Tau Leptons at HERAL. Lindfeld on behalf of the experiments H1 and ZEUS a ∗

aPhysics Institute, University of Zurich,Winterthurer Str. 190, CH-8057 Zurich, Switzerland

We review recent analyses at HERA dealing with a tau lepton in the final state of ep-collisions. At the ZEUS experiment, asearch for events containing isolated tau leptons and missing transverse momentum in addition to that due to the tau decay wasperformed. The analysis, based on isolated tracks from hadronic tau decays and the exploitation of the internal jet structure,yields 3 tau candidates where 0.40+0.12

−0.13 are expected from the Standard Model. With the H1 detector, the production of high-PT isolated tau leptons was studied using typical one-prong hadronic tau decays as identification. In addition, an analysis oftau lepton pairs in elastic photon-photon collisions was performed. Furthermore, a search for doubly charged Higgs bosonsincludes also like-sign tau lepton pairs in the final state. Finally, the tau lepton was studied by both experiments in searches fornew physics as a decay product of a resonantly produced leptoquark violating the conservation of lepton flavor.

1. Introduction

According to the Standard Model, the tau leptoncan only be rarely observed in the final state of ep-collions at HERA. Nevertheless, various theoreticalextensions of the Standard Model predict significantrates for final states with a tau lepton. This gives mo-tivation for recent searches with the experiments H1and ZEUS to include tau channels. In this note wereview these analyses with a focus on their particulartau channel.

2. The HERA Collider and the experiments H1and ZEUS

The Hadron-Electron Ring Accelerator (HERA),located at the DESY research site in Hamburg, Ger-many, is a unique storage ring of its kind, where elec-trons2 and protons are accelerated and stored at an en-ergy of 27.6 GeV and 920 GeV respectively. At twointeraction points, the experiments H1 and ZEUS de-tect the ep-collisions at a rate of 10.5 MHz and a cen-ter of mass energy of

√s = 320 GeV. A detailed de-

scription of the H1 and ZEUS detectors can be foundin [1,2].

∗c/o DESY, 1/222e, Notkestr. 85, 22607 Hamburg, Germany2HERA can run with either an electron or positron beam

3. Recent analyses with tau leptons

3.1. Isolated tau leptons and missing PT

The H1 and ZEUS collaborations have both pre-viously reported the observation of events with iso-lated high energy electrons and muons in events withsignificant missing transverse momentum, P miss

T , [3–6]. The dominant Standard Model (SM) contribu-tion to this topology is the production of real W ±

bosons with subsequent leptonic decay. Such eventscan also be a signature of new phenomena beyond theStandard Model, for example the production of sin-gle top quarks via Flavor Changing Neutral Currents(FCNC) [7,8]. Production of stop quarks in R-parityviolating (Rp) SUSY models [9] with subsequenttwo-body decay (e.g. τ → τb) or Rp-conservingthree-body decay modes (τ → τ ντ b, τντ b) are alsopotential sources.

3.1.1. ZEUSUsing an integrated luminosity of 130 pb−1, the

search for tau leptons is concentrated on hadronic taudecays. The narrow, “pencil-like” shape and the lowcharged-particle multiplicity of the tau jets were usedto distinguish them from quark- and gluon-inducedjets.

The internal jet structure is generally well de-scribed by the Monte Carlo (MC) simulations [10].Here, it was characterised by six observables, i.e. thefirst and second moments of both the longitudinal and

Nuclear Physics B (Proc. Suppl.) 144 (2005) 315–322

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radial extensions, the number of subjets and the in-variant mass of the jets. The inclusive CC DIS datasample is well described by the MC CC events and thedifference in the shapes between tau jets and quark- orgluon-induced jets is evident in all six variables.

To seperate the signal from the background, the sixjet-shape variables were combined in a discriminant,D, with

D =ρsig(�x)

ρsig(�x) + ρbg(�x),

where ρsig(�x) and ρbg(�x) are the density functionsin the six-dimensional jet-shape space of the signaland the background events, respectively. The result-ing distribution of D in Figure 1 shows a clear seper-ation of the signal dominant at large discriminant val-ues (D → 1). The observed distribution dominant atlow discriminant values (D → 0) is well reproducedby the CC DIS simulation.

The final cut on the discriminant of D > 0.95leaves 3 tau candidate events where 0.40+0.12

−0.13 are ex-pected from the Standard Model. Figure 2 shows thedistribution of the discriminant before the final cutand the phad

T -distribution after D > 0.95 is applied.In particular the two events with highest phad

T couldserve as candidates for FCNC with single top produc-tion. Figure 3 illustrates one of these events.

