onsdag 9 februari 2011

EDM reveals - antimatter without MSSM and Higgs?

Electrons have a secret? (This is a short variant of the article, look for more details at the link.)

The shape of the electron. This is one of the most sensitive probes of the difference between matter and antimatter.

We are looking for a deviation from spherical that would indicate a difference in the laws of physics that govern matter and antimatter. This would show that our current theory of particle physics, the Standard Model is incomplete. We know that any deviation from roundness is going to be tiny.
Detecting an electric dipole moment (EDM) would mean electrons, which buzz about atomic nuclei, have internal structure. So far we've checked the roundness of the electron to an incredible degree of precision: the equivalent would be measuring the diameter of the earth to better than the width of one human hair. And so far, we've seen no evidence of non-roundness. Measuring the inner shape of the famous particle could help solve a cosmic mystery, where's all the antimatter? By carefully measuring the shape of the electron, through a particular property known as the electric dipole moment, which would mean that the electron has some kind of internal structure, a bizarre concept for a particle that is supposed to buzz around the nuclear hearts of atoms and molecules with its mass concentrated into an essentially sizeless point. Although no one has yet measured the electron's electric dipole moment, researchers think it should exist and could be within reach of today's modern laboratory setups.

You can learn about the techniques we use, our latest results, what we're planning, and who we are. Trapped polar molecules may be used to measure the electric dipole moment of the electron, a search for physics beyond the standard model. By trapping chiral molecules, it should be possible to study the role of the weak interaction in the emergence of homochirality in biological molecules. Once we have trapped the molecules we hope to cool them further, bringing them into the ultracold regime. Ultracold polar molecules offer a new dimension in cold atom physics, primarily due to their long-range dipole-dipole interactions. By varying the switching timing, the length of the lenses can be varied, which affects the number of stable trajectories along the guide. The electrode geometry was chosen to provide a strongly harmonic field variation; transverse confinement in the high-field Stark limit can be treated as that of a harmonic potential.

"There are good theoretical reasons to think that it isn't too far away," says physicist Larry Hunter of Amherst College in Massachusetts, who has been hunting the electron's electric dipole moment since the 1980s. "What has made us all dedicate our lives to it is the real good chance that something might emerge soon." Within the next few months, scientists at Imperial College London are expected to report the latest limit on the size of the electron electric dipole moment, the first such improvement in a decade.

Physicists suspect that electric dipole moments exist because they allow particles to violate what's known as time-reversal symmetry. Although symmetry sounds like a good thing, scientists know that processes involving other particles (such as B mesons) behave differently whether running forward or backward, a violation of time-reversal symmetry. In order for this to happen, the electron (and other fundamental particles) must have an internal structure, something an electric dipole moment can reveal.

Imagine that the electron has "a cloud of stuff following it and blow that electron cloud up to the size of Earth, and extra positive charge would appear as a tiny dent on the north pole while extra negative charge would be a tiny bulge on the south pole. Given current limits, the size of that dent or bulge would correspond to adding or subtracting no more than about one-thousandth the width of a human hair from either end of the planet.

Reversing time (by switching the direction of a particle's spin) changes the direction of the magnetic dipole (blue) but not the electric dipole (green). This means that possessing an electric dipole moment would give particles a way to violate time-reversal symmetry. Whether a property known as the particle's spin responds differently when the field is switched on in different orientations, which would mean the electron possesses an electric dipole moment. Seeing that difference is the hard part.

Our most recent published measurement of the electron edm is (-0.2 ± 3.2) 10-26 e.cm. [You can find the paper on our publications page]. The current limit has already ruled out the simplest version of a popular idea known as supersymmetry, which tries to explain the cosmic matter/antimatter imbalance by suggesting that every particle has an as-yet-unseen "superpartner." If researchers can push the limit to 10–29, that would rule out another extension to the standard model that tries to solve the matter problem by postulating multiple kinds of the particle known as the Higgs boson, which Europe's Large Hadron Collider was designed to detect. In 2002, Commins' team published the most stringent limit yet: 1.6 × 10–27. The standard model predicts that the electron's electric dipole moment is less than 10–38 in units of electron charge times centimeters.

