A joint Fermilab/SLAC publication

LHC set to gain major ground in Higgs search

03/16/11
This plot shows the confidence with which the CMS collaboration should be able to observe the Higgs with different amounts of collision data. Image courtesy of the CMS Collaboration.

This plot shows the confidence with which the CMS collaboration should be able to observe the Higgs with different amounts of collision data. Image courtesy of the CMS Collaboration.

The Standard Model Higgs is running out of places to hide, fast.

LHC experiments will reduce the range in which it could be lurking by about 80 percent by the end of this year, according to projections.

That’s if the LHC meets minimum expectations by collecting 1 inverse femtobarn of data in 2011.

If the CMS and ATLAS experiments collect 5 inverse femtobarns of data, which they hope to do by the end of 2012, they will either see signs of the Standard Model Higgs or know with 95 percent certainty that the model is wrong. If they can collect twice that much, or 10 inverse femtobarns, experiments will be able to claim either discovery or nonexistence of the Higgs across the majority of its expected mass range.

“We are entering a very exciting time in physics,” said physicist Vivek Sharma, Higgs search leader for the CMS experiment, at the Rencontres de Moriond conference on Monday.

Scientists theorize the Higgs boson is the reason particles have mass. Funny thing is, physicists don’t know how much mass the Higgs itself has. But they have a good idea what the Higgs would look like if they created it with a particle collider, so they’re trying to make Higgs particles of different masses.

It won't be enough to catch just one Higgs event. When physicists find a new particle, they use statistics to be sure they’re really seeing the thing they’re after. Once scientists have enough data, they can determine whether they have seen signs of the Higgs in a certain mass range or whether they can cross that area off of the map.

Physicists measure particle masses in units of energy called giga-electron volts, or GeV. They do this because, according to Einstein, energy is the same as mass multiplied by a constant, the speed of light squared. When high-energy particles collide in a particle accelerator, energy converts briefly to mass, creating new particles.

“The larger energy you have, the more massive particles you can create,” Sharma said. That’s why the LHC, which accelerates beams of protons up to 3.5 TeV and will eventually accelerate them to 7 TeV, can summon heavier particles than the Tevatron, which accelerates particle beams to about 1 TeV.

This plot shows the ATLAS collaboration's projected ability to rule out Higgs bosons of different masses by the end of 2011. Wherever a line dips below the dotted line at 1, ATLAS will be able to exclude the Higgs at that mass. Image courtesy of the ATLAS Collaboration.

This plot shows the ATLAS collaboration\'s projected ability to rule out Higgs bosons of different masses by the end of 2011. Wherever a line dips below the dotted line at 1, ATLAS will be able to exclude the Higgs at that mass. Image courtesy of the ATLAS Collaboration.

The Standard Model (or Standard Model-like) Higgs mass range LHC experiments use falls between 114-600 GeV. Tevatron experiments have already ruled out the range between 158-173 GeV with a certainty of 95 percent. According to projections, LHC experiments should rule out the range between 120-530 GeV with the same certainty after each collecting 1 inverse femtobarn of data. This will reduce the portion of the Standard Model Higgs mass range in which experiments have not ruled out the Higgs with 95 percent certainty by about 80 percent.

“But we are not just in the business of excluding the Higgs,” Sharma said. “We are in the business of discovering it.”

Since physicists’ searches involve statistics, their results are expressed in degrees of certainty. They measure that certainty in units called sigma, represented as the Greek letter &#963.

In this case, a result with a 3&#963 level of certainty is 99.7 percent sure to be correct. If scientists have 3&#963 certainty that they’ve found a new particle, they say they’ve found evidence of that particle. Results with a 5&#963 level of certainty are 99.9999 percent sure to be correct. If scientists have 5&#963 certainty that they’ve found a new particle, they consider that a discovery.

Recording just 1 inverse femtobarn of data, the minimum expected in 2011, the ATLAS and CMS experiments will be able to see evidence of the Higgs if it’s hiding in a mass range spanning an impressive 340 GeV, between 135 and 475 GeV, and will be able to discover the Higgs if it’s between 152 and 175.

This is a huge leap forward, but scientists hope to do even better. The more data the CMS and ATLAS experiments collect, the more sensitive they will be to excluding, finding signs of or discovering the Higgs.

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