Dimuon Production Research Articles

Rho-meson resonance broadening in QCD at finite temperature

Thermal finite energy QCD sum rules for the vector current correlator are used to study quark-gluon deconfinement. Assuming $\ensuremath{\rho}$-meson saturation of the correlator in the hadronic sector, and the operator product expansion in QCD, we obtain the temperature behavior of the resonance parameters (coupling, mass, and width) and of the leading vacuum condensates, as well as the perturbative QCD threshold in the complex squared energy plane. The results are consistent with quark-gluon deconfinement at a critical temperature ${T}_{c}\ensuremath{\simeq}197\text{ }\text{ }\mathrm{MeV}$. The temperature dependence of the $\ensuremath{\rho}$-meson width is of importance for current experiments on dimuon production in nuclear collisions.

Light vector meson production in pp collisions at [formula omitted

The ALICE experiment has measured low-mass dimuon production in pp collisions at s=7 TeV in the dimuon rapidity region 2.5<y<4. The observed dimuon mass spectrum is described as a superposition of resonance decays (η,ρ,ω,η′,ϕ) into muons and semi-leptonic decays of charmed mesons. The measured production cross sections for ω and ϕ are σω(1<pt<5 GeV/c,2.5<y<4)=5.28±0.54(stat)±0.49(syst) mb and σϕ(1<pt<5 GeV/c,2.5<y<4)=0.940±0.084(stat)±0.076(syst) mb. The differential cross sections d2σ/dydpt are extracted as a function of pt for ω and ϕ. The ratio between the ρ and ω cross section is obtained. Results for the ϕ are compared with other measurements at the same energy and with predictions by models.

Charm dimuon production in neutrino-nucleon interactions in the NOMAD experiment

We present our new measurement of charm dimuon production in neutrino-iron interactions based upon the full statistics collected by the NOMAD experiment. After background subtraction we observe 15,340 charm dimuon events, providing the largest sample currently available. The analysis exploits the large inclusive charged current sample (about 9 million events after all analysis cuts) to constrain the total systematic uncertainty to ∼2%. The extraction of strange sea and charm production parameters is also discussed.

Low mass dimuon production in p–A collisions at GeV with NA60

The NA60 experiment has studied low-mass muon pair production in proton–nucleus collisions with a system of Be, Cu, In, W, Pb and U targets using a 400 GeV/c proton beam at the CERN SPS. The mass spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons η, ρ, ω, η′ and ϕ. A new high-precision measurement of the electromagnetic transition form factors of the η and ω mesons is presented, complemented with a measurement of the temperature parameter of the ρ meson in cold nuclear matter. The pT spectra for the ω and ϕ mesons are extracted in the full pT range accessible, up to pT = 2 GeV/c. The nuclear dependence of the production cross sections for the η, ω and ϕ mesons has been investigated in terms of the power law σpA∝Aα, and the α parameter was studied as a function of pT.

Hidden SUSY at the LHC: the light higgsino-world scenario and the role of a lepton collider

While the SUSY flavor, CP and gravitino problems seem to favor a very heavy spectrum of matter scalars, fine-tuning in the electroweak sector prefers low values of superpotential mass \mu. In the limit of low \mu, the two lightest neutralinos and light chargino are higgsino-like. The light charginos and neutralinos may have large production cross sections at LHC, but since they are nearly mass degenerate, there is only small energy release in three-body sparticle decays. Possible dilepton and trilepton signatures are difficult to observe after mild cuts due to the very soft p_T spectrum of the final state isolated leptons. Thus, the higgsino-world scenario can easily elude standard SUSY searches at the LHC. It should motivate experimental searches to focus on dimuon and trimuon production at the very lowest p_T(\mu) values possible. If the neutralino relic abundance is enhanced via non-standard cosmological dark matter production, then there exist excellent prospects for direct or indirect detection of higgsino-like WIMPs. While the higgsino-world scenario may easily hide from LHC SUSY searches, a linear e^+e^- collider or a muon collider operating in the \sqrt{s}\sim 0.5-1 TeV range would be able to easily access the chargino and neutralino pair production reactions.

