arXiv subjects
Publications and source records attributed to Nihar Ranjan Sahoo.
These proceedings discuss recent jet measurements by the STAR experiment at RHIC to study jet substructure in p+p and jet quenching in Au+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 200 GeV. Furthermore, STAR's future plans for precision jet measurements with the upcoming data-taking periods in 2023-2025 are presented.
Integration of conventional mid to long-wavelength infrared polarizers with chip-scale platforms is restricted by their bulky size and complex fabrication. Van der Waals materials based polarizer can address these challenges due to its non-lithographic fabrication, ease of integration with chip-scale platforms, and room temperature operation. In the present work, mid-IR optical response of the sub-wavelength thin films of $α$-MoO$_3$ is investigated for application towards high temperature mid-IR transmission and reflection type thin film polarizer. To our knowledge, this is the first report of above room temperature mid-IR optical response of $α$-MoO$_3$ to determine the thermal stability of the proposed device. We find that our $α$-MoO$_3$ based polarizer retains high extinction ratio with peak value exceeding 10 dB, up to a temperature of 140$^{\circ}$C. We explain our experimental findings by natural in-plane hyperbolic anisotropy of $α$-MoO$_3$ in the mid-IR, high temperature X-ray diffraction and Raman spectroscopic measurements. This work opens up new avenues for naturally in-plane hyperbolic van der Waals thin-films to realize sub-wavelength IR optical components without lithographic constraints.
We present the semi-inclusive measurement of charged jets recoiling from direct-photon and $π^{0}$ triggers in central Au+Au collisions at $\sqrt{s_{\mathrm {NN}}}$ = 200 GeV, using a dataset with integrated luminosity 13 $\mathrm {nb^{-1}}$ recorded by the STAR experiment in 2014. The photon and $π^{0}$ triggers are selected within transverse energy ($\rm E_\mathrm{T}^\mathrm{trig}$) between 9 GeV and 20 GeV. Charged jets are reconstructed with the anti-$k_{\mathrm{T}}$ algorithm with resolution parameters R = 0.2 and 0.5. A Mixed-Event technique developed previously by STAR is used to correct the recoil jet yield for uncorrelated background, enabling recoil jet measurements over a broad $\rm p_{T,jet}$ range. We report fully corrected charged-jet yields recoiling from direct-photon and $π^{0}$ triggers for the above two jet radii and also discuss the jet R dependence of in-medium parton energy loss at the top RHIC energy.
Recently, in-plane biaxial hyperbolicity has been observed in $α$-MoO${_3}$ --a van der Waal crystal-- in the mid-infrared frequency regime. Here, we present a comprehensive theoretical analysis of thin film $α$-MoO${_3}$ for application to two mid-IR photonic devices -- a polarizer and a waveplate. We show the possibility of a significant reduction in the device footprint while maintaining an enormous extinction ratio from $α$-MoO${_3}$ based polarizers in comparison with that of conventional polarizers. Secondly, we carry out device optimization of $α$-MoO${_3}$ based waveplates with subwavelength thickness. We explain our results using natural in-plane hyperbolicity of $α$-MoO${_3}$ via analytical and full wave simulations. This work will build a foundation for miniaturization of mid-infrared photonic devices by exploiting the optical anisotropy of $α$-MoO${_3}$.
We studied the centrality selection effect on cumulants (up to fourth order) and the cumulants ratios of net-proton multiplicity distributions in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 7.7, 19.6 and 200 GeV from UrQMD model. The net-proton cumulants are calculated with collision centralities by using charged particle multiplicity from different pesudorapidity ($η$) region. By comparing the results from various collision centralities, we found that the autocorrelation effects are not significant in the results with collision centralities "refmult-3" and "refmult-2", which are using mid-rapidity charged particles but excluding (anti-)protons and analysis region, respectively. Furthermore, due to the contributions of spectator protons, we observed poor centrality resolution when using charged particles at forward $η$ region at low energies. This work can serve as a baseline for centrality selection of future fluctuations analysis in relativistic heavy-ion collisions.
We report the first measurement of fully corrected semi-inclusive charged recoil jets for both direct-photon ($γ_{dir}$) and $π^{0}$ triggers, within two trigger transverse energy ranges between 9 GeV and 15 GeV, in central Au+Au collisions at $\sqrt {s_{\rm NN}}$ = 200 GeV using data with an integrated luminosity of 13 $\rm nb^{-1}$ collected by the STAR experiment. A comparison between $π^{0}$-triggered recoil jets in p+p collisions and PYTHIA is discussed. A comparison is also presented between the suppression of $γ_{dir}$- and $π^{0}$-triggered recoil jets in central Au+Au collisions with respect to their corresponding PYTHIA references. A strong and similar level of suppression is observed in recoil-jet yields as a function of jet transverse momentum for $γ_{dir}$+jet and $π^{0}$+jet.
We present two measurements related to jet production in p+p and central Au+Au collisions at $\sqrt{{s}_{NN}}$=200 GeV. Firstly, a study of semi-inclusive charged recoil jets coincident with high-$\rm{p_{T}}$ direct-photon and neutral pions, and secondly, the hadron correlations with respect to reconstructed di-jet triggers. Indication of medium effects is observed by comparing $γ$+jet and $π^{0}$+jet measurements. The di-jet+hadron study shows signs that the medium-induced modifications of an imbalanced set of di-jets with "hard cores" primarily affect the recoil jet.
Hard probes created through large momentum transfers are used to study the properties of QCD matter created in heavy-ion collisions, by comparing the measurements to those in p+p collisions. Jets, and the "quenching" or suppression of jets in the medium created in heavy-ion collisions, are studied through various different observables. We present the most recent measurements from $\sqrt{s_{NN}}$ = 200 GeV Au+Au collisions, with p+p collisions as the reference, by the STAR Collaboration. The observables are semi-inclusive charged jets and di-jet transverse momentum imbalance. Additionally, correlation measurements of direct photon-hadron and neutral pion-hadron are presented and discussed.
