Jet physics at the LHC with ALICE

In central Pb-Pb collisions at the LHC, jet rates are expected to be high at energies at which ALICE can reconstruct jets over the background of the underlying event. This will open the possibility to quantify the effect of partonic energy loss through medium induced gluon radiation, jet quenching, by detailed measurement of the modification of the longitudinal and transverse structure of identified jets. In order to obtain probes sensitive to the properties of the QCD medium, it is mandatory to measure the high- parton fragments together with the low- particles from the radiated gluons. Hence, the excellent charged particle tracking capabilities of ALICE combined with the proposed electromagnetic calorimeter for ALICE, EMCAL, represent an ideal tool for jet quenching studies at the LHC. Electronic Supplementary Material Supplementary material is available for this article at PACS: 25.75.Nq, 24.85. + p, 13.87.-a     

Jet measurements by ALICE at LHC

Jets are collimated sprays of particles originating from fragmentation of high energy partons produced in a hard collision. They are an important diagnostic tool in studies of the Quark Gluon Plasma (QGP). The modification of the jet fragmentation pattern and its structure is a signature for the influence of hot and dense matter on the parton fragmentation process. Jet measurements in proton-proton collisions provide a baseline for similar measurements in heavy-ion collisions, while studies in proton-nucleus system allow to estimate cold nuclear matter effects. Here we present jet studies in different colliding systems (p–p, p–Pb, Pb–Pb) performed by the ALICE collaboration at LHC energies. Results on jet spectra, cross sections, nuclear modification factors, jet structure and other kinematic observables will be presented.     

Cold nuclear matter effects on dijet productions in relativistic heavy-ion reactions at LHC

We investigate the cold nuclear matter (CNM) effects on dijet productions in high-energy nuclear collisions at LHC with the next-to-leading order perturbative QCD. The nuclear modifications for dijet angular distributions, dijet invariant mass spectra, dijet transverse momentum spectra and dijet momentum imbalance due to CNM effects are calculated by incorporating EPS, EKS, HKN and DS parametrization sets of parton distributions in nucleus. It is found that dijet angular distributions and dijet momentum imbalance are insensitive to the initial-state CNM effects and thus provide optimal tools to study the final-state hot QGP effects such as jet quenching. On the other hand, the invariant mass spectra and the transverse momentum spectra of dijet are generally enhanced in a wide region of the invariant mass or transverse momentum due to CNM effects with a feature opposite to the expected suppression because of the final-state parton energy loss effect in the QGP. The difference of EPS, EKS, HKN and DS parametrization sets of nuclear parton distribution functions is appreciable for dijet invariant mass spectra and transverse momentum spectra at p+Pb collisions, and becomes more pronounced for those at Pb+Pb reactions.     

Jet quenching pattern at LHC in PYQUEN model

The first LHC data on high transverse momentum hadron and dijet spectra in PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed in the frameworks of PYQUEN jet quenching model. The presented studies for the nuclear modification factor of high-p _T hadrons and the imbalance in dijet transverse energy support the supposition that the intensive wide-angular (“out-of-cone”) medium-induced partonic energy loss is seen in central PbPb collisions at the LHC.     

Estimating nonlinear effects in forward dijet production in ultra-peripheral heavy ion collisions at the LHC

Using the framework that interpolates between the leading power limit of the color glass condensate and the high energy (or \(k_{T}\)) factorization we calculate the direct component of the forward dijet production in ultra-peripheral \(\mathrm {Pb}\)–\(\mathrm {Pb}\) collisions at CM energy \(5.1\,\mathrm {TeV}\) per nucleon pair. The formalism is applicable when the average transverse momentum of the dijet system \(P_{T}\) is much bigger than the saturation scale \(Q_{s}\), \(P_{T}\gg Q_{s}\), while the imbalance of the dijet system can be arbitrary. The cross section is uniquely sensitive to the Weizsäcker–Williams (WW) unintegrated gluon distribution, which is far less known from experimental data than the most common dipole gluon distribution appearing in inclusive small-x processes. We have calculated cross sections and nuclear modification ratios using WW gluon distribution obtained from the dipole gluon density through the Gaussian approximation. The dipole gluon distribution used to get WW was fitted to the inclusive HERA data with the nonlinear extension of unified BFKL + DGLAP evolution equation. The saturation effects are visible but rather weak for realistic \(p_{T}\) cut on the dijet system, reaching about 20% with the cut as low as \(6\,\mathrm {GeV}\). We find that the LO collinear factorization with nuclear leading-twist shadowing predicts quite similar effects.     

