In semi-inclusive deep inelastic scattering of polarized leptons on a transversely polarized target eight target transverse spin-dependent azimuthal modulations are allowed. In the QCD parton model half of these asymmetries can be interpreted within the leading order approach and the other four are twist-three contributions. The first two leading twist asymmetries extracted by HERMES and COMPASS experiments are related: one to the transversity distribution and the Collins effect, the other to the Sivers distribution function. These results triggered a lot of interest in the past few years and allowed the first extractions of the transversity and the Sivers distribution functions of nucleon. The remaining six asymmetries were obtained by the COMPASS experiment using a 160 GeV/c longitudinally polarized muon beam and transversely polarized deuteron and proton targets. Here we review preliminary results from COMPASS proton data of 2007.
After reviewing the most important COMPASS results on transverse spin effects in SIDIS, I will present the recent work we have done on weighted Sivers asymmetries and "transversity induced" $Λ$ polarisation. Using the high statistics data collected in 2010 on a transversely polarised proton target COMPASS has evaluated the transverse momentum weighted Sivers asymmetries both in $x$- and in $z$-bins. The results are also compared with the standard unweighted asymmetries. Using the whole data sample collected over the years on transversely polarised deuteron and proton targets COMPASS has measur…
The production of vector mesons in SIDIS is a particularly interesting channel to study the polarized fragmentation and related phenomena. Preliminary COMPASS results for the first measurement of inclusive $ρ^0$ Collins and Sivers transverse-spin asymmetries are presented here for the first time. The analysis is based on the data-set collected by COMPASS in $2010$ using a $160\, \rm{GeV}/c$ $μ^+$ beam and a transversely polarized $NH_3$ target. The asymmetries are extracted as function of Bjorken-$x$, of the total transverse momentum of the oppositely charged hadron pair and of the fraction of…
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 $π^-$) 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 $π^-$ beam on polarized $NH_3$. The experimental results obtained by COMPASS for azimuthal…
COMPASS is a high-energy physics experiment operating at the SPS at CERN. Wide physics program of the experiment comprises study of hadron structure and spectroscopy with high energy muon and hadrons beams. As for the muon-program, one of the important objectives of the COMPASS experiment is the exploration of the transverse spin structure of the nucleon via spin (in)dependent azimuthal asymmetries in single-hadron production in deep inelastic scattering of polarized leptons off transversely polarized target. For this purpose a series of measurements were made in COMPASS, using 160 GeV/c longi…
In this review a first comprehensive study of azimuthal asymmetries in semi-inclusive deep inelastic muoproduction of hadron pairs off longitudinally polarized protons at COMPASS is presented. The study is based on data taken in 2007 and 2011, obtained by impinging a high-energetic $μ^+$ beam of $160$ GeV/$\it{c}$, respectively $200$ GeV/$\it{c}$, momentum on a solid ammonia target. The discussion is focused on both leading and subleading longitudinal target-spin-dependent asymmetries arising in the di-hadron SIDIS cross section, addressing the role of spin-orbit couplings and quark-gluon corr…
In general, eight target transverse spin-dependent azimuthal modulations are allowed in semi inclusive deep inelastic scattering of polarized leptons on a transversely polarized target. In the QCD parton model four of these asymmetries can be interpreted within the leading order approach. Two of them, namely Collins and Sivers effects were already measured by HERMES and COMPASS experiments. Other two leading twist and remaining four azimuthal asymmetries which can be interpreted as twist-three contributions have been measured for the first time in COMPASS using a 160 GeV/c longitudinally polar…
Preliminary results obtained by the COMPASS experiment for longitudinal-spin-dependent azimuthal asymmetries in single-hadron muoproduction off protons are presented for the first time. The analysis was carried out on the full data sample collected by COMPASS with longitudinally polarized proton ($NH_3$) target in 2007 and 2011 using 160~\gvc and 200~\gvc muon beams, correspondingly. Within QCD parton model approach extracted asymmetries are giving access to the specific convolutions of certain transverse-momentum-dependent \textit{twist-2} and \textit{higher twist} parton distribution functio…
