STAR Newsletter

February 2017 edition

Contents:

• Quark Matter 2017 in a Flash!
• From the Spokesperson
• Software & Computing Update
• STAR Council News
• Evidence for Charm Quark Thermalization at RHIC

A note from the editor: as a collaboration-wide communication tool, this newsletter is set up to allow comments (subject to moderation against abuse), and all STAR Collaborators are welcome to do so! Please keep in mind that some content (including all comments) may be considered internal to the Collaboration and only accessible when logged into Drupal. Documentation is available here.

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Quark Matter 2017 in a Flash!
(Guannan Xie - Student, USTC)

Quark Matter 2017 was the XXVI^th international conference on ultrarelativistic heavy-ion collisions, held in Chicago from February 5^th to February 11^th, 2017. The focus of the discussions for this conference series is on fundamental understanding of strongly interacting matter at extreme conditions formed in ultrarelativistic heavy-ion collisions which relate to the state of the early universe.

During the conference, STAR had 19 talks (18 pre-scheduled talks + 1 flash talk) + 39 posters. And I am the lucky one who was selected for the flash talk!

My analysis mainly focuses on the Λ_C reconstruction in Au+Au collisions at √s_NN = 200 GeV using the STAR Heavy Flavor Tracker. Λ_C is the lightest charmed baryon with a mass close to the D⁰ meson and has an extremely short lifetime (cτ ~ 60 μm) which makes it a most challenging measurement in heavy-ion collisions due to the huge combinatorial background. From our previous measurements in heavy-ion collisions, we observed an enhancement for the baryon-to-meson ratio in light hadrons and strange hadrons. Then a follow-up question is, "How about the baryon-to-meson ratio in heavy quarks, such as Λ_C/D⁰?" Different models predict different levels of enhancement in the Λ_C/D⁰ (baryon-to-meson) ratio depending on the degree of charm quark thermalization in the medium and how the coalescence mechanism is implemented. Actually, our result is the first measurement of Λ_C in heavy-ion collisions and we observed a clear Λ_C/D⁰ ratio enhancement compare to PYTHIA prediction. And the level of this ratio is similar to light-flavor hadrons.

Best Regards,
Guannan



From the Spokesperson
(Zhangbu Xu - Spokesperson)

Dear Collaborators:

There have been two big events in February 2017: the beginning of RHIC Run 17 and the Quark Matter 2017 Conference in Chicago.

In addition to the news releases related to the conference by BNL and CERN, STAR has shown, just to name a few, the major discovery of Global Hyperon Polarization (Letter to NATURE under news embargo), the first observation of charmed baryons (Λ_C) in heavy-ion collisions, new ways of probing the Chiral Magnetic Effect, separation of Υ states at RHIC, the systematic study of multiple harmonic correlations and flow, new BES-I results and fixed-target program, and different ways of studying reconstructed jets in heavy-ion collisions. A special congratulation goes to Guannan Xie, a student from USTC, who was awarded a plenary flash talk for the poster presentation on Λ_C observation and enhancement in Au+Au collisions [see Guannan's contribution to this Newsletter]. The whole package (plenary preview talk, 17 parallel talks, 1 flash talk and 39 posters) represents an enormous amount of hard work from the entire Collaboration and those involved in the preparation (detector, software, analysis), presentation, vetting, and commenting. I would like to congratulate all the speakers and poster presenters who have represented our collaboration extremely well at this conference. Our talks and posters are available here, and pictures of all speakers are available here

Two weeks before the Quark Matter conference, we submitted 6 scientific papers for journal publications, which was about half of the average journal submissions per year. All the results (semi-inclusive jet, BES-I spectra, harmonic decomposition, D⁰ elliptic flow [see Xin's contribution to this Newsletter] and Global Hyperon Polarization) from those papers were highlighted by our speakers and other presenters throughout the Quark Matter conference.

