Hadron Level Research Articles

An update of muon capture on hydrogen

The successful precision measurement of the rate of muon capture on a proton by the MuCap Collaboration allows for a stringent test of the current theoretical understanding of this process. Chiral perturbation theory (χPT), which is a low-energy effective field theory (EFT) that preserves the symmetries and the pattern of symmetry breaking in the underlying theory of QCD, offers a systematic framework for describing μp capture and provides a basic test of QCD at the hadronic level. We describe how this effective theory with no free parameters reproduces the measured capture rate. A recent study has addressed new sources of uncertainties that were not considered in the previous works, and we review to what extent these uncertainties are now under control. Finally, the rationale for studying muon capture on the deuteron and some recent theoretical developments regarding this process are discussed.

Oriented event shapes at N3LL + $ \mathcal{O}\left( {\alpha_{\mathrm{S}}^2} \right) $

We analyze oriented event-shapes in the context of Soft-Collinear Effective Theory (SCET) and in fixed-order perturbation theory. Oriented event-shapes are distributions of event-shape variables which are differential on the angle θ T that the thrust axis forms with the electron-positron beam. We show that at any order in perturbation theory and for any event shape, only two angular structures can appear: F 0 = 3/8 (1 + cos2 θ T ) and F 1 = (1 − 3 cos2 θ T ). When integrating over θ T to recover the more familiar event-shape distributions, only F 0 survives. The validity of our proof goes beyond perturbation theory, and hence only these two structures are present at the hadron level. The proof also carries over massive particles. Using SCET techniques we show that singular terms can only arise in the F 0 term. Since only the hard function is sensitive to the orientation of the thrust axis, this statement applies also for recoil-sensitive variables such as Jet Broadening. We show how to carry out resummation of the singular terms at N3LL for Thrust, Heavy-Jet Mass, the sum of the Hemisphere Masses and C-parameter by using existing computations in SCET. We also compute the fixed-order distributions for these event-shapes at $ \mathcal{O}\left( {{\alpha_{\mathrm{S}}}} \right) $ analytically and at $ \mathcal{O}\left( {\alpha_{\mathrm{S}}^2} \right) $ with the program Event2.

Determination of the CMSSM parameters using neural networks

In most (weakly interacting) extensions of the Standard Model the relation mapping the parameter values onto experimentally measurable quantities can be computed (with some uncertainties), but the inverse relation is usually not known. In this paper we demonstrate the ability of artificial neural networks to find this unknown relation, by determining the unknown parameters of the constrained minimal supersymmetric extension of the Standard Model (CMSSM) from quantities that can be measured at the LHC. We expect that the method works also for many other new physics models. We compare its performance with the results of a straightforward \chi^2 minimization. We simulate LHC signals at a center of mass energy of 14 TeV at the hadron level. In this proof-of-concept study we do not explicitly simulate Standard Model backgrounds, but apply cuts that have been shown to enhance the signal-to-background ratio. We analyze four different benchmark points that lie just beyond current lower limits on superparticle masses, each of which leads to around 1000 events after cuts for an integrated luminosity of 10 fb^{-1}. We use up to 84 observables, most of which are counting observables; we do not attempt to directly reconstruct (differences of) masses from kinematic edges or kinks of distributions. We nevertheless find that m_0 and m_{1/2} can be determined reliably, with errors as small as 1% in some cases. With 500 fb^{-1} of data tan\beta as well as A_0 can also be determined quite accurately. For comparable computational effort the \chi^2 minimization yielded much worse results.

Tt̄ pair hadroproduction in association with a heavy boson at the NLO QCD accuracy + Parton Shower

The PowHel framework allows to make predictions of total and differential cross-sections of multiparticle hadroproduction processes at both NLO QCD accuracy and NLO QCD matched to Parton Shower, on the basis of the interface between the P0WHEG-B0X and HELAC-NL0 codes. It has already been applied to study several processes involving a tt̄ pair in association with a third particle or hadronic jet. Our most recent predictions concern tt̄V hadroproduction (with V = W or Z), at both parton and hadron level, by considering different decay channels (hadronic and leptonic) of the heavy particles. In particular, we show the results of our phenomenological analyses under the same system of cuts also recently adopted by the CMS collaboration at LHC.

