35 research outputs found
Violations of Bjorken scaling in inclusive e+e- annihilation
We discuss the application of renormalization-group techniques to inclusive e+e- annihilation. It is shown by a modest extension of Mueller's techniques that annihilation structure functions have a behavior completely analogous to electroproduction structure functions: Their moments scale for large virtual photon mass, and this scaling is described by "anomalous dimensions" which have a singularity structure and general form very similar to the usual anomalous dimension, though there is no simple relation between the two. We show how information about the structure functions can be deduced from the moments and how, in appropriate limits, deviations from Bjorken scaling can be interpreted in terms of an underlying field theory
Quantifying selection in immune receptor repertoires
The efficient recognition of pathogens by the adaptive immune system relies
on the diversity of receptors displayed at the surface of immune cells. T-cell
receptor diversity results from an initial random DNA editing process, called
VDJ recombination, followed by functional selection of cells according to the
interaction of their surface receptors with self and foreign antigenic
peptides. To quantify the effect of selection on the highly variable elements
of the receptor, we apply a probabilistic maximum likelihood approach to the
analysis of high-throughput sequence data from the -chain of human
T-cell receptors. We quantify selection factors for V and J gene choice, and
for the length and amino-acid composition of the variable region. Our approach
is necessary to disentangle the effects of selection from biases inherent in
the recombination process. Inferred selection factors differ little between
donors, or between naive and memory repertoires. The number of sequences shared
between donors is well-predicted by the model, indicating a purely stochastic
origin of such "public" sequences. We find a significant correlation between
biases induced by VDJ recombination and our inferred selection factors,
together with a reduction of diversity during selection. Both effects suggest
that natural selection acting on the recombination process has anticipated the
selection pressures experienced during somatic evolution
Model calculations of electroproduction and inclusive annihilation cross sections
We have studied "deep-inelastic electroproduction" and "inclusive e+e- annihilation" in massless φ4 theory in the ladder approximation. The relevant Bethe-Salpeter-type equations can be completely solved and the physically important asymptotic limits are studied. The behavior of the moments of the structure functions is analyzed, and the "anomalous dimensions" which govern the asymptotic power behavior of the moments in the two cases are found. The anomalous dimensions are quite different in general, but are simply related in the weak-coupling limit
OLGA: fast computation of generation probabilities of B- and T-cell receptor amino acid sequences and motifs
Motivation: High-throughput sequencing of large immune repertoires has
enabled the development of methods to predict the probability of generation by
V(D)J recombination of T- and B-cell receptors of any specific nucleotide
sequence. These generation probabilities are very non-homogeneous, ranging over
20 orders of magnitude in real repertoires. Since the function of a receptor
really depends on its protein sequence, it is important to be able to predict
this probability of generation at the amino acid level. However, brute-force
summation over all the nucleotide sequences with the correct amino acid
translation is computationally intractable. The purpose of this paper is to
present a solution to this problem.
Results: We use dynamic programming to construct an efficient and flexible
algorithm, called OLGA (Optimized Likelihood estimate of immunoGlobulin
Amino-acid sequences), for calculating the probability of generating a given
CDR3 amino acid sequence or motif, with or without V/J restriction, as a result
of V(D)J recombination in B or T cells. We apply it to databases of
epitope-specific T-cell receptors to evaluate the probability that a typical
human subject will possess T cells responsive to specific disease-associated
epitopes. The model prediction shows an excellent agreement with published
data. We suggest that OLGA may be a useful tool to guide vaccine design.
Availability: Source code is available at https://github.com/zsethna/OLG
Towards the large N limit of pure N=1 super Yang Mills
We find the gravity solution corresponding to a large number of NS or D
fivebranes wrapped on a two sphere so that we have pure super
Yang-Mills in the IR. The supergravity solution is smooth, it shows confinement
and it breaks the chiral symmetry in the appropriate way. When the
gravity approximation is valid the masses of glueballs are comparable to the
masses of Kaluza Klein states on the fivebrane, but if we could quantize
strings on this background it looks like we should be able to decouple the KK
states.Comment: latex, 12 pages. v2: Reference to a forthcoming paper added. v3
references and clarifications adde
General Composite Higgs Models
We construct a general class of pseudo-Goldstone composite Higgs models,
within the minimal SO(5)/SO(4) coset structure, that are not necessarily of
moose-type. We characterize the main properties these models should have in
order to give rise to a Higgs mass around 125 GeV. We assume the existence of
relatively light and weakly coupled spin 1 and 1/2 resonances. In absence of a
symmetry principle, we introduce the Minimal Higgs Potential (MHP) hypothesis:
the Higgs potential is assumed to be one-loop dominated by the SM fields and
the above resonances, with a contribution that is made calculable by imposing
suitable generalizations of the first and second Weinberg sum rules. We show
that a 125 GeV Higgs requires light, often sub-TeV, fermion resonances. Their
presence can also be important for the models to successfully pass the
electroweak precision tests. Interestingly enough, the latter can also be
passed by models with a heavy Higgs around 320 GeV. The composite Higgs models
of the moose-type considered in the literature can be seen as particular limits
of our class of models.Comment: 51 pages, 12 figures, 5 appendices; v2: Corrected estimates of \delta
g_b in appendix B, references fixed, several minor improvements; v3: minor
improvements, to appear in JHE
The 4D Composite Higgs
We propose a four dimensional description of Composite Higgs Models which
represents a complete framework for the physics of the Higgs as a
pseudo-Nambu-Goldstone boson. Our setup captures all the relevant features of
5D models and more in general of composite Higgs models with partial
compositeness. We focus on the minimal scenario where we include a single
multiplet of resonances of the composite sector, as these will be the only
degrees of freedom which might be accessible at the LHC. This turns out to be
sufficient to compute the effective potential and derive phenomenological
consequences of the theory. Moreover our simplified approach is well adapted to
simulate these models at the LHC. We also consider the impact of non-minimal
terms in the effective lagrangian which do not descend from a 5D theory and
could be of phenomenological relevance, for example contributing to the
S-parameter.Comment: 28 pages, 7 figures. v2) minor changes, references added, to appear
in JHE