Any non-zero measurement of the electron edm would be clear evidence for physics beyond the Standard Model. Low-energy particle edm measurements can be viewed as complementary to high-energy accelerator experiments.
The minimal supersymmetric standard model, or MSSM (red), is a standard model extension that holds that every elementary particle has a “superpartner.” One of the simplest versions has been ruled out by the current limit. Another version (blue) of MSSM that sets a parameter dubbed phi to a different value is still a possibility, but it too may be ruled out when researchers lower the bar further. The multi-Higgs model and left-right symmetric models are also ruled out if the new limit is achieved.

What's more, the electric dipole interaction has a property that makes it particularly interesting: it violates time reversal symmetry. That's because the dipole must lie either parallel or anti-parallel to the spin, but the choice of one over the other violates time reversal symmetry. Direct observation of microscopic time-reversal asymmetry would be a profoundly interesting result.

Measurements of particle interactions at the smallest scales seem to indicate that matter and anitmatter are treated in a rather symmetric way. Andrei Sakharov showed that CP-violation is needed to explain the matter dominance of the universe. The degree of CP-violation known in particle physics falls well short of that which would be needed. So, one might reasonably expect that there must be extra CP-violating mechanisms at work - looking for them seems like a good idea. Our measurement of the electron edm could shed light on these mechanisms.

To measure this tiny interaction we use a neat trick. We choose a pair of energy levels that are shifted oppositely by the edm interaction. This gives us a powerful "common mode rejection" of many deleterious effects. We measure the energy difference between our two chosen levels directly using quantum interference. We prepare a coherent superposition of the two energy levels of interest and measure the phase evolution of this superposition interferometrically. We call this technique spin interferometry, and it is described in detail in many of our publications.

An example of interference from two molecular energy levels that have different energies in a tiny magnetic field.
See the cutting edge of research in molecule cooling, manipulation and trapping to build a foundation for the next generation of edm experiment.

From article comments: If the electron has no internal structure, and consists of a real point charge, its energy has to be infinite! And it is impossible to explain its 1/2 spin. (Just claiming that the spin is "intrinsic" is no explanation...)

There already exists a model of the electron that explains its charge and spin, based just on the particles structure/topology and else classical physics (google "neoclassical atom").

Watching An Atom's Electrons Move in Real Time
Physicists have used ultrashort flashes of laser light to directly observe the movement of an atom’s outer electrons for the first time - a process called attosecond absorption spectroscopy.

Researchers were able to time the oscillations between simultaneously produced quantum states of valence electrons with great precision. These oscillations drive electron motion. “This revealed details of a type of electronic motion – coherent superposition – that can control properties in many systems,” says Stephen Leone.

A classical diagram of a krypton atom (background) shows its 36 electrons arranged in shells. Researchers have measured oscillations of quantum states (foreground) in the outer orbitals of an ionized krypton atom, oscillations that drive electron motion.
The crucial role of coherent dynamics in photosynthesis as an example of its importance, (the Graham Fleming group) noting that “the method developed by our team for exploring coherent dynamics has never before been available to researchers. It’s truly general and can be applied to attosecond electronic dynamics problems in the physics and chemistry of liquids, solids, biological systems, everything.” - Leone. Theoretically, however, with longer ionization pulses the production of the ions gets out of phase with the period of the electron wave-packet motion. After just a few cycles of the pump pulse, the coherence is washed out. Thus, says Leone, “Without very short, attosecond-scale probe pulses, we could not have measured the degree of coherence that resulted from ionization.”

Electrons can also be used to create an hologram of the atom.
Scientists have created holograms of atoms using laser-driven electron motion, which could lead to a new type of ultra-fast photoelectron spectroscopy. In the future, this type of holography could enable scientists to study the structures of molecules in a more direct way than before. Ymkje Huismans from the FOM-Institute AMOLF in Amsterdam, The Netherlands, and an international research team have published their study in a recent issue of . They have experimentally demonstrated is that it is possible to make holograms by taking an electron out of a molecule and, using a laser field, redirect the electron toward the molecule.

The scientists beamed an intense infrared laser light at an atom or molecule, which resulted in the atom or molecule becoming ionized and releasing an electron. The laser field causes the liberated electron to oscillate away from and toward the ion. Sometimes, an electron and ion collide, releasing a very short burst of radiation.

Because the is fully coherent, meaning it always has the same phase, the scientists realized that they could apply holographic techniques to record information about the ion and electron. The key to holographic electron imaging is to observe the interference between a reference wave (which is emitted by the electron and doesn’t interact with the ion) and a signal wave (which scatters off the ion and encodes its structure). When the reference wave and signal wave interfere on a detector, the encoded information about the electron and ion is stored and can be viewed in the future. As the scientists explained, the result is a hologram of an atom produced by its own electrons.

