Periodic Reporting for period 5 - QCDforfuture (QCD for the Future of Particle Physics)
Reporting period: 2023-01-01 to 2023-11-30
The momentous discovery of the Higgs boson in 2012 marked the start of a new era in particle physics. The energy of collisions at the Large Hadron Collider (LHC) has since increased allowing us to probe fundamental physics at an energy scale which has been out of reach until now. This presents a challenge to particle theory to keep pace with these developments, and respond to the fact that Standard Model interactions will have different features in this new energy range. We must understand these differences in order to extract as much information as possible from LHC data, and in particular to identify any signs of new physics. My framework, High Energy Jets, is the only tool of its kind to include the dominant high-energy corrections to all orders in the strong coupling within a flexible tool which can be adapted to a wide range of experimental analyses.
The high-energy corrections I have been able to describe had already been shown to be necessary to describe data at lower collisions energies. The priority of the project was to build on this proof-of-concept to develop more accurate and more widely-applicable predictions for the LHC. This work was structured in two programmes: the first of these was based on the construction of a publicly available fully-flexible event generator while the second programme concentrated on developing theoretical understanding of scattering amplitudes in the high energy limit. There has been natural interplay between these two with new theory calculations being incorporated into the event generator and numerical results suggesting new theoretical calculations to perform.
A particular priority during the project has been providing predictions for collisions which create a Higgs boson alongside colour-charged particles, which is an important process for determining its properties. Due to the behaviour of the scattering amplitudes (which give the probabilities for these to occur), in addition to high-energy corrections my framework can include finite quark mass and loop propagator effects for higher numbers of colour-charged particles than any other method.
Highlights on the side of increased accuracy are the inclusion of a class of next-to-leading log processes and the construction of a mechanism to increase fixed-order accuracy to next-to-leading order for distributions. Both are included in the latest release of the public code, HEJ v2.2 available at https://hej.hepforge.org(opens in new window). Throughout the grant, there has been an overhaul of this code and several version releases with accompanying documentation.
We were asked to provide predictions for two forthcoming LHC analyses by the ATLAS collaboration. The final publications have yet to appear, but they should teach us about the behaviour of QCD in the new energy regime which is now being tested.
The high-energy corrections I have been able to describe had already been shown to be necessary to describe data at lower collisions energies. The priority of the project was to build on this proof-of-concept to develop more accurate and more widely-applicable predictions for the LHC. This work was structured in two programmes: the first of these was based on the construction of a publicly available fully-flexible event generator while the second programme concentrated on developing theoretical understanding of scattering amplitudes in the high energy limit. There has been natural interplay between these two with new theory calculations being incorporated into the event generator and numerical results suggesting new theoretical calculations to perform.
A particular priority during the project has been providing predictions for collisions which create a Higgs boson alongside colour-charged particles, which is an important process for determining its properties. Due to the behaviour of the scattering amplitudes (which give the probabilities for these to occur), in addition to high-energy corrections my framework can include finite quark mass and loop propagator effects for higher numbers of colour-charged particles than any other method.
Highlights on the side of increased accuracy are the inclusion of a class of next-to-leading log processes and the construction of a mechanism to increase fixed-order accuracy to next-to-leading order for distributions. Both are included in the latest release of the public code, HEJ v2.2 available at https://hej.hepforge.org(opens in new window). Throughout the grant, there has been an overhaul of this code and several version releases with accompanying documentation.
We were asked to provide predictions for two forthcoming LHC analyses by the ATLAS collaboration. The final publications have yet to appear, but they should teach us about the behaviour of QCD in the new energy regime which is now being tested.
The study of the production of a Higgs boson plus dijets is particularly important at the LHC as it allows for a measurement of the coupling of the Higgs boson to other gauge bosons, critical to testing the Higgs mechanism of symmetry breaking. This directly addresses the question of whether the particle discovered really is the particle predicted by Higgs and others. However, theoretical predictions to compare to data are difficult for this process. The typical experimental selection cuts applied in these studies enhance the high-energy corrections uniquely described in my framework High Energy Jets (HEJ). For both of these reasons it has been a major focus within the grant period.
In a series of papers we have provided predictions which contain the dominant set of these high-energy corrections (so-called "leading log" or LL corrections). We began using the same approximations for intermediate particles as those required in traditional methods, but then, given the properties of the high-energy limit, were able to remove these approximations. Our predictions can therefore describe collisions for higher numbers of colour-charged particles than any other. We find a significant impact of these corrections with estimates of the impact of selection cuts to suppress the QCD process being twice as effective than previous calculations suggested.
