High Invariant Mass Research Articles

Hunting for Bileptons at Hadron Colliders.

I review possible signals at hadron colliders of bileptons, namely doubly charged vectors or scalars with lepton number L=±2, as predicted by a 331 model, based on a SU(3)c×SU(3)L×U(1)X symmetry. In particular, I account for a version of the 331 model wherein the embedding of the hypercharge is obtained with the addition of three exotic quarks and vector bileptons. Furthermore, a sextet of SU(3)L, necessary to provide masses to leptons, yields an extra scalar sector, including a doubly charged Higgs, i.e., scalar bileptons. As bileptons are mostly produced in pairs at hadron colliders, their main signal is provided by two same-sign lepton pairs at high invariant mass. Nevertheless, they can also decay according to non-leptonic modes, such as a TeV-scale heavy quark, charged 4/3 or 5/3, plus a Standard Model quark. I explore both leptonic and non-leptonic decays and the sensitivity to the processes of the present and future hadron colliders.

Probing parton distribution functions at large x via Drell-Yan forward-backward asymmetry

The forward-backward asymmetry of the Drell-Yan process in dilepton decays at high invariant masses can be used to probe the parton distribution functions at large x. The behavior of three modern parton distribution function sets (CT18NNLO, MSHT20, and NNPDF4.0) are compared, and updated under various scenarios via eump using proton-proton collision pseudodata generated at s=13 TeV with 3000 fb−1 of integrated luminosity. Published by the American Physical Society 2024

S -wave contribution to rare D0→π+π−ℓ+ℓ− decays in the standard model and sensitivity to new physics

Physics of the up-type flavor offers unique possibilities of testing the standard model (SM) compared to the down-type flavor sector. Here, we discuss SM and new physics (NP) contributions to the rare charm-meson decay D0→π+π−ℓ+ℓ−. In particular, we discuss the effect of including the lightest scalar isoscalar resonance in the SM picture, namely, the f0(500), which manifests in a big portion of the allowed phase space. Other than showing in the total branching ratio at an observable level of about 20%, the f0(500) resonance manifests as interference terms with the vector resonances, such as at high invariant mass of the leptonic pair in distinct angular observables. Recent data from LHCb optimize the sensitivity to P-wave contributions that we analyze in view of the inclusion of vector resonances. We propose the measurement of alternative observables that are sensitive to the S-wave and are straightforward to implement experimentally. This leads to a new set of null observables that vanish in the SM due to its gauge and flavor structures. Finally, we study observables that depend on the SM interference with generic NP contributions from semileptonic four-fermion operators in the presence of the S-wave. Published by the American Physical Society 2024

Predictions of the ratio and asymmetry probes of the invisible Z-boson decay

Higher-order predictions through the combined accuracy including next-to-leading order (NLO) electroweak (EW) and next-to-NLO (NNLO) quantum chromodynamics (QCD) corrections in underlying perturbation theories are presented thoroughly for the invisible decay of the Z boson into neutrino pair relative to its decay into charged-lepton pair (leptonic decay). The combined NNLO QCD+NLO EW predictions are achieved based on the fully-differential calculations of cross sections of both the invisible and leptonic processes in proton-proton (pp) collisions at 13 TeV center-of-mass energy. Differential distributions of cross-section ratios of the invisible process to the leptonic process are presented as a function of the transverse momentum of the Z boson . For the first time, the predictions for differential distributions of cross-section asymmetries between the invisible process and the leptonic process are presented in bins of the . The cross-section ratio and asymmetry distributions, which are referred to as the invisible probes, are considered to be important for controlling the invisible process by the leptonic process of the Z boson and probing deviation from the Standard Model (SM) for new-physics searches. The predictions are extensively presented beyond the Z-boson mass resonance region to assess the potential of the invisible ratio and asymmetry probes for new-phenomena searches in high-invariant mass region of the lepton-pair final states. Various tests with threshold requirements of transverse momenta of neutrino pair and leptons are performed to assess the impact on the combined predictions. The invisible ratio and asymmetry probes are proposed to be important probes for indirect searches of new-physics scenarios.

