LEP Measurements Research Articles

The automation of SMEFT-assisted constraints on UV-complete models

The ongoing Effective Field Theory (EFT) program at the LHC and elsewhere is motivated by streamlining the connection between experimental data and UV-complete scenarios of heavy new physics beyond the Standard Model (BSM). This connection is provided by matching relations mapping the Wilson coefficients of the EFT to the couplings and masses of UV-complete models. Building upon recent work on the automation of tree-level and one-loop matching in the SMEFT, we present a novel strategy automating the constraint-setting procedure on the parameter space of general heavy UV-models matched to dimension-six SMEFT operators. A new Mathematica package, match2fit, interfaces MatchMakerEFT, which derives the matching relations for a given UV model, and SMEFiT, which provides bounds on the Wilson coefficients by comparing with data. By means of this pipeline and using both tree-level and one-loop matching, we derive bounds on a wide range of single- and multi-particle extensions of the SM from a global dataset composed by LHC and LEP measurements. Whenever possible, we benchmark our results with existing studies. Our framework realises one of the main objectives of the EFT program in particle physics: deploying the SMEFT to bypass the need of directly comparing the predictions of heavy UV models with experimental data.

Type III seesaw with R-parity violation in light of m (CDF)

Motivated by the recently reported measurement of the $W$ boson mass $M_W = 80.4335 \pm 0.0094$ GeV by the CDF collaboration, we propose a type III seesaw extension of the minimal supersymmetric standard model (MSSM) which also includes an R-parity violating term. Without taking potential SUSY radiative corrections into account, we show that the CDF measurement of $M_W$ and the LEP measurement of the $\rho$ parameter can be simultaneously accommodated at the $2 \sigma$ level. A long-lived gravitino in a few GeV mass range is a unique viable dark matter candidate in this framework.

Interpreting LHCb’s Λb→Λcτν¯ measurement and puzzles in semileptonic Λb decays

Normalizing the recent LHCb measurement of $\Lambda_b \to \Lambda_c \tau \bar\nu$ to the standard model (SM) prediction for the $\Lambda_b \to \Lambda_c \mu \bar\nu$ rate, instead of a LEP measurement, provides a more consistent comparison with the SM prediction for the lepton flavor universality ratio $R(\Lambda_c)$. This modestly increases $R(\Lambda_c)$ compared to the quoted LHCb result, such that it no longer hints at a suppression compared to the SM, which would be hard to accommodate in new physics scenarios that enhance $R(D^{(*)})$. We point out that the fraction of excited states in inclusive semileptonic $\Lambda_b$ decay may be significantly greater than the corresponding fraction in $B$ decays. Possible implications are speculated upon.

Omniphobic membrane with nest-like re-entrant structure via electrospraying strategy for robust membrane distillation

Omniphobic membranes have acquired some recognitions for alleviating the membrane wetting in the robust membrane distillation because of the super-repulsion towards different surface tension liquids. In this study, the Ti-CNFs/PVDF composite membrane with omniphobicity and high permeate flux was fabricated via electrospraying multilevel heat-conducting nanofibers on the PVDF membrane and fluorosilane treatment simultaneously with one-step strategy. In detail, carbon nanofibers decorated by TiO2 with hydrothermal treatment were immobilized on the surface by the adhesive attraction of PDMS to establish the re-entrant structure. Meanwhile, 1H,1H,2H,2H-perfluorodecyltriethoxysilane was presented in the spraying liquid to gain the low surface energy for the surface. Additionally, the composite nanofibers played a role in mitigating the temperature polarization. A series of characterizations on membrane morphologies, wetting behaviors, structural peculiarities and stable properties were investigated systematically. The results showed that the re-entrant morphology consisted of the intersecting nanofibers was constructed skillfully, looking like the nest's hierarchical structure. The composite membrane displayed outstanding contact angles of 172.0 ± 1.8°, 170.5 ± 3.5°, 160.1 ± 2.6°, 165.0 ± 3.0°, 162.5 ± 4.5°, 151.0 ± 1.5°, 153.5 ± 3.5° and 118.0 ± 3.0° towards DI water, 3.5 wt% NaCl, 0.4 mM SDS, 0.4 mM CTAB, 0.4 mM Tween-20, ethanol, soybean milk and dodecane, respectively. The modified membrane presented a 167% LEP measurement of the pristine membrane and satisfactory chemical and thermal stabilities along with silver mirror phenomena. Furthermore, the omniphobic membrane exhibited anti-fouling and anti-wetting performances with high and stable permeate fluxes (36.3 kg/m2·h) as well as a nearly 100% salt rejection rate in the consecutive DCMD tests, where HA and diverse surfactants were added to the brine feed. This suggests the omniphobic membrane as a promising alternative for wastewater treatment in membrane distillation.

