11,311 research outputs found
Quantum Mechanics on Noncommutative Spacetime
We consider electrodynamics on a noncommutative spacetime using the
enveloping algebra approach and perform a non-relativistic expansion of the
effective action. We obtain the Hamiltonian for quantum mechanics formulated on
a canonical noncommutative spacetime. An interesting new feature of quantum
mechanics formulated on a noncommutative spacetime is an intrinsic electric
dipole moment. We note however that noncommutative intrinsic dipole moments are
not observable in present experiments searching for an EDM of leptons or nuclei
such as the neutron since they are spin independent. These experiments are
sensitive to the energy difference between two states and the noncommutative
effect thus cancels out. Bounds on the noncommutative scale found in the
literature relying on such intrinsic electric dipole moment are thus incorrect.Comment: 8 page
Resurrecting the Dead Cone
The dead cone is a well-known effect in gauge theories, where radiation from
a charged particle of mass m and energy E is suppressed within an angular size
of m/E. This effect is universal as it does not depend on the spin of the
particle nor on the nature of the gauge interaction. It is challenging to
directly measure the dead cone at colliders, however, since the region of
suppressed radiation either is too small to be resolved or is filled by the
decay products of the massive particle. In this paper, we propose to use jet
substructure techniques to expose the dead cone effect in the strong-force
radiation pattern around boosted top quarks at the Large Hadron Collider. Our
study shows that with 300/fb of 13-14 TeV collision data, ATLAS and CMS could
obtain the first direct evidence of the dead cone effect and test its basic
features.Comment: 12 pages, 12 figures; v2: references added; v3: approximate version
to appear in PR
Tracking down hyper-boosted top quarks
The identification of hadronically decaying heavy states, such as vector
bosons, the Higgs, or the top quark, produced with large transverse boosts has
been and will continue to be a central focus of the jet physics program at the
Large Hadron Collider (LHC). At a future hadron collider working at an
order-of-magnitude larger energy than the LHC, these heavy states would be
easily produced with transverse boosts of several TeV. At these energies, their
decay products will be separated by angular scales comparable to individual
calorimeter cells, making the current jet substructure identification
techniques for hadronic decay modes not directly employable. In addition, at
the high energy and luminosity projected at a future hadron collider, there
will be numerous sources for contamination including initial- and final-state
radiation, underlying event, or pile-up which must be mitigated. We propose a
simple strategy to tag such "hyper-boosted" objects that defines jets with
radii that scale inversely proportional to their transverse boost and combines
the standard calorimetric information with charged track-based observables. By
means of a fast detector simulation, we apply it to top quark identification
and demonstrate that our method efficiently discriminates hadronically decaying
top quarks from light QCD jets up to transverse boosts of 20 TeV. Our results
open the way to tagging heavy objects with energies in the multi-TeV range at
present and future hadron colliders.Comment: 19 pages + appendices, 17 figures; v2: added references, updated
cross section tabl
Please Hold me Up: Why Firms Grant Exclusive Dealing Contracts
Why do irreplaceable firms with a choice of suppliers or customers deliberately expose themselves to the threat of hold up by contracting ex ante to deal with only one of them? Our explanation revolves around the multiple equilibria intrinsic to situations of unverifiable investment and many traders. Exclusive dealing eliminates inefficient equilibria in which too many firms invest too little. The enhanced ex post bargaining power of the chosen firm is beneficial for incentives whilst the distributional impact is more than offset in the ex ante negotiations over which this firm obtains the access privilege.exclusive dealing, hold-up, renegotiation
Exclusive Contracts Foster Relationship-Specific Investment
Exclusive contracts prohibit one or both parties from trading with anyone else. Contrary to earlier findings, notably Segal and Whinston (2000), we show that investments that are specific to the contracted parties may be encouraged. Results depend on the nature of the investments and whether the bargaining is cooperative or non-cooperative. The major part of the analysis show that exclusive contracts designed to 'assure' the supply of essential inputs promote investment. Infinite penalties for breach, even if ex post renegotiable, may result in excessive investment, in which case a finite penalty for breach achieves the first-best outcome.exclusive dealing, non-cooperative bargaining, liquidated damages, renegotiation, resale
Induction of donor-specific transplantation tolerance to skin and cardiac allografts using mixed chimerism in (A + B → A) in rats
Mixed allogeneic chimerism (A + B → A) was induced in rats by reconstitution of lethally irradiated LEW recipients with a mixture of T-cell depleted (TCD) syngeneic and TCD allogeneic ACI bone marrow. Thirty-seven percent of animals repopulated as stable mixed lymphopoietic chimeras, while the remainder had no detectable allogeneic chimerism. When evaluated for evidence of donor-specific transplantation tolerance, only those recipients with detectable allogeneic lymphoid chimerism exhibited acceptance of donor-specific skin and cardiac allografts. Despite transplantation over a major histocompatibility complex (MHC)- and minor-disparate barrier, animals accepted donor-specific ACI skin and primarily vascularized cardiac allografts permanently, while rejecting third party Brown Norway (BN) grafts. The tolerance induced was also donor-specific in vitro as evidenced by specific hyporeactivity to the allogeneic donor lymphoid elements, yet normal reactivity to MHC-disparate third party rat lymphoid cells. This model for mixed chimerism in the rat will be advantageous to investigate specific transplantation tolerance to primarily vascularized solid organ grafts that can be performed with relative ease in the rat, but not in the mouse, and may provide a method to study the potential existence of organ- or tissue-specific alloantigens in primarily vascularized solid organ allografts. © 1993
Top B Physics at the LHC
In top-pair events where at least one of the tops decays semi-leptonically,
the identification of the lepton charge allows to tag not only the top charge
but also that of the subsequent b quark. In cases where the b also decays
semi-leptonically, the charge of the two leptons can be used to probe CP
violation in heavy flavor mixing and decays. This strategy to measure CP
violation is independent of those adopted so far in experiments, and can
already constrain non Standard Model sources of CP violation with current and
near future LHC data. To demonstrate the potential of this method we construct
two CP asymmetries based on same-sign and opposite-sign leptons and estimate
their sensitivities. This proposal opens a new window for doing precision
measurements of CP violation in b and c quark physics via high p_T processes at
ATLAS and CMS.Comment: 5 pages, 2 figures. v2: Slightly improved the b-charge association
analysis; some discussions and clarifications added; fixed typos in Eqs.
(29)-(32). Matches published versio
Fast simulation of the CEPC detector with Delphes
Fast simulation tools are highly appreciated in particle physics
phenomenology studies, especially in the exploration of the physics potential
of future experimental facilities. The Circular Electron Positron Collider is a
proposed Higgs and Z factory that can precisely measure the Higgs boson
properties and the electroweak precision observables. A fast-simulation toolkit
dedicated to the CEPC detector has been developed using Delphes. The comparison
shows that this fast simulation tool is highly consistent with the full
simulation, on a set of benchmark distributions. Therefore, we recommend this
fast simulation toolkit for CEPC phenomenological investigations
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