Knocking on Heaven's Door - Lisa Randall [59]
The total proton charge doesn’t determine the particles that get made, since the rest of the proton just goes forward, avoiding the collision. The pieces of the protons that don’t collide carry away the rest of the net proton charges, which just disappear down the beam pipe. This was the subtle answer to the question the Paduan mayor asked, which was where the proton charges go during an LHC collision. It has to do with the composite nature of the proton and the high energy that guarantees that only the smallest elements we know of—quarks and gluons—directly collide.
Because only pieces of the proton collide and those pieces can be virtual particles that collide with net zero charge, the choice of proton-proton versus proton-antiproton collider is not so obvious. Whereas in the past, it was worth the sacrifice at lower-energy colliders to make antiprotons in order to guarantee interesting events, at LHC energies that’s not such an obvious choice. At the high energies the LHC will achieve, a significant fraction of the energy of the proton is carried by sea quarks, antiquarks, and gluons.
LHC physicists and engineers made the design choice to collide together two proton beams, rather than a proton and an antiproton beam. 34 This makes generating high luminosity—that is, a higher number of events—a far more accessible goal. It’s considerably easier to make proton beams than antiproton beams.
So—rather than a proton-antiproton collider—the LHC is a proton-proton collider. With its many collisions—more readily achievable with protons colliding with protons—it has enormous potential.
CHAPTER SEVEN
THE EDGE OF THE UNIVERSE
On December 1, 2009, I reluctantly woke up at 6:00 A.M. at the Marriott near the Barcelona airport in order to catch a plane. I was visiting to attend the Spanish premiere of a small opera—for which I’d written a libretto—about physics and discovery. The weekend had been enormously satisfying, but I was exhausted and eager to get home. However, I was briefly delayed by a lovely surprise.
The lead story in the newspaper that the hotel provided at my door that morning was “Atom-smasher Sets Record Levels.” Rather than the usual headline reporting a horrible disaster or some temporary curiosity, a story about the record energies that the Large Hadron Collider had achieved a couple of days before was the most important news of the day. The excitement in the article about the milestone for the LHC was palpable.
A couple of weeks later, when the two high-energy beams of protons actually collided with each other, the New York Times ran a front-page news article titled “Collider Sets Record, and Europe Takes U.S.’s Lead.”35 The record energy reported by the earlier news was now on track to be only the first of a series of milestones to be set by the LHC during this decade.
The LHC is now probing the tiniest distances ever studied. At the same time, satellite and telescope observations are exploring the largest scales in the cosmos, studying the rate at which its expansion accelerates and investigating details of the relic cosmic microwave background radiation left over from the time of the Big Bang.
We currently understand a lot about the makeup of the universe. Yet as with most progress, further questions have emerged as our knowledge has grown. Some have exposed crucial gaps in our theoretical frameworks. Nonetheless, in many cases, we understand the nature of the missing links well enough to know what we need to look for and how.
So let’s take a closer look at what’s on the horizon—what experiments are out there and what we anticipate they might find. This chapter is about some of the chief questions and physics investigations that the rest of the book will explore.
REACHING