Physics 7802.01 Introduction

Physics 7802.01 Introduction

P780.02 Spring 2003 L12 Richard Kass Weak Interactions & Neutral Currents Until the the mid-1970s all known weak interaction processes could be described by the exchange of a charged, spin 1 boson, the W boson. The initial and final state quarks or leptons differed by one unit of electric charge. Weak interactions mediated by a W-boson are called charged current interactions. A key prediction of the Glashow-Weinberg-Salam model was the existence of weak interactions mediated by the Z 0, a neutral vector boson. In the GWS model the Z0 did not change the flavor of the lepton or quark. Recall: the GIM mechanism was invented to eliminate (first order) neutral current reactions where the flavor of the quark or lepton changed. The measured branching fractions of K0+- and K++e provided evidence for the absence of strangeness changing neutral currents. M&S CH 9.1 BR ( K 0 ) 7 10 9 10 8 0.64 BR( K ) + Charged current u - + W+ strangeness changing neutral current ??0 s d s P780.02 Spring 2003 L12 Richard Kass Weak Interactions & Neutral Currents

The GIM mechanism eliminated strangeness changing neutral currents by adding a fourth quark (charm): sin d d cos s sin c d d ?? 0 s s ??0 c cos c s If we add the two amplitudes together we get: d d d d cos 2 c s s sin 2 c [d s s d ] cos c sin c s s d d sin 2 c s s cos 2 c [d s s d ] cos c sin c d d s s d d s s The strangeness changing terms are gone but there are still neutral current terms! In 1973 the first experimental evidence for neutral currents was found using the reaction: e e This reaction cannot occur by W exchange! P780.02 Spring 2003 L12 Richard Kass How do we produce a neutrino beam? Where do the neutrinos come from ? Proton-nucleon collisions produce lots of 's and K's which decay into neutrinos. s and K's and K's which decay into neutrinos. s which decay into neutrinos. Neutrino Beam Anti-Neutrino Beam Branching Ratio 0.999 ee ee 10-4 K K 0.63

Kee Kee 10-5 K K 0.03 Kee Kee 0.05 ee Also get neutrinos from decay: muon ee have branching ratios of 100%. Rough calculation of /e ratio: a) neglect neutrinos from muon decay. b)# assume many pions in aproton-nucleus ( we produce ) (K 10X ) as 10BR ( as) kaons BR(K ) BR( K ocollision. ) # e #

o ( e ) (K e ) 10BR( e e ) BR(K e e ) BR(K e e ) 10 0.999 0. 63 0. 03 10. 7 210 # e 10 10 4 10 5 0. 05 . 05 Relatively easy to make a beam of high energy 's and K's which decay into neutrinos. s, but hard to make a "pure" beam of high energy e's and K's which decay into neutrinos. s. To make a pure beam of e's and K's which decay into neutrinos. s use nuclear beta decay which is below the energy threshold to produce muons. P780.02 Spring 2003 L12 Richard Kass Neutrino Induced Neutral Current Interactions Many other examples of neutral current interactions were discovered in the 1970s W exchange (charged current): Z exchange (neutral current): +N -+X +N +X Neutrino cross section for neutral currents predicted to be 1/3 of charged current cross section . Typical layout for a neutrino beam line. Observation of Neutrino-Like Interactions Without Muon Or Electrons in the Gargamelle Neutrino Experiment PL, V45, 1973. CC= charged current event (e or in final state) NC= neutral current event (no e or in final state) Gargamelle was the name of the bubble chamber (BC). Expect neutrino interactions to be uniformly distributed in BC. Background from neutrons and K0s expected mostly in front of BC. Data in good agreement with expectation of Standard Model. NC 1 20 4 2 sin W sin W 0.21 0.03 CC 2 27 NC 1

