Dimuon Spectrum Research Articles

Search for the decay B0→ ϕμ+μ−

A search for the decay B0→ ϕμ+μ− is performed using proton-proton collisions at centre-of-mass energies of 7, 8, and 13 TeV collected by the LHCb experiment and corresponding to an integrated luminosity of 9 fb−1. No evidence for the B0→ ϕμ+μ− decay is found and an upper limit on the branching fraction, excluding the ϕ and charmonium regions in the dimuon spectrum, of 4.4 × 10−3 at a 90% credibility level, relative to that of the {B}_s^0 → ϕμ+μ− decay, is established. Using the measured {B}_s^0 → ϕμ+μ− branching fraction and assuming a phase-space model, the absolute branching fraction of the decay B0→ ϕμ+μ− in the full q2 range is determined to be less than 3.2 × 10−9 at a 90% credibility level.

Hunting for B^+rightarrow K^+ tau ^+tau ^- imprints on the B^+ rightarrow K^+ mu ^+mu ^- dimuon spectrum

We investigate the possibility of indirectly constraining the B^{+}rightarrow K^{+}tau ^+tau ^- decay rate using precise data on the B^{+}rightarrow K^{+}mu ^+mu ^- dimuon spectrum. To this end, we estimate the distortion of the spectrum induced by the B^{+}rightarrow K^{+}tau ^+tau ^-rightarrow K^{+} mu ^+mu ^- re-scattering process, and propose a method to simultaneously constrain this (non-standard) contribution and the long-distance effects associated to hadronic intermediate states. The latter are constrained using the analytic properties of the amplitude combined with data and perturbative calculations. Finally, we estimate the sensitivity expected at the LHCb experiment with present and future datasets. We find that constraints on the branching fraction of O(10^{-3}), competitive with current direct bounds, can be achieved with the current dataset, while bounds of O(10^{-4}) could be obtained with the LHCb upgrade-II luminosity.

High- $$p_T$$ p T dilepton tails and flavor physics

We investigate the impact of flavor-conserving, non-universal quark-lepton contact interactions on the dilepton invariant mass distribution in p~p rightarrow ell ^+ ell ^- processes at the LHC. After recasting the recent ATLAS search performed at 13 TeV with 36.1 fb^{-1} of data, we derive the best up-to-date limits on the full set of 36 chirality-conserving four-fermion operators contributing to the processes and estimate the sensitivity achievable at the HL-LHC. We discuss how these high-p_T measurements can provide complementary information to the low-p_T rare meson decays. In particular, we find that the recent hints on lepton-flavor universality violation in b rightarrow s mu ^+ mu ^- transitions are already in mild tension with the dimuon spectrum at high-p_T if the flavor structure follows minimal flavor violation. Even if the mass scale of new physics is well beyond the kinematical reach for on-shell production, the signal in the high-p_T dilepton tail might still be observed, a fact that has been often overlooked in the present literature. In scenarios where new physics couples predominantly to third generation quarks, instead, the HL-LHC phase is necessary in order to provide valuable information.

Finite Temperature QCD Sum Rules: A Review

The method of QCD sum rules at finite temperature is reviewed, with emphasis on recent results. These include predictions for the survival of charmonium and bottonium states, at and beyond the critical temperature for deconfinement, as later confirmed by lattice QCD simulations. Also included are determinations in the light-quark vector and axial-vector channels, allowing analysing the Weinberg sum rules and predicting the dimuon spectrum in heavy-ion collisions in the region of the rho-meson. Also, in this sector, the determination of the temperature behaviour of the up-down quark mass, together with the pion decay constant, will be described. Finally, an extension of the QCD sum rule method to incorporate finite baryon chemical potential is reviewed.

Precision study of the η → μ + μ − γ and ω → μ + μ − π 0 electromagnetic transition form-factors and of the ρ → μ + μ − line shape in NA60

The NA60 experiment studied low-mass muon pair production in proton-nucleus (p-A) collisions using a 400~GeV proton beam at the CERN SPS. The low-mass dimuon spectrum is well described by the superposition of the two-body and Dalitz decays of the light neutral mesons $\eta$, $\rho$, $\omega$, $\eta'$ and $\phi$, and no evidence of in-medium effects is found. A new high-precision measurement of the electromagnetic transition form factors of the $\eta$ and $\omega$ was performed, profiting from a 10~times larger data sample than the peripheral In-In sample previously collected by NA60. Using the pole-parameterisation $|F(M)|^2 = (1 -M^2/\mathrm{\Lambda}^2)^{-2}$ we find $\mathrm{\Lambda}_\eta^{-2} = 1.934\ \pm\ 0.067$~(stat.) $\pm\ 0.050$~(syst.)~(GeV/$c^2$)$^{-2}$ and $\mathrm{\Lambda}_\omega^{-2} = 2.223\ \pm\ 0.026$~(stat.) $\pm\ 0.037$~(syst.)~(GeV/$c^2$)$^{-2}$. An improved value of the branching ratio of the Dalitz decay $\omega \to \mu^+\mu^-\pi^0$ is also obtained, with $BR(\omega \to \mu^+\mu^-\pi^0) = [1.41~\pm~0.09~\mathrm{(stat.)}$ $\pm~0.15~\mathrm{(syst.)}] \times 10^{-4}$. Further results refer to the $\rho$ line shape and a new limit on $\rho/\omega$ interference in hadron interactions.

