Heavy Gluon Research Articles

Consistency and interpretation of the LHC dijet excesses

ATLAS observed a limit for the cross section of dijet resonances, which is weaker than expected for a mass slightly below $\ensuremath{\approx}1\text{ }\text{ }\mathrm{TeV}$. In addition, CMS reported hints for the (nonresonant) pair production of dijet resonances $X$ via a particle $Y$ at a very similar mass range with a local (global) significance of $3.6\ensuremath{\sigma}$ ($2.5\ensuremath{\sigma}$) at ${m}_{X}\ensuremath{\approx}950\text{ }\text{ }\mathrm{GeV}$. In this article, we show that, using the preferred range for ${m}_{X}$ from the ATLAS analysis, one can reinterpret the CMS analysis of didijets in terms of a resonant search with $Y\ensuremath{\rightarrow}XX$, with a significantly reduced look-elsewhere effect, finding an excess for ${m}_{Y}\ensuremath{\approx}3.6\text{ }\text{ }\mathrm{TeV}$ with a significance of $4.0\ensuremath{\sigma}$ ($3.2\ensuremath{\sigma}$) locally (globally). We present two possible UV completions capable of explaining the (di)dijet excesses, one containing two scalar diquarks, the other one involving heavy gluons based on an $SU(3{)}_{1}\ifmmode\times\else\texttimes\fi{}SU(3{)}_{2}\ifmmode\times\else\texttimes\fi{}SU(3{)}_{3}$ gauge symmetry, spontaneously broken to $SU(3)$ color. In the latter case, nonperturbative couplings are required, pointing toward a composite or extradimensional framework. In fact, using 5D anti--de Sitter space-time, one obtains the correct mass ratio for ${m}_{X}/{m}_{Y}$, assuming the $X$ is the lowest-lying resonance, and predicts a third (di)dijet resonance with a mass around $\ensuremath{\approx}2.2\text{ }\text{ }\mathrm{TeV}$.

Precision axion physics with running axion couplings

We study the renormalization group running of axion couplings while taking into account that the Standard Model can be extended to its supersymmetric extension at a certain energy scale below the axion decay constant. We then apply our results to three different classes of axion models, i.e. KSVZ-like, DFSZ-like, and string-theoretic axions, and examine if string-theoretic axions can be distinguished from others by having a different pattern of low energy couplings to the photon, nucleons and electron. We find that the low energy couplings of string-theoretic axions have a similar pattern as those of KSVZ-like axions but yet reveal a sizable difference which might be testable in future axion search experiments. We also note that the coupling of KSVZ-like QCD axions to the electron is dominated by a three-loop contribution involving the exotic heavy quark, gluons, top quark and Higgs field.

Transferability of deep learning models in searches for new physics at colliders

In this work we assess the transferability of deep learning models to detect beyond the standard model signals. For this we trained Deep Neural Networks on three different signal models: $tZ$ production via a flavour changing neutral current, pair-production of vector-like $T$-quarks via standard model gluon fusion and via a heavy gluon decay in a grid of 3 mass points: 1, 1.2 and 1.4 TeV. These networks were trained with $t\bar{t}$, $Z$+jets and dibosons as the main backgrounds. Limits were derived for each signal benchmark using the inference of networks trained on each signal independently, so that we can quantify the degradation of their discriminative power across different signal processes. We determine that the limits are compatible within uncertainties for all networks trained on signals with vector-like $T$-quarks, whether they are produced via heavy gluon decay or standard model gluon fusion. The network trained on flavour changing neutral current signal, while struggling the most on the other signals, still produce reasonable limits. These results indicate that deep learning models are capable of providing sensitivity in the search for new physics even if it manifests itself in models not assumed during training.

A fragmentation-based study of heavy quark production

Processes involving heavy quarks are a crucial component of the LHC physics program, both by themselves and as backgrounds for Higgs physics and new physics searches. In this work, we critically reconsider the validity of the widely-adopted approximation in which heavy quarks are generated at the matrix-element level, with special emphasis on the impact of the collinear logarithms associated with final-state heavy quark and gluon splittings. Our study, based on a perturbative fragmentation-function approach, explicitly shows that neglecting the resummation of collinear logarithms may yield inaccurate predictions, in particular when observables exclusive in the heavy quark degrees of freedom are considered. Our findings motivate the use of schemes which encompass the resummation of final-state collinear logarithms.

