Quark-quark and quark-lepton searches for compositenss at the Fermilab
Tevatron are summarized. These are of the contact-interaction variety where
sqrt(s-hat) < the hypothesized mass scales, Lambda. Tevatron experiments limit
a variety of compositeness phenomena in the range from 1.9 to 8.3 TeV. These
limits result from measurements of: the Ht spectrum (D0), dijet mass (D0),
dijet angular distribution (CDF, D0), drell-yan production (CDF, D0), and the
Neutral Current to Charged Current ratio (CCFR/NuTeV).Comment: Presented at VIIIth RENCONTRES DE BLOIS, with recent updates added. 7
  Figure

Green, J. Andrew

English

arXiv

The existence of three families of quarks and leptons suggests the possibility of a substructure for these objects. The hypothetical constituents known generically as preons, interact via a new strong interaction called Metacolor. The characteristic energy scale, {Lambda}, for the new interactions is, of course, unknown. The strength of the interactions through a contact term can be written as {cflx s}/({alpha}{sub S}{Lambda}{sup 2}), where {cflx s} is the square of the energy in the center of mass frame of the (normal) interacting partons, and {alpha}{sub S} is the QCD coupling. The first limit on the size of the atomic nucleus was obtained by Geiger and Mardsen in the Rutherford scattering of {alpha} particles from nuclei. In an analogous way, the authors can set a limit on the size of quarks and leptons by observing the scattering of the highest energy quarks and antiquarks at the Fermilab Tevatron at {bar p}p center-of-mass energy of 1.8 TeV for collider experiments, and proton beam energy of 0.8 TeV for fixed-target experiments. The collider detectors at Fermilab, CDF and D0, have performed searches for compositeness, and this paper gives a summary of those searches. Those detectors are general-purpose, have nearly 4{pi} acceptance, and measure lepton and jet energies to high precision. In addition, the neutrino detector, CCFR, which utilized the 800 GeV proton line at Fermilab has performed a compositeness search

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F Fermi National Accelerator LaboratoryFERMILAB-Conf-00/088-ED0Searches for Compositeness at the TevatronJ. Andrew GreenIowa State UniversityAmes, Iowa 50010Fermi National Accelerator LaboratoryP.O. Box 500, Batavia, Illinois 60510May 2000Presented at and to appear in the Published Proceedings of Rencontres de Blois,Blois, France, June 27-July 3, 1999Operated by Universities Research Association Inc. under Contract No. DE-AC02-76CH03000 with the United States Department of EnergyDisclaimerThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any oftheir employees, makes any warranty, expressed or implied, or assumes any legal liability orresponsibility for the accuracy, completeness, or usefulness of any information, apparatus,product, or process disclosed, or represents that its use would not infringe privately ownedrights. Reference herein to any specic commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the United States Government or any agencythereof. The views and opinions of authors expressed herein do not necessarily state or reectthose of the United States Government or any agency thereof.DistributionApproved for public release; further dissemination unlimited.Copyright NoticationThis manuscript has been authored by Universities Research Association, Inc. under con-tract No. DE-AC02-76CH03000 with the U.S. Department of Energy. The United StatesGovernment and the publisher, by accepting the article for publication, acknowledges thatthe United States Government retains a nonexclusive, paid-up, irrevocable, worldwide licenseto publish or reproduce the published form of this manuscript, or allow others to do so, forUnited States Government Purposes.Searches for Compositeness at theTevatronJ. Andrew GreenIowa State UniversityAmes, Iowa 50010, USAagreen@fnal.govon behalf of Fermilab.Presented at Rencontres de Blois June 1, 1999, with morerecent searches now included.A. IntroductionThe existence of three families of quarks and leptonssuggests the possibility of a substructure for these objects[1]. The hypothetical constituents known generically aspreons, interact via a new strong interaction called