| Xibai's profileNautilusPhotosBlogLists |  Tools  Help |
|
November 12 The Mystery of Higgs BosonOne of the most fundamental issues in high energy physics - the origin of mass...
The Search for the Higgs Boson
The Higgs boson is a hypothetical particle to account for the origin of mass. An overview of the Higgs mechanism is presented here, with emphasis on the production of the Higgs boson and searches for the Higgs at LEP. Indication of the production of a Higgs boson at 115GeV has emerged from LEP but further evidence is needed before any conclusion can be made.
The Origin of the Higgs
One of the central questions in today’s research in particles physics focuses on a hypothetical particle called the Higgs boson (H), which if found to exist, may explain the origin of mass. The Higgs boson is closely associated with the electroweak theory, developed and incorporated into the standard model by Abdus Salam and Steven Weinberg in the 1960s. In this theory electromagnetic forces are seen as due to the exchange of photons and the weak forces, which govern radioactivity, are due to the exchange of W and Z particles. Unlike the massless photon, the W and Z are massive particles, weighing 80 and 91 GeV respectively. That is, over 100 time the mass of a proton.
The predictions of the electroweak theory have been verified to very high precision by various experiments. However, there is still one puzzle: the electromagnetic and weak interactions are known to arise from a common symmetry called SU(2)L×SU(1)Y symmetry group and this symmetry indicates that the W and Z particles are massless, just as the photon, which is clearly not true. To solve the puzzle there must be something in addition to the standard model to break the symmetry and give the particles their mass. The simplest solution was introduced by Peter Higgs of Edinburgh University in 1964. Based on the earlier works of Julian Schwinger, Phil Anderson and others, he proposed a new field which permeates all of space and interacts with other particles. The interaction slows the particles down and gives them a mass, the stronger the interaction, the heavier the particles become.
According the wave-particle duality, all quantum fields have a particle associated with them. The particle associated with the Higgs field is called the Higgs boson, corresponding to the disturbance in the field. Further calculation showed that the Higgs boson is spin zero and has neutral charge.
Although the Higgs mechanism itself does not predicts the mass of the Higgs boson, it can be inferred from the precise measurement of the mass of the W, Z and top quark. Combined Data from the Stanford Linear Collider (SLC), the Fermilab and the European Laboratory of Particle Physics (CERN) suggests that the mass of the Higgs boson should be less than 170 GeV, if it is to be consistent with all existing theories.
Search for the Higgs at LEP Physicists have been hunting for the Higgs boson for twenty years, but so far no Higgs were detected. One of the reasons is that the Higgs are very massive. It requires an enormous amount of energy to create it and such an event usually cannot be distinguished from the huge background of other processes. The most thorough search of the Higgs boson so far has been carried out at the Large Electron-Positron (LEP) Collider at CERN. Opened in 1989, the LEP collider is the biggest scientific instrument in the world. The main accelerator (a cyclotron) is built in an underground tunnel 27 kilometres in circumference and consists of four huge detectors: ALEPH, L3, DELPHI and OPAL. Apart from searching for the Higgs boson, particle physicists there also carried out extensive research on the detail properties of the W, Z and top quark. The principle mechanism of producing the Higgs boson at LEP is the so-called the Higgsstrahlung process, i.e. e+ + e- → Z + H.
Both the Z and H are very short-lived and can only be identified by their decay products. The main decay mode for the Higgs is a pair of bottom quark (b) and anti bottom quark (b-), while the Z decays into a quark anti-quark pair (qq-). The main background processes are
e+ + e- → Z + Z and e+ + e- → W+ + W- This was well understood in the early runs of the LEP. The Higgs boson can also be produced in the fusion of W+ and W-, i.e. e+ + e- → W+ + W- and then W+ + W- → H + ve+ + ve- where ve is a electron neutrino and ve- its anti particle. However, this process is quite insignificant in the energy region in which LEP is operated. 2) 2) e+ + e- → (Z → vv-) + (H → bb-). This is the so called missing energy states and occurs with a ratio of 17%. It only has two b jets and the resolution for the Higgs mass is 3 GeV. 3) 3) e+ + e- → (Z → ee-, uu-) + (H → bb-). The ratio is low (6%) but the resolution can be increased to 1.5 GeV since has a low background. 4) 4) e+ + e- → (Z → qq-, ττ) + (H → bb-, ττ). The branching ratio for this process is about 10%. Figure. 1 Computer reconstruction of one of the candidate Higgs events recorded by ALEPH. All four jets are originated from b quarks and anti-quarks. The blue and green jets are believed to have come from the decay of a Higgs boson. (This graph is reproduced from M. Riordan et al., Science 291, 259 (2001))
Final Results from LEP
The LEP was originally designed to have maximum collision energy of 200 GeV and experiments carried out prior to the year 2000 did not reveal any evidence of the production of the Higgs boson. However, in 2000, LEP’s final year of operation, engineers were able to optimize the accelerator and pushed the collision energy up to an unprecedented 206 GeV. Then intriguing results began to appear.
