1,809 research outputs found
Classification of Flipped SU(5) Heterotic-String Vacua
We extend the classification of the free fermionic heterotic-string vacua to
models in which the SO(10) GUT symmetry at the string scale is broken to the
flipped SU(5) subgroup. In our classification method, the set of basis vectors
defined by the boundary conditions which are assigned to the free fermions is
fixed and the enumeration of the string vacua is obtained in terms of the
Generalised GSO (GGSO) projection coefficients entering the one-loop partition
function. We derive algebraic expressions for the GGSO projections for all the
physical states appearing in the sectors generated by the set of basis vectors.
This enables the analysis of the entire string spectrum to be programmed in to
a computer code therefore, we performed a statistical sampling in the space of
2^{44} (approximately 10^{13}) flipped vacua and scanned up to 10^{12}
GGSO configurations. For that purpose, two independent codes were developed
based on JAVA and FORTRAN95. All the results presented here are confirmed by
the two independent routines. Contrary to the corresponding Pati-Salam
classification, we do not find exophobic flipped SU(5) vacua with an odd number
of generations. We study the structure of exotic states appearing in the three
generation models that additionally contain a viable Higgs spectrum. Moreover,
we demonstrate the existence of models in which all the exotic states are
confined by a hidden sector non-Abelian gauge symmetry as well as models that
may admit the racetrack mechanism.Comment: Minor changes (Version 2) - 51 pages - 3 figures - Added
acknowledgement
Efficient Kidney Exchange: Coincidence of Wants in a Structured Market
Patients needing kidney transplants may have willing donors who cannot donate to them because of blood or tissue incompatibility. Incompatible patient-donor pairs can exchange donor kidneys with other such pairs. The situation facing such pairs resembles models of the “double coincidence of wants,” and relatively few exchanges have been consummated by decentralized means. As the population of available patient-donor pairs grows, the frequency with which exchanges can be arranged will depend in part on how exchanges are organized. We study the potential frequency of exchanges as a function of the number of patient-donor pairs, and the size of the largest feasible exchange. Developing infrastructure to identify and perform 3-way as well as 2-way exchanges will have a substantial effect on the number of transplants, and will help the most vulnerable patients. Larger than 3- way exchanges have much smaller impact. Larger populations of patient- donor pairs increase the percentage of patients of all kinds who can find exchanges.
Kidney Exchange
Most transplanted kidneys are from cadavers, but there are also substantial numbers of transplants from live donors. Recently, there have started to be kidney exchanges involving two donor-patient pairs such that each donor cannot give a kidney to the intended recipient because of immunological incompatibility, but each patient can receive a kidney from the other donor. Exchanges are also made in which a donor-patient pair makes a donation to someone on the queue for a cadaver kidney, in return for the patient in the pair receiving the highest priority for a compatible cadaver kidney when one becomes available. We explore how such exchanges can be arranged efficiently and incentive compatibly. The problem resembles some of the housing' problems studied in the mechanism design literature for indivisible goods, with the novel feature that while live donor kidneys can be assigned simultaneously, the cadaver kidneys must be transplanted immediately upon becoming available. In addition to studying the theoretical properties of the design we propose for a kidney exchange, we present simulation results suggesting that the welfare gains would be substantial, both in increased number of feasible live donation transplants, and in improved match quality of transplanted kidneys.
Pairwise Kidney Exchange
In connection with an earlier paper on the exchange of live donor kidneys (Roth, S”nmez, and šnver 2004) the authors entered into discussions with New England transplant surgeons and their colleagues in the transplant community, aimed at implementing a Kidney Exchange program. In the course of those discussions it became clear that a likely first step will be to implement pairwise exchanges, between just two patient-donor pairs, as these are logistically simpler than exchanges involving more than two pairs. Furthermore, the experience of these surgeons suggests to them that patient and surgeon preferences over kidneys should be 0-1, i.e. that patients and surgeons should be indifferent among kidneys from healthy donors whose kidneys are compatible with the patient. This is because, in the United States, transplants of compatible live kidneys have about equal graft survival `robabilities, regardless of the closeness of tissue types between patient and dOnor (unless there is a rare perfect match). In the present paper we show that, although thd pairwise constraint eliminates some potential exchanges, there is a wide class of constrained-efficient mechanisms 4hat are strategy-proof when patient-donor pairs and surgeons have 0-1 preferences. This class of meahanisms includes deterministic mechanisms that would accomodate the kinds of priority setting that organ banks currently use for the allocation of cadaver organs, as well as stochastic mechanisms that allow considerations of distributive justice to be addressed.
