Leading Baryons at HERA

Transcription

1 Leading Baryons at HERA R.Sacchi Univ. of Torino and INFN On behalf of the ZEUS and H1 Introduction and models Data sets LB normalized cross sections in DIS and γp LN + dijets in γp Comparisons with models F LB and pion structure function Summary Legenda: LP = Leading Proton LN = Leading Neutron LB = Leading Baryon OPE = One Pion Exchange γp = photoproduction Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 1

2 Introduction Events with LB are a large fraction of the HERA cross-section Production mechanism is still unclear Models: Lepton variables: Q, W, x, y p,n Standard fragmentation LB from hadronization of p remenant standard MC generators Virtual particle exchange π, IR, IP p,n LB variables: x L = E LB /E p t = (p-p ) LP: neutral iso-scalar iso-vector (π, IR, IP) LN: charged iso-vector (π +, ρ +, a +...) In the dominant process (e.g. OPE) cross section factorizes dσ(γ*p nx) = f (x π/p L,t) dσ(γ*π π X) LB production independent on photon vertex probe structure function of the exchanged particle Models predict factorization violation (absorption) Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008

3 d Alesio and Pirner, EPJ A7 (000) 109 the larger the photon, the fewer the n s detected (more absorption in γp than DIS) the smaller the nπ system (r nπ ), the fewer the n s detected (more absorption at high p T, low x L ) Absorption from additional pomeron exchange: (Kaidalov), Khoze, Martin, Ryskin (Nikolaev, Speth, Zakharov, hep-ph/970890) EPJ C 47 (006) 385; A B Absorption models: LN via π exchange EPJ C 48 (006) 797; evaluate correction due to enhanced absorptive diagrams B (~ 15%) show importance of distortion of energy spectra (migration) due to rescattering for x L < 0.8 include effects of ρ, a exchange estimate the gap survival factor S (important for LHC!) which takes into account that rescattering may populate the rapidity gap with secondary particles carrying away energy from the leading neutron Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 3

4 LB detectors (H1 & ZEUS) Z = 106m Forward Neutron Calorimeter (ZEUS+H1) p beam Leading Proton Spectrometer (ZEUS) acceptance window θ n <0.75 mrad θ=0 o ZEUS FNC + FNT Pb-sci. calorimeters + veto m σ/e = 70%/ E ( 63%/ E 3%for H1) e-scale accuracy ± % (± 0%/ E for H1) Position reconstruction accuracy σ X,Y = 0.3 cm σ θ = µrad allows to measure neutron p T 6 stations with µstrip detectors hit position resolution 30 µm σ X L < 1% σ P T ~ few MeV momentum accuracy <1% For both, p T resolution is dominated by p T spread of p-beam ( MeV) Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 4

5 LB detectors: acceptance Limited by beam apertures and detector size (LPS) Integrating over the azimuth: LPS: s1-s3 and s4-s6 have very little overlap p T range varies with x L ; for x L >0.6, p T <0.5 GeV FNC: acceptance 15 % FNC θ n <0.75 mrad restricted to θ n <0.75 mrad p T range increases with x L ; for x L >0.6, θ n < 0.75 mrad acceptance 5 % Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 5

6 Main Issues in LB analyses Measure x L, p T spectra of LB for various reactions; Verify absorption effects and test vertex factorization; Compare to hadronization/particle exchange/absorption models In LN production, F LN and F π In the following: ZEUS: Leading neutron energy and p T distributions in DIS and γp at HERA (Nucl. Phys. B 776 (007) 1-37) ZEUS: Leading proton production in DIS at HERA (Submitted for publication) ZEUS: Measurement of dijet cross section for events with LN in γp at HERA (EPS07 #6) H1: Leading neutron production in DIS at HERA (ICHEP08 #864) Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 6

7 DATA Sets For all samples: neutron: x L > 0. θ n < 0.75 mrad p T < x L GeV Deep Inelastic Scattering (DIS): proton: x L > 0.3 p T < 0.5 GeV 40 pb -1, Q > GeV (ZEUS, LN) 1.8 pb -1, Q > 3 GeV (ZEUS, LP) 1 pb -1, Q > 6 GeV (H1, LN) HERAII Photoproduction (γp): 6 pb -1, Q < 0.0 GeV Dijets in photoproduction: 40 pb -1, Q < 1 GeV, 130 < W < 80 GeV E T jet1 > 7.5 GeV, E T jet > 6.5 GeV -1.5 < η jet1, <.5 DIS and γp have very different inclusive cross sections σ inc for sensible comparisons look at σ LN /σ inc Additional benefit: systematic uncertainties of central detector cancel Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 7

