Searches for Long-lived Particles at the Tevatron Collider Page: 2 of 15
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2 T. Adams
from stable particles that escape without interacting. However, as is shown in this
review, the multipurpose design of the Tevatron detectors, DO and CDF, allows
a broader range of searches to be conducted, namely for long-lived, but unstable
particles. Many possibilities exist that give rise to unique signatures.
1.1. Theoretical Motivation
A wide variety of theoretical models, including large classes based on supersymmetry
(SUSY), allow for long-lived particles. The simplest SUSY variation is R-parity
violation which allows the lightest supersymmetric particle (LSP) to decay purely
to non-SUSY particles.1 The rate of such decays is generally determined by one or
more couplings. Numerous searches at both the LEP and Tevatron colliders have
looked for (nearly) prompt R-parity violating processes, however they are insensitive
to longer lifetimes where the coupling is small.2,3,45,6,7
Gauge mediated supersymmetry breaking (GMSB) models, in which the su-
persymmetry breaking is mediated by gauge fields other than gravity, generally
have a light (MeV) gravitino LSP and all other supersymmetric particles decay via
cascades to the next-to-lightest supersymmetric particle (NLSP). The coupling to
gravity can determine how quickly the NLSP decays to its non-SUSY partner and
the gravitino. If the symmetry breaking scale is small (larger coupling), the lifetime
is short. Conversely, if the scale is large, this results in a long-lived NLSP that can
decay either within the detector or beyond it.8 Favored NLSP candidates include
the neutralino (xy) or the stau (T).
Long lifetimes can also arise from small mass splittings between the NLSP and
the LSP.9 If the mass difference (MNLSP - MLSP) is small enough, phase space
suppression will impede the decay of the NLSP. One particular model of interest
here is inspired by anomaly mediated supersymmetry breaking (AMSB) with a
chargino NLSP and a mass splitting < 150 MeV.
Split supersymmetry, where SUSY scalars are much heavier than SUSY fermions,
can suppress the decay rate of gluinos giving them long lifetimes."," They may live
long enough to hadronize into "R-hadrons," colorless, bound states of the gluino,
quarks, and gluons.12 Nuclear interactions can cause some neutral R-hadrons to
become charged, leading to a number of interesting signatures within the detector.13
There has been recent discussions of a class of models referred to as "hidden
valley" models which can yield new phenomena resulting in long-lived particles
through reduced coupling to standard model (SM) particles.141516 The hidden
valley structures are very general and can be added to many beyond the standard
model (BSM) theories. Most interestingly, they can give rise to significantly different
phenomenology from that commonly associated with the original BSM model.14 An
example is a light Higgs boson that decays to two neutral, long-lived particles that
further decay to b-jets.15 These "hidden valley" models provide a timely cautionary
tale: if one only looks where the light shines brightest, one risks missing a possible
spectacular prize in the shadows.
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Adams, T. Searches for Long-lived Particles at the Tevatron Collider, article, February 1, 2008; Batavia, Illinois. (https://digital.library.unt.edu/ark:/67531/metadc902662/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.