Light Quark Jets Research Articles

Accessing CKM suppressed top decays at the LHC

We propose an extension of the existing experimental strategy for measuring branching fractions of top quark decays, targeting specifically t\to j_q Wt→jqW, where j_qjq is a light quark jet. The improved strategy uses orthogonal bb- and qq-taggers, and adds a new observable, the number of light-quark-tagged jets, to the already commonly used observable, the fraction of bb-tagged jets in an event. Careful inclusion of the additional complementary observable significantly increases the expected statistical power of the analysis, with the possibility of excluding |V_{tb}|=1|Vtb|=1 at 95\%95% C.L. at the HL-LHC, and accessing directly the standard model value of |V_{td}|^2+|V_{ts}|^2|Vtd|2+|Vts|2.

Mass suppression effect in QCD radiation and hadron angular distribution in jet* *Supported by the National Science Foundation of China (12235008, 12275156, 12275157, 12321005)

The finite mass of the heavy quark suppresses the collimated radiations; this is generally referred to as the dead cone effect. In this paper, we study the distribution of hadron multiplicity over the hadron opening angle with respect to the jet axis for various jet flavors. The corresponding measurement can be the most straightforward and simplest approach to explore the dynamical evolution of the radiations in the corresponding jet, which can expose the mass effect. We also propose a transverse energy-weighted angular distribution, which sheds light on the interplay between perturbative and non-perturbative effects in the radiation. Through Monte-Carlo simulations, our calculations show that the dead cone effect can be clearly observed by finding the ratio between the b and light-quark (inclusive) jets; this is expected to be measured at the LHC in the future.

Light quark jet quenching in higher-derivative gravity

We study finite-coupling corrections on the energy loss of light quarks in strongly coupled {mathcal {N}}=4 super Yang–Mills (SYM) plasma. We perform the analysis by computing the stopping distance of an image jet induced by a massless source field, characterized by a massless particle falling along the null geodesic in Einstein gravity with curvature-squared (R^2) corrections. It turns out that the stopping distance in the R^2 theories can be larger or smaller than its SYM counterpart depending on the higher-derivative coefficients. Moreover, we evaluate the stopping distance in the Gauss–Bonnet background and find that increasing lambda _textrm{GB} (a dimensionless parameter in Gauss–Bonnet gravity) leads to a decrease in the stopping distance, thus enhancing the energy loss of light quarks, in agreement with previous findings for the drag force, jet quenching parameter, and the instantaneous energy loss of light quarks using shooting strings.

Improving heavy dijet resonance searches using jet substructure at the LHC

The search for new physics at high energy accelerators has been at the crossroads with very little hint of signals suggesting otherwise. The challenges at a hadronic machine such as the LHC is compounded by the fact that final states are swamped with jets which one needs to understand and unravel. A positive step in this direction would be to separate the jets in terms of their gluonic and quark identities, much in a similar spirit of distinguishing heavy quark jets from light quark jets that has helped in improving searches for both neutral and charged Higgs bosons at the LHC. In this work, we utilise this information using the jet substructure techniques to comment on possible improvements in sensitivity as well as discrimination of new resonances in the all hadronic mode that would be crucial in pinning down new physics signals at HL-LHC, HE-LHC and any future 100 TeV hadron collider.

Transverse momentum broadening of a jet in quark-gluon plasma: an open quantum system EFT

We utilize the technology of open quantum systems in conjunction with the recently developed effective field theory for forward scattering to address the question of massless jet propagation through a weakly-coupled quark-gluon plasma in thermal equilibrium. We discuss various possible hierarchies of scales that may appear in this problem, by comparing thermal scales of the plasma with relevant scales in the effective field theory. Starting from the Lindblad equation, we derive and solve a master equation for the trans- verse momentum distribution of a massless quark jet, at leading orders both in the strong coupling and in the power counting of the effective field theory. Markovian approximation is justified in the weak coupling limit. Using the solution to the master equation, we study the transverse momentum broadening of a jet as a function of the plasma temperature and the time of propagation. We discuss the physical origin of infrared sensitivity that arises in the solution and a way to handle it in the effective field theory formulation. We suspect that the final measurement constraint can only cut-off leading infrared singularities and the solution to the Markovian master equation resums a logarithmic series. This work is a stepping stone towards understanding jet quenching and jet substructure observables on both light and heavy quark jets as probes of the quark-gluon plasma.

