DOE OSTI2017
The Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) near Geneva, Switzerland, is now the highest energy accelerator in the world, colliding protons with protons. On July 4, 2012, the two general-purpose experiments, ATLAS and the Compact Muon Solenoid (CMS) experiment, announced the observation of a particle consistent with the world’s most sought-after particle, the Higgs boson, at a mass of about 125 GeV (approximately 125 times the mass of the proton). The Higgs boson is the final missing ingredient of the standard model, in which it is needed to allow most other particles to acquire mass through the mechanism of electroweak symmetry breaking. We are members of the team in the CMS experiment that found evidence for the Higgs boson through its decay to two photons, the most sensitive channel at the LHC. We are proposing to carry out studies to determine whether the new particle has the properties expected for the standard model Higgs boson or whether it is something else. The new particle can still carry out its role in electroweak symmetry breaking but have other properties as well. Most theorists think that a single standard model Higgs boson cannot be the complete solution – there are other particles needed to answer some of the remaining questions, such as the hierarchy problem. The particle that has been observed could be one of several Higgs bosons, for example, or it could be composite. One model of physics beyond the standard model is supersymmetry, in which every ordinary particle has a superpartner with opposite spin properties. In supersymmetric models, there must be at least five Higgs bosons. In the most popular versions of supersymmetry, the lightest supersymmetric particle does not decay and is a candidate for dark matter. This proposal covers the period from June 1, 2013, to March 31, 2016. During this period the LHC will finally reach its design energy, almost twice the energy at which it now runs. We will be able to study the Higgs boson at the current LHC energy using about three times as much data as were used to make the observation. In 2013 the LHC will shut down to make preparations to run at its design energy in 2015. During the shutdown period, we will be preparing upgrades of the detector to be able to run at the higher rates of proton-proton collisions that will also be possible once the LHC is running at design energy. The upgrade on which we are working, the inner silicon pixel tracker, will be installed in late 2016. Definitive tests of whether the new particle satisfies the properties of the standard model Higgs boson will almost certainly require both the higher energy and the larger amounts of data that can be accumulated using the higher rates. Meanwhile we will use the data taken during 2012 and the higher energy data starting in 2015 to continue to search for beyond-the-standard-model physics such as supersymmetry and heavy neutrinos. We have already made such searches using data since the LHC started running. We are discussing with theorists how a 125-GeV Higgs modifies such models. Finding such particles will probably also require the higher energy and larger amounts of data beginning in 2015. The period of this proposal promises to be very exciting, leading to new knowledge of the matter in the Universe.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