1,721,084 research outputs found
Exotic Hadron Spectroscopy
No full textSince ten years ago experiments have been observing a host of exotic states decaying into heavy quarkonia. The interpretation of most of them still remains uncertain and, in some cases, controversial. Notwithstanding, a considerable progress has been made on the quality of the experimental information available and a number of ideas and models have been put forward to explain the observations. We will review the most promising theoretical interpretations of exotic states, in particular we will discuss the nature of the charmonium-like resonances in the region 3850−4050 MeV, and propose a new mechanism to explain prompt X(3872) production cross section at hadron colliders. Finally, the phenomenology of doubly charmed states is commented
Gauge theories in anti-selfdual variables
No full textSome years ago the Nicolai map, viewed as a change of variables from the gauge connection in a fixed gauge to the anti-selfdual part of the curvature, has been extended by the first named author to pure Yang-Mills from its original definition in N = 1 supersymmetric Yang-Mills. We study here the perturbative one-particle irreducible effective action in the anti-selfdual variables of any gauge theory, in particular pure Yang-Mills, QCD and N = 1 supersymmetric Yang-Mills. We prove that the one-loop one-particle irreducible effective action of a gauge theory mapped to the anti-selfdual variables in any gauge is identical to the one of the original theory. This is due to the conspiracy between the Jacobian of the change to the anti-selfdual variables and an extra functional determinant that arises from the non-linearity of the coupling of the anti-selfdual curvature to an external source in the Legendre transform that defines the one-particle irreducible effective action. Hence we establish the one-loop perturbative equivalence of the mapped and original theories on the basis of the identity of the one-loop one-particle irreducible effective actions. Besides, we argue that the identity of the perturbative one-particle irreducible effective actions extends order by order in perturbation theory
Study of B → Kππγ decays
No full textIn b → sγ transitions, the Standard Model predicts that B0 () decays are related predominantly to the presence of right- (left-) handed photons in the final state. Therefore, the mixing-induced CP asymmetry in B → fCPγ decays is expected to be small, thus being any sizeable observed asymmetries due to New Physics. In this analysis, we extract information about the Kππ resonant structures by means of an amplitude analysis of the mKππ and mKπ spectra in B+ → K+π-π+γ. The results are used, assuming isospin symmetry, to extract the mixing-induced CP parameters of the process B0 → K0sρ0γ from the time-dependent analysis of B0 → K0sπ+π-γ decays without an explicit amplitude analysis of this mode
Exotic hadrons as Feshbach resonances
No full textSince ten years ago a host of exotic resonances have challenged the usual quarkonium picture. A number of ideas have been put forward to explain these new states, but a comprehensive framework is still missing. We review the estimates for prompt production cross sections at hadron collider, and show that the interpretation of exotic states in terms of hadron molecules is not favored. We show a recently proposed model to solve this paradox and to explain the nature of above-threshold states in terms of Feshbach resonances
XYZ: Four quark states?
Full text linkThe observation of many unexpected states decaying into heavy quarkonia has challenged the usual QQ̄ interpretation. We will discuss the nature of some of the charmonium-like resonances recently observed by BES III and LHCb, and their identification according to the compact tetraquark model. We also comment the production of light nuclei in hadron collisions and the relevance for the physics of the X(3872)
Exotic hadron spectroscopy
Full text linkSince the discovery of the X(3872), a decade ago, more than 20 new charmonium-like resonances have been registered. Most of them have features which do no match what expected from standard charmonium theory. A few resonances have been found in the beauty sector too. Some authors just claim that most of the so called XYZ states are not even resonances but kind of effects of kinematical or dynamical origin, due to the intricacies of strong interactions. According to them, data analyses are naïvely describing and fitting as resonances what are indeed the footprints of such
complicated effects. On the other hand, the X(3872), for example, is an extremely narrow state, Gamma ~ 1 MeV, and it is very difficult, in our understanding, to imagine how this could
be described with some sort of strong rescattering mechanism. We do not know of other clear examples of such phenomena in the field of high-energy physics and in this thesis we will give little space to this kind of interpretations, which we can barely follow. We shall assume instead that what experiments agree to be a resonance is indeed a resonance. Moreover, we find very confusing the approach of mixing the methods proper of nuclear theory to discuss what we learned with the observations of XY Z resonances especially at Tevatron and LHC. It is true that X seems to be an extreme version of deuterium as its mass happens to be fine-tuned on the value of the D0 D0*bar threshold, but one cannot separate this observation from the fact that X is observed at CMS after imposing kinematical transverse momentum cuts as large as pT ~ 15 GeV on hadrons produced. Is there any evidence of a comparable prompt production of deuterium within the same kinematical cuts, in the same experimental conditions? The ALICE experiment could provide in the near future a compelling measurement of this latter rate (and some preliminary estimates described in the text are informative of what the result will be). Some of the XYZ, those happening to be close to some threshold, are interpreted as loosely-bound molecules, regardless of the great difficulties in explaining their production mechanisms in high energy hadron collisions. Some of them are described just as bound hadron molecules, once they happen to be below a close-by open flavor meson threshold. Other ones, even if sensibly above the close-by thresholds, have been interpreted as molecules as well: in those cases subtle mistakes in the experimental analysis of the mass have been advocated.
