683 research outputs found
String Phase Transitions in a Strong Magnetic Field
We consider open strings in an external constant magnetic field . For an
(infinite) sequence of critical values of an increasing number of (highest
spin component) states lying on the first Regge trajectory becomes tachyonic.
In the limit of infinite all these states are tachyons (with a common
tachyonic mass) both in the case of the bosonic string and for the
Neveu-Schwarz sector of the fermionic string. This result generalizes to
extended object the same instability which occurs in ordinary non-Abelian gauge
theories. The Ramond states have always positive square masses as is the case
for ordinary QED. The weak field limit of the mass spectrum is the same as for
a field theory with gyromagnetic ratio for all charged spin states.
This behavior suggests a phase transition of the string as it has been argued
for the ordinary electroweak theory.Comment: 8 page
Supergravity
We formulate pure supergravity as a scale invariant theory built only
in terms of superfields describing the geometry of curved superspace. The
standard supergravity duals are obtained in both "old" and "new" minimal
formulations of auxiliary fields. These theories have massless fields in de
Sitter space as they do in their non supersymmetric counterpart. Remarkably,
the dual theory of supergravity in the new minimal formulation is an
extension of the Freedman model, describing a massless gauge field and a
massless chiral multiplet in de Sitter space, with inverse radius proportional
to the Fayet-Iliopoulos term. This model can be interpreted as the "de-Higgsed"
phase of the dual companion theory of supergravity.Comment: 12 page
The economic effects of violent conflict: evidence from asset market reactions
This paper studies the effects of conflict onset on asset markets applying the event study methodology. We consider a sample of 112 conflicts during the period 1974-2004 and find that a sizeable fraction of them had a significant impact on stock market indices and on major commodity prices. Furthermore, our results suggest that we are more likely to see investor reactions in response to conflicts that occur in highly polarized settings, possibly because the expected duration and intensity of the conflict is higher.Stock exchanges ; Prices
Diamonds are forever, wars are not. Is conflict bad for private firms?
This paper studies the relationship between civil war and the value of firms in a poor, resource abundant country using microeconomic data for Angola. We focus on diamond mining firms and conduct an event study on the sudden end of the conflict, marked by the death of the rebel movement leader in 2002. We find that the stock market perceived this event as âbad newsâ rather than âgood newsâ for companies holding concessions in Angola, as their abnormal returns declined by 4 percentage points. The event had no effect on a control portfolio of otherwise similar diamond mining companies. This finding is corroborated by other events and by the adoption of alternative methodologies. We interpret our findings in the light of conflict-generated entry barriers, government bargaining power and transparency in the licensing process.Microeconomics ; Mineral industries
Bots increase exposure to negative and inflammatory content in online social systems
Societies are complex systems which tend to polarize into sub-groups of
individuals with dramatically opposite perspectives. This phenomenon is
reflected -- and often amplified -- in online social networks where, however,
humans are no more the only players, and co-exist alongside with social bots,
i.e., software-controlled accounts. Analyzing large-scale social data collected
during the Catalan referendum for independence on October 1, 2017, consisting
of nearly 4 millions Twitter posts generated by almost 1 million users, we
identify the two polarized groups of Independentists and Constitutionalists and
quantify the structural and emotional roles played by social bots. We show that
bots act from peripheral areas of the social system to target influential
humans of both groups, bombarding Independentists with violent contents,
increasing their exposure to negative and inflammatory narratives and
exacerbating social conflict online. Our findings stress the importance of
developing countermeasures to unmask these forms of automated social
manipulation.Comment: 8 pages, 5 figure
Food, Migration, and Identity: Halal food and Muslim immigrants in Italy
Recent events have solidified an Islamic identity for North African immigrants in Italy. Parts of the Italian population have transformed their general prejudice against immigrants into an anti-Islamic sentiment. This sentiment is caused by a clash of created identities that do not necessarily correspond to ethnic groups. Food is an important aspect of personal identity, and in the case of Muslim immigrants, religious dietary guidelines make certain foods legal (halal) and other ones illegal (haram). Does halal food unite Muslim immigrants or are other aspects of personal identity that are more important? Halal butcher shops have become the symbol of this cultural identity, where both religion and national origin mix. I argue that immigrants recreate a personal and ethnic Muslim identity through food
Supersymmetry Breaking by Higher Dimension Operators
We discuss a supersymmetry breaking mechanism for N = 1 theories triggered by
higher dimensional op- erators. We consider such operators for real linear and
chiral spinor superfields that break superymmetry and reduce to the
Volkov-Akulov action. We also consider supersymmetry breaking induced by a
higher dimensional operator of a nonminimal scalar (complex linear) multiplet.
The latter differs from the stan- dard chiral multiplet in its auxiliary
sector, which contains, in addition to the complex scalar auxiliary of a chiral
superfield, a complex vector and two spinors auxiliaries. By adding an
appropriate higher di- mension operator, the scalar auxiliary may acquire a
nonzero vev triggering spontaneous supersymmetry breaking. We find that the
spectrum of the theory in the supersymmetry breaking vacuum consists of a free
chiral multiplet and a constraint chiral superfield describing the goldstino.
Interestingly, the latter turns out to be one of the auxiliary fermions, which
becomes dynamical in the supersymmetry breaking vacuum. In all cases we are
considering here, there is no sgoldstino mode and thus the goldstino does not
have a superpartner. The sgoldstino is decoupled since the goldstino is one of
the auxiliaries, which is propagating only in the supersymmetry breaking
vacuum. We also point out how higher dimension operators introduce a potential
for the propagating scalar of the theory.Comment: 28 page
Minimal Supergravity Models of Inflation
We present a superconformal master action for a class of supergravity models
with one arbitrary function defining the Jordan frame. It leads to a
gauge-invariant action for a real vector multiplet, which upon gauge fixing
describes a massive vector multiplet, or to a dual formulation with a linear
multiplet and a massive tensor field. In both cases the models have one real
scalar, the inflaton, naturally suited for single-field inflation. Vectors and
tensors required by supersymmetry to complement a single real scalar do not
acquire vev's during inflation, so there is no need to stabilize the extra
scalars which are always present in the theories with chiral matter multiplets.
The new class of models can describe any inflaton potential which vanishes at
its minimum and grows monotonically away from the minimum. In this class of
supergravity models one can fit any desirable choice of inflationary parameters
n_s and r.Comment: 25 pages, 5 figures, the version to appear in Phys. Rev.
Higher Order Corrections in Minimal Supergravity Models of Inflation
We study higher order corrections in new minimal supergravity models of a
single scalar field inflation. The gauging in these models leads to a massive
vector multiplet and the D-term potential for the inflaton field with a
coupling g^{2} ~ 10^{-10}. In the de-Higgsed phase with vanishing g^2, the
chiral and vector multiplets are non-interacting, and the potential vanishes.
We present generic manifestly supersymmetric higher order corrections for these
models. In particular, for a supersymmetric gravity model -R+ R^2 we derive
manifestly supersymmetric corrections corresponding to R^n. The dual version
corresponds to a standard supergravity model with a single scalar and a massive
vector. It includes, in addition, higher Maxwell curvature/scalar interaction
terms of the Born-Infeld type and a modified D-term scalar field potential. We
use the dual version of the model to argue that higher order corrections do not
affect the last 60 e-foldings of inflation; for example the \xi R^4 correction
is irrelevant as long as \xi< 10^{24}.Comment: 25 pages, the version to appear in JCA
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