physics at hera - desy · 2009. 8. 18. · zeus (9407 data): Λ> 3.8 8.9 tev katja krüger physics...
TRANSCRIPT
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Physics at HERA
Katja Krüger KirchhoffInstitut für Physik
H1 Collaborationemail: [email protected]
Summer Student Lectures1013 August 2009
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Overview Part 3
● Polarization– CC and Polarization
– Proton Spin Measurements by HERMES
● Exotics– Model Dependent Searches– Model Independent Searches
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Polarization
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Polarization @ HERA
● transverse polarization builds up in ~40 minutes through synchrotron radiation (SokolovTernov effect)
● spin rotators flip transverse longitudinal before experiments and back after
Pe =N RH−N LHN RHN LH
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Polarization @ HERA
spin rotator
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CC & Polarization
● CC cross section depends on longitudinal electron/positron polarization Pe
● reason: W boson couples only to lefthanded (LH) particles and righthanded (RH) antiparticles:
d 2CC±
dx dQ2Pe ≈ 1±Pe
GF2
4 x⋅ MW2MW2 Q2
2
⋅YW 2±
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CC: Polarization Dependence
● Standard Modell expectation:
● experimental result: (H1)
CC− Pe=1 = 0
CC Pe=−1 = 0
CC− 1 =−0.9±2.9stat
±1.9syst±1.9pol pb
CC −1 =−3.9±2.3stat
±0.7syst±0.8pol pb
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Electroweak Parameters: Z0 Couplings
1 0.5 0 0.5 1
1
0.5
0
0.5
1PDF (prel.)uvu ZEUSpola
total uncert. uncorr. uncert.
H1 prel. (HERA I+II 9505)
SM CDF LEP
1 0.5 0 0.5 1
1
0.5
0
0.5
1
ua
uv
68% CL
ZEUS
1 0.5 0 0.5 1
1
0.5
0
0.5
1PDF (prel.)dvd ZEUSpola
total uncert. uncorr. uncert.
H1 prel. (HERA I+II 9505)
SM CDF LEP
1 0.5 0 0.5 1
1
0.5
0
0.5
1
da
dv
68% CL
ZEUS
polarization also allows better sensitivity to vector and axialvector couplings of up and downtype quarks to the Z0
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The spin of the proton
● spin: very important quantity in quantum physics with properties of angular momentum
● spin½ paricles (fermions): – fundamental constituents of matter: quarks, leptons
– proton, neutron
● spin½ responsible for stability of matter (Pauliprinciple): „No two spin½ particles can occupy a state where all quantum numbers are identical.“
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Spin and Magenetic Moment
slides by K. Rith
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Constituent Quark Model
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Quark helicity distributions
q+(x) = q↑↑ q–(x) = q↑↓
proton
quark
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EMC result of g1(x)
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Nucleon Spin in QCD
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Asymmetry A1=g1/F1
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g1(x)
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Determination of ∆∑
∆∑ = 0.330 ± 0.025 (exp.) ± 0.011 (theory) ± 0.028 (evol.)
most precise determinationcomes from deuteron data
for comparison:EMC: ∆∑ = 0.12 ± 0.09 ± 0.14
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Determination of ∆g/g
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Nucleon Spin
● origin still unclear– ∆q cotributes ~1/3
– ∆g contribution seems to be very small➔ but very low x not yet measured
● contribution of orbital angular momentum?➔ Deeply Virtual Compton Scattering,
Generalised Parton Distributions
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Generalised Parton Distributions
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Transversity
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Exotics orBeyond the Standard Modell
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New Particles
many theories predict more particles than the SM:● SUSY:
– every Standard Model particle has a supersymmetric partner
– fermion partners are bosons, boson partners fermions
● leptoquarks– particle with lepton and quark properties
– can be produced resonantly in ep collisions
● ... exited fermions, contact interactions, large extradimensions ...
but experimentally search also modelindependent!
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Leptoquarks● can look the same as NC or CC process● MLQ2 = (xP + k)2 = xs● compare measured cross section with
SM expectation● derive limits on coupling
/ GeVLQM50 100 150 200 250 300
Eve
nts
/ 20
GeV
1
10
210
310
410
/ GeVLQM50 100 150 200 250 300
Eve
nts
/ 20
GeV
1
10
210
310
410
NC, P=27%H1 data (prelim.)SMSM uncertainty
/ GeVLQM50 100 150 200 250 300
Eve
nts
/ 20
GeV
1
10
210
310
/ GeVLQM50 100 150 200 250 300
Eve
nts
/ 20
GeV
1
10
210
310
CC, P=27%H1 data (prelim.)SMSM uncertainty
e e,,(µ)
q q,q'
LQk
xP
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Limits on Leptoquarks
100 150 200 250 300 350 400
210
110
1
/ GeVLQM
λ
d)νu, (e0,LS
u) (e0,RS
d) (e0,RS~
d)νu, d, e (e1,LS
d)νu, (e0,LS
u) (e0,RS
d) (e0,RS~
d)νu, d, e (e1,LS
d)νu, (e0,LS
u) (e0,RS
d) (e0,RS~
d)νu, d, e (e1,LS
d)νu, (e0,LS
u) (e0,RS
d) (e0,RS~
d)νu, d, e (e1,LS
Exclu
ded
at 95
% C
.L.
H1 preliminary
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SUSY● R parity violation: single SUSY particle can
be produced
● limits depend on many parameters (masses, couplings)
● example: stop
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+
Contact Interactions● New interactions at higher scale (Λ >> √s) can be
effectively described at lower energies as 4fermion eeqq Contact Interactions
● Reminder: before W and Z0 were discovered, weak interactions (Λ M≈ W) were described as 4fermion Contact Interactions with Fermi constant GF=g
2/M2W➔ Contact Interactions would modify the DIS cross section
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Contact Interactions
● No sign for Contact Interactions found
● masses much larger than √s excludedZEUS (9407 data):ZEUS (9407 data): ΛΛ> 3.8 8.9 TeV> 3.8 8.9 TeV
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General Searches
● idea: new particles have typically large mass
➔ final state should contain particles with large transverse momentum from the decay– jets
– electrons
– muons
– photons
– neutrinos (missing transverse momentum)
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General Searches
every channel in reasonable agreement with the standard model
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MultiLeptonsin HERA1 a small excess of di and trielectron events at high transverse momenta observed by H1
combined HERA (H1+ZEUS HERA1&2) data show no significant excess
first combined paper by ZEUS & H1
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Isolated Leptons and Missing PT
● spectacular events
● excess in HERA1 data at large transverse momenta of the hadronic system (PTX) seen by H1
e
X
Xe
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Isolated Leptons and Missing PT
● no excess in e– data● e+:H1+ZEUS combined:
1.9 excess● H1 alone: 2.4 excess
?