Search for Leptoquarks and Compositeness at D0

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Search for Leptoquarks and Compositeness at D0
Maxim Titov (on behalf of D0 Collaboration)
University of Freiburg
European Physics Conference (EPS 2005), Lissabon,
21. 07. 2005
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Tevatron D0 Detector Luminosity in Run II

• Run II started March 2001
• Higher energy (1.8 TeV -gt 1.96 TeV)
• Higher antiproton intensity
• (66 bunches -gt 3636 bunches)

Acceptance Electrons h lt 3.0 Muons h lt
2.0 Jets h lt 4.2
1 fb-1
Results presented in this talk are based on
integrated luminosity of 200 400 pb-1 More
than double of this dataset is already on the tape
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Leptoquarks Phenomenology

• Leptoquarks (LQ) predicted to exist in various SM
  extensions (e.g. GUT, technicolor,
• SUSY with R-parity violation, composite models,
  superstring-inspired E6-models)
• Connection of lepton and quark sector
• (color-triplet field, fractional electric charge,
  both lepton and quark numbers)
• HERA BuchmullerRucklWyler (BRW) Minimal LQ
  Model (Phys. Lett.B191(1987) 442)
• (7 scalar and 7 vector leptoquarks with fermion
  numbers F - (3B L) 0 or 2)
• TEVATRON LQ pair production does not depend on
  its electroweak properties ?
• No need to use specific model, but number of
  constraints on LQ are coming from
• Atomic parity violation (APV) experiments
• Baryon and Lepton number conservation (to avoid
  rapid proton decays)
• Family diagonal LQ couples to a single
  leptonic and quark generation (to avoid FCNC)
• Chiral coupling (to avoid deviations from
  universality in leptonic p decays)

D0 Limits for scalar leptoquarks are only
presented in this talk Scalar leptoquarks are
less model dependent typically have lower
production cross section ? limits could be valid
for vector LQ
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Scalar Leptoquarks at Tevatron
Scalar LQ Production (sNLO 0.3 pb at MLQ 200
GeV)

• Scalar LQ pair production
• Quark-antiquark annihilation and gluon fusion
• (qq annihilation is dominant for MLQ gt 100 GeV)
• Phys.Rev Lett.79(1997)341 Phys.Rev.D59,015001(199
  8)
• The gluon-leptoquark coupling is simply
• given by the strong coupling
• Cross section is independent of Yukawa
• leptoquark-lepton-quark coupling (lLQ)
• ( lLQ contributes only 1 of total cross
• section in t-channel of qq annihilation)

LQ Decay
LQ experimental signatures 1st generation
2e2j, e2jMET, 2j MET 2nd generation 2m2j,
m2jMET, 2j MET Combined D0-CDF Run I 1st
generation LQ limit MLQ1 gt 242 GeV for b
1.0 (hep-ex/9810015)
b LQ branching fraction to charged lepton and
quark
5
Pair Production of 2nd Generation Leptoquarks
NEW ! ! !
Run II D0 analysis have been performed in mmqq
channel (294 pb-1)
Selection 2m (ETgt 15 GeV) 2 jets (ETgt 25GeV)
Z veto Main SM backgrounds Z/Drell Yan jets,
ttbar
Only the combination with the smaller mass
difference min(M1(mj)-M2(mj)) of the two LQ
candidates in the event is chosen
ST ET(j1) ET(j2) ET(m1) ET(m2)
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Pair Production of 2nd Generation Leptoquarks
NEW ! ! !
Sensitivity to LQ decays is studied using a
two-dimensional distribution Scalar sum of
transverse energies (ST) vs. Dimuon invariant
mass Mmm
All events are arranged in 4 bins ? choice of
binning follows the ratio of the expected signal
over background (S/B) for a leptoquark mass of
240 GeV
Modified frequentist approach
The observed limit is calculated using CLS
CLSB / CLB ? (T. Junk, NIMA434(1999) 435)
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D0 Run III Combined Scalar LQ Limits (2nd
generation)
The mass limit is extracted from the
intersection of the lower edge of the s(NLO)
with the observed upper bound to the cross
section
D0 Run II Limits (294 pb-1) MLQ gt 247 GeV (mmjj
channel, b 1.0) MLQ gt 182 GeV (mmjj channel, b
0.5)
D0 Combined (Run I Run II) (D0 Note 4829)
MLQ gt 251 GeV (b 1.0) MLQ gt 204 GeV (b 0.5)

