arXiv subjects

Shou-hua Zhu

Publications and source records attributed to Shou-hua Zhu.

At least 37 records · Page 2Linked to original sources

arXiv2016

Higgs-$μ$-$τ$ Coupling at High and Low Energy Colliders

There is no tree-level flavor changing neutral current (FCNC) in the standard model (SM) which contains only one Higgs doublet. If more Higgs doublets are introduced for various reasons, the tree level FCNC would be inevitable except extra symmetry was imposed. Therefore FCNC processes are the excellent probes for the physics beyond the SM (BSM). In this paper, we studied the lepton flavor violated (LFV) decay processes $h\rightarrowμτ$ and $τ\rightarrowμγ$ induced by Higgs-$μ$-$τ$ vertex. For $τ\rightarrowμγ$, its branching ratio is also related to the $ht\bar{t}$, $hτ^+τ^-$ and $hW^+W^-$ vertices. We categorized the BSM into two scenarios for the Higgs coupling strengths near or away from SM. For the latter scenario, we took the spontaneously broken two Higgs doublet model (Lee model) as an example. We considered the constraints by recent data from LHC and B factories, and found that the measurements gave weak constraints. At LHC Run II, $h\rightarrowμτ$ will be confirmed or set stricter limit on its branching ratio. Accordingly, $\textrm{Br}(τ\rightarrowμγ)\lesssim\mathcal{O}(10^{-10}-10^{-8})$ for general chosen parameters. For the positive case, $τ\rightarrowμγ$ can be discovered with $\mathcal{O}(10^{10})$ $τ$ pair samples at SuperB factory, Super $τ$-charm factory and new Z-factory. The future measurements for $\textrm{Br}(h\rightarrowμτ)$ and $\textrm{Br}(τ\rightarrowμγ)$ will be used to distinguish these two scenarios or set strict constraints on the correlations among different Higgs couplings, please see Table II in the text for details.

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arXiv2016

A Loop-philic Pseudoscalar

We construct a weakly-coupled renormalizable model to explain the $750\mbox{GeV}$ diphoton excess. The $750\mbox{GeV}$ resonance (denoted as $X(750)$) is interpreted as a pseudoscalar coming from a complex singlet. The model also naturally provides a dark matter candidate. One most attractive feature of the model is that decays of $X(750)$ are all loop-induced so the diphoton rate is not diluted by unwanted tree level branching fractions. Relevant Yukawa interactions need not to be tuned to near non-perturbative region to explain the rate. The model is highly predictive, including the pseudoscalar nature of $X(750)$, and two nearly mass-degenerate exotic quarks carrying electric charge $5/3$ and $2/3$, respectively. Rich phenomenology is expected with respect to collider searches, flavor physics and dark matter detection, if $X(750)$ can be pinned down by future LHC experiments.

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arXiv2015

Invisible Higgs Decay at the LHeC

The possibility that the 125 GeV Higgs boson may decay into invisible non-standard-model (non-SM) particles is theoretically and phenomenologically intriguing. In this letter we investigate the sensitivity of the Large Hadron Electron Collider (LHeC) to an invisibly decaying Higgs, in its proposed high luminosity running mode. We focus on the neutral current Higgs production channel which offers more kinematical handles than its charged current counterpart. The signal contains one electron, one jet and large missing energy. With a cut-based parton level analysis, we estimate that if the $hZZ$ coupling is at its standard model (SM) value, then assuming an integrated luminosity of $1\,\mbox{ab}^{-1}$ the LHeC with the proposed 60 GeV electron beam (with $-0.9$ polarization) and 7 TeV proton beam is capable of probing $\mathrm{Br}(h\rightarrow E\!\!\!\!/_T)=6\%$ at $2σ$ level. Good lepton veto performance (especially hadronic $τ$ veto) in the forward region is crucial to the suppression of the dominant $Wje$ background. We also explicitly point out the important role that may be played by the LHeC in probing a wide class of exotic Higgs decay processes and emphasize the general function of lepton-hadron colliders in precision study of new resonances after their discovery in hadron-hadron collisions.

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arXiv2015

Anomalous Higgs-top Coupling Pollution on Triple Higgs Coupling Extraction at Future High-Luminosity Electron-Positron Collider

One of the most challenging tasks for future high-luminosity electron-positron colliders is to extract Higgs triple coupling. It was proposed that this can be carried out via the precisely measuring the cross section of ZH associated production up to $0.4\%$. In this paper, we example the possible heavy pollution from Higgs-top anomalous coupling. Our numerical results show that the pollution is small for $\sqrt{s}_{e^+e^-}= 240 GeV$. However for the higher energy collider, pollution is sizable, which should be taken into account. We further explored the possibility to measure CP-violated Higgs top coupling, via the forward-backward asymmetry $A_{FB}$ for the process $e^+e^- \rightarrow ZH$. The asymmetry can reach $0.7\%$ which is comparable to the precision of cross section measurement.