Transverse momentum spectra at threshold for groomed heavy quark jets

We present the transverse momentum spectrum for a heavy hadron at threshold in a groomed jet initiated by a heavy quark. The cross section is doubly differential in the energy fraction of an identified heavy hadron in the jet and its transverse momentum measured with respect to the groomed (recoil free) jet axis. The grooming is implemented using a soft-drop grooming algorithm and helps us in mitigating the effects of Non-Global logarithms and pile up. For the particular case of a B meson, we identify two distinct regimes of the transverse momentum spectrum and develop an EFT within the formalisms of Soft Collinear Effective Theory (SCET) and Heavy Quark Effective Theory (HQET) for each of these regions. We show how each region can be matched smoothly into the other to provide a prediction for the perturbative transverse momentum spectrum. The EFT also predicts the scaling behavior of the leading non-perturbative power corrections and implements a simple shape function to account for hadronization. We work in the threshold region where the heavy hadron carries most of the energy of the jet since in this regime, we have a very good discriminating power between heavy quark and gluon initiated jets. We observe that the shape of the spectrum is independent of the energy of the jet over a large range of transverse momentum. We propose that this spectrum can be used as a probe of evolution for heavy quark TMD fragmentation function. At the same time, it can be treated as a jet substructure observable for probing Quark-Gluon Plasma (QGP).

Search for single production of a vector-like quark via a heavy gluon in the 4b final state with the ATLAS detector in pp collisions at s=8 TeV

A search is performed for the process $pp \to G^* \to B_H\bar b/\bar B_H b \to H b \bar b \to b\bar bb\bar b$, predicted in composite Higgs scenarios, where $G^*$ is a heavy colour octet vector resonance and $B_H$ a vector-like quark of charge $-1/3$. The data were obtained from $pp$ collisions at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.5 fb$^{-1}$, recorded by the ATLAS detector at the LHC. The largest background, multijet production, is estimated using a data-driven method. No significant excess of events with respect to Standard Model predictions is observed, and upper limits on the production cross section times branching ratio are set. Comparisons to the predictions from a specific benchmark model are made, resulting in lower mass limits in the two-dimensional mass plane of $m_{G^*}$ vs. $m_{B_H}$.

Interpretation of vector-like quark searches: heavy gluons in composite Higgs models

Pair production of new vector-like quarks in pp collisions is considered model independent as it is usually dominated by QCD production. We discuss the interpretation of vector-like quark searches in the case that QCD is not the only relevant production mechanism for the new quarks. In particular we consider the effect of a new massive color octet vector boson with sizeable decay branching ratio into the new quarks. We pay special attention to the sensitivity of the Large Hadron Collider experiments, both in run-1 and early run-2, to differences in the kinematical distributions from the different production mechanisms. We have found that even though there can be significant differences in some kinematical distributions at the parton level, the differences are washed out at the reconstruction level. Thus, the published experimental results can be reinterpreted in models with heavy gluons by simply rescaling the production cross section.

Z-peaked excess from heavy gluon decays to vectorlike quarks

A 3 sigma excess has been recently announced by ATLAS in events with Z-peaked dilepton pairs, jets, and large transverse missing energy. We interpret this finding in the context of composite Higgs / RS theories. We find that composite Higgs theories with custodial symmetry protection to the $Zb\bar{b}$ coupling predict a significant contribution to $ZZbb$ (and to $hhbb$) final states coming from heavy gluon decays to pairs of bottom-partner vector-like quarks. The heavy gluon to vector-like quarks signal is largely accepted by the ATLAS selection if one of the $Z$ boson in the $ZZbb$ final state decays leptonically and the other to neutrinos. For a bottom partner of $\sim$900 GeV, we find that the ATLAS excess can be reproduced by composite Higgs models, in an experimentally allowed parameter space, for heavy gluon masses roughly in a range 1.87 - 2.15 TeV and for heavy gluon couplings to light quarks within $\sim(0.3-0.65) g_S$. We briefly discuss the implication of this result for future experimental tests.

