Measurement of Collins Asymmetries in the inclusive production of hadron pairs

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1 Measurement of Collins Asymmetries in the inclusive production of hadron pairs INFN-University of Ferrara For the BaBar Collaboration SIF-Società Italiana di Fisica XCVI Congresso Nazionale Bologna, Se embre 2010 Page 1

2 Outline Physics introduction Analysis reference frame Analysis strategy Raw asymmetries and Double Ratio Plans and Conclusion Page 2

3 Physics introduction: Transversity distribution Unpolarized distribution, helicity distribution and transversity distribution functions contain basic and necessary information for a full understanding of the quark structure at leading twist order. Transversity (h 1 ) remains the less know distribution due to its chiral odd nature it appears with another chiral-odd function in semi inclusive DIS experiment. The most promising approach is the double transverse spin asymmetries in Drell-Yan pp interaction but, at present, the antiproton polarization efficiency is very low. Page 3

4 Collins FF in e + e - annihilation SIDIS: Semi Inclusive Deep Inelastic Scattering e + e - h 1 h 2 X e + e - q q h 1 FF FF h 2 The Collins Asymmetries are proportional to: H 1 (z 1 ) x H 1 (z 2 ) where H 1 is the Collins FF Factorization theorem: Transversity Distribution Function Collins Fragmentation Function Transverse Single Spin Asymmetries: A T h 1 xh 1 Collins Effect: Fragmentation of a quark q with Spin S q into a spinless hadron h carries an azimuthal dependence (k x p h, )*S q Belle: Phys. Rev. D78,032011(2008) Anselmino et al: Phys.Rev. D75, hep-ex Page 4

5 Collins FF at BaBar e + e - ππx Number density for finding an hadron h produced from a transversely polarized quark. Quark spin direction unknown: measurement of Collins function in one hemisphere is not possible; Correlation between two hemispheres results in cos(φ 1 + φ 2 ) (or cos(2φ 0 ) ) modulation of the observed di-hadron yield. Page 5

6 RF12: rust Reference Frame Daniel Boer Nucl. Phys. B 806(2009) [arxiv: v2] φ 1 +φ 2 or thrust reference frame φ 2 P h2t P h2 θ 1 e+ e- P h1 P h1t φ 1 Thrust (qq axis appox.) θ 1 : angle between the lepton axis and the thrust axis; φ 1,2 : azimuthal angles of the two hadrons between the scattering plane and the transverse hadrons momenta P hit. Page 6

7 RFM0: 2φ 0 Reference Frame Daniel Boer Nucl. Phys. B 806(2009) [arxiv: v2] 2φ 0 reference frame θ 2 : angle between the lepton axis and the second hadron momenta P h2 ; φ 0 : angle between the plane spanned by one hadron momentum and the lepton momenta and the transverse momentum of the second hadron respect to the first hadron momentum P h2 e+ θ 2 φ 0 P h1t e- P h1 Page 7

8 What we can measure. Accessing the Collins Asymmetries: measurement of cos(φ 1 +φ 2 ) (or cos(2φ 0 )) normalized modulation of hadron pair (N(φ 1 +φ 2 ), N(2φ 0 )). Normalized distribution: where φ α =φ 1 +φ 2 or φ α =2φ 0 Collins asymmetries In the two hemispheres, we can detect two pions with the same sign(π + π + or π - π - ) or with opposite sign (π + π - or π - π + ) The favored fragmentation process describes the fragmentation of a quark of flavor q into an hadron with a valence quark of the same flavor (u π +, d π -.) and unfavored for u π -, d π +. : UnLike-sign π pair (UL) R UL Like-sign π pair (L) R L Charged-sign π pair (C) R C (fav x fav)+(dis x dis) (fav x dis)+(dis x fav) (fav+dis)x(fav+dis) Page 8

9 Analysis strategy 1) Preselection: a) Multi-hadron events (number of tracks > 2); b) Visible energy: E vis > 7 GeV; c) Two jets events: Thrust>0.8 eliminate the ϒ(4S) contribution; z=2e h /Q >0.2 (fractional energy of pions); Select tracks from primary vertex; MC BB cc uds ττ Preliminary on-peak data sample MC uds MC ττ Preliminary on-peak data sample ETC*,TRENTO, June I. Garzia Page 9

10 Analysis strategy 2) assume the thrust axis as qq direction; 3) separate events in two hemispheres according to thrust axis; 4) select pions in opposite hemispheres; 5) measure the corresponding azimuthal angles (φ i ); 6) Fit the raw asymmetries taking care of the detector acceptance cos(θ)=(p x n x +P y n y +P z n z )/ P ) p 2 x e + z θ 2 p 1 θ 1 e - thrust y Cos(θ 1,2 ): witouh cut data Cos(θ 1,2 ): with cut data hemi2 hemi1 hemi2 hemi1 Page 10

