Searching for the Higgs boson - new LHC results

Some of you may have heard that 2 of the experiments at CERN (the European particle collider built to find the Higgs boson).

So, what is the Higgs boson?  Well, in the standard model of particle physics, the Higgs boson is the particle which couples to everything else and gives those particles mass.  It is a strange notion of quantum field theory and may be difficult to understand.  Basically, the language of particle physics is quantum field theory.  Quantum field theory tells us interactions between particles are mediated by bosons.  For a more concrete example, electromagnetic interactions are mediated by photons (the interaction (or gauge) boson of quantum electrodynamics - the quantum field theory that describes electromagnetic interactions).  So, when two like charges repel each other - they two charges are exchanging a bunch of photons to accomplish that interaction.  I hope that helps in some way.

Here is a summary of particle interactions in the standard model:

There is a great video put out by Fermilab (a US particle physics lab) on searching for the Higgs and how it is done:


So, recent results by two different experiments at CERN, ATLAS and CMS, both have constrained the mass of the Higgs to a much smaller region.  CMS gives the limits of 115-127 GeV/c² and ATLAS gives the limits of 116-131 GeV/c². 

For those with a little more interest here are some plots from Sean Carrol's blog (he borrowed from others, so follow the trail down the rabbit hole if you like!).

 Here are the ATLAS preliminary results:

And the CMS preliminary results:

The way you read these plots is this: Any time the black dotted line dips below 1 on the vertical scale, these mass regions for the Higgs are excluded.  Anytime the black line dotted line rises above the yellow shaded region, there is increased confidence the Higgs may be hiding in there.

ATLAS is seeing a peak at approximately 126 GeV/c², while CMS is observes a peak at about 124 GeV/c².

Guido Tonelli, the Spokesperson for CMS said this at the CERN Higgs Seminar:

"…we observe in our data a modest excess of events between 115 and 127 GeV that appears, quite consistently, in five independent channels. The excess is most compatible with a SM Higgs hypothesis in the vicinity of 124 GeV and below, but the statistical significance (2.6 sigma local and 1.9 sigma global after correcting for the LEE in the low mass region) is not large enough to say anything conclusive."

While Fabiola Gianotti, the ATLAS Spokesperson said this (also at the CERN Higgs Seminar):

"We observe an excess of events around m_H ~ 126 GeV:   local significance of 3.6 sigma, with contributions from the  H –>2 gammas (2.8 sigma), H –> ZZ –>4l (2.1 sigma), H –> WW –> lvlv (1.4 sigma), SM Higgs expectation: 2.4 sigma local –> observed excess compatible with signal strength, the global significance (taking account Look-Elsewhere-Effect) is ~2.3 sigma"

So, the big take away is that the LHC is closing in on the Higgs, but there is still a lot of data to collect next year.  It should also be noted that the LHC just finished the 2011 runs about 6 weeks ago.  So these results are very preliminary, but also very promising!

Earth's Antimatter Radiation Belt

A great paper came out on the arXiv last week: O. Adriani et al., The discovery of geomagnetically trapped cosmic ray antiprotons, arXiv:1107.4882v1. Here is the abstract:

The existence of a significant flux of antiprotons confined to Earth's magnetosphere has been considered in several theoretical works. These antiparticles are produced in nuclear interactions of energetic cosmic rays with the terrestrial atmosphere and accumulate in the geomagnetic field at altitudes of several hundred kilometers. A contribution from the decay of albedo antineutrons has been hypothesized in analogy to proton production by neutron decay, which constitutes the main source of trapped protons at energies above some tens of MeV. This Letter reports the discovery of an antiproton radiation belt around the Earth. The trapped antiproton energy spectrum in the South Atlantic Anomaly (SAA) region has been measured by the PAMELA experiment for the kinetic energy range 60--750 MeV. A measurement of the atmospheric sub-cutoff antiproton spectrum outside the radiation belts is also reported. PAMELA data show that the magnetospheric antiproton flux in the SAA exceeds the cosmic-ray antiproton flux by three orders of magnitude at the present solar minimum, and exceeds the sub-cutoff antiproton flux outside radiation belts by four orders of magnitude, constituting the most abundant source of antiprotons near the Earth.

The red data points are the measured antiproton flux in the South Atlantic Anomaly region.

 So, most people have heard of the Van Allen radiation belts.  These are composed protons and electrons trapped by the Earth's magnetic field at a certain distance above the Earth's surface.  The proton component is thought to be due to the decay of neutrons produced by galactic cosmic ray interactions with the Earth's atmosphere.  The cartoon description is as follows: A galactic cosmic ray (fully ionized, highly energetic heavy ion) and an atmospheric molecule experience a nuclear interaction.  A neutron is produced which decays into a proton (and an electron and an antineutrino).  In addition, there will be a contribution from direct proton production from the nuclear interaction - that is ion + neutral nucleus --> protons + lots of other stuff.  These are the basic process which produce the protons that are trapped in the Van Allen belts. 

The production of the antiprotons is not from decay, but only from the direct interaction.  With enough energy (which cosmic rays have - there are particles in the cosmic ray spectrum with energies orders of magnitude above that CERN uses), antiprotons will be produced in the nuclear interactions.  Some of these will escape the atmosphere and should become trapped in the Earth's geomagnetic field.  Now the PAMELA experiment, as reported in this article, have confirmed this experimentally!

Introduction and Brief History of Particle Physics

There is a really great article over at Physics Today written by Steven Weinberg on particle physics entitled Particle physics, from Rutherford to the LHC. 

From the article:

It is clearly necessary to go beyond the standard model. There is a mysterious spectrum of quark and lepton masses and mixing angles that we have been staring at for decades, as if they were symbols in an unknown language, without our being able to interpret them. Also, something beyond the standard model is needed to account for cosmological dark matter. 

It is now widely understood that the standard model is just an effective field theory (see the box above), the low-energy limit of some more fundamental theory involving a scale of mass much larger than the masses with which we are familiar. That means we should expect the standard model to be supplemented with interactions that are not renormalizable in the usual sense—in fact, with all interactions allowed by symmetry principles—but suppressed by denominators proportional to powers of the large new mass. Infinities are still absorbed in a redefinition of the constants of the theory, but the number of constants that need to be redefined is no longer finite.

 The box he is talking about is here:

There is a great layman's introduction to effective field theories - which I find very interesting and did part of my dissertation on.

 

Major Discovery at CERN!

The Hugs has been found, the Hugs has been found!  The Hugs Boson has been discovered at CERN!  It is a very snuggle particle!

Sorry all you poor particle physicists, you have been looking in the wrong spot all along.  So sorry!  Have a hug instead?!?!

Geek humor, gotta love it!