Discovering a Higgs boson decaying to four jets in supersymmetric cascade decays
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Bellazzini, Brando↗
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Early LHC phenomenology of Yukawa-bound heavy QQ mesons
Current limits from the LHC on fourth generation quarks are already at the unitarity bound of 500 GeV or so. If they exist, the strong Yukawa couplings are turning nonperturbative, and may form bound states. We study the domain of m{sub b'} and m{sub t'} in the range of 500 to 700 GeV, where we expect binding energies are mainly of Yukawa origin, with QCD subdominant. To be consistent with electroweak precision tests, the t' and b' quarks have to be nearly degenerate, exhibiting a new 'isospin'. Comparing relativistic expansion with a relativistic bound state approach, we find the most interesting is the production of a color octet, isosinglet vector meson (a 'gluon-prime') via qq-bar{yields}{omega}{sub 8}. Leading decay modes are {pi}{sub 8}{sup {+-}}W{sup {+-}}, {pi}{sub 8}{sup 0}Z{sup 0}, and constituent quark decay, with qq and tt-bar' and bb' subdominant. The color octet, isovector pseudoscalar {pi}{sub 8} meson decays via constituent quark decay, or to Wg. These decay rates are parameterized by the decay constant, the binding energy and mass differences, and V{sub tb'}. For small V{sub t'b}, one could have a spectacular signal of WWg, where a soft W accompanies a very massive Wg pair. In general, however, one has high multiplicity signals with b, W, and t jet substructures that are not so different from the t't-bar' and b'b-bar' search.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for t(t)over-bar resonances in the lepton plus jets final state with ATLAS using 4.7 fb(-1) of pp collisions at root s=7 TeV
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72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measuring $CP$ violation in $h$ → $τ$ + $τ$ – at colliders
We investigate the LHC and Higgs Factory prospects for measuring the CP phase in the Higgs-tau-tau coupling. Currently this phase can be anywhere between 0° (CP even) and 90° (CP odd). A new, ideal observable is identified from an analytic calculation for the τ ± → ρ ± ν → π ± π 0 ν channel. It is demonstrated to have promising sensitivity at the LHC and superior sensitivity at the ILC compared to previous proposals. Furthermore, our observable requires the reconstruction of the internal substructure of decaying taus but does not rely on measuring the impact parameter of tau decays. It is the first proposal for such a measurement at the LHC. For the 14 TeV LHC, we estimate that about 1 ab –1 data can discriminate CP-even versus CP-odd at the 5σ level. With 3 ab –1 , the CP phase should be measurable to an accuracy of ~11°. At an e + e – Higgs Factory, we project that a 250 GeV run with 1 ab –1 luminosity can measure the phase to ~4.4° accuracy.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Boosted di-boson from a mixed heavy stop
The lighter mass eigenstate ($\widetilde{t}_1$) of the two top squarks, the scalar superpartners of the top quark, is extremely difficult to discover if it is almost degenerate with the lightest neutralino ($\widetilde{\chi}_1^0$), the lightest and stable supersymmetric particle in the R-parity conserving supersymmetry. The current experimental bound on $\widetilde{t}_1$ mass in this scenario stands only around 200 GeV. For such a light $\widetilde{t}_1$, the heavier top squark ($\widetilde{t}_2$) can also be around the TeV scale. Moreover, the high value of the higgs ($h$) mass prefers the left and right handed top squarks to be highly mixed allowing the possibility of a considerable branching ratio for $\widetilde{t}_2 \to \widetilde{t}_1 h$ and $\widetilde{t}_2 \to \widetilde{t}_1 Z$. In this paper, we explore the above possibility together with the pair production of $\widetilde{t}_2$ $\widetilde{t}_2^*$ giving rise to the spectacular di-boson + missing transverse energy final state. For an approximately 1 TeV $\widetilde{t}_2$ and a few hundred GeV $\widetilde{t}_1$ the final state particles can be moderately boosted which encourages us to propose a novel search strategy employing the jet substructure technique to tag the boosted $h$ and $Z$. The reconstruction of the $h$ and $Z$ momenta also allows us to construct the stransverse mass $M_{T2}$ providing an additional efficient handle to fight the backgrounds. We show that a 4--5$\sigma$ signal can be observed at the 14 TeV LHC for $\sim$ 1 TeV $\widetilde{t}_2$ with 100 fb$^{-1}$ integrated luminosity.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Building a better boosted top tagger
Distinguishing hadronically decaying boosted top quarks from massive QCD jets is an important challenge at the Large Hadron Collider. In this paper we use the power-counting method to study jet substructure observables designed for top tagging, and gain insight into their performance. We introduce a powerful new family of discriminants formed from the energy correlation functions which outperform the widely used N-subjettiness. These observables take a highly nontrivial form, demonstrating the importance of a systematic approach to their construction.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Measurement of the differential cross-section of highly boosted top quarks as a function of their transverse momentum in s = 8 TeV proton-proton collisions using the ATLAS detector
