Person:

Schwartz, Matthew

Having pure samples of quark and gluon jets would greatly facilitate the study of jet properties and substructure, with many potential standard model and new physics applications. To this end, we consider multijet and jets (+ X) samples, to determine the purity that can be achieved by simple kinematic cuts leaving reasonable production cross sections. We find, for example, that at the 7TeV LHC, the (pp \to \gamma +2)jets sample can provide 98% pure quarkjets with 200 GeV of transverse momentum and a cross section of 5 pb. Toget 10 pb of 200 GeV jets with 90% gluon purity, the (pp \to 3) jets sample can be used. (b+2)jets is also useful for gluons, but only if the b-tagging is very efficient.

The LHC will have unprecedented sensitivity to flavor-changing neutral current (FCNC) top quark decays, whose observation would be a clear sign of physics beyond the standard model. Although many details of top flavor violation are model dependent, the standard model gauge symmetries relate top FCNCs to other processes, which are strongly constrained by existing data. We study these constraints in a model independent way, using a low energy effective theory from which the new physics is integrated out. We consider the most important operators which contribute to top FCNCs and analyze the current constraints on them. We find that the data rule out top FCNCs at a level observable at the LHC due to most of the operators comprising left-handed first or second generation quark fields, while there remains a substantial window for top decays mediated by operators with right-handed charm or up quarks. If FCNC top decays are observed at the LHC, such an analysis may help decipher the underlying physics.

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 (\sim 2) improvement in (S/\sqrt{B}) for jet (p_T \ge 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 (\sim 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-je)t tagging algorithm is publicly available at http://jets.physics.harvard.edu/wtag.

Starting from a factorization theorem in Soft-Collinear Effective Theory, the thrust distribution in e(^+)e(^-) collisions is calculated including resummation of the next-to-next-to-next-to leading logarithms. This is a significant improvement over previous calculations which were only valid to next-to-leading logarithmic order. The fixed-order expansion of the resummed result approaches the exact fixed-order distribution towards the kinematic endpoint. This close agreement provides a verification of both the effective field theory expression and recently completed next-to-next-to-leading fixed order event shapes. The resummed distribution is then matched to fixed order, resulting in a distribution valid over a large range of thrust. A fit to ALEPH and OPAL data from LEP 1 and LEP 2 produces (\alpha_s(m_Z))= 0.1172 (\pm) 0.0010 (\pm) 0.0008 (\pm) 0.0012 (\pm) 0.0012, where the uncertainties are respectively statistical, systematic, hadronic, and perturbative. This is one of the world's most precise extractions of (\alpha_s) to date.

A systematic method for optimizing multivariate discriminants is developed and applied to the important example of a light Higgs boson search at the Tevatron and the LHC. The Significance Improvement Characteristic (SIC), defined as the signal efficiency of a cut or multivariate discriminant divided by the square root of the background efficiency, is shown to be an extremely powerful visualization tool. SIC curves demonstrate numerical instabilities in the multivariate discriminants, show convergence as the number of variables is increased, and display the sensitivity to the optimal cut values. For our application, we concentrate on Higgs boson production in association with a W or Z boson with (H \rightarrow b\bar{b}) and compare to the irreducible standard model background, (Z/W + b\bar{b}). We explore thousands of experimentally motivated, physically motivated, and unmotivated single variable discriminants. Along with the standard kinematic variables, a number of new ones, such as twist, are described which should have applicability to many processes. We find that some single variables, such as the pull angle, are weak discriminants, but when combined with others they provide important marginal improvement. We also find that multiple Higgs boson-candidate mass measures, such as from mild and aggressively trimmed jets, when combined may provide additional discriminating power. Comparing the significance improvement from our variables to those used in recent CDF and D(\varnothing) searches, we find that a 10-20% improvement in significance against (Z/W + b\bar{b}) is possible. Our analysis also suggests that the (H + W/Z) channel with (H \rightarrow b\bar{b}) is also viable at the LHC, without requiring a hard cut on the (W/Z) transverse momentum.

