Away-side Yield Research Articles

Di-$\pi^0$ correlations in p+p, p+Al and p+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV at STAR

The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-\pi^0π0 produced at forward pseudorapidity (2.6<\eta<4.02.6<η<4.0) in p+p+pp, p+p+\mathrm{Al}Al and p+p+\mathrm{Au}Au collisions at a center-of-mass energy per nucleon-nucleon pair of 200 GeV. A clear suppression of the correlated away-side yields is observed in p+p+\mathrm{Au}Au and for the first time in p+p+\mathrm{Al}Al collisions, compared with the pp++pp data. The enhanced suppression found in p+p+\mathrm{Au}Au with respect to p+p+\mathrm{Al}Al collisions exhibits the saturation scale (Q_s^2Qs2) dependence on the mass number AA. The observed suppression of back-to-back pairs as a function of event activity and transverse momentum points to non-linear gluon dynamics arising at high parton densities.

Probing Jet Modification in Small Systems via π0-hadron Correlations in PHENIX

Previous measurements in small systems indicate that the modifications of jets relative to p+p collisions is small. However, more differential measurements may reveal if any modifications do exist. Di-hadron correlations may serve as a proxy for jets as the observed hadron is likely to originate from the same parton as the jet. In these proceedings, we compare π0-hadron correlations measured with PHENIX in small systems to those observed in p+p collisions to probe any initial state effects. Several systematic uncertainties cancel when comparing the ratio of the away-side yield to the near-side yield in d+Au and 3He+Au systems to p+p, revealing a quantifiable modification in these systems. In addition, pout distributions for the p+A collisions are measured. An increasing trend in the width of the pout distributions in p+Al and p+Au compared to p+p is observed as a function of Ncoll, the number of binary nucleon-nucleon collisions. The potential implications of these measurements will be discussed.

THE ONSET OF JET QUENCHING PHENOMENON

The aim of this study is to set a baseline for the jet quenching measurements of the Quark Gluon Plasma (QGP) formed in the large system size Nucleus-Nucleus (A-A) at top central collisions, via studying simulated small system size, Nucleon-Nucleon (N-N) collisions. The proton-proton (p-p) collisions were simulated using PYTHIA, at center of mass energies $\sqrt{s_{_{NN}}} = 200$ $GeV$ and $\sqrt{s_{_{NN}}} = 13$ $TeV$ corresponding to the available energies at the current collider experiments; the Relativistic Heavy Ion Collider (RHIC), and the Large Hadron Collider (LHC). At both energies, the two-particle azimuthal correlation functions have been considered, and the yield associated with the high transverse momentum ($p_{_{T}}$) particles were extracted at its near-side ($\Delta\phi \approx 0$) and away-side ($\Delta\phi \approx \pi$) at mid pseudo rapidity ($|\eta| \le 2$). The ratio between the near-side yields in the high multiplicity events to these of the low multiplicity events ($I_{_{HL}}^{^{N}}$), as well as, the ratio of the away-side yields ($I_{_{HL}}^{^{A}}$) were calculated at both energies as a function of the hadron fractional energy $z_{_{T}}$ of the high-$p_{_{T}}$ particle. At both energies, the values of $I_{_{HL}}^{^{N}}$ and $I_{_{HL}}^{^{A}}$ were less than unity, and of trivial dependence on $z_{_{T}}$. The values of $I_{_{HL}}^{^{A}}$ are always less than these of $I_{_{HL}}^{^{N}}$ at the same multiplicity and energy, and both quantities show a pattern of systematic decreases with the multiplicity. Such multiplicity dependence cannot be used neither to exclude the jet quenching nor to prove it in the high multiplicity events in p-p collisions, as the suppressions have been found at both sides, near and away of the high-$p_{_{T}}$ particle.

Onset of the Jet Quenching Phenomenon

The aim of this study is to set a baseline for the jet quenching measurements of the Quark Gluon Plasma (QGP) formed in the large system size Nucleus-Nucleus (A-A) at top central collisions, via studying simulated small system size, Nucleon-Nucleon (N-N) collisions. The proton-proton (p-p) collisions were simulated using PYTHIA, at center of mass energies $\sqrt{s_{_{NN}}} = 200$ $GeV$ and $\sqrt{s_{_{NN}}} = 13$ $TeV$ corresponding to the available energies at the current collider experiments; the Relativistic Heavy Ion Collider (RHIC), and the Large Hadron Collider (LHC). At both energies, the two-particle azimuthal correlation functions have been considered, and the yield associated with the high transverse momentum ($p_{_{T}}$) particles were extracted at its near-side ($\Delta\phi \approx 0$) and away-side ($\Delta\phi \approx \pi$) at mid pseudo rapidity ($|\eta| \le 2$). The ratio between the near-side yields in the high multiplicity events to these of the low multiplicity events ($I_{_{HL}}^{^{N}}$), as well as, the ratio of the away-side yields ($I_{_{HL}}^{^{A}}$) were calculated at both energies as a function of the hadron fractional energy $z_{_{T}}$ of the high-$p_{_{T}}$ particle. At both energies, the values of $I_{_{HL}}^{^{N}}$ and $I_{_{HL}}^{^{A}}$ were less than unity, and of trivial dependence on $z_{_{T}}$. The values of $I_{_{HL}}^{^{A}}$ are always less than these of $I_{_{HL}}^{^{N}}$ at the same multiplicity and energy, and both quantities show a pattern of systematic decreases with the multiplicity. Such multiplicity dependence cannot be used neither to exclude the jet quenching nor to prove it in the high multiplicity events in p-p collisions, as the suppressions have been found at both sides, near and away of the high-$p_{_{T}}$ particle.

