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NOTE:  Results announced Friday morning.   A new diary will be posted at a little after noon Friday, although it will assume the reader has read this.

The most important particle physics conference in over 30 years begins later this week in Grenoble.   Last year, the Large Hadron Collider (LHC) began operation in Geneva.   The initial run in 2010 went well, and many papers were written based on the results.  In some cases, the results from the LHC gave more information than all previous accelerators combined.   At the Europhysics conference beginning Thursday, the LHC experimenters will report the results of this year's run (up through two weeks ago), and they have forty times as much data as in 2010.   The tension in the field has been growing, and extraordinary results are expected.  In particular, the search for "the Holy Grail" of particle physics, the Higgs boson, will cover much more territory than ever before.    If there is no evidence for the Higgs found, the particle's existence could be ruled out within a few months.   In this diary, I'm going to describe the Higgs boson and explain why it is so important to our understanding of Nature.

First, a little background.   I'm a theoretical particle physicist at a research university, and have written over a hundred published papers, many of which deal with the Higgs boson (and models which have several such particles).   My career began in 1978, just after a revolution in the field which took place between 1971 and 1975.

Prior to 1971, it was known that there are three forces in Nature (besides gravity, which I won't discuss here):  the electromagnetic force, the weak force (which is responsible for a type of radioactive decay and all interactions involving neutrinos), and the strong force (which holds protons and neutrons together in a nucleus).    Although there was a very solid theory of electromagnetism (called QED), nobody had any understanding of the strong interaction, and the weak interaction theory seemed to lead to nonsensical predictions.

In 1971-1973, a new theory was developed which unified the electromagnetic and weak forces together into a single interaction called the "electroweak force".   This theory was dramatically confirmed in 1974 and 1976 in experiments at Stanford.    In addition, in 1973-1974, a new theory of the strong interactions (called QCD) was developed, and it was so simple and beautiful that it was accepted immediately, and was also tested over the next few years and shown to be correct.  Together, these theories constitute what is called "The Standard Model".    A nice chart showing all of the particles and interactions of the Standard Model can be found at
http://www.cpepweb.org/...      The most important particles are the quarks, combinations of which make up protons and neutrons (the heaviest two are called the top and bottom quark), leptons, which include the electron and neutrino, and the W and Z bosons, which mediate the electroweak force (in the same way that photons mediate the electromagnetic force).

Over the past 35 years, the Standard Model has been experimentally tested to  great precision, with many dozens of predictions now verified to at least a part in a thousand.   It is an extraordinary achievement.   But there is one missing piece, and it is the strangest and most uncertain aspect of the entire Standard Model, and that is the Higgs boson.  

So what is the Higgs boson, and why is it so important?

One of the long-standing mysteries in physics is the origin of mass.   While most people tend to think of mass in terms of gravity, it is actually independent of gravity (in zero gravity conditions, it is still harder to push Rush Limbaugh, who has a large mass, than a ping-pong ball, which has a small mass).    In particle physics, we measure mass in units of "electron volts".   An electron-volt is a unit of energy, and Einstein's famous relation between energy and mass  relates a particle's mass to its energy.  All of the known elementary particles have masses, which have a huge range of values, ranging from fractions of an electron volt for neutrinos up to around a hundred billion electron volts for the heaviest quark (the top quark).    The values of all of the masses are found at http://www.cpepweb.org/... .  Nobody knows why the masses have the values they do.   One could just say "well, that's the way things are".   But there is a problem---without a Higgs boson, the mathematics of the Standard Model would predict that the masses of the fundamental elementary particles are zero.

So what is the Higgs?  Physicists assume that everywhere in space there is a field called a "Higgs field".    On Earth, we all know that everywhere in space there is something called a "magnetic field".  The Higgs field is similar, but it has no preferred direction and is constant everywhere in the Universe.   When particles move through the Higgs field, they interact with it and the main effect of this interaction is to give these particles a mass.     One can think of this situation as being like a person walking through a swimming pool filled with water.   As the person walks in the pool, the water makes it harder to move---it makes them feel heavier than they would on dry land.    With a real swimming pool, of course, you can climb out and walk normally.  But the Higgs field is everywhere in the Universe, and particles can't "escape".   All particle masses arise from the particle's interaction with the Higgs field, and heavy particles interact more strongly with the field than light particles do.

The Higgs boson is "an excitation" of the Higgs field.  It arises naturally from the existence of the Higgs field.   It would correspond to waves inside the swimming pool.  

What is the mass of the Higgs?  Alas, the theory doesn't specify it precisely, although physicists expect that its mass cannot be much less than 50 GeV or more than 500 GeV (where "GeV" means a billion electron volts, roughly the mass of a proton)

How does one detect Higgs bosons?

