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  • 1.
    Arhrib, Abdesslam
    et al.
    Abdelmalek Essaadi University, Morocco.
    Benbrik, Rachid
    Cadi Ayyad University and Faculté Polydisciplinaire de Safi, Morocco.
    Enberg, Rikard
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Klemm, William
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics. School of Physics and Astronomy, University of Manchester, UK.
    Moretti, Stefano
    School of Physics and Astronomy, University of Southampton, UK.
    Munir, Shoaib
    Korea Institute for Advanced Study, Seoul, Korea.
    Identifying a light charged Higgs boson at the LHC Run II2017In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 774, p. 591-598Article in journal (Refereed)
    Abstract [en]

    We analyse the phenomenological implications of a light Higgs boson, h, within the CP-conserving 2-Higgs Doublet Model (2HDM) Type-I, for the detection prospects of the charged H± state at Run II of the Large Hadron Collider (LHC), assuming s√=13 TeV as energy and O(100 fb−1) as luminosity. When sufficiently light, this h state can open up the bosonic decay channel H±→W±(∗)h, which may have a branching ratio significantly exceeding those of the H±→τν and H±→cs channels. We perform a broad scan of the 2HDM Type-I parameter space, assuming the heavier of the two CP-even Higgs bosons, H, to be the observed SM-like state with a mass near 125 GeV. Through these scans we highlight regions in which mH±<mt+mb that are still consistent with the most recent limits from experimental searches. We find in these regions that, when the H±→W±(∗)h decay mode is the dominant one, the h can be highly fermiophobic, with a considerably large decay rate in the γγ channel. This can result in the total cross section of the σ(pp→H±h→W±(∗)+4γ) process reaching up to O(100 fb). We therefore investigate the possibility of observing this spectacular signal at the LHC Run II.

  • 2. Bickelhaupt, F Matthias
    et al.
    Nagle, Jeffrey K
    Klemm, William L
    Role of s-p orbital mixing in the bonding and properties of second-period diatomic molecules.2008In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 112, no 11, p. 2437-46Article in journal (Refereed)
    Abstract [en]

    Qualitative molecular orbital theory is widely used as a conceptual tool to understand chemical bonding. Symmetry-allowed orbital mixing between atomic or fragment orbitals of different energies can greatly complicate such qualitative interpretations of chemical bonding. We use high-level Amsterdam Density Functional calculations to examine the issue of whether orbital mixing for some familiar second-row homonuclear and heteronuclear diatomic molecules results in net bonding or antibonding character for a given molecular orbital. Our results support the use of slopes of molecular orbital energy versus bond distance plots (designated radial orbital-energy slope: ROS) as the most useful criterion for making this determination. Calculated atomic charges and frontier orbital properties of these molecules allow their acid-base chemistry, including their reactivities as ligands in coordination chemistry, to be better understood within the context of the Klopman interpretation of hard and soft acid-base theory. Such an approach can be extended to any molecular species.

  • 3. Buckley, Matthew R.
    et al.
    Heinemann, Beate
    Klemm, William
    Murayama, Hitoshi
    Quantum Interference Effects Among Helicities at LEP-II and Tevatron2008In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 77, no 11, p. 113017-Article in journal (Refereed)
    Abstract [en]

    A completely model-independent method of obtaining information on the spin using the quantum interference effect among various helicity states was proposed in a recent paper. Here we point out that this effect should be demonstrable in the existing data on e-e+→W+W- at LEP-II and pp̅ →Z0+j at Tevatron.

  • 4. Buckley, Matthew R.
    et al.
    Murayama, Hitoshi
    Klemm, William
    Rentala, Vikram
    Discriminating spin through quantum interference2008In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 78, no 1, p. 014028-Article in journal (Refereed)
    Abstract [en]

    Many of the proposed solutions to the hierarchy and naturalness problems postulate new “partner” fields to the standard model particles. Determining the spins of these new particles will be critical in distinguishing among the various possible standard model extensions, yet proposed methods rely on the underlying models. We propose a new model-independent method for spin measurements which takes advantage of quantum interference among helicity states. We demonstrate that this method will be able to discriminate scalar particles from higher spin states at a linear collider.

