aa r X i v : . [ h e p - e x ] O c t Proceedings of the XXIX PHYSICS IN COLLISION 1
Searches for New Physics with High Energy Colliders
Emmanuel Sauvan (1) CPPM, CNRS/IN2P3 et Aix-Marseille Universit´e, 163 Av. de Luminy, F-13288 Marseille, France Abstract
Recent experimental results of searches for new phe-nomena performed at high energy colliders are reviewed.The results reported are based on data samples of up to1 fb − and 4 fb − collected at HERA and at the Teva-tron, respectively. No significant evidence for physicsbeyond the Standard Model has been found and limitsat the 95% confidence level have been set on the massand couplings of several possible new particles.
1. Introduction
For more than a century, high-energy collisions of par-ticles have been a golden method of investigating thestructure of matter. Together with precision studies ofheavy meson decays, primarily at lower energy colliders,these experiments have lead to the consolidation of theStandard Model (SM) of strong, weak and electroweakinteractions. Although remarkably confirmed by presentexperimental results, the SM remains unsatisfactory andincomplete. Many questions are unexplained by the SM.For example, the SM does not explain the quantisationof the electromagnetic charge, or the observed replica-tion of the three fermion families. It explains neitherthe origin of fermion masses nor the observed hierarchybetween them. No candidate for the dark matter existsin the SM. Many models of new physics have been pro-posed to address these issues, the most popular amongthem being the supersymmetry.The corrections to the Higgs mass indicate that newphysics may be at an energy scale of the order of 1 TeV. Itwould therefore be possible to see effects beyond the SMat present high-energy colliders. They indeed providehigh sensitivity to new phenomena, allowing new massiveparticles to be directly produced or the effect of newinteractions interfering with SM processes to be studied.At HERA (Hadron Electron Ring Anlage) electrons(or positrons) collide with protons at a centre-of-massenergy of √ s ≃
320 GeV. During the two running periodsof HERA from 1994 to 2000 and from 2003 to 2007,respectively, the H1 and ZEUS experiments have eachrecorded ∼ . − of data in total, shared between e + p and e − p collision modes. These high energy electron-proton interactions provide a testing ground for the SM,complementary to e + e − and p ¯ p scattering studied at theLEP and at the Tevatron, respectively.The Tevatron proton-antiproton collider delivers datato the CDF and DØ experiments with a collision energyin the centre-of-mass of 1 .
96 TeV. Since the start of itssecond running phase, a total integrated luminosity of ∼ − has been collected by each experiment and upto ∼ − are presently used to search for new phe-nomena. Before the start of the Large Hadron Collider(LHC), the Tevatron is the only high-energy collider tak-ing data, with steadily increasing integrated luminosi-ties.
2. Model-Driven Searches
The observed replication of three fermion families mo-tivates the possibility of a new scale of matter yet unob-served. In the line of historical scattering experiments whichlead to the successive discoveries of the different sub-structures of matter, the nucleus, nucleons and quarks,scattering of point-like electrons on quarks at HERA canbe used to search for possible substructure of quarks.A finite charge radius for the quark would modify the dσ/dQ neutral current (NC) deep-inelastic scattering(DIS) cross section of the electron on a quark with aform factor type term (1 − R q / Q ) , where R q isthe root-mean-square radius of the electroweak chargeof the quark. At the highest Q values, no deviationsbetween the data and the SM prediction, derived fromthe DGLAP evolution of parton density functions de-termined at lower Q , was observed by the H1 andZEUS experiments, using their full data set [1, 2]. Themost stringent constraint on the the quark radius of R q < .
63 10 − m is derived by the ZEUS experi-ment [2].An unambiguous signature for a new scale of mat-ter would be the direct observation of excited states offermions ( f ∗ ), via their decay into a gauge boson and afermion. Effective models describing the interaction ofexcited fermions with standard matter have been pro-posed [3, 4, 5]. In the models [3, 4] the interaction ofan f ∗ with a gauge boson is described by a magneticcoupling proportional to 1 / Λ where Λ is a new scale.Proportionality constants f , f ′ and f s result in differentcouplings to U (1), SU (2) and SU (3) gauge bosons.The H1 experiment has carried out searches for ex-cited neutrinos ( ν ∗ ), electrons ( e ∗ ) and quarks ( q ∗ ) us-ing its full data set [6, 7, 8]. The total luminosity anal-ysed amount to up to 475 pb − . The new bounds onthe ν ∗ and e ∗ masses obtained as a function of f / Λ arepresented in Figure 1.(a) and (b), under the assump-tions f = − f ′ and f = + f ′ , respectively. Assuming f / Λ = 1 /M ν ∗ and f = − f ′ , masses below 213 GeVare ruled out for ν ∗ . Excited electrons of mass below272 GeV are excluded if we assume f / Λ = 1 /M e ∗ and f = + f ′ . Searches for q ∗ performed at HERA are com-plementary to Tevatron results, since at a p ¯ p colliderexcited quarks are dominantly produced in a quark-gluon fusion mechanism, which requires f s = 0. For f / Λ = 1 /M q ∗ and f = f ′ and f s = 0 excited quarkswith a mass below 252 GeV are excluded by H1. Asobserved in Figure 1., the H1 analysis has probed newparameter space regions, and the limits set extend pre-vious bounds reached at LEP and Tevatron colliders. An intriguing characteristic of the Standard Model isthe observed symmetry between the lepton and quarksectors. New symmetries connecting the two sectorsare therefore introduced in various unifying theories be-yond the SM, leading to the appearance of leptoquarks(LQs). LQs are new scalar or vector color-triplet bosons,carrying a fractional electromagnetic charge and both abaryon and a lepton number. Several types of LQs mightexist, differing in their quantum numbers. A classifica-tion of LQs has been proposed by Buchm¨uller, R¨uckl andWyler (BRW) [10] under the assumption that LQs have c (cid:13) * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 320 ] - [ G e V L f / -3 -2 -1 * Mass [GeV] n
100 120 140 160 180 200 220 240 260 280 300 320 ] - [ G e V L f / -3 -2 -1 ) -1 p, 184 pb - * at HERA (e n Search for f = - f’ L3 H1 = 1 / M L f / * n H1 (a) e* Mass [GeV]
100 120 140 160 180 200 220 240 260 280 300 ] - [ G e V L f / -4 -3 -2 -1 e* Mass [GeV]
100 120 140 160 180 200 220 240 260 280 300 ] - [ G e V L f / -4 -3 -2 -1 e* Mass [GeV]
100 120 140 160 180 200 220 240 260 280 300 ] - [ G e V L f / -4 -3 -2 -1 ) -1 Search for e* at HERA (475 pb f = + f’ H1 LEP (direct) LEP (indirect)H1) -1 Tevatron (1 fb = 1 / M L f / e* (b) q* Mass [GeV]
100 150 200 250 300 350 ] - [ G e V L f / -4 -3 -2 -1 q* Mass [GeV]
100 150 200 250 300 350 ] - [ G e V L f / -4 -3 -2 -1 q* Mass [GeV]
100 150 200 250 300 350 ] - [ G e V L f / -4 -3 -2 -1 ) -1 Search for q* at HERA (475 pb = 0 s f = f’, f H1 DELPHI )=1 g q fi * BR(q H1 = 1 / M L f / q* (c) Fig. 1.
