The God Particle

The new, ‘tantalizing hints’ of the ‘God particle’

Physicists excitedly announce that they are closing in on the mysterious Higgs boson — which is believed to have played a key role in our universe’s creation

In this artist's rendering, two photons (red) smash into one another, and the wreckage (yellow) is where researchers are searching for the mysterious "God particle."

In this artist’s rendering, two photons (red) smash into one another, and the wreckage (yellow) is where researchers are searching for the mysterious “God particle”.

Photo: CERN: Thomas McCauley, Lucas Taylor SEE ALL 7 PHOTOS

After months of subatomic particle smashing, two teams of researchers at CERN, the European Center for Nuclear Research, announced Tuesday morning that they’ve each recorded hints of the elusive Higgs boson, better known as the universe-creating “God particle.” Here’s what you should know:

What exactly is this “God particle”?
The Higgs boson is an elusive, subatomic particle “that was predicted to exist nearly 50 years ago,” says Alok Jha at Britain’s Guardian. And though scientists have been hunting for evidence of the particle for decades, they’ve had “no solid proof that it was real.” Still, physicists have long believed that the mysterious “God particle” was an essential building block for the universe.

What exactly is this “God particle” supposed to do?
Immediately after the universe was created by the Big Bang nearly 14 billion years ago, “all the particles in the cosmos weighed nothing at all and zipped around chaotically at the speed of light,” Jha says. Researchers believe that a mere trillionth of a second later, a field of “God particles” somehow “switched on,” acting as a “cosmic glue” to slow other particles down and give them mass, thus allowing for the eventual formation of stars and planets. Without the Higgs field to give objects mass, all those particles would still be flying around, and the universe as we know it wouldn’t exist.

So what did researchers discover?
The Higgs is quite difficult to find, largely because if it exists, it disappears or transforms into another particle so quickly that it can’t be measured. But two giant teams of CERN scientists announced on Tuesday that they’d seen “tantalizing hints” of the particle after “sifting debris from high-energy proton collisions,” says Dennis Overbye at The New York Times. The two teams, consisting of roughly 3,000 physicists each, came to these results independently, which is giving other scientists hope. The CERN scientists saw bumps in their data that helped them narrow down the atomic weight of the “putative particle,” which, if it exists, weighs between 115 to 127 billion electron volts — well more than 100 times as much as a proton.

What now?
The two teams expect to have enough data to make a definitive conclusion by summer 2012. “The simple fact that” both teams “seem to be seeing a data spike at the same mass has been enough to cause enormous excitement in the particle physics community,” says Paul Rincon at BBC News. “Finding the Higgs would be one of the biggest scientific advances of the last 60 years.”


The Higgs boson (sometimes nicknamed the “God particle” in popular media) is a hypothetical massive elementary particle that is predicted to exist by the Standard Model (SM) of particle physics. The Higgs boson is an integral part of the theoretical Higgs mechanism. If shown to exist, it would help explain why other particles can have mass.[Note 2] It is the only predicted elementary particle that has not yet been observed in particle physics experiments.[1] Theories that do not need the Higgs boson also exist and would be considered if the existence of the Higgs Boson were ruled out. They are described as Higgsless models.

If shown to exist, the Higgs mechanism would also explain why the W and Z bosons, which mediate weak interactions, are massive whereas the related photon, which mediates electromagnetism, is massless. The Higgs boson is expected to be in a class of particles known as scalar bosons. (Bosons are particles with integer spin, and scalar bosons have spin 0.)

Experiments attempting to find the particle are currently being performed using the Large Hadron Collider (LHC) atCERN, and were performed at Fermilab’s Tevatron until its closure in late 2011. Some theories suggest that any mechanism capable of generating the masses of elementary particles must be visible at energies below 1.4 TeV;[2]therefore, the LHC is expected to be able to provide experimental evidence of the existence or non-existence of the Higgs boson.[3]

On 12 December 2011, the ATLAS collaboration at the LHC found that a Higgs mass in the range from 145 to 206 GeV was excluded at the 95% confidence level.[4] On 13 December 2011, experimental results were announced from the ATLAS and CMS experiments, suggesting that if the Higgs Boson exists, it is probably limited to a range of 115–130 GeV at the 3.6 sigma level (ATLAS) or 117–127 GeV at the 2.6 sigma level (CMS), and indicating possible scope for a 124 GeV (CMS) or 125-126 GeV (ATLAS) Higgs. As of 13 December 2011, a joint estimate is not available.[5][6][7][8]

Contents

Origin of the theory

Five of the six 2010 APS J.J. Sakurai Prize winners. From left to right: KibbleGuralnikHagen,Englert, and Brout.