D

Figure 1. Distribution of the tau discriminant, D, forinclusive CC DIS data events (dots) from e+p-datataken in the years 1999-2000 only, a simulation ofCC DIS events (shaded) and the simulation of the di-rect W -production signal, W → τν, with subsequenthadronic tau decay (hatched).

Figure 2. Distribution of (a) the tau discriminant,− log(1−D), before the cut on D > 0.95 and (b) thehadronic transverse momentum, phad

T , after applyingthe cut D > 0.95. The dashed line represents the nor-malised distribution of the single-top MC, includingall decay channels of the W boson.

3.1.2. H1The H1 collaboration presents a search for events

with tau leptons using the H1 detector with the datacollected between 1996 and 2000, corresponding toan integrated luminosity of 108 pb−1. The hadronicdecays of tau leptons are searched for in eventswith significant missing transverse momentum. Thissearch aims to complement the previous observationof events with electrons or muons and significantPmiss

T . Due to the difficult tau lepton identification,more powerful background supression is required.

The identification of events with missing transversemomentum is based on the following observables thatquantify the momentum imbalance in the event:

• PmissT , the net transverse momentum.

• P caloT , the net transverse momentum mea-

sured from all energy deposits recorded in thecalorimeters only.

• E − Pz =∑

i Ei(1 − cos θi), where Ei

and θi denote the energy and polar angle

L. Lindfeld / Nuclear Physics B (Proc. Suppl.) 144 (2005) 315–322316

Figure 3. A tau-candidate event with phadT = 38 GeV

and pτ−jetT = 41 GeV in the ZEUS detector.

of each particle in the event detected in themain detector. For an event where only mo-mentum in the proton direction is undetected,E − Pz = 2Ee = 55 GeV (Ee = 27.5 GeV isthe electron beam energy). This quantity givesa measure of the longitudinal momentum bal-ance.

The hadronic final state (HFS) is measured by com-bining calorimeter energy deposits with low momen-tum tracks. Identified isolated electrons or muons areexcluded from the hadronic final state. The calibra-tion of the hadronic energy scale is made by compar-ing the transverse momentum of the precisely mea-sured scattered electron to that of the HFS in a largeNC event sample. Particles from the reconstructedHFS are combined into jets using an inclusive kT al-gorithm [11] with a minimum PT of 4 GeV.

The tau candidate forms a narrow hadronic jet as-sociated to one isolated charged track measured in theinner tracker. The present analysis restricts the searchfor hadronic tau decays to the “one-prong” decays,which make up about 50% of all tau decays. The sizeof the hadronic jet is estimated using the jet radiusRjet, defined as the energy weighted average distancein the η−φ plane between the jet axis and the hadronsmaking up the jet:

Rjet =1

Ejet

∑

i

Ei

√∆η(jet, i)2 + ∆φ(jet, i)2

where the sum runs over all hadrons that belong to thecandidate hadronic jet. The isolation of the tau can-didate is measured by the distance in η − φ plane to

the closest hadronic jet (Djet) or to the closest track(Dtrack). The remaining hadronic system after ex-cluding the tau jet candidate, is denoted by X here-after.

The selection of events with isolated tau can-didates and missing transverse momentum is per-fomed in three steps. At first, the data is prese-lected with a minimum calorimetric tranverse mo-mentum, P calo

T , of 12 GeV and a longitudinal im-balance E − Pz < 45 GeV. At least one jet withPT > 7 GeV and a topological acoplanarity are fur-ther preselection criteria. Then, the candidate tau jetis required to have a PT > 7 GeV and to be in the po-lar angle range 20◦ < θ < 120◦. The jet is requiredto contain only one charged track and to be isolatedfrom other jets or charged tracks: Djet > 1 andDtrack > 1. Finally, narrow pencil-like jets are se-lected by the requirement Rjet < 0.12. The acopla-narity ∆φ(jet, X) between the tau candidate momen-tum and the momentum of the remaining measuredparticles in the event must be below 170◦. In order tofurther supress the mainly low PT background, fur-ther cuts P calo

T > 20 GeV and Ptrack > 5 GeV areapplied.

After the final selection, five events are observedin the data sample compared to a SM expectation of5.8 ± 1.4. The data events are in the very low P X

T

region. In the region at large hadronic momentumPX

T > 25 GeV no events are observed. Table 1 sum-marizes the results of the search in comparison withthe ZEUS experiment and previous searches for iso-lated electrons and muons.

