From Don Lincoln at FNAL: “Eight is enough”

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October 7, 2016

FNAL Don Lincoln
Don Lincoln

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In the search for new physics, no stone can be left unturned. In this analysis, scientists looked for a new particle that underwent a cascade of decays, resulting in eight distinct particles. No image credit.

While the LHC was built for many purposes, one of the key reasons it was created was to investigate the most energetic collisions possible. The basic idea is that high-energy collisions have the best chance of unveiling phenomena never before observed.

In particle physics, low-energy things happen all the time, while high-energy things are extremely rare. To give a sense of scale, the lowest-energy collisions that we study occur about a billion times more often than the highest-energy collisions we can create.

Now if you want to study very high-energy things, you want to use the strongest force available to you. That’s because a strong force makes many collisions, and if you make many collisions, you are more likely to see the rare and very high-energy type of event you are looking for. In particle physics terminology, that means that you need to use events that use the strong nuclear force.

That works out as a good strategy at the LHC because the LHC collides protons together, and protons are full of quarks and gluons, both of which interact via the strong force. The basic idea is that a quark or gluon from one proton will interact with a quark or gluon from the other proton, merge into some new and undiscovered particle, and then decay and be observed in the detector. Now the CMS experiment has already looked into the case where this new particle decayed directly into two ordinary particles.

CERN/CMS Detector
CERN/CMS Detector

It has also looked into the case where the new particle decayed into two new (but different) particles that then each decayed into two ordinary particles. In this scenario, there would be four ordinary particles hitting the detector. Neither of these analyses led to the discovery of new physics.

However, there is no reason that these should be the only two possible scenarios. It could be that LHC collisions would make one new particle, which then decayed into two new but lower-mass particles, each of which subsequently decayed into two more new and even lighter particles, resulting in four, which each finally decayed into two ordinary particles, producing eight. Thus the CMS experiment went looking for events in which eight ordinary particles simultaneously hit the detector.

CMS was searching for particles called “jets,” which are actually collections of even more particles, but we can use algorithms to reduce a jet to looking like a single particle. So they were looking for events that produced eight jets.

So far so good. The problem with this analysis arises because, even without new physics, the strong nuclear force makes lots of events in which there are eight or more jets, so it is pretty hard to identify events with eight jets that are made by new physics. But there is one saving grace. The collisions in which eight jets are made by ordinary physics have the same basic distribution of total energy as the ones in which only two jets are made. So they use the well-understood two-jet data to make predictions of eight-jet data and then compare it to the measurements all eight-jet data. If too many eight-jet events are found, then maybe they’ve made a discovery.

Sadly, no excess was found. But this was a clever technique and one that might well be worth pursuing in the future. The most recent paper was for data recorded at a collision energy of eight trillion electronvolts of energy (back in 2012), and we’ve recorded data with 13 trillion electronvolts. Maybe with the new data, this technique will lead to a different result.

See the full article here .

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Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
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