Three-loop soft function for heavy-to-light quark decays

We compute the 1-jettiness soft function for the decay of a heavy quark into a light quark jet plus colorless particles at three-loop order in soft-collinear effective theory. The 1-jettiness measurement fixes the total small light-cone momentum component of the soft radiation with respect to the jet direction. This soft function is a universal ingredient to the factorization of heavy-to-light quark decays in the limit of small 1-jettiness. Our three-loop result is required for resummation at the N3LL′ level, e.g. near the endpoint in the photon energy spectrum of the B → Xsγ decay. It is also a necessary ingredient for future calculations of fully-differential heavy-to-light quark decay rates at N3LO using the N -jettiness subtraction method, e.g. for semileptonic top decays. Using our result for the soft anomalous dimension we confirm predictions on the universal infrared structure of QCD scattering amplitudes with a massive external quark at three loops.

Measurement of Charged Hadron Production in Z-Tagged Jets in Proton-Proton Collisions at sqrt[s]=8 TeV.

The production of charged hadrons within jets recoiling against a Z boson is measured in proton-proton collision data at sqrt[s]=8 TeV recorded by the LHCb experiment. The charged-hadron structure of the jet is studied longitudinally and transverse to the jet axis for jets with transverse momentum p_{T}>20 GeV and in the pseudorapidity range 2.5<η<4. These are the first measurements of jet hadronization at these forward rapidities and also the first where the jet is produced in association with a Z boson. In contrast to previous hadronization measurements at the Large Hadron Collider, which are dominated by gluon jets, these measurements probe predominantly light-quark jets which are found to be more longitudinally and transversely collimated with respect to the jet axis when compared to the previous gluon dominated measurements. Therefore, these results provide valuable information on differences between quarks and gluons regarding nonperturbative hadronization dynamics.

Extracting a short distance top mass with light grooming

We propose a kinematic method based on a factorization formula for precisely measuring the top quark mass $m_t$ in $pp$ collisions using boosted top jets with light soft drop grooming. By using light grooming, which is an order of magnitude less aggressive than typical grooming, we retain a universal description of the top mass scheme and decay effects, while still effectively removing soft contamination from the top jet. We give field theory results for the hadronization corrections for jets induced by a heavy top quark, showing they are described by a universal hadronic parameter that also appears for groomed light quark jets. An important phenomenological application of our results is that one can obtain $m_t$ in a short distance scheme by fitting the hadron level jet mass distributions, predicted by our factorization formula, to data or by Monte-Carlo calibration. The peaked distributions for $pp$ and $e^+e^-$ collisions are similar, up to sensitivity to underlying event which is significantly reduced by soft drop. Since soft drop implies that the $t$ and $\bar t$ jet masses each can be independently measured, the analysis enables the use of lepton+jet samples.

Gluon emission from heavy quarks in dense nuclear matter

We study the medium-induced gluon emission process experienced by a hard jet parton propagating through the dense nuclear matter in the framework of deep inelastic scattering off a large nucleus. We work beyond the collinear rescattering expansion and the soft gluon emission limit, and derive a closed formula for the medium-induced single gluon emission spectrum from a heavy or light quark jet interacting with the dense nuclear medium via transverse and longitudinal scatterings. Without performing the collinear rescattering expansion, the medium-induced gluon emission spectrum is controlled by the full distribution of the differential elastic scattering rates between the propagating partons and the medium constituents. We further show that if one utilizes heavy static scattering centers for the traversed nuclear matter and takes the soft gluon emission limit, our result can reduce to the first order in opacity Djordjevic-Gyulassy-Levai-Vitev formula.

Measurement of jet substructure observables in tt¯ events from proton-proton collisions at s=13 TeV

A measurement of jet substructure observables is presented using tt¯ events in the lepton+jets channel from proton-proton collisions at s=13 TeV recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 fb−1. Multiple jet substructure observables are measured for jets identified as bottom, light-quark, and gluon jets, as well as for inclusive jets (no flavor information). The results are unfolded to the particle level and compared to next-to-leading-order predictions from powheg interfaced with the parton shower generators pythia 8 and herwig 7, as well as from sherpa 2 and Dire 2. A value of the strong coupling at the Z boson mass, αS(mZ)=0.115−0.013+0.015, is extracted from the substructure data at leading-order plus leading-log accuracy.15 MoreReceived 22 August 2018DOI:https://doi.org/10.1103/PhysRevD.98.092014Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.© 2018 CERN, for the CMS CollaborationPhysics Subject Headings (PhySH)Physical SystemsTop quarkTechniquesHadron collidersParticle decaysParticles & Fields

Identification of hadronically decaying W bosons and top quarks using multivariate techniques at ATLAS

By colliding protons and examining particles emitted from the collisions, the Large Hadron Collider aims to study the interactions of quarks and gluons at the highest energy accessible in a controlled experimental way. In such collisions, W bosons or top quarks which have TeV scale momentum can be accessible. Reconstructing such boosted jets are becoming important. In particular, the ability to identify original particle that decays to quarks against normal QCD jets plays a central role in various searches at high energy scale. This is typically done by the use of a single physically motivated observable constructed from the constituents of the jet. In this contribution, multiple complementary observables are combined using boosted decision trees and neural networks to increase the ability to distinguish W bosons and top quarks from light quark jets in the ATLAS experiment.

