Particle Accelerators and Detectors

Contents

Cosmic Rays

		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, cosmic rays bombard the Earth at a rate of about 1 particle/km2-sec. At ultra-high energies,
Figure 01a Cosmic Rays, Origins of [view large image]	Figure 01b Spectrum [view large image]	above 1019 ev, the rate falls to less than 1 particle/km2-year.

Detection of cosmic rays:


• 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 and could be conceivably

Figure 02a Cosmic Rays Detectors [view large image]

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

surface detector, the secondary particles produce faint flashes in the purified water. The fluorescence detector measures the glow emitted by nitrogen atoms after colliding with the secondary particles. The picture on the lower right shows both kinds of detectors installed at the Auger Observatory, which will try to shed light on the mysterious UHECRs.

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

1. 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.
2. The absence of surplus anti-protons is explained by the heavy mass of the mediating boson, which can decay into electrons and positrons only.
3. The 511 Kev photons observed by INTEGRAL can be identified to the annihilation of the electron-positron pair.
4. 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.

Two 2013 reports suggest strongly that the UHECRs were originated from the shock wave associated with supernova explosion. Gamma-ray data on 2 supernova remnants, called IC443 and W44, from NASA's Fermi space telescope found abundance of gamma-ray photons in the range of energies expected from pion decay. The pion is produced by collision of high energy proton with matter. In the other study, the spectrum from supernova remnant 1006 reveals the presence of high energy protons in the shock wave front.

Figure 02f Supernova Shockwave

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-line accelerators suffer from the

Figure 03 Van de Graaff Generator [view large image]

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 accelerate them.
Figure 04 Cyclotron [view large image]	Figure 05 Synchrotron [view large image]

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 with the debris after impact on the target. In a collider, on the

Figure 06 Collider [view large image]

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 produced them, other things being equal

Figure 07 High Energy Collision [view large image]

- 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 strong interaction)

Figure 08 Top Quark Events [view large image]

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

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.

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.

An Example - The LHC (Large Hadron Collider)

		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 designed to collide two counter
Figure 16 LHC [view large image]	Figure 17 LHC Layout [view large image]	rotating beams of protons or heavy ions. Proton-proton collisions are foreseen at an energy of 7 TeV per beam.

			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
Figure 18 LHC Beam [view large image]	Figure 19 Magnet [view large image]	Figure 20 Beampipe [view large image]	LHC 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
	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 or 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

only a proportion of the total proton energy (the two Higgs bosons production modes are shown in Figure 21a). 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

Figure 21a Higgs Boson Production Modes [view large image]

collision energies of 14 Tev, which are 7 times larger than those of any previous accelerator.

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 21b. 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 would be more difficult

Figure 21b Higgs Boson Decay Modes [view large image]

(due to more debris particles) if the Higgs boson has mass at the lower end. Experimentalists have been working on the assumption that the Higgs boson lies somewhere between 114 and 175 GeV. In the lower mass range, the Higgs boson decays most probably into bottom quark and antibottom quark (Figure 21b).

In addition to the official list above, There is also a speculative laundry list that LHC may be able to illuminate:


• 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]

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 quicker option is the only one on the

Figure 23 LHC, Problems with [view large image]

table for now. Hopefully, it will be restarted in late July 2009 as announced on December 5, 2008.

"excess events" have been observed in both CMS (Figure 24) and ATLAS within a mass range of 130 - 150 Gev - right in the middle of the prediction for the mass of the Higgs particle. But physicists familiar with the experiments urge caution. The new data are a long way from a discovery. The disappointment in the conference is prompted by no hint of new physics. LHC failed to find trace of any supersymmetry particles or additional dimensions. Physicists are now waiting for collection of more data and the results of the full energy (14 Tev) collisions to see if it could turn up something entirely new (in its 20-year lifetime).

Figure 24 Particle Collision at CMS [view large image]

		is likely to be at the lower mass end of the energy spectrum, perhaps between about 115 and 145 Gev. Those lower energy ranges will require more data to find a signal because it is harder to tell a true one from other particles that produce similar tracks. It may be possible to rule out the Higgs in some regions by the end of 2011, but confirmation of a discovery, which requires more data, will probably have to wait until the end of 2012.
Figure 25 Higgs Searching [view large image]	Figure 26 Finding Higgs [view large image]	Figure 26 shows the opinions of some theoretical physicists on the odds of finding the Higgs particle. The 5 means 10-7 probability due to random fluctuation - an almost certainty.

LHC Update 3 - The first sign of new physics at LHC showed up in the LHCf detector (Figure 27, and see Table 02) involving Do and anti-Do decay (into either a pion and anti-pion or kaon and anti-kaon). By comparing the decay products from Do and anti-Do, it is found that there is a difference of 0.8%. That disparity is 8 times predicted by the standard model, and could be explained by new theories including supersymmetry. The virtual super-partners could boost the asymmetry (of the matter/anti-matter process) to as much as 1%, enough to account for the LHCf measurements.

