We investigate the possibility to measure the top quark mass using the collider variable $m_{T2}$ at the LHC experiment. Monte Carlo studies of $m_{T2}$ are performed with the events corresponding to the dilepton decays of $t\bar{t}$ produced at the LHC with 10 $fb^{-1}$ integrated luminosity. Our analysis suggests that the top quark mass can be determined by the $m_{T2}$ variable alone with a good precision at the level of 1 GeV.
A new process with the lepton flavor violation (LFV) is presented in the setup for LHC. The LFV is induced by the one-loop effect through Higgs bosons in the framework of the type-III two Higgs doublet model. It is demonstrated that the vast of parameter space in the Yukawa sector could be accessed by current and future LHC experiments.
In Split-Supersymmetry models, where the only non-Standard Model states produceable at LHC-energies consist of a gluino plus neutralinos and charginos, it is conventionally accepted that only mass differences among these latter are measureable at the LHC. The present work shows that application of a simple `Kinematic Selection' technique allows full reconstruction of neutralino and chargino masses from one event, in principle. A Monte Carlo simulation demonstrates the feasibilty of using this technique at the LHC.
Following recent ATLAS and CMS publications we interpret the results of their Higgs searches in terms of Standard Model operators. For a Higgs mass of 125 GeV we determine several Higgs couplings from 2011 data and extrapolate the results towards different scenarios of LHC running. Even though our analysis is limited by low statistics we already derive meaningful constraints on modified Higgs sectors.
We show that simultaneous precision measurement of the CP-violating phase in time-dependent Bs --> J/psi phi study and the Bs --> mu+ mu- rate, together with measuring m_t' by direct search at the LHC, would determine V_{t's}^*V_{t'b} and therefore the b --> s quadrangle in the four-generation standard model. The forward-backward asymmetry in B --> K* l+ l- provides further discrimination.
We show in the framework of Higgs triplet model that the lower bound of the neutrino mass is obtained if we can measure $\frac{Γ(Δ^{--}\to ee)}{Γ(Δ^{--}\to μμ)}$ at the LHC.
We consider the ratio between the double and single Higgs production cross sections and examine the prospect of measuring the trilinear Higgs self-coupling using this observable. Such a ratio has a reduced theoretical (scale) uncertainty than the double Higgs cross section. We find that with 600/fb, the 14 TeV LHC can constraint the trilinear Higgs self coupling to be positive, and with 3000/fb one could measure it with a +30 % {-20 %}) accuracy.
In this paper we revisit the idea of measuring the magnetic dipole moments of the charm baryons and, in particular, of charmed Lambda by studying the spin precession induced by the strong effective magnetic field inside the channels of a bent crystal. We present a detailed sensitivity study showing the feasibility of such an experiment at the LHC in the coming years.
Diffractive physics program for ATLAS and CMS is discussed with emphasis on measurements of central exclusive processes. At low luminosities, a L1 trigger based on requiring rapidity gaps can be used, while at high luminosities, the use of proton taggers proposed to be placed at 220 m and 420 m from the interaction point is foreseen.
