The H1 and ZEUS Collaborations at the HERA electron-proton

collider are publishing their final analyses based on the full collected statistics and on the combination of their data sets. These results are an important input for the

determination of the proton structure to be used for the predictions of proton-proton processes at the LHC. The most recent results obtained at HERA on inclusive, jet and heavy flavour cross-section and their impact on the determination of the proton structure are discussed. The proton parton distribution functions determined using the HERA data as sole input are presented. Predictions for cross-sections at the LHC based on these PDFs are shown

Turcato, Monica

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Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-11027-4Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011Latest results from HERA and their impact on the LHCM. Turcato(∗)Institut fu¨r Experimentalphysik, Universita¨t Hamburg - Luruper Chaussee 14922761 Hamburg, Germany(ricevuto il 29 Luglio 2011; pubblicato online il 25 Ottobre 2011)Summary. — The H1 and ZEUS Collaborations at the HERA electron-protoncollider are publishing their final analyses based on the full collected statistics andon the combination of their data sets. These results are an important input for thedetermination of the proton structure to be used for the predictions of proton-protonprocesses at the LHC. The most recent results obtained at HERA on inclusive, jetand heavy flavour cross-section and their impact on the determination of the protonstructure are discussed. The proton parton distribution functions determined usingthe HERA data as sole input are presented. Predictions for cross-sections at theLHC based on these PDFs are shown.PACS 13.60.Hb – Total and inclusive cross sections (including deep-inelastic pro-cesses).1. – IntroductionAfter the end of the data taking at the electron-proton collider HERA in June 2007,the two Collaborations H1 and ZEUS are finalising their analyses using the full availabledata statistics. The main goal of the analyses is a deeper understanding of QCD andin particular of the proton parton distribution functions (PDFs). The best possibledetermination of the proton PDFs is a hot topic at the moment due to their impact onthe predictions of cross-sections at the LHC. The HERA data cover a kinematic regionin Bjorken x corresponding to the rapidity plateau for the LHC processes. Therefore aprecise measurement of the proton PDFs in the HERA region provides through DGLAPevolution accurate PDFs for the LHC regimes.The cross-sections of inclusive neutral- (NC) and charged-current (CC) deep inelastice±p scattering (DIS) interactions provide an accurate determination of the valence- andthe sea-quark content of the proton, as well as of the gluon. A further improvement of theunderstanding of the proton PDFs comes from other more exclusive QCD processes, like(∗) E-mail: monica.turcato@desy.dec© Societa` Italiana di Fisica 70LATEST RESULTS FROM HERA AND THEIR IMPACT ON THE LHC 71H1 and ZEUSx = 0.02   (x300.0)x = 0.032   (x170.0)x = 0.05   (x90.0)x = 0.08   (x50.0)x = 0.13   (x20.0)x = 0.18   (x8.0)x = 0.25   (x2.4)x = 0.40   (x0.7)x = 0.65Q2/ GeV2σr,NC(x,Q2)HERA I NC e+pHERA I NC e-pHERAPDF1.0   e+pHERAPDF1.0   e-p±10-210-111010 210 310 2 10 3 10 4 10 5Tevatron Jet Cross Sections2.0 < |yjet| < 2.4 (x 10-9)1.6 < |yjet| < 2.0 (x 10-6)1.2 < |yjet| < 1.6 (x 10-3)0.8 < |yjet| < 1.20.4 < |yjet| < 0.8 (x 103)|yjet| < 0.4 (x 106)D0 RunIIHERAPDF1.0Cone R=0.7  - fastNLO(+ non-perturbative corr.)PT jet [Gev/c]d2σ/dyjet dp T jet [nb/(Gev/c)]10-1510-1210-910-610-3110 310 610 80 100 200 300 400 500 600 700Fig. 1. – Left: HERAI combined NC cross-sections for e−p and e+p interactions. Right: jetproduction at D0 compared with theoretical predictions obtained using HERAPDF1.0.jet and heavy flavour production. The study of such processes allows on the one hand totest theoretical predictions based on fits from inclusive data only. On the other hand, asalso those measurements are reaching a very high precision, they can provide importantinput to the QCD fits. For this reason, the H1 and ZEUS Collaborations are combiningtheir results in order to achieve the best possible precision of the measurements. In thispaper a selection of the most recent H1 and ZEUS combined results is illustrated.2. – Inclusive measurementsInclusive NC and CC DIS cross-sections are measured at HERA to investigate theproton structure and to determine the proton PDFs. A combination of the H1 and ZEUSinclusive cross-sections based on the data collected between 1994 and 2000 (HERAIrunning) has been published [1].The combination of the data sets was done using a χ2 minimisation method [1]. Theχ2 function takes into account the correlated systematic