Drell-Yan Reaction Research Articles

Development of a precise time and position resolution TOF-tracker MRPC for the [formula omitted]20 beam line at J-PARC

We are developing time-of-flight (TOF) multi-gap Resistive Plate Chambers (MRPCs) which perform both timing and hit position measurements, the TOF-tracker MRPC, for muon detection to measure exclusive Drell–Yan reaction (π−p→γ∗n→μ+μ−n) at J-PARC’s hadron hall. Here we report on the development of a prototype TOF-tracker MRPC with a sensitive area of 500×1000mm2 and its performance in an electron-positron particle beam at the SPring-8 synchrotron in Japan.

Atomic mass, Bjorken variable, and scale dependence of quark transport coefficient in Drell-Yan process for proton incident on nucleus* *Supported in part by the National Natural Science Foundation of China (11975090, 11575052).

By means of the nuclear parton distributions determined without the fixed-target Drell-Yan experimental data and the analytic expression of quenching weight based on the BDMPS formalism, next-to-leading order analyses were performed on the Drell-Yan differential cross section ratios from the Fermilab E906 and E866 collaborations. It was found that the results calculated only with the nuclear effects of the parton distribution were not in agreement with the E866 and E906 experimental data. The incoming parton energy loss effect cannot be ignored in the nuclear Drell-Yan reactions. The predicted results indicate that, with the quark transport coefficient as a constant, the suppression due to the target nuclear geometry effect is approximately% for the quark transport coefficient. It was shown that we should consider the target nuclear geometry effect in studying the Drell-Yan reaction on nuclear targets. On the basis of the Bjorken variable and scale dependence of the quark transport coefficient, the atomic mass dependence was incorporated. The quark transport coefficient was determined as a function of the atomic mass, Bjorken variable, and scale by the global fit of the experimental data. The determined constant factor of the quark transport coefficient is GeV/fm. It was found that the atomic mass dependence has a significant impact on the constant factor in the quark transport coefficient in cold nuclear matter.

First Measurement of the Z→μ^{+}μ^{-} Angular Coefficients in the Forward Region of pp Collisions at sqrt[s]=13 TeV.

The first study of the angular distribution of μ^{+}μ^{-} pairs produced in the forward rapidity region via the Drell-Yan reaction pp→γ^{*}/Z+X→ℓ^{+}ℓ^{-}+X is presented, using data collected with the LHCb detector at a center-of-mass energy of 13TeV, corresponding to an integrated luminosity of 5.1 fb^{-1}. The coefficients of the five leading terms in the angular distribution are determined as a function of the dimuon transverse momentum and rapidity. The results are compared to various theoretical predictions of the Z-boson production mechanism and can also be used to probe transverse-momentum-dependent parton distributions within the proton.

Transverse momentum and transverse momentum distributions in the MIT bag model

The typical transverse momentum of a quark in the proton is a basic property of any QCD based model of nucleon structure. However, calculations in phenomenological models typically give rather small values of transverse momenta, which are difficult to reconcile with the larger values observed in high energy experiments such as Drell-Yan reactions and Semi-inclusive deep inelastic scattering. In this letter we calculate the leading twist transverse momentum dependent distribution functions (TMDs) using a generalization of the Adelaide group's relativistic formalism that has previously given good fits to the parton distributions. This enables us to examine the kT dependence of the TMDs in detail, and determine typical widths of these distributions. These are found to be significantly larger than those of previous calculations. We then use TMD factorization in order to evolve these distributions up to experimental scales where we can compare with data on 〈kT〉 and 〈kT2〉. Our distributions agree well with this data.

An effective field theory treatment of the production and annihilation of magnetic monopoles and their relic abundance

We revisit the thermal production and annihilation of magnetic monopoles and their relic abundance in order to gain a deeper physical interpretation on the monopole phenomenology predicted from the Baines et al.’s effective field theory, recently proposed in the description of monopole pair production via Drell–Yan and photon fusion processes. In this sense, we use of the vacuum cross sections for the Drell–Yan reactions derived within the mentioned framework to evaluate the cross section averaged over the thermal distribution associated to other particles that constitute the hot medium where the monopoles propagate. In the considered range of monopole mass with spin-zero and spin-half, our findings suggest that the thermally averaged cross sections for the pair production are highly suppressed, while at higher temperatures those for the annihilation of lighter pairs reach larger magnitudes. Besides, we observe that smaller temperature leads to a rate of annihilation for scalar monopoles smaller than the one for fermionic monopoles, which might be interpreted as a theoretical evidence of a more pronounced stability for spin-zero and heavier monopoles. Then we input these thermally averaged cross sections into the kinetic equation that describes the evolution of the monopole abundance via an extension of a freeze-out theory. Our results infer that heavier monopoles achieve the equilibrium at earlier stages of the expansion, and consequently at higher temperatures. In addition, larger monopole masses produce higher values of the relic abundance. Besides, the results indicate that the abundance does not behave differently for spin-zero and spin-half relic monopoles.