11 Raw Asymmetries: RF12 R L R UL Run1-Run6 OffPeak uds events R L R UL MC: Collins Asymmetry not implemented. VERY PRELIMINARY data L~45 fb 10.54GeV Fit function: a+b*cos(φ 1 +φ 2 ) a is consistent with 1 b is the raw asymmetry parameter (acceptance effects and radiative effects are included) Page 11

12 Double Ratio In order to eliminate the acceptance effects and the first order radiative effects, we can construct a double ratio, e.g. R L /R U L (the asymmetries generated by QCD radiative events and acceptance effects do not depend on the charge combination of pion pair): It will be consistent with unity within the statistical error It contains only the Collins asymmetry and higher order radiative effects. The same parametrization is true for other double ratio (e.g: R L /R C ). Page 12

13 Double Ratio: thrust RF 10.54GeV e + e - qqbar (q=u,d,s) VERY PRELIMINARY data L~45 fb 10.54GeV Collins asymmetries not implemented in MC simulation we expect P 1 consistent with zero. Preliminary data: cc and ττ contributions not subtracted; errors due to systematic effects not included. The acceptance effects and the first order radiative effects cancel. Page 13

14 MC binned asymmetries A 0 A 12 A 0 A 12 σ light q 2.09 nb σ cc 1.30 nb σ bb 1.05 nb The Babar detector collected about 432 fb -1 at ϒ(4S) and 45 fb -1 below the ϒ(4S) resonance (10.54GeV) ( 10 9 events for light quark); A 0 A 12 A 0 A 12 Measurement of the Collins FF (H(z)) in small z i intervalls. Monte Carlo: we expect zero asymmetry in each z bin Page 14

15 Competing sources to the asymmetries The competing sources for the azimuthal correlation are: γ-z interference: does not lead to any significant additional φ dependence (it only modifies the θ distribution with a forward-backward asymmetry of a few percent) (hep-ph/ v2); week decay: parity violation can lead to azimuthal correlation between hadrons in opposite hemispheres, e.g. τ πν (E vis >7GeV); GLUON RADIATIVE EFFECTS: It is the dominant Collins-like background contribution. It originates from low energetic gluon radiation e + e - qqg which does not manifest itself in a third jet but creates azimuthal asymmetries The differenzial cross section is proporzional to D 1 (z) and D 1 (z) for a given combination of hadrons it s necessary to construct quantities in which the gluon radiation component is largely absent (Double Ratio). Page 15

16 Outlook Measurement of charm contribution to the asymmetries and correction of data (e + e - cc ~ 40% of the total cross section); τ + τ - contribution; Bottom background; Evaluate the sistematic effects which influence the asymmetry,e.g. : Estimate higher order radiative effects; Possible charge dependence of the detector response; Beam polarization measurements; Page 16

17 Conclusion We have shown that this measurement is also feasible with BaBar data and we expect the first results for pions ready in a few months. Thanks for your attention Page 17

18 Backup slides Page 18

19 B-Factories The B-factories, PEPII and KEK, consist of asymmetric electron-positron beams. The center of mass energy is selected to be on the peak on the ϒ(4S) meson (10.58Gev/c 2 ), which decay more than 96% into BB meson pairs. Physics program: The principal physics goal of the B-factories is to test the Kobayashi-Maskawa picture of CP violation by measuring the angle of the unitary triangle; precision measurements of decays of bottom and charm mesons of the τ lepton; search for rare processes; the very clean environment make BaBar suitable also for inclusive studies of hadron production: Study of the azimuthal asymmetries in the inclusive production of two back-to-back hadron in the reaction: e + e - h 1 h 2 X Page 19

20 PEP-II B-Factory SLAC: Stanford Linear Accelerator Center (California,USA) PEP lenght m; HER (High Energy Ring): 9.0 GeV e - ; LER (Low Energy Ring): 3.1 GeV e + ; Project Luminosity: 3*10 33 cm -2 s -1 BaBar detector Page 20

21 BaBar detector DIRC 144 quartz bars 1.5T Solenoid ElectroMagnetic Calorimeter 6580 CsI(Tl) crystals e + (3.1GeV) e - (9GeV) Drift CHamber 40 stereo layers Instrumented Flux Return 19 RPCs layers (Run1-Run3) 12 LSTs layers (Run4-Run7) Silicon Vertex Tracker 5 layers of double sided silicon strips Page 21

22 BaBar Luminosity Accelerator parameters: e GeV e GeV s = GeV βγ=0.56 Integrated Luminosity: ϒ(4S) GeV Page 22