Particle Accelerators and Detectors

LHC (Large Hadron Collider) + Update 2022

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
Figure 16 LHC [view large image]	Figure 17 LHC Layout [view large image]	LHC is 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
Figure 18 LHC Beam [view large image]	Figure 19 Magnet [view large image]	Figure 20 Beampipe [view large image]	energy to damage the 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.

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 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

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

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.

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. 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 (due to more debris particles) if the Higgs boson has mass at the lower end. Experimentalists

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

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 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 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.

Figure 23 LHC, Problems with [view large image]

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

"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,

		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 D^o and anti-D^o decay (into either a pion and anti-pion or kaon and anti-kaon). By comparing the decay products from D^o and anti-D^o, 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 cross section in the standard model excluding the Higgs'.

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

. 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 (

) has 68% CL, the yellow one (

) 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

Figure 30 Beyond Standard Model [view large image]

dimension) were also proposed to resolve the problem (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 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 involve 2

(photon) and 4l (lepton) as the final products.

Figure 31 Data for H ZZ* 4l Decay Channel

See the LHC Home page for more details, and a brief review of "Scattering Cross Section and Decay Width".

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.

Figure 32 Data from Tevatron [view large image]

Such difference makes 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).

			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 some decay modes (see Figure 35, which also shows the % probability of the decay mode and a flowchart for future development).
Figure 33 Peter Higgs	Figure 34 Bump of Destiny	Figure 35 Beyond Standard Model [view large image]

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.

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.

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.

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 E_T,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 other value, it implies that the
Figure 37 LHC Update 10 [view large image]	Figure 38 Higgs Signature	boson does 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").

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 B_s 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 B_s meson decays) - another un-favorable omen for SUSY. As LHC will continue to smash protons until 17 December 2012 (see down-time works).

Figure 39 LHCb Search for SUSY [view large image]

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 B_s 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 B_s 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.

	The parent particle would have no electrical charge. Its spin could be 2 or 0 It may not be an elementary particle if the spin is 0 It could be a composite particle with parts bound by an entirely new fifth force. It doesn't correspond to any supersymmetric particles. There should be other decay products associated with such massive particle, but none were found.
Figure 40 A Blip at 750 Gev [view large image]	Anyway LHC will start taking more data in April, 2016. The bump should disappear in the summer if it is just a statistical fluke.

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.

[2022 Update]

In the Standard Model (SM) of particle physics, the top quark is the elementary particle with the largest mass and the largest Yukawa coupling g_t = m_t/v with the Higgs boson, where m_t is the mass and v the vacuum expectation value of the Higgs field. Its mass m_t is a fundamental parameter of the SM, it helps to check out the internal consistency of the theroy at the electroweak scale. However, the value of m_t is slightly different depending on different kinds of measurement. The LHC reports a coupling strength of 1.16 indicating no deviations with respect to the SM expectations (see "Recent measurements of the top-quark mass and Yukawa coupling using the ATLAS and CMS detectors at the LHC").
Measurement to greater precision for the mass of the Higgs boson (125.35 � 0.15 Gev), top quark (171.77 � 0.38 Gev) and W boson (80.370 � 0.019 Gev) are important for constraining the Standard Model as a stable theory, and Existence of the Universe.
Precise measurements on Higgs by both ATLAS and CMS were found to be in excellent agreeent with the predictions of SM.
Measurement of Higgs self-coupling (see "Does the Higgs talk to itself?", and "Extending the reach on Higgs� self-coupling").
Much precise measurements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix improves our understanding of CP-violation.
The Beauty quark decay probabilities to electron and muon seems to be different violating prediction by the Standard Model (see "Intriguing new result from the LHCb experiment at CERN").
Absence of any sign of super-symmetry becomes a curse or blessing (depending on point of view).

3rd Run

Finding New Physics (NP) has now become the primary mission. Interest is focusing on persistent bump of the "number-of-events vs collision-energy" graph.

Learning more about the Higgs boson (may offers hint to NP) :

More precision measurements of parameter such as interaction strength.
Check out if it interacts in the same way with particles lighter than the heaviest one.
Direct measurement of its lifetime, which has been obtained indirectly to be 2.1x10^-22 sec via the uncertainly relation tE ~ h.
More on the so-called "trilinear-self-coupling" k in V(h) = m_hh² + kh³ + h⁴, where k

See "Happy birthday, Higgs boson! What we do and don�t know about the particle"

Figure 41 LHC 3rd Run
[view large image]

Further investigation of the difference in Beauty quark decay probabilities to electron and muon in violation predicted by the Standard Model (see "Intriguing new result from the LHCb experiment at CERN").

Heavy-ion collision allows investigation into the "quark-gluon plasma" state in early Universe.

See "The Higgs boson turns ten", and "Particle physics isn�t going to die � even if the LHC finds no new particles".

[End of 2022 Update]