Running after Diphoton

A very plausible explanation for the recently observed diphoton excess at the 13 TeV LHC is a (pseudo)scalar with mass around 750 GeV, which couples to a gluon pair and to a photon pair through loops involving vector-like quarks (VLQs). To accommodate the observed rate, the required Yukawa couplings tend to be large. A large Yukawa coupling would rapidly run up with the scale and quickly reach the perturbativity bound, indicating that new physics, possibly with a strong dynamics origin, is near by. The case becomes stronger especially if the ATLAS observation of a large width persists. In this paper we study the implication on the scale of new physics from the 750 GeV diphoton excess using the method of renormalization group running with careful treatment of different contributions and perturbativity criterion. Our results suggest that the scale of new physics is generically not much larger than the TeV scale, in particular if the width of the hinted (pseudo)scalar is large. Introducing multiple copies of VLQs, lowing the VLQ masses and enlarging VLQ electric charges help reduce the required Yukawa couplings and can push the cutoff scale to higher values. Nevertheless, if the width of the 750 GeV resonance turns out to be larger than about 1 GeV, it is very hard to increase the cutoff scale beyond a few TeVs. This is a strong hint that new particles in addition to the 750 GeV resonance and the vector-like quarks should be around the TeV scale.Comment: 19 pages, 6 figures; v3: corrected Eq.(2.6) and (3.1), updated reference

Potential precision of a direct measurement of the Higgs boson total width at a muon colliderr

In the light of the discovery of a 126 GeV Standard-Model-like Higgs boson at the LHC, we evaluate the achievable accuracies for direct measurements of the width, mass, and the s-channel resonant production cross section of the Higgs boson at a proposed muon collider. We find that with a beam energy resolution of R=0.01% (0.003%) and integrated luminosity of 0.5 fb^{-1} (1 fb^{-1}), a muon collider would enable us to determine the Standard-Model-like Higgs width to +/- 0.35 MeV (+/- 0.15 MeV) by combining two complementary channels of the WW^* and b\bar b final states. A non-Standard-Model Higgs with a broader width is also studied. The unparalleled accuracy potentially attainable at a muon collider would test the Higgs interactions to a high precision.Comment: 7 pages, 5 figures. Version appeared on Physical Review

The Fate of Long-Lived Superparticles with Hadronic Decays after LHC Run 1

Supersymmetry searches at the LHC are both highly varied and highly constraining, but the vast majority are focused on cases where the final-stage visible decays are prompt. Scenarios featuring superparticles with detector-scale lifetimes have therefore remained a tantalizing possibility for sub-TeV SUSY, since explicit limits are relatively sparse. Nonetheless, the extremely low backgrounds of the few existing searches for collider-stable and displaced new particles facilitates recastings into powerful long-lived superparticle searches, even for models for which those searches are highly non-optimized. In this paper, we assess the status of such models in the context of baryonic R-parity violation, gauge mediation, and mini-split SUSY. We explore a number of common simplified spectra where hadronic decays can be important, employing recasts of LHC searches that utilize different detector systems and final-state objects. The LSP/NLSP possibilities considered here include generic colored superparticles such as the gluino and light-flavor squarks, as well as the lighter stop and the quasi-degenerate Higgsino multiplet motivated by naturalness. We find that complementary coverage over large swaths of mass and lifetime is achievable by superimposing limits, particularly from CMS's tracker-based displaced dijet search and heavy stable charged particle searches. Adding in prompt searches, we find many cases where a range of sparticle masses is now excluded from zero lifetime to infinite lifetime with no gaps. In other cases, the displaced searches furnish the only extant limits at any lifetime.Comment: 36 pages, 10 figures, plus appendix and reference

Field-based branch prediction for packet processing engines

Network processors have exploited many aspects of architecture design, such as employing multi-core, multi-threading and hardware accelerator, to support both the ever-increasing line rates and the higher complexity of network applications. Micro-architectural techniques like superscalar, deep pipeline and speculative execution provide an excellent method of improving performance without limiting either the scalability or flexibility, provided that the branch penalty is well controlled. However, it is difficult for traditional branch predictor to keep increasing the accuracy by using larger tables, due to the fewer variations in branch patterns of packet processing. To improve the prediction efficiency, we propose a flow-based prediction mechanism which caches the branch histories of packets with similar header fields, since they normally undergo the same execution path. For packets that cannot find a matching entry in the history table, a fallback gshare predictor is used to provide branch direction. Simulation results show that the our scheme achieves an average hit rate in excess of 97.5% on a selected set of network applications and real-life packet traces, with a similar chip area to the existing branch prediction architectures used in modern microprocessors

Light scalar mesons and charmless hadronic Bc→SP,SVB_c \to SP, SV decays in the perturbative QCD approach

