Towards the Finals - Kirchhoff Institut Für Physik models, verify predicted signatures and phase space. • leptoquarks • lepton flavour violation • excited fermions • anomalous

Martin Wessels, DESY

Searches at HERATowards the Finals

Heidelberg, October 2, 2007

ATLAS Analysemeeting

Martin Wessels, DESY Searches at HERA - Towards the Finals Page 2

HERA

Ecms = 320 (300) GeV2

Hadron-Elektron Ring Anlage19.10.1991 – 30.06.2007

H1 ZEUS

city of Hamburg

HSV stadium

airport

two colliding experiments


Data SamplesH1 HERA I (1992-2000)• e-p data ~ 14 pb-1

• e+p data ~ 105 pb-1

H1 HERA II (2003-2007)• e-p data ~ 170 pb-1

• e+p data ~ 190 pb-1

& long. polarized lepton beam

Low Energy Run (2007)• EP = 460 / 575 GeV• measurement of FL

H1 HERA I+II: 0.48 fb-1

e-p: 0.18 fb-1 e+p: 0.30 fb-1

HERA Luminosity: ~ 0.5 fb-1 per experiment


ep BasicsNeutral Current: eP→ eX

Charged Current: eP→ νX

Q2 = -q2 = (k-k’)2

• virtuality• resolving power

Q2 ~ 0→ Photoproduction

Q2 >~ 4 GeV2

→ deep inelastic scattering (DIS)


H1 Detector


Neutral Current DIS

Neutral Current DIS: eP → eX• electron and hadronic system X

balanced in transverse momentum PT

hadronic system X is well collimated bundle of particles → jet

Q2 = 25030 GeV2

e P

jet


Charged Current DIS

Charged Current DIS: eP → νX• unbalanced hadronic system X,

neutrino escapes detection, missing transverse momentum PT

miss

Q2 = 21475 GeV2

e P

jet


Photoproduction

Dijets in photoproduction: eP→ j j• scattered electron leaves the

detector undetected under very small scattering angles

Q2 ~ 0

jet

jet

QCD Compton Boson-Gluon Fusion


Defects of the Standard Model• Parameter problem

Why does the SM need at least 18 independent parameters?

• Problem of fermion generationsWhy exactly 3 generations?

• Problem of charge quantisationWhy is the electron charge exactly threefold the quark charge?

• Hierarchy problemWhy does EW symmetry breaking occur at MW~102 GeV and not near the Planck scale MP~1019 GeV?

• Problem of unificationIs there a simple group framework unifying all particle interactions?

Solutions to all question might come in Theory of Everything TOE, but not enough understanding yet to make clear experimental predictions.

“Hence” a lot of extensions to the SM has been constructed.

HERA well suited to test a large variety of these models beyond the SM

• centre of mass energy up to 320 GeV

• lepton and baryon number in initial state ideally suited to look for particles coupling to eq pairs


Searches at HERAModel dependent searches

Test models, verify predicted signatures and phase space.

• leptoquarks

• lepton flavour violation

• excited fermions

• anomalous top production

• doubly charged Higgs

• Supersymmetry

Model independent searches

Compare data and SM prediction, reveal anomalies above small SM contribution.

• isolated lepton events

• multi lepton events

• general search

Searches in inclusive DIS

Precision measurements allow for stringent limits on new physics.

• NC: quark radius, CI, extra dimensions

• CC: polarisation dependencered covered in

following


Quark RadiusForm factor analysis• finite size of leptons and/or quarks• “classical” form factor f(Q2) at vertices

diminish SM cross section at high Q2

HERA I+II e-p 435 pb-1Rq< 0.74 10-18 m

dσdQ2 =

dσSM

dQ2 f2e (Q

2)f2q (Q2)

with f(Q2) = 1− 16R

2Q2


Leptoquarks

Motivation

• symmetry of quark and lepton generations raises question of direct interactions → Leptoquarks

• scalar or vector bosons with Lepton (L) and Baryon (B) number

• Fermion number F = 3B + LF=0/2 different sensitivity to e+/e- p

Production and decay

• MLQ<Ecm resonant production dominates (s-channel)