Run 17 has started with the RHIC machine cool down on February 6^th. We commissioned for a few days and physics data-taking was declared on February 24^th. Our main physics goal is “Probing the "color" interactions among quarks tests a theoretical concept of nature's strongest force to pave a way toward mapping protons' 3D internal structure” with high-statistics of single spin asymmetries of W-boson, Forward Drell-Yan and direct photon measurements, a DOE milestone (HP13). New detector systems, which have been installed for Run 17 are: FMS poster shower detector, FMS UV radiation curing system, new QT boards for MTD, 1/8^th of Event-Plane Detector, eTOF prototype modules, forward calorimeter prototype modules, and RHICf. The operation and computing teams have been preparing for this Run together with the subsystem experts (thank you!).

A call for the annual Beam Use Request (BUR) has been sent from the BNL ALD to RHIC collaborations on February 22^nd, 2017:
In the next weeks, we will assemble a BUR writing committee and a review committee for the Forward Upgrade proposal. The goal is to provide convincing scientific arguments for isobar collisions in Run 18, and strong programs for BES-II and the Cold QCD plan.

Sincerely,

Zhangbu Xu



Software & Computing Update
(Jérôme Lauret - S&C Leader)

Near the end of February, the S&C Team management met with the PWGCs for the second "PWGC / S&C Exchange meeting". The discussion was productive and we covered lots of ground, including progress and status since the last meeting we held, an overall status of data production, recent developments in tracking and vertex-finding, as well as the potential to run a massive data production on NERSC/Cori as part of the recently acquired 25 million hours allocation. We also asked of the PWGCs to prepare a new data production plan and review the datasets on distributed disks.

For the data production, the good news is that we are doing very well: most of the 2016 Au+Au 200 GeV that was requested have been produced and we even started to produce the AuAu200_production2_2016 trigger setup. As a reminder, we have a Production Dataset Inventory page showing what is complete and what not. From the past samples, we re-discussed the fate of the st_sst sream (it will be produced as the rest of the data) as well as the Run 2015 p+p 200 GeV st_ssdmb stream, which we will process with the latest calibrations available and a different tracking tune. The Run 16 200 GeV d+Au is also moving forward (in fact, at the time of this newsletter it is already 22% done). Finally, we are targetting the whole st_mtd stream as a sample to process on our NERSC/Cori allocation - despite a new production plan, slow event processing is best for this High Performance Computing infrastructure (where IO capabilities are limited).

For the NERSC/Cori exercise, we are also in the process of upgrading and tuning our grid gatekeeper infrastructure. At this point, the data transfer rate tests from BNL to NERSC seems to show a performance above 600 MB/sec (see Fig. 3). We are working with our NERSC colleague to understand the link speed in the reverse direction (which is not symetrical). In all cases, the good news is that the transfer rate is above requirement needs and we should be in good shape to re-start the full end-to-end production workflow.

Now that Run 17 is in full ramp-up, many members of the team are also focused and shifting attention to its support. All databse daemons and information gathering processes were restored and FastOffline plus automated QA & calibrations resumed. We wish all a productive shift and as usual, if there are Run-related problems, please use the rts Hypernews forum for communication.



STAR Council News
(Olga Evdokimov - Council Chair)

The STAR Council is called to deal with general collaboration issues, such as organizing collaboration governance (through bylaw adoption or amendments), establishing and overseeing publication policies, admission of new Institutional members, etc. Another important responsibility of the STAR Council is the election of the STAR Spokesperson. The Spokesperson, elected for a nominal term of 3 years, carries major responsibilities within the Collaboration in terms of scientific, technical and managerial concerns, ensuring efficient operations of the experiment, facilitating detector upgrades, as well as disseminating STAR’s physics results in the timeliest fashion possible.

Needleless to say, the Council takes election matters very seriously and currently is preparing to run the next Spokesperson election at the upcoming Collaboration meeting at BNL during the week of May 15^th-20^th, 2017. Following the procedures prescribed by the Collaboration bylaws, I have established a nomination committee for this election, and the STAR Council has approved it. The nomination committee, consisting of Michael Cherney, Flemming Videbaek, Jaroslav Bielcik, and myself, is now seeking candidates and soliciting nominations for the Spokesperson position. The voting procedures for this election were set forth in accordance with STAR bylaws, and are effectively unchanged from what was used in the last election three years ago.

