Describing the VBS processes
12-17.11.2017in Nikhef, Amsterdam
Study of WZ VBS processes
19.11.-14.12.2017in ITKP, Dresden
Study of novel analysis directly related to VBS, aiming at separating Higgs couplings to longitudinally and transversally polarised W bosons
21-26.01.2018in CERN, Geneva, Switzerland
Study of statistical procedure of extracting WZ VBS cross sections through a fit and use this information further to extract aQGCs and contribute to the combination of the ATLAS and CMS
25-30.03.2018in ITKP, Dresden
Implementation and Investigation of EFT Models
12.03-06.04.18in Milano, Italy
Study on separating Higgs couplings to longitudinally and transversally polarised W bosons, implementation of statistical procedure
12-27.04.2018in CERN, Geneva, Switzerland
Studies on the extracting longitudinal polarization through angular distributions
5-10.08.2018in in Ljubljana, Slovenia
Investigation of VBF forward jet tagging techniques
20-29.09.2018in Sheffield, UK
W boson reconstruction with neural network
03.12-08.12.2018from Ljubljana to Pavia
VBS analysis in the CMS experiment
20.02-6.03.2019from Milano to CERN
Improvement and refinement of tools towards the computation and precise description of vector-boson scattering at the LHC
6-12.01.2019from Wuerzburg to Cambridge
Global fit on the parameters of Dimension-6 or Dimension-8 operators of the SM Effective Field Theory
13.02-27.02.2019from Thessaloniki to CERN
Electroweak corrections for VBS at the LHC
18.03-22.03.2019from Cambridge to Wuerzburg
VBS studies using same-sign WW events using full Run-II LHC data
25.02-14.03.2019from Budapest to Antwerp
Color Evolution in VBS/VBF
25.03-14.04.2019from Karlsruhe to Vienna
BDT and DNN multivariate analysis techniques for VBF jets
30.03-07.04.2019from Istanbul to Helsinki
Rapidity-dependent, Dynamic Jet Vetoes in VBS Searches for Type II Seesaw Scalars
03.04-14.04.2019from Louvain-la-Neuve to Ljubljana
Comparing the SMEFT and eCHL in the context of VBS experimental analyses
25.05-10.06.2019from Durham to Madrid
VBS ZZ to 4l with CMS
24.07-05.08.2019from Split to Palaiseau
New techniques in VBS study part 2
18.08-23.08.2019from Pavia to Zurich
Run 2 Analysis Collaboration of Ioannis Karkanias with ATLAS group of LAPP Annecy
09.11-09.12.2019from Thessaloniki to Annecy
Advance Techniques to Maximize the Information Content of Multivariate and Multiparameter Estimation in the Framework of SMEFT Approach
06-13.02.2020from Thessaloniki to Sheffield
Extensions of the Same-Sign WW VBS analysis
15-29.02.2020from Milano to CERN
New techniques in VBS study part 3
29.03-05.04.2020from Pavia to Zurich
November 2017 - Precise predictions for vector-boson scattering at the LHC
The Large Hadron Collider (LHC) is the largest physics experiment in the world
and is based in Geneva (Switzerland). It is colliding bits of matter at very high
energy in order to produce elementary particles. These particles are numerous
and each of them have different characteristics that allow them to be "seen" in
different parts of the detectors built around the interaction points of the colliding
beams. The task of the physicists working in particle physics is to verify if the
theoretical predictions match the experimental measurements.
If they don't, this might be a sign of unknown mechanisms.
The collision of two protons (the tiny bits of matter) can produce many particles in
different ways and these are called processes. The processes with only two
particles in the final states are easier to compute with high precision than theones with high multiplicity. On the experimental side, the processes with low
multiplicity happen more often and are thus measured more precisely due to
higher statistics. On the other hand, processes with high multiplicity are more
difficult to compute theoretically and are also more complex to see
experimentally. In order to accumulate enough statistics one should then wait
longer and collide protons at higher energy.
The latter is actually happening at the LHC where now protons are collided at a
centre-of-mass energy of 13 TeV. This has never been achieved before and this
means that experimental collaboration (ATLAS and CMS) are now able to measure
very rare processes with up to 6 particles in the final state.
Among these processes, key processes for the understanding of the Standard
Model of particle physics are processes that involve the internal scattering of
bosons (see Figure).