Figure 27 LHCf Detector [view large image]

	LHC Update 4 - The ATLAS and CMS teams announced on December 13, 2011 that they have narrowed down the Higgs mass to 125-126 Gev (3.6), and 124 Gev (2.6) respectively. The result is consistent with the prediction from either the standard model or more likely form its extension with supersymmetry.
Figure 28 Data from ATLAS and CMS [view large image]	Figure 28 shows the number of events registered by the ATLAS and CMS detectors as function of Higgs mass. Followings are translations of jargons used by the experimentalists in LHC:

• Energy scale - It portrays the various cross sections as a function of the Higgs mass (in Gev).



• Combined (or integrated) L over time - It is related to the intensity of collisions. In term of the inverse femtobarn (fb^-1), it is the number of particle collision events per femtobarn (a cross-section area of 10^-39 cm²).



• Observed cross section - The observed cross section = N/L, where N is the number of events and L is the combined L.



• /_SM - It is the observed cross section divided by the expected Higgs cross section in the standard model.



• Solid dark curve - This curve is the observed cross section divided by _SM, i.e., /_SM. If its value is equal to or greater than 1.0 and also somewhat above the dotted curve, then there might be a hint that the Higgs exists with a mass at that value. It turns out that most of the Higgs events involve the previously neglected two photons decay.



• Darker dotted curve - It shows the expected number of events based on the background of similar-looking events produced by something other than a Higgs decay (also in unit of _SM).



• Horizontal line - This line with /_SM = 1 is used to exclude the Higgs mass on the range of energy with /_SM 1.



• CL (Confidence level) - It is defined as the frequency of the observed intervals (in samples) containing the adopted mean value. A mass is said to be ¡§excluded at 95% confidence level¡¨if the Standard Model cross section _SM with that mass is higher than the observed data at least 95% of the time in a set of repeated experiments.




Standard Deviation - It is denoted as n. In this case, larger +n signifies the fact that the observed cross section deviates further away from the non-Higgs background and represents a more genuine Higgs event (Figure 29). Event on the green band (1) has 68% CL, the yellow one (2) has 95% CL to be related to Higgs particles. These bands help to estimate the significance of the observed data (the probability of being a Higgs signal).

Figure 29 Standard Deviation

The standard model used to predict a Higgs mass in the order of 100 Gev. However, its extension - the Grand Unified Theory (GUT) - adds much heavier particles into the scheme. The Higgs mass becomes very heavy as the result of interacting with the heavy virtual particles from the GUT (Figure 30, a) in contradiction with the expected Higgs mass of about 125 Gev. This is known as the "hierarchy problem"¶. The theory of supersymmetry was introduced to address this problem as the virtual super-partners tend to restore the Higgs mass back to the hundred Gev range (Figure 30, b). Other more exotic ideas such as "technicolor" (involving new force) and "extra-dimension" (involving additional dimension) were also proposed to resolve the problem

Figure 30 Beyond Standard Model [view large image]

(Figure 30, c and d, also see another version of the "(Warped) 5-D Theory"). It is believed that such theories can be tweaked to produce a theoretical Higgs mass compatible with observation.

LHC Update 5 - According to a blog in Nature, both the ATLAS and CMS teams submitted the results of their latest analyses on February 2012. By taking into account another possible kind of Higgs decay, the CMS graph boosts the deviation to 3.1 at 125 Gev. Taken together with data from the other detector, ATLAS claims an overall signal at about 4.3. In other words, the signal has about a 99.996% chance of being right (it requires 99.9999% to qualify for an scientific discovery). Figure 31 plots the data from ATLAS for just one particular H ZZ* 4l decay channel, where Z* denotes the virtual Z boson, and the 4l represents 2 pairs of either e-e+ or -+. It also shows a large deviation at 125 Gev with 2.1. Meanwhile, scientists are meeting to decide at what power to run the collider this coming year. The latest rumours are that the machine will push from 7 to 8 Tev, and will also increase its luminosity (the number of collisions per pass). In the low mass range of

Figure 31 Data for H ZZ* 4l Decay Channel

~ 100 Gev, the dominant decay channels are those involve 2 (photon) and 4l (lepton) as the final products. See the LHC Home page for more details.

LHC Update 6 (march 7, 2012) - This update is actually not from LHC. It comes from the now defunct Tevatron at Fermilab in Batavia, Illinois - the last hooray from the once great US particle collider. The final analyse of data collected over the years (starting from 2002 to September 2011 when the machine was shut down) reveals an excess of events by Higgs with a mass between 116 and 127 Gev (Figure 32). The excess had a statistical significance of 2.6 sigma. Meaning there is about a 0.5% probability that the result is due to chance. At Tevatron's 2 Tev colliding energy, the Higgs decay mainly into a bottom and antibottom quark, whereas the LHC is sensitive to the production of two photons and other decay modes. Such difference makes

Figure 32 Data from Tevatron [view large image]

the results from the two machines complementary. Some physicists lament that the Tevatron may be able to find the Higgs if the funding is not cut by the US Department of Energy.

