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The LHC as a photon collider | CMS Experiment

The LHC as a photon collider | CMS Experiment | Tout est relatant |
The Large Hadron Collider is known for smashing together protons. The energy from these collisions gets converted into matter, producing new particles that allow us to explore the nature of our Universe. The protons are not fired at one another individually; instead, they are circulated in bunches inside the LHC, each bunch containing some 100 billion (100,000,000,000) particles. When two bunches cross each other in the centre of CMS, a few of the protons — around 25 or so — will collide with one another. The rest of the protons continue flying through the LHC unimpeded until the next time two bunches cross.

Sometimes, something very different happens. As they fly through the LHC, the accelerating protons radiate photons, the quanta of light. If two protons going in opposite directions fly very close to one another within CMS, photons radiated from each can collide together and produce new particles, just as in proton collisions. The two parent protons remain completely intact but recoil as a result of this photon-photon interaction: they get slightly deflected from their original paths but continue circulating in the LHC. We can determine whether the photon interactions took place by identifying these deflected protons, thus effectively treating the LHC as a photon collider and adding a new probe to our toolkit for exploring fundamental physics.

This kind of proton-tagging has not been possible at the LHC so far. But a new project called the CMS-TOTEM Precision Proton Spectrometer (CTPPS) will soon enable us to study these rare collisions. The project brings together the CMS and TOTEM collaborations, which had previously worked together during the proton-lead collisions of 2013. The CTPPS will be located on either side of CMS, 200 metres away from the interaction point at the centre of the detector.
The physics case for studying photon collisions

The physics of photon collisions has been a topic of some interest for many decades. Indeed, a special meeting in 1978 discussed the prospects of such collisions at LEP, the LHC’s predecessor, which collided electrons with positrons from 1989 until 2000.

“These collisions are very clean as we’re colliding photons, which are elementary particles and not composite ones like protons,” notes Joao Varela, former Deputy Spokesperson for CMS, who is heading the CTPPS project. “It was first proposed to do this type of physics at the LHC with CMS many years ago but the project didn’t materialise then.”

One objective of the CTPPS project is to enable CMS to study quartic gauge couplings. These are interactions where the two photons annihilate upon collision to produce two W bosons: one gets four gauge bosons at the same vertex (see Feynman diagram above). “With the CTPPS, CMS can study whether the distributions and production rates of these interactions are compatible with the Standard Model or not with two orders of magnitude better sensitivity than before,” says Varela.

By locating the CTPPS at 200 metres away from the collision point, it is possible to study a mass region above 200 GeV. If there are new particles with these high masses, the CTPPS also improves CMS’s discovery potential. Varela adds, “Recently, there were two proposals in CMS and one in TOTEM to build such a spectrometer, and we put them together into a single project.”
Design and operation of the spectrometer

The CTPPS relies on objects called “Roman Pots”, which are TOTEM’s speciality. They are cylinders that allow one to move small detectors into the vacuum of the LHC in such a way that there are detectors inside the beam pipe a mere 2 mm from the beam. The tracking detectors of the CTPPS are quite small, with a surface area of 2 cm2. There will be two stations located 10 metres apart on either side of the collision point. Six planes of silicon pixels on each station will detect the track of the flying protons to give direction information. The magnetic field of the LHC’s quadrupoles will serve as the field for the CTPPS.

Once the CTPPS tags deviated protons involved in photon interactions, the CMS detector will collect the data from the collisions themselves, with information about the tagged protons embedded in the same dataset.

The Roman Pots of TOTEM are designed to operate under special LHC runs with a small number of collisions per second. The physics goals of the CTPPS will require the Roman Pots to operate during normal CMS data taking, with the LHC providing an even higher number of collisions per second from 2015 onwards. Before collecting data for physics analyses, the CMS and TOTEM teams will need to demonstrate that this operation is possible and that the CTPPS detectors can be brought very close to the beam without disrupting the beam in the process.

“One of the reasons I joined this project,” says Varela, “is to have the possibility of having detector development in a time scale that is in my lifetime. The LHC Phase 1 work is mostly done, while the Phase 2 R&D is for longer-term projects. With the CTPPS, we can make small prototypes and put them in the LHC, and start collecting data relatively quickly.”

The CMS-TOTEM Precision Proton Spectrometer will go into production in 2016.
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Future LHC super-magnets pass muster

Future LHC super-magnets pass muster | Tout est relatant |

In the past four years, scientists at the Large Hadron Collider have accomplished unprecedented feats of physics, all with their particle accelerator working at half its design capacity.

The future is looking even brighter, literally.

Last week the US LHC Accelerator Research Program, or LARP, successfully tested a new type of magnet required to boost the power of the LHC—or the luminosity of its particle beams—by a factor of 10.

LARP is a collaboration among the US Department of Energy’s Brookhaven, Fermi, Lawrence Berkeley and SLAC national laboratories, working in partnership with CERN.

The improved magnets are one of the most critical components in a series of LHC upgrades that will be implemented over the next ten years. In the accelerator, magnets squeeze and focus beams of charged particles, directing them to a point of high-energy collision inside a detector. The new magnets, along with other upgrades, will allow the LHC to collect a larger amount of data at higher energies, making it possible to search for more massive potentially hidden particles than ever before.

Lucio Rossi, leader of the high luminosity project at CERN, says the improved LHC could illuminate unexplored corners of physics.

If you enter a dark room with only a candle, the room will be dim, and the candle will soon burn out, he says. But if you have a high-powered flashlight, not only can you see more of the room, but you also have enough time to get a good look around.

“Thanks to this magnet, we will have more collisions, more statistics and more rare events,” Rossi says. “If there is physics beyond the Standard Model, these magnets will shed light on it.”

Like the magnets that currently steer particles through the LHC, the new magnets are superconducting. A superconductor is a material that allows electric current to flow without resistance, creating a strong magnetic field.

The current LHC magnets are made of a metal alloy called niobium titanium. While they have performed remarkably well, there’s a limit to the amount of magnetic field they can sustain—and they’ve gone almost as far as they can go.

For the LHC to continue pushing the boundaries of high-energy physics, physicists plan to switch to magnets made out of niobium tin. Niobium tin has a greater tolerance to heat than niobium titanium, which means it has a larger window of superconductivity and can sustain a higher magnetic field longer.

However, there’s a catch; although niobium tin is a better superconducting material, it’s brittle and sensitive to strain.

“Think of a steel wire you would use for home repairs,” says Berkeley Lab’s GianLuca Sabbi, who directed the development of the new magnets. “You can bend it, and it won’t break. This is the case for niobium titanium, but niobium tin is more like glass.”

This presents some serious technical challenges because making a traditional superconducting magnet requires drawing the alloy into thin wires, gathering those wires into high current cables and then tightly winding them into an accelerator coil. If scientists took these steps, niobium tin would shatter.

US LHC Accelerator Research Program scientists get around this issue by following a clever recipe. First, they coil the “raw” ingredients of niobium tin—the metals that combine to create it—and then put the whole device into a special furnace for a high temperature heat treatment, which melds the components into a superconductor with the desired shape already intact.

At this point, it becomes sensitive to strain, so the scientists fill all the gaps and voids with an epoxy, which glues the brittle material together, providing the support the fragile wires need to withstand the harsh environment of the LHC.

The new technology has applications beyond high-energy physics. Plans are already in motion to incorporate these magnets into medical practices such as imaging and cancer treatment.

As the LHC continues to be streamlined, physicists hope to see further beyond the veil, piecing together the truth behind dark matter, dark energy, extra dimensions and other mysteries. At this scale of luminosity, previously undiscovered particles may even begin to appear.
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