STAR TPC Research Articles

MWPC prototyping and performance test for the STAR inner TPC upgrade

A new prototype of STAR inner Time Projection Chamber (iTPC) MWPC sector has been fabricated and tested in an X-ray test system. The wire chamber built at Shandong University has a wire tension precision better than 6% and wire pitch precision better than 10 μm. The gas gain uniformity and energy resolution are measured to be better than 1% (RMS) and 20% (FWHM), respectively, using an55Fe X-ray source. The iTPC upgrade project is to replace all 24 STAR TPC inner sectors as a crucial detector upgrade for the RHIC beam energy scan phase II program. The test results show that the constructed iTPC prototype meets all project requirements.

Proton/pion ratios in Δϕ with respect to a jet in sNN=200GeV Au + Au collisions at STAR

Inclusive Proton/pion ratios show an enhancement at intermediate transverse momentum (P_{T} ~ 1.5 - 4.0 GeV/c) in central sqrt{s_{NN}} = 200 GeV Au+Au collisions compared to peripheral Au+Au and p+p collisions. This effect suggests a production mechanism, different from fragmentation, which is consistent with coalescence and recombination models. A high E_{T} trigger particle selects a surface-biased jet, which is measured to have a similar p_{T} distribution as a p+p jet. This jet is used to enhance the quenching effects of the recoiling, medium traversing one. We reconstruct the trigger jet using the FASTJET algorithm, with a E_{T} (p_{T}) cut of 3.0 GeV(/c) on the towers(tracks) in order to reduce the heavy-ion collision background. The particle identification of tracks with p_{T} up to ~ 2.8 GeV/c is obtained by taking advantage of STAR TOF and TPC detectors with full azimuth coverage. Correlations in \Delta\phi between jets and identified hadrons are presented, and the particle ratios in different regions of azimuth are measured. Particle ratios associated with the trigger jet vs. the recoil jet, and comparisons to inclusive particle ratios can help to distinguish between jet-related (vacuum and medium-modified) and bulk-related contributions to the ratios enhancement.

Isolation of Flow and Nonflow by Two- and Multi-Particle Cumulant Measurements of vn in sNN=200 GeV Au + Au Collisions by STAR

We apply a data-driven method to STAR Au + Au collisions at sNN=200 GeV to isolate Δη-dependent and Δη-independent correlations by using two- and four-particle Q-cumulant vn measurements. The Δη-independent part, dominated by flow, is found to be η-independent within the STAR TPC of ±1 unit of pseudo-rapidity. The Δη-dependent part may be associated to nonflow, and is responsible for the Δη drop in the measured two-particle v2 cumulant. We combine our result to four- and six-particle cumulants to gain further insights on the nature of flow fluctuations.

Leading hadron PID effects in di-hadron angular correlations from STAR

We present preliminary results of a di-hadron correlation analysis with identified triggers, measured as a function of relative azimuth (Δϕ) and pseudorapidity (Δη). The relativistic rise in the momentum dependence of the ionization energy loss of particles measured in the STAR TPC is used to statistically separate charged pions from kaons and protons. On the near side, the long range Δη correlation known as the ridge is revealed to have a strong trigger-type dependence, providing new constraints for its theoretical description. The jet-like cone structure above the ridge, while also trigger-dependent, shows little modification compared to d+Au measurements, challenging expectations from recombination models.

The study of longitudinal properties of the charge balance function

The charge balance function is studied in Au + Au collisions at \(\surd \overline {s_{NN} } \) GeV using the STAR detector at the Relativistic Heavy-Ion Collider. Within the acceptance of the STAR TPC, it is found that the balance function is independent of the position of pseudorapidity windows with equal-size. It is also demonstrated for the first time at RHIC that the balance function divided by a scaling factor related to pseudorapidity is independent of the width and position of the pseudorapidity window. Moreover, Such a scaled balance function is further found to exist in the subset of particles with the same restricted transverse-momentum range.

Azimuth Quadrupole Systematics in Au-Au Collisions

We have measured pt-dependent two-particle number correlations on azimuth and pseudorapidity for eleven centralities of = 62 and 200 GeV Au-Au collisions at STAR. 2D fits to these angular correlations isolate the azimuth quadrupole amplitude, denoted 2v22{2D}(pt), from localized same-side correlations. Event-plane V2(pt) measurements within the STAR TPC acceptance can be expressed as a sum of the azimuth quadrupole and the quadrupole component of the same-side peak. V2{2D}(pt) can be transformed to reveal quadrupole pt spectra which are approximately described by a fixed transverse boost and universal Levy form nearly independent of centrality. A parametrization of V2{2D}(pt) can be factored into centrality and pt-dependent pieces with a simple pt dependence above 0.75 GeV/c. Results from STAR are compared to published data and model predictions.

