arXiv subjects

Li-Gang Xia

Publications and source records attributed to Li-Gang Xia.

14 recordsLinked to original sources

arXiv2025

A model of mass generation

In this work, we build a model to combine the mass generated from the Higgs mechanism and that from the dynamical chiral symmetry breaking mechanism. This is motivated by the fermion mass hierarchy that the neutrino mass is smaller than the charged lepton mass and the charged lepton mass is smaller than the quark mass. Since they participate different interactions, it is natural to conjecture that interactions contribute to the fermion mass. This conjecture could be explained via the dynamical chiral symmetry breaking mechanism with assuming the existence of a non-perturbative regime. In addition, this model predicts a different ratio of fermion Yukawa coupling to the Higgs self coupling, which could be verified in the near future.

physics.gen-ph↗

arXiv2021

$t$-channel Higgs production and constraints on off-shell Higgs couplings

This note is to record an unsucessful idea. I tried hard to persuade myself to write this note as I have spent a significant fraction of time during this summer. For Higgs resonant production and decaying in the $s$ channel, $i \to H \to f$, the cross section is proportional to $y_i^2y_f^2/\Gamma_H$. Thus there is a multi-solution ambiguity on the couplings due to uncertain knowledge on Higgs width. In this work, we study the process $g+q \to g+q+l^++l^-$ with the $t$-channel Higgs boson contribution included. As Higgs boson is not on-shell, this process can be used to measure the Higgs couplings without the ambiguity. Based on the performance of the ATLAS detector in Run~II, the coupling product $y_gy_Z$ can be potentially constrained to be less than 18 times the Standard-Model value for a $p$-$p$ dataset of 140~fb$^{-1}$ at $\sqrt{s}=13$~TeV. Assuming the equality of on-shell couplings and off-shell couplings and combining the on-shell measurements, the width of the Higgs boson is constrained to be less than 1.3~GeV, which is similar to that in the direct measurement. In addition, the interference effect is also analyzed.

hep-ph↗

arXiv2021

Improved Asymptotic Formulae for Statistical Interpretation Based on Likelihood Ratio Tests

In this work, we attempt to refine the classic asymptotic formulae to describe the probability distribution of likelihood-ratio statistical tests. The idea is to split the probability distribution function into two parts. One part is universal and described by the asymptotic formulae. The other part is case-dependent and is estimated explicitly using a 6-bin model proposed in this work. The latter is similar to performing toy simulations and can therefore predict the discrete structures in the probability distributions. The new asymptotic formulae provide a much better differential description of the test statistics. This improved performance is demonstrated in two toy examples for common likelihood ratio statistics.

physics.data-an↗

arXiv2020

Search for Low-Mass Leptoquarks using the $llqq$ final states in Pb-Pb Ultra-Peripheral Collisions

After a review over past experiments and theoretical requirements from low-energy flavour physics, we argue that the possibility of low-mass leptoquarks (LQ) cannot be fully excluded due to the assumptions made in the measurements. Therefore we propose to search for the pair production of low-mass leptoquarks in Pb-Pb ultra-peripheral collisions with the least model dependence (assuming a small coupling constant $\lesssim0.1$). There are a couple of advantages: 1) high photon flux provides high production rate for low mass LQs; 2) the background contamination is much lower than that in $p$-$p$ collisions. The analysis strategy permits a leptoquark to decay to all possible lepton-plus-quark modes. Taking the scalar LQ, $S_3$, with an electric charge $|q|=\frac{4}{3}e$, as example, the mass point of 100 GeV can be excluded at the 95~\% confidence level using a dataset of 4~pb$^{-1}$ Pb-Pb ultra-peripheral collisions at $\sqrt{s}=5.02$ TeV and the performance of the ATLAS detector in Run~2. The proposed method also applies to searching for high-mass LQs in $p$-$p$ collisions as long as the LQ pair production mechanism dominates.

hep-ph↗

arXiv2020

Enhanced inequalities about arithmetic and geometric means

For $n$ positive numbers ($a_k$, $1\leq k \leq n$), enhanced inequalities about the arithmetic mean ($A_n \equiv \frac{\sum_ka_k}{n}$) and the geometric mean ($G_n\equiv \sqrt[n]{\Pi_ka_k}$) are found if some numbers are known, namely, \begin{equation} \frac{G_n}{A_n} \leq (\frac{n-\sum_{k=1}^mr_k}{n-m})^{1-\frac{m}{n}}(\Pi_{k=1}^mr_k)^{\frac{1}{n}} \:, \nonumber \end{equation} if we know $a_k=A_nr_k$ ($1\leq k\leq m\leq n$) for instance, and \begin{equation} \frac{G_n}{A_n} \leq \frac{1}{(1-\frac{m}{n})\Pi_{k=1}^mr_k^{\frac{-1}{n-m}}+\frac{1}{n}\sum_{k=1}^mr_k} \: ,\nonumber \end{equation} if we know $a_k=G_nr_k$ ($1\leq k\leq m \leq n$) for instance. These bounds are better than those derived from S.~H.~Tung's work [1].