Exclusion limits on the coupling f/ Λ as a function ofthe mass of the excited neutrino (a), electron (b) and quarks(c). The new limits set by H1 are represented by the shadedarea. Values of the couplings above the curves are excluded. pure chiral couplings to SM fermions of a given family.The interaction of the LQ with a lepton-quark pair is ofYukawa or vector nature and is parametrised by a cou-pling λ . Leptoquarks decay into a quark and a chargedor neutral lepton, with branching fractions β and (1 − β ),respectively.
200 250 300 350 400 -2 -1 / GeV LQ M l d) n u, - (e S H1 prelim. single LQH1 (94-00) single LQD0 pair prod.L3 indir. limit
200 250 300 350 400 -2 -1 E x c l u d e d ) -1 Leptoquark Search, HERA I+II (449 pb (a) (GeV) LQ M
160 180 200 220 240 260 280 300 320 e q ) fi = BR ( L Q b DØ 1fb Scalar leptoquark ee jj jj n e jj nn - D Ø pb (b) Fig. 2. (a) Exclusion limit set by H1 on the coupling λ as afunction of the mass for S ,L leptoquarks.(b) Limits on themass of first generation scalar LQs as a function of β obtainedby DØ [11]. The observed limit is represented by the full lineand the expected limit by the dot-dashed line. In ep collisions at HERA, first generation LQs mightbe singly produced from the fusion of the incoming lep-ton with a quark from the proton. LQs might thereforebe observed as a resonant peak in the lepton-jet massdistribution of neutral or charge current DIS events. Nosuch signal has been observed by the H1 experiment ina search using up to 449 pb − of data and limits on theLQ mass depending on the λ coupling have been set [9](see figure 2.(a)).At the Tevatron, LQs might be pair produced in q ¯ q interactions via their coupling to gluons. Depending ontheir decay, the possible final states are ℓ + ℓ − q ¯ q , ℓνq ¯ q or ν ¯ νq ¯ q with respective branching fractions β , 2 β (1 − β ) or(1 − β ) . Searches have been performed by the DØ Col-laboration for LQs of all three generations. For the firstgeneration, the search was performed in the eejj and eνjj channels and LQs with a mass below 299 GeV havebeen excluded for β = 1 [11]. The limit on the LQ massas a function of β is shown in figure 2.(b). Lowest valuesof β are probed in a search for two acoplanar jets and /E T [12]. Leptoquarks decaying with a large branchingratio in νq are not easily probed at the Tevatron due tolarge background. In such cases, if λ is reasonably large,HERA experiments can still provide a better sensitivity.A search for second generation LQs has also been per-formed by DØ in the µµjj and µνjj channels [13]. Themass limit obtained for β = 1 is 316 GeV. Third gen-eration LQs were searched for in the τ τ bb channel, withone τ decaying to a muon and the other to hadrons. The mass limit obtained for β = 1 is 210 GeV [14]. Third gen-eration LQs are also searched via their decay into ν ¯ νb ¯ b in acoplanar jet topologies, using heavy flavour tagging.A mass limit of 252 GeV for β = 0 is reached in this caseby DØ using 4 fb − of data [15]. Supersymmetry (SUSY) is one of the most attrac-tive extension of the SM. It allows for the unificationof the four known forces at the GUT scale, it is theonly non-trivial extension of the Lorentz-Poincar´e groupand provides a possible candidate for the dark matter,the lightest supersymmetric particle (LSP). In SUSY,every SM particle has a superpartner differing in spinby 1 /
2. The SUSY partners are assigned an R -parity R P = ( − (3 B + L +2 S ) = −
1, in contrast to the SM par-ticles of R P = +1. In most cases R -parity is assumedto be conserved and sparticles are therefore produced inpair and decay to SM particles and to the LSP. Sincethe LSP is stable and weakly interacting, the final stateexhibits missing energy.Since superpartners have not been yet observed, SUSYcannot be an exact symmetry and different breakingmechanisms have been proposed. Under certain assump-tions they allow to reduce the large number ( > m , a com-mon gaugino mass m / , a common trilinear coupling A , the ratio β of the vacuum expectation values of thetwo Higgs doublets and the sign of µ , the supersymmet-ric mass term. Searches for squarks and gluinos
