17983245
Boosted decision trees, a powerful event classifier
Consider the problem of classification of events between signal and background, given a number of particle identification (PID) variables. A decision tree is a sequence of binary splits of the data. To train the tree a set of known training events are used. The results are measured using a separate set of known testing events. Consider all of the data to be on one node. The best PID variable and best place on that variable to split the data into separate signal and background is found. There are then two nodes. The process is repeated on these new nodes and is continued until a given number of final nodes (called “leaves”) are obtained, or until all leaves are pure or until a node has too few events. There are several popular criteria to determine the best PID variable and best place on which to split a node. The gini criterion is used here. Suppose that event i has weight Wi. The purity P of a node is defined as the weight of signal events on the node divided by the total weight of events on that node. For a given node: gini = P (1 − P ) ∑ i Wi. gini is zero for P = 1 or P = 0. The best split is chosen as the one which minimizes gini. The next node to split is chosen by finding that node whose splitting maximizes the change in gini. In this way a decision tree is built. Leaves with P ≥ 0.5 are signal leaves and the rest are background leaves. Decision trees are powerful, but unstable. A small change in the training data can produce a large change in the tree. This is remedied by the use of boosting. For boosting, the training events which were misclassified (a signal event fell on a background leaf or vice versa) have their weights increased (boosted), and a new tree is formed. This procedure is then repeated for the new tree. In this way many trees are built up. The score from the mth individual tree Tm is taken as +1 if the event falls on a signal leaf and −1 if the event falls on a background leaf. The final score is taken as a weighted sum of the scores of the individual leaves. Two methods for boosting are considered here. The first is called AdaBoost. Define errm = weight misclassified/total weight for tree m. Let αm = β log [(1 − errm)/errm], where β is a constant. In the statistical literature β has been taken as one, but for the MiniBooNE experiment, β = 0.5 has been found to be the optimum value. The misclassified events have their weight multiplied by em . The weights are then renormalized so the sum of all of the training event weights is one. The final score is T = ∑Ntree m=1 αmTm. The second method of boosting considered here is called -boost or, sometimes, shrinkage. Misclassified events have their weight multiplied by e , where is a constant. For the MiniBooNE experiment, = 0.03 has been optimum. (The results vary only mildly as β or are changed a bit.) The final score is T = ∑Ntree m=1 Tm. -boost changes weights a little at a time, while AdaBoost can be shown to try to optimize each change in weights to minimize e where T is the score and y is +1 for a signal event and −1 for a background event. The optimum value is F = log prob/(1 − prob), where prob is the probability that y = 1, given the observed PID variables. In practice, for MiniBooNE, the two boosting methods have performed almost equally well. Boosting is described as using many weak classifiers to build a strong classifier. This is seen in Figure 1. After the first few trees, the misclassification fraction for an individual tree is above 40%. In the MiniBooNE experiment some hundreds of possible PID variables have been suggested. The most powerful of these have been selected by accepting those which are used most often as the splitting variable. Some care needs to be taken as some-
18986664
Discovery Potential of the Standard Model Higgs Boson Through H -> WW Decay Mode with the ATLAS Detector at LHC
We report results of a study of the Standard Model Higgs boson discovery potential through the W-pair leptonic decay modes with the ATLAS detector at LHC at 14 TeV center-of-mass energy. We used MC samples with full detector simulation and reconstruction of the ATLAS experiment to estimate the ATLAS detection sensitivity for the reaction of pp -> H -> WW -> e\nu \mu\nu with no hard jet or two hard jets in the final states. The prospects for the Higgs boson searches at ATLAS are presented, including trigger efficiencies and data-driven methods to estimate the backgrounds using control samples in data. With 10/fb of integrated luminosity, one would expect to discover a Standard Model Higgs boson with ATLAS detector in Higgs boson mass range 135 < m_H < 190 GeV. If the Higgs boson does not exist, we will extend and confirm the exclusion produced by the Tevatron Higgs boson search result, which has ruled out the Higgs boson mass range of 160 < m_H < 170 GeV at 95% confidence level. If Higgs boson would be discovered, ATLAS could measure its mass with a precision of about 2 and 7 GeV for Higgs boson mass around 160 GeV and 130 GeV, respectively.
