arXiv subjects

Michael Spannowsky

Publications and source records attributed to Michael Spannowsky.

At least 235 records · Page 13Linked to original sources

arXiv2014

Finding physics signals with event deconstruction

We develop a matrix element based reconstruction method called event deconstruction. The method uses information from the hard matrix element and a parton shower to assign probabilities to whether a final state was initiated by a signal or background process. We apply this method in the signal process of a Z' decaying to boosted top quarks in an all hadronic final state and discuss envisioned improvements of the method. We find that event deconstruction can considerably improve on existing reconstruction techniques.

hep-ph↗

arXiv2014

Tagging highly boosted top quarks

For highly energetic top quarks, the products of the decay t->bqq' are collimated. The 3-prong decay structure can no longer be resolved using calorimeter information alone if the particle jet separation approaches the calorimeter granularity. We propose a new method, the HPTTopTagger, that uses tracks of charged particles inside a fat jet to find top quarks with transverse momentum pT >1 TeV. The tracking information is complemented by the calorimeter measurement of the fat jet energy to eliminate the sensitivity to jet-to-jet fluctuations in the charged-to-neutral particle ratio. We show that with the HPTTopTagger, a leptophobic narrow-mass Z' boson of mass 3 TeV could be found using 300/fb of 14 TeV LHC data.

hep-ph↗

arXiv2014

Non-standard top substructure

The top quark, being the heaviest particle of the Standard Model, is a prime candidate of where physics beyond the SM might currently hide before our eyes. There are many natural extensions of the SM that rely on top compositeness, and the top quark could follow the paradigm of revealing a substructure when it is probed at high enough momentum transfers. Observing high $p_T$ top final states naturally drives us towards boosted hadronic analyses that can be tackled efficiently with jet substructure techniques. In this paper we analyse the prospects of constraining exemplary non-standard QCD top interactions in this kinematical regime. We correctly include QCD modifications to additional gluon emission off the boosted top quark and keep track of the modified top tagging efficiencies. We conclude that non-standard top QCD interactions can be formidably constrained at the LHC 14 TeV. Experimental systematic uncertainties are a major obstacle of the described measurement. Unless significantly improved for the 14 TeV run, they will saturate the direct sensitivity to non-resonant BSM top physics at luminosities of around 100/fb.

hep-ph↗

arXiv2013

Further on up the road: $hhjj$ production at the LHC

A measurement of the $hh+2j$ channel at the LHC would be particularly thrilling for electroweak physics. It is not only the leading process which is sensitive to the $W^+W^- hh$ and $ZZhh$ interactions, but also provides a potentially clear window to study the electroweak symmetry-breaking sector by probing Higgs-Goldstone interactions through the weak boson fusion component of the scattering process. Until now, a phenomenologically complete analysis of this channel has been missing. This is mostly due to the high complexity of the involved one-loop gluon fusion contribution and the fact that a reliable estimate thereof cannot be obtained through simplified calculations in the $m_t\to \infty$ limit. In particular, the extraction of the Higgs trilinear coupling from this final state rests on a delicate $m_t$-dependent interference pattern which is not captured in an effective field theory approach. In this paper, we report on the LHC's potential to access di-Higgs production in association with two jets in a fully-showered hadron-level analysis. Our study includes the finite top and bottom mass dependencies for the gluon fusion contribution. On the basis of these results, we also comment on the potential sensitivity to the trilinear Higgs and $VV^\dagger hh$ ($V=W^\pm,Z$) couplings that can be expected from measurements of this final state.

hep-ph↗

arXiv2013

Di-Higgs final states augMT2ed -- selecting $hh$ events at the high luminosity LHC

Higgs boson self-interactions can be investigated via di-Higgs ($pp\to hh+X$) production at the LHC. With a small ${\cal{O}}(30)$ fb Standard Model production cross section, and a large $t\bar t$ background, this measurement has been considered challenging, even at a luminosity-upgraded LHC. We demonstrate that by using simple kinematic bounding variables, of the sort already employed in existing LHC searches, the dominant $t\bar t$ background can be largely eliminated. Simulations of the signal and the dominant background demonstrate the prospect for measurement of the di-Higgs production cross section at the 30% level using 3/ab of integrated luminosity at a high-luminosity LHC. This corresponds to a Higgs self-coupling determination with 60% accuracy in the $b\bar b τ^+τ^-$ mode, with potential for further improvements from e.g. subjet technologies and from additional di-Higgs decay channels.

hep-ph↗

arXiv2013

Pinning down Higgs triplets at the LHC

Extensions of the Standard Model Higgs sector involving weak isotriplet scalars are not only benchmark candidates to reconcile observed anomalies of the recently discovered Higgs-like particle, but also exhibit a vast parameter space, for which the lightest Higgs' phenomenology turns out to be very similar to the Standard Model one. A generic prediction of this model class is the appearance of exotic doubly charged scalar particles. In this paper we adapt existing dilepton+missing energy+jets measurements in the context of SUSY searches to the dominant decay mode $H^{\pm\pm}\to W^\pm W^\pm$ and find that the LHC already starts probing the model's parameter space. A simple modification towards signatures typical of weak boson fusion searches allows us to formulate even tighter constraints with the 7 TeV LHC data set. A corresponding analysis of this channel performed at 14 TeV center of mass energy will constrain the model over the entire parameter space and facilitate potential $H^{\pm\pm}\to W^\pm W^\pm$ discoveries.

