Higgs boson

Day two. I’ve got enough time to myself so I’m going back to study deep physics. Perhaps I’m not going to develope a succesful professional career, but maybe some day I’ll grab the Nobel Prize …  let’s talk about Higgs boson, what it is and what we’re talking about.

The Higgs boson is a hypothetical elementary particle that is predicted to exist by the Standard Model (SM) of particle physics. The Higgs field is a hypothetical, ubiquitous quantum field that has a non-zero value in its ground state. This non-zero value explains why fundamental particles such as quarks and electrons have mass. The Higgs boson is an elementary excitation of the Higgs field above its ground state.

The existence of the Higgs boson is predicted by the Standard Model to explain how spontaneous breaking of electroweak symmetry (the Higgs mechanism) takes place in nature, which in turn explains why other elementary particles have mass. Its discovery would further validate the Standard Model as essentially correct, as it is the only elementary particle predicted by the Standard Model that has not yet been observed in particle physics experiments. The Standard Model completely fixes the properties of the Higgs boson, except for its mass. It is expected to have no spin and no electric or colour charge, and it interacts with other particles through weak interaction and Yukawa interactions. Alternative sources of the Higgs mechanism that do not need the Higgs boson are also possible and would be considered if the existence of the Higgs boson were ruled out. They are known as Higgsless models.

Experiments to find out whether or not the Higgs boson exists are currently being performed using the Large Hadron Collider (LHC) at CERN, and were performed at Fermilab’s Tevatron until its closure in late 2011. Mathematical consistency of the Standard Model requires that any mechanism capable of generating the masses of elementary particles become visible at energies above 1.4 TeV; therefore, the LHC (designed to collide two 7-TeV proton beams) is expected to be able to answer the question of whether or not the Higgs boson actually exists.

In December 2011, the two main experiments at the LHC (ATLAS and CMS) both reported independently that their data hints at a possibility the Higgs may exist with a mass around 125 GeV/c² (about 133 proton masses, on the order of 10⁻²⁵ kg). They also reported that the original range under investigation has been narrowed down considerably and that a mass outside approximately 115–130 GeV/c² is almost ruled out. No conclusive answer yet exists, although it is expected that the LHC will provide sufficient data by the end of 2012 for a definite answer.

Particle physicists believe matter to be made from fundamental particles whose interactions are mediated by exchange particles known as force carriers. At the start of the 1960s a number of these particles had been discovered or proposed, along with theories suggesting how they relate to each other. However these theories were known to be incomplete. One omission was that they could not explain the origins of mass as a property of matter. Goldstone’s theorem, relating to continuous symmetries within some theories, also appeared to rule out many obvious solutions.

The Higgs mechanism is a process by which vector bosons can get rest mass without explicitly breaking gauge invariance. The proposal for such a spontaneous symmetry breaking mechanism was originally suggested in 1962 by Philip Warren Anderson and developed into a full relativistic model in 1964 independently and almost simultaneously by three groups of physicists: by François Englert and Robert Brout; by Peter Higgs;and by Gerald Guralnik, C. R. Hagen, and Tom Kibble (GHK). Properties of the model were further considered by Guralnik in 1965 and by Higgs in 1966. The papers showed that when a gauge theory is combined with an additional field which spontaneously breaks the symmetry group, the gauge bosons can consistently acquire a finite mass. In 1967, Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the breaking of the electroweak symmetry, and showed how a Higgs mechanism could be incorporated into Sheldon Glashow’s electroweak theory, in what became the Standard Model of particle physics.

The latest update of the ATLAS searches for the Standard Model Higgs boson was presented at a CERN seminar on December 13, 2011.  As stated in the CERN press release, the new ATLAS and CMS results are “sufficient to make significant progress in the search for the Higgs boson, but not enough to make any conclusive statement on the existence or non-existence of the elusive Higgs. Tantalising hints have been seen by both experiments in the same mass region, but these are not yet strong enough to claim a discovery.”

“We have restricted the most likely mass region for the Higgs boson to 115-130 GeV, and over the last few weeks we have started to see an intriguing excess of events in the mass range around 125 GeV,” explained ATLAS experiment spokesperson Fabiola Gianotti. “This excess may be due to a fluctuation, but it could also be something more interesting. We cannot conclude anything at this stage. We need more study and more data. Given the outstanding performance of the LHC this year, we will not need to wait long for enough data and can look forward to resolving this puzzle in 2012.”

The CMS experiment also has updated their results in this same low mass region.