The researchers also developed theoretical models to simulate their measurements, confirming that the hologram had stored spatial and temporal information about the and ions. By using the holographic structures to develop a new kind of ultra-fast , researchers could be able to directly measure electron and movements on the attosecond timescale.

Do electrons hold the key to matter/antimatter asymmetry?Sizing up the electron. Science News, Feb. 12, 2011 Sizing up the electron

Eleftherios Goulielmakis, Zhi-Heng Loh, Adrian Wirth, Robin Santra, Nina Rohringer, Vladislav Yakovlev, Sergey Zherebtsov, Thomas Pfeifer, Abdallah Azzeer, Matthias Kling, Stephen Leone, and Ferenc Krausz, “Real-time observation of valence electron motion,” Nature, 466, 739-743 (5 August 2010). Abstract.

Y. Huismans, et al. “Time-Resolved Holography with Photoelectrons.” Science Express. 16 December 2010. DOI:10.1126/science.1198450

Electron EDM. Centre for Cold Matter, Imperial College, London.

The Fleming Group 2010:
  1. Spectroscopic elucidation of uncoupled transition energies in the major photosynthetic light-harvesting complex, LHCII, G.S. Schlau-Cohen, T.R. Calhoun, N.S. Ginsberg, M. Ballottari, R. Bassi, G.R. Fleming, PNAS 107 (30), 13276-13281, JUL 2010.
  2. Quantum coherence and its interplay with protein dynamics in photosynthetic electronic energy transfer, A. Ishizaki, T.R. Calhoun, G.S. Schlau-Cohen, G.R. Fleming, PHYSICAL CHEMISTRY CHEMICAL PHYSICS 12 (27), 7319-7337, 2010.
  3. Quantum entanglement in photosynthetic light-harvesting complexes, M. Sarovar, A. Ishizaki, G.R. Fleming, K.B. Whaley, NATURE PHYSICS 6 (6), 462-467, JUN 2010.
  4. Branching Relaxation Pathways from the Hot S2 State of 8′-apo-β-caroten-8′-al, Y. Pang, G.R. Fleming, PHYSICAL CHEMISTRY CHEMICAL PHYSICS 12 (25), 6782-6788, 2010.
  5. Quantum superpositions in photosynthetic light harvesting: delocalization and entanglement, A. Ishizaki, G.R. Fleming, NEW JOURNAL OF PHYSICS 12 (5), 055004, May 2010.
  6. Ultrafast Spectroscopy of Midinfrared Internal Exciton Transitions in Separated Single-Walled Carbon Nanotubes, J. G. Wang, M. W. Graham,Y.-Z. Ma, G. R. Fleming, R. A. Kaindl, PHYSICAL REVIEW LETTERS 104 (17), APR 30 2010.
  7. Exciton annihilation and dephasing dynamics in semiconducting single-walled carbon nanotubes, M. W. Graham, Y.-Z. Ma, G. R. Fleming, A.A. Green, M. C. Hersam, Ultrafast Phenomena in Semiconductors and Nanostructure materials, Proceedings of SPIE, 7600-7613, 2010.
  8. Unusual Relaxation Pathway from the Two-Photon Excited First Singlet State of Carotenoids, Y. Pang, G. A. Jones, M. A. Prantil, G. R. Fleming, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 132 (7), 2264-2273, FEB 24 2010.
Uppdated 4.3.11.

17 kommentarer:

  1. http://news.discovery.com/space/could-the-higgs-be-hiding-in-graphene.html

    Pablo San-Jose, Francisco Guinea, and Jose Gonzalez at Madrid's Institute for Material Science propose - in a new paper in Physical Review Letters - that their graphene model loosely mimics how the Higgs field condenses, thereby giving rise to the symmetry breaking that may have split the electroweak force into the weak and electromagnetic forces in the early stages of our universe.

    Why are scientists so excited about a 2D sheet of carbon? Graphene has quantum superpowers! Much has been made of the material's potential for creating ultrafast molecular-scale transistor, especially the fact that the electrons in graphene zip along at the speed of light, as if they had no mass - contrary to special relativity, which says no object with even the tiniest bit of mass can exactly reach the speed of light.