Measuring tau-polarisation in Neutralino2 decays at the LHC
我们展示了如何在LHC上测量具有保守R宇称的最小超对称性的衰变链chi 20-> stau 1 tau -> tau tau chi 10中的两个平均tau偏振之和。这是通过利用可见的di-tau质谱的偏振依赖性来实现的。这样的测量提供了有关所涉及的超对称性粒子的耦合的信息，并允许更精确地确定di-tau质量终点。如果可以区分不同的τ衰变模式，则可以进一步改进偏振和终点测量。
Precision measurements of top-associated Higgs production are an important ingredient to unravel the $\mathcal{CP}$ nature of the Higgs boson. In this work, we constraint the $\mathcal{CP}$ nature of the top-Yukawa coupling taking into account all relevant inclusive and differential Higgs boson measurements. Based upon this fit, we show that it is crucial to disentangle single- and di-top-associated Higgs production for tightening indirect constraints on a $\mathcal{CP}$-odd top-Yukawa coupling in the future. In this context, we propose an analysis strategy for measuring $tH$ production at the…
In Two-Higgs-Doublet models, the conditions for CP violation can be expressed in terms of invariants under U(2) rotations among the two SU(2) Higgs doublet fields. In order to design a strategy for measuring the invariants we express them in terms of observables, i.e., masses and couplings of scalar bosons. We find amplitudes directly sensitive to the invariants. Observation of the Standard-Model-like Higgs boson at the LHC severely constrains the models. In particular, in the model with Z_2 symmetry imposed on dimension-4 terms (in order to eliminate tree-level flavour-changing neutral curren…
Heavy Majorana neutrinos beyond the standard model can simultaneously explain the origin of tiny neutrino masses and matter-antimatter asymmetry in our Universe. The existence of heavy Majorana neutrinos will also lead to lepton number violation and the rare lepton-number-violating $W$ decays are possible. With contributions from two different Majorana neutrinos, a nonzero CP asymmetry may be generated from the rate difference between $W$ decay and its CP-conjugate process. The aim of this paper is to investigate the prospects for measuring CP violation in rare $W$ decays via Majorana neutrino…
Long-lived charged massive particles (CHAMPs) appear in various particle physics models beyond the Standard Model. In this Letter, we discuss the prospects for studying the stopping and decaying events of such long-lived CHAMPs at the LHC detectors, and show that the lifetime measurement (and the study of decay products) is possible with the LHC detectors for a wide range of the lifetime O(0.1)-O(10^{10}) sec, by using periods of no $pp$ collision. Even a short lifetime of order one second can be measured by (i) identifying the stopping event with the online Event Filter, (ii) immediately maki…
We address the question of how to determine the stop mixing angle and its CP-violating phase at the LHC. As an observable we discuss ratios of branching ratios for different decay modes of the light stop ~t_1 to charginos and neutralinos. These observables can have a very strong dependence on the parameters of the stop sector. We discuss in detail the origin of these effects. Using various combinations of the ratios of branching ratios we argue that, depending on the scenario, the observable may be promising in exposing the light stop mass, the mixing angle and the CP phase. This will, however…
In supergravity where the gravitino is the lightest supersymmetric particle (LSP), the next-to-lightest supersymmetric particle (NLSP) decays to the gravitino with a naturally long lifetime (10^4 - 10^8). However, cosmological constraints favour charged sleptons with lifetimes below a year as the natural NLSP candidate. For this scenario we report a method to accurately determine the slepton lifetime and SUSY cross-section from observation of the decays of sleptons trapped in the material comprising the main detector (ATLAS, CMS). A measurement of the lifetime to 5% is possible after 3 years a…
Composite Higgs models are an attractive scenario, where the discovered Higgs boson is regarded as a Nambu-Goldstone boson associated with spontaneous breakdown of a global symmetry of more fundamental theory. This class of models predicts violation of perturbative unitarity at high energies, and new resonances are expected to appear around TeV scale to maintain the unitarity, while a sizable phase shift is predicted in certain scattering amplitude. We investigate the new resonance scale from the phase shift by drawing analogies with pion physics in QCD. The detectability of the phase shift at…
We consider the electroweak production of a top quark in association with a photon at the LHC to probe the electroweak top quark couplings (t$\bar{\text{t}}γ$) as well as the triple gauge boson couplings (WW$γ$). The study is based on the modifications of the t$\bar{\text{t}}γ$ and WW$γ$ interactions via heavy degrees of freedom in the form of dimension-six operators which we add to the standard model Lagrangian. A binned angular asymmetry in single top quark plus photon events and cross section ratio are proposed to probe the anomalous t$\bar{\text{t}}γ$ and WW$γ$ couplings. It is shown that …
We study the mechanism of single top production at the LHC in the framework of an effective electroweak Lagrangian, analyzing the sensitivity of different observables to the magnitude of the effective couplings that parametrize new physics beyond the Standard Model. The observables relevant to the distinction between left and right effective couplings involve in practice the measurement of the spin of the top and this can be achieved only indirectly by measuring the angular distribution of its decay products. We show that the presence of effective right-handed couplings implies that the top is…
The identity of Dark Matter (DM) is one of the most captivating topics in particle physics today. The R-parity conserving Minimal Supersymmetric Standard Model (MSSM), which naturally provides a DM candidate in the form of the lightest neutralino ($\tildeχ_{1}^{0}$), is used as a benchmark scenario to show that a measurement of $Ω_{\tildeχ_{1}^{0}}h^{2}$ can be achieved from measurements at the CERN Large Hadron Collider. Focus is placed on compressed mass spectra regions, where the mass difference between the $\tildeχ_{1}^{0}$ and the $\tildeτ_{1}$ is small and where the $\tildeτ_{1}$-$\tilde…
Supersymmetry with heavy scalars is a model where at the LHC we have to rely on rate measurements to determine the parameters of the underlying new physics. For this example we show how to properly combine rate measurements with kinematic endpoints, taking into account statistical, systematic and theory uncertainties. Provided we observe a sizeable number of events the LHC should be able to determine many model parameters with small enough error bars to for example test unification patterns.