CDF Run II (200 pb-1) (CDF Note 7216) MLQ gt
241 GeV (b 1.0) MLQ gt 175 GeV (b 0.5)
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D0 Run III Combined Scalar LQ Limits (1st
generation)

• D0 Run II Limits (252 pb-1)
• MLQ gt 241 GeV (eejj channel, b 1.0)
• MLQ gt 218 GeV (combined eejj enjj
• channels for b 0.5)

eejj

• D0 Combined (Run I Run II)
• (Phys. Rev.D71,071104(2005))
• MLQ gt 256 GeV (eejj, b 1.0)
• MLQ gt 234 GeV (combined
• eejj enjj. b 0.5)

Combined result as a function of b
Also CDF in Run II using eeqq, enqq nnqq
channels (200 pb-1) (hep-ex/0506074) MLQ gt 236
GeV (b 1.0) MLQ gt 205 GeV (b 0.5)
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Contact Interaction Compositeness Phenomenology
New Physics appear from the interference of any
new particle field (M L) associated to a
characteristic energy scale (L2 gtgt s) with the
SM field
New physics
Four-fermion contact interaction can be applied
to Compositeness, Leptoquarks, New gauge
bosons by an appropriate choice of coefficients
hAB
SM
Quark and leptons might be composite objects and
bound states of more fundamental constituents
(preons) Quark-Lepton compositeness can be
expressed through effective coupling
coefficients, which depends on the ratio of
coupling constant g0 over scale of compositeness
L hAB g02 / L2AB (hAB chiral
structure of the interation A,B ? L(eft) -
R(ight) quark/lepton helicities) At L2 ltlt s,
multiple fermion production processes will
dominate over SM two-body fermion scattering
processes At L2 gtgt s, flavor diagonal contact
interaction will modify SM cross-section for
elastic fermion-fermion scattering
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Quark-Lepton Compositeness at Hadron Colliders
Quark Compositeness is most sensitive to the
deviation in production of high-transverse
momentum jets relative to SM predictions Quark-Le
pton Compositeness would modify Standard Model
Drell-Yan (DY) cross-section for lepton pair
production at large invariant masses
Quark-Lepton Composite Models The modified
dilepton cross-section is controlled by
compositeness scale L and interference sign (I)
with SM
(I hAB is the interference of DY and contact
term, C is the pure contact term)
Limits are obtained independently for each
separate channel (LL, RR, RL, LR, VV, AA) of
contact interaction Lagrangian and L hAB -1 ?
constructive interference L- hAB 1 ?
destructive interference
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Quark-Lepton Compositeness Searches
Run II D0 Analysis have been performed in pp ?
qq ? g/Z ? mm (L 406 pb-1) and pp ? qq ? g/Z ?
ee (L 271 pb-1)
Di-muon Channel 2 isolated m (pT gt 15 GeV),
cosmic veto and Mmm gt 50 GeV Di-electron
Channel 2 electrons (pT gt 25 GeV), Mee gt120 GeV
SM CI (L 2 TeV)
Z? ee
SM Monte Carlo
cos q
Mee
Mmm
Z? mm
g/Z? mm A 95 CL upper limit on L is computed
from 2-dimen. distributions (Mmm vs. scattering
angle cos q) using DATA, background and signal
MC g/Z ? ee Limit is calculated using a
Bayesian analysis of the shape of the mass
distribution of events
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Compositeness Searches Experimental Status
D0 Run II Results in di-electron (L 271 pb-1)
and di-muon channels (L 406 pb-1)
Limits of Compositeness Scale L
Lower L Limit
Upper L Limit
Parity violating terms (LL, RR, LR, RL) are
constrained by APV experiments with L gt 11 TeV
HERA Phys. Lett. 568(203)35, hep-ex/9905039
LEP Phys. Lett.B489(2000) 81, Eur. Phys.
J.C12(2000) 183, Eur. Phys. J.C.13(2000) 553,
Eur. Phys. J.C.11(1999) 383 http//lepewwg.web.ce
rn.ch/LEPEWWG/lep2 CDF Phys. Rev.
Lett.79(12)(1997)2198 Phys. Rev. Lett.
78,(1997)4307, D0 D0 Note 4552, Phys. Rev.
Lett. 82(1999)2457, APV Phys. Lett.B480(2000)149
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Summary and Outlook