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arXiv2014

Charged Higgs Boson: Tracer of the Physics beyond Standard Model

Charged Higgs boson can exist in many physics beyond the standard models (BSM) and it is the obvious BSM signal. We briefly describe why the 125GeV scalar discovered at the LHC must have (heavy) companion: the charged Higgs boson, in a new paradigm. We then focus on the charged Higgs phenomenology, especially on how to measure $\tanβ$ precisely utilizing the top quark polarization information.

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arXiv2014

A New Paradigm: Role of Electron-positron and Hadron Colliders

In 2012, a light scalar boson (denoted as H(125) in this paper) was discovered at the LHC. We explore the possible correlation between the lightness of H(125) and the smallness of CP-violation based on the Lee model, namely the spontaneous CP-violation two-Higgs-doublet-model. It is a new way to understand why H(125) is light. Based on this we propose that it is the much heavier scalar bosons, instead of the H(125), which need to be understood. This opens a new paradigm that one tries to understand the electro-weak symmetry breaking and CP violation. For the new paradigm, similar to many other physics beyond the standard model, one need both electron-positron and higher energy hadron collider, as well as the low energy experiments, in order to pin down the whole picture.

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arXiv2014

On Extra Neutral and Charged Higgs bosons at the LHC

The properties of 125 GeV new particle, which was discovered in 2012 at the Large Hadron Collider (LHC), are found to be consistent with those of the Higgs boson in the standard model (SM). Hereafter the new particle is dubbed as SM-like Higgs boson. However there is still spacious room for physics beyond the SM (BSM) due to the limited energy and luminosity of the LHC. With more data, experiments will scrutinize whether the new particle is indeed the SM one or not. At the same time, one believes that discovery of the SM-like Higgs boson is just the start of the new era of particle physics. The predominant topic is whether there are others new Higgs bosons as speculated in various BSM models. In this short review we willdescribe the current status of Higgs physics at the LHC and several BSM models which contain more Higgs sectors. In literature there are numerous studies on extended Higgs sector and a comprehensive review is beyond the scopeof this review. Instead, we will present two latest studies on Higgs physics: (1) how to detect the charged Higgs boson and measure $\tanβ$ after including the top polarization information, and (2) how todiscover the extra neutral Higgs boson via the pair production of SM-like Higgs boson.

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arXiv2013

Complex Vacuum and Lightness of Higgs Boson

The CP violation observed in K- and B-mesons can be successfully accommodated via the CKM matrix in the standard model. However the additional CP violation is required in order to induce the matter and anti-matter asymmetry observed in the Universe. The additional CP violation can be induced by complex vacuum, namely the spontaneous CP violation. In this paper we reveal the intimate connection of the spontaneous CP violation and the lightness of the Higgs boson, which was recently discovered by ATLAS and CMS at the LHC.

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arXiv2013

Discovering extra Higgs boson via pair production of the SM-like Higgs bosons

In the standard model (SM), pair production rate of Higgs boson at the Large Hadron Collider (LHC) is quite low. One usually think that it is extremely important for the measurement of triple Higgs coupling at the high luminosity LHC. In this paper, we propose to search for the extra Higgs boson (denoted as $S$) utilizing pair production of the SM-like Higgs boson ($H$) which was discovered in July, 2012.The pair production of $H$ can be huge due to the resonant production of heavy scalar $S$, namely $PP \rightarrow S \rightarrow HH$. The couplings of $H$ with weak gauge boson are similar to ones in the SM and it implies that the couplings between $S$ and gauge bosons are likely suppressed. Provided that $S$ is heavy enough, the decay into weak gauge bosons may not be the dominant modes. Instead $S$ can decay into a pair of $H$ and offer the promising channel to discover it. In this paper, we studied the 5 promising decay modes of $H$, i.e. $b\bar{b}$, $WW^*$, $ZZ^*$, $γγ$ and $τ^+τ^-$, and simulated the signals and backgrounds for the 15 combination modes for $HH$ at the LHC with $\sqrt{s}= 14$ TeV and integrated luminosity $\mathcal{L}=1000fb^{-1}$. We found that with the help of suitable selection rules, very good signal to background ratio $S/B$ can be archived in many decay channels, for example $b\bar b+ (WW^*, ZZ^*, γγ, τ^+τ^-)$, $WW^*+(WW^*, ZZ^*, γγ, τ^+τ^-)$ and $τ^+τ^- +(γγ, τ^+τ^-)$. For the detailed results please refer to Table \ref{tab:discovery} in the text. On the contrary, $b\bar b b\bar b$ mode is less important due to the huge QCD background. However if one has excellent control on light jet mis-tagging, the $b\bar b b\bar b$ mode can be promising to discover the extra Higgs boson due to its the largest branching ratio compared to other modes.

