Stransverse Mass Research Articles

MTN is all you need: Production of multiple semi-invisible resonances at hadron colliders

The stransverse mass variable [Formula: see text] was originally proposed for the study of hadron collider events in which [Formula: see text] parent particles are produced and then decay semi-invisibly. Here we consider the generalization to the case of [Formula: see text] semi-invisibly decaying parent particles. We introduce the corresponding class of kinematic variables [Formula: see text] and illustrate their mathematical properties. Many of the celebrated features of the [Formula: see text] kinematic endpoint are retained in this more general case, including the ability to measure the mass of the invisible daughter particle from the stransverse mass kink. We describe and validate a numerical procedure for computing [Formula: see text] in practice. We also identify the configurations of visible momenta which result in nontrivial ([Formula: see text]) values, and derive a pure phase-space estimate for the fraction of such events for any N.

Measurements of W^{+}W^{-} production in decay topologies inspired by searches for electroweak supersymmetry

This paper presents a measurement of fiducial and differential cross-sections for W^{+}W^{-} production in proton–proton collisions at sqrt{s}=13 TeV with the ATLAS experiment at the Large Hadron Collider using a dataset corresponding to an integrated luminosity of 139 fb^{-1}. Events with exactly one electron, one muon and no hadronic jets are studied. The fiducial region in which the measurements are performed is inspired by searches for the electroweak production of supersymmetric charginos decaying to two-lepton final states. The selected events have moderate values of missing transverse momentum and the ‘stransverse mass’ variable m_{textrm{T2}}, which is widely used in searches for supersymmetry at the LHC. The ranges of these variables are chosen so that the acceptance is enhanced for direct W^{+}W^{-} production and suppressed for production via top quarks, which is treated as a background. The fiducial cross-section and particle-level differential cross-sections for six variables are measured and compared with two theoretical SM predictions from perturbative QCD calculations.

Is the machine smarter than the theorist: Deriving formulas for particle kinematics with symbolic regression

We demonstrate the use of symbolic regression in deriving analytical formulas, which are needed at various stages of a typical experimental analysis in collider phenomenology. As a first application, we consider kinematic variables like the stransverse mass, $M_{T2}$, which are defined algorithmically through an optimization procedure and not in terms of an analytical formula. We then train a symbolic regression and obtain the correct analytical expressions for all known special cases of $M_{T2}$ in the literature. As a second application, we reproduce the correct analytical expression for a next-to-leading order (NLO) kinematic distribution from data, which is simulated with a NLO event generator. Finally, we derive analytical approximations for the NLO kinematic distributions after detector simulation, for which no known analytical formulas currently exist.

Deep-learned event variables for collider phenomenology

The choice of optimal event variables is crucial for achieving the maximal sensitivity of experimental analyses. Over time, physicists have derived suitable kinematic variables for many typical event topologies in collider physics. Here, we introduce a deep-learning technique to design good event variables, which are sensitive over a wide range of values for the unknown model parameters. We demonstrate that the neural networks trained with our technique on some simple event topologies are able to reproduce standard event variables like invariant mass, transverse mass, and stransverse mass. The method is automatable and completely general and can be used to derive sensitive, previously unknown, event variables for other, more complex event topologies.

Prospects for discovery and spin discrimination of dark matter in Higgs portal DM models and their extensions at 100 TeV pp collider

We study the discovery and discriminating prospects of the Higgs portal dark matter (DM) models for scalar, fermion and vector DM and their extensions in proton–proton (pp) collisions. The tbar{t}+DM associated production in dileptonic final states is considered, in which the stransverse mass of two leptons is found to be effective in suppressing the Standard Model backgrounds along with the missing transverse energy and the angle between two leptons. The distributions of missing transverse energy and polar angle between two leptons are used for a discrimination of the spin nature of DM. For the proposed benchmark points, the discovery/exclusion can be made with an integrated luminosity less than 1 ab^{-1} given a 1% systematic uncertainty, while the spin discrimination require integrated luminosity of a few O(10) ab^{-1} given a 0.5% systematic uncertainty. The DM phenomenology is also discussed. A consistent DM candidate can be obtained either by extending our model where the Higgs portal couples to excited dark states that decay into DM, or modifying the coupling form into pseudoscalar.

Measurement of the top quark mass in the dileptonic tt¯ decay channel using the mass observables Mbℓ , MT2 , and Mbℓν in pp collisions at s=8 TeV

A measurement of the top quark mass (M[t]) in the dileptonic t t-bar decay channel is performed using data from proton-proton collisions at a center-of-mass energy of 8 TeV. The data was recorded by the CMS experiment at the LHC and corresponding to an integrated luminosity of 19.7 +/- 0.5 inverse femtobarns. Events are selected with two oppositely charged leptons (l = e, mu) and two jets identified as originating from b quarks. The analysis is based on three kinematic observables whose distributions are sensitive to the value of M[t]. An invariant mass observable, M[bl], and a `stransverse mass' observable, M[T2], are employed in a simultaneous fit to determine the value of M[t] and an overall jet energy scale factor (JSF). A complementary approach is used to construct an invariant mass observable, M[blnu], that is combined with M[T2] to measure M[t]. The shapes of the observables, along with their evolutions in M[t] and JSF, are modeled by a nonparametric Gaussian process regression technique. The sensitivity of the observables to the value of M[t] is investigated using a Fisher information density method. The top quark mass is measured to be 172.22 +/- 0.18 (stat) +0.89/-0.93 (syst) GeV.