8 DIS: normalized cross sections vs x L dσ LB /dx L normalized to the inclusive DIS cross sections LP p T <0.5 GeV non-diffractive part LN p T <0.476 x L GeV clear diffractive peak x L 1 proton yield flat below x L 0.95 consistent with earlier low-q data LN yield 0 al kinematic limit x L 1 below x L 0,7 yield drops due to decreasing of p T range Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 8

9 DIS: normalized cross sections vs x L Comparison LP LN yields Restricted to a common p T range where the detector acceptances overlap X LP For pure isovector (e.g. pion) particle exchange one expects LP = ½ LN p T < 0.04 GeV LN data suggest LP LN Additional isoscalar exchanges are needed to account for the observed LP rates Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 9

10 DIS: p T distributions LP LN Note: varying p T ranges Well described by exponential fall-off in p T intercepts and slopes Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

11 DIS p T distributions: slopes and intercept Fit LB data to 1 d b(x LB L ) pt inc σ dp = a(x L ) TdxL slopes b(x L ) intercepts a(x L ) σ e LN LP LN ZEUS ZEUS LP intercepts not shown but ~ flat vs. x L clear different trends for LP - LN similar magnitude for x L ~ 0, where π exch. dominates LN intercepts fall with x L plateau for 0,4 < x L < 0,8 Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

12 DIS (3 Q bins) and γp Q dependence: LN production x L distributions slopes b(x L ) ZEUS γp ZEUS yield increase from γp to DIS small but significant increase from midto high-q DIS slopes equal γp slopes higher for 0,6<x L <0,9 suggests violation of vertex factorization Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 1

13 Q dependence: LP production Average LP yield vs. Q P T < 0,5 GeV different x L ranges 0,3<x L <0,9 γp 0,6<x L <0,9 Steady rise with Q also observed. Results shown are consistent with absorptive effects: smaller Q larger γ* transv. size more absorption Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

14 Ratio γp/dis LN yields Comparison γp/dis: LN x L spectra All DIS Q > GeV combined together Ratio rises with increasing x L Consistent with absorption picture: n-π densities in trasv. plane from D Alesio and Pirner average n-π separation r nπ increases with x L small n-π separation larger absorption Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

15 Comparison γp/dis: LN p T distributions Small but clear slope difference DIS-γp at intermediate x L consistent with absorption: large p T small r nπ more abs. fewer LN at large p T steeper slope Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

16 Dijet photoproduction with LN Absorption effects increase from high Q γp i.e. from hard soft scale A hard scale in γp can be introduced by requiring high E T jets E T,1 > 7,5 GeV Still signs of absorption? Note: E T, > 6,5 GeV Factorization breaking observed in diffractive dijet photoproduction. (see Armen s talk ) look at preliminary (007) results from ZEUS! Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

17 Dijet photoproduction with LN: p T distributions 1 dσ σ dp inc LN TdxL Normalized to the inclusive dijet cross section Well described by exponential fall-off in p T with single slope σ 1 dσ LB b(x L ) pt inc dp ~ TdxL e Note: varying p T ranges Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

18 Dijet photoproduction with LN vs. DIS LN p T dist. fitted with exponential slope b ZEUS x L distr.show suppression at high x L! ZEUS dijet γp DIS DIS dijet γp Slopes have similar magnitude Statistics limits any conclusion on possible differences Suggestive of phase space limitation: dijets in the final state leave little room for energetic high-x L neutrons Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

19 LN in dijet γp vs. DIS: kinematic constraints Consider X BP = fraction of p-energy available for LN production down the beam pipe x L < X BP = 1-(E+P Z )/E P X BP dist. is different in DIS and dijet γp: inc. dijet γp much less energy available in dijet γp for LN production inc. DIS Test: reweight DIS LN x L dist. to match the X BP dist. in dijet γp: high x L mostly gone DIS dijet γp large low x L seen in γp without jets not there Tentatively: differences in the x L spectra due to kin. suppression. More accurate MC studies of kinematic effects ongoing... Hard to draw any conclusion on absorption! Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

20 Comparisons with models Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 0

21 Model comparisons: DIS LP Std. fragmentation MCs Model with multiple exchanges LP yield π IR πn IP p T slopes p T slopes good description of diff. peak but all fail at low x L slopes too low at low x L Good description of LP yield and slope by adding different exch. iso-scalar reggeon domininant at intermediate x L Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 1