hep-ph↗

arXiv2013

Searching for Charged Higgs Boson in Polarized Top Quark

The charged Higgs boson is quite common in many new physics models. In this study we examine the potential of observing a heavy charged Higgs boson in its decay mode of top-quark and bottom-quark in the Type-II Two-Higgs-Doublet-Model. In this model, the chirality structure of the coupling of charged Higgs boson to the top- and bottom-quark is very sensitive to the value of $\tanβ$. As the polarization of the top-quark can be measured experimentally from the top-quark decay products, one could make use of the top-quark polarization to determine the value of $\tanβ$. We preform a detailed analysis of measuring top-quark polarization in the production channels $gb\to tH^-$ and $g\bar{b}\to \bar{t}H^+$. We calculate the helicity amplitudes of the charged Higgs boson production and decay.Our calculation shows that the top-quark from the charged Higgs boson decay provides a good probe for measuring $\tanβ$, especially for the intermediate $\tanβ$ region. On the contrary, the top-quark produced in association with the charged Higgs boson cannot be used to measure $\tanβ$ because its polarization is highly contaminated by the $t$-channel kinematics.

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arXiv2012

Constraints on the Universal Varying Yukawa Couplings: from SM-like to Fermiophobic

Varying the Standard Model (SM) fermion Yukawa couplings universally by a generic positive scale factor ($F_{Yu}$), we study the phenomenological fit to the current available experimental results for the Higgs boson search at hadron colliders. We point out that the Higgs production cross section and its decay branching ratio to $γγ$ can be varied oppositely by $F_{Yu}$ to make their product almost invariant. Thus, our scenario and the SM Higgs are indistinguishable in the inclusive $H\to γγ$ channel. The current measurements on direct Yukawa coupling strength in the $H\to b\bar{b}/ττ$ channel are not precise enough to fix the scale factor $F_{Yu}$. The most promising is the vector-boson-fusion channel in which the CMS has already observed possible suppression effect on the Yukawa couplings. Further more, the global $χ^2$ fit of the experimental data can get the optimal value by introducing a suppression factor $F_{Yu}\sim1/2$ on the SM Yukawa couplings.

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arXiv2012

A simple algorithm for automatic Feynman diagram generation

An algorithm for the automatic Feynman diagram (FD) generation is presented in this paper. The algorithm starts directly from the definition formula of FD, and is simple in concept and easy for coding. The symmetry factor for each FD is naturally generated. It is expected to bring convenience for the researchers who are studying new calculation techniques or making new calculation tools and for the researchers who are studying effective field theory. A C-program made from the algorithm is also presented, which is short, fast, yet very general purpose: it receives arbitrary user defined model and arbitrary process as input and generates FD's at any order.

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arXiv2012

Toward the Natural and Realistic NMSSM with and without R-Parity

From the current ATLAS and CMS results on Higgs boson mass and decay rates, the NMSSM is obviously better than the MSSM. To explain the fine-tuning problems such as gauge hiearchy problem and strong CP problem in the SM, we point out that supersymmetry does not need to provide a dark matter candidate, i.e., R-parity can be violated. Thus, we consider three kinds of the NMSSM scenarios: in Scenarios I and II R-parity is conserved and the lightest neutralino relic density is respectively around and smaller than the observed value, while in Scenario III R-parity is violated. To fit all the experimental data, we consider the χ^2 analyses, and find that the Higgs boson mass and decay rates can be explained very well in these Scenarios. Considering the small χ^2 values and fine-tuning around 2-3.7% (or 1-2%), we obtain the viable parameter space with light (or relatively heavy) supersymmetric particle spectra only in Scenario III (or in Scenarios I and II). Because the singlino, Higgsinos, and light stop are relatively light in general, we can relax the LHC supersymmetry search constraints but the XENON100 experiment gives a strong constraint in Scenarios I and II. In all the viable parameter space, the anomalous magnetic moment of the muon (g_μ - 2)/2 are generically small. With R-parity violation, we can increase (g_μ - 2)/2, and avoid the contraints from the LHC supersymmetry searches and XENON100 experiment. Therefore, Scenario III with R-parity violation is more natural and realistic than Scenarios I and II.