uncertainties for cross-sectionmeasurements thus achieving a reduction of the systematic uncertainties in addition tothe statistical. The combined cross-sections are everywhere significantly more precisethan the individual measurements. The total uncertainty of the combined measurementsis typically smaller than 2% for 3 < Q2 < 500GeV2 and reaches 1% for 20 < Q2 <100GeV2, where Q2 is the negative four-momentum squared of the intermediate boson.In fig. 1 (left) the combined HERAI NC cross-sections are shown separately for thee−p and the e+p data samples, and compared to theoretical predictions based on theHERAPDF1.0 PDF set (described below).The HERAI combined H1 and ZEUS inclusive cross-sections were used as the sole in-put to extract the HERAPDF1.0 PDF set [1]. The data at low x determine the sea-quarkand the gluon distributions while the large-x data constrain the up and down valencequark distributions. Due to the accuracy of the input data, the HERAPDF1.0 PDFshave a precision of the order of a percent in the medium-x region, which corresponds72 M. TURCATOH1 and ZEUSHERA Structure Functions Working GroupJune 2010x = 0.02   (x300.0)x = 0.032   (x170.0)x = 0.05   (x90.0)x = 0.08   (x50.0)x = 0.13   (x20.0)x = 0.18   (x8.0)x = 0.25   (x2.4)x = 0.40   (x0.7)x = 0.65Q2/ GeV2σr,NC(x,Q2)HERA I+II NC e+p (prel.)HERA I+II NC e-p (prel.)HERAPDF1.0   e+pHERAPDF1.0   e-p±10-210-111010 210 310 2 10 3 10 4 10 5| < 0.3y|1.510.5 [GeV]Tp210 310| < 2.1y1.2 < |1.510.5| < 1.2y0.8 < |1.510.5| < 0.8y0.3 < |1.510.5Ratio wrt CTEQ 6.6 PreliminaryATLAS statistical errorData withuncertaintiesSystematic=7 TeVs-1 dt=37 pbL ∫    jets,  R=0.4tanti-kNon-pert. corr.×NLO pQCD CTEQ 6.6MSTW 2008NNPDF 2.1HERAPDF 1.5Fig. 2. – Left: HERAI+II combined NC cross-sections for e−p and e+p interactions. Right:ATLAS inclusive jet double-differential cross-sections as a function of the jet pT in differentcentral regions of rapidity |y|.to the rapidity plateau region for the LHC measurements. To test the goodness of thePDFs in a kinematic region not covered by the data used to extract them, theoreticalpredictions for jet production at the Tevatron were produced. The description of thedata obtained for, i.e. jet production at D0 [2] is good, as shown in fig. 1 (right).The statistical precision of the combined HERAI data is limited in the high-x andhigh-Q2 regions. The precision of the data in this region can be significantly improvedby adding to the combination the NC and CC cross-sections measured using the datacollected in the HERAII running period (2003–2007). This corresponds to a three-foldincrease in the overall statistics and roughly a 10-fold increase in the e−p data sample.The combination of the HERAI and a partial [3, 4] HERAII data sample has beenrecently released as preliminary result [5]. The only missing data are the ZEUS NC e+pcross-sections for HERAII [6] which were released after this combination was performed.The improved precision of the HERAI+II combined data is clearly visible when fig. 1(left) is compared to fig. 2 (left), where the HERAI and HERAI+II NC cross-sectionsare shown. The uncertainties are visibly reduced in the high-x and high-Q2 regions.The data are compared with theoretical predictions based on the HERAPDF1.0, whichdescribe the data well.The HERAI+II combined cross-sections have been used as the sole input to extractthe HERAPDF1.5 PDF set [7]. Predictions based on the HERAPDF1.5 are comparedto jet cross-sections measured at ATLAS [8] in fig. 2 (right). The predictions based onthe HERA data alone are able to describe well jet production at ATLAS.3. – Jet and heavy flavour productionAlthough very precise proton PDFs can be obtained by using only inclusive NC andCC cross-section measurements, the study of jet and heavy flavour production can im-prove the picture. In this section two recent results obtained using jet and charm datain the PDF fits are discussed.LATEST RESULTS FROM HERA AND THEIR IMPACT ON THE LHC 73)Z(MSα0.114 0.116 0.118 0.12 0.122 0.124 0.126   min2 χ - 2 χ05101520H1 and ZEUS (prel.)HERAPDF1.5fHERAPDF1.6HERAPDF Structure Function Working Group    March 2011  / GeV modelcm1.2 1.4 1.6 1.8 / nbZσ26272829303132 RT standard RT optimised ACOT-fullχ S-ACOT- ZMVFNS ) = 7 TeVsZ ((prel.)cc2HERAPDF1.0 + FHERA Inclusive Working Group         August 2010Fig. 3. – Left: Δχ2 distribution as a function of αs(MZ) for the fit to the inclusive (HERA-PDF1.5f) and inclusive+jet data samples (HERAPDF1.6). Right: Z production cross-sectionat the LHC for√s = 7TeV as a function of mmodelc .3.1. Jet production and αs. – The data sample used for the extraction of the HER-APDF1.5 PDF set was enlarged to include jet cross-section measurements [9], and thePDF set HERAPDF1.6 was extracted [10]. The use of jet cross-sections allows the si-multaneous determination in the fit of