I R F U Nucleon structure studies with the COMPASS experiment at CERN Stephane Platchkov Institut de Recherche sur les lois Fondamentales de l’Univers CEA/IRFU, Saclay, France (for the COMPASS Collaboration) International Conference on New Frontiers in Physics Kolymbari, Aug.23-30, 2015

I R F U Nucleon structure studies with COMPASS  COMPASS I Longitudinally polarized DIS and SIDIS Transversely polarized SIDIS  COMPASS II Deeply-Virtual Compton Scattering (DVCS) Massive lepton pairs from Drell-Yan process  Not covered in this talk Hadron spectroscopy, etc… ► Talk by F. Nerling this afternoon S. PlatchkovICNFP, Kolymbari

I R F U COMPASS – physics and tools 1/2 S. PlatchkovICNFP, Kolymbari from Bacchetta Deep Inelastic Scattering (DIS) Semi-Inclusive DIS (L or T) Drell-Yan process

I R F U COMPASS – physics and tools 2/2 S. PlatchkovICNFP, Kolymbari Transversity Momentum Distributions: TMD (x,k T ): probe the transverse parton momentum dependence Generalized Parton Distributions : GPD (x,b T ): probe the transverse parton distance dependence Semi-Inclusive DIS Drell-Yan process Deeply Virtual Compton Scattering

I R F U COMPASS – a fixed target experiment at CERN A very versatile setup Several beams available: µ +, µ -, h+, h -, e - => Several ways of probing the nucleon structure S. PlatchkovICNFP, Kolymbari m Energy: 100 – 200 GeV Intensity: up to 10 9 /spill Large acceptance, PID detectors Several particles in the final state Large (1.2 m) polarized target

I R F U COMPASS – polarized target S. PlatchkovICNFP, Kolymbari Two 60 cm oppositely polarized cells Target cells ( 2 x 60 cm) Superconducting solenoid (2.5T) High magnetic field High field uniformity Very low temperature L or T polarization

I R F U Interaction due to one single photon Scattering from nearly free partons Structure functions depend on x only (Bjorken, 1968) Q 2 dependence is governed by the QCD evolution Deep-Inelastic Lepton Scattering S. PlatchkovICNFP, Kolymbari

I R F U Unpolarized measurements and QCD fits S. PlatchkovICNFP, Kolymbari

I R F U Polarized structure function g 1 (x) – world data S. PlatchkovICNFP, Kolymbari Data are used as input to a global QCD fit

I R F U Reminder: the proton spin problem S. PlatchkovICNFP, Kolymbari Naive quark model :  = 1.0 Relativistic quark model:  ≈ 0.6 Experiment:  ≈ 0.3

I R F U x  q(x) xGxG xsxsxdxdxuxu  Inputs: world data, functional forms, assume SU(3) ►  G is determined through DGLAP evolution (NLO) COMPASS NLO pQCD fit to g 1 (x) S. PlatchkovICNFP, Kolymbari

I R F U  Inputs: world data, functional forms, assume SU(3) ►  G is determined through DGLAP evolution (NLO) x  q(x) xGxG xsxsxdxdxuxu COMPASS NLO pQCD fit to g 1 (x) S. PlatchkovICNFP, Kolymbari

I R F U Contribution of gluons to the nucleon spin Gluons: spin 1, no charge Tool : Photon-Gluon Fusion (PGF) Identify the PGF process? 1. Detect charmed quarks: clean signature, but limited statistics “Open Charm” method 2. Detect light quarks: high statistics, but large physical background “Hadron production” method S. PlatchkovICNFP, Kolymbari Rely on a Monte-Carlo estimate of the background

I R F U  G/G results S. PlatchkovICNFP, Kolymbari  G/G = 0.113±0.038 (stat)±0.035(sys) Open charm World direct  G/G extraction (LO) Hadron production

I R F U Polarized Semi-Inclusive DIS (SIDIS) S. PlatchkovICNFP, Kolymbari

I R F U SIDIS asymmetries: World proton data S. PlatchkovICNFP, Kolymbari COMPASS preliminary

I R F U Polarized PDFs as determined by pSIDIS S. PlatchkovICNFP, Kolymbari

I R F U The strange quark puzzle S. PlatchkovICNFP, Kolymbari

I R F U  s puzzle: what about Fragmentation Functions? S. PlatchkovICNFP, Kolymbari An independent measurement of D 1 (z,Q 2 ): hadron multiplicities

I R F U Kaon ( K + ) multiplicities M K (z): in 9 x bins S. PlatchkovICNFP, Kolymbari

I R F U Kaon Fragmentation Function (COMPASS fits) S. PlatchkovICNFP, Kolymbari

I R F U Transversity structure functions  Transversity? ► Transversely polarized quarks in a transversely polarized nucleon (to the direction of the virtual photon) Third distribution function (leading twist) S. PlatchkovICNFP, Kolymbari