At the Tevatron, squarks and gluinos are expected tobe copiously produced via the strong interaction.Due to the large production cross section, the inclu-sive production of squarks and gluinos is one of the mostpromising discovery channels for SUSY at the Tevatron.The cascade decays of the produced squark and gluinogive rise to final states with two to four jets and /E T ,depending whether the squark mass is smaller, equal orgreater than the gluino mass. The CDF and DØ Col-laborations have performed searches optimised for thethree cases and using ∼ − of data [16, 17]. Asno particular excess is observed, limits are set in thetwo-dimensional plane ( m ˜ q , m ˜ g ) in the context of themSUGRA model (see figure 3.). Gluinos with a massbelow 308 GeV are now excluded for all squark massesand squarks lighter than 380 GeV are excluded for allgluinos massesIn the case of the third generation squarks, light masseigenstates ˜ t and ˜ b are possible, in case of a largemixing between the right-handed and left-handed weakeigenstates. Sbottom and stop may be produced in pairvia q ¯ q annihilation or gluon-gluon fusion and the pro-duction cross section mainly depends on their mass.They subsequently decay into a quark and the LSP, p ¯ p → ˜ t ¯˜ t → c ˜ χ ¯ c ˜ χ and p ¯ p → ˜ b ¯˜ b → b ˜ χ ¯ b ˜ χ , lead-ing to final states with two acoplanar charm- or b -jetswith high /E T . In a recent stop search in this channel,the CDF Collaboration exploits charm tagging of jets toreduce the background from b -jets [18]. The sensitivityachieved with 2 . − of data and the absence of signalallows to exclude ˜ t masses up to 180 GeV. The exclu- Gluino Mass (GeV) S qu a r k M ass ( G e V ) -1 DØ, L=2.1 fb <0 m =0, =3, A b tan UA UA LEP CD F I BD Ø I A DØ IB
DØ II no mSUGRAsolution – c~ LEP2 – l~ LEP2
Fig. 3.
Limits on the squark and gluino masses obtained byDØ [17] and by earlier experiments. The thick line corre-sponds to the limit obtained with nominal choices of renor-malisation and factorisation scales and of parton distributionfunctions. The band delimited by the two dashed lines repre-sents the uncertainty associated to different choices of scalesand of PDFs. sion limit obtained on the stop mass depending on theneutralino mass is presented in figure 4.(a). It extendsbeyond the LEP reach. No signal of sbottom produc-tion was also observed [15, 19]. The exclusion area forthe sbottom obtained by the DØ Collaboration is shownin figure 4.(b). It is the most sensitive such analysis todate.
Searches for charginos and neutralinos
Chargino and neutralino production in p ¯ p collisions ismediated by electroweak interactions and has thereforea low production cross section. They are best searchedat the Tevatron in the associated production channel, q ¯ q → W ∗ → ˜ χ ± ˜ χ → ℓℓℓν ˜ χ ˜ χ . It leads to a veryclean experimental signature with three leptons, of littleenergy, and /E T , in a low background environment. Be-cause of the small leptonic branching fractions, severalfinal states need to be combined. Both CDF and DØexperiments found no evidence for excess of events withthis signature and set limits on the mSUGRA parame-ters m and m / , extending the parameter space probedby LEP experiments (see figure 5.) [20, 21]. Gauge Mediated SUSY Breaking
In the gauge mediated SUSY breaking (GMSB) sce-nario, the gravitino is the LSP and the next-to-lightestSUSY particle (NLSP) may be the lightest neutralino,which decays to a gravitino and a photon, ˜ χ → γ ˜ G . Atthe Tevatron, pair production of SUSY particles decay-ing to a neutralino NLSP with negligible life time wouldtherefore lead to final states with two acoplanar photonsand /E T . Searches for such events have been performed bythe CDF and DØ Collaborations but no excess was ob-served over the backgrounds coming mainly from photonmisidentification of from fake /E T [25, 26]. The processis then used to set a limit on the GMSB scale Λ, whichalso gives the scale of the gaugino masses. For a ˜ χ life-time of 0 ns, ˜ χ masses below 149 GeV are excluded bythe analysis of the CDF experiment, based on 2 . − of data [25]. The domain in the plane of the neutralinomass and lifetime excluded by this analysis is comparedto previous existing limits in the figure 6.. ] Stop Mass [GeV/c
60 80 100 120 140 160 180 ] N eu t r a li no M a ss [ G e V / c ] Stop Mass [GeV/c
60 80 100 120 140 160 180 ] N eu t r a li no M a ss [ G e V / c Observed Limit (95% CL)) s – Expected Limit ( ] Stop Mass [GeV/c
60 80 100 120 140 160 180 ] N eu t r a li no M a ss [ G e V / c c + m c~ = m t ~ m c~ + m b + m W = m t ~ m o = 56 q LEP o = 0 q LEP -1 CDF 295 pb -1 DØ 995 pb
CDF Run II Preliminary -1 L dt=2.6 fb (cid:242) (a)
Sbottom Mass (GeV)0 50 100 150 200 250 N e u t r a li no M ass ( G e V ) N e u t r a li no M ass ( G e V ) N e u t r a li no M ass ( G e V ) ObservedExpected ) -1 D0 Run II Preliminary (4 fb
D0Run I -1
92 pb CDFRun I -1
88 pb CDFRun IIa -1
295 pb D0Run IIa -1
310 pbLEP =208 GeVs ) c ) = M ( b ) + M ( b ~ M ( Sbottom Mass (GeV)0 50 100 150 200 250 N e u t r a li no M ass ( G e V ) (b) Fig. 4.
