QCD Physics Research Articles

Is It Hard Yet? The Qualitative Agreement of pQCD Energy Loss with RHIC and LHC Data

Heavy flavor research is a vigorous and active topic in high-energy QCD physics. Comparing theoretical predictions to data as a function of flavor provides a unique opportunity to tease out properties of quark-gluon plasma. We explicitly demonstrate this utility with energy loss predictions based on the assumption of 1) a weakly-coupled plasma weakly coupled to a high-pT probe using pQCD and 2) a strongly-coupled plasma strongly coupled to a high-pT probe using AdS/CFT; we find that while the former enjoys broad qualitative agreement with data, it is difficult to reconcile the latter with experimental measurements.

Heavy Quark Production and Energy Loss

Heavy flavor research is a vigorous and active topic in high-energy QCD physics. Comparing theoretical predictions to data as a function of flavor provides a unique opportunity to tease out properties of quark-gluon plasma. We explicitly demonstrate this utility with energy loss predictions based on the assumption of 1) a weakly-coupled plasma weakly coupled to a high-pT probe using pQCD and 2) a strongly-coupled plasma strongly coupled to a high-pT probe using AdS/CFT; we find that while the former enjoys broad qualitative agreement with data, it is difficult to reconcile the latter with experimental measurements.

Emergent Higgs from hidden dimensions

The Higgs mechanism well describes the electroweak symmetry breaking in nature. We consider a possibility that the microscopic origin of the Higgs field is UV physics of QCD. We construct a UV complete model of a higher dimensional Yang-Mills theory as a deformation of a deconstructed (2,0) theory in six dimensions, and couple the top and bottom (s)quarks to it. We see that the Higgs fields appear as magnetic degrees of freedom. The model can naturally explain the masses of the Higgs boson and the top quark. The rho meson-like resonances with masses such as 1 TeV are predicted.

Hadron physics in holographic QCD

Hadron physics deals with the study of strongly interacting subatomic particles such as neutrons, protons, pions and others, collectively known as baryons and mesons. Physics of strong interaction is difficult. There are several approaches to understand it. However, in the recent years, an approach called, holographic QCD, based on string theory (or gauge-gravity duality) is becoming popular providing an alternative description of strong interaction physics. In this article, we aim to discuss development of strong interaction physics through QCD and string theory, leading to holographic QCD.

Azimuthal correlations in top pair decays and the effects of new heavy scalars

We present the azimuthal correlations in the decay products of the top quark pair created in the process $gg\ensuremath{\rightarrow}t\overline{t}\ensuremath{\rightarrow}b\overline{b}{W}^{+}{W}^{\ensuremath{-}}$ calculated in leading-order QCD. We then discuss deviations in this distribution that could arise from new physics contributions, focusing, in particular, on the case of a new heavy scalar coupling to the top quark and produced via the gluon fusion mechanism. Within this scenario, we consider two cases: one in which the top quark is the only particle with a significant coupling to the scalar and another in which there are additional heavy fermions that modify the effective coupling of the scalar to gluons. We discuss the effects on the overall cross section and azimuthal correlations of the top quark decay products, with particular emphasis on the effect of interference between the QCD and new physics contributions and on the existence of $CP$-nonconserving couplings.

Phenomenology of High Gluon Density QCD and Heavy-Ion Physics at ISMD 2011: $x$ Smaller than Ever!

I provide a brief summary of the theory presentations at ISMD 2011 related to the phenomenology of small-x QCD evolution and its application to particle production and fluctuations in high-energy hadron and heavy-ion collisions. I also mention some challenges for quantitative phenomenology which emerged from the LHC, such as understanding the long-range "ridge" in high-multiplicity p+p collisions, the transverse momentum distributions in p+p at semi-hard pT, and the origin and scale of density fluctuations in the initial state of heavy-ion collisions.

QCD and Top physics studies in proton-proton collisions at 7 TeV centre-of-mass energy with the ATLAS detector

Beginning from 2010, ATLAS collected proton-proton collision data at a centre-of-mass energy of 7 TeV, accumulating by the Spring of 2011 an integrated luminosity of ∼200 pb−1. The high energy gives the possibility to produce a sizeable amount of top quarks, allowing for measurements of top quark properties and searches for heavy exotic resonances decaying to top-antitop pairs. Measurements of jet cross-sections confirm the predictions of perturbative QCD at the high energy of the Large Hadron Collider.

An Introduction to AdS/CFT Correspondence for Phenomenologists

We give a brief introduction to the AdS/CFT correspondence and its application to QCD physics, especially its application in the study of quark-gluon-plasma (QGP) formed in the relativistic heavy ion collision (RHIC). This review is based on the talks given in several schools and programs for the phenomenologists working on nuclear physics and particle physics.

