Drell-Yan Experiments Research Articles

Pion valence quark distributions from maximum entropy method

Valence quark distributions of pion at very low resolution scale Q02∼0.1 GeV2 are deduced from a maximum entropy method, under the assumption that pion consists of only a valence quark and a valence anti-quark at such a low scale. Taking the obtained initial quark distributions as the nonperturbative input in the modified Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (with the GLR-MQ-ZRS corrections) evolution, the generated valence quark distribution functions at high Q2 are consistent with the measured ones from a Drell-Yan experiment. The maximum entropy method is also applied to estimate the valence quark distributions at relatively higher Q2=0.26GeV2. At this higher scale, other components (sea quarks and gluons) should be considered in order to match the experimental data. The first three moments of pion quark distributions at high Q2 are calculated and compared with the other theoretical predictions.

Lepton angular distributions of fixed-target Drell-Yan experiments in perturbative QCD and a geometric approach

The lepton angular distributions of the Drell-Yan process in fixed-target experiments are investigated by NLO and NNLO perturbative QCD. We present the calculated angular parameters $\lambda$, $\mu$, $\nu$ and the degree of violation of the Lam-Tung relation, $1-\lambda-2\nu$, for the NA10, E615 and E866 experiments. Predictions for the ongoing COMPASS and SeaQuest experiments are also presented. The transverse momentum ($q_T$) distributions of $\lambda$ and $\nu$ show a clear dependence on the dimuon mass ($Q$) while those of $\mu$ have a strong rapidity ($x_F$) dependence. Furthermore, $\lambda$ and $\nu$ are found to scale with $q_T/Q$. These salient features could be qualitatively understood by a geometric approach where the lepton angular distribution parameters are expressed in terms of the polar and azimuthal angles of the "natural axis" in the dilepton rest frame.

Measurement of qT-weighted TSAs in 2015 COMPASS Drell–Yan data

In the polarised Drell–Yan experiment at the COMPASS facility at CERN the beam of negatively-charged pions with 190GeV/c momentum and intensity about 108 pions/s interacted with transversely polarised NH3 target. Muon pairs produced in Drell–Yan process (DY) were detected. Recently, the first ever Transverse Spin Asymmetries (TSAs) measurement in DY has been presented by COMPASS. A complementary analysis of the TSAs weighted by powers of the dimuon transverse momentum qT are presented. In the Transverse Momentum Dependent (TMD) PDF formalism, the qT-weighted TSAs can be written in terms of products of the TMD PDFs of two colliding hadrons, unlike the conventional TSAs, which are their convolutions over quarks transverse momenta. The results are compared in a straightforward way with the weighted Sivers asymmetry in the SIDIS process, released by COMPASS in 2016.

Transverse pion structure beyond leading twist in constituent models.

The understanding of the pion structure as described in terms of transverse-momentum-dependent parton distribution functions (TMDs) is of importance for the interpretation of currently ongoing Drell–Yan experiments with pion beams. In this work we discuss the description of pion TMDs beyond leading twist in a pion model formulated in the light-front constituent framework. For comparison, we also review and derive new results for pion TMDs in the bag and spectator model.

COMPASS Polarized Target for Pion-induced Drell–Yan Experiment

The first ever polarized Drell–Yan (DY) measurement is under preparation at COMPASS experiment at CERN. One of the key parts is the low-temperature polarized target. Modifications are required to cope with the intense pion beam that will be used. Solid NH3 will serve as a transversely-polarized target. Polarization is expected to be up to 90%. Two 55 cm long target cells give the target volume of about 690 cm3. The data taking is expected to start on fall 2014 and to continue in 2015 (approximately 180 days). Current status of the target, the modifications and future plans are presented.