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arXiv2012

Cosmic Rays during BBN as Origin of Lithium Problem

There may be non-thermal cosmic rays during big-bang nucleosynthesis (BBN) epoch (dubbed as BBNCRs). This paper investigated whether such BBNCRs can be the origin of Lithium problem or not. It can be expected that BBNCRs flux will be small in order to keep the success of standard BBN (SBBN). With favorable assumptions on the BBNCR spectrum between 0.09 -- 4 MeV, our numerical calculation showed that extra contributions from BBNCRs can account for the $^7$Li abundance successfully. However $^6$Li abundance is only lifted an order of magnitude, which is still much lower than the observed value. As the deuteron abundance is very sensitive to the spectrum choice of BBNCRs, the allowed parameter space for the spectrum is strictly constrained. We should emphasize that the acceleration mechanism for BBNCRs in the early universe is still an open question. For example, strong turbulent magnetic field is probably the solution to the problem. Whether such a mechanism can provide the required spectrum deserves further studies.

astro-ph.CO↗

arXiv2012

Four Jets as a Probe of O(100 GeV) Physics beyond Standard Model at Hadron Colliders

O(100 GeV) physics beyond the standard model (BSM) could be overlooked provided that it is hidden in the untouched Higgs sector or jets. The top quark forward-backward asymmetry measurements and di-jet bump, which is observed in the associated production with charged lepton plus missing energy (supposed arising from W decay), may indicate the existence of a new color-octet axial-vector Z_C with a mass about 145 GeV. Here Z_C only decays into two jets. In this paper we investigated the possibility to discover Z_C pair via analyzing the four jets as the final states, which are heavily polluted by huge QCD background. Our simulation showed that, however, both Tevatron and LHC have the excellent chance to discover Z_C through analyzing the four jets events in the current accumulated data.

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arXiv2011

New color-octet axial vector boson revisited

In this paper we re-examine how to utilize the previous proposed color-octet axial-vector boson $Z_{\rm C}$ to explain the $3.4σ$ anomaly of $t\bar t$ forward-backward (FB) asymmetry $A_{\rm FB}$ for $m_{t\bar t}> 450$GeV observed by CDF. Our numerical results indicate that the best-fit parameters are $g_A^q=0.07$, $g_A^Q=3$, and $M_{\rm C}=440\text{GeV}$, which are obtained by fitting the mass dependent $A_{\rm FB}$ and total cross section data provided by a recent CDF measurement. Here $g_A^q (g_A^Q)$ and $M_{\rm C}$ are the axial couplings among $Z_{\rm C}$ with the first two (the third) generation quarks, and $Z_{\rm C}$ mass respectively. We also calculate one-side forward-backward asymmetry $A_{\rm OFB}$ for top and bottom quark pair production at the LHC, focusing on the new contributions from $Z_{\rm C}$. Our studies show that $A_{\rm OFB}$ can be utilized to measure the properties of new particle $Z_{\rm C}$.

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arXiv2011

$\mathscr{O}(100 GeV)$ Deci-weak $W^\prime/Z^\prime$ at Tevatron and LHC

Recently Tevatron released their measurements on invariant mass spectrum of electron/positron, as well as the di-jet arising from WW+WZ production with one W leptonically decay. Though the statistics is not significant, there are two bumps around 240 GeV and 120-160 GeV respectively. We proposed that the two bumps correspond to the extra light gauge bosons $Z^\prime$ and $W^\prime$, which couple with quarks with the deci-weak strength. In this brief report, we also simulated di-jet invariant mass distribution at the current running LHC.

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arXiv2011

New Color-Octet Vector Boson Revisit

Motivated by CDF recent measurements on di-jet invariant mass spectrum where di-jet is associated production with charged leptons ($e/μ$) and missing energy, we re-examine the previous proposed massive color-octet axial-vector-like boson $Z_c$. Our simulation showed that the di-jet bump around 120-160 GeV can be induced by $Z_c$ with effective coupling $g_{Z_c q \bar q} =0.2 g_s$ (q represents the quark other than top and $g_s$ is the strong coupling constant). Moreover our numerical investigation indicated that the top quark forward-backward asymmetry $A_{FB}^t$ can be reproduced without distorting shape of differential cross section $dσ/d M_{t\bar t}$, provided that the $Z_c$ and top quark coupling is appropriately chosen ($g_{Z_c t \bar t} \simeq 4.5 g_{Z_c q \bar q}$). Our results also showed that the theoretical $A_{FB}^t$ as functions of $Δy$ and $M_{t\bar t}$ can be consistent with data within $1σ$ and $1.8σ$ respectively.

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