A search for partonic collective effects inside jets produced in proton-proton collisions is performed via correlation measurements of charged constituents using the CMS detector at the CERN LHC. The analysis uses data collected at a center-of-mass energy of $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. Jets are reconstructed with the anti-$k_\mathrm{T}$ algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity $\lvert\eta\rvert$$\lt$ 1.6. Two-particle correlations among the charged constituents within the jets are studied as functions of the particles' azimuthal angle and pseudorapidity separations ($\Delta\phi^*$ and $\Delta\eta^*$) in a jet coordinate basis, where constituents' $\eta^*$, $\phi^*$ are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to $N_\text{ch}^\mathrm{j}$$\approx$ 100. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for $\lvert\Delta\eta^*\rvert$$\gt$ 2. For low-multiplicity jets, the long-range elliptic anisotropic harmonic, $v^*_2$, is observed to decrease with $N_\text{ch}^\mathrm{j}$. This trend is well described by Monte Carlo event generators. However, a rising trend for $v^*_2$ emerges at $N_\text{ch}^\mathrm{j}$$\gtrsim$ 80, hinting at a possible onset of collective behavior, which is not reproduced by the models tested. This observation yields new insights into the dynamics of parton fragmentation processes in the vacuum.
Examples of two-particle angular correlations projected onto 1D $\Delta\phi^*$ for $\abs{\Delta\eta^*}>2$.
A continuous evolution of extracted two-particle Fourier coefficients $V^*_{N\Delta}$ as a function of $N_{ch}^{j}$.
The single-particle elliptic anisotropies $v^*_2$, as a function of $N_{ch}^{j}$.
In relativistic heavy-ion collisions, a global spin polarization, $P_\mathrm{H}$, of $\Lambda$ and $\bar{\Lambda}$ hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing $P_\mathrm{H}$ with decreasing $\sqrt{s_{NN}}$. A splitting between $\Lambda$ and $\bar{\Lambda}$ polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of $\sqrt{s_{NN}}=19.6$ and $27$ GeV in the RHIC Beam Energy Scan Phase II using the STAR detector, with an upper limit of $P_{\bar{\Lambda}}-P_{\Lambda}<0.24$% and $P_{\bar{\Lambda}}-P_{\Lambda}<0.35$%, respectively, at a 95% confidence level. We derive an upper limit on the na\"ive extraction of the late-stage magnetic field of $B<9.4\cdot10^{12}$ T and $B<1.4\cdot10^{13}$ T at $\sqrt{s_{NN}}=19.6$ and $27$ GeV, respectively, although more thorough derivations are needed. Differential measurements of $P_\mathrm{H}$ were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of $|y|<1$ and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.
The first-order event-plane resolution determined by the STAR EPD as a function of collision centrality is roughly doubled in comparison to previous analyses using the STAR BBC. We see $R_{\rm EP}^{(1)}$ peak for mid-central collisions.
The mid-central $P_{\rm H}$ measurements reported in this work are shown alongside previous measurements in the upper panel, and are consistent with previous measurements at the energies studied here. The difference between integrated $P_{\bar{\Lambda}}$ and $P_{\Lambda}$ is shown at $\sqrt{s_{\rm{NN}}}$=19.6 and 27 GeV alongside previous measurements in the lower panel. The splittings observed with these high-statistics data sets are consistent with zero. Statistical uncertainties are represented as lines while systematic uncertainties are represented as boxes. The previous $P_{\bar{\Lambda}}-P_{\Lambda}$ result at $\sqrt{s_{\rm NN}}=7.7$ GeV is outside the axis range, but is consistent with zero within $2\sigma$.
$P_{\rm H}$ measurements are shown as a function of collision centrality at $\sqrt{s_{\rm NN}}$=19.6 and 27 GeV. Statistical uncertainties are represented as lines while systematic uncertainties are represented as boxes. $P_{\rm H}$ increases with collision centrality at $\sqrt{s_{\rm NN}}$=19.6 and 27 GeV, as expected from an angular-momentum-driven phenomenon.
Global polarizations ($P$) of $\Lambda$ ($\bar{\Lambda}$) hyperons have been observed in non-central heavy-ion collisions. The strong magnetic field primarily created by the spectator protons in such collisions would split the $\Lambda$ and $\bar{\Lambda}$ global polarizations ($\Delta P = P_{\Lambda} - P_{\bar{\Lambda}} < 0$). Additionally, quantum chromodynamics (QCD) predicts topological charge fluctuations in vacuum, resulting in a chirality imbalance or parity violation in a local domain. This would give rise to an imbalance ($\Delta n = \frac{N_{\text{L}} - N_{\text{R}}}{\langle N_{\text{L}} + N_{\text{R}} \rangle} \neq 0$) between left- and right-handed $\Lambda$ ($\bar{\Lambda}$) as well as a charge separation along the magnetic field, referred to as the chiral magnetic effect (CME). This charge separation can be characterized by the parity-even azimuthal correlator ($\Delta\gamma$) and parity-odd azimuthal harmonic observable ($\Delta a_{1}$). Measurements of $\Delta P$, $\Delta\gamma$, and $\Delta a_{1}$ have not led to definitive conclusions concerning the CME or the magnetic field, and $\Delta n$ has not been measured previously. Correlations among these observables may reveal new insights. This paper reports measurements of correlation between $\Delta n$ and $\Delta a_{1}$, which is sensitive to chirality fluctuations, and correlation between $\Delta P$ and $\Delta\gamma$ sensitive to magnetic field in Au+Au collisions at 27 GeV. For both measurements, no correlations have been observed beyond statistical fluctuations.