Overview of the COMPASS results on the nucleon spin

The COMPASS experiment [COMPASS, P. Abbon et al., The COMPASS experiment at CERN, Nucl. Inst. Meth. A577, 455 (2007)] at CERN is one of the leading experiments studying the nucleon spin. These studies are being carried on since 2002, by measuring hadrons produced in deep inelastic scattering (DIS) of 160 GeV/c and 200 GeV/c polarised muons off different polarised targets (NH3 for polarised protons and 6LiD for polarised deuterons). One of the main goals is to determine how the total longitudinal spin projection of the nucleon, 1/2, is distributed among its constituents, quarks and gluons. We review here the recent results on the quark and gluon helicities obtained by COMPASS. The other major goal, whose fulfilment is needed for a complete understanding of the nucleon spin, is the determination of the transverse momentum dependent parton distributions (TMDs). Regarding this topic, the latest results on the Collins and Sivers asymmetries will be shown. The former is sensitive to the transverse spin structure of the nucleon, while the latter reflects the correlations between the quarks transverse momentum and the nucleon spin. This overview will conclude with a summary of the approved plans of COMPASS for the near future: the study of TMDs with a pioneering polarised Drell-Yan experiment and the measurement of generalised parton distributions (GPDs).

Understanding the large-distance behavior of transverse-momentum-dependent parton densities and the Collins-Soper evolution kernel

There is considerable controversy about the size and importance of nonperturbative contributions to the evolution of transverse-momentum-dependent (TMD) parton distribution functions. Standard fits to relatively high-energy Drell-Yan data give evolution that when taken to lower Q is too rapid to be consistent with recent data in semi-inclusive deeply inelastic scattering. Some authors provide very different forms for TMD evolution, even arguing that nonperturbative contributions at large transverse distance b_T are not needed or are irrelevant. Here, we systematically analyze the issues, both perturbative and nonperturbative. We make a motivated proposal for the parameterization of the nonperturbative part of the TMD evolution kernel that could give consistency: with the variety of apparently conflicting data, with theoretical perturbative calculations where they are applicable, and with general theoretical nonperturbative constraints on correlation functions at large distances. We propose and use a scheme- and scale-independent function A(b_T) that gives a tool to compare and diagnose different proposals for TMD evolution. We also advocate for phenomenological studies of A(b_T) as a probe of TMD evolution. The results are important generally for applications of TMD factorization. In particular, they are important to making predictions for proposed polarized Drell-Yan experiments to measure the Sivers function.

Nuclear effects in the proton-deuteron Drell-Yan process

We compute the nuclear corrections to the proton-deuteron Drell-Yan cross section for inclusive dilepton production, which, when combined with the proton-proton cross section, is used to determine the flavor asymmetry in the proton sea, $\overline{d}\ensuremath{\ne}\overline{u}$. In addition to nuclear smearing corrections that are known to be important at large values of the nucleon's parton momentum fraction ${x}_{N}$, we also consider dynamical off-shell nucleon corrections associated with the modifications of the bound nucleon structure inside the deuteron, which we find to be significant at intermediate and large ${x}_{N}$ values. We also provide estimates of the nuclear corrections at kinematics corresponding to existing and planned Drell-Yan experiments at Fermilab and J-PARC which aim to determine the $\overline{d}/\overline{u}$ ratio for $x\ensuremath{\lesssim}0.6$.

