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Register a new userVector boson production in proton-lead and lead-lead collisions at the LHC and its impact on nCTEQ15 PDFs
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We provide a comprehensive comparison of W/Z vector boson production data in proton-lead and lead-lead collisions at the LHC with predictions obtained using the nCTEQ15 PDFs. We identify the measurements which have the largest potential impact on the PDFs, and estimate the effect of including these data using a Monte Carlo reweighting method. We find this data set can provide information about both the nuclear corrections and the heavy flavor (strange) PDF components. As the proton flavor determination is dependent on nuclear corrections (from heavy target DIS, for example), this information can also help improve the proton PDFs.
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If the fundamental planck scale is near a TeV, then parton collisions with high enough center-of-mass energy should produce black holes. The production rate for such black holes at LHC has been extensively studied for the case of a proton-proton collision. In this paper, we extend this analysis to a lead-lead collision at LHC. We find that the cross section for small black holes which may in principle be produced in such a collision is either enhanced or suppressed, depending upon the black hole mass. For example, for black holes with a mass around 3 TeV we find that the differential black hole production cross section, dsigma/dM, in a typical lead-lead collision is up to 90 times larger than that for black holes produced in a typical proton-proton collision. We also discuss the cross-sections for `string ball production in these collisions. For string balls of mass about 1 (2) TeV, we find that the differential production cross section in a typical lead-lead collision may be enhanced by a factor up to 3300 (850) times that of a proton-proton collision at LHC.
We compare predictions of nCTEQ15 nuclear parton distribution functions with proton-lead vector boson production data from the LHC. We select data sets that are most sensitive to nuclear PDFs and have potential to constrain them. We identify the kinematic regions and flavours where these data can bring new information and will have largest impact on the nuclear PDFs. Finally, we estimate the effect of including these data in a global analysis using a reweighting method.
We present a systematic theoretical analysis of the ALICE measurement of low-$p_T$ direct-photon production in central lead-lead collisions at the LHC with a centre-of-mass energy of $sqrt{s_{NN}}=2.76$ TeV. Using next-to-leading order of perturbative QCD, we compute the relative contributions to prompt-photon production from different initial and final states and the theoretical uncertainties coming from independent variations of the renormalisation and factorisation scales, the nuclear parton densities and the fragmentation functions. Based on different fits to the unsubtracted and prompt-photon subtracted ALICE data, we consistently find an exponential, possibly thermal, photon spectrum from the quark-gluon plasma (or hot medium) with slope $T=304pm 58$ MeV and $309pm64$ MeV at $p_Tin[0.8;2.2]$ GeV and $p_Tin[1.5;3.5]$ GeV as well as a power-law ($p_T^{-4}$) behavior for $p_T>4$ GeV as predicted by QCD hard scattering.
The first observation of $Z$ boson production in proton-lead collisions at a centre-of-mass energy per proton-nucleon pair of $sqrt{s_{NN}}=5~text{TeV}$ is presented. The data sample corresponds to an integrated luminosity of $1.6~text{nb}^{-1}$ collected with the LHCb detector. The $Z$ candidates are reconstructed from pairs of oppositely charged muons with pseudorapidities between 2.0 and 4.5 and transverse momenta above $20~text{GeV}/c$. The invariant dimuon mass is restricted to the range $60-120~text{GeV}/c^2$. The $Z$ production cross-section is measured to be begin{eqnarray*} sigma_{Ztomu^+mu^-}(text{fwd})&=&13.5^{+5.4}_{-4.0}text{(stat.)}pm1.2text{(syst.)}~text{nb} end{eqnarray*} in the direction of the proton beam and begin{eqnarray*} sigma_{Ztomu^+mu^-}(text{bwd}) & =&10.7^{+8.4}_{-5.1}text{(stat.)}pm1.0text{(syst.)}~text{nb} end{eqnarray*} in the direction of the lead beam, where the first uncertainty is statistical and the second systematic.
The extraction of the strange quark parton distribution function (PDF) poses a long-standing puzzle. Measurements from neutrino-nucleus deep inelastic scattering (DIS) experiments suggest the strange quark is suppressed compared to the light sea quarks, while recent studies of W/Z boson production at the LHC imply a larger strange component at small x values. As the parton flavor determination in the proton depends on nuclear corrections, e.g. from heavy-target DIS, LHC heavy ion measurements can provide a distinct perspective to help clarify this situation. In this investigation we extend the nCTEQ15 nPDFs to study the impact of the LHC proton-lead W/Z production data on both the flavor differentiation and nuclear corrections. This complementary data set provides new insights on both the LHC W/Z proton analyses and the neutrino-nucleus DIS data. We identify these new nPDFs as nCTEQ15WZ. Our calculations are performed using a new implementation of the nCTEQ code (nCTEQ++) based on C++ which enables us to easily interface to external programs such as HOPPET, APPLgrid and MCFM. Our results indicate that, as suggested by the proton data, the small x nuclear strange sea appears larger than previously expected, even when the normalization of the W/Z data is accommodated in the fit. Extending the nCTEQ15 analysis to include LHC W/Z data represents an important step as we advance toward the next generation of nPDFs.
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