There is one new proposal in connection with the Spokesperson election, which is currently under Council deliberation. A modification to our bylaws was proposed by two STAR member Institutions to allow a team of two people to seek election for the office as co-Spokespeople. The proponents of this proposal argue that such a change, tested by other RHIC collaborations, could bring in the advantage of a combined skill set, while alleviating individual obligation constraints, thus enabling a more effective operation as well as broadening the potential candidate pool. The issues that are now carefully being weighted in this Council discussion are whether current management structure has some deficiencies or if similar advantages could be achieved through delegation of power. Also under consideration are the potential impacts on several aspects of publication policy, which currently cites a single Spokesperson as the final authority.

I will keep you updated on these developments. Sincerely,

Olga Evdokimov


Evidence for Charm Quark Thermalization at RHIC
(Xin Dong - LBNL)

STAR has recently submitted its first paper to Phys. Rev. Letters (arXiv:1701.06060) on measurements enabled by the Heavy Flavor Tracker (HFT) high-resolution silicon detector system. The results are the first from a detector based on Monolithic Active Pixel Sensor (MAPS) technology in a collider environment and are the first measurements of D⁰ elliptic flow, v₂, in Au+Au collisions at √s_NN = 200 GeV.

Heavy flavor quarks, due to their large masses, are considered to offer unique information about QGP dynamics in heavy-ion collisions. A measurement of heavy flavor hadron v₂, particularly in the low-to-intermediate p_T region, will provide us a better understanding of medium thermalization, and can help quantitatively measure the heavy quark diffusion coefficient – one of the intrinsic transport parameters of the QGP.

The HFT consists of three subsystems: one layer of Silicon Strip Detector (SSD), one layer of Intermediate Silicon Tracker (IST) and two layers of silicon PiXeL (PXL) detectors. The HFT-PXL detector is the first application of the MAPS technology in a collider experiment. Its unique features include fine pixel size and thin material budget which provides superior track pointing resolution for charged particles over a broad momentum range. The HFT was designed for precision measurements of charmed hadron production via topological reconstruction of displaced vertices in heavy-ion collisions. The HFT was installed and taken physics data during RHIC Runs 2014-2016. The dataset used in the PRL was about 1.1B minimum-bias-triggered Au+Au 200 GeV events taken in 2014. Fig. 4 (right) shows the track pointing resolution in the transverse plane as a function of particle momentum (or transverse momentum) at mid-rapidity from experiments at RHIC and the LHC.

Fig. 5 (left) shows the v₂ normalized by the number-of-constituent-quark (n_q) vs. the transverse kinetic energy (also normalized by n_q) for D⁰ mesons from this measurement and other light hadron results. With unprecedented precision, the result shows that D⁰ v₂ follows the same trend as light hadrons with this scaling. In the low p_T region, this indicates a clear mass ordering for light hadrons and D⁰ mesons. In the intermediate p_T region, the magnitude of the D-meson v₂ is the same as light mesons. This result suggests that charm quarks have gained a similar amount of collectivity in these collisions as light mesons.

Fig. 5 (right) shows the D-meson v₂ compared to various theoretical model calculations. One interesting observation is that the measured D-meson v₂ can be well-described by a 3D viscous hydrodynamic model calculation, which indicates that charm quarks may have reached local thermal equilibrium. The precision of the current data allows us to distinguish different models but there are non-trivial differences between different models that need to be settled. One important physics goal, for the future, is to constrain the temperature dependence of the heavy quark diffusion coefficient parameter via joint investigations between theorists and experimentalists.

The D⁰ v₂ results, together with other heavy flavor results from STAR (e.g. the enhancement observed in the D_s and Λ_C production in mid-central Au+Au collisions as well as the charm/bottom-separated single-electron R_AA measurements) have been reported and highlighted in the recent Quark Matter 2017 conference in Chicago in February [see Guannan's contribution to this Newsletter]. These measurements strongly suggest that charm quarks may have reached thermalization in the Au+Au collisions at RHIC energy.

The paper was made possible with significant contributions from colleagues at BNL, CCNU, KSU, LBNL, MIT, Purdue, SINAP, UIC, USTC, and UT Austin, with critical support from the STAR operation and computing teams as well as the paper's GPC.

Previous Edition: December 2016