Symmetry energy softening in nuclear matter with non-nucleonic constituents

We study the trend of the nuclear symmetry energy in relativistic mean-field models with appearance of the hyperon and quark degrees of freedom at high densities. On the pure hadron level, we focus on the role of $\Lambda$ hyperons in influencing the symmetry energy both at given fractions and at charge and chemical equilibriums. The softening of the nuclear symmetry energy is observed with the inclusion of the $\Lambda$ hyperons that suppresses the nucleon fraction. In the phase with the admixture of quarks and hadrons, the equation of state is established on the Gibbs conditions. With the increase of the quark volume fraction in denser and denser matter, the apparent nuclear symmetry energy decreases till to disappear. This softening would have associations with the observations which need detailed discriminations in dense matter with the admixture of new degrees of freedom created by heavy-ion collisions.

Limits on lepton flavor violation fromμ−−e−conversion

We revisit the status of Lepton Flavor Violating (LFV) \mu e qq contact interactions from the view point of \mu - e -conversion in nuclei. We consider their contribution to this LFV process via the two mechanisms on the hadronic level: direct nucleon and meson exchange ones. In the former case the quarks are embedded directly into the nucleons while in the latter in mesons which then interact with nucleons in a nucleus. We revise and in some cases reevaluate the hadronic parameters relevant for both mechanisms and calculate the contribution of the above mentioned contact interactions in coherent \mu - e -conversion in various nuclei. Then we update the existing upper bounds and also derive new ones for the scales of the \mu e qq contact interactions from the experimental limits on the capture rates of \mu - e -conversion. We compare these limits with the ones derived in the literature from other LFV processes and comment on the prospects of LHC searches related to the contact \mu e qq interactions.

SANC integrator in the progress: QCD and EW contributions

Modules and packages for the one-loop calculations at the partonic level represent the first level of SANC output computer product. The next level represents Monte Carlo integrator mcsanc, realizing fully differential hadron level calculations (convolution with PDF) for the HEP processes at LHC. In this paper we describe the implementation into the framework mcsanc first set of processes: DY NC, DY CC, $$f_1 \bar f'_1 \to HW^ \pm (Z)$$ and single top production. Both EW and QCD NLO corrections are taken into account. A comparison of SANC results with those existing in the world literature is given.

Light stop searches at the LHC with monojet events

We consider light top squarks (stops) in the minimal supersymmetric Standard Model at the Large Hadron Collider. Here, we assume that the lightest neutralino is the lightest supersymmetric particle (LSP) and the lighter stop is the next-to-LSP. Stop pair production is difficult to probe at the Large Hadron Collider for small stop-LSP mass splitting. It has been shown previously that even in this case stop detection is possible if one considers stop pair production in association with one hard jet. We reconsider this supersymmetric monojet signature and go beyond previous works by including the full Standard Model background and optimizing the cuts, working at the hadron level and including detector effects. As a result, a larger portion of the stop-LSP mass plane becomes accessible to monojet searches.

Consistent simulation of nonresonant diphoton production at hadron collisions with a custom-made parton shower

We have developed a Monte Carlo event generator for non-resonant diphoton ($\gamma\gamma$) production at hadron collisions in the framework of GR@PPA, which consistently includes additional one-jet production. The jet-matching method developed for initial-state jet production has been extended to the final state in order to regularize the final-state QED divergence in the $qg \rightarrow \gamma\gamma + q$ process. A QCD/QED-mixed parton shower (PS) has been developed to complete the matching. The PS has the capability of enforcing hard-photon radiation, and small-$Q^{2}$ photon radiations that are not covered by the PS are supplemented by using a fragmentation function. The generated events can be passed to general-purpose event generators in order to perform the simulations down to the hadron level. Thus, we can simulate the isolation requirements that must be applied in experiments at the hadron level. The simulation results are in reasonable agreement with the predictions from RESBOS and DIPHOX. The simulated hadron-level events can be further fed to detector simulations in order to investigate the detailed performance of experiments.

Measurement of the Z/γ* + b-jet cross section in pp collisions at $ \sqrt {s} = 7 $ TeV

Abstract The production of b jets in association with a Z/γ* boson is studied using proton-proton collisions delivered by the LHC at a centre-of-mass energy of 7 TeV and recorded by the CMS detector. The inclusive cross section for Z/γ* + b-jet production is measured in a sample corresponding to an integrated luminosity of 2.2 fb−1. The Z/γ* + b-jet cross section with Z/γ* → ℓℓ (where ℓℓ = ee or μμ) for events with the invariant mass 60 < M ℓℓ < 120 GeV, at least one b jet at the hadron level with p T > 25 GeV and |η| < 2.1, and a separation between the leptons and the jets of ΔR > 0.5 is found to be $ {5}.{84}\pm 0.0{8}\left( {{\text{stat}}.} \right)\pm 0.{72}\left( {{\text{syst}}.} \right)_{ - 0.55}^{ + 0.25} $ (theory) pb. The kinematic properties of the events are also studied and found to be in agreement with the predictions made by the MadGraph event generator with the parton shower and the hadronisation performed by pythia.