Measurement of two-particle correlations with respect to second- and third-order event planes in Au + Au collisions at sNN=200 GeV

We present measurements of azimuthal correlations of charged hadron pairs in $\sqrt{s_{_{NN}}}=200$ GeV Au$+$Au collisions for the trigger and associated particle transverse-momentum ranges of $1<p_T^t<10$~GeV/$c$ and $0.5<p_T^a<10$~GeV/$c$. After subtraction of an underlying event using a model that includes higher-order azimuthal anisotropy $v_2$, $v_3$, and $v_4$, the away-side yield of the highest trigger-\pt ($p_T^t>4$~GeV/$c$) correlations is suppressed compared to that of correlations measured in $p$$+$$p$ collisions. At the lowest associated particle $p_T$ ($0.5<p_T^a<1$ GeV/$c$), the away-side shape and yield are modified relative to those in $p$$+$$p$ collisions. These observations are consistent with the scenario of radiative-jet energy loss. For the low-$p_T$ trigger correlations ($2<p_T^t<4$ GeV/$c$), a finite away-side yield exists and we explore the dependence of the shape of the away-side within the context of an underlying-event model. Correlations are also studied differentially versus event-plane angle $\Psi_2$ and $\Psi_3$. The angular correlations show an asymmetry when selecting the sign of the difference between the trigger-particle azimuthal angle and the $\Psi_2$ event plane. This asymmetry and the measured suppression of the pair yield out of plane is consistent with a path-length-dependent energy loss. No $\Psi_3$ dependence can be resolved within experimental uncertainties.

PHENIX results on direct photon–hadron correlations

Direct photon-hadron correlations are a golden channel to study parton in-medium energy loss in QGP. The modification of the effective fragmentation function for the away-side jet can be measured by comparing integrated away-side yields of direct photon-hadron pairs in heavy ion collisions to those in p+p. We measured per-trigger-yield of associated hadrons in Au+Au collisions and observed that there is a suppression compared to p+p for higher momentum fraction (zT) hadrons. This can be explained by the opacity of the hot medium: energetic partons from the initial hard scattering lose energy while traversing it. A yield enhancement on the other hand has been found at low zT (high ξ). Medium response is likely to be responsible for the enhanced production of these lower momentum particles.The same measurement is done in d+Au collisions and the result suggests that no modification to the fragmentation function is observed, given the current uncertainties.

Minijet analysis with two-particle correlations in p–Pb collisions with ALICE at LHC

Two-particle angular correlations between unidentified charged trigger and associated particles are measured by the ALICE detector in p–Pb collisions. The correlations are expressed as associated yield per trigger particle. Near-side and away-side per-trigger yields are studied as a function of multiplicity class. From the near-side and away-side yields, the uncorrelated seeds are calculated, which are proportional to the number of multiple parton interactions (MPIs) in Pythia. The uncorrelated seeds are also studied as a function of the number of binary collisions N coll from Glauber model in order to relate them to the number of hard scatterings.

Direct photon-hadron correlations with the PHENIX detector

Direct photon tagged jets, in the form of photon-hadron correlations, are well suited to provide unique insight into how jets interact with the quark gluon plasma. Since photons do not interact strongly with medium produced at RHIC, at leading order the measured photon momentum approximately balances that of the away-side parton. The modification to the effective fragmentation function can be measured by comparing integrated away-side yields in direct photon-hadron correlations in Au+Au collisions to those in p+p. By varying the away-side integration range, the angular dependence of the observed modification can also be studied. Direct photon-hadron correlations have been measured with PHENIX in p+p and Au+Au using a statistical subtraction technique to remove the decay photon contribution from the inclusive photon-hadron correlations, with an additional isolation cut applied in p+p to reduce uncertainties. Recently published results showing a significant modification to the fragmentation function with an azimuthal angle dependence indicative of broadening are discussed in the context of related theoretical models and complementary results from other experiments.