One of the important objectives of the COMPASS experiment (CERN, SPS north area) is the exploration of transverse spin structure of nucleon via study of spin (in)dependent azimuthal asymmetries with semi-inclusive deep inelastic scattering (SIDIS) processes and recently also with Drell-Yan (DY) reactions. In the past twelve years series of measurements were made in COMPASS, using 160 GeV/c longitudinally polarized muon beam and polarized $^6LiD$ and $NH_3$ targets. Drell-Yan measurements with high energy (190 GeV/c) pion beam and transversely polarized $NH_3$ target started in 2014 with a pilo…
Exploration of transverse spin structure of the nucleon via study of the spin (in)dependent azimuthal asymmetries in semi-inclusive deep inelastic scattering (SIDIS) and Drell-Yan (DY) reactions is one of the main aspects of the broad physics program of the COMPASS experiment (CERN, Switzerland). In past decade COMPASS has collected a considerable amount of polarized deuteron and proton SIDIS data, while recent 2014 and 2015 runs were dedicated to the Drell-Yan measurements. Results on SIDIS azimuthal effects provided so far by COMPASS play an important role in general understanding of the thr…
One of the important objectives of the COMPASS experiment (SPS, CERN) \cite{Abbon:2007pq} is the exploration of the transverse spin structure of the nucleon via spin dependent azimuthal asymmetries in single-hadron production in deep inelastic scattering of polarized leptons off transversely polarized targets. For this purpose a series of measurements were made in COMPASS, using 160 GeV/c longitudinally polarized muon beam and transversely polarized $^6LiD$ (in 2002, 2003 and 2004) and $NH_3$ (in 2007 and 2010) targets. In the past few years considerable theoretical interest and experimental e…
COMPASS is a high-energy physics experiment operating on the M2 beam line at the SPS at CERN. Using high energy muon and hadron beams the experiment covers broad range of physics aspects in the field of the hadron structure and spectroscopy. One of the important objectives of the COMPASS experiment is the exploration of transverse spin structure of the nucleon via study of spin (in)dependent azimuthal asymmetries with semi-inclusive deep inelastic scattering (SIDIS) processes and starting from 2014 also with Drell-Yan (DY) reactions. Experimental results obtained by COMPASS for azimuthal effec…
COMPASS is a fixed-target high energy physics experiment at the SPS at CERN \cite{Abbon:2007pq}. One of the important objectives of the experiment is the exploration of the transverse spin structure of the nucleon via spin dependent azimuthal asymmetries in single-hadron production in deep inelastic scattering of polarized leptons off transversely polarized target. For this purpose a series of measurements were made in COMPASS, using 160 GeV/c longitudinally polarized muon beam and transversely polarized $^6LiD$ (in 2002, 2003 and 2004) and $NH_3$ (in 2007 and 2010) targets. Till now main atte…
Azimuthal Asymmetries in unpolarized SIDIS can be used to probe the transverse momentum of quarks inside the nucleon. Furthermore, they give access to the so-far unmeasured Boer-Mulders function. We report on the first measurement of azimuthal asymmetries of the SIDIS cross section from scattering of muons off a deuteron target.
The investigation of transverse spin and transverse momentum effects in the nucleon is one of the key physics programs of the COMPASS experiment at CERN. COMPASS investigates these effects scattering 160 GeV/c muons off a fixed NH3 or 6LiD target. The azimuthal asymmetries which appear in the cross-section of semi-inclusive deep-inelastic scattering on an unpolarized target have been measured. These asymmetries give insight into the intrinsic transverse momentum of the quarks in the nucleon by the Cahn effect and into a possible correlation between transverse momentum and transverse spin. New …
Azimuthal asymmetries in unpolarized SIDIS can be used to probe the transverse momentum of the quarks inside the nucleon. Furthermore they give access to the so-far unmeasured Boer-Mulders function. We report on the extraction of these asymmetries from the COMPASS data taken with a 160 GeV/c $μ^+$ beam impinging on a deuteron target. This asymmetries have been extracted separately for positive and negative hadrons, and the results have been compared with theoretical predictions.