117928786
Measurement of the $W^+W^-$ Cross Section in 7 TeV pp Collisions with the ATLAS Detector
15769713
A Complete Classification of D-branes in Type IIB Plane Wave Background
We give a complete classification of D-branes in the type IIB plane wave background using Green-Schwarz superstring theory. We find all possible supersymmetric oblique and curved D-branes and consider their intersections. The supersymmetries preserved by various configurations of these D-branes are also identified. In particular, we show that D+-branes of type (+,−, n, n) for n = 1, 2, 3, 4 preserve 4 dynamical supersymmetries by introducing gauge field excitations and newly discovered oblique D5and D7-branes also preserve four or two dynamical supersymmetries with gauge field condensates. Furthermore we find new curved D-branes preserving four dynamical supersymmetries.
119256430
Remnants, fermions’ tunnelling and effects of quantum gravity
A bstractThe remnants are investigated by fermions’ tunnelling from a 4-dimensional charged dilatonic black hole and a 5-dimensional black string. Based on the generalized uncertainty principle, effects of quantum gravity are taken into account. The quantum numbers of the emitted fermions affects the Hawking temperatures. For the black hole, the quantum gravity correction slows down the increase of the temperature, which leads to the remnant left in the evaporation. For the black string, the existence of the remnant is determined by the quantum gravity correction and effects of the extra compact dimension.
17081750
String Field Theory and Tachyon Dynamics
In this thesis we present some works done during my doctoral studies. These results focus on two directions. The first one is motivated by tachyon dynamics in open string theory. We calculate the stress tensors for the p-adic string model and for the pure tachyonic sector of open string field theory (OSFT). We give the energy density of lump solutions and attempt to evaluate the evolution of the pressure in rolling tachyon solutions. We discuss the relevance of the pressure calculation for the identification of the large time solution with a gas of closed strings. In the second direction, we give some results in closed string field theory (CSFT). We considered marginal deformations in CSFT. The marginal parameter, called a, is that associated with the dimension-zero primary operator cWcX&X. We use this marginal operator to test the quartic structure of CSFT and the feasibility of level expansion. We check the vanishing of the effective potential for a. In the level expansion the quartic terms generated by the cubic interactions must be cancelled by the elementary quartic interaction of four marginal operators. We confirm this prediction, thus giving evidence that the sign, normalization, and region of integration Vo, 4 for the quartic vertex are all correct. This is the first calculation of an elementary quartic amplitude for which there is an expectation that can be checked. We also extend the calculation to the case of the four marginal operators associated with two space coordinates. We then try to search a critical point of the tachyon potential in CSFT. We include the tachyon, the dilaton, and massive fields in the computation. Some evidence is found for the existence of a closed string tachyon vacuum. It seems that this critical point becomes more shallow when higher level contributions are considered. We also relate fields in the sigma model and those in CSFT. Moreover, large dilaton deformations are studied numerically. Finally, we use the low-energy effective field equations that couple gravity, the dilaton, and the bulk closed string tachyon to study the end result of the physical decay process associated with the instability of closed string tachyon. We establish that whenever the tachyon induces the rolling process, the Einstein metric undergoes collapse while the dilaton rolls to strong coupling. Some more general potentials and the possible cosmological application are discussed. Thesis Supervisor: Barton Zwiebach Title: Professor of Physics
119410655
Exact Seiberg–Witten map, induced gravity and topological invariants in non-commutative field theories
Abstract We revisit the exact Seiberg–Witten (SW) map on Dirac–Born–Infeld actions, making a connection with the deformation quantization scheme. The picture on field dependent induced gravity from non-commutativity becomes more transparent in the context of deformation quantization. We also find an exact SW map for an adjoint scalar field, consistent with that deduced from RR couplings of unstable non-BPS D-branes. The dual description via the exact SW map can again be interpreted as the ordinary field theory coupling to gravity induced by gauge fields. Using the exact SW maps, we further discuss several aspects of topological invariants in non-commutative (NC) gauge theory. Especially, it is shown that the K-theory class on NC instantons is mapped to the usual second Chern class via exact SW map and it leads to an exact SW map between commutative and NC Chern–Simons terms.
119468158
The nonperturbative closed string tachyon vacuum to high level
We compute the action of closed bosonic string field theory at quartic order with fields up to level ten. After level four, the value of the potential at the minimum starts oscillating around a nonzero negative value, in contrast with the proposition made in [5]. We try a different truncation scheme in which the value of the potential converges faster with the level. By extrapolating these values, we are able to give a rather precise value for the depth of the potential.