Ulrich Husemann Yale University Outline of the Talk

Experimental Particle Physics Seminar University of Pennsylvania, October 17, 2007

Search for Flavor Changing Neutral Currents in Top Quark Decays at CDF

What are Flavor Changing Neutral Currents?

The CDF Experiment at the Tevatron

Top Quark Physics at CDF

Search for FCNC in Top Quark Decays

Summary & Conclusions

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 2 Outline of the Talk

What are Flavor Changing Neutral Currents?

The CDF Experiment at the Tevatron

Top Quark Physics at CDF

Search for FCNC in Top Quark Decays

Summary & Conclusions

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 3 Standard Model of Particle Physics

Matter in the standard model: 12 fermions in three generations Six quarks and their anti-particles Six leptons and their anti-particles Forces in the standard model: Strong force (carrier: gluon) Electroweak force (carriers: photon, W± bosons, Z boson) Interactions can be described by “currents” coupling to gauge bosons, e.g. electromagnetic current

electron electron

[Fermilab Visual Media Service] photon

L em = qe¯γ eAµ jem Aµ − µ ≡ µ Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 4 Flavor Changing Neutral Currents

Flavor changing neutral current (FCNC) q q’ interactions: Flavor Changing Transition from a quark of flavor A and charge Q Neutral to quark of flavor B with the same charge Q ,g,Z,H Current Examples: b → sγ, t → cH, …

1960s: only three light quarks (u,d,s) known, mystery in neutral kaon system:

W d d + 0 – d 8 K π 10 times K0 smaller than…? W+ u ! – π+ W

Solution: “GIM Mechanism” (Glashow, Iliopoulos, Maiani, 1970) Fourth quark needed for cancellation in box diagram: prediction of charm quark Cancellation would be exact if all quarks had the same mass: estimate of charm quark mass

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 5 FCNC in the Standard Model (I)

Standard model: no FCNC at Lagrangian level Massless theory: weak neutral current is flavor-diagonal 1 4 1 2 JNC = J3 2sin2 θ jem = u¯ γ (1 γ ) sin2 θ γ u d¯ γ (1 γ ) sin2 θ γ d µ µ − W µ 2 µ − 5 − 3 W µ − 2 µ − 5 − 3 W µ ! " ! " Quark masses via Higgs mechanism: Eigenstates of electroweak interactions are not mass eigenstates αβ α β αβ ¯ α β 1 αβ α β 1 αβ ¯ α β LYuk = mu u¯" u" m d" d" fu u¯" h(x)u" f d" h(x)d" + h.c. − L R − d L R − √2 L R − √2 d L R Mass Terms Higgs Couplings

Unitary transformation of Lagrangian to mass basis, i.e. for physical particles: u u† u† u u¯L = u¯L! UL uR = UR uR! mu = UL mu! UR ¯ ¯ d d† d† d dL = dL! UL dR = UR dR! md = UL md! UR Kinetic terms: unchanged Higgs couplings proportional to mass terms: no flavor changing Higgs couplings Neutral currents have same structure as kinetic terms: unchanged → no FCNC

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 6 FCNC in the Standard Model (II)

Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix (obtained from transformation of charged current to mass basis):

CC 1 u† d J = u¯! γ (1 γ ) d! = u¯! γ d! = u¯ U γ U d = u¯ γ V d , µ 2 µ − 5 L µ L L L µ L L L µ CKM L ! " 1.5 excluded at CL > 0.95 CKM matrix: unitary 3×3 matrix excluded area has CL > 0.95 % Vud Vus Vub 1 m & m V = V V V with sin2$ & s & d CKM  cd cs cb &md Vtd Vts Vtb 0.5 † †  ' # # VCKM VCKM = VCKM VCKM = 1 K · · % $ " 0

Vub/Vcb yields unitarity relations, + B * ) ( e.g. the unitary triangle of -0.5 'K flavor physics (1st vs. 3rd column) # -1 V ∗ Vub +V ∗ Vcb +V ∗Vtb = 0 ud cd td CKM sol. w/ cos2$ < 0 f i t t e r (excl. at CL > 0.95) FPCP 2007 % or (used in top FCNC): -1.5 -1 -0.5 0 0.5 1 1.5 2 Vcd∗ Vtd +Vcs∗Vts +Vcb∗ Vtb = 0 !

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 7 FCNC in the Standard Model (III)

FCNC are allowed via higher order mechanisms such as penguin diagrams, Top FCNC Penguin c,u but heavily suppressed t W+ Suppression mechanism 1: GIM Penguin matrix element depends on b,s,d universal functions of single parameter 2 2 xi = mi /mW /Z M ∝ F(xd) Vcd∗ Vtd + F(xs) Vcs∗Vts + F(xb) Vcb∗ Vtb, Compare to CKM unitarity relation: Universal Function for t → γc V ∗ Vtd +V ∗Vts +V ∗ Vtb = 0 10–6 cd cs cb ) i 0 [J.A. Aguilar-Saavedra, Re F Exact cancellation if masses of F(x Acta Phys. Polor B35 Im F b, s, and d quarks were the same –2 (2004) 2695] Quark masses more similar for down-type –4 than for up-type: top FCNC more strongly suppressed than bottom FCNC, e.g. –6 –14 –4 BR(t → Zq) ≈ 10 vs. BR(b→ sγ) ≈ 10 –8 Suppression mechanism 2: smallness of relevant CKM matrix elements 0 0.2 0.4 0.6 0.8 1.0 xi = mi /mW V ∗ V 0.002, V ∗V 0.04, V ∗ V 0.04 | cd td| ≈ | cs ts| ≈ | cb tb| ≈ Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 8 FCNC & New Physics

