arXiv subjects

Adam Falkowski

Publications and source records attributed to Adam Falkowski.

At least 73 records · Page 4Linked to original sources

arXiv2013

Direct detection of Higgs-portal dark matter at the LHC

We consider the process in which a Higgs particle is produced in association with jets and show that monojet searches at the LHC already provide interesting constraints on the invisible decays of a 125 GeV Higgs boson. Using the existing monojet searches performed by CMS and ATLAS, we show the 95% confidence level limit on the invisible Higgs decay rate is of the order of the total Higgs production rate in the Standard Model. This limit could be significantly improved when more data at higher center of mass energies are collected, provided systematic errors on the Standard Model contribution to the monojet background can be reduced. We also compare these direct constraints on the invisible rate with indirect ones based on measuring the Higgs rates in visible channels. In the context of Higgs portal models of dark matter, we then discuss how the LHC limits on the invisible Higgs branching fraction impose strong constraints on the dark matter scattering cross section on nucleons probed in direct detection experiments.

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arXiv2013

Higgs At Last

We update the experimental constraints on the parameters of the Higgs effective Lagrangian. We combine the most recent LHC Higgs data in all available search channels with electroweak precision observables from SLC, LEP-1, LEP-2, and the Tevatron. Overall, the data are perfectly consistent with the 126 GeV particle being the Standard Model Higgs boson. The Higgs coupling to W and Z bosons relative to the Standard Model one is constrained in the range [0.98,1.09] at 95% confidence level, independently of the values of other Higgs couplings. Higher-order Higgs couplings to electroweak gauge bosons are also well constrained by a combination of LHC Higgs data and electroweak precision tests.

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arXiv2012

Data driving the top quark forward--backward asymmetry with a lepton-based handle

We propose that, within the standard model, the correlation between the $t\bar{t}$ forward--backward asymmetry $A_{t\bar t}$ and the corresponding lepton-based asymmetry $A_l$ -- at the differential level -- is strong and rather clean both theoretically and experimentally. Hence a combined measurement of the two distributions as a function of the lepton $p_T$, a direct and experimentally clean observable, would lead to a potentially unbiased and normalization-free test of the standard model prediction. To check the robustness of our proposal we study how the correlation is affected by mis-measurement of the $t\bar t$ system transverse momenta, acceptance cuts, scale dependence and compare the results of MCFM, POWHEG (with & without PYTHIA showering), and SHERPA's CSSHOWER in first-emission mode. We find that the shape of the relative differential distribution $A_{l} (p^{l}_{T}) [A_{t\bar{t}} (p^l_T)]$ is only moderately distorted hence supporting the usefulness of our proposal. Beyond the first emission, we find that the correlation is not accurately captured by lowest-order treatment. We also briefly consider other differential variables such as the system transverse mass and the canonical $t\bar t$ invariant mass. Finally, we study new physics scenarios where the correlation is significantly distorted and therefore can be more readily constrained or discovered using our method.

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arXiv2012

Non-thermal Dark Matter Production from the Electroweak Phase Transition: Multi-TeV WIMPs and "Baby-Zillas"

Particle production at the end of a first-order electroweak phase transition may be rather generic in theories beyond the standard model. Dark matter may then be abundantly produced by this mechanism if it has a sizable coupling to the Higgs field. For an electroweak phase transition occuring at a temperature T_EW ~ 50-100 GeV, non-thermally generated dark matter with mass M_X > TeV will survive thermalization after the phase transition, and could then potentially account for the observed dark matter relic density in scenarios where a thermal dark matter component is either too small or absent. Dark matter in these scenarios could then either be multi-TeV WIMPs whose relic abundace is mostly generated at the electroweak phase transition, or "Baby-Zillas" with mass M_GUT >> M_X >> v_EW that never reach thermal equilibrium in the early universe.

