Study of Inclusive Jets Production in ep Interactions at HERA

Transcription

1 HEP 003 Europhysics Conference in Aachen, Germany Study of Inclusive Jets Production in ep Interactions at HERA Mónica Luisa Vázquez Acosta Universidad Autónoma de Madrid On behalf of the ZEUS & H1 Collaborations Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003-1

2 Jet physics at HERA The HERA collider provides a unique laboratory for the study of the hadronic final state Jet data are very precise at high transverse energy where the experimental uncertainties and non-perturbative effects are small. This allows Precision tests of perturbative QCD Constrains on proton and photon parton distribution functions Study the size of the theoretical uncertainties for further improvements Extraction of QCD parameters where the uncertainties are small Study of subprocesses proportional to α s or higher powers Explore the low Q transition region where non-perturbative effects become important Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003 -

3 HERA Kinematics Negative squared 4-momentum transfer Q q = (k k jorken scaling variable Q x p q Deep Inelastic Scattering: Neutral Current: e + p e + Q >> 1 GeV X ( γ,z 0 Inelasticity p q y p k Charged Current: e + p ν e X (W ± Total hadronic centre-of-mass energy Photoproduction: Q ~ 0 GeV W (q + p = ys Q Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003-3

4 Inclusive Jet Photoproduction at HERA In photoproduction the photon has low virtuality, at HERA Q ~ 10-3 GeV the high transverse energy s provide a hard scale High E T production is described up to O (α α S in the SM by Direct γ is point-like Resolved γ is hadron-like Inclusive production does not restrict the phase space of the second reduced theoretical uncertainties in the NLO QCD predictions the information of the event kinematics is reduced Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003-4

5 Inclusive Jet Cross Sections in Photoproduction ZEUS Collaboration - hep-ex/01064 ex/01064 Cross sections for s with 17 < 14 < E T W < 95GeV, 1 < η γp < 93 GeV, Q <.5 1 GeV Good agreement of NLO QCD calculation with the measured cross section over 5 orders of magnitude Small theoretical uncertainties uncertainty due to missing higher orders in the perturbative series ~10% photon and proton PDFs ~ 5% Theoretical calculation from Klasen, Kleinwort and Kramer - Eur. Phys. J. Direct C (1998 Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003-5

6 Measurement of α s in Photoproduction ZEUS Collaboration - hep-ex/01064 ex/01064 α s (M Z values determined from a QCD fit to dσ/de T in different E T regions Small experimental uncertainties Theoretical error dominates Running of α s in a single measurement Consistent with recent determination of ethke A χ fit to all the E t regions gives a value of α S (M Z of: ethke: hep-ex/01101 α s (M Z = 0.14 ± (stat (exp (th. Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003-6

7 Scaling Violations in Photoproduction ZEUS Collaboration - hep-ex/01064 ex/01064 Scaling hypothesis: the scaled invariant cross section ( E T 4 E dp x 3 d σ dp dp averaged over < η < γ p 0 as a function of should be independent of W γp y z η x T ET W 169 < W < γ p 191 GeV 40 < W < γ p 70 GeV Effectively equivalent to running with two different beam energies NLO QCD calculations are consistent with the data Theoretical uncertainties reduced Scaling violation due to: parton evolution and running of α s First observation of scaling violations in photoproduction Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003-7

8 Inclusive Jet Cross Sections in PHP H1 Collaboration - hep-ex/ ex/ W γ p Agreement with NLO QCD very good All predictions obtained using different proton (MRST99,CTEQ5 and photon (GRV,AFG PDFs agree with the data E T Theoretical calculation from Frixione, Ridolfi - Nucl. Phys. 507 ( Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003-8

9 Comparison of γp p with pp H1 Collaboration - hep-ex/ ex/ Scaled cross section: independent of energy up to scaling violations S(x T 3 ET π de d T σ dη ; x T E T W γp x T < 0. shape similar for γp and pp resolved photon ~ hadron x T > 0. γp harder than pp spectrum enhanced quark density in the resolved photon w.r.t. a hadron dominance of direct point-like photon Confirmation of the dual nature of the photon Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003-9

