Prospects For LHC Research Articles

Low-energy probes of the small cosmic microwave background amplitude in models of the radiative Higgs mechanism

Abstract The small cosmic microwave background (CMB) amplitude As ≃ 10−9 (or small temperature fluctuation δT/T ≃ 10−5) typically requires an unnaturally small effective coupling of an inflaton λϕ ∼ 10−14. In models with non-minimal coupling ξ, extra suppression of the amplitude, e.g. by the inflaton’s large field values, usually allows λϕ to be much larger, but at the price of ξ ≫ 1. Although the difficulties have not been strictly quantified, models with λϕ ≪ 1 or ξ ≫ 1 are harder to build. We show that the absence of new physics signals at TeV scale can suggest a relatively small $\xi \lesssim {\cal O}(1\mbox{--}100)$ with $\lambda _\phi \lesssim {\cal O}(10^{-4}\mbox{--}10^{-8})$, while constraining larger ξ with larger λϕ more strongly. Above all, this is possible by a connection between low- and high-energy physics that can be made in scenarios where the U(1)X Higgs is an inflaton at a high scale while its renormalization running also induces the Coleman–Weinberg mechanism for the electroweak symmetry breaking at a low scale. The best TeV-scale signals are Z′ resonances and Higgs signal strengths. We further find the connection particularly useful since the Z′ mass is upper bounded in order to produce the correct As and the weak scale simultaenously. Utilizing the intriguing upper bounds, we work out the prospects for LHC 13 and 100 TeV pp colliders probing the parameter space of the small CMB amplitude in such a model.

Improving third-generation leptoquark searches with combined signals and boosted top quarks

Search strategies for the third-generation leptoquarks (LQs) are distinct from other LQ searches, especially when they decay to a top quark and a $\tau$ lepton. We investigate the cases of all TeV-scale scalar and vector LQs that decay to either a top-tau pair (charge-$1/3$ and $5/3$ LQs) or a top-neutrino pair (charge-$2/3$ LQs). One can then use the boosted top (which can be tagged efficiently using jet-substructure techniques) and high-$p_{\rm T}$ $\tau$ leptons to search for these LQs. We consider two search channels with either one or two taus along with at least one hadronically decaying boosted top quark. We estimate the high luminosity LHC (HL-LHC) search prospects of these LQs by considering both symmetric and asymmetric pair and single production processes. Our selection criteria are optimised to retain events from both pair and single production processes. The combined signal has better prospects than the traditional searches. We include new three-body single production processes to enhance the single production contributions to the combined signal. We identify the interference effect that appears in the dominant single production channel of charge-$1/3$ scalar LQ ($S^{1/3}$). This interference is constructive if $S^{1/3}$ is weak-triplet and destructive, for a singlet one. As a result, their LHC prospects differ appreciably.

Top quark pair production in the exclusive processes at the LHC

We analyze the LHC prospects for measurements of the $t\bar{t}$ pair produced exclusively in photon-photon or semi-exclusively in photon-Pomeron and Pomeron-Pomeron processes using protons tagged in forward proton detectors on both sides of the interaction point. These processes are interesting from the point of view of a possible measurement of the top quark mass and constraining models used in Beyond Standard Model physics. Focusing on the semi-leptonic channel, $t\bar{t}\rightarrow jjbl\nu_l\bar{b}$, making use of the exclusive nature of the final state, together with the use of timing information provided by forward proton detectors, relevant exclusive and inclusive backgrounds are studied in detail for different luminosity (or pile-up) scenarios and found to be important for further considerations. While good prospects are found for observing the signal, the top quark mass measurement turns out not to be competitive with measurements in inclusive channels.

LHC searches for Dark Matter in compressed mass scenarios: challenges in the forward proton mode

We analyze in detail the LHC prospects at the center-of-mass enery of sqrt{s} = 14 TeV for charged electroweakino searches, decaying to leptons, in compressed supersymmetry scenarios, via exclusive photon-initiated pair production. This provides a potentially increased sensitivity in comparison to inclusive channels, where the background is often overwhelming. We pay particular attention to the challenges that such searches would face in the hostile high pile-up environment of the LHC, giving close consideration to the backgrounds that will be present. The signal we focus on is the exclusive production of same-flavour muon and electron pairs, with missing energy in the final state, and with two outgoing intact protons registered by the dedicated forward proton detectors installed in association with ATLAS and CMS. We present results for slepton masses of 120–300 GeV and slepton-neutralino mass splitting of 10–20 GeV, and find that the relevant backgrounds can be controlled to the level of the expected signal yields. The most significant such backgrounds are due to semi-exclusive lepton pair production at lower masses, with a proton produced in the initial proton dissociation system registering in the forward detectors, and from the coincidence of forward protons produced in pile-up events with an inclusive central event that mimics the signal. We also outline a range of potential methods to further suppress these backgrounds as well as to enlarge the signal yields.

Constraining top quark flavor violation and dipole moments through three and four top quark productions at the LHC

In this paper, we examine the sensitivity of the three-top quark production at the LHC to the top quark flavor-changing neutral currents (FCNC) as well as the sensitivity of the four-top production to the strong and weak dipole moments of the top quark. Upper limits at $95\%$ CL on the branching fractions of $\mathcal{B}(t\rightarrow qX)$, where $X = g, Z, \gamma, H$ and $q=u,c$, are set by performing an analysis on three-top events in the same-sign dilepton channel. We consider the main sources of the background processes and a realistic detector simulation is performed at the center-of-mass energy of 14 TeV. In the second part of this work, based on the recent upper limits which have been set on the four-top quark cross section by the ATLAS and CMS collaborations from 13 TeV data, we constrain the top quark strong and weak dipole moments. The bounds on the top quark dipole moments are presented using the future LHC prospects for four-top quark cross section measurement.