The differential cross-section for pair production of top quarks with high transverse momentum is measured in 20.3 fb -1 of proton-proton collisions at a center-of-mass energy of 8 TeV. The measurement is performed for t$\bar{t}$t events in the lepton + jets channel. The cross-section is reported as a function of the hadronically decaying top quark transverse momentum for values above 300 GeV. The hadronically decaying top quark is reconstructed as an anti- k t jet with radius parameter R = 1.0 and identified with jet substructure techniques. The observed yield is corrected for detector effects to obtain a cross-section at particle level in a fiducial region close to the event selection. A parton-level cross-section extrapolated to the full phase space is also reported for top quarks with transverse momentum above 300 GeV. The predictions of a majority of next-to-leading-order and leading-order matrix-element Monte Carlo generators are found to agree with the measured cross-sections.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Amplitude analysis of B - → D + π - π - decays
The Dalitz plot analysis technique is used to study the resonant substructures of B - → D + π - π - decays in a data sample corresponding to 3.0 fb -1 of pp collision data recorded by the LHCb experiment during 2011 and 2012. A model-independent analysis of the angular moments demonstrates the presence of resonances with spins 1, 2 and 3 at high D + π - mass. The data are fitted with an amplitude model composed of a quasi-model-independent function to describe the D + π - S wave together with virtual contributions from the D*(2007) 0 and B* 0 states, and components corresponding to the D$*\atop{2}$(2460) 0 , D$*\atop{1}$(2680) 0 , D$*\atop{3}$(2760) 0 and D$*\atop{2}$(3000) 0 resonances. The masses and widths of these resonances are determined together with the branching fractions for their production in B- → D + π - π - decays. The D + π - S wave has phase motion consistent with that expected due to the presence of the D$*\atop{0}$(2400) 0 state. These results constitute the first observations of the D$*\atop{3}$(2760) 0 and D$*\atop{2}$(3000) 0 resonances, with significances of 10σ and 6.6σ, respectively.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for anomalous electroweak production of W W / W Z in association with a high-mass dijet system in p p collisions at s = 8 TeV with the ATLAS detector
A search is presented for anomalous quartic gauge boson couplings in vector-boson scattering. Here, the data for the analysis correspond to 20.2 fb -1 of √ s = 8 TeV pp collisions and were collected in 2012 by the ATLAS experiment at the Large Hadron Collider. The search looks for the production of WW or WZ boson pairs accompanied by a high-mass dijet system, with one W decaying leptonically and a W or Z decaying hadronically. The hadronically decaying W/Z is reconstructed as either two small-radius jets or one large-radius jet using jet substructure techniques. Constraints on the anomalous quartic gauge boson coupling parameters α 4 and α 5 are set by fitting the transverse mass of the diboson system, and the resulting 95% confidence intervals are - 0.024 < α 4 < 0.030 and - 0.028 < α 5 < 0.033 .
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for low-mass resonances decaying into bottom quark-antiquark pairs in proton-proton collisions at s = 13 TeV
A search for narrow, low-mass, scalar, and pseudoscalar resonances decaying to bottom quark-antiquark pairs is presented. The search is based on events recorded in $\sqrt{s} =$ 13 TeV proton-proton collisions with the CMS detector at the LHC, collected in 2016, and corresponding to an integrated luminosity of 35.9 fb -1 . The search selects events in which the resonance would be produced with high transverse momentum because of the presence of initial- or final-state radiation. In such events, the decay products of the resonance would be reconstructed as a single large-radius jet with high mass and two-prong substructure. A potential signal would be identified as a narrow excess in the jet invariant mass spectrum. No evidence for such a resonance is observed within the mass range from 50 to 350 GeV, and upper limits at 95% confidence level are set on the product of the cross section and branching fraction to a bottom quark-antiquark pair. Lastly, these constitute the first constraints from the LHC on exotic bottom quark-antiquark resonances with masses below 325 GeV.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Search for γ -ray emission from dark matter particle interactions from the Andromeda and Triangulum galaxies with the Fermi Large Area Telescope
The Andromeda (M31) and Triangulum (M33) galaxies are the closest Local Group galaxies to the Milky Way, being only 785 and 870 kpc away. These two galaxies provide an independent view of high-energy processes that are often obscured in our own Galaxy, including possible signals of dark matter (DM) particle interactions. The Fermi Large Area Telescope (Fermi-LAT) preliminary eight year list of sources includes both M31, which is detected as extended with a size of about 0.4°, and M33, which is detected as a pointlike source. The spatial morphology of M31 γ-ray emission could trace a population of unresolved sources and energetic particles originating in sources not related to massive star formation. Alternatively, the γ-ray emission could also be an indication of annihilation or decay of DM particles. We investigate these two possibilities using almost 10 years of data from the Fermi-LAT. An interpretation that involves only a DM γ-ray emission is in tension with the current limits from other searches, such as those targeting Milky Way dwarf spheroidal galaxies. When we include a template of astrophysical emission, tuned on γ-ray data or from observations of these galaxies in other wavelengths, we do not find any significant evidence for a DM contribution and we set limits for the annihilation cross section that probe the thermal cross section for DM masses up to a few tens of GeV in the b¯b and τ + τ – channels. For models where the DM substructures have masses above 10 –6 solar masses our limits probe the DM interpretation of the Fermi-LAT Galactic center excess. In conclusion, we provide also the lower limit for the DM lifetime assuming the same spatial models of the DM distribution in M31 and M33.