FCNC are enhanced in many models c,u of physics beyond the SM t W+ Enhancement mechanisms: FCNC interactions at tree level b,s,d Weaker GIM cancellation by new /Z particles in loop corrections Examples: New quark singlets: Z couplings not flavor-diagonal → tree level FCNC Model BR(t Zq) → 14 Two Higgs doublet models: modified Standard Model O(10− ) 4 Higgs mechanism q = 2/3 Quark Singlet O(10− ) 7 Flavor changing Higgs couplings allowed Two Higgs Doublets O(10− ) 6 at tree level MSSM O(10− ) 5 Virtual Higgs in loop corrections R-Parity violating SUSY O(10− ) Supersymmetry: gluino/neutralino [after J.A. Aguilar-Saavedra, and squark in loop corrections Acta Phys. Polor B35 (2004) 2695]

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 9 FCNC & New Physics

FCNC are enhanced in many models + c,u of physics beyond the SM t HW?+ Enhancement mechanisms: FCNC interactions at tree level b,s,d Weaker GIM cancellation by new /Z particles in loop corrections Examples: New quark singlets: Z couplings not flavor-diagonal → tree level FCNC Model BR(t Zq) → 14 Two Higgs doublet models: modified Standard Model O(10− ) 4 Higgs mechanism q = 2/3 Quark Singlet O(10− ) 7 Flavor changing Higgs couplings allowed Two Higgs Doublets O(10− ) 6 at tree level MSSM O(10− ) 5 Virtual Higgs in loop corrections R-Parity violating SUSY O(10− ) Supersymmetry: gluino/neutralino [after J.A. Aguilar-Saavedra, and squark in loop corrections Acta Phys. Polor B35 (2004) 2695]

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 9 FCNC & New Physics

FCNC are enhanced in many models + c,u of physics beyond the SM t HWq??+ Enhancement mechanisms: FCNC interactions at tree level b,s,dg, 0 ? Weaker GIM cancellation by new /Z particles in loop corrections Examples: New quark singlets: Z couplings not flavor-diagonal → tree level FCNC Model BR(t Zq) → 14 Two Higgs doublet models: modified Standard Model O(10− ) 4 Higgs mechanism q = 2/3 Quark Singlet O(10− ) 7 Flavor changing Higgs couplings allowed Two Higgs Doublets O(10− ) 6 at tree level MSSM O(10− ) 5 Virtual Higgs in loop corrections R-Parity violating SUSY O(10− ) Supersymmetry: gluino/neutralino [after J.A. Aguilar-Saavedra, and squark in loop corrections Acta Phys. Polor B35 (2004) 2695]

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 9 Experimental Tests of FCNC

Experimental tests of FCNC interactions: sensitive probes of new physics Any signal above SM expectations would indicate new physics Measurements constrain allowed phase space for new physics models

Two types of searches for FCNC in the top sector: Search for single top production (LEP, HERA, DØ) Search for top quark decay via FCNC (CDF)

Experiments usually report limits on Branching fractions for specific processes, e.g. BR(t → Zq) Coupling parameters of effective Lagrangian, e.g. for tZq coupling g µ Leff = κ xL q¯LγµtL + xR q¯RγµtR Z + ... −2cosθW · · · · ! "

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 10 Previous Searches for Top FCNC

CDF Run I search: Tevatron q F. Abe et al., PRL 80 (1998) 2525. W– + – p Signature: Z → l l + 4 jets (1 b-jet) q Limit on BR(t→Zq): 33% g q LEP searches: t P. Achard et al. (L3), Phys. Lett. B549 (2002) 290. – G. Abbiendi et al. (Opal), Phys. Lett. B521 (2001) 181. Z l J. Abdallah et al. (Delphi), Phys. Lett. B590 (2004) 21. + A. Heister et al. (Aleph), Phys. Lett. B453 (2002) 173. l

Hadronic top decay (4 jets) or semileptonic top e– LEP decay (2 jets & lepton) */Z* Very similar results among all LEP experiments, best limit on BR(t→Zq):13.7% (L3) e+ t HERA searches: A. Aktas et al. (H1), Eur. Phys. J. C33 (2004) 9. HERA S. Chekanov et al. (ZEUS), Phys. Lett. B559 (2003) 153. e– Hadronic top decay (3 jets) or semileptonic top * decay (lepton & jet) t Most sensitive to tγq vertex, preference for u u,c over c quarks (proton sea) p

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 11 Best Limits 2006The H1 Collaboration: Search for single top quark production in ep collisions at HERA 21 |

0.5 | 1.01 channel, shows good agreement with the expectation for tZu Excluded by H1 tuZ

|v mt = 170 GeV Standard Model processes within the uncertainties. |v m = 175 GeV 0.4 t In order to extract the top quark production cross sec- 0.80.8 mt = 180 GeV tion, a multivariate likelihood analysis is performed in addi-