This preliminary analysis completes the picture ofthe searches for events with isolated leptons and miss-ing transverse momentum at HERA. The status af-ter HERA I data taking is summarised in Table 1.While H1 reports events with electrons and muonsin the high P X

T region in excess of the SM predic-tion, ZEUS observes good agreement with the SM.In turn, ZEUS observes two tau events at large P X

T

for 0.20 ± 0.05 expected. The new H1 preliminarysearch for τ + P miss

T events reveals no candidate atlarge P X

T . In the tau channel, the H1 analysis has a Wdetection efficiency which is roughly two times that ofthe ZEUS analysis in the high-P X

T region. The back-ground suppression of the ZEUS analysis is roughlytwo times stronger than that of the H1 analysis in thesame region.

L. Lindfeld / Nuclear Physics B (Proc. Suppl.) 144 (2005) 315–322 317

Ele

ctro

nM

uon

Tau

(H1:

108

pb−

1)

1994

-200

0e±

pob

s./e

xp.

obs.

/exp

.ob

s./e

xp.

(W±

cont

rib.

)(W

±co

ntri

b.)

(W±

cont

rib.

)

Full

Sam

ple

11/1

1.54

±1.

508

/2.9

4±0.

505

/5.8

1±1.

36

(71%

)(8

6%)

(15%

)

H1

PX T

>25G

eV

5/1

.76±

0.30

6/1

.68±

0.30

0/0

.53±

0.10

(82%

)(8

8%)

(49%

)

118.4

pb−

1P

X T>

40G

eV

3/0

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0.13

3/0

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0.14

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0.05

(80%

)(9

2%)

(54%

)

Full

Sam

ple

24/2

0.6

+1

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4.6

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1.9+

0.6

−0

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0+0

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−0

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(17%

)(1

6%)

(49%

)

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US

PX T

>25G

eV

2/2

.90+

0.5

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0.3

25

/2.7

5+0

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−0

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)(5

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2

(61%

)(6

1%)

(71%

)

Tabl

e1

Sum

mar

yof

the

resu

ltsof

sear

ches

for

even

tsw

ithis

olat

edle

pton

s,m

issi

ngtr

ansv

erse

mom

entu

man

dla

rge

PX T

atH

ER

A.

3.2. Tau pairs at H1As a supplementary investigation of the ability of

the H1 detector to detect tau leptons, a preliminarysearch for elastic tau–pair production in ep collisionsis performed. Events with semi-leptonic tau decaysare selected by requiring one identified electron ormuon and one hadronic tau candidate (1-prong or 3-prong) with PT > 2 GeV. The hadronic tau signa-ture is verified by a neural network algorithm, basedmainly on the hadronic cluster shape and trained dif-ferently for one- or three-prong candidates. In orderto select elastic events, events with additional energydeposits or tracks that cannot be interpreted as beingdue to the scattered electron are rejected.

With this selection, 15 events are observed with op-posite charges of the two tau leptons, compatible withan expectation of 17.6± 3.9 events, dominated by thetau–pair process. A selected data event is presented inFigure 4. In the event sample with like-sign charges,dominated by NC background, 1 event is observed foran expectation of 1.9 ± 1.2.

The results of the preliminary search for elastictau–pair production are in agreement with the expec-tation and demonstrate the ability of the H1 detectorto detect tau leptons.

Run 248568 Event 27325 Class: 4 11 12 13 15 16 19 20 24 28 29 .

Z

e+p −→ e+ τ+ τ− (X)

µ+ νµντ ντ h−h+h−

−→ −→ TAU-PAIR CANDIDATE

h−h+h−

e+

µ+µ+

h−h+h−

e+

H1

Figure 4. Tau–pair candidate event with one tau lep-ton decaying leptonically to a muon, and the othertau lepton decaying to three charged hadrons (3-prongtopology). The scattered positron is also detected inthe backward calorimeter.

3.3. Search for H++ → τ+τ+ at H1Doubly charged Higgs bosons (H∓∓) appear

in several extensions to the Standard Model andcan in principle be light enough to be detectableat HERA. Examples are provided by some Left-Right Symmetry (LRS) models [12–14], where theextended symmetry SU(2)L × SU(2)R × U(1)B−L

is spontaneously broken to the SM symmetrySU(2)L × U(1)Y by a SU(2)R triplet of scalarfields, whose neutral component acquires a non-vanishing vacuum expectation value (vev).