Energy and system size dependence of charged particle elliptic flow and v2/ε scaling

We report measurements of charged particle elliptic flow at mid-rapidity in Au+Au and Cu+Cu collisions at and 200 GeV. Using correlations between main STAR TPC and forward TPCs ensures minimal bias due to non-flow effects. We further investigate the effect of flow fluctuations on v2/ε scaling studying initial geometry eccentricity fluctuations in the Monte Carlo Glauber model. It is found that accounting for the effect of flow fluctuations improves v2/ε scaling.

Correcting for distortions due to ionization in the STAR TPC

Physics goals of the STAR Experiment at RHIC in recent (and future) years drive the need to operate the STAR TPC at ever higher luminosities, leading to increased ionization levels in the TPC gas. The resulting ionic space charge introduces field distortions in the detector which impact tracking performance. Further complications arise from ionic charge leakage into the main TPC volume from the high gain anode region. STAR has implemented corrections for these distortions based on measures of luminosity, which we present here. Additionally, we highlight a novel approach to applying the corrections on an event-by-event basis applicable in conditions of rapidly varying ionization sources.

Pion, kaon and (anti-)proton production in Au+Au collisions at

We report on preliminary results of pion, kaon and (anti-)proton transverse momentum spectra (−0.5 < y < 0) in Au+Au collisions at using the STAR detector at RHIC. The particle identification (PID) is achieved by a combination of the STAR TPC and the new TOF detectors, which allow a PID coverage in transverse momentum (pT) up to 7 GeV/c for pions, 3 GeV/c for kaons and 5 GeV/c for (anti-)protons.

Measurement of open charm production in d+Au collisions at

We present the first comprehensive measurement of D0, D+, D*+ and their charge conjugate states at mid-rapidity in d+Au collisions at using the STAR TPC. The directly measured open charm multiplicity distribution covers a broad transverse momentum region of 0 < pT < 11 GeV/c. The measured dN/dy at mid-rapidity for D0 is 0.0265 ± 0.0036 (stat.), ± 0.0071 (syst.), and the measured D*+/D0 and D+/D0 ratios are approximately equal with a magnitude of 0.40 ± 0.09 (stat.) ± 0.13 (syst.). The total cross section per nucleon–nucleon collision extracted from this study is 1.18 ± 0.21 (stat.) ± 0.39 (syst.) mb. The direct measurement of open charm production is consistent with STAR single electron data. This cross section is higher than expectations from PYTHIA and other pQCD calculations. The measured pT distribution is harder than the pQCD prediction using the Peterson fragmentation function.

Measurement of jet modification at RHIC

Charged hadrons (0.15 < p⊥ < 4 GeV/c) associated with a large p⊥ trigger particle (4 < ptrig⊥ < 6 GeV/c) are statistically reconstructed in the large acceptance STAR TPC for p+p and Au+Au collisions at GeV. Preliminary results on transverse momentum distributions of these hadrons are presented, separately on the near and the away side from the trigger particle. An increase in the multiplicity and total scalar p⊥ of these hadrons from p+p to central Au+Au collisions is observed. The away-side distribution is found to be significantly softened in central Au+Au with respect to p+p collisions. The results are consistent with modification of jets in heavy-ion collisions at RHIC.

Elliptic flow from two- and four-particle correlations in Au+Au collisions atsNN=130GeV

Elliptic flow holds much promise for studying the early-time thermalization attained in ultrarelativistic nuclear collisions. Flow measurements also provide a means of distinguishing between hydrodynamic models and calculations which approach the low density (dilute gas) limit. Among the effects that can complicate the interpretation of elliptic flow measurements are azimuthal correlations that are unrelated to the reaction plane (non-flow correlations). Using data for Au + Au collisions at sqrt{s_{NN}} = 130 GeV from the STAR TPC, it is found that four-particle correlation analyses can reliably separate flow and non-flow correlation signals. The latter account for on average about 15% of the observed second-harmonic azimuthal correlation, with the largest relative contribution for the most peripheral and the most central collisions. The results are also corrected for the effect of flow variations within centrality bins. This effect is negligible for all but the most central bin, where the correction to the elliptic flow is about a factor of two. A simple new method for two-particle flow analysis based on scalar products is described. An analysis based on the distribution of the magnitude of the flow vector is also described.

