PoS(2008LHC)045. Tevatron QCD physics. Robert Hirosky. University of Virginia

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1 evatron QCD phsics Robert Hirosk Universit of Virginia he Run evatron program at Fermilab is producing a wide range of QCD results, confronting new corners of the Standard Model with precision data, exploring the limits of perturbative theories and non-perturbative models, and performing direct searches for evidence of new phsics. his talk summarizes recent results presented b the CDF and DØ Collaborations. PoS(8LHC)5 8 Phsics at LHC September 9 - October 8 Split, Croatia Speaker. For the CDF and DØ Collaborations. c Copright owned b the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.

2 evatron QCD phsics Robert Hirosk. Overview his talk summarizes recent QCD phsics results from the CDF and DØ experiments at Fermilab, corresponding to integrated luminosities of up to f b for pp collisions at s =.96 ev. he evatron QCD measurements confront Standard Model processes with unprecidented precision and are providing some of the most stringent limits on possible new phsics scenarios.. High p jets he evatron data provide unique and increasingl precise measures of the strong interaction at large values of Björken-x and momentum exchange (Q ) in parton-parton interactions. Both CDF and DØ have measured the differential cross sections for inclusive jet production as a function of jet transverse momentum (P ) in various ranges of jet rapidit () [],[]. he DØ measurement is shown in Fig.. he smooth curves are predictions from NLO perturbative QCD, using the CEQ6.5M parton distributions []. Also included are non-perturbative corrections for contributions due to the underling event and hadronization effects calculated using PYHIA [] and tune QW [5] for the underling event model. Figure shows the ratios of the DØ data to theor in the various rapidit regions. he shaded bands show sstematic uncertainties on the data. he uncertaint in the theor, determined b varing the factorization and renormalization scales (p / <= µ R, µ F <= p ) is approximatel 5%. Of particular note in this figure is that the experimental uncertainties in the measurement are substantiall less than those from the input PDFs, indicating that the evatron data will offer significant constraints on future PDF models. hese data are included in a recent analsis [6] b the MSW group and are leading to a significant reduction of their high-x gluon distribution in the global analsis. he value of the evatron data for constraining PDF models remains substantial well into the LHC era. While a relativel larger percentage of high-x events are gluon initiated at the LHC, the spectrum of parton momentum fractions is significantl softened at LHC energies. If one compares jet spectra using the invariant measure x = p / s, the evatron cross sections are approximatel times higher for all x and times higher for x >.5. Effectivel the LHC would need to acquire an integrated luminosit of around 6 f b to match the same partonic integrated luminosit at high x as the evatron will reach with 8 f b. DØ has presented the first measurement of dijet angular distributions for dijet masses exceeding ev/c. Dijet angular distributions are measured in terms of the variable χ di jet = exp( ). At leading order in dijet production this is related to the partonic center of mass scattering angle via as: χ di jet = +cosθ cosθ. Rutherford scattering produces a flat χ distribution, Standard Model QCD is relativel flat, while man new phsics signatures produce signals more strongl peaked at low values of χ. Figure shows normalized differential χ di jet distributions ( dσ σ dχ di jet ) measured b DØ in various ranges of the invariant mass of the dijet sstem. Predictions for Standard Model QCD processes are shown in the solid line. Also shown are expectations for various new phsics scenarios. Observing good agreement between Standard Model expectations and the data, limits are set in several new phsics scenarios. hese are summarized in able. A CDF dijet analsis compares the measured dijet mass spectrum to a smooth ansatz function (see Fig. ). In this analsis a bump hunt is performed to search for indications of resonant structures in the dijet PoS(8LHC)5

