分类: 物理学 >> 基本粒子与场物理学 分类: 物理学 >> 核物理学 提交时间: 2016-06-16
摘要: 质量作为物质最基本的属性,其起源一直是物理学研究的根本问题之一。根据物质层次的划分,理解可见宇宙物质 (能量) 组成的关键在于理解核子~(基态重子)~的质量。现代粒子物理与原子核物理研究指出,基态重子质量的绝大部分来自强相互作用。然而,由于色禁闭现象,强相互作用的基本理论—量子色动力学—在低能区无法解析求解,导致很长一段时间里对基态重子质量的研究不得不借助于各种唯象模型。二十一世纪以来,随着计算机运算能力的发展和算法的持续改进,格点量子色动力学模拟取得了令人瞩目的成绩,使得人类从第一原理出发计算基态重子的质量,进而定量地理解质量的起源成为可能。另一方面,受到计算资源的限制,目前绝大部分格点量子色动力学模拟必须采用比物理值大的轻夸克质量、不足够大的盒子体积和不足够小的格点间距。因而,为了从格点量子色动力学模拟中提取感兴趣的观测量的物理值,必须对格点数据进行如下三种延拓:手征延拓将轻夸克质量延拓到物理值 (即 $m_q\rightarrow m_q^\mathrm{phys.}$),有限体积修正将有限的四维空间延拓到无穷大 (即~$V=L^4\rightarrow\infty$),连续性延拓将有限格点间距延拓到零 (即 $a\rightarrow0$)。手征微扰理论为开展这些延拓提供了必要的理论基础。作为量子色动力学的低能有效理论,手征微扰理论原则上可以模型无关地描述强相互作用物理。但随着手征阶数的升高,仅仅依靠实验数据无法完全确定理论中未知的低能常数。高统计量的格点量子色动力学模拟数据的出现为解决这一难题提供了新的思路,从而使得基于高阶手征微扰理论的研究成为可能。本文将简要介绍当前基于协变重子手征微扰理论对基态八重态重子质量及格点量子色动力学模拟数据的研究。
分类: 物理学 >> 核物理学 提交时间: 2016-08-15
摘要: The quark-gluon plasma is studied via medium-induced changes to correlations between jets and charged particles in PbPb collisions compared to pp reference data. This analysis uses data sets from PbPb and pp collisions with integrated luminosities of 166 inverse microbarns and 5.3 inverse picobarns, respectively, collected at sqrt(s[NN]) = 2.76 TeV. The angular distributions of charged particles are studied as a function of relative pseudorapidity (Delta eta) and relative azimuthal angle (Delta phi) with respect to reconstructed jet directions. Charged particles are correlated with all jets with transverse momentum (pt) above 120 GeV, and with the leading and subleading jets (the highest and second-highest in pt, respectively) in a selection of back-to-back dijet events. Modifications in PbPb data relative to pp reference data are characterized as a function of PbPb collision centrality and charged particle pt. A centrality-dependent excess of low-pt particles is present for all jets studied, and is most pronounced in the most central events. This excess of low-pt particles follows a Gaussian-like distribution around the jet axis, and extends to large relative angles of Delta eta approximately 1 and Delta phi approximately
分类: 物理学 >> 核物理学 提交时间: 2016-08-15
摘要: Measurements of the top quark-antiquark (t t-bar) spin correlations and the top quark polarization are presented for t t-bar pairs produced in pp collisions at sqrt(s) = 8 TeV. The data correspond to an integrated luminosity of 19.5 inverse femtobarns collected with the CMS detector at the LHC. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a bottom quark. The spin correlations and polarization are measured from the angular distributions of the two selected leptons, both inclusively and differentially, with respect to the invariant mass, rapidity, and transverse momentum of the t t-bar system. The measurements are unfolded to the parton level and found to be in agreement with predictions of the standard model. A search for new physics in the form of anomalous top quark chromo moments is performed. No evidence of new physics is observed, and exclusion limits on the real part of the chromo-magnetic dipole moment and the imaginary part of the chromo-electric dipole moment are evaluated.
