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[Assistant-faculty] Higgs
Dear all,
As you are surely all aware, High Energy Physics is in really exciting
times with the huge quantity of high quality data collected this year at
the energy frontier. Many of you asked me about rumors concerning a Higgs
discovery at the LHC, the last missing piece of the Standard Model of
particle physics and one of the justification for the building of the LHC.
Both ATLAS and CMS experiments just made their public presentations of
their most up-to-date results. I though you might be interested in knowing
the final outcomes, so here they are:
We have not found the Higgs yet. We haven't completely exclude its
existence either. Present analyses are still preliminary, but good enough
to tell us that we will need the complete 2012 dataset to make stronger
statements.
These are the official statements from the CERN DG. Now, for those
interested, I explain below in slightly more details why there is an
excitement about these results and why we can only make such open
conclusions. If you have questions, don't hesitate to pass by my office
for a chat!
I wish you all happy holidays!
Hugo
Why rumors of discovery?
-----------------------------------
A challenge-
The Higgs has low cross section compared to most of the other
Standard Model (SM) processes, and decay to various
final states with a small probability for each, so it needs a lot of
good data and clever analysis strategy in order to simply
be sensitive to its existence. Both ATLAS and CMS experiments, reach
this sensitivity with 2011 data and this is impressive!
Methodology-
Predictions for SM contributions in every single channels (final
states) in which we can find the Higgs have been made for
each experiments and compared to observations. Potential deviations
between the predictions and the observations are
compared to Higgs expectation for various Higgs mass hypothesis. The
absence of a deviation results in an exclusion of the
Higgs in the mass ranges for which the Higgs signal would be
sufficient. Excesses consistent with Higgs expectation in a
mass range for which the cross section is substantial would support a
discovery hypothesis. A claim for a Higgs discovery
can be made, if (this is somewhat arbitrary, but generally
accepted in the community):
-Data feature an excess of 5 standard deviation over SM expectations and
in agreement with total Higgs expectations
-Each of the channel in which the number of expected Higgs event is
sufficient should present an individual 3sigma excess
-All excesses are in the same mass window (same Higgs mass hypothesis)
Results-
Both ATLAS and CMS have been able to exclude the Higgs for a very
wide range of the Higgs mass hypothesis.
According to ATLAS data analyses: A Higgs is excluded at 95% CL if its
mass is: 131 GeV < MH < 600 GeV
According to CMS data analyses: A Higgs is excluded at 95% CL if its
mass is: 127 GeV < MH < 600 GeV
These are the official preliminary results. So why is there an excitement?
What the ATLAS-CMS difference means?
This is well answered when looking at expectations, i.e. How much we would
a priori be able to exclude the Higgs
if we assume that no Higgs is there and that data follows our predictions
without statistical fluctuations:
ATLAS expectations: Exclude the Higgs at 95% CL if its mass is: 125 GeV
< MH < 520 GeV
CMS expectations: Exclude the Higgs at 95% CL if its mass is: 117 GeV <
MH < 545 GeV
So the excitement comes from the fact that in both experiments, the
observed exclusions are worse than the
expectations because excessed were observed! In ATLAS, a total excess of
3.6 sigma with respect to SM
expectations has been observed, while in CMS, the excess if of 2.6 sigma.
In both cases, the largest deviations
lie at a mass around 124 to 126 GeV. The fact that both experiments
observed similar excesses in similar mass
range is responsible for the excitement… and it is very exciting!
Why only an open conclusion?
-------------------------------------------
Many caveats come with the above results and these caveats refrain both
collaboration from making a discovery
claim.
1- Statistical significance:
The statistical significance of the excesses is too low for a
discovery. Data can fluctuate at a 2.6 sigma level
with a probability of 0.5%. This is frequent. Also, given the wide
range of channels and
masses considered, the probability to observe a 2.6 sigma deviations
is bigger. If we include global effects (trial
factors), the deviation drops to 1.9 sigma in CMS and 2.4 sigma in
ATLAS. There is 3% of the chances of a
fluctuation of SM data to accommodate the CMS excess. This is far too
large for claiming a discovery. (Note
However that ATLAS and CMS results haven't been combined yet).
2- Many channels:
The above numbers are for the sum of many various channels that all
contribute only a little to the final
excess. Most of these channels (WW, tautau, bb+W/Z) don't even
concentrate excesses in 1 or 2 close
mass bins, but spread excesses through various Higgs mass
hypotheses. They contribute to between 1 to
1.4 sigma which is of small significance. The ZZ channel is a really
clean signal (golden discovery channel)
as there are few SM events that involve 4-charged leptons in the
final state with invariant masses of 2 Zs
combined to give the Higgs mass. Only few excess events were
observed in the mass range of 120-130
GeV (3 in ATLAS, 2 in CMS), for another small significance. Finally,
the gamma-gamma channel is the weaker
channel, but both experiments are well tooled to study it (high
photon energy resolution, high gamma-Pi0
Separation, etc). In both experiments, the largest contribution to
the deviation come from this channel.
In ATLAS, gamma-gamma channel yield, alone, a 2.6 sigma deviation.
Basically, we see that
one relatively large excess in a peak shape get enhanced by small
insignificant excesses at same mass
hypothesis. A discovery would require significant excess (3 sigma?)
from each channel. This is also
Important to confirm we found a Higgs and not some other new states.
This will take a lot of time!
3- Other excesses:
Masses down to 131 GeV have been excluded by both experiments but
excesses were nevertheless
observed at higher masses. For example, the largest deviations in the
ZZ channel in ATLAS is 2.5 sigma
at MH=240 GeV. The deviation at 125 GeV is only 1.4 sigma. Small
excesses observed in the mass range
where the larger gamma-gamma excess has been observed are therefore
not unique and populate
excluded mass regions too. These small additions around 125 GeV might
be simple statistical boost.
4- Too high signal in gamma-gamma:
In contrary to the other channel, the excess in the gamma-gamma
channel is too high to be only due to
a Higgs itself. This channel has the lowest production rate (times
decay probability) and expectations of
excesses are small. There are therefore either due to a large Higgs
fluctuation contributing to the data,
to a large SM contribution to a smaller Higgs signal fluctuation, or
it could even be due to another type
of new particle responsible for such excess (non-standard Higgs???,
way too early to speculate). A Higgs
discovery must agree with Higgs expectations. Here it does not. In
CMS, they checked what would be
their excess in this channel if they were eliminating all the
sub-channels with less good precision and keep
only their good central-central channel. They found an excess
compatible with SM expectations, but that
would by the same token reduce the overall significance of the
excess. In ATLAS, the uncertainty band just
covers the Higgs expectation too. In both cases, their final results
are nevertheless still dominated by a
contribution higher than what the Higgs in the gamma-gamma channel
would give.
All these reasons contribute to refrain the collaborations of saying that
they found the Higgs. However, the
excesses are there, similar for both experiments and are responsible for
worse exclusions than expected. The
Higgs is therefore alive, and next year, we should be able to close this
discussion! Stay tuned…
----------------------------------------------------------------
Prof. Pierre-Hugues Beauchemin
Department of Physics & Astronomy
Tufts University
4 Colby Street,
Medford, MA, 02155
Phone: (617) 627-2968
Fax: (617) 627-3744