• The performance of Tevatron improves steadily
  allowing to test experimentally
• wider range of new phenomena searches for an
  ultimate theory
• No evidence for leptoquarks and compositeness
  has been observed
• Combined D0 Run I Run II Limits for scalar
  leptoquarks allow to exclude
• 1st LQ generation up to 256 GeV (for b 1)
• 2nd LQ generation up to 251 GeV (for b 1)
• D0 Run II Limits from 4.2 Tev to 9.8 TeV (for
  different chirality models) are the
• most stringent limits in the dimuon channel for
  the compositeness scale
• Start counting on gt 1 fb-1 data sample
• ? a new era for searches at hadron colliders

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Backup Slides
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Pair Production of 1st Generation Leptoquarks
Run II D0 analysis have been performed in eeqq
and enqq channels (252 pb-1)
Selection 2 Electrons and 2 Jets 2j (ETgt20 GeV)
2e (ETgt25 GeV) Z-veto Main Backrounds
ZDYjets, multi-jets ST ET(j1) ET(j2)
ET(e1) ET(e2)gt450 GeV
Selection 1 Electron, 2 Jets and Missing ET 2j
(ETgt25 GeV) e (ETgt25 GeV) MET gt 30 GeV
W-veto Main backroundsW2jets,multi-jets,
ttbar ST ET(j1) ET(j2) ET(e1) METgt330 GeV
1 event (DATA) vs 0.5 - 0.1 events expected 30
signal efficiency (MLQ 240 GeV)
1 event (DATA) vs 3.6 - 1.2 events expected 18
signal efficiency (MLQ 200 GeV)
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Scalar Leptoquarks in the Acoplanar Jet Topology
Run II D0 analysis have been performed in nnqq
channels (85 pb-1)
SM bkg 38.4 -3.7 QCD 3.1 - 2.0 Total bkg
41.5 - 4.2 DATA events 44
D0 Run I Limit(Phys. Rev. D64(2001)092004)
LQ mass range 85 -109 GeV is excluded By the Run
II Analysis
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Leptoquark Searches Experimental Status
LEP Limits http//lepewwg.web.cern.ch/LEPEWWG/lep
2
H1 Collaboration, hep-ex/0506044
b 1 (LQ branching fraction to charged lepton
and quark)
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Quark-Lepton Compositeness Dimuon Channel
Run II D0 Analysis have been performed in pp ?
qq ? g/Z ? mm (L 406 pb-1)
Selection 2 isolated m (pT gt 15 GeV), cosmic
veto and Mmm gt 50 GeV Backgrounds tt ? mm and
bb production
cos(Q) - scattering angle, relative to the
direction of the boost of the dimuon system
A 95 CL upper limit on composite scale L is
computed from the fit, using DATA, background
and signal MC 2-dimensional distributions (Mmm vs
cos q) ? better confidence limits on L than
using only 1-dim Mmm
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Quark-Lepton Compositeness Dielectron Channel
Run II D0 Analysis have been performed in pp ?
qq ? g/Z ? ee (L 271 pb-1)
Selection 2 electrons (pT gt 25 GeV), Mee gt120
GeV Backgrounds multijet and g/jet events ?
estimated from the same data sample
Mee
Bayesian Method to set limit on L (separately
for each chirality channel)
To get a 95 CL limit on L