High-multiplicity Final States Research Articles

Machine learning templates for QCD factorization in the search for physics beyond the standard model

High-multiplicity all-hadronic final states are an important, but difficult final state for searching for physics beyond the Standard Model. A powerful search method is to look for large jets with accidental substructure due to multiple hard partons falling within a single jet. One way for estimating the background in this search is to exploit an approximate factorization in quantum chromodynamics whereby the jet mass distribution is determined only by its kinematic properties. Traditionally, this approach has been executed using histograms constructed in a background-rich region. We propose a new approach based on Generative Adversarial Networks (GANs). These neural network approaches are naturally unbinned and can be readily conditioned on multiple jet properties. In addition to using vanilla GANs for this purpose, a modification to the traditional WGAN approach has been investigated where weight clipping is replaced by drawing weights from a naturally compact set (in this case, the circle). Both the vanilla and modified WGAN approaches significantly outperform the histogram method, especially when modeling the dependence on features not used in the histogram construction. These results can be useful for enhancing the sensitivity of LHC searches to high-multiplicity final states involving many quarks and gluons and serve as a useful benchmark where GANs may have immediate benefit to the HEP community.

Lattice-friendly gauge completion of a composite Higgs with top partners

We give an explicit example of a composite Higgs model with a pseudo-Nambu-Goldstone Higgs in which the top Yukawa coupling is generated via the partial compositeness mechanism. This mechanism requires composite top partners which are relatively light compared to the typical mass scale of the strongly coupled theory. While most studies of the phenomenology of such models have focused on a bottom-up approach with a minimal effective theory, a top-down approach suggests that the theory should contain a limit in which an unbroken global chiral symmetry protects the mass of the top partners, and the spectrum of the partners satisfies ‘t Hooft matching conditions. We find that the relatively light fermions and pseudo-Goldstone bosons fall into complete multiplets of a large approximate global symmetry, and that the spectrum of particles lighter than a few TeV is non-minimal. Our example illustrates the likely features of a such a composite Higgs theory and also serves as an example of a non-chiral theory with a possible solution to the ‘t Hooft matching conditions. We find in this example that for some low-energy parameters in the effective theory the top partners can decay into high-multiplicity final states, which could be difficult for the Large Hadron Collider (LHC) to constrain. This may potentially allow for the top partners to be lighter than those in more minimal models.

Search for black holes and sphalerons in high-multiplicity final states in proton-proton collisions at sqrt{s}=13 TeV

A search in energetic, high-multiplicity final states for evidence of physics beyond the standard model, such as black holes, string balls, and electroweak sphalerons, is presented. The data sample corresponds to an integrated luminosity of 35.9 fb−1 collected with the CMS experiment at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV in 2016. Standard model backgrounds, dominated by multijet production, are determined from control regions in data without any reliance on simulation. No evidence for excesses above the predicted background is observed. Model-independent 95% confidence level upper limits on the cross section of beyond the standard model signals in these final states are set and further interpreted in terms of limits on semiclassical black hole, string ball, and sphaleron production. In the context of models with large extra dimensions, semiclassical black holes with minimum masses as high as 10.1 TeV and string balls with masses as high as 9.5 TeV are excluded by this search. Results of the first dedicated search for electroweak sphalerons are presented. An upper limit of 0.021 is set at 95% confidence level on the fraction of all quark-quark interactions above the nominal threshold energy of 9 TeV resulting in the sphaleron transition.

Clockwork/linear dilaton: structure and phenomenology

The linear dilaton geometry in five dimensions, rediscovered recently in the continuum limit of the clockwork model, may offer a solution to the hierarchy problem which is qualitatively different from other extra-dimensional scenarios and leads to distinctive signatures at the LHC. We discuss the structure of the theory, in particular aspects of naturalness and UV completion, and then explore its phenomenology, suggesting novel strategies for experimental searches. In particular, we propose to analyze the diphoton and dilepton invariant mass spectra in Fourier space in order to identify an approximately periodic structure of resonant peaks. Among other signals, we highlight displaced decays from resonantly-produced long-lived states and high-multiplicity final states from cascade decays of excited gravitons.

Rare Z boson decays to a hidden sector

We demonstrate that rare decays of the Standard Model $Z$ boson can be used to discover and characterize the nature of new hidden-sector particles. We propose new searches for these particles in soft, high-multiplicity leptonic final states at the Large Hadron Collider. The proposed searches are sensitive to low-mass particles produced in $Z$ decays, and we argue that these striking signatures can shed light on the hidden-sector couplings and mechanism for mass generation.