Observation of electroweak W+W− pair production in association with two jets in proton-proton collisions at [formula omitted

An observation is reported of the electroweak production of a W+W− pair in association with two jets, with both W bosons decaying leptonically. The data sample corresponds to an integrated luminosity of 138fb−1 of proton-proton collisions at s=13TeV, collected by the CMS detector at the CERN LHC. Events are selected by requiring exactly two opposite-sign leptons (electrons or muons) and two jets with large pseudorapidity separation and high dijet invariant mass. Events are categorized based on the flavor of the final-state leptons. A signal is observed with a significance of 5.6 standard deviations (5.2 expected) with respect to the background-only hypothesis. The measured fiducial cross section is 10.2±2.0fb and this value is consistent with the standard model prediction of 9.1±0.6fb.

Threshold enhanced cross sections for colorless productions

We study the threshold effect for neutral and charged Drell-Yan productions, associated production of Higgs boson with a massive vector boson and Higgs production in bottom quark annihilation at LHC to the third order in QCD. Using the third order soft-virtual results for these processes and exploiting the universality of the threshold logarithms, we extract the process-dependent coefficients for these processes and resum large threshold logarithms to next-to-next-to-next-to leading logarithmic (N$^3$LL) accuracy. By matching our results to the recently available N$^3$LO results, we provide the most precise theoretical predictions for these processes. We present numerical results for invariant mass distribution and total production cross-sections. We find the conventional scale uncertainties of about $0.4\%$ at N$^3$LO level in the fixed order results get reduced to as small as less than $0.1\%$ at N$^3$LO+N$^3$LL level in the high invariant mass region.

Single-pion production in electron-proton interactions

This paper presents an extension of the MK single-pion production model [Kabirnezhad, Phys. Rev. D 97, 013002 (2018); Phys. Rev. D 102, 053009 (2020)] to high hadron invariant mass ($W$) and high momentum transfer (${Q}^{2}$) to conform to the predictions of perturbative QCD due to the quark-hadron duality evidence. New form factors for several resonances and nonresonant background in the electron-nucleon cross sections are determined taking into account the experimental data and improved evaluation techniques. Fits to electron-proton scattering data are used to constrain free parameters and to assign the related uncertainties of the model. The results from this work can be used to determine the vector current in the corresponding neutrino-nucleon cross sections, which is an important input for event generators in long-baseline neutrino-oscillation measurements.

Mixed QCD-electroweak corrections to dilepton production at the LHC in the high invariant mass region

We compute mixed QCD-electroweak corrections to the neutral-current Drell-Yan production of a pair of massless leptons in the high invariant mass region. Our computation is fully differential with respect to the final state particles. At relatively low values of the dilepton invariant mass, mℓℓ ∼ 200 GeV, we find unexpectedly large mixed QCD-electroweak corrections at the level of −1%. At higher invariant masses, mℓℓ ∼ 1 TeV, we observe that these corrections can be well approximated by the product of QCD and electroweak corrections. Hence, thanks to the well-known Sudakov enhancement of the latter, they increase at large invariant mass and reach e.g. −3% at mℓℓ = 3 TeV. Finally, we note that the inclusion of mixed corrections reduces the theoretical uncertainty related to the choice of electroweak input parameters to below the percent level.

Multi-lepton probes of new physics and lepton-universality in top-quark interactions