BR(t → Zq) tion to the cut-based analyses. The top signal contribution Excluded by ZEUS 0.3 0.60.6 in each channel is determined in a maximum-likelihood fit to the likelihood discriminator distributions. The results from the hadronic channel do not rule out a single top in- Excluded by CDF 0.2 0.40.4 terpretation of the candidates observed in the electron and muon channels. For the combination of the electron, muon and hadronic channels a cross section for single top pro- Excluded by L3 +0.15 0.1 0.20.2 duction of σ = 0.29 0.14 pb at √s = 319 GeV is obtained. This result is not in−contradiction with limits obtained by other experiments. The addition of a contribution from a 0.0 0.00 model of anomalous single top production yields a better 0.0 0.01 0.02 0.03 0.04 0.05 0.00 0.20.2 0.40.4 0.60.6 0.80.8 1.01 description of the data than is obtained with the Standard |κ | BR(t→γq) |κ ttuγuγ| Model alone. Fig. 8. Exclusion limits at the 95% confidence level on the Assuming that the small number of top candidates are anomalous tqγ magnetic coupling κtuγ and the vector coupling the result of a statistical fluctuation, exclusion limits for the vtuZ obtained at the TeVatron (CDF experiment [38]), LEP (L3 single top cross section of σ < 0.55 pb at √s = 319 GeV and The H1 result caused some excitement:experiment is shown, which currently gives the best limit of the for the anomalous tuγ coupling of κtuγ < 0.27 are also LEP experiments [40]) and HERA (H1 and ZEUS experiments). derived at the 95 % confidence level.| The| HERA bounds Abstract. [...] In the leptonic channel, 5 events are found while 1.31 ± 0.22 events are The anomalous couplings to the charm quark are neglected extend into a region of parameter space so far not covered expected from the Standard Model background. In the hadronic channel, no excess above the κtcγ = vtcZ = 0. The error band on the H1 limit shows the by experiments at LEP and the TeVatron. expectation for Standard Model processes is found.uncertain [...]ty on the coupling κtuγ induced by a variation of the nominal top quark mass by 5 GeV ± Acknowledgements. We are grateful to the HERA machine Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 12 group whose outstanding efforts have made this experiment constraints on κtuγ and vtuZ . The limit on the anomalous possible. We thank the engineers and technicians for their work coupling κtuγ obtained in the present analysis significantly in constructing and maintaining the H1 detector, our funding improves the CDF and LEP upper bounds if the vector agencies for financial support, the DESY technical staff for 1 continual assistance, and the DESY directorate for support coupling vtuZ is not too large . The error band on the H1 limit represents the uncertainty induced by a variation of and for the hospitality which they extend to the non-DESY members of the collaboration. We thank A.S. Belyaev, P. Bock, the nominal top quark mass of mt = 175 GeV by 5 GeV in the analysis. Other theoretical errors are neglected.± The K.-P. Diener and M. Spira for useful discussions and fruitful collaboration. H1 results on single top production are not in contradiction with the limits set by other experiments. References 9 Summary 1. T. Stelzer, Z. Sullivan, S. Willenbrock, Phys. Rev. D 56, 5919 (1997) [hep-ph/9705398] A search for single top production is performed using 2. S. Moretti, K. Odagiri, Phys. Rev. D 57, 3040 (1998) the data sample collected by the H1 experiment between [hep-ph/9709435] 1994 and 2000, corresponding to a total luminosity of 3. D. Atwood, L. Reina, A. Soni, Phys. Rev. D 53, 1199 118.3 pb 1. This search is motivated by the previous ob- (1996) [hep-ph/9506243] − 4. G.M. de Divitiis, R. Petronzio, L. Silvestrini, Nucl. Phys. servation of events containing an isolated lepton, missing B 504, 45 (1997) [hep-ph/9704244] transverse momentum and large hadronic transverse mo- 5. R.D. Peccei, X. Zhang, Nucl. Phys. B 337, 269 (1990) mentum, a topology typical of the semileptonic decay of 6. H. Fritzsch, D. Holtmannspotter, Phys. Lett. B 457, 186 the top quark. (1999) [hep-ph/9901411] In a cut-based analysis, 5 events are selected as top 7. C. Adloff et al. [H1 Collaboration], Eur. Phys. J. C 5, quark candidate decays in the leptonic channel. The predic- 575 (1998) [hep-ex/9806009] tion for Standard Model processes is 1.31 0.22 events. The 8. V. Andreev et al. [H1 Collaboration], Phys. Lett. B 561, ± analysis of multi-jet production at high PT , correspond- 241 (2003) [hep-ex/0301030] ing to a search for single top production in the hadronic 9. S. Chekanov et al. [ZEUS Collaboration], Phys. Lett. B 559, 153 (2003) [hep-ex/0302010] 1 The change of the product σep etX t bW due to a non- 10. T. Han, J.L. Hewett, Phys. Rev. D 60, 074015 (1999) → × B → zero vtuZ coupling has been calculated in LO with ANOTOP [hep-ph/9811237] for κtuγ = 0.27. It is negligible in the region vtuZ < 0.3 where 11. E.E. Boos et al., SNUTP-94-116 [hep-ph/9503280]; in: the present analysis improves on the CDF and LEP limits. The Proceedings of the Xth Int. Workshop on High En- sensitivity to the vector coupling vtuZ has also been studied ergy Physics and Quantum Field Theory, QFTHEP-95, by the ZEUS collaboration [9]. Moscow (1995), Eds. B. Levtchenko, V. Savrin, p 101 DØ 2007: Single Top via FCNC

45 DØ 230 pb–1 Study single Top production via FCNC: 40 FCNC ( 10) 35 SM tb+tqb 30 tt ! " Events / 0.04 25 W+jets 20 multijet " 15 g 10 u,c 5 t 0 0 0.5 1 p NN output )

–2 Artificial neural network to discriminate –1 95% CL eV 0.003 DØ 230 pb signal from background

(T 90% CL

2 68% CL Λ ) World’s best limit on t-c-g and / g u 0.002 t-u-g couplings (κ/Λ)2 → previous limits ( κ improved by order of magnitude 0.001 c 2 2 κg/Λ < 0.023TeV− (95% C.L.) u 2 2 0 κg /Λ < 0.0014TeV− (95% C.L.) 0 0.01 0.02 0.03 0.04 ! " c 2 –2 (κg / Λ) (TeV ) [V!. M. Abazov" et al., hep-ex/0702005, submitted to PRL]

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 13 Outline of the Talk

What are Flavor Changing Neutral Currents?

The CDF Experiment at the Tevatron

Top Quark Physics at CDF

Search for FCNC in Top Quark Decays

Summary & Conclusions

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 14 Tevatron Run II: 2001–2009 (2010?)