These are generically dubbed
vector-boson scattering (VBS)
processes and have been measured
for the first time a few months ago.
The study of these processes
constitutes the main focus of the
VBSCan COST action.
In this exciting time for particle
physicists exploring these new
territories, it is very important to have precise and reliable predictions for such
processes. During this short term scientific mission (STSM), several theorists have
gathered at the institute Nikhef in Amsterdam (the Netherlands).
This week of work has been devoted to performing comparisons of various
This allows to infer whether approximations commonly used are reasonable or not
and to investigate whether there are ambiguities between different predictions,
especially when parton showers are used.
Such a work is very important for experimental collaborations as this provides
them with indications on how to use and interpret theoretical calculations with
their intrinsic errors.
In that way, theoretical predictions are used appropriately by experimental
physicists so that they can make the most of the data of the LHC. A report to be
published in a peer-reviewed jorunal is in progress and will become public in the
December 2017 - Study of WZ VBS processes
How probably is it that the observed events stem indeed from the process that we are interested in?
Can we increase the ratio of signal (what we are interested in) to the background (what we are not interested in)?
These were the questions, Despoina Sampsonidou from Aristotle University of Thessaloniki tackled together with scientists at the Technical University TU Dresden, visiting the Institute IKTP from November 19th to December 14th 2017.
Together they concentrated on the study of the W±Z production via Vector Boson Scattering (VBS) in the ATLAS experiment.
ATLAS is one of the four major experiments at the Large Hadron Collider (LHC) at CERN.
It is a general-purpose particle physics experiment run by an international collaboration and, together with CMS, is designed to exploit the full discovery potential and the huge range of physics opportunities that the LHC provides.
The VBS production is a rare but at the same time a very important set of processes that it will be possible to measure at LHC.
It manifests itself as two high-energetic, well-separated jets in the detector, produced together with two vector bosons (dibosons).
So in the final state of the event, apart from the the two jets, the decay products of the two vector bosons are present and can be used to select the signal events instead of uninteresting background.
The study of such states may provide evidence for the existence of New Physics, which will manifest itself at energies higher than the ones available in the LHC collisions currently.
The 13 TeV center-of-mass energy, as well as the high luminosity in Run2 at LHC, which will continue for one more year, significantly enhances the sensitivity of these processes.
Thus, it allows us to test the predictions of the Standard Model to a high precision and consequently look for deviation from it.
Despoina’s visit allowed a close collaboration with the TU Dresden team in a time period crucial for the next publication.
The results of the work performed during the STSM were presented in the TU Dresden group meeting.
January and April 2018 - Study of novel analysis directly related to VBS, aiming at separating Higgs couplings to longitudinally and transversally polarised W bosons
A precise measurement of the Higgs couplings to W and Z bosons is one of the aims for VBScan researchers being involved in the ATLAS experiment. They analyse processes, in which the Higgs boson is produced in the fusion of
(W and Z) and decays to a pair of W bosons, both of which further decay to electrons, muons and neutrinos (see picture).
They measure individual Higgs coupling strengths to longitudinally and transversally polarised bosons. The Standard Model predicts these parameters to be equal and finely-tuned in order to restore unitarity in the scattering of
bosons (and to ensure, the universe is stable). Therefore, any observed deviations from the SM couplings allow for a direct test of the electroweak symmetry breaking mechanism. If an anomalous coupling is observed, this would hint at
physics in the interactions between the Higgs and W and Z bosons or indicate the composite nature of either the Higgs or electroweak bosons. To best measure these processes and interpret the results, the VBScan researchers prepared a
for statistical analysis together with statistics experts at CERN and implemented a maximum likelihood fit for the statistical interpretation of results using morphing techniques.
March 2018 - Study of statistical procedure of extracting WZ VBS cross sections through a fit and use this information further to extract aQGCs and contribute to the combination of the ATLAS and CMS
Statistical analysis is a bundle of tools to determine a parameter or investigate whether a certain model or hypothesis can be confirmed or not. In short: It helps to answer our research questions - especially in cases, where our own
might trick us or where the answer is not very clear. VBS network member Jon Butterworth has described this nicely in his
In order to learn how to use statistical tools for their analysis, researchers from the TU Dresden and the Aristotle University of Thessaloniki teams met in Dresden to exchange tools and knowledge - hands-on. Sitting in front of a
together with a cup of coffee next to you and a friendly colleague and having immediate feedback, when something does not work as you think it should!