79 ASTRONOMY AND ASTROPHYSICS↗
Stokes Phenomenon and Schwinger Vacuum Pair Production in Time-Dependent Laser Pulses
Particle production due to external fields (electric, chromoelectric, or gravitational) requires evolving an initial state through an interaction with a time-dependent background, with the rate being computed from a Bogoliubov transformation between the in and out vacua. When the background fields have temporal profiles with substructure, a semiclassical analysis of this problem confronts the full subtlety of the Stokes phenomenon: WKB solutions are only local, while the production rate requires global information. We give a simple quantitative explanation of the recently computed [Phys. Rev. Lett. 102, 150404 (2009)] oscillatory momentum spectrum of e{sup +}e{sup -} pairs produced from vacuum subjected to a time-dependent electric field with subcycle laser pulse structure. This approach also explains naturally why for spinor and scalar QED these oscillations are out of phase.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Seeing in Color: Jet Superstructure
A new class of observables is introduced which aims to characterize the superstructure of an event, that is, features, such as color flow, which are not determined by the jet four-momenta alone. Traditionally, an event is described as having jets which are independent objects; each jet has some energy, size, and possible substructure such as subjets or heavy flavor content. This description discards information connecting the jets to each other, which can be used to determine if the jets came from decay of a color-singlet object, or if they were initiated by quarks or gluons. An example superstructure variable, pull, is presented as a simple handle on color flow. It can be used on an event-by-event basis as a tool for distinguishing previously irreducible backgrounds at the Tevatron and the LHC.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Spin and the Honeycomb Lattice: Lessons from Graphene
A model of electrons hopping from atom to atom in graphene's honeycomb lattice gives low-energy electronic excitations that obey a relation formally identical to a 2+1 dimensional Dirac equation. Graphene's spin equivalent, 'pseudospin', arises from the degeneracy introduced by the honeycomb lattice's two inequivalent atomic sites per unit cell. Previously it has been thought that the usual electron spin and the pseudospin indexing the graphene sublattice state are merely analogues. Here we show that the pseudospin is also a real angular momentum. This identification explains the suppression of electron backscattering in carbon nanotubes and the angular dependence of light absorption by graphene. Furthermore, it demonstrates that half-integer spin like that carried by the quarks and leptons can derive from hidden substructure, not of the particles themselves, but rather of the space in which these particles live.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Bulk Dirac Points in Distorted Spinels
Here, we report on a Dirac-like Fermi surface in three-dimensional bulk materials in a distorted spinel structure on the basis of density functional theory as well as tight-binding theory. The four examples we provide in this Letter are BiZnSiO 4 , BiCaSiO 4 , BiAlInO 4 , and BiMgSiO 4 . A necessary characteristic of these structures is that they contain a Bi lattice which forms a hierarchy of chainlike substructures, with consequences for both fundamental understanding and materials design.
36 MATERIALS SCIENCE↗
Energy Scan of the e + e - → h b ( n P ) π + π - ( n = 1 , 2) Cross Sections and Evidence for Υ ( 11020 ) Decays into Charged Bottomoniumlike States
With data collected with the Belle detector at the KEKB asymmetric-energy e + e - collider, we measure the energy dependence of the e + e - → h b ( n P ) π + π - ( n = 1 , 2) cross sections from thresholds up to 11.02 GeV. We find clear Υ ( 10860 ) and Υ ( 11020 ) peaks with little or no continuum contribution. We study the resonant substructure of the Υ ( 11020 ) → h b ( n P ) π + π - transitions and find evidence that they proceed entirely via the intermediate isovector states Z b ( 10610 ) and Z b ( 10650 ) . The relative fraction of these states is loosely constrained by the current data: The hypothesis that only Z b ( 10610 ) is produced is excluded at the level of 3.3 standard deviations, while the hypothesis that only Z b ( 10650 ) is produced is not excluded at a significant state.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
Empirical Determination of Dark Matter Velocities Using Metal-Poor Stars
The Milky Way dark matter halo is formed from the accretion of smaller subhalos. These sub-units also harbor stars—typically old and metal-poor—that are deposited in the Galactic inner regions by disruption events. In this report, we show that the dark matter and metal-poor stars in the Solar neighborhood share similar kinematics due to their common origin. Using the high-resolution eris simulation, which traces the evolution of both the dark matter and baryons in a realistic Milky Way analog galaxy, we demonstrate that metal-poor stars are indeed effective tracers for the local, virialized dark matter velocity distribution. The local dark matter velocities can therefore be inferred from observations of the stellar halo made by the Sloan Digital Sky Survey within 4 kpc of the Sun. This empirical distribution differs from the standard halo model in important ways and suggests that the bounds on the spin-independent scattering cross section may be weakened for dark matter masses below ~10 GeV. Data from Gaia will allow us to further refine the expected distribution for the smooth dark matter component, and to test for the presence of local substructure.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