3 evatron QCD phsics Robert Hirosk d (pb/gev) σ/dp d 7 DØ Run II <. (x) s =.96 ev L =.7 fb R cone =.7 NLO pqcd +non-perturbative corrections CEQ6.5M µ = µ = p R F.< <.8 (x6).8< <. (x8).< <.6 (x).6< <. (x).< <. 5 6 p 6 (GeV) Figure : DØ inclusive jet cross section versus P jet measured in bins of η jet []. he solid lines represent NLO PQCD predictions with nonperturbative corrections. data / theor DØ Run II.5 L =.7 fb..5 <..5. R cone =.7 NLO scale uncertaint.5. < <.6. 5 NLO pqcd µ = µ = p R F +non-perturbative corrections. < <.8 CEQ6.5M with uncertainties MRS.6 < <. 5 Data Sstematic uncertaint.8 < <.. < <. 5 p (GeV) Figure : Data divided b theor for the DØ inclusive jet cross section measurements []. Sstematic uncertainties on the data are shown b shaded bands. NLO PQCD scale uncertainties are shown at the bottom of each panel. Solid(dashed) lines represent the cenrtal prediction(error band) using the CEQ6.5 PDF set. he predictions using MRS PDFs is shown b the dotted lines. mass spectrum. No significant resonances are observed and this analsis proceeds to determine exclusion regions for various new phsics scenarios. A sampling of these is given in able. composite ev A.D.D. excited color octet axi-gluon quarks extra large quark techni-rho & coloron [7] dims. extra [] [] [] [8] dims. [9] units of ev units of GeV/c CDF Exclusions 8-8 () 6 () 65 () DØ Limits Λ M S M c.58 (). ().56 () PoS(8LHC)5 able : Examples of excluded regions and limits on new phsics processes from the CDF and DØ dijet data. () Most stringent limits. () Best limits from a hadron collider. () Best limit from a single proces at a hadron collider.. Photons (+jets) he measurement of photon production is of particular interest in understanding QCD processes. As opposed to partons that carr color, a photon produced via parton interactions can reach a detector unaltered b subsequent interactions, thus serving as a direct probe of the hard scattering process. Furthermore, with a sufficient theoretical framework, photons can be used as a direct probe of the momentum fractions carried b gluons in p p scattering. In these analsis a combination of calorimeter and track-based isolation requirements are tpicall applied to reduce backgrounds from photons produced via jet fragmentation. Both CDF and DØ have performed a variet of photon analses in Run. Figure 5 shows the preliminar CDF measure of the differential cross section for inclusive photon production as a function of photon p. Both DØ and CDF measurements of this process are found to be in agreement for < p < GeV/c, however

4 evatron QCD phsics Robert Hirosk Figure : DØ χ di jet distributions measured in ranges of dijet invariant mass. he solid line shows the expectation from Standard Model QCD, also shown are expectations for several new phsics scenarios: quark compositeness with Λ=.5 ev (dashed); ADD Large Extra Dimensions, M s =.55 ev (dotted); and ev Extra Dimensions, M c =. ev (dot-dash). (Data - Fit) / Fit (Data - Fit) / Error )] dσ / dmjj [pb/(gev/c CDF Run II Preliminar,. fb χ / ndf 6.6 / 7 Prob M [GeV/c ] jj M [GeV/c jj ] 6 8 M [GeV/c jj ] Figure : CDF dijet mass spectrum (top). Errors are statistical onl. he spectrum is well represented b a smooth ansatz function (solid line). Comparing the fit to the data, no significant indication of resonant structure is observed (middle, bottom). PoS(8LHC)5 data and theor show shape differences approaching the lower p range of the measurements. In order to better isolate kinematical regions that ma contribute to a greater extent to the observed discrepancies with theor, DØ has measured photon+jet production in regions subdivided b the locations of the leading photon and leading jet. Centrall produced photons are selected requiring <. and event classes are defined with respect to the leading jet: central ( jet <.8 ), forward (.5< jet <.5 ) and same side ( jet > ) versus opposite side ( jet < ) pairs. Figure 6 shows the DØ measurements compared to NLO PQCD calculations. Photon plus heav flavor is an important signature in various extensions to the Standard Model, examples include echnicolor, SUSY, th generation fermions, and an excited b-quark. Photons plus b-flavor jets also serve as a probe of the b-content of the proton. With increasing data the evatron experiments are gaining access to observe photon plus heav flavor processes. CDF has performed two measurements: a b-tagging analsis using their inclusive photon data set shown in Fig. 7 (top) and an analsis using a speciall triggered data set using their silicon vertex detector to select events with a displaced vertex as well as a photon candidate, see Fig. 7 (bottom). Both analses are in reasonable agreement with leading order predictions modeled b PYHIA. DØ has a measurement of the the triple differential cross sections d σ/(d p d jet ) for both + c and +b as a function p. he results, shown in Fig. 8, are in agreement with NLO PQCD predictions