    On a less cosmic scale, some version of spontaneous symmetry breaking appears to be a crucial component in many basic physical processes, including simple phase transitions, such as the critical temperature/pressure point where water turns into ice.

    It also occurs in graphene: it's responsible for the transition of the material from flat to rippled when graphene is compressed.

    And this, apparently, bears a striking resemblance to the symmetry breaking in the early universe that led to the electroweak force splitting in two. Physicists generally describe this process in terms of the Higgs field - the vibrations of which are associated with the Higgs boson - shifting from a high energy state down to its ground state. Perhaps this explanation from a recent article in Physics World will help with a handy visualization:

    There are several different models for the Higgs mechanism.The graphene model maps nicely onto one possible model, but it might not be the model that ends up being correct. It's definitely an example of spontaneous symmetry breaking, but, as noted previously, this happens all the time in physics, and has been observed repeatedly (e.g. in ferromagnets). There's nothing particularly "higgs-y" about graphene compared to those other examples.

    Oh well. On the upside, it's still pretty nifty in its own right. And graphene can now join superconductors in the category of Cool Materials That Generate Mass with a Higgs-Like Mechanism in the Solid State. Yes, that's right, a similar effect also occurs in superconductors via something called the Meissner effect. A magnetic field can only penetrate a short distance inside a superconductor, and this effect "is equivalent to saying that the photons propagating in the superconductor have acquired a mass," according to Armitage. (Photons, remember, are usually massless particles.)

    And that, for Armitage, is the most relevant aspect of the Spanish paper. "It's incredibly interesting that nature repeats on the large scale - superconductors and graphene - and the small scale (particle physics)," he says. "The same general ideas repeat themselves in different forms."

  2. No SuSy. No love for low scale supersymmetry at the LHC, ATLAS.

    I should also say that this is only one of many searches for new physics (including many other SUSY searches).

  3. http://matpitka.blogspot.com/2011/02/good-bye-large-extra-dimension-and-mssm.html


  4. Look also at

    Lubos has finally began to speak :)

    Talk by Alessandro Strumia.


  5. To get the list complete I must add a post of Lubos on Atlas SUSY, the paper is published in Phys. Rev. Lett soon.


    There are a lot more yet to come.

  6. Resonaances write Monday, 21 February 2011, More SUSY limits.


    These days new experimental results pop up like mushrooms with the peak expected middle March at the Moriond conference. Last week new results from LHC SUSY searches were presented at the Aspen conference, both by CMS and ATLAS. The latest additions are the jets+MET search from ATLAS, and the photons+MET and dileptons+MET searches from CMS. The new ATLAS search provides the current best limits on the mSUGRA parameter space.

    Unfortunately, for the moment ATLAS and CMS present their theoretical interpretations only in this obscure and contrived way.

    SUSY searches are in fact searches for squarks and gluinos. That's because only superpartners carrying the QCD charge have had a chance to be produced at the LHC in reasonable quantities.

    With several final states already covered, and more to appear soon, it's getting harder to avoid the stringent LHC limits in most popular SUSY scenario. Nevertheless, the possibility of sub-TeV superpartners has not been completely excluded yet. Firstly, uncolored superpartners are not constrained by the LHC. Furthermore, gluinos and/or squarks with masses 500 GeV or less are still allowed as long as the mass splitting with the lightest neutralino is small enough, such that the supersymmetric events fail the missing energy cuts. Stops, that is the scalar superpartners of the top quark, are even less constrained due to the smaller production cross section and the pesky t-tbar background. As a last resort one can turn to R-parity violating scenarios which are not constrained by the current LHC searches.

    See also Not Even Wrong

    http://arxiv.org/abs/1102.3386 Supersymmetry from the top down.

  7. Tommaso Dorigo has also spoken. LHC Excludes SUSY Theories, Theorists Clinch Hands.



    CMS, whose result is also shown as a black curve in the plot above, studied events just featuring missing transverse energy and jets. This is known to be a priori the best search channel for "inclusive" Supersymmetric signatures at hadron colliders: not requiring a lepton increases backgrounds, but accepts the most signal. The reason for the large acceptance to signal events is that squarks and gluinos are coloured objects, and thus can be copiously pair-produced in the interaction of two regular hadrons. The chain of decays of pairs of such particles ends up producing jets and, if R-parity is conserved, a couple of neutralinos -particles that escape undetected and produce an imbalance in the transverse energy detected by the experiment. (I should mention that the ATLAS search is also sensitive to squarks and gluinos, but only when a chargino is produced in the decay chain, yielding an additional charged lepton. In this sense, the ATLAS search is more "exclusive", but that does not mean it should be more sensitive).