A complete solution classification and unified algorithmic treatment for the one- and two-step asymmetric S-transverse mass M ˜ T 2 $$ {\tilde{M}}_{\mathrm{T}2} $$ event scale statistic

The MT2 or "s-transverse mass" statistic was developed to associate a parent mass scale to a missing transverse energy signature, given that escaping particles are generally expected in pairs, while collider experiments are sensitive to just a single transverse momentum vector sum. This document focuses on the generalized extension of that statistic to asymmetric one- and two-step decay chains, with arbitrary child particle masses and upstream missing transverse momentum. It provides a unified theoretical formulation, complete solution classification, taxonomy of critical points, and technical algorithmic prescription for treatment of the MT2 event scale. An implementation of the described algorithm is available for download, and is also a deployable component of the author's selection cut software package AEACuS (Algorithmic Event Arbiter and Cut Selector). Appendices address combinatoric event assembly, algorithm validation, and a complete pseudocode.

Boosted dibosons from mixed heavy top squarks

The lighter mass eigenstate (${\stackrel{\texttildelow{}}{t}}_{1}$) of the two top squarks, the scalar superpartners of the top quark, is extremely difficult to discover if it is almost degenerate with the lightest neutralino (${\stackrel{\texttildelow{}}{\ensuremath{\chi}}}_{1}^{0}$), the lightest stable supersymmetric particle in the R-parity conserving supersymmetry. The current experimental bound on ${\stackrel{\texttildelow{}}{t}}_{1}$ mass in this scenario stands only around 200 GeV. For such a light ${\stackrel{\texttildelow{}}{t}}_{1}$, the heavier top squark (${\stackrel{\texttildelow{}}{t}}_{2}$) can also be around the TeV scale. Moreover, the high value of the Higgs ($h$) mass prefers the left- and right-handed top squarks to be highly mixed, allowing the possibility of a considerable branching ratio for ${\stackrel{\texttildelow{}}{t}}_{2}\ensuremath{\rightarrow}{\stackrel{\texttildelow{}}{t}}_{1}h$ and ${\stackrel{\texttildelow{}}{t}}_{2}\ensuremath{\rightarrow}{\stackrel{\texttildelow{}}{t}}_{1}Z$. In this paper, we explore the above possibility together with the pair production of ${\stackrel{\texttildelow{}}{t}}_{2}$ ${\stackrel{\texttildelow{}}{t}}_{2}^{*}$, giving rise to the spectacular $\mathrm{\text{diboson}}+\mathrm{\text{missing}}$ transverse energy final state. For an approximately 1 TeV ${\stackrel{\texttildelow{}}{t}}_{2}$ and a few hundred GeV ${\stackrel{\texttildelow{}}{t}}_{1}$ the final state particles can be moderately boosted, which encourages us to propose a novel search strategy employing the jet substructure technique to tag the boosted $h$ and $Z$. The reconstruction of the $h$ and $Z$ momenta also allows us to construct the stransverse mass ${M}_{T2}$, providing an additional efficient handle to fight the backgrounds. We show that a $4--5\ensuremath{\sigma}$ signal can be observed at the 14 TeV LHC for $\ensuremath{\sim}1\text{ }\text{ }\mathrm{TeV}$ ${\stackrel{\texttildelow{}}{t}}_{2}$ with $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity.

The stransverse mass, M T2, in special cases

This document describes some special cases in which the stransverse mass, MT2, may be calculated by non-iterative algorithms. The most notable special case is that in which the visible particles and the hypothesised invisible particles are massless -- a situation relevant to its current usage in the Large Hadron Collider as a discovery variable, and a situation for which no analytic answer was previously known. We also derive an expression for MT2 in another set of new (though arguably less interesting) special cases in which the missing transverse momentum must point parallel or anti parallel to the visible momentum sum. In addition, we find new derivations for already known MT2 solutions in a manner that maintains manifest contralinear boost invariance throughout, providing new insights into old results. Along the way, we stumble across some unexpected results and make conjectures relating to geometric forms of M_eff and H_T and their relationship to MT2.

Amplification of endpoint structure for new particle mass measurement at the LHC

We introduce a new collider variable, MCT2, named as constransverse mass. It is a mixture of `stransverse mass(MT2)' and `contransverse mass(MCT)' variables, where the usual endpoint structure of MT2 distribution can be amplified in the MCT2 basis by large Jacobian factor which is controlled by trial missing particle mass. Thus the MCT2 projection of events increases our observability to measure several important endpoints from new particle decays, which are usually expected to be buried by irreducible backgrounds with various systematic uncertainties at the LHC. In this paper we explain the phenomenology of endpoint amplification in MCT2 projection, and describe how one may employ this variable to measure several meaningful mass constraints of new particles.

Inclusive transverse mass analysis for squark and gluino mass determination

We propose an inclusive analysis of a stransverse mass (mT2) using a hemisphere method. The hemisphere method is an algorithm to group collinear and high pT particles and jets, assuming that there are two of such groups in a event. An inclusive mT2 is defined as a function of an unknown LSP mass, two hemisphere momenta, and missing transverse momentum. Kinematical end points of mT2 distributions provide information on sparticle masses. We perform a Monte Carlo simulation to study the inclusive mT2 distribution at the LHC. We show that the end point of the inclusive mT2 distribution as a function of a test LSP mass has a kink-like structure around the true LSP mass. We find that the inclusive analysis is useful for obtaining information on the heaviest of squark/gluino.