Observation of a Resonance inB+→K+μ+μ−Decays at Low Recoil

A broad peaking structure is observed in the dimuon spectrum of B+ → K+ μ+ μ- decays in the kinematic region where the kaon has a low recoil against the dimuon system. The structure is consistent with interference between the B+ → K+ μ+ μ- decay and a resonance and has a statistical significance exceeding six standard deviations. The mean and width of the resonance are measured to be 4191(-8)(+9) MeV/c2 and 65(-16)(+22) MeV/c2, respectively, where the uncertainties include statistical and systematic contributions. These measurements are compatible with the properties of the ψ(4160) meson. First observations of both the decay B+ → ψ(4160)K+ and the subsequent decay ψ(4160) → μ+ μ- are reported. The resonant decay and the interference contribution make up 20% of the yield for dimuon masses above 3770 MeV/c2. This contribution is larger than theoretical estimates.

A first look at open charm production in Indium-Indium collisions at SPS energies

NA60 is an experiment at the CERN-SPS devoted to the study of dimuon production in heavy-ion and proton-nucleus collisions. The main topics under study are low mass vector meson production, J/\(\psi\) production and suppression, and the sources of the dimuon continuum in the mass range 1.2-2.7 GeV/c2. In 2003, NA60 collected \(\sim 230\) million dimuon events from Indium-Indium collisions. We present preliminary results of the analysis of this data sample in view of measuring the open charm contribution to the dimuon spectrum. Although we are still working on the final background subtraction procedure, we can already demonstrate that the detector performance is good enough to allow the separation of prompt dimuons from muon pairs originating in distant D \(\overline{\rm D}\) decays.

Excess of continuum dimuon production at masses between threshold and the $J/\Psi$ in S–W interactions at 200 GeV $/c$ /nucleon

Results are presented on dimuon production for invariant masses ranging from the dimuon threshold up to the $J/\Psi$ meson. Proton-tungsten and sulphur-tungsten interactions at 200 GeV $/c$ /nucleon were measured over a large kinematic region, using the HELIOS/3 dimuon spectrometer at the CERN SPS. In the continuum regions between the dimuon threshold and the $\rho/\omega$ mesons, and between the $\phi$ and $J/\Psi$ mesons, an excess is observed in S–W interactions relative to minimum bias p–W interactions. The observed excess is continuous over the explored mass range and has no apparent resonant structure. In the low mass region the dimuon yield increases by $(76\pm 4)$ % of the corresponding p–W dimuon spectrum, whereas in the higher mass region the excess amounts to $2.4\pm 0.3$ times the p–W yield. The observed excess for the low mass region follows an exponential transverse mass distribution with an average inverse slope parameter T of $(190\pm5)$ MeV $/c^2$ , constant for all but the lowest charged multiplicity interval and consistent with the slope of the excess in the higher mass region. In the invariant mass range of 1.35–2.5 GeV $/c^2$ the continuum dimuon mass distribution observed in p–W interactions can be quantitatively understood as a sum of three expected contributions (vector meson decays, semileptonic charm decays and Drell-Yan process), but both in central and in minimum bias S–W interactions an excess is observed with respect to these sources which does not depend on centrality, nor very strongly on the transverse momentum.

Excess of continuum dimuon production at masses between threshold and the

Results are presented on dimuon production for invariant masses ranging from the dimuon threshold up to the $J/\Psi$ meson. Proton-tungsten and sulphur-tungsten interactions at 200 GeV $/c$ /nucleon were measured over a large kinematic region, using the HELIOS/3 dimuon spectrometer at the CERN SPS. In the continuum regions between the dimuon threshold and the $\rho/\omega$ mesons, and between the $\phi$ and $J/\Psi$ mesons, an excess is observed in S–W interactions relative to minimum bias p–W interactions. The observed excess is continuous over the explored mass range and has no apparent resonant structure. In the low mass region the dimuon yield increases by $(76\pm 4)$ % of the corresponding p–W dimuon spectrum, whereas in the higher mass region the excess amounts to $2.4\pm 0.3$ times the p–W yield. The observed excess for the low mass region follows an exponential transverse mass distribution with an average inverse slope parameter T of $(190\pm5)$ MeV $/c^2$ , constant for all but the lowest charged multiplicity interval and consistent with the slope of the excess in the higher mass region. In the invariant mass range of 1.35–2.5 GeV $/c^2$ the continuum dimuon mass distribution observed in p–W interactions can be quantitatively understood as a sum of three expected contributions (vector meson decays, semileptonic charm decays and Drell-Yan process), but both in central and in minimum bias S–W interactions an excess is observed with respect to these sources which does not depend on centrality, nor very strongly on the transverse momentum.

The ϱ-peak in the dimuon spectrum as a clock for fireball lifetimes in relativistic nuclear collisions

We discuss the modification of the dipleton yield from vector mesons inside a nuclear fireball. We argue that the relative magnitude of the ϱ peak in the dilepton spectrum determines the lifetime of the hadron gas phase in nuclear collisions with an accuracy of about 1 fm/ c. We also find that the φ (ω + ϱ) ratio measured in the dimuon channel in 200 A GeV O + U and S + U collisions points towards a rather short-lived hadronic phase in these collisions.

Charged-gluon effects in dilepton production

The high-mass dimuon spectrum in proton-nucleus collisions and their transverse-momentum distributions are studied in the broken color gauge theory of Pati and Salam. Using the distribution function determined by Feynman and Field in deep-inelastic lepton-hadron scattering, the impulse approximation in the Drell-Yan scheme combined with the broken gauge theory of Pati and Salam fits the recent experimental data beautifully.