Theoretical analysis of a possible observation of the chiral magnetic effect in Au+Au collisions within the RHIC beam energy scan program

In terms of the hadron-string-dynamics (HSD) approach we investigate the correlation function in the azimuthal angle $\psi$ of charged hadrons that is expected to be sensitive to a signal of local strong parity violation. Our analysis of Au+Au collisions is based on the recent STAR data within the RHIC Beam-Energy-Scan (BES) program. The HSD model reasonably reproduces STAR data for $\sqrt{s_{NN}}=$7.7 GeV, while there are some deviations from the experiment at the collision energy of 11.5 GeV and an increase of deviations between theory and experiment at $\sqrt{s_{NN}}=$39 GeV. For reference, the results for $\sqrt{s_{NN}}=$ 200 GeV are given as well. The role of the retarded electromagnetic field is discussed and a compensation effect for the action of its electric and magnetic components is pointed out. We conclude that the recent RHIC BES data at $\sqrt{s_{NN}}=$7.7 and 11.5 GeV can be understood on the hadronic level without involving the idea of a strong parity violation; however, at $\sqrt{s_{NN}}\sim$40 GeV and above one needs to take into consideration explicit partonic (quark-qluon) degrees-of-freedom for a proper treatment of the dynamics.

Dynamical implications of gluonic excitations in meson-meson systems

We study meson-meson interactions using an extended $q^2\bar{q}^2(g)$ basis that allows calculating coupling of an ordinary meson-meson system to a hybrid-hybrid one. We use a potential model matrix in this extended basis which at quark level is known to provide a good fit to numerical simulations of a $q^2\bar{q}^2$ system in pure gluonic theory for static quarks in a selection of geometries. We use a combination of resonating group method formalism and Born approximation to include the quark motion using wave functions of a $q\bar{q}$ potential within a cluster. This potential is taken to be quadratic for ground states and has an additional smeared $\frac{1}{r}$ (Gaussian) for the matrix elements between hybrid mesons. For the parameters of this potential, we use values chosen to 1) minimize the error resulting from our use of a quadratic potential and 2) best fit the lattice data for differences of $\Sigma_{g}$ and $\Pi_{u}$ configurations of the gluonic field between a quark and an antiquark. At the quark (static) level, including the gluonic excitations was noted to partially replace the need for introducing many-body terms in a multi-quark potential. We study how successful such a replacement is at the (dynamical) hadronic level of relevance to actual hard experiments. Thus we study effects of both gluonic excitations and many-body terms on mesonic transition amplitudes and the energy shifts resulting from the second order perturbation theory (i.e. from the respective hadron loops). The study suggests introducing both energy and orbital excitations in wave functions of scalar mesons that are modelled as meson-meson molecules or are supposed to have a meson-meson component in their wave functions.

Ks0andΛproduction inppinteractions ats=0.9and 7 TeV measured with the ATLAS detector at the LHC

The production of KS0 and Λ hadrons is studied in pp collision data at s=0.9 and 7 TeV collected with the ATLAS detector at the LHC using a minimum-bias trigger. The observed distributions of transverse momentum, rapidity, and multiplicity are corrected to hadron level in a model-independent way within well-defined phase-space regions. The distribution of the production ratio of Λ¯ to Λ baryons is also measured. The results are compared with various Monte Carlo simulation models. Although most of these models agree with data to within 15% in the KS0 distributions, substantial disagreements are found in the Λ distributions of transverse momentum.17 MoreReceived 5 November 2011DOI:https://doi.org/10.1103/PhysRevD.85.012001This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2012 CERN, for the ATLAS Collaboration

Monte Carlo Event Generators

We describe progress in the development of Monte Carlo event generators for the full simulation of collider physics events on the hadron level. We briefly comment on all areas of simulation but focus on the matching of higher order perturbative matrix elements and developments in multiple partonic interaction models.