    Allanach uses the SUSY limits set by CMS to constrain the full space of parameters with a global fit to particle physics and astrophysics observables.

    Tommaso ends with: if we see nothing in three years or so there will be a rush to abandon the sinking ship... The next few years are going to be quite interesting!!


    And note the discussion of so many Anonymous Heroes. They has come forth now in masses. Are their existence induced by the courageous results?

  8. So has he spoken.

    Lubos, we wait for your qualified analysis of the situation. As an expert you can certainly point to the errors and open questions in this SUSY analysis. We are all truly interested. Certainly also fans to the left or to the right :) If you are left, maybe I can be right, to keep the balance. Ulla.

    Ulla | 02/23/11 | 16:20 PM

    The early searches by CMS and ATLAS changed the best fit parameters of the supersymmetric standard model in its four subversions just by a dozen of percent in average. See my review of 3 articles that incorporate all the findings into a new fit:


    To suggest that low-energy SUSY has been ruled out would be preposterous.

    Luboš Motl | 02/23/11 | 16:34 PM

    See also http://vixra.org/abs/1102.0034

    Ben Allanach: Impact Of The CMS Supersymmetry Search On Global Supersymmetric Fits.

    I noticed a recent article of his in the arxiv, and asked him to report on it here, given the interest that the recent LHC results have stirred in the community. He graciously agreed.... So let us hear it from him!

  9. Not Even Wrong
    Implications of Initial LHC Searches for Supersymmetry

  10. http://www.symmetrymagazine.org/breaking/2011/03/02/lhc-publishes-first-higgs-measurements/

    no evidence of the Higgs in their dataset from 2010. The latest result explores an exotic version of the Higgs that proposes an extra generation of fundamental particles exists.
    Physicists predict that a fourth family may exist, opening the door to new physics models, including an alternative form of the Higgs. The CMS result allows physicists to exclude the Higgs mass range of 144-207 GeV/c2 for physics models that include the proposed fourth family of particles.

    Read the CMS statement, http://cms.web.cern.ch/cms/News/2011/HiggsSearchWW/HiggsSearchWW_Feb.pdf

  11. New Tight SUSY Exclusion From ATLAS, Tommaso Dorigo.

    Theory: If R-parity is conserved, as in most versions of SUSY, sparticles can only decay yielding other sparticles, such that the lightest of them is perfectly stable, and constitutes a perfect candidate to explain the dark matter in the universe.

    the Higgs boson may be as light as we expect it to be (and as light as to make electroweak fits to observed standard model parameters look as good as they do) only if a score of quantum corrections to its mass produced by virtual particles coupling to it exactly cancel their total contribution. SUSY particles do that for free, since for every particle in the SM producing a contribution to the Higgs mass, a SUSY sparticle produces a variation of the opposite sign.

    we may one day explain all the forces of nature as the low-energy manifestations of a single interaction.

    Practice: I am actually convinced that SUSY is not there to be found, so I am prepared to see more and more of the parameter space being eaten up by experimental searches at LHC; the game will more or less go as follows: CMS excludes some region, ATLAS then excludes a bit more, then CMS takes revenge and extends the exclusion region with an improved analysis, then ATLAS does it, etcetera. This sort of game has been going on for quite a while at the Tevatron, and now that the players have changed the rules remain the same.


  12. TGD

  13. http://marcofrasca.wordpress.com/2011/03/01/back-to-earth/

    Beautiful theory collides with smashing particle data
    But, as often discussed in this blog, there is another way out saving both Higgs and supersymmetry. All the analysis carried out so far about Higgs field are from small perturbation theory and small couplings: This is the only technique known so far to manage a quantum field theory. If the coupling of the Higgs field is large, the way mass generation could happen is different being with a Schwinger-like mechanism. This imposes supersymmetry on all the particles in the model. http://arxiv.org/abs/1007.5275 (Marco Frasca (2010). Mass generation and supersymmetry) there is no parameter space to be constrainted for fine tuning to be avoided and this is a nice result indeed.
    http://arxiv.org/abs/1102.2357v1 Atlas
    http://arxiv.org/abs/1101.1628v1 CMS

    "Plenty of things will change if we fail to discover SUSY," says Lester. Theoretical physicists will have to go back to the drawing board and find an alternative way to solve the problems with the standard model. That's not necessarily a bad thing, he adds: "For particle physics as a whole it will be really exciting."