As a result the field of the theoretical description of XYZ states appears as an heterogeneous mixture of ad-hoc explanations, mainly post-dictions and contradictory statements which is rather confusing to the experimental community and probably self-limiting in the direction of making any real progress. It is our belief instead that a more simple and fundamental dynamics is at work
in the hadronization of such particles. More quark body-plans occur with respect to usual mesons and baryons: compact tetraquarks. The diquark-antidiquark model in its updated version, to be described in Chapter 7, is just the most simple and economical description (in terms of new states predicted) that we could find and we think that the recent confirmation of Z(4430) + especially, and of some more related charged J^PG = 1++ states, is the smoking gun for the intrinsic validity of this idea. The charged Z(4430) was the most uncomfortable state for the molecular interpretation for at least two reasons: i) it is charged and molecular models have never provided any clear and consistent prediction about charged states; ii) it is far from open charm thresholds. However, if what observed (by Belle first and confirmed very recently by LHCb) is not an “effect” but a real resonance, we should find the way to explain and put it in connection to all other ones. The Z(4430) appears extremely natural in the diquark-antidiquark model, which in general was the only approach strongly suggesting the existence of charged states years before their actual discovery. We think otherwise that open charm/bottom meson thresholds should likely play a role in the formation of XY Z particles. We resort to the Feshbach resonance mechanism, as mediated by some classic studies in atomic physics, to get a model on the nature of this role. The core of our preliminary analysis is the postulated existence of a discrete spectrum of compact tetraquark levels in the fundamental strong interaction Hamiltonian. The occurrence of open charm/beauty meson thresholds in the vicinity of any of these levels might result in an enhanced probability of resonance formation
H → γγ: a comment on the indeterminacy of non-gauge-invariant integrals
No full textWe reanalyze the recent computation of the amplitude of the Higgs boson decay into two photons presented by Gastmans et al. [1, 2]. The reasons for why this result cannot be the correct one have been discussed in some recent papers. We address here the general issue of the indeterminacy of integrals with four-dimensional gauge-breaking regulators and to which extent it might eventually be solved by imposing physical constraints. Imposing gauge invariance as the last step upon Rξ-gauge calculations with four-dimensional gauge-breaking regulators, allows us to recover the well known H→γγ result. However we show that in the particular case of the unitary gauge, the indeterminacy cannot be tackled in the same way. The combination of the unitary gauge with a cutoff regularization scheme turns out to be non-predictive. © 2013 Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Sciences and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd
Oscillation shaping in uncertain linear plants with nonlinear PI control: Analysis and experimental results
No full textLinear systems controlled by a nonlinear version of the PI algorithm are under
study. The modified PI controller in question is known in the literature as the Super–Twisting
(STW) algorithm (see Levant (1993)), and it belongs to the family of second order sliding
mode controllers. The considered closed–loop system exhibits self–sustained stable oscillations
(chattering) when the relative degree of the linear plant is higher than one (see Boiko and
Fridman (2005)) and it is the task of the present paper to present a systematic yet simple
procedure for tuning the STW algorithm parameters in order to obtain pre–specified frequency
and magnitude of the resulting chattering oscillation. The proposed methodology is based on
the Describing Function (DF) approach. The approach is theoretically illustrated and verified
by means of, both, simulation analysis and experiments carried out by making references to a
DC motor
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