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BSM constraints from model-independent measurements: A Contur Update

Mon, 11/02/2019 - 02:46

by: Butterworth, J.M.
Particle-level measurements, especially of differential cross-sections, made in fiducial regions of phase-space have a high degree of model-independence and can therefore be used to give information about a wide variety of Beyond the Standard Model (BSM) physics implemented in Monte Carlo generators, using a broad range of final states. The Contur package is used to make such comparisons. We summarise a snapshot of current results for a number of BSM scenarios; a UV complete model in which the global Baryon-number minus Lepton-number symmetry is gauged; several Dark Matter simplified models, and two generic light scalar models.

Monte Carlo event generators for high energy particle physics event simulation

Wed, 06/02/2019 - 03:05

by: Buckley, Andy
Monte Carlo event generators (MCEGs) are the indispensable workhorses of particle physics, bridging the gap between theoretical ideas and first-principles calculations on the one hand, and the complex detector signatures and data of the experimental community on the other hand. All collider physics experiments are dependent on simulated events by MCEG codes such as Herwig, Pythia, Sherpa, POWHEG, and MG5_aMC@NLO to design and tune their detectors and analysis strategies. The development of MCEGs is overwhelmingly driven by a vibrant community of academics at European Universities, who also train the next generations of particle phenomenologists. The new challenges posed by possible future collider-based experiments and the fact that the first analyses at Run II of the LHC are now frequently limited by theory uncertainties urge the community to invest into further theoretical and technical improvements of these essential tools. In this short contribution to the European Strategy Update, we briefly review the state of the art, and the further developments that will be needed to meet the challenges of the next generation.

Full NLO corrections to 3-jet production and R32 at the LHC

Wed, 06/02/2019 - 03:00

by: Reyer, Max
We present the evaluation of the complete set of NLO corrections to three-jet production at the LHC. To this end we consider all contributions of $\mathcal{O}(\alpha_s^n\alpha^m)$ with $n+m=3$ and $n+m=4$. This includes in particular also subleading Born contributions of electroweak origin, as well as electroweak virtual and QED real-radiative corrections. As an application we present results for the three- over two-jet ratio $R_{32}$. While the impact of non-QCD corrections on the total cross section is rather small, they can exceed $-10\%$ for high jet transverse momenta. The $R_{32}$ observable turns out to be very stable against electroweak corrections, receiving absolute corrections below $5\%$ even in the high-$p_T$ region.

Soft modifications to jet fragmentation in high energy proton-proton collisions

Wed, 23/01/2019 - 03:04
LU-TP 19-05

by: Bierlich, Christian
The discovery of collectivity in proton-proton collisions, is one of the most puzzling outcomes from the two first runs at LHC, as it points to the possibility of creation of a Quark-Gluon Plasma, earlier believed to only be created in heavy ion collisions. One key observation from heavy ion collisions is still not observed in proton-proton, namely jet-quenching. In this letter it is shown how a model capable of describing soft collective features of proton-proton collisions, also predicts modifications to jet fragmentation properties. With this starting point, several new observables suited for the present and future hunt for jet quenching in small collision systems are proposed.

Hard diffraction in photoproduction

Thu, 17/01/2019 - 03:12
LU TP 19-06

by: Helenius, Ilkka
We present a new framework for modeling hard diffractive events in photoproduction, implemented in Pythia 8. The model is an extension of the model for hard diffraction with dynamical gap survival in $pp$ and $p\bar{p}$ collisions proposed in 2015, now also allowing for other beam types. It thus relies on several existing ideas: the Ingelman-Schlein approach, the framework for multiparton interactions and the recently developed framework for photoproduction in $\gamma p$, $\gamma\gamma$, $ep$ and $e^+e^-$ collisions. The model proposes an explanation for the observed factorization breaking in photoproduced diffractive dijet events at HERA, showing an overall good agreement with data. The model is also applicable to ultraperipheral collisions with $pp$ and $pPb$ beams, and predictions are made for such events at the LHC.

Top-quark effects in diphoton production through gluon fusion at NLO in QCD

Fri, 21/12/2018 - 03:21

by: Maltoni, Fabio
At hadron colliders, the leading production mechanism for a pair of photons is from quark-anti-quark annihilation at the tree level. However, due to large gluon-gluon luminosity, the loop-induced process $gg\to \gamma \gamma$ provides a substantial contribution. In particular, the amplitudes mediated by the top quark become important at the $t \bar t$ threshold and above. In this letter we present the first complete computation of the next-to-leading order (NLO) corrections (up to $\alpha_S^3$) to this process, including contributions from the top quark. These entail two-loop diagrams with massive propagators whose analytic expressions are unknown and have been evaluated numerically. We find that the NLO corrections to the top-quark induced terms are very large at low diphoton invariant mass $m(\gamma \gamma)$ and close to the $t \bar t$ threshold. The full result including five massless quarks and top quark contributions at NLO displays a much more pronounced change of slope in the $m(\gamma \gamma)$ distribution at $t \bar t$ threshold than at LO and an enhancement at high invariant mass with respect to the massless calculation.

