Diffractive and Electromagnetic Processes at the LHC Trento, January 4-8, 2010 Leading Baryon Production at HERA Armen Bunyatyan Outline: Leading Protons and Neutrons in DIS Leading Neutrons in photoproduction of jets Leading Baryons and Cosmic Rays
Introduction scale for secondary particle production decreases from Q 2 in current region (or high γ * P T jets if Q 2 ~0) to a soft Current Current hadronic scale (proton region jet fragmentation region) Proton remnant p,n Significant fraction of ep scattering events contains in the final state a leading proton or neutron which carry a substantial portion of the energy of the incoming proton: e+p � e’+n+X or e’+p+X Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 2
Introduction Production mechanism of leading baryons: p,n exchange of virtual particle ‘conventional’ fragmentation of • LP: neutral iso-scalar,iso-vector proton remnant (e.g. Lund string) ( π ,IR,IP) • LN: charged iso-vector ( π +, ρ +,a 2 ..) Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 3
Kinematics and Vertex factorisation k’ ep � e’NX k Lepton variables: q e π � e’X Q 2 =-(k-k’) 2 x=Q 2 /(2p•q) Leading baryon variables: p � π N x L =E LB /E p t=(p-p LB ) 2 (or p 2 T,LB ) In the exchange model the cross sections factorise, e.g. for one pion exchange σ (ep → e’NX) = f π /p (x L ,t) × σ (e π→ e’X) f π /p (x L ,t) - pion flux: σ (e π � e’X) - cross-section probability to emit pion from the photon with given x L ,t of e π scattering -Leading Baryon production independent from photon vertex -probe structure of exchanged particle -factorisation violation predicted– absorption/rescattering Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 4
H1 and ZEUS detectors for leading baryons y ZEUS FNC+FNT H1 FNC ZEUS LPS acceptance window θ < 0.75÷0.8 mrad 6 stations with μ strip detectors 14 towers, 17x15 grid hit position resolution ~30 μ m of the FNT hodoscopes , σ X L <1%, σ PT ~few MeV σ E /E ≈ 0.7/ √ E σ E /E ≈ 0.63/ √ E ⊕ 2% position resolution 2-3mm momentum accuracy <1% Acceptance limited by beam apertures and detector size p T resolution is dominated by p T spread of proton beam (50-100 MeV) Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 5
Cross sections vs x L normalised to σ DIS (1/ σ DIS ×d σ /dx L ) Leading protons: (JHEP 0906:074,2009) • diffractive peak at x L =1 ; flat at x L <0.95 Leading neutrons: (Nucl.Phys.B776(2007)1) • yield � 0 as x L � 1 ; Leading protons • drop at x L < 0.7 due to drop in acceptance p T 2 <0.5 GeV 2 2 range restrict to the same p T • measurement: LP ~ 2·LN • for pure isovector particle exchange (e.g. LP pion) one expects LP = ½ · LN � more isoscalar exchanges contribute Leading neutrons to the LP rates LN p T 2 <0.04 GeV 2 p T 2 <0.476·x L 2 GeV 2 Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 6
2 and x L Double differential cross sections vs p T JHEP 0906:074,2009 Nucl.Phys.B776(2007)1 Exponential behavior LP LN d 2 σ 2 b(x )p − ~ a(x ) e L T L dx dp 2 L T slope b(x L ) LP LN x L • different behavior for LP and LN • similar around x L ~0.7 Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 7
Comparison with fragmentation and exchange models: Leading Protons in DIS JHEP 0906:074,2009 IP IR πΔ cross-section πΝ p T slope p T slope standard fragmentation MC models don’t good description by exchange models describe the data out of the diffractive peak isoscalar reggeon dominant at slopes too low at low x L intermediate x L Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 8
Comparison with fragmentation and exchange models: Leading Neutrons in DIS • all standard fragmentation models Nucl.Phys.B776(2007)1 underestimate the neutron yield at high x L cross-section • LEPTO-SCI better for x L shape, but not for the slope • RAPGAP- π - exchange describes data well for x L >0.6, underestimate data at lower x L • Mixture of RAPGAP- π -exchange and standard fragmentation (e.g. DJANGO-CDM) gives the best description of the data DESY-09-185 p T slope Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 9