Long-range angular correlations on the near and away side in p–Pb collisions at [formula omitted

Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p–Pb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5<pT,assoc<pT,trig<4 GeV/c. The correlations are measured over two units of pseudorapidity and full azimuthal angle in different intervals of event multiplicity, and expressed as associated yield per trigger particle. Two long-range ridge-like structures, one on the near side and one on the away side, are observed when the per-trigger yield obtained in low-multiplicity events is subtracted from the one in high-multiplicity events. The excess on the near-side is qualitatively similar to that recently reported by the CMS Collaboration, while the excess on the away-side is reported for the first time. The two-ridge structure projected onto azimuthal angle is quantified with the second and third Fourier coefficients as well as by near-side and away-side yields and widths. The yields on the near side and on the away side are equal within the uncertainties for all studied event multiplicity and pT bins, and the widths show no significant evolution with event multiplicity or pT. These findings suggest that the near-side ridge is accompanied by an essentially identical away-side ridge.

Direct photon-hadron correlations measured with the PHENIX detector

Photon tagged jets are an excellent probe of the hot dense matter created in heavy ion collisions at RHIC. Since photons interact weakly and are not modified by the strongly coupled medium, the measured photon momentum approximately balances that of the away-side parton. Therefore, the modification of the fragmentation function can be obtained by comparing the spectra of the away-side yields in direct photon-hadron correlations in Au+Au collisions to those in p+p. Direct photon-hadron correlations are measured with the PHENIX detector in p+p and Au+Au collisions at s N N = 200 GeV at RHIC. We employ a statistical subtraction method to remove the contribution of decay photons from the inclusive photon-hadron correlations. For the p+p analysis, we developed event by event techniques to improve the systematic uncertainty in the baseline measurement. Compared to previous measurements, this Au+Au result extends to higher ξ (lower z T ) where a medium-induced modification of jet fragmentation is observed.

Universal Flow-Driven Conical Emission in Ultrarelativistic Heavy-Ion Collisions

The double-peak structure observed in soft-hard hadron correlations is commonly interpreted as a signature for a Mach cone generated by a supersonic jet interacting with the hot and dense medium created in ultrarelativistic heavy-ion collisions. We show that it can also arise due to averaging over many jet events in a transversally expanding background. We find that the jet-induced away-side yield does not depend on the details of the energy-momentum deposition in the plasma, the jet velocity, or the system size. Our claim can be experimentally tested by comparing soft-hard correlations induced by heavy-flavor jets with those generated by light-flavor jets.

Scaling behavior of the azimuthal and centrality dependencies of jet production in heavy-ion collisions

For heavy-ion collisions at RHIC a scaling behavior is found in the dependencies on azimuthal angle $\phi$ and impact parameter $b$ for pion production at high $p_T$ essentially independent of the hadronization process. The scaling variable is in terms of a dynamical path length $\xi$ that takes into account detailed properties of geometry, medium density and probability of hard scattering. It is shown in the recombination model how the nuclear modification factor depends on the average $\bar\xi(\phi,b)$. The data for $\pi^0$ production at $p_T=$ 4-5 and 7-8 GeV/c at RHIC are shown to exhibit the same scaling behavior as found in the model calculation. Extension to back-to-back dijet production has been carried out, showing the existence of $\bar\xi$ scaling also in the away-side yield per trigger. At LHC the hard-parton density can be high enough to realize the likelihood of recombination of shower partons arising from neighboring jets. It is shown that such 2-jet recombination can cause strong violation of $\bar\xi$ scaling. Furthermore, the large value of $R_{AA}$ that exceeds 1 can become a striking signature of such a hadronization process at high energy.

Modified fragmentation function in heavy ion collisions at RHIC via direct γ-jet measurements

The presented results are the first measurements at RHIC for direct γ-charged hadron azimuthal correlations in heavy ion collisions. We use these correlations to study the color charge density of the medium through the medium-induced modification of high-p T parton fragmentation. Azimuthal correlations of direct photons at high transverse energy (8<p T <16 GeV) with away-side charged hadrons of transverse momentum (3<p T <6 GeV/c) have been measured over a broad range of centrality for Au+Au collisions and p+p collisions at \(\sqrt{s_{NN}}=200\) GeV in the STAR experiment. A transverse shower shape analysis in the STAR Barrel Electromagnetic Calorimeter Shower Maximum Detector is used to discriminate between the direct photons and photons from the decays of high p T π 0. The per-trigger away-side yield of direct γ is smaller than from π 0 trigger at the same centrality class. Within the current uncertainty the I CP of direct γ and π 0 are similar.

Low- and intermediate-pT di-hadron distributions in Au + Au collisions at GeV from STAR

We present a study of low- and intermediate-pT correlated azimuthal angular distributions in Au+Au collisions at from STAR. The near side associated yields in Au+Au collisions are found to be strongly enhanced, due to contributions from large Δη. The enhancement is reduced for high ptrigT. We show a strong broadening and enhancement of the away-side yield. The evolution of the away-side shape may be explained as the sum of a broad structure from bulk response and a narrow peak from jet fragmentation.