GR@PPA 2.8: Initial-state jet matching for weak-boson production processes at hadron collisions

The initial-state jet matching method introduced in our previous studies has been applied to the event generation of single W and Z production processes and diboson (W+W−, WZ and ZZ) production processes at hadron collisions in the framework of the GR@PPA event generator. The generated events reproduce the transverse momentum spectra of weak bosons continuously in the entire kinematical region. The matrix elements (ME) for hard interactions are still at the tree level. As in previous versions, the decays of weak bosons are included in the matrix elements. Therefore, spin correlations and phase-space effects in the decay of weak bosons are exact at the tree level. The program package includes custom-made parton shower programs as well as ME-based hard interaction generators in order to achieve self-consistent jet matching. The generated events can be passed to general-purpose event generators to make the simulation proceed down to the hadron level. Program summaryProgram title: GR@PPA 2.8Catalogue identifier: ADRH_v3_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADRH_v3_0.htmlProgram obtainable from: CPC Program Library, Queenʼs University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 112 146No. of bytes in distributed program, including test data, etc.: 596 667Distribution format: tar.gzProgramming language: Fortran; with some included libraries coded in C and C++Computer: AllOperating system: Any UNIX-like systemRAM: 1.6 Mega bytes at minimumClassification: 11.2Catalogue identifier of previous version: ADRH_v2_0Journal reference of previous version: Comput. Phys. Comm. 175 (2006) 665External routines: Bash and Perl for the setup, and CERNLIB, ROOT, LHAPDF, PYTHIA according to the userʼs choice.Does the new version supersede the previous version?: No, this version supports only a part of the processes included in the previous versions.Nature of problem: We need to combine those processes including 0 jet and 1 jet in the matrix elements using an appropriate matching method, in order to simulate weak-boson production processes in the entire kinematical region.Solution method: The leading logarithmic components to be included in parton distribution functions and parton showers are subtracted from 1-jet matrix elements. Custom-made parton shower programs are provided to ensure satisfactory performance of the matching method.Reasons for new version: An initial-state jet matching method has been implemented.Summary of revisions: Weak-boson production processes associated with 0 jet and 1 jet can be consistently merged using the matching method.Restrictions: The built-in parton showers are not compatible with the PYTHIA new PS and the HERWIG PS.Unusual features: A large number of particles may be produced by the parton showers and passed to general-purpose event generators.Running time: About 10 min for initialization plus 25 s for every 1k-event generation for W production in the LHC condition, on a 3.0-GHz Intel Xeon processor with the default setting.

Extraction of top backgrounds in the Higgs boson search with theH→WW(*)→ℓℓ+ETmissdecay with a full-jet veto at the LHC

The search for a Standard Model Higgs boson with the $H\to WW^\ast\to \ell\ell + E_{\rm T}^{\rm miss}$ decay and the application of a full-jet veto yields a strong sensitivity in the mass range $130<m_H<200$ GeV. One of the residual backgrounds is related to processes involving top quarks. A method for the extraction of top backgrounds is evaluated at the parton and hadronic levels based on the data-driven extraction of the jet veto survival probability. The uncertainty of the proposed method is about 15%.

Gauge invariant two-photon exchange contributions in [formula omitted

The gauge invariant two-photon exchange (TPE) contributions in e−π+→e−π+ are discussed at hadronic level. The contact term is added to keep the full amplitude gauge invariant by two methods: one is to multiply form factors with the amplitude for point-like particles and another is to construct a gauge invariant Lagrangian. The practical calculations show the TPE contributions by these two methods are almost the same, while the later method is favored when extending the discussion to processes including two charged finite-size particles like ep→enπ+.

Top quark pair production in association with a jet at NLO accuracy with parton showering

We compute the production cross section of a top–antitop pair in association with a jet at hadron colliders at next-to-leading order accuracy matched with parton shower algorithms to make predictions at the hadron level. The parton shower allows for including the decay of the top quarks at the leading order accuracy. We use a framework based on three well established numerical codes, the POWHEG BOX, used for the calculation of the cross section, HELAC, which generates the matrix elements for the Born-level, real emission and the virtual part, and finally a parton shower program, such as PYTHIA or HERWIG, which generate the parton-shower and hadronization.

Open charm effects ine+e−→J/ψη,J/ψπ0, andϕηc

We propose to study the open charm effects in $e^+ e^-\to J/\psi\eta$, $J/\psi\pi^0$ and $\phi\eta_c$. We show that the exclusive cross section lineshapes of these processes would be strongly affected by the open charm effects. Since the final state light meson productions are through soft gluon radiations, we assume a recognition of this soft process via charmed meson loops at hadronic level. A unique feature among these three reactions is that the $D\bar{D^*}+c.c.$ open channel is located in a relatively isolated energy, i.e. $\sim 3.876$ GeV, which is sufficiently far away from the known charmonia $\psi(3770)$ and $\psi(4040)$. Therefore, the cross section lineshapes of these reactions may provide an opportunity for singling out the open charm effects with relatively well-defined charmonium contributions. In particular, we find that reaction $e^+ e^-\to J/\psi\pi^0$ is sensitive to the open charm $D\bar{D^*}+c.c.$ Due to the dominance of the isospin-0 component at the charmonium energy region, we predict an enhanced model-independent cusp effect between the thresholds of $D^0\bar{D}^0+c.c.$ and $D^+ D^{*-}+c.c.$ This study can also help us to understand the X(3900) enhancement recently observed by Belle Collaboration in $e^+ e^-\to D\bar{D}+c.c.$