分类: 物理学 >> 核物理学 提交时间: 2016-08-15
摘要: Inclusive jet production in pPb collisions at a nucleon-nucleon (NN) center-of-mass energy of sqrt(s[NN]) = 5.02 TeV is studied with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 35 inverse nanobarns is analyzed. The jet transverse momentum spectra are studied in seven pseudorapidity intervals covering the range -2.0 < eta[CM] < 1.5 in the NN center-of-mass frame. The jet production yields at forward and backward pseudorapidity are compared and no significant asymmetry about eta[CM] = 0 is observed in the measured kinematic range. The measurements in the pPb system are compared to reference jet spectra obtained by extrapolation from previous measurements in pp collisions at sqrt(s) = 7 TeV. In all pseudorapidity ranges, nuclear modifications in inclusive jet production are found to be small, as predicted by next-to-leading order perturbative QCD calculations that incorporate nuclear effects in the parton distribution functions.
分类: 物理学 >> 核物理学 提交时间: 2016-08-15
摘要: A measurement of the B0s decay parameters in the B0s→J/ψϕ channel using an integrated luminosity of 14.3 fb−1 collected by the ATLAS detector from 8 TeV pp collisions at the LHC is presented. The measured parameters include the CP-violating phase ϕs, the decay width Γs and the width difference between the mass eigenstates ΔΓs. The values measured for the physical parameters are statistically combined with those from 4.9 fb−1 of 7 TeV data, leading to the following:
分类: 物理学 >> 核物理学 提交时间: 2016-08-15
摘要: This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of τ→3μ. A method utilising the production of τ leptons via W→τν decays is used. This method is applied to the sample of 20.3 fb−1 of pp collision data at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment at the LHC in 2012. No event is observed passing the selection criteria, and the observed (expected) upper limit on the τ lepton branching fraction into three muons, Br(τ→3μ), is 3.76×10−7 (3.94×10−7) at 90% confidence level.
分类: 物理学 >> 核物理学 提交时间: 2016-08-16
摘要: The CDEX-1 experiment conducted a search of low-mass (< 10 GeV/c2) Weakly Interacting Massive Particles (WIMPs) dark matter at the China Jinping Underground Laboratory using a p-type point-contact germanium detector with a fiducial mass of 915 g at a physics analysis threshold of 475 eVee. We report the hardware set-up, detector characterization, data acquisition and analysis procedures of this experiment. No excess of unidentified events are observed after subtraction of known background. Using 335.6 kg-days of data, exclusion constraints on the WIMP-nucleon spin-independent and spin-dependent couplings are derived.
分类: 物理学 >> 核物理学 提交时间: 2016-08-16
摘要: A measurement of the forward-backward asymmetry A[FB] of oppositely charged lepton pairs (mu mu and e e) produced via Z/gamma* boson exchange in pp collisions at sqrt(s) = 8 TeV is presented. The data sample corresponds to an integrated luminosity of 19.7 inverse femtobarns collected with the CMS detector at the LHC. The measurement of A[FB] is performed for dilepton masses between 40 GeV and 2 TeV and for dilepton rapidity up to 5. The A[FB] measurements as a function of dilepton mass and rapidity are compared with the standard model predictions.
分类: 物理学 >> 核物理学 提交时间: 2016-08-16
摘要: A new search signature for excited leptons is explored. Excited muons are sought in the channel pp→μμ∗→μμ jet jet, assuming both the production and decay occur via a contact interaction. The analysis is based on 20.3 fb−1 of pp collision data at a centre-of-mass energy of s√ = 8 TeV taken with the ATLAS detector at the Large Hadron Collider. No evidence of excited muons is found, and limits are set at the 95% confidence level on the cross section times branching ratio as a function of the excited-muon mass mμ∗. For mμ∗ between 1.3 TeV and 3.0 TeV, the upper limit on σB(μ∗→μqq¯) is between 0.6 and 1 fb. Limits on σB are converted to lower bounds on the compositeness scale Λ. In the limiting case Λ=mμ∗, excited muons with a mass below 2.8 TeV are excluded. With the same model assumptions, these limits at larger μ∗ masses improve upon previous limits from traditional searches based on the gauge-mediated decay μ∗→μγ.