Elliptic flow in Au+Au collisions at [formula omitted] GeV

We report the elliptic flow of charged and identified particles at mid-rapidity in Au+Au collisions at $\sqrt{s_{_{NN}}}=130$ GeV using the STAR TPC at RHIC. The integrated elliptic flow signal, $v_2$, for charged particles reaches values of about 0.06, indicating a higher degree of thermalization than at lower energies. The differential elliptic flow signal, $v_2$($p_t$) up to 1.5 GeV/$c$, shows a behavior expected from hydrodynamic model calculations. Above 1.5 GeV/$c$, the data deviate from the hydro predictions; however the $v_2$($p_t$) is still large, suggesting finite asymmetry for the products of hard scattering. For the identified particles, elliptic flow as a function of $p_t$ and centrality differ significantly for particles of different masses. This dependence can be accounted for in hydrodynamic models, indicating that the system created shows a behavior consistent with collective hydrodynamical flow.

Resonance studies at STAR

We report on the observed signals of ${K^{\star0}(892)}\to\pi K$ and $\phi(1020)\to K^{+}K^{-}$ using the mixed-event method with powerful statistics from the large acceptance and highly efficient STAR TPC. Preliminary results from the first observation of such states from the year-one STAR data in $\sqrt{s_{NN}}=130$ GeV Au-Au collisions are presented. The $K^{\star0}/h^{-}$ ratios with an assumed $K^{\star0}$ $p_{T}$ inverse slope of 300MeV are compatible with that from pp at ISR. For 14% central Au+Au collisions, we observe $K^{\star0}/h^{-}=0.060\pm0.007(stat)$ and $\bar{K^{\star0}}/h^{-}=0.058\pm0.007(stat)$. We show that $\bar{\Lambda}/\Lambda=0.77\pm0.07(stat)$ from this method is consistent with the measurement via decay topology.

A TPC for measuring high multiplicity events at RHIC

The Relativistic Heavy Ion Collider ran for the first time in the summer of 2000 with Au beams circulating at S NN =130 GeV and at approximately 10% of the machine's final design luminosity. We gathered data with the STAR Time Projection Chamber for 2 months and recorded events over a wide dynamic range of particle momenta and multiplicities. We reconstructed tracks with momenta from as low as 100 MeV/ c to above 3 GeV/ c, and we observed collisions with multiplicities ranging from 1 or 2 tracks per event in ultraperipheral collisions to 3000 tracks per event in central collisions. I will report on the design and performance of the STAR TPC in this demanding environment.

Identified Particle Elliptic Flow inAu+AuCollisions atsNN=130GeV

We report first results on elliptic flow of identified particles at mid-rapidity in Au+Au collisions at $\sqrt{s_{_{NN}}}=130$ GeV using the STAR TPC at RHIC. The elliptic flow as a function of transverse momentum and centrality differs significantly for particles of different masses. This dependence can be accounted for in hydrodynamic models, indicating that the system created shows a behavior consistent with collective hydrodynamical flow. The fit to the data with a simple model gives information on the temperature and flow velocities at freeze-out.

Design and implementation of the STAR experiment's DAQ

The STAR experiment is one of the two large detectors currently being built at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, Upton, U.S.A. The major issue of STAR's DAQ is the large amount of data that has to be processed as fast as possible. The required data rate is of the order of 90 Gbits/s which has to be processed and scaled down to about 15 MBytes/s and stored to tape or other permanent archiving media. To be able to do so the STAR DAQ uses a custom built ASIC which preprocesses the raw data for later use by a software Level 3 trigger. The Level 3 trigger selects events to be archived depending on physics criteria based upon the particle track information extracted during Level 3 processing. The design presented is a massively parallel multiprocessor system which consists of front end microprocessors hierarchically organized within a VME crate system. Each VME crate contains 6 custom made Receiver Boards with 3 Intel I960HD processors per board for a total of 18 processors per crate. The STAR's TPC detector uses 24 such crates and the SVT detector will use 4 crates for a total of 504 microprocessors.

STAR TPC at RHIC

Design information, construction methods and testing results are given for the STAR TPC which will be installed at the BNL RHIC collider.