hep-ph↗

arXiv2013

Boosting Top Partner Searches in Composite Higgs Models

Fermionic third generation top partners are generic in composite Higgs models. They are likely to decay into third generation quarks and electroweak bosons. We propose a novel cut-and-count-style analysis in which we cross correlate the model-dependent single and model-independent pair production processes for the top partners $X_{5/3}$ and $B$. In the class of composite Higgs models we study, $X_{5/3}$ is very special as it is the lightest exotic fermion. A constraint on the mass of $X_{5/3}$ directly extends to constrains on all top partner masses. By combining jet substructure methods with conventional reconstruction techniques we show that in this kind of final state a smooth interpolation between the boosted and unboosted regime is possible. We find that a reinterpretation of existing searches can improve bounds on the parameter space of composite Higgs models. Further, at 8 TeV a combined search for $X_{5/3}$ and $B$ in the $l+\rm{jets}$ final state can be more sensitive than a search involving same-sign dileptons.

hep-ph↗

arXiv2013

Dark Sector spectroscopy at the ILC

Recent studies have shown that searches in the mono-photon and missing energy final state can be used to discover dark matter candidates at the ILC. While an excess in this final state over the Standard Model background would indicate the existence of a dark sector, no detailed information about the internal structure of this sector can be inferred. Here, we demonstrate how just a few observables can discriminate between various realizations of dark sectors, including e.g. the spin of mediators.

hep-ph↗

arXiv2013

LHC probes the hidden sector

In this note we establish LHC limits on a variety of benchmark models for hidden sector physics using 2011 and 2012 data. First, we consider a "hidden" U(1) gauge boson under which all Standard Model particles are uncharged at tree-level and which interacts with the visible sector either via kinetic mixing or higher dimensional operators. Second, we constrain scalar and pseudo-scalar particles interacting with the Standard Model via dimension five operators and Yukawa interactions, in particular including so-called axion-like particles. In both cases we consider several different final states, including photons, electrons, muons and taus, establishing new constraints for a range of GeV to TeV scale masses. Finally, we also comment on particles with electric charges smaller than e that arise from hidden sector matter.

hep-ph↗

arXiv2013

Triplet Higgs boson collider phenomenology after the LHC

ATLAS and CMS have discovered a Standard Model Higgs-like particle. One of the main discovery channels is the Higgs decay to two photons, which, at the moment, seems to be considerably enhanced over the Standard Model expectation. Models with additional charged matter coupling to the Higgs sector can enhance or decrease the Higgs to two photons branching ratio. We take this as motivation to confront the so-called Georgi-Machacek model of Higgs triplets with the results of recent searches for a SM Higgs boson performed at the LHC. We also investigate the model in regions of the allowed parameter space relevant for a SM-like phenomenology. The Georgi-Machacek model avoids tree-level issues of the T parameter, while offering a vastly modified Higgs phenomenology compared to the Standard Model. This comes at the price of introducing another fine-tuning problem related to electroweak precision measurements. We investigate the collider phenomenology of the Georgi-Machacek model in the light of existing collider constraints beyond any effective field theory approximation and contextualize our findings with electroweak precision constraints.

hep-ph↗

arXiv2013

Making The Most Of MET: Mass Reconstruction From Collimated Decays

At hadron colliders invisible particles $χ$ can be inferred only through observation of the transverse component of the vectorial sum of their momenta -- missing $E_T$ or MET -- preventing reconstruction of the masses of their mother particles. Here we outline situations where prior prejudice about the event kinematics allows one to make the most of MET by decomposing it into its expected sum of transverse contributions, each of which may be promoted to a full four-momentum approximating the associated $χ$. Such prejudice arises when all $χ$ in the event are expected to be light and (anti-)parallel to a visible object, due to spin-correlations, back-to-back decays or boosted decays. We focus on the last of these, with boosted semi-invisibly decaying neutralinos widely motivated in supersymmetry (in the presence of light gravitinos, singlinos, photini or pseudo-goldstini), and demonstrate our simple method's ability to reconstruct sharp mass peaks from the MET decomposition.

hep-ph↗

arXiv2013

The shape of spins

After the discovery of a Higgs-like particle at the LHC, the determination of its spin quantum numbers across different channels will be the next step in arriving at a more precise understanding of the new state and its role in electroweak symmetry breaking. Event shape observables have been shown to provide extremely sensitive observables for the discrimination of the scalar Higgs boson's CP quantum numbers as a consequence of the different radiation patterns of Higgs production via gluon fusion vs. weak boson fusion in the $pp\to X+2j$ selection. We show that a similar strategy serves to constrain the spin quantum numbers of the discovered particle as a function of the involved couplings. We also discuss the prospects of applying a similar strategy to future discoveries of Higgs-like particles.