Figure 1
Figure 2ab
Figure 2c
The deconfined quark-gluon plasma (QGP) created in relativistic heavy-ion collisions enables the exploration of the fundamental properties of matter under extreme conditions. Non-central collisions can produce strong magnetic fields on the order of $10^{18}$ Gauss, which offers a probe into the electrical conductivity of the QGP. In particular, quarks and anti-quarks carry opposite charges and receive contrary electromagnetic forces that alter their momenta. This phenomenon can be manifested in the collective motion of final-state particles, specifically in the rapidity-odd directed flow, denoted as $v_1(\mathsf{y})$. Here we present the charge-dependent measurements of $dv_1/d\mathsf{y}$ near midrapidities for $\pi^{\pm}$, $K^{\pm}$, and $p(\bar{p})$ in Au+Au and isobar ($_{44}^{96}$Ru+$_{44}^{96}$Ru and $_{40}^{96}$Zr+$_{40}^{96}$Zr) collisions at $\sqrt{s_{\rm NN}}=$ 200 GeV, and in Au+Au collisions at 27 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The combined dependence of the $v_1$ signal on collision system, particle species, and collision centrality can be qualitatively and semi-quantitatively understood as several effects on constituent quarks. While the results in central events can be explained by the $u$ and $d$ quarks transported from initial-state nuclei, those in peripheral events reveal the impacts of the electromagnetic field on the QGP. Our data put valuable constraints on the electrical conductivity of the QGP in theoretical calculations.
Directed flow of $p$ and $\bar{p}$ vs rapidity in Au+Au 200 GeV 50-80% centrality.
Directed flow of $p$ and $\bar{p}$ vs rapidity in Zr+Zr and Ru+Ru 200 GeV (combined) 50-80% centrality.
Directed flow of $p$ and $\bar{p}$ vs rapidity in Au+Au 27 GeV 50-80% centrality.
Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange, a full understanding of the strong interaction remains a major challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distances on the order of the size of the proton. Here we show that in relativistic heavy-ion collisions, where quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely $\phi$ and $K^{*0}$, emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for $\phi$ is unexpectedly large, while that for $K^{*0}$ is consistent with zero. The observed spin-alignment pattern and magnitude for the $\phi$ cannot be explained by conventional mechanisms, while a model with a connection to strong force fields, i.e. an effective proxy description within the Standard Model and Quantum Chromodynamics, accommodates the current data. This connection, if fully established, will open a potential new avenue for studying the behaviour of strong force fields.
Global spin alignment of $\phi$ and $K^{*0}$ vector mesons in heavy-ion collisions. The measured matrix element $\rho_{00}$ as a function of beam energy for the $\phi$ and $K^{*0}$ vector mesons within the indicated windows of centrality, transverse momentum ($p_T$) and rapidity ($y$). The open symbols indicate ALICE results for Pb+Pb collisions at 2.76 TeV at $p_{T}$ values of 2.0 and 1.4 GeV/c for the $\phi$ and $K^{*0}$ mesons, respectively, corresponding to the $p_{T}$ bin nearest to the mean $p_{T}$ for the 1.0 – 5.0 GeV/$c$ range assumed for each meson in the present analysis. The red solid curve is a fit to data in the range of $\sqrt{s_{NN}} = 19.6$ to 200 GeV, based on a theoretical calculation with a $\phi$-meson field. Parameter sensitivity of $\rho_{00}$ to the $\phi$-meson field is shown in Ref.5. The red dashed line is an extension of the solid curve with the fitted parameter $G_s^{(y)}$. The black dashed line represents $\rho_{00}=1/3.$
Global spin alignment of $\phi$ and $K^{*0}$ vector mesons in heavy-ion collisions. The measured matrix element $\rho_{00}$ as a function of beam energy for the $\phi$ and $K^{*0}$ vector mesons within the indicated windows of centrality, transverse momentum ($p_T$) and rapidity ($y$). The open symbols indicate ALICE results for Pb+Pb collisions at 2.76 TeV at $p_{T}$ values of 2.0 and 1.4 GeV/c for the $\phi$ and $K^{*0}$ mesons, respectively, corresponding to the $p_{T}$ bin nearest to the mean $p_{T}$ for the 1.0 – 5.0 GeV/$c$ range assumed for each meson in the present analysis. The red solid curve is a fit to data in the range of $\sqrt{s_{NN}} = 19.6$ to 200 GeV, based on a theoretical calculation with a $\phi$-meson field. Parameter sensitivity of $\rho_{00}$ to the $\phi$-meson field is shown in Ref.5. The red dashed line is an extension of the solid curve with the fitted parameter $G_s^{(y)}$. The black dashed line represents $\rho_{00}=1/3.$
Example of combinatorial background subtracted invariant mass distributions and the extracted yields as a function of $\cos \theta^*$ for $\phi$ and $K^{*0}$ mesons. \textbf{a)} example of $\phi \rightarrow K^+ + K^-$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{b)} example of $K^{*0} (\overline{K^{*0}}) \rightarrow K^{-} \pi^{+} (K^{+} \pi^{-})$ invariant mass distributions, with combinatorial background subtracted, integrated over $\cos \theta^*$; \textbf{c)} extracted yields of $\phi$ as a function of $\cos \theta^*$; \textbf{d)} extracted yields of $K^{*0}$ as a function of $\cos \theta^*$.
