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The Higgs Particle

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what is it, and why do we badly need it?

higgsWolfgang Bietenholz
We sketch in simple terms the concept of the Higgs mechanism, and its importance in particle physics.
To the best of our knowledge, the world consists of only very few types of elementary particles, the smallest entities of matter, which are indivisible.
They are described successfully by the Standard Model (SM) of particle physics, a great scienti c achievement of the 20th century.
All phenomena observed so far with elementary particles are compatible with the SM, which made a large number of correct predictions.
However, there is one particle that the SM needs in order to work, which has not been safely observed yet: the famous Higgs particle.
The Large Hadron Collider at CERN near Geneva is intensively searching for it.
In December 2011 it reported rst hints of an observation, which have meanwhile been further substantiated, but a rigorous veri cation takes more time…
Read more at http://arxiv.org/pdf/1304.2423v1.pdf

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April 21, 2013 at 8:13 pm

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Higgs boson is too saintly and supersymmetry too shy

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by Michael Slezak
HOPES of using the Higgs boson and the elegant theory of supersymmetry as shortcuts to discovering the mysteries of the universe are evaporating fast. That’s the verdict of a major update from the Large Hadron Collider in CERN, near Geneva, Switzerland – the first since a boson resembling the Higgs was spotted there earlier this year.

“If our understanding of nature is correct, then the details of what happens next are more complicated than we had hoped,” says Matthew Walker of CMS, one of the major LHC detectors.

In July, when CMS and its sister detector, ATLAS, announced the discovery of the boson, anomalies in the data hinted at physics beyond the standard model, the well-established description of the universe’s particles and forces.

Such new physics is urgently needed because the standard model contains no mention of dark matter, makes incorrect predictions about the universe’s antimatter and requires awkward “fine-tuning” to incorporate the Higgs mass reported in July.

The Higgs isn’t searched for directly, but spotted via a slew of particles that the standard model predicts it decays to. One anomaly in July’s particle debris was insufficient tau leptons, which could have implied the existence of non-standard particles (see “diagram”).

But on 14 November, armed with twice as much data, CMS and ATLAS researchers told the Hadron Collider Physics Symposium in Kyoto, Japan, that the number of taus has crept up, removing the hint of deviant physics. CMS also reported a signal suggesting that the boson behaves the same when viewed in a mirror, giving it the property of positive parity, which the standard model also predicts.

There’s still one anomaly left. In July, the newly discovered boson seemed to decay twice as often as predicted into pairs of photons, which could be the signature of an extra, non-standard particle, or of a non-standard Higgs. If that anomaly disappears too, the probable Higgs boson will look very standard indeed, which is strange because of all the known possible extensions to the standard model, none predicts a completely standard Higgs.

One explanation could lie in a theory called the Neutrino Minimal Standard Model (nuMSM), in which dark matter is actually a neutrino and the Higgs behaves so similarly to the standard model that the differences would be unobservable. Instead nuMSM might be discovered via space-based detectors that look for its proposed dark-matter particles, but it’s a long shot. Just because it seems to fit right now doesn’t mean nuMSM is the most plausible scenario, says Raymond Volkas of the University of Melbourne, Australia.

As if the boson’s good behaviour wasn’t frustrating enough, the LHC’s searches for particles predicted by supersymmetry (SUSY) have turned up nothing. As SUSY – which proposes a heavier superpartner for each known particle – extends the standard model to include dark matter and other omissions, this failure deals a further blow to possible sources of new physics at the LHC. It is also stoking exchanges between SUSY supporters and sceptics (see “SUSY no-show fuels debate”).

SUSY particles could show up at the higher LHC energies scheduled for 2014, after its year-long planned rest next year. But that is cold comfort to those hoping to have gleaned clues already. “I would, as a hunter of new physics, have liked to see it different to what we have now,” says Albert De Roeck of CMS. “But the data is the data.”
Read more: www.newscientist.com

Written by physicsgg

November 23, 2012 at 1:11 pm

Posted in High Energy Physics

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Why do physicists care so much about finding the Higgs boson?

