Higgs in the Large Hadron Collider

1. Higgs in the Large Hadron Collider Joe Mitchell Advisor: Dr. Chung Kao

2. Outline • The Setup • Standard Model • What is the Higgs Particle? • The Large Hadron Collider • Detector • Finding the Higgs Particle • Programs • Results http://atlas.ch/photos/events-simulated-higgs-boson.html http://atlas.ch/photos/full-detector-cgi.html

3. The Setup • Particle collider useful to find new particles and high energy effects • Smash particles at high speed, for high energy interactions • Look at events, or collisions, with large difference between signal and background • Simulate these events with and without new particle • Compare these with experiment to see which is closer • Particle collision: • http://hands-on-cern.physto.se/ani/acc_lhc_atlas/lhc_atlas.swf

4. The Standard Model: Matter Neutron Proton • The most modern verified theory about the makeup and interactions of matter • Matter made of 12 fermions, 5 bosons, and their antiparticles • What particles form the proton? • Two up quarks and one down quark are the proton’s “valence quarks” • Gluons traveling between these quarks at the speed of light • Give rise to “sea quarks” that diverge from the gluons, then merge back into gluons u g u g q http://www2.slac.stanford.edu/vvc/theory/fundamental.html q g d

5. The Standard Model: Interactions Electron repulsion • Interaction of matter is field interaction • Field interactions approximated by particle interactions • Each interaction mediated by a boson, or force carrier • Gives the type of interaction: strong, EM, or weak • Interaction with more particles less likely to occur • Interactions described by Lagrangian of the particle fields • Particle interaction given by perturbing the Lagrangian around low potential Space Time Potential Low Energy Interactions e- e- Field Strength γ • Ground state normally where fields are zero, but Higgs field different • Higgs field has a vacuum expectation value, so perturb around this value μ- μ-

6. Why Higgs? • Main incentive is Electroweak Unification • Weak force makes the Lagrangian unrenormalizable because of W and Z masses • To fix this: γ, W+, W-, and Z are at high energies mixed together to be new fields W1, W2, W3, and B • To solve mass problem, Higgs field hypothesized, with a nonzero vacuum expectation value (VEV) • Higgs field has a zeroth order coupling to all particles involved in the Electroweak Interaction • Coupling acts as a mass for all of these particles • However, W3 and B mix to form a particle with no Higgs coupling (γ) and an orthogonal particle (Z) • In simplest form, unifies EM and weak forces, not strong force • Also provides a convenient way to introduce and perhaps explain particle mass γ Higgs Interaction Potential Low Energy Interactions W1 W2 W3 B Field Strength W+ W- Z Gauge Particles Physical Particles http://www.particleadventure.org/frameless/masses.html

7. Bosons The Higgs Field Fermions • Higgs boson has a zeroth order interaction, unlike all other particles • VEV means Higgs field interacts with a particle even when the Higgs particle is uninvolved • This constant interaction gives a kind of inertia to particle, difficult to change momentum • Interacts with 12 of the particles • All fermions except the three neutrinos • W± and Z • Does not interact with photon • Does not interact with gluon • Interacts with itself Higgs Z Top W Bottom Charm Tau Strange Muon Down Up Electron Photon νμ ντ νe Neutrinos Gluon Higgs boson Massive particle Same particle

8. The Large Hadron Collider CMS detector ATLAS detector http://cmsinfo.cern.ch/outreach/CMSmedia/CMSphotos.html http://atlas.ch/photos/full-detector.html

9. Detector Cross Section http://atlas.ch/photos/events-general-detection.html

10. Detector Detector Cross Section • The detector of a particle collider must distinguish between the different particles • Has many components designed for this • Still ambiguous, so many quark interactions lumped in to the category of “jet” • Measures many properties of each particle in the collision: • Momentum perpendicular to beam pipe, PT • Angles of momentum • Charge • Energy • Another property of an event is missing transverse energy, MET • Sum of momenta perpendicular to beam pipe should be zero as it is initially • Extra visible particle momentum called MET • MET equals the invisible transverse momentum http://www.particleadventure.org/frameless/end_view.html

11. Finding the Higgs Particle Signal Interaction • Higgs boson interacts primarily with W± particles, so look for events with these • However, W± particles decay before reaching the detector • Cut out events in which the output particles are unlikely to have come from W± • This includes a like-sign dilepton cut • Two leptons of same sign and either another lepton or a pair of jets with opposite sign • Use this cut and other standard cuts to remove many background events and few signal events d d u Proton u Interesting Particles u d u Proton d Keep the events of this kind

12. Additional Cuts • Like sign dilepton cut does not exclude Z and γ events • How to reconstruct the event from the decay products? • Conservation of energy => invariant mass of two decay particles equals mass of mother particle • Check that the invariant mass of opposite sign leptons is not around Mγ or MZ • Could also have events with just a single W decay, rather than three • If there is only one W decay, then there is only one neutrino contributing to MET • Check that the invariant mass of MET and each lepton is not around MW • Invariant mass with MET unmeasurable, momentum along beam pipe unknown • Use similar property called transverse mass Mother particle Mass: M

13. Programs • Several programs are used to simulate collisions in a particle collider • MadGraph: Generates scattering amplitude and evaluates cross section for a specified interaction • Pythia: Generates final states for high energy detector from MadGraph input • PGS: Simple but realistic detector which mimics output of experimental data from Pythia input • ROOT: Code for graphics and analysis of Pythia or PGS data Lepton 1 Lepton 2 Lepton 3

14. Results • Check the programs independently • Check that MadGraph is generating correct scattering amplitudes, by analytical computation • Check that Pythia works using MadGraph and FORTRAN • Check that PGS works by applying realistic cuts • Generate events, signal and background • FORTRAN program and Pythia/PGS • Check that the method is consistent with the results of a CDF paper • “Search for the Wh Production Using High-pT Isolated Like-Sign Dilepton Events in Run-II with 2.7 fb-1” • Paper concentrates on TeVatron rather than LHC • Optimize cuts • Cut many background events and few signal events • Optimize cuts for a Higgs boson with a mass of 160 GeV • Future: MT2 can investigate events with two invisible particles

15. Questions? http://atlas.ch/photos/detector-site-underground.html