Efficient Kidney Exchange: Coincidence of Wants in a Structured Market
Patients needing kidney transplants may have willing donors who cannot donate to them because of blood or tissue incompatibility. Incompatible patient-donor pairs can exchange donor kidneys with other such pairs. The situation facing such pairs resembles models of the "double coincidence of wants," and relatively few exchanges have been consummated by decentralized means. As the population of available patient-donor pairs grows, the frequency with which exchanges can be arranged will depend in part on how exchanges are organized. We study the potential frequency of exchanges as a function of the number of patient-donor pairs, and the size of the largest feasible exchange. Developing infrastructure to identify and perform 3-way as well as 2-way exchanges will have a substantial effect on the number of transplants, and will help the most vulnerable patients. Larger than 3-way exchanges have much smaller impact. Larger populations of patient-donor pairs increase the percentage of patients of all kinds who can find exchanges.
Non-Tachyonic Semi-Realistic Non-Supersymmetric Heterotic String Vacua
The heterotic--string models in the free fermionic formulation gave rise to
some of the most realistic string models to date, which possess N=1 spacetime
supersymmetry. Lack of evidence for supersymmetry at the LHC instigated recent
interest in non-supersymmetric heterotic-string vacua. We explore what may be
learned in this context from the quasi--realistic free fermionic models. We
show that constructions with a low number of families give rise to
proliferation of a priori tachyon producing sectors, compared to the
non--realistic examples, which typically may contain only one such sector. The
reason being that in the realistic cases the internal six dimensional space is
fragmented into smaller units. We present one example of a quasi--realistic,
non--supersymmetric, non--tachyonic, heterotic--string vacuum and compare the
structure of its massless spectrum to the corresponding supersymmetric vacuum.
While in some sectors supersymmetry is broken explicitly, i.e. the bosonic and
fermionic sectors produce massless and massive states, other sectors, and in
particular those leading to the chiral families, continue to exhibit fermi-bose
degeneracy. In these sectors the massless spectrum, as compared to the
supersymmetric cases, will only differ in some local or global U(1) charges. We
discuss the conditions for obtaining at the massless level in these
models. Our example model contains an anomalous U(1) symmetry, which generates
a tadpole diagram at one loop-order in string perturbation theory. We speculate
that this tadpole diagram may cancel the corresponding diagram generated by the
one-loop non-vanishing vacuum energy and that in this respect the
supersymmetric and non-supersymmetric vacua should be regarded on equal
footing. Finally we discuss vacua that contain two supersymmetry generating
sectors.Comment: 31 pages. 10 tables. Minor corrections. Tables are amended. Published
versio
Kidney Exchange
Most transplanted kidneys are from cadavers, but there are also substantial numbers of transplants from live donors. Recently, there have started to be kidney exchanges involving two donor-patient pairs such that each donor cannot give a kidney to the intended recipient because of immunological incompatibility, but each patient can receive a kidney from the other donor. Exchanges are also made in which a donor- patient pair makes a donation to someone on the queue for a cadaver kidney, in return for the patient in the pair receiving the highest priority for a compatible cadaver kidney when one becomes available. We explore how such exchanges can be arranged efficiently and incentive compatibly. The problem resembles some of the "housing" problems studied in the mechanism design literature for indivisible goods, with the novel feature that while live donor kidneys can be assigned simultaneously, the cadaver kidneys must be transplanted immediately upon becoming available. In addition to studying the theoretical properties of the design we propose for a kidney exchange, we present simulation results suggesting that the welfare gains would be substantial, both in increased number of feasible live donation transplants, and in improved match quality of transplanted kidneys.
Changing the Boston School Choice Mechanism
In July 2005 the Boston School Committee voted to replace the existing Boston school choice mechanism with a deferred acceptance mechanism that simplifies the strategic choices facing parents. This paper presents the empirical case against the previous Boston mechanism, a priority matching mechanism, and the case in favor of the change to a strategy-proof mechanism. Using detailed records on student choices and assignments, we present evidence both of sophisticated strategic behavior among some parents, and of unsophisticated strategic behavior by others. We find evidence that some parents pay close attention to the capacity constraints of different schools, while others appear not to. In particular, we show that many unassigned students could have been assigned to one of their stated choices with a different strategy under the current mechanism. This interaction between sophisticated and unsophisticated players identifies a new rationale for strategy-proof mechanisms based on fairness, and was a critical argument in Boston's decision to change the mechanism. We then discuss the considerations that led to the adoption of a deferred acceptance mechanism as opposed to the (also strategy-proof) top trading cycles mechanism.
Classification of SU(4) X SU(2) X U(1) Heterotic-String Models
The free fermionic construction of the heterotic string in four dimensions
produced a large space of three generation models with the underlying
embedding of the Standard Model states. The symmetry is broken to a
subgroup directly at the string scale. Over the past few years free fermionic
models with the Pati-Salam and flipped subgroups have been classified.
In this paper we extend this classification program to models in which the
symmetry is broken at the string level to the (SU421) subgroup. The subspace of free fermionic models
that we consider corresponds to symmetric orbifolds. We provide a general argument that shows that this
class of SU421 free fermionic models cannot produce viable three generation
models.Comment: Publication Version (Notes: 16 Pages & References added
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