Progress Report 1:
Search for Low Mass Strings at the LHC with CMS
Andy Yen
Mentor: Harvey Newman
Co-Mentor: Marat Gataullin
I
Background/Overview
The LHC is first and foremost, a discovery machine. By being the first
accelerator to probe deep into the multi-TeV scale, it promises to revolutionize our
current understanding of elementary particles. While extraordinarily accurate, the
best current model, the Standard Model (SM) of Particle Physics, is far from
complete as there are numerous observed physical phenomena which is it unable to
explain. As many theoretical papers have hypothesized, there are plenty of reasons
to believe in the existence of new physics beyond that of the current SM. One such
alternate model is superstring theory. Superstring theory attempts explain all the
fundamental forces and particles of nature by modeling them as vibrations of tiny
supersymmetric strings. By utilizing extra degrees of freedom in the form of extra
spacetime dimensions, superstring theory also attempts to incorporate quantum
gravity to create a grand unified theory. Superstring theory became connected to
particle phenomenology with the development of the concept of the D-brane, a class
of extended objects upon which open strings can end with Dirichlet boundary
conditions. It has recently been demonstrated that signals for superstring theory
may be detected at the LHC provided the fundamental mass scale is sufficiently low
[1-2].
The mass scale (MS) of these fundamental strings may be as low as a couple
TeV provided that spacetime extends into large extra dimensions [1].
Coincidentally, this extension into extra dimensions also provides a mechanism for
solving the hierarchy problem, i.e. why the weak force is 1032 times stronger than
gravity [3]. MS gives a lower bound on the collider center of mass energy (√s) above
which Regge resonance can occur leading to the onset of string effects. As LHC is
expected to have √s=14 TeV, the condition √s > Ms is clearly satisfied. Theorists
have recently demonstrated that a consequence of TeV scale D-brane physics is that
a “new physics” signal associated with low mass scale string theory can be detected
at the LHC at 5 sigma significance for MS up to 2.3 TeV with 100 fb-1 of integrated
luminosity [2]. The “new physics” signal results from a scattering process that
occurs on the (color) U(3) stack of D-branes. Of much more consequence is the fact
that the amplitudes of this scattering are model independent. The calculations hold
for arbitrary compactifications of superstring theory ranging from four to ten
dimensions, including those that break symmetry [2]. Hence, the results that can
be obtained will be very general and valid for a wide range of superstring scenarios.
As seen in Figure 1, the string cross section increases dramatically as Ms
decreases. Thus, for a lower value of Ms around 1 TeV, we would expect
approximately 100 events for 200 pb-1 of integrated luminosity. Based on the
theoretical cross section calculations (Eqn. 1), distinct bumps in the signal invariant
mass distribution are expected for Ms < 3 TeV [5]. As these resonances are
expected to show up in the high invariant mass region where there is very little
Standard Model background, the direct photon signal from low mass strings is a
very promising early discovery channel.
Figure 1: Cross sections for gluon fusion into gamma + jet for varying Ms.
Eqn. 1
II
Progress/Challenges:
The first challenge in this project is figuring out how to generate the
simulated events that are needed to develop an analysis in the direct photon low
mass strings channel. This task is particularly difficult because the low mass string
model has not yet been incorporated into any of the existing particle physics
generators such as PYTHIA or CompHEP. In the first week, significant progress
was made in this area. By collaborating with theorists at the University of
Mississippi, University of Wisconsin Madison, and Northeastern University, we
were able to obtain a Fortran source code which allowed us to successfully interface
with PYTHIA and generate some low mass strings events for the gg→gγ signal.
Following that, the next challenge is to correctly perform the hadronization by
taking into account the non-standard color flow that results because the gg→gγ
process is not present at tree level in the Standard Model. Our discussions with the
theorists have indicated that the impact of this color flow related correction is very
small and for a first study in the LHC startup scenario, it will not significantly alter
our results. At the time of writing, the signal and irreducible standard model
background processes have been generated and hadronized in PYTHIA/CMSSW.
These events have also been successfully propagated through the CMS detector
using CMSSW and successfully outputted through the Caltech data analysis
framework (CITAnalyzer). Thus, the first CMS low mass strings simulated data
has been successfully produced.
While a lot of progress has been made much more rapidly than initially
expected, a lot of challenges remain. First, the existing theoretical papers have only
calculated differential cross sections for a small section of the parameter space. To
do a more complete analysis, we will have to add to their calculations and include a
larger minimum Pt, string mass scale, and coupling constant range. From an
experimental and detector standpoint, an analysis at CMS requires that we take
into account jet fakes and detector efficiency, something the theorists did not have
to consider in their analysis. Thus, in addition to the main irreducible backgrounds
gq→γq and qq→γg, we also need to take into account a very significant QCD dijet
background as well as other Brem processes. The effects of final and initial state
radiation will also have to be looked at. Finally, there are difficulties involving high
energy jet and photon reconstruction. The reconstruction efficiencies and sample
purity need to be examined along with their associated systematic errors.
III
Goals/Conclusion
The immediate next step is to validate and verify the simulated low mass
string events. To do this, I will examine the generator level information from the
data that we produced and see if it agrees with the result from theoretical papers.
If this generator level data is consistent, I will begin full scale production of events
with full detector simulation. I anticipate approximately 1 million signal and
irreducible background events will have to be generated for this analysis. After
this, I will commence a study of which additional backgrounds are needed for this
analysis and how large their contributions are.
Once production of all the signal and background simulated data is
completed, I will write a selection algorithm and do a bump-hunting search to try
and see if such string resonances are visible at CMS. In the event that such
resonances cannot be seen, I will use a search strategy based on trying to find an
excess of high Pt direct photon events that also contain a tagged jet. Lastly, I will
estimate the amount of integrated luminosity needed before a discovery can be
claimed. This will let us gauge whether the low mass strings scenario can be
considered an early discovery channel.
IV
References
[1] L. Anchordoqui et al., Dijet Signals for Low Mass Strings at the Large Hadron
Collider. Phys. Rev. Lett. 101, 241803 (2008).
[2] L. Anchordoqui et al., Direct Photon as probes of low mass strings at the CERN
LHC. Phys. Rev. D. 78, 016005 (2008).
[3] L. Randall, R. Sundrum. Large Mass Hierarchy from a Small Extra Dimension.
Phys. Rev. Lett. 83, 3370-3373 (1999).
[4] T. Banks, A. Zaks, Nucl. Phys. B. 206, 23 (1982)
[5] A. Roy, M. Cavagliá. String Resonances at the Large Hadron Collider.
arXiv:0902.1473v1
[6] http://root.cern.ch/