The ATLAS and CMS collaborations recently announced the observation of the associated production of the Higgs boson with a top quark pair ($t\bar tH$) at the LHC. This process depends directly on the the top quark Yukawa coupling and provides access to its properties. In particular, a CP-odd component is allowed in models beyond the Standard Model with extended Higgs sectors. Studies of the feasibility of such a measurement at the 13 TeV LHC were carried out, in the $H\rightarrow b\bar b$ decay channel and with the $t\bar t$ system decaying through the semileptonic and dileptonic channels. Fas…
Determination of trilinear Higgs coupling ($λ_{HHH}=κλ_{HHH}^{\rm SM}$) through Higgs pair productions is a major motivation for the LHC high luminosity phase. We perform a detailed collider simulation to explore the potential of measuring $λ_{HHH}$ in the $VHH$ ($V=W,Z$) production at the HL-LHC. We find that the trilinear Higgs coupling in the SM ($λ_{HHH}^{\rm SM}$) could be measured at the level of $1.3σ$. Combining with the gluon fusion, vector boson fusion and $t\bar{t}HH$ channels, $λ_{HHH}^{\rm SM}$ is expected to be measured at the level of $3.13σ$. If no evidence of Higgs pair produc…
The existence of heavy neutrinos mediating neutrino masses via a type-I seesaw can be directly probed at the LHC, or indirectly in rare lepton flavor-violating processes. The synergy between these two approaches requires a direct measurement of the heavy neutrino couplings. We discuss a strategy to perform such measurements at the LHC in the context of left-right symmetric models, which is also applicable to other models implementing a type-I or type-III seesaw. We demonstrate that the ambiguities in the determination of the heavy neutrino mixing parameters can be resolved by performing an exc…
A long-range ``dark force'' has recently been proposed to mediate the dark matter (DM) annihilation. If DM particles are copiously produced at the Large Hadron Collider (LHC), the light dark force mediator will also be produced through radiation. We demonstrate how and how precise we can utilize this fact to measure the coupling constant of the dark force. The light mediator's mass is measured from the ``lepton jet'' it decays to. In addition, the mass of the DM particle is determined using the MT2 technique. Knowing these quantities is critical for calculating the DM relic density.
The LHC offers a unique opportunity to investigate an ample spectra of phenomenous ranging from the Electro-Weak (EW) to the QCD sector of the Standard Model (SM). Among the quantities which can be measured in the LHC experiments are the CP and production asymmetries for several particles in a wide variety of decay modes. In this work we discuss about the interplay between production and CP asymmetries for particles produced in proton-proton interactions and the effects of one on the measurement of the other. This kind of effects are not present in flavor symmetric machines like the Tevatron o…
Many methods have been developed for measuring the mass of invisible particles that only use kinematic information available at hadron colliders. Because a particle is identified by its mass, these methods are critical when distinguishing between dark matter and fake dark matter, where a neutrino or other massless states can mimic a dark-matter signal. However, the uncertainty associated with measuring the mass of an invisible particle could be so large that it is indistinguishable from a neutrino. Monte Carlo is used to estimate lower bounds on how heavy an invisible particle must be in order…
The Cabibbo-Kobayashi-Maskawa (CKM) element $V_{ts}$, representing the coupling between the top and strange quarks, is currently best determined through fits based on the unitarity of the CKM matrix, and measured indirectly through box-diagram oscillations, and loop-mediated rare decays of the $B$ or $K$ mesons. It has been previously proposed to use the tree level decay of the $t$ quark to the $s$ quark to determine $|V_{ts}|$ at the LHC, which has become a top factory. In this paper, we extend the proposal by performing a detailed analysis of measuring $t \to sW$ in dileptonic $t\bar{t}$ eve…
