arXiv subjects

Ambresh Shivaji

Publications and source records attributed to Ambresh Shivaji.

33 records · Page 2Linked to original sources

arXiv2015

Constraints on CP-violating gauge-Higgs operators

We consider the most general set of $SU(2) \times U(1)$ invariant CP-violating operators of dimension six, which contribute to $VVh$ interactions ($V = W, Z, γ$). Our aim is to constrain any CP-violating new physics above the electroweak scale via the effective couplings that arise when such physics is integrated out. For this purpose, we use, in turn, electroweak precision data, global fits of Higgs data at the Large Hadron Collider and the electric dipole moments of the neutron and the electron. We thus impose constraints mainly on two-parameter and three-parameter spaces. We find that the constraints from the electroweak precision data are the weakest. Among the existing Higgs search channels, considerable constraints come from the diphoton signal strength. We note that potential contribution to $h \rightarrow γZ$ may in principle be a useful constraining factor, but it can be utilized only in the high energy run. The contributions to electric dipole moments mostly lead to the strongest constraints, though somewhat fine-tuned combinations of more than one parameter with large magnitudes are allowed. We also discuss constraints on gauge boson trilinear couplings which depend on the parameters of the CP-violating operators .

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arXiv2015

NLO QCD corrections to the resonant Vector Diquark production at the LHC

With the upcoming run of the Large Hadron Collider (LHC) at much higher center of mass energies, the search for Beyond Standard Model (BSM) physics will again take center stage. New colored particles predicted in many BSM scenarios are expected to be produced with large cross sections thus making them interesting prospects as a doorway to hints of new physics. We consider the resonant production of such a colored particle, the diquark, a particle having the quantum number of two quarks. The diquark can be either a scalar or vector. We focus on the vector diquark which has much larger production cross section compared to the scalar ones. In this work we calculate the next-to-leading order (NLO) QCD corrections to the on-shell vector diquark production at the LHC produced through the fusion of two quarks as well as the NLO corrections to its decay width. We present full analytic results for the one-loop NLO calculation and do a numerical study to show that the NLO corrections can reduce the scale uncertainties in the cross sections which can be appreciable and therefore modify the expected search limits for such particles. We also use the dijet result from LHC to obtain current limits on the mass and coupling strengths of the vector diquarks.

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arXiv2014

Gluon Fusion Contribution to $VHj$ Production at Hadron Colliders

We study the associated production of an electroweak vector boson and the Higgs boson with a jet via gluon-gluon fusion. At the leading order, these processes occur at one-loop level. The amplitudes of these one-loop processes are gauge invariant and finite. Therefore, their contributions towards the corresponding hadronic cross sections and kinematic distributions can be calculated separately. We present results for the Large Hadron Collider and its discussed upgrades. We find that the gluon-gluon one-loop process gives dominant contribution to the $γH j$ production. We observe a destructive interference effect in the $gg\to Z H j$ amplitude. We also find that in the high transverse momentum and central rapidity region, the $ZHj$ production cross section via gluon-gluon fusion becomes comparable to the cross section contributions coming from quark-quark and quark-gluon channels.

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arXiv2014

$ttH$ Anomalous Coupling in Double Higgs Production

We study the effects of top-Higgs anomalous coupling in the production of a pair of Higgs boson via gluon fusion at the Large Hadron Collider (LHC). The introduction of anomalous $ttH$ coupling can alter the hadronic double Higgs boson cross section and can lead to characteristic changes in certain kinematic distributions. We perform a global analysis based on available LHC data on the Higgs to constrain the parameters of $ttH$ anomalous coupling. Possible overlap of the predictions due to anomalous $ttH$ coupling with those due to anomalous trilinear Higgs coupling is also studied. We briefly discuss the effect of the anomalous $ttH$ coupling on the $HZ$ production via gluon fusion which is one of the main backgrounds in the $HH \to γγb {\bar b}$ channel.

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arXiv2014

Searching for an elusive charged Higgs at the Large Hadron Collider

We study the signals for a "fermiophobic" charged Higgs boson present in an extension of the standard model with an additional Higgs doublet and right handed neutrinos, responsible for generating Dirac-type neutrino masses. We study the pair production of the charged Higgs at the Large Hadron Collider (LHC), which can be relatively light and still allowed by experimental data. The charged Higgs decays dominantly into a $W$ boson and a very light neutral scalar present in the model, which decays invisibly and passes undetected. We find that the signal for such a charged Higgs is overwhelmed by the standard model background and will prove elusive at the 8 TeV run of the LHC. We present a cut-flow based analysis to pinpoint a search strategy at the 14 TeV run of the LHC which can achieve a signal significance of 5$σ$ for a given mass range of the charged Higgs.

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arXiv2013

Effect of Anomalous Couplings on the Associated Production of a Single Top Quark and a Higgs Boson at the LHC

We consider the production of a single top quark in association with a Higgs boson at the LHC. In particular, we compute the cross sections for the processes $ p p \to t h j$, $t h b$, $t h W$, $t h j j$, $t h j b$, $t h W j$, $t h W b$ in the presence of the anomalous $Wtb, WWh$ and $tth$ couplings. We find that the anomalous $Wtb$ and $tth$ couplings can enhance the cross sections significantly. We also analyze a few signatures and show that, if these couplings are indeed anomalous, then with enough data, one should be able to observe the production of the Higgs boson in association with single top quark.

