Prof. Julia Thom, physics, presented results from the Large Hadron Collider’s first year of operation on Sept. 19 in Schwartz Auditorium. The LHC at the European Organization of Nuclear Research, also known as CERN, is meant to address the fundamentals of physics, according to the organization’s website. So far, the LHC has found no evidence of two important theories in physics, the Higgs boson and Supersymmetry.
The LHC accelerates protons to 99.999999 percent of the speed of light in a 27-km long circular tunnel beneath the Franco-Swiss border and it collides them head-on 20 million times a second. It is the largest and most expensive scientific experiment in history, with about 10,000 scientists from more than 100 countries participating. The superconducting magnets in the accelerator use 60 tons of liquid helium and 11,000 tons of liquid nitrogen and during operation store as much energy as 2.4 tons of TNT.
The two main goals of the LHC are to experimentally verify the existence of the Higgs boson and to find signatures of Supersymmetry, abbreviated SUSY. “At the LHC, we’ve seen no sign of the Higgs, no sign of SUSY where it’s accessible to us so far and no other new physics –– yet… That in itself is very interesting. What seems most natural to us may be excluded,” Thom said. “We are deep into the most interesting time in particle physics in decades. We are still in the first year of a 20-year program.”
According to the Standard Model of particle physics, which has been one of the most significant theories in history, the Higgs boson is responsible for all the observed masses of fundamental particles.
“The Higgs field permeates the vacuum, and other particles couple to it to effectively become massive. Think of a bead being pulled through honey –– it appears that the bead is really heavy, but it’s due to the honey,” Thom said. “It is central to our current understanding of physics, but has not been confirmed experimentally.”
Thom presented data from the LHC so far, in which the Higgs theory has not yet been found.
“This leaves a very small window, in the mass range where the Higgs is hardest to detect, as the most likely hiding place [for the Higgs boson],” Thom said. “We will have to wait till the end of the year to exclude [completely] or discover the Standard Model Higgs. Either way, it will have far-reaching implications for science, and will be extremely interesting,” she added.
The Higgs mechanism has some theoretical problems. “The Higgs mass receives radiative corrections due to quantum loops that make it diverge … New physics [at the scale probed by the LHC] has been postulated to solve this problem,” Thom said. Furthermore, the current theory does not account for dark matter. Many cosmological and astrophysical observations suggest that about 83 percent of the matter in the universe is dark, meaning that it neither emits nor scatters light, but its gravitational effects on ordinary visible matter is evident.
SUSY is an extension to the current Standard Model that attempts to solve such problems. It implies the existence of yet-undiscovered “super-partners” to the known fundamental particles.
“This theory is very powerful –– in it, all the different forces can be unified, general relativity [gravity] can be incorporated and dark matter can be explained,” Thom said.
There are many different versions of SUSY theories. Dark matter might be made up of one or more of the SUSY particles, which the LHC aims to find.
No evidence for any SUSY particles has been found yet and the LHC data so far show excellent agreement with the Standard Model calculations over a wide parameter range.
“A year ago, all data available at the time suggested that super-partners may be [relatively] light … Many of us expected an early discovery,” Thom said. Although the results are surprising, Thom said that SUSY is not dead by any means. “It is true that we are starting to exclude some SUSY scenarios, but that does not mean that we can exclude SUSY in general,” she added.
Thom, along with five other Cornell professors, works on the Compact Muon Solenoid detector at the LHC, probing interactions occurring on small length scales of about one ten-thousandth of the radius of the proton.
“The performance is exceeding all expectations… We now [already] have a data set that is about half of what we’ve integrated at the Tevatron [the previous biggest collider near Chicago] for years and years,” Thom said.
The CMS detector measures energy and momentum of the particles produced as a result of the high-energy proton-proton collisions. Charged particles are tracked by their deflection in a high magnetic field. Cornell has contributed significantly to the “silicon pixel and strip tracker” that measures the particle trajectories in 3-D with high resolution. “We have worked on data acquisition and analysis software, detector commissioning, and are deeply involved in run management, data management, data analysis and upgrade plans,” said Thom. The LHC will be ramped up in 2014 to double the current energy of the proton beams.
Original Author: Vivek Venkataraman