22 research outputs found
Highlights from COMPASS in hadron spectroscopy
Since Quantum Choromdynamics allows for gluon self-coupling, quarks and
gluons cannot be observed as free particles, but only their bound states, the
hadrons. This so-called confinement phenomenon is responsible for of the
mass in the visible universe. The measurement of the hadron excitation spectra
therefore gives valuable input for theory and phenomenology to quantitatively
understand this phenomenon. One simple model to describe hadrons is the
Constituent Quark Model (CQM), which knows two types of hadrons: mesons,
consisting of a quark and an antiquark, and baryons, which are made out of
three quarks. More advanced models, which are inspired by QCD as well as
calculations within Lattice QCD predict the existence of other types of
hadrons, which may be e.g. described solely by gluonic excitations (glueballs)
or mixed quark and gluon excitations (hybrids). In order to search for such
states, the COMPASS experiment at the Super Proton Synchrotron at CERN has
collected large data sets, which allow to study the light-quark meson and
baryon spectra in unmatched precision. The overview shown here focuses on the
light meson sector, presenting a detailed Partial-Wave Analysis of the
processes: and . A
new state, the with is observed. Its Breit-Wigner
parameters are found to be in the ranges: and
. In the same analysis, a signal in a wave
with is observed. A resonant origin of this signal would not be
explicable within the CQM. In addition to this possibility of an exotic state,
a possible non resonant origin of this signal is discussed.Comment: Proceedings of the International Conference Dark Matter, Hadron
Physics and Fusion Physics Messina (Italy) - September 24-26, 2014, 8 pages,
16 figure
Recent progress in the partial-wave analysis of the diffractively produced final state at COMPASS
The COMPASS spectrometer at CERN has collected a large data set for
diffractive three-pion production of exclusive events. Based on
previous conventional Partial-Wave Analyses (PWA), we performed a `freed-isobar
PWA' on the same data, removing model assumptions on the dynamic isobar
amplitudes for dominating waves. In this analysis, we encountered continuous
mathematical ambiguities, which we were able to identify and resolve. This
analysis gives an unprecedented insight in the interplay of and
dynamics in the process. As an example we show results for a spin-exotic wave
wave.Comment: Proceedings for the 15th International Workshop on Meson Physics
(MESON 2018) from June 7th to 12th 2018 in Krak\'ow, Poland. Five pages, four
figure
The inclusive decay b→ccs¯ revisited
The inclusive decay rate b →ccs¯ is enhanced considerably due to perturbative QCD corrections. We recalculate the dominant part of the NLO–QCD corrections, because they cannot be reconstructed from the literature and we give the full expressions in this paper. Further we include some previously neglected corrections originating from penguin diagrams. Combined with the impressive progress in the accurate determination of input parameters like charm quark mass, bottom quark mass and CKM parameters, this enables us to make a very precise prediction of the corresponding branching ratioBr(b→ccs) ¯ =(23±2)%. This result is an essential ingredient for a model and even decay channel independent search for new physics effects in B decays
Generalised Known Kinematics (GKK) An Approach for Kinematic Observables in Pair Production Events with Decays Involving Invisible Particles
Many analyses in high energy physics are limited due to missing kinematic
information of known invisible particles in the detector, for example
neutrinos. The undetected particle carries away momentum and energy
information, preventing the full reconstruction of such an event. In this
paper, we present a method to handle this missing information, referred to as
the Generalised Known Kinematics (GKK) approach. It is based on constructing
event-by-event probability density distributions that describe the physically
allowed kinematics of an event. For GKK we take into account the available
kinematic information and constraints given by the assumed final state. Summing
these event-wise distributions over large data sets allows the determination of
parameters that influence the event kinematics, such as particle masses, which
are otherwise obscured by the missing information on the invisible final-state
particles. The method is demonstrated in simulation studies with events in collisions at the (4S) resonance,
presenting a new, promising approach for the measurement of the lepton
mass.Comment: Second Versio
Coherent photo-thermal noise cancellation in a dual-wavelength optical cavity for narrow-linewidth laser frequency stabilisation
Optical resonators are used for the realisation of ultra-stable frequency
lasers. The use of high reflectivity multi-band coatings allows the frequency
locking of several lasers of different wavelengths to a single cavity. While
the noise processes for single wavelength cavities are well known, the
correlation caused by multi-stack coatings has as yet not been analysed
experimentally. In our work, we stabilise the frequency of a nm and a
nm laser to one mirror pair and determine the residual-amplitude
modulation (RAM) and photo-thermal noise (PTN). We find correlations in PTN
between the two lasers and observe coherent cancellation of PTN for the