Z ′ bosons at colliders: a Bayesian viewpoint

We revisit the CDF data on di-muon production to impose constraints on a large class of Z' bosons occurring in a variety of E_6 GUT based models. We analyze the dependence of these limits on various factors contributing to the production cross-section, showing that currently systematic and theoretical uncertainties play a relatively minor role. Driven by this observation, we emphasize the use of the Bayesian statistical method, which allows us to straightforwardly (i) vary the gauge coupling strength, g', of the underlying U(1)'; (ii) include interference effects with the Z' amplitude (which are especially important for large g'); (iii) smoothly vary the U(1)' charges; (iv) combine these data with the electroweak precision constraints as well as with other observables obtained from colliders such as LEP 2 and the LHC; and (v) find preferred regions in parameter space once an excess is seen. We adopt this method as a complementary approach for a couple of sample models and find limits on the Z' mass, generally differing by only a few percent from the corresponding CDF ones when we follow their approach. Another general result is that the interference effects are quite relevant if one aims at discriminating between models. Finally, the Bayesian approach frees us of any ad hoc assumptions about the number of events needed to constitute a signal or exclusion limit for various actual and hypothetical reference energies and luminosities at the Tevatron and the LHC.

Low mass dimuon production in proton-nucleus collisions at 400 GeV/ c

The NA60 experiment has studied low-mass muon pair production in proton-nucleus collisions with a system of Be, Cu, In, W, Pb and U targets using a 400 GeV/ c proton beam at the CERN SPS. The mass spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons η, ρ, ω, η ′ and ϕ new measurement of the electromagnetic transition form factors of the η and ω was performed. The values found are Λ η − 2 = 1.950 ± 0.059 (stat.) ± 0.042 (syst.) ( GeV / c 2 ) − 2 and Λ ω − 2 = 2.241 ± 0.025 (stat.) ± 0.028 (syst.) ( GeV / c 2 ) − 2 . They agree with the previous available measurements, improving their uncertainty thanks to the higher statistics, and confirm the discrepancy with the prediction of the Vector Meson Dominance model in the case of the electromagnetic form factor of the ω meson. The comparison of the data to an improved calculation including also a direct point-like term is discussed. The nuclear dependence of the production cross sections of the ω, ρ and ϕ mesons was also studied. An increase of the ϕ production with respect to the ω is observed from p-Be to p-U collisions. Parameterizing the nuclear dependence of the production cross sections with the power law σ pA ∝ A α , one finds α ω = 0.841 ± 0.014 (stat.) ± 0.030 (syst.) and α ϕ = 0.906 ± 0.011 (stat.) ± 0.025 (syst.) . A possible bias induced by the use of the light Be target is found. If it is excluded, the results are α ω = 0.801 ± 0.037 (stat.) ± 0.041 (syst.) and α ϕ = 0.963 ± 0.026 (stat.) ± 0.017 (syst.) , showing an increase of α ϕ and a decrease of α ω .

Neutrino dimuon production and the dynamical determination of strange parton distributions

Utilizing recent neutrino dimuon production measurement from NuTeV the assumptions on the determination of the strangeness content of the nucleon within the dynamical approach to parton distributions are investigated. The data are found to be in good agreement with the predictions derived from our (GJR08) dynamical parton distributions, which have been generated entirely radiatively starting from vanishing strange input distributions at an optimally chosen low resolution scale. Further, the data induce an asymmetry in the strange sea which is found to be small and positive in agreement with previous results.

Dimuon production by laser-wakefield accelerated electrons

We analyze ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ pair production generated by high-energy electrons emerging from a laser-wakefield accelerator. The ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ pairs are created in a solid thick high-$Z$ target, following the electron accelerating plasma region. Numerical estimates are presented for 1 to 10 GeV electron beams which are expected to be reliable in the nearest future. Reactions induced by the secondary bremsstrahlung photons dominate the dimuon production. According to our estimates, a 20 pC electron bunch with energy of 1 (10) GeV may create about 100 (5000) muon pairs. The produced ${\ensuremath{\mu}}^{\ifmmode\pm\else\textpm\fi{}}$ can be used in studying various aspects of muon-related physics in tabletop installations. This may be considered as an important step towards the investigation of more complicated elementary processes induced by laser-driven electrons.