math.GM↗

arXiv2019

F2F, a model-independent method to determine the mass and width of a particle in the presence of interference

It is generally believed that any particle to be discovered will have a TeV-order mass. Given its great mass, it must have a large decay width. Therefore, the interference effect will be very common if they and the Standard-Model (SM) particles contribute to the same final state.However, the interference effect could make a new particle show up not like a resonance, and it is difficult to search and measure its properties. In this work, a model-independent method, F2F (Fit To Fourier coefficients), is proposed to search for an unknown resonance and to determine its mass ($M$) and width ($\Gamma$) in the presence of interference. Basically we express the sum of reosnant signal and the interference as a cosine Fourier series and relate the Fourier coefficients with the mass and width. The relation is based on the general propagator form, $1/(x^2-M^2+iM\Gamma)$. Thus it does not need any signal model. Toy experiments show that the obtained mass and width agree well with the inputs with a similar precision as using an explicit signal model. We also show that we can apply this method to measure the Stardard-Model Higgs width and to make statistic interpretation in searching for new resonance allowing for interference.

hep-ph↗

arXiv2018

Understanding the boosted decision tree methods with the weak-learner approximation

Two popular boosted decsion tree (BDT) methods, Adaptive BDT (AdaBDT) and Gradient BDT (GradBDT) are studied in the classification problem of separating signal from background assuming all trees are weak learners. The following results are obtained. a) The distribution of the BDT score is approximately Gaussian for both methods. b) With more trees in training, the distance of the expectaion of score distribution between signal and background is larger, but the variance of both distributions becomes greater at the same time. c) Extenstion of the boosting mechanism in AdaBDT to any loss function is possible. d) AdaBDT is shown to be equivalent to the GradBDT with 2 terminal nodes for a decision tree. In the field high energy physics, many applications persue the best statistical significance. We also show that the maximization of the statistical significance is closely related to the minimization of the loss function, which is the target of the BDT algorithms.

physics.data-an↗

arXiv2018

QBDT, a new boosting decision tree method with systematic uncertainties into training for High Energy Physics

A new boosting decision tree (BDT) method, QBDT, is proposed for the classification problem in the field of high energy physics (HEP). In many HEP researches, great efforts are made to increase the signal significance with the presence of huge background and various systematical uncertainties. Why not develop a BDT method targeting the significance directly? Indeed, the significance plays a central role in this new method. It is used to split a node in building a tree and to be also the weight contributing to the BDT score. As the systematical uncertainties can be easily included in the significance calculation, this method is able to learn about reducing the effect of the systematical uncertainties via training. Taking the search of the rare radiative Higgs decay in proton-proton collisions $pp \to h + X \to \gamma\tau^+\tau^-+X$ as example, QBDT and the popular Gradient BDT (GradBDT) method are compared. QBDT is found to reduce the correlation between the signal strength and systematical uncertainty sources and thus to give a better significance. The contribution to the signal strength uncertainty from the systematical uncertainty sources using the new method is 50-85~\% of that using the GradBDT method.

physics.data-an↗

arXiv2018

Study of constraint and impact of a nuisance parameter in maximum likelihood method

Maximum likelihood method is widely used for parameter estimation in high energy physics. To consider various systematic uncertainties, tens of or even hundreds of nuisance parameters (NP) are introduced in a likelihood fit. The constraint of a nuisance parameter and its impact on the parameter of interest (POI) will be the main concerns for a precise measurement. A fit involving many parameters is usually slow and it is even more time-consuming to investigate why a parameter is over-constrained or has a large impact. In this paper, we are trying to understand the reasons behind and provide simple formulae to estimate the constraint and impact directly.

physics.data-an↗

arXiv2017

A theorem about two-body decay and its application for a doubly-charged boson $H^{\pm\pm}$ going to $\tau^{\pm}\tau^{\pm}$

In a general decay chain $A\to B_1B_2\to C_1C_2\ldots$, we prove that the angular correlation function $I(\theta_1,\theta_2,\phi_+)$ in the decay of $B_{1,2}$ is irrelevant to the polarization of the mother particle $A$ at production. This guarantees that we can use these angular distributions to determine the spin-parity nature of $A$ without knowing its production details. As an example, we investigate the decay of a potential doubly-charged boson $H^{\pm\pm}$ going to same-sign $\tau$ lepton pair.

