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Particle Accelerators and Detectors


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,
LHC LHC Layout 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
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Figure 17 LHC Layout
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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).
LHC Beam LHC Beampipe 1 LHC Beampipe 2 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
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Figure 19 Magnet [view large image]

Figure 20 Beampipe
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LHC 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 the "CERN Home Page" to obtain more information about LHC.

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

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
Production of Higgs Boson particles, and the collisions actually occur between constituent quarks and gluons, each carrying 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 wider range and with 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.

In layman's language, the Higgs field is similar to a calm sea where the fish swim about without noticing the medium surrounding them. The creation of Higgs boson is like a tsunami (a high energy disturbance) making everybody aware of its presence. The analogy to the virtual Higgs particles (from quantum fluctuations) could be related to the undulated sea wave bobbling up and down un-predictably. Peter Higgs himself prefers the analogy of light refraction in a medium with no energy loss in the process. BTW, the Higgs particle is usually referred to as "God particle" by the media via the title of Lederman's famous or infamous book "The God Particle". He meant Goddamn to express the frustration in finding this particle. The current name stuck when the publisher refused to use the offensive language. But such name often evokes mental image of religious overtone.

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 21b shows the theoretical predictions of the decay percentages for the various primary decay modes.
Signature of Higgs Boson 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

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


difficult (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).

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 Ep, the centre-of-mass energy Ecm=2Ep, and the energy in the laboratory frame Elab=Ecm2/2mp. Thus, for Ep=7 Tev, Ecm=14 Tev, and Elab=105 Tev.

LHC Projected Discovery 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
Magnet, Damaged 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.

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.

LHC resumed operation on November 2009. It will run for two years accelerating protons to 3.5 Tev before entering a year-long shutdown around 2012 for upgrades needed to bring the machine up to a total collision energy of 14 Tev. This schedule seems to lend support to the allegation of faulty design.

LHC Update 1 - LHC started to run its experiments in earnest in earlier 2011. It is reported in a conference in July 2011 that
LHC Update 1 "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]

LHC Update 2 - By August 2011 the "excess events" has dropped from a significant level of 99% to 95% with additional data, and the improved understanding of the other processes that can make W bosons. Many physicists believe that the Higgs, if it exits,
LHC Update 2 Finding Higgs 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 Detector 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]

Data from ATLAS and CMS 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:
Beyond SM 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.

It has been shown in 2004 that the hierarchy problem is more general. It is not merely a consequence of GUT. The Higgs mass can be reduced to the range of 100 Gev only through a highly contrived fine tuning process within the frame work of the standard model. A latest study reported just before the December 13, 2011 announcement shows that the superstring theory (or M-theory) predicts a Higgs mass of about 125 Gev (see Nature - World View, 22-29 December 2011). The same theory also yields a spectrum of super partners, which could be detected in 2012 by LHC. The theory of superstring finally may have come of age.

LHC Update05 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 ~ 100 Gev, the dominant decay channels are those

Figure 31 Data for H ZZ* 4l Decay Channel

involve 2 (photon) and 4l (lepton) as the final products. See the LHC Home page for more details, and a brief review of "Scattering Cross Section and Decay Width".


LHC Update06 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.

Meanwhile, both the CMS and ATLAS teams have since refined their analyses by taking into account the 3 other channels, i.e., the 2W, 2, and a pair of bottom-antibottom quark, resulting in slightly weaker signals as reported at the same conference.

LHC Update 7 (July 4, 2012) - With the personal appearance of Peter Higgs (the next Nobel Prize winner in physics ? Figure 33), CERN announced that both the ATLAS and CMS experiments have analyzed trillions of proton-proton collisions. The result strongly indicates a new particle at the 125-126 Gev range with properties consistent with the Higgs boson (Figure 34).
Peter Higgs Bump of Destiny Beyond SM 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).
Meanwhile two days earlier, the now defunct Fermilab announced that their data collected over the last 10 years strongly indicates the existence of the Higgs boson.

P.S. The "binding the earth together" reference can be better understood by alluding to the case when there is no Higgs field. In such circumstance, all particles would have zero mass and move with the speed of light. It would be very difficult to maintain a coherent entity (such as the earth) with so many runaway particles. The exception would be something like the laser with all the photons moving as a whole at the speed of light.

LHC Update 8 (July 31, 2012) - By including the W boson decay channel, ATLAS announced the improvement of statistical significance for the Higgs signal from 5 sigma to nearly 6 sigma, meaning the chance of it being due to background processes has been reduced to 2 in a billion.