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By Daisy Yuhas
If you’ve read anything about the Higgs boson, you probably know that this particle is special because it can explain how fundamental particles acquire mass. Specifically, evidence of the boson is evidence that an omnipresent Higgs field exists—one that slows particles down and makes them heavy.

But there’s a misconception that sometimes creeps into this explanation. The Higgs field does not explain the origin of all mass. “Many uninformed physicists have been saying that for years,” says theoretical physicist Chris Quigg of Fermi National Accelerator Laboratory.

“We have actually understood the source of most of the mass in the proton [for example] for some time,” Quigg says. Most mass—including your own—comes from the strong force, a force of nature that keeps the nucleus of atoms bound together.

If you’re concerned that you’ve been hoodwinked into celebrating a particle with slight implications, fear not. “It’s actually much more exciting than that, and when you hear the whole story, the symmetries and laws, it’s more amazing to think that this works,” says Columbia University physicist Tim Andeen. There’s a more complex question involved, one that has grander implications for the way the forces of nature work. The Higgs mass-giving mechanism is key to explaining a mystery called “spontaneous electroweak symmetry breaking.” Before your eyes glaze over, let’s break down that vocabulary a bit.

When you think of symmetry, you probably think of beautiful faces, Classical architecture, and an Art 101 course on drawing. Physicists think of symmetry in terms of sameness. For example, because we can do an experiment in two different places and get the same result, we know that the universe is spatially symmetric. The laws of physics don’t change with time, either, which means the universe is temporally symmetric. These symmetries are inextricably linked to the laws of the universe, such as the conservation of momentum and energy. If you hit upon a symmetry, there must be a law of conservation accompanying it—and vice-versa.

“Spontaneous,” in this case, implies a certain amount of random chance—nothing has been predetermined. Imagine going to a dinner party and sitting down at a round table, only to realize that no one is sure whether they should take the bread roll to the left or bread roll to the right. At some point, someone will just grab a roll—spontaneously breaking the symmetry of the place settings.

Electroweak refers to two forces of nature: the electromagnetic force, which unites electricity and magnetism, and the weak force, which governs radioactive decay. Why have these two forces been squelched together? Physicists have long been attempting to unify the forces of nature, both because a single description is more simple and elegant, and because at very high energies, these forces appear to become one single force.

In the 1960s, many physicists were working on reconciling the electromagnetic and weak forces into a unified theory. Three physicists in particular—Sheldon Lee Glashow, Abdus Salam, and Steven Weinberg—developed something called “electroweak theory,” which neatly extended our understanding of electromagnetism to incorporate the weak force.

Yet the theorists hit a snag. In order to combine these forces, they needed to introduce a set of force-carrying particles for the weak force to complement the photon—the mass-less particle that carries the electromagnetic force. For the electromagnetic and weak forces to unify, their force carriers would have to be symmetric. In consequence, none of them should have any mass.

But it turned out that the weak force–carrying particles (the W and Z bosons) did have mass. In fact, they were quite heavy. The Higgs mechanism (which had been proposed and developed by several theorists in addition to Peter Higgs) offers the solution. It suggests that when the Higgs field interacts with the W and Z bosons, the Higgs field spontaneously breaks the symmetry that would have kept the W and Z massless. It masks their true massless nature. Finding the Higgs, therefore, is part of a larger quest to unify the forces of nature, with implications that sweep across the laws that govern the universe.

There’s another reason to care about the Higgs boson. Quigg has written extensively on the consequences of a Higgs-free universe. He points out that when Glashow, Salam and Weinberg were working out how the Higgs mechanism might help unify electromagnetism and the weak force, they realized that in addition to giving mass to the heavy force-carriers, the Higgs might also give mass to other fundamental particles. The electron, therefore, would owe its mass to the Higgs field. Without that mass, electrons wouldn’t hook up with nuclei to form atoms. “That would mean no valence bonding, so much of chemistry, essentially all, would vanish,” Quigg says. “Therefore no solid structures and no template for life.”

