Some 100 metres underground, on the 27-kilometre ring of the Large Hadron Collider (LHC), sit four experiments the size of buildings. ATLAS and CMS are general-purpose detectors designed to investigate a wide range of physics phenomena from Higgs bosons to dark matter; ALICE specializes in studying quark-gluon plasma – a state of matter thought to have existed moments after the Big Bang – and LHCb is investigating the difference between matter and antimatter by analysing beauty quarks.
But these are not the only experiments at the world’s most powerful particle accelerator. Three smaller experiments – TOTEM, LHCf and MoEDAL – will be among those searching for new physics when data taking begins, in early June, at the LHC’s new energy frontier of 13 teraelectronvolts (TeV).
The TOTEM experiment takes precise measurements of protons as they emerge from collisions in the LHC at small angles to the beampipe. This region is known as the ‘forward’ direction. TOTEM detectors on both sides of the interaction point at CMS are spread across a total distance of almost half a kilometre. For the LHC’s second run, the TOTEM and CMS collaborations plan to coordinate the use of their detectors to perform combined measurements with unprecedented accuracy.
“TOTEM will continue to give insights on the structure of the proton, as well as diffractive processes relevant in forward and cosmic-ray physics,” says TOTEM spokesperson Simone Giani. “Combining TOTEM data with those of CMS will allow measurements of ‘missing energy’ with discovery potential in a phase space not accessible to former experiments.”
The Large Hadron Collider forward (LHCf) experiment measures neutral particles emitted at nearly zero degrees to the direction of the proton beam. Because these ‘very forward’ particles carry a large fraction of the collision energy, they are important for understanding the development of showers of particles produced in the atmosphere by high-energy cosmic rays. To measure these particles, two detectors, Arm1 and Arm2, sit along the LHC beamline, at 140 metres either side of the ATLAS collision point.
“Since their birth around 2004 the LHCf detectors have been upgraded year after year, in such a way that their performance and radiation hardness have been greatly improved in view of the 13 TeV proton-proton run,” says Lorenzo Bonechi, who leads a team for the LHCf collaboration in Florence, Italy. “The LHCf results at 7 TeV collisions are in good agreement with model predictions for forward photon and neutral pion productions but not for forward neutrons. The operation at 13 TeV collisions give us a great opportunity to confirm the results with collisions of about factor four higher energy in the laboratory frame and to test models more precisely than at 7 TeV.”
MoEDAL, the LHC’s newest experiment, is designed to search for highly ionizing avatars of new physics such as magnetic monopoles. Its physics programme defines numerous scenarios that yield insights into such questions as: are there extra dimensions or new symmetries; does magnetic charge exist; and what is the nature of dark matter.
The largely passive MoEDAL detector, deployed at Point 8 on the LHC ring, has a dual nature. First, it acts like a giant camera, comprised of over 200 square metres of nuclear track detectors – analysed offline by ultra-fast scanning microscopes – sensitive only to new physics. Second, with roughly one tonne of trapping detectors, it is able to capture particle messengers of physics beyond the Standard Model for further study.
“The MoEDAL experiment will begin to take data for the first time in June 2015,” says MoEDAL spokesperson James Pinfold. “Any MoEDAL discovery would have a revolutionary impact comparable to that of the Higgs boson.”
With data taking to start in early June, LHC experiments large and small are rearing to explore new frontiers in physics.