Meta-color. The characteristic energy scale, , for the newinteractions is, of course, unknown. The strength of theinteractions through a contact term can be written asŝ=(S2), where ŝ is the square of the energy in the cen-ter of mass frame of the (normal) interacting partons,and S is the QCD coupling.The rst limit on the size of the atomic nucleus wasobtained by Geiger and Mardsen in the Rutherford scat-tering of  particles from nuclei. In an analogous way,we can set a limit on the size of quarks and leptons byobserving the scattering of the highest energy quarks andantiquarks at the Fermilab Tevatron at pp center-of-massenergy of 1.8 TeV for collider experiments, and protonbeam energy of 0.8 TeV for xed-target experiments. Thecollider detectors at Fermilab, CDF and D, have per-formed searches for compositeness, and this paper gives asummary of those searches. Those detectors are general-purpose, have nearly 4 acceptance, and measure lep-ton and jet energies to high precision. In addition, theneutrino detector, CCFR, which utlilized the 800 GeVproton line at Fermilab has performed a compositenesssearch.B. Contact Interactions: Indirect Searches forCompositenessSeveral previous searches for compositeness relied ondirect searches for excited quark states [2]. The searchesthat we discuss assume thatpŝ is small compared to thecharacteristic mass scale, . These are generically called\contact interaction" searches, and each interaction maybe regarded as a contact term in the Lagrangian, whichhas the formL =22(LL(fLfL)(fLfL)+LR(fLfL)(fRfR)+2RL(fRfR)(fLfL));(1)where fL;R are the left and right-handed chiral compo-nents of the the quark or lepton, and ij is the sign ofeach term, where ij = +1 corresponds to destructive,and ij =  1 to constructive interference [1].Only leading-order calculations are available for com-positeness. Hence, the following approximate relation isused to calculate the correction for higher-order contri-butions:( = X )LO( =1 )LO  ( =1 )NLOorNLL; (2)where  =1 , corresponds to the standard model.The next-to-leading order (NLO) or next-to-leadinglog (NLL) cross-sections are calculated using the eventgenerators jetrad [3] or the Monte Carlo written byHamburg et al [4,5], respectively. The presense of anycompositness is expected to contribute to an increase incross section at large jet and lepton transverse momenta.Discussed in this paper are the most up-to-date quark-quark and quark-lepton contact interaction searches. Ofcourse, excellent lepton-lepton searches have been accom-plished as well [6].C. Quark Compositenss1. Limits from Dijet Mass MeasurementsD has measured [7] the ratio of the dijet mass spec-trum for jets at pseudorapidities jjetj < 0:5 relative tojets at 0:5 < jjetj < 1:0. The resulting cancellation ofsystematic uncertainties results in an absolute systematicuncertainty of < 8% of the ratio.Predictions for quark compositeness are obtained usingthe LO pythia event generator [8]. The ratios of theseLO predictions with, and without compositeness, areused to scale the jetrad NLO predictions (see Eq. 2),and are compared to data in Fig.1. Limits on models withcolor singlet (octet), vector, or axial contact interactionswere set using an analytic LO calculation [9] rather thanpythia. The resulting limits are given in Table I. Modelsof quark compositeness with a contact interaction scale2.4 TeV for quark-quark interactions are excluded at the95% CL. The limits on the scale of  V8 can be convertedinto limits on a avor-universal coloron [10] resulting in1a 95% CL of MC=cot > 837GeV=c2, where MC is themass of the coloron and cot a parameter of the theory.The comparison is given in Fig. 2.0.50.7511.251.5200 400 600 800M (GeV/c2)Cross section ratioJETRAD: CTEQ3M, µ = 0.5ET    max,  sep=1.3Λ+ =1.5 TeVΛ+ =2.0 TeVΛ+ =2.5 TeVΛ+ =3.0 TeVDØ DataFIG. 1. The ratio of cross sections for jets with jjetj < 0:5to jets with 0:5 < jjetj < 1:0 for data (solid circles) andtheory (dierent ). The full error bars show the statisticaland systematic uncertainties added in quadrature, and thecrossbars show the size of the statistical uncertainties.TABLE I. 95% condence limits in TeVfor dierent compositeness scale for dier-ent models calculated using an analytic LOprediction [9] (see text).Model +  LL (left-left isoscalar) 2.7 2.4V (vector singlet) 3.2 3.1A (axial singlet) 3.2 3.1V8 (vector octet) 2.0 2.3A8 (axial octed) 2.1 2.12. Limits from The Dijet Angular DistributionAt small center of mass scattering angles, ?, thedijet angular distribution predicted by leading orderQCD is proportional to the Rutherford cross section:d̂ij=d cos ?  