Exclusion limits on the stop (a) and sbottom (b) massesas a function of the neutralino mass obtained by CDF andDØ, respectively. ) (GeV/c m20 40 60 80 100 120 140 160 180 ) ( G e V / c / m ) ) ( G e V / c – c~ M ( Observed LimitExpected LimitLEP direct limit ) > 0 m =0, ( =3, A b mSugra tan ) – c ~ m( » ) c ~ m( ) t ~ ) > m( R m ~ ), m( R e ~ m( -1 CDF Run II Preliminary, 3.2 fb ) (GeV/c m ) c ~ ) < m ( t ~ m ( ) c ~ ) > m ( t ~ m ( (a) (GeV) m ( G e V ) / m (GeV) m ( G e V ) / m DØ observed limitDØ expected limitCDF observed) -1 limit (2.0 fb LEP Chargino LimitLEPSleptonLimit -1 DØ, 2.3 fb mSUGRA > 0 m = 0, = 3, A b tan c~ – c~ Search for ) c~ M ( » ) l ~ M ( ) – c~ M ( » ) n~ M ( (GeV) m ( G e V ) / m (b) Fig. 5.
Regions in the ( m , m / ) plane excluded by the CDF(a) and DØ (b) experiments in the trilepton search for elec-troweak gauginos. Fig. 6.
Excluded domain in the ˜ χ mass and lifetime plane de-rived from the search for GMSB neutralino NLSP performedby CDF. R -parity violation In searches discussed so far, R -parity was assumed tobe conserved. If R -parity violation (RPV) is allowed, ˜ ν τ sneutrinos might be singly produced at the Tevatron viaa non-zero λ ′ Yukawa coupling to d -quarks, followedby decays into two eµ , eτ or µτ leptons. This wouldproduce very clean signatures with a two-lepton reso-nance on a low SM background environment. The eµ channel has been investigated by the DØ Collaborationusing 4 . − of data [22] and, in the absence of a sig-nal, limits on the mass of ˜ ν τ as a function of the Yukawacouplings λ ′ and λ have been set (see figure 7.). Asimilar search has been done by the CDF Collaboration,looking also at the µτ and eτ channels, but no deviationfrom the SM expectation was observed [23]. (GeV) t n~ M
100 150 200 250 300 ’ l % C L =0.005 l =0.01 l =0.02 l =0.07 l preliminary -1 DØ, 4.1 fb
Fig. 7.
Limits on RPV couplings as a function of the sneutrinomass obtained by DØ. If R -parity is not conserved, squarks could also be res-onantly produced at HERA, similarly to leptoquarks.In addition to the “LQ-like” decays into eq and νq thesquarks also undergo cascade decays via gauginos, lead-ing to a large number of possible final states. A re-cent search for first and second generation squarks hasbeen done by the H1 Collaboration using its full datasample with an integrated luminosity of 438 pb − [24].Almost all possible final states have been searched for,taking into account direct and indirect R -parity violat-ing decay modes. In the absence of a signal, mass de-pendent limits on the RPV couplings λ ′ jk have beenderived within a phenomenological version of the min-imal supersymmetric model. An example of limits ob-tained for d -type squarks is shown in figure 8.. For aYukawa coupling λ ′ j ( λ ′ k ) of electromagnetic strength, λ ′ j = √ πα em ≃ . u -type ( d -type) squarks up tomasses ∼
275 GeV ( ∼
290 GeV) are excluded. [GeV]
Squark M
100 120 140 160 180 200 220 240 260 280 −3 −2 −1 [GeV] Squark M
100 120 140 160 180 200 220 240 260 280 −3 −2 −1 ’ l ( k = , ) ) n bb ( ’ l (CCU) ’ l [GeV] Squark M
100 120 140 160 180 200 220 240 260 280 −3 −2 −1 Unconstrained MSSM ) −1 p 183 pb − SUSY at HERA (e p RSearch for Squarks in E x c l u d e d a t % C L E xc l ud e d i n p a r t o f p a r a m e t e r s p ace = 2 b tan < 300 GeV m −300 < < 350 GeV
70 < M =90 GeV slepton
M > 30 GeV imposed
LSP M H1 Preliminary [GeV]
Squark M
100 120 140 160 180 200 220 240 260 280 −3 −2 −1 [GeV] Squark M
100 120 140 160 180 200 220 240 260 280 −3 −2 −1 [GeV] Squark M
100 120 140 160 180 200 220 240 260 280 −3 −2 −1 [GeV] Squark M
100 120 140 160 180 200 220 240 260 280 −3 −2 −1 Fig. 8.
Exclusion limits on RPV d -type squark production ob-tained by the H1 experiment. A scan of the SUSY parameterspace has been performed. The dark shaded domain is ruledout for any value of the parameters. Models based on large extra-dimensions (LED), suchas the model of Arkani-Hamed, Dimopolous and Dvali(ADD) [27] try to adress the hierarchy problem betweenthe Planck and the electroweak scales. In these mod-els, gravity is allowed to propagate in a 4 + n D dimen-sional bulk space-time, while the remainder of the SMfields are confined in the 4 d world volume. The extra n dimensions are compactified with a radius R . In thesimplest model, the fundamental Planck scale M D inthe 4 + n D space is then related to the Planck scaleby M P l = 8 πR n D M n D +2 D . If the size of the extra-dimensions is small, M D could be of the order of 1 TeVand its effects could be visible at present colliders. Sinceit propagates in the extra-dimension, the graviton ob-served in 4 d manifests itself as a tower of Kaluza-Klein(KK) excitations which form a quasi-continuum in mass.In p ¯ p collisions, such KK gravitons can be directlyproduced together with a quark, a gluon or a photonand escape into the bulk space, leading to large miss-ing transverse energy. The corresponding signatures arethen a mono-jet, or a mono-photon, with /E T . Using2 fb − of data, CDF has recently published an analy-sis combining searches for mono-jet and mono-photontopologies and set limits on the effective Plank scale be-tween M D > . M D > .