The measurement of direct photons from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=39$ and 62.4 GeV in the transverse-momentum range $0.4<p_T<3$ Gev/$c$ is presented by the PHENIX collaboration at the Relativistic Heavy Ion Collider. A significant direct-photon yield is observed in both collision systems. A universal scaling is observed when the direct-photon $p_T$ spectra for different center-of-mass energies and for different centrality selections at $\sqrt{s_{_{NN}}}=62.4$ GeV is scaled with $(dN_{\rm ch}/d\eta)^{\alpha}$ for $\alpha=1.21{\pm}0.04$. This scaling also holds true for direct-photon spectra from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV measured earlier by PHENIX, as well as the spectra from Pb$+$Pb at $\sqrt{s_{_{NN}}}=2760$ GeV published by ALICE. The scaling power $\alpha$ seems to be independent of $p_T$, center of mass energy, and collision centrality. The spectra from different collision energies have a similar shape up to $p_T$ of 2 GeV/$c$. The spectra have a local inverse slope $T_{\rm eff}$ increasing with $p_T$ of $0.174\pm0.018$ GeV/$c$ in the range $0.4<p_T<1.3$ GeV/$c$ and increasing to $0.289\pm0.024$ GeV/$c$ for $0.9<p_T<2.1$ GeV/$c$. The observed similarity of low-$p_T$ direct-photon production from $\sqrt{s_{_{NN}}}= 39$ to 2760 GeV suggests a common source of direct photons for the different collision energies and event centrality selections, and suggests a comparable space-time evolution of direct-photon emission.
$R_{\gamma}$ for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For $62.4$ GeV also centrality bins of 0-20% (c) and 20-40% (d) are shown. Data points are shown with statistical (bar) and systematic uncertainties (box)
$R_{\gamma}$ for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For $62.4$ GeV also centrality bins of 0-20% (c) and 20-40% (d) are shown. Data points are shown with statistical (bar) and systematic uncertainties (box)
Direct photon spectra for minimum bias (0-86%) Au+Au collision at $\sqrt{s_{NN}} = 62.4$ GeV (a) and $39$ GeV (b). For $62.4$ GeV also centrality bins of 0-20% (c) and 20-40% (d) are shown. Data points are shown with statistical and systematic uncertainties, unless the central value is negative (arrows) or is consistent with zero within the statistical uncertainties (arrows with data point). In these cases upper limit with CL = 95$%$ are given.
The PHENIX collaboration presents first measurements of low-momentum ($0.4<p_T<3$ GeV/$c$) direct-photon yields from Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=39 and 62.4 GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=200. Analyzing the photon yield as a function of the experimental observable $dN_{\rm ch}/d\eta$ reveals that the low-momentum ($>$1\,GeV/$c$) direct-photon yield $dN_{\gamma}^{\rm dir}/d\eta$ is a smooth function of $dN_{\rm ch}/d\eta$ and can be well described as proportional to $(dN_{\rm ch}/d\eta)^\alpha$ with $\alpha{\sim}$1.25. This new scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and Large Hadron Collider, for centrality selected samples, as well as for different, $A$$+$$A$ collision systems. At a given beam energy the scaling also holds for high $p_T$ ($>5$\,GeV/$c$) but when results from different collision energies are compared, an additional $\sqrt{s_{_{NN}}}$-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.
Direct photon spectra(Physical Review C87, 054907 (2013)) normalized by $(dN_{ch}/d\eta)^{1.25}$ for in p+p at $\sqrt{s_{NN}}$= 200 GeV.
Direct photon spectra(Physics Letters B94, 106 (1980)) normalized by $(dN_{ch}/d\eta)^{1.25}$ for in p+p at $\sqrt{s_{NN}}$= 62.4 GeV.
Direct photon spectra(Nucl. Part. Phys. 23, A1 (1997) and Sov. J. Nucl. Phys. 51, 836 (1990)) normalized by $(dN_{ch}/d\eta)^{1.25}$ for in p+p at $\sqrt{s_{NN}}$= 63 GeV.
A search for physics beyond the Standard Model, in final states with at least one high transverse momentum charged lepton (electron or muon) and two additional high transverse momentum leptons or jets, is performed using 3.2 fb$^{-1}$ of proton--proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015 at $\sqrt{s}=13$ TeV. The upper end of the distribution of the scalar sum of the transverse momenta of leptons and jets is sensitive to the production of high-mass objects. No excess of events beyond Standard Model predictions is observed. Exclusion limits are set for models of microscopic black holes with two to six extra dimensions.