Proton-antiproton collider: √s = 1.96 TeV Fermi National Accelerator Laboratory – Aerial View 36×36 bunches, collisions every 396 ns Record instantaneous peak luminosity: 292 µb–1 s–1 Tevatron (1 µb–1 s–1 = 1030 cm–2 s–1) 2 km Luminosity goal: 5.5–6.5 fb–1 of integrated luminosity by 2009, running in 2010 currently [Fermilab Visual Media Service] under discussion Two multi-purpose detectors: CDF and DØ

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 15 Tevatron Performance

Tevatron continues to perform very well: More than 3 fb–1 delivered up to Summer 2007 shutdown More than 2.5Year fb2002–1 recorded 2003by CDF 2004 2005 2006 2007 Month1 4 7 10 1 4 7 101 4 7 1 4 7101 7 101 4 ) )

–1 3.5

-1 3500 2002 2003 2004 2005 2006 2007 3.25 fb–1 30003.0 2.66 fb–1 25002.5

20002.0

15001.5 Integrated Luminosity (fb

Total Luminosity (pb 10001.0

5000.5 Delivered To tape 00 10001000 20002000 30003000 4000 5000 TevatronStore Store Number Number

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 16 Solenoid Magnet The CDF II Detector

Hadronic Wall Calorimeter

Protons

Antiprotons y

z x

[CDF]

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 17 Solenoid Magnet The CDF II Detector

Solenoid Magnet Hadronic Wall Calorimeter

Protons

Antiprotons y

z x

[CDF] Intermediate Silicon Layers

Central Outer Tracker SVX II & Layer 00

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 17 Solenoid Magnet The CDF II Detector

Hadronic Wall Calorimeter Central Calorimeter (Em/Had)

Plug Calorimeter (Em/Had) Solenoid Magnet Hadronic Wall Calorimeter

Protons

Antiprotons y

z x

[CDF] Intermediate Silicon Layers

Central Outer Tracker SVX II & Layer 00

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 17 Solenoid Magnet The CDF II Detector

Central Muon Detector

Hadronic Wall Calorimeter Central Calorimeter (Em/Had)

Plug Calorimeter (Em/Had) Solenoid Magnet Hadronic Wall Calorimeter Forward Muon Detector

Protons

Antiprotons y

z Luminosity Monitor x [CDF] Intermediate Silicon Layers Time of Flight Detector

Central Outer Tracker SVX II & Layer 00

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 17 Hadron Collider Kinematics

Cylindrical coordinate system: θ: polar angle w.r.t. to proton direction ϕ: azimuthal angle Pseudorapidity: η = −lntan(θ/2) Transverse energy: ! ET = ∑ Ei(sinθi,φi) cal towers Missing transverse energy (“MET”): ! = − ! − E/T ∑ET ∑ p!T jets leptons

Interaction Point z

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 18 Outline of the Talk

What are Flavor Changing Neutral Currents?

The CDF Experiment at the Tevatron

Top Quark Physics at CDF

Search for FCNC in Top Quark Decays

Summary & Conclusions

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 19 The Discovery of the Top Quark

Brief history of top Brief historyThe of D topis quarkcov ery quark discovery: discovery: 1977: Υ discovery – of the Top Quark bottom quark Finding the sixth quark involved the world’s 1980s: Searches for “light” top most energetic collisions and a cast of thousands

(mass smaller by Tony M. Liss and Paul L. Tipton than W boson mass) as isospin partner of bottom at PETRA, SppS, LEP, CDF Run 0 1992/3: Tevatron Run I starts, first indications for top quark production March 2, 1995: CDF and DØ announce VIOLENT COLLISION between a proton and an antiproton (center) creates a top quark (red) and an antitop (blue). These decay to other top quark discovery particles, typically producing a number of jets [Scientific American, September 1997] and possibly an electron or positron.

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 20 N

n March 1995 scientists gathered quarks are the building blocks of mat- must exist since 1977, when its partner, A M D

at a hastily called meeting at Fer- ter. The lightest quarks, designated “up” the bottom, was discovered. But the top O O G

milab—the Fermi National Accel- and “down,” make up the familiar pro- proved exasperatingly hard to ﬁnd. Al- L E A H

I C

erator Laboratory in Batavia, Ill., near tons and neutrons. Along with the elec- though a fundamental particle with no I Chicago—to witness a historic event. In trons, these make up the entire periodic discernible structure, the top quark M back-to-back seminars, physicists from table. Heavier quarks (such as the charm, turns out to have a mass of 175 billion rival experiments within the lab an- strange, top and bottom quarks) and electron volts (GeV)—as much as an nounced the discovery of a new particle, leptons, though abundant in the early atom of gold and far greater than most the top quark. A decades-long search moments after the big bang, are now theorists had anticipated. The proton, for one of the last missing pieces in the commonly produced only in accelera- made of two ups and one down, has a Standard Model of particle physics had tors. The Standard Model describes the mass of just under 1 GeV. (The electron come to an end. interactions among these building blocks. volt is a unit of energy, related to mass The top quark is the sixth, and quite It requires that leptons and quarks each via E = mc 2.) possibly the last, quark. Along with come in pairs, often called generations. Creating a top quark thus required leptons—the electron and its relatives— Physicists had known that the top concentrating immense amounts of en-

2 The top is heavy: mt ≈ 170 GeV/c (40×mb, approx. mass of gold atom) Mass close to scale of electroweak symmetry breaking (EWSB), top Yukawa coupling f ≈1: = v v LLYY,ukt ,t =f f tLt¯tLRtR mmt ttLt¯tLRtR √√22 ≡≡ (vacuum expectation value of Higgs field: v/√2 ≈ 178 GeV) → Important role in EWSB models Top is the only “free” quark: lifetime shorter than hadronization time