    Strumia, A. Preprint at http://arxiv.org/abs/1101.2195 (2011).

  14. Electroweak BuZZ http://www.fnal.gov/pub/today/archive_2011/today11-03-10.html

    The production of two Z bosons in a single collision is among the rarest phenomena expected to occur at the Tevatron. It is about 20,000 times rarer than finding a single W boson. Exploiting the entire Run II DZero data set netted just 10 events of this kind.

    The weak nuclear force is the weakest interaction that physicists at the Tevatron have successfully studied. Like all subatomic forces, we can understand it as being caused by the exchange of force carrying particles, specifically the W and Z bosons. These bosons were discovered in 1983, netting a quick Nobel Prize the following year.

    The W and Z bosons are physical particles that are seen as a consequence of the unification of the weak and electromagnetic forces. The unification theory initially predicts that these force carriers are massless, but become massive by interacting with the hypothetical Higgs boson. So the study of the W and Z bosons is tied together with the Higgs search.

    During the course of the following 25 years, physicists observed other interesting types of events containing W and Z bosons, each rarer than the ones before. In 2008, the DZero experiment observed the production of pairs of Z bosons. This phenomena is very rare; it’s about 20,000 times harder to produce pairs of Z bosons than it is to produce a single W boson.

    Since the discovery, DZero has accumulated almost four times as much data and extended the analysis. To give a sense of the rarity of the phenomenon and difficulty of analysis, when the entire data set we’ve collected over the last eight years was exploited, we observed a grand total of ten particle collisions in which two Z bosons were created.

    Ten is a small number, but it’s enough to start studying things in detail, including such things as the energy and direction of the Z bosons and their decay products. We even evaluated the data to see if there was evidence for a new type of particle produced that decayed into Z bosons.

    The data was consistent with the Standard Model, which is bad news for people searching for new physical phenomena, but crucial input for our Higgs boson searches. You see, if the Higgs boson is heavy, it will decay into two Z bosons. - Don Lincoln

    a flavor change from rare decay.


  15. http://www.science20.com/quantum_diaries_survivor/more_susy_forecasts_recent_atlas_results-77056



  16. http://blogs.uslhc.us/the-road-to-the-higgs-boson
    http://arxiv.org/pdf/1102.5429v2 a very nice representation of one of the trajectories that the LHC can follow to a discovery of the Higgs boson (should it actually exist).

    events that have two high-momentum leptons with opposite electric charge, where here we define leptons as electrons or muons. In proton-proton collisions, the production of even one high-momentum lepton is already unusual, and two is quite interesting. There are a variety of physics processes that can lead to this. The most common, by far, is the decay of a Z boson; this was easily observed last summer. Another process is the decay of a pair of top quarks; a few percent of the time both will decay to a lepton. Top quarks are produced only about one sixth as often as Z’s, so that takes a bit more data to find.

    The next process that can lead to two leptons is the direct production of a pair of W bosons, which happens about four times less frequently than top-pair production. This process is what is observed in the paper; there are a total of thirteen candidate WW events

    another process that can lead to two leptons is the production of a Higgs boson that would be heavy enough to decay to a pair of W’s. should the Higgs be sufficiently heavy, a decay to WW is quite common and the two-lepton signature is quite clean.

    we would need a factor of ten more data to have a hope of seeing a Higgs boson this way. But it’s worth making an effort, and thus the paper sets upper limits on the production rate of a standard model Higgs. It’s not competitive with the limits that have been set by the Tevatron experiments, but it establishes that it is possible to do this analysis at the LHC.

    There is one more trick that the paper pulls out. We are used to thinking of having three generations of quarks and leptons in our world. There is nothing to suggest that this isn’t so, but if there were a fourth generation of particles that were very, very heavy, it would be very hard to know about it because they would be out of the reach of our current experiments. But if such a scenario were true, it turns out that Higgs particles would be produced at a much greater rate at the LHC. The fact that no Higgs signal is observed in this paper tells us that this scenario is unlikely.

    there is no theory that predicts a value, so how you might look for a Higgs depends what mass you might think it has.

  17. http://www.physorg.com/news/2011-03-large-hadron-collider-world-machine.html