near the sense wire and thus the current is proportional to the number of electrons produced by the available energy of the incident particle). As in the Geiger counter, a particle leaves a trace of ions and electrons, which drift toward the case or the nearest wire, respectively. By marking |
Figure 11a Wire Chamber |
off the wires which had a pulse of current, one can see the particle's path. Several planes of wires with different orientations are used to determine the position of the particle very accurately. With the multiwire chamber the data handling capacity increased dramatically. |
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A drift chamber has a similar construction, but with the wires in the central plane spaced farther apart. Varying voltages applied to the cathode wires produce a field in which ionization electrons drift at a constant velocity towards the nearest sense wire. The drift time, measured by an electronic "stopwatch" started by a signal from a scintillator, is directly related to the distance between the track of the particle and the wire that produces a signal. This greatly increases the accuracy of the path reconstruction. Figure 11b shows a schematic diagram of a drift chamber and the actual construction of the |
Figure 11b Drift Chamber |
wires that make up the drift chamber. See also bottom right diagram in Figure 11a. |
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Figure 11c Vertex Detection [view large image] |
pairs of equal and opposite charges. |
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Figure 12 ALEPH Detector |
Figure 13 Side-view [view large image] |
bends the particle's path; the curvature of the path belps identify the particle's charge and momentum. |
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Figure 14a Cap End |
Figure 14b ALEPH Event [view large image] |
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Figure 15 shows an example of particles generated after a collision in diagram a, and the actual display observed in diagram b: 1. Magnetic field bends the low and medium energy charged particles into curved trajectories. 2. The filled-in dots represent the charged particles detected by the ionization detectors. 3. The smaller size ellipses show the showers (secondary particles) detected by the ECAL. |
Figure 15 Detection Display [view large image] |
4. The larger size ellipses show the showers detected by the HCAL. 5. Three additional dots are from the muon chamber at the outer fringe of the detector. 6. The neutrino, which disappears without a trace, can be accounted for from missing energy. |
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while the LHC construction forged ahead regardless. The LHC is being built in a circular tunnel 27 km in circumference. The tunnel is buried around 50 to 175 m. underground. It straddles the Swiss and French borders on the outskirts of Geneva (see Figure 16). It planned to circulate the first beams in May 2008. First collisions at high energy are expected mid-2008 with the first results from the experiments soon after. The LHC is |
Figure 16 LHC |
Figure 17 LHC Layout |
designed to collide two counter rotating beams of protons or heavy ions. Proton-proton collisions are foreseen at an energy of 7 TeV per beam. |
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The designated luminosity of 1034 protons/sec-cm2 translates as 2808 bunches, each containing 1.15x1011 protons with a transverse beam size of 16x10-4cm, and bunch length of 7.5 cm. Since the injected beam already has sufficient energy to damage the LHC |
Figure 18 LHC Beam |
Figure 19 Magnet [view large image] |
Figure 20 Beampipe |
equipment, the stability of the whole complex is critical during the 8 minutes needed to fill the LHC completely. |
| Detector | Name | Colliding Particle |
Energy | Experiment | Theory |
|---|---|---|---|---|---|
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ALICE (A Large Ion Collider Experiment) |
Lead | 2.8 Tev per nucleon | To study the physics of strongly interacting matter at extreme energy densities, where the formation of a new phase of matter, the quark-gluon plasma, is expected. | It is related to the key issues in QCD for the understanding of confinement and of chiral-symmetry restoration. |
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ATLAS (A large Toroidal LHC ApparatuS) |
Proton1 | 7 Tev / proton | The ATLAS detector will search for new discoveries (such as the Higgs2) in the head-on collisions of protons. | It will explore the fundamental nature of matter and the basic forces that shape our universe. |
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CMS3 (Compact Muon Solenoid) |
Proton | 7 Tev / proton | The CMS detector will search for new discoveries (such as the Higgs) in the head-on collisions of protons. | It will explore the fundamental nature of matter and the basic forces that shape our universe. |
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LHCb (Large Hadron Collider beauty) |
Proton | 7 Tev / proton | It undertakes precision studies of the decays of particles that contain heavy flavours of quarks (charm and beauty). | It will search for new particles beyond the Standard Model, and will cast more light on the subtle difference between matter and antimatter that is manifest in CP violation. |
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LHCf (Large Hadron Collider forward) |
Proton maybe Lead | 105 Tev in lab. frame4 | Measurement of 0 production cross section in the very forward region at LHC. |
To study ultra-high energy cosmic rays by the Simulation of an atmospheric shower due to a 107 Tev proton. |
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TOTEM (Total Cross Section, ... etc. at LHC) |
Proton | 7 Tev / proton | It is dedicated to the measurement of total cross section, elastic scattering and diffractive processes at the LHC. | Studying of QCD by the diffractive processes. |
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The signature actually depends on the secondary decay products which can be a pair of jets, or pairs of leptons, or pair of photons (via the annihilation of the quark antiquark pair) as shown in Figure 21. Although there are 109 proton-proton interactions per second, it is estimated that less than 10 Higgs events will be produced per day. On March13 2009, the Fermilab announced that the Tevatron has failed to find Higgs particle in the 160 - 170 Gev range. It means the discovery |
Figure 21 Higgs Boson Decay Modes [view large image] |
would be more difficult (due to more debris particles) if the Higgs boson has mass at the higher end. Experimentalists have been working on the assumption that the Higgs boson lies somewhere between 114 and 175 GeV. |
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On September 10, 2008 a lone beam of protons will complete the first lap around the LHC. It will take another two months before attempt to be made for proton beams collision. It will take years to analyse and verify (double check) the data. Thus, discovery of new particles could be years away. The short-term schedule is to use the initial data to calibrate the detectors. After the shut down at the end of 2008, it will be ready by March 2009 to pack even more protons into the beams and ramp them up to the maximum collision of 14 Tev. Figure 22 shows the tentative timeline for discovery in the bottom. The structure on top portrays the various devices for boosting up the velocity of the proton to 99.9999991% of the speed of light. |
Figure 22 Timeline [view large image] |
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fast enough, this led to an explosive burst of pressure that damaged neighboring sections of the machine. Engineers plan to address this problem by improving the pressure relief system, which includes increasing the number of valves. Meanwhile 53 of the 10,000 magnets used to guide the proton beam will be replaced or repaired. In order to provide enough lead-time for preventing similar incident, instruments will be installed to measure millivolt changes in the electrical bus — indicative of an impending failure — allowing enough lead time to divert the thousands of amps coursing through the machine's cables. Engineers are also looking at the possibility of detecting tiny increases in the temperature of liquid helium around the wire - another warning sign. It is estimated that the cost for repairs and modifications will amount to US$29 million. The decision now is whether to install this upgrade all round the LHC's 27-kilometer ring, or in stages. It will take about a year to upgrade the whole ring. It seems the |
Figure 23 LHC, Problems with [view large image] |
quicker option is the only one on the table for now. Hopefully, it will be restarted in late July 2009 as announced on December 5, 2008. |