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arXiv2012

Higgs After the Discovery: A Status Report

Recently, the ATLAS and CMS collaborations have announced the discovery of a 125 GeV particle, commensurable with the Higgs boson. We analyze the 2011 and 2012 LHC and Tevatron Higgs data in the context of simplified new physics models, paying close attention to models which can enhance the diphoton rate and allow for a natural weak-scale theory. Combining the available LHC and Tevatron data in the ZZ* 4-lepton, WW* 2-lepton, diphoton, and b-bbar channels, we derive constraints on the effective low-energy theory of the Higgs boson. We map several simplified scenarios to the effective theory, capturing numerous new physics models such as supersymmetry, composite Higgs, dilaton. We further study models with extended Higgs sectors which can naturally enhance the diphoton rate. We find that the current Higgs data are consistent with the Standard Model Higgs boson and, consequently, the parameter space in all models which go beyond the Standard Model is highly constrained.

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arXiv2012

Interpreting the Higgs

The LHC and Tevatron Higgs data are interpreted as constraints on an effective theory of a Higgs boson with mass close to 125 GeV. We focus on the diphoton, ZZ*, WW* channels at the LHC, and the b-bbar channel at the Tevatron, which are currently the most sensitive probes of a Higgs with such a mass. Combining the available data in these channels, we derive the preferred regions of the parameter space of the effective theory. We further provide the mapping between the effective theory and the relevant Higgs event rates, facilitating future extraction of the preferred region by the ATLAS and CMS collaborations.

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arXiv2012

Interpreting LHC Higgs Results from Natural New Physics Perspective

We analyze the 2011 LHC Higgs data in the context of simplified new physics models addressing the naturalness problem. These models are expected to contain new particles with sizable couplings to the Higgs boson, which can easily modify the Higgs production cross sections and branching fractions. We focus on searches in the Higgs to 4 leptons and Higgs to diphoton channels, in the latter case including the vector boson fusion production mode. Combining the available ATLAS and CMS data in these channels, we derive constraints on an effective low-energy theory of the Higgs boson. We then map several simplified scenarios to the effective theory, capturing numerous natural new physics models such as supersymmetry and Little Higgs, and extract the constraints on the corresponding parameter space. We show that simple models where one fermionic or one scalar partner is responsible for stabilizing the Higgs potential are already constrained in a non-trivial way by LHC Higgs data.

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arXiv2012

What if the Higgs couplings to W and Z bosons are larger than in the Standard Model?

We derive a general sum rule relating the Higgs coupling to W and Z bosons to the total cross section of longitudinal gauge boson scattering in I=0,1,2 isospin channels. The Higgs coupling larger than in the Standard Model implies enhancement of the I=2 cross section. Such an enhancement could arise if the Higgs sector is extended by an isospin-2 scalar multiplet including a doubly charged, singly charged, and another neutral Higgs.

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arXiv2011

If no Higgs then what?

In the absence of a Higgs boson, the perturbative description of the Standard Model ceases to make sense above a TeV. Heavy spin-1 fields coupled to W and Z bosons can extend the validity of the theory up to higher scales. We carefully identify regions of parameter space where a minimal addition - a single spin-1 custodial SU(2) triplet resonance - allows one to retain perturbative control in all channels. Elastic scattering of longitudinal W and Z bosons alone seems to permit a very large cut-off beyond the Naive Dimensional Analysis expectation. We find however that including scattering of the spin-1 resonances then leads to an earlier onset of strong coupling. Most importantly for LHC searches, we define a self-consistent set-up with a well-defined range of validity without recourse to unitarization schemes whose physical meaning is obscure. We discuss the LHC phenomenology and the discovery reach for these electroweak resonances and mention the possibility of a nightmare scenario with no Higgs nor resonance within the LHC reach. Finally, we discuss the effects of parity breaking in the heavy resonance sector which reduces the contributions to the S parameter.