Background fit results for regions SR-2TeV ( sumPT > 2 TeV) and SR-3TeV ( sumPT > 3 TeV) for the electron and muons channels. The errors shown are the statistical plus systematic uncertainties. The uncertainty in the total background count includes correlations between nuisance parameters and so does not reflect a quadrature sum of the uncertainties in the individual background components.
The sumPT distribution in the W+jets control region (electron channel). Expected background yields are given along with the total background uncertainty. The ttbar, W+jets and Z+jets backgrounds are normalised by the factors 0.95, 0.81 and 1.01 as obtained from the background likelihood fit. The single-top-quark and diboson background normalisations are taken from the simulation. The multijet background is obtained using a data-driven method. Additionally, the likelihood fit may constrain nuisance parameters for certain systematic uncertainties, altering the normalisation and shape of some of the distributions.
The sumPT distribution in the W+jets control region (muon channel). Expected background yields are given along with the total background uncertainty. The ttbar, W+jets and Z+jets backgrounds are normalised by the factors 0.95, 0.81 and 1.01 as obtained from the background likelihood fit. The single-top-quark and diboson background normalisations are taken from the simulation. The multijet background is obtained using a data-driven method. Additionally, the likelihood fit may constrain nuisance parameters for certain systematic uncertainties, altering the normalisation and shape of some of the distributions.
An inclusive search for a new-physics signature of lepton-jet resonances has been performed by the ATLAS experiment. Scalar leptoquarks, pair-produced in $pp$ collisions at $\sqrt{s}$ = 13 TeV at the Large Hadron Collider, have been considered. An integrated luminosity of 3.2 fb$^{-1}$, corresponding to the full 2015 dataset was used. First (second) generation leptoquarks were sought in events with two electrons (muons) and two or more jets. The observed event yield in each channel is consistent with Standard Model background expectations. The observed (expected) lower limits on the leptoquark mass at 95% confidence level are 1100 GeV and 1050 GeV (1160 GeV and 1040 GeV) for first and second generation leptoquarks, respectively, assuming a branching ratio into a charged lepton and a quark of 100%. Upper limits on the aforementioned branching ratio are also given as a function of leptoquark mass. Compared with the results of earlier ATLAS searches, the sensitivity is increased for leptoquark masses above 860 GeV, and the observed exclusion limits confirm and extend the published results.
Normalisation factors for the main backgrounds obtained from the combined fit in each of the channels. The total uncertainty is given.
Search for the first generation leptoquarks (LQs). Event yields in the Z control region (CR), ttbar CR and in the signal region (SR). Each CR is treated as one bin in the profile likelihood fit. The SR is split to 7 bins according to $m_{\text{LQ}}^{\text{min}}$ for the fit. The table below shows the total number of events in each CR. For the SR, it shows the number of events per 100 GeV as a function of $m_{\text{LQ}}^{\text{min}}$. The background expectations are scaled by a scale factor extracted from the fit. However, the uncertainties shown are the pre-fit ones. The data event yield uncertainty is statistical (gaussian). The background uncertainty consists of all the experimental and theoretical components summed in quadrature. The uncertainty of the fit-extracted background scale factor is also added in quadrature.
Search for the second generation leptoquarks (LQs). Event yields in the Z control region (CR), ttbar CR and in the signal region (SR). Each CR is treated as one bin in the profile likelihood fit. The SR is split to 7 bins according to $m_{\text{LQ}}^{\text{min}}$ for the fit. The table below shows the total number of events in each CR. For the SR, it shows the number of events per 100 GeV as a function of $m_{\text{LQ}}^{\text{min}}$. The background expectations are scaled by a scale factor extracted from the fit. However, the uncertainties shown are the pre-fit ones. The data event yield uncertainty is statistical (gaussian). The background uncertainty consists of all the experimental and theoretical components summed in quadrature. The uncertainty of the fit-extracted background scale factor is also added in quadrature.
Measurements of midrapidity charged particle multiplicity distributions, $dN_{\rm ch}/d\eta$, and midrapidity transverse-energy distributions, $dE_T/d\eta$, are presented for a variety of collision systems and energies. Included are distributions for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$, 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 GeV, Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ and 62.4 GeV, Cu$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, U$+$U collisions at $\sqrt{s_{_{NN}}}=193$ GeV, $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV, and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Centrality-dependent distributions at midrapidity are presented in terms of the number of nucleon participants, $N_{\rm part}$, and the number of constituent quark participants, $N_{q{\rm p}}$. For all $A$$+$$A$ collisions down to $\sqrt{s_{_{NN}}}=7.7$ GeV, it is observed that the midrapidity data are better described by scaling with $N_{q{\rm p}}$ than scaling with $N_{\rm part}$. Also presented are estimates of the Bjorken energy density, $\varepsilon_{\rm BJ}$, and the ratio of $dE_T/d\eta$ to $dN_{\rm ch}/d\eta$, the latter of which is seen to be constant as a function of centrality for all systems.