1 1 1 1 [Fermilab Visual Media Services] τ =1 ≈ . −<1 < 1 ≈ . −1 τ = Γ ≈(11.55GGeVeV) ΛQCD ≈ 0(.02GeV2GeV) Γ ΛQCD → No spectroscopy of bound states → Spin transferred to decay products

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 21 Top Pair Production at the Tevatron

Top production is rare: one top quark pair 85% qq → tt: produced every 10 billion collisions 1 t Total Inelastic 1 x 1010 –2 10 q - mb p 10–4 b 6 x 106

10–6 - b 15% gg → tt:

Cross Section (barns) W 10–8 4000 400 ! t - nb Z g 10–10 t 1 g 10–12 - pb Single Top 0.3 p "

–14 10 Higgs (ZH + WH) g - fb 10–16 100 120 140 160 180 200 t Higgs Mass (GeV/c2) p g

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 22 Analyzing Top Quark Events

W– → hadrons τ μ e Top decay in the Standard Model: t → Wb (BR ≈ 100%) tt decay signatures Lepton + Jets Lepton+ τ

(S/B ≈ 1) characterized by W decays: All Hadronic All-Hadronic (45% of all decays) (S/B ≈ 0.04)

hadrons Lepton+Jets (30% of all decays) Dilepton (5% of all decays) Main background process:

τ Lepton+ τ production of W bosons in association with Jets

e Lepton + Jets Dilepton → μ + + (S/B ≈ 1) (S/B ≈ 3) tt events contain two b quarks:

W b quark identification (“b-tagging”) crucial

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 23 Top Pair Production Cross Section

CDF Run II Preliminary (1.12 fb–1) Data SecVtx b-tagging algorithm: based on Top (8.2 pb) EW & Single Top significance of 2D impact parameter 1000 W+Light Flavor Single Non-W Tracks W+Charm y 800 SecVtx Tag W+Bottom

600 z 400 Secondary x Vertex Lxy 200 Number of Tagged Events Protons Antiprotons 0 1 2 3 4 5 Primary Number of Jets Vertex CDF Run II Preliminary (1.12 fb–1) Data Top (8.8 pb) CDF’s single most precise top cross EW & Single Top 80 W+Light Flavor section measurement: Lepton + Jets Non-W W+Charm channel with SecVtx b-tags W+Bottom 60 Results (CDF Public Note 8795) Double 40 SecVtx Tag Single σtt = 8.2 ± 0.5 (stat) ± B-Tag 0.8 (syst) ± 0.5 (lum) pb 20

Double σtt = 8.8 ± 0.8 (stat) ± 0 2 3 4 5 B-Tag 1.2 (syst) ± 0.5 (lum) pb Number of Double-Tagged Events Number of Jets Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 24 CDF’s Top Properties Program

“Zweifle an allem wenigstens einmal, und wäre es auch der From top discovery in 1995 Satz: zwei mal zwei ist vier” to precision physics in 2007: (G. F. Lichtenberg)

Dataset: 1000s of top events Isospin Mass & cross section very partner of the b quark? precisely measured Always a W boson? Evidence for single top production Production Mechanism? Broad program to study properties of the top quark: production, decay, quantum numbers, … Always a b quark? Measurements of top properties try to answer: Electroweak V–A interaction? Is the top really the Standard Model top?

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 25 Outline of the Talk

What are Flavor Changing Neutral Currents?

The CDF Experiment at the Tevatron

Top Quark Physics at CDF

Search for FCNC in Top Quark Decays

Summary & Conclusions

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 26 Top FCNC Search: Roadmap

Basic question: how often do top quarks decay into Zq? Z Decay Modes: → set limit on branching fraction 20% BR(t → Zq) Z → νν Z → ee/µµ 7% Selection of decay channels for 3% Z → ττ 70% tt → Zq Wb: Z → hadrons Z → charged leptons: very clean signature, lepton trigger W → hadrons: large branching fractions, no neutrinos → event can by fully reconstructed W Decay Modes: 21% Final signature: Z + ≥4 jets W → lν 11% Analysis Outline: W → τν 68% I. Baseline Event Selection W → hadrons II. Initial Background Estimate III. Optimization of Event Selection IV. Systematic Uncertainties V. Final Limit Calculation

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 27 Blind Analysis

Event signature: Z → l+ l– + 4 jets Motivation for blind analysis: avoid biases by looking into the data too early Blinding & unblinding strategy: Initial blinded region: Z + ≥ 4 jets Later: add control region in Z + ≥ 4 jets from kinematic constraints Optimization of event selection, prediction of backgrounds, and systematic uncertainties on data control regions and Monte Carlo (MC) simulation only Very last step: “opening the box”, i.e. look into signal region in data

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 28 Simulation of FCNC Signal

Monte Carlo (MC) simulation of FCNC decay t → Zq with PYTHIA positively t → Zq vertex unknown to PYTHIA charged Decay generated flat in cos θ* lepton top boost cos * (angle between top boost direction direction and lepton of same charge sign from Z Z decay, in Z rest frame) negatively charged lepton

Solution: reweight according to expectation from standard model Higgs mechanism:

dσ 0 3 2 3 2 + 3 2 = f 1 cos(θ ∗) + f − (1 cos(θ ∗)) + f (1 + cos(θ ∗)) dcos(θ ∗) · 4 − · 8 − · 8 with f 0 = 0.65 (“longitudinal),! f – = "0.35(“left-handed”), f + = 0 (“right-handed”) Main FCNC signal sample: one top decays t → Zc, other decays t → Wb Additional sample required for decay t → Zu Additional sample for “double FCNC” events, i.e. both tops decay via FCNC t → Zq

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 29 Search for FCNC: Ingredients

q W– p q g q t – Z l l+

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 30 Search for FCNC: Ingredients

q W– p q g q t l– Two leptons Z (ee or µµ) with opposite + charge, form l Z boson