Using recently developed techniques for computing event shapes with Soft-Collinear Effective Theory, LEP event shape data is used to derive strong model-independent bounds on new colored particles. In the effective field theory computation, colored particles contribute in loops not only to the running of ({\alpha}_s) but also to the running of hard, jet and soft functions. Moreover, the differential distribution in the effective theory explicitly probes many energy scales, so event shapes have strong sensitivity to new particle thresholds. Using thrust data from ALEPH and OPAL, colored adjoint fermions (such as a gluino) below 51.0 GeV are ruled out to 95% confidence level. This is nearly an order-of-magnitude improvement over the previous model-independent bound of 6.3 GeV.

A method is introduced for distinguishing top jets (boosted, hadronically decaying top quarks) from light quark and gluon jets using jet substructure. The procedure involves parsing the jet cluster to resolve its subjets, and then imposing kinematic constraints. With this method, light quark or gluon jets with (p_T \simeq) 1 TeV can be rejected with an efficiency of around 99% while retaining up to 40% of top jets. This reduces the dijet background to heavy (t\bar{t}) resonances by a factor of ~10,000, thereby allowing resonance searches in (t\bar{t}) to be extended into the all-hadronic channel. In addition, top-tagging can be used in (t\bar{t}) events when one of the tops decays semi-leptonically, in events with missing energy, and in studies of b-tagging efficiency at high (p_T).

The resummed differential thrust rate in (e^+e^-) annihilation is calculated using Soft-Collinear Effective Theory (SCET). The resulting distribution in the two-jet region ((T \sim 1)) is found to agree with the corresponding expression derived by the standard approach. A matching procedure to account for finite corrections at (T < 1) is then described. There are two important advantages of the SCET approach. First, SCET manifests a dynamical seesaw scale (q = p^2/Q) in addition to the center-of-mass energy (Q) and the jet mass scale (p \sim Q\sqrt{(1 - T)}). Thus, the resummation of logs of (p/q) can be cleanly distinguished from the resummation of logs of (Q/p). Second, finite parts of loop amplitudes appear in specific places in the perturbative distribution: in the matching to the hard function, at the scale (Q), in matching to the jet function, at the scale p, and in matching to the soft function, at the scale (q). This allows for a consistent merger of fixed order corrections and resummation. In particular, the total NLO (e^+e^-) cross section is reproduced from these finite parts without having to perform additional infrared regulation.

The hemisphere soft function is calculated to order (\alpha_s^2). This is the first multi-scale soft function calculated to two loops. The renormalization scale dependence of the result agrees exactly with the prediction from effective field theory. This fixes the unknown coefficients of the singular parts of the two-loop thrust and heavy-jet mass distributions. There are four such coefficients, for 2 event shapes and 2 color structures, which are shown to be in excellent agreement with previous numerical extraction. The asymptotic behavior of the soft function has double logs in the (C_F C_A) color structure, which agree with non-global log calculations, but also has sub-leading single logs for both the (C_F C_A) and (C_F T_F n_f) color structures. The general form of the soft function is complicated, does not factorize in a simple way, and disagrees with the Hoang-Kluth ansatz. The exact hemisphere soft function will remove one source of uncertainty on the (\alpha_s) fits from (e^+e^-) event shapes.

The production of hard photons in hadronic collisions is studied using Soft-Collinear Effective Theory (SCET). This is the first application of SCET to a physical, observable cross section involving energetic partons in more than two directions. A factorization formula is derived which involves a non-trivial interplay of the angular dependence in the hard and soft functions, both quark and gluon jet functions, and multiple partonic channels. The relevant hard, jet and soft functions are computed to one loop and their anomalous dimensions are determined to three loops. The final resummed inclusive direct photon distribution is valid to next-to-next-to-leading logarithmic order (NNLL), one order beyond previous work. The result is improved by including non-logarithmic terms and photon isolation cuts through matching, and compared to Tevatron data and to fixed order results at the Tevatron and the LHC. The resummed cross section has a significantly smaller theoretical uncertainty than the next-to-leading fixed-order result, particularly at high transverse momentum.