The feasibility studies of the measurement of the central exclusive jet production at the LHC using the proton tagging technique are presented. In order to reach the low jet-mass region, single tagged events were considered. The studies were performed at the c.m. energy of 14 TeV and the ATLAS detector, but are also applicable for the CMS-TOTEM experiments. Four data-taking scenarios were considered: AFP and ALFA detectors as forward proton taggers and $β^*$ = 0.55 m and $β^*$ = 90 m optics. After the event selection, the signal-to-background ratio ranges between 5 and $10^4$. Finally, the exp…
The energy asymmetry in top-antitop-jet production is an observable of the top charge asymmetry designed for the LHC. We perform a realistic analysis in the boosted kinematic regime, including effects of the parton shower, hadronization and expected experimental uncertainties. Our predictions at particle level show that the energy asymmetry in the Standard Model can be measured with a significance of $3σ$ during Run 3, and with more than $5σ$ significance at the HL-LHC. Beyond the Standard Model the energy asymmetry is a sensitive probe of new physics with couplings to top quarks. In the frame…
Assuming the Higgs triplet model, we obtain the bounds on the absolute neutrino mass and the unknown MNS parameters by measuring decay processes of doubly charged particles at the CERN LHC. Majorana CP violating phases affect the prediction rather seriously, which is served to restrict these phases.
We propose a direct measurement of the CKM element $V_{td}$ at the LHC. Taking profit of the imbalance between $d$ and $\bar d$ quark content in the proton, we show that a non-zero $V_{td}$ induces a charge asymmetry in the $tW$ associated production. The main backgrounds to this process, $t\bar t$ production, and $tW$ associated production mediated by $V_{tb}$, give charge symmetric contributions at leading order in QCD. Therefore, using specific kinematic features of the signal, we construct a charge asymmetry in the di-lepton final state which, due also to a reduction of systematic uncertai…
Elastic light-by-light scattering, $γγ\toγγ$, can be measured in electromagnetic interactions of lead (Pb) ions at the Large Hadron Collider (LHC) and Future Circular Collider (FCC), using the large (quasi)real photon fluxes available in ultraperipheral collisions. The $γγ\toγγ$ cross sections for diphoton masses m$_{γγ}>$ 5 GeV in pp, pPb, and PbPb collisions at LHC ($\sqrt{\rm s_{_{NN}}}$ = 5.5, 8.8, 14 TeV) and FCC ($\sqrt{\rm s_{_{NN}}}$ = 39, 63, 100 TeV) center-of-mass energies are presented. The measurement has controllable backgrounds in PbPb collisions, and one expects about 70 and 25…
Majorana neutrinos in the seesaw model can have sizable mixings through which they can be produced at the Large Hadron Collider (LHC) and show a remarkable Lepton Number Violating (LNV) signature. In this article we study the LNV decay of the W boson via two almost degenerate heavy on-shell Majorana neutrinos $N_j$, into three charged leptons and a light neutrino. We consider the scenario where the heavy neutrino masses are within $1$ GeV $\leq M_N \leq 10$ GeV. We evaluated the possibility to measure a LNV oscillation process in such a scenario, namely, the modulation of the quantity $d Γ/d L…
The search for a new source of CP violation is one of the most important endeavors in particle physics. A particularly interesting way to perform this search is to probe the CP phase in the $hττ$ coupling, as the phase is currently completely unconstrained by all existing data. Recently, a novel variable $Θ$ was proposed for measuring the CP phase in the $hττ$ coupling through the $τ^\pm \to π^\pm π^0 ν$ decay mode. We examine two crucial questions that the real LHC detectors must face, namely, the issue of neutrino reconstruction and the effects of finite detector resolution. For the former, …
We discuss the possibility of measuring leading neutron production at the LHC. These data could be used to extract from it $π^+ p$ and $π^+π^+$ cross-sections. In this note we give some estimates for the case of elastic cross-sections and discuss related problems and prospects.
The discriminators of quark-gluon jets developed for $pp$ collisions at the LHC can be used to measure the gluon jet fraction in a jet sample. It allows to measure various characteristics of the quark and gluon jets. The methodology of such measurements for modern hadron colliders is discussed.
