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PHYS 5326 – Lecture #5 QCD Evolution of PDFs Measurement of Sin2qW

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1 PHYS 5326 – Lecture #5 QCD Evolution of PDFs Measurement of Sin2qW
Wednesday, Jan. 29, 2003 Dr. Jae Yu QCD Evolution of PDFs Measurement of Sin2qW Formalism of Sin2qW in n-N DIS Improvements in Sin2qW Interpretation and Link to Higgs Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

2 Factorization } s=f*sp nm,(`nm) k k’ m-, (m+) W+(W-) pm, qm EHad P
q=k-k’ q, (`q) xP Partonic hard scatter sp Non-perturbative, infra-red part f Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

3 DGLAP QCD Evolution Equations
The evolution equations by Dokshitzer-Gribov-Lipatov-Altarelli-Parisi provide mechanism to evolve PDF’s to any kinematic regime or momentum scale Pij(x/y): Splitting function that is the probability of parton i with momentum y get resolved as parton j with momentum x<y Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

4 Feynman Diagrams for Parton Splitting
LO: O(as) NLO: O(as2) Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

5 Electroweak Threory Standard Model unifies Weak and EM to SU(2)xU(1) gauge theory Weak neutral current interaction Measured physical parameters related to mixing parameters for the couplings Neutrinos in this picture are unique because they only interact through left-handed weak interactions  Probe weak sector only Less complication in some measurements, such as proton structure Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

6 sin2qW and n-N scattering
In the electroweak sector of the Standard Model, it is not known a priori what the mixture of electrically neutral electomagnetic and weak mediator is This fractional mixture is given by the mixing angle Within the on-shell renormalization scheme, sin2qW is: Provides independent measurement of MW & information to pin down MHiggs via higher order loop corrections, in comparable uncertainty to direct measurements Measures light quark couplings  Sensitive to other types (anomalous) of couplings In other words, sensitive to physics beyond SM  New vector bosons, compositeness,n-oscillations, etc Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

7 Higher Order Corrections
LO GSW requires three parameters: a, GF and MZ Higher order corrections bring in dependence two additional parameters: MTop and MHiggs n m W t q q’ `b n Z t `t q aW aW aW aW aW aW aW aW Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

8 How is sin2qW measured? Cross section ratios between NC and CC proportional to sin2qW Llewellyn Smith Formula: Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

9 Original Experiments Conventional neutrino beam from p/k decays
Focus all signs of p/k for neutrinos and antineutrinos Only nm in the beam (NC events are mixed) Very small cross section  Heavy neutrino target ne are the killers (CC events look the same as NC events) Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

10 How Can Events be Separated?
Event Length x-view y-view Nothing is coming in!!! Charged Current Events m x-view y-view Nothing is coming in!!! Nothing is going out!!! Neutral Current Events Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

11 Experimental Variable
Define an Experimental Length variable Distinguishes CC from NC experimentally in statistical manner Compare experimentally measured ratio to theoretical prediction of Rn Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

12 Past Experimental Results
The yellow band represents a correlated uncertainty!! Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

13 Improvements on Measurements
Asses the uncertainties from previous measurements Determine what the sources of largest theoretical and experimental uncertainties are Provide new methods to reduce large uncertainties Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

14 sin2qW Theoretical Uncertainty
Significant correlated error from CC production of charm quark (mc) modeled by slow rescaling mechanism Suggestion by Paschos-Wolfenstein by separating n and`n beams: Reduce charm CC production error by subtracting sea quark contributions Only valence u, d, and s contributes while sea quark contributions cancel out Massive quark production through Cabbio suppressed dv quarks only Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

15 Homework Assignments Draw a few additional Feynman diagrams for higher order GSW corrections to n-N scattering at the same order as those on pg 7 in this lecture Due: One week from today, Wed., Feb. 5 Wednesday, Jan. 29, 2003 PHYS 5326, Spring 2003 Jae Yu

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