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    Eilam Gross: Higgs - The Best There Is, For Now
    By Tommaso Dorigo | December 14th 2011 10:05 AM | 14 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    Eilam Gross is a professor of Physics at the Weizmann Institute of Science of Rehovot, Israel, and a distinguished member of the ATLAS collaboration. That makes him a competitor, since I work for the other experiment around the ring, CMS. But Eilam is also a colleague, especially since we are members of the Statistics Committees of our respective experiments and we cooperate in a joint group to try and converge on common practices for statistical procedures in data analysis at the LHC. Ah, and- I forgot to mention he is the convener of the ATLAS Higgs group| So I am very pleased to feature his own take on the LHC results on Higgs searches...

    “The God Particle,” as the Higgs boson is often called, comes from the title of the book by Nobel laureate Leon Lederman that deals with the search for the elusive particle. This particle, according to the Standard Model of Particle Physics, is responsible for giving mass to all of the elementary particles in nature.

    The mass of an electron determines the size of a hydrogen atom; ultimately the size of atoms ensures, amazingly enough, our existence. Maybe this is the reason the Higgs has been treated with almost mystical reverence in the mass media.

    The Higgs boson is the only one of the elementary particles making up the Standard Model of Particle Physics that has not yet been discovered. Its importance can’t be denied. Many scientists believe that the Standard Model will stand or fall on the discovery of Higgs boson particles orproof that they don’t exist.

    Three weeks ago, I attended a conference  in Paris. Nearly everyone who is involved in the search for this particle was there. I was the guest of my friend Marumi Kado. At some point after a wonderful, wine-filled dinner, Marumi suggested a glass of grappa. “You must,” he said. “It is Prime Uve, the best there is.” A few hours later, in the wee hours of the morning, Marumi nudged me awake from a deep sleep and said: “Eilam, do you want to see a Higgs?” Of course, I jumped up immediately: “What? Where?” “The computer has stopped running; here are the results,” he said. On his computer screen were images from ATLAS. We were all in shock. Something was out of the ordinary at a mass of 126 GeV (a unit of mass close to that of a proton). Definitely significant – 3.6 standard deviations. We couldn’t believe our eyes – we looked at the screen for ages before we started to digest what we were seeing. For the past three weeks, the entire Higgs search team in the ATLAS experiment have checked and rechecked the results from every possible angle. We checked for errors… for bugs in the program.

    In 2011 the LHC particle accelerator in Geneva collided over 300 trillion (a million million) protons in two opposing beams. All of that enormous energy (7 trillion electron volts) went into the effort to produce the Higgs boson. In each collision, other similar particles are created and there is no way to foresee what will be found. Quantum field theory enables us to predict the chances of a certain particle being created. The calculations show that the likelihood of getting a Higgs out of a particular collision is so small that over the course of a year we can’t see the signs of more than a hundred or so (with a mass of 126 GeV).

    In the figure above we see the result of a collision that looks like a Higgs. But is this a Higgs particle? The problem is that we don’t know enough to conclusively identify it. It could look like hundreds or thousands of other particles produced in the collisions. How can we tell?

    We don’t know for sure! What we can do is count how many of the collisions outcome look similar to a Higgs, and we can calculate how many of these we expect to see in the Standard Model without the Higgs. On the basis of observation, we can compute the probability that the number of collisions that result in particles similar to a Higgs boson will fit the Standard Model (without the Higgs). The numerical value of that probability is called p0.

    If there is no Higgs, we would expect that value to bearound 0.5 (50%), i.e., the same as the chance that the flip of a coin will come up heads.

    But if the Higgs exists, we should get “extra” collision results with a mass in the range that the Higgs might have. Of course, even if there is no Higgs, there could be statistical fluctuations affecting the experimental results. But the chances of such fluctuations are low (that is, they have a small p0).

    In the following illustration we see the results of the measurement of the ATLAS detectors in CERN (as they were presented by ATLAS spokeswoman Fabiola Gianotti).

    In most of the mass range (between 110 and 150 GeV) the p0 value is around 0.5, as expected if there were no Higgs. But right around 126 GeV, the p0 probability drops to 0.00019, that is, there is a chance of about 1 in 5000 that the “extra” events observed at this specific mass are the result of a statistical fluke rather than the creation of a Higgs boson.

    Moreover, had we seen such a peak at a different mass it would also been considered as a possible Higgs signal. This increases the chances that the observed “extra” events are the result of a statistical fluke. (This is called the “Look Elsewhere Effect”).

    Does all of this prove the existence of the Higgs boson? Everyone must judge for him or herself the significance of those two hundredths of a percent.

    My opinion is – maybe. Maybe yes. Maybe no. It depends.

