Physics Results on the Tagged Structure Functions at HERA

Transcription

1 Physics Results on the Tagged Structure Functions at HERA DESY on behalf of H1 and ZEUS Thomas Jefferson National Accelerator Facility Newport News, VA January Page 1 Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014

2 HERA, the H1 and the ZEUS Detector Three periods of data taking: HERA I : HERA II : Low proton energy: ~10 pp 1 ~380 pp 1 ~65 pp 1 E p = 90 GeV E e = 7.5 GeV s = 4E E p e = 318 GeV e P Inner tracking system Superconducting solenoid Calorimeter Iron yoke Muon chambers inside/outside yoke Forward muon detector P ZEUS Detector plus Forward proton and neutron spectrometers about 100m downstream e H1 Detector Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page

3 The Proton Taggers at HERA I Z=96 m HERA magnets P-beam direction Position resolution -3 mm H1 FPS module Scintillating fibers Silicon strip detectors Z=6 m Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 3

4 Kinematics of DIS Events Inclusive nondiffractive deep inelastic scattering (DIS) events Diffractive DIS events Proton or dissociative system, measured in FPS/LPS s = ( k+p) ( ) Q = -q = - k-k Q x = p q W = (q+p) y= pq pk center of mass energy squared photon-proton center of mass sqared virtuality, size of the probe x: fraction of the proton momentum carried by the struck parton Y: inelasticity, fraction of the electron momentum carried by the virtual photon For diffractive events in addition: M x t = (p-p ) M +Q (p-p ) q x = = x IP pq W +Q Q x Q β = = = (p-p ) q x M +Q IP mass of the diffractive system x four-momentum transfer squared at the proton vertex X momentum fraction of the proton carried by the Pomeron fraction of the Pomeron momentum which enters the hard scattering Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 4

5 The tagged Proton Signal x L p = 1- x p IP x L 1 -> elastically (diffractively) scattered proton t p = - x T L (1 x x L L ) M p Detecting the elastically scattered Proton is the only method to measure the four-momentum transfer, t, at the proton vertex. Events with x L < 1 originate from mainly from proton dissociative processes. The resolution in x L determines the background under the elastic peak. x L Identification of Diffractive Events Three methods have been used at HERA to select diffractive events: large rapidity gap selection -> p-dissociation background and Regge-contributions large acceptance; M X method -> no Regge-contributions but p-dissociation background, large acceptance; detecting proton at x L 1 -> almost no background from p-dissociation but Regge-contributions, very small acceptance. Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 5

6 Inclusive DIS Proton Structure Functions d σ γ p dxdq = πα x Q em 4 r [ 1 (1 y) ] σ ( x, Q ) xf 3 can safely be neglected at not too high Q for HERA data σ ( x, Q r ) = F ( x, Q F ( x, Q ) sizeable only at high y L For HERA data typically: Q y = x s 0.1 σ D(4) r σ Analogous to inclusive DIS: d 4 σ dq dtdxip dβ = παem r βq D( 4 ) [ 1 ( 1 y) ] σ (Q,t,x, β ) D(3) r Inclusive Diffractive Proton Structure Functions If t is not measured and integrated over: y ) - 1+ (1- y) D(4) y D(4) ( Q,t, xip, β ) = F ( Q,t, xip, β ) - F (,t,, ) L Q xip β 1+ (1- y) D(3) y D(3) ( Q, xip, β ) = F ( Q, xip, β ) - FL ( Q, xip, β ) 1+ (1- y) IP Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 6

7 Regge fit: Proton tagged DIS Cross-sections from H1 Pomeron contribution Reggeon contribution Assumption: Regge/vertex factorization DESY Eur.Phys.J. C71(011) 1578 α ( ) = α (0) + α IP t IP IPt Q 5 9 from the parametrization of the pion trajectory 15 6 Input from other Measurements: Fit Results α (0) = 0.50 α B IR IR IR = 0.3GeV = 1.6 GeV GeV Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 7

8 p T -Dependence of leading Proton Production DESY JHEP06 (009) 74 DESY Eur.Phys.J. C71 (011) 1578 dσ = dp T A e ( Bp T ) Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 8

9 Proton tagged DIS Cross-sections from H1 H1 FPS data from HERA II ; comparison with fit from LRG data DESY Eur.Phys.J. C71 (011) 1578 DPDF fit: pqcd fit to earlier LRG dataset, Eur.Phys.J. C50 (006). Dashed line shows the diffractive (Pomeron) contribution. For x IP > 10 - considerable contributions from Regge exchanges are present. Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 9

10 Ratio of Proton tagged Events at Low x L to Inclusive DIS Events DESY JHEP06 (009) 74 Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 10

11 pqcd Analysis of Diffractive Data The concept of diffractive parton distribution functions (DPDF) σ D(3) r D(3) y D(3) ( Q, xip, β ) = F ( Q, x, β IP ) - FL ( Q, xip, β ) 1+ (1- y) QCD-factorization theorem for diffractive DIS: Regge-factorization assumption: DPDFs obey DGLAP evolution ; Parametrisation of DPDFs at Q 0 = 1.8 GeV e 1 z Additional factor to ensure that distributions vanish for z 1. included Fit the DGLAP evolution with these parameterizations to inclusive diffractive data 9 parameters left free in fit: Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 11

12 pqcd Analysis of Diffractive Data by ZEUS LRG sample: < Q < 305 GeV, 40 < W < 40 GeV, < M X < 5 GeV, < x IP < 0.0 LPS sample: < Q < 10 GeV, 40 < W < 40 GeV, < M X < 40 GeV, 0.00 < x IP < 0.1 Samples are corrected for proton dissociation where necessary. Standard Fit: A g, B g, C g as free parameters ZEUS DPDF S LPS Data: DESY Nucl. Phys. B 816 (009) pqcd analysis: DESY Nucl. Phys B 831 (010) Data used in fit: ZEUS Coll. Nucl.Phys. B 816, 1 (009) Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 1