At the tree level, doubly charged Higgs bosonscouple to charged leptons and to other Higgs andgauge bosons. Couplings to quarks are not allowedby charge conservation. Their coupling to chargedleptons can be generally described by the Lagrangian

L = hL,Rij H−−lciPL,Rlj + h.c. ,

L. Lindfeld / Nuclear Physics B (Proc. Suppl.) 144 (2005) 315–322318

where i, j = e, µ, τ are lepton generation indices,PL,R = (1 ∓ γ5)/2, l are the charged lepton fieldsand the superscript c denotes the charge conjugatespinors. The Yukawa couplings hL,R

ij are free param-eters of the model.

Since the production processes at HERA I3 areinsensitive to the chirality of the lepton fields, thegeneric case of an either right-handed or left-handedcoupling, hij , is considered here.

The tau channel contribution for the Higgs decayin this analysis is restricted to the diagonal couplinghττ in the Lagrangian. The analysed e+p H1 dataset of the period 1999-2000 corresponds to an inte-grated luminosity of 64.5 pb−1. Therefore, the pro-duction process e+p → e−H++X followed by a de-cay H++ → τ+τ+ is searched for.

The selection of ττ final states requires two high-PT tau candidates in the central detector region. Thetau identification is based on charged tracks measuredin the central drift chamber with transverse momentaof P τ1

T > 10 GeV and P τ2T > 5 GeV. The lep-

tonic tau decays τ → eνeντ and τ → µνµντ arereconstructed by identifying the electron in the LArcalorimeter or the muon in the central muon detec-tor, respectively. Remaining tau candidates are re-constructed as hadronic tau decay, if at least 40% ofthe charged track momentum is reconstructed in thehadronic calorimeter. Further track isolation criteriaand different additional phase space cuts for each de-cay mode are applied to reduce mainly photoproduc-tion and NC DIS background. Finally, the Higgs massis reconstructed via the invariant mass of the tau-tausystem, Mττ , by imposing

∑i Ei − P i

z = 2Ee andassuming the missing neutrinos to be boosted into thedirection of the tau tracks with minimal missing tran-verse momentum in the event.

At this stage, one event with Mττ = 85 GeV andtwo hadronic one-prong tau decays is selected, wherea total SM background of 2.12 ± 0.32 is expected.However, the two tracks are found to have differentsigns of charge. Imposing a like-sign tau pair as a finalcut removes this event and also remaining backgroundfrom γ → τ+τ− processes.

Therefore, no candidate event for a doubly chargedHiggs is found in the tau channel, where 1.03 ± 0.19

3data taking period 1994-2000 with longitudinally unpolarisedbeams

are expected from SM. Together with the other decaychannels of the doubly charged Higgs boson, Figure5 shows the resulting upper limits at 95% confidencelevel on the product of the H∓∓ production cross-section and the decay branching ratio as a function ofthe doubly charged Higgs mass.

0.04

0.05

0.06

0.070.080.09

0.1

0.2

0.3

0.4

80 90 100 110 120 130 140 150MH (GeV)

σ *B

R (

pb)

H1 Higgs search: H±± limits

H1 Preliminary

H±±→ µ±µ±

H±±→ e±e±

H±±→ e±µ±

H±±→ τ±τ±

Figure 5. Upper limits at 95% confidence levelon σ(e∓p → e±H∓∓X) × BR(H∓∓ → l∓∓) as afunction of the doubly charged Higgs mass.

3.4. Search for lepton flavor violation via lepto-quarks decaying to a tau lepton and a quark

In the SM, all known interactions involving leptonsconserve the lepton flavors individually. However,from a theoretical point of view, there is no under-lying gauge symmetry supporting this experimentalobservation. In fact, experimental evidence for leptonflavor violation (LFV) in atmospheric neutrino oscil-lations has already been reported in 1998 [15].

In e+p collisions at HERA, a LFV process can in-duce the appearence of a muon or tau instead of thepositron in the final state. A convenient concept toexplain such exotic signatures is the exchange of aleptoquark (LQ). Leptoquarks couple to both quarksand leptons and can therefore be resonantly producedin ep-collisions at HERA. Here, we focus on thesearches for LFV processes, where the exchange ofa leptoquark leads to a tau in the final state.

L. Lindfeld / Nuclear Physics B (Proc. Suppl.) 144 (2005) 315–322 319

Leptoquarks are color triplet scalar or vectorbosons, carrying both lepton (L) and baryon (B) num-ber. The fermion number F = L + 3 B is conservedand takes values of F = 2 for e−q and F = 0 for e+qstates. A detailed overview of the Buchmuller-Ruckl-Wyler (BRW) effective model using the Aachen nota-tion for the leptoquark coupling to u and d quarks canbe found elsewhere [16,17,21].

