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News from the Nuclear Science Division at Berkeley Lab
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Nuclear Science Division Newsletter
In this issue:April, 2017

Heavy quarks go with the QGP flow in STAR HFT

STAR has submitted its first paper to Phys. Rev. Letter (arXiv:1701.06060) using data from the Heavy Flavor Tracker (HFT) high-resolution silicon detector system (right). These are the first results from a detector based on Monolithic Active Pixel Sensor (MAPS) technology in a collider environment and are the first measurement of D0 elliptic flow in Au+Au collisions at √sNN = 200 GeV. The elliptic flow, v2 is the second harmonic coefficient in the azimuthal Fourier decomposition of the particle momentum distribution in heavy-ion collisions. The strong collective v2 observed for light flavor particles was a key piece of evidence of the formation of the Quark-Gluon Plasma (QGP).

In the new result, separated vertices from D0 meson decays were reconstructed with the help of the HFT detector. The D0 signal-to-noise was much higher than in pre-HFT analyses. The figure compares the elliptic anisotropy parameter v2 divided by the number-of-constituent-quark (nq, 2 for mesons and 3 for baryons) vs. the transverse kinetic energy (mT-m0, where mT = √pT2 + m02) also normalized by nq for D0 mesons from this measurement and other light hadron results. The data clearly shows that D0 v2 follows the same trend as light hadrons with this scaling. In the low pT region, the data show a mass ordering for light hadrons and D0 mesons, clear evidence that D0 mesons behave hydrodynamically. At intermediate pT, the magnitude of the D0 v2 is the same as light mesons. This suggests that, in these collisions, charm quarks have a similar level of collectivity as light mesons and flow with the QGP medium. This is unique information on the charm quark thermalization, and sheds light on the intrinsic transport parameters of the QGP medium that is produced at RHIC.

The elliptic flow (v2) of charmed mesons (D0 ) follows constituent quark scaling, just like light quark mesons.

The analysis was led by colleagues from the RNC group together with collaborators from Kent State University, Purdue University and other STAR institutions.

Majorana demonstrates results

Figure 1: Summed energy spectrum of 76-Ge-enriched detectors. The 2νββ continuous spectrum is clearly visible and there is one event in the 400-keV-wide monitoring window centered at the Q value of 2039 keV.

The Majorana Demonstrator (MJD) project aims to demonstrate the feasibility to construct a future ton-scale 76Ge-based neutrinoless double-beta decay (0νββ) experiment. One of its main goals is to show the background in a 4-keV wide 0νββ region of interest (ROI) to be < ~3 counts/t-y, which would scale to ~1 count/t-y in a ton-sized experiment. MJD updated its background measurement result recently. With an exposure of 1.39 kg-y in its Data Sets 3 and 4, which are the first data when both ultra-clean detector modules were running simultaneously. After all analysis cuts, MJD saw 1 count in a 400-keV-wide monitoring window centered at the Q-value of 76Ge decay at 2039 keV (Fig. 1). NSD scientists acquired the 76Ge-enriched p-type point-contact germanium detectors; and developed and fabricated the world’s cleanest front-end electronics used in MJD, which also have superior noise and spectroscopic characteristics. These features allow the choice of a very narrow 2.8-keV-wide ROI for 0νββ searches. Under the assumption of a flat background spectrum, the background rate in the 400-keV monitoring window translates to 5.1+8.9-3.2 counts/t-y (68% CL) in the 2.8-keV-wide ROI, or a background index of 1.8+3.1-1.1 x 10-3/ keV-t-y (68% CL). More exposure is required to firmly establish the background level, but this preliminary result is already more than an order of magnitude better than all other operating experiments that do not use germanium-diode technology (Fig. 2). MJD has also demonstrated the best energy resolution in the ROI for all operating experiments.

Figure 2. Background index and full-width-at-half-maximum energy resolution in the 0νββ ROI for different experiments. The gray diagonal lines indicate the integral count range in the 0νββ ROI (assumed to be 1 FWHM).

The European GERDA experiment and MJD employ different strategies to shield external backgrounds in their respective setups. GERDA uses a liquid argon active veto, while MJD uses a graded copper-lead passive shield. The latest MJD results indicate that by combining its clean-material technology with GERDA’s superior background rejection, it would be possible to reduce the background to ~0.1 count/ROI-t-y in a future experiment. The two collaborations, along with other international groups with expertise in 0νββ and gamma-ray tracking and spectroscopy have recently form a new collaboration that will pursue a ton-scale experiment with the combined technology. NSD is a founding member of this new experiment LEGEND, and will be a leader in the development and delivery of germanium detectors and front-end electronics — NSD’s two principal contributions to MJD.

