Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://nuclphys.sinp.msu.ru/conf/lpp14/250809/Schneekloth.pdf Äàòà èçìåíåíèÿ: Thu Mar 25 10:51:46 2010 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 00:45:59 2012 Êîäèðîâêà: IBM-866

Electroweak Physics and Searches for New Physics at HERA
Uwe Schneekloth DESY On behalf of the H1 and ZEUS Collaborations 14th Lomonosov Conference on Elementary Particle Physics 25.08.2009


Outline
Introduction Electroweak physics
Neutral and charged current DIS cross sections Unpolarized and polarized DIS cross sections Interference structure function Combined Electroweak-QCD fits Isolated lepton events
single W production cross section

Multi-lepton final states

l + l

-

cross section

Physics beyond the Standard Model
Single top production Quark radius, contact interactions Excited fermions Leptoquarks Individual analyses of H1 and ZEUS General searches experiments and recently significant

Conclusions
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improvement by combining both experiments
Electroweak Physics and Searches at HERA 2


The HERA ep Collider at DESY
World's only electron proton collider, in operation 1992-2007
HERA-I (1992-2000)
§ ~130 pb-1 per exp., (90% e+p)

HERA-II (2003-07)
§ Luminosity upgrade § Longitudinal e polarization (avg. 30%-40%) Total integrated luminosity ~0.5 fb-1 per experiment

eí p e+ p
13x increase in e-p statistics

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Electroweak Physics and Searches at HERA

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H1 and ZEUS Experiments
H1
Large general purpose collider experiments
Asymmetric design 4 coverage Excellent electromagnetic and hadronic calorimeters

ZEUS

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Electroweak Physics and Searches at HERA

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Deep Inelastic Scattering at HERA
Neutral Current y, Q2 x

Charged Current

ep X

Q2 í virtuality of exchange boson x í Bjorken scaling variable y í Inelasticity

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Unpolarized DIS Cross Sections
Neutral Current
d/dQ2 (pb/GeV2)

Electroweak unification
10 0 10

HERA I & II
+

10

H1 e p NC 94-07 (prel.) H1 e p NC 94-07 (prel.)

Charged Current

1 10-1

ex cha

ZEUS e p NC 06-07 (prel.) ZEUS e p NC 05-06

+

nge

SM e p NC (HERAPDF 0.1) SM e p NC (HERAPDF 0.1)

+

3 10-3

H1 e p CC 03-04 (prel.) H1 e p CC 2005 (prel.)
-

+

~0

~ 1/Q4 at Q2>MW

2
5 10-5

ZEUS e p CC 06-07 (prel.) ZEUS e p CC 04-06 SM e p CC (HERAPDF 0.1) SM e p CC (HERAPDF 0.1)
+ -

+

Z and W dominate exchange
. .

Excellent agreement between data y P and SM over large kinematic range 7 -7 10 (many orders of magnitude) Electroweak unification at Q2 = MW2 Q2 (resolving) power up to 40000 GeV2, corresponds to spatial resolution 1/Q ~ 10-18m
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< 0.9 e=0

103

104

Q2 (GeV2)

mW

2

Electroweak Physics and Searches at HERA

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Neutral Current Cross Section
A closer look at the neutral current cross section in (x, Q2)

F 2

~

x

F3

~

Cross section parametrized using generalized structure functions related to quark/gluon density distributions in proton F
2

exchange dominant contribution, Z interference depends on polarization (axial-vector coupling to Z ae large)

xF

3

Z interference / Z exchange, depends on beam lepton charge (vector coupling to Z ve small)

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Unpolarized NC Cross Sections
Measured using 170 pb-1 of HERA-II data Good agreement with SM (ZEUS-JETS) over large kinematic range

Dependence on beam charge apparent: e-p cross section larger at high Q2 Cross section difference gives interference xF3Z
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e- p e+p


Interference Structure Function xF
Charge asymmetry observed: Exploit difference in e-p/e+p cross sections to measure xF3

3

Z

Z interference dominates in HERA kinematic range: Measure "interference structure function"

Z

0.8

ZEUS
ZEUS NC, e‘p (233.1pb 1), Q2=5000 GeV
_ 2

xF3

0.6

BCDMS, ‘N, 40
2

0.4

All measurements extrapolated to Q2=5000 GeV2 Measured as function of x Result in good agreement with standard model expectation

0.2

0

10-1

x

1

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Polarized NC Cross Sections: Parity Violation
Polarization asymmetries:
Measurement of Z interference term in F2 HERA polarization asymmetries

~ aev

q

Observation of parity violation in NC e‘p scattering down to 10-18m Direct measurement of electroweak SM effects Combined measurement increases statistics (prelim.)