Energy dependence of strangeness production and event-byevent fluctuations

We review the energy dependence of strangeness production in nucleus-nucleus collisions and contrast it with the experimental observations in pp and p-A collisions at LHC energies as a function of the charged particle multiplicities. For the high multiplicity final states the results from pp and p-Pb reactions systematically approach the values obtained from Pb-Pb collisions. In statistical models this implies an approach to the thermodynamic limit, where differences of mean multiplicities between various formalisms, such as Canonical and Grand Canonical Ensembles, vanish. Furthermore, we report on event-by-event net-proton fluctuations as measured by STAR at RHIC/BNL and by ALICE at LHC/CERN and discuss various non-dynamical contributions to these measurements, which should be properly subtracted before comparison to theoretical calculations on dynamical net-baryon fluctuations.

Search for black holes and other new phenomena in high-multiplicity final states in proton–proton collisions at [formula omitted

A search for new physics in energetic, high-multiplicity final states has been performed using proton–proton collision data collected with the CMS detector at a center-of-mass energy of 13TeV and corresponding to an integrated luminosity of 2.3fb−1. The standard model background, dominated by multijet production, is determined exclusively from control regions in data. No statistically significant excess of events is observed. Model-independent limits on the product of the cross section and the acceptance of a new physics signal in these final states are set and further interpreted in terms of limits on the production of black holes. Semiclassical black holes and string balls with masses as high as 9.5TeV, and quantum black holes with masses as high as 9.0TeV are excluded by this search in the context of models with extra dimensions, thus significantly extending limits set at a center-of-mass energy of 8TeV with the LHC Run 1 data.

Non-factorizable photonic corrections to resonant production and decay of many unstable particles

Electroweak radiative corrections to the production of high-multiplicity final states with several intermediate resonances in most cases can be sufficiently well described by the leading contribution of an expansion about the resonance poles. In this approach, also known as pole approximation, corrections are classified into separately gauge-invariant factorizable and non-factorizable corrections, where the former can be attributed to the production and decay of the unstable particles on their mass shell. The remaining non-factorizable corrections are induced by the exchange of soft photons between different production and decay subprocesses. We give explicit analytical results for the non-factorizable photonic virtual corrections to the production of an arbitrary number of unstable particles at the one-loop level and, thus, deliver an essential building block in the calculation of next-to-leading-order electroweak corrections in pole approximation. The remaining virtual factorizable corrections can be obtained with modern automated one-loop matrix-element generators, while the evaluation of the corresponding real photonic corrections can be evaluated with full matrix elements by multi-purpose Monte Carlo generators. Our results can be easily modified to non-factorizable QCD corrections, which are induced by soft-gluon exchange.

Stealth QCD-like strong interactions and thett¯asymmetry

We show that a new strongly interacting sector can produce large enhancements of the t-tbar asymmetries at the Tevatron. The Standard Model is extended by a new vector-like flavor triplet of fermions and one heavy scalar, all charged under a hypercolor gauge group SU(3) HC. This simple extension results in a number of new resonances. The predictions of our model are rather rigid once a small number of UV parameters are fixed, since all the strong dynamics can be directly taken over from our understanding of QCD dynamics. Despite the rather low hypercolor confinement scale of ~100 GeV, the new strongly interacting sector is stealth. It is shielded from present direct and indirect NP searches since the light resonances are QCD singlets, whereas the production of the heavier QCD colored resonances leads predominantly to high multiplicity final states. Improved searches can potentially be devised using top tagged final states or decays into a small number of hypercolor pions.

Heavy flavor and dark sector

We consider some contributions to rare processes in $B$ meson decays from a Dark Sector containing 2 light unstable scalars, with large couplings to each other and small mixings with Standard Model Higgs scalars. We show that existing constraints allow for an exotic contribution to high multiplicity final states with a branching fraction as large as $\mathcal{O}(10^{-4})$, and that exotic particles could appear as narrow resonances or long lived particles which are mainly found in high multiplicity final states from $B$ decays.

A swarm of Bs

New physics signals containing five or more b-tagged jets, but without MET or leptons, could realistically be sitting within the current 8 TeV LHC data set without receiving meaningful constraints from any of the existing LHC searches at either ATLAS or CMS. This work provides several examples of simple, motivated models that yield final states containing many b-jets. To study the potential for uncovering new physics in these high b-jet multiplicity channels, this paper focuses on a natural supersymmetry scenario where each of the pair-produced stops decays to an on-shell chargino, which subsequently decays via an MFV-motivated, R-parity violating coupling. This gives rise to an eight-jet final state containing six b-quarks. Although no public measurements exist, estimates indicate that the standard model backgrounds in high b-jet multiplicity channels should be very small. To circumvent the background uncertainty, an asymmetric method is presented that utilizes two different techniques to conservatively exclude or to discover new physics in high b-jet multiplicity final states.

