Transverse Mass Distributions Research Articles

Observing many-body effects on lepton pair production from low mass enhancement and flow at RHIC and LHC energies

The ρ spectral function at finite temperature calculated using the real-time formalism of thermal field theory is used to evaluate the low mass dilepton spectra. The analytic structure of the ρ propagator is studied and contributions to the dilepton yield in the region below the bare ρ peak from the different cuts in the spectral function are discussed. The space-time integrated yield shows significant enhancement in the region below the bare ρ peak in the invariant mass spectra. It is argued that the variation of the inverse slope of the transverse mass (M T ) distribution can be used as an efficient tool to predict the presence of two different phases of the matter during the evolution of the system. The sensitivities of the effective temperature obtained from the slopes of the M T spectra to the medium effects are studied.

Impact of the parton distribution function uncertainties on the measurement of theWboson mass at the Tevatron and the LHC

We study at a quantitative level the impact of the uncertainties on the value of the W boson mass measured at hadron colliders due to: the proton parton distribution functions (PDFs), the value of the strong coupling constant alpha_S and the value of the charm mass used in the PDF determination. The value of the W boson mass is extracted, by means of a template fit technique, from the lepton-pair transverse mass distribution measured in the charged current Drell-Yan process. We study the determination of M_W at the Tevatron and at the LHC with 7 and 14 TeV of center-of-mass energy in a realistic experimental setup. The analysis has been done at the Born level using the event generator HORACE and at NLO-QCD using the event generators DYNNLO and ResBos. We consider the three global PDF sets, CTEQ6.6, MSTW2008 and NNPDF2.1. We estimate that the total PDF uncertainty on M_W is below 10 MeV both at the Tevatron and at the LHC for all energies and final states. We conclude that PDF uncertainties do not challenge a measurement of the W boson mass at the level of 10 MeV accuracy.

Search for a [formula omitted] boson decaying to a muon and a neutrino in pp collisions at [formula omitted

A new heavy gauge boson, W′, decaying to a muon and a neutrino, is searched for in pp collisions at a centre-of-mass energy of 7 TeV. The data, collected with the CMS detector at the LHC, correspond to an integrated luminosity of 36 pb−1. No significant excess of events above the standard model expectation is found in the transverse mass distribution of the muon–neutrino system. Masses below 1.40 TeV are excluded at the 95% confidence level for a sequential standard-model-like W′. The W′ mass lower limit increases to 1.58 TeV when the present analysis is combined with the CMS result for the electron channel.

Transverse mass distributions in central heavy-ion collisions at high energies

The transverse mass spectra of protons, pions, kaons, Lambda and Antilambda produced in central nucleus-nucleus collisions at high energies are described by using one-temperature and two-temperature emission pictures. The calculated results are compared and found to be in good agreement with the experimental data of the E895, E866 and E917 Collaborations measured in central Au-Au collisions at the Alternating Gradient Synchrotron (AGS) energies and the NA49 Collaboration measured in central Pb-Pb collisions at the Super Proton Synchrotron (SPS) energies. It is demonstrated that the transverse mass distributions of protons, kaons, Lambda and Antilambda, except for Lambda hyperons produced in central Pb-Pb collisions at 158 A GeV, can be described by using the one-temperature emission picture, and for pions, we need to use the two-temperature emission picture.

Search for a heavy gauge boson [formula omitted] in the final state with an electron and large missing transverse energy in pp collisions at [formula omitted

A search for a heavy gauge boson W′ has been conducted by the CMS experiment at the LHC in the decay channel with an electron and large transverse energy imbalance ETmiss, using proton–proton collision data corresponding to an integrated luminosity of 36 pb−1. No excess above standard model expectations is seen in the transverse mass distribution of the electron-ETmiss system. Assuming standard-model-like couplings and decay branching fractions, a W′ boson with a mass less than 1.36 TeV/c2 is excluded at 95% confidence level.