22 Comparisons of DIS LN with MC models Comparison with several MC models all std. frag. predict to few n and too low x L; LEPTO-SCI correct yield but no x L dependence of slopes; LN yield RAPGAP-π ~ OK in x L >0,6 Best description found with mixture DJANGO and RAPGAP-π slope intercept Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008

23 Comparisons of DIS LN with pion exch. models Regge factorization: p T distribution is determined solely by the pion flux f π/p (x L,t) dσ dx γ*p nx L dt = f π/p (x L,t) σ γ*π ( (1 x )W,Q ) L various parametrizations of f π/p compare to DIS slope vs x L (only best agreeing models) all models give too large b(x L ) pion exchange alone do not describe the p T dist. Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 3

24 Expectations from Comparison with pion exch. + absorption geometrical absorption model (D Alesio and Pirner); Regge-based model with multi-pomeron exchanges (Nikolaev, Speth, Zakharov) Note on energy dependence: Different dependence of σ γπ on c.m. energy in DIS and γp. γπ s =(1-x L )W Assuming same power-law dependence of γp and γπ cross-section, at fixed W σ γπ γπ /σ γ*π ~ (1-x L )-0,13 Scaling absorption ratio by (1-x L ) -0,13 from ZEUS measurements Ratio γp/dis vs. x L better agreement with data Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 4

25 Comparison with enhanced absorption model + secondary exchanges Model of Kaidalov, Khoze, Martin, Ryskin (KKMR): γp neutron yield rescattering on intermediate partons in central rapidity region; migration of LN after rescattering prediction of ~ 50% abs. loss in γp related to gap survival prob. S extended (KMR) to include exchange of secondary (ρ,a ) reggeons. Good description of LN yield in γp Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 5

26 Comparison with enhanced absorption model + secondary exchanges Absorption + migration with pion exchange alone do not describe LN slopes in DIS: too large values; no turnover at large x L Adding (ρ,a ) exchanges good description of slopes vs x L achieved Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 6

27 LB structure functions and F π Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 7

28 LN structure function F LN Semi-inclusive LB s.f. can be defined analogous to proton s.f. F LN LN(4) e.g. F ( Q, x, xl) = F ( Q, x, xl, t) dt x Look at ratio F LN (Q,x,x L )/F (Q,x) ~ flat in all x or Q bins same behaviour of F and F LN vs (x,q ) LN production mechanism independent of (x,q ) factorization as in exch. models Q 6<Q <100 GeV ; 0<p T <0, GeV x L bin used for π str. function Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 8

29 LP structure function F LP Similar behaviour for LP structure function F LP r LP() LP() F ( x, Q ) ( x, Q ) = F ( x, Q ) diff. peak excluded r LP() approx. constant vs. x and Q with average value ~ 0,4 same trand as F observed Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 9

30 Pion structure function F π Assuming factorization: F LN (Q,x,x L,t) = f π/p (x L,t) F π (Q,β) use F LN x L =0,73 take f (x π/p L,t) from Regge exch. model and integrate over t β=x/(1-x L ) fraction of π-momentum carried by parton Γ π = fπ/p(xl = 0,73, t) dt then extract F π = F LN /Γπ Results compared with: expectations from additive quark model F π = /3 F Pion p.d.f. parametrizations (GRV, ABFKW shown here) Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

31 Pion structure function F π F π vs β at fixed Q Data slightly below expectations absorptive effects may play a role β also uncertainties in pion flux need to be considered. work ongoing... Leading Baryons at HERA - New Trends in HERA Physics October 5 10,

32 Summary LB is a good ground to study soft vs. hard physics Precise measurements of LB x L and p T presented in γp, DIS, γp+dijet; suppression in low x L, high p T, in agreement with absorption; reintroducing hard scale in γp with high E T jets: absorption effect? MCs with `standard` fragmentation do not describe the data multiple exch. model describes LP data, but pure OPE fails to describe LN slopes better description including ρ, a exchanges The neutron energy spectrum in γp compatible with effects of absorption and migration as calculated by Kaidalov, Khoze, Martin and Ryskin gap survival factor S ~ 0.5 has been evaluated For LN production, extract pion structure F π (x,q ) Leading Baryons at HERA - New Trends in HERA Physics October 5 10, 008 3