• MLQ>Ecm u-channel contributes, transition to contact interaction

• LFC: 1st generation lepton in initial and final state→ interference with NC/CC DIS

• LFV: μ or τ in final stateλij: Yukawa coupling, family indices i j

Martin Wessels, DESY Page 13

LeptoquarksNC topology (ep → e j)

CC topology (ep → ν j)

• no evidence for signal

• set limits on F=2 LQs (BRW model)

Scalars

Vectors→ for λ~0.3 mass exclusion ~280 GeV

HERA II e-p 92 pb-1


Leptoquarks

increased HERA II e-p statistics improves limits in resonant region

full e-p data set still to be analysed (factor 1.5 luminosity)

LQ at HERA: single production


Excited LeptonsMotivation and Production

• unambiguous signature for new matter substructure→ direct observation of excited states

• predicted by compositeness models • lepton-boson resonances

Decay and Analysis

• de-excitation by emission of γ, Z or W bosons

• NC/CC DIS like final states plus various Z/W decay products (ll, qq)

• resonances in inv. mass

excited neutrinos ν*

νγ, eW, νZresonances

excited electrons e*

eγ, νW, eZresonances


Excited Leptons

good agreement between data and SM in all channelsset exclusion regions (f/λ, Ml)

Search for excited neutrinos ν* HERA I+II e-p 184 pb-1

ν* cross section much larger for e-p

almost all decay topologies investigated


Excited Leptons

good agreement between data and SM in all channelsset exclusion regions (f/λ, Ml)

HERA I+II e±p 435 pb-1

all data at Ecm=320 GeVSearch for excited electrons e*

only hadronic decays of W/Z analysed so far


Excited Leptons

presently most stringent world limits

excited neutrinos ν* excited electrons e*

• large improvement compared to HERA I• for masses beyond the LEP reach best

sensitivity achieved so far

• a new territory explored• best sensitivity for intermediate e*

masses

Page 19

Isolated Leptons and PTmiss

Topology:

• high PT isolated lepton (e, μ)• large missing transverse momentum PT

miss

• (PT of hadronic system X)

Standard Model:

• real W production with leptonic decay• usually soft hadronic system• total cross section about 1 pb

W Monte Carlo event

ep → l + PTmiss (+ jet)

ep → l + W± + X

lepton

PTmiss

jet, PTX



HERA I (118 pb-1, e+p dominated)Phys. Lett. B561 (2003) 241, 01/03

• observation of increased event rate with PT

X > 25 GeV• 10 events observed, 2.9 ± 0.5 expected• 3 sigma deviation

example of an atypical eventwith PT

X > 25 GeV



e-p: good agreement with SM

HERA I+II: 478 pb-1

e-p e+p

e+p: excess at high PTX less

pronounced wrt. HERA I



H1 excess remains at high PTX in e+p data at 3.0 σ level

not confirmed by ZEUS

• good overall agreement

• excess in both lepton channels for e+p


Isolated τ Leptons and PTmiss

• search for events with high PT isolated τ and PTmiss

• τ’s identified in the hadronic one-prong decay channel (→ “jets with single tracks in CC events”)

e-p e+p

τ lepton

PTmiss

jet, PTX

ν

HERA I+II: 471 pb-1

good agreement with SM in e-p and e+p data set


overall good agreement between data and SM

full sample: 87 / 92.7±11.2PT

X>25 GeV: 29 / 25.3±3.2

e+p (0.58 fb-1) full sample: 57 / 53.1±6.4PT

X>25 GeV: 23 / 14.6±1.9

H1/ZEUS Combination

e-pe+p

e±p e±p

Exploit full HERA luminosity to investigate puzzle on isolated lepton events

→ H1+ZEUS (0.97 fb-1)

• common phase space

• combine data and SM

beautiful measurement of W production … and excess remains at 2 sigma, still


Anomalous Single Top Production

• high PTX abnormal for W production

• but typical for top decay t → b W

• but tiny SM top production cross section at HERA < 1 fb

→ anomalous top production via flavour changing neutral current (FCNC)?

