Download 2009S-FindingHiggs

Higgs in the Large Hadron Collider
Joe Mitchell
Advisor: 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-oncern.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
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
+2/3
-1/3
-1
0
Charge (e)
http://www2.slac.stanford.edu/vvc/theory/fundamental.html
Neutron
Proton
•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
q
g
d
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
•Ground state normally where fields Interaction
Bosons
are zero, but Higgs field different
Acts on
•Higgs field has a vacuum expectation
Strength
value, so perturb around this value
Macroscopic
Role
Electron repulsion
e-
e-
Space
γ
Time
μ-
μ-
Potential
Low Energy
Interactions
Field Strength
EM
Weak
Strong
γ
W+, W-, Z
G
Charged
All
Quarks
Mid
Weakest
Strongest
Organizes
Structures
Many
Decays
Binds
Nuclei
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
Gauge Particles
W 1 W2
W3 B
γ Z
W+ WPhysical Particles
Higgs Interaction
Potential
Low Energy
Interactions
Field Strength
•Also provides a convenient way to
introduce and perhaps explain particle mass
http://www.particleadventure.org/frameless/masses.html
The Higgs Field
Fermions
•Higgs boson has a zeroth order
interaction, unlike all other particles
•All fermions except the three neutrinos
•W± and Z
•Does not interact with photon
•Does not interact with gluon
•Interacts with itself
Charm
Tau
Strange
Muon
Down
Up
Electron
νμ
Neutrinos
Massive particle
Same particle
Higgs
Bottom W
νe
Higgs boson
Z
Top
•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
Bosons
Photon
ντ
Gluon
The Large Hadron Collider
CMS detector
http://cmsinfo.cern.ch/outreach/CMSmedia/CMSphotos.html
ATLAS detector
http://atlas.ch/photos/full-detector.html
http://atlas.ch/photos/events-general-detection.html
Detector Cross Section
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
http://www.particleadventure.org/frameless/end_view.html
•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
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
u
Interesting
Particles
d
d
u
Proton
d
Keep the events of this kind
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
Mother particle
•Could also have events with just a single W Mass: M
M12  E1  E2  M
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
E1
E2
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