Particle Accelerators and Detectors

Cosmic Rays
Accelerators
Detectors
An Example - The LHC (Large Hadron Collier)

Cosmic Rays

Cosmic rays are energetic particles come from outer space traveling near the speed of light. Before the development of particle accelerators, cosmic rays provided physicists with their only sources of high-energy particles to study. Although low-energy cosmic rays are emitted from the Sun, the origin of the highest energy cosmic rays is one of the outstanding puzzles in astrophysics. These particles (mostly protons, but including heavier atomic nuclei and gamma rays) have energies of up to 10⁸ TeV. The sources of high-energy cosmic rays could be supernovae, blackholes, or defects in space-time (Figure 01a). The UHECRs (ultra-high-energy cosmic rays) events are rare, but they should not be here at all; because within a few hundred million years at most, they should be slowed down by successive collisions with the omnipresent photons of the cosmic microwave background. They could be produced nearby, e.g., they could be originated within some 150 million lightyears of

		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.

Low energy cosmic rays don't have enough energy to penetrate the earth's atmospher all the way to the surface. They have to be detected by instruments on satellite (Figure 02a).

Although secondary high-energy cosmic rays still don't have enough energy to reach the surface, they can generate Cerenkov radiation detectable by Cerenkov detector on the ground (Figure 02a). Cerenkov radiation is the bluish light produced by charged particles moving through a transparent medium at a speed greater than the speed of light in the medium. This is the optical equivalent of a sonic boom. The greater than light speed is possible because the speed of light is c/n in the medium, where n is the index of refraction, and is always greater than one. Therefore, the speed of light in the medium is lower than that in the vacuum

Figure 02a Cosmic Rays Detectors [view large image]

and could be conceivably lower than that for the high-energy cosmic rays.

The secondary ultra-high energy cosmic rays can be detected on the ground by either the surface detectors, or the fluorescence detectors (Figure 02a). In the

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
[view large image]

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.

		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 [view large image]	Figure 02e Dark Matter Experiments	boson that weighs about the same as a proton. It claims to reconcile the many facets of the observations:

The new dark force boosts the annihilation rate of slow-moving WIMPs giving a surplus of electrons and positrons. Yet it has no effect on much faster particles that filled the early universe leaving plenty of WIMPs around today.
The absence of surplus anti-protons is explained by the heavy mass of the mediating boson, which can decay into electrons and positrons only.
The 511 Kev photons observed by INTEGRAL can be identified to the annihilation of the electron-positron pair.
The scenario favours the DAMA detectors, which use material with heavier nuclei (e.g., iodine). While much lighter germanium and silicon is inside CDMS, which detects no signal of any WIMP.

NASA's Fermi satellite, launched in 2008, could confirm with a high degree of accuracy the excess of electrons over a wide range of energy. If WIMPs really are 600 times as massive as protons, there should be an abrupt drop in the number of electrons above a certain energy threshold. It should also be able to see gamma rays that are produced when WIMPs annihilate. The energies of these rays should differentiate the KK particles from neutralinos. Fermi will even be able to pinpoints the gamma ray sources in the sky. If it detects a big nearby clump, then neutralinos will be back in favours as the claims against them assume that dark matter is evenly spread throughout the galactic halo. Of course, the LHC will render a big helping hand if such force carrier is detected there.

An April 2009 report by the Fermi team indicates that there is no bump in the 300 - 800 Gev range of the cosmic electrons. In July 2009, the same Fermi team reports detection of a spike in cosmic gamma ray at 100 Gev.

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Accelerators

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.

		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 [view large image]	Figure 05 Synchrotron [view large image]	accelerate them.

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
[view large image]

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.

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.

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

Table 01 High Energy Accelerators

Note: CERN = European Organization for Nuclear Research, BNL = Brookhaven National Laboratory,
FNAL = Fermi National Accelerator Laboratory.

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Detectors

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.

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.

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
[view large image]

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.

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
[view large image]

wires that make up the drift chamber. See also bottom right diagram in Figure 11a.