Read more: blogs.scientificamerican.com

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November 23, 2012 at 8:13 am

Posted in High Energy Physics

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Demystifying the Higgs Boson with Leonard Susskind

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Professor Susskind presents an explanation of what the Higgs mechanism is, and what it means to “give mass to particles.” He also explains what’s at stake for the future of physics and cosmology.


http://youtu.be/JqNg819PiZY

Written by physicsgg

November 20, 2012 at 9:36 pm

Posted in High Energy Physics

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The large hadron collider and the Higgs boson

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UCL lunch hour lecture

UCL runs a series of public lectures at lunchtime. On Tuesday I gave one of these, about the news from the energy frontier, including the discovery on the fourth of July this year. Here is the recording.

For the past two years, until the end of last month, I was convener of one of the big analysis groups at the ATLAS detector on the Large Hadron Collider at CERN, Geneva. This meant I was travelling a lot. So whenever I was asked to do something in London, I would say “not until October”.

So October has been predictably busy with engagements in the UK, often at UCL. As well as the debate on Monday, I gave one of our well-known public lunch hour lectures on Tuesday this week. We were full, some people were turned away, but it was live streamed and here is the recording in case you’d like to see it.


http://youtu.be/Z7YRDE4IG6w

There’s some overlap of material (especially jokes) with the Royal Institution evening discourse I gave in February. Except at UCL I didn’t have the demonstrations – but I did have a new boson!

guardian.co.uk

Written by physicsgg

October 25, 2012 at 10:09 am

Posted in High Energy Physics

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Rolling in the Higgs (Adele Parody)

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A Capella Science – Rolling in the Higgs (Adele Parody)

http://youtu.be/VtItBX1l1VY

Written by physicsgg

August 29, 2012 at 1:33 pm

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The beauty of the Higgs boson

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Albert Einstein’s equation of general relativity is a thing of dazzling beauty. Photograph: Bettmann/CORBIS

Jeff Forshaw
The dust is beginning to settle – a new particle has been discovered using the Large Hadron Collider. Discovering new particles of nature is not an everyday occurrence and we are reasonably entitled to proclaim that this is the arrival of the Higgs. We aren’t certain, though: more careful examination of the particle’s properties is needed before we can be – we want to know that it has spin zero and that it couples to other particles with a strength that is in proportion to their mass. Answers to those questions and to many others will follow over the coming months and years. This is all very important – but why? Why is the discovery of a new type of particle something to get so excited about?

The best way to appreciate the beauty of a discovery is to get stuck in, learn some mathematics and see those dazzling equations in all their glory. Examples include Einstein’s equation of general relativity, Dirac’s equation for the electron and the Lagrangian at the heart of the standard model of particle physics. But it is possible to get the gist of what a physicist means when they speak of a beautiful theory without the hard work. Before doing that let’s be clear – this is a kind of life-changing beauty. This is not titillation and it is not a conceit of the human mind – it leaves everyone who has studied these things with an overwhelming sense that the natural world operates according to some beautiful rules and that we are very fortunate to be able to appreciate them. To spend time contemplating this is thrilling. We believe that these are universal rules that would also be uncovered by sufficiently intelligent aliens on a distant planet: we are discovering something at the heart of things.

The situation is extreme enough for greats such as Einstein and Hawking to invoke God. But they were certainly using the word to express the intimate relationship between the human mind and the glorious intelligibility of the universe. It feels like a personal thing – like we are relating to something very special. This is the sense in which Hawking once spoke of knowing the mind of God, but it doesn’t really have anything to say about the existence or not of a creator, and Id be surprised if science will ever have anything much to say about that.