Hyperon production in Ar + KCl collisions at 1.76A GeV

We present transverse momentum spectra, rapidity distribution and multiplicity of Lambda-hyperons measured with the HADES spectrometer in the reaction Ar(1.76A GeV)+KCl. The yield of Xi- is calculated from our previously reported Xi-/(Lambda+Sigma0) ratio and compared to other strange particle multiplicities. Employing a strangeness balance equation the multiplicities of the yet unmeasured charged Sigma hyperons can be estimated. Finally a statistical hadronization model is used to fit the yields of pi-, K+, K0s, K-, phi, Lambda and Xi-. The resulting chemical freeze-out temperature of T=(76+-2) MeV is compared to the measured slope parameters obtained from fits to the transverse mass distributions of the particles.

Transverse mass distributions of protons produced in heavy-ion collisions at high energies

The transverse mass distributions of protons produced in Au-Au collisions at 8 A GeV and Pb-Pb collisions at 158 A GeV are calculated by using the Monte Carlo method in the framework of the multisource ideal gas model. It is found that our calculated results are in agreement with the experimental data in nucleus-nucleus collisions at high energies.

Transverse mass and invariant mass observables for measuring the mass of a semi-invisibly decaying heavy particle

Formulae are derived for the positions of end-points in the invariant mass and transverse mass distributions obtained from the products of heavy states decaying to pairs of semi-invisibly decaying lighter states. Formulae are derived both for the special case where the two decay chains are identical and the more general case where they are different. The formulae are tested with a simple case study of heavy SUSY higgs particles decaying to gauginos at the LHC.

Strangeness production within parton–hadron–string dynamics (PHSD)

Parton-hadron–string dynamics (PHSD) transport approach consistently simulates the full evolution of a relativistic heavy ion collision from the initial hard scatterings and string formation through the dynamical deconfinement phase transition to quark gluon plasma (QGP) to the hadronization and to the subsequent interactions in the hadronic phase. The transport theoretical description of quarks and gluons is based on a dynamical quasiparticle model for partons matched to reproduce lattice QCD results in thermodynamic equilibrium. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark–antiquark pairs or three quarks (antiquarks). Studying Pb+Pb reactions from 40 to 158 A GeV, we find that at most 40% of the collision energy is stored in the dynamics of the partons. This implies a large fraction of non-partonic, i.e. hadronic or string-like matter, which can be viewed as a hadronic corona; thus, neither hadronic nor purely partonic models can be employed to extract physical conclusions in comparing model results with data. On the other hand, comparing the PHSD results to those of the hadron-string dynamics (HSD) approach without the phase transition to QGP, we observe that the existence of the partonic phase has a sizeable influence on the transverse mass distribution of final kaons due to the repulsive partonic mean fields. Furthermore, we find a significant effect of the QGP on the production of multi-strange antibaryons due to a slightly enhanced pair production in the partonic phase from massive time-like gluon decay and to a more abundant formation of strange antibaryons in the hadronization process.

ϕ-meson production in In–In collisions at [formula omitted]: Evidence for relics of a thermal phase

Yields and transverse mass distributions of the ϕ-mesons reconstructed in the ϕ→μ+μ− channel in In+In collisions at Elab=158A GeV are calculated within an integrated Boltzmann+hydrodynamics hybrid approach based on the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model with an intermediate hydrodynamic stage. The analysis is performed for various centralities and a comparison with the corresponding NA60 data in the muon channel is presented. We find that the hybrid model, that embeds an intermediate locally equilibrated phase subsequently mapped into the transport dynamics according to thermal phase-space distributions, gives a good description of the experimental data, both in yield and slope. On the contrary, the pure transport model calculations tend to fail in catching the general properties of the ϕ meson production: not only the yield, but also the slope of the mT spectra, compare poorly with the experimental observations at top SPS energies.