LHC constraints on Mini-Split anomaly and gauge mediation and prospects for LHC 14 and a future 100 TeV pp collider

Stringent experimental constraints have raised the lower limit on the masses of squarks to TeV levels, while compatibility with the mass of the Higgs boson provides an upper limit. This two-sided bound has lead to the emergence of Mini-Split theories where gauginos are not far removed from the electroweak scale while scalars are somewhat heavier. This small hierarchy modifies the spectrum of standard anomaly and gauge mediation, leading to Mini-Split deflected anomaly and gauge mediation models. In this paper, we study LHC constraints on these models and their prospects at LHC 14 and a 100 TeV collider. Current constraints on their parameter space come from ATLAS and CMS supersymmetry searches, the known mass of the Higgs boson, and the absence of a color-breaking vacuum. Prospects at LHC 14 and a 100 TeV collider are obtained from these same theoretical constraints in conjunction with background estimates. As would be expected from renormalization group effects, a slightly lighter third generation of squarks is assumed. Higgsinos have masses similar to those of the scalars and are at the origin of the deflection.

LHC prospects for minimal decaying dark matter

We study the possible signals at LHC of the minimal models of decaying dark matter. Those models are characterized by the fact that DM interacts with SM particles through renormalizable coupling with an additional heavier charged state. Such interaction allows to produce a substantial abundance of DM in the early Universe via the decay of the charged heavy state, either in- or out-of-equilibrium. Moreover additional couplings of the charged particle open up decay channels for the DM, which can nevertheless be sufficiently long-lived to be a good DM candidate and within reach of future Indirect Detection observations. We compare the cosmologically favored parameter regions to the LHC discovery reach and discuss the possibility of simultaneous detection of DM decay in Indirect Detection.

Implications of gauge-mediated supersymmetry breaking with vectorlike quarks and a∼125 GeVHiggs boson

We investigate the implications of models that achieve a Standard Model-like Higgs boson of mass near 125 GeV by introducing additional TeV-scale supermultiplets in the vectorlike $\mathbf{10}+\overline{\mathbf{10}}$ representation of $SU(5)$, within the context of gauge-mediated supersymmetry breaking. We study the resulting mass spectrum of superpartners, comparing and contrasting to the usual gauge-mediated and constrained minimal supersymmetric Standard Model scenarios, and discuss implications for LHC supersymmetry searches. This approach implies that exotic vectorlike fermions ${t}_{1,2}^{\ensuremath{'}}$, ${b}^{\ensuremath{'}}$, and ${\ensuremath{\tau}}^{\ensuremath{'}}$ should be within the reach of the LHC. We discuss the masses, the couplings to electroweak bosons, and the decay branching ratios of the exotic fermions, with and without various unification assumptions for the mass and mixing parameters. We comment on LHC prospects for discovery of the exotic fermion states, both for decays that are prompt and for those that are nonprompt on detector-crossing time scales.

Higgs mass, muong−2, and LHC prospects in gauge mediation models with vectorlike matters

Recently the ATLAS and CMS collaborations presented preliminary results of Standard Model Higgs searches and reported excesses of events for a Higgs boson at 124-126 GeV. Such a Higgs mass can be naturally realized, simultaneously explaining the muon g-2 anomaly, in gauge-mediated SUSY breaking models with extra vector-like matters. Upper bounds are obtained on the gluino mass, m_{\tilde g}\lesssim 1.2 (1.8) TeV, and on the extra vector-like quark mass, M_{Q'} \lesssim 1.0 (1.8) GeV, in the parameter region where the Higgs boson mass is 124-126 GeV and the muon g-2 is consistent with the experimental value at the 1 sigma (2 sigma) level. The LHC prospects are explored in the parameter region. It is found that some of the regions are already excluded by the LHC, and most of the parameter space is expected to be covered at \sqrt{s} = 14 TeV. A study on the extra vector-like quarks, especially current bounds on their masses and prospects for future searches, is also included.

Dilaton constraints and LHC prospects

The Standard Model Higgs searches using the first 1-2 fb-1 of LHC data can be used to put interesting constraints on new scalar particles other than the Higgs. We investigate one such scenario in which electroweak symmetry is broken via strongly coupled conformal dynamics. This scenario contains a neutral scalar dilaton---the Goldstone boson associated with spontaneously broken scale invariance---with a mass below the conformal symmetry breaking scale and couplings to Standard Model particles similar (but not identical) to those of the Standard Model Higgs boson. We translate the LEP and LHC Higgs limits to constrain the dilaton mass and conformal breaking scale. The conformal breaking scale f is constrained to be above 1 TeV for dilaton masses between 145 and 600 GeV, though it can be as low as 400 GeV for dilaton masses below 110 GeV. We also show that (i) a dilaton chi with mass below 110 GeV and consistent with the LEP constraints can appear in gg --> chi --> gamma gamma with a rate up to ~10 times the corresponding Standard Model Higgs rate, and (ii) a dilaton with mass of several hundred GeV is much narrower than the corresponding Standard Model Higgs, leading to improved search sensitivity in chi --> ZZ --> 4l.

LHC prospects in searches for neutral scalars in pp → γγ + jet

At hadron colliders, the γγ + jet channel provides a larger signal-to-background ratio in comparison with the inclusive γγ channel in hunting for scalars uncharged under the SM gauge group. By using NLO results for the SM Higgs boson production and corresponding background, we estimate LHC prospects in searches for the radion and sgoldstino in the γγ + jet channel. Three-body final-state kinematics allows for refined cuts. We have found that this channel could be comparable with the γγ channel in searches for new physics.

Boson competition lifts prospects for LHC

Boson competition lifts prospects for LHC