The elusive gluon

We study the phenomenology of vector resonances in the context of natural composite Higgs models. A mild hierarchy between the fermionic partners and the vector resonances can be expected in these models based on the following arguments. Both direct and indirect (electroweak and flavor precision) constraints on fermionic partners are milder than the ones on spin one resonances. Also the naturalness pressure coming from the top partners is stronger than that induced by the gauge partners. This observation implies that the search strategy for vector resonances at the LHC needs to be modified. In particular, we point out the importance of heavy gluon decays (or other vector resonances) to top partner pairs that were overlooked in previous experimental searches at the LHC. These searches focused on simplified benchmark models in which the only new particle beyond the Standard Model was the heavy gluon. It turns out that, when kinematically allowed, such heavy-heavy decays make the heavy gluon elusive, and the bounds on its mass can be up to 2TeV milder than in the simpler models considered so far for the LHC14. We discuss the origin of this difference and prospects for dedicated searches.

Possible Effects of Fierz Transformations on Vacua of Some four Fermion Interaction Models

A theoretical research on possible effects of the Fierz transformations on the ground states (vacua) of some 2-flavor and $N_c$-color four-fermion (quark) interaction models has been systematically conducted. It has been shown that, based on the known criterions of the interplay between the antiquark-quark ($\bar{q}$-$q$) and diquark ($q$-$q$) condensates, in 4D space-time, for the given $\bar{q}$-$q$ channel couplings with chiral symmetry and from the heavy gluon exchange, the effects of the Fierz transformations are not enough to change the feature that the models' vacua could only be in the pure $\bar{q}$-$q$ condensate phases. However, for a given pure scalar $q$-$q$ channel coupling with the strength $H_S$, the Fierz transformations will lead to the nontrivial effect that the model's vacuum could be in the expected $q$-$q$ condensate phase only if $N_c<9$ and $H_S$ is small, and as the increase of $N_c$ and $H_S$, the vacuum will get first in a coexistence phase with $q$-$q$ and $\bar{q}$-$q$ condensates then in a pure $\bar{q}$-$q$ condensate phase until $N_c\rightarrow\infty$. Similar conclusions are also drawn from relevant four-fermion interaction models in 2D and 3D space-time. The general significance of the research is indicated.

New Higgs production mechanism in composite Higgs models

Composite Higgs models are only now starting to be probed at the Large Hadron Collider by Higgs searches. We point out that new resonances, abundant in these models, can mediate new production mechanisms for the composite Higgs. The new channels involve the exchange of a massive color octet and single production of new fermion resonances with subsequent decays into the Higgs and a Standard Model quark. The sizable cross section and very distinctive kinematics allow for a very clean extraction of the signal over the background with high statistical significance. Heavy gluon masses up to 2.8 TeV can be probed with data collected during 2012 and up to 5 TeV after the energy upgrade to $\sqrt{s}=14$ TeV.

Heavy-light decay topologies as a new strategy to discover a heavy gluon

We study the collider phenomenology of the lightest Kaluza-Klein excitation of the gluon, G*, in theories with a warped extra dimension. We do so by means of a two-site effective lagrangian which includes only the lowest-lying spin-1 and spin-1/2 resonances. We point out the importance of the decays of G* to one SM plus one heavy fermion, that were overlooked in the previous literature. It turns out that, when kinematically allowed, such heavy-light decays are powerful channels for discovering the G*. In particular, we present a parton-level Montecarlo analysis of the final state Wtb that follows from the decay of G* to one SM top or bottom quark plus its heavy partner. We find that at \sqrt{s} = 7 TeV and with 10 fb^{-1} of integrated luminosity, the LHC can discover a KK gluon with mass in the range M_{G*} = (1.8 - 2.2) TeV if its coupling to a pair of light quarks is g_{G*qqbar} = (0.2-0.5) g_3. The same process is also competitive for the discovery of the top and bottom partners as well. We find, for example, that the LHC at \sqrt{s} = 7 TeV can discover a 1 TeV KK bottom quark with an integrated luminosity of (5.3 - 0.61) fb^{-1} for g_{G*qqbar} = (0.2-0.5) g_3.

