arXiv subjects

Rick S. Gupta

Publications and source records attributed to Rick S. Gupta.

34 records · Page 2Linked to original sources

arXiv2018

The Nelson-Barr Relaxion

Cosmological relaxation models in which the relaxion is identified with the QCD axion, generically fail to account for the smallness of the strong CP phase. We present a simple alternative solution to this "relaxion CP problem" based on the Nelson-Barr mechanism. We take CP to be a symmetry of the UV theory, and the relaxion to have no anomalous coupling with QCD. The non-zero vacuum expectation value of the relaxion breaks CP spontaneously, and the resulting phase is mapped to the Cabibbo-Kobayashi-Maskawa phase of the Standard Model. The extended Nelson-Barr quark sector generates the relaxion "rolling" potential radiatively, relating the new physics scale with the relaxion decay constant. With no new states within the reach of the LHC, our relaxion can still be probed in a variety of astrophysical and cosmological processes, as well as in flavor experiments.

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arXiv2017

Phenomenology of relaxion-Higgs mixing

We show that the relaxion generically stops its rolling at a point that breaks CP leading to relaxion-Higgs mixing. This opens the door to a variety of observational probes since the possible relaxion mass spans a broad range from sub-eV to the GeV scale. We derive constraints from current experiments (fifth force, astrophysical and cosmological probes, beam dump, flavour, LEP and LHC) and present projections from future experiments such as NA62, SHiP and PIXIE. We find that a large region of the parameter space is already under the experimental scrutiny. All the experimental constraints we derive are equally applicable for general Higgs portal models. In addition, we show that simple multiaxion (clockwork) UV completions suffer from a mild fine tuning problem, which increases with the number of sites. These results favour a cut-off scale lower than the existing theoretical bounds.

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arXiv2016

Interpreting a 750 GeV Diphoton Resonance

We discuss the implications of the significant excesses in the diphoton final state observed by the LHC experiments ATLAS and CMS around a diphoton invariant mass of 750 GeV. The interpretation of the excess as a spin-zero s-channel resonance implies model-independent lower bounds on both its branching ratio and its coupling to photons, which stringently constrain dynamical models. We consider both the case where the excess is described by a narrow and a broad resonance. We also obtain model-independent constraints on the allowed couplings and branching fractions to final states other than diphotons, by including the interplay with 8 TeV searches. These results can guide attempts to construct viable dynamical models of the resonance. Turning to specific models, our findings suggest that the anomaly cannot be accounted for by the presence of only an additional singlet or doublet spin-zero field and the Standard Model degrees of freedom; this includes all two-Higgs-doublet models. Likewise, heavy scalars in the MSSM cannot explain the excess if stability of the electroweak vacuum is required, at least in a leading-order analysis. If we assume that the resonance is broad we find that it is challenging to find a weakly coupled explanation. However, we provide an existence proof in the form of a model with vectorlike quarks with large electric charge that is perturbative up to the 100 TeV scale. For the narrow-resonance case a similar model can be perturbative up to high scales also with smaller charges. We also find that, in their simplest form, dilaton models cannot explain the size of the excess. Some implications for flavor physics are briefly discussed.

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arXiv2016

Is the Relaxion an Axion?

We consider the recently proposed cosmological relaxation mechanism where the hierarchy problem is ameliorated, and the electroweak scale is dynamically selected by a slowly rolling axion field. We argue that, in its simplest form, the construction breaks a gauge symmetry that always exists for pseudo-Nambu-Goldstone bosons (in particular the axion). The small parameter in the relaxion model is therefore not technically natural as it breaks a gauge symmetry rather than global symmetries only. The consistency of the theory generically implies that the cutoff must lie around the electroweak scale, but not qualitatively higher. We discuss several ways to evade the above conclusion. Some of them may be sufficient to increase the cutoff to the few-TeV range (and therefore may be relevant for the little-hierarchy problem). To demonstrate the ideas in a concrete setting we consider a model with a familon, the Nambu-Goldstone boson of a spontaneously broken chiral flavor symmetry. The model has some interesting collider-physics aspects and contains a viable weakly interacting dark matter candidate.