the PDFs and of the strong coupling constant, αs.This was not possible in the previous fits for the extraction of the HERAPDF1.0 and1.5, as the correlation between the gluon PDF and αs(MZ) is too strong when inclusivedata alone are used.A new parameterisation of the PDFs was introduced in order to allow a greaterflexibility in the gluon density. This helps to avoid a parameterisation bias when moredata which are sensitive to the gluon distribution, like jet production, are included.The fit to the NC and CC data included in HERAPDF1.5, but using the new PDFparameterisation, is referred to as HERAPDF1.5f.The αs dependence of the fits to the inclusive and the inclusive+jet data is wellillustrated in fig. 3 (left), where the χ2 of the HERAPDF1.5f and HERAPDF1.6 withfree αs is shown as a function of αs(MZ). The HERAPDF1.5f fit shows a shallowminimum, while the HERAPDF1.6 fit with the addition of the jet data provides a strongconstraint on αs.3.2. Charm production. – The combined H1 and ZEUS measurement of the charmcontribution to the proton structure function F2, F cc¯2 [11], was used to investigate thePDF fit formalism, in particular the role of the charm mass parameter in the differentmodels [12] used by different groups of fitters. In each model, the onset of the heavyquarks is controlled by the parameters mmodelc,b . The charm data are sensitive to the valueof mmodelc and to the scheme used for the treatment of heavy quarks in the PDF fits.For each of the different heavy-flavour schemes, a fit to the combined F cc¯2 data togetherwith the published combined HERAI data was performed. In each implementation anoptimal value of mmodelc was determined as that corresponding to the best description ofthe data. The obtained optimal values for mmodelc show a sizable spread, from 1.26 to1.68GeV. However, the description of the data is satisfactory in each of the schemes aslong as the optimal charm mass parameter is used.74 M. TURCATOPDFs obtained from fits with different mmodelc were used to predict W and Z produc-tion cross-sections at the LHC. A sizable spread in the predictions was observed for eachmodel when mmodelc was varied between 1.2 and 1.8GeV, as well as when consideringdifferent schemes at a fixed value of mmodelc . This spread is significantly reduced whenthe optimal value of mmodelc is used in each model, as shown in fig. 3 (right).The inclusion of the F cc¯2 measurements therefore helps to reduce in a model-independent way the uncertainties on the W and Z production cross-sections at theLHC due to the heavy flavour treatment in the PDF fit.4. – ConclusionsThe ZEUS and H1 Collaborations are combining their data in order to achieve thebest possible precision in the measurement of inclusive neutral- and charged-currentDIS cross-sections, jets and heavy flavour processes. Precise sets of parton distributionfunctions have already been published using the data collected in the HERAI data-takingperiod. Now a better precision is provided by including the latest HERA data and byimproving the fit also with the help of jets and heavy flavour measurements. A preciseinput for the determination of the proton PDFs for cross-section predictions at the LHCis provided.∗ ∗ ∗The author would like to thank the organisers for the invitation and for having madeher participation to the conference possible.REFERENCES[1] Aaron F. D. et al. (H1 and ZEUS Collaborations), JHEP, 1001 (2010) 109.[2] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. Lett., 101 (2008) 062001.[3] Aaron F. D. et al. (H1 Collaboration), H1prelim-09-042; H1prelim-09-043.[4] Chekanov S. et al. (ZEUS Collaboration), Eur. Phys. J. C, 62 (2009) 625; 61 (2009)223; Abramowicz H. et al. (ZEUS Collaboration), Eur. Phys. J. C, 70 (2010) 945.[5] Aaron F. D. et al. (H1 and ZEUS Collaborations), H1prelim-10-141, ZEUS-prel-10-017.[6] Abramowicz H. et al. (ZEUS Collaboration), ZEUS-prel-11-003.[7] Aaron F. D. et al. (H1 and ZEUS Collaborations), H1prelim-10-142, ZEUS-prel-10-018.[8] ATLAS Collaboration, ATLAS-CONF-2001-047.[9] Aaron F. D. et al. (H1 Collaboration), Eur. Phys. J. C, 65 (2010) 363; 67 (2010) 1;Chekanov S. et al. (ZEUS Collaboration), Phys. Lett. B, 547 (2002) 164; Nucl. Phys.B, 765 (2007) 1.[10] Aaron F. D. et al. (H1 and ZEUS Collaborations), H1prelim-11-034, ZEUS-prel-11-001.[11] Aaron F. D. et al. (H1 and ZEUS Collaborations), H1prelim-09-171, ZEUS-prel-09-015.[12] Tung W. K. et al., JHEP, 02 (2007) 053; Kramer M., Olness F. I. and Soper D. E.,Phys. Rev. D, 62 (2000) 096007; Thorne R. S. and Roberts R. G., Phys. Rev. D, 57(1998) 6871; Ball R. D. et al., arXiv 1002:4407.

Latest results from HERA and their impact on the LHC

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Latest results from HERA and their impact on the LHC

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