MathQCDSR: A Mathematica package for QCD sum rules calculations

We present a software package written in Mathematica for standard QCD sum rules calculations. Two examples are given to demonstrate how to use the package. One is for the mass spectrum of octet baryons from two-point correlation functions; the other for the magnetic moments of octet baryons in the external-field method. The free package FeynCalc is used to handle the gamma-matrix algebra. In addition to two notebooks for the construction of the QCD sum rules, two corresponding notebooks are provided for a Monte Carlo-based numerical analysis, complete with in-line graphical display of sum rule matching, error distributions, and scatter plots for correlations. Program summaryProgram title: MathQCDSRCatalogue identifier: AEJA_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEJA_v1_0.htmlProgram obtainable from: CPC Program Library, Queenʼs University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 93 897No. of bytes in distributed program, including test data, etc.: 631 481Distribution format: tar.gzProgramming language: MathematicaComputer: PCs and WorkstationsOperating system: Any OS that supports Mathematica. The package has been tested under Windows XP, Macintosh OS X, and LinuxClassification: 11.5External routines: FeynCalc (http://www.feyncalc.org/). It is a freely available Mathematica package for high-energy physics calculations. Here it is used primarily to handle gamma-matrix algebra.Nature of problem: The QCD sum rule method is a nonperturbative approach to solving quantum chromodynamics (QCD), the fundamental theory of the strong force. The approach establishes a direct link between hadron phenomenology and the QCD vacuum structure via a few QCD parameters called vacuum condensates and susceptibilities. It has been widely applied in nuclear and particle physics to gain insight into various aspects of strong-interaction physics.Solution method: First, QCD sum rules are constructed by evaluating correlation functions from two perspectives. On the quark level, it leads to a function of QCD parameters and the Borel mass parameter M. On the hadronic level, it leads to a function of phenomenological parameters and the same M. By numerically matching the two sides over a range in M, the phenomenological parameters can be extracted. The construction involves a large amount of gamma-matrix algebra, Fourier transform, and Borel transform. The matching usually involves searching for minimum χ2. We employ a Monte Carlo-based procedure to perform the analysis which allows for realistic error estimates.Restrictions: The package deals with only standard (SVZ) QCD sum rule calculations. It can be easily adapted to handle other variants of the method (like finite-energy sum rules). Due to the use of FeynCalc, two of the notebooks (qcdsr2pt-construction.nb and qcdsr3pt-construction.nb) only run on version 6.0 of Mathematica. The other two can run on any version.Additional comments: The package consists of the following 4 notebooks:•qcdsr2pt-construction.nb – This notebook constructs the QCD sum rules for octet baryon masses and outputs them, one particle at a time, to disk in plain text for analysis. For reference, we include all the output files (named Mass-*.txt) as part of the package, totaling 8 files in about 20 lines. The user should generate the outputs on their own computer and check against the supplied ones.•qcdsr2pt-analysis.nb – This notebook reads and analyzes the QCD sum rules produced by qcdsr2pt-construction.nb. The user can save the graphics in the analysis to disk in a variety of formats.•qcdsr3pt-construction.nb – This notebook constructs the QCD sum rules for the octet baryon magnetic moments and outputs them, one particle at a time, to disk in plain text for analysis. Again, for reference, we include all the output files (named Mag-*.txt), totaling 24 files in about 400 lines. The user should generate the outputs on their own computer and check against the supplied ones to make sure the program is running properly.•qcdsr3pt-analysis.nb – This notebook reads and analyzes the QCD sum rules produced by qcdsr3pt-construction.nb. The user can save the graphics in the analysis to disk in a variety of formats. Each notebook can be run separately, apart from the simple interface between the construction and analysis programs via plain text files written to disk.Running time: For mass calculations, qcdsr2ptconstruction.nb and qcdsr2pt-analysis.nb take about a minute each to run on a laptop. For magnetic moment calculations, qcdsr3pt-construction.nb can take up to 10 minutes for a given particle, and qcdsr3pt-analysis.nb typically a few minutes, depending on the number of Monte Carlo samples.