These processes can also be mediated by squarks inRp-violating SUSY models. Further details on thesemodels and on the cross-section calculation can befound in [18]. Although there are strong constraintsfrom low energy experiments [19] on some of theLFV processes, HERA has a unique discovery poten-tial for some cases where high generations of quarksare involved.

3.4.1. ZEUSThis analysis supplements previous searches for

LFV by the ZEUS collaboration [20]. The search isperformed using data of e−p- and e+p-collisions col-lected in the years 1994 to 2000 at a centre-of-massenergy,

√s, of 300 GeV and 318 GeV corresponding

to an integrated luminosity of altogether 130 pb−1.A preselection for events with the reaction

e±p → τ±X is mainly based on a CC DIS triggerand a requirement of P miss

T > 12 GeV. The analysiscovers hadronic and leptonic decays of the tau leptonand imposes individual cuts for these channels. In thehadronic selection electrons with E e > 10 GeV arevetoed, ET > 50 GeV and 15 < E − Pz < 60 GeVis applied. At least one tau jet candidate which fulfillsP τ

T > 15 GeV, 15◦ < θτ < 164◦, f τ−jetem < 0.95,

f τ−jetem + ltfτ−jet < 1.6, D > 0.9 and |∆φ| < 20◦ is

required4.The selection for tau leptons decaying to elec-

trons (muons) involves P missT > 15 GeV (20 GeV),

15 < E − Pz < 60 GeV, veto energy de-posits in the rear calorimeter (RCAL) withE > 7 GeV, Pmiss

T /√

ET > 2.5√

GeV (3√

GeV)and 8◦ < θe < 125◦ ( 8◦ < θµ < 164◦). The dis-criminant variable for the selection of the hadronictau decays is identical to the one previously discussedin Section 3.1.1 and well described by the simulation.

After the final cut on the acoplanarity, |∆φ| < 20◦,no candidate event is found for any of the three chan-

4fem = electromagnetic energy fraction, ltf = leading track frac-tion, |∆φ| = acoplanarity in azimuth between Pmiss

T and τ -jet

φ∆0 20 40 60 80 100 120 140 160 180

Eve

nts

10-2

10-1

1

10

102

p 94-00±ZEUS (prel.) e

Background MC

m=240 (GeV)τLFV

ZEUS

Figure 6. Distribution of the acoplanarity in azimuthbetween P miss

T and τ -jet, |∆φ|, after the cut on thediscriminant, D > 0.9.

nels, while 1.7 ± 0.4 are predicted by SM simulation(see Figure 6). Limits on λeq = λτq and λeq ×

√βτq,

i.e. the product of the coupling at resonant leptoquarkproduction and the branching ratio for the decay pro-cess LQ → τq, for all 14 kinds of leptoquarks inthe BRW model are derived and shown for LQs withF = 0 in Figure 7.

3.4.2. H1In this analysis, the search for leptoquarks is pre-

formed in e+p collisions only and therefore only lep-toquarks with F = 0 are considered. For the de-termination of the signal detection efficiencies theLEGO [21] event generator is used and the completeH1 detector response is simulated. The contributionsfrom several SM background processes which maymimic the signal through measurement fluctuationsare evaluated in this analysis. These processes includeNC DIS, lepton pair production, W-production, pho-toprodution and charged current deep-inelastic scat-tering (CC DIS) modelled by the generators describedin [22–28].

The search for leptoquarks possessing couplings toa third generation lepton leading to τ + q final statesis restricted to the hadronic decays of the tau. Thehadronic decays of a high PT tau lead to a typical sig-nature of a high PT “pencil-like” τ -jet. This τ -jet ischaracterised by a narrow shape in the calorimeter andlow track multiplicity, i.e. one to three tracks in theidentification cone of the jet. The neutrinos from thedecay of the tau are boosted with the hadrons in thedirection of the τ -jet. Therefore, the signal topology

L. Lindfeld / Nuclear Physics B (Proc. Suppl.) 144 (2005) 315–322320

ZEUS

(GeV)LQM150 200 250 300

qτβ× 1

eqλ

10-3

10-2

10-1

1

L1/2S~

R1/2SL1/2S

p 94-00+a) ZEUS (prel.) e

excluded at 95% C.L.