分类: 物理学 >> 核物理学 提交时间: 2016-08-16
摘要: A search for a massive resonance W' decaying into a W and a Higgs boson in the l nu b anti-b (l = e, mu) final state is presented. Results are based on data corresponding to an integrated luminosity of 19.7 inverse femtobarns of proton-proton collisions at sqrt(s) = 8 TeV, collected using the CMS detector at the LHC. For a high-mass ( greater than 1 TeV) resonance, the two bottom quarks coming from the Higgs boson decay are reconstructed as a single jet, which can be tagged by placing requirements on its substructure and flavour. Exclusion limits at 95% confidence level are set on the production cross section of a narrow resonance decaying into WH, as a function of its mass. In the context of a little Higgs model, a lower limit on the W' mass of 1.4 TeV is set. In a heavy vector triplet model that mimics the properties of composite Higgs models, a lower limit on the W' mass of 1.5 TeV is set. In the context of this model, the results are combined with related searches to obtain a lower limit on the W' mass of 1.8 TeV, the most restrictive to date for decays to a pair of standard model bosons.
分类: 物理学 >> 核物理学 提交时间: 2016-08-17
摘要: The decorrelation in the azimuthal angle between the most forward and the most backward jets (Mueller-Navelet jets) is measured in data collected in pp collisions with the CMS detector at the LHC at sqrt(s) = 7 TeV. The measurement is presented in the form of distributions of azimuthal-angle differences, Delta(phi), between the Mueller-Navelet jets, the average cosines of (pi-Delta(phi)), 2(pi-Delta(phi)), and 3(pi-Delta(phi)), and ratios of these cosines. The jets are required to have transverse momenta, pT, in excess of 35 GeV and rapidities, |y|, of less than 4.7. The results are presented as a function of the rapidity separation, Delta(y), between the Mueller-Navelet jets, reaching Delta(y) up to 9.4 for the first time. The results are compared to predictions of various Monte Carlo event generators and to analytical predictions based on the DGLAP and BFKL parton evolution schemes.
分类: 物理学 >> 核物理学 提交时间: 2016-08-17
摘要: A search for Lorentz- and CPT-violating signals in the double beta decay spectrum of 136Xe has been performed using an exposure of 100 kg⋅yr with the EXO-200 detector. No significant evidence of the spectral modification due to isotropic Lorentz-violation was found, and a two-sided limit of −2.65×10−5GeV展开 -->
分类: 物理学 >> 核物理学 提交时间: 2016-08-17
摘要: A search for the pair production of top squarks, each with R-parity-violating decays into two Standard Model quarks, is performed using 17.4 fb−1 of s√ = 8 TeV proton-proton collision data recorded by the ATLAS experiment at the LHC. Each top squark is assumed to decay to a b- and an s-quark, leading to four quarks in the final state. Background discrimination is achieved with the use of b-tagging and selections on the mass and substructure of large-radius jets, providing sensitivity to top squark masses as low as 100 GeV. No evidence of an excess beyond the Standard Model background prediction is observed and top squarks decaying to b¯s¯ are excluded for top squark masses in the range 100≤mt~≤315 GeV at 95% confidence level.
分类: 物理学 >> 核物理学 提交时间: 2016-08-17
摘要: We present an exploratory lattice study for the two-photon decay of ηc using Nf=2 twisted mass lattice QCD gauge configurations generated by the European Twisted Mass Collaboration. Two different lattice spacings of a=0.067fm and a=0.085fm are used in the study, both of which are of physical size of 2fm. The decay widths are found to be 1.025(5)KeV for the coarser lattice and 1.062(5)KeV for the finer lattice respectively where the errors are purely statistical. A naive extrapolation towards the continuum limit yields Γ≃1.122(14)KeV which is smaller than the previous quenched result and most of the current experimental results. Possible reasons are discussed.
分类: 物理学 >> 核物理学 提交时间: 2016-08-17
摘要: In this exploratory lattice study, low-energy near threshold scattering of the (D¯1D∗)± meson system is analyzed using lattice QCD with Nf=2 twisted mass fermion configurations. Both s-wave (JP=0−) and p-wave (JP=1+) channels are investigated. It is found that the interaction between the two charmed mesons is attractive near the threshold in both channels. This calculation provides some hints in the searching of resonances or bound states around the threshold of (D¯1D∗)± system.