New study of the Boer-Mulders function: Implications for the quark and hadron transverse momenta

In series of papers the Boer-Mulders function for a given quark flavour has been extracted: (i) from data on semi-inclusive deep inelastic scattering, using the simplifying, but theoretically inconsistent, assumption that it is proportional to the Sivers function for each quark flavour and (ii) from data on Drell-Yan reactions. In earlier papers, using the semi-inclusive deep inelastic COMPASS deuteron data on the $\langle\cos\phi_h\rangle$ and $\langle\cos 2 \phi_h\rangle$ asymmetries, we extracted the collinear $x_{_{\!B}}$ dependence of the Boer-Mulders function for the sum of the valence quarks $Q_V=u_V + d_V$ using a small number of model dependent assumptions and found a significant disagreement with the analysis in (i). In the present paper, we provide a more complete analysis of the semi-inclusive deep inelastic scattering reaction, including a discussion of higher twist and interaction-dependent terms, and also a comparison with the Boer-Mulders function extracted from data on the Drell-Yan reaction. We confirm that the proportionality relation of the BM function to the Sivers function, for each quark flavour, fails badly, but find that it holds rather well if applied to the nonsinglet valence-quark combination, $Q_V$. We also find good agreement with the results of the Drell-Yan analysis. Furthermore, we obtain interesting information on the quark transverse momentum densities in the nucleon and on the hadron transverse momentum dependence in quark fragmentation.

Study ofW±andZboson production in proton-lead collisions at the LHC with Kulagin-Petti nuclear parton distributions

We present a detailed study of the (pseudo)rapidity distributions of massive vector bosons produced in p+p and p+Pb collisions at the LHC within the next-to-leading order approximation in perturbative quantum chromodynamics. In particular, we discuss the impact of different cold nuclear matter effects on this process using the nuclear parton distributions calculated from the microscopic model developed by Kulagin and Petti (KP). This model was successfully applied to study nuclear effects in the deep-inelastic scattering and the Drell-Yan reactions off various (fixed) target nuclei. Results are compared with the recent CMS and ATLAS p+Pb data with \sqrt{s}=5.02 TeV per two colliding nucleons. We found an excellent agreement between the predictions of the KP model and the recent LHC data on W+- and Z0 production in p+Pb collisions, including the differential cross sections, the forward-backward asymmetries, and W charge asymmetry. We also discuss the sensitivity of the current and future LHC data to the underlying mechanisms responsible for the nuclear modifications of parton density functions.

New results on transverse spin asymmetries from COMPASS

One of the important objectives of the COMPASS experiment is the exploration of transverse spin structure of nucleon via spin (in)dependent azimuthal asymmetries in semi-inclusive deep inelastic scattering (SIDIS) of polarized leptons (and soon also Drell-Yan (DY) reactions with $\pi^-$) off transversely polarized target. For this purpose a series of measurements were made in COMPASS, using 160 GeV/c longitudinally polarized muon beam and polarized $^6LiD$ and $NH_3$ targets and are foreseen with 190 GeV/c $\pi^-$ beam on polarized $NH_3$. The experimental results obtained by COMPASS for azimuthal effects in SIDIS play an important role in the general understanding of the three-dimensional nature of the nucleon and are widely used in theoretical analyses and global data fits. Future first ever polarized DY-data from COMPASS compared with SIDIS results will open a new chapter probing general principles of QCD TMD-formalism. In this review main focus will be given to the very recent COMPASS results obtained for SIDIS transverse spin asymmetries from four "Drell-Yan" $Q^2$-ranges.