Jets in a strongly coupled anisotropic plasma

In this paper, we study the dynamics of the light quark jet moving through the static, strongly coupled {mathcal {N}}=4, anisotropic plasma with and without charge. The light quark is presented by a 2-parameters point-like initial condition falling string in the context of the AdS/CFT. We calculate the stopping distance of the light quark in the anisotropic medium and compare it with its isotropic value. We study the dependency of the stopping distance to the both string initial conditions and background parameters such as anisotropy parameter or chemical potential. Although the typical behavior of the string in the anisotropic medium is similar to the one in the isotropic AdS-Sch background, the string falls faster to the horizon depending on the direction of moving. Particularly, the enhancement of quenching is larger in the beam direction. We find that the suppression of stopping distance is more prominent when the anisotropic plasma have the same temperature as the isotropic plasma.

Time Dependent q̂ from AdS/CFT

We present the first ever AdS/CFT calculation of q̂ for a light quark jet as a function of position or, equivalently, time. Our result does not suffer from the gamma factor blow up of the usual time-independent AdS/CFT heavy quark setup and is qualitatively similar to, but differs by ∼O(1) factor from, the light flavor result of Liu, Rajagopal, and Wiedemann. Our findings can be immediately implemented into any q̂-based energy loss model.Our q̂ derivation relies on our calculation of the average distance squared, s2(t), travelled by the endpoint of a string falling in an AdS3-Schwarzschild spacetime. The early time behavior is ballistic, s2(t)∼t2, but the late time behavior is the usual diffusive Brownian motion, s2(t)∼t. These late time dynamics are universal and depend only on the near-horizon physics, which allows us to generalize our results to arbitrary dimensions and thus make contact with the physics explored by RHIC and LHC.Additionally, we find that AdS/CFT predicts angular ordering for radiation in medium, just as in vacuum, and in contradistinction to weak-coupling, with its anti-angular ordering prediction. Finally, our results also imply, sensibly, that AdS/CFT predicts a smooth interpolation between the angular correlations of open heavy flavor and light flavor observables.

Time Dependent from AdS/CFT

We present the first ever AdS/CFT calculation of for a light quark jet as a function of position or, equivalently, time. Our result does not suffer from the gamma factor blow up of the usual time-independent AdS/CFT heavy quark setup and is qualitatively similar to, but about a factor of 2/3 larger than, the light flavor result from Liu, Rajagopal, and Wiedemann. Our findings can be immediately implemented into any -based energy loss model.Our derivation relies on our calculation of the average distance squared, s2(t), travelled by the endpoint of a string falling in an AdS3-Schwarzschild spacetime. The early time behavior is ballistic, s2(t) ∼ t2, but the late time behavior is the usual diffusive Brownian motion, s2(t) ∼ t. These late time dynamics are universal and depend only on the near-horizon physics, which allows us to generalize our results to arbitrary dimensions and thus make contact with the physics explored by RHIC and LHC.Additionally, we find that AdS/CFT predicts angular ordering for radiation in a medium, just as in vacuum, and in contradistinction to weak-coupling. Finally, our results also imply, sensibly, that AdS/CFT predicts a smooth interpolation between the angular correlations of open heavy flavor and light flavor observables.

Jet substructures of boosted polarized hadronic top quarks

We study jet substructures of a boosted polarized top quark, which undergoes the hadronic decay $t\to b u\bar d$, in the perturbative QCD framework, focusing on the energy profile and the differential energy profile. These substructures are factorized into the convolution of a hard top-quark decay kernel with a bottom-quark jet function and a $W$-boson jet function, where the latter is further factorized into the convolution of a hard $W$-boson decay kernel with two light-quark jet functions. Computing the hard kernels to leading order in QCD and including the resummation effect in the jet functions, we show that the differential jet energy profile is a useful observable for differentiating the helicity of a boosted hadronic top quark: a right-handed top jet exhibits quick descent of the differential energy profile with the inner test cone radius $r$, which is attributed to the $\mbox{V-A}$ structure of weak interaction and the dead-cone effect associated with the $W$-boson jet. The above helicity differentiation may help to reveal the chiral structure of physics beyond the Standard Model at high energies.