			The level of significance is 5 sigma, which merits the status of scientific discovery. The complete analysis is expected to be published around the end of July. Then it could be sure that the particle has spin 0 and whether super-symmetry is required to explain the result. Right now many physicists are pinning their hope for a new theory on the abnormal rate of decay in
Figure 33 Peter Higgs [view large image]	Figure 34 Bump of Destiny	Figure 35 Beyond Standard Model [view large image]	some decay modes (see Figure 35, which also shows the % probability of the decay mode and a flowchart for future development).

Followings are comments by some prominent politicians on the God particle after the CERN announcement :
• Speaking to a crowd of blue-collar workers in Ohio, President Barack Obama hailed the scientific news from Geneva as "a great moment in history, not unlike my rescue of the auto industry." The physicists who made the discovery, Obama noted, all had health insurance. He then added that there have been more Higgs bosons discovered during his administration than during those of both George Bushes combined.
• Mitt Romney today denounced Barack Obama for allowing Europe to beat the United States at particle physics research. Under his administration, Romney vowed, "All particles that bind the earth together will be discovered in America, by Americans and for Americans."
• Gov. Chris Christie today called for the privatization of the Higgs boson. "Binding the earth together is something that could be handled much more efficiently by the for-profit sector," the Republican governor and deeply available vice-presidential prospect said. "Auctioning off the rights to the Higgs boson will create American jobs and balance American budgets."
• Donald Trump told reporters that "my people in Hong Kong" have uncovered evidence that America's failure to take the lead in subatomic particle research was because of a conspiracy between the Obama administration and unnamed Chinese industrialists. He also said that he had invited the Higgs boson to be a contender on "All-Star Celebrity Apprentice", but he wants to check out its birth certificate first.

LHC Update 9 (March, 2012) - LHC has published a report about the search for SUSY particles - the supersymmetric partners of the normal particles. Analysis of the data indicates that the searches have failed so far. The details are summarized in a table (Figure 36). The blue bars represent the lower mass limit (in Gev or Tev) for the partners according to different theoretical models, which produce various primary (and secondary) decay products shown on the left of the graph (SS = same sign, OS = opposite sign, j = jet) with a sample illustration on the right side. The ET,miss term denotes the missing transverse energy to the stable lightest SUSY particle (LSP). The acronyms for the various theoretical models are briefly explained below :

Figure 36 Search for SUSY [view large image]

• MSUGRA - Minimal Supergravity = gauging (N = 1, i.e., 1 SUSY generator) supersymmetry in standard model (SM).
• CMSSM - Constrained Minimal Supersymmetric extension of Standard Model (MSSM). Constraint means making certain assumptions about the free parameters.
• Pheno model - Phenomenological model (calculation of detailed predictions for high energy experiment from theory).
• Gluino med - Gluino mediated supersymmetry breaking (to explain the mass difference between particle and superpartner).
• GMSB - Gauge mediated supersymmetry breaking.
• Direct xx - Direct xx (e.g., direct gaugino = DG ) pair production.
• AMSB - Anomaly-mediated supersymmetry breaking.
• SMP - (Quasi)Stable massive particles.
• R-hadrons - Gluino, stop.
• RPV - R-parity violation process (bilinear - neutrinoless double beta decay).
• R-parity - In MSSM, the conservation of baryon number B, and lepton number L separately is replaced by the conservation of the R-parity P_R = (-1)^2s+3B+L with spin s (P_R = +1 for particle, = -1 for superpartner). It implies that superpartners would be created in pair from particle collision, and a superpartner must decay into another superpartner. Thus, the lightest superpartner (LSP) cannot decay, since there is no other superpartner that it can decay into (see insert in Figure 36). However, some sypersymmetric models such as the GMSB predict the violation of the R-parity.

		LHC Update 10 (November, 2012) - The CMS and ATLAS teams presented more data at a symposium held in Kyoto, Japan. Figure 37 shows the results from both teams. The ratio of observation to expectation (on the SM) is denoted by /SM or . A value of zero indicates no activity on that particular channel, contrary to expectation. If the value is one then it means the channel behaves exactly as predicted by SM. For any
Figure 37 LHC Update 10 [view large image]	Figure 38 Higgs Signature [view large image]	other value, it implies that the boson does not agree with SM prediction. The diagram shows that most channels decay as expected (within the error bar) except

The LHCb team (see Table 02) announced in the same occasion that it has failed to detect any sign for supersymmetry. They measured the extremely rare process in which a Bs meson decays into a muon-antimuon pair. According to the theory of supersymmetry, the rate could increases by a factor of about 100. But the result agrees with the prediction of the Standard Model (about once for every 300 million Bs meson decays) - another un-favorable omen for SUSY. As LHC

Figure 39 LHCb Search for SUSY [view large image]

will continue to smash protons until 17 December 2012 (see down-time works), there will be another chance in March 2013 to provide more news on its fate.