  • 5. Chen, Chuan-Ren
    et al.
    Klemm, William
    Rentala, Vikram
    Wang, Kai
    Color Sextet Scalars at the CERN Large Hadron Collider2009In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 79, no 5, p. 054002-Article in journal (Refereed)
    Abstract [en]

    Taking a phenomenological approach, we study a color sextet scalar at the LHC. We focus on the QCD production of a color sextet pair Φ6Φ̅ 6 through gg fusion and qq̅ annihilation. Its unique coupling to ψ̅ cψ allows the color sextet scalar to decay into same-sign diquark states, such as Φ6→tt/tt*. We propose a new reconstruction in the multijet plus same-sign dilepton with missing transverse energy samples (bb+ℓ±±+E̸T+Nj, N≥6) to search for on-shell ttt̅ t̅ final states from sextet scalar pair production. Thanks to the large QCD production, the search covers the sextet mass range up to 1 TeV for 100  fb-1 integrated luminosity.

  • 6. Cho, Won Sang
    et al.
    Klemm, William
    Nojiri, Mihoko M.
    Mass measurement in boosted decay systems at hadron colliders2011In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 84, no 3, p. 035018-Article in journal (Refereed)
    Abstract [en]

    We report a new possibility of using the MCT2 (Constransverse mass) variable for mass measurement in single step decay chains involving missing particles with moderate transverse momentum. We show that its experimental feasibility is enhanced compared to the corresponding MT2-kink method. We apply this method to reconstruct a pair of chargino decay chains.

  • 7.
    Enberg, Rikard
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Klemm, William
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics. School of Physics and Astronomy, University of Manchester, UK.
    Moretti, Stefano
    School of Physics and Astronomy, University of Southampton, UK.
    Munir, Shoaib
    Korea Institute for Advanced Study, Seoul, Korea.
    Electroweak production of Higgs boson pairs in 2HDMs2017Conference paper (Other academic)
    Abstract [en]

    One of the main features of a Two-Higgs Doublet Model (2HDM) is the presence of two additional neutral Higgs states, besides the one mimicking the ~125 GeV state observed at the LHC. The three Higgs bosons of a 2HDM can be produced at the LHC either singly via gluon fusion or in pairs with each other. When analyzing their pair production, the emphasis is laid on gluon-initiated processes, and the electroweak (EW) production is generally not treated on the same footing, assuming its contribution to be highly subleading. We show here that when the sum of the masses of the lightest scalar and pseudoscalar Higgs bosons in the Type-I 2HDM is smaller than the Z-boson mass, their EW pair production can dominate over QCD pair production by orders of magnitude.

  • 8.
    Enberg, Rikard
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Klemm, William
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics. School of Physics and Astronomy, University of Manchester, UK.
    Moretti, Stefano
    School of Physics and Astronomy, University of Southampton, UK.
    Munir, Shoaib
    Korea Institute for Advanced Study, Seoul, Korea.
    Electroweak production of light scalar-pseudoscalar pairs from extended Higgs sectors2017In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 764, p. 121-125Article in journal (Refereed)
    Abstract [en]

    In models with extended Higgs sectors, it is possible that the Higgs boson discovered at the LHC is not the lightest one. We show that in a realistic model (the Type I 2-Higgs Doublet Model), when the sum of the masses of a light scalar and a pseudoscalar (h and A) is smaller than the Z boson mass, the Electroweak (EW) production of an hA pair, which is generally neglected, can dominate over QCD production by orders of magnitude. This is because in the gg-initiated process, hA production via a resonant Z in the s-channel is prohibited according to the Landau-Yang theorem, which is not the case for the qq-initiated process. We explore the parameter space of the model to highlight regions giving such hA solutions while being consistent with all constraints from collider searches, b-physics and EW precision data. We also single out a few benchmark points to discuss their salient features, including the hA search channels that can be exploited at Run II of the LHC.