Results on angular distributions of thermal dileptons in nuclear collisions

The NA60 experiment at the CERN SPS has studied dimuon production in 158 AGeV In-In collisions. The strong pair excess above the known sources found in the mass region 0.2 < M < 2.5 GeV has been previously interpreted as thermal radiation. In this paper results on the associated angular distributions for M < 1 GeV , as measured in the Collins-Soper reference frame, are presented. The structure function parameters λ, μ, ν are consistent with zero and the projected polar and azimuth angle distributions are uniform. The absence of any polarization is consistent with the interpretation of the excess dimuons as thermal radiation from a randomized system.

Probing theZ′ sector of the minimalB−Lmodel at future Linear Colliders in thee+e−→ μ+μ−process

We study the capabilities of future electron-positron Linear Colliders, with centre-of-mass energy at the TeV scale, in accessing the parameter space of a $Z'$ boson within the minimal $B-L$ model. In such a model, wherein the Standard Model gauge group is augmented by a broken $U(1)_{B-L}$ symmetry -- with $B(L)$ being the baryon(lepton) number -- the emerging $Z'$ mass is expected to be in the above energy range. We carry out a detailed comparison between the discovery regions mapped over a two-dimensional configuration space ($Z'$ mass and coupling) at the Large Hadron Collider and possible future Linear Colliders for the case of di-muon production. As known in the literature for other $Z'$ models, we confirm that leptonic machines, as compared to the CERN hadronic accelerator, display an additional potential in discovering a $B-L$ $Z'$ boson as well as in allowing one to study its properties at a level of precision well beyond that of any of the existing colliders.

Precision determination of electroweak parameters and the strange content of the proton from neutrino deep-inelastic scattering

We use recent neutrino dimuon production data combined with a global deep-inelastic parton fit to construct a new parton set, NNPDF1.2, which includes a determination of the strange and antistrange distributions of the nucleon. The result is characterized by a faithful estimation of uncertainties thanks to the use of the NNPDF methodology, and is free of model or theoretical assumptions other than the use of NLO perturbative QCD and exact sum rules. Better control of the uncertainties of the strange and antistrange parton distributions allows us to reassess the determination of electroweak parameters from the NuTeV dimuon data. We perform a direct determination of the | V cd | and | V cs | CKM matrix elements, obtaining central values in agreement with the current global CKM fit: specifically we find | V cd | = 0.244 ± 0.019 and | V cs | = 0.96 ± 0.07 . Our result for | V cs | is more precise than any previous direct determination. We also reassess the uncertainty on the NuTeV determination of sin 2 θ W through the Paschos–Wolfenstein relation: we find that the very large uncertainties in the strange valence momentum fraction are sufficient to bring the NuTeV result into complete agreement with the results from precision electroweak data.

First Results on Angular Distributions of Thermal Dileptons in Nuclear Collisions

The NA60 experiment at the CERN Super Proton Synchrotron has studied dimuon production in 158A GeV In-In collisions. The strong excess of pairs above the known sources found in the complete mass region 0.2<M<2.6 GeV has previously been interpreted as thermal radiation. We now present first results on the associated angular distributions. Using the Collins-Soper reference frame, the structure function parameters lambda, mu, and nu are measured to be zero, and the projected distributions in polar and azimuth angles are found to be uniform. The absence of any polarization is consistent with the interpretation of the excess dimuons as thermal radiation from a randomized system.

Highlights from the NA60 experiment

NA60 measured dimuon production in p–A and In–In collisions at the CERN SPS. This paper presents a high statistics measurement of ϕ meson production in In–In collisions at 158 A GeV. Both the transverse momentum, rapidity, decay angular distributions and the absolute yield were measured as a function of centrality. The results are compared to previous measurements in order to shed light on the long standing ϕ puzzle. In addition, highlights on η meson production and on the dimuon excess below the J/ψ mass are presented.