Multi-channel search for squarks and gluinos in [Formula: see text]pp collisions with the ATLAS detector at the LHC.

A search for supersymmetric particles in final states with zero, one, and two leptons, with and without jets identified as originating from b-quarks, in 4.7 fb−1 of sqrt{s}=7mbox{ TeV} pp collisions produced by the Large Hadron Collider and recorded by the ATLAS detector is presented. The search uses a set of variables carrying information on the event kinematics transverse and parallel to the beam line that are sensitive to several topologies expected in supersymmetry. Mutually exclusive final states are defined, allowing a combination of all channels to increase the search sensitivity. No deviation from the Standard Model expectation is observed. Upper limits at 95 % confidence level on visible cross-sections for the production of new particles are extracted. Results are interpreted in the context of the constrained minimal supersymmetric extension to the Standard Model and in supersymmetry-inspired models with diverse, high-multiplicity final states.

Jet substructure by accident

Abstract We propose a new search strategy for high-multiplicity hadronic final states. When new particles are produced at threshold, the distribution of their decay products is approximately isotropic. If there are many partons in the final state, it is likely that several will be clustered into the same large-radius jet. The resulting jet exhibits substructure, even though the parent states are not boosted. This “accidental” substructure is a powerful discriminant against background because it is more pronounced for high-multiplicity signals than for QCD multijets. We demonstrate how to take advantage of accidental substructure to reduce backgrounds without relying on the presence of missing energy. As an example, we present the expected limits for several R-parity violating gluino decay topologies. This approach allows for the determination of QCD backgrounds using data-driven methods, which is crucial for the feasibility of any search that targets signatures with many jets and suppressed missing energy.

Gaugephobic Higgs signals at the LHC

The Gaugephobic Higgs model provides an interpolation between three different models of electroweak symmetry breaking: Higgsless models, Randall-Sundrum models, and the Standard Model. At parameter points between the extremes, Standard Model Higgs signals are present at reduced rates, and Higgsless Kaluza-Klein excitations are present with shifted masses and couplings, as well as signals from exotic quarks necessary to protect the Zb coupling. Using a new implementation of the model in SHERPA, we show the LHC signals which differentiate the generic Gaugephobic Higgs model from its limiting cases. These are all signals involving a Higgs coupling to a Kaluza-Klein gauge boson or quark. We identify the clean signal pp → W(i) → WH mediated by a Kaluza-Klein W, which can be present at large rates and is enhanced for even Kaluza-Klein numbers. Due to the very hard lepton coming from the W± decay, this signature has little background, and provides a better discovery channel for the Higgs than any of the Standard Model modes, over its entire mass range. A Higgs radiated from new heavy quarks also has large rates, but is much less promising due to very high multiplicity final states.

A Very High Event Rate Data Acquisition System in Fastbus

A very high event rate Data Acquisition System is described. This DAS is planned for NA36, an SPS heavy ion experiment at CERN that uses calorimetry, wire chambers and a TPC to look for strangeness production enhancement as a signature of the quark-gluon plasma. Because of the high multiplicity final states and the fact that such states have never been seen, real time event selection is impractical and the DAS must record unusual amounts of data. The purpose is to organize and record 1000 events of 20 K byte average size during the 2 sec beam spill and to archive these events during the 10 seconds between spills. Full scale system testing is scheduled for June 1986, so considerable use of FASTBUS modules, developed at other institutions, is made in this design. The overall system block diagram is shown in Fig. 1.

Correlations in the final states ppπ +π −π +π − and ppπ +π −π +π −π +π − from 28.5 GeV/ c pp collisions

We compare various invariant mass and angular distributions from the final states ppπ +π −π +π − and ppπ +π −π +π −π +π − with the predictions of two models. The essential input to these models is the correct single-particle distributions with energy and momentum conservation constraints taken into account. The data is from 28.5 GeV/ c pp interactions in the BNL 80-inch bubble chamber. Substantial agreement between the data and the models is obtained inferring that the kinematic constraints of energy and momentum conservation play a strong role in determining correlations between particles in these high multiplicity final states.

Evidence for strong clustering effects in high-multiplicity final states at 30 GeV

A statistical analysis of the event-to-event fluctuations of the langitudinal-momentum distributions is applied to 2-, 3-, 4- and 6-pion production data in 28 GeV c proton-nucleon collisions, as well as to several models. The results indicate that a dominant feature of these final states is the formation of high-multiplicity clusters of particles.