nm coating. We show that the fractional frequency instability of the
nm laser is limited by RAM at . The instability of the
nm laser is at close to the thermal noise limit of
.Comment: 17 pages, 5 figure
Light-Meson Spectroscopy at COMPASS
The goal of the COMPASS experiment at CERN is to study the structure and spectroscopy of hadrons. The two-stage spectrometer has large acceptance and covers a wide kinematic range for charged as well as neutral particles allowing to access a wide range of reactions. Light mesons are studied with negative (mostly ) and positive (, ) hadron beams with a momentum of . The light-meson spectrum is measured in different final states produced in diffractive dissociation reactions with squared four-momentum transfer to the target between and . The flagship channel is the final state, for which COMPASS has recorded the currently world's largest data sample. These data not only allow us to measure the properties of known resonances with high precision, but also to search for new states. Among these is a new axial-vector signal, the , with unusual properties. The findings are confirmed by the analysis of the final state.The goal of the COMPASS experiment at CERN is to study the structure and spectroscopy of hadrons. The two-stage spectrometer has large acceptance and covers a wide kinematic range for charged as well as neutral particles allowing to access a wide range of reactions. Light mesons are studied with negative (mostly ) and positive (, ) hadron beams with a momentum of . The light-meson spectrum is measured in different final states produced in diffractive dissociation reactions with squared four-momentum transfer to the target between and . The flagship channel is the final state, for which COMPASS has recorded the currently world's largest data sample. These data not only allow us to measure the properties of known resonances with high precision, but also to search for new states. Among these is a new axial-vector signal, the , with unusual properties. The findings are confirmed by the analysis of the final state.The goal of the Compass experiment at CERN is to study the structure and spectroscopy of hadrons. The two-stage spectrometer has large acceptance and covers a wide kinematic range for charged as well as neutral particles allowing to access a wide range of reactions. Light mesons are studied with negative (mostly π−) and positive (p, π+) hadron beams with a momentum of 190 GeV/c.The light-meson spectrum is measured in different final states produced in diffractive dissociation reactions with squared four-momentum transfer t to the target between 0.1 and 1.0 (GeV/c)2. The flagship channel is the π−π+π− final state, for which Compass has recorded the currently world’s largest data sample. These data not only allow us to measure the properties of known resonances with high precision, but also to search for new states. Among these is a new axial-vector signal, the a1(1420), with unusual properties. The findings are confirmed by the analysis of the π−π0π0 final state
Model dependence of the signal
Using the large C OMPASS data set on diffractive three pion production, we investigate the contradictory observations reported by previous experiments on the existence of a resonance signal in the spin-exotic wave with spin, parity and charge conjugation quantum numbers 1−+ . We identify a strong dependence of the result on the employed analysis model as the cause and derive a model tuned to minimize these effects. Additionally, we study the robustness of our analysis model using the approach of freed-isobar partial-wave analysis
Freed-Isobar Partial-Wave Analysis
The main goal of hadron spectroscopy is the identification and classification of bound states of the strong interaction. Excited hadronic states are extremely short-lived and usu- ally decay into lighter hadrons. A common method to ex- trac the hadron resonances from the measured kinematic distributions of the decay products is partial-wave anal- ysis (PWA). The conventional PWA method for decays of light mesons into multi-body final states usually relies on the isobar model, describing the decay process as a series of two-particle decays. Hereby additional interme- diary hadronic states appear, called isobars. The decays of these states are described by dynamic amplitudes that have to be known beforehand and therefore may intro- duce a model bias. In this thesis, we develop a novel approach, in which the parametrizations for the dynamic isobar amplitudes are replaced by step-like functions to extract the dynamic amplitudes from the data, thereby greatly reducing the model dependence of the results. In this approach, which we call freed-isobar PWA, we en- counter continuous mathematical ambiguities caused by exact cancellations between different amplitudes and show ways to resolve them. We apply the freed-isobar PWA method to a data set for the process π − p → π − π + π − p, collected by the Compass experiment in 2008. We extract the dynamic isobar ampli- tudes for 24 partial waves, resolved in bins of the invariant mass of the 3π system and the four-momentum transfer between beam and target. We analyze the extracted dy- namic isobar amplitudes with simple Breit-Wigner and Flatté models and obtain new insights in the interplay between the two- and three-particle dynamics of the three- pion system