Stealth gluons at hadron colliders

We find that a heavy gluon G of mass 800–900 GeV with small, mostly axial-vector couplings to the light quarks and relatively large vector and axial-vector couplings to the top quark can explain the tt¯ forward–backward asymmetry observed at the Tevatron with no conflict with other top-quark or dijet data. The key ingredient is a complete treatment of energy-dependent width effects and a new decay mode G→qQ, where q is a standard quark and Q a vector-like quark of mass 400–600 GeV. We show that this new decay channel makes the heavy gluon invisible in the tt¯ invariant mass distribution and discuss its implications at the Tevatron and the LHC.

Gluon excitations intt¯production at hadron colliders

We argue that a relatively light massive gluon with mass <= 1 TeV, small purely axial couplings to light quarks and sizable vector and axial couplings to the top quark can reproduce the large forward-backward asymmetry observed at the Tevatron without conflicting with the t tbar and the dijet invariant mass distributions measured at the Tevatron and the LHC. We show that realistic Higgsless models with warped extra dimensions naturally fulfil all the necessary ingredients to realize this scenario. While current data is unable to discover or exclude these heavy gluons with masses 850 GeV, they should be observed at the (7 TeV) LHC with a luminosity of the order of 300 pb^{-1}.

On the existence of angular correlations in decays with heavy matter partners

If heavy partners of the Standard Model matter fields are discovered at the LHC it will be imperative to determine their spin in order to uncover the underlying theory. In decay chains, both the spin and the mass hierarchy of all particles involved can influence the resulting angular correlations. We present the necessary conditions for decays involving the matter partners to exhibit angular correlations. In particular we find that when the masses are not degenerate, a heavy fermionic sector always displays angular correlations in cascade decays. When the masses are closely degenerate the size of spin effects is controlled by a global phase parameter. In a large region of the parameter space, correlations are strongly suppressed. In other regions, such as in UED model where this phase is fixed by 5-d Lorentz invariance, the correlations are pronounced. In addition, we show that in certain cases one may even have enough information to determine the spin of other heavy partners involved in the decay (such as the Lightest Stable Partner or a heavy gluon).

Energy loss of gluons, baryons andk-quarks in an Script N = 4 SYM plasma

We consider different types of external color sources that move through a strongly-coupled thermal N=4 super-Yang-Mills plasma, and calculate, via the AdS/CFT correspondence, the dissipative force (or equivalently, the rate of energy loss) they experience. A bound state of k quarks in the totally antisymmetric representation is found to feel a force with a nontrivial k-dependence. Our result for k=1 (or k=N-1) agrees at large N with the one obtained recently by Herzog et al. and Gubser, but contains in addition an infinite series of 1/N corrections. The baryon (k=N) is seen to experience no drag. Finally, a heavy gluon is found to be subject to a force which at large N is twice as large as the one experienced by a heavy quark, in accordance with gauge theory expectations.

It is a gluino

For a long time it has been known that the like-sign dilepton signature can help establish the existence of a gluino at the LHC. To unambiguously claim that we see a strongly interacting Majorana fermion---which we could call a gluino---we need to prove that it is indeed a fermion. We propose how to extract this information from a different gluino-decay cascade which is also used to measure its mass. Looking only at angular correlations we distinguish a universal extra dimensional interpretation assuming a bosonic heavy gluon from supersymmetry with a fermionic gluino. Assuming a supersymmetric interpretation, we show how the same angular correlations can be used to study the left-right nature of the sfermions appearing in the decay chain.

A GLUON PLASMA GIANT RESONANCE AND COLOR-SINGLETNESS AT FINITE TEMPERATURE

By imposing the SU(3) color-singletness constraint on a gluonic system, a heavy gluon–plasma giant resonance is shown to arise at finite temperature. This is made possible through the proper incorporation of the finite size correction brought in by the color-singletness restriction.

A Gluon Plasma Giant Resonance at Finite Temperature

It is shown that a heavy gluon giant resonance state which is highly collective exists in a gluon plasma. This arises when the finite size effects are taken properly into account.

MODEL FOR GLUON PROPAGATOR FROM ANALYSIS IN RENORMALIZATION GROUP

The gluon propagator is considered on the basis of one-loop calculations in the renormalization group. To describe a set of nonperturbative phenomena such as heavy quark potential, gluon and quark condensates, constituent masses of light quarks, this analysis allows one to propose a model for the gluon propagator in the vicinity of ir region.