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 perturbativ
e 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.
Next-to-leading order (NLO) QCD predictions for the production of heavy quarks in proton-proton collisions are presented within three different approaches to quark mass, resummation and fragmentation effects. In particular, new NLO and parton shower
simulations with POWHEG are performed in the ALICE kinematic regime at three different centre-of-mass energies, including scale and parton density variations, in order to establish a reliable baseline for future detailed studies of heavy-quark suppression in heavy-ion collisions. Very good agreement of POWHEG is found with FONLL, in particular for centrally produced D^0, D^+ and D^*+ mesons and electrons from charm and bottom quark decays, but also with the generally somewhat higher GM-VFNS predictions within the theoretical uncertainties. The latter are dominated by scale rather than quark mass variations. Parton density uncertainties for charm and bottom quark production are computed here with POWHEG for the first time and shown to be dominant in the forward regime, e.g. for muons coming from heavy-flavour decays. The fragmentation into D_s^+ mesons seems to require further tuning within the NLO Monte Carlo approach.
We present a rigorous 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 NLO QCD, we compute the relative contribut
ions 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 T = 304 pm 58 MeV and 309 pm 64 MeV for the effective temperature of the quark-gluon plasma (or hot medium) at p_T in [0.8;2.2] GeV and p_T in [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.
We present an extensive analysis of gauge-mediated supersymmetry breaking models with minimal and non-minimal flavour violation. We first demonstrate that low-energy, precision electroweak, and cosmological constraints exclude large collider-friendly
regions of the minimal parameter space. We then discuss various possibilities how flavour violation, although naturally suppressed, may still occur in gauge-mediation models. The introduction of non-minimal flavour violation at the electroweak scale is shown to relax the stringent experimental constraints, so that benchmark points, that are also cosmologically viable, can be defined and their phenomenology, i.e. squark and gaugino production cross sections with flavour violation, at the LHC can be studied.
We present an exploratory study of gaugino-pair production in polarized and unpolarized hadron collisions, focusing on the correlation of beam polarization and gaugino/Higgsino mixing in the general Minimal Supersymmetric Standard Model. Helicity-dep
endent cross sections induced by neutral and charged electroweak currents and squark exchanges are computed analytically in terms of generalized charges, defined similarly for chargino-pair, neutralino-chargino associated, and neutralino-pair production. Our results confirm and extend those obtained previously for negligible Yukawa couplings and nonmixing squarks. Assuming that the lightest chargino mass is known, we show numerically that measurements of the longitudinal single-spin asymmetry at the existing polarized pp collider RHIC and at possible polarization upgrades of the Tevatron or the LHC would allow for a determination of the gaugino/Higgsino fractions of charginos and neutralinos. The theoretical uncertainty coming from factorization scale and squark mass variations and the expected experimental error on the lightest chargino mass is generally smaller than the one induced by the polarized parton densities, so that more information on the latter would considerably improve on the analysis.
We improve the theoretical predictions for the production of extra neutral gauge bosons at hadron colliders by implementing the Z bosons in the MC@NLO generator and by computing their differential and total cross sections in joint p_T and threshold r
esummation. The two improved predictions are found to be in excellent agreement with each other for mass spectra, p_T spectra, and total cross sections, while the PYTHIA parton and ``power shower predictions usually employed for experimental analyses show significant shortcomings both in normalization and shape. The theoretical uncertainties from scale and parton density variations and non-perturbative effects are found to be 9%, 8%, and less than 5%, respectively, and thus under good control. The implementation of our improved predictions in terms of the new MC@NLO generator or resummed K factors in the analysis chains of the Tevatron and LHC experiments should be straightforward and lead to more precise determinations or limits of the Z boson masses and/or couplings.
We present a precision calculation of the transverse-momentum and invariant-mass distributions for supersymmetric particle pair production at hadron colliders, focusing on Drell-Yan like slepton pair and slepton-sneutrino associated production at the
CERN Large Hadron Collider. We implement the joint resummation formalism at the next-to-leading logarithmic accuracy with a process-independent Sudakov form factor, thus ensuring a universal description of soft-gluon emission, and consistently match the obtained result with the pure perturbative result at the first order in the strong coupling constant, i.e. at O(alpha_s). We also implement three different recent parameterizations of non-perturbative effects. Numerically, we give predictions for ~e_R ~e_R^* production and compare the resummed cross section with the perturbative result. The dependence on unphysical scales is found to be reduced, and non-perturbative contributions remain small.
We present an extensive analysis of squark and gaugino hadroproduction and decays in non-minimal flavour violating supersymmetry. We employ the so-called super-CKM basis to define the possible misalignment of quark and squark rotations, and we use ge
neralized (possibly complex) charges to define the mutual couplings of (s)quarks and gauge bosons/gauginos. The cross sections for all squark-(anti-)squark/gaugino pair and squark-gaugino associated production processes as well as their decay widths are then given in compact analytic form. For four different constrained supersymmetry breaking models with non-minimal flavour violation in the second/third generation squark sector only, we establish the parameter space regions allowed/favoured by low-energy, electroweak precision, and cosmological constraints and display the chirality and flavour decomposition of all up- and down-type squark mass eigenstates. Finally, we compute numerically the dependence of a representative sample of production cross sections at the LHC on the off-diagonal mass matrix elements in the experimentally allowed/favoured ranges.