  • 9.
    Enberg, Rikard
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Klemm, William
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics. School of Physics and Astronomy, University of Manchester, UK.
    Moretti, Stefano
    School of Physics and Astronomy, University of Southampton, UK.
    Munir, Shoaib
    Korea Institute for Advanced Study, Seoul, Korea.
    Electroweak production of multiple (pseudo)scalars in the 2HDM2019In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 79, article id 512Article in journal (Refereed)
    Abstract [en]

    The two-Higgs Doublet Model (2HDM) is the most minimal extension of the Standard Model (SM) containing extra Higgs doublet fields. Given the multiplicity of Higgs states in a 2HDM, its Higgs potential is significantly more involved than the SM one. Importantly, it contains a multitude of Higgs triple self-couplings, unlike the SM, which only has one. These interactions are key to understanding the phenomenology of the 2HDM, as they uniquely determine the form of the potential. Several studies analysing the prospects of measuring these couplings at the Large Hadron Collider (LHC) have found them to be quite low generally. However, such studies have largely concentrated on Higgs pair-production induced by gluon-gluon scattering, either via direct annihilation or followed by their splitting into b-(anti)quark pairs, which in turn annihilate leaving behind spectator b-(anti)quarks. Both of these channels are therefore governed by QCD dynamics. We compare here the yields of such channels to those initiated by (primarily) valence quarks, which involve Electro-Weak (EW) interactions only, for neutral multi-Higgs final states. We find that EW production can be dominant over QCD production for certain final state combinations. We also illustrate that charged final states, which can only be produced via EW modes, could serve as important probes of some H± triple couplings, that are inaccessible in QCD-induced processes, during Run 2 and 3 of the LHC. Our analysis covers regions of the parameter space of the Type-I 2HDM that have escaped the most up-to-date experimental constraints coming from EW precision data, LHC measurements of the 125 GeV Higgs boson properties, searches for additional Higgs states, and flavour physics.

  • 10.
    Enberg, Rikard
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Klemm, William
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics. School of Physics and Astronomy, University of Manchester, UK.
    Moretti, Stefano
    School of Physics and Astronomy, University of Southampton, UK.
    Munir, Shoaib
    Korea Institute for Advanced Study, Seoul, Korea.
    Signatures of the Type-I 2HDM at the LHC2018Conference paper (Other academic)
    Abstract [en]

    One of the simplest extensions of the Standard Model (SM) is the two-Higgs-doublet model (2HDM), which contains two neutral Higgs bosons, in addition to a 125 GeV one, and a charged pair. At the Large Hadron Collider (LHC), gluon-induced processes are generally the most important modes for the resonant production of the SM-like Higgs boson as well as its pair-production, and it is generally considered to be the case also for an additional neutral Higgs boson possibly existing in nature. We show that for certain parameter configurations in the Type-I 2HDM, electroweak pair-production of the neutral Higgs states can dominate over the QCD-initiated production. Moreover, it is possible for the pair-production of the charged Higgs state along with a neutral one, which can only take place electroweakly, to have a substantial cross section. We delineate such 2HDM parameter space regions through its comprehensive numerical scanning, requiring their consistency with the most relevant theoretical and experimental constraints. We also highlight some specific di-Higgs signatures that can be probed at the LHC in order to establish the Type-I 2HDM as the underlying new physics model.

  • 11.
    Enberg, Rikard
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Klemm, William
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics. School of Physics and Astronomy, University of Manchester, UK.
    Moretti, Stefano
    School of Physics and Astronomy, University of Southampton, UK.
    Munir, Shoaib
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics. Korea Institute for Advanced Study, Seoul, Korea.
    Wouda, Glenn
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    H± in the W±h channel at the LHC Run 22015Conference paper (Other academic)
    Abstract [en]

    We analyse the discovery prospects of the charged Higgs boson, H±, via its decay in the W±h channel in the minimal supersymmetric Standard Model (MSSM) as well as several 2-Higgs Doublet Models (2HDMs). h, the lightest scalar Higgs boson in these models, is identified with the recently discovered ∼125 GeV state, Hobs, at the Large Hadron Collider (LHC). We find that, while it provides an important input in the kinematic selection of signal events, the measured Hobs mass renders this channel inaccessible in the MSSM. In the 2HDMs though, through a dedicated signal-to-background analysis for the pp→t(b¯)H−→ℓ±νℓjjbbb¯(b¯)+h.c process, we establish that some parameter space regions will be testable at the LHC with s√=14 TeV.

