Higgs Boson's like Boson Found at CERN : A Ground Breaking Discovery in Particle Physics

Fig : A proton-proton collision event in the CMS experiment producing two high-energy photons (red towers). This is what we would expect to see from the decay of a Higgs boson but it is also consistent with background Standard Model physics processes. © CERN 2012

By CERN, Geneva

Published on 4 July 2012. 

At a seminar held at CERN today as a curtain raiser to the year’s major particle physics conference, ICHEP2012 in Melbourne, the ATLAS and CMS experiments presented their latest preliminary results in the search for the long sought Higgs particle. Both experiments observe a new particle in the mass region around 125-126 GeV.

“We observe in our data clear signs of a new particle, at the level of 5 sigma, in the mass region around 126 GeV. The outstanding performance of the LHC and ATLAS and the huge efforts of many people have brought us to this exciting stage,” said ATLAS experiment spokesperson Fabiola Gianotti, “but a little more time is needed to prepare these results for publication.”

"The results are preliminary but the 5 sigma signal at around 125 GeV we’re seeing is dramatic. This is indeed a new particle. We know it must be a boson and it’s the heaviest boson ever found,” said CMS experiment spokesperson Joe Incandela. “The implications are very significant and it is precisely for this reason that we must be extremely diligent in all of our studies and cross-checks."

“It’s hard not to get excited by these results,” said CERN Research Director Sergio Bertolucci. “ We stated last year that in 2012 we would either find a new Higgs-like particle or exclude the existence of the Standard Model Higgs. With all the necessary caution, it looks to me that we are at a branching point: the observation of this new particle indicates the path for the future towards a more detailed understanding of what we’re seeing in the data.”

The results presented today are labelled preliminary. They are based on data collected in 2011 and 2012, with the 2012 data still under analysis.  Publication of the analyses shown today is expected around the end of July. A more complete picture of today’s observations will emerge later this year after the LHC provides the experiments with more data.

The next step will be to determine the precise nature of the particle and its significance for our understanding of the universe. Are its properties as expected for the long-sought Higgs boson, the final missing ingredient in the Standard Model of particle physics? Or is it something more exotic? The Standard Model describes the fundamental particles from which we, and every visible thing in the universe, are made, and the forces acting between them. All the matter that we can see, however, appears to be no more than about 4% of the total. A more exotic version of the Higgs particle could be a bridge to understanding the 96% of the universe that remains obscure.

“We have reached a milestone in our understanding of nature,” said CERN Director General Rolf Heuer. “The discovery of a particle consistent with the Higgs boson opens the way to more detailed studies, requiring larger statistics, which will pin down the new particle’s properties, and is likely to shed light on other mysteries of our universe.”

Positive identification of the new particle’s characteristics will take considerable time and data. But whatever form the Higgs particle takes, our knowledge of the fundamental structure of matter is about to take a major step forward.

Cracks in the standard model

The latest results from the BaBar experiment may suggest a surplus over standard model predictions of a type of particle decay called “B to D-star-tau-nu.” In this conceptual art, an electron and positron collide, resulting in a B meson (not shown) and an antimatter B-bar meson, which then decays into a D meson and a tau lepton as well as a smaller antineutrino. Credit: Greg Stewart, SLAC National Accelerator Laboratory

Published by Science and Technology Facilities Council, United Kingdom
Published: June 19, 2012

Recently analyzed data from BaBar, a high-energy physics experiment in the U.S., may suggest possible flaws in the standard model of particle physics — the reigning description of how the universe works on subatomic scales. The data from BaBar, a particle accelerator at the U.S. Department of Energy’s (DOE) SLAC National Accelerator Laboratory, which was built by 10 countries including the United Kingdom, show that a particular type of particle decay happens more often than the standard model says it should.

The data refers to a particle called the B-bar meson that decays into a D meson, an antineutrino, and a tau lepton. This particular decay of a B meson should, theoretically, only happen in one in every 100 cases, but the new results from BaBar show it is happening too often. While the level of certainty of the difference, or excess, is not enough to claim a break from the standard model, the results are a potential sign of something amiss and are likely to impact existing theories.

“The excess over the standard model prediction is exciting,” said Michael Roney from the University of Victoria in Canada. “The results are significantly more sensitive than previously published studies of these decays. But before we can claim an actual discovery, other experiments have to replicate it and rule out the possibility this isn’t just an unlikely statistical fluctuation.”

“This result is very interesting, and, if confirmed, could be a sign of physics beyond the standard model,” said Adrian Bevan from Queen Mary, University of London.

“Our current theory about the fundamental forces of the universe, which has been around for nearly 40 years, is beginning to show signs of failure,” said Fergus Wilson from STFC’s Rutherford Appleton Laboratory. “Just as exciting, our new measurement indicates that any replacement theory will need to be more exotic and complex than we could have hoped or imagined. Although we must not jump to conclusions based on just one measurement, this new result is one of the most compelling yet. It follows on from previous indications recently reported by us, all of which point in the same direction.”