The ALEPH installed in the now de-activated LEP at CERN is a good example to illustrate the construction of the modern particle detector. The various parts are shown in Figures 11c, 12, 13 and 14a with a brief description in the followings:

Vertex detector (VDET) - It is a high resolution position detector for identifying very short-lived particles. This detector consists of two concentric arrays of silicon wafers surrounding the beam pipe. It is designed to identify as closely as possible the location of the collision. Particles traveling through the thin chips leave behind small electric charges in the squares they cross. The location of these deposits can be recorded electronically and a computer can "connect the dots" to reconstruct the tracks of all the particles through the layers. Since the electronic squares are so small, they allow measurement of the charged particle position with microscopic accuracy (about 200 millionths of an inch). By drawing each path back to where it meets with one or more other paths we can find the position where any given charged particle is created (the vertex), since charged particles are never created alone, but always in

Figure 11c Vertex Detection [view large image]

pairs of equal and opposite charges.

		Inner Tracking Chamber (ITC) - It is a large drift chamber crisscrossed with 960 sense wires arranged in 8 concentric cylindrical layers. Whenever a charged particle passes near one of the wire, the wire's electrical properties change, and a computer records the changes. The active length of the chamber is 2 meters and extends in radius between 16 and 26 cm from the beam line. A strong magnetic field
Figure 12 ALEPH Detector [view large image]	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.

		Time projection chamber (TPC) - This is for measuring three dimensional coordinate at many points along a charged particle track. Having 3-dimensional information is important when there are large numbers of tracks within a small angular cone. The transverse coordinates are determined by wire proportional chambers at the ends of the TPC while the longitudinal (z) coordinate is obtained from the time it takes charges to drift to the ends of the TPC. This is essentially a larger drift chamber with a total length of 4 meters.
Figure 14a Cap End [view large image]	Figure 14b ALEPH Event [view large image]

Superconducting magnet coil - The superconducting coils provide a very uniform (<0.2%) magnetic field of 15000 gauss magnetic field in the z direction along the axis of the device. The current in the coil is about 5000 A. The coils are cooled by liquid Helium. Two correction coils serve to improve the uniformity of the field.
Calorimeters - The electromagnetic calorimeter (ECAL) is a 45 layer lead/proportional chamber sandwich to measure the energy of both charged and neutral particles (e.g., electron and photon) by dissipating the energy in successive secondary inter-actions in some dense medium. The hadron calorimeter (HCAL) is located further out. It consists of a series of iron plates separated by gas counters, also returns the magnetic flux. The whole is surrounded by an additional muon detection system, the Muon Chamber, of two double layers of streamer tubes. Finally, important for precise cross section measurement is the highly segmented Luminosity Calorimeter composed of twelve-layer tungsten/silicon sandwiches that surround the beam pipe at each end. Figure 14b shows a Higgs candidate event with particle tracks; while the particle energies are represented by histograms in different colour. Only neutrinos run the gauntlet and escape completely, without registering in any of the layers. Their presence can be deduced from the missing energy they carry away. Many particles don't live long enough to reach the tracking chamber. They can only be detected indirectly, by identifying the particles they decay into, and then working backward to find the charge, and spin of the parent particle.

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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An Example - The LHC (Large Hadron Collider)

The idea of building the LHC dates back at least to 1977. Research and development started on the LHC magnets in 1988 - a year after the SCC (Superconducting Super Collider) was endorsed by the US. The SCC was eventually cancelled in 1993,

		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 [view large image]	Figure 17 LHC Layout [view large image]	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.

As shown in Figure 17, the two proton beams rotate in opposite directions around the ring, crossing at the designated interaction regions (IRs). Four of these (IR1, IR2, IR5 and IR8) contain the various experiments (ATLAS, LHCf, ALICE, CMS, TOTEM and LHCb). IR4 contains the radio-frequency (RF) acceleration equipment, and IR3 and IR7 contain equipment for collimation and for protecting the machine from stray beam particles. IR6 houses the beam abort system, where the LHC beam can be extracted from the machine and its energy absorbed safely.

The beam of the LHC starts off in a 50 Mev linear accelerator, LINAC2. The beam is accelerated further by another three existing rings built in the 1970's (Figure 18). The beam is transferred from the SPS to the LHC in bunch pattern (packages).

			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 [view large image]	Figure 19 Magnet [view large image]	Figure 20 Beampipe [view large image]	equipment, the stability of the whole complex is critical during the 8 minutes needed to fill the LHC completely.

Figure 19 shows the cross-section of the two-in-one design for the main LHC magnets. In the centre are the two beam pipes, separated by 194 mm. The superconducting coils (red) are held in place by collars (green) and surrounded by the magnet yoke (yellow). Together, these form the cold mass of the magnet, which is insulated in a vacuum vessel (outer blue circle) to minimize heat uptake from the surroundings. Figure 20 is a picture of the beam pipe in the LHC's main tunnel. It houses the two particle streams that zoom in opposite directions at near the speed of light.