Spin correlations in the Drell–Yan process, parton entanglement, and other unconventional QCD effects

We review ideas on the structure of the QCD vacuum which had served as motivation for the discussion of various non-standard QCD effects in high-energy reactions in articles from 1984 to 1995. These effects include, in particular, transverse-momentum and spin correlations in the Drell–Yan process and soft photon production in hadron–hadron collisions. We discuss the relation of the approach introduced in the above-mentioned articles to the approach, developed later, using transverse-momentum-dependent parton distributions (TDMs). The latter approach is a special case of our more general one which allows for parton entanglement in high-energy reactions. We discuss signatures of parton entanglement in the Drell–Yan reaction. Also for Higgs-boson production in pp collisions via gluon–gluon annihilation effects of entanglement of the two gluons are discussed and are found to be potentially important. These effects can be looked for in the current LHC experiments. In our opinion studying parton-entanglement effects in high-energy reactions is, on the one hand, very worthwhile by itself and, on the other hand, it allows to perform quantitative tests of standard factorisation assumptions. Clearly, the experimental observation of parton-entanglement effects in the Drell–Yan reaction and/or in Higgs-boson production would have a great impact on our understanding how QCD works in high-energy collisions.

Polarized hadron pair production from electron-positron annihilation

We study the production of two almost back-to-back hadrons from the annihilation of an electron and a positron, allowing for the polarization of all particles involved. In particular, we conduct a general (model-independent) structure function decomposition of the cross section for the case ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}\ensuremath{\rightarrow}{h}_{a}{h}_{b}X$. Moreover, using the parton model we calculate the relevant structure functions in terms of twist-2 transverse momentum dependent (TMD) fragmentation functions (FFs). We also give results for the situation ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{Z}^{*}\ensuremath{\rightarrow}{h}_{a}{h}_{b}X$ (including $\ensuremath{\gamma}\text{\ensuremath{-}}Z$ interference) within this model. This is the first time a complete framework has been presented for the examination of TMD FFs within ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{h}_{a}{h}_{b}X$. We also specify certain parts of our analysis that hold for the triple-polarized semi-inclusive deep-inelastic scattering process and for di-hadron fragmentation. Furthermore, we give an explicit prescription of how our work can be translated to the Drell-Yan reaction, which provides, for the first time, full results for double-polarized Drell-Yan that include electroweak effects. We further discuss the relevance of our ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{h}_{a}{h}_{b}X$ results for future experiments at ${e}^{+}{e}^{\ensuremath{-}}$ machines.

Results and perspective on TMDs and GPDs at COMPASS

COMPASS is a fixed target experiment at the CERN SPS dedicated to the study of the nucleon structure in the framework of the most complete descriptions based on transverse momentum dependent parton distribution functions (TMDs) and generalised parton distributions (GPDs). TMDs have been accessed so far with semi-inclusive deep inelastic reactions using a 160GeV muon beam off transversely polarized targets and they will be further investigated with Drell-Yan reactions using a 190GeV pion beam off a transversely polarized proton target. GPDs will be studied with deeply virtual Compton scattering and hard exclusive meson production using muon beams. A selection of the results as well as the projections for the future program are discussed.

Soft and collinear factorization and transverse momentum dependent parton distribution functions

In this work we consider how a parton distribution function, with an explicit transverse momentum dependence can be properly defined in a regularization-scheme independent manner. We argue that by considering a factorized form of the transverse momentum dependent spectrum for the production of a heavy lepton pair in Drell–Yan reaction, one should first split the relevant soft function into two boost invariant contributions. When those soft contributions are added to the pure collinear contributions, well-defined hadronic matrix elements emerge, i.e., the transverse momentum dependent distributions. We also perform a comparison with Collinsʼ definition.

Single-spin asymmetries in semi-inclusive deep inelastic scattering and Drell-Yan processes

We examine in detail the diagrammatic mechanisms which provide the change of sign between the single transverse spin asymmetries measured in semi-inclusive deep inelastic scattering (SIDIS) and in the Drell-Yan process (DY). This asymmetry is known to arise due to the transverse spin dependence of the target proton combined with a $T$-odd complex phase. Using the discrete symmetry properties of transverse spinors, we show that the required complex phase originates in the denominators of rescattering diagrams and their respective cuts. For simplicity, we work in a model where the proton consists of a valence quark and a scalar diquark. We then show that the phases generated in SIDIS and in DY originate from distinctly different cuts in the amplitudes, which at first appears to obscure the relationship between the single-spin asymmetries in the two processes. Nevertheless, further analysis demonstrates that the contributions of these cuts are identical in the leading-twist Bjorken kinematics considered, resulting in the standard sign-flip relation between the Sivers functions in SIDIS and DY. Physically, this fundamental, but yet untested, prediction occurs because the Sivers effect in the Drell-Yan reaction is modified by the initial-state ``lensing'' interactions of the annihilating antiquark, in contrast to the final-state lensing which produces the Sivers effect in deep inelastic scattering.