Chargino reconstruction in supersymmetry with long-lived staus

We consider a supersymmetric (SUSY) scenario including right-handed neutrinos, one of whose scalar superpartners is the lightest SUSY particle (LSP). The distinguishing feature in the collider signal of SUSY in such a case is not missing energy but a pair of charged tracks corresponding to the next-to- lightest SUSY particle, when it is, as in the case considered, a stau. Following up on our recent work on neutralino reconstruction in such cases, we explore the possibility of reconstructing charginos, too, through a study of transverse mass distributions in specified final states. The various steps of isolating the transverse momenta of neutrinos relevant for this are outlined, and regions of the parameter space where our procedure works are identified.

Direct Measurement of theWBoson Width

We present a direct measurement of the width of the W boson using the shape of the transverse mass distribution of W->enu candidates selected in 1 fb-1 of data collected with the D0 detector at the Fermilab Tevatron collider in ppbar collisions at sqrt{s}=1.96 TeV. We use the same methods and data sample that were used for our recently published W boson mass measurement, except for the modeling of the recoil, which is done with a new method based on a recoil library. Our result, 2.028 +- 0.072 GeV, is in agreement with the predictions of the standard model.

Parton–hadron–string dynamics: An off-shell transport approach for relativistic energies

The dynamics of partons, hadrons and strings in relativistic nucleus–nucleus collisions is analyzed within the novel Parton–Hadron–String Dynamics (PHSD) transport approach, which is based on a Dynamical QuasiParticle Model (DQPM) for partons matched to reproduce recent lattice-QCD results – including the partonic equation of state – in thermodynamic equilibrium. Scalar- and vector-interaction densities are extracted from the DQPM as well as effective scalar- and vector-mean fields for the partons. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark–antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energy–momentum conservation. Since the dynamical quarks and antiquarks become very massive close to the phase transition, the formed resonant ‘pre-hadronic’ color-dipole states ( q q ¯ or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. The PHSD approach is applied to nucleus–nucleus collisions from 20 to 160 A GeV in order to explore the space–time regions of ‘partonic matter’. We find that even central collisions at the top-SPS energy of 158 A GeV show a large fraction of non-partonic, i.e. hadronic or string-like matter, which can be viewed as a hadronic corona. This finding implies that neither hadronic nor only partonic ‘models’ can be employed to extract physical conclusions in comparing model results with data. On the other hand – studying in detail Pb + Pb reactions from 40 to 158 A GeV – we observe that the partonic phase has a very low impact on rapidity distributions of hadrons but a sizeable influence on the transverse mass distribution of final kaons due to the repulsive partonic mean fields. Furthermore, we find a significant effect on the production of multi-strange antibaryons due to a slightly enhanced s s ¯ pair production in the partonic phase from massive time-like gluon decay and a larger formation of antibaryons in the hadronization process.

Evidence for the production of thermal dimuons with masses above [formula omitted] in 158A GeV In-In collisions

The yield of 1 < M < 2.5 GeV / c 2 dimuons produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, dimuons from Drell-Yan and from the decays of D meson pairs. These sources properly account for the dimuons produced in p-A collisions. We show that this excess in 158 A GeV In-In collisions is consists of prompt muon pairs. Furthermore, the transverse mass distribution of the excess is well described by an exponential function, with T eff ∼ 190 MeV , independent of mass and significantly lower than the T eff found at M < 1 GeV / c 2 , rising there up to 250 MeV due to radial flow. This suggests the emission of thermal dimuons above 1 GeV / c 2 from a largely partonic source, when radial flow has not yet built up.

Evidence for the production of thermal muon pairs with masses above 1 GeV/c 2 in 158 A GeV indium-indium collisions

The yield of muon pairs in the invariant mass region 1<M<2.5 GeV/c 2 produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of prompt muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are independent of mass and significantly lower than those found at masses below 1 GeV/c 2, rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/c 2 to be of largely partonic origin, when radial flow has not yet built up.

