4,205 research outputs found
Report of the Snowmass Subgroup on Extra Dimensions
In this report we summarize the work performed at Snowmass 2001 on the
physics of extra dimensions. We divide these analyses into the following
classes: searches for extra dimensional phenomena, identification of specific
new physics scenarios, studies of black hole production and non-commutative
QED.Comment: Contributed to APS / DPF / DPB Summer Study on the Future of Particle
Physics (Snowmass 2001), Snowmass, Colorado, 30 Jun - 21 Jul 200
Revealing the large extra dimension effective interaction at an e^+e^- collider with polarized beams
Contactlike nonstandard effective interactions can be revealed through
deviations of observables from the Standard Model predictions. If such
deviations were observed, the relevant source should be identified among the
possible models that could explain them. We estimate the expected
discrimination reach on the ADD model of gravity in large compactified extra
dimensions, against the compositeness-inspired four-fermion contact
interaction. As basic observables we take the differential cross sections for
fermion-pair production at a 0.5-1 TeV electron-positron linear collider with
both beams longitudinally polarized. For the four-fermion contact interaction,
we assume a general linear combination of the individual models with definite
chiralities, and arbitrary coupling constants. Accordingly, the estimated
identification reach on the ADD scenario can be considered as
`model-independent'. We emphasize the substantial role of beams polarization in
enhancing the identification sensitivity.Comment: 14 pages, 2 figure
Competitive Exclusion and Limiting Similarity: A Unified Theory
Robustness of coexistence against changes of parameters is investigated in a model-independent manner through analyzing the feed-back loop of population regulation. We define coexistence as fixed point of the community dynamics with no population having zero size. It is demonstrated that the parameter range allowing coexistence shrinks and disappears when the Jacobian of the dynamics decreases to zero. A general notion of regulating factors/variables is introduced. For each population, its 'impact' and 'sensitivity' niches a re defined as the differential impact on, and the differential sensitivity towards, the regulating variables, respectively. Either similarity of the impact niches, or similarity of the sensitivity niches, result in a small Jacobian and in a reduced likelihood of coexistence. For the case of a resource continuum, this result reduces to the usual "limited niches overlap" picture for both kinds of niche. As an extension of these ideas to the coexistence of infinitely many species, we demonstrate that Roughgarden's example for coexistence of a 'continuum' of populations is structurally unstable
Unique Identification of Graviton Exchange Effects in e^+ e^- Collisions
Many types of new physics can lead to contact interaction-like modifications
in e^+ e^- processes below direct production threshold. We examine the
possibility of uniquely identifying the effects of graviton exchange, which are
anticipated in many extra dimensional theories, from amongst this large set of
models by using the moments of the angular distribution of the final state
particles. In the case of the e^+ e^- --> f bar{f} process we demonstrate that
this technique allows for the unique identification of the graviton exchange
signature at the 5 sigma level for mass scales as high as 6 sqrt(s). The
extension of this method to the e^+ e^- --> W^+ W^- process is also discussed.Comment: 21 pages, 3 figs, LaTe
Transverse Polarization Signatures of Extra Dimensions at Linear Colliders
If significant longitudinal polarization of both the electrons and positrons
becomes feasible at a future linear collider(LC), it may be possible to use
spin rotators to produce transversely polarized beams. Using the transverse
polarization of both beams, new azimuthal spin asymmetries can be formed which
will be sensitive probes for new physics beyond the Standard Model. Here we
demonstrate that these asymmetries are particularly sensitive to the exchange
of Kaluza-Klein towers of gravitons, or other spin-2 fields, that are predicted
to exist in higher dimensional theories which address the hierarchy problem.
These new asymmetries are shown to be able to extend the search reach for such
new physics by more than a factor of two, provide an additional tool for
isolating the signatures for spin-2 exchange up to mass scales in excess of
, and can be used to help differentiate among the proposed solutions
to the hierarchy problem below the production threshold for new particles.Comment: 22 pages, 6 figs. LaTe
Tests of model of color reconnection and a search for glueballs using gluon jets with a rapidity gap
Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from
hadronic Z0 decay events produced in e+e- annihilations. A subsample of these
jets is identified which exhibits a large gap in the rapidity distribution of
particles within the jet. After imposing the requirement of a rapidity gap, the
gluon jet purity is 86%. These jets are observed to demonstrate a high degree
of sensitivity to the presence of color reconnection, i.e. higher order QCD
processes affecting the underlying color structure. We use our data to test
three QCD models which include a simulation of color reconnection: one in the
Ariadne Monte Carlo, one in the Herwig Monte Carlo, and the other by Rathsman
in the Pythia Monte Carlo. We find the Rathsman and Ariadne color reconnection
models can describe our gluon jet measurements only if very large values are
used for the cutoff parameters which serve to terminate the parton showers, and
that the description of inclusive Z0 data is significantly degraded in this
case. We conclude that color reconnection as implemented by these two models is
disfavored. The signal from the Herwig color reconnection model is less clear
and we do not obtain a definite conclusion concerning this model. In a separate
study, we follow recent theoretical suggestions and search for glueball-like
objects in the leading part of the gluon jets. No clear evidence is observed
for these objects.Comment: 42 pages, 18 figure
Scaling violations of quark and gluon jet fragmentation functions in e+e- annihilations at sqrt(s) = 91.2 and 183-209 GeV
Flavour inclusive, udsc and b fragmentation functions in unbiased jets, and
flavour inclusive, udsc, b and gluon fragmentation functions in biased jets are
measured in e+e- annihilations from data collected at centre-of-mass energies
of 91.2, and 183-209 GeV with the OPAL detector at LEP. The unbiased jets are
defined by hemispheres of inclusive hadronic events, while the biased jet
measurements are based on three-jet events selected with jet algorithms.
Several methods are employed to extract the fragmentation functions over a wide
range of scales. Possible biases are studied in the results are obtained. The
fragmentation functions are compared to results from lower energy e+e-
experiments and with earlier LEP measurements and are found to be consistent.
Scaling violations are observed and are found to be stronger for the
fragmentation functions of gluon jets than for those of quarks. The measured
fragmentation functions are compared to three recent theoretical
next-to-leading order calculations and to the predictions of three Monte Carlo
event generators. While the Monte Carlo models are in good agreement with the
data, the theoretical predictions fail to describe the full set of results, in
particular the b and gluon jet measurements.Comment: 46 pages, 17 figures, Submitted to Eur. Phys J.
A study of charm production in beauty decays with the OPAL detector at LEP
Using an inclusive method, BR(b -> D\bar{D}X) has been measured in hadronic
Z^0 decays with the OPAL detector at LEP. The impact parameter significance of
tracks opposite tagged b-jets is used to differentiate b -> D\bar{D}X decays
from other decays. Using this result, the average number of charm and
anti-charm quarks produced per beauty quark decay, n_c, is determined.Comment: 20 pages, 4 figure
Search for the Standard Model Higgs Boson with the OPAL Detector at LEP
This paper summarises the search for the Standard Model Higgs boson in e+e-
collisions at centre-of-mass energies up to 209 GeV performed by the OPAL
Collaboration at LEP. The consistency of the data with the background
hypothesis and various Higgs boson mass hypotheses is examined. No indication
of a signal is found in the data and a lower bound of 112.7GeV/C^2 is obtained
on the mass of the Standard Model Higgs boson at the 95% CL.Comment: 51 pages, 21 figure
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