5 evatron QCD phsics Robert Hirosk Figure 5: CDF measurement of inclusive photon production versus p, for η <.. Data(theor) values are shown b the solid(open) smbols. Figure 6: Data/theor ratios for the DØ photon+jet measurements. Cross section ratios for photon+jets production are plotted versus p and further subdivided into kinematical regions based on the locations of the photon and leading jet. over the full range of + b data. In the case of + c agreement is onl observed for p < 5 GeV with an increasing divergence from predictions at higher p. his difference is somewhat reduced using a PDF model including intrinsic charm [].. W/Z +jets W/Z plus jet events are analogous to plus jets in that the daughter particles from the vector boson decas ield a direct probe of the underling dnamics of the parton scattering. hese final states are also important signatures, as well as backgrounds, to a variet of Standard Model and new phsics processes. Results from W/Z plus jet measurements at the evatron are covered in detail in S. Lammer s contributions to these proceedings. Here we discuss a single measurement of the p spectrum of Z-bosons at DØ. High p Z production is dominated b the associated emission of one or more hard partons. his process is well modeled in PQCD. For p Z around GeV/c and lower at the evatron, the Z is primaril balanced b soft gluon emission. his process can be described b using gluon re-summation techniques. DØ has performed a new measurement of the p Z to extract the main coefficient in the BLNY parametrization [] as used in the RESBOS monte carlo. An earlier measurement [5] of the p Z spectrum was restricted b detector resolution, limiting the accurac with which the observed spectrum could be used to extract model parameters. An updated analsis uses a new technique based on event quantities, primaril sensitive to angular resolutions of the detector, reducing overall sstematic uncertainties in the analsis. he new analsis is performed for Z s decaing into either electron or muons pairs. Rather than measure p Z directl, it is instead decomposed into orthogonal components according to the event axis, ˆt = p p / p p, where p, represent the individual lepton momenta. Z p = p + p is then decomposed into components transverse to the axis, a = p Z ˆt, and aligned with the axis, a L = p Z ˆt. At low p Z the uncertaint on a scales as the uncertaint on the individual lepton p s multiplied b PoS(8LHC)5 5

6 evatron QCD phsics Robert Hirosk (pb/gev) + b jet) /E σ ( R(j-)>.7, η() <., η(j) <.5, E 6 (j)> GeV CDF Run II preliminar Data (L = 8 pb ) Pthia MC 6 8 Photon E (GeV) Figure 7: CDF measurements of dσ(+b-jet) +b-jet production. op: de measured in inclusive +jet sample with application of b tagging. Bottom: he same measured using a speciall triggered data sample requiring identification of a displaced vertex. ) (pb/gev) jet d d (dp σ d DØ Run II Preliminar L int =. fb (x.) CEQ 6.6M, µ =p R,F,f NLO pqcd b jet > b jet <.8 <. b jet p > 5 GeV 6 8 p (GeV) b jet + b jet < ) (pb/gev) jet d d (dp σ d - - DØ Run II Preliminar L int =. fb (x.) CEQ 6.6M, µ = p R,F,f NLO pqcd c jet <.8 <. c jet p > 5 GeV 6 8 p (GeV) c jet c jet + c jet Figure 8: DØ measurements of the differential cross sections for +b-jet (left) and +c-jet (right) production, > < d σ d p d d jet. Measurements (smbols) are shown in two kinematic regions, jet > and jet <. Error bars include all uncertainties. he NLO theoretical predictions calculated using the CEQ6.6M PDFs [6] are shown b the dotted lines. the sine of a small angle. he technique is shown schematicall in Fig. 9. Using this method DØ extracts the single best measure of the value g in the BLNY parametrization: g =.6 ±.(exp) ±.(PDF) [Preliminar]. PoS(8LHC)5 5. Diffractive phsics and the underling event As part of a program of diffractive phsics measurements, CDF has performed a variet of analses requiring a forward proton tag in downstream Roman-pot detectors. hese include a recent measure of events with a W or Z boson produced via a diffracive process. Ultimatel such measurements can lead to an improved understanding of the diffractive structure function. he ratios of diffractive to non-diffractive W/Z production in the region of fractional momentum loss of the scattered proton. < ξ <. and -momentum transfer squared t < are found to be R W = [.97 ±.5(stat) ±.(sst)]% and R Z = [.85 ±.(stat) ±.(sst)]%. Results from the W analsis are shown in Fig.. CDF has presented studies of the event topolog in Drell-Yan lepton-pair production in protonantiproton collisions at.96 ev. he direction of the lepton-pair is used to define three regions of η φ space on a event b event basis; toward, awa and transverse to the vector sum of the lepton-pair. he transverse region is ver sensitive to the underling event. he data are corrected to the particle level and are then compared with the PYHIA une AW. he properties of the underling event are examined as a function of the transverse momentum of the leptonpair and the data are also compared with previous measurements of the underling event in high 6