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arXiv2013

A Note on the IR Finiteness of Fermion Loop Diagrams

We show that the most general fermion loop diagram is finite in both soft and collinear regions and therefore, it's IR finite. We use this result to express the IR singular structure of a box scalar integral in terms of the IR singular structure of reduced triangle scalar integrals.

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arXiv2013

Gluon Fusion Processes at One-loop within the Standard Model and Beyond

In this thesis, we have studied certain gluon fusion processes which proceed via quark loop diagrams at the leading order. The fact that these gluon-gluon channel processes are independent processes, their contributions towards the total/differential hadronic cross sections can be calculated separately. We have considered the production of a pair of electroweak vector bosons in association with a jet via gluon fusion within the standard model. These processes were not accessible at earlier hadron colliders such as the Tevatron. Therefore, observation of these rare processes at the LHC will be a test of the standard model itself. Like the di-vector boson production via gluon fusion processes, these processes are also important backgrounds for many new physics signals, and the standard model Higgs boson signal at the LHC. These leading order gluon fusion processes contribute to the corresponding hadronic processes at the next-to-next-to-leading order in α_s. We have taken a model of extra-dimensions, the ADD (Arkani-Hamed, Dimopoulos and Dvali) model, as the possible candidate of new physics at the LHC. This model tries to solve the hierarchy problem of the standard model by proposing large extra space dimensions which may be accessible at TeV scale. We have considered the direct production of KK-gravitons (GKK) in association with an electroweak boson (H/γ/Z) via gluon fusion. These processes contribute to the corresponding hadronic processes at the next-to-leading order in α_s. Many interesting issues related to the fermion loop amplitudes have also been discussed.

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arXiv2013

Production of $γZ g$ and associated processes via gluon fusion at hadron colliders

We present results for the process $gg \to γZ g$ within the standard model at hadron colliders. Due to large gluon flux at the LHC and the proposed HE-LHC, this process can have significant cross section at these machines. Several thousand such events have already been produced at the LHC according to the standard model predictions. This process can also be a background to new physics signals. We discuss some of the important features of this process related to the structure of its amplitude. We also compute the total cross section at the hadron colliders and give some important kinematic distributions. A comparison of our results is made with the corresponding NLO calculation using MCFM program. We briefly discuss the production of $γγg $ and $γγZ$ via gluon fusion. The amplitude of these processes are closely related to the $gg \toγZ g$ amplitude.

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arXiv2012

Di-Vector Boson + Jet Production via Gluon Fusion at Hadron Colliders

We study the production of two electro-weak vector bosons in association with a jet via gluon fusion. In particular we consider the production of $γZg$, $ZZg$ and $W^{+}W^{-}g$ at hadron colliders and compute their cross-sections. Such processes have already produced large number of events at the Large Hadron Collider (LHC) by now. These processes can be background to the Higgs boson production and a number of beyond the standard model scenarios. Therefore it is important to know the values of their contribution. The calculation is based on conventional Feynman diagram approach. In particular we find that the process $g g \to Z Z g$ can make a significant contribution to the process $p p \to Z Z + {\rm jet}$.

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arXiv2012

Multi Vector Boson Production via Gluon Fusion at the LHC

At the LHC, the processes with several particles in the final state occur routinely. These processes provide a new domain to test the Standard Model of particle physics. Due to large gluon luminosity at the LHC, gluon initiated processes will be quite important. We study the production of multi-vector bosons via gluon fusion. In particular we consider the process $ g g \to g γZ $ and compute its cross-section at hadron colliders.

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arXiv2012

Associated Production of a KK-Graviton with a Higgs Boson via Gluon Fusion at the LHC

In order to solve the hierarchy problem, several extra-dimensional models have received considerable attention. We have considered a process where a Higgs boson is produced in association with a KK-graviton ($G_{\rm KK}$) at the LHC. At the leading order, this process occurs through gluon fusion mechanism $gg \to h G_{\rm KK}$ via a quark loop. We compute the cross section and examine some features of this process in the ADD model. We find that the quark in the loop does not decouple in the large quark-mass limit just as in the case of $gg\to h$ process. We compute the cross section of this process for the case of the RS model also. We examine the feasibility of this process being observed at the LHC.

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arXiv2011

Production of a KK-graviton and a Vector Boson in ADD Model via Gluon fusion

In the models with large extra-dimensions, we examine the production of a vector boson (γ/ Z) in association with the Kaluza-Klein (KK) modes of the graviton via gluon fusion. At the leading order, the process takes place through quark-loop box and triangle diagrams and it is ultraviolate finite. We report the results for the LHC. We also discuss the issues of anomaly and decoupling of heavy quarks in the amplitude.

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arXiv2011

Production of a KK-graviton in association with a boson via gluon fusion at the LHC

We discuss the processes where a KK-graviton ($G$) of the extra-dimensional models is produced in association with a color singlet boson via gluon fusion at the LHC. In particular, we consider the processes $gg \to h G$, $γG$, $Z G$. These processes occur at one-loop through box and triangle diagrams. The cross-section for the process $gg \to γG$ vanishes at the one-loop level. It can be understood by introducing the charge conjugation transformations of the KK-graviton. The processes $gg \to h G$, $Z G$ can be observed at the LHC if the KK-graviton and the Higgs boson exist with appropriate properties.

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arXiv2010

Simple Analysis of IR Singularities at One Loop

In this article, we explore the structure of IR singularity of Feynman diagrams at one loop via power counting in loop momentum. The emphasis is on many known results which follow from this simple analysis.

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