Jet shape modification at LHC energies by JEWEL

Jet shape measurements are employed to explore the microscopic evolution mechanisms of parton-medium interaction in ultra-relativistic heavy-ion collisions. In this study, jet shape modifications are quantified in terms of the fragmentation function F(z), relative momentum p_T~(rel), density of charged particles p(r), jet angularity girth, jet momentum dispersion p_T~(disp),and LeS ub for proton-proton(pp) collisions at 0.9, 2.76. 5.02, 7,and 13 TeV, as well as for lead-lead collisions at 2.76 TeV and 5.02 TeV by JEWEL. A differential jet shape parameter Dgirth is proposed and studied at a smaller jet radius r 0.3. The results indicate that the medium has the dominant effect on jet shape modification, which also has a weak dependence on the center-of-mass energy. Jet fragmentation is enhanced significantly at very low z 0.02, and fragmented jet constituents are linearly spread to larger jet-radii for p_T~(rel) 1. The waveform attenuation phenomena is observed in p_T~(rel),girth, and Dgirth distributions. The results obtained for D_(girth) from pp to Pb + Pb, where the wave-like distribution in pp collision is ahead of Pb + Pb collisions at small jetradii, indicates a strong medium effect.     

FCNC-induced heavy-quark events at the LHC from supersymmetry

We analyze the production and subsequent decay of the neutral Higgs bosons h≡h⁰,H⁰,A⁰$h\equiv h^0,H^0,A^0$ of the MSSM into electrically neutral quark pairs of different flavors (qq^′≡tc,bs$q{q}^{\prime }\equiv tc,bs$, depending on h  ) at the LHC, i.e. σ(pp→h→qq^′)$\sigma (pp\to h\to q{q}^{\prime })$, and compare with the direct FCNC production mechanisms σ(pp→qq^′)$\sigma (pp\to q{q}^{\prime })$. The cross-sections are computed in the unconstrained MSSM with minimal flavor-mixing sources and taking into account the stringent bounds from b→sγ$b\to s\gamma$. We extend the results previously found for these FCNC processes, which are singularly uncommon in the SM. Specifically, we report here on the SUSY-EW part of σ(pp→h→qq^′)$\sigma (pp\to h\to q{q}^{\prime })$ and the SUSY-QCD and SUSY-EW contributions to σ(pp→bs)$\sigma (pp\to bs)$. In this way, the complete map of MSSM predictions for the qq^′$q{q}^{\prime }$-pairs produced at the LHC becomes available. The upshot is that the most favorable channels are: (1) the Higgs boson FCNC decays into bs, and (2) the direct production of tc   pairs, both of them at the ∼1 pb$\sim 1\text{pb}$ level and mediated by SUSY-QCD effects. If, however, the SUSY-QCD part is suppressed, we find a small SUSY-EW yield for σmax_{(pp→h→tc)}∼¹⁰⁻⁴ pb$\sigma {(pp\to h\to tc)}_{\max }\sim {10}^{\mathrm{-4}}\text{pb}$ but, at the same time, σmax_{(pp→h→bs)}∼0.1–1 pb$\sigma {(pp\to h\to bs)}_{\max }\sim 0.1\text{–}1\text{pb}$, which implies a significant number (∼¹⁰⁴–¹⁰⁵$\sim {10}^4\text{–}{10}^5$) of bs   pairs per 100 fb⁻¹$100{\text{fb}}^{\mathrm{-1}}$ of integrated luminosity.     

On resonance search in dilepton events at the LHC

The main distribution for a bump search is the dilepton-invariant-mass distribution with appropriate cut on an absolute value of pseudorapidity difference Δη ≡ |η₁ ? η₂| between the two leptons. The back-ground from the Standard Model Drell-Yan process contributes mainly to the central pseudorapidity region Δη ≈ 0. By contrast, the excited bosons lead to a peak at Δη ≈ 1.76. We show that this property allows one to enhance the significance of their bump search by means of the new cut optimization. Nevertheless, in order to confirm an observation of the bump and reveal the resonance nature, other angular distributions should be used in addition.     

Charged Top-Pion Production Associated with a Gluon Jet at the LHC

The charged top-pions have been predicted by the topcolour-assisted technicolour （TC2） model and the observation of the charged top-pions can be regarded as the robust evidence of the model. We study the charged top-pion and gluon associated production at the LHC, i.e., pp → gIIe^＋. The cross section of this process can reach tens of pb, even hundreds ofpb, and there will be enough gIIt^＋ events produced at the LHC. Because the gIIt^＋ production is a flavour-changing （FC） process, the SM background can be greatly depressed. Furthermore, there exists an FC decay mode cb for the charged top-pions, and such decay mode can provide us with the typical signals to detect the charged top-pions. Therefore, it is hopeful to find the charged top-pions via the gIIe^＋ production at the LHC.     