We report the results of $γ_{\rm dir}$ and $π^{0}$-hadron azimuthal correlations as a measure of the away-side jet-like correlated yields in central Au+Au and p+p collisions at $\sqrt{s_{_{NN}}}$=200 GeV in the STAR experiment. The charged-hadron per-trigger yields at mid-rapidity with respect to high-$p_{T}$ $γ_{\rm dir}$ and $π^{0}$ in central Au+Au collisions are compared with p+p collisions. Within uncertainties, the same $z_{T}$ (~$p_{T}^{\mathrm{assoc}}$ / $p_{T}^{\mathrm{trig}}$) dependence of the suppression is observed for $γ_{\rm dir}$ and $π^{0}$ - triggers. The results are compared with energy-loss model predictions. The $γ-\rm jet$ measurements can provide further understanding on the redistribution of in-medium energy loss. Ongoing $γ-\rm jet$ studies in the STAR experiment are also discussed.
Susceptibilities of conserved quantities, such as baryon number, strangeness and electric charge are sensitive to the onset of quantum chromodynamics (QCD) phase transition and are expected to provide information on the matter produced in heavy-ion collision experiments. A comprehensive study of the second-order diagonal susceptibilities and cross correlations has been made within a thermal model approach of the hadron resonance gas (HRG) model as well as with a hadronic transport model, UrQMD. We perform a detailed analysis of the effect of detector acceptances and choice of particle species in the experimental measurements of the susceptibilities for heavy-ion collisions corresponding to \sNN = 4 GeV to 200 GeV. The transverse momentum cutoff dependence of suitably normalised susceptibilities are proposed as useful observables to probe the properties of the medium at freezeout.
We report new results of $γ_{dir}$+hadron and $π^{0}$+hadron azimuthal correlations as a measure of the away-side jet-like correlated yields in central Au+Au and p+p collisions at $\sqrt{s_{NN}}$ = 200 GeV in the STAR experiment from years 2011 and 2009 of data taking, respectively. The charged-hadron per-trigger yields at mid-rapidity $(|η| < 1)$ and for transverse momenta $p_{T}^{assoc} > 1.2$ ~GeV/$c$ associated with $γ_{dir}$~ and $π^{0}$~(for triggers $|η| < $0.9, 12 $< p_{T}^{trig} <$ 20 GeV/$c$) in central Au+Au collisions are compared with p+p collisions. The $z_{T}$ ($= \frac{p_{T}^{assoc}}{p_{T}^{trig}}$) dependence, now extending down to $z_{T}$=0.1, of the suppression of the away-side associated yields is presented. We observed that the suppression of away-side associated yields tends to vanish at lower \zT. The dependence of the suppression on both $p_{T}^{assoc}$ and $p_{T}^{trig}$ is also discussed. Finally, these results are compared with various model predictions.
The dynamical charge fluctuations have been studied in ultra-relativistic heavy-ion collisions by using hadronic model simulations, such as UrQMD and HIJING. The evolution of fluctuations has been calculated at different time steps during the collision as well as different observation window in pseudorapidity (\DelEta). The final state effects on the fluctuations have been investigated by varying $\bigtriangleupη$~ and the time steps with the aim of obtaining an optimum observation window for capturing maximum fluctuations. It is found that $\bigtriangleupη$~ between 2.0 and 3.5 gives the best coverage for the fluctuations studies. The results of these model calculations for Au+Au collisions at $\sqrt{s_{\rm NN}}$~=~7.7 to 200~GeV and for Pb+Pb collisions at 2.76 TeV are presented and compared with available experimental data from the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC).
Event-by-event fluctuations of global observables in relativistic heavy-ion collisions are studied as probes for the QCD phase transition and as tools to search for critical phenomena near the phase boundary. Dynamical fluctuations in mean transverse momentum, identified particle ratios and conserved quantities (such as net-charge, net-baryon) are expected to provide signatures of a de-confined state of matter. Non-monotonic behavior in the higher-moments of conserved quantities as a function of beam energy and collision centrality are proposed as signatures of the QCD critical point. To study the QCD phase transition and locate the critical point, the STAR experiment at RHIC has collected a large amount of data for Au+Au collisions from $\sqrt{s_{NN}} = 7.7-200$~GeV in the RHIC Beam Energy Scan (BES) program. We present the recent beam energy scan results on dynamical fluctuations of particle ratios and two-particle transverse momentum correlations at mid-rapidity. Higher-moments of the net-charge and net-proton multiplicity distributions as a function of beam energy will be presented. We give a summary of what has been learnt so far and future prospectives for the BES-II program.
We report the higher order moments of the net-charge multiplicity distributions for the Au+Au collisions at \sNN = 7.7, 11.5, 19.6, 27, 39, 62.4 and 200 GeV in the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). The energy and centrality dependence of higher moments and their products (such as $σ^2/M$, $Sσ$ and $κσ^{2}$) are presented. The data are also compared to Poisson expectations and Hadron Resonance Gas model calculations.
The Beam Energy Scan program has been undertaken at the Relativistic Heavy Ion Collider (RHIC) to search for the QCD critical point. The presence of the critical point is expected to lead to non-monotonic behavior of several quantities. Here we report the result of higher moments of net-charge distributions for Au+Au collisions at $\sqrt{s_{NN}}$ = 39 GeV as measured by the STAR experiment. The STAR results are compared with results from HIJING event generator and Hadron Resonance (HRG) Models.