Leading Proton production rate in DIS Rates to inclusive DIS Structure function F 2 LP(2) ⎡ ⎤ 2 → 2 2 d σ (ep eNX) 4 πα y = − + ⋅ LP(2) 2 1 y F (Q , x) ⎢ ⎥ 2 2 4 dQ dx xQ 2 ⎣ ⎦ r LP(2) is approximately constant vs Same trend as inclusive F 2 is observed x and Q 2 with average value ~0.24 Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 10
Leading Neutron production rate in DIS: F 2 LN(3) (Q 2 ,x,x L ) to F 2 (Q 2 ,x) ratio 6< Q 2 <100 GeV 2 , p T < 0.2 GeV F 2 (Q 2 ,x) from the H1-2000-PDF parameterisation σ → 3 ( ) d ep eNX = 2 dQ dx dx L ⎡ ⎤ πα 2 2 4 y = − + 2 1 ( , , ) LN ⎢ ⎥ y F Q x x 2 4 2 L ⎣ ⎦ xQ F 2 LN (Q 2 ,x,x L )/F 2 (Q 2 ,x) is mostly flat in Q 2 and x i.e. LN production rate, kinematics is approx. independent of (Q 2 ,x) � consistent with factorisation, limiting fragmentation (overall suppression of LN events is also possible) Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 11
F 2 LN(3) (Q 2 , β ,x L ): factorisation properties In particle exchange picture expect proton vertex factorisation: LN(3) (Q 2 , β ,x L ) ~ f(x L )×F 2 LN(2) (Q 2 , β ) F 2 β =x/(1-x L ) - fraction of exchange’s momentum carried by the struck quark LN(3) (Q 2 , β ,x L ) ~ β - λ F 2 λ is almost independent of x L � consistent with vertex factorisation Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 12
Estimate the Pion structure function from F 2 LN (Q 2 ,x,x L ) DESY-09-185 within π + -exchange model we may try to estimate F 2 π from measured F 2 LN : π β = Γ ⋅ β ( 3 ) 2 2 ( , , ) ( ) ( , ) LN F Q x x F Q π 2 2 L L where β =x/(1-x L ) - fraction of pion momentum carried by struck quark (i.e. x Bj for pion) Γ π (x L ) is integrated over t pion flux ∫ Γ = = f ( x 0 . 73 , t ) dt π π / p L use pion flux parameterisation (Holtmann et al.): g 2 1 t ⎛ m 2 t ⎞ − − p ππ f (1 x ) exp ⎜ R 2 ⎟ π = − ⋅ − ⎜ ⎟ L π n + 2 π 4 π (m 2 t) 2 1 x π /p − − ⎝ ⎠ π L Data are sensitive to the parameterisations of the pion structure function (constrained for x>0.1 from the fixed target experiments). Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 13
Estimate the Pion structure function from F 2 LN (Q 2 ,x,x L ) DESY-09-185 within π -exchange model we can π from measured F 2 estimate F 2 LN : π β = Γ ⋅ β ( 3 ) 2 2 ( , , ) ( ) ( , ) LN F Q x x F Q π 2 2 L L where β =x/(1-x L ) Γ π (x L ) is integrated over t pion flux ∫ Γ = = ( 0 . 73 , ) f x t dt π π / p L use pion flux expression (Holtmann et al.): g 2 m t 1 t ⎛ 2 ⎞ − − p ππ f (1 x ) exp ⎜ R 2 ⎟ = − ⋅ − π ⎜ ⎟ L π n + 2 π 4 π (m 2 t) 2 1 x π /p − − ⎝ ⎠ π L LN dependence on x and Q 2 similar to proton, � F 2 � universality of hadron structure at low x LN / Γ below the F 2 π and F 2 � in absolute values F 2 However: large uncertainty of pion flux normalisation: choice of pion flux (formfactor), absorption/rescattering, background… Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 14
Comparison of p T slope of Leading Neutrons with pion exchange models in π -exchange picture σ (ep → e’nX) = f π /p (x L ,t) × σ (e π + → e’X) 2 (or t ) distribution is determined solely p T by pion flux 2 g − 1 t 2 = p ππ − ⋅ 1 - 2 α (t) f (1 x ) F(x , t) π /p L L − 2 2 2 π 4 π (m t) π many parameterizations of pion flux f π /p (x L ,t) in literature compare measured p T slope b(x L ) with models (shown best agreeing models) - reasonable agreement in shape but not in absolute values: all give too large b(x L ) - π -exchange models alone don’t describe p T 2 distribution Armen Bunyatyan Leading Baryon Production at HERA Trento, 4-8 January 2010 15
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