the elusive Higgs boson

All theorized particles in the Standard Model of Particle Physics has been found except the Higgs boson. A confirmation of the existence of this particle will complete the theoretical description of over half a century of experimental and theoretical particle physics work. Yesterday, the combined data from the ATLAS (4.9 fb-1) and CMS experiments were shown in a public seminar from CERN hinting that the Higgs exists with a mass of about 126GeV(as a comparison, the mass of the proton is 0.938GeV).

Invariant mass distribution for the inclusive data sample, overlaid with the sum of the background-only fits in different categories described in Sections 3 and 4 in the ATLAS paper and the signal expectation for a mass hypothesis of 120 GeV corresponding to the SM cross section. The figure below displays the residual of the data with respect to the background-only fit sum. Source: ATLAS Collaboration

 As shown in the figure above, the dotted lines show how the Higgs would modify the Standard Model background.  It is a very minor modification so it requires high sensitivity of the detector.

The observed and expected 95% confidence level limits, normalised to the SM Higgs boson cross sections, as a function of the hypothesized Higgs boson mass. Source: ATLAS Collaboration.

We only see a 3.6σ(ATLAS) and 2.6σ(CMS) above the expected SM background for the Higgs mass of 126GeV in the figure above.  A 5σ significance is required to claim a discovery.

(to answer Mr. Hawkin's question) Do I want the Higgs to exist? Well, its kind of nice to have a theory being proven correct by experiment especially it involved the hard work of many generations of physicists.  Having the Higgs to exist at the low mass region of 115-150GeV would also be nice.  There will be other problems that physicists could work on and more work for me to do, supersymmetry maybe?

On the other hand, no Higgs means a totally different theory to explain where mass comes from.  Although there are already many theories like Technicolor or Higgless SM models, we will always have more work to do.

What is particle physics?

Today, I boarded a flight to Geneva from Copenhagen, and arrived at CERN roughly about noon time.  The weather was the total opposite of the Lund's where it has been cloudy, rainy and windy the past few weeks if not most of summer.  The sun was terrific and warm which reminded me so much about the trip I had in Italy.

CERN is located in the part of Geneva called Meyrin, which is really at the border with France.  For the uninitiated, CERN is a major research center for major discoveries in the field of particle physics.  In the past, it has contributed to the discovery of

• 1973: The discovery of neutral currents in the Gargamelle bubble chamber.
• 1983: The discovery of W and Z bosons in the UA1 and UA2 experiments.
• 1989: The determination of the number of light neutrino families at the Large Electron–Positron Collider (LEP) operating on the Z boson peak.
• 1995: The first creation of antihydrogen atoms in the PS210 experiment.
• 1999: The discovery of direct CP-violation in the NA48 experiment.
• 2010: The isolation of 38 atoms of anti-hydrogen.
• 2011: Maintaining anti-hydrogen for over 15 minutes.

source: wikipedia

These experiments have increased our knowledge of what forms matter and how they interact with each other.  Current experiments that have been followed by the media are ATLAS, CMS, ALICE amongst others.  They are discovery machines that find new physics, which sometimes mean new particles or new properties of matter.  The Higgs boson is one of the most talked about but however till now, it has not been discovered yet.  Other hypothetical particles waiting to be discovered are like the sparticles; neutralinos, charginos and squarks.

The CERN research complex looks like an industrial area with a smattering of workshops and warehouses.  Ocassionally you find some art pieces, which are actually decommissioned detector pieces like the Gargamelle bubble chamber.  There are also two beam pipes used as banner placeholders and you will see in the pictures.  In 2000, the Swiss built and object for the World Expo in Hannover.  This object has then been transplanted to the CERN site and is called the Globe of Science and Innovation, and it one of the donated art pieces which is slowly becoming a landmark icon (according to the CERN site, but if you visit, it definitely catches your eye orders of magnitude higher than the boring 70's type architecture of the CERN buildings).

The CERN signboard outside Entrance B

Where all the accelerator actions happens.

... so the beam from SPS goes to LEP then LHC ...

All is calm in the control room.  No beams running yet.

Sample beam pipe or rather drift tubes.

The cafeteria that is chaotic at lunch time.  Reminds me much about the canteens in NUS.

This picture and the previous one was once a piston activated bubble chamber.

The interior of the bubble chamber, which is a particle detector.

The Gargamelle detector.  The one that discovered the neutral current.

A Van deGraff generator.  The first step in accelerating charged particles in the old days.

The Globe of Science and Innovation on the opposite side of the road from CERN.

Can you see Mont Blanc.

The ATLAS control room building.  You can tell from the mural of the cross section of the detector on the wall.