A SVD accelerated kernel-independent fast multipole method and its application to BEM

The kernel-independent fast multipole method (KIFMM) proposed in [1] is of almost linear complexity. In the original KIFMM the time-consuming M2L translations are accelerated by FFT. However, when more equivalent points are used to achieve higher accuracy, the efficiency of the FFT approach tends to be lower because more auxiliary volume grid points have to be added. In this paper, all the translations of the KIFMM are accelerated by using the singular value decomposition (SVD) based on the low-rank property of the translating matrices. The acceleration of M2L is realized by first transforming the associated translating matrices into more compact form, and then using low-rank approximations. By using the transform matrices for M2L, the orders of the translating matrices in upward and downward passes are also reduced. The improved KIFMM is then applied to accelerate BEM. The performance of the proposed algorithms are demonstrated by three examples. Numerical results show that, compared with the original KIFMM, the present method can reduce about 40% of the iterating time and 25% of the memory requirement.Comment: 19 pages, 4 figure

Explanation of the ATLAS Z-peaked excess by squark pair production in the NMSSM

The ATLAS collaboration recently reported a $3\sigma$ excess in the leptonic-$Z+jets+E_{T}^{miss}$ channel. We intend to interpret this excess by squark pair production in the Next-to-Minimal Supersymmetric Standard Model (NMSSM). The decay chain we employ is $\tilde{q} \to q \tilde{\chi}_2^0 \to q \tilde{\chi}_1^0 Z$, where $\tilde{\chi}_1^0$ and $\tilde{\chi}_2^0$ denote the lightest and the next-to-lightest neutralinos with singlino and bino as their dominant components respectively. Our simulations indicate that after considering the constraints from the ATLAS searches for $jets + E_{T}^{miss}$ signal the central value of the excess can be obtained for $m_{\tilde{q}} \lesssim 1.2 {\rm TeV}$, and if the constraint from the CMS on-$Z$ search is further considered, more than 10 signal events are still attainable for $m_{\tilde{q}} \lesssim 750 {\rm GeV}$. Compared with the interpretation by gluino pair production, the squark explanation allows for a significantly wider range of $m_{\tilde{q}}$ as well as a less compressed SUSY mass spectrum. We also show that the squark explanation will be readily tested at the initial stage of the 14 TeV LHC.Comment: 19 pages, 4 figure

Top quark forward-backward asymmetry at the Tevatron: a comparative study in different new physics models

The top quark forward-backward asymmetry A_{FB}^t measured at the Tevatron is above the Standard Model prediction by more than 2-sigma deviation, which might be a harbinger for new physics. In this work we examine the contribution to A_{FB}^t in two different new physics models: one is the minimal supersymmetric model without R-parity (RPV-MSSM) which contributes to A_{FB}^t via sparticle-mediated t-channel process d d_bar-> t t_bar; the other is the third-generation enhanced left-right model (LR model) which contributes to A_{FB}^t via Z'-mediated t-channel or s-channel processes. We find that in the parameter space allowed by the tt_bar production rate and the tt_bar invariant mass distribution at the Tevatron, the LR model can enhance A_{FB}^t to within the 2-sigma region of the Tevatron data for the major part of the parameter space, and in optimal case A_{FB}^t can reach 12% which is slightly below the 1-sigma lower bound. For the RPV-MSSM, only in a narrow part of the parameter space can the \lambda'' couplings enhance A_{FB}^t to within the 2-sigma region while the \lambda' couplings just produce negative contributions to worsen the fit.Comment: Version in PRD (RPV-MSSM lambda" effects added

Explanation of the ATLAS Z-peaked excess in the NMSSM

Recently the ATLAS collaboration reported a $3\sigma$ excess in the leptonic-$Z+jets+E_{T}^{miss}$ channel. This may be interpreted in the Next-to-Minimal Supersymmetric Standard Model (NMSSM) by gluino pair production with the decay chain $\tilde{g} \to q \bar{q} \tilde{\chi}_2^0 \to q \bar{q} Z \tilde{\chi}_1^0$, where $\tilde{\chi}_1^0$ and $\tilde{\chi}_2^0$ denote the lightest and the next-to-lightest neutralinos with singlino and bino as their dominant components respectively. After exploring the relevant parameter space of the NMSSM by considering the constraints from the ATLAS searches for $jets + E_{T}^{miss}$ signals, we conclude that the NMSSM is able to explain the excess at $1 \sigma$ level with the number of the signal events reaching its measured central value in optimal cases, and the best explanation comes from a compressed spectrum such as $m_{\tilde{g}} \simeq 650 {\rm GeV}$, $m_{\tilde{\chi}_2^0} \simeq 565 {\rm GeV}$ and $m_{\tilde{\chi}_1^0} \simeq 465 {\rm GeV}$. We also check the consistency of the ATLAS results with the null result of the CMS on-$Z$ search. We find that under the CMS limits at $95\%$ C.L., the event number of the ATLAS on-$Z$ signal can still reach 11 in our scenario, which is about $1.2 \sigma$ away from the measured central value.Comment: 18 pages, 2 figure