Fluctuations and asymmetric jet events in PbPb collisions at the LHC

Recent LHC results concerning full jet-quenching in Pb Pb collisions have been presented in terms of a jet asymmetry parameter, measuring the imbalance between the transverse momenta of leading and subleading jets. We examine the potential sensitivity of this distribution to fluctuations from the heavy-ion background. Our results suggest that a detailed estimate of the true fluctuations would be of benefit in extracting quantitative information about jet quenching. We also find that the apparent impact of fluctuations on the jet asymmetry distribution can depend significantly on the choice of low-p _t threshold used for the simulation of the hard pp events.     

Jet flavor tomography of quark gluon plasmas at RHIC and LHC

A new Monte Carlo model of jet quenching in nuclear collisions, CUJET1.0, is applied to predict the jet flavor dependence of the nuclear modification factor for fragments f=π,D,B,e(-) from quenched jet flavors g,u,c,b in central collisions at RHIC and LHC. The nuclear modification factors for different flavors are predicted to exhibit a novel level crossing pattern over a transverse momentum range 5相似文献   

Probing vector-like top partner from same-sign dilepton events at the LHC

The standard model is a successful theory but is lacking a mechanism for neutrino mass generation as well as a solution to the naturalness problem. In the models that are proposed to simultaneously solve the two problems, heavy Majorana neutrinos and top partners are usually predicted to lead to a new decay mode of the top partner mediated by the heavy Majorana neutrinos: $Tto b,{W}^{+}to b,{{ell }}^{+}{{ell }}^{+}qbar{q^{prime} }$. In this paper, we will study the observability of such a new signature via the pair production process of a top partner ${pp}to Tbar{T}to 2b+{{ell }}^{pm }{{ell }}^{pm }+4j$ in a model independent way. By performing Monte Carlo simulations, we present the 2σ exclusion limits of the top partner mass and mixing parameters at the HL-LHC.     

Wedgebox analysis of four-lepton events from neutralino pair production at the LHC

A ‘wedgebox’ plot is a two-dimensional scatter-plot of two invariant mass quantities. Here pp→e⁺e^-μ⁺μ^-+≠E signature LHC events are analyzed by plotting the di-electron invariant mass versus the di-muon invariant mass. Data sets of such events are obtained across the MSSM input parameter space in realistic event-generator simulations, including cuts designed to remove SM backgrounds. Their study reveals several general features. Firstly, regions in the MSSM input parameter space where a sufficient number of events are expected so as to be able to construct a clear wedgebox plot are delineated. Secondly, the presence of box shapes on a wedgebox plot either indicates the presence of heavy Higgs bosons’ decays or restricts the location to a quite small region of low μ and M₂ values, ≲ 200 GeV, a region denoted as the ‘lower island’. In this region, wedgebox plots can be quite complicated and change in pattern rather quickly as one moves around in the (μ,M₂) plane. Thirdly, direct neutralino pair production from an intermediate Z^0* may only produce a wedge shape since only decays can contribute significantly. And fourthly, a double-wedge or wedge-protruding-from-a-box pattern on a wedgebox plot, which results from combining a variety of MSSM production processes, yields three distinct observed endpoints, almost always attributable to decays, which can be utilized to determine a great deal of information about the neutralino and slepton mass spectra and related MSSM input parameters. Wedge and double-wedge patterns are seen in wedgebox plots in another region of higher μ and M₂ values, denoted as the ‘upper island’. Here the pattern is simpler and more stable as one moves across the (μ,M₂) input parameter space.     

Observation of a centrality-dependent dijet asymmetry in lead-lead collisions at sqrt[S(NN)] =2.76 TeV with the ATLAS detector at the LHC

By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per-nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse energies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium.     

Search for supersymmetry at the LHC in events with jets and missing transverse energy

A search for events with jets and missing transverse energy is performed in a data sample of pp collisions collected at √s=7 TeV by the CMS experiment at the LHC. The analyzed data sample corresponds to an integrated luminosity of 1.14 fb(-1). In this search, a kinematic variable α(T) is used as the main discriminator between events with genuine and misreconstructed missing transverse energy. No excess of events over the standard model expectation is found. Exclusion limits in the parameter space of the constrained minimal supersymmetric extension of the standard model are set. In this model, squark masses below 1.1 TeV are excluded at 95% C.L. Gluino masses below 1.1 TeV are also ruled out at 95% C.L. for values of the universal scalar mass parameter below 500 GeV.     

Diffractive dijet cross sections in photoproduction at HERA

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon () and pomeron () momentum participating in the production of the dijet system. The observed dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section increases as increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron. Received: 20 April 1998 / Published online: 2 July 1998