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 30 Search for FCNC: Ingredients

q W– p q g

Additional jet, can be q combined with leptons t to form top quark l– Two leptons Z (ee or µµ) with opposite + charge, form l Z boson

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 30 Search for FCNC: Ingredients

q – Two quark jets W form W boson p q g

Additional jet, can be q combined with leptons t to form top quark l– Two leptons Z (ee or µµ) with opposite + charge, form l Z boson

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 30 Search for FCNC: Ingredients

q – Two quark jets W form W boson p q Jet from b quark, can g be combined with W to form top quark

Additional jet, can be q combined with leptons t to form top quark l– Two leptons Z (ee or µµ) with opposite + charge, form l Z boson

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 30 Z Boson Reconstruction

Simple trigger: single electron or muon, transverse momentum >18 GeV/c Sharp Z resonance, good lepton momentum resolution 2 2 → cut on lepton pair invariant mass: 76 GeV/c < Mll < 106 GeV/c Enhancing the Z acceptance: BremsstrahlungZ mass from Recovery: TCE-TRK Z Dielectrons → e track Tracking systems have better 2 c CDF II Preliminary 1.12 fb–1 Data coverage than calorimeter and (with track correction) 1500 Data muon detectors: allow second (no track correction) lepton to be isolated track 76 GeV/c2 106 GeV/c2 → doubles acceptance w.r.t. 1000 standard lepton selection Entries per GeV/ Electron tracks lose momentum via 500 bremsstrahlung: correct track 0 momentum with calorimeter energy 60 80 100 120 → 3% more dielectron pairs Dielectron Mass (GeV/c2)

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 31 Adding Jets Jet Jet FCNC: four jet assignments 1 b-jet from t → Wb decay Lepton Jet 2 jets from subsequent W decay 1 jet from t → Zq decay Lepton For all 12 possible combinations Jet of first four jets in the event: Pick combination with lowest 1. Combine jets #1 and #2 to W, 2 mW,rec mW, calculate invariant mass mW,rec χ2 = − PDG + σ 2. Vary momenta of jets #1 and #2 ! W,rec " 2 within their resolutions to match mt Wb,rec mt,PDG → − + PDG W mass (“fix W mass”) σt Wb ! → " 2 3. Add jet #3 to fixed W, calculate mt Zq,rec mt,PDG → − invariant mass mt→Wb,rec σt Zq ! → " 4. Vary momenta of leptons within Widths reflect mass resolutions their resolutions to match PDG Z mass (“fix Z mass”) as measured in MC simulation: 2 σW,rec = 15 GeV/c , 2 5. Add jet #4 to fixed Z, calculate σt→Wb,rec = 24 GeV/c invariant mass mt→Zq,rec 2 σt→Zq,rec = 21 GeV/c

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 32 Expected Backgrounds

How do you search for a signal that is likely not there? Understand the background! Standard model processes that can mimic Z + ≥4 jets signature: Z+Jets: Z boson production in association with jets → dominant background for top FCNC search, most difficult to estimate Standard model tt production → small background Dibosons: WZ and ZZ diboson production → small background W+Jets, WW: negligible Top FCNC background estimate: mixture of data driven techniques and MC predictions

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 33 Expected Backgrounds

How do you search for a signal that is likely not there? Standard Model tt Production Understand the background! Small background: no real Z, need Standard model processes that extra jets from gluon radiation and/or can mimic Z + ≥4 jets signature: “fake lepton” Z+Jets: Z boson production in Dilepton channel association with jets (tt → Wb Wb → lνb lνb): → dominant background for top dilepton invariant mass can fall FCNC search, most difficult to into Z mass window estimate Lepton+Jets channel Standard model tt production → small background (tt → Wb Wb → lνb qq’b): misreconstruct one jet as a lepton Dibosons: WZ and ZZ diboson (“fake”), invariant mass of lepton production → small background and fake lepton can fall into Z W+Jets, WW: negligible mass window Top FCNC background estimate: Large fraction of heavy flavor jets: mixture of data driven techniques more important in b-tagged samples and MC predictions Estimated from MC simulation

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 33 Expected Backgrounds

How do you search for a signal that is likely not there? Diboson Production: WZ, ZZ Understand the background! Small background (similar in size to Standard model processes that standard model tt production) can mimic Z + ≥4 jets signature: Small cross section but real Z Z+Jets: Z boson production in Need extra jets from gluon association with jets radiation → dominant background for top FCNC search, most difficult to ZZ: Heavy flavor contribution from estimate Z→bb decay Standard model tt production Estimated from MC simulation → small background Dibosons: WZ and ZZ diboson production → small background W+Jets, WW: negligible Top FCNC background estimate: mixture of data driven techniques and MC predictions

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 33 Z+Jets Production Z+Jets Spectrum

MC tool for Z+Jets: ALPGEN CDF II Preliminary 1.12 fb–1 Data 105 MC Simulation Modern MC generator for multiparticle Entries final states 104

“MLM matching” prescription to 3 10 Blinded remove overlap between jets from matrix element and partons showers 102 Comparing ALPGEN with data: 0 2 4 Leading order generator: no absolute Z+Jets SpectrumNumber of Jets prediction for cross section 2 CDF II Preliminary 1.12 fb–1 Ratio Data/MC ALPGEN Underestimate of number of events Uncertainties with large jet multiplicities, large 1.5 uncertainties Ratio Data/MC Our strategy: only shapes of Blinded kinematic distributions from MC, 1 normalization from control samples in data 0.5 0 2 4 Number of Jets

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 34 Kinematic Constraints

Mass χ2: combination of mass constraints – best discriminator 2 2 2 2 mW,rec mW,PDG mt Wb,rec mt,PDG mt Zq,rec mt,PDG χ = − + → − + → − σW,rec σt Wb σt Zq ! " ! → " ! → " Transverse mass: FCNC top decays are more central than Z+jets 2 2 M = E !p T ∑ T − ∑ T ! Jet transverse" # energies" :# FCNC signal has four “hard” jets, background processes: jets have to come from gluon radiation

2 FCNC FCNC Mass χ 5000 Transverse Mass Area Z+Jets Area Z+Jets 10000 Z c+Jets Z c+Jets Z b+Jets 4000 Z b+Jets 8000 3000 6000

4000 2000 Normalized to Equal Normalized to Equal 2000 1000

0 0 0 2 4 6 8 10 0 100 200 300 400 500 600 2 Transverse Mass (GeV)

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 35 To B-Tag or not to B-Tag?

