Session LB: Heavy Ions and Jets IV

Energy Loss in Di-jet Production in DIS in the Color Glass Condensate Formalism

Studies of energy loss in Deep Inelastic Scattering (DIS) serve as an important diagnostic tool for understanding the properties of cold nuclear matter. Here we present our results for the energy loss of a di-jet (two partons) produced in DIS using the Color Glass Condensate description of a large nucleus. Spinor helicity methods are used to evaluate the Dirac matrices which simplifies the calculation tremendously.   

Gluon saturation search using direct $\gamma$+hadron correlations in LHCb

Previous DIS results from HERA show a fast increasing of the gluon density in protons towards small Bjorken-$x$ fractional momentum. The size of gluons is inversely proportional to the $Q^2$ momentum transfer of the process. Hence, gluons from processes where $Q^2(x)$ is smaller than $Q_S^2(x)$ are supposed to be saturated. The scale $Q^2_S(x)$ in proton and nucleus are still to be determined experimentally. The LHCb experiment is a single arm detector in LHC with vertexing, tracking, $p$, $K$, $pi$, $e$, $\mu$ identification and calorimetry in the region 1.6$< \eta <$ 4.9, which can access $x \sim 10^{-6}-10^{-4}$, up to two orders of magnitude smaller than HERA. A direct probing of small-$x$ and small $Q^2$ gluons can be performed with direct $\gamma$+hadron correlation measurements. This talk is going to report the status of the analysis efforts aimed of finding evidences of gluon saturation and its scale $Q^2_S(x)$ at LHCb. Future detector improvements in gluon saturation and heavy ion measurements with real-time analysis and a soft particle tracker in LHCb are also going to be discussed.   

Photon-jet angular correlations in proton-nucleus collisions

We calculate the double differential cross section for production of a photon and a quark jet in high energy proton-nucleus collisions in a new formalism that includes contributions of both small and large x gluons of the target. This allows us to investigate photon-jet angular correlations in both low and high transverse momentum region which would help clarify the role of saturation dynamics in the current experiments at RHIC and the LHC.   

Isolated Photon-Hadron Correlations in pp and p--Pb Collisions at $\sqrt{s_\mathrm{NN}}$= 5.02 TeV Measured with the ALICE Detector

The measurement of isolated photon-tagged correlations of jets and jet fragments is a promising channel for the study of partonic energy loss in heavy-ion collisions. Photons are measured in the ALICE EMCal. We use a combination electromagnetic shower-shape information and isolation criteria obtained with the ALICE ITS to reduce the large background from meson decays and fragmentation photons. We present isolated photon-hadron correlations and yields of charged hadrons in $\sqrt{s_{\rm NN}} = $ 5.02 TeV pp and p—Pb collisions in an unexplored kinematic range: [12-40] GeV/$c$ for the photon $p_{\rm T}$ and [0.7-10] GeV/$c$ for the charged track $p_{\rm T}$. We report the first measurement of photon-tagged parton fragmentation in p--Pb at the LHC. We show the ratio of fragmentation measurements in pp and p--Pb is consistent with unity, constraining the impact of cold nuclear matter effects on parton fragmentation.   

Jet and Di-jet Underlying Event in p+Au Collisions at $\sqrt{s_{NN}}$ = 200 GeV at STAR

Proton-ion collisions have been included in runs at the LHC and RHIC in addition to proton-proton and heavy-ion collisions as a means of studying cold nuclear matter (CNM) effects. These asymmetric systems have yielded some unexpected trends, notably in measurements of jet nuclear modification factors at different centralities. Detectors at forward/backward rapidity have been used to measure quantities as a proxy for centrality or event activity (EA) in p+Au collisions in order to avoid auto-correlations resulting from EA and physical observables both being measured at mid-rapidity. In this talk, we show correlations of backward-rapidity (Au-going) event activity with mid-rapidity underlying event (UE) in p+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV measured with the STAR detector. We present UE measurements and show the trends of UE in relation to EA for high-transverse momentum ($p_{\mathrm{T}}$) jet events vs. di-jet events. Additionally, we will study collision kinematics via observables including jet pseudorapidity ($\eta$) and jet $p_{\mathrm{T}}$ as a means of investigating the initial hard scatterings of partons in these events.   

Di-hadron correlations in p+p, p+Au and p+Al collisions at STAR

Two-particle azimuthal correlation has been proposed to be one of the most direct and sensitive channels to access the underlying nonlinear gluon dynamics. In hadron collisions at RHIC, forward particle production probes gluons at small x where the gluon densities rise sharply. During the 2015 RHIC run, STAR has collected data for measuring azimuthal correlations of neutral pions detected with the Forward Meson Spectrometer (FMS, 2.6 $\leq$ $\eta$ $\leq$ 4.0) in $p$+$p$, $p$+Au and $p$+Al collisions at $\sqrt{s_{NN}}$ = 200 GeV. An analysis update, which will measure the di-hadron correlations as a function of $A$ and transverse momenta of both the trigger $\pi^{0}$ (1.4 GeV/c $<$ $p_{T}$ $<$ 5 GeV/c) and the associated back-to-back $\pi^{0}$ (1 GeV/c $<$ $p_{T}$ $<$ 2.8 GeV/c), will be presented.   

Isolated photon production in pp and p-Pb collisions at LHC with ALICE

Photons are unique probes to study QCD processes since they do not interact strongly. Isolated photon production in pp collisions is one of the most clear tests of hard QCD processes and nuclear parton distribution functions. Photon measurement in pA collisions provides the opportunity to measure initial geometrical scaling and possible modifications of the nucleon structure function in nuclei. The ALICE collaboration has collected data on pp and p--Pb collisions at several colliding energies. ALICE is able to measure isolated photons down to small $p_{\mathrm{T}} \sim$ 10 GeV/$c$, thus extending previous measurements down to small $x$. In this talk, the isolated photon distributions measured in pp and p--Pb collisions at $\sqrt{s}_{\mathrm{NN}}$ = 5.02 TeV will be presented.   

Recent Photon, Jet, and Two-particle Correlations Results at RHIC

I will discuss recent developments in jet physics generally at RHIC including new detectors, and results on photons, jets, and two-particle correlations.   

Toward Mueller-Tang Jets at Next-to-Leading Order

We present recent theoretical progress in the description of the production of two jets separated by a rapidity interval void of radiation. This process is known as Mueller-Tang jets. This process is of high interest, as one might be able to investigate Balitsky-Fadin-Kuraev-Lipatov (BFKL) dynamics which emerges in the high-energy limit of quantum chromodynamics (QCD). In these regimes, the color-singlet exchange is favored over other color representations as they tend to be associated with large amount of radiation as the rapidity difference between the jets increases. On the experimental side, fair agreement has been found between BFKL predictions and Tevatron data. The CMS experiment has presented results at collision energies of 7 and 13 TeV. However, no clear-cut evidence of the BFKL dynamics can be claimed yet. It is possible that this is due to an incomplete theoretical description in existing models for the comparison with data, rather than the validity of the process itself to test BFKL predictions. In this talk, we present progress toward such a task. We recall some of the difficulties encountered along the way, as well as strategies to overcome these.