Transverse energy in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV
Multiplicity in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV
Transverse energy in Au+Au collisions at $\sqrt{s_{NN}}$ = 130 GeV
Measurements of the fractional momentum loss ($S_{\rm loss}\equiv{\delta}p_T/p_T$) of high-transverse-momentum-identified hadrons in heavy ion collisions are presented. Using $\pi^0$ in Au$+$Au and Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=62.4$ and 200 GeV measured by the PHENIX experiment at the Relativistic Heavy Ion Collider and and charged hadrons in Pb$+$Pb collisions measured by the ALICE experiment at the Large Hadron Collider, we studied the scaling properties of $S_{\rm loss}$ as a function of a number of variables: the number of participants, $N_{\rm part}$, the number of quark participants, $N_{\rm qp}$, the charged-particle density, $dN_{\rm ch}/d\eta$, and the Bjorken energy density times the equilibration time, $\varepsilon_{\rm Bj}\tau_{0}$. We find that the $p_T$ where $S_{\rm loss}$ has its maximum, varies both with centrality and collision energy. Above the maximum, $S_{\rm loss}$ tends to follow a power-law function with all four scaling variables. The data at $\sqrt{s_{_{NN}}}$=200 GeV and 2.76 TeV, for sufficiently high particle densities, have a common scaling of $S_{\rm loss}$ with $dN_{\rm ch}/d\eta$ and $\varepsilon_{\rm Bj}\tau_{0}$, lending insight on the physics of parton energy loss.
Global variables for Au+Au collisions at RHIC from PHENIX.
Global variables for Au+Au collisions at RHIC from PHENIX.
Global variables for Cu+Cu collisions at RHIC from PHENIX.
Using data collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider, we measure the energy dependence of the $e^+e^- \to h_b(nP)\pi^+\pi^-$ $(n=1,2)$ cross sections from thresholds up to $11.02\,$GeV. We find clear $\Upsilon(10860)$ and $\Upsilon(11020)$ peaks with little or no continuum contribution. We study the resonant substructure of the $\Upsilon(11020) \to h_b(nP)\pi^+\pi^-$ transitions and find evidence that they proceed entirely via the intermediate isovector states $Z_b(10610)$ and $Z_b(10650)$. The relative fraction of these states is loosely constrained by the current data: the hypothesis that only $Z_b(10610)$ is produced is excluded at the level of 3.3 standard deviations, while the hypothesis that only $Z_b(10650)$ is produced is not excluded at a significant level.
Center-of-mass energies, integrated luminosities and Born cross sections for all energy points. The first uncertainty in the energy is uncorrelated, the second is correlated. The three uncertainties in the cross sections are statistical, uncorrelated systematic and correlated systematic.
We report the measurement of cumulants ($C_n, n=1\ldots4$) of the net-charge distributions measured within pseudorapidity ($|\eta|<0.35$) in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=7.7-200$ GeV with the PHENIX experiment at the Relativistic Heavy Ion Collider. The ratios of cumulants (e.g. $C_1/C_2$, $C_3/C_1$) of the net-charge distributions, which can be related to volume independent susceptibility ratios, are studied as a function of centrality and energy. These quantities are important to understand the quantum-chromodynamics phase diagram and possible existence of a critical end point. The measured values are very well described by expectation from negative binomial distributions. We do not observe any nonmonotonic behavior in the ratios of the cumulants as a function of collision energy. The measured values of $C_1/C_2 = \mu/\sigma^2$ and $C_3/C_1 = S\sigma^3/\mu$ can be directly compared to lattice quantum-chromodynamics calculations and thus allow extraction of both the chemical freeze-out temperature and the baryon chemical potential at each center-of-mass energy.
Efficiency corrected cumulants of net-charge distributions as a function of $\langle N_{part} \rangle$ from Au+Au collisions at different collision energies.
Efficiency corrected cumulants of net-charge distributions as a function of $\langle N_{part} \rangle$ from Au+Au collisions at different collision energies.
Efficiency corrected cumulants of net-charge distributions as a function of $\langle N_{part} \rangle$ from Au+Au collisions at different collision energies.
We present measurements of Collins asymmetries in the inclusive process $e^+e^- \rightarrow h_1 h_2 X$, $h_1h_2=KK,\, K\pi,\, \pi\pi$, at the center-of-mass energy of 10.6 GeV, using a data sample of 468 fb$^{-1}$ collected by the BaBar experiment at the PEP-II $B$ factory at SLAC National Accelerator Center. Considering hadrons in opposite thrust hemispheres of hadronic events, we observe clear azimuthal asymmetries in the ratio of unlike- to like-sign, and unlike- to all charged $h_1 h_2$ pairs, which increase with hadron energies. The $K\pi$ asymmetries are similar to those measured for the $\pi\pi$ pairs, whereas those measured for high-energy $KK$ pairs are, in general, larger.
Light quark ($uds$) Collins asymmetries obtained by fitting the U/L and U/C double ratios as a function of ($z_1$,$z_2$) for kaon pairs. In the first column, the $z$ bins and their respective mean values for the kaon in one hemisphere are reported; in the following column, the same variables for the second kaon are shown; in the third column the mean value of $\sin^2\theta_{th}/(1+\cos^2\theta_{th})$ is summarized, calculated in the RF12 frame; in the last two columns the asymmetry results are summarized. The mean values of the quantities reported in the table are calculated by summing the corresponding values for each $KK$ pair and dividing by the number of $KK$ pairs that fall into each ($z_1$,$z_2$) interval. Note that the $A^{UL}$ and $A^{UC}$ results are strongly correlated since they are obtained by using the same data set.
