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Zhenyu Han

Publications and source records attributed to Zhenyu Han.

At least 37 records · Page 2Linked to original sources

Boosting Searches for Natural SUSY with RPV via Gluino Cascades

In the presence of even minuscule baryonic R-parity violation, the stop can be the lightest superpartner and evade LHC searches because it decays into two jets. In order to cover this interesting possibility, we here consider new searches for RPV stops produced in gluino cascades. While typical searches for gluinos decaying to stops rely on same-sign dileptons, the RPV cascades usually have fewer hard leptons, less excess missing energy, and more jets than R-parity conserving cascades. If the gluino is a Dirac fermion, same-sign dilepton signals are also often highly depleted. We therefore explore search strategies that use single-lepton channels, and combat backgrounds using HT, jet counting, and more detailed multijet kinematics or jet substructure. We demonstrate that the stop mass peaks can be fully reconstructed over a broad range of spectra, even given the very high jet multiplicities. This would not only serve as a "double-discovery" opportunity, but would also be a spectacular confirmation that the elusive top-partner has been hiding in multijets.

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(Light) Stop Signs

Stop squarks with a mass just above the top's and which decay to a nearly massless LSP are difficult to probe because of the large SM di-top background. Here we discuss search strategies which could be used to set more stringent bounds in this difficult region. In particular, we note that both the rapidity difference Delta y(t,tbar) and spin correlations (inferred from, for example, Delta phi(l+,l-)) are sensitive to the presence of stops. We emphasize that systematic uncertainties in top quark production can confound analyses looking for stops, making theoretical and experimental progress on the understanding of Standard Model top production at high precision a very important task. We estimate that spin correlation alone, which is relatively robust against such systematic uncertainties, can exclude a 200 GeV stop at 95% confidence with 20 fb^-1 at the 8 TeV LHC.

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Tracking the Identities of Boosted Particles

We show that the tracking system in a collider detector can be used to efficiently identify boosted massive particles from their QCD backgrounds. We examine variables defined with tracking information which are sensitive to jet radiation patterns, including charged particle multiplicity and N-subjettiness. These variables are barely correlated with variables sensitive to the hard splitting scale in the jet, such as the filtered jet mass. Therefore these two kinds of variables should be combined to optimize the discriminating power. We illustrate the method with $W$ jet tagging. It is shown that for jet PT=500 GeV, one can gain a factor of 1.6 in statistical significance by combining filtered jet mass and charged multiplicity, over filtered mass alone. Adding N-subjettiness increases the factor to 1.8.

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Improving the Top Quark Forward-Backward Asymmetry Measurement at the LHC

At the LHC, top quark pairs are dominantly produced from gluons, making it difficult to measure the top quark forward-backward asymmetry. To improve the asymmetry measurement, we study variables that can distinguish between top quarks produced from quarks and those from gluons: the invariant mass of the top pair, the rapidity of the top-antitop system in the lab frame, the rapidity of the top quark in the top-antitop rest frame, the top quark polarization and the top-antitop spin correlation. We combine all the variables in a likelihood discriminant method to separate quark-initiated events from gluon-initiated events. We apply our method on models including G-prime's and W-prime's motivated by the recent observation of a large top quark forward-backward asymmetry at the Tevatron. We have found that the significance of the asymmetry measurement can be improved by 10% to 30%. At the same time, the central values of the asymmetry increase by 40% to 100%. We have also analytically derived the best spin quantization axes for studying top quark polarization as well as spin-correlation for the new physics models.

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Top condensation as a motivated explanation of the top forward-backward asymmetry

Models of top condensation can provide both a compelling solution to the hierarchy problem as well as an explanation of why the top-quark mass is large. The spectrum of such models, in particular topcolor-assisted technicolor, includes top-pions, top-rhos and the top-Higgs, all of which can easily have large top-charm or top-up couplings. Large top-up couplings in particular would lead to a top forward-backward asymmetry through $t$-channel exchange, easily consistent with the Tevatron measurements. Intriguingly, there is destructive interference between the top-mesons and the standard model which conspire to make the overall top pair production rate consistent with the standard model. The rate for same-sign top production is also small due to destructive interference between the neutral top-pion and the top-Higgs. Flavor physics is under control because new physics is mostly confined to the top quark. In this way, top condensation can explain the asymmetry and be consistent with all experimental bounds. There are many additional signatures of topcolor with large tu mixing, such as top(s)+jet(s) events, in which a top and a jet reconstruct a resonance mass, which make these models easily testable at the LHC.