Gauge/string duality applied to heavy-ion collisions: limitations, insights and prospects**Plenary talk at ‘Quark Matter 2011’, Annecy, France.

Over the last decade a fruitful interplay has developed between analyses of strongly coupled non-Abelian plasmas via the gauge/string duality and the phenomenology of the quark–gluon plasma created in heavy-ion collisions. The reasons why the gauge/string duality is not a precision tool for QCD physics at present are reviewed, with emphasis on conceptual issues. It is then argued that, nevertheless, the duality can provide valuable insights at both the quantitative and the qualitative levels. This is illustrated with a few examples and concluded with a brief discussion of future prospects.

Effective axial-vector coupling of gluon as an explanation to the top quark asymmetry

We explore the possibility that the large $t\overline{t}$ forward-backward asymmetry measured by the CDF detector at Tevatron could be due to a universal effective axial-vector coupling of gluon. Using an effective field theory approach we show model independently how such a log-enhanced coupling occurs at 1-loop level. The interference with QCD gluon vector coupling naturally induces the observed positive forward-backward $t\overline{t}$ asymmetry that grows with $t\overline{t}$ invariant mass and is consistent with the cross section measurements. This scenario does not involve new flavor changing couplings nor operators that interfere with QCD, and, therefore, is not constrained by the LHC searches for 4-quark contact interactions. We predict top quark polarization effects that grow with energy and allow to test this scenario at the LHC. Our proposal offers a viable alternative to new physics scenarios that explain the $t\overline{t}$ forward-backward asymmetry anomaly with the interference between QCD and tree level new physics amplitudes.

Exclusive quarkonium production in pp collisions

Central exclusive production (CEP) processes in high-energy proton – (anti)proton collisions offer a very promising framework within which to study both novel aspects of QCD and new physics signals. Among the many interesting processes that can be studied in this way, those involving the production of heavy ( c , b ) quarkonia states are of particular importance, serving as ‘standard candle’ processes with which we can benchmark predictions for new physics CEP at the CERN Large Hadron Collider, as well as being of interest in their own right. We report on our recent quantitative studies of exclusive heavy quarkonia ( χ and η ) production, and make cross section predictions for the LHC, Tevatron and RHIC. We also examine the possibility of observing quarkonium CEP at RHIC with tagged forward protons, for which measurements of the outgoing proton momenta would provide interesting spin-parity information about the centrally produced state.

FIRST RESULTS OF THE CMS EXPERIMENT ON QCD PHYSICS

Results presented here constitute the first achievements of the rich QCD physics program of the CMS experiment at LHC. Using data from proton-proton collisions at center-of-mass energies of 0.9, 2.36 and 7 TeV, measurements of charged particle yields, Bose-Einstein correlations and underlying event features have been produced. These are among the first results concerning the properties and interactions of charged hadrons at the new energy frontier accessible at LHC.

Small- x physics with the ALICE experiment at the CERN-LHC

High energy p–p, p–Pb and Pb–Pb collisions at the CERN-LHC offer unprecedented opportunities for studying wide variety of physics at small Bjorken- x. Here we discuss capabilities of the ALICE experiment at the CERN-LHC for probing small- x QCD physics. A new forward electromagnetic calorimeter is being proposed as an ALICE upgrade to explore the small- x region in detail.

Recent results on QCD and heavy ion physics from CMS

Recent results on QCD and heavy ion physics from CMS

EXPERIMENTAL OVERVIEW AND CHALLENGE IN STRANGENESS NUCLEAR PHYSICS — STRANGENESS IN THE PAST AND COMING DECADES

A great progress has been made in strangeness nuclear physics in the past decade. Examples are; 1) The "hyperfine" structure of hypernuclei were measured with the Hyperball, and ΛN spin dependent interactions in p -shell hypernuclei were determined. 2) The "complete measurements" of the weak decay of hypernuclei were made and the np ratio puzzle in the non-mesonic decay was solved. 3) The discovery of a clean event of "Lambpha" and determination of its binding energy concluded that the Λ-Λ interaction is weak attractive. However, we still have important questions to be answered in this field, especially in relation with QCD and nuclear physics. For the future strangeness nuclear physics, we have and will have facilities such as JLab, SPring-8, Daphne, J-PARC, FAIR. We discuss experimental challenges in the strangeness nuclear physics and related fields in the next decade.