Light quark ($uds$) Collins asymmetries obtained by fitting the U/L and U/C double ratios as a function of ($z_1$,$z_2$) for kaon pairs. In the first column, the $z$ bins and their respective mean values for the kaon in one hemisphere are reported; in the following column, the same variables for the second kaon are shown; in the third column the mean value of $\sin^2\theta_{2}/(1+\cos^2\theta_{2})$ is summarized, calculated in the RF0 frame; in the last two columns the asymmetry results are summarized. The mean values of the quantities reported in the table are calculated by summing the corresponding values for each $KK$ pair and dividing by the number of $KK$ pairs that fall into each ($z_1$,$z_2$) interval. Note that the $A^{UL}$ and $A^{UC}$ results are strongly correlated since they are obtained by using the same data set.
Light quark ($uds$) Collins asymmetries obtained by fitting the U/L and U/C double ratios as a function of ($z_1$,$z_2$) for $K\pi$ hadron pairs. In the first column, the $z$ bins and their respective mean values for the hadron ($K$ or $\pi$) in one hemisphere are reported; in the following column, the same variables for the second hadron ($K$ or $\pi$) are shown; in the third column the mean value of $\sin^2\theta_{th}/(1+\cos^2\theta_{th})$ is summarized, calculated in the RF12 frame; in the last two columns the asymmetry results are summarized. The mean values of the quantities reported in the table are calculated by summing the corresponding values for each $K\pi$ pair and dividing by the number of $K\pi$ pairs that fall into each ($z_1$,$z_2$) interval. Note that the $A^{UL}$ and $A^{UC}$ results are strongly correlated since they are obtained by using the same data set.
We have studied the dependence of azimuthal anisotropy $v_2$ for inclusive and identified charged hadrons in Au$+$Au and Cu$+$Cu collisions on collision energy, species, and centrality. The values of $v_2$ as a function of transverse momentum $p_T$ and centrality in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV and 62.4 GeV are the same within uncertainties. However, in Cu$+$Cu collisions we observe a decrease in $v_2$ values as the collision energy is reduced from 200 to 62.4 GeV. The decrease is larger in the more peripheral collisions. By examining both Au$+$Au and Cu$+$Cu collisions we find that $v_2$ depends both on eccentricity and the number of participants, $N_{\rm part}$. We observe that $v_2$ divided by eccentricity ($\varepsilon$) monotonically increases with $N_{\rm part}$ and scales as ${N_{\rm part}^{1/3}}$. The Cu$+$Cu data at 62.4 GeV falls below the other scaled $v_{2}$ data. For identified hadrons, $v_2$ divided by the number of constituent quarks $n_q$ is independent of hadron species as a function of transverse kinetic energy $KE_T=m_T-m$ between $0.1<KE_T/n_q<1$ GeV. Combining all of the above scaling and normalizations, we observe a near-universal scaling, with the exception of the Cu$+$Cu data at 62.4 GeV, of $v_2/(n_q\cdot\varepsilon\cdot N^{1/3}_{\rm part})$ vs $KE_T/n_q$ for all measured particles.
$v_2$ for inclusive charged hadrons in Au+Au at $\sqrt{s_{NN}}$ = 200 GeV.
$v_2$ for inclusive charged hadrons in Au+Au at $\sqrt{s_{NN}}$ = 200 GeV.
$v_2$ for inclusive charged hadrons in Au+Au at $\sqrt{s_{NN}}$ = 200 GeV.
We present measurements of the inclusive production of antideuterons in $e^+e^-$ annihilation into hadrons at $\approx 10.58 \mathrm{\,Ge\kern -0.1em V}$ center-of-mass energy and in $\Upsilon(1S,2S,3S)$ decays. The results are obtained using data collected by the BABAR detector at the PEP-II electron-positron collider. Assuming a fireball spectral shape for the emitted antideuteron momentum, we find $\mathcal{B}(\Upsilon(1S) \to \bar{d}X) = (2.81 \pm 0.49 \mathrm{(stat)} {}^{+0.20}_{-0.24} \mathrm{(syst)})/! \times /! 10^{-5}$, $\mathcal{B}(\Upsilon(2S) \to \bar{d}X) = (2.64 \pm 0.11 \mathrm{(stat)} {}^{+0.26}_{-0.21} \mathrm{(syst)})/! \times /! 10^{-5}$, $\mathcal{B}(\Upsilon(3S) \to \bar{d}X) = (2.33 \pm 0.15 \mathrm{(stat)} {}^{+0.31}_{-0.28} \mathrm{(syst)})/! \times /! 10^{-5}$, and $\sigma (e^+e^- \to \bar{d}X) = (9.63 \pm 0.41 \mathrm{(stat)} {}^{+1.17}_{-1.01} \mathrm{(syst)}) \mbox{\,fb}$.
The rate of antideuteron production from the decay of UPSILON(3S).
The rate of antideuteron production from the decay of UPSILON(2S).
The rate of antideuteron production from the decay of UPSILON(1S).