1=sin4(?=2). It is useful to measure theangular distribution in the variable , rather than cos?,where  = (1 + cos ?)=(1  cos ?) = ej1 2j. The vari-able  facilitates comparison of theory with data. Theangular distribution from composite quarks in quark-quark interactions is isotropic. Hence, the measure-ment of the dijet angular distribution is sensitive to thepresence of such new physics. The quantity measuredin the dijet angular analysis is 1=N(dN=d), which ismeasured in bins of dijet mass MJJ. Figure 3 depictsthe angular distribution as measured [11] by D forMJJ > 625GeV=c2.To remove the point to point correlated errors, thedistribution can be described by a single parameter,51015200 1 2 3Mc (TeV/c2)cot2 θMc/cotθ> 837 GeV/c2FIG. 2. Limits for the coloron parameter space: col-oron mass Mc vs. mixing parameter cot . The darkshaded region shows the 95% CL exclusion region from theD measurement of the ratio of cross sections in Fig. 1(Mc= cot  > 837GeV=c2). The lightly shaded region showsthe area excluded by the D dijet angular distribution(Mc= cot  > 759GeV=c2). The horizontally hatched regionat large cot  is not allowed in this version of the model.The diagonally hatched region is excluded by the value of (Mc= cot  > 450GeV=c2). The cross{hatched region at lowcot2  is excluded by the CDF search for new particles decay-ing to dijets.R=N( < X)=N(X <  < max), which is the ratio ofthe number of events with  < X to the number betweenX <  < max. The CDF [12] analysis usesX = 2:5, andD [11] uses X = 4. Figure 4 exhibits R measured byCDF as a function ofM for two dierent renormalizationscales, along with the predictions for dierent compos-iteness scales. The dijet angular distribution from QCDwas calculated using the jetrad event generator. Thepredictions for quark compositeness scale are obtainedusing a LO analytic calculation [1]. The ratio of theseLO predictions with compositeness, to the LO with nocompositeness, is used again to scale the jetrad NLOpredictions.Analysis of the CDF data exludes models with +LL <2.1 TeV and  LL < 2.2 TeV. In addition, D uses theirmeasurements to place limits on the production of col-orons, requiring Mc= cot  > 759GeV=c2 (see Fig. 2).3. Limits from Large HTD has also used the HT =PEjetsT variable to setlimits on quark compositeness based on [13]. By treat-ing the event as a whole, this analysis complements theother searches for compositeness. It studies the com-20.050.10.150.20 2.5 5 7.5 10DØ DataNLO Predictionµ=0.5ET    maxΛ+ = 2.0 TeVΛ+ = 1.8 TeVΛ+ = 1.6 TeV 1σSystematic Errorχ1/N dN/dχM (GeV/c2) > 635FIG. 3. D dijet angular distribution compared to the-ory for dierent quark compositeness scales.The errors on thepoints are statistical and the band represents the correlated 1 standard deviation systematic uncertainty.positeness of left-handed quarks in the left-left isoscalarcontact term of the Lagrangian given in [1]. The scaleparameter for this term is LL.The events are chosen to have HT > 500 GeV, wellabove the contribution expected from top quarks thatpeaks near 2mt  350 GeV. Jets with ET > 20 GeVand jjetj < 3:0 are included in the calculation of HT foreach event. In addition, the events are required to have atleast one jet with ET > 115 GeV. Other cuts are appliedto reduce the instrumental backgrounds, e.g. from mis-vertexing. The only important backgrounds consideredare from such instrumental sources.Figure 5 displays the fractional deviation between thedata and the Monte Carlo for the CTEQ4M PDF [14]with renormalization scale  = EmaxT =2. For LL valuesbetween 1:4 and 7:0 TeV, pythia [8] is used to simulatethe eects of quark compositeness to leading order. Thisleading order calculation is scaled using Eq. 2, wherejetrad is used to compute the NLO component. Theresults for composite quarks relative to expectations fromthe standard model are shown in Fig. 5 for LL = 1:7; 2:0and 2:5 TeV. The ratios are found to be independent ofthe pythia renormalization scale.As seen in Fig. 5, quark compositeness would producea rise in the cross section as a function of HT . Changesin renormalization scale aect the absolute cross section,but not the shape of the HT distribution. Cross sectionscalculated using CTEQ4M or MRST [15] PDFs dier innormalization but only slightly in shape.The measured HT distribution above 500 GeV is wellmodeled by the jetrad (NLO QCD) event generator.No evidence is found for compositeness in quarks, andthe data are used to set limits on compositeness. AsFIG. 