Search for SUSY 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]

    LHC Update 9 Higgs Signature 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 other value, it implies that the boson does

    Figure 37 LHC Update 10
    [view large image]

    Figure 38 Higgs Signature [view large image]

    not agree with SM prediction. The diagram shows that most channels decay as expected (within the error bar) except the 2 photons channel.
    This slight deviation seems to be the only data (up to now) pointing to new physics. The combined ratio from both team equals to 1.09 - indicating almost complete agreement with the Standard Model. Furthermore, observation of all 5 modes is compatible with a spin-zero particle, and measurement of the direction of decay fragments confirms that the boson has positive parity as demanded by SM. Thus, the signature for Higgs is almost complete as shown by Figure 38. However, CMS has started to look for other bosons with masses beyond 600 Gev, the current excluded limit. If something turns up there, then it means there is multiple Higgs bosons (up to 5) as predicted by supersymmetry. Meanwhile, the LHC has been scheduled to shut down on February 2013 for upgrade to 14 Tev collision energy keeping particle physicists in suspended animation for two years until early 2015. If the current situation persists then they would be hard pressed to explain why the mass of Higgs is not at the Planck scale of 1019 Gev because the Higgs obtains its mass through the surrounding virtual particles (this is known as the "hierarchy problem"). One way to reduce the mass to 100 Gev range is through cancellation with the spartners in the theory of supersymmetry (see also "beyond SM").

    LHCb Search for SUSY 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 will continue to smash protons until 17 December 2012

    Figure 39 LHCb Search for SUSY [view large image]

    (see down-time works), there will be another chance in March 2013 to provide more news on its fate.


    At the HEP (High Energy Physics) meeting 2013 July 18-24 in Stockholm, a CERN team confirmed that the Bs mesons decay is in agreement with the prediction by the Standard Model. Report by the CMS and LHCb collaborations in a June 2015 Letter to Nature confirms the previous conclusions, i.e., the joint analysis of the 2011, and 2012 data produces results which are consistent with the "Standard Model".

    Note : There are many kinds of B meson, all of them contain an anti-bottom quark. The one pairing with a strange quark is the Bs meson. See more about B Meson in a Wikipedia article.

    LHC Update 11 - At a conference on 14 March 2013, the ATLAS and CMS collaborations presented new results that further elucidate the particle discovered last year. Having analyzed two and a half times more data than was available for the discovery announcement in July 2012, they find that the new particle is looking more and more like a Higgs boson. They have compared a number of options for the spin-parity of this particle, and these all prefer no spin and positive parity. In addition, the latest data indicate that the boson decays into the tau leptons as predicted, and also dampen earlier hints that the boson decays into pairs of photons more often than the standard model allows. No evidence yet points to theories beyond the standard model such as supersymmetry. It remains an open question, however, whether this is the Higgs boson of the Standard Model of particle physics, or possibly the lightest of several bosons predicted in some theories that go beyond the Standard Model.

    LHC Update 12 - The 2013 Nobel prize for physics has been awarded to Peter Higgs and Francois Englert (on October 8) for "The theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle." Actually, there were 6 theorists proposing such mechanism in 1964. Higgs was the only author, at the time, to allude to the heavy boson that the theory implied. Englert was the first to publish the idea. Even none of the experimenters at LHC share the prize, they were happy just the same considering the award as a recognition for their work.

    LHC Update 13 - The existence of Pentaquark is confirmed by the LHCb experiment in July 2015.

    LHC Update 14 - The 2015 data collected by both ATLAS and CMS indicate a blip or bump at a total energy of 750 Gev from two photons decays (Figure 40). The odd for such occurrence is about one in several hundred (~ coin flips of 9-10 tails in a row, i.e., ~ 1/29 = 1/256). A scientific discovery requires an odd of 1 in 3.4 million (5 sigma). The same figure also reveals that starting from ~ 600 Gev the margin of error becomes relatively large comparing to the data points at lower energy. Nevertheless, it prompts more than 100 possible explanations by theorists with more to come. Followings are some interesting comments about the blip. An interim report on March 17, 2016 indicates that significance of the CMS bump has increased slightly from 1.2 to 1.6 sigma with 22% additional data; while the result from ATLAS remains the same.

    Representatives from ATLAS and CMS revealed on August 5, 2016 that the 750 Gev bump has faded to almost nothing after incorporating almost five times the amount of data used previously, i.e., the hint of a new discovery was just a statistical fluctuation.

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