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W-jet Tagging: Optimizing the Identification of Boosted Hadronically-Decaying W Bosons

A method is proposed for distinguishing highly boosted hadronically decaying W's (W-jets) from QCD-jets using jet substructure. Previous methods, such as the filtering/mass-drop method, can give a factor of ~2 improvement in S/sqrt(B) for jet pT > 200 GeV. In contrast, a multivariate approach including new discriminants such as R-cores, which characterize the shape of the W-jet, subjet planar flow, and grooming-sensitivities is shown to provide a much larger factor of ~5 improvement in S/sqrt(B). For longitudinally polarized W's, such as those coming from many new physics models, the discrimination is even better. Comparing different Monte Carlo simulations, we observe a sensitivity of some variables to the underlying event; however, even with a conservative estimates, the multivariate approach is very powerful. Applications to semileptonic WW resonance searches and all-hadronic W+jet searches at the LHC are also discussed. Code implementing our W-jet tagging algorithm is publicly available at http://jets.physics.harvard.edu/wtag

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Missing Momentum Reconstruction and Spin Measurements at Hadron Colliders

We study methods for reconstructing the momenta of invisible particles in cascade decay chains at hadron colliders. We focus on scenarios, such as SUSY and UED, in which new physics particles are pair produced. Their subsequent decays lead to two decay chains ending with neutral stable particles escaping detection. Assuming that the masses of the decaying particles are already measured, we obtain the momenta by imposing the mass-shell constraints. Using this information, we develop techniques of determining spins of particles in theories beyond the standard model. Unlike the methods relying on Lorentz invariant variables, this method can be used to determine the spin of the particle which initiates the decay chain. We present two complementary ways of applying our method by using more inclusive variables relying on kinematic information from one decay chain, as well as constructing correlation variables based on the kinematics of both decay chains in the same event.

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Accurate Mass Determinations in Decay Chains with Missing Energy: II

We discuss kinematic methods for determining the masses of the particles in events at a hadron collider in which a pair of identical particles is produced with each decaying via a series of on-shell intermediate beyond-the-SM (BSM) particles to visible SM particles and an invisible particle (schematically, pp -> ZZ + jets with Z -> Aa -> Bba -> Ccba -> ... -> cba... + N where a,b,c,... are visible SM particles or groups of SM particles, A,B,C,... are on-shell BSM particles and N is invisible). This topology arises in many models including SUSY processes such as squark and gluino pair production and decay. We present the detailed procedure for the case of Z -> 3 visible particles + N and demonstrate that the masses obtained from the kinematic procedure are independent of the model by comparing SUSY to UED.

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Measuring the Dark Force at the LHC

A long-range ``dark force'' has recently been proposed to mediate the dark matter (DM) annihilation. If DM particles are copiously produced at the Large Hadron Collider (LHC), the light dark force mediator will also be produced through radiation. We demonstrate how and how precise we can utilize this fact to measure the coupling constant of the dark force. The light mediator's mass is measured from the ``lepton jet'' it decays to. In addition, the mass of the DM particle is determined using the MT2 technique. Knowing these quantities is critical for calculating the DM relic density.

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A Unified Dark Matter Model in sUED

We propose a dark matter model with standard model singlet extension of the universal extra dimension model (sUED) to explain the recent observations of ATIC, PPB-BETS, PAMELA and DAMA. Other than the standard model fields propagating in the bulk of a 5-dimensional space, one fermion field and one scalar field are introduced and both are standard model singlets. The zero mode of the new fermion is identified as the right-handed neutrino, while its first KK mode is the lightest KK-odd particle and the dark matter candidate. The cosmic ray spectra from ATIC and PPB-BETS determine the dark matter particle mass and hence the fifth dimension compactification scale to be 1.0-1.6 TeV. The zero mode of the singlet scalar field with a mass below 1 GeV provides an attractive force between dark matter particles, which allows a Sommerfeld enhancement to boost the annihilation cross section in the Galactic halo to explain the PAMELA data. The DAMA annual modulation results are explained by coupling the same scalar field to the electron via a higher-dimensional operator. We analyze the model parameter space that can satisfy the dark matter relic abundance and accommodate all the dark matter detection experiments. We also consider constraints from the diffuse extragalactic gamma-ray background, which can be satisfied if the dark matter particle and the first KK-mode of the scalar field have highly degenerate masses.

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Top-antitop and Top-top Resonances in the Dilepton Channel at the CERN LHC

We perform a model-independent study for top-antitop and top-top resonances in the dilepton channel at the Large Hadtron Collider. In this channel, we can solve the kinematic system to obtain the momenta of all particles including the two neutrinos, and hence the resonance mass and spin. For discovering top-antitop resonances, the dilepton channel is competitive to the semileptonic channel because of the good resolution of lepton momentum measurement and small standard model backgrounds. Moreover, the charges of the two leptons can be identified, which makes the dilepton channel advantageous for discovering top-top resonances and for distinguishing resonance spins. We discuss and provide resolutions for difficulties associated with heavy resonances and highly boosted top quarks.

