Search results for "Observable"
showing 10 items of 634 documents
Laser control for the optimal evolution of pure quantum states
2005
Starting from an initial pure quantum state, we present a strategy for reaching a target state corresponding to the extremum (maximum or minimum) of a given observable. We show that a sequence of pulses of moderate intensity, applied at times when the average of the observable reaches its local or global extremum, constitutes a strategy transferable to different control issues. Among them, post-pulse molecular alignment and orientation are presented as examples. The robustness of such strategies with respect to experimentally relevant parameters is also examined.
Scale-free relaxation of a wave packet in a quantum well with power-law tails
2013
We propose a setup for which a power-law decay is predicted to be observable for generic and realistic conditions. The system we study is very simple: A quantum wave packet initially prepared in a potential well with (i) tails asymptotically decaying like ~ x^{-2} and (ii) an eigenvalues spectrum that shows a continuous part attached to the ground or equilibrium state. We analytically derive the asymptotic decay law from the spectral properties for generic, confined initial states. Our findings are supported by realistic numerical simulations for state-of-the-art expansion experiments with cold atoms.
Constraints on jet quenching in p–Pb collisions at sNN=5.02 TeV measured by the event-activity dependence of semi-inclusive hadron-jet distributions
2018
The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high-transverse momentum trigger hadron in p–Pb collisions at √sNN = 5.02 TeV. Jets are reconstructed from charged-particle tracks using the anti-kT algorithm with resolution parameter R = 0.2 and 0.4. A data-driven statistical approach is used to correct the uncorrelated background jet yield. Recoil jet distributions are reported for jet transverse momentum 15 < pch T,jet < 50 GeV/c and are compared in various intervals of p–Pb event activity, based on charged-particle multiplicity and zero-degree neutral energy in the forward (Pb-going) direction. The semi-inclusive ob…
Measurement of angular correlations of jets at s=1.96 TeV and determination of the strong coupling at high momentum transfers
2012
We present a measurement of the average value of a new observable at hadron colliders that is sensitive to QCD dynamics and to the strong coupling constant, while being only weakly sensitive to parton distribution functions. The observable measures the angular correlations of jets and is defined as the number of neighboring jets above a given transverse momentum threshold which accompany a given jet within a given distance Delta-R in the plane of rapidity and azimuthal angle. The ensemble average over all jets in an inclusive jet sample is measured and the results are presented as a function of transverse momentum of the inclusive jets, in different regions of Delta-R and for different tran…
Comparison study of medium-modified QCD shower evolution scenarios
2009
The computation of hard processes in hadronic collisions is a major success of perturbative Quantum Chromodynamics (pQCD). In such processes, pQCD not only predicts the hard reaction itself, but also the subsequent evolution in terms of parton branching and radiation, leading to a parton shower and ultimately to an observable jet of hadrons. If the hard process occurs in a heavy-ion collision, a large part of this evolution takes place in the soft medium created along with the hard reaction. An observation of jets in heavy-ion collision thus allows a study of medium-modified QCD shower evolution. In vacuum, Monte-Carlo (MC) simulations are well established tools to describe such showers. Fo…
Medium-modified Jet Shapes and other Jet Observables from in-medium Parton Shower Evolution
2009
The suppression of large transverse momentum hadrons in heavy-ion (A-A) collisions as compared to their scaled expectation from proton-proton collisions due to the interaction of hard partons with the hot and dense QCD medium in A-A collisions is experimentally a well established phenomenon. Focusing on leading hadrons produced in hard processes, the medium effect appears as energy loss. Beyond that, the question is how the lost energy is redistributed in the medium. With increased experimental statistics and most importantly the kinematic range of the LHC, studying the properties of full jets rather than leading hadrons is becoming feasible. On the theory side, analytic models and Monte-Ca…
Physics at MAMI C
2005
The physics motivation for the energy increase of the Mainz Microtron from 885 to 1500 MeV (MAMI C) is presented. It is argued that the low energy regime in hadron physics where the perturbative approach to QCD is not applicable is the most relevant to further understand this peculiar theory. However, the investigation of the most significant observables needs excellent experimental precision and an intensive interaction with theory. As examples the recent measurement of the form factor of the neutron and the pion polarisability are presented. They show that the mentioned conditions are given at MAMI and will be extended by the energy increase.
ELFE: An electron laboratory for europe
2000
Abstract After about 10 years of discussions the physics case for ELFE, the Electron Laboratory For Europe, has become much clearer. The paper gives a personal view of the salient physics visible today. It is argued that QCD cannot be taken as the the “theory of strong interactions” as long as it cannot describe the hadronic world at low energies. New theoretical tools as “semi-exclusive reactions” and “off-forward-parton distributions” are promising new insight into hadrons by means of new observables. It is shown how connection of the new observables to QCD can be made. The most important parameters for an ELFE following from these conjectures are given allowing an experimental study of t…
Parton shower evolution in a 3-d hydrodynamical medium
2008
We present a Monte Carlo simulation of the perturbative Quantum Chromodynamics (pQCD) shower developing after a hard process embedded in a heavy-ion collision. The main assumption is that the cascade of branching partons traverses a medium which (consistent with standard radiative energy loss pictures) is characterized by a local transport coefficient qhat which measures the virtuality per unit length transferred to a parton which propagates in this medium. This increase in parton virtuality alters the development of the shower and in essence leads to extra induced radiation and hence a softening of the momentum distribution in the shower. After hadronization, this leads to the concept of a…
Initial state in heavy ion collisions
2016
Abstract We briefly review advances in understanding the initial stages of a heavy ion collision. In particular the focus is on moving from parametrizing the initial state to calculating its properties from QCD, consistently with the description of hard probes and dilute-dense scattering experiments. Modeling the event-by-event fluctuating nuclear geometry in initial state calculations has significantly improved in recent years. We also discuss prospects of directly seeing effects of particle correlations created in the initial state in the experimental observables.