 Search for heavy metastable particles decaying to quark pairs in
proton antiproton collisions at s**1/2 =1.96 TeV.
Shawn Kwang
Mel Shochet
Enrico Fermi Institute


Abtract

We report
on an analysis searching for heavy particles that are metastable
and decay into quark pairs with a macroscopic lifetime (c*tau ~ 1 cm)
at a displaced vertex, which in term decay into quark pairs. We
use a data driven background approach, where we build
probability density functions to model Standard Model secondary
vertices from known processes in order to estimate the
background contribution from the Standard Model. No
statistically significant excess is observed above the
background. Limits on the production cross section in a Hidden
Valley benchmark phenomenology are set for various Higgs boson
masses as well as metastable particle masses and lifetimes.

Hidden Valley Model


This analysis serches for
metastable particles decaying at a displaced vertex. However, a
phenomenological theory is useful for a benchmark. The Hidden Valley
(HV) phenomenology provides a framework in which we can generate signal
MC, search for discriminants, optimize our search, and compare results.

See the Description of the analysis for diagrams of
Hidden Valley decay

Signal Search
Results

A counting experiment is
performed using the variables developed for this search. The resulting number
of expected signal MC, background estimate, and number of observed
events is presented below. Two searches were performed, a low HV mass
search and a high HV mass search.

The background estimate is the
mean of a distribution of the number of events that pass the analysis
cuts, performed manycal_dis/ (10,000) times. The uncertainties on the
background estimate, as well as systematic uncertianties on the signal
MC are shown the table below.



Pvalues and Limit
Calcuation

We calculate
the pvalues for this search. No statistically significant excess is
observed. Since we do not observe a statistically significant excess we
proceed to set a limit on the production cross section times branching
ratio of the Hidden Valley model for the particular masses and lifetimes
we studied. The counting experiment was performed with a small discrete
number of events, where the background estimate is less than one. Thus
the expected limit can only fluctuate up (from 0). The result is that
the negative sigma expected limits will be identical to the median
limit.
Graphs of the limit calculations are shown below.



Observed and Expected limit with +1 & +2 sigma bands for signal
MC with HV mass 20 GeV/c^2.
Download: eps png svg

Observed and Expected limit with +1 & +2 sigma bands for signal
MC with HV mass 40 GeV/c^2.
Download: eps png svg

Observed and Expected limit with +1 & +2 sigma bands for signal
MC with HV mass 40 GeV/c^2.
Download: eps png svg 
Observed and Expected limit with +1 & 2 sigma bands for signal
MC for differing HV particle lifetimes.
Download: eps png svg