94 TeV for a numberof extra dimension ranging from 2 to 6 [28]. A simi-lar search for mono-photon events was performed by theDØ Collaboration in 2 . − of data, leading to lim-its on M D of 0 .
97 TeV and 0 .
831 TeV for n D = 2 and n D = 6, respectively [29].An alternative way to search for large extra-dimensions is to look for the effect of graviton exchangeand its interference with SM processes. An effective cou-pling of the form λ/M S , with a parameter λ which is ex-pected to be close to one, needs to be introduced to per-form calculations. Therefore, such effects due to gravitonexchanges allows to probe an effective scale M S , but notthe fundamental scale M D . Nevertheless, this effectivescale M S should not differ too much from M D . At theTevatron, such indirect effects may induce an enhance-ment of the production cross section of fermion or bosonpairs. This was searched for in the ee and γγ final statesby the DØ experiment [30]. Combining both analysesthe constraint M S > .
62 TeV is obtained, using theGRW formalism [31]. A recent analysis from the DØ ex-periment uses high P T di-jet events and the jet angulardistributions, via the variable χ = exp | y − y | , where y and y are the rapidities of the two jets, to constraint the existence of extra-dimensions [32]. A lower limit of M S > .
66 TeV is obtained, which is the most stringentbound so far.At HERA, after summing the effects of graviton ex-citations in the extra-dimensions, the graviton contribu-tion could be visible as a contact interaction contributingto the eq → eq scattering. The effect could therefore bevisible in the NC DIS cross section, introducing termsof the form η G = ± /M S in the Lagrangian. This wassearched for by the ZEUS experiment using its full datasample [2]. A good agreement of the measured cross sec-tion with the SM expectation is observed, allowing to seta lower limit of M S > .
94 TeV.
3. Model-Independent Searches
A large variety of possible extensions to the SM ex-ists, which often predict similar experimental signatures.Searches for new physics presented above compare datato the predictions of these specific models. A comple-mentary approach which now tends to develop is fol-lowed in signature based searches. In this case differ-ences between data and SM expectations are looked forin various event topologies. For a given final state, in theabsence of deviations, limits can then be set on differentexotic models. As another advantage, such model inde-pendent analyses do not rely on any a priori definitionof expected signatures for exotic phenomena. Therefore,they also address the important question of whether un-expected phenomena might occur through a new pat-tern, not predicted by existing models. Following thisapproach, signals for new phenomena can be searchedfor through resonances in invariant mass distributions oftwo SM particles, in final states corresponding to rareSM processes or in topologies containing a certain typeof particle, as e.g. a photon. More generally, a scan athigh transverse momenta of all possible final states canalso be performed.
At the Tevatron, resonances from new particles havebeen searched for in invariant mass distributions of var-ious di-particles combinations. A peak in the di-leptonmass distribution could for instance sign the presenceof an extra Z ′ boson. Recent measurements of the di-electron [33] and di-muon [34] invariant mass distribu-tions performed by the DØ and CDF Collaborations arepresented in figure 9.. An excess in the e + e − invariantmass distribution at ∼
240 GeV was reported by theCDF Collaboration with a significance of 2 . σ [35]. Nosuch excess is observed in the recent analysis from DØwhich uses a larger data sample of 3 . − [33]. A goodagreement between data and the SM expectation is alsoseen in the di-muon mass spectrum measured by CDF. Itis presented in terms of the variable 1 /m µµ to better fitto the resolution of the trackers which is Gaussian in in-verse momentum. Assuming a Z ′ boson having the samecouplings to SM fermions as the Z boson, a lower limit of1 .
03 TeV was set on the Z ′ mass. From the di-electronmass spectrum, Z ′ bosons masses below 950 GeV areexcluded by DØ.The invariant mass distribution of di-jets has also beeninvestigated by the CDF Collaboration [36]. No devia-tion from NLO pQCD predictions was observed, allowingto exclude, for instance, the presence of W ′ or Z ′ bosonswith masses below 840 and 740 GeV, respectively.A search for resonances decaying into a pair of gaugebosons, W + W − or W ± Z , has been recently performedby the CDF experiment in final states where one W bo-son decays into an electron and the other boson into two (GeV) ee M
200 400 600 800 1000 E ve n t s / G e V dataDrell-YanInstrumentalOther SM D0 Run II Preliminary, 3.6fb (a) ) -1 (TeV -1 mm m0 5 10 - E ve n t s / ( . T e V ) -3
10 1 DataTotal backgroundDrell-YanHadron fakesCosmic raysWWtt (b)
Fig. 9.
Invariant mass of electron pairs in DØ data compared tothe SM expectation (a). Distribution of the di-muon inversemass in CDF data (b). jets. Invariant mass distributions of the reconstructedboson pair are presented in figure 10., when the recon-structed invariant mass of the two jets is constrained tobe around the nominal W or Z boson masses, respec-tively. No significant excess over the SM prediction isobserved. WW Invariant Mass (GeV)0 100 200 300 400 500 600 700 800 900 1000 E ve n t s / G e V b i n xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Data σ ± Bkgd xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Z’ signal xxxxxxxx
W+JetsttWWQCDOthers -1 CDF run II Preliminary 2.9fbfor 400-700GeV Z’Et cuts optimized 600GeV Z’ (a)
WZ Invariant Mass (GeV)0 100 200 300 400 500 600 700 800 900 1000 E ve n t s / G e V b i n xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Data σ ± Bkgd xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
W’ signal xxxxxxxx
W+JetsttWWQCDOthers -1 CDF run II Preliminary 2.9fbfor 400-700GeV W’Et cuts optimized 600GeV W’ (b)
Fig. 10.