Asymmetries of quark sea in nucleon

The effects of in determining the flavor structure of the octet baryons have been investigated in the chiral constituent quark model. The chiral constituent quark model is able to qualitatively generate the requisite amount of quark sea and is also known to provide a satisfactory explanation of the spin and related issues in the nonperturbative regime. The phenomenological implications of the quark sea asymmetries in the nucleon have been investigated to understand the importance of quark sea at lower values of the Bjorken scaling variable. After the first direct evidence for the point-like constituents in the nucleon, identified as the valence quarks with spin-1/2 in the naive constituent quark model (NQM) (1-3), a lot of experiments have been conducted to probe the structure of the in the deep inelastic scattering (DIS) experiments. Surprisingly, the DIS results in the early 80's (4) indicated that the valence quarks of the carry only about 30% of its spin and is referred to as the proton spin crisis in the NQM. These results provided the first evidence for the being composed of three valence quarks surrounded by an indistinct sea of quark-antiquark pairs (henceforth referred to as the sea). In the present day, the study of the composition of hadrons can be said to be primarily the study of the quark sea and gluons and is considered as one of the active areas in hadronic physics. The conventional expectation that the quark sea perhaps can be obtained through the perturbative production of the quark-antiquark pairs by gluons produces nearly equal numbers of ¯ u and ¯ d. Until early 90's a symmetric sea w.r.t. ¯ u and ¯ d was assumed, however, the famous New Muon Collaboration in 1991 (5) established the quark sea asymmetry of the unpolarized quarks in the case of nucleon by measuring ¯ d − ¯ u giving first clear evidence for the nonperturbative origin of the quark sea. This was later confirmed by the Drell-Yan experiments (6) which measured a large quark sea asymmetry ratio ¯ d/¯ u reminding us that the study of the quark sea is intrinsically a nonperturbative phenomena and it is still a big challenge to perform these calculations from the first principles of QCD. The chiral constituent quark model (CQM) (7) can yield an adequate description of the quark sea generation through the chiral fluctuations. The basic idea is based on the possibility that chiral symmetry breaking takes place at a distance scale much smaller than the confinement scale. In this region, the effective degrees of freedom are the valence quarks and the internal Goldstone bosons (GBs) which are coupled to the valence quarks (8-10) allowing a simple and intuitive method to investigate

Effects of azimuth-symmetric acceptance cutoffs on the measured asymmetry in unpolarized Drell-Yan fixed-target experiments

Fixed-target unpolarized Drell-Yan experiments often feature an acceptance depending on the polar angle of the lepton tracks in the laboratory frame. Typically leptons are detected in a defined angular range, with a dead zone in the forward region. If the cutoffs imposed by the angular acceptance are independent of the azimuth, at first sight they do not appear dangerous for a measurement of the cos(2\phi)-asymmetry, relevant because of its association with the violation of the Lam-Tung rule and with the Boer-Mulders function. On the contrary, direct simulations show that up to 10 percent asymmetries are produced by these cutoffs. These artificial asymmetries present qualitative features that allow them to mimic the physical ones. They introduce some model-dependence in the measurements of the cos(2\phi)-asymmetry, since a precise reconstruction of the acceptance in the Collins-Soper frame requires a Monte Carlo simulation, that in turn requires some detailed physical input to generate event distributions. Although experiments in the eighties seem to have been aware of this problem, the possibility of using the Boer-Mulders function as an input parameter in the extraction of Transversity has much increased the requirements of precision on this measurement. Our simulations show that the safest approach to these measurements is a strong cutoff on the Collins-Soper polar angle. This reduces statistics, but does not necessarily decrease the precision in a measurement of the Boer-Mulders function.

Comparison of kaon and pion valence quark distributions in a statistical model

We have calculated the Bjorken-x dependence of the kaon and pion valence quark distributions in a statistical model. Each meson is described by a Fock state expansion in terms of quarks, antiquarks and gluons. Although Drell–Yan experiments have measured the pion valence quark distributions directly, the kaon valence quark distributions have only been deduced from the measurement of the ratio u¯K(x)/u¯π(x). We show that, using no free parameters, our model predicts the decrease of this ratio with increasing x.

A Drift Chamber for a Muon Spectrometer and Measurement of Sea Quark Flavor Asymmetry in the Proton at E906/SeaQuest Drell-Yan Experiment

Fermilab E-906/SeaQuest is a particle physics experiment which will use Drell-Yan process to measure the contributions of antiquarks to the structure of the proton or neutron and how this structure is modified when the proton or neutron is included within an atomic nucleus. It is part of a series of fixed target Drell-Yan experiments. The Drell-Yan process occurs in high energy hadron–hadron scattering. It takes place when a quark of one hadron and an antiquark of another hadron annihilate, creating a virtual photon which then decays into a pair of oppositely-charged muons. The kinematics of these myons allow a direct relation to the antiquark structure of the proton. In addition the E-906/SeaQuest experiment can also examine the modifications to the antiquark structure of the proton from nuclear binding or access transverse momentum dependent parton distribution functions. The E906/SeaQuest experiment will use a 120 GeV proton beam extracted from the Fermilab Main Injector. This beam energy provides a tremendously increasing cross section compared to past Drell-Yan experiments. Taking the reduction of background events due to J/Ψ decays into account a gain in statistics by a factor of 50 compared to the latest Drell-Yan experiments is expected. The spectrometer is currently being assembled at Fermilab. It is expected to be commissioned in spring 2011 and collect data for two years. The group of Tokyo Institute of Technology is among Japanese collaborators in charge of the tracking stations in the E-906/SeaQuest experiment. In this report we will give an overview and a status report of the ongoing commissioning of the Drift Chambers (DC).