We explore the sensitivity to new physics (NP) in the associated production of top-quarks with leptons pp→tt¯ℓ+ℓ−, which leads to the multi-leptons signals pp→nℓ+jets+E̸T, where n=2,3,4. The NP is parameterized via 4-Fermi effective tt¯ℓ+ℓ− contact interactions of various types, which are generated by multi-TeV heavy scalar, vector or tensor exchanges in tt¯→ℓ+ℓ−; we focus on the case of ℓ=e,μ. We match the 4-Fermi ttℓℓ terms to the SMEFT operators and also give examples of specific underlying heavy physics that can generate such terms. Analysis of the SM signals and corresponding backgrounds shows that the di-lepton and tri-lepton channels are much better probes of the effective tt¯ℓ+ℓ− 4-Fermi terms than the four-lepton one at the 13 TeV LHC. Therefore, the best sensitivity is obtained in the di- and tri-lepton channels, for which the dominant background pp→tt¯ and pp→WZ, respectively, can be essentially eliminated after applying the 2ℓ and 3ℓ selections and a sufficiently high invariant mass selection for the opposite sign same flavor (OSSF) lepton-pair. We explore two cases: lepton flavor universal (LFU) NP where the ttee and ttμμ contact interactions are of same size and LFU violating (LFUV) NP, where the scale of the ttμμ terms is assumed to be much lower. We show that in both cases it is possible to obtain new 95% CL bounds on the scale of the ttℓℓ contact interactions at the level Λ≳2−3 TeV, which are considerably tighter than the current bounds on these 4-Fermi terms.

Hadronization model tuning in genie v3

The GENIE neutrino Monte Carlo describes neutrino-induced hadronization with an effective model, known as AGKY, which is interfaced with PYTHIA at high invariant mass. Only the low-mass AGKY model parameters were extracted from hadronic shower data from the FNAL 15 ft and BEBC experiments. In this paper, the first hadronization tune on averaged charged multiplicity data from deuterium and hydrogen bubble chamber experiments is presented, with a complete estimation of parameter uncertainties. A partial tune on deuterium data only highlights the tensions between hydrogen and deuterium datasets.

Potential and limitations of machine-learning approaches to inclusive |Vub| determinations

The determination of |Vub| in inclusive semileptonic B → Xuℓν decays will be among the pivotal tasks of Belle II. In this paper we study the potential and limitations of machine-learning approaches that attempt to reduce theory uncertainties by extending the experimentally accessible fiducial region of the B → Xuℓν signal into regions where the B → Xcℓν background is dominant. We find that a deep neural network trained on low-level single particle features offers modest improvement in separating signal from background, compared to BDT set-ups using physicist-engineered high-level features. We further illustrate that while the signal acceptance of such a deep neural network deteriorates in kinematic regions where the signal is small, such as at high hadronic invariant mass, neural networks which exclude kinematic features are flatter in kinematics but less inclusive in the sampling of exclusive hadronic final states at fixed kinematics. The trade-off between these two set-ups is somewhat Monte Carlo dependent, and we study this issue using the multipurpose event generator Sherpa in addition to the widely used B-physics tool EvtGen.

Scalar-pseudoscalar pair production at the Large Hadron Collider at NLO+NLL accuracy in QCD * *Supported in part by the National Natural Science Foundation of China (11775211, 12061141005, 11805001, 11935001) and the CAS Center for Excellence in Particle Physics (CCEPP)

We thoroughly investigate both transverse momentum and threshold resummation effects on scalar-pseudoscalar pair production via quark-antiquark annihilation at the Large Hadron Collider at QCD NLO+NLL accuracy. A factorization method is introduced to properly supplement the soft-gluon (threshold) resummation contribution from parton distribution functions to the resummed results obtained by the Collins-Soper-Sterman resummation approach. We find that the impact of the threshold-resummation improved PDFs is comparable to the resummation effect of the partonic matrix element and can even predominate in high invariant mass regions. Moreover, the loop-induced gluon-gluon fusion channel in the type-I two-Higgs-doublet model is considered in our calculations. The numerical results show that the electroweak production via quark-antiquark annihilation dominates over the gluon-initiated QCD production by orders of magnitude.

Search for resonant and nonresonant new phenomena in high-mass dilepton final states at sqrt{s} = 13 TeV

A search is presented for physics beyond the standard model (SM) using electron or muon pairs with high invariant mass. A data set of proton-proton collisions collected by the CMS experiment at the LHC at sqrt{s} = 13 TeV from 2016 to 2018 corresponding to a total integrated luminosity of up to 140 fb−1 is analyzed. No significant deviation is observed with respect to the SM background expectations. Upper limits are presented on the ratio of the product of the production cross section and the branching fraction to dileptons of a new narrow resonance to that of the Z boson. These provide the most stringent lower limits to date on the masses for various spin-1 particles, spin-2 gravitons in the Randall-Sundrum model, as well as spin-1 mediators between the SM and dark matter particles. Lower limits on the ultraviolet cutoff parameter are set both for four-fermion contact interactions and for the Arkani-Hamed, Dimopoulos, and Dvali model with large extra dimensions. Lepton flavor universality is tested at the TeV scale for the first time by comparing the dimuon and dielectron mass spectra. No significant deviation from the SM expectation of unity is observed.