We report results on studies of the e+e- annihilation into three-body Y(nS)pi+pi- (n=1,2,3) final states including measurements of cross sections and the full amplitude analysis. The cross sections measured at sqrt(s)=10.865 GeV and corrected for the initial state radiation are sigma(e+e-=>Y(1S)pi+pi-)=(2.27+-0.12+-0.14) pb, sigma(e+e-=>Y(2S)pi+pi-)=(4.07+-0.16+-0.45) pb, and sigma(e+e-=>Y(3S)pi+pi-)=(1.46+-0.09+-0.16) pb. Amplitude analysis of the three-body Y(nS)pi+pi- final states strongly favors I^G(J^P)=1^+(1^+) quantum-number assignments for the two bottomonium-like Zb+- states, recently observed in the Y(nS)pi+- and hb(mP)pi+- (m=1,2) decay channels. The results are obtained with a $121.4 1/fb data sample collected with the Belle detector at the KEKB asymmetric-energy e+e- collider.
The measured cross section and visible cross section for the three-body transition E+ E- --> UPSILON(1S) PI+ PI-.
The measured cross section and visible cross section for the three-body transition E+ E- --> UPSILON(2S) PI+ PI-.
The measured cross section and visible cross section for the three-body transition E+ E- --> UPSILON(3S) PI+ PI-.
Measurements of the midrapidity transverse energy distribution, $d\Et/d\eta$, are presented for $p$$+$$p$, $d$$+$Au, and Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV and additionally for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=62.4$ and 130 GeV. The $d\Et/d\eta$ distributions are first compared with the number of nucleon participants $N_{\rm part}$, number of binary collisions $N_{\rm coll}$, and number of constituent-quark participants $N_{qp}$ calculated from a Glauber model based on the nuclear geometry. For Au$+$Au, $\mean{d\Et/d\eta}/N_{\rm part}$ increases with $N_{\rm part}$, while $\mean{d\Et/d\eta}/N_{qp}$ is approximately constant for all three energies. This indicates that the two component ansatz, $dE_{T}/d\eta \propto (1-x) N_{\rm part}/2 + x N_{\rm coll}$, which has been used to represent $E_T$ distributions, is simply a proxy for $N_{qp}$, and that the $N_{\rm coll}$ term does not represent a hard-scattering component in $E_T$ distributions. The $dE_{T}/d\eta$ distributions of Au$+$Au and $d$$+$Au are then calculated from the measured $p$$+$$p$ $E_T$ distribution using two models that both reproduce the Au$+$Au data. However, while the number-of-constituent-quark-participant model agrees well with the $d$$+$Au data, the additive-quark model does not.
Et EMC distributions for sqrt(sNN) = 62.4 GeV Au+Au collisions shown in 5% wide centrality bins.
Et EMC distributions for sqrt(sNN) = 62.4 GeV Au+Au collisions shown in 5% wide centrality bins.
Et EMC distributions for sqrt(sNN) = 62.4 GeV Au+Au collisions shown in 5% wide centrality bins.
Measurements of transverse-single-spin asymmetries ($A_{N}$) in $p$$+$$p$ collisions at $\sqrt{s}=$62.4 and 200 GeV with the PHENIX detector at RHIC are presented. At midrapidity, $A_{N}$ is measured for neutral pion and eta mesons reconstructed from diphoton decay, and at forward rapidities, neutral pions are measured using both diphotons and electromagnetic clusters. The neutral-pion measurement of $A_{N}$ at midrapidity is consistent with zero with uncertainties a factor of 20 smaller than previous publications, which will lead to improved constraints on the gluon Sivers function. At higher rapidities, where the valence quark distributions are probed, the data exhibit sizable asymmetries. In comparison with previous measurements in this kinematic region, the new data extend the kinematic coverage in $\sqrt{s}$ and $p_T$, and it is found that the asymmetries depend only weakly on $\sqrt{s}$. The origin of the forward $A_{N}$ is presently not understood quantitatively. The extended reach to higher $p_T$ probes the transition between transverse momentum dependent effects at low $p_T$ and multi-parton dynamics at high $p_T$.
Neutral pion $A_N$ at $\sqrt{s} = 62.4$ GeV as a function of $x_F$ in pseudorapidity $3.1 < |\eta| < 3.5$, with statistical and systematic uncertainties.
Neutral pion $A_N$ at $\sqrt{s} = 62.4$ GeV as a function of $x_F$ in pseudorapidity $3.5 < |\eta| < 3.8$, with statistical and systematic uncertainties.
Neutral pion $A_N$ at $\sqrt{s}$ = 62.4 GeV as function of transverse momentum $p_T$.
Inclusive production cross sections of $\pi^\pm$, $K^\pm$ and $p\bar{p}$ per hadronic $e^+e^-$ annihilation event in $e^+e^-$ are measured at a center-of-mass energy of 10.54 GeV, using a relatively small sample of very high quality data from the BaBar experiment at the PEP-II $B$-factory at the SLAC National Accelerator Laboratory. The drift chamber and Cherenkov detector provide clean samples of identified $\pi^\pm$, $K^\pm$ and $p\bar{p}$ over a wide range of momenta. Since the center-of-mass energy is below the threshold to produce a $B\bar{B}$ pair, with $B$ a bottom-quark meson, these data represent a pure $e^+e^- \rightarrow q\bar{q}$ sample with four quark flavors, and are used to test QCD predictions and hadronization models. Combined with measurements at other energies, in particular at the $Z^0$ resonance, they also provide precise constraints on the scaling properties of the hadronization process over a wide energy range.