4. CDF measurement of R as a function of dijetinvariant mass.seen in Table II, the limits are not greatly aected bychoice of PDF. The depencence of the 95% CL limit isalso studied as a function of renormalization scale, andshows little eect on the limits. The limits for  LLarenot given, but are nearly identical (slightly higher thanon +LL).TABLE II. The 95% CL lower limits on quark composite-ness scale in LL( TeV) for dierent s (CTEQ4A1-5) and dif-ferent gluon content (MRSTGU and MRSTGD). The renor-malization scale is EmaxT =2.PDF +LLPDF +LLPDF +LLCTEQ4A1 2.0 CTEQ4A2 2.0 CTEQ4M 1.9CTEQ4A4 1.9 CTEQ4A5 1.9MRSTGU 2.0 MRSTGD 2.1 MRST 2.0D. Quark-Lepton Compositeness1. Neutrino-Nucleon ScatteringThe CCFR experiment used the Fermilab 800GeVproton beam directed at a Beryllium-Oxide targetto produce a beam of neutrinos (86% , 12% , and2% e(e)) with a mean energy of 125 GeV. The 's areproduced from decays of secondaries downstream fromthe BeO target. The CCFR detector was a 3 3 18mneutrino sampling calorimeter with the ability to resolveminimum-ionizing tracks (mostly from muons) and deter-mine shower energy. The CCFR experiment ran from1984 to 1988 and  1  106  scatterings pass typicalanalysis cuts. This detector has evolved to be what is3FIG. 5. Comparison of the measured HT distributionwith jetrad (CTEQ4M and a renormalization scale of = EmaxT =2). The errors on the points are statistical, and theerror band represents the highly-correlated systematic uncer-tainty due to the jet energy scale. The superimposed curvescorrespond to expectations for three compositeness scales.now known as NuTeV, which used an experimental pro-gram of switching between  and  beams to drasticallyreduce systematic errors and improve measurements ofquantities such as sin2W , and MW [16].The primary goal of these detectors was to do high-statistics studies of charged and neutral current inter-actions via N scattering. Most of these studies relyon measuring the ratio of the Neutral Current (NC) toCharged Current (CC) interactions seen in the detector.The NC interactions produce a compact shower of energy,while the CC interactions produce a shower, with a long(> 2m) minimum-ionizing trail left by the  or . Animportant correction to the ratio comes from the e(e)ux component, where the CC interaction produces e'sthat, unlike 's, cannot as easily be distinguished fromthe hadronic shower in a full range of energies. Suchevents can fake the NC signature.The Rmeas = NC=CC ratio is used to measure thecoupling, , of the Z0 boson to quarks, where the errorin the comparison of Rmeas to theory is about 0.6%.With this measurement, CCFR has set limits on quark-neutrino contact interactions [17]. Table III shows theselimits.The NuTeV collaboration will soon provide compli-mentary and more precise information which can furtherconstrain these contact interactions [16].TABLE III. 95% CL lower limits on mass scales of newqq contact terms from CCFR.Interaction + (TeV)   (TeV)LL 4.7 5.1LR 4.2 4.4RL 1.3 1.8RR 3.9 5.2VV 8.0 8.3AA 3.7 5.92. Drell-Yan ProductionBoth CDF [18] and D [19] have placed limits on thecombined quark-electron compositeness [20] scale by ana-lyzing Drell-Yan dilepton production.CDF has measured the Drell-Yan cross section for bothelectrons and muons, satisfying jj < 1:0 for each lepton(see Fig. 6). The backgrounds to the signal from QCDjets mimicking electrons, fromW+W , + , and heavyquark production, are estimated using data, and subse-quently subtracted. The Drell-Yan production cross sec-tion is based on an NLL calculation [5], with and the datanormalized to the prediction in the mass range between50 and 150 GeV/c2. The data are then used to placelimits on the combined quark-electron compositeness (seeTable IV).FIG. 