Strangeness Production in DIS at HERA HERA kinematics

Strangeness Production in DIS at HERA • • HERA kinematics Strange Particles (K 0 s, Λ) Production in ep Collisions Differential cross-sections asymmetry Ratios of differential cross sections (Λ /K 0 s, K 0 s/h±) • Summary Grażyna Nowak IFJ PAN Krakow Grażyna Nowak representing Strangeness Production in DIS at HERA Collaboration DIS 2009 Madrid 26 -30. 04. 2009

e±p kinematics at HERA ep center of mass energy hadronic final state mass exchanged momentum (squared) inelasticity variable Hadrons: light (u, d) strange (s) Bjorken scaling variable heavy (c, b) Measurement: Neutral Current (NC) Deep Inelastic Scattering (DIS) 2 < Q 2 < 100 Ge. V 2 non-perturbative hadronisation process leading to hadronic final state 0. 1 < y < 0. 6 kinematics from scattered electron HERA I data int. luminosity of ≈ 50 pb-1 2 Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

Processes for Strangeness Production in ep Scattering Dominant: hadronisation (non-perturbative process) LUND string fragmentation model strangeness suppression factor λs=P(s)/P(q) more parameters for baryon production: diquark suppression factor strange diquark suppression factor hard processes: QPM Grażyna Nowak boson-gluon fusion Strangeness Production in DIS at HERA λqq=(P(qq)/P(q) λsq=(P(sq)/P(qq))/(P(s)/P(q)) decays of heavy quarks DIS 2009 Madrid 26 -30. 04. 2009

QCD Models for DIS ep Interactions first order QCD Matrix Element parton shower ARIADNE LEPTO MEPS Matrix Element+ Parton Shower DGLAP – strong ordering in k. T of emitted partons CDM Color Dipole Model dipoles radiate independently no ordering in k. T of emitted partons Both are interfaced to JETSET for hadronisation (Lund string model) with s=0. 286, qq=0. 108, sq=0. 690 (e+e- ALEPH tuned parameters) only s varied for comparison of the predictions Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

Observation of K 0 s , Λ 0. 5 < pt < 3. 5 Ge. V K 0 s , Λ signals ~ 213 000 K 0 s -1. 3 < η < 1. 3 ~22 000 Λ Grażyna Nowak Strangeness Production in DIS at HERA ~20 000 DIS 2009 Madrid 26 -30. 04. 2009

K 0 2 s differential production cross-sections in LAB frame in Q , x , η , p. T Overall agreement with LO Monte Carlo predictions CDM with s = 0. 3 MEPS with s = 0. 22 Theory(MC pred. )/Data points Difficulties: at low x , at low p. T , shape of η Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

Λ differential production cross-sections in LAB frame in Q 2, x , p. T, η Overall agreement with LO Monte Carlo predictions CDM and MEPS with s = 0. 3 (strange baryon production depends not only on s , but also on qq, sq ) Difficulties: at low x , at low p. T , shape of η Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

K 0 Breit differential x-sections in Breit frame in p , x T p s current struck quark xp. Breit = (2 ph. Breit) Q target proton remnants QPM picture is modified by higher orders CDM and MEPS with s = 0. 3 describe the differential x-sections current region less sensitive to s with respect to the laboratory frame or target hemisphere Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

Λ differential production cross-sections in Breit frame in xp. Breit , p. TBreit current target Q struck quark proton remnants QPM picture is modified by higher orders CDM and MEPS with s = 0. 3 describe the differential cross-sections current region less sensitive to s with respect to the laboratory frame or target hemisphere Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

AΛ= Breit frame Laboratory frame Test of transfer of the baryon number from the proton beam to the final state strange particles AΛ≈ 0 No evidence of baryon number transfer is visible in the measured data Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

Sensitivity of K 0 s, Λ Production to Proton Parton Density Functions CDM predictions with s = 0. 286 K 0 s Λ PDF parametrisations: CTEQ 6 L, H 12000 L 0 GRV-94 LO K 0 s , Λ η distributions: no sensitivity to PDF Λ Grażyna Nowak K 0 s Strangeness Production in DIS at HERA Q 2 distributions: different predictions for different PDFs DIS 2009 Madrid 26 -30. 04. 2009

Ratio of production cross sections: strange baryons/strange mesons LAB frame CDM describes the data MEPS below the data difficulties: at high Q 2 η shape no sensitivity to s , as expected Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

Ratio of production cross sections: strange baryons/strange mesons Breit frame distributions in the target and current hemispheres reasonably well described by CDM and MEPS models very weak dependence on s Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

Ratio of production cross sections: strange mesons/charged hadrons LAB h± in the same kinematic region as K 0 s ratio rises strongly with p. T , ~constant as a function of x, Q 2, η MEPS favours a lower value of s= 0. 22 over the full phase space CDM at low Q 2 better with s=0. 3, at high Q 2 prefered lower s=0. 22 difficulties : at low x, at low pt, large positive η Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009

Summary • K 0 s and Λ production in laboratory frame: reasonable good description by LO Monte Carlos (CDM, MEPS) CDM with s = 0. 3 best , but • • both models have difficulties at small x, low p. T , η shape • K 0 s and Λ production in Breit frame: well described by both the MEPS and CDM predictions with s = 0. 3 ratios of production cross-sections: Λ/K 0 s better described by CDM with s = 0. 3 K 0 s/charged hadrons better described by MEPS with s = 0. 22 No single combination of model and s describes all data in the measured region • no indication of baryon number transfer is observed Grażyna Nowak Strangeness Production in DIS at HERA DIS 2009 Madrid 26 -30. 04. 2009