A beautiful piece of physics is elegant. An elegant theory has the capacity to explain many apparently different things simultaneously – it means that rather than needing a library full of textbooks to explain the workings of the universe we can manage with just one book. In fact the situation is better than that – the fundamental equations that underpin all known natural phenomena can be written down on the back of an envelope. That is really true – the nature of light, the workings of the sun, the laws of electricity and magnetism, the explanation for atoms, gravity and much more can all be expressed with breathtaking economy. It is like we are in the business of discovering the rules of an elaborate game and we have figured out that they are really very simple, despite the rich variety of phenomena we see around us. Uncovering the rules of the game is exciting, and maybe one day we will know all of the rules accessible to us – that is what people are referring to when they speak about a “theory of everything”. It sounds very arrogant to speak about a theory of everything but those in pursuit of it are not so dumb. They are well aware that knowing the rules is not the whole story. A child can know the rules of chess but exploiting them to produce a classic game is far from easy. This is an illustration of how simple rules can lead to something very complicated. The study of complex phenomena and their emergence is another very exciting area of modern physics.

Beautiful physics is also compelling. It is as if nature possesses a kind of perfection that is guiding us in our pursuit of the rules of the game. The result is that we very often have little or no choice when figuring out what equations to write down. That is a very satisfying situation to be in. It means that when we try to figure out an equation to describe something important, such as how an electron behaves, instead of saying, “Well… the equation might look like this… or maybe it looks like that… or…” we have no choice and nature simply screams out at us: “The equation simply must look like this.” Dirac’s beautiful equation is just like that – it describes the electron and predicts the existence of its anti-matter partner, the positron. Our understanding of the origins of inter-particle interactions (aka force) is like this too – starting from a very dull theory in which particles do not interact with one another (so no stars or people) and the idea that nature is symmetric in a certain way we are absolutely compelled to introduce interactions into the theory – the symmetry forces our hand and dictates how the theory should look. Symmetry is so often the device that leads to elegant and compelling theories. A snowflake is symmetric – if I draw part of one you could probably do a good job of sketching the rest. Likewise equations can be symmetric, which means we only need part of one in order to figure out the rest. In the case of particle interactions, symmetry means we can infer their necessary existence starting from the simpler equations that describe a world without any interactions at all… and that really is beautiful.

The genius of Peter Higgs and the other physicists who proposed the existence of the Higgs boson was to take the idea of symmetry seriously. The same symmetry that gives us “for free” the theory of inter-particle interactions also appears, at first glance, to predict that nature’s elementary particles should all be without mass. That is flatly wrong and we are faced either with ditching a symmetry that has delivered so much (although that was not known when the Higgs pioneers were beavering away in the early 1960s) or figuring out an ingenious solution.

-=-The Higgs idea is that solution – it says empty space is jammed full of Higgs particles that deflect otherwise massless particles as they move – the more a particle is jiggled by the Higgs particles the more it has mass. As a result, the fundamental equations maintain their precious symmetry while the particles gain mass. Faith in the idea that nature’s laws should be elegant and compelling has, yet again, delivered insight. The Higgs discovery is the jewel in the crown of particle physics and a worthy testament to nature’s astonishing beauty.
Read more: www.guardian.co.uk

Written by physicsgg

August 5, 2012 at 11:33 am

Posted in High Energy Physics

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ATLAS: 5.9 Sigma For A 126 GeV Higgs !

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By Tommaso Dorigo
ATLAS has just released a note which summarizes the searches for the standard model Higgs boson in 7-TeV and 8-TeV data. Since July 4th the main improvement is the addition of the WW channel, which had not been shown back then. With it, the combined local significance of the 126 GeV Higgs boson excess in the WW, ZZ, and γγ channels grows to 5.9 standard deviations. In the words of a Facebook friend who’s in ATLAS: “if this is not a discovery, I don’t know what is”.

So, due congratulations to my ATLAS colleagues for this new important document. The paper is indeed full of detail about the searches, answering many of the questions that the format of the July 4th event did not allow to be asked.

The most important measurements, those of mass and cross sections, are summarized in the figure on the right, which shows the 1-sigma contours for the different final states: the WW measurement is the one which extends the most in the horizontal axis, because of the large indetermination in the mass due to the escaping neutrino pair.

The three measurements are consistent with each other, and the global signal strength is measured at 1.4+-0.3 times the standard model predictions for a 126 GeV Higgs boson…..
Read more: www.science20.com

Written by physicsgg

July 31, 2012 at 9:16 pm

Posted in High Energy Physics

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