The study of the spin structure of the nucleon and of the effects rising from the quarks transverse momentum are part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS. The azimuthal asymmetries which appear in the cross-section of SIDIS off unpolarized targets can give insights on the intrinsic momentum structure of the nucleon and on the possible correlation between transverse spin and transverse momentum of the quarks. Here we present the new results for these asymmetries obtained from the COMPASS data collected with a 160 GeV/c positive muon beam impinging on …
The study of transverse spin and transverse momentum effects is an important part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS taking data since 2002. The studies are carried on by measuring the hadrons produced in deep inelastic scattering (DIS) of 160 GeV/c muons off different targets. Among the possible asymmetries in the hadron azimuthal distributions, particularly interesting are the Collins and Sivers asymmetries which the COMPASS Collaboration has measured using transversely polarised deuteron and proton targets. The new results for charged pions and k…
Azimuthal asymmetries measured in unpolarized semi-inclusive deep inelastic scattering bring important information on the inner structure of the nucleons, and can be used both to estimate the average quark transverse momentum k_T and to access the so-far unmeasured Boer-Mulders functions. COMPASS results using part of the 2004 data collected with a 6LiD target and a 160 GeV positive muon beam are presented separately for positive and negative hadrons.
Muon pairs created in the Drell-Yan process of a negative pion beam of 190 GeV/c momentum impinging on a transversely polarised NH$_3$ target were detected with the COMPASS spectrometer at CERN in 2015. In addition to the extraction of the transverse spin asymmetries (TSAs) from these data, a complementary analysis of the TSAs weighted by powers of the dimuon momentum $q_T$ has been performed and is presented in this paper. In the transverse momentum dependent parton distribution functions (TMD PDFs) formalism, the $q_T$-weighted TSAs can be interpreted in terms of products of the TMD PDFs of …
COMPASS is a fixed-target experiment on the SPS M2 beamline at CERN. Its LiD target can be polarised both longitudinally and transversally with respect to the longitudinally polarised 160 GeV/c muon beam. Approximately 20% of the beam-time in 2002, 2003 and 2004 was spent in the transverse configuration, allowing the first measurement of both the Collins and Sivers asymmetries on a deuterium target. First results from the the transverse data of the COMPASS run in 2002 are reported here.
Transverse spin physics is an important part of the scientific programme of the COMPASS experiment at CERN. The analysis of the data taken with the target polarized orthogonally to the 160 GeV/c muon beam momentum has allowed to measure for the first time the Collins and Sivers asymmetries of the deuteron. Both for the positive and the negative hadrons produced in semi-inclusive DIS the measured asymmetries are small and, within errors, compatible with zero. New results for charged pions and kaons are presented here.
New results on single spin asymmetries of charged hadrons produced in deep-inelastic scattering of muons on a transversely polarised LiD target are presented. The data were taken in the years 2002, 2003 and 2004 with the COMPASS spectrometer using the muon beam of the CERN SPS at 160 GeV/c. Preliminary results are given for the Sivers asymmetry and for all the three ``quark polarimeters'' presently used in COMPASS to measure the transversity distributions. The Collins and the Sivers asymmetries for charged hadrons turn out to be compatible with zero, within the small (~1%) statistical errors, …
In the list of the main items studied by the CERN COMPASS experiment there are the transverse spin and momentum effects visible in the azimuthal distributions of hadrons produced in the deep inelastic scattering. In the years 2002-2004 COMPASS has collected data with a 6LiD target with the polarization oriented transversely with respect to the muon beam direction for about 20% of the running time; in 2007, COMPASS has used for the first time a proton NH_3 target with the data taking time equally shared between longitudinal and transverse polarization of the target. After reviewing the results …
In semi-inclusive DIS of polarized leptons on a transversely polarized target eight azimuthal modulations appear in the cross-section. Within QCD parton model four azimuthal asymmetries can be interpreted at leading order, two of them being the already measured Collins and Sivers asymmetries. The other two leading twist asymmetries, related to different transverse momentum dependent quark distribution functions, and also additional four asymmetries which can be interpreted as twist-three contributions have been measured for the first time at COMPASS, using a 160 GeV/c longitudinally polarized …