In this work, based on the assumption of two-quark structure of the scalars, the charmless hadronic $B_c \to SP, SV$ decays(here, $S$, $P$, and $V$ denote the light scalar, pseudoscalar, and vector mesons, respectively) are investigated by employing the perturbative QCD(pQCD) factorization approach.From our numerical evaluations and phenomenological analysis, we find that (a) the pQCD predictions for the {\it CP}-averaged branching ratios(BRs) of the considered $B_c$ decays vary in the range of $10^{-5}$ to $10^{-8}$, which will be tested in the ongoing LHCb and forthcoming Super-B experiments, while the {\it CP}-violating asymmetries for these modes are absent naturally in the standard model because only one type tree operator is involved; %% (b) analogous to $B \to K^* \eta^{(\prime)}$ decays, $Br(B_c \to \kappa^+ \eta) \sim 5 \times Br(B_c \to \kappa^+ \eta^\prime)$ in the pQCD approach, which can be understood by the constructive and destructive interference between the $\eta_q$ and $\eta_s$ contributions to the $B_c \to \kappa^+ \eta$ and $B_c \to \kappa^+ \eta^\prime$ decays, however, $Br(B_c \to K_0^*(1430) \eta)$ is approximately equal to $Br(B_c \to K_0^*(1430) \eta')$ in both scenarios because the factorizable contributions from $\eta_s$ term play the dominant role in the considered two channels; %% (c) if $a_0(980)$ and $\kappa$ are the $q\bar q$ bound states, the pQCD predicted BRs for $B_c \to a_0(980) (\pi, \rho)$ and $B_c \to \kappa K^{(*)}$ decays will be in the range of $10^{-6} \sim 10^{-5}$, which are within the reach of the LHCb experiments and could be measured in the near future; and %% (d) for the $a_0(1450)$ and $K_0^*(1430)$ channels, the BRs for $B_c \to a_0(1450) (\pi, \rho)$ and $B_c \to K_0^*(1430) K^{(*)}$ modes in the pQCD approach are found to be $(5 \sim 47) \times 10^{-6}$ and $(0.7 \sim 36) \times 10^{-6}$, respectively.Comment: 24 pages, 1 figure, accepted for publication in Physical Review

Axial-vector f1(1285)−f1(1420)f_1(1285)-f_1(1420) mixing and Bs→J/ψ(f1(1285),f1(1420))B_s \to J/\psi (f_1(1285), f_1(1420)) decays

Inspired by the very recent LHCb measurements of $B_s \to J/\psi f_1(1285)$ and the good agreement between the perturbative QCD predictions and the data for many $B \to J/\psi V$ decays, we here investigate the $B_s \to J/\psi f_1(1285)$ and $B_s \to J/\psi f_1(1420)$ decays for the first time by employing the perturbative QCD (pQCD) approach, in which the $1^3P_1$ states $f_1(1285)$ and $f_1(1420)$ are believed to be the mixture of flavor singlet $f_1$ and octet $f_8$ or of quark-flavor states $f_{1q}$ and $f_{1s}$. We show that the pQCD predictions for the branching ratio of $B_s \to J/\psi f_1(1285)$ agree well with the data within errors for the mixing angle $\theta_{^3P_1} \approx 20^\circ (\phi_{^3P_1} \approx 15^\circ)$ between $f_1 (f_{1q})$ and $f_8 (f_{1s})$ states. Furthermore, the branching ratio of $B_s \to J/\psi f_1(1420)$ and the large transverse polarization fractions in these two considered channels are also predicted and will be tested by the LHC and the forthcoming Super-B factory experiments. Based on the decay rates of $B_s \to J/\psi f_1(1285)$ and $B_s \to J/\psi f_1(1420)$ decay modes predicted in the pQCD approach, the extracted mixing angle between $f_1(1285)$ and $f_1(1420)$ is basically consistent with currently available experimental measurements and lattice QCD analysis within still large theoretical errors.Comment: 10 pages, 1 figure, 1 table; Contents improved and references added; Accepted for publication as a Brief Report in Physical Review

Study of the pure annihilation Bc→A2A3B_c \to A_2 A_3 decays

In this work, we calculate the {\it CP}-averaged branching ratios and the polarization fractions of the charmless hadronic $B_c \to A_2 A_3$ decays within the framework of perturbative QCD(pQCD) approach, where $A$ is either a light $^3P_1$ or $^1P_1$ axial-vector meson. These thirty two decay modes can occur through the annihilation topology only. Based on the perturbative calculations and phenomenological analysis, we find the following results: (a) the branching ratios of the considered thirty two $B_c\to A_2 A_3$ decays are in the range of $10^{-5}$ to $10^{-8}$; (b) $B_c\to a_1 b_1$, \ov{K}_1^0 K_1^+ and some other decays have sizable branching ratios and can be measured at the LHC experiments; (c) the branching ratios of $B_c \to A_2(^1P_1) A_3(^1P_1)$ decays are generally much larger than those of $B_c \to A_2(^3P_1) A_3(^3P_1)$ decays with a factor around (10 $\sim$ 100); (d) the branching ratios of B_c \to \ov{K}_1^0 K_1^+ decays are sensitive to the value of $\theta_K$, which will be tested by the running LHC and forthcoming SuperB experiments; (e) the large longitudinal polarization contributions govern most considered decays and play the dominant role.Comment: 19 pages, 1 eps figure. arXiv admin note: some text overlap with arXiv:1003.392

Multi-engine packet classification hardware accelerator

As line rates increase, the task of designing high performance architectures with reduced power consumption for the processing of router traffic remains important. In this paper, we present a multi-engine packet classification hardware accelerator, which gives increased performance and reduced power consumption. It follows the basic idea of decision-tree based packet classification algorithms, such as HiCuts and HyperCuts, in which the hyperspace represented by the ruleset is recursively divided into smaller subspaces according to some heuristics. Each classification engine consists of a Trie Traverser which is responsible for finding the leaf node corresponding to the incoming packet, and a Leaf Node Searcher that reports the matching rule in the leaf node. The packet classification engine utilizes the possibility of ultra-wide memory word provided by FPGA block RAM to store the decision tree data structure, in an attempt to reduce the number of memory accesses needed for the classification. Since the clock rate of an individual engine cannot catch up to that of the internal memory, multiple classification engines are used to increase the throughput. The implementations in two different FPGAs show that this architecture can reach a searching speed of 169 million packets per second (mpps) with synthesized ACL, FW and IPC rulesets. Further analysis reveals that compared to state of the art TCAM solutions, a power savings of up to 72% and an increase in throughput of up to 27% can be achieved