Latest results of the CMS experiment

The Compact Muon Solenoid (CMS) experiment at CERN's Large Hadron Collider (LHC) has started to collect and analyze proton-proton collisions at = 7 TeV. At the time of this conference, CMS had collected approximately 300 nb−1 of collision data, including small samples at lower energies of 0.9 TeV and 2.36 TeV. These samples allowed some precise studies of QCD and heavy flavour physics, the first cross section measurements of the W and Z bosons at 7 TeV, the first hints of reobservation of the top quark, and even the start of the exploration of the allowed parameter space of some new physics models. The results presented here demonstrate good detector performance and illustrate well understood physics signatures that may lead in the near future to discovery physics at the LHC.

The latest results from the ATLAS experiment

The ATLAS detector at the LHC is a general purpose experiment for studying p + p collisions at TeV energies. This talk presents early ATLAS physics measurements, based on O(10 nb−1) of data from the first half year of LHC running. Topics include minimum bias and soft QCD physics, W and Z observation, and various measurements of detector performance.

Experimental Projects for Spectroscopy of Pionic Atoms and N*(1535) in Nuclei

Chiral symmetry and its spontaneous breakdown play an important role as a link between QCD and nuclear physics in the lowest energy region of strong interaction where the quark and gluon degrees of freedom are hidden and pseudo-scalar mesons of massless Nambu-Goldstone bosons appear.1) Chiral condensate 〈qq〉 is known to parametrize the symmetry breaking, and is theoretically calculated on the plane of temperature and density. The theory tells that beyond certain energy density, the chiral condensate is expected to vanish and the symmetry restores.2) There are several experimental approaches to understand the chiral dynamics especially on its relation to the mechanism of hadron mass generation. One approach utilizes head-on collisions of heavy ions with relativistically high energies in order to reach and explore extremely high temperature region.3) The other takes examination of hadrons in nuclear matter which is regarded as extremely high density material.4)–8) The above theory predicts 30% reduction of the chiral condensate at the normal nuclear density to that in vacuum.9)–11) This article briefly describes two experimental projects to study the chiral dynamics both in the meson and in the baryon sectors,12) namely, “Precision spectroscopy of pionic atoms”13) and “N∗(1535) production and its in-medium spectroscopy”.14) The former pursues high precision systematic spectroscopy of deeply bound pionic atoms∗) in the RI beam factory in RIKEN. The systematic spectroscopy will serve unique and high quality information on the in-medium strong interaction strength between the pion and nuclei, which improves the precision of the quantitative estimation of the chiral condensate at the normal nuclear density.5) The latter, “N∗(1535) production and its in-medium spectroscopy”, has its own particular interest.14),15) According to the theories, N∗(1535) is a candidate of chiral partner of nucleons,12),16),17) and thus, the mass difference of the N∗(1535) and nucleon is reduced as the chiral symmetry restores. In-medium N∗(1535) spectroscopy will provide strong evidence of the hadron mass generation through direct measurement of the mass difference through possible level crossing between N∗(1535)-hole and η modes in the nuclear medium.18),19)

Standard candle central exclusive processes at the Tevatron and LHC

Central exclusive production (CEP) processes in high-energy proton -- (anti)proton collisions offer a very promising framework within which to study both novel aspects of QCD and new physics signals. Among the many interesting processes that can be studied in this way, those involving the production of heavy (c,b) quarkonia and gamma gamma states have sufficiently well understood theoretical properties and sufficiently large cross sections that they can serve as `standard candle' processes with which we can benchmark predictions for new physics CEP at the CERN Large Hadron Collider. Motivated by the broad agreement with theoretical predictions of recent CEP measurements at the Fermilab Tevatron, we perform a detailed quantitative study of heavy quarkonia (chi and eta) and gamma gamma production at the Tevatron, RHIC and LHC, paying particular attention to the various uncertainties in the calculations. Our results confirm the rich phenomenology that these production processes offer at present and future high-energy colliders.

Color superconductivity at largeN: A new hope

At zero density, the `t Hooft large N_c limit often provides some very useful qualitative insights into the non-perturbative physics of QCD. However, it is known that at high densities the `t Hooft large N_c world looks very different from the N_c=3 world, which is believed to be in a color superconducting phase at high densities. At large N_c, on the other hand, the DGR instability causes a chiral-density wave phase to dominate over the color superconducting phase. There is an alternative large N_c limit, with the quarks transforming in the two-index antisymmetric representation of the gauge group, which at N_c=3 reduces to QCD but looks quite different at large N_c. We show that in this alternative large N_c limit, the DGR instability does not occur, so that it may be plausible that the ground state of high-density quark matter is a color superconductor even when N_c is large. This revives the hope that a large N_c approximation might be useful for getting some insights into the high-density phenomenology of QCD.