Di-boson invariant mass distributions for
W W (a) and
W Z (b) resonance searches, respectively, measured by CDFand compared to SM background expectations. Expected W ′ and Z ′ signals with a mass of 600 GeV are added on top ofthe background in (a) and (b), respectively. The production of W bosons in ep collisions at HERAhas a cross-section of about 1 pb. The leptonic decay ofthe W leads to events with an isolated high transversemomentum lepton (electron, muon or tau) and missingtotal transverse momentum. Of particular interest areevents with a hadronic system of large transverse mo-mentum ( P XT ). A larger than expected rate of high P XT events is observed by the H1 experiment [38] in the elec-tron and muon channels. In the analysis of all availabledata, which amounts to a total luminosity of 478 pb − ,18 events are observed at P XT >
25 GeV for a SM ex-pectation of 13 . ± .
2. Amongst them only 1 event isobserved in e − p collisions, compared to a SM expecta-tion of 5 . ± .
91, while 17 events are observed in the e + p data for an expectation of 8 . ± . [GeV] XT P E ve n t s -1 [GeV] XT P E ve n t s -1 SMSM Signal ) -1 H1+ZEUS (0.59 fb p + e Preliminary [GeV] XT P E ve n t s -1 (a) [GeV] XT P E ve n t s -1 [GeV] XT P E ve n t s -1 SMSM Signal ) -1 H1+ZEUS (0.39 fb p - e Preliminary [GeV] XT P E ve n t s -1 (b) Fig. 11.
Hadronic transverse momentum distribution of isolatedlepton events observed by H1 and ZEUS in e + p (a) and e − p (b) data samples. The total SM expectation is represented bythe open histograms and the contribution from W productionby the hatched histograms. common phase space [39]. The combined data set corre-sponds to a total integrated luminosity of 0 .
98 fb − . Atotal of 81 events containing an isolated electron or muonand missing transverse momentum are observed in thedata, compared to a SM expectation of 87 . ± .
5. At P XT >
25 GeV, a total of 29 events are observed com-pared to a SM prediction of 24 . ± .
2. In this kinematicregion, 23 events are observed in the e + p data comparedto a SM prediction of 14 . ± .
9. Seventeen of the 23 dataevents are observed in H1 data. The observations in the e + p and e − p data sets are presented in figure 11. wherethe P XT distributions of both data sets are displayed. The main production mechanism for multi-leptonevents at HERA is photon-photon collisions. All eventtopologies with high transverse momentum electrons andmuons have been investigated by the H1 and ZEUS ex-periments [40] using a total luminosity of 0 .
94 fb − . Themeasured yields of di-lepton and tri-lepton events are ingood agreement with the SM prediction, except in thetail of the distribution of the scalar sum of transversemomenta of the leptons, P P T (see figure 12.). In e + p collisions, 7 data events with at least two high P T leptonsare observed with P P T >
100 GeV compared to a SMprediction of 1 . ± .
17, corresponding to a probabilityof 0 . e − p collisionsfor a similar SM expectation of 1 . ± . The CDF experiment has developed a series of model-independent signature based searches, looking for de-viations from the SM in various final state topologiescontaining a photon. Such analyses are based on theability to identify and define a priori objects which canbe charged leptons, jets or heavy flavour quarks, elec- [GeV] T P S E ve n t s -2 -1 [GeV] T P S E ve n t s -2 -1 SMSM Pair Prod. ) -1 H1+ZEUS (0.56 fb p+e
Multi-Leptons at HERA [GeV] T P S E ve n t s -2 -1 [GeV] T P S E ve n t s -2 -1 SMSM Pair Prod. ) -1 H1+ZEUS (0.38 fb e p - [GeV] T P S E ve n t s -2 -1 [GeV] T P S E ve n t s -2 -1 SMSM Pair Prod. ) -1 H1+ZEUS (0.94 fb p – e (a)(b)(c) Fig. 12.