10 Jet production in DIS Jets in Deep Inelastic Scattering e + p e + ( ν e + (s + X is described in the SM O (α S by reit Frame Jets with high E T in the reit frame: orn Supress orn contribution (the current quark has no E T Lowest order contributions QCD-Compton and boson-gluon fusion Directly sensitive to QCD subprocesses at O (α S QCD-Compton Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19,

11 Inclusive s in NC DIS at low Q H1 Collaboration - hep-ex/00609 ex/00609 ackward Central Forward Low Q region (5 < Q < 100 GeV At large virtualities Q 15 GeV data agrees well with NLO QCD predictions At low virtualities discrepancies are seen and are studied in more detail Good agreement of NLO QCD with data in backward and central regions Large NLO corrections for low E T and in the forward region: NLO/LO~5 NLO predictions below data for E T < 0 GeV in the forward region Discrepancy Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19,

12 Inclusive s in NC DIS: forward region H1 Collaboration - hep-ex/00609 ex/00609 Forward region: 1.5 < η lab <.8 NLO predictions up to 50% lower than data where NLO corrections are largest Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19, 003-1

13 Azimuthal asymmetry of s in NC DIS ZEUS Collaboration - hep-ex/ ex/ No cuts in the laboratory frame E > 8GeV, < η < 1.8 T, Q > 15 GeV, 0.7 < cos γ < 0.5 Distribution of the azimuthal angle in the reit frame between the lepton scattering plane and the plane defined by the and the incoming proton direction LO pqcd predicts for NC DIS at Q << M Z dσ dφ = A + cos ( φ + C cos( φ In γ * q qg the outgoing g(q preferentially appears at φ=0(π In γ*g qq the φ dependence is dominated by the cosφ term dσ For an inclusive measurement: = A + C cos( φ dφ Small experimental and theoretical uncertainties NLO QCD calculations describe the measurements very well Normalised cross section Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19,

14 Azimuthal asymmetry of s in NC DIS ZEUS Collaboration - hep-ex/ ex/ To quantify the asymmetry and the dependence with Q a fit was performed to the data and the QCD calculation using the functional form 1 dσ σ d φ = 1 π [ 1 + f cos( φ + f cos( φ ] 1 Normalised inclusive cross section folded about π The asymmetry is predicted to decrease as Q increases due to the progressive decline of the γ*g qq process NLO QCD describes the data well LO QCD predicts a larger asymmetry at low Q f 1 is consistent with 0 f decreases with Q Precise test of pqcd prediction for the azimuthal asymmetry Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19,

15 Inclusive s in Charged Current DIS ZEUS Collaboration - hep-ex/ ex/ e + p ν e + + Test flavour changing electroweak theory and QCD in one type of events Analysis peformed in the laboratory frame X 1. NLO QCD calculations (MEPJET. Matrix elements+parton showers (ARIADNE MC have been compared to the data oth the NLO QCD and MC calculations describe well the data Mirkes and Zeppenfeld, Phys. Lett. 380 ( Lönnblad, Z. Phys. C 65 ( Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19,

16 Jet Substructure of inclusive s in CC DIS ZEUS Collaboration - hep-ex/ ex/ Mean sub multiplicity NC CC The measurement is sensitive to the value of α S (M Z A χ fit to the region E t > 5 GeV where the parton-to-hadron corrections are <10% gives a value of α S (M Z of: α S (M Z = 0.10 ± (stat (syst (th. QCD does not distinguish if W (CC or γ (NC is exchanged Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19,

17 Summary and Outlook HERA is now producing a wealth of precision data at high E T in NC and CC deep inelastic scattering, photoproduction and the transition region. Many extractions of the QCD coupling constant α S with a precision competitive with the world average. The running behaviour of α S is clearly seen. At high E T in inclusive cross sections, theoretical uncertainties are small and the theoretical predictions are able to reproduce cross sections over many orders of magnitude. At lower Q theoretical uncertainties are dominant theoretical developments are needed. Experimental precision and coverage of data is now very good time to include the HERA data in global PDF fits. Inclusive Jets in ep Interactions at HERA, Mónica Vázquez Acosta (UAM HEP 003 Europhysics Conference in Aachen, July 19,