Broken Symmetry: W, Z and Higgs Bosons at D0 The Collider Experiments High Energy Group.
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Transcript of Broken Symmetry: W, Z and Higgs Bosons at D0 The Collider Experiments High Energy Group.
Broken Symmetry:W, Z and Higgs Bosons at D0
The Collider Experiments High Energy Group
Outline
• Physics introduction• W mass measurement• Higgs boson searches• Our group• What do grad students do?
– Atlas or D0. Which is right for you?
What’s 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)
What’s 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: , W, Z).
and minimal gauge theories require the bosons to be massless, e.g. m = MW = MZ = 0.
but we know from experiment, m = 0 MW = 80.399 +- 0.025 GeV, MZ = 91.1876 +- 0.0021 GeV
electroweak symmetry is broken (EWSB)
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
W mass measurements• In the absence of Higgs discovery
– Use internal consistency of the S.M. to constrain mH. Biggest uncertainty from Mt, MW…
W Wt
b
W WH
W
In SM, MW/MZ = cosW
sinW = f(GF,aEM,MZ,R) R = c1(Mt/MZ)2 + c2ln(MH/MW)
And these constraints give
• Experimental issues: W-> ev
– It’s true that, M2 = (Ee+Ev)2 – (pe+pv)2, but..
Electron
MET
Measuring the W mass
Measuring the W mass
– We cannot measure neutrino. Infer from momentum conservation, but
– We cannot constrain pv along beamProtons are bags of quarks, not
fundamental…Instead measure pT
e, 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
Luckily, there’s the Z boson
• Z->ee, so can measure mass directly– Use this to calibrate our simulation
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
Does the W pT matter?
g
The recoil resolution and model affects the pT
v and mT variables!
We can test our simulation using Z’s…
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…
Measuring the W mass • Having tuned up the simulation, what
do we see?
http://www-d0.fnal.gov/Run2Physics/ WWW/results/prelim/EW/E27/
Measuring the Mass
• Our brand new result:
• Uncertainties:
MW = 80.401 +- 0.044 GeV
Limited by stats in control samples!
The world’s best measurement
Measuring the Mass
• Where to now?– Beginning next round with 4x the data.
Finish in <1 yr if all goes well.
– Then on to the final version with full data, at least another 2x improvementFinish in 2012/13?
• Expect to share the world’s best measurement for 10 years (or forever?)– Room for a student to work on this. A
great thesis topic!
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?
What do we know?
Reminder from earlier mH > 114 GeV, butalso probably mH < 160 GeV (or so)
At D0: Lots of stuff in the way…
BF
Rates for different processes
And then the HiggsWH: e/ bb
bbqq’ e/ W(e/)W(e/)
ZH: ee/ bb bb bbqq
ttH: lb qq’b bb
gg→H: W(e/)W(e/)gg (+ 2 jets)
WW →H: (+ 2 jets)
Two regimes: mH < 135 GeV: H->bb, needs additional info mH > 135 GeV: H->WW/ZZ, stands alone
Low Higgs Mass, mH < 135 GeV
• Cannot get sufficient S/B with only Higgs, so add something else at a price in rate
EVENT DISPLAYq
q
W/Z
W/Z
H bb
l, l, , l,
time
Low Mass (con’t)
• Use the W or Z as a “tag” to reject background:– W->lv or Z->ll, vv– Look for lepton, or missing energy or
both,
• and then also for the Higgs decay– But don’t see bb quarks. – See “jets”, or streams, of particles– Reconstruct Higgs mass (limited res.)
Mass Reconstruction (con’t)
Do you see the Higgs? I don’t…
Low Mass• Use advanced computing techniques
– H matrix, neural nets, boosted decision trees, …
• As well as physics insight– Better resolution (Strauss…)
High Mass, mH > 135 GeV
• Here, H->WW, then Wlv with l=e,– Very low backgrounds
primarily straight WW from SMDifferent spin structure, so use
angles, pT’s
– Very low rate, so need efficiency!
q
q
t
t
HtW
W
l
l’,
High Mass (con’t)
But it’s still difficult!
High Mass (con’t)
So, again, be smarter
And where we stand now
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
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…
1/5 of “official” D0
Our SBU D0 Group
• People– 3+ faculty
Grannis, Hobbs, McCarthy, Rijssenbeek
– 2+ post docs (long term)– 2-4 graduate students
Not shown: RM, MR
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
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.)
Recent History
• We’ve had or are about to have– 6 theses on Higgs (or related topics)
Zdrazil, Mutaf, Dong, Desai, Herner, Strauss
– 2 theses on W massGuo, Guo
• And both topics are going strong at D0 for another 3-4 yrs.
SBU: Atlas or D0
• Both are very interesting, and faculty in both. Which one?– Personal choice
Why D0? long-term important science running very well! “guaranteed” timeline interested in detailed work at a mature exp. stay in U.S.
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
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?