分类: 物理学 >> 核物理学 提交时间: 2016-08-17
摘要: In recent years, lots of studies of charmonium decays have been performed at BESIII based on large data samples of J/ψ, ψ(3686) and ψ(3770). Recent results in searches for radiative transitions of ψ(3770) and rare phenomena in charmonium decays, and studies of light hadrons structures and properties will be presented.
分类: 物理学 >> 核物理学 提交时间: 2016-08-17
摘要: The number of charged particles inside jets is a widely used discriminant for identifying the quark or gluon nature of the initiating parton and is sensitive to both the perturbative and non-perturbative components of fragmentation. This paper presents a measurement of the average number of charged particles with pT>500 MeV inside high-momentum jets in dijet events using 20.3 fb−1 of data recorded with the ATLAS detector in pp collisions at s√=8 TeV collisions at the LHC. The jets considered have transverse momenta from 50 GeV up to and beyond 1.5 TeV. The reconstructed charged-particle track multiplicity distribution is unfolded to remove distortions from detector effects and the resulting charged-particle multiplicity is compared to several models. Furthermore, quark and gluon jet fractions are used to extract the average charged-particle multiplicity for quark and gluon jets separately.
分类: 物理学 >> 核物理学 提交时间: 2016-08-17
摘要: Based on a sample of (1310.6±10.5)×106 J/ψ events collected with the BESIII detector operating at the BEPCII storage ring, a partial wave analysis of the decay J/ψ→γϕϕ is performed in order to study the intermediate states. Results of the partial wave analysis show that the structures are predominantly 0−+ states. The existence of the η(2225) is confirmed, and its resonance parameters are measured. Two additional pseudoscalar states, the η(2100) with a mass of 2050+30−24+75−26~MeV/c2 and a width of 250+36−30+181−164~MeV/c2 and the X(2500) with a mass of 2470+15−19+101−23~MeV/c2 and a width of 230+64−35+56−33~MeV/c2, are observed. In addition to these three pseudoscalar states, the scalar state f0(2100), and three tensor states, the f2(2010), f2(2300) and f2(2340), are observed in the process J/ψ→γϕϕ. The product branching fractions B(J/ψ→γX)×B(X→ϕϕ) are reported.
分类: 物理学 >> 核物理学 提交时间: 2016-08-18
摘要: Charged-particle distributions are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV, using a data sample of nearly 9 million events, corresponding to an integrated luminosity of 170 μb−1, recorded by the ATLAS detector during a special Large Hadron Collider fill. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity and the dependence of the mean transverse momentum on the charged-particle multiplicity are presented. The measurements are performed with charged particles with transverse momentum greater than 500 MeV and absolute pseudorapidity less than 2.5, in events with at least one charged particle satisfying these kinematic requirements. Additional measurements in a reduced phase space with absolute pseudorapidity less than 0.8 are also presented, in order to compare with other experiments. The results are corrected for detector effects, presented as particle-level distributions and are compared to the predictions of various Monte Carlo event generators.
分类: 物理学 >> 核物理学 提交时间: 2016-08-18
摘要: A search for supersymmetry in hadronic final states with highly boosted W bosons and b jets is presented, focusing on compressed scenarios. The search is performed using proton-proton collision data at a center-of-mass energy of 8 TeV, collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 19.7 inverse femtobarns. Events containing candidates for hadronic decays of boosted W bosons are identified using jet substructure techniques, and are analyzed using the razor variables M[R] and R^2, which characterize a possible signal as a peak on a smoothly falling background. The observed event yields in the signal regions are found to be consistent with the expected contributions from standard model processes, which are predicted using control samples in the data. The results are interpreted in terms of gluino-pair production followed by their exclusive decay into top squarks and top quarks. The analysis excludes gluino masses up to 1.1 TeV for light top squarks decaying solely to a charm quark and a neutralino, and up to 700 GeV for heavier top squarks decaying solely to a top quark and a neutralino.