In a further paper, we produced a calculation of LL effects in our flexible framework for the first time for an event with only one colour-charged particle. The method which allows that also allows an extension to higher-multiplicity processes.
A related important process is same-sign W-pair production with two jets. This is a background to the Higgs processes above and also offers the opportunity to probe unitarity in WW-scattering. Again experimental selection cuts enhance the high-energy logarithmic effects which motivated us to study this process. We demonstrated a significant effect on the predicted shape of a number of key distributions and it has been included in the latest code release.
Important steps were made in improving the predictions away from the strict limit which is controlled by the all-order corrections. This is necessary to address regions of phase space where there is not one single dominant effect. We did this by including a set of processes which are formerly subdominant but can be numerically significant (NLL processes) and by developing a method to increase fixed-order accuracy to one higher order (NLO). We demonstrated the improvement of the description of data from collisions producing a W boson with jets of coloured particles, and have now made the two strands of improvements available in the public code.
It has been mentioned above, but a significant output of this project has been a publicly-available well-documented event generator which is flexible enough to adapt to different experimental analyses as these arise.
An important strand of this programme has been in the theoretical understanding of the scattering amplitudes which are the starting point for all predictions of the kind described above. We have also contributed theoretical results to the community who study the high energy limit of scattering amplitudes as a topic in its own right. We have calculated new components of the so-called BFKL equation at next-to-next-to-leading order. We have also developed understanding in how to disentangle two types of contributions in Regge theory.
In addition to the peer-reviewed publications which have arisen from the work above, these results have been presented by myself and my team at many conferences and workshops.
In a series of papers we have provided predictions which contain the dominant set of these high-energy corrections (so-called "leading log" or LL corrections). We began using the same approximations for intermediate particles as those required in traditional methods, but then, given the properties of the high-energy limit, were able to remove these approximations. Our predictions can therefore describe collisions for higher numbers of colour-charged particles than any other. We find a significant impact of these corrections with estimates of the impact of selection cuts to suppress the QCD process being twice as effective than previous calculations suggested.
In a further paper, we produced a calculation of LL effects in our flexible framework for the first time for an event with only one colour-charged particle. The method which allows that also allows an extension to higher-multiplicity processes.
A related important process is same-sign W-pair production with two jets. This is a background to the Higgs processes above and also offers the opportunity to probe unitarity in WW-scattering. Again experimental selection cuts enhance the high-energy logarithmic effects which motivated us to study this process. We demonstrated a significant effect on the predicted shape of a number of key distributions and it has been included in the latest code release.
Important steps were made in improving the predictions away from the strict limit which is controlled by the all-order corrections. This is necessary to address regions of phase space where there is not one single dominant effect. We did this by including a set of processes which are formerly subdominant but can be numerically significant (NLL processes) and by developing a method to increase fixed-order accuracy to one higher order (NLO). We demonstrated the improvement of the description of data from collisions producing a W boson with jets of coloured particles, and have now made the two strands of improvements available in the public code.
It has been mentioned above, but a significant output of this project has been a publicly-available well-documented event generator which is flexible enough to adapt to different experimental analyses as these arise.
An important strand of this programme has been in the theoretical understanding of the scattering amplitudes which are the starting point for all predictions of the kind described above. We have also contributed theoretical results to the community who study the high energy limit of scattering amplitudes as a topic in its own right. We have calculated new components of the so-called BFKL equation at next-to-next-to-leading order. We have also developed understanding in how to disentangle two types of contributions in Regge theory.
In addition to the peer-reviewed publications which have arisen from the work above, these results have been presented by myself and my team at many conferences and workshops.
The achievements described above all contributed new results to the theoretical particle physics community, and have provided theoretical predictions which no other tool/framework are able to compute due to the exclusive event-by-event implementation. In that sense, they represent signficant research advances and have been recognised by requests to provide predictions for experimental analyses, peer-reviewed publications, conference proceedings and invites to talks.
I would again highlight the new predictions provided for collisions which produce a Higgs boson along with colour-charged particles. No other framework is able to provide predictions including finite quark mass and loop propagator effects. Even the standard leading-order calculations stall at 3 jets, where in principle our framework has no upper limit. It therefore provides a window into the size of these effects and the impact of the approximations which other tools are forced to make.
I would again highlight the new predictions provided for collisions which produce a Higgs boson along with colour-charged particles. No other framework is able to provide predictions including finite quark mass and loop propagator effects. Even the standard leading-order calculations stall at 3 jets, where in principle our framework has no upper limit. It therefore provides a window into the size of these effects and the impact of the approximations which other tools are forced to make.