COMPASS is a fixed-target experiment in operation at the CERN North Area (SPS, M2 beam-line) since 2002. An important part of the broad physics programme of the experiment is dedicated to the exploration of the transverse spin-structure of the nucleon studying target transverse spin dependent azimuthal asymmetries (TSAs) arising in the Semi-Inclusive DIS (SIDIS) and Drell-Yan (DY) cross-sections. In addition to those measurements, COMPASS has recently studied also the TSAs weighted by powers of the hadron transverse momentum (in SIDIS) and virtual photon transverse momentum, $q_T$ (in DY). In …
COMPASS is a fixed target experiment presently running at CERN. In 2002, 2003, and 2004 the experiment used a 160 GeV polarized muon beam coming from SPS and scattered off a $^6$LiD target. The nucleons in the target can be polarized either longitudinally or transversely with respect to the muon beam and 20% of the running time has been devoted to transverse polarization. From the transverse polarization data collected in 2002, which correspond to a total integrated luminosity of about 200 pb$^{-1}$, the Collins and the Sivers asymmetries have been determined separately and the preliminary res…
COMPASS is a fixed target experiment at CERN investigating the spin structure of the nucleon and performing hadron spectroscopy. The transverse spin structure of the nucleon is studied in semi-inclusive deep-inelastic scattering of 160 GeV/c muons off a transversely polarized proton or deuteron target. In 2002-2005, a transversely polarized 6LiD, and in 2007 a transversely polarized NH3 target were used. To get access to the transversity distribution, different single-spin asymmetries have been measured: the Collins asymmetry, the hadron-pair asymmetry and the transverse lambda polarization. I…
Successful realization of polarized Drell-Yan physics program is one of the main goals of the second stage of the COMPASS experiment. Drell-Yan measurements with high energy (190 GeV/c) pion beam and transversely polarized NH3 target have been initiated by a pilot-run in the October 2014 and will be followed by 140 days of data taking in 2015. In the past twelve years COMPASS experiment performed series of SIDIS measurements with high energy muon beam and transversely polarized deuteron and proton targets. Results obtained for Sivers effect and other target transverse spin dependent and unpola…
Generalized parton distributions (GPDs) provide a new and powerful framework for a complete description of the nucleon structure. They can provide a three-dimensional picture of how the quarks and gluons form a nucleon. GPDs can be probed experimentally in hard exclusive meson production or deeply virtual Compton scattering (DVCS). The COMPASS experiment at CERN is a unique place to study these reactions. At COMPASS, a high energy polarized positive or negative muon beam is scattered off a polarized or unpolarized fixed target. First results for exclusive rho^0 meson production are shown. The …
COMPASS is a fixed target experiment at CERN. Part of its physics programme is dedicated to study the transverse spin and the transverse momentum structure of the nucleon using SIDIS. For these measurements, data have been collected using transversely polarised proton and deuteron targets. A selection of recent measurements of azimuthal asymmetries using data collected with transversely polarised protons is presented.
COMPASS is a running fixed-target experiment at the CERN SPS with a rich physics program focused on nucleon spin structure and on hadron spectroscopy. One of the main goals of the spin program is the measurement of the transverse spin effects in semi-inclusive DIS off transversely polarised nucleons. In the years 2002, 2003 and 2004 data have been taken using a 160 $GeV/c$ naturally polarised $μ^{+}$ beam and a deuterium target ($^{6}LiD$) transversely polarised respect to the beam direction. In 2007 the run year has been devoted to collect data with a proton ($NH_{3}$) target. The preliminary…
The COMPASS experiment at CERN, as part of its programme addresses the exploration of the transverse spin structure of the nucleon by measuring spin (in)dependent azimuthal asymmetries in semi-inclusive DIS and, recently, also in Drell-Yan processes. Between 2002 and 2010 COMPASS performed a series of SIDIS measurements, using a longitudinally polarized muon beam impinging on transversely polarized $^6LiD$ or $NH_3$ targets. Drell-Yan measurements with a $π^-$ beam interacting with a transversely polarized $NH_3$ target started with the 2015 run and will be continued in 2018. In this Letter th…
We consider the single-spin azimuthal asymmetries recently measured at the HERMES experiment for charged pions produced in semi-inclusive deep inelastic scattering of leptons off longitudinally polarized protons. Guided by the experimental results and assuming a vanishing twist-2 transverse quark spin distribution in the longitudinally polarized nucleon, denoted as ``reduced twist-3 approximation'', a self-consistent description of the observed single-spin asymmetries is obtained. In addition, predictions are given for the z dependence of the single target-spin asymmetry.