13 Diffractive Parton Density Functions (DPDF) from ZEUS Diffractive quark density functions from the ZEUS DPDF S and DPDF C fits. Diffractive gluon density functions from the ZEUS DPDF S and DPDF C fits. Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 13

14 Combination of H1 FPS and ZEUS LPS Data Choose a common (x,q )-grid, if necessary swim data using fit of diffractive structure functions. Then combine data, taking into account correlations of systematic errors. DESY Eur.Phys.Journal C7(01) 175 Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 14

15 Results from Events with a Leading Neutron X n Final state neutron in the proton fragmentation system Leading neutron production via an exchange process. X n One pion exchange (OPE) is the dominating mechanism for the production of neutron tagged events. x LDistributions There is no elastic (diffractive) peak present. DESY Nucl.Phys.B776(007) 1-37 DESY Eur.Phys.J. C68 (010) 381 Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 15

16 Comparison of Leading Proton and Leading Neutron Cross-sections DESY JHEP06 (009) 74 The leading neutron rate is roughly a factor of two lower than the leading proton rate for x L <1. No diffractive processes, including dissociative events, contribute to leading neutron production. Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 16

17 Leading Neutron Cross-sections and Structure Functions The semi-inclusive leading neutron cross-section can be written as, neglecting the longitudinal structure function. DESY Eur.Phys.J. C68 (010) 381 The neutron tagged structure function has the same Q dependence as the inclusive structure function. Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 17

18 The Pion Structure Function DESY Eur.Phys.J. C68 (010) 381 Assuming the validity of the vertex factorisation: with, e.g. (various parametrisations exist) Then one can write: with might be interpreted as the pion structure function. The data are compared to two different parametrisatons of the pion structure function: Eur.Phys.J.C10 (1999), Phys.Lett. B 33 (1989), and the H1 parametrisation of the proton structure function scaled by a factor /3. Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 18

19 Summary Despite very low acceptance of the proton taggers a large amount of proton tagged data has been collected. The proton tagged data are mainly diffractive events for < For > 10 - Regge-exchange process contribute considerably. x IP Proton tagged data can be described well by Regge theory inspired fits assuming vertex factorization. pqcd fits including Regge contributions deliver the Pomeron structure function assuming vertex factorization. From inclusive diffractive data measured with the LRG method and leading proton tagged events diffractive parton distribution functions can be extracted. x IP Neutron tagged data are dominated by pion exchange processes. They are not diffractive. Assuming vertex factorization and taking the pion flux factor from hadronic scattering data the pion structure function can be extracted. Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 19

20 Backup Slides

21 Results from the VFPS of H1 VFPS results are compared to Data from the FPS, LRG selected data, and DPDF fit. The LRG data and the DPDF fit are scaled by a factor of 0.81 for the proton-dissociative contributions H1 prelim

22 Diffractive Longitudinal Structure Function F L D σ D(3) r D(3) y D(3) ( Q, xip, β ) = F ( Q, xip, β ) - F (,, ) L Q xip β 1+ (1- y) Measurements of σ r D done so far in kinematical regions where F L D can be neglected or it is corrected for by taking F L D from pqcd predictions. F L D is strongly correlated to the diffractive gluon density Need measurements of σ r D for different values of y at fixed necessary to measure F L D directly (x,q ) y = Q 1 x β s IP vary center of mass energy squared s = 4 E E e p D D σ FL = r ( y / Y+ ) 1 with Y = + 1+ (1 y) Keep electron energy E e, vary proton energy E p E p = 90 GeV, 575 GeV, 460 GeV

23 Measurement of the Diffractive Longitudinal Structure Function F L D by H1 DESY Phys.J. C7 (01)

24 Ratio of LPS-tagged to LRG selected diffractive data DESY Nucl.Phys.B816(009) 1-61 LRG selected data are plotted before subtraction of proton-dissociative background. The ratio is independent of x IP, Q, and β. r = 0.76 ± 0.01( stat.) ( syst.) The difference of r to one is due to the proton-dissociative contributions included in the LRG data. Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 11

25 The M X Method to extract Diffractive DIS Events DESY Nucl.Phys. B173 (005) 3-80

26 Results from the M X method DESY Nucl.Phys. B800 (008) 1-76 Vertex factorization assumption: F ( x, β, Q ) = f ( x ) F ( β, Q IP Consequence: IP IP IP ) For fixed β, the Q dependence should be independent of x IP. Within a column of fixed β, x IP dependence results only in scale factors. Visible are scaling violations: At high x IP the structure function falls with Q whereas at low x IP it rises with Q.

27 Leading Neutron Cross-section and Structure Function, cont. DESY Eur.Phys.J. C68 (010) 381 In the OPE model, the cross-section for leading neutron production factorises: The pion flux factor can be derived from hadron scattering data. Various parametrisations exist, e.g.: In the OPE model, β can be interpreted as the momentum fraction of the pion which takes part in the hard interaction with the virtual photon. x x β = = 1 xl xπ Fit: Exploring Hadron Structure with Tagged Structure Functions, JLAB January 16-18, 014 Page 18

28 Inclusive DIS PDFs from H1-ZEUS combined data Parton distribution functions from HERAPDF1.0 at Q = 1.9 GeV and Q = 10 GeV For xu v, xd v, xs(sea) and xg.