Andrea Linville
Office of Science, SULI Program 2009
Stanford Linear Accelerator
August 13, 2009
 Objective
 Standard
Model
 What
is SUSY?
 What
will we calculate?
 Results
 The
Future

Use parton-model methods to predict
the stop squark production crosssection in proton-proton collisions at
LHC energies.



Fermions (spin ½)
Bosons (integer spin)
= force mediators
Elusive Higgs boson
◦ Generates masses of
leptons and W, Z

SM is incomplete
◦ Dark matter?
◦ Hierarchy problem?
◦ Matter/antimatter
imbalance?

Each SM particle has a
superpartner
◦ difference: ½ unit of spin

SM fermions → SUSY bosons

SM bosons → SUSY fermions


SUSY is a broken
symmetry
Sparticle masses depend
on SUSY breaking model
(unknown)
.
.
 .Parton Distribution Functions (PDFs)
.
.
Qμ = muon charge
Qe = electron charge
gL = xw – ½
g R = xw
(xw = weak mixing angle)
α = fine structure constant
GF = Fermi coupling constant
MZ = mass of Z boson
ΓZ = decay width of Z boson

Modifications made to equations:
◦ Replace electron charge with quark charge
◦ Separate components for:
 u, c, t quarks (+2/3 charge)
 d, s, b quarks (-1/3 charge)
◦ Sum over quark flavors


Partons: quasi-free pointlike structures that make up
hadrons
PDFs describe the probability density for finding a
parton with a given fraction of the total momentum

Convenient definitions:
◦
= Q: scattering energy/invariant mass of the products
◦
: proton center-of-mass energy (“machine” energy)

The equation we integrate:

Monte Carlo integration algorithm

3 possible initial states:
◦.
◦.
◦.
◦ Each initial state needs a separate
cross-section equation…

but…
where
αs = strong coupling constant
m1, m2 = masses of produced squarks
s = scattering energy

was calculated to be 61.6 fb

Reminder: 1 barn = 10-24 cm2

Invariant dimuon mass: 500 GeV

This is the order of magnitude expected.
∘ Greater contribution from sea
quarks as scattering energy increases
∘ At low energies, the sea quarks
are confined within hadrons
∘ The probability densities of b and t
were exactly zero at these energies





Resonance at 91
GeV ±0.5, ≈ MZ
(91.2 GeV)
Expected: 1fb at
M =500 GeV
Calculated: 0.96 fb
Data taken every
1 GeV
Machine energy:
14 TeV (LHC)



Maximum crosssection: 0.27 nb
More difficult to
produce stops with
increasing stop mass
We examined only
leading order (LO)
Feynman diagrams



can be used to establish a
lower bound on stop mass, once we have
an experimental bound on σ
Similarly, if stops are observed, σ can
determine stop particle masses
Our results are useful for:
◦ Interpreting data
◦ Preparing new experimental searches for
SUSY

My mentor, JoAnne Hewett

Tom Rizzo

Michael Peskin

Theory graduate students: John Conley,
Randy Cotta, and Jamie Gainer


S. Dawson, E. Eichten, and C. Quigg. “Search for supersymmetric
particles in hadron-hadron collisions,” Physical Review D, vol. 31,
no. 7, pp. 1581-1637, 1 April 1985.
W. Beenakker, M. Krämer, T. Plehn, M. Spira, and P.M. Zerwas.
“Stop Production at Hadron Colliders,” Nuclear Physics B, vol.
515, pp. 3-14, 1998.

M. Schott. “Z Boson Production at LHC with First Data,” The 2007

Elsevier publishing, Physics Letters B, vol. 667, Issues 1-5, 18
September 2008.
Europhysics Conference on High Energy Physics Journal of
Physics, Conference Series 110, 2008.