Shortly before the scheduled shutdown, the ALEPH experiment reported on 5 September that they observed a few events which could be interpreted as the production of an 114GeV Higgs boson. This result prompted the CERN board to extend the running of the LEP for another six weeks to the end of November. The final outcome is interesting: Analysis of the data reveals an excess of events consistent with the production of a Higgs boson at 115GeV and incompatible with backgrounds by 3 standard deviation (Figure. 2). That is, the probability that such events are due to the background fluctuation is 4 percent. When the data from DELPHI, L3 and OPAL are added, the significance decreases to 2 standard deviation. With 95% C.L (confidence level), the lower limit of the mass is set to 114GeV. This is not a conclusive result, since a valid claim of a new particle usually requires 5 standard deviation. But it gives us some hint that we are on the right track and there is indeed something out there to be discovered.
Figure. 2 The signals against the mass of the Higgs boson (mh) for (a) NN stream and (b) cut stream. The expectation value of the backgrounds is represented by the dashed line and the shaded areas correspond to 1 and 2 standard deviation respectively. The solid line is the result obtained from the data. Note the large deviation at mh = 115 GeV. (This graph is taken from ALEPH Collaborations, Phys. Lett. B 526 191 (2002)) The Tevatron and the LHC With the closing down of CERN’s LEP collider, the centre of Higgs research shifted to the Tevatron collider at Fermilab. The proton anti-proton (pp-) collider now runs at collision energy up to 1.8 TeV and may produce a Higgs and a W boson in a proton anti-proton collision p+ + p- → W + H, with H decays into a pair of b quark and anti b quark and W → u + v or W → e + v. So far the data samples from Run I are not enough to make a discovery. In March 2001 the newly upgraded Run II began to collect data, but it will take sometime to improve the sensitivity of the devices. By 2006 the Tevatron should be able to see Higgs evidence up to 180 GeV. If nothing is found at the Tevatron, then our best hope lies in the Large Hadron Collider (LHC) at CERN. Now being installed on the site of the original LEP tunnel, the huge proton-proton collider is expected to produce data from 2007. It can reach collision energy of 14TeV and search for the Higgs over a wide range from 100 GeV to 1TeV. The dominant production channel for the Higgs at LHC would be Gluon fusion, i.e. gg → H. At very high energy regions WW and ZZ fusions also become important. Prospects Most theorists are convinced that the Higgs boson will be discovered sometime in this decade at the Tevatron or LHC. However if no Higgs is found, then it would be even more interesting because something must do the job of the Higgs. There are already several alternative theories, one of which is the popular idea of “supersymmetry”. It suggests the existence of several Higgs boson and a new type of strong interaction. In addition, more advanced linear accelerators such as the TeV Energy Superconducting Linear Accelerator (TESLA) in Germany are being designed. They might provide more striking evidence for the Higgs boson. References 1. P. W. Higgs, Phys. Lett. 12, 132 (1964) 2. R. Cahn, Rep. Prog. Phys. 52, 389 (1989) 3. M. Riordan et al., Science 291, 259 (2001) 4. ALEPH Collaboration, Phys. Lett. B 495 1 (2000) 5. ALEPH Collaboration, Phys. Lett. B 526 191 (2002) 6. K. Hagiwara et al., Phys. Rev. D 66, 010001 (2002) 7. J. Glanz, Science 284, 2079 (1999) 8. J. Thompson, LEP’s Glorious Decade, Frontier 9, 19 (2000) 9. B. Murray, Hunting the Higgs, Frontier 14, 19 (2002) 10. S. Schmaltz, Introducing the Little Higgs, Phys. World, November 2002 11. G. Collins, Higgs Won’t Fly, Scientific American, February 2001 12. S. Tomlin, Nature 409, 754 (2001) Comments (4)
TrackbacksWeblogs that reference this entry
|
|
|