Search for azimuthal asymmetries in semi-inclusive production of charged hadrons by 160 GeV muons on the longitudinally polarized deuterium target, has been performed using the 2002- 2004 COMPASS data. The observed asymmetries integrated over the kinematical variables do not depend on the azimuthal angle of produced hadrons and are consistent with the ratio $g_1^d(x)/f_1^d(x)$. The asymmetries are parameterized taking into account possible contributions from different parton distribution functions and parton fragmentation functions depending on the transverse spin of quarks.They can be modulat…
The investigation of transverse spin and transverse momentum effects in deep inelastic scattering is one of the key physics programs of the COMPASS collaboration. Three channels have been analyzed at COMPASS to access the transversity distribution function: The azimuthal distribution of single hadrons, involving the Collins fragmentation function, the azimuthal dependence of the plane containing hadron pairs, involving the two-hadron interference fragmentation function, and the measurement of the transverse polarization of Lambda hyperons in the final state. Azimuthal asymmetries in unpolarize…
The exploration of the transverse spin structure of the nucleon by measuring spin (in)dependent azimuthal asymmetries in semi-inclusive DIS (SIDIS) and in Drell-Yan processes is one of the main objectives of the COMPASS experiment at CERN (SPS, M2 beamline). During the first phase of the experiment (2002-2011) a series of SIDIS measurements were performed, using a longitudinally polarized muon beam impinging on transversely polarized $^6$LiD or NH$_3$ targets. As a part of the COMPASS-II programme, in 2015 and 2018 the experiment performed Drell-Yan measurements with a $π^-$ beam interacting w…
The study of transverse spin and transverse momentum effects is an important part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS. For these studies a 160 GeV/c momentum muon beam is scattered on a transversely polarized nucleon target, and the scattered muon and the forward going hadrons produced in DIS processes are reconstructed and identified in a magnetic spectrometer. The measurements have been performed on a deuteron target in 2002, 2003 and 2004, and on a proton target in 2007 and 2010. The results obtained for the Collins and Sivers asymmetries from the…
New results on single spin asymmetries of identified charged pion and kaon pairs produced in deep-inelastic scattering of muons on a transversely polarized 6LiD target are presented. The data were taken in the years 2003 and 2004 with the COMPASS spectrometer at CERN with a 160 GeV muon beam from the CERN SPS accelerator. The asymmetries can be interpreted in the context of transversity as a convolution of the chiral-odd interference fragmentation function $H_1^\sphericalangle$ with the transverse spin distribution of quarks $Δ_Tq(x)$. The measured azimuthal target spin asymmetries on the deut…
We present results for all leading-twist azimuthal spin asymmetries in semi-inclusive lepton-nucleon deep-inelastic scattering due to T-even transverse-momentum dependent parton distribution functions on the basis of a light-cone constituent quark model. Attention is paid to discuss the range of applicability of the model, especially with regard to the scale dependence of the observables and the transverse-momentum dependence of the distributions. We find good agreement with available experimental data and present predictions to be further tested by future CLAS, COMPASS and HERMES data.