I R F U Transverse Momentum Dependent PDFs S. PlatchkovICNFP, Kolymbari

I R F U Transverse (Collins) asymmetries S. PlatchkovICNFP, Kolymbari K- K+

I R F U Transversity QCD fit S. PlatchkovICNFP, Kolymbari

I R F U Generalized Parton Distributions (GPD) S. PlatchkovICNFP, Kolymbari GPD: correlation between the long. momentum x and the transverse position b T Measured in exclusive reactions 4 GPDs: Unpolarized target: GPD H Polarized target: GPD E

I R F U DVCS and BH cross section for µ + and µ - Cross section for µp -> µpγ DVCS and BH (known) processes:  COMPASS beams: opposite charge/spin Charge-and-Spin Sum Charge-and-Spin Difference s S. PlatchkovICNFP, Kolymbari DVCS BH

I R F U DVCS run – main new equipment S. PlatchkovICNFP, Kolymbari x2 m 2 electromagnetic calorimeter, ECAL0 4.0 m longTime-Of-Flight detector: 24 inner and 24 outer slabs 50 m ECAL1 ECAL2 2.5 m long LH target ECAL0

I R F U DVCS – the COMPASS x B regions – SIMULATION S. Platchkov ICNFP, Kolymbari DVCS BH DVCS BH

I R F U Test run – 4 days with a 40 cm long H 2 target DVCS – the COMPASS x B regions – REAL DATA S. PlatchkovICNFP, Kolymbari DVCS BH dominance Interference DVCS dominance 2009 data

I R F U DVCS – SUM of  + and  - cross sections S. PlatchkovICNFP, Kolymbari Expected statistics in 2x6 months of data taking

I R F U  Drell-Yan cross section:  Features (parton model): Cross section depends on τ = M 2 /s Convolution of quark and antiquark PDFs Can be used to determine PDFs in , K, p Transverse momentum of µµ pair is small No fragmentation process  Confirmed in QCD Assumptions: factorization Polarized (+ unpolarized) Drell-Yan measurements S. PlatchkovICNFP, Kolymbari Ito et al. PRD 23(1981)604. (from Kenyon, RPP, 1982) FERMILAB: non-DY contribution

I R F U Hadron (pion + kaon + antiproton) beam Transversely polarized NH 3 target Large muon angular acceptance With a negative pion beam: u /u annihilation COMPASS acceptance Dominated by valence quarks (x ≥ 0.1) COMPASS exclusive setup advantages S. PlatchkovICNFP, Kolymbari

I R F U  Full formalism for two spin ½ hadrons  COMPASS: access 4 TMDs: Boer-Mulders, Sivers, Pretzelosity, Transversity  Access 4 TMDs – asymmetry modulations: DY (polarized) cross section expansion S. PlatchkovICNFP, Kolymbari Arnold, Metz and Schlegel, Phys. Rev. D79 (2009) Boer-Mulders Sivers Pretzelosity Transversity Worm-Gear

I R F U Transverse Momentum Dependent PDFs S. PlatchkovICNFP, Kolymbari Sivers: correlation between the quark transverse momentum and the nucleon transverse spin (polarized nucleon) Boer-Mulders: correlation between the quark transverse spin and transverse momentum (unpolarized nucleon)

I R F U TMDs in Drell-Yan and SIDIS  SIDIS vs TMD SIDIS: TMD and FF Drell-Yan: two TMDs  Factorization TMDs (unlike PDFs) can be process dependent (“non-universality”) Opposite sign in SIDIS and DY processes: S. PlatchkovICNFP, Kolymbari Collins, Soper, Sterman, Adv. Ser. High En Phys. 5, 1988.

I R F U  Test setup (3 days in 2009) 190 GeV negative pion beam, I ≤ 1.5x10 7 /s (instead of 10 8 /s) “poor-man” hadron absorber ( concrete and steel) two polyethylene target cells preliminary DY trigger  Results Count rate confirmed Mass resolution as expected Good vertex resolution Low background at high masses Drell-Yan – test data taking S. PlatchkovICNFP, Kolymbari

I R F U Polarized Drell-Yan – expected results S. PlatchkovICNFP, Kolymbari SiversBoer-Mulders PretzelosityTransversity 140 days of data pions/spill 2 x 55 cm NH 3 target 4 < M mm < 9 GeV

I R F U Summary  COMPASS is the largest fixed-target experiment at CERN  Unique combination of hadron and muon beams of both polarities  COMPASS has a very versatile experimental setup  Rich physics program dedicated to both nucleon structure and hadron spectroscopy studies  Present schedule 2015 : Drell - Yan data taking (1 st “year” ≈ 140 days) 2016 : DVCS data taking 2017 : DVCS data taking 2018 : Drell-Yan data taking (2 nd year) ICNFP, Kolymbari 2015S. Platchkov 39