Supersymmetric backgrounds to standard model calibration processes at the CERN LHC

One of the first orders of business for LHC experiments after beam turn-on will be to calibrate the detectors using well-understood standard model (SM) processes such as $W$ and $Z$ production and $t\overline{t}$ production. These familiar SM processes can be used to calibrate the electromagnetic and hadronic calorimeters, and also to calibrate the associated missing transverse energy signal. However, the presence of new physics may already affect the results coming from these standard benchmark processes. We show that the presence of relatively low mass supersymmetry (SUSY) particles may give rise to significant deviations from SM predictions of $Z+\mathrm{\text{jets}}$ and $W+\mathrm{\text{jets}}$ events for jet multiplicity $\ensuremath{\ge}4$ or $\ensuremath{\ge}5$, respectively. Furthermore, the presence of low mass SUSY may cause nonstandard deviations to appear in top quark invariant and transverse mass distributions. Thus, effects that might be construed as detector malperformance could in fact be the presence of new physics. We advocate several methods to check when new physics might be present within SM calibration data.

Transverse momentum and transverse mass distributions of charged hadrons produced in Au–Au collisions at high energies

The transverse momentum distribution and the transverse mass distribution of charged hadrons produced in nucleus–nucleus collisions at high energies are described by using a two-cylinder model. The results calculated by the model are compared and found to be in agreement with the experimental data of the STAR and E895 Collaborations, measured in Au–Au collisions at the relativistic heavy ion collider (RHIC) and alternating-gradient synchrotron (AGS) energies, respectively. In the energy range concerned, the excitation degree of emission source close to the central axis of cylinders increases obviously with the collision centrality and incident energy increasing, but it does not show any obvious change with the increase of the (pseudo)rapidity in central collisions. The excitation degree of emission source close to the side-surface of cylinders does not show any obvious change with the collision centrality, the (pseudo)rapidity, and the incident energy increasing.

W production and properties at CDF

I present measurements of the W boson charge asymmetry and the W boson width (Γ W) using 350 pb-1 of CDF Run II data. The charge asymmetry is the first direct measurement, which uses a new technique to reconstruct the W rapidity by constraining its mass; the result will further constrain Parton Distribution Functions in future fits. The width measurement relies on a fit to the W transverse mass distribution. We measure Γ W = 2032 ± 71 MeV which is in good agreement with the Standard Model prediction.

Search for a standard model Higgs boson in CMS via vector boson fusion in the H→WW→lνlν channel

We present the potential for discovering the standard model Higgs boson produced via the vector-boson fusion mechanism. We considered the decay of Higgs bosons to the W+W- final state, with both W-bosons subsequently decaying leptonically. The main background is tt̄ produced in association with one or more jets. This study is based on a full simulation of the CMS detector. The result is that a signal of 5σ significance can be obtained with an integrated luminosity of 12–72 fb-1 for Higgs boson masses in the range 130<mH< 200 GeV. In addition, the major background can be measured directly to 7% from the data with an integrated luminosity of 30 fb-1. We also suggest a method to determine the Higgs mass using template transverse mass distributions.

The determination of the helicity ofW′ boson couplings at the LHC*

Apart from its mass and width, the most important property of a new charged gauge boson, W', is the helicity of its couplings to the SM fermions. Such particles are expected to exist in many extensions of the Standard Model. In this paper we explore the capability of the LHC to determine the W' coupling helicity at low integrated luminosities in the ℓ + ETmiss discovery channel. We find that measurements of the transverse mass distribution, reconstructed from this final state in the W−W' interference region, provides the best determination of this quantity. To make such measurements requires integrated luminosities of ∼10(60) fb−1 assuming MW' = 1.5(2.5) TeV and provided that the W' couplings have Standard Model magnitude. This helicity determination can be further strengthened by the use of various discovery channel leptonic asymmetries, also measured in the same interference regime, but with higher integrated luminosities.