Early SUSY discovery at LHC via sparticle cascade decays to same-sign and multimuon states

In the very early stages of LHC running, uncertainties in detector performance will lead to large ambiguities in jet, electron and photon energy measurements, along with inferred missing transverse energy ETmiss. However, muon detection should be quite straightforward, with the added benefit that muons can be reliably detected down to transverse energies of order 5 GeV. Supersymmetry discovery through multimuon channels has been extensively explored in the literature, but always relying on hard ETmiss cuts. Here, we quantify signal and background rates for same-sign (SS) dimuon and multimuon production at the LHC without any ETmiss cuts. The LHC, operating at s=10 TeV, should be able to discover a signal over expected background consistent with gluino pair production for mg˜≲450 (550) GeV in the SS dimuon plus ⩾4 jets state with just 0.1 (0.2) fb−1 of integrated luminosity.

ϕ meson production in In–In collisions and the ϕ puzzle

The NA60 experiment measured dimuon production in In–In collisions at 158 A GeV. This paper presents a high statistics measurement of ϕ → μμ with the specific objective to provide an insight into the ϕ puzzle, i.e. the difference in the inverse T slopes and absolute yields measured by NA49 and NA50 in the kaon and lepton channel, respectively. Transverse momentum distributions were studied as a function of centrality. The slope parameter T shows a rapid increase with centrality, followed by a saturation. Variations of T with the fit range of the order of 15 MeV were observerd, possibly as a consequence of radial flow. The ϕ meson yield normalized to the number of participants increases with centrality and is consistently higher than the yield measured by the NA49 experiment at any centrality.

Latest results from NA60

NA60 is a fixed target experiment at the CERN SPS studying dimuon production in different colliding systems: In-In at 158 A·GeV incident energy and p-A collisions at 158 and 400 GeV. A strong excess of dimuon pairs has been observed in the region of the invariant mass spectrum below the J/ψ. The high resolution of the NA60 detector allowed to measure the in-medium modifications of the ρ spectral function and to clarify the origin of the dimuon excess in the region above the ϕ. The J/ψ suppression has been studied in In-In and p-A collisions at 158 GeV incident energy. The latest results on these topics are addressed in this paper.

Evidence for Radial Flow of Thermal Dileptons in High-Energy Nuclear Collisions

The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158A GeV In-In collisions. An excess of pairs above the known meson decays has been reported before. We now present precision results on the associated transverse momentum spectra. The slope parameter Teff extracted from the spectra rises with dimuon mass up to the rho, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.

J/ψ production in In–In and p–A collisions

The NA60 experiment studies dimuon production in In–In and p–A collisions at the CERN SPS. We report recent results on J/ψ production, measured through its muon pair decay. As a function of centrality, we show that in In–In the J/ψ yield is suppressed beyond expectations from nuclear absorption. We present also for the first time results on J/ψ production in p–A collisions at 158 GeV, the same energy of the nucleus–nucleus data. For both p–A and In–In we show preliminary results on ψ′ suppression. Finally, we have studied the kinematical distributions of the J/ψ produced in In–In collisions. We present results on transverse momentum and rapidity, as well as on the angular distribution of the J/ψ decay products.

New results from NA60 and other SPS experiments

NA60 has been the last heavy ion experiment running at the SPS. It has performed measurements on dimuon production with an indium beam at 158 AGeV/c and with a proton beam at 158 and 400 GeV/c, focusing on (i) the study of the ρ spectral function in nuclear collisions, (ii) the unambiguous determination of the open charm yield and the quantitative assessment of the production of thermal radiation in the intermediate mass region, and (iii) the study of the J/ψ suppression in a collision system other than Pb–Pb. This paper provides an overview of the latest results. In addition, several other SPS experiments are still producing physics results. Interesting highlights from WA98, NA45 and NA49 on (i) nuclear modification factors of neutral and charged pions, (ii) three particle correlations at high transverse momentum and (iii) net baryon distributions will be addressed at the end of the paper.