The quarks that miss

The magenta arrows indicate the paths of the protons before collision, the four red lines are particles resulting from direct quark or gluon annihilation, while much of the yellow and blue are fragments of the protons that missed each other.

In his freshman physics lectures, Richard Feynman compared the principle of energy conservation to a child playing with blocks. At the end of the day, the child seemed to have fewer blocks than he was given at the beginning of the day―until his mother looked hard enough and found them hidden under the bed. Similarly, we believe that energy cannot be created or destroyed because whenever some energy appears to be missing, we eventually find it hiding in another form.

When the LHC collides protons, the resulting energy forms new particles. The new particles are usually known types, like W and Z bosons, though new types, such as Higgs bosons, would also be produced if they exist. However, only a fraction of the collision energy produces particles in this way. It even varies from one event to another–many collisions convert only a tenth or a hundredth of the impact energy into new particles, while some rare events convert nearly all of it.

Where is the energy hiding? The key is that protons are composed of more fundamental particles–quarks and gluons– that collide. When two protons pass through each other, usually only one quark or gluon from each actually annihilate to make new particles. The rest of the fragments are deflected and carry most of the energy as a spray of particles close to the beamline.

In every collision event, the shrapnel from these near-misses overlaps the products of the direct impact. This complicates the data, but physicists are able to untangle them. The near-misses are interesting in their own right: a recent CMS paper presents a study of the number and energy of these particles. A thorough understanding of the quarks that miss improves the accuracy of identifying the quarks that hit, as well as the exact points in space where each collision took place. It also yields deeper insight into the inner structure of protons, which are perhaps the most familiar and yet most complex particles in physics.

Jim Pivarski

The U.S. physicists pictured above contributed to this analysis, which was an international effort.
The CMS collaboration has long been committed to share its exciting findings with the public. After years of service, Dan Karmgard has stepped down as the US CMS Education and Outreach Coordinator. He has been replaced by Don Lincoln.