Measuring QCD Splittings with Invertible Networks

QCD splittings are among the most fundamental theory concepts at the LHC. We show how they can be studied systematically with the help of invertible neural networks. These networks work with sub-jet information to extract fundamental parameters from jet samples. Our approach expands the LEP measurements of QCD Casimirs to a systematic test of QCD properties based on low-level jet observables. Starting with an toy example we study the effect of the full shower, hadronization, and detector effects in detail.

Testing the applicability limits of a membrane distillation process with ceramic hydrophobized membranes: The critical wetting temperature

The new concept of the critical wetting temperature (TC wetting) of a hydrophobic membrane is introduced. It represents the maximum temperature at which a hydrophobic membrane can operate in no-wetting conditions with a specific liquid. The concept relies on the theoretical premises of the Laplace-Young equation.The existence of the critical wetting temperature is experimentally documented for the case of carbon-based titania membranes hydrophobized with FAS, by measuring the minimum Liquid Entry Pressure values as a function of temperature, in the range from 20 °C to 136 °C. In that case, the TCwetting was obtained close to 130 °C with pure water.The new parameter should be included in the membrane characterization protocols, since the evaluation of TC wetting is crucial to define which are the applicability limits of a membrane contactor. In alternative to various LEP measurements as a function of temperature, the measure of the minimum Liquid Entry Temperature at very low pressures can represent a more efficient method for the TCwetting evaluation.

Improved Bhabha cross section at LEP and the number of light neutrino species

In e+e− collisions, the integrated luminosity is generally measured from the rate of low-angle Bhabha interactions e+e−→e+e−. In the published LEP results, the inferred theoretical uncertainty of ±0.061% on the predicted rate is significantly larger than the reported experimental uncertainties. We present an updated and more accurate prediction of the Bhabha cross section in this letter, which is found to reduce the Bhabha cross section by about 0.048%, and its uncertainty to ±0.037%. When accounted for, these changes modify the number of light neutrino species (and its accuracy), as determined from the LEP measurement of the hadronic cross section at the Z peak, to Nν=2.9963±0.0074. The 20-years-old 2σ tension with the Standard Model is gone.

A self-calibrating, double-ratio method to test tau lepton universality in W boson decays at the LHC

Measurements in W^+W^- events at LEP2 and in B hadron semileptonic decays at B factories and LHCb provide intriguing hints of a violation of lepton universality in the charged current coupling of tau leptons relative to those for electrons and muons. We propose a novel, self-calibrating method to test tau lepton universality in W boson decays at the LHC. We compare directly the ratio of the numbers of selected ell tau _{mathrm {,had}} and emu final states in di-leptonic top quark pair events with that in Z/gamma ^*rightarrow tau tau events. Here ell =e {mathrm {or}} mu and tau _{mathrm {,had}} is a candidate semi-hadronic tau decay. This “double-ratio” cancels to first order sensitivity to systematic uncertainties on the reconstruction of e, mu , and tau leptons, thus improving very significantly the precision to which tau lepton universality can be tested in W boson decay branching ratios at the LHC. Using particle-level Monte Carlo events, and a parameterised simulation of detector performance, we demonstrate the effectiveness of the method and estimate the most significant residual sources of uncertainty arising from experimental and phenomenological systematics. Our studies indicate that a single LHC experiment precision on the tau lepton universality test of around 1.4% is achievable with a data set of int !! L, hbox {,d}t = 140 mathrm {fb}^{-1} at {sqrt{s}}~=~13~hbox {TeV}. This would improve significantly upon the precision of 2.5% on the four-experiment combined LEP2 measurements. If the central value of the proposed new measurement were equal to the central value of the LEP2 measurement this would yield an observation of BSM physics at a significance level of around 5sigma .

Scalar phenomenology in type-II seesaw model

In this work we study the viable parameter space of the scalar sector in the type-II seesaw model. In identifying the allowed parameter space, we employ constraints from low energy precision measurements, theoretical considerations and the 125-GeV Higgs data. These tools prove effective in constraining the model parameter space. Moreover, the triplet also offers a rich collider phenomenology from having additional scalars that have unique collider signatures. We find that direct collider searches for these scalars can further probe various parts of the viable parameter space. These parts can be parametrized by the electroweak scalar triplet vacuum expectation value, the mass splitting of the singly- and doubly-charged scalars, and the doubly-charged Higgs mass. We find that different regions of the viable parameter space give rise to different collider signatures, such as the same-sign dilepton, the same-sign W and the multilepton signatures. By investigating various LEP and LHC measurements, we derive the most updated constraints over the whole range of parameter space of the type-II seesaw model.