The Higgs boson is predicted by the Standard Model.   Via the Higgs field, it gives mass to the fundamental particles.  It is so short-lived that it decays almost instantly, and the experiment can only observe the particles that it decays into.   The Higgs boson is expected to decay in several distinct combinations of particles, and what is most intriguing about these results is that small excesses of events are seen in more than one such decay mode and in more than one experiment.

To identify and discover the Higgs Boson will take an enormous amount of data because the Higgs boson is very rarely produced.  A definitive statement on the existence or non-existence of the Higgs is not likely until later in 2012.

Discovery of the Higgs boson would be the first step on the path to many other new advances.

Experimental limits from ATLAS on Standard Model Higgs production in the mass range 110-150 GeV. The solid curve reflects the observed experimental limits for the production of Higgs of each possible mass value (horizontal axis). The region for which the solid curve dips below the horizontal line at the value of 1 is excluded with a 95% confidence level (CL). The dashed curve shows the expected limit in the absence of the Higgs boson, based on simulations. The green and yellow bands correspond (respectively) to 68%, and 95% confidence level regions from the expected limits.

The Standard Model does not predict the mass of the Higgs boson, but does  predict the production cross section once the mass is known. The “cross section”  is the likelihood of a collision event of a particular type.

ATLAS uses plots like this one to seek hints for the Higgs boson and also to exclude regions of mass where the Higgs is very unlikely to be found.   This example is not real data, but is a simplified plot to show how we interpret the results of our searches for the Higgs boson. The vertical axis shows, as a function of the Higgs mass, the Higgs boson production  cross-section that we exclude, divided by the expected cross section for Higgs  production in the Standard Model at that mass.  This is indicated by the solid black line.

This shows a 95% confidence level, which in effect means the certainty that a  Higgs particle with the given mass does not exist.  The dotted black line shows  the median (average) expected limit in the absence of a Higgs.  The green and yellow bands  indicate the corresponding 68% and 95% certainty of those values.

If the solid black line dips below the value of 1.0 as indicated by the red line,  then we see from our data that the Higgs boson is not produced with the expected  cross section for that mass. This means that those values of a possible Higgs mass  are excluded with a 95% certainty.  In this example, two regions would be ruled out at 95% certainty: approximately 135-225 GeV and 290-490 GeV.

If the solid black line is above 1.0 and also somewhat above the dotted black line (an excess),  then there might be a hint that the Higgs exists with a mass at that value.  If the solid black line  is at the upper edge of the yellow band, then there may be 95% certainty that this is above  the expectations.  It could be a hint for a Higgs boson of that mass, or it could be a sign of background processes or of systematic errors that are not well understood.  In this example, there is an excess and the solid black line is above 1.0 between about 225 and 290 GeV, but the  excess has not reached a statistically significant level.

The red-gray shaded regions show what is excluded.  The “bump” near a mass of 250 GeV could be a slight hint of a Higgs boson in this fictional example.

This plot shows hypothetical data and expectations that could be used in setting the limits shown in Figure A.

The green curve shows (fictional) predicted results if there were a Higgs boson in addition to all the usual backgrounds.  It could also represent the predictions of some other new physics.  The dashed black curve shows what is expected from all background processes without a Higgs or some new physics.   The black points show the hypothetical data.

In this case, the data points are too low to explain the Higgs boson hypothesis (or whatever new physics the green curve represents), so we can rule out that hypothesis.

Nonetheless the data points are higher than the expectations for the background processes. This could yield an excess such as shown on the left in Figure A.  There are three possible explanations for this excess:

1. It is a statistical fluctuation above the expected background processes.
2. It is a systematic problem due to an imperfect understanding of the background processes.
3. The excess is due to some different new physics (than that hypothesized) that would predict a smaller excess.

If instead, the black points lay close to the green curve, that could be evidence for the discovery of the Higgs boson (if it were statistically significant).

If the black points lay on or below the dashed black curve (the expected background), then there is no evidence for a Higgs boson and depending on the statistical significance, the Higgs boson might be ruled out at the corresponding mass.

 

ATLAS event containing four muons. This event is consistent with coming from two Z particles decaying: both Z particles decay to two muons each. Such events are produced by Standard Model processes without Higgs particles. They are also a possible signature for Higgs particle production, but many events must be analysed together in order to tell if there is a Higgs signal. This view (the original picture has been removed) is a zoom into the central part of the detector. The four muons are picked out as red tracks. Other tracks and deposits of energy in the calorimeters are shown in yellow.

After reading this, all of us must have a perfect knowledge about Higgs Boson, as clear as crystal is! So I’m glad to introduce you in this magnificient world.

Talent always is above the mediocrity.