The BaBar experiment, which collected data from 1999 to 2008, was designed to explore various mysteries of particle physics, including why the universe contains matter but no antimatter. Data from the collaboration, which includes 75 institutions from Canada, France, Germany, Italy, Norway, Russia, Spain, the United Kingdom, and the U.S., helped confirm a matter-antimatter theory for which two researchers won the 2008 Nobel Prize in physics. At its peak, some 90 British particle physicists and engineers from 11 institutions took part in the experiment.

Researchers continue to apply BaBar data to a variety of questions in particle physics. “This result will help guide teams of researchers looking for potentially related new physics effects at the Large Hadron Collider and at other particle physics labs around the world,” said Bevan.

“If the excess decays shown are confirmed, it will be exciting to figure out what is causing it," said Abner Soffer from Tel Aviv University in Israel. “Other theories involving new physics are waiting in the wings, but the BaBar results already rule out one important model called the Two Higgs Doublet Model. We hope our results will stimulate theoretical discussion about just what the data are telling us about new physics.”

The researchers also hope their colleagues in the Belle collaboration, which studies the same types of particle collisions, see something similar. "If they do, the combined significance could be compelling enough to suggest how we can finally move beyond the standard model,” said Roney.

The God Particle

The Higgs boson (The God Particle), also BEH Mechanism, is a hypothetical massive scalar elementary particle predicted to exist by the Standard Model of particle physics. It is the only Standard Model particle not yet observed. An experimental observation of it would help to explain how otherwise massless elementary particles cause matter to have mass. More specifically, the Higgs boson would explain the difference between the massless photon and the relatively massive W and Z bosons. Elementary particle masses, and the differences between electromagnetism (caused by the photon) and the weak force (caused by the W and Z bosons), are critical to many aspects of the structure of microscopic (and hence macroscopic) matter; thus, if it exists, the Higgs boson is an integral and pervasive component of the material world.

No experiment has yet directly detected the Higgs boson; the Large Hadron Collider (LHC) at CERN, which came on line on 10 September 2008, is expected to provide experimental evidence that will confirm or reject the particle's existence when fully operational in 2009. The Higgs mechanism, which gives mass to vector bosons, was theorized in August 1964 by François Englert and Robert Brout ("boson scalaire"); in October of the same year by Peter Higgs, working from the ideas of Philip Anderson; and independently by Gerald Guralnik, C. R. Hagen, and Tom Kibble, who worked out the results by the spring of 1963. The three papers written on this discovery by Guralnik, Hagen, Kibble, Higgs, Brout, and Englert were each recognized as milestone papers by Physical Review Letters 50th anniversary celebration. Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The electroweak theory predicts a neutral particle whose mass is not far from that of the W and Z bosons.

The Higgs boson particle is one quantum component 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 which should have mass, including the Higgs boson itself. In particular, the acquisition of a non-zero vacuum expectation value spontaneously breaks electroweak gauge symmetry, which scientists often refer to as the Higgs mechanism. This is the simplest mechanism capable of giving mass to the gauge bosons while remaining compatible with gauge theories. In essence, this field is analogous to a pool of molasses that “sticks” to the otherwise massless fundamental particles which travel through the field, converting them into particles with mass which 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 are massless and 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 value of the Higgs boson mass. If the mass of the Higgs boson 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 around one TeV; beyond this point, the Standard Model becomes inconsistent without such a mechanism because unitarity is violated in certain scattering processes. Many models of Supersymmetry predict that the lightest Higgs boson (of several) will have a mass only slightly above the current experimental limits, at around 120 GeV or less.



photo: 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 may decay into top anti-top quark pairs.

Experimental Search:



photo: A Feynman diagram of one way the Higgs boson may be produced at the LHC. Here, two gluons decay into a top/anti-top pair which then combine to make a neutral Higgs.

As of 2008, the Higgs boson has not been observed experimentally, despite large efforts invested in accelerator experiments at CERN and Fermilab. The non-observation of clear signals leads to an experimental lower bound for the Standard Model Higgs boson mass of 114 GeV/c² at 95% confidence level. A small number of events were recorded by experiments at LEP collider at CERN that could be interpreted as resulting from Higgs bosons, but the evidence is inconclusive. The Large Hadron Collider (LHC), due to begin proper experimentation in 2009 after initial calibration, is expected to be able to confirm or reject the existence of the Higgs boson. The LHC has had trouble with a number of magnets in its initial calibration and startup phase and has been delayed 2 months due to these problems.

Precision measurements of electroweak observables exclude a Standard Model Higgs boson mass of 170 GeV/c² at the 95% confidence level as of August 2008 (incorporating an updated measurement of the top quark and W boson masses). Experiments searching for the Higgs boson are ongoing at the Fermilab Tevatron. The limits on the production cross section of the Higgs boson set by the on-going Tevatron searches are now less than a factor of 1.5 away from Standard Model predictions in the mass range where the Higgs boson primarily decays to an on-shell W boson and an off-shell W boson. There have been optimistic articles about potential evidence of the Higgs Boson, but no evidence is yet compelling enough to convince the scientific community as a whole.