Here is the difficulty in producing and detecting a Higgs boson.    To produce a Higgs boson, physicists must provide a huge amount of energy in a small volume, thereby producing heavy particles which will decay or emit Higgs bosons.  

The diagrams below show how a Higgs boson is produced at an accelerator with two colliding beams of protons.   The protons collide with each other.  A proton is a very complicated object, made of quarks, antiquarks and particles called "gluons", which hold the quarks together.     In the top two diagrams, two gluons, one from each beam combine to produce top quarks, which then combine to make a Higgs.    In the bottom two, quarks combine to make W bosons, which then combine to make a Higgs.  

Photobucket

Because Higgs bosons are unstable, they then decay right away into other heavy particles, which can be detected.  The most interesting decays are into a pair of W's, a pair of Z's or a pair of "bottom" quarks.    W's decay most of the time into two quarks or into an electron and a neutrino.   Z's decay most of the time into two quarks or an electron-antielectron pair.  
A simulated event is shown in the figure.

Photobucket

This is an event for a Higgs produced by the lower left diagram above, with a Z and a Higgs produced.  The Z decays into two electrons and the Higgs decays into two bottom quarks.   Each of the quarks turns into a spray of particles (called a "jet") and decades of experience have shown that experimenters can identify a particular jet as having originated from a bottom quark.   This is schematic, of course.  In an actual event, there are lots of tracks whose energies need to be measured.   But measuring the energies of the jets and electrons precisely will pin down their origin.

Picking a Higgs out of the data is like finding a needle in a large haystack, since there are predicted to be trillions of collisions for every Higgs produced, but the properties of the Higgs in the Standard Model are so well-determined that physicists are confident that the Higgs will be discovered with enough data.   Sufficient data will be collected this year, and may already have been collected.

What if we look and we don't find a Higgs?

Of course, it would mean that the Standard Model is wrong.   But there is a much more serious problem if there is no Higgs.   For technical reasons, even in other theories besides the Standard Model, the nonexistence of a Higgs would mean  a serious problem with quantum mechanics itself.   If asked, I will explain this in the comments.  Now, there could be many Higgs bosons, they could be composite (made of more fundamental particles), they could have properties somewhat different than expected, but a Higgs boson must exist or the basic foundations of modern theoretical physics would be endangered.  

So what is the current status, and what happens this week?

The current status of Higgs searches is as follows:   At the Tevatron accelerator in Illinois, physicists have ruled out the possibility of a Higgs with a mass which is less than 110 GeV.   They also ruled out a Higgs with a mass between 158 and 176 GeV.    At the LEP accelerator in Geneva ten years ago, a Higgs below 114.5 GeV was excluded.   So the remaining possibilities are between 114.5 - 158 GeV or 176-500 GeV.

At the LHC, the total intensity collected by each detector is measured in a strange unit called "inverse femtobarns", or fb^-1.         Given the total intensity, we can predict how many Higgs bosons will be produced and what the backgrounds (which can look like a Higgs signal) are.    If the Higgs boson is not detected at a particular intensity, then one can determine what mass range can be excluded.  The results are

Intensity                        Mass range excluded
0.5 fb^-1                           136 - 185
1.0 fb^-1                           128 - 400
2.0 fb^-1                           123 - 500
5.0 fb^-1                           114.5- 600

In 2010, each of the two detectors at the LHC collected 0.04 fb^-1, so we learned nothing about the Higgs.   As of two weeks ago, they had collected 1.3 fb^-1.     It is the analyses of these data  that will be reported in a week.  If the experimenters see no Higgs, then the region from 125 -450 will be excluded, leaving only a narrow window between 114.5 and 125 GeV.   For technical theoretical reasons, most particle theorists would expect the Higgs mass to be between 115 and 135 (I can discuss that in the comments).    

The LHC is continuing to run, and the experimenters expect to reach an intensity of 5 fb^-1 within the next two months.  If they exclude the entire allowed region from 114.5 to 600 GeV, the Standard Model will be dead.  It is make-or-break time...

The first three days (Thursday-Saturday) of the conference are small parallel talks, and it is likely we will get some clue as to the results. The big LHC talks are Monday morning (European time).   I may post a brief diary giving the results then.   The entire particle physics community is extremely tense and excited right now....

UPDATE: 10:30 PM.   Good night all.   I've been amazed by all of the interest.  I'll be up before 6:00 in the morning, and will answer any questions/comments that come up then.

Originally posted to science on Mon Jul 18, 2011 at 02:29 PM PDT.

Also republished by J Town, SciTech, and Science Matters.

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