Future exploration of the nucleon structure at COMPASS

Up to now, COMPASS experiment [1] essentially focussed, in it's program with muon beams, on studying aspects of the longitudinal momentum structure of the nucleon in the collinear approximation, like Δq(x) and ΔG/G(x) [2, 3, 4]. However, quarks can also have intrinsic transverse momentum in the nucleon, which give rise to a new class of Transverse Momentum Distribution (TMD) Parton Distribution Functions. As an example, Sievers function has been measured by both COMPASS and HERMES to be non zero on the proton, paving thus the way for more precise investigations[5, 6].It is precisely the goal of the new COMPASS phase II proposal [7] to investigate in more detail new transverse description of the nucleon structure. Deeply Virtual Compton Scattering (DVCS) will allow studies in the transverse space via Generalized Parton Distributions (GPDs). Transverse Mometum Dependent PDFs will essentially be studied in Drell-Yan (DY) reaction and SIDIS, and some universality arguments in QCD imply different signs for Sievers and Boer-Mulders functions in DY and SIDIS.

Fields with continuously distributed mass

We describe local field theories with continuously distributed mass. Such models can be realized as models in d > 4 space-time with Poincare invariance only in four-dimensional space-time. We discuss some possible phenomenological consequences. Namely, we show that the Higgs boson phenomenology in the Standard Model extension with continuously distributed Higgs boson mass can differ in a drastic way from the standard Higgs boson phenomenology that makes the Higgs boson discovery at the Large Hadron Collider (LHC) an extremely difficult task. We also study the LHC discovery potential for Z′ models with continuously distributed mass for √2 = 7, 10 and 14 TeV center-of-mass energies. One of possible LHC signatures for such models is the existence of a broad resonance structure in the Drell-Yan reaction pp → Z′ + … → l + l − + ….

LHC Z' DISCOVERY POTENTIAL FOR MODELS WITH CONTINUOUSLY DISTRIBUTED MASS

We study the Large Hadron Collider (LHC) discovery potential for Z' models with continuously distributed mass for [Formula: see text], 10 and 14 TeV center-of-mass energies. One of the possible LHC signatures for such models is the existence of broad resonance structure in Drell–Yan reaction pp →Z' + ⋯→l+l- + ⋯.

Determining the saturation scale in nuclei

The saturation momentum seen in the nuclear infinite-momentum frame is directly related to transverse momentum broadening of partons propagating through the medium in the nuclear rest frame. Calculation of broadening within the color dipole approach, including the effects of saturation in the nucleus, gives rise to an equation which describes well the data on broadening in the Drell-Yan reaction and heavy quarkonium production.

Experimental Studies of Hadronization and Parton Propagation in the Space-Time Domain

Over the past decade, new data have become available from DESY, Jefferson Lab, Fermilab, and RHIC that connect to parton propagation and hadron formation. Semi-inclusive DIS on nuclei, the Drell-Yan reaction, and heavy-ion collisions all bring different kinds of information on parton propagation within a medium, while the most direct information on hadron formation comes from the DIS data. Over the next decade one can hope to begin to understand these data within a unified picture. We briefly survey the most relevant data and the common elements of the physics picture, then highlight the new Jefferson Lab data from CLAS, and close with prospects for the future.

Dynamic versus Static Hadronic Structure Functions

“Static” structure functions are the probabilistic distributions computed from the square of the light-front wavefunctions of the target hadron. In contrast, the “dynamic” structure functions measured in deep inelastic lepton-hadron scattering include the effects of rescattering associated with the Wilson line. Initial- and final-state rescattering, neglected in the parton model, can have a profound effect in QCD hard-scattering reactions, producing single-spin asymmetries, diffractive deep inelastic scattering, diffractive hard hadronic reactions, the breakdown of the Lam-Tung relation in Drell-Yan reactions, nuclear shadowing, and non-universal nuclear antishadowing—novel leading-twist physics not incorporated in the light-front wavefunctions of the target computed in isolation. I also review how “direct” higher-twist processes – where a proton is produced in the hard subprocess itself – can explain the anomalous proton-to-pion ratio seen in high centrality heavy ion collisions.

LHC SIGNATURES FOR Z' MODELS WITH CONTINUOUSLY DISTRIBUTED MASS

We discuss phenomenological consequences of renormalizable Z' models with continuously distributed mass. We point out that one of the possible LHC signatures for such model is the existence of broad resonance in Drell–Yan reaction pp →Z' →l+l-.