    First, we must see what the results are from the competition– CMS. Unfortunately, CMS did not measure the statistical fluctuations. They measured two statistical anomalies that may be evidence for the Higgs: one witha mass of 119 GeV and another with a mass of 124 GeV, both with a significance of about 1%. That is, the chance of getting the CMS results without the Higgs is not that low (1 in 100).

    But it is likely that if we combine the two results, we might perhaps see the first signs of a Higgs in one of two possible places: 119 GeV and around 125 GeV with a relatively high level of confidence.

    Will we ever conclusively find the Higgs?

    It seems that the year 2012 will reveal it to us. In April 2012, the collider will start up again, apparently at a higher energy and greater collision frequency.

    We can expect that the summer of 2012 will be a summer of tidings.

    Comments

    fundamentally
    Now, what to do with theories that don't require the Higgs and still support the rest of the standard model. I learned one thing from the results. The rest of the standard model is well established! Or am I wrong? Is SUSY lost in the dark?
    If you think, think twice
    The Standard Model does contain the Higgs field and has to contain the Higgs field. What is well established is that the Standard Model without any Higgs field is an inconsistent theory producing nonsensical answers such as probabilities (for collisions between pairs of W-bosons, among more complicated things) that are outside the 0% - 100% interval. That just can't happen in reality, so the Higgsless theory has to be wrong.

    You can't just brutally remove the Higgs from the Standard Model. If it were possible, be sure that physicists would have thought about it as well. The Higgs boson has a very important and well-understood role in the Standard Model; it's a prediction of the Standard Model. A single Higgs boson is the minimal choice - and it defines the term "Standard Model" - but there may be more complicated options.

    A Higgs boson of mass 125 GeV which was near-discovered by the LHC in 2011 makes it almost inevitable that there has to be something else that prevents the vacuum from instability - and the something else must look very much like stop squarks and higgsinos. In other words, a 125 GeV Higgs makes supersymmetry much more likely to be true than it was before.

    More generally, masses below 135 GeV at least mildly favor supersymmetry while masses of the lightest Higgs above 135 GeV would almost completely rule out supersymmetry. Higgs masses below 125 GeV or so favor "simple field-theoretical minimal supersymmetric models" such as MSSM with near-degenerate masses of the superpartners; Higgs masses above 125 GeV or so favor "extended, grand-unified, stringy, hidden, hierarchical-mass-boasting, or otherwise non-minimal models of SUSY". The LHC data seem to place us exactly on this boundary between "simple visible SUSY" and "hidden or fancier or stringy SUSY".

    fundamentally
    @Lubos

    I repeat this from an earlier thread:

    I discovered a scheme in which massive elementary particles can be identified by a coupled pair of sign flavours of a quaternionic probability amplitude distribution (QPAD). At the same time the corresponding equation of motion (in quaternionic format) reads:

    ∇ψˣ = m ψʸ
    Here ∇ is the quaternionic nabla, m is a coupling factor and {ψˣ, ψʸ} forms the pair of sign flavors of a QPAD ψ.

    Multiplying both sides of the equation with ψʸ* and then integrate over the full parameter space, leads to an equation for the coupling factor m.

    ∫˯ψʸ*∇ψˣ dV =m ∫˯ψʸ*ψʸ dV= m∫˯|ψ|² dV=mg

    Here g is a real and positive constant.

    It means that m, which usually plays the role of mass in equation of motion, must be seen as a coupling constant.
    The scheme does not explain the existence of generations, but it explains the different categories of elementary particle types: electron (positron), neutrino, down quark, up quark, W bosons and Z boson.

    The Kerr-Newmann metric relates local curvature with some local properties, such as mass (or instead coupling factor m?), electric charge and angular momentum, including spin. If the coupling factor plays the actual role, then this would mean that gravitation and inertia are determined by the above properties of the particles and not by an external Higgs. However, I can imagine that the Higgs plays a role in creating the higher generations.
    See: http://www.crypts-of-physics.eu/Quaternionic_continuity_equation_for_charges.pdf for the relation between the sign flavors in {ψˣ, ψʸ} and particle properties.

    Hans
    I am a retired, but not tired physicist.

    If you think, think twice
    You say that "Something was out of the ordinary at a mass of 126 GeV (a unit of mass close to that of a proton)". But the mass of a proton is .9383 GeV/c^2. Should you not have said that the putative Higgs is about 125 more massive than a proton?

    Bonny Bonobo alias Brat
    On his computer screen were images from ATLAS. We were all in shock. Something was out of the ordinary at a mass of 126 GeV (a unit of mass close to that of a proton). Definitely significant – 3.6 standard deviations. We couldn’t believe our eyes – we looked at the screen for ages before we started to digest what we were seeing.
    Any chance of you posting those images here please or links to them, or is that the only one?

