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The most important discovery in particle physics in the past 40 years was announced last July 4th.   The long-sought Higgs boson was discovered.    This diary is to update you all on the current status and future prospects.

Two years ago (July 2011), I  posted a diary here describing the beginning of the search at the Large Hadron Collider (LHC), at CERN in Geneva, followed up with a couple of other diaries over the next few months and then finally posted a diary here on "discovery day" a year ago.   I was very gratified by the response - far above anything I expected.    

You will not learn what the Higgs boson is in this diary.    For a brief discussion, go to the "discovery day" diary, and for a more substantial introduction, go to the first diary linked above.    In the earlier diaries, I explained that the Higgs field is responsible for the masses of all fundamental particles, and that without the Higgs field, the laws of physics would be much simpler.  Recently, I did hear (in an article by a friend, Frank Wilczek) of another analogy that might explain why this discovery is so much more important than just the discovery of "a new particle".  

Imagine a planet composed entirely of water.  A fish civilization develops and begins to learn about chemistry and physics.   Since the water is always around them, they don't even know it is there.   They would find that something as simple as the laws of motion would be incredibly complicated --  boxlike fish would move more slowly, torpedo like fish would move more quickly, some fish drift down, dead fish drift up, etc.   It would seem to be a mess.  Then a very smart fish, Fish Newton, makes a suggestion.  Suppose there is a substance everywhere called "Ocean".   All of inertia is due to the interaction of objects with Ocean.   If Ocean were to completely disappear, the laws of physics would become incredibly simple.    Fish Newton would propose that if you could put a lot of energy into a small volume, you might shake loose a molecule of Ocean, and in that way detect its presence.  That would be the "Ocean boson".

The Ocean is the Higgs field.  While the Higgs boson isn't exactly the same as a "molecule" of Higgs field (no analogy is perfect), it does represent direct evidence that the Higgs field is real.

The discovery a year ago today established that there really is an Ocean.  The Ocean is real.   This is hugely important for our understanding of Nature.

A description of the current status of the Higgs, and of prospects for the future, is below the Great Orange Croissant.

The properties of the Higgs are completely determined.   There is no wiggle room (if the Standard Model is correct).   One property is its spin, predicted to be zero.  Over the past year, it has now been conclusively demonstrated that it is zero.  This surprised nobody.

The other main properties are the way in which the Higgs decays.   There are several possible particles that it can decay into.

In the Standard Model of particle physics, there are six types of quarks (the particles that make up protons and neutrons):  up, down, strange, charm, bottom and top.  There are three charged leptons:  electron, muon, tau.   And then there are force carriers:  the gluon mediates the strong force, the W & Z mediate the weak force and the photon (also called a gamma ray) mediates the electromagnetic force.

These particles have all been seen and studied and are understood.  The Standard Model gives a very explicit prediction of how the Higgs boson will decay.  Any deviation in these predictions means that the Standard Model is wrong.

The main ways in which the Higgs decays are completely predicted and are as follows:

56.1% of the time:   a bottom-antibottom quark pair
23.1% of the time:   a W-boson pair
8.5% of the time:     a gluon pair
6% of the time:        a tau-lepton pair
3.3% of the time:     a charm-anticharm quark pair
2.9% of the time:     a Z-boson pair
0.2% of the time:     a photon pair.

Unfortunately, the gluon pair and charm-anticharm pair are completely swamped by huge backgrounds -- processes that don't involve Higgs bosons also produce the same thing.  So one can never see those.    A year ago, it looked as if the decay into a bottom-antibottom pair would be very difficult, but perhaps not impossible, to detect.   In addition, W's and tau-leptons decay into particles that can't all be detected, and so one can't measure them with very high sensitivity.   So it turns out that the Z-boson pair and the photon pair are the easiest ways to detect the Higgs.   (Also, Z-bosons are roughly ten times harder to see than photons, so the sensitivity to the two is very similar).   Those two signals were the basis of the discovery announcement last July.

The LHC ran through December, and then shut down for a two-year long upgrade.  It will start again in early 2015, and should get 10 times the amount of data during the next two years.   It took a few months for the Higgs data to be analyzed, and the results have been announced.   Experimenters managed an amazing tour-de-force and extracted the bottom-antibottom, tau pair and W pair signals.    Note the huge variation in rates in the above table, and yet all of the rates are in perfect agreement with the Standard Model.    The results for the ratio of the measured value to the expected value are (this is for the CMS detector - the other detector is similar):

bottom quark pair:     1.15   +-  0.50
tau lepton pair:          1.10    +-  0.4
W pair                       0.68   +-  0.20
Z  pair                       0.92   +-  0.28
photon pair                0.77   +-  0.27

The first number should be 1.00 if the Standard Model is correct.   The second number is the "one-sigma" experimental error.   This means that for any measurement, there is a 68% chance that it will be within this error, and a 95% chance that it will be within twice the error.      One sees excellent agreement, with no significant deviations.

What next?

Although the Standard Model is extraordinarily successful, it must not be the whole story.  There are several question that it does not address (and I don't have room to go into detail about these now):

1.  Dark matter exists.  It dominates the matter in the Universe.  The Standard Model contains no dark matter candidate.
2.  There is matter in the Universe, but little antimatter.   The Standard Model can't explain this.
3.  There is something called dark energy, causing the Universe to expand rapidly.  There is no explanation.
4.  The scale of the force of gravity is much much higher than the Higgs mass, but they should be comparable.  There is no explanation.
5.  The values of many of the fundamental masses are not given in the Standard Model.

There are many, many extensions of the Standard Model that address some or all of these questions.   I've worked on many of them.   Many of them predict a Higgs boson.  But they all have the following in common:  (a) there are new particles that the LHC might be able to see and (b) the above ratios will deviate somewhat (often at the 5-10% level).    The objective of the LHC in the rest of this decade will be to look for (a) these new particles and (b) measure the above ratios more precisely.

Most physicists believe that they will see new things when they turn on again in 2015.   If nothing new is seen by 2020 or so, then we will hit the "nightmare scenario" in which  the above questions (and others) simply can't be experimentally measured.   In 2025, a Higgs factory will start in Japan, which will measure the ratios to 1% accuracy.    But we will still  be worried --- if everything fits what we predicted, what else is left?    I don't expect that to happen....but time will tell.

Originally posted to science on Wed Jul 03, 2013 at 02:46 PM PDT.

Also republished by SciTech and Community Spotlight.

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