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Unpolarized Charged Current Cross Section
Measured charged current cross sections for e+p/e-p data (HERA-I, unpolarized) Good agreement with SM over large kinematic range (shown here: HERAPDF0.1) Sensitive to flavors of partons in proton p ~ (uud) at high x

d
e+p e- p

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Polarized Charged Current Cross Section
Different HERA-II data sets allow measuring CC cross section as function of polarization Pe positive, negative and zero

Fundamental feature of SM directly visible: Only L(R)-handed (anti)particles interact weakly

Right-handed currents forbidden in SM. At P =-1 (extrapolated): MWR > 208 GeV (H1)
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Combined Electroweak-QCD Fits

íí All these measurements are used to extract 5 PDFs (g,u,u,d,d) and weak couplings to Z0 (au,ad,vu,vd) simultaneously NC: Z interference / Z exchange sensitive to au,ad and can resolve signs of couplings CC: flavor sensitivity helps to disentangle u, d-quarks Precision competitive with LEP and Tevatron results Most precise value for u-coupling to Z comes from HERA
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Isolated Lepton Events with Missing pT
Motivation: Main Standard Model process for high PT isolated lepton with missing PT is single W production Other SM process have smaller cross sections Measure single W production (~1.3pb) Search for physics beyond the Standard Model Experimental signature:

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Isolated Lepton Events with Missing pT
Look for events with isolated, high-PT lepton (e, ), missing PT and hadronic system (PTX)

e‘p

H1+ZEUS prel. e‘p e + total pTX>25GeV

Data 81 29

SM prediction 87.8‘10.6 24.0‘3.2

In general, good agreement with SM prediction

H1+ZEUS prel. e+p

Data 53 23

SM prediction 49.8‘6.2 14.0‘1.9

e+p

e + total pTX>25GeV

Excess of H1 e+p data at large PTX, small SM expectation Not confirmed by ZEUS analysis Excess remains in common phase space of combined analysis (1.9)
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Single W Production Cross Section
High purity of ~75% of W production Clear Jacobian peak Strong evidence for W production

mW Cross section measurement in common (H1,ZEUS) phase space W branching ratio of leptonic decays used to calculate full cross section Measurement done differentially as function of hadronic transverse momentum Inclusive single W production = 1.07 ‘ 0.16(stat.) ‘ 0.08(sys.) pb In good agreement with SM prediction 1.26 ‘ 0.19 pb (EPVEC at NLO)
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Multi-Lepton Production
Motivation
Main Standard Model process with multi-lepton is process QED cross section well known, modelled using GRAPE Any deviation, indication of new phenomena

Signature Results
Events with 2 or more isolated high-PT leptons (e or ) H1 and ZEUS combined results (0.94 fb-1) In general, observed number of events in good agreement with SM expectation

PT>100 GeV:

e+p data: 7 obs/1.94‘0.17 exp (excess of 2.6 ) e-p data: 0 obs/1.19‘0.12 exp
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Measurement of l l Cross Section
+-
Two-photon channels used to measure the H1 + ZEUS weighted average cross section for e and pair production

Differential cross sections lepton and invariant mass Total visible cross section good agreement with SM

measured as function of PT of leading of lepton pair 0.66‘0.03(stat.)‘0.03(sys.) pb in prediction 0.69‘0.02 pb (GRAPE)

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Search for Single-top Production
Motivation
Strongly suppressed within Standard Model (< 1fb GIM mechanism). Any observation clear indication of new physics. Single-top production through flavor-changing neutral current (FCNC) Several theories beyond the SM predict FCNC Most sensitive to tuV (charm PDF of proton small at high x) Effective anomalous coupling at t-u- or t-u-Z vertex