0.2 Advantage of requiring b-tag: FCNC Z+Jets Better discrimination against main Z c!+Jets Z+jets background 0.15 Z b"+Jets SM t# (heavy flavor backgrounds rather Diboson small: SM tt, Zbb + jets) 0.1 ≥ 1 Loose Disadvantage: 0.05 Reduction of data sample size Normalized to Unit Area SecVtx Tag 0 Solution: use both! 0 2 4 6 8 10 Split sample in tagged and anti-tagged 2

Optimize cuts individually for tagged FCNC and anti-tagged samples Z+Jets 0.15 Z c+Jets Z b+Jets Combine samples in limit calculation SM t Diboson Main difficulty of this approach: 0.1 event migration between samples Exactly 0 Loose 0.05 Systematics may be correlated or anti- Normailzed to Unit Area SecVtx Tags correlated between samples 0 0 2 4 6 8 10 Taken into account in limit calculation 2

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 36 Optimization of Event Selection

Question: best choice for cut values? Final Event Selection Goal: derive limit on branching Kinematic Variable Optimized Cut Z Mass [76,106] GeV/c2 fraction of FCNC process t → Zq ∈ Leading Jet ET > 40 GeV No prediction for amount of signal: Second Jet ET > 30 GeV “signal over background” et al. do not Third Jet ET > 20 GeV work Fourth Jet ET > 15 GeV Transverse Mass > 200 GeV Solution: optimize cuts for best χ2 < 1.6 (b-tagged) < 1.35 (anti-tagged) expected limit (assuming no signal) ! P(n n ) Lim(n A,n ) ∑ obs| back · obs| back Expected 95% C.L. Upper Limit nobs onBayesian BR(t→ ExpectedZq): 7.1% Limit ±Distribution 3.0% P: Poisson probability L: any limit calculation method 0.2

Our analysis: faster objective Bayesian LEP Limit (L3): limits for optimization, “better” Feldman- 13.7% @ 95% C.L. Cousins limits for final result (both 0.1 including systematic uncertainties) Normalized to Unit Area 0 Correlations among variables: 0 0.1 0.2 0.3 multi-dimensional optimization Expected Limit

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 37 Background: Putting it all Together

Total background prediction from Fit to High !2 Tail control region in data: 130 ± 28 events CDF II Preliminary 1.12 fb–1

Tail of mass χ2 distribution Entries 10 Average of cuts at √χ2 = 3.0, 3.2 144.2 total events Tagging rate: 15% ± 4% 5 Ignoring bins 1..16 Tail of mass χ2: 16% ± 7% (small sample → large uncertainties) MC prediction of tagging rate: 11% 0 (but: 30% too low for Z+≤ 3 Jets) 0 2 4 6 8 10 2 Template fit of MC tagging probabilities vs. number of jets: 14% Source Without b-tag Loose SECVTX b-tag Z+Jets 123.3 28 17.6 6 ± ± Standard Model tt 2.4 0.3 1.7 0.2 ± ± Diboson (WZ, ZZ) 4.3 0.2 0.7 0.1 ± ± WW, W+Jets < 0.1 negligible Total Backgrounds: 130 28 20 6 ± ± Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 38 Acceptance Algebra: Catch 22?

Question: how to get from event counts to limit on BR(t→Zq)? Circular dependency #1: Limit calculation requires knowledge of signal acceptance, but signal acceptance depends on limit Signal Limit on Circular dependency #2: Measure limit on Acceptance ? BR(t→Zq) fraction of tt production cross section, but cross section changes with changing FCNC contribution Solution: “running acceptance” – functional form of above dependencies implemented in limit machinery Signal acceptance dynamically adjusted as tt Production Limit on a function of BR(t→Zq) Cross Section ? BR(t→Zq) Signal normalized to measured tt production cross section measurement tt cross section re-interpreted as a function of BR(t→Zq) to allow for FCNC contribution

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 39 Acceptance Algebra: Details

Signal count: probability for one B B(t Zc) = 1 B(t Wb) Z ≡ → − → or both tops to decay via FCNC A FCNC Acceptance WZ ≡ P(tt ZcWb, ZcZc,...) AZZ Double FCNC Acceptance → ≡ ALJWW L+J Acceptance for SM tt Normalization to double-tagged ≡ ALJ L+J Acceptance for FCNC tt cross section measurement: WZ ≡ A L+J Acceptance for Double FCNC LJZZ ≡ Double-tagged: smallest overlap KZZ/WZ AZZ/AWZ between acceptances ≡ RWZ/WW ALJWZ /ALJWW Luminosity uncertainties cancel, ≡ RZZ/WW ALJ /ALJ other uncertainties reduced ≡ ZZ WW Acceptance Master Formula:

N = [(P(tt WbZc) A ) + (P(tt ZcZc) A )] σ ¯ L dt signal → · WZ → · ZZ · tt · ! . . . 1/2 page of algebra. . .