Flavor tagging TeV jets for physics beyond the Standard Model

We present a new scheme for tagging boosted heavy flavor jets called “µ x tagging.” At the LHC, the primary method to tag b -jets relies on tracking their charged constituents. However, when highly boosted, track-based b -tags lose efficiency, and the probability to mistag light jets rises dramatically. Using muons from B hadron decay and defining a particular combination “x ” of angular information and boost estimation, we find fairly flat efficiencies to tag b -jets, c -jets, light-quark jets, and light-heavy jets (containing B hadrons from gluon splitting) of ϵ b = 14%, ϵ c = 6:5%, ϵ light–light = 0:1%, and ϵ light-heavy = 0:5%, respectively. We demonstrate the usefulness of this new scheme by showing the reach for discovery of a leptophobic Z′ → bb in the dijet channel.

Associated jet and subjet rates in light-quark and gluon jet discrimination

We show that in studies of light quark- and gluon-initiated jet discrimination, it is important to include the information on softer reconstructed jets (associated jets) around a primary hard jet. This is particularly relevant while adopting a small radius parameter for reconstructing hadronic jets. The probability of having an associated jet as a function of the primary jet transverse momentum ($p_T$) and radius, the minimum associated jet $p_T$ and the association radius is computed upto next-to-double logarithmic accuracy (NDLA), and the predictions are compared with results from Herwig++, Pythia6 and Pythia8 Monte Carlos (MC). We demonstrate the improvement in quark-gluon discrimination on using the associated jet rate variable with the help of a multivariate analysis. The associated jet rates are found to be only mildly sensitive to the choice of parton shower and hadronization algorithms, as well as to the effects of initial state radiation and underlying event. In addition, the number of $k_T$ subjets of an anti-$k_T$ jet is found to be an observable that leads to a rather uniform prediction across different MC's, broadly being in agreement with predictions in NDLA, as compared to the often used number of charged tracks observable.

Light-quark and gluon jet discrimination in [Formula: see text] collisions at [Formula: see text] with the ATLAS detector.

A likelihood-based discriminant for the identification of quark- and gluon-initiated jets is built and validated using 4.7 fb^{-1} of proton–proton collision data at sqrt{s}=7 mathrm { TeV} collected with the ATLAS detector at the LHC. Data samples with enriched quark or gluon content are used in the construction and validation of templates of jet properties that are the input to the likelihood-based discriminant. The discriminating power of the jet tagger is established in both data and Monte Carlo samples within a systematic uncertainty of approx 10–20 %. In data, light-quark jets can be tagged with an efficiency of approx 50,% while achieving a gluon-jet mis-tag rate of approx 25,% in a p_{mathrm {T}} range between 40mathrm { GeV} and 360mathrm { GeV} for jets in the acceptance of the tracker. The rejection of gluon-jets found in the data is significantly below what is attainable using a Pythia 6 Monte Carlo simulation, where gluon-jet mis-tag rates of 10 % can be reached for a 50 % selection efficiency of light-quark jets using the same jet properties.

Holographic light quark jet quenching at RHIC and LHC via the shooting strings

A new shooting string holographic model of jet quenching of light quarks in strongly coupled plasmas is presented to overcome the phenomenological incompatibilities of previous falling string holographic scenarios that emerged when confronted with the recent LHC data. This model is based on strings with finite momentum endpoints that start close to the horizon and lose energy as they approach the boundary. This framework is applied to compute the nuclear modification factor RAA of light hadrons at RHIC and LHC, showing that this model improves greatly the comparison with the recent light hadron suppression data. The effects of the Gauss-Bonnet quadratic curvature corrections to the AdS5 geometry further improve the agreement with the data.

Searching for W′ bosons through decays to boosted-top and boosted-bottom jets

We propose a novel model-independent method to search for ${W}^{\ensuremath{'}}$ bosons at the Large Hadron Collider by looking at dijets where one jet is identified as a boosted-top jet and another jet is tagged as a boosted-bottom jet. Performing a detector-simulated signal and background study, we demonstrate that the reach in effective coupling ${g}^{\ensuremath{'}}$ is improved over existing analysis methods by up to factor of 2 for ${W}^{\ensuremath{'}}$ masses above 1.5 TeV, and the reach in mass is extended by 600 GeV to 2.6 TeV at an 8-TeV Large Hadron Collider. We introduce a method for distinguishing high-energy $b$ jets from light-quark jets and describe an important set of backgrounds to top jets. We propose a series of data-driven samples that might be used to measure the efficiencies for these new backgrounds in the LHC data.