Leff = eet t

i q

g tu uA + t vtuZ uZ + h.c. 2 cos W


magnetic
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vector coupling
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Electroweak Physics and Searches at HERA


Search for Single-top Production
Decay modes:
Standard Model leptonic (BR 32%): t bW, W l isol.lepton, jet, pT hadronic (BR 68%): t bW, W qïq 3 jets, mW, mtop Flavor Changing Neutral Currents t u n-jets (+lepton pairs) tu vtuZ t uZ0
miss

excluded domain

new ZEUS prel



tu < 0.13

No significant excess in the signal region, upper limits are set on anomalous coupling tu
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Are Quarks Elementary?
Quark substructure can be detected by measuring spatial distribution of quark charge. If quark has finite radius, cross section will decrease as probe penetrates into it.
Rq is rms of electroweak charge
2 d d SM Rq 2 1 - = Q 2 2 dQ dQ 6 2

ZEUS
10 1.1 1.05 1 0.95 0.9 10 1
3

assuming point-like electron ZEUS: Rq < 0.62 10-18m H1: Rq < 0.74 10-18m

N/Nexp

Limit on quark size (95%CL),

ZEUS (prel.) e‘p 0.44 fb-1 2 Rq = (0.63 10-16cm)2
2 Rq = -(0.57 10
4

-16

cm)

2

10

Use similar fit for limits on contact interactions and large extra dimensions
ZEUS prel.: > 3.8 í 8.9 TeV (95%C.L.)

Quark Radius Limits (prel.)
10
3

10

4

Q2 (GeV2)

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Excited Fermions
Excited fermions would be signature of compositeness Compositeness could explain 3 families and mass hierarchy Excitation/de-excitation described by effective Lagrangian:

[f, f' and fs are the couplings to the SM gauge groups]
Example: production and decay of e*

M

e

22


Excited Fermions
H1: all possible decay channels studied. No deviation from SM expectation limits set on f/ (@ 95% CL)

e

*





q

*

f/ limits can be translated into mass limits assuming f/ = 1/Mf*

Me* >272 GeV M* >213 GeV Mq* >252 GeV (assuming fs=0)

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Leptoquarks
Leptoquarks appear in many SM extensions Couple to both electrons and quarks and carry SU(3) color, fractional electric charge, baryon (B) and lepton (L) number - Fermion number: F= 3B+L = 0, 2 LQs model are explored in Buchm?ller-R?ckl-Wyler (BRW) framework (14 different LQ types, which couple to electron) We search for LQ decaying into e-jet or -jet:

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Leptoquarks
Events / 20 GeV

Full statistics analyzed by H1 (prel. results) Search for all 14 LQ types No deviation from SM limits set on coupling at 95% CL

10

4

Events / 20 GeV

Leptoquark Search, HERA I+II

Leptoquark Search, HERA I+II

H1
e‘p (449 pb-1)

103 102 10
CC

H1
e‘p (428 pb-1)

10

3

102 10 1 100
NC

NC
H1 Data (prelim.) SM SM Uncertainty

CC
H1 Data (prelim.) SM SM Uncertainty

1
250 300 MLQ / GeV

150

200

100

150

200

250

300 MLQ / GeV

Example: Exclusion limits on scalar F=0 and F=2 LQs

HERA limits are complementary to LEP and Tevatron

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General Searches
H1 performed a model independent, generic search in final states with 2 high-PT objects: PT > 20GeV 10o < < 140o Classified by final state Standard model predictions for all HERA processes considered: NC and CC DIS, photoproduction, lepton pair production, W-production, QEDC Good agreement of event yields with SM expectation. All deviations compatible with statistical fluctuations.

e,²,jets,,

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Summary
Full HERA data (1994-2007) being analyzed Recently, combined analysis in order to improve errors/sensitivity Study of deep inelastic scattering processes NC/CC cross sections Precision of tests of Standard Model Observation of electroweak effects Combined QCD and EW fits performed Rare electroweak processes investigated Single W production cross section Lepton-pair production cross section Search for new physics Single top production Overall good agreement with the SM: Exclusion limits set, competitive to Quark radius LEP and TEVATRON Excited fermions Stringent limits on excited fermions, Leptoquarks anomalous productions General searches
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