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Minimal Kinematic Constraints and MT2

We clarify the relation between the variable MT2 and the method of kinematic constraints, both of which can be used for mass determination in events with two missing (dark matter) particles at hadron colliders. We identify a set of minimal kinematic constraints, including the mass shell conditions for the missing particles and their mother particles, as well as the constraint from the measured missing transverse momentum. We show that MT2 is the boundary of the mass region consistent with the minimal constraints. From this point of view, we also obtained a more efficient algorithm for calculating MT2. When more constraints are available in the events, we can develop more sophisticated mass determination methods starting from the MT2 constraint. In particular, we discuss cases when each decay chain contains two visible particles.

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Effective Theories and Electroweak Precision Constraints

This is a pedagogical and self-contained review on obtaining electroweak precision constraints on TeV scale new physics using the effective theory method. We identify a set of relevant effective operators in the standard model and calculate from them corrections to all major electroweak precision observables. The corrections are compared with data to put constraints on the effective operators. Various approaches and applications in the literature are reviewed.

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Accurate Mass Determinations in Decay Chains with Missing Energy

Many beyond the Standard Model theories include a stable dark matter candidate that yields missing / invisible energy in collider detectors. If observed at the Large Hadron Collider, we must determine if its mass and other properties (and those of its partners) predict the correct dark matter relic density. We give a new procedure for determining its mass with small error.

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Mass Determination in SUSY-like Events with Missing Energy

We describe a kinematic method which is capable of determining the overall mass scale in SUSY-like events at a hadron collider with two missing (dark matter) particles. We focus on the kinematic topology in which a pair of identical particles is produced with each decaying to two leptons and an invisible particle (schematically, $pp\to YY+jets$ followed by each $Y$ decaying via $Y\to \ell X\to \ell\ell'N$ where $N$ is invisible). This topology arises in many SUSY processes such as squark and gluino production and decay, not to mention $t\anti t$ di-lepton decays. In the example where the final state leptons are all muons, our errors on the masses of the particles $Y$, $X$ and $N$ in the decay chain range from 4 GeV for 2000 events after cuts to 13 GeV for 400 events after cuts. Errors for mass differences are much smaller. Our ability to determine masses comes from considering all the kinematic information in the event, including the missing momentum, in conjunction with the quadratic constraints that arise from the $Y$, $X$ and $N$ mass-shell conditions. Realistic missing momentum and lepton momenta uncertainties are included in the analysis.

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Higgs boson from the meta-stable SUSY breaking sector

We construct a calculable model of electroweak symmetry breaking in which the Higgs doublet emerges from the meta-stable SUSY breaking sector as a pseudo Nambu-Goldstone boson. The Higgs boson mass is further protected by the little Higgs mechanism, and naturally suppressed by a two-loop factor from the SUSY breaking scale of 10 TeV. Gaugino and sfermion masses arise from standard gauge mediation, but the Higgsino obtains a tree-level mass at the SUSY breaking scale. At 1 TeV, aside from new gauge bosons and fermions similar to other little Higgs models and their superpartners, our model predicts additional electroweak triplets and doublets from the SUSY breaking sector.

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Electroweak Constraints on Effective Theories

We discuss electroweak constraints on TeV scale extensions of the standard model. To obtain model-independent results, effective theory approach is adopted. Constraints are given on arbitrary linear combinations of a set of dimension-6 operators that respect the SM gauge symmetry, as well as CP, lepton and baryon number conservation. Applications of the results are also discussed.

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Electroweak Constraints on Effective Theories with U(2)xU(1) Flavor Symmetry

In a previous analysis presented in hep-ph/0412166, electroweak constraints were given on arbitrary linear combinations of a set of dimension-6 operators. Flavor universality and thus U(3)^5 flavor symmetry were assumed for the operators. In this article, we expand the analysis to account for the flavor-dependent theories that distinguish the third generation of fermions from the light two generations. We still assume flavor universality for the light two generations to avoid large FCNCs. Consequently, the U(3)^5 flavor symmetry is relaxed to [U(2)xU(1)]^5, and therefore more operators are added. We calculate the corrections to electroweak precision observables, assuming arbitrary coefficients for the operators. The corrections are combined with the standard model predictions and the experimental data to obtain the chi^2 distribution as a function of the operator coefficients. We apply our result to constrain two flavor-dependent extensions of the standard model: the simplest little Higgs model and a model with an SU(2)xSU(2)xU(1) gauge group.

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