Distribution of the scalar sum of the lepton transversemomenta for multi-lepton events recorded by H1 and ZEUScompared to SM expectations, for e + p (a), e − p (b) and e ± p (b) collisions. troweak gauge bosons, or non-interacting particles iden-tified by /E T , independently of the considered final state.Different event topologies with two photons and athird object ( e , µ , τ , γ or /E T ) have been investigatedby CDF in a data sample of 2 fb − of integrated lu-minosity [41]. No deviations from the SM expectationswere observed in these final states.Recently, γbj/E T [42] and ℓbj/E T [43] topologies havealso been investigated by CDF. Numbers of data eventsobserved in each channel were found to be in good agree-ment with SM expectations. The extension of signature based searches are globalmodel independent searches, extending to all possibletopologies at high P T . In this approach all events areclassified into exclusive event classes according to thenumber and types of objects detected in the final state.A comparison is then performed between the numbersof events in each class and the expectations from all SMprocesses.Such a broad range signature based search has beenpioneered by the H1 Collaboration using data from thefirst running phase of HERA, which were mainly from e + p collisions [44]. It has been recently updated to thefull data set of H1, which amount to an integrated lumi-nosity of 463 pb − and includes e − p collision data [45].All final states containing at least two objects ( e , µ , j , γ , ν ) with P T >
20 GeV in the polar angle range10 ◦ < θ < ◦ are investigated. The observed andpredicted event yields in each channel are presented infigure 13.(a) and (b) for e + p and e − p collisions, respec-tively. The good agreement observed between data andSM prediction demonstrates the good understanding ofthe detector and of the contributions of the SM back-grounds. j-je-j-j m -j n n e-e-e m e- m - m -j g -e g m - g n - g g - g j-j-je-j-j-j-j m -j-j n e-e-j n e-e-e-e-e-j m - m m - m e- n - m - m -j m e- -j n e- -j n - m n - m e- -j-j g -e-j g -j n - g e-j-j-j-j-j-j n -j-j n - g -j-j n e- -j-j-e g -j n e-e- -j n - m e-j-j-j-je-j-j-j-j-j-j-j-j n j-j-j-j-jj-je-j-j m -j n n e-e-e m e- m - m -j g -e g m - g n - g g - g j-j-je-j-j-j-j m -j-j n e-e-j n e-e-e-e-e-j m - m m - m e- n - m - m -j m e- -j n e- -j n - m n - m e- -j-j g -e-j g -j n - g e-j-j-j-j-j-j n -j-j n - g -j-j n e- -j-j-e g -j n e-e- -j n - m e-j-j-j-je-j-j-j-j-j-j-j-j n j-j-j-j-jj-je-j-j m -j n n e-e-e m e- m - m -j g -e g m - g n - g g - g j-j-je-j-j-j-j m -j-j n e-e-j n e-e-e-e-e-j m - m m - m e- n - m - m -j m e- -j n e- -j n - m n - m e- -j-j g -e-j g -j n - g e-j-j-j-j-j-j n -j-j n - g -j-j n e- -j-j-e g -j n e-e- -j n - m e-j-j-j-je-j-j-j-j-j-j-j-j n j-j-j-j-j SMH1 Data -2 -1
10 1 10
10 ) -1 p, 285 pb + H1 General Search at HERA (e
Events H1 (a) j-je-j-j m -j n n e-e-e m e- m - m -j g -e g m - g n - g g - g j-j-je-j-j-j-j m -j-j n e-e-j n e-e-e-e-e-j m - m m - m e- n - m - m -j m e- -j n e- -j n - m n - m e- -j-j g -e-j g -j n - g e-j-j-j-j-j-j n -j-j n - g -j-j n e- -j-j-e g -j n e-e- -j n - m e-j-j-j-je-j-j-j-j-j-j-j-j n j-j-j-j-jj-je-j-j m -j n n e-e-e m e- m - m -j g -e g m - g n - g g - g j-j-je-j-j-j-j m -j-j n e-e-j n e-e-e-e-e-j m - m m - m e- n - m - m -j m e- -j n e- -j n - m n - m e- -j-j g -e-j g -j n - g e-j-j-j-j-j-j n -j-j n - g -j-j n e- -j-j-e g -j n e-e- -j n - m e-j-j-j-je-j-j-j-j-j-j-j-j n j-j-j-j-jj-je-j-j m -j n n e-e-e m e- m - m -j g -e g m - g n - g g - g j-j-je-j-j-j-j m -j-j n e-e-j n e-e-e-e-e-j m - m m - m e- n - m - m -j m e- -j n e- -j n - m n - m e- -j-j g -e-j g -j n - g e-j-j-j-j-j-j n -j-j n - g -j-j n e- -j-j-e g -j n e-e- -j n - m e-j-j-j-je-j-j-j-j-j-j-j-j n j-j-j-j-j SMH1 Data -2 -1
10 1 10
10 ) -1 p, 178 pb - H1 General Search at HERA (e
Events H1 (b) Fig. 13.
The data and the SM expectation in event classes in-vestigated by the H1 general search. Only channels with ob-served data events or a SM expectation greater than one eventare displayed. The results are presented separately for e + p (a) and e − p (b) collision modes. In each channel, a systematic scan of distributions ofthe invariant mass, M all , and of the scalar sum of trans- verse momenta, P P T of all identified objects has beenperformed to look for regions of largest deviations to theSM. A statistical analysis is then used to quantify thesignificance of the observed deviations. The largest de-viation is found in e + p data, in the e - e channel whichcorresponds to the topology of multi-lepton events. Itcorresponds to a probability of 0.0035. The probabil-ity to observe a SM fluctuation with that significance orhigher for at least one event class is 12%. In addition,the final state topologies are also evaluated in terms ofangular distributions and energy sharing between finalstate particles, with a good agreement observed betweendata and the SM expectation.A similar approach is followed at the Tevatron by theCDF and DØ experiments. The CDF Collaborationconsidered 399 final states containing electrons, muons,taus, photons, jets, b -jets and /E T in a data sample of2 fb − [46]. The shapes of 19650 distributions have beenstudied using a dedicated algorithm, called VISTA [47],and discrepancies between data and the SM expectationhave been found in 555 of them. The origin of these dis-crepancies is however attributed to an inadequate mod-eling of soft QCD effects in the simulation, rather thanto signs of new physics. In a second step, high P T tails ofthe distributions were investigated using the SLEUTH algorithm [47]. No excess beyond what is expected fromstatistics was observed. Using a similar framework basedon
