arXiv subjects

Tevong You

Publications and source records attributed to Tevong You.

49 records · Page 3Linked to original sources

arXiv2016

The Price of an Electroweak Monopole

In a recent paper, Cho, Kim and Yoon (CKY) have proposed a version of the SU(2) $\times$ U(1) Standard Model with finite-energy monopole and dyon solutions. The CKY model postulates that the effective U(1) gauge coupling $\to \infty$ very rapidly as the Englert-Brout-Higgs vacuum expectation value $\to 0$, but in a way that is incompatible with LHC measurements of the Higgs boson $H \to γγ$ decay rate. We construct generalizations of the CKY model that are compatible with the $H \to γγ$ constraint, and calculate the corresponding values of the monopole and dyon masses. We find that the monopole mass could be $< 5.5$ TeV, so that it could be pair-produced at the LHC and accessible to the MoEDAL experiment.

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arXiv2015

Sensitivities of Prospective Future e+e- Colliders to Decoupled New Physics

We explore the indirect sensitivities to decoupled new physics of prospective precision electroweak measurements, triple-gauge-coupling measurements and Higgs physics at future $e^+e^-$ colliders, with emphasis on the ILC250 and FCC-ee. The Standard Model effective field theory (SM EFT) is adopted as a model-independent approach for relating experimental precision projections to the scale of new physics, and we present prospective constraints on the Wilson coefficients of dimension-6 operators. We find that in a marginalised fit ILC250 EWPT measurements may be sensitive to new physics scales $Λ= \mathcal{O}(10)$~TeV, and FCC-ee EWPT measurements may be sensitive to $Λ= \mathcal{O}(30)$~TeV. The prospective sensitivities of Higgs and TGC measurements at the ILC250 (FCC-ee) are to $Λ= \mathcal{O}(1)$~TeV ($Λ= \mathcal{O}(2)$~TeV).

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arXiv2014

The Effective Standard Model after LHC Run I

We treat the Standard Model as the low-energy limit of an effective field theory that incorporates higher-dimensional operators to capture the effects of decoupled new physics. We consider the constraints imposed on the coefficients of dimension-6 operators by electroweak precision tests (EWPTs), applying a framework for the effects of dimension-6 operators on electroweak precision tests that is more general than the standard $S,T$ formalism, and use measurements of Higgs couplings and the kinematics of associated Higgs production at the Tevatron and LHC, as well as triple-gauge couplings at the LHC. We highlight the complementarity between EWPTs, Tevatron and LHC measurements in obtaining model-independent limits on the effective Standard Model after LHC Run~1. We illustrate the combined constraints with the example of the two-Higgs doublet model.

hep-ph↗

arXiv2014

Complete Higgs Sector Constraints on Dimension-6 Operators

Constraints on the full set of Standard Model dimension-6 operators have previously used triple-gauge couplings to complement the constraints obtainable from Higgs signal strengths. Here we extend previous analyses of the Higgs sector constraints by including information from the associated production of Higgs and massive vector bosons (H+V production), which excludes a direction of limited sensitivity allowed by partial cancellations in the triple-gauge sector measured at LEP. Kinematic distributions in H+V production provide improved sensitivity to dimension-6 operators, as we illustrate here with simulations of the invariant mass and $p_T$ distributions measured by D0 and ATLAS, respectively. We provide bounds from a global fit to a complete set of CP-conserving operators affecting Higgs physics.

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arXiv2013

Enhancement of Majorana Dark Matter Annihilation Through Higgs Bremsstrahlung

For Majorana dark matter, gauge boson bremsstrahlung plays an important role in enhancing an otherwise helicity-suppressed s-wave annihilation cross-section. This is well known for processes involving a radiated photon or gluon together with a Standard Model fermion-antifermion pair, and the case of massive electroweak gauge bosons has also recently been studied. Here we show that internal Higgs bremsstrahlung also lifts helicity suppression and could be the dominant contribution to the annihilation rate in the late Universe for dark matter masses below ~1 TeV. Using a toy model of leptophilic dark matter, we calculate the annihilation cross-section into a lepton-antilepton pair with a Higgs boson and investigate the energy spectra of the final stable particles at the annihilation point.

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arXiv2013

Constraining Higgs Properties in Associated Production

The Higgs boson H produced in association with a vector boson V=W, Z is the main production mechanism for searches at the Tevatron and forms an important part of Higgs analyses at the LHC. We show here that the V+H invariant mass distribution and the energy dependence of associated production provide powerful ways of constraining Higgs properties, such as its spin-parity and dimension-6 operator coefficients.

hep-ph↗

arXiv2013

Updated Global Analysis of Higgs Couplings

There are many indirect and direct experimental indications that the new particle H discovered by the ATLAS and CMS Collaborations has spin zero and (mostly) positive parity, and that its couplings to other particles are correlated with their masses. Beyond any reasonable doubt, it is a Higgs boson, and here we examine the extent to which its couplings resemble those of the single Higgs boson of the Standard Model. Our global analysis of its couplings to fermions and massive bosons determines that they have the same relative sign as in the Standard Model. We also show directly that these couplings are highly consistent with a dependence on particle masses that is linear to within a few %, and scaled by the conventional electroweak symmetry-breaking scale to within 10%. We also give constraints on loop-induced couplings, on the total Higgs decay width, and on possible invisible decays of the Higgs boson under various assumptions.