Within the LO QCD parton model of SIDIS with unintegrated quark distribution and fragmentation functions we study the transverse momentum and azimuthal dependencies of the double spin asymmetries $A_{LT}$ and $A_{LL}$. For later we include ${\cal O}(k_{\perp}/Q)$ kinematic corrections, which induce an azimuthal modulation of the asymmetry, analogous to the Cahn effect in unpolarized SIDIS. We show that a study of these asymmetries allows to extract the transverse momentum dependence of the unintegrated helicity distribution function $g_{1L}^q(x,k_\perp)$ and $g_{1T}^q(x,k_\perp)$. This report …
The theoretical status on transverse momentum dependent factorization in semi-inclusive DIS and Drell-Yan is not clear in contrast to claims in the literature in which gauge links only at one loop were considered explicitly. Recent obtained results beyond this order questions the validity of these claims and will be briefly discussed. Possible input from experiments to solve these matters will be outlined.
Using a previous extraction of the quark Boer-Mulders distributions from semiinclusive deep inelastic scattering data, we fit the unpolarized Drell-Yan data on the $\cos 2 ϕ$ asymmetry, determining the antiquark Boer-Mulders distributions. A good agreement with the data is found in the region of low $q_T$, where the transverse-momentum factorization approach applies.
Measuring single spin asymmetries in semi-inclusive deep-inelastic scattering (SIDIS) on a transversely polarized target gives a handle to investigate the transversity distribution and transverse momentum dependent distribution functions. In the years 2002, 2003 and 2004 COMPASS took data with a transversely polarized deuteron target and in the year 2007 with a proton target. Three channels for accessing transversity have been analysed. Azimuthal asymmetries in the production of hadron pairs, involving the polarized two hadron interference fragmentation function, azimuthal asymmetries in the p…
The SIDIS transverse spin asymmetries weighted with powers of $P_T$, the hadron transverse momentum in the $γN$ reference system,have been introduced already twenty years ago and are considered quite interesting. While the amplitudes of the modulations in the azimuthal distribution of the hadrons are the ratios of convolutions over transverse momenta of the transverse--momentum dependent parton distributions and of the corresponding fragmentation functions, and can be evaluated analytically only making assumptions on the transverse--momentum dependence of these functions, the weighted asymmetr…
The COMPASS Collaboration is measuring the asymmetries in the azimuthal distributions of positive and negative hadrons produced in Deep Inelastic Scattering (DIS) on unpolarized protons. The data have been collected in 2016 and 2017 with a 160 GeV/$c$ muon beam scattering off a liquid hydrogen target. The amplitudes of three modulations, $A_{UU}^{\cosϕ_h}$, $A_{UU}^{\cos 2ϕ_h}$ and $A_{LU}^{\sinϕ_h}$ are measured as function of the Bjorken variable $x$, of the fraction of virtual photon energy carried by the hadron $z$, and of the hadron transverse momentum with respect to the virtual photon $…
Unpolarised semi-inclusive DIS is receiving a growing interest as a powerful tool to access poorly known universal functions that play a key role in many processes, in particular in the study of the spin structure of the nucleon. These functions can be investigated through experimental observables such as hadron multiplicities in the collinear and transverse framework, the dihadron multiplicities, the azimuthal asymmetries and some others. New results on these observables by the COMPASS experiment at CERN will be shown and discussed.
By measuring transverse single spin asymmetries one has access to the transversity distribution function $Δ_T q(x)$ and the transverse momentum dependent Sivers function $q_0^T(x,\vec{k}_T)$. New measurements from identified hadrons and hadron pairs, produced in deep inelastic scattering of a transversely polarized $^6LiD$ target are presented. The data were taken in 2003 and 2004 by the COMPASS collaboration using the muon beam of the CERN SPS at 160 GeV/c, resulting in small asymmetries.
Some properties of intrinsic transverse momentum dependent nucleon distribution functions are considered in the simple quark-diquark model. The transverse target polarization dependent asymmetries for SIDIS are calculated and compared with recent results of COMPASS. The model describes well the measured asymmetries. Generalization of quark-diquark model for Sivers function is discussed.