VISTA and
SLEUTH algorithms, the DØ exper-iment investigated 180 final states containing at leastone lepton, using ∼ − of data [48]. Only four out ofthe 180 final states present a statistically significant dis-crepancy between data and SM expectations. However,the observed deviations are attributed to difficulties inmodelling the SM background or the detector responsein the given final states and do not point to new physics. Despite intensive searches performed in data of highenergy colliders, no convincing sign of physics beyondthe SM has been found so far.Two years after the end of HERA running, the H1and ZEUS Collaborations are delivering their final re-sults based on an integrated luminosity of 1 fb − , com-bining their data in common analyses. The most signifi-cant deviation to the SM expectation observed in HERAdata concerns intriguing multi-lepton events in e + p data.At the Tevatron, many different models of new physicsare tested with ever increasing sensitivity. Complement-ing to these model-dependent searches, more generaltests of the SM validity at the highest energies are alsobeing developed through signature based searches. But,so far, no discoveries are to be reported and stringentconstraints on models are set.Most likely, new physics will remain hidden until theadvent of the LHC era. References [1] H1 Coll., H1prelim-07-141, August 2007.[2] ZEUS Coll., ZEUS-prel-09-013, July 2009.[3] K. Hagiwara, D. Zeppenfeld and S. Komamiya, Z.Phys. C (1985) 115.[4] F. Boudjema, A. Djouadi and J. L. Kneur, Z. Phys.C (1993) 425.[5] U. Baur, M. Spira and P. M. Zerwas, Phys. Rev. D (1990) 815.[6] F. D. Aaron et al. [H1 Collaboration], Phys. Lett.B (2008) 382 [arXiv:0802.1858].[7] F. D. Aaron et al. [H1 Collaboration], Phys. Lett.B (2008) 131 [arXiv:0805.4530]. [8] F. D. Aaron et al. [H1 Collaboration], Phys. Lett.B (2009) 335 [arXiv:0904.3392].[9] H1 Coll., H1prelim-07-164, August 2007.[10] W. Buchm¨uller, R. R¨uckl and D. Wyler, Phys. Lett.B , (1987) 442; Err. ibid. B , (1999) 320.[11] V. Abazov [D0 Collaboration], Submitted to Phys.Lett. B, arXiv:0907.1048.[12] V. M. Abazov et al. [DØ Collaboration], Phys. Lett.B (2008) 357 [arXiv:0808.0446].[13] V. M. Abazov et al. [DØ Collaboration], Phys. Lett.B (2009) 224 [arXiv:0808.4023].[14] V. M. Abazov et al. [DØ Collaboration], Phys. Rev.Lett. (2008) 241802 [arXiv:0806.3527].[15] DØ Coll., Conference Note 5931-CONF, July 2009.[16] T. Aaltonen et al. [CDF Collaboration], Phys. Rev.Lett. (2009) 121801 [arXiv:0811.2512].[17] V. M. Abazov et al. [DØ Collaboration], Phys. Lett.B (2008) 449 [arXiv:0712.3805].[18] CDF Coll., CDF Note 9834, July 2009.[19] CDF Coll., CDF Note 9969, August 2009.[20] V. M. Abazov et al. [D0 Collaboration], Phys. Lett.B (2009) 34 [arXiv:0901.0646].[21] CDF Coll., CDF Note 9817, June 2009.[22] DØ Coll., Conference Note 5894-CONF, March2009.[23] CDF Coll., CDF Note 9930, July 2009.[24] H1 Coll., H1prelim-09-061, July 2009.[25] CDF Coll., CDF Note 9625, April 2009.[26] V. M. Abazov et al. [DØ Collaboration], Phys. Lett.B (2008) 856 [arXiv:0710.3946].[27] N. Arkani-Hamed, S. Dimopoulos and G. R. Dvali,Phys. Lett. B (1998) 263 [hep-ph/9803315].[28] T. Aaltonen et al. [CDF Collaboration], Phys. Rev.Lett. (2008) 181602 [arXiv:0807.3132].[29] DØ Coll., Conference Note 5729-CONF, July 2008.[30] V. M. Abazov et al. [DØ Collaboration], Phys. Rev.Lett. (2009) 051601 [arXiv:0809.2813][31] G. F. Giudice, R. Rattazzi and J. D. Wells, Nucl.Phys. B (1999) 3 [hep-ph/9811291].[32] V. M. Abazov et al. [DØ Collaboration],arXiv:0906.4819.[33] DØ Coll., Conference Note 5923-CONF, June 2009.[34] T. Aaltonen et al. [CDF Collaboration], Phys. Rev.Lett. (2009) 091805 [arXiv:0811.0053].[35] T. Aaltonen et al. [CDF Collaboration], Phys. Rev.Lett. (2009) 031801 [arXiv:0810.2059].[36] T. Aaltonen et al. [CDF Collaboration], Phys. Rev.D (2009) 112002 [arXiv:0812.4036].[37] CDF Coll., CDF Note 9730, April 2009.[38] F. D. Aaron et al. [H1 Collaboration], Accepted forpublication in Eur. Phys. J. C, arXiv:0901.0488.[39] H1 an ZEUS Coll., H1prelim-09-161, ZEUS-prel-09-014 ,July 2009.[40] F. D. Aaron et al. [H1 and ZEUS Collaborations],JHEP 10 (2009) 013 [arXiv:0907.3627].[41] CDF Coll., CDF Note, August 2007.[42] T. Aaltonen et al. [CDF Collaboration], Phys. Rev.D (2009) 052003 [arXiv:0905.0231].[43] T. Aaltonen et al. [CDF Collaboration], Phys. Rev.D (2009) 011102 [arXiv:0906.0518].[44] A. Aktas et al. [H1 Collaboration], Phys. Lett. B (2004) 14 [hep-ex/0408044][45] F. D. Aaron et al. [H1 Collaboration], Phys. Lett.B (2009) 257 [arXiv:0901.0507].[46] T. Aaltonen et al. [CDF Collaboration], Phys. Rev.D (2009) 011101 [arXiv:0809.3781].[47] T. Aaltonen et al. [CDF Collaboration], Phys. Rev.D78