    My latest forum article 'Australian Researchers Discover Potential Blue Green Algae Cause & Treatment of Motor Neuron Disease (MND)&(ALS)' Parkinsons's and Alzheimer's can be found at http://www.science20.com/forums/medicine
    he's probably referring to the "Local P-Value" plot as shown above

    The Stand-Up Physicist
    Let's say we get to the summer of 2012, and we can conclude there is something real at 125.3 +/- .2 GeV to make up a number and how well it is known based on the data at hand. What additional tests are done to confirm the particle is the Higgs and not an omission from the code? The only test I was able to come up with was for spin: if the spin is determined from the data and it is 2 or 4, then the valid signal is not the Higgs. Another commenter said a spin 0 should be simple enough to distinguish from other even spin particles.

    Are there any other measurements that can be used to confirm the Higgs label to the signal?
    If they get answers in diphoton, ZZ and WW channels that are independntly consistent with a standard model higgs that will make a string case. That should be possible by the end of next year. To check directly that it is spin zero rather than spin 2 they need to show that it also decays as predicted into two fermions, e.g. bb or 2 tau. That will take a lot longer.

    The Stand-Up Physicist
    Thanks for such a specific answer, I know which channels to look for in the future, bb or 2 tau :-)

    So lets imagine a future where the photon channel brings in the best numbers, all the way past 6 sigma, the ZZ and WW channels follow behind, just clearing the bar at 5 sigmas. The bb and 2 tau signals are there, but only a little above 2 - I am guessing there is both a huge background and low Higgs boson production rate. The Higgs boson does not add mass to anything, but the Higgs field it this future has been confirmed experimentally.

    One think I have not heard discussed was the links between a Higgs field and general relativity. I probably don't hang around with the right people :-) I think of the Higgs field as that which makes inertial mass, all those molasses analogies. General relativity is based on the weak and strong equivalence principles. I am not seeing why the Higgs field would lead necessarily to either equivalence principles. This far on the outside, the hunt for the Higgs boson to confirm the Higgs field felt like a necessary fix for the standard model that was independent of general relativity. Is that the right impression to take on the theory side?
    The connection between the "would be" SM Higgs and General Relativity is not understood. What is known today is that SM Higgs is inconsistent with the cosmological constant by many orders of magnitude. It is also known that this puzzle is not the only one: vacuum stability, fine tuning, breaking of conformal symmetry by Higgs mass, gauge hierarchy problem, fermion family problem, neutrino masses and mixing are also on the list.

    The Stand-Up Physicist
    Cool! Lots of thinking to be done.
    It is hard to bet, but I'd say the detectors were still not 100% understood, and there can still be unaccounted systematical errors. Not to mention the rush and risk with the high luminosity (12 pile-up events at average at the end = about half of the integrated lumi is affecte. Very ugly :) ).

    It's a very nice christmas gift, but I can't wait to see what they show in Moriond. I think the nice message from this hard work is that the Higgs boson was excluded from a very large range.

    Let me tell you why it is not possible for Higgs boson to be there, because there cannot be any fields in a realistic understanding of the natural world. Fields were devised in the times of Maxwell to comprehend pre quantum phenomena. Every event has to have a particle/wave explanation, no field would fill in the details where a postulation is weak. It's a non quantum sub ev world out there. Gravitation and mass are due to a very different form of particle or particles, no resemblance with Higgs. Look for DCE research in Sweden, if you want to see the shape of the things to come. Eventually STR will be marginalized and space and mass will be seen as interchangeable.

    Then you cannot even remove liquid helium from the LHC tunnel and talk of removing water from the pond, then your pond is full of magnetic fields and the distortions that the Earth's gravitation brings to the surrounding space. The physical world is much more complex than a number of particles put together like bricks to define it's existence. A copper atom sized Higgs boson is as laughable as the Earth centred universe of the Ptolemic construct. The Church and its professional scientists spent centuries to not only defend it but to take it to greater mathematical heights before finally crashing.

    Finally, faster than light Neutrinoes and Higgs both cannot coexist -- either one has to be wrong. It's DCE research and superluminal speed which has the potential of breaking current scientific barriers, rather than finding a nebulous statistical dual peak for a Higgs, which well could be due to many other anomalies, one that LHC could not decipher is that of the UFOs.

    Will changing the energy level next year complicate the effort to combine all the data since the system parameters have changed? For instance, if we do go to 8TeV or similiar, the production rate of all the particles then change as well etc. Won't this interfer with efforts to get a coherent set of 10fb-1 data (guess) instead of two sets of 5fb-1 data that alone are not enough to make a discovery?