Drell-Yan Experiment: Studying Anti-Quarks in the Proton

The internal structure of the proton is one of the most vital topic in the present hadron physics. At a high-energy scale the proton reveals its complicated parton structure which is beyond the scope of the Quark Parton Model (QPM), i.e. quarks (q), anti-quarks (q̄) and gluons (g). Gluons and anti-quarks are dynamically created via q → q + g and g → q + q̄, respectively. The dynamics of the proton structure is being investigated in both experiment and theory.The Drell-Yan process in proton+proton collisions is the reaction in which a quark in one proton and an anti-quark in the other proton annihilate into a virtual photon and then create a muon pair: q + q̄ → γ* → μ+ + μ−. This process is suited to study the anti-quarks in the proton, and many experiments measuring the Drell-Yan process have been carried out in the past 20 years. The E906/SeaQuest experiment at Fermi National Accelerator Lab (FNAL) in USA is a Drell-Yan experiment being prepared now to explore a new kinematic region with very high statistics. The main purpose of this experiment is to precisely measure the asymmetry of ū and d̄ distributions in the proton at the xBj range from 0.25 to 0.45.In this paper, the anti-quark flavor asymmetry in the proton and the past Drell-Yan experiments are summarized, and the status of the E906 experiment is reported.

Prospects of polarized fixed target Drell-Yan experiments

It has been proposed that the Siverse transverse single spin asymmetry in Drell-Yan production in transversely polarized p+p collisions would have an opposite sign compared to what has been observed in the polarized Semi-Inclusive Deep Inelastic Scattering (SIDIS) experiments. Experimental confirmation or disproval of this prediction would provide a novel fundamental test of QCD and shed new light on our theoretical understanding of the transverse spin physics phenomena. We discuss the prospects and physics sensitivities of polarized fixed target Drell-Yan experiments that could utilize the existing proton and other hadron beams at Fermilab, and polarized proton beams at RHIC with a polarized solid proton and/or neutron target option. We show that if realized, the new experiments would provide critical measurements of not only the sign change (or not) of Sivers functions, but also the information of quark and antiquark's Sivers distributions over a wide kinematic range.

Opportunities with Drell-Yan Scattering at Fermilab

The proton is a composite object made of fundamental, strongly-interacting quarks. Many of the features of the proton can be described by a simple picture based on three valence quarks bound by the exchange of gluons. However, protons are much more complex objects with the vast majority of their mass dynamically generated by the strong force and manifest in the sea quark and gluon distributions. While deep inelastic scattering cannot differentiate between quark and antiquarks, the Drell-Yan process inherently involves the antiquark distributions of either the beam or target hadron. By measuring Drell-Yan scattering, the Fermilab E-906/SeaQuest experiment will study the sea quarks in the proton and in nuclei. The primary goals of this measurement include elucidating the anti-d to anti-u quark asymmetry in the proton and a study of the EMC effect in sea quarks. With the same data, the angular distribution of Drell-Yan scattering will be studied. Previous measurements of these distributions have been interpreted as evidence for transverse momentum dependent Boer-Mulders h⊥1 distribution; although, pion and proton induced Drell-Yan data disagree. The increased statistical precision of the anticipated data will allow for better extraction of the angular distributions. To accomplish these goals, the experiment will use a 120 GeV proton beam extracted from the Fermilab Main Injector. While the experiment will be taking advantage of equipment from earlier Drell-Yan experiments, the changes in kinematics of the experiment require several, significant upgrades to the spectrometer. The collaboration expects to begin data collection in late fall of 2010.