  • 12.
    Enberg, Rikard
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Klemm, William
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Moretti, Stefano
    School of Physics and Astronomy, University of Southampton, UK.
    Munir, Shoaib
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Wouda, Glenn
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Search for Charged Higgs bosons via decays to W± and a 125 GeV Higgs at the Large Hadron Collider2014Conference paper (Other academic)
    Abstract [en]

    The recent observation of a 125 GeV neutral Higgs boson (Hobs) provides additional input for charged Higgs boson searches in the H±→W±Hobs decay channel at the Large Hadron Collider (LHC). We reassess the discovery potential in this channel, which is important for H± heavier than the top quark mass. When Hobs decays to a bb¯ pair, knowledge of the Higgs mass aids in the kinematic selection of signal events. We perform a signal-to-background analysis to demonstrate the LHC prospects for charged Higgs discovery in the resulting channel pp→t(b¯)H−→ℓ±νℓjjbbb¯(b¯)+h.c. for standard (300 fb−1) and high (3000 fb−1) luminosities at design energy, s√=14TeV. We find that regions of the parameter space of several two-Higgs doublet models, consistent with constraints from LHC Higgs searches and b-physics observables, are testable in this channel.

  • 13. Evans, Jason L.
    et al.
    Feldstein, Brian
    Klemm, William
    Murayama, Hitoshi
    Yanagida, Tsutomu T.
    Hermitian Flavor Violation2011In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 703, no 5, p. 599-605Article in journal (Refereed)
    Abstract [en]

    The fundamental constraint on two Higgs doublet models comes from the requirement of sufficiently suppressing flavor-changing neutral currents. There are various standard approaches for dealing with this problem, but they all tend to share a common feature; all of the Higgs doublets couple very weakly to the first generation quarks. Here we consider a simple two Higgs doublet model which is able to have large couplings to the first generation, while also being safe from flavor constraints. We assume only that there is an SUf(3)SU(3)f flavor symmetry which is respected by the couplings of one of the Higgs doublets, and which is broken by Hermitian Yukawa couplings of the second doublet. As a result of the large permitted couplings to the first generation quarks, this scenario may be used to address the excess in W+dijetW+dijet events recently observed by CDF at the Tevatron. Moreover, Hermitian Yukawa coupling matrices arise naturally in a broad class of solutions to the strong CP problem, providing a compelling context for the model.

  • 14.
    Feldstein, Brian
    et al.
    Institute for the Physics and Mathematics of the Universe, University of Tokyo.
    Klemm, William
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, High Energy Physics.
    Large Mixing Angles From Many Right-Handed Neutrinos2012In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 85, no 5, p. 053007-Article in journal (Refereed)
    Abstract [en]

    A beautiful understanding of the smallness of the neutrino masses may be obtained via the seesaw mechanism, whereby one takes advantage of the key qualitative distinction between the neutrinos and the other fermions: right-handed neutrinos are gauge singlets, and may therefore have large Majorana masses. The standard seesaw mechanism, however, does not address the apparent lack of hierarchy in the neutrino masses compared to the quarks and charged leptons, nor the large leptonic mixing angles compared to the small angles of the Cabibbo-Kobayashi-Maskawa matrix. In this paper, we will show that the singlet nature of the right-handed neutrinos may be taken advantage of in one further way in order to solve these remaining problems: Unlike particles with gauge interactions, whose numbers are constrained by anomaly cancellation, the number of gauge singlet particles is essentially undetermined. If large numbers of gauge singlet fermions are present at high energies-as is suggested, for example, by various string constructions-then the effective low-energy neutrino mass matrix may be determined as a sum over many distinct Yukawa couplings, with the largest ones being the most important. This can reduce hierarchy, and lead to large mixing angles. Assuming a statistical distribution of fundamental parameters, we will show that this scenario leads to a good fit to low-energy phenomenology, with only a few qualitative assumptions guided by the known quark and lepton masses. The scenario leads to predictions of a normal hierarchy for the neutrino masses, and a value for the vertical bar m(ee)vertical bar mass matrix element of about 1-6 meV.

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