Higgs in the Large Hadron Collider

Joe MitchellAdvisor: Dr. Chung Kao

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

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

The Standard Model: Matter•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

http://www2.slac.stanford.edu/vvc/theory/fundamental.html

Fermions Up-Like Quarks

Down-Like Quarks

Charged Lepton

Neutrino

Generation 1 u d e- νe

Generation 2 c s μ- νμ

Generation 3 t b τ- ντ

Charge (e) +2/3 -1/3 -1 0

u

u

d

g

g

g

q

q

Proton Neutron

The Standard Model: Interactions•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

Interaction EM Weak Strong

Bosons γ W+, W-, Z G

Acts on Charged All Quarks

Strength Mid Weakest Strongest

MacroscopicRole

Organizes Structures

Many Decays

Binds Nuclei

•Ground state normally where fields are zero, but Higgs field different•Higgs field has a vacuum expectation value, so perturb around this value

e- e-

μ-μ-

γ

Electron repulsion

Time

Space

Potential

Field Strength

Low EnergyInteractions

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

W3W1 W2 B

W+ W- Z

Gauge Particles

Physical Particles

Higgs Interaction

Potential

Field Strength

Low EnergyInteractions

http://www.particleadventure.org/frameless/masses.html

γ

The Higgs Field•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

Massive particle

Higgs boson

Same particle

Up

Down Muon

Electron

Strange

CharmBottom W

ZHiggs

Top

Tau

νμντνe

Photon

Gluon

Fermions Bosons

Neutrinos

The Large Hadron Collider

CMS detector ATLAS detectorhttp://cmsinfo.cern.ch/outreach/CMSmedia/CMSphotos.html http://atlas.ch/photos/full-detector.html

Detector Cross Section

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Detector•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

Detector Cross Section

Finding the Higgs Particle•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

Signal Interaction

Keep the events of this kind

d

uu

d

du

u

d

Proton

Proton

InterestingParticles

Mother particleMass: M

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

1E

2EMEEM 2112

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

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

Questions?

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