Two paths towards precision at a very high energy lepton collider

We illustrate the potential of a very high energy lepton collider (from 10 to 30 TeV center of mass energy) to explore new physics indirectly in the vector boson fusion double Higgs production process and in direct diboson production at high energy. Double Higgs production is found to be sensitive to the anomalous Higgs trilinear coupling at the percent level, and to the Higgs compositeness ξ parameter at the per mille or sub-per mille level thanks to the measurement of the cross-section in the di-Higgs high invariant mass tail. High energy diboson (and tri-boson) production is sensitive to Higgs-lepton contact interaction operators at a scale of several tens or hundred TeV, corresponding to a reach on the Higgs compositeness scale well above the one of any other future collider project currently under discussion. This result follows from the unique capability of the very high energy lepton collider to measure Electroweak cross-sections at 10 TeV energy or more, where the effect of new physics at even higher energy is amplified. The general lesson is that the standard path towards precision physics, based on measurements of high-statistics processes such as single and double Higgs production, is accompanied at the very high energy lepton collider by a second strategy based on measurements at the highest available energy.

New flavor physics in di- and trilepton events from single-top production at the LHC and beyond

The associated production of a single-top with opposite-sign same-flavor (OSSF) di-leptons, $pp \to t \ell^+ \ell^-$ and $ pp \to t \ell^+ \ell^- + j$ ($j=$light jet), can lead to striking tri-lepton $pp \to \ell^\prime \ell^+ \ell^- + X$ and di-lepton $pp \to \ell^+ \ell^- + j_b + X$ ($j_b=b$-jet) events at the LHC, after the top decays. Although these rather generic multi-lepton signals are flavor-blind, they can be generated by new 4-Fermi flavor changing (FC) $u_i t \ell \ell$ scalar, vector and tensor interactions ($u_i \in u,c$), which we study in this paper; we match the FC $u_i t \ell \ell$ 4-Fermi terms to the SMEFT operators and also to different types of FC underlying heavy physics. The main backgrounds to these di- and tri-lepton signals arise from $t \bar t$, $Z$+jets and $VV$ ($V=W,Z$) production, but they can be essentially eliminated with a sufficiently high invariant mass selection on the OSSF di-leptons, $m_{\ell^+ \ell^-}^{\tt min}(OSSF) > 1$ TeV; the use of $b$-tagging as an additional selection in the di-lepton final state case also proves very useful. We find, for example, that the expected 95\% CL bounds on the scale of a tensor(vector) $u t \mu \mu$ interaction, with the current $\sim 140$ fb$^{-1}$ of LHC data, are $\Lambda < 5(3.2) $ TeV or $\Lambda < 4.1(2.7)$ TeV, if analyzed via the di-muon $\mu^+ \mu^- + j_b$ signal or the $e \mu^+ \mu^-$ tri-lepton one, respectively. The expected reach at the HL-LHC with 3000 fb$^{-1}$ of data is $\Lambda < 7.1(4.7)$ TeV and $\Lambda < 2.4(1.5)$ TeV for the corresponding $u t \mu \mu$ and $c t \mu \mu$ operators. We also study the potential sensitivity at future 27 TeV and 100 TeV high-energy LHC successors and also discuss the possible implications of this class of FC 4-Fermi effective interactions on lepton non-universality tests at the LHC.

Charm physics confronts high-pT lepton tails

We present a systematic survey of possible short-distance new-physics effects in (semi)leptonic charged- and neutral-current charmed meson decays. Using the Standard Model Effective Field Theory (SMEFT) to analyze the most relevant experimental data at low and high energies, we demonstrate a striking complementarity between charm decays and high invariant mass lepton tails at the LHC. Interestingly enough, high-pT Drell-Yan data offer competitive constraints on most new physics scenarios. Furthermore, the full set of correlated constraints from K, π and τ decays imposed by SU(2)L gauge invariance is considered. The bounds from D(s) decays, high-pT lepton tails and SU(2)L relations chart the space of the SMEFT affecting semi(leptonic) charm flavor transitions.

Resummed inclusive cross-section in ADD model at N3LL

We present three loop soft-plus-virtual (SV) corrections to the spin-2 production at the Large Hadron Collider (LHC). For this calculation, we make use of the recently computed quark and gluon three loop form factors for the spin-2 production, the universal soft-collinear coefficients as well as the mass factorization kernels. The SV coefficients are presented up to next-to-next-to-next-to leading order (N3LOsv). We also use these coefficients at three loops to compute the resummed prediction for inclusive cross-section to next-to-next-to-next-to leading logarithmic accuracy (N3LL) matched to N3LOsv. We use the standard technique to derive the Mellin N-dependent coefficients and also the N-independent coefficients to achieve the resummation using the minimal prescription matching procedure. Considering the spin-2 propagator in the large extra dimensional (ADD) model, we also study the numerical impact of these three-loop SV corrections as well as the resummed predictions on the di-lepton invariant mass distribution at the 13 TeV LHC. We find that the conventional scale uncertainties in the N3LOsv +N3LL resummed results substantially get reduced to as low as 2% in the high invariant mass region. We also estimate the PDF uncertainties in our predictions that will be useful in the experimental searches for large extra dimensions.

CP violation in three-body B decays: a model ansatz

The mechanism of CP violation remains one of the puzzles in particle physics. Key to understanding this phenomenon are nonleptonic B decays, especially multibody decays which exhibit large CP asymmetries in various regions of phase space. A full QCD- based theoretical description of these decays is still missing, requiring the use of models to fit the data. In this paper, we suggest a model ansatz which reflects the underlying physics and the known mechanism of CP violation via the CKM matrix. In addition, since CP violation is driven by the interference between amplitudes with and without valence charm quarks, we argue that the opening of the open-charm threshold may play an important role in generating CP violation in the high invariant mass region. We present a natural extension of the isobar model to incorporate these effects. We suggest an analysis of nonleptonic three-body B decay data including this extension, which would be interesting as it may give new hints to the sources of CP violation both at low and high invariant mass.

Testing the origin of the \u201cf_1(1420)\u201d with the {bar{K}} p rightarrow Lambda (Sigma ) K{bar{K}} pi reaction

We study the {bar{K}} p rightarrow Y K{bar{K}} pi reactions with {bar{K}} = {bar{K}}^0, K^- and Y=Sigma ^0, Sigma ^+, Lambda , in the region of K{bar{K}} pi invariant masses of 1200-1550 MeV. The strong coupling of the f_1(1285) resonance to K^* {bar{K}} makes the mechanism based on K^* exchange very efficient to produce this resonance observed in the K{bar{K}} pi invariant mass distribution. In addition, in all these reactions one observes an associated peak at 1420 MeV which comes from the K^* {bar{K}} decay mode of the f_1(1285) when the K^* is placed on shell at higher invariant masses. We call the attention to the possibility that the peaks observed in other reactions where the “f_1(1420)” is observed have a similar origin.

Quantum Critical Higgs: from AdS5 to colliders

We examine distinctive signatures of Quantum Critical Higgs models at the LHC and future higher energy colliders. In these models the Higgs boson is part of a conformal sector that is softly broken at a threshold scale, and generically the scaling dimension of the Higgs is larger than in the Standard Model. In particular we examine the gg → H → ZZ, gg → H → γγ, and gg → Z → H Z channels to see how the cross sections deviate from the Standard Model in the high invariant mass region. In order to perform the calculations we use 5D duals of Quantum Critical Higgs models using the AdS/CFT correspondence, with a soft wall to break the conformal symmetry.