We investigate the Collins azimuthal asymmetry of hadrons produced inside jets in transversely polarized proton-proton collisions. Recently, the quark transversity distributions and the Collins fragmentation functions have been extracted within global analyses from data of the processes semi-inclusive deep inelastic scattering and electron-positron annihilation. We calculate the Collins azimuthal asymmetry for charged pions inside jets using these extractions for RHIC kinematics at center-of-mass energies of 200 and 500 GeV. We compare our results with recent data from the STAR Collaboration a…
The measurement of azimuthal modulations in hadron leptoproduction on unpolarized nucleons allows to get information on the intrinsic transverse momentum $\langle k_T^2 \rangle$ of quarks in a nucleon through both the Cahn effect and the Boer-Mulders function. We have compared the azimuthal asymmetries in the cross section of $160\, \rm{GeV}/c$ muons scattered off an unpolarised deuteron target as measured by COMPASS with a Monte Carlo program, based on the ${}^3P_0$ model of quark anti-quark pair production at string breaking, which accounts for the Cahn effect. Large differences have been ob…
We analytically calculate weighted azimuthal asymmetries in semi-inclusive lepton-nucleon deep-inelastic scattering and Drell-Yan processes, using transverse-momentum-dependent partonic densities obtained in a diquark spectator model. We compare the asymmetries with available preliminary experimental data, in particular for the Collins and the Sivers effect. We make predictions for other cases of interest in running and planned experiments.
The investigation of transverse spin and transverse momentum effects in deep inelastic scattering is one of the key physics programs of the COMPASS collaboration. In the years 2002-2004 COMPASS took data scattering 160 GeV muons on a transversely polarized 6LiD target. In 2007, a transversely polarized NH3 target was used. Three different channels to access the transversity distribution function have been analyzed: The azimuthal distribution of single hadrons, involving the Collins fragmentation function, the azimuthal dependence of the plane containing hadron pairs, involving the two-hadron i…
We consider high-mass systems of two or more particles that are produced by QCD hard scattering in hadronic collisions. We examine the azimuthal correlations between the system and one of its particles. We point out that the perturbative QCD computation of such azimuthal correlations and asymmetries can lead to divergent results at fixed perturbative orders. The fixed-order divergences affect basic (and infrared safe) quantities such as the total cross section at fixed (and arbitrary) values of the azimuthal-correlation angle $φ$. Examples of processes with fixed-order divergences are heavy-qu…
In 2016 and 2017, the COMPASS Collaboration at CERN collected a large sample of DIS events with a longitudinally polarized 160 GeV/c muon beam scattering off a liquid hydrogen target. Part of the collected data has been analyzed to extract preliminary results for the amplitudes of the modulations in the azimuthal angle of the charged hadrons and for their transverse-momentum distributions. These observables give relevant information for the study of the transverse momentum and spin structure of the nucleon. Similarly to the results obtained in COMPASS with a deuteron target, the azimuthal asym…
We perform a calculation of Sivers function in a light-cone SU(6) quark-diquark model with both scalar diquark and vector diquark spectators. We derive the transverse momentum dependent light-cone wave function of the proton by taking into account the Melosh-Wigner rotation. By adopting one-gluon exchange, we obtain a non-vanishing Sivers function of $down$ quark from interference of proton spin amplitudes. We analyze the $\frac{|P_{h\perp}|}{M}$ weighted Sivers asymmetries in $π^+$, $π^-$ and $π^0$ electroproduction off transverse polarized proton target, averaged and not averaged by the kine…
In 2016 and 2017 COMPASS has collected a considerable amount of deep inelastic scattering events with a 160 GeV/c muon beam and a liquid hydrogen target. An analysis of 4 % of these data has allowed to extract preliminary results on the amplitudes of three azimuthal modulations, $A_{UU}^{\cosϕ_h}$, $A_{UU}^{\cos2ϕ_h}$ and $A_{LU}^{\sinϕ_h}$. The first two are particularly important since they carry information on the intrinsic transverse momentum $\vec{k}_T$ of the quarks and on the correlations between the quark spin and $\vec{k}_T$, expressed by the Boer-Mulders TMD PDFs. They show strong ki…