20 4 2 sin W sin W 0.45 0.09 CC 2 9 expect : NC NC CC CC Distributions along beam axis P780.02 Spring 2003 L12 Richard Kass Weak Interactions & Neutral Currents By 1983 CERN was able to produce real Zs and Ws and measure their properties: mass, branching fractions, and decay distributions. By 1989 accelerators that could produce Z 0s via e+e-Z were online: SLC @ SLAC and LEP @ CERN By 1999 CERN could produce Ws via e+e-W+W-. The model of Glashow, Weinberg and Salam becomes the Standard Model. In the standard model the coupling of the Z to quarks and leptons is different than the coupling of the W to quarks and leptons. The W has a V-A coupling to quarks and leptons: The Z coupling to quarks and leptons is much more complicated than the Ws: cvfcafI3fI3f Qfsin2W I3 is the third component of weak isospin W 28 degrees f=fermion type M&S 9.2.1, C.6.3 W is the Weinberg angle The Weinberg angle is a fundamental parameter of the standard model. It relates the strength of the W and Z boson couplings to the EM coupling: gEM=gWsinW gEM=gZsinWcosW M&S use gEM=gZcosW, eq. 9.8. However, this is NOT the factor that would be used to describe coupling of the Z boson to fermions described on the next slide. The correct factor is g EM=gZsinWcosW. P780.02 Spring 2003 L12 Richard Kass Weak Interactions & Neutral Currents The standard model gives us the following for the Z boson coupling to a fermion anti-fermion pair: cvfcafI3fI3f Qfsin2W f fermion (f) cv ca I3

Q gZcvfcaf e, , 1/2 1/2 1/2 0 e, , -1/2+2sin2W -1/2 -1/2 -1 u, c, t 1/2-4/3sin2W 1/2 1/2 2/3 d, s, b -1/2-2/3sin2W -1/2 -1/2 -1/3 In the standard model the masses of the W and Z are related by: MW=MZcosW 2 1/ 2 2 gW MW GF 2 2GF sin W 1/ 2 37.4 GeV/c2 sinW f Z0 First order calculation Interesting new development (2001) with measurement of sin2W. World average for sin2W =0.22270.0003 (mainly from LEP @ CERN) NuTeV result for sin2W = 0.22770.0013(stat) 0.0009(syst) NuTeV is a Fermilab neutrino experiment. Their result for sin2W disagrees with previous measurements, may point to physics beyond standard model.

Recently Viewed Presentations

  • Henri Rousseau  To learn about the life and

    Henri Rousseau To learn about the life and

    The starter for this next lesson was a thinking skills "bridge map" where they had to find a synonym for the 5 items featured. (These were all provided by the identifying key words activity at the end of the previous...
  • GOES-R Program PDR Brief Template

    GOES-R Program PDR Brief Template

    GOES-R Calibration Definition. Calibration: The process to determine factors for converting and correcting raw detector measurements into science data units (e.g., radiance) with the specified level of accuracy.
  • Embryology and Histology of Thyroid and Parathyroid glands

    Embryology and Histology of Thyroid and Parathyroid glands

    pharyngeal pouches endoderm, initially hollow and then solidifying with cell proliferation. Interestingly, the inferior parathyroid originates from the third pharyngeal pouch and the superior arises from the fourth pharyngeal pouch. The adult anatomical position is the opposite of the fetal...
  • COLUMBUS STATE COMMUNITY COLLEGE Presidents Cabinet Strategic Planning

    COLUMBUS STATE COMMUNITY COLLEGE Presidents Cabinet Strategic Planning

    Current Environment Assessment Dr. Harrison's Environmental Scan Interviews with 12 local business, community and educational leaders National studies and initiatives Achieving the Dream Reclaiming the American Dream (AACC) Time is the Enemy (Complete College America) A Stronger Nation through Higher...
  • CS412 Computer Networks

    CS412 Computer Networks

    CS 313 Introduction to Computer Networking & Telecommunication DSL and Cable Chi-Cheng Lin, Winona State University * Topics Digital Subscriber Line Cable ADSL versus Cable * Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL.
  • The Triple Aim - Family Voices of CA

    The Triple Aim - Family Voices of CA

    Bridging The Gap:Transition from Pediatric to Adult Care For Young Adults with Childhood Onset Chronic Disease. Build and support self-management skills. T. eam-up providers to match care to changing patient needs. G. uide patients & families through service changes to...
  • Industry Sponsorships Highlight your company's name as a

    Industry Sponsorships Highlight your company's name as a

    Industry Sponsorships Highlight your company's name as a leader in Language Technologies. GOLD SILVER BRONZE Sponsor a specific event: Opening Reception
  • Rock Detectives - Science Spot

    Rock Detectives - Science Spot

    Get a piece of "earth" and glue on page ____ Find the other students in your class with the same piece. Examine the picture to identify key features for clues to its formation and history.