hep-ph↗

arXiv2017

Analytical Forms for Cross Sections of Di-lepton Production from $e^+e^-$ Collisions around the $J/\psi$ Resonance

A detailed theoretical derivation of the cross sections of $e^+e^- \to e^+e^-$ and $e^+e^- \to \mu^+\mu^-$ around the $J/\psi$ resonance is reported. The resonance and interference parts of the cross sections, related to $J/\psi$ resonance parameters, are calculated. Higher-order corrections for vacuum polarization and initial-state radiation are considered. An arbitrary upper limit of radiative correction integration is involved. Full and simplified versions of analytic formulae are given with precision at the level of 0.1% and 0.2%, respectively. Moreover, the results obtained in the paper can be applied to the case of the $\psi(3686)$ resonance.

hep-ph↗

arXiv2016

A mass reconstruction technique for a heavy resonance decaying to $\tau^+\tau^-$

For a resonance decaying to $\tau^+\tau^-$, it is difficult to reconstruct its mass accurately because of the presence of neutrinos in the decay products of the $\tau$ leptons. If the resonance is heavy enough, we show that its mass can be well determined by the momentum component of the $\tau$ decay products perpendicular to the velocity of the $\tau$ lepton, $p_{\perp}$, and the mass of the visible/invisible decay products, $m_{vis/inv}$, for $\tau$ decaying to hadrons/leptons. By sampling all kinematically allowed values of $p_{\perp}$ and $m_{vis/inv}$ according to their joint probability distributions determined by the MC simulations, the mass of the mother resonance is assumed to lie at the position with the maximal probability. Since $p_{\perp}$ and $m_{vis/inv}$ are invariant under the boost in the $\tau$ lepton direction, the joint probability distributions are independent upon $\tau$'s origin. Thus this technique is able to determine the mass of an unknown resonance with no efficiency loss. It is tested using the MC simulations of the physics processes $ pp \to Z/h(125)/h(750)+X\to \tau\tau+X$ at 13~TeV. The ratio of the full width at half maximum and the peak value of the reconstructed mass distribution is found to be 20\%-40\% using the information of missing transverse energy.

hep-ex↗

arXiv2015

Search for Flavor Changing Neutral Currents in $t\to H c, H\to \tau\tau$ Decay at the LHC

The prospects of searching for the flavor changing neutral current effect in the decay of $t\to H c, H\to \tau\tau$ are investigated with the simulated $p-p$ collision data for the ATLAS detector at the LHC, where the Higgs mass is assumed to be 125~GeV. A fit based on the constraints from the Higgs mass and the tau decay kinematics is performed for each event, which improves significantly the Higgs and top mass reconstruction and helps the signal-background separation. Boosted Decision Trees discriminants are developed to achieve an optimal sensitivity of searching for the FCNC signal. An expected upper limit of the branching ratio $\mathcal{B}(t\to H c)$ at $95\%$ confidence level of 0.25\% is obtained with a data set of 100~fb$^{-1}$ at $\sqrt{s}=13$ TeV during the LHC Run-2 period.

hep-ph↗

arXiv2015

Study of $\psi(3770)$ decaying to Baryon anti-Baryon Pairs

To study the decays of $\psi(3770)$ going to baryon anti-baryon pairs ($B\bar{B}$), all available experiments of measuring the cross sections of $e^+e^- \to B\bar{B}$ at center-of-mass energy ranging from 3.0 GeV to 3.9 GeV are combined. To relate the baryon octets, a model based on the SU(3) flavor symmetry is used and the SU(3) breaking effects are also considered. Assuming the elctric and magnetic form factors are equal ($|G_E|=|G_M|$), a global fit including the interference between the QED process and the resonant process is performed. The branching fraction of $\psi(3770) \to B\bar{B}$ is determined to be $(2.4\pm0.8\pm0.3)\times10^{-5}$, $(1.7\pm0.6\pm0.1)\times10^{-5}$, $(4.5\pm0.9\pm0.1)\times10^{-5}$, $(4.5\pm0.9\pm0.1)\times10^{-5}$, $(2.0\pm0.7\pm0.1)\times10^{-5}$, and $(2.0\pm0.7\pm0.1)\times10^{-5}$ for $B=p, \Lambda, \Sigma^+, \Sigma^0, \Xi^-$ and $\Xi^0$, respectively, where the first uncertainty is from the global fit and the second uncertainty is the systematic uncertainty due to the assumption $|G_E|=|G_M|$. They are at least one order of magnitude larger than a simple scaling of the branching fraction of $J/\psi\to B\bar{B}$.

hep-ph↗