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arXiv2013

Associated Production Evidence against Higgs Impostors and Anomalous Couplings

There is still no proof that the new particle $X$ recently discovered by the ATLAS and CMS Collaborations indeed has spin zero and positive parity, as confidently expected. We show here that the energy dependence of associated $W/Z + X$ production would be much less for a $J^P = 0^+$ boson with minimal couplings, such as the Higgs boson of the Standard Model, than for a spin-two particle with graviton-like couplings or a spin-zero boson with non-minimal couplings. The $W/Z + (X \to {\bar b}b)$ signal apparently observed by the CDF and D0 Collaborations can be used to predict the cross section for the same signal at the LHC that should be measured under the spin-two and different spin-zero hypotheses. The spin-two prediction exceeds by an order of magnitude the upper limits established by the ATLAS and CMS Collaborations, which are consistent with the minimal $0^+$ prediction, thereby providing {\it secunda facie} evidence against spin-two Higgs impostors. Similar analyses of energy dependences provide evidence against $0^-$ impostors, non-minimal scalar boson couplings, including the best LHC limits on dimension-six operators. Comparing the LHC vector boson fusion cross sections at 7 and 8 TeV in the centre of mass provides additional but weaker evidence in favour of the identification of the $X$ particle as a $J^P = 0^+$ boson with minimal couplings.

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arXiv2012

Prima Facie Evidence against Spin-Two Higgs Impostors

The new particle X recently discovered by the ATLAS and CMS Collaborations is widely expected to have spin zero, but this remains to be determined. The leading alternative is that X has spin two, presumably with graviton-like couplings. We show that measurements of the X particle to pairs of vector bosons constrain such scenarios. In particular, a graviton-like Higgs impostor in scenarios with a warped extra dimension of AdS type is prima facie excluded, principally because they predict too small a ratio between the X couplings to WW and ZZ, compared with that to photons. The data also disfavour universal couplings to pairs of photons and gluons, which would be predicted in a large class of graviton-like models.

hep-ph↗

arXiv2012

Distinguishing `Higgs' Spin Hypotheses using gamma gamma and WW* Decays

The new particle X recently discovered by the ATLAS and CMS Collaborations in searches for the Higgs boson has been observed to decay into gamma gamma, ZZ* and WW*, but its spin and parity, J^P, remain a mystery, with J^P = 0^+ and 2^+ being open possibilities. We use PYTHIA and Delphes to simulate an analysis of the angular distribution of gg to X to gamma gamma decays in a full 2012 data set, including realistic background levels. We show that this angular distribution should provide strong discrimination between the possibilities of spin zero and spin two with graviton-like couplings: ~ 3 sigma if a conservative symmetric interpretation of the log-likelihood ratio (LLR) test statistic is used, and ~ 6 sigma if a less conservative asymmetric interpretation is used. The WW and ZZ couplings of the Standard Model Higgs boson and of a 2^+ particle with graviton-like couplings are both expected to exhibit custodial symmetry. We simulate the present ATLAS and CMS search strategies for X to WW* using PYTHIA and Delphes, and show that their efficiencies in the case of a spin-two particle with graviton-like couplings are a factor ~ 1.9 smaller than in the spin-zero case. On the other hand, the ratio of X_{2^+} to WW* and ZZ* branching ratios is larger than that in the 0^+ case by a factor ~ 1.3. We find that the current ATLAS and CMS results for X to WW* and X to ZZ* decays are compatible with custodial symmetry under both the spin-zero and -two hypotheses, and that the data expected to become available during 2012 are unlikely to discriminate significantly between these possibilities.

hep-ph↗

arXiv2012

A Fast Track towards the `Higgs' Spin and Parity

The LHC experiments ATLAS and CMS have discovered a new boson that resembles the long-sought Higgs boson: it cannot have spin one, and has couplings to other particles that increase with their masses, but the spin and parity remain to be determined. We show here that the `Higgs' + gauge boson invariant-mass distribution in `Higgs'-strahlung events at the Tevatron or the LHC would be very different under the J^P = 0+, 0- and 2+ hypotheses, and could provide a fast-track indicator of the `Higgs' spin and parity. Our analysis is based on simulations of the experimental event selections and cuts using PYTHIA and Delphes, and incorporates statistical samples of `toy' experiments.

hep-ph↗

arXiv2012

Global Analysis of the Higgs Candidate with Mass ~ 125 GeV

We analyze the properties of the Higgs candidate with mass ~ 125 GeV discovered by the CMS and ATLAS Collaborations, constraining the possible deviations of its couplings from those of a Standard Model Higgs boson. The CMS, ATLAS and Tevatron data are compatible with Standard Model couplings to massive gauge bosons and fermions, and disfavour several types of composite Higgs models unless their couplings resemble those in the Standard Model. We show that the couplings of the Higgs candidate are consistent with a linear dependence on particle masses, scaled by the electroweak scale ~ 246 GeV, the power law and the mass scale both having uncertainties ~ 20%.

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arXiv2012

Global Analysis of Experimental Constraints on a Possible Higgs-Like Particle with Mass ~ 125 GeV

We perform a global analysis of the constraints on a possible Higgs-like particle with mass ~ 125 GeV that are provided by the ATLAS, CDF, CMS and D0 experiments. We combine the available constraints on possible deviations from the Standard Model Higgs couplings to massive vector bosons and to fermions, considering also the possibilities of non-standard loop-induced couplings to photon and gluon pairs. We analyze the combined constraints on pseudo-dilaton scenarios and on some other scenarios in which the possible new particle is identified as a pseudo-Nambu-Goldstone boson in a composite electroweak symmetry-breaking sector.

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