New directions for jet measurements

High energy collisions of all kinds generate “jets,” which are correlated sprays of particles generated by scattered quarks and gluons. Nuclear physicists use jets as a tomographic probe of the Quark-Gluon Plasma (QGP) generated in nuclear collisions at RHIC and the LHC. Jets in these collisions plough through the QGP and interact with it, before flying off to the detector. This process, called “jet quenching,” modifies jet properties dramatically, and has produced some of the most striking measurements of the QGP.

Figure 1. Jet momentum for data (stars) compared with the mixed event background (hashed region). An excess from jets is visible for pT > 15 GeV/c.

Jet measurements are challenging. The experimentalist’s task for accurate jet measurements is to cluster the particles in a cone containing the jet spray, a process called “jet reconstruction,” without including particles from other, background processes in the same collision. This task is especially complex in nuclear collisions, where background processes generate thousands of particles. Jet quenching was initially discovered at RHIC using a simpler measurement of single high momentum particles, which are indirect jet messengers. More recently, reconstructed jet measurements have been carried out in heavy ion collisions at the LHC, focusing on very energetic jets to minimize the large background effects. The RHIC jet energy range is much more limited, and accurate jet measurements in RHIC heavy ion collisions require new approaches to tackle the jet background problem head-on.
The STAR collaboration has recently released a paper presenting a new approach to jet measurements in heavy ion collisions (https://arxiv.org/abs/1702.01108), based on analysis carried out in the RNC group. The huge backgrounds to the jet signal are measured using a sophisticated “event mixing technique.” This method enables the first measurement of jets in heavy ion collisions over the complete range of jet energies at RHIC, even for jet reconstruction with large cone radius, providing qualitatively new ways of studying jet quenching.

The red points in Fig. 1 show the distribution of reconstructed jet momentum from all measured jets in the data, both signal and background. The shaded region shows the background distribution. It matches the red points precisely on the left side of the distribution, which is dominated by background, validating the method. The difference of the two distributions gives the jet signal, which is used to measure quenching effects. Comparison of these results to measurements at the LHC show that jet quenching at RHIC is a factor of two smaller, providing the first direct comparison of jet quenching at the two colliders.

Figure 2 The azimuthal separation between reconstructed jets and the high-momentum trigger hadrons, compared with a pQCD model.

Figure 2 shows another observable in the analysis, the azimuthal angular separation between a reconstructed jet and a high-momentum trigger hadron. This distribution is sensitive to the possible deflection of very low momentum jets due to scattering in the QGP, an important process that has never been observed. The grey points are measured data, while the red band is a “Reference” distribution due to all other processes. There is a hint of angular broadening in the data, but with very limited statistical significance. These data establish the method to carry out these unique measurements. New analyses, with much higher statistics are now in progress by members of RNC together along with other STAR collaborators.

Fragments

About a dozen NSDers journeyed to Chicago last February, for Quark Matter ’17, joining about 750 colleagues for a week of discussions on diverse topics involving heavy-ion collisions. One of the highlights of QM’17 was the presentation of STAR HFT results, including studies of heavy quark flow (see the NSD newsletter cover story) and the first observation of Λc+ decays in heavy ion collisions. NSDers presented three plenary talks: Spencer Klein (Ultra-peripheral collisions), Alex Schmah (STAR Overview) and Xin Dong (heavy flavor – experiment) while NSD affiliate graduate student Guanan Xie was given a ‘flash talk’ to orally present his poster on Λc+ production in STAR.

Dr. Lukas Hehn joined the Majorana group as a postdoctoral fellow in January 2017. A recent PhD recipient from Karlsruhe Institute of Technology in Germany, Dr. Hehn searched for low-mass weakly interacting massive particles (WIMPs) in the EDELWEISS-III experiment. At LBNL, he is participating in data analysis in the Majorana project and the development of the next-generation 76Ge neutrinoless double-beta decay experiment LEGEND.

Several NSDers reached important service milestones this year having completed 5, 10, 150, 20 or 25 years at the University of California:

Brian Fujikawa – 25 years
Mario Cromaz – 20 years
Jane Toby McCall – 20 years
Janilee Benitez – 10 years
Xin Dong – 10 years

Congratulations to all of them!

Newsletter Notes
Please send any comments, including story suggestions to Spencer Klein at srklein@lbl.gov.
 
Previous issues of the newsletter are available at:
https://commons.lbl.gov/display/nsd/NSD+Newsletter.






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