Table 02 below summarizes the various experiments at LHC. Detail of each experiment is accessible by clicking its "acronym" in column 2 (Name); also check out its "Home Page" to obtain detailed information about LHC.

Name	Colliding Particle	Energy	Experiment	Theory
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.
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.
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.
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.
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.
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.

Table 02 LHC Experiments

¹ In principle, colliders can be designed for many different particle species. The LEP (the predecessor of LHC) used leptons in the form of electron and positron as colliding particles. Since leptons are elementary particles, the centre-of-mass collision energies are precisely defined and therefore are well suited to high-precision experiments. On the other hand, the hadrons (proton in LHC) that are smashed together are composite particles, and the collisions actually occur between constituent quarks and gluons, each carrying only a proportion of the total proton energy. The centre-of-mass energy of these collisions can vary significantly, so they are not as well suited for high-precision experiments. The hadron colliders, however, offer tremendous potential for the discovery of as-yet unknown particles, because they admit the possibility of collision over a wide range much higher energies than is otherwise possible. LHC performance envisages roughly 30 million proton-proton collisions per second, spaced by intervals of 25 ns, with centre-of-mass collision energies of 14 Tev, which are 7 times larger than those of any previous accelerator.

² The Higgs particles only survive for about 10^-25 sec. This is far too brief a lifetime to leave a measurable trail. Instead, the presence of a Higgs particle will be inferred by the particles into which it decays. Since it is believed that the Higgs boson interacts preferentially with heavy particles, it will decay into pair of the heavier particles such as the top quark, bottom quark, the W or Z boson. Figure 21 shows the theoretical predictions of the decay percentages for the various primary decay modes.

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.

³ Although the designs for ATLAS and CMS are quite different, they are looking for exactly the same things. The reason is a fundamental principle of science: Experimental results must always be confirmed through duplication. Since there is no other collider that can reach the energy attained by the LHC, so to prevent any embarrassing excursions into the scientific wilderness, CERN decided to build two detectors with independent teams, each to check the results of the other. As the exact properties of the Higgs are unknown, two different designs also allows CERN to hedge its bets.

⁴ If the kinetic energy is much larger than the rest mass energy, then in term of the proton energy E_p, the centre-of-mass energy E_cm=2E_p, and the energy in the laboratory frame E_lab=E_cm²/2m_p. Thus, for E_p=7 Tev, E_cm=14 Tev, and E_lab=10⁵ Tev.

New dimensions - By discovery of a mini black hole or missing energy from gravitational leakage, we can probe into the new dimensions in additional to the familiar three: "length, width, height". It will give indirect evidence to support the theory of superstring.
Dark matter - Many of the proposed solutions are related to the supersymmetric particles. It is certainly possible that such particles will emerge from the collisions of the LHC if the theory is correct.
Dark energy - We haven't a clue as to what it is. It might not be dark, and it might not be energy. But its identification should be an important LHC objective.
Right-handed neutrinos - A heavy right-handed neutrino is required to reconcile with the new discovery of neutrino mass. The LHC may have enough energy to create such particle. The discovery of such particle at LHC would resolve the dilemma of the excess of matter over antimatter.
Compositeness - All the standard model particles have zero radius and so no substructure. As smaller domain can be probed by more energetic particles, the LHC may discover that the elementary particles is not so elementary after all.
Technicolour - It is a quantum field theory postulated as an alternative to the Higgs hypothesis for explaining the masses of the W and Z particles. Technicolour predicts a new strong force and a large number of new particles, whose signatures could stand out above backgrounds of the complex collisions that the LHC will produce.

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]

Nine days after its September 10 startup, the machine is badly damaged. The problem arose when an electrical fault punched a hole in the enclosure containing cryogenic liquid helium, causing it to vaporize (Figure 23). Because the gas could not escape

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.

On February 9, 2009 CERN announced a further delay, citing additional safety protocols and complex repair schedules as the reasons. The machine will not run until later October. With a short technical stop over Christmas, the LHC will run through to autumn 2010, but it will accelerate its protons to just 5 Tev reducing the chance of finding something new.

Another announcement by CERN on August 6, 2009 stated that the LHC will start to produce data by Christmas, 2009 with energies at about half of that originally designed for and may not reach the peak energy unitl 2010 - if at all.