6. The Drell-Yan cross section as measured by CDFfor muons (crosses) and electrons (diamonds). The data arenormalized between 50 and 150 GeV/c2 to the standardmodel cross-section. The curves are based on standard modelNLL calculation and .D has also measured the Drell-Yan cross section forelectrons satisfying either jj < 1:1 or 1:5 < jj < 2:5(Fig. 7). D measures the contribution to the cross sec-tion from misidentied QCD events, and other processes,using a combination of data and pythia Monte Carlo.The production cross section for Drell-Yan is calculated4FIG. 7. The electron-electron cross section as measured byD, and expected for Drell-Yan and Drell-Yan including con-tact interactions (both including backgrounds). Error barsindicate statistical errors.TABLE IV. The 95% CL lower limit on the com-bined quark-lepton compositeness scale  for dierentcontact-interaction models.CDF DModel +qe  qe +q  q +ql  ql +qe  qeLL 2.5 3.7 2.9 4.2 3.1 4.3 3.3 4.2LR 2.8 3.3 3.1 3.7 3.3 3.9 3.4 3.6RL 2.9 3.2 3.2 3.5 3.3 3.7 3.3 3.7RR 2.6 3.6 2.9 4.0 3.0 4.2 3.3 4.0VV 3.5 5.2 4.2 6.0 5.0 6.3 4.9 6.1AA 3.8 4.8 4.2 5.4 4.5 5.6 4.7 5.5using an NNLO prediction [4]. Limits are placed on var-ious models of quark-electron compositeness (Table IV).D and CDF rule out models of quark-electron com-positeness with interaction scales below 2.5 to 6.3 TeV,depending on the details of the model.E. ConclusionThe Fermilab experiments, CDF, D, and CCFR haveshown that predictions from the Standard Model are inexcellent agreement with the data, and there is no evi-dence for compositeness in quarks below a scales from2-8 TeV.The next run of the Tevatron, beginning in early 2001,will reach far greater luminosities, and provide moreopportunity for nding new physics at higher mass scales.F. AcknowledgementsThe author would like to acknowledge assistance fromIain Bertram in the preparation of the talk and this pro-ceeding.[1] E. Eichten, K. Lane, and M.E. Peskin, Phys. Rev. Lett.50, 811 (1983);E. Eichten, I. Hinchclie, K. Lane, and C. Quigg, Rev.Mod. Phys. 56, 579 (1984); Addendum - ibid. 58 1065;K. Lane, hep-ph/9605257 (1996).[2] CDF Collaboration, F. Abe et al., Phys. Rev. D 55, 5263(1997), hep-ex/9702004;CDF Collaboration , F. Abe et al., Phys. Rev. Lett. 74,3538 (1995), hep-ex/9501001 ;D Collaboration, B. Abbott et al., FERMILAB-CONF-97-356-E;L3 Collaboration, Adriani et al., Phys. Rep. 236, 1(1993), ;ALEPH Collaboration, Decamp et al., Phys. Rep. 216,253 (1992);[3] W. Giele, E. Glover, and D. Kosower, Nucl. Phys. B 403,633 (1993), hep-ph/9302225.[4] R. Hamburg, W.L. Van Neervan and T. Maatsura, Nucl.Phys. B 359, 343 (1991),[5] A.D. Martin et al., Phys. Lett. B 354, 155 (1995), hep-ph/9502336;P.J. Sutton et al., Phys. Rev. D 45, 2349 (1992).[6] OPAL Collaboration, G. Abbiendi et al., Eur. Phys. J. C6, 1 (1999), hep-ex/9808023.[7] D Collaboration, B. Abbot et al., Phys. Rev. Lett. 82,2457 (1999), hep-ex/9807014.[8] T. Sjostrand, Comp. Phys. Comm. 82, 74 (1994). pythia5.7 only contains the left-left isoscalar model of quarkcompositeness.[9] R.S. Chivukula, A.G. Cohen and E.H. Simmons, Phys.Lett. B 380 92, (1996), hep-ph/9603311.[10] I.A. Bertram and E.H. Simmons, Phys. Lett. B 443, 347(1998), hep-ph/9809472.[11] D Collaboration, B. Abbott et al., Phys. Rev. Lett. 80,666 (1998), hep-ex/9707016.[12] CDF Collaboration, F. Abe et al., Phys. Rev. Lett. 77,5336 (1996), erratum | ibid. 78, 4307 (1997), hep-ex/9609011.CDF Collaboration, F. Abe et al., Phys. Rev. Lett. 69,2896 (1992).CDF Collaboration, F. Abe et al., Phys. Rev. Lett. 62,3020 (1989).[13] D Collaboration, B. Abbot et al., submitted to Phys.Rev. D, hep-ex/9912023.[14] H.L. Lai et al., Phys. Rev. D 55, 1280 (1997), hep-ph/9606399.[15] A.D. Martin et al., Eur. Phys. J. C 4 463 (1998), hep-ph/9803445.5[16] NuTeV Collaboration, G.P. Zeller et al., hep-ex/9906024;K. McFarland et al., hep-ex/9806013.[17] CCFR Collaboration, K. McFarland et al., Eur. Phys. J.C 1, 509 (1998), hep-ex/9701010.[18] CDF Collaboration, F. Abe et al., Phys. Rev. Lett. 79,2198 (1997).[19] D Collaboration, B. Abbott et al., hep-ex/9812010,Submitted to Phys. Rev. Lett.[20] T. Lee, Phys. Rev. D 55, 2591 (1997), hep-ph/9605429.6

Searches for Compositeness at the Tevatron

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