April 2018 - Implementation and Investigation of EFT Models
Vector-boson scattering (VBS) processes are an ideal tool to study the nature of electroweak symmetry breaking and test the electroweak sector of the Standard Model (SM) to high precision. The non-Abelian nature of the weak
predicts the presence of triple and quartic interactions of electroweak gauge
bosons. Measuring VBS processes and testing the theoretical predictions is one of the main motivations for the current run-2 and future runs of the Large Hadron Collider (LHC) at CERN. So far, no significant deviations from the SM
have been established. Hence, it is reasonable to assume that any effects from physics beyond the SM are associated with an energy scale larger than the one currently probed by experiments. Also, modelling new-physics effects should
large variety of morespecific, UV-complete models. An ideal tool to perform this task are Effective Field Theories (EFTs), which provide a well motivated and consistent framework. It is an expansion in terms of higher-dimensional
consisting of the SM fields, with associated Wilson coefficients and corresponds to integrating out any new-physics degrees of freedom.
The meeting in Milano allowed researchers from various VBScan member states to start a comparison between different codes providing theoretical predictions for VBS processes in the framework of EFTs. This important step makes sure,
consistent results can be achieved. Implementation choices otherwise could lead to non-agreeing results when generators, important tools in aQGC determinations, are used with their default settings. Another focus of the meeting were
phenomenological studies to judge the sensitivity of VBS processes.
August 2018 - Studies on the extracting longitudinal polarization through angular distributions
One of the key topics in the VBSCan COST Action is the feasibility of separation between longitudinal and transverse component of the W boson in the VBS full-leptonic channel. This channel is the most promising channel in terms of
discrimination, despite its low cross section.
Researchers from Ljubljana and and Pavia, with two PhD students taking the lead in these investigations, studied, how to extract the W boson polarization in VBS scattering. One of the investigated techniques to measure the W boson
includes extracting polarization fractions from the angular distribution of leptons in the W rest frame. In principle nothing is knows about the polarization of a W boson, it can only be reconstruct through the angular distributions
decay products. The reconstruction of the W rest frame represents a challenge from experimental point of view.
Longitudinal and transverse polarization vectors depend on the reference frames in which they are defined. The polarization of a massive vector boson is the reflection of its spin–1 particle nature. The orientation of the boson spin
as a three–dimensional spin vector, resulting (quantum– mechanically) in three independent spin states: for each boson momentum, the spin projection along an arbitrary axis has three possible eigenvalues, 0, ±1 (helicities). In
polarization fractions of W bosons are frame dependent. Actually, this dependence can be more or less evident, depending on the considered process.
In events generated with PHANTOM Monte Carlo framework sign ambiguity in longitudinal neutrino momentum reconstruction can be partially resolved by setting some selection criteria, this applies both to semi-leptonic and
PhD students determined thresholds for selection by visualizing in a 2d plot the number of events as a function of relative error and specific discriminating variable, such as the scalar product of the neutrino three-momentum, with
Then they evaluated the impact of the detector resolution on the full-leptonic polarized sample (transverse and longitudinal polarization). Looking at the results, it can be said that in semi-leptonic channel the reconstruction of
reference frame can be obtained up to a sign ambiguity. The use of selection criteria represents the most promising way to select correct solution. In full-leptonic case instead the reconstruction is worse with respect to
The the effect of detector smearing, especially in the fully-leptonic channel, is sub leading.
September 2018 - Investigation of VBF forward jet tagging techniques
Vector boson scattering (VBS) and vector boson fusion (VBF) are important processes to study at the Large Hadron Collider (LHC) at CERN for both the ATLAS and the CMS experiments. VBS and VBF are characterized by two high energetic
forward region of the detector, produced in association with one or two vector bosons, respectively. VBS is a rare but also the key process to experimentally probe the Standard Model (SM) nature of electroweak symmetry breaking
Higgs boson. Researchers from Sheffield University (UK) and Piri Reis University (Tuzla, near Istanbul) studied the of the kinematic properties of different VBS and VBF processes.
VBF Z production is an important benchmark process as it permits the study of the jet activity in a relatively large event sample. The researchers aim to identify common features in the VBS Z and other channels, mainly central and
region jets kinematic variables such as eta, phi, mass and transverse energy, in order to determine some universal cuts.