Differential cross section for prompt PI+-, K+- and PBAR/P production.
Differential cross section for conventional PI+-, K+- and PBAR/P production.
Integrated cross sections for prompt PI+-, K+- and PBAR/P production. The second (sys) error is the uncertainty due to the model dependence of the extrapolation.
Measurements of inclusive differential cross sections for charged pion and kaon production in electron-positron annihilation have been carried out at a center-of-mass energy of Q = 10.52 GeV. The measurements were performed with the Belle detector at the KEKB electron-positron collider using a data sample containing 113 million e+e- -> qqbar events, where q={u,d,s,c}. We present charge-integrated differential cross sections d\sigma_h+-/dz for h+- = pi+-, K+- as a function of the relative hadron energy z = 2*E_h / sqrt{s} from 0.2 to 0.98. The combined statistical and systematic uncertainties for pi+- (K+-) are 4% (4%) at z ~ 0.6 and 15% (24%) at z ~ 0.9. The cross sections are the first measurements of the z-dependence of pion and kaon production for z > 0.7 as well as the first precision cross section measurements at a center-of-mass energy far below the Z^0 resonance used by the experiments at LEP and SLC.
Measured charged-integrated differential cross sections for charged pion and kaon production as a function of the fractional hadron energy Z (=2*Eh/sqrt(s)).
The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at sqrt(s_NN)=200 GeV. The p_T of the photon is an excellent approximation to the initial p_T of the jet and the ratio z_T=p_T^h/p_T^\gamma is used as a proxy for the jet fragmentation function. A statistical subtraction is used to extract the direct photon-hadron yields in Au+Au collisions while a photon isolation cut is applied in p+p. I_ AA, the ratio of jet fragment yield in Au+Au to that in p+p, indicates modification of the jet fragmentation function. Suppression, most likely due to energy loss in the medium, is seen at high z_T. The fragment yield at low z_T is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.
Direct photon-hadron pair per-trigger yields vs Delta-phi (Au+Au and p+p)
Integrated per-trigger yields and I_AA vs xi
Integrated per-trigger yields and I_AA vs xi
We present measurements of the J/psi invariant yields in sqrt(s_NN)=39 and 62.4 GeV Au+Au collisions at forward rapidity (1.2<|y|<2.2). Invariant yields are presented as a function of both collision centrality and transverse momentum. Nuclear modifications are obtained for central relative to peripheral Au+Au collisions (R_CP) and for various centrality selections in Au+Au relative to scaled p+p cross sections obtained from other measurements (R_AA). The observed suppression patterns at 39 and 62.4 GeV are quite similar to those previously measured at 200 GeV. This similar suppression presents a challenge to theoretical models that contain various competing mechanisms with different energy dependencies, some of which cause suppression and others enhancement.
Estimates used for the 39- and 62.4-GeV $J/\psi$ $p$+$p$ cross sections along with their uncertainties.
$J/\psi$ invariant yields are shown for Au+Au collisions at 39 and 62.4 GeV as a function of the number of participating nucleons.
$J/\psi$ invariant yields are shown for Au+Au collisions at 39 and 62.4 GeV as a function of the number of participating nucleons.
Neutral-pion, pi^0, spectra were measured at midrapidity (|y|<0.35) in Au+Au collisions at sqrt(s_NN) = 39 and 62.4 GeV and compared to earlier measurements at 200 GeV in the 1<p_T<10 GeV/c transverse-momentum (p_T) range. The high-p_T tail is well described by a power law in all cases and the powers decrease significantly with decreasing center-of-mass energy. The change of powers is very similar to that observed in the corresponding p+p-collision spectra. The nuclear-modification factors (R_AA) show significant suppression and a distinct energy dependence at moderate p_T in central collisions. At high p_T, R_AA is similar for 62.4 and 200 GeV at all centralities. Perturbative-quantum-chromodynamics calculations that describe R_AA well at 200 GeV, fail to describe the 39 GeV data, raising the possibility that the relative importance of initial-state effects and soft processes increases at lower energies. A conclusion that the region where hard processes are dominant is reached only at higher p_T, is also supported by the x_T dependence of the x_T-scaling power-law exponent.
INVARIANT YIELDS
INVARIANT YIELDS
INVARIANT YIELDS
Unpolarized cross sections and double-helicity asymmetries of single-inclusive positive and negative charged hadrons at midrapidity from p+p collisions at sqrt(s)=62.4 GeV are presented. The PHENIX measurements for 1.0 < p_T < 4.5 GeV/c are consistent with perturbative QCD calculations at next-to-leading order in the strong coupling constant, alpha_s. Resummed pQCD calculations including terms with next-to-leading-log accuracy, yielding reduced theoretical uncertainties, also agree with the data. The double-helicity asymmetry, sensitive at leading order to the gluon polarization in a momentum-fraction range of 0.05 ~< x_gluon ~< 0.2, is consistent with recent global parameterizations disfavoring large gluon polarization.
Fit-function parameters for the efficiency curves for different hadron species.
Fit-function parameters for relative fractions of different species in the hadron mix.
Systematic uncertainties of cross-section measurements from various sources.
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