The sixth of the 2010 APS J.J. Sakurai Prize winners: Peter Higgs 2009

The three papers written on this discovery were each recognized as milestone papers during Physical Review Letters‘s 50th anniversary celebration.[13] While each of these famous papers took similar approaches, the contributions and differences between the 1964 PRL symmetry breaking papers are noteworthy. These six physicists were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.[14]The Higgs mechanism is a process by which vector bosons can get a mass. It was proposed in 1964 independently and almost simultaneously by three groups of physicists: François Englert and Robert Brout;[9] by Peter Higgs[10] (inspired by ideas of Philip Anderson[11]); and by Gerald GuralnikC. R. Hagen, and Tom Kibble.[12]

The 1964 PRL papers by Higgs and by Guralnik, Hagen, and Kibble (GHK) both displayed equations for the field that would eventually become known as the Higgs boson. In the paper by Higgs the boson is massive, and in a closing sentence Higgs writes that “an essential feature” of the theory “is the prediction of incomplete multiplets of scalar and vector bosons”. In the model described in the GHK paper the boson is massless and decoupled from the massive states. In recent reviews of the topic, Guralnik states that in the GHK model the boson is massless only in a lowest-order approximation, but it is not subject to any constraint and it acquires mass at higher orders. Additionally, he states that the GHK paper was the only one to show that there are no massless Nambu-Goldstone bosons in the model and to give a complete analysis of the general Higgs mechanism.[15][16]Following the publication of the 1964 PRL papers, the properties of the model were further discussed by Guralnik in 1965 and by Higgs in 1966.[17][18]

Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The Higgs mechanism not only explains how the electroweak vector bosons get a mass, but predicts the ratio between the W bosonand Z boson masses as well as their couplings with each other and with the Standard Model quarks and leptons. Many of these predictions have been verified by precise measurements performed at the LEP and the SLC colliders, thus confirming that the Higgs mechanism takes place in nature.[19]

The Higgs boson’s existence is not a strictly necessary consequence of the Higgs mechanism: the Higgs boson exists in some but not all theories which use the Higgs mechanism. For example, the Higgs boson exists in the Standard Model and the Minimal Supersymmetric Standard Model yet is not expected to exist in Higgsless models, such as Technicolor. A goal of the LHC and Tevatron experiments is to distinguish among these models and determine if the Higgs boson exists or not.

Theoretical overview

A one-loop Feynman diagram of the first-order correction to the Higgs mass. The Higgs boson couples strongly to the top quark so it might decay into top–anti-top quark pairs if it were heavy enough.

The Higgs boson particle is the quantum of the theoretical Higgs field. In empty space, the Higgs field has an amplitude different from zero; i.e. a non-zero vacuum expectation value. The existence of this non-zero vacuum expectation plays a fundamental role; it gives mass to every elementary particle that couples to the Higgs field, including the Higgs boson itself. The acquisition of a non-zero vacuum expectation value spontaneously breaks electroweak gauge symmetry. This is the Higgs mechanism, which is the simplest process capable of giving mass to the gauge bosons while remaining compatible with gauge theories. This field is analogous to a pool of molasses that “sticks” to the otherwise massless fundamental particles that travel through the field, converting them into particles with mass that form (for example) the components of atoms.

In the Standard Model, the Higgs field consists of two neutral and two charged component fields. Both of the charged components and one of the neutral fields are Goldstone bosons, which act as the longitudinal third-polarization components of the massive W+, W, and Z bosons. The quantum of the remaining neutral component corresponds to the massive Higgs boson. Since the Higgs field is a scalar field, the Higgs boson has no spin, hence no intrinsic angular momentum. The Higgs boson is also its own antiparticle and is CP-even.

The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and 180 GeV/c2, then the Standard Model can be valid at energy scales all the way up to the Planck scale (1016 TeV). Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model. The highest possible mass scale allowed for the Higgs boson (or some other electroweak symmetry breaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such a mechanism, because unitarity is violated in certain scattering processes. There are over a hundred theoretical Higgs-mass predictions.[20]

Extensions to the Standard Model including supersymmetry (SUSY) predict the existence of families of Higgs bosons, rather than the one Higgs particle of the Standard Model. Among the SUSY models, in the Minimal Supersymmetric Standard Model (MSSM) the Higgs mechanism yields the smallest number of Higgs bosons: there are two Higgs doublets, leading to the existence of a quintet of scalar particles, two CP-even neutral Higgs bosons h and H, a CP-odd neutral Higgs boson A, and two charged Higgs particles H±. Many supersymmetric models predict that the lightest Higgs boson will have a mass only slightly above the current experimental limits, at around 120 GeV/c2 or less.

“The God particle”

The Higgs boson is often referred to as “the God particle” by the media,[51] after the title of Leon Lederman‘s book, The God Particle: If the Universe Is the Answer, What Is the Question?[52] Lederman initially wanted to call it the “goddamn particle,” because “nobody could find the thing.”[53]; but his editor would not let him.[54]While use of this term may have contributed to increased media interest in particle physics and the Large Hadron Collider,[52] many scientists dislike it, since it overstates the particle’s importance, not least since its discovery would still leave unanswered questions about the unification of QCD, the electroweak interaction and gravity, and the ultimate origin of the universe.[51] A renaming competition conducted by the science correspondent for the British Guardian newspaper chose the name “thechampagne bottle boson” as the best from among their submissions: “The bottom of a champagne bottle is in the shape of the Higgs potential and is often used as an illustration in physics lectures. So it’s not an embarrassingly grandiose name, it is memorable, and [it] has some physics connection too.”[55]

Notes

  1. ^ This upper bound for the Higgs boson mass is a prediction within the minimal Standard Model assuming that it remains a consistent theory up to the Planck scale. In extensions of the SM, this bound can be loosened or, in the case of supersymmetry theories, lowered. The lower bound which results from direct experimental exclusion by LEP is valid for most extensions of the SM, but can be circumvented in special cases. [1]
  2. ^ The masses of composite particles such as the proton and neutron would only be partly due to the Higgs mechanism, and are already understood as a consequence of the strong interaction.

References

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Further reading

External links

 

As in

http://theweek.com/article/index/222407/the-new-tantalizing-hints-of-the-god-particle

http://www.wikipedia.com

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