Pion-induced Drell-Yan processes and the flavor-dependent EMC effect

Pion-induced Drell-Yan processes are proposed as a promising tool with which to measure the flavor dependence of the European Muon Collaboration (EMC) effect, that is, the flavor-dependent modification of quark distributions in the nuclear medium. Existing pionic Drell-Yan data are compared with calculations using a recent model for nuclear quark distributions that incorporates flavor-dependent nuclear effects. While no firm conclusions can yet be drawn, we find that existing Drell-Yan data likely imply a flavor dependence of the EMC effect. We demonstrate that pion-induced Drell-Yan experiments on nuclear targets can access new aspects of the EMC effect not probed in deep inelastic scattering and can therefore provide important new constrains on the nuclear quark distributions. Predictions for possible future pion-induced Drell-Yan experiments are also presented.

Measuring the flavor asymmetry in the sea quarks of the proton with the Drell–Yan process

The proton is a composite object made of fundamental, strongly interacting quarks. Many of the features of the proton can be described by a simple picture based on three “valence” quarks bound by the exchange of gluons. However, protons are much more complex objects with the vast majority of their mass dynamically generated by quantum chromodynamics (QCD). This mass manifests itself through a “sea” of gluons and quark–antiquark pairs. By measuring Drell–Yan scattering, the Fermilab E-906/Drell–Yan experiment will study the sea quark distribution in the proton and, in particular, the unusually large asymmetry between anti-up and anti-down quarks measured by earlier Drell–Yan experiments. This asymmetry cannot simply be generated through pair creation, but rather indicates an underlying, fundamental antiquark component in the proton. Using the same technique, the E-906/SeaQuest experiment will also investigate the differences between the antiquark distributions of the free proton and a proton bound in a nucleus. Nuclear binding is expected to modify the quark distributions and it has long been known that the overall quark distributions are different (the EMC effect). Surprisingly, present data suggest, however, that the antiquark distributions are not modified. To accomplish these goals, the experiment will use a 120 GeV proton beam extracted from the Fermilab Main Injector. While the experiment will be taking advantage of equipment from earlier Drell–Yan experiments, the changes in kinematics of the experiment require several, significant upgrades to the spectrometer. Japanese institutions, including Tokyo Institute of Technology, KEK, RIKEN, Kyoto University and Yamagata University, are making substantial contributions to this upgrade. The collaboration expects to begin data collection in Summer, 2010.

HADRON STRUCTURE STUDY IN FORTHCOMING DRELL–YAN EXPERIMENTS: COMPASS PROJECT AT CERN

The study of Drell–Yan (DY) processes involving the collision of an (un)polarised hadron beam on an (un)polarised proton target can result in a fundamental improvement of our knowledge on the transverse momentum dependent (TMDs) parton distribution functions (PDFs) of hadrons. The production mechanism of J/ψ and J/ψ - DY duality can also be addressed. One of the forthcoming polarised DY experiments (COMPASS (SPS, CERN)) is discussed in this context. The most important features of this project are briefly reviewed, as well as its sensitivity to the various transverse momentum dependent spin asymmetries.

A scheme for fast exploratory simulation of azimuthal asymmetries in Drell–Yan experiments at intermediate energies: The DY_AB Monte Carlo event generator

In this note I report and discuss the physical scheme and the main approximations used by the event generator code DY_AB. This Monte Carlo code is aimed at preliminary simulation, during the stage of apparatus planning, of Drell–Yan events characterized by azimuthal asymmetries, in experiments with moderate center of mass energy s ⪡ 100 GeV .

Implications of the nuclear EMC effect

The discovery more than twenty years ago, by the EMC Collaboration, that the deep-inelastic-scattering DIS structure functions are influenced by the nuclear environment stunned the nuclear physics community and brought quarks and gluons into the field with great impact. A great length of time has passed, but despite a semi-infinite number of papers on the subject, there is no explanation that is universally accepted. Many models (related in one way or another to QCD) have been successful in reproducing data for deep inelastic scattering on nuclear targets, but fewer have described both the DIS and nuclear Drell-Yan experiments. Although there are some positive indications, no model has been used to predict correctly and unambiguously new independent phenomena. We review the history and discuss the best experimental prospects for future discovery.