Broken Symmetry: W, Z and Higgs Bosons at D0

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Broken SymmetryW, Z and Higgs Bosons at D0
The Collider Experiments High Energy Group
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Outline

• Physics introduction
• W mass measurement
• Higgs boson searches
• Our group
• What do grad students do?
• Atlas or D0. Which is right for you?

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Whats the Physics Motivation?
The goal of particle physics is to understand the
universe at the smallest distances and,
equivalently, the highest energies.
And now a (partial) picture, the standard
model including an H (not shown)
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Whats the Physics Motivation?
Many talks begin with The standard model is
extremely successful, and it is.
Interestingly, the SM is a gauge theory and
therefore has matter particles (fermions)
interacting via force carrier particles
(gauge bosons g, W, Z). and minimal gauge
theories require the bosons to be
massless, e.g. mg MW MZ 0. but we know
from experiment, mg 0
MW 80.399 - 0.025 GeV, MZ
91.1876 - 0.0021 GeV ? electroweak
symmetry is broken (EWSB)
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How is this dealt with?
Introduce a new scalar field (particle) into the
theory. When put in properly, gauge invariance
is maintained, and (some) bosons must get
mass. This is the Higgs mechanism nice
theory, but the Higgs boson has not been
found! Is it right? We are working on two areas
directly related to understanding this symmetry
breaking and the Higgs mechanism 1.
Measuring the W boson mass, 2. Searching
for the Higgs
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W mass measurements

• In the absence of Higgs discovery
• Use internal consistency of the S.M. to constrain
  mH. Biggest uncertainty from Mt, MW

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And these constraints give
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Measuring the W mass

• Experimental issues W-gt ev
• Its true that, M2 (EeEv)2 (pepv)2, but..

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Measuring the W mass

• We cannot measure neutrino. Infer from momentum
  conservation, but
• We cannot constrain pv along beam
• Protons are bags of quarks, not fundamental
• Instead measure pTe, pTv, mT and infer MW
• We cannot analytically predict these
  distributions, need a simulation
• Much of the work goes into developing simulation
• Use data control samples to calibrate

T subscript means perp. to beam
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Luckily, theres the Z boson

• Z-gtee, so can measure mass directly
• Use this to calibrate our simulation

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But many other effects

• Trigger
• e identification
• What happens with W pT? (recoil)
• Beam luminosity increasing
• W (and Z) production details
• Bremstrahlung

This is a measurement at roughly 1 part in 5000.
Everything must be done extremely thoroughly
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Does the W pT matter?
g
The recoil resolution and model affects the pTv
and mT variables!
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We can test our simulation using Zs
The Z pT is very well measured from the ee pair.
For W, we cannot do this and must use rest of
the detector. This plot compares the rest in
Z events to the well measured ee pair in Z
events
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Measuring the W mass

• Having tuned up the simulation, what do we see?

http//www-d0.fnal.gov/Run2Physics/
WWW/results/prelim/EW/E27/
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Measuring the Mass

• Our brand new result
• Uncertainties

MW 80.401 - 0.044 GeV
The worlds best measurement
Limited by stats in control samples!
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Measuring the Mass

• Where to now?
• Beginning next round with 4x the data.
• Finish in lt1 yr if all goes well.
• Then on to the final version with full data, at
  least another 2x improvement
• Finish in 2012/13?
• Expect to share the worlds best measurement for
  10 years (or forever?)
• Room for a student to work on this. A great
  thesis topic!

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But, really, just Find the Higgs!

• or whatever is responsible for EWSB
• This is a major component of the current D0
  research program
• Many people, but still many opportunities
• Unlike W mass, Higgs is very low S/B
• The whole issue is needles in haystacks
• Better look in all the possible haystacks
• What is already known?

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What do we know?
Reminder from earlier mH gt 114 GeV, but also
probably mH lt 160 GeV (or so)
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At D0 Lots of stuff in the way
BF
Rates for different processes
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And then the Higgs
WH e/mn bb tn bb qq tt e/mn W(e/m)W(e/m)
ZH ee/mm bb nn bb tt bb qq tt ttH lnb
qqb bb gg?H W(e/m)W(e/m) gg tt ( 2
jets) WW ?H tt ( 2 jets)
Two regimes mH lt 135 GeV H-gtbb, needs
additional info mH gt 135
GeV H-gtWW/ZZ, stands alone
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Low Higgs Mass, mH lt 135 GeV
EVENT DISPLAY

• Cannot get sufficient S/B with only Higgs, so add
  something else at a price in rate

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Low Mass (cont)

• Use the W or Z as a tag to reject background
• W-gtlv or Z-gtll, vv
• Look for lepton, or missing energy or both,
• and then also for the Higgs decay
• But dont see bb quarks.
• See jets, or streams, of particles
• Reconstruct Higgs mass (limited res.)

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Mass Reconstruction (cont)
Do you see the Higgs? I dont
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Low Mass

• Use advanced computing techniques
• H matrix, neural nets, boosted decision trees,
• As well as physics insight
• Better resolution (Strauss)

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High Mass, mH gt 135 GeV

• Here, H-gtWW, then W?lv with le,m
• Very low backgrounds
• primarily straight WW from SM
• Different spin structure, so use angles, pTs
• Very low rate, so need efficiency!

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High Mass (cont)
But its still difficult!
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High Mass (cont)
So, again, be smarter
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And where we stand now
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What next?

• Will continue to add data. 3x more?
• And improve analysis techniques

We are doing much more than simply adding data.
Getting smarter all the time
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An aside about Fermilab D0

• Fermilab Tevatron
• ppbar accelerator
• ECM 1.96 TeV
• 60 through running
• Near Chicago

• D0
• detector at FNAL
• broad purpose HEP program 600 collabs.

Both Tevatron and D0 are running very well
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1/5 of official D0
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Our SBU D0 Group

• People
• 3 faculty
• Grannis, Hobbs,
• McCarthy, Rijssenbeek
• 2 post docs (long term)
• 2-4 graduate students

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Students?

• Typically, reside at FNAL
• After 2 years for classes
• 3 years for thesis
• Technical work
• shifts, computing, detector hardware
• Thesis analysis
• Start with a small, self-contained study
• and apply it to an analysis and complete the full
  analysis on a given data set

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Students?

• Analysis groups include collaborators at other
  institutions (e.g. W mass)
• so although D0 is big, really work with 4-10
  people on a daily basis.

About eight of these folks are finishing
working on this topic (graduating, new job,
other exp.)
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Recent History

• Weve had or are about to have
• 6 theses on Higgs (or related topics)
• Zdrazil, Mutaf, Dong, Desai, Herner, Strauss
• 2 theses on W mass
• Guo, Guo
• And both topics are going strong at D0 for
  another 3-4 yrs.

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SBU Atlas or D0

• Both are very interesting, and faculty in both.
  Which one?
• Personal choice

Why Atlas (see earlier talk)? the
up-and-coming thing brand new, so learn how
detector really works highest energies
ever, so good discovery potential. live
in France/Switzerland
Why D0? long-term important science running
very well! guaranteed timeline
interested in detailed work at a mature
exp. stay in U.S.
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Summary

• The D0 experiment is doing fundamental work, and
  SBU towards EWSB study
• W boson mass
• Higgs search
• to illuminate basic issues at the interface of
  theory and experiment
• Do gauge theories really work as we think?
• What is the structure of matter and the
  interactions that govern it?