• but could not explain e+/e- difference

H1 observes excess of events with isolated high PT lepton (e, μ) and large PT

miss at high PTX in e+p scattering

κtuγ anomalous magnetic couplingνtuγ anomalous Z vector coupling


Anomalous Single Top Production• analysis of full H1 HERA I+II isolated

lepton events– signal: ep → e t (→ bW → blν)– main bgr: W production

• reconstruction of top candidate

• multi-variate method to discriminate top and SM background

• extraction of limit on signal cross section from the discriminator spectra

σ(ep→etX) < 0.16 pb (95% CL)κtuγ < 0.14 (95% CL)

presently most stringent limit on κtuγ

HERA I+II: 482 pb-1


Multi Lepton Events

• search for events with at least 2 isolated high PT leptons (e, μ)

• dominant SM contribution: two photon interaction

If maybe anomalous l-ν production (W), what about l-l final states (Z)?

γγ → l+l−


Multi Lepton Events

HERA I: 115 pb-1, e+p dominatedEur Phys J C31 (2003) 17, 07/03

• overall good agreement with SM prediction,measurement of cross section

• but also interesting events at high inv. mass


e-pe+p

e±p

Multi Lepton Events

overall good agreement with SM

all 4 events with ΣPT>100 GeV in e+p data

HERA I+II: 459 pb-1


2

3

4

5

General Search for New Phenomenamany dedicated searches for new physics

• investigate all final states at high PT in a single coherent analysis

• BSM independent search for deviations

common phase space: PT>20 GeVelectrons, muons, photons, jets, neutrinos→ exclusive event classes

global statistical interpretation → check SM consistency

overall good agreement with SMsome discrepancies (isolated leptons)

HERA I 117 pb-1

But: Are we missing something?


General Search for New Phenomena

HERA II 337 pb-1

overall event yields in good agreement

e+p e-p



dedicated statistical algorithm:

systematic search for deviations between data and SM predictionin differential (1-dim) distributions with high sensitivity to BSM signals

Mall : invariant mass of objectsPPT : sum of transverse momenta

1. regions of most interestsearch region of largest deviation in given histogram

2. event class of most interestweigh up significance of deviations found

3. global significancedecide if “event class of most interest” is interesting at all


General Search for New Phenomena1. Region of most interest• Scan all regions with size ≥ resolution and

calculate probability p that data agrees with SM.

2. Event Class of most interest• What is the probability to observe a deviation

with p<pmin?

• Dice hypothetical histograms Hhyp according to SM and run the algorithm to find pmin

hyp.

3. Global significance• what would be the outcome if we could redo the

experiment?

• replace data histograms with MC pseudo-data and determine according MCE values.

deficit region

Nobs = 1NSM = 6.4± 3.9pmin = 0.10

P̂ =numHhyp with p

hypmin

<pdatamin

tot numHhyp

is measure for significance of pmindata

P̂e−j = 0.85

P̂MCE =numHhyp with p

hypmin

<ppseudomin

tot numHhyp

P̂

P̂

P̂



e+p e-pall event classes have systematically been scanned for deviations

allows quantification of overall agreement between HERA data and SM



good agreement with SM for both HERA II e+p and e-p

μ-j-νHERA I: -log = 3P̂


SummaryHERA data taking came to a successful end after 15 years of operation, delivering 0.5 fb-1 ep data to each colliding experiment.

Precision analyses of these data will teach us about the content of the proton, and help a lot of understanding physics at hadron colliders.

Searches for new physics in ep collisions exploiting the full HERA statistics are performed, but the SM seems to hold … once more.

Even after 1 fb-1 of data, mystery of isolated lepton events at HERA remains unsolved - Future will tell, whether it was a fairly large statistical fluctuation or the hint of new physics.

There is still work to do, so stay tuned for the unexpected … and good luck to the TeVatron and LHC


Thank You

Last HERA beam dump: 23h30, June 30, 2007

Documents

Towards the Finals - Kirchhoff Institut Für Physik models, verify predicted signatures and phase space. • leptoquarks • lepton flavour violation • excited fermions • anomalous