7 evatron QCD phsics Robert Hirosk Entries x (Data-MC)/MC DØ Preliminar fb χ = / Data Z µµ (GeV) a Figure 9: op: Variables used in DØ P Z analusis. Bottom: he a distribution for di-muon events. he solid line shows the prediction from RESBOS with a tuned value of the g parameter. transverse momentum jet production. he goal is to improve understanding and modeling of high energ collider events. he method studies charged particle p densities in the above regions. In the awa region, p densit increases with the p of the lepton pair, whereas the p densit appears flat with respect to leption pair p in the transverse or awa regions. Results are summarized in Fig. Number of Charged Particles Densit Figure : CDF Measurement of Diffractive W. Left: M W calculated using the Roman-pot track to reconstruct the kinematics of the neutrino. Right: he reconstructed ξ distribution in the calorimeter for all events with a Roman-pot track, those consistent with a single interaction and missing energ due to a neutrino, and those with reconstructed W mass in the range of 5 GeV/c. All hree Regions Charged Particle Densit: dn/dηdφ CDF Run Preliminar p >.5 GeV/c and η < ransverse: PYHIA une AW ransverse: Data oward:pyhia une AW oward: Data Awa: PYHIA une AW Awa: Data 7 < M ll < GeV/c L~.7 fb ransverse Momentum of Lepton Pair (GeV/c) PoS(8LHC)5 6. Conclusion QCD analses are a broad component of the CDF and DØ phsics programs. As such, the are providing fundamental insights into light/heav flavor and gluon PDFs. he are making increasingl sensetive statements about the interpla of perturbative and nonperturbative models and the limitations of possible new phsics processes. Precise Figure : CDF measurement of the underling event in Drell-Yan lepton-pair production. Charged particle multiplicit densities measured in three regions for electron and muon combined data (using tracks with p >.5 GeV/c and η < ). Lines represent PYHIA une AW predictions. he data are corrected back to particle level (errors include both statistical and sstematic uncertaint). measures of benchmark processes at the evatron will also speed validation of an new phsics signatures at the LHC. he full evatron data set will be -8 times higher than most results in this talk. hese data will dominate our models of the high-x gluon for some time and further improve our understading of rarer Standard Model processes well into the LHC era. 7

8 evatron QCD phsics Robert Hirosk References [] CDF Collaboration, Measurement of the inclusive jet cross section at the Fermilab evatron p anti-p collider using a cone-based jet algorithm, Phs. Rev. D 78, 56 (8), [hep-ex/5]. [] DØ Collaboration, Measurement of the Inclusive Jet Cross Section in p p Collisions at s =.96eV, PRL, 6 (8), [arxiv.org:8.]. [] W.K. ung et al., JHEP 7, 5 (7); J. Pumplin et al., JHEP 7, (); D. Stump et al., JHEP, 6 (). []. Sjöstrand et al., Comp. Phs. Comm. 5, 8 (). [5] R. Field in: M.G. Albrow et al. [evlhc QCDWorking Group], arxiv:hep-ph/6. [6] G. Watt, A. D. Martin, W. J. Stirling, R. S. horne, arxiv: [7] E. Eichten, I. Hinchliffe, K. D. Lane and C. Quigg, Super Collider Phsics, Rev. Mod. Phs. 56, 579 (98) [Addendum-ibid. 58, 65 (986)]; P. Chiappetta and M. Perrottet, Possible bounds on compositeness from inclusive one jet production in large hadron colliders, Phs. Lett. B 5, 89 (99); K. D. Lane, Electroweak and flavor dnamics at hadron colliders, arxiv:hep-ph/ [8] K. R. Dienes, E. Dudas and. Gherghetta, Grand unification at intermediate mass scales through extra dimensions, Nucl. Phs. B 57, 7 (999); A. Pomarol and M. Quiros, he standard model from extra dimensions, Phs. Lett. B 8, 55 (998); K. Cheung and G. Landsberg, Kaluza-Klein states of the standard model gauge bosons: Constraints from high energ experiments, Phs. Rev. D 65, 76 (). [9] N. Arkani-Hamed, S. Dimopoulos and G. R. Dvali, he hierarch problem and new dimensions at a millimeter, Phs. Lett. B 9, 6 (998); D. Atwood, S. Bar-Shalom and A. Soni, Dijet production at hadron colliders in theories with large extra dimensions, Phs. Rev. D 6, 568 (). [] U. Baur, I. Hinchliffe and D. Zeppenfeld, Int. J. Mod. Phs. A, 85 (987); U. Baur, M. Spira and P. M. Zerwas, Phs. Rev. D, 85 (99). [] K. D. Lane and M. V. Ramana, Phs. Rev. D, 678 (99); E. Eichten and K. D. Lane, Phs. Lett. B 7, 9 (99); K. Lane and S. Mrenna, Phs. Rev. D 67, 5 (). [] J. L. Hewett and. G. Rizzo, Phs. Rept. 8, 9 (989). [] J. Pumplin et al., Phs. Rev. D 75, 59 (7). [] F. Landr et al., Phs. Rev. D 67, 76 (). [5] DØ Collaboration, Phs. Rev. L., (8). [6] P. Nadolsk et al., Implications of CEQ global analsis for collider observables, Phs. Rev. D78, (8). PoS(8LHC)5 8