(GeV)LQM150 200 250 300

qτβ× 1

eqλ

10-3

10-2

10-1

1

R0,VL

0VR0V~

L1V

p 94-00+b) ZEUS (prel.) e

excluded at 95% C.L.

(GeV)LQM150 200 250 300

1eq

λ

10-3

10-2

10-1

1

e Xτ→B eπ→τ

ννπ→K

L1/2S~

p 94-00+c) ZEUS (prel.) e

j q τλ=1eqλ

(GeV)LQM150 200 250 300

1eq

λ

10-3

10-2

10-1

1

e τ→B eπ→τ

K e→τ

R0V

p 94-00+d) ZEUS (prel.) e

j q τλ=1eqλ

Figure 7. Top: Limits on λeq×√

βτq, i.e. the productof the coupling at resonant leptoquark production andthe branching ratio for the decay process LQ → τq, at95% C.L. as a function of LQ mass. Bottom: Limitson particular LQs with βτq = 0.5. Here, only theresults for LQs with F = 0 from e+p-data are shown.

is a dijet event with no leptons. The missing trans-verse momentum in the event carried by the neutrinois aligned with the second highest PT jet (jet2), whichhas to fulfil the τ -jet criteria.

The following selection criteria are applied for highPT dijet events with no lepton: No e± or µ± found, atleast 2 jets with P jet1

T > 25 GeV, P jet2T > 15 GeV,

7◦ < θjet1,jet2 < 145◦ and f jet1,jet2em < 0.95. The

subsequent selection of events with jet2 fulfilling τ -jetcriteria involves P miss

T > 20 GeV, 1 ≤ N jet2tracks ≤ 3,

no tracks in an outer cone in the ηφ-plane aroundjet2 with 0.12 < Dtrack < 1.0, M jet2 < 7 GeV,Rjet2 < 0.12 and |φjet2 − φmiss| < 30◦. One dataevent is found as a high PT tau candidate from a LQdecay compared to a SM expectation of 0.56 ± 0.16.This SM background is dominated by NC DIS andphotoproduction. The selection efficiency for theLFV signal of scalar LQs decaying to a τ + q finalstate varies between 12% at a LQ mass of 100 GeVand 24% at 200 − 250 GeV. For vector LQs the effi-

ciency rises up to 32% and falls steeply near the kine-matic limit of

√s = 319 GeV to become stable at

10% above the kinematic limit.

100 150 200 250 300 350-310

-210

-110

1

/ GeVLQM

eqλ =qτλ

u,d)+ (eR1/2S

u)+ (eL1/2S

d)+ (eL

1/2S~ Exclu

ded at

95%

C.L

.

= 0.5qτ →LQ BR

H1 Preliminary

100 150 200 250 300 350-310

-210

-110

1

/ GeVLQM

eqλ =qτλ

u,d)+ (eL1V

u)+ (eR0V~

d)+ (eR0V

d)+ (eL0V Exc

luded

at 95

% C

.L.

= 0.5qτ →LQ BR

H1 Preliminary

Figure 8. Limits on the coupling constant strength λ lq

at 95% C.L. as a function of LQ mass for scalar (left)and vector (right) LQs in the tau decay channels.

In order to set limits on the signal cross-section,the mass spectra are scanned for signals using a slid-ing mass window with optimised borders. Within thiswindow the number of data events, background eventsand the selection efficiency are used to calculate anupper limit on the signal at a 95% confidence level(CL) [29]. These limits are converted into limits onthe coupling λτq. The obtained limits are shown inFigure 8 for scalar and vector LQs with an assumedLFV branching ratio of BRLQ→τq = 0.5.

4. Summary

We reviewed the latest analyses at the HERA ex-periments H1 and ZEUS dealing with a tau lepton inthe final state. The difficult identification of tau lep-tons in the final state of ep-collisions was successfullyperformed in all analyses.

The three tau candidate events with significantmissing transverse momentum found with the ZEUSdetector go along with the previously reported eventswith isolated high energy electrons and muons. TheSM prediction of W -production can not be signifi-cantly ruled out. Furthermore, the H1 experiment ob-serves no such tau candidate events with high P had

T .Limits on theories beyond the Standard Model

could be set, extended or improved in searches fordoubly charged Higgs bosons and lepton flavor vio-lation by both experiments H1 and ZEUS.

L. Lindfeld / Nuclear Physics B (Proc. Suppl.) 144 (2005) 315–322 321

We thank the organisers of the TAU’04 workshopfor the invitation and their hospitality.

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