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arXiv2011

Spinning the Top

We propose to measure the threshold lepton asymmetry, that is the forward-backward asymmetry of the charged lepton in ttbar events near the production threshold. At threshold top quark pairs are produced in an s-wave. Angular momentum conservation then implies that the top spins equal the spin of the initial state which - in the case of quarks - is uniquely fixed by the chirality of the initial quarks. Thus measuring final state top spins determines the chirality of the quarks which produced them. Information about the top spins can be extracted by measuring the angular distribution of the charged lepton in semileptonic or dileptonic decays of the top pair. One such distribution, the threshold lepton asymmetry, vanishes in tree-level QCD but is non-zero if new physics modifies the relative contribution of right-handed and left-handed quarks to top pair production. This is interesting because realistic models addressing the anomalous ttbar asymmetry have chiral couplings to light quarks. Models with identical ttbar asymmetries at the Tevatron can be distinguished by their threshold lepton asymmetries which range between plus and minus 25% in realistic models.

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arXiv2011

Asymmetric Dark Matter from Leptogenesis

We present a new realization of asymmetric dark matter in which the dark matter and lepton asymmetries are generated simultaneously through two-sector leptogenesis. The right-handed neutrinos couple both to the Standard Model and to a hidden sector where the dark matter resides. This framework explains the lepton asymmetry, dark matter abundance and neutrino masses all at once. In contrast to previous realizations of asymmetric dark matter, the model allows for a wide range of dark matter masses, from keV to 10 TeV. In particular, very light dark matter can be accommodated without violating experimental constraints. We discuss several variants of our model that highlight interesting phenomenological possibilities. In one, late decays repopulate the symmetric dark matter component, providing a new mechanism for generating a large annihilation rate at the present epoch and allowing for mixed warm/cold dark matter. In a second scenario, dark matter mixes with the active neutrinos, thus presenting a distinct method to populate sterile neutrino dark matter through leptogenesis. At late times, oscillations and dark matter decays lead to interesting indirect detection signals.

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arXiv2010

Discovering Higgs Decays to Lepton Jets at Hadron Colliders

The Higgs boson may decay predominantly into a hidden sector, producing lepton jets instead of the standard Higgs signatures. We propose a search strategy for such a signal at hadron colliders. A promising channel is the associated production of the Higgs with a Z or W, where the dominant background is Z or W plus QCD jets. The lepton jets can be discriminated from QCD jets by cutting on the electromagnetic fraction and charge ratio. The former is the fraction of jet energy deposited in the electromagnetic calorimeter and the latter is the ratio of energy carried by charged particles to the electromagnetic energy. We use a Monte Carlo description of detector response to estimate QCD rejection efficiencies of order 10^-3 per jet. The expected Higgs mass reach is about 155 GeV at the Tevatron with 10 fb^-1 of data and about 135 GeV at the 7 TeV LHC with 1 fb^-1.

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arXiv2010

Unburied Higgs

Many models of physics beyond the Standard Model yield exotic Higgs decays. Some of these, particularly those in which the Higgs decays to light quarks or gluons, can be very difficult to discover experimentally. Here we introduce a new set of jet substructure techniques designed to search for such a Higgs when its dominant decay is into gluons via light, uncolored resonances. We study this scenario in both V+h and tt+h production channels, and find both channels lead to discovery at the LHC with more than 5 sigma significance at 100 inverse femtobarn.

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arXiv2010

Hidden Higgs Decaying to Lepton Jets

The Higgs and some of the Standard Model superpartners may have been copiously produced at LEP and the Tevatron without being detected. We study a novel scenario of this type in which the Higgs decays predominantly into a light hidden sector either directly or through light SUSY states. Subsequent cascades increase the multiplicity of hidden sector particles which, after decaying back into the Standard Model, appear in the detector as clusters of collimated leptons known as lepton jets. We identify the relevant collider observables that characterize this scenario, and study a wide range of LEP and Tevatron searches to recover the viable regions in the space of observables. We find that the Higgs decaying to lepton jets can be hidden when the event topology mimics that of hadronic backgrounds. Thus, as many as 10^4 leptonic Higgs and SUSY decays may be hiding in the LEP and Tevatron data. We present benchmark models with a 100 GeV Higgs that are consistent with all available collider constraints. We end with a short discussion of strategies for dedicated searches at LEP, the Tevatron and the LHC, that allow for a discovery of the Higgs or SUSY particles decaying to lepton jets.