hep-ph↗

arXiv2013

Emergence of the Electroweak Scale through the Higgs Portal

Having discovered a candidate for the final piece of the Standard Model, the Higgs boson, the question remains why its vacuum expectation value and its mass are so much smaller than the Planck scale (or any other high scale of new physics). One elegant solution was provided by Coleman and Weinberg, where all mass scales are generated from dimensionless coupling constants via dimensional transmutation. However, the original Coleman-Weinberg scenario predicts a Higgs mass which is too light; it is parametrically suppressed compared to the mass of the vectors bosons, and hence is much lighter than the observed value. In this paper we argue that a mass scale, generated via the Coleman-Weinberg mechanism in a hidden sector and then transmitted to the Standard Model through a Higgs portal, can naturally explain the smallness of the electroweak scale compared to the UV cutoff scale, and at the same time be consistent with the observed value. We analyse the phenomenology of such a model in the context of present and future colliders and low energy measurements.

hep-ph↗

arXiv2012

On jet mass distributions in Z+jet and dijet processes at the LHC

The mass distribution of jets produced in hard processes at the LHC plays an important role in several jet substructure related studies involving both Standard Model and BSM physics, especially in the context of boosted heavy particle searches. We compute analytically the jet-mass distribution for both Z+jet and dijet processes, for QCD jets defined in the anti-k_t algorithm with an arbitrary radius R, to next-to-leading logarithmic accuracy and match our resummed calculation to full leading-order results. We note the important role played by initial state radiation (ISR) and non-global logarithms explicitly computed here for the first time for hadron collider observables, as well as the jet radius dependence of these effects. We also compare our results to standard Monte Carlo event generators and discuss directions for further studies and phenomenology.

hep-ph↗

arXiv2012

Partially (in)visible Higgs decays at the LHC

Both Atlas and CMS have reported a discovery of a Standard Model-like Higgs boson $H$ of mass around 125 GeV. Consistency with the Standard Model implies the non-observation of non-SM like decay modes of the newly discovered particle. Sensitivity to such decay modes, especially when they involve partially invisible final states is currently beyond scrutiny of the LHC. We systematically study such decay channels in the form of $H\rightarrow AA\rightarrow jets+missing energy$, with $A$ a light scalar or scalar, and analyze to what extent these exotic branching fractions can be constrained by direct measurements at the LHC. While the analysis is challenging, constraints as good as ${BR}\lesssim 10%$ can be obtained.

hep-ph↗

arXiv2012

Enhanced Gamma Ray Signals in Cosmic Proton-Wimp Collisions Due to Hadronization

In this paper, we investigate the gamma ray signal produced from dark matter collisions with high energy cosmic protons. Notably, we extend past results by including important hadronization effects. Showering and hadronization produces a high multiplicity of photons from the decays of hadrons, whose rate is not suppressed by the fine structure constant. Notably, proton remnants that do not participate in hard scattering, can produce a large rate of photons in the forward direction. These effects significantly enhance the photon rate and alter the energy and angular distributions compared to previous results which used only parton level calculations. Due to this modification, the gamma ray signal from the nearby active galactic nuclei Centaurus A is potentially testable in future Fermi-LAT and HESS measurements, for a dark matter mass and coupling consistent with current XENON100 bounds.

astro-ph.HE↗

arXiv2012

Polishing a shiny Higgs with matrix elements

After the recent discovery of a Standard Model Higgs boson-like particle at the LHC, the question of its couplings to known and unknown matter is eminent. In this letter, we present a method that allows for an enhancement in S/B(irreducible) of the order of 100% in $pp\to (h\to γγ) jj$ for a center of mass energy of 8 and 14 TeV. This is achieved by applying the matrix element method. We discuss the implications of detector resolution effects and various approximations of the involved event simulation and reconstruction. The matrix element method provides a reliable, stable, and efficient handle to separate signal from background, and the gluon and weak boson fusion components involved in this process. Employing this method, a more precise Higgs boson coupling extraction can be obtained, and our results are of immediate relevance for current searches.

hep-ph↗

arXiv2012

New Physics in LHC Higgs boson pair production

Multi-Higgs production provides a phenomenologically clear window to the electroweak symmetry breaking sector. We perform a comprehensive and comparative analysis of new electroweak physics effects in di-Higgs and di-Higgs+jet production. In particular, we discuss resonant di-Higgs phenomenology, which arises in the Higgs portal model and in the MSSM at small tan beta, and non-resonant new physics contributions to di-Higgs production in models where the newly discovered Higgs candidate is interpreted as a pseudo-Nambu-Goldstone boson. We show that, for all these scenarios, a measurement of the di-Higgs and di-Higgs+jet final states provides an accessible and elaborate handle to understand electroweak symmetry breaking in great detail.

hep-ph↗