A 2 (1 BZ) + K / BZ WZ · − ZZ WZ · = BZ (NLJ BLJ) 2 2 · − · ALJww · (1 Bz) +" 2Bz (1 Bz) Rwz/ww +#Bz Rzz/ww Acc. − · − · · L+J yield Ratio “Running” Acceptance Correction

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 40 Acceptance Algebra: Details

N = [(P(tt WbZc) A ) + (P(tt ZcZc) A )] σ ¯ L dt signal → · WZ → · ZZ · tt · ! . . . 1/2 page of algebra. . .

A 2 (1 BZ) + K / BZ WZ · − ZZ WZ · = BZ (NLJ BLJ) 2 2 · − · ALJww · (1 Bz) +" 2Bz (1 Bz) Rwz/ww +#Bz Rzz/ww Acc. − · − · · L+J yield Ratio “Running” Acceptance Correction

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 40 Signal Systematics

Signal systematic evaluated for acceptance ratio AWZ/ALJ Distinguish uncertainties: correlated or anti-correlated between selections Correlated: shift anti-tagged & tagged selection into same direction (e.g. lepton SF) Anti-correlated: shift anti-tagged & tagged into opposite directions (e.g. b-tagging)

Systematic Uncertainty Base Selection (%) Anti-Tagged (%) Loose Tag (%) Lepton Scale Factor 0.5 0.5 0.5 Trigger Efﬁciency 0.2 0.2 0.2 Jet Energy Scale 3.1 2.6 1.9 ISR/FSR 1.3 2.6 6.5 Helicity Re-Weighting 3.5 3.4 3.2 Parton Distribution Functions 0.9 0.9 0.9 Total Correlated 5.0 5.1 7.5 B-Tagging Scale Factor 10.2 16.3 5.5 Mistag αβ Correction 0.6 1.0 0.4 B(t Zc) versus B(t Zu) 0.0 4.0 4.0 → → Total Anti-Correlated 10.2 16.8 6.8

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 41 Signal Systematics

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 41 Background Systematics

Background systematics FCNC dominated by yield Z+Jets uncertainties 0.15 Z c+Jets Z b+Jets Total background yield: SM t 0.1 Diboson 130 ± 28 (21.5% relative uncertainty) 0.05 Tagging rate: 15% ± 4% (relative uncertainty: 26.7% Normalized to Unit Area 0 tagged, 4.7% anti-tagged) 0 2 4 6 8 10 Remaining uncertainties: !2 efficiency of χ2 cut Systematic Uncertainty Anti-Tagged (%) Loose Tag (%) Ratio of events with Lepton Scale Factor < 0.1 < 0.1 √χ2 < 1.6 (signal region) vs. Trigger Efﬁciency < 0.1 < 0.1 √χ2 > 3.0 (control region) Jet Energy Scale 5.1 2.1 B-Tagging Scale Factor < 0.1 0.3 Dominated by choice of MC Mistag αβ Correction 0.2 0.4 generator and jet energy ALPGEN MC Generator 10.0 5.9 scale Total Uncertainty 11.2 6.3

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 42 Background Systematics

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 42 The World’s Best Limit on BR(t → Zq)

Opening the box with 1.12 fb–1 Selection Observed Expected Event yield consistent with Base Selection 141 130 28 background only ± Base Selection (Tagged) 17 20 6 ± Fluctuated about 1σ high: slightly Anti-Tagged Selection 12 7.7 1.8 ± unlucky Tagged Selection 4 3.2 1.1 ± Result: The World’s Best Limit! Mass !2 (95% C.L. Upper Limit)

B(t Zq) < 10.6%@ 95% C.L. CDF II Preliminary Data → FCNC Signal (10.6%) L dt = 1.12 fb–1 20 Total Background

Entries Total Syst. Uncertainties Expected limit: 7.1% ± 3.0% 2 Cut 15 Tagged Anti-Tagged 25% better than L3 (13.7%) Selection Selection 3x better than CDF Run I (33%) 10

Above results assumes mt = 175 5 2 2 GeV/c , limit at mt = 170 GeV/c : BR(t → Zq) < 11.2% @ 95% C.L. 0 0 2 4 6 8 0 2 4 6 8 Update with 2 fb–1 and improved 2 method in the works

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 43 Top FCNC Searches at the LHC

Large Hadron Collider (LHC): ATLAS Detector Proton-proton collider at 14 TeV center-of-mass energy (CERN) Two multi-purpuse experiments: ATLAS and CMS First data expected in 2008 (2009?)

Recent ATLAS study on [J. Carvalho 1 sensitivity for top FCNC q) CDF ZEUS Improvement of current limits on BR 10–1 (q=u only) 95% C.L.

BR(t EXCLUDED LEP –1 (t→Zq) by 2–3 orders of magnitude CDF (2 fb )? REGIONS et al. Entering interesting regime of 10–4 to 10–2 10–5: exclusion of first theoretical , SN-A models 10–3 ATLAS (10 fb–1) TLAS-2007-059] Caveat: background model ZEUS –4 (q=u only) 10 (630 pb–1) Existing MC tools not tuned to new ATLAS –1 energy regime 10–5 (100 fb ) Tevatron experience: obtain 10–5 10–4 10–3 10–2 10–1 1 backgrounds from data BR(tq)

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 44 Summary and Conclusions

Top flavor changing neutral current decays Extremely rare in the standard model Enhanced in theories beyond the standard model → any signal would indicate new physics First Tevatron Run II search for FCNC t → Zq in top quark decays Event signature: Z + ≥ 4 jets Main background process: standard model Z + jets production Mass χ2 to separate signal from background No evidence for top FCNC found World’s best limit: BR(t→Zq) < 10.6% at 95% C.L. Working on improvements, stay tuned!

Exp. Particle Physics Seminar, Penn, 10/17/07– U. Husemann: Search for Top FCNC 45