Exploring new physics through contact interactions in lepton pair production at a linear collider Page: 1 of 6
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Exploring New Physics Through Contact Interactions in Lepton Pair Production
at a Linear Collider
CERN, CH-1211 Geneva 23, Switzerland
Theory Group, Lawrence Berkeley National Lab, Berkeley, CA 94720, USA
(Dated: July 10, 2004)
If a contact interaction type correction to a Standard Model process is observed, studying its
detailed properties can provide information on the fundamental physics responsible for it. Assuming
that such a correction has been observed in lepton pair production at a 500 GeV - 1 TeV linear
collider, we consider a few possible models that could explain it, such as theories with large and TeV-
scale extra dimensions and models with lepton compositeness. We show that using the measured
O cross-sections and angular distributions, these models can be distinguished with a high degree of
All known solutions to the gauge hierarchy problem of the Standard Model (SM) require the appearance of
Cl new particles at energy scales around 1 TeV. It is not guaranteed, however, that these new particles can be
produced directly at the proposed 500 GeV linear collider (LC). Only for supersymmetric theories are there
strong arguments that at least some superpartners should be kinematically accessible at such a collider . In
the case of composite Higgs models and models with extra dimensions, the situation is far less certain. It is
possible that all the new states predicted in these theories are too heavy and cannot appear in the final state
at a 500 GeV LC. In fact, for models with large extra dimensions , current experimental constraints most
likely rule out the possibility that string Regge excitations could be lighter than 500 GeV. In this case, the
only direct effect of extra dimensions would be the enhanced rate of events with missing energy due to graviton
emission. These events, however, provide only very limited amount of information about the fundamental
theory. Moreover, this signature could be mimicked by gravitino emission processes in certain supersymmetric
$L models, so one would need additional handles to disentangle the underlying physics . In this situation, it is
i important to look for indirect effects of new physics, that is, the effects of new heavy particles appearing as
q) virtual states. For example, processes such as Bhabha scattering or other lepton pair production,
e e -e ee p p+ ,j+ (1)
could receive an additional contribution from the exchange of a heavy state X. Because such additional con-
tributions come from short-distance physics and do not possess poles in the accessible range of any kinematic
Ct variables, they are referred to as contact interactions. By carefully examining the total cross section and angular
distribution of these processes, it should be possible to not only find deviations from the Standard Model, but
also gain some information about the nature of the state X, such as its spin and couplings.
In this report, we will assume that the cross section of process (1) was found to deviate from the Standard
Model prediction. We will then consider several possible explanations for this deviation, such as models with
lepton substructure, models with TeV-scale strings, and models in which gauge fields can propagate in the extra
dimensions. Our main goal is to determine how well one can discriminate between these possibilities, given the
measurement of the total cross section and angular distributions of the final-state particles.
II. MODELS WITH CONTACT INTERACTIONS
The unpolarised cross section formula for Bhabha scattering can be written in the form
dcos ra2 [v2(ALL12 +ARR12)+ 2t2ARL, 2 +2s2 ARLJt2] , (2)
*Gabriella.Pasztor@cern.ch; On leave of absence from KFKI RMKI, Budapest, Hungary.
tmeperelstein A lbl.gov
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Perelstein, Maxim & Pasztor, Gabriella. Exploring new physics through contact interactions in lepton pair production at a linear collider, article, December 4, 2001; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc784665/m1/1/: accessed February 17, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.