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-boso
n.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/l
  hc_atlas.swf

4
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 protons
  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/fundament
al.html
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
Field Strength

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

6
Why Higgs?

• Main incentive is Electroweak Unification
• Weak force makes the Lagrangian unrenormalizable
  because of W and Z masses
• To fix this ?, 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 (?) 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

?
Higgs Interaction
Potential
Low Energy Interactions
Field Strength
http//www.particleadventure.org/frameless/masses.
html
7
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

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.ht
ml
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_vie
w.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
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 ?
  events
• How to reconstruct the event from the decay
  products?
• Conservation of energy gt invariant mass of two
  decay particles equals mass of mother particle
• Check that the invariant mass of opposite sign
  leptons is not around M? 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.h
tml