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arXiv2009

Charming Higgs

We present a simple supersymmetric model where the dominant decay mode of the lightest Higgs boson is h->2eta->4c where eta is a light pseudoscalar and c is the charm quark. For such decays the Higgs mass can be smaller than 100 GeV without conflict with experiment. Together with the fact that both the Higgs and the pseudoscalar eta are pseudo-Goldstone bosons, this resolves the little hierarchy problem.

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arXiv2009

Buried Higgs

We present an extension of the MSSM where the dominant decay channel of the Higgs boson is a cascade decay into a four-gluon final state. In this model the Higgs is a pseudo-Goldstone boson of a broken global symmetry SU(3)-> SU(2). Both the global symmetry breaking and electroweak symmetry breaking are radiatively induced. The global symmetry breaking pattern also implies the existence of a light (few GeV) pseudo-Goldstone boson eta which is a singlet under the standard model gauge group. The h -> eta eta branching fraction is large, and typically dominates over the standard h -> b b decay. The dominant decay of eta is into two gluons, while the decays to photons, taus or lighter standard model flavors are suppressed at the level of 10^(-4) or more. With h-> 4 jets as the dominant decay, the Higgs could be as light as 78 GeV without being detected at LEP, while detection at the LHC is extremely challenging. However many of the super- and global symmetry partners of the standard model particles should be easily observable at the LHC. Furthermore, the LHC should be able to observe a "wrong Higgs" that is a 300-400 GeV heavy Higgs-like particle with suppressed couplings to W and Z that by itself does not account for electroweak precision observables and the unitarity of WW scattering. At the same time, the true Higgs is deeply buried in the QCD background.

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arXiv2009

Dark Matter Through the Neutrino Portal

We consider a model of dark matter whose most prominent signature is a monochromatic flux of TeV neutrinos from the galactic center. As an example of a general scenario, we consider a specific model where the dark matter is a fermion in the adjoint representation of a hidden SU(N) gauge group that confines at GeV energies. The absence of light fermionic states in the dark sector ensures stability of dark matter on cosmological time scales. Dark matter couples to the standard model via the neutrino portal, that is, the singlet operator H L constructed from the Higgs and lepton doublets, which is the lowest dimensional fermionic singlet operator in the standard model. This coupling prompts dark matter decay where the dominant decay channel has one neutrino (and at least one dark glueball) in the final state. Other decay channels with charged standard model particles involve more particles in the final state and are therefore suppressed by phase space. In consequence, the standard indirect detection signals like gamma-ray photons, antiprotons and positrons are suppressed with respect to the neutrino signal. This coupling via the neutrino portal is most robustly constrained by Super-Kamiokande, which restricts the dark matter lifetime to be larger than 10^25 seconds. In the near future, the scenario will be probed by the new generation of neutrino telescopes. ANTARES will be sensitive to a dark matter lifetime of order 10^26 seconds, while IceCube/DeepCore can probe a lifetime as large as 10^27 seconds.

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arXiv2009

Electroweak Precision Observables and the Unhiggs

We compute one-loop corrections to the S and T parameters in the Unhiggs scenario. In that scenario, the Standard Model Higgs is replaced by a non-local object, called the Unhiggs, whose spectral function displays a continuum above the mass gap. The Unhiggs propagator has effectively the same UV properties as the Standard Model Higgs propagator, which implies that loop corrections to the electroweak precision observables are finite and calculable. We show that the Unhiggs is consistent with electroweak precision tests when its mass gap is at the weak scale; in fact, it then mimics a light SM Higgs boson. We also argue that the Unhiggs, while being perfectly visible to electroweak precision observables, is invisible to detection at LEP.

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