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arXiv2014

BSM Primary Effects: The complete set of predictions from the dimension-6 BSM Lagrangian

We present a physical parameterization of the leading effects beyond the SM (BSM), that give us, at present, the best way to constrain heavy new-physics at low-energies. We call these effects that constrain all possible interactions at the dimension 6 level, BSM Primary effects; there are 8 primaries related to Higgs physics, 3 related to Triple Gauge Couplings and 7 related to Z- pole measurements at LEP. Starting from these experimentally measurable deformations (and not operators), we construct the dimension 6 Lagrangian in a bottom up way. We, thus, show that other BSM effects are not independent from the primary ones and we provide the explicit correlations. We also discuss the theoretical expectation for the size of these BSM primaries in some well-motivated BSM theories.

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arXiv2014

BSM Primary Effects

Using the predictive power of the effective field theory approach, we present a physical parametrization of the leading effects beyond the SM (BSM), that give us at present the best way to constrain heavy new-physics at low-energies. We show that other BSM effects are not independent from these ones, and we provide the explicit correlations. This information is useful to know where to primarily look for new physics in future experiments, and to know how this new physics is related to previous measurements, most importantly in electroweak-symmetry breaking processes or Higgs physics.

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arXiv2014

Scaling and tuning of EW and Higgs observables

We study deformations of the SM via higher dimensional operators. In particular, we explicitly calculate the one-loop anomalous dimension matrix for 13 bosonic dimension-6 operators relevant for electroweak and Higgs physics. These scaling equations allow us to derive RG-induced bounds, stronger than the direct constraints, on a universal shift of the Higgs couplings and some anomalous triple gauge couplings by assuming no tuning at the scale of new physics, i.e. by requiring that their individual contributions to the running of other severely constrained observables, like the electroweak oblique parameters or $Γ(h \rightarrow γγ)$, do not exceed their experimental direct bounds. We also study operators involving the Higgs and gluon fields.

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arXiv2013

BSM Theories Face Higgs Coupling data

We discuss how much Higgs couplings (including the Higgs self coupling) can deviate from their Standard Model values, in different Beyond Standard Model (BSM) theories, if no other BSM states are accessible at the LHC. Then, we focus on supersymmetric theories and show that there is a connection between the mechanism to raise the Higgs mass and the pattern of Higgs coupling deviations.

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arXiv2013

Physics at the CLIC e+e- Linear Collider -- Input to the Snowmass process 2013

This paper summarizes the physics potential of the CLIC high-energy e+e- linear collider. It provides input to the Snowmass 2013 process for the energy-frontier working groups on The Higgs Boson (HE1), Precision Study of Electroweak Interactions (HE2), Fully Understanding the Top Quark (HE3), as well as The Path Beyond the Standard Model -- New Particles, Forces, and Dimensions (HE4). It is accompanied by a paper describing the CLIC accelerator study, submitted to the Frontier Capabilities group of the Snowmass process.

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arXiv2013

How well do we need to measure the Higgs boson mass and self-coupling?

Much of the discussion regarding future measurements of the Higgs boson mass and self-coupling is focussed on how well various collider options can do. In this article we ask a physics-based question of how well do we need colliders to measure these quantities to have an impact on discovery of new physics or an impact in how we understand the role of the Higgs boson in nature. We address the question within the framework of the Standard Model and various beyond the Standard Model scenarios, including supersymmetry and theories of composite Higgs bosons. We conclude that the LHC's stated ability to measure the Higgs boson to better than 150 MeV will be as good as we will ever need to know the Higgs boson mass in the foreseeable future. On the other hand, we estimate that the self-coupling will likely need to be measured to better than 20 percent to see a deviation from the Standard Model expectation. This is a challenging target for future collider and upgrade scenarios.

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arXiv2012

SUSY Faces its Higgs Couplings

In supersymmetric models, a correlation exists between the structure of the Higgs sector quartic potential and the coupling of the lightest CP-even Higgs to fermions and gauge bosons. We exploit this connection to relate the observed value of the Higgs mass ~ 125 GeV to the magnitude of its couplings. We analyze different scenarios ranging from the MSSM with heavy stops to more natural models with additional non-decoupling D-term/F-term contributions. A comparison with the most recent LHC data, allows to extract bounds on the heavy Higgs boson masses, competitive with bounds from direct searches.

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arXiv2012

How well do we need to measure Higgs boson couplings?

Most of the discussion regarding the Higgs boson couplings to Standard Model vector bosons and fermions is presented with respect to what present and future collider detectors will be able to measure. Here, we ask the more physics-based question of how well do we need to measure the Higgs boson couplings? We first present a reasonable definition of "need" and then investigate the answer in the context of various highly motivated new physics scenarios: supersymmetry, mixed-in hidden sector Higgs bosons, and a composite Higgs boson. We find the largest coupling deviations away from the SM Higgs couplings that are possible if no other state related to EWSB is directly accessible at the LHC. Depending on the physics scenario under consideration, we find targets that range from less than 1% to 10% for vector bosons, and from a few percent to tens of percent for couplings to fermions.

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arXiv2012

Theory and LHC Phenomenology of Classicalon Decays

It has been recently proposed by Dvali et al that high energy scattering in non-renormalizable theories, like the higgsless Standard Model, can be unitarized by the formation of classical configurations called classicalons. In this work we argue that classicalons should have analogs of thermodynamic properties like temperature and entropy and perform a model-independent statistical mechanical analysis of classicalon decays. We find that, in the case of massless quanta, the decay products have a Planck distribution with an effective temperature T~1/r_* where r_* is the classicalon radius. These results, in particular a computation of the decay multiplicity, N_*, allow us to make the first collider analysis of classicalization. In the model for unitarization of WW scattering by classicalization of longitudinal Ws and Zs we get spectacular multi-W/Z final states that decay into leptons, missing energy and a very high multiplicity (at least 10) of jets. We find that for the classicalization scale, M_* = v=246 GeV (M_*=1 TeV) discovery should be possible in the present 7 TeV (14 TeV) run of the LHC with about 10 /fb (100 /fb) data. We also consider a model to solve the hierarchy problem, where the classicalons are configurations of the Higgs field which decay into to multi-Higgs boson final states. We find that, in this case, for M_*=500 GeV (M_*=1 TeV), discovery should be possible in the top fusion process with about 10 /fb (100 /fb) data at 14 TeV LHC.

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arXiv2012

Higgs boson search significance deformations due to mixed-in scalars

The existence of exotic scalars that mix with the Standard Model (SM) Higgs boson can affect Higgs boson phenomenology in a multitude of ways. We consider two light Higgs bosons with shared couplings to SM fields and with masses close to each other, in the range where the h \to WW \to l νl νis an important search channel. In this channel, we do not find the dilution of significance of the `SM-like' Higgs boson that is naively expected because of the mixing. This is because of leakage of events from the decay of the other scalar into its signal region. Nevertheless, we show that the broadening of the h\to WW \to l νl νsignificance plots of Standard Model Higgs boson searches could indicate the first evidence of the the extra scalar state.

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arXiv2011

Probing Quartic Neutral Gauge Boson Couplings using diffractive photon fusion at the LHC

A complete list of operators contributing at the lowest order to Quartic Neutral Gauge Boson Couplings involving photons and Z-bosons, is presented. We show that, for the couplings we consider, the lowest order contribution is from dimension 8 operators in the case when a light Higgs is present and from dimension 6 operators in the higgsless case where electroweak symmetry is non-linearly realized. We also show that these operators are generated by exchange of the Kaluza-Klein partners of the graviton in extra-dimensional models. We then explore the possibility of probing these couplings in the diffractive photon fusion processes pp(γγ\to γγ)pp and pp(γγ\to ZZ)pp at the 14 TeV LHC. We find that the γγγγ-coupling can be probed most sensitively and values as small as 1/(1.8 TeV)^{4} can be measured. For the γγZZ-coupling, values as small as 1/(850 GeV)^{4} and 1/(1.9 TeV)^2 can be probed in the light Higgs and higgsless cases respectively, which is an improvement by orders of magnitude over existing limits.

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arXiv2010

Next Generation Higgs Bosons: Theory, Constraints and Discovery Prospects at the Large Hadron Collider

Particle physics model building within the context of string theory suggests that further copies of the Higgs boson sector may be expected. Concerns regarding tree-level flavor changing neutral currents are easiest to allay if little or no couplings of next generation Higgs bosons are allowed to Standard Model fermions. We detail the resulting general Higgs potential and mass spectroscopy in both a Standard Model extension and a supersymmetric extension. We present the important experimental constraints from meson-meson mixing, loop-induced $b\to sγ$ decays and LEP2 direct production limits. We investigate the energy range of valid perturbation theory of these ideas. In the supersymmetric context we present a class of examples that marginally aids the fine-tuning problem for parameter space where the lightest Higgs boson mass is greater than the Standard Model limit of 114 GeV. Finally, we study collider physics signatures generic to next generation Higgs bosons, with special emphasis on $Ah\to hhZ\to 4b+2l$ signal events, and describe the capability of discovery at the Large Hadron Collider.

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