Finite Quark-Mass Effects in Higgs Boson Production With Dijets at Large Energies

Thu, 20/12/2018 - 04:09
DESY 18-217

by: Andersen, Jeppe R.
The production of a Higgs boson in association with at least two jets receives contributions both from the fusion of weak vector bosons (VBF) and from QCD processes, especially gluon fusion (GF). The former process is important for measuring the coupling of the Higgs boson to weak bosons, whereas the latter process plays an important role in determining any $CP$-admixtures in the Higgs sector. In this paper we go beyond the current state-of-the-art for fixed order calculations of the GF process (i.e. one loop $H+2j$ including full quark mass effects) by including the all-order effects in leading $\log(\hat s/p_t^2)$, together with full quark mass and loop-propagator kinematic effects. We calculate the mass-dependent components and implement the resummation within the framework of High Energy Jets. The high energy effects suppress the prediction compared to fixed order at large $\Delta y_{12}$ and $m_{jj}$ (and therefore within the usual VBF cuts of widely separated jets), just as found in the limit of $m_t\to \infty$. The mass dependence is more significant than at fixed order, because the systematic inclusion of the leading logarithms in $\hat s/p_t^2$ results in a hardening of the transverse momentum of the Higgs boson, which in turn probes in more detail the loop-structure of the coupling. In particular, the full mass dependence reduces the cross section within VBF cuts by 11\% compared to a calculation based just on the infinite top mass limit, but the impact of the bottom quark remains small. This all implies that the gluon-fusion contribution within VBF-cuts is less severe than current estimates suggest.

Evaluating multi-loop Feynman integrals numerically through differential equations

Mon, 10/12/2018 - 15:04

by: Mandal, Manoj K.
The computation of Feynman integrals is often the bottleneck of multi-loop calculations. We propose and implement a new method to efficiently evaluate such integrals in the physical region through the numerical integration of a suitable set of differential equations, where the initial conditions are provided in the unphysical region via the sector decomposition method. We present numerical results for a set of two-loop integrals, where the non-planar ones complete the master integrals for $gg\to\gamma\gamma$ and $q\bar{q}\to\gamma\gamma$ scattering mediated by the top quark.

Probing the scalar potential via double Higgs boson production at hadron colliders

Fri, 30/11/2018 - 03:41

by: Borowka, Sophia
We present a sensitivity study on the cubic and quartic self couplings in double Higgs production via gluon fusion at hadron colliders. Considering the relevant operators in the Standard Model Effective Field Theory up to dimension eight, we calculate the dominant contributions up to two-loop level, where the first dependence on the quartic interaction appears. Our approach allows to study the independent variations of the two self couplings and to clearly identify the terms necessary to satisfy gauge invariance and to obtain UV-finite results order by order in perturbation theory. We focus on the $b \bar b \gamma \gamma$ signature for simplicity and provide the expected bounds for the cubic and quartic self couplings at the 14 TeV LHC with 3000 fb$^{-1}$ (HL-LHC) and for a future 100 TeV collider (FCC-100) with 30 ab$^{-1}$. We find that while the HL-LHC will provide very limited sensitivity on the quartic self coupling, precision measurements of double Higgs production at a FCC-100 will offer the opportunity to set competitive bounds. We show that combining information from double and triple Higgs production leads to significantly improved prospects for the determination of the quartic self coupling.

LHC Constraints on a $B-L$ Gauge Model using Contur

Thu, 29/11/2018 - 03:12

by: Amrith, S.
The large and growing library of measurements from the Large Hadron Collider has significant power to constrain extensions of the Standard Model. We consider such constraints on a well-motivated model involving a gauged and spontaneously-broken $B-L$ symmetry, within the Contur framework. The model contains an extra Higgs boson, a gauge boson, and right-handed neutrinos with Majorana masses. This new particle content implies a varied phenomenology highly dependent on the parameters of the model, very well-suited to a general study of this kind. We find that existing LHC measurements significantly constrain the model in interesting regions of parameter space. Other regions remain open, some of which are within reach of future LHC data.

Kinematic strangeness production in cluster hadronization

Tue, 27/11/2018 - 03:40

by: Duncan, Cody B.
We present a modification to the non-perturbative strangeness production mechanisms in the Monte-Carlo event generator Herwig in order to make the processes more dynamic and collective. We compare the model to a series of observables for soft physics at both LEP and LHC.