Novel mechanism for suppression of heavy flavored mesons in heavy ion collisions

Production of heavy flavored hadrons from fragmentation of heavy quarks represents an alternative probe for a medium created after heavy ion collisions. We demonstrate that observed strong suppression of heavy flavored D and B mesons, produced with high transverse momenta pT, is caused by final state interactions with such a medium. The space-time pattern of hadronization of a highly virtual heavy quark is controlled predominantly by intensive gluon radiation, which is ceased at a short time scale in accordance with perturbative QCD calculations and LEP measurements of the fragmentation functions. However, production of heavy flavored hadrons lasts a long time due to prompt multiple breakups of produced colorless (pre)hadrons in the medium. This fact together with the specific shape of heavy quark fragmentation function, peaked at large z, allows to explain the observed strong suppression of D and B mesons in a good accord with data.

Preparation and characterization of novel modified PVDF-HFP/GO/ODS composite hollow fiber membrane for Caspian Sea water desalination

In this Study, a novel modified PVDF-HFP/GO/ODS hollow fiber membrane fabricated by dry/wet jet spinning. The dope solution containing 12wt% polymer, and different amounts of graphene oxide (GO) nanosheets (0, 1, 3 and 5%) were prepared in NMP as a solvent. Then the composite membranes were silanized using ODS in acidic condition. Various techniques such as SEM, AFM, FTIR, LEP, contact angle (CA) and tensile measurement tests were used to investigate the effects of modifications on the structure, surface chemistry, and performance of the membranes. The modified membrane exhibits a permeation flux of 34.1±1.1kgm−2h−1. Furthermore, the contact angle and LEP were increased from 115°±1.5 and 112±4.5kPa for PVDF-HFP membrane to 162°±1.7 and 181±5.3kPa for modified PVDF-HFP/GO/ODS membrane, respectively. The incorporation of GO in the membrane matrix induces the hierarchy roughness on the membrane surface and provides reactive hydroxyl sites for further chemical modification. Desalination of Caspian Sea water (CSW) was conducted using prepared membranes through direct contact membrane distillation (DCMD). Results showed that salt precipitation and permeation flux reduction partially occurred after 10days for modified membranes. Finally, the membranes recovered by chemical cleaning in HCl solution (pH=5).

Inclusive semitauonic B decays to order [formula omitted

We calculate the decay width and the τ-lepton energy distribution as well as relevant moments for inclusive B¯→Xcτν¯τ process including power corrections up to order ΛQCD3/mb3 and QCD corrections to the partonic level. We compare the result with the sum of the standard-model predictions of the branching fractions of the exclusive semileptonic B¯→(D,D⁎,D⁎⁎)τν¯τ decays as well as with the relevant experimental data. Our prediction is in agreement with the LEP measurement and is consistent with the standard-model calculation of the exclusive modes. We discuss the impact from physics beyond the Standard Model.

Novel scenario for production of heavy flavored mesons in heavy ion collisions

The observed strong suppression of heavy flavored hadrons produced with high $p_T$, is caused by final state interactions with the created dense medium. Vacuum radiation of high-pT heavy quarks ceases at a short time scale, as is confirmed by pQCD calculations and by LEP measurements of the fragmentation functions of heavy quarks. Production of a heavy flavored hadrons in a dense medium is considerably delayed due to prompt breakup of the hadrons by the medium. This causes a strong suppression of the heavy quark yield because of the specific shape of the fragmentation function. The parameter-free description is in a good accord with available data.

Application of Salt Additives and Response Surface Methodology for Optimization of PVDF Hollow Fiber Membrane in DCMD and AGMD Processes

In this study, the influence of the salts as an additive on the performance of the membrane was investigated and an extensive work was performed to optimize PVDF hollow fiber membranes through a response surface methodology (RSM). The prepared membranes were characterized by SEM, contact angle and LEP measurement. Then, the RSM was used for the optimization of surface pore size, porosity and hydrophobicity of the synthesized hollow fiber at different conditions (polymer concentration, salt concentrations, and air gap). Under MD conditions (feed concentration, 100 mg/l; feed temperature 80 °C, and cooling temperature 15 °C), the optimum membrane was compared with the virgin one in the same condition. In addition, the influence of distillate flux at different feed concentrations and temperatures was evaluated. The results show that the optimum hollow fiber membrane was fabricated in the polymer concentration of 22 %w/w, BaCl2 concentration of 2.9 %w/w and an air gap of 34.5 cm. Consequently, the optimum fiber was examined for the desalination of water with 35, 50 and 70 g/l salt concentration by DC and AG membrane distillation. Our findings show that the distillate flux with the salt rejection of 99.9% was increased to 46% and 31% for DCMD and AGMD, respectively.

Precision Drell-Yan measurements at the LHC and implications for the diphoton excess

Precision measurements of the Drell-Yan (DY) cross section at the LHC constrain new physics scenarios that involve new states with electroweak (EW) charges. We analyze these constraints and apply them to models that can address the LHC diphoton excess at 750 GeV. We confront these findings with LEP EW precision tests and show that DY provides stronger constraints than the LEP data. While 8 TeV data can already probe some parts of the interesting region of parameter space, LHC14 results are expected to cover a substantial part of the relevant terrain. We derive the bounds from the existing data, estimate LHC14 reach and compare them to the bounds one gets from LEP and future FCC-ee precision measurements.

Combining LEP and LHC to bound the Higgs width

The correlation of on- and off-shell Higgs boson production at the LHC in gg→h⁎→ZZ has been used to bound the Higgs width. We propose an alternative complementary constraint which is only possible through the combination of LEP and LHC measurements. Precision electroweak measurements at LEP allow for the determination of indirect constraints on Higgs couplings to vector bosons by considering one-loop processes involving virtual Higgs exchange. As the indirect constraint is model dependent we will consider two specific models which modify the Higgs couplings and width, and our results will apply specifically to these models. By combining these LEP constraints with current LHC 8 TeV Higgs measurements a stronger limit on the Higgs width can be achieved than with LHC data alone. Looking to the future, a more robust constraint can be achieved by correlating LEP measurements with WBF Higgs production followed by Higgs decays to WW and ZZ. We will discuss the model dependence of this method in comparison to other proposed methods.

Measuring polarization of light quarks at ATLAS and CMS

Polarization of strange quarks is preserved to a high degree when they hadronize into Lambda baryons, as observed in Z decays at LEP. This opens up the possibility for ATLAS and CMS to use strange-quark polarization measurements as a characterization tool for new physics scenarios that produce such quarks. Measurements in ttbar samples would be useful for obtaining additional information about the polarization transfer from the strange quark to the Lambda baryon. Already with 100/fb in Run 2, ttbar samples in ATLAS and CMS become competitive in sensitivity with the Z samples of the LEP experiments. Moreover, while the LEP measurements were done inclusively over all quark flavors, which makes their interpretation dependent on various modeling assumptions, ttbar events at the LHC offer multiple handles for disentangling the different contributions experimentally. We also discuss the possibility of measuring polarizations of up and down quarks.

Color reconnection at future e $$^{+}$$ + e $$^{-}$$ - colliders

The effects of color reconnection (CR) at $${\mathrm e}^+{\mathrm e}^-$$ colliders are revisited, with focus on recently developed CR models. The new models are compared with the LEP2 measurements for $${\mathrm e}^+{\mathrm e}^- \rightarrow {\mathrm W}^+{\mathrm W}^- \rightarrow {\mathrm q}_1 \overline{\mathrm q}_2 {\mathrm q}_3 \overline{\mathrm q}_4$$ and found to lie within their limits. Prospects for constraints from new high-luminosity $${\mathrm e}^+{\mathrm e}^-$$ colliders are discussed. The novel arena of CR in Higgs decays is introduced, and it is illustrated by shifts in angular correlations that would be used to set limits on a potential CP-odd admixture of the 125 GeV Higgs state.

Fragmentation fractions of $$c$$ c and $$b$$ b quarks into charmed hadrons at LEP

The fragmentation fractions of c and b quarks into the weakly decaying charmed hadrons D0, D+, D_s+ and Lambda_c+, and into the charmed vector meson D*+ have been derived from the LEP measurements and averaged. The c quark fragmentation fractions represent probabilities to hadronise as a given charmed hadron, while the b quark fragmentation fractions are defined as sums of probabilities to hadronise as a particular charmed hadron or its antiparticle.

Measurement of ratios of fragmentation fractions for bottom hadrons inpp¯collisions ats=1.96 TeV

This paper describes the first measurement of b-quark fragmentation fractions into bottom hadrons in Run II of the Tevatron Collider at Fermilab. The result is based on a 360 pb-1 sample of data collected with the CDF II detector in p-pbar collisions at sqrt{s}=1.96 TeV. Semileptonic decays of B0, B+, and B_s mesons, as well as Lambda_b baryons, are reconstructed. For an effective bottom hadron p_T threshold of 7 GeV/c, the fragmentation fractions are measured to be f_u/f_d=1.054 +/- 0.018 (stat) +0.025-0.045(sys) +/- 0.058 (Br), f_s/(f_u+f_d)=0.160 +/- 0.005 (stat) +0.011-0.010 (sys) +0.057-0.034 (Br), and f_{Lambda_b}/(f_u+f_d)=0.281\pm0.012 (stat) +0.058-0.056 (sys) +0.128-0.086 (Br), where the uncertainty (Br) is due to uncertainties on measured branching ratios. The value of f_s/(f_u+f_d) agrees within one standard deviation with previous CDF measurements and the world average of this quantity, which is dominated by LEP measurements. However, the ratio f_